HSLA Steels 2015, Microalloying 2015 & Offshore Engineering Steels 2015

During the last decade, the adjustment and upgrade of steel product structures always be very important tasks in China’s iron and steel industry. Since there is a fast growth of steel production in China, a series of research achievements in the technology area of HSLA steels have been made and applied successfully in the actual production, and thereby promoted a rapid development and application of China’s HSLA Steel products. However, The China’s iron and steel industry is now facing the excess production capacity and under pressure from respects of resource, energy and environment, therefore, it would be an effective way to realize the sustainable development in China’s iron and steel industry by strengthening the applications of HSLA steels continuously and positively.

The role of microalloying has changed over the years as the range of products where used has expanded and the production processes have improved. When used in the early ferrite-pearlite steels, its chief function was grain refinement and precipitation hardening. Later, as the quest for higher strength led to bainitic and martensitic microstructures, the role of microalloying changed to grain refinement and transformation control. Later still microalloying has been successfully applied in martensitic and the advanced high strength steels grades for automobiles. The goal of this paper is to follow this changing role of microalloying in both conventional and advanced steels from the 1960s until today.

Rails generally do not have a homogeneous austenite grain structure across their sections because the degree of plastic strain achieved during hot-rolling depends on location. Here we explore a philosophy in which niobium microalloying may be introduced in order to thermomechanically process the material so that pancaked and refined austenite grains may eventually be achieved in the critical regions of the rail. The essential principle in alloy design involves the avoidance of coarse niobium carbide precipitates in the regions of the steel that contain chemical segregation caused by non-equilibrium solidification. Both pearlitic and cementite-free bainitic rails have been studied. The work is of generic value to the design of high-carbon microalloyed steels.

This paper presents the advantages of replacing mild steel with high strength niobium microalloyed steel in the structure of Electric Super Scooters, Electric Cargo Motorcycles and Solar Charging Stations. The Mini-Fleet-in-a-Box concept was developed by Current Motor to guarantee mobility, efficiency and solar generated electricity. With the adoption of niobium microalloyed high strength steel for more than 90% of the Super Scooter and Motorcycle structures, it was possible to redesign the frame, resulting in a 31% weight reduction and a very modern and functional body. Together with a new powertrain, these changes were responsible for increasing Motorcycle autonomy by more than 15%, depending on average speed. The new frame design reduced the number of high strain points in the frame, increasing the safety of the project. The Solar Charging Station was built using the container concept and designed with high strength niobium microalloyed steel, which resulted in a weight reduction of 25%. CBMM’s facility in Araxá, Brazil was selected in the second half of 2013 as the demonstration site to test the efficiency of the Super Scooter and Solar Charging Station. Each Super Scooter has run more than 2,000 km maintenance-free with an autonomy of more than 100 km per charge.

In the four decades since the Conference Microalloying ’75 was held in Washington, DC, there has been an avalanche of development and application in almost all steel products but most notably in automotive, plate and linepipe steels. Revolutions in steelmaking, casting, rolling and accelerated cooling have all contributed to the adoption of this versatile, economical class of steel. Today it is estimated that fully seventeen percent of all steel produced relies on microalloying additions and related metallurgical technology.This paper describes and summarizes four decades of evolution of HSLA steels since 1975 with special reference to linepipe and plate.

Two major drivers for the use of newer steels in the automotive industry is fuel efficiency and increased safety performance. Fuel efficiency is mainly a function of weight of steel parts, which in turn, is controlled by gauge and design. Safety is determined by the energy absorbing capacity of the steel used to make the part. All of these factors are incentives for the U.S. automakers to use both Highly Formable and Advanced High Strength Steels (AHSS) to replace the conventional steels used to manufacture automotive parts in the past.Highly Formable Steels are generally ultra-low carbon steels fully or partially stabilized by micro-alloying elements such as Ti or Nb. AHSS is a general term used to describe various families of steels. The most common AHSS is the dual-phase steel that consists of a ferrite-martensite microstructure. These steels are characterized by high strength, good ductility, low tensile to yield strength ratio and high bake-hardenability. Another class of AHSS is the complex-phase or multi-phase steel which has a complex microstructure consisting of various phase constituents and a high yield to tensile strength ratio. Transformation Induced Plasticity (TRIP) steels is another class of AHSS steels finding interest among the U.S. automakers. Finally, martensitic steels with very high strengths are also in use for certain parts. The most recent initiative in the area of AHSS is the so-called “3rd Generation” AHSS. These steels are designed to fill the region between the dual-phase/TRIP and the Twin Induced Plasticity (TWIP) steels with very high ductility at strength levels comparable to the conventional AHSS. Premier among this new class of AHSS is the Quenching and Partitioning (Q&P) steels. These steels offer higher ductility than the dual-phase steels of similar strengths or similar ductility as the TRIP steels at higher strengths. Enhanced Q&P steel is one method to achieve this target. Other ideas include TRIP assisted dual phase steels, high manganese steels and carbide-free bainitic (CFB) steels. Finally the post hardened steels (PHS) are an important component of the strategy of future vehicles.Niobium is an integral part of the design of most of these new steels. In this paper, some of the above families of advanced high strength steels with micro-alloying additions, utilized in the automotive industry will be discussed.

Reforma 509 is a high-rise building located in the heart of the Central Business District of Mexico City. The building is comprised of office, hotel, residential and parking and forms part of a cluster of tall buildings in the area. If completed today, Reforma 509 would be the tallest building in Mexico, at 238m. All of the building’s gravity and lateral (wind and seismic) loads are carried by an architecturally expressed perimeter frame that is formed from highly efficient Steel Reinforced Concrete (SRC) columns coupled together by steel tube perimeter bracing. This paper investigates the implications of substituting a grade 50 (fy=345 MPa) carbon steel with a higher strength micro-alloyed grade 70 (fy=480 MPa) steel in the design of Reforma 509.This paper has shown that potential savings in material are possible if high strength steel is economically viable and can be competitively tendered in both the primary frame and connections.

A survey is given on the various reasons for microallying of engineering steels. In case hardening steels microalloying improves toughness and fatigue properties mainly due to a smaller and more homogeneous prior austenite grain size. Niobium additions enable higher process efficiency by high temperature carburizing. In forging steels microalloying controls the phase transformation during cooling after forging and enables shorter process routes.

Research efforts in the development of high strength steels during the past 4 decades have led to a bundle of new steels which provide not only high strength to the designer, but also have excellent toughness, low transition temperature behavior in view to cold climate application and good weldability. However, application of such modern steels in many cases seems to be restricted which means that advantages resulting from light weight design such as lower working costs, lower transportation weights and lower energy consumption cannot be fully realized. Reasons for that are manifold and can either be technically justified (e.g. for highly fatigue loaded structures as bridges) or may result from lack of design knowledge, fabrication knowledge with respect to weldability and heat treatment, a diffuse understanding of the toughness issue and finally lack of appropriate standards.To overcome such obstacles worldwide research has been performed on modern structural integrity concepts and weldability of HSLA steels for long time. Taking account of such research, which has led to practical welding aspects for such steels (such as t8/5 concept), it has become possible to produce safe weldments and to derive toughness requirement in a realistic and safe way.As a result of such efforts, new toughness oriented design methods have been included in European design rules like EN 1993–1–10 (Eurocode 3) and EN 13445 (unfired pressure vessels). They provide better opportunities for the application of modern fine grain high strength steel within the legal frame of such highly safety demanding public areas.The contribution will reflect upon this development with respect to a discussion of the material properties, the effect of fabrication on the latter and the effect of both on the necessary design assumptions. Specifically the derivation of toughness requirements that is closely related to the discussion for low temperature applications is presented. Examples of application close this contribution.

China now experiences a fast development in shipbuilding and marine steel, the self-developed steel could meet the needs among 95% domestic clients. But in the items of some special high-end products, there are still certain gaps with advanced foreign countries, and these are mainly high-quality and strong-capacity products, large-size products, low-temperature products, and anti-corrosion, anti-fatigue and high-level failure arrest products. In the present paper, some domestic research and development (R& D) results in industry have been introduced, also, it points out that to eliminate these gaps, should rely on both the tech-progress in iron and steel industry, shipbuilding, and marine steel industry, and the establishment and improvement of the R& D system in researching, manufacturing, testing, producing and application.

Multi-pass deformation simulation tests were performed on V-N microalloyed steels with different nitrogen addition by using a Gleeble-3800 thermo-mechanical simulator and the corresponding continuous cooling transformation (CCT) diagrams were determined by thermal dilation method and metallographic method. The deformed austenite transformation behavior and resultant microstructure of the tested steels were studied. Furthermore, the effect of nitrogen addition on the transformation behavior and microstructure evolution was analyzed. The results show that the transformed microstructures in the three tested steels are ferrite, pearlite and bainite respectively while the transformation temperatures are not the same. For the two tested steel with higher nitrogen additions, higher ferrite start temperature and critical cooling rates are observed. Furthermore, an increase in nitrogen addition leads to increasing quantities of ferrite and the transformed ferrite is smaller in size. The hardness test results reveal that the hardness number increases with increasing nitrogen addition at low cooling rate while the value tends to be smaller due to increasing nitrogen addition at high cooling rate. Therefore, the hardness number of the steel with high nitrogen addition is not so sensitive to cooling rate as that of the steel with low nitrogen addition.

Nb is the key element in thermo-mechanically controlled processing of high strength low alloy steels due to its enormous effect on delaying the recrystallization of deformed austenite. Similarly Nb in solution delays the austenite-ferrite transformation, thereby further promoting microstructural refinement and high strength at low carbon levels. Mo is frequently added to further increase the hardenability. The aim of this study is to quantitatively describe the synergistic effect of Nb and Mo on the microstructure evolution in HSLA steels during austenite decomposition. Continuous cooling transformation studies on steels with varying Nb and Mo levels were conducted and the resulting microstructures were characterized with EBSD. The addition of both Nb and Mo lowers the transformation temperature and promotes the formation of complex bainitic transformation products that have been distinguished and categorized based on their orientation relationship with the parent austenite grain. In particular, microstructure refinement has been quantified in terms of high angle grain boundary length per unit area. In addition the fraction and size of carbon-rich martensite/austenite constituents have been characterized and correlated with the bainitic structure as a function of the transformation temperature.

Decomposition of supercooled austenite in continuous cooling transformation process of a Mn-Mo-Ni low alloy steel was evaluated by dilatometric measurements, light microscopy, electron backscatter diffraction, and microhardness testing and other methods. The results show that at the cooling rates of 1°C/s or below, ferrite initially formed and continuously rejecting C into the untransformed austenite, which transforms to C-rich lower bainite at lower temperature, resulting in ferrite-bainite dual-phase microstructures. At the cooling rates between 1 °C/s and 5 °C/s, the successive transformation products are bainite ferrite, upper bainite and lower bainite, and islands of carbon-enriched austenite transform to martensite (plus retained austenite) at low temperatures. The upper bainite and martensite dual-phase microstructures are formed at the range of 5 °C/s to 50 °C/s with a lower M_s. When cooling rates greater than approximately 50 °C/s, the microstructure are martensite and retained austenite at room temperature. With the increase of cooling speed, the residual austenite content increased before decreased at the cooling speed of 2 °C/s, which is probably associated with the incompleteness of bainite transformation.

Effect of a high magnetic field of 8T on the pearlite transformation was investigated in a Fe-0.4mass%C steel. It was found that a magnetic field of 8T can promote the pearlite transformation, reduce the final pearlite fraction and interlamellar spacing. The mechanism of field-effect was discussed by analyzing the influences of high magnetic field on the Fe-C phase diagram and the eutectoid temperature.

Utilizing high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD), chemical composition characteristics and thermal stability of reverted Austenite (RA) in 9%Ni steel processed by quench-lamellarize-temper (QLT) heat-treatment has been investigated. The results showed that Austenite-stabilizing elements, such as Ni, Mn, are inclined to concentrate in reverted Austenite and distribute unevenly in it, higher near the boundary as high as 21% and lower in the centre. It was also found that RA has good heat and mechanical stability in 9%Ni steel but likely loses its stability and transform to Martensite undergoing both ultra low temperature and mechanical loading, accordingly inducing plasticity unceasingly.

For more than thirty years, CALPHAD based tools have been successfully applied within the steel industry. Such tools are useful from an alloy design perspective and as a way of improving understanding of existing steels and process optimization. This presentation focuses on the thermodynamic modeling and precipitation simulation of high strength, low alloy (HSLA) steels. Thermodynamic calculations using Thermo-Calc can be used to predict the underlying thermodynamics of multicomponent alloys of industrial importance; phase equilibrium and phase transformation information by taking into account compositional and temperature variations, e.g., the effect of an alloying element on the austenite to ferrite transformation temperature; segregation during solidification; driving force for nucleation; stability of precipitates; etc. Additionally, precipitation plays an important role in industrial steels, especially for strengthening HSLA steels. The precipitation kinetics of alloy carbides, i.e., NbC, is simulated using TC-PRISMA, a computational tool that is integrated with Thermo-Calc and DICTRA together with additional interface property data. Properties such as interfacial energies between matrix and precipitate phases can be estimated by using an approximation model which has been implemented in TC-PRISMA.

Based on Tрновский’s displacement field, equivalent strain expression was derived. And according to the dynamic recrystallization (DRX) critical strain, DRX process was determined by equivalent strain. It was found that equivalent strain distribution in deformed specimen is inhomogeneous, and it increases with increasing true strain. Under a certain true strain, equivalent strains at the center, demisemi radius or on tangential plane just below the surface of the specimen are higher than the true strain. Thus, micrographs at those positions can not exactly reflect the true microstructures under the certain true strain. With increasing strain rate, the initial and finish time of DRX decrease. The frozen microstructures of 20Mn23AlV steel with the experimental condition validate the feasibility of predicting DRX process by equivalent strain.

Recently, we have being focused on improving the strength without sacrificing ductility of High-strength low-alloy (HSLA) steels by designing nanostructures. Several developments have been obtained, summarized as the following three parts: (a) Depressively nanoscale precipitates: A ferritic steel with finely dispersed precipitates reveals a yield strength of 760 MPa, approximately three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels, and its ultimate tensile strength reaches 850 MPa with an elongation-to-failure value of 18%. The finely dispersed TiC precipitates in the matrix provide matrix strengthening. The estimated magnitude of precipitation strengthening is around 458 MPa. The effects of the particle size, particle distribution and intrinsic particle strength have been investigated through dislocation dynamics (DD) simulations. The DD results show that strengthening is not only a function of the density of the nano-scale precipitates but also of their size. (b) Ultrafinely ferritic plate: An interstitial-free (IF) steel sheet with a cold-rolling reduction of 75% shows a high tensile strength (710MPa) while preserving a considerable plastic strain (13%). The ductility recovery with increasing the rolling reduction up to 75% is related with the decreasing both in lamellar spacings and cell blocks sizes. (c) Parallel nano-laminated austenite: A composite microstructure consisting of ferrite, bainitic ferrite (BF) laths and retained austenite (RA) platelets has been found for the steel with a chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (in mass fraction), processed with annealing and bainitic holding. The sample annealed at 820oC (for 120s) and partitioned at 400oC (for 300s) has the best combination of ultimate tensile strength (UTS, ~682 MPa) and elongation to failure (~70%) with about 26% of BF plates 16% RA in its microstructure.

Instead of off-line quenching and tempering (QT), on-line ausforming (non-recrystallization controlled roling) and direct quenching (DQ) was employed to improve the toughness of low alloy steel with yield strength of 900 MPa Grade. Low carbon content ensured a high level of upper shelf energy, while the fine-grained martensite substructures decreased the ductile-to-brittle transition temperature and compensated the strength loss due to carbon reduction. Two mechanisms for the refinement of martensite substructure were proposed: one was the austenite grain refinement in the thickness direction, and the other was the self-accommodation of martensite variants due to austenite grain hardening. More than 200 J of Charpy V-notch impact absorbed energy at 233 K was obtained in the industrial ausformed plate, which was about the double in the traditional QT steel with the same strength grade.

The effect of Mo addition on the precipitation of carbides both in austenite and ferrite of Nb-bearing HSLA steel were investigated by stress relaxation and transmission electron microscopy (TEM). Experimental results showed that the addition of ~0.2 wt.% Mo into Nb-bearing steel slightly accelerated the precipitation kinetics of Nb carbides both in austenite and ferrite. The particles, (Nb, Mo)C, precipitated in Nb-Mo-bearing steel exhibit superior coarsening resistance compared to that of NbC particles in Nb-bearing steel. And the difference of carbides precipitated in anstenite and ferrite of Nb-Mo-bearing steel was discussed.

A cost effective alloy design based on niobium microalloying was developed by Shagang for the production of 32 mm gage K60-E2 steel plate for manufacturing LSAW pipes of 1422 mm diameter. Two metallurgical challenges are (i) to meet enhanced CTOD toughness and (ii) to ensure consistent DWTT performance in accordance with the specification laid out by Gazprom Vniigaz.Nano-scale precipitate engineering of TiN-NbC composite offers a novel method based on microstrutural engineering for integrated design and processing to refine the austenite grain size in upstream processing of heavy gage plate before pancaking in order to improve DWTT performance. Accelerated growth of NbC precipitates engineered at austenite grain boundary is effective in catalyzing the nucleation of acicular ferrite. The increase in acicular ferrite fraction in target microstructure is the key to achieve enhanced CTOD toughness in base plate.This paper deals with the application of nano-scale precipitate engineering of TiN-NbC composite precipitates in plate rolling in order to obtain consistent fracture toughness at low temperature in 32 mm gage plate of K60-E2 grade, as measured by CTOD and DWTT performance.

The microstructure, mechanical property and precipitation behavior in a low carbon Nb-Ti micro-alloyed steel were investigated using dilatometer, optical microscopy and transmission electron microscope. The results show that the microstructure of the experimental steel treated by isothermal quenching process mainly consists of ferrite and martensite. The volume fraction of ferrite increases with a decrease in the isothermal temperature. It is found that both of interphase precipitation and supersaturated precipitation would appear in the samples treated by isothermal quenching process. Along with the isothermal temperature decreasing, the precipitation state changes from interphase precipitation to supersaturated precipitation. The interphase precipitation of these carbides with different row spacing and different orientation in ferrite grains, is related to the mobility of interfaces during γ/α transformation based on ledge mechanism. In addition to {110}_α plane suggested by the ledge mechanism, the planar sheets of interphase precipitation are also found to be parallel with{035}_a planes. Moreover, the interphase precipitation carbides were identified to have a NaCl-type crystal structure with a lattice parameter of 0.432 nm and obey the Baker-Nutting (B-N) orientation relationship with respect to ferrite matrix. The contribution of the precipitation hardening to the yield strength of the experiment steel has been estimated to be ~337 MPa at 620 °C, based on Orowan mechanism.

International Standard ISO 6892–2 maintains that strain rate variations can induce the measurement uncertainty of mechanical properties in tensile testing, which will imperil the reliability of tension tests. In this paper, the measurement uncertainty of reduction in area relative to strain rate, called as strain rate embrittlement, is first described experimentally for some micro-alloy steels. Then the measurement uncertainty is clarified based on microscopic theory of elastic deformation in metals. It is shown that the elastic deformation of tension test induces the segregation of impurities to grain boundaries and the relevant embrittlement which produces the measurement uncertainty of reduction in area in tensile testing. This work gives a theoretical basis for correcting the present tension testing system to avoid the measurement uncertainty of mechanical property.

The influence of Nb, Mo, Cr and B on phase transformations and mechanical properties are studied in a 0.15C-2.0Mn-0.3Si-0.020Ti dual phase steel separately and in combination. The formation and decomposition of austenite together with recrystallization of ferrite are evaluated by dilatometry and constructed CCT-diagrams in laboratory processed cold rolled material cooled after full austenitization and from intercritical temperature range. The effect of alloying elements on formation of austenite through their effect on initial hot rolled structure is taken into account. The interpretation of phase transformations during heating and cooling is supported by metallography. The effect of alloying elements on mechanical properties and structure are evaluated by annealing simulations. It has been shown that mechanical properties are strongly influenced by alloying additions such as Nb, Mo, Cr and B through their effect on ferrite formation during continuous cooling and corresponding enrichment of remaining austenite by carbon. Depending on combined effect of these alloying elements, different phase transformations can be promoted during cooling. This allows controlling of final microstructural constituents and mechanical properties.

Mn-Mo-Ni low-alloy steels are widely used in the manufacture of nuclear pressure vessels. This article studied the influence of carbon content and cooling rate on the toughness of Mn-Mo-Ni low-alloy steel. Experiment results indicated that the toughness of this low-alloy steel imporved as the carbon content decreases or the cooling rate increases. Detailed microscopic analysis showed that though more ferrite forms as carbon content goes down, the size of carbides become smaller which decreases the possibility of formation of crack. The growth of cooling rate results in the transition from bainite to martensite, and the decrease in size of both carbides and lath, which make respectively the formation and propagation of crack more difficult. This study provides basis for determining composition and processing of Mn-Mo-Ni low-alloy steels with high toughness.

Deformation processes of microalloyed steels, both during hot and cold processing, are characterized by complex interactions between fine strain-induced precipitates and deformation activated movement of dislocations. Disperse particles that are either forming during the deformation at elevated temperature or are already present in the material at ambient temperature can significantly affect also other strengthening mechanisms e.g. dislocation and grain size strengthening. In the current work, quantitative and qualitative analysis of the effects of second phase particles in fcc and bcc microalloyed structures subjected to severe plastic deformation (SPD) processing was performed with respect to the inhomogeneity of microstructure evolution and properties. Observed inhomogeneities of deformation mechanisms, work hardening and microstructure refinement of microalloyed ferrite much more pronounced than in the case of microalloyed austenite. Discussion of the mechanical and microstructural reasons of such differences is also presented. Conclusions regarding effects of applied complex deformation history on microstructure morphology and properties were used as a validation of performed computer modelling.

The use of niobium as a microalloying element in high strength steels is now well-known and widely adopted. However, with recent advances in thermo-mechanical processing, inclusive of ultrafast cooling technology, and modifications in alloy design, niobium continues to provide significant mechanical property and processing benefits. With this in perspective, we describe here the footprint of niobium in the new generation of high strength microalloyed steels that involve low carbon-based alloy design approach. In this regard, we underscore the effectiveness of niobium in low carbon-Nb (or low C-Nb-Ti) microalloyed steels in terms of obtaining superior mechanical properties and cost-effectiveness, using recent experiences in the processing of microalloyed steels. Also, elucidated are precipitation and microstructural characteristics when niobium is used as a standalone microalloying element or in conjunction with titanium and molybdenum.

We examined the effects of micro-texture and grain shape on the delamination of hot-rolled high strength low alloyed (HSLA) steel sheets through a series of uniaxial tensile tests. Prior to testing, the HSLA steel sheets were hot-rolled at different finish rolling temperatures to obtain two specimens with different microstructures.Results show that the depth of delamination in HSLA steel decreased as the finish rolling temperature increased. The depth of delamination was reduced by about 46%.It has been found that the increasing finish rolling temperature decreases a rolling texture. In addition, the control of finish rolling temperature leads to a reduction of elongated grains. From an industrial standpoint, it is possible to improve a delamination resistance for the HSLA steel sheets with the texture controlled rolling process

The microstructure and mechanical properties of an HB400 grade abrasion resistant steel was studied with an emphasis on the initial microstructure before intercritical heat treatment. The result shows that the initial microstructure, before intercritical quenching (Q’), has a great effect on the final microstructure and mechanical properties. The hardness of samples with two initial microstructures, i.e. bainite and martensite, is almost the same, but the toughness and wear resistance of the latter are much higher than that of the former, after intercritical quenching plus tempering (Q’+T) process. The one with bainite as the initial microstructure has a final microstructure of block-type tempered martensite and ferrite with martensite islands inside. Cracks may easily occur at the boundaries of martensite islands and pass through the low angle grain boundaries in the ferrite. By contrast, the one with martensite as the initial microstructure has a uniformly distributed tempered martensite and needle-like ferrite. The boundaries between them are high angle grain boundaries, and hence cracks may be arrested or deflected, resulting in a higher toughness.

The recrystallisation behaviour of medium carbon steels with dilute Nb addition was investigated by means of plane strain compression tests and the observation of prior austenite microstructures during different deformation conditions. It was found that complete suppression of recrystallisation did not occur in the deformation temperature range investigated. At lower deformation temperatures, partial recrystallisation occurred in the higher Nb sample. This gives the potential to obtain a full suppression of recrystallisation at lower deformation temperatures.

In-situ material characterization methods such as High Temperature Confocal Laser Scanning Microscopy and hot-stage Electron Back Scattered Diffraction allow direct observation of microstructural processes during simulated thermal treatment. In this paper V-bearing micro-alloyed low carbon steel with and without Mo addition is studied using a combination of in-situ methods. It is concluded that addition of 0.2 wt.% Mo into V-bearing steel restricts austenitic grain growth as well as limitation of formation of both Allotriomorphic and Widmanstatten ferrite through a coupled solute drag effect.

The behavior of austenite grain growth and the solution of Ti were researched by heated in an electric furnace at the temperature of 850 °C to 1250 °C for 30 min. The precipitates of Ti during the heating process were examined by transmission electron microscopy (TEM). The results indicate that the tested steel has two grain coarsening temperatures of austenite grains with the increase of heating temperature, 1050 °C and 1250 °C respectively, which corresponds to the solution temperature of two precipitates of Ti, but lower than the value of the solution temperature. It is found that the activation energy of grain growth reduced from 26,561 J/mol to 239.8 KJ/mol with the dissolving of TiC precipitates by analyzing two stages of austenite grain growth process.

Thick gauge strip production using thin slab casting and direct rolling (TSDR) technologies show important challenges due to the lack of high total reduction from slab to final strip. In these cases, a correct design of the reductions in the first rolling passes is an important tool to achieve an optimized through-thickness homogenization in the final product. There are metallurgically key thicknesses where an optimized per pass reduction should be taken to result in an optimized homogenization of the through-thickness austenite grain (austenite conditioning) setting up the stage for the final metallurgy and ferrite grain transformation. This proper per pass reduction design can be called taking a “critical reduction at a critical thickness”. This paper shows how the combination of finite element analysis and microstructural evolution models help understand the strain distribution after each rolling pass and the improvement in strain penetration to the centerline area when an optimum reduction schedule is selected. The analysis, applied to a niobium microalloyed grade, provides useful information for, not only mean grain size refinement in the centerline area, but also an improvement in the homogenization of the grain size distribution. Following these concepts of taking a “critical reduction at a critical thickness”, an improvement in both strength and toughness properties could be achieved in the final TSDR rolled product. In fact this very basic metallurgical principle can and has been applied to the metallurgy of producing plate, traditional hot strip coil and long product section sizes.

X90 pipeline steel requires ultra low for sulfur content and gas content in the smelting process. The secondary refining process is very important for X90 pipeline in smelting process and the control of cleanliness is the key for the secondary refining process in the steelmaking process for Pretreatment of hot metal → LD → LF refining → RH refining → Calcium treatment → CC. In the current paper, the cleanliness control method of secondary refining was analyzed for the evolution of non-metallic inclusions in the secondary refining prcess and related changes for composition in steel. The size, composition and the type of the non-metallic inclusions were analyzed by aspex explorer automated scanning electron microscope in X90 pipeline samples for 20mm * 25mm * 25mm by the line cutting. The results show that the number of non-metallic inclusions in steel decrease from the beginning of the LF refining to the RH refining. In the composition of the Non-metallic inclusions, the initial non-metallic inclusions of alumina is converted to two comple-type non-metallic inclusions. Most of them, the non-metallic inclusions were composed by the calcium aluminate and CaS. The others are that the spinel is the core, peripheral parcels calcium aluminate nonmetallic inclusions for complex-type non-metallic inclusions. For the size of the non-metallic inclusions, the non-metallic inclusions for size larger than 100µm is converted to 5 ~ 20µm based small size non-metallic inclusions. While the S content of the steel decreased from 0.012% to 0.0012% or less, Al content is kept at between 0.025% to 0.035% and the quality for the casting slab satisfies the requirement of the steel. The ratings for various types of the non-metallic inclusions are 1.5 or less. The control strategy for the inclusions in 90 pipeline is small size, diffuse distribution and little amount of the deformation after rolling. On the contrary, the specific chemical composition of the inclusions is not important, single component in the inclusions is better.

The flow stress under typical hot coiling conditions has been quantified for low C and Nb-Ti (V) microalloyed steels. The equation is intended for calculating the amount of bending torque required during coiling, particularly for thick, microalloyed skelp, so as to optimally adjust the coiler tension settings. Simplified rolling, runout table (ROT), and coiling simulations, typical for 10mm strip were performed. A small plastic strain was applied during the coiling stage to study flow behaviour in the ferrite and two-phase regions. The flow stress equation developed is applicable for the following chemistry limits: 0–0.09%Nb, 0.25–1.59%Mn, 0.06–0.16%C, 0–0.25%Si, 0–0.02%Ti and 0–0.07%V. Dilatometry measurements showed that, at high coiling temperatures, a significant fraction of untransformed austenite was often present during coiling. The most significant variables contributing to flow stress in the coiling region in decreasing order are: strain, Nb content, temperature and Mn content.

A helical magnetic field stirrer that can generate both rotating and traveling forces is designed in this paper. The electromagnetic field and the flow field generated by the new equipment are simulated and compared with those generated by the traditional one. It can be seen that with the helical stirrer, the mass and heat transfer can be enhanced in three dimensions, while the traditional one can drive the melt flow only in two dimensions. In simulation, with the helical stirrer, the temperature gradient is almost 18.5% lower than that with the traditional one. The applied feasibility of the helical stirrer in the continuous casting has been analyzed also. Compared with the traditional stirrer, the helical one has the similar dimension and power consumption, it can be utilized instead of the traditional one in the mold, second cooling zone and ending stirring in continuous casting.

Microstructure evolution during hot rolling depends on several processing variables. These are the furnace soaking temperature and, for each pass in the schedule, the temperatures, the strains, the strain rates. Finally the cooling rate during transformation is also of importance. These variables will determine the amount of Nb in solution prior to rolling, the condition of the austenite during processing, the amount of accumulated strain in the austenite prior to transformation to ferrite and the room temperature microstructure. The phases and constituents present at room temperature will largely define the mechanical properties of the products.Although the basics of the physical metallurgy involved in the determination of austenite and then ferrite grain sizes are well known, the interrelation between the physical phenomena during rolling is complex in nature. Models of microstructure evolution will help to better understand the metallurgy of the thermomechanical process and give the opportunity to optimize or improve an existing schedule.In this paper, modelling of microstructure evolution is presented as a practical expert tool to be used in the design and improvement of hot rolling schedules. First, the fundamentals of the model are introduced. Then, a case study emphasizing applications to a Steckel Mill is described to illustrate how to use the model from the perspective of schedule design and schedule improvements. Finally, “what if scenarios are presented and discussed aiming at increasing familiarity of end users with the model.

Vanadium microalloying is highly effective in high strength strip steels produced by thin slab casting and direct rolled process. Because of the high solubility of V(C,N) in austenite, vanadium is likely to remain in solution during casting, equalisation and rolling. Vanadium microalloyed steels have better hot ductility and are less prone to transverse cracking than niobium containing steels. Despite a coarse as-cast austenite grain size before rolling, significant grain refinement can be achieved in vanadium microalloyed steels by repeated recrystallization during rolling, resulting in a fine uniform ferrite microstructure in final strip. Almost all vanadium present in microalloyed steels is available to precipitate in ferrite as very fine particles, contributing to precipitation strengthening. Vanadium microalloyed steels show less sensitivity to rolling process variables and exhibit excellent combination of strength and toughness.

Rolling force model is very important during the rolling process. The accuracy of the model not only affect the formulation of rolling process, the equipment designing, but also involve with the thickness precision of the product. SIMS formula is adopted to establish the rolling force model. The parameters of the model are obtained by using the fitting method[l]. After that, the factors of the formula affected the rolling force are analyzed, the result shows that the steel rolling temperature is the key influence factors of rolling force, and it indicates that the key factor of the stability of rolling process is temperature.By the same time, the reference yield stress of different steel grades are calculated by using the regression method. The pipeline steel economical X90, deep seabed pipeline X70Mo, High-rise building steel Q420GJC, Bridge plate Q420qE, Ship plate EQ51, Saw steel 50Mn2V, High strength steel A514Gr.S is respectively calculated. The results indicate that no obvious difference about reference yield stress of each steel grade in the condition of high temperature. It suggests that the value of rolling foce is not very closely related with the steel grade when large reduction is taken at the stage of rough rolling, the main influence factor of the rolling force is temperature factor.

A new generation of higher strength cold rolled HSLA is studied. Two typical chemical compositions are investigated. The two concepts are based on a partially recovered microstructure with some recrystallized ferrite and few martensite/austenite islands. The microstructural evolutions of these cold rolled grades along the annealing are presented. It is shown that the level of recrystallization achieved at the end of the annealing controls the final mechanical properties. Additionally, depending on the chemical composition, more or less MA constituents are present at the end of the annealing. One concept is designed for the applications that only need higher strength. The second concept is designed for the applications that need a high yield strength/tensile strength ratio.

Ultra-fast cooling (UFC) has been increasingly applied in industry, but accompanying with great changes of rolling strategy. It is therefore of importance to evaluate the characteristics of steels produced by UFC as compared to those processed by conventional accelerated cooling (ACQ. The present study examines the microstructure through thickness and centerline segregation of solute elements between UFC and ACC steels, both of which were rolled at a final rolling temperature at around non-recrystallized temperature. UFC steel showed the pronounced microstructural transition from lath-type bainite with Widmanstätten ferrite at subsurface to acicular ferrite in an average size of ~5 µm dispersed with degenerate pearlite in the interior. In contrast, ACC steel had the homogeneous microstructure through the thickness, which was distinguished with coarser polygonal ferrite grains and pearlite nodules. Moreover, the centerline segregation was significantly suppressed by applying UFC at a higher cooling rate of 40 K/s compared to 17K/s for ACC steel. The significant differences in the microstructure and centerline segregation caused by various cooling rate is discussed from the view of γ→α transformation.

The differences of microstructure and property of non-quenched and tempered bainitic steel under different finish rolling temperatures was analyzed by the methods of color metallographic, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that the sample with finish rolling temperature 870°C, has high yield strength, tensile strength and impact energy. Comparing with finish rolling temperature 750°C and 800°C, the sample has a higher content of bainite phase and bainitic lath is smaller at finish rolling temperature 870°C. Meanwhile, the percentage of retained austenite is 15% and retained austenite has dispersed distribution. It indicates that, when the finish rolling temperature is higher, the experimental steel has fine phase composition, microstructure distribution and excellent mechanical properties.

A new V-Ti-Mo complex micro-alloyed hot-rolled high strength sheet steel was developed by controlling a thermomechanical controlled processing (TMCP) schedule, in particular with variants in coiling temperature. At coiling temperature of 600 °C, the experimental steel has the best mechanical properties: ultimate tensile strength (UTS) 1000 MPa, yield strength (YS) 955 MPa and elongation (EL) 17%. Furthermore, the contributions of precipitation hardening and grain refinement hardening were high to about 310 and 361 MPa, respectively. The steels developed were mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in contribution of precipitation hardening increment caused a significant difference in YS.

A phase field model has been developed to simulate microstructure evolution during intercritical annealing. The model describes ferrite recrystallization, intercritical austenite formation and decomposition. In particular, the overlap of ferrite recrystallization and austenite formation for sufficiently fast line speeds has been taken into account in the model. The model has been benchmarked and validated with experimental data for a DP600 steel. Further, simulations have been performed with a systematic variation of processing parameters, i.e. line speed and intercritical holding temperature. Processing maps have been constructed based on these simulations providing martensite fraction and ferrite grain size as a function of line speed and intercritical temperature for the investigated DP600 steel. This study demonstrates the phase field approach as a promising tool to develop through-process models for advanced high strength steels.

Before being subjected to water quenched, 16mm WDL690D steel was heated at 910°C, 930°C, 950°C and 970°C for 20min, 40min and 60min, respectively. Microstructures of these samples after heat treatment were observed by optical microscopy and SEM. Rather uniform and fine martensitic laths could be observed from the samples which were heated at 950°C for 40min and then water quenched. And then the samples with fine structure were tempered at 600°C, 640°C, and 680°C for 50min, 80min, 100min, and 120min and then air cooling, respectively. The tensile properties at room temperature, and impact properties at different temperatures of the tempered samples at different heat treatment process parameters were measured. The metallography and SEM results showed that fairly uniform microstructure without obvious coarsening grain could be found from the samples tempered at 640°C for 80min. Moreover, the samples tempered at 640°C for 80min exhibited rather high hardness, strength, ductility and toughness properties, which had an excellent combination of strength and toughness. Based on all the results, the optimal parameters of heat treatment for WDL690D steel were determined to be heated at 950°C for 40min then water quenched and tempered at 640°C for 80min, which resulted in the finest microscopic structure and the best mechanical properties of the steel.

To develop thickness>60mm, yield strength>550MPa high-strength steel plate, the effects of heat treatment process on the microstructure and mechanical properties of plate specimens were investigated on the same controlled rolling and cooling process. The research results showed that, a single time-quenching and tempering heat treatment were conducted to obtain the bainite + free ferrite and residual austenite, the grain size was large and the impact toughness was low. Twice-quenching and a tempering heat treatment could refine the microstructure grain size and increase the soft ferrite proportion to obviously improve the mechanical properties and impact toughness. Taking reasonable intercritical quenching after tempering, the mechanical properties of plate are significantly improved.

A Quenching & Self Tempering (QST) treatment is one of advanced thermo-mechanically controlled processes to develop high strength and good toughness H shaped beams. However, the QST has sometimes caused their abnormal H shape called “off-squareness” due to non-uniform rapid cooling. In this paper, a correlation between the non-uniform rapid cooling and shape deformation is observed by QST test equipment. In order to understand the mechanism, deformation behaviors are analyzed by a commercial software package, DEFORM, coupled with thermal expansion and volumetric shrinkage during phase transformation in accelerated cooling, its reliability was evaluated with direct measure of deformation.

Potential benefits of vanadium in heat treated medium carbon bar steels are considered by reviewing examples from several recent studies. Emphasis is placed on microstructural development, strength, and fatigue performance after conventional and induction heat treating. Potential areas for future research are identified.

An new method is proposed to predict in sequence the phase transformation behavior and the depth of the decarburized ferrite layer with simultaneous scaling of a steel during cooling process from the measured temperature history. This proposed method successively couples a set of mathematical models describing the temperature evolution, phase transformation, decarburization and oxidation. The dilatometry experiment was performed to verify the proposed method. Results reveal that the measured depth of the decarburized ferrite layer agreed well with the predicted depth. The predicted pearlite volume fraction nearly coincided with the measured value as well.

We introduce a new strengthening heat treatment of a Ni-Cr-Mo-V alloyed spring steel by partial isothermal salt-bath and subsequent air-cooling and tempering. Detailed isothermal treatments were made at temperatures below or above the Ms point (230°C). The salt bath time was controlled between 10 and 80 minutes. Through the new treatment, the candidate steel developed ultrahigh tensile strength ~2,100 MPa, yield strength ~1,800 MPa, elongation 8–10 %, hardness 580–710 HV, and V-notch Charpy toughness 10–12 J. Optical and electron microscopic observations and X-ray diffraction revealed multi-phase microstructures of bainitic/martensitic ferrites, fine carbide precipitates and retained austenite. Carbon partitioning during the bainitic/martensitic transformation was investigated for its remarkable influence on the strengthening mechanism.

Niobium is not currently added to ferritic steels with the specific objective of improving weldability and is more likely to be present to harness its combined beneficial effects on strength and toughness. Nevertheless, as carbon levels in many classes of HSLA steel, are progressively reduced, there is an increasing awareness that, amongst the microalloying elements, niobium is uniquely placed to deliver the mechanical property combinations that modern specifications demand and simultaneously deliver a ‘bonus’ by way of enhanced weldability.It has taken some time to achieve the realisation of niobium’s full potential as lack of understanding and ‘decades old’ adverse mythology have sometimes, inadvertently, contributed to unwarranted niobium restrictions in specifications for modern low carbon steels for wide ranging applications from high pressure gas transmission pipe to structural steels for use in critical offshore applications.This paper briefly explores the origins of negative mythology and sets out to dispel any lingering concerns before embracing the reality of today’s technological advances that have enabled niobium to command the attention it so richly deserves; and yes, there is magic too because, in lower carbon steels, niobium truly does deliver specific, unexpected, weldability benefits.In fact the biggest outstanding issue, which still threatens to restrict the use of higher levels of niobium for the benefit of steelmakers and fabricators, is the failure of world wide specifications to keep pace with rapidly changing technology and progressive metallurgical thinking. We cannot afford to ignore the wind of change and, hopefully, ongoing developments will soon permit us to universally embrace the future.

Orthogonal experimental design is used to study the effects of alloying elements Ti, Nb and Zr on strength, toughness of base metal and quantity of M-A constituent in HAZ of ship plate. The results show that Nb has the greatest influence on strength and toughness of base metal. Strength is enhanced intensively with the increase of Nb quantity. Ti has the greatest impact on toughness in HAZ. Toughness in HAZ is the highest when Ti/N is appropriate. Nb is adverse to toughness in HAZ. Superfluous Ti and Nb increase quantity of M-A constituent in HAZ. Moderate Ti (0.012~0.016%) and Nb 0.015~0.025%) should be used in ship steel design for high heat input.

A microalloy addition of Ti is currently the option of choice to minimise austenite grain coarsening in the weld HAZ through the grain boundary pinning action of TiN precipitation. High thermal stability of the TiN precipitates provides effective control of boundary migration compared to other microalloy additions but steelmaking controls required to produce the optimum precipitate size distribution can be difficult to achieve consistently.Increased additions of Nb in modern high temperature processed (HTP) pipeline steels have demonstrated increased control of HAZ microstructures with improved fracture toughness. The present paper details the microstructure property relationship of two pipe steel grades with different alloy designs. Thermo-mechanical simulation techniques were utilized to evaluate the critical coarse-grained HAZ (CGHAZ), and the inter-critically (IC) reheated CGHAZ. Simulations were calibrated using real weld thermal cycles, to quantify the influence of alloy design and specifically the role of Nb in weld zone properties.The results reveal that the fracture toughness of the simulated CGHAZ in the HTP steel is superior to that of a lower Nb, Ti microalloyed pipeline steel grade. Toughness was related to the subtle difference in the bainitic HAZ microstructure and most importantly a difference in austenite grain size. As a result, improvements in the ICCGHAZ are expected. Further work is required to elucidate the mechanisms involved.

690MPa high strength steel with low yield ratio for building structure has been developed successfully by micro-alloying design and thermal-mechanical controlled processing in WISCO. The mechanical properties, weldabilities and microstructure of the based steel were studied by means of mechanical property test and microstructural analysis with optical microscope and transmission electron microscope. The results show that the mechanical properties meet the requirement of specification for developed steel, the microstructure of the based steel primarily comprised of fine lath and granular bainite, small fraction of ferrite and martensite—austenite constituent. During the thermal cycle, the austenite grain growth, microstructure coarsening, increasing of volume fraction and grain size of M-A constituent, and blocky M-A constituent in HAZs are the main reasons for deteriorating low temperature toughness. The plates of developed steel have high strength, high toughness, lower yield ratio and good weldability, and the plates will be the suitable high strength materials for fabricating high-rise, super high-rise building structures and large span projects.

Effect of Mo, Ni, and Cr on impact property of pipe seam and heat-affected zone (HAZ) of X80 steel was investigated by thermal simulation test and butt welding test. The results showed that, there was an obvious relationship between strip’s composition and the toughness of weld and HAZ, the more content of Mo, Ni and less of Cr in the strip matrix, the better of impact toughness of weld and HAZ. Metallographic microscope was used to compare microstructures of welding specimens, every welded seam microstructure was mainly acicular ferrite (AF) and a little volume of proeutectoid ferrite (PF), and with some granular precipitations on original austenite grain boundary, the difference was that there were more PF and less precipitations of the specimen with more content of Mo, Ni and less of Cr in the strip matrix. Because of the high price of Mo and Ni, alloy design must be considered comprehensively with the cost and property requirements in the production.

In the shipbuilding industry, the welding efficiency of the ship plate not only has a great effect on the construction cost of the ship, but also affects the construction speed and determines the delivery cycle. The steel plate used for large heat input welding was developed sufficiently. In this paper, the composition of the steel with a small amount of Nb, Ti and large amount of Mn had been designed in micro-alloyed route. The content of C and the carbon equivalent were also designed to a low level. The technology of oxide metallurgy was used during the smelting process of the steel. The rolling technology of TMCP was controlled at a low rolling temperature and ultra-fast cooling technology was used, for the purpose of controlling the transformation of the microstructure. The microstructure of the steel plate was controlled to be the mixed microstructure of low carbon bainite and ferrite. Large amount of oxide particles dispersed in the microstructure of steel, which had a positive effects on the mechanical property and welding performance of the steel. The mechanical property of the steel plate was excellent and the value of longitudinal Akv at -60 °C is more than 200 J. The toughness of WM and HAZ were excellent after the steel plate was welded with a large heat input of 100–250 kJ/cm. The steel plate processed by mentioned above can meet the requirement of large heat input welding.

In multi-pass welding process, various thermal cycle of both weld metal (WM) and heat affected zone (HAZ) will be subjected several times. This will make the initial microstructure occur an irreversible transformation. As the transformed microstructure become extremely complex, the mechanical properties, especially the low temperature toughness are very much fluctuant. In this research, the microstructure and low temperature toughness of WM obtained from a real multi-pass weld joint (up to 55 mm) by submerged arc welding have been elaborated. The results indicated that the necklace-type coarse martensite-austenite (M-A) constituent formed in interlayer heat affected zone (IHAZ) of WM and the impact energy of WM at -40 °C was only 39 J. Furthermore, by conventional tempering with holding time of 30 min, the toughness of WM can’t be effectively improved. However, by a new developed heat treatment process, the toughness of WM could be significantly improved, and it is believed to be caused by the composition of weld metal and the post-welding heat treatment process. It also shows that the decomposition of M-A constituent and formation of the retained austenite are the mechanism of the improvement of low temperature toughness.

Effect of heat-affected zone (HAZ) on spot weldability in automotive hot stamping (HS) steel sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 3 mm/min, using spot welded test specimen (Parallel length: 60 mm, Width: 20 mm, Thickness: 1.4 mm, Tab: 20×20 mm). The spot welding test was carried out using spot welded test specimen with welding current (I) of 6.3 kA to 9.5 kA. Hardness was measured with the dynamic ultra micro Vickers hardness tester. In HS steel, has very high strength of 1 500 MPa, tensile strength (TS) and total elongation (TEl) of the spot welded test specimen of HS steel were lower than those of base metal test specimen. The spot welded test specimen broke in the weld. The Vickers hardnesses (HVs) of base metal and fusion zone of hot stamping steel were around HV500. In addition, the hardness of HAZ was under HV300. The difference of hardness between fusion zone and HAZ was around HV200. The hardness distribution acted as a notch. On the other hand, in dual phase (DP) steel, has low strength of 590 MPa, the TS of spot welded test specimen of DP steel was the same as the base metal test specimen because of the breaking of base metal. The TEl of the spot welded test specimen of DP steel was smaller than that of base metal test specimen. In the spot welded test specimen of DP steel, the hardness of base metal was around HV200 and the fusion zone was around HV500. The hardness distribution did not act as a notch. The difference in hardness between base metal and HAZ acted on a crack initiation at HAZ softening.

In the present study, we have analyzed a rupture incident on converter trunnion in a certain factory and it adopts disc spring for buffer. Crack propagation paths indicate that this is caused by torsional fatigue. Detection results show large vibration speed of tilting mechanism appeared in the smelting process and vibration reduction performance of spring buffer is poor. Trunnion is made of 18MnMoNb low alloy steel, and bainite is its main structure. Martensite, lath bainite, ferrite and even widmanstatten transformations will occur during welding repair. Strict preheating and post weld heat treatment (PWHT) should be adopted to avoid welding cracks. Thermal cycle simulation shows preheat temperature higher than 210°C can reduce lath bainite formation and refine microstructure effectively, the temperature of PWHT should exceed 600°C to ensure the transformation of lath bainite to granular bainite.

The heavy plate used for offshore structure is one of the important strategic products. In recent years, there is an increasing demand for heavy shipbuilding steel plate with excellent weldability in high heat input welding. During the thermal cycle, the microstructure of the heat affected zone (HAZ) of plates was damaged, and this markedly reduced toughness of HAZ. So, how to improve the toughness of HAZ has been a key subject in the fields of steel research. Oxide metallurgy is considered as an effective way to improve toughness of HAZ, because it could be used to retard grain growth by fine particles, which are stable at the high temperature.The high strength steel plate, which satisfies the low temperature specification, has been applied to offshore structure. Excellent properties of the plates and welded joints were obtained by oxide metallurgy technology, latest controlled rolling and accelerated cooling technology using Ultra-Fast Cooling (an on-line accelerated cooling system). The 355MPa-grade high strength steel plates with normalizing condition were obtained, and the steels have excellent weldability with heat input energy of 79~287kJ/cm, and the nil ductility transition (NDT) temperature was -70°C, which can satisfy the construction of offshore structure in cold regions.

Wheel steel products of Han Steel occurred welding cracking when using in a wheel factory, by analyzing the crack in the wheel steel weld cracking with microstructure analysis and spectrum analysis, test results showed the grain in heat affect zone serious grow, and the user at the end of the flash-butt quenched from a high temperature to room temperature at welding seam, larger cooling rate to generate sufficiently large quenching stress, increased the risk of cracking along the grain boundary. When the stress reaches a certain level, there will be a greater area of the grain cracks at the location of welding seam, eventually leading to weld cracking. We develop measures for improvement to solving this problem, we suggest that the cooling mode at welding seam should be slow cooling or air cooling after the rim welding process, welding current range is 7800 ~ 9500 amps, upsetting time is 0.2 seconds, these measures can improve the welding quality of wheel steel products and reduce the risk of welding cracks.

For the purpose of obtaining the optimal microstructures and mechanical properties of the CGHAZ under high input welding, continuous cooling transformation diagrams of the coarse grain heat-affected zone (CGHAZ) and the corresponding microstructures were investigated for a E36 class V-N-Ti, V-Ti, and Nb-Ti shipbuilding steels. The results indicated that the CGHAZ continuous transformation behaviors of Nb-Ti and V-Ti steel were similar, but the V-retard phenomenon was not as apparent as that of Nb. In addition, the cooling rate of ferrite transformation of V-Ti steel was higher than that of Nb-Ti steel. The nitrogen addition in the V-Ti steel enhanced the ferrite transformation, since that increasing the nitrogen could obtain fine (Ti, V)(C, N) particles and refine the original austenite size, which can promote the ferrite nucleation. The bainite transformation range of V-N-Ti steel was obviously lower than that of Nb-Ti, V-Ti steel at the t_8/5≥100s.

Continuous cooling transformation diagrams and microstructures of coarse grain heat-affected zone (CGHAZ) of V-N-Ti and Nb-V-Ti microalloyed steels at different cooling time form 800°C to 500°C (t8/5) were comparatively investigated. With the increase of t8/5, the microstructure of V-N-Ti steel changes from martensite and lath bainite to ferrite, pearlite, lath bainite and granular bainite, however, lath bainite and granular bainite are the main microstructures of Nb-V-Ti steel, and hardness of the CGHAZ of both steels are decreased, but the CGHAZ of Nb-V-Ti steel has a higher hardness than that of V-N-Ti steel. Necklace-like M-A constituents along prior austenite grain boundaries are observed in CGHAZ of Nb-V-Ti steel at all of the t8/5 which could deteriorate the impact toughness. As the simulated HAZ continuous cooling transformation diagrams (SHCCT) are determined, the average transformation start temperature of V-N-Ti steel is nearly 100°C higher than Nb-V-Ti steel, the reason of which is (Ti, V)N particles did not dissolve during the welding thermal circle, and have an effect of pinning on prior austenite grain boundaries and induced the formation of ferrite at prior austenite grain boundaries.

Cold rolled steels with various vanadium and nitrogen levels have been treated to simulate the application of galvanizing and galvannealing to hardened martensitic microstructures. Strength levels were raised 100–150MPa by alloying with vanadium, which mitigates the effect of tempering. This opens the way for new ultra-high strength steels with corrosion resistant coatings produced by hot dip galvanising.

Ultra-high strength steels are materials of considerable interest for automotive and structural applications and are increasingly being used in those areas. Higher strength, however, makes steels more prone to hydrogen embrittlement (HE). The effects of Nb and other alloying elements on the hydrogen-induced delayed fracture resistance of cold rolled martensitic steels with ultra-high strength 2000 MPa were studied using an acid immersion test, thermal desorption analysis (TDA) and measuring of permeation. The microstructure was characterized by high resolution field emission Scanning Electron Microscopy (SEM) with Electron Backscattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM). It was shown that the combined addition of Nb significantly improved the delayed fracture resistance of investigated steel. The addition of Nb to alloyed martensitic steels resulted in very apparent grain refinement of the prior austenite grain size. The Nb microalloyed steel contained a lower diffusible hydrogen content during thermal desorption analysis as compared to the base steel and had a higher trapped hydrogen amount after charging. The reason that Nb improved the delayed fracture resistance of steels can be attributed mostly to both hydrogen trapping and grain refinement.

Shougang Group has developed advanced high-strength wear-resistance steel of NM450 by using Nb-Mo-Cr-Ti-B micro-alloyed design and two-stage rolling and quenching and tempering process. In spite of high strength, the toughness and weldability of the steel was improved by reducing the carbon equivalent. For the high-strength wear-resistant steel, the yield strength is higher than 1100MPa, the tensile strength is higher than 1350MPa, the elongation is greater than 15%, the low temperature(-40°C) impact energy value is not less than 60J,and the Brinell hardness value of surface is more than 430HB. The abrasion resistance of NM450 is five times of that of the Q235.The high-strength wear-resistance steel is successfully applied in Shougang SGE150® heavy dump body to replace the plain carbon steel Q235, resulting in the reduction of body weight, the improvement of the life of the mine car and the reduction of the transportation costs.

Increasing demands with regard to performance of special steels necessitate the further development of process technology as well as steel metallurgy. The present paper exemplifies recent developments based on alloy modifications with microalloying elements enhancing the performance of special steel types. The focus is thereby on molybdenum alloyed carburizing grades that are typically applied e.g. in the powertrain of vehicles, machinery and power generation equipment. A dedicated addition of niobium enables better mechanical properties and higher carburizing temperatures. Simultaneously the carburizing time can be shortened and distortion by heat treatment is reduced, as the microstructure remains more homogeneous. Further development potential offered by microalloying in carburizing steels is seen in an improved toughness due to the finer sized grain structure allowing to produce higher strength and more economical steel. These measures when applied in new steel concepts enable significant cost savings along the entire manufacturing chain especially in the production of larger transmissions. Finally, additional cost savings can be expected by a reduction and optimization of processes and procedures at the end-user.

Effects of galvanizing simulation parameters on microstructures and mechanical properties of Ti-microalloyed cold rolled hot-dip galvanizing DP980 steel were investigated in this study by optical microscopy (OM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and tensile test. Moreover, the precipitation behavior of Ti in the experimental steel was also studied. The results show that, as the heating temperature increases, the tensile strength of experimental galvanizing DP980 steel decreases while the yield ratio and elongation of the steel are enhanced. The microstructures of experimental steels exhibit typical dual phase steel character and the volume fractions of MA islands are almost 30%. In addition, lots of nano-sized TiC precipitates can be found in the ferrite grains.

Press hardening has become the state-of-art technology in the car body manufacturing to enhance safety standard and to reduce CO₂ emission of new vehicles. However the delayed cracking due to hydrogen embrittlement remains to be a critical issue. Generally press hardening steel is susceptible to hydrogen embrittlement due to ultra-high strength and martensitic microstructure. The hydrogen charging tests clearly demonstrate that only a few ppm of diffusible hydrogen is sufficient to cause such embrittlement. Currently the hydrogen embrittlement cannot be detected in the press hardened components and the embitteled components could collapse in the crash situation with fatal consequences arisen through dramatic loss in both strength and ductility. This paper introduces a new metallurgical solution to increase the resistance to hydrogen embrittlement of conventional press hardening steel based on 22MnB5 by Nb microalloying. In the hydrogen embrittlement and permeation tests the impact of Nb microalloying on the hydrogen embrittlement behavior was investigated under different hydrogen charging conditions and constant load. The test results revealed that Nb addition increases the resistance to hydrogen embrittlement due to reduced hydrogen diffusivity. The focus of this paper is to investigate the precipitation behavior of microalloying elements by using TEM and STEM and to find out the mechanisms leading to higher performance against hydrogen embrittlement of Nb alloyed steels.

Hot-dip galvanizing dual phase steel DP600 steel grade with low Si was produced by steel plant and experiments by simulating galvanizing thermal history. The microstructure was observed and analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of different annealing temperatures on the microstructure and mechanical properties of dual-phase steel was also discussed. The experimental results show that the dual-phase steel possesses excellent strength and elongation that match EN10346 600MPa standards. The microstructure is ferrite and martensite. TEM micrograph shows that white ferrite with black martensite islands inlay with a diameter of around 1um and the content of 14~18%. The volume will expand and phase changing take the form of shear transformation when ferrite converted to martensite. So there are high density dislocations in ferrite crystalline grain near martensite. The martensite content growing will be obvious along with annealing temperature going up. But the tendency will be weak when temperature high.

When producing high surface quality galvanizing steel coils for automobile use, there are always many light spots on the surface since Hansteel CGL No.1 has been put into operation. The defect samples were analyzed by SEM and EDS. The result shows that cause for light spot is not only one. There are more Mn and P in high strength auto sheet, which can result in difficulty to be cleaned off the oxide on the hot rolled coils, so the defects coming. This is why the defects come with high strength auto sheet. When coils galvanized, the defects can’t be covered up. To the contrary, the defects will be more obvious when zinc growing on the surface. And sometimes zinc or residue can adhere to work rolls when strips passing through SPM. The deposits then press normal coating. So the light spots come more. When the defect comes from pressing, there is no defect on steel base. The causation is found and measures were taken including high pressure cleaning equipments adopted. Result shows that the defects disappeared.

In this paper, against the actual operating condition of the low alloy CrMo wear-resistant steel liner plate, the influence of Nb on the microstructure and properties of the low alloy CrMo steel were investigated based on the reasonable chemical composition design, got excellent combination of hardness and impact toughness to reach the requirements of liner. It is found that, with the treatment of 920°C(8h)+550°C(20h), adding 0.03wt% Nb could refine and homogenize the grain structure and the lamella spacing of matrix is smaller. Tensile strength and elongation are similar in both alloys with and without Nb, while increases hardness by 16%, which is benefit to the wear resistance.

Lightweight wheels can reduce weight about 100kg for commercial vehicles, and it can save energy and reduce emission, what’s more, it can enhance the profits for logistics companies. The development of lightweight commercial vehicle wheels is achieved by the development of new steel for rim, the process optimization of flash butt welding, and structure optimization by finite element methods. Niobium micro-alloying technology can improve hole expansion rate, weldability and fatigue performance of wheel steel, and based on Niobium micro-alloying technology, a special wheel steel has been studied whose microstructure are Ferrite and Bainite, with high formability and high fatigue performance, and stable mechanical properties. The content of Nb in this new steel is 0.025% and the hole expansion rate is ≥ 100%. At the same time, welding parameters including electric upsetting time, upset allowance, upsetting pressure and flash allowance are optimized, and by CAE analysis, an optimized structure has been attained. As a results, the weight of 22.5in×8.25in wheel is up to 31.5kg, which is most lightweight comparing the same size wheels. And its functions including bending fatigue performance and radial fatigue performance meet the application requirements of truck makers and logistics companies.

Dual phase steel is widely used in today’s car body manufacturing. Its characteristics of high n-value and good elongation (A80) are the basis of good press formability. However, practical experience has shown unexpected failure in forming operations where tight bending, stretch flanging or hole expansion are predominant. The inhomogeneous microstructure of soft ferrite and hard martensite in combination with highly localized straining is the origin of these problems. Furthermore, weldability and delayed cracking have been experienced to cause problems in ultra-high strength DP steel. Refinement and homogenization of the two-phase microstructure as well as lowering of the carbon content have been identified as remedies to the mentioned problems. However, mill processing of DP steel with reduced carbon content is more difficult especially for the higher strength levels. Niobium microalloying proved to be very effective in increasing the processing window of low-carbon DP steels besides of its natural effect of refining the microstructure. Meanwhile the production of niobium microalloyed DP steel has been established in several markets including China. The paper details the fundamentals, demonstrates respective production concepts and presents examples of application of Nb-microalloyed DP steels.

Automotive engine exhaust gas after combustion of fuel, and the gas will be liquefied in the rear of automotive exhaust system. A lot of corrosive anions existing in the condensate make corrosion of the exhaust system materials. Therefore, once pitting perforation, automotive exhaust system will fail directly. In 1980s, automotive exhaust manifold was made of Si-Mo ductile iron, mufflers and the tail pipe were made of carbon steel or aluminized steel. But with higher emission standards carried out, the improvement of engine performance and the higher exhaust temperature as well as the needs of the automotive light-weighting, we need the higher corrosion resistance of the material for automotive exhaust systems to meet the requirements.With the improvement of oil refining technology, the quality of domestic oil will be better. The S% has reduced greatly and has reached the level of developed countries. With the environmental protection requirements more stringent, domestic fuel quality will be improved in the next period. There will be less SO_X, NO_X and other harmful gases in the exhaust gas so that the corrosive anions will be reduced greatly. Based on the fact, Efficient, low-cost and low-Cr 409 stainless steel will have a broader marke to take place of 429 steel stainless or 439 stainless steel . The study has found that new 409Nb-Ti stainless steel perform better in high temperature resistance, anti-condensate corrosion property, intergranular corrosion resistance and pitting potential than the traditional 409Ti.In particular,409Ti-0.3%Nb has the best high-temperature property. Therefore, the research will pay more attention to 409Ti-0.3%Nb.

For galvanized or galvannealed steels to be commercially successful, they must exhibit several attributes: (i) easy and inexpensive processing in the hot mill, cold mill and on the coating line, (ii) high strength with good formability and spot weldability, and (iii) good corrosion resistance, especially after cold forming. For good corrosion resistance, the coating must have sufficient coverage, be of uniform thickness, and most importantly, the coating must survive the cold stamping or forming operation. The purpose of this paper is to present research aiming at improving the steel substrate, such that high strength can be obtained while maintaining good global formability (tensile ductility), local formability (sheared-edge ductility), and good spot weldability. It is well-known that the strength of DP steels is controlled by several factors, including the amount of martensite found in the final microstructure. Recent research has revealed that the amount of austenite formed during intercritical annealing can be strongly influenced by the annealing temperature and the pre-annealing conditions of the hot band (coiling temperature) and cold band (% cold reduction). Current experiments have explored the combination of pre-annealing conditions and four annealing practices to help define the best practice to optimize the strength-formability balance in these higher strength DP steels. The steels used in these experiments contained (i) low carbon content for good spot weldability, (ii) the hardenability additions Mo and Cr for strength, and (iii) V for grain refinement, precipitation hardening and temper resistance. When processed correctly, these steels exhibited UTS levels up to 1000MPa, total elongation to 25%, reduction in area to 45%, and Hole Expansion Ratios to 50%. The results of this program will be presented and discussed.

Cold rolled high strength low alloy steel is widely applied in the automotive parts due to its excellent formability and weldability. In this paper, the steel grade HC260LA according to European Norm was developed with batch annealing process. With commercial C-Mn mild steel as a benchmark, three different groups of chemistry namely C-Mn-Si, C-Mn-Nb-Ti and C-Mn-Nb were compared in terms of yield-tensile strength (Y/T) ratio. Microstructure and mechanical properties were characterized as well. Based on industrial production results, chemistry and detailed process parameters for batch annealing were identified. In the end the optimal Y/T ratio was proposed for this steel grade under batch annealing process.

Due to composition segregation and cooling speed, streamline or banded structure were often obtained in the thermal forming parts along the direction of parts forming. Generally speaking, banded structure doesn’t decrease the longitudinal mechanical properties, so the secondary banded structure can’t get enough attention. The effect of secondary banded structure on the fatigue properties of micro alloyed DG20Mn and 35CrMo steel was investigated using the axial tensile fatigue test of stress ratio of 0.1. The result shows that secondary banded structure was obtained in the center of the steel parts, because of the composition segregation and the lower cooling rate in center part of steel. Secondary banded structure has no significant effect on axial tensile properties of both DG20Mn and 35CrMo, but decreases the axial tensile fatigue performance of DG20Mn steel. This study suggests that under the high cyclic tensile stress, multi-source damage cracks in steel initiated by large strain of pearlite of secondary banded structure, which is larger than damage strain, is the major factor of the decrease of fatigue life of steel.

Automobile axle housing is the basic element to install the main reducing gear, differential mechanism, semi-axis, wheel hub and suspension. The main function of automobile axle housing is to support the automobile quality with driven axle, fix the driving wheel relative axial position and bear the driving wheel transmission force during the automobile running. Axle housing steel with the thickness of not less than 12mm is produced by the thermal forming method, which is to heat the plate to 830 degree and hold some time, then thermal forming, and cool to room temperature. The steel plate should maintain the original strength and good ductility and toughness requirements with thermal forming process.In China, automobile manufacturing plants commonly use Q345B to produce automobile axle housing steel. Now, for reducing vehicle weight, it is tried to reduce steel thickness by improving steel strength. But from the test results, it is shown that the ordinary 600C steel strength is decreased significantly after thermal forming process, which causes the car safety problems. So, there is an urgent to develop a kind of economic 600C type plate, which has a tensile strength of not less than 600MPa and zero impact power of not less than 54J.Three types of designed materials, which have component systems of Nb-V, Ti and V-N are studied. The mechanical properties and welding performances of three experimental steel plates can meet requirements. But with the process of thermal forming and slow cooling to room temperature, only V-N micro-alloyed steel properties reach the user requirements. The precipitations before and after thermal forming are analyzed. The main reason for the material strength could meet the requirements is that VN precipitation is not easy to be coarsened in the thermal forming processes.

According to the technical specifications of 700MPa high strength automotive axle tube steel, a low cost of 0.07%C+1.5%Mn+0.05%Nb+0.10%Ti was designed. The high strength mainly relies on grain refinement strengthening and precipitation strengthening. The recrystallization, precipitation, and CCT curves of the 700MPa grade axle tube steel were studied in order to determine a reasonable TMCP process. By controlling the low level segregation band, low level of C and N content, 700MPa grade high strength automotive axle tube steel is successfully developed with excellent mechanical property, welding property, flattening and flaring property, torsion fatigue property, static torsional property and surface quality.

Effect of rare earth element Ce on behavior of high temperature deformation of IF steel was discussed. Different Ce content in IF steel were heated to 1250°C for 5 min with a heating rate of 10°C/s at vacuum conditions, then cooled to 1100°C, 1000°C, 900°C, 800°C for 30s with a cooling rate of 5°C/s and compressed by 50% with a strain rate of 10−2 s−1 and 1 s−1 by using a Gleeble-1500D thermo-mechanical simulation machine to get the true stress-strain curves. Finally, in order to observe the compressed microstructure, compressed sample were cut apart along longitudinal direction of the sheets. Results show that with a low rate of deformation of IF Steel, the dynamic recrystallization is the main softening mechanism, at a high rate of deformation dynamic recovery is the main softening mechanism. Deformation resistance of IF steel with an increasing rare earth Ce content has an inhibitory effect on recrystallization.

Handan Iron and Steel production of high-strength structural car steel QStE500TM thin gauge products using Nb + Ti composite strengthening, with a small amount of Cr element to improve its hardenability, the process parameter control is inappropriate with Nb + Ti complex steel, it is easy to produce in the mixed crystal phenomenon, resulting in decreasing the toughness and uneven performance. In this paper, Gleeble 3500 thermal simulation testing machine for high-strength structural steel car QStE500TM product deformation austenite recrystallization behavior research, determined completely recrystallized, partial recrystallization and non-recrystallization region, provide theoretical basis and necessary data for reasonable controlled rolling process for production.

In the study a low carbon niobium bearing high strength F-B dual phase automobile steel with high hole expansion property has been investigated. Steels of different chemical composition have been investigated by simulation experiments of controlled rolling and cooling process to study the influences of chemical elements, especially for C,Nb and Ti, and cooling pattern on the mechanical properties, flangeability and microstructure of strips. So-called 3-stages cooling pattern was adopted in simulation experiments, combining ultra fast cooling in first stage, air cooling in middle stage and fast cooling in the last stage, and at the end of run-out table the temperature of rolled pieces drop to below Bs point. Optical microstructure and SEM morphology have been observed. Results indicate that it is possible to obtain dual phase microstructure of polygonal ferrite plus bainite in adopting 3-stages cooling pattern. The low temperature coiling method using 3-step controlled cooling pattern after hot rolling is effective to produce low carbon Nb bearing steel with high balance of strength-ductility-flangeability, in addition, higher carbon content of steel tend to be detrimental to flangeability of steel, due to much carbide precipitation at ferrite boundary. Based on the results of simulation experiments mill trial has been carried out and hot rolled high strength steel with tensile strength higher as 600Mpa and hole expansion ratio higher as 100% has been developed successfully.

Hydrocarbon pipelines transporting compressible products like methane or high-vapor-pressure (HVP) liquids under supercritical conditions can be susceptible to long-propagating failures. As the unplanned release of such hydrocarbons can lead to significant pollution and/or the horrific potential of explosion and/or a very large fire, design criteria to preclude such failures were essential to environmental and public safety. Thus, technology was developed to establish the minimum arrest requirements to avoid such failures shortly after this design concern was evident. Soon after this technology emerged in the early 1970sit became evident that its predictions were increasinglynon-conservative as the toughness of line-pipe steel increased. A second potentially critical factor for what was a one-dimensional technology was that changes in steel processing led to directional dependence in both the flow and fracture properties. While recognized, this dependence was tacitly ignored in quantifying arrest, as were early observations that indicated propagating shear failure was controlled by plastic collapse rather than by fracture processes.This paper considers the effects of process-induced directional properties and orientational effects for steels. Because it will take some years before plastic-collapse-based technology develops to predict arrest and characterize steel resistance, these aspects are considered for both flow and fracture. This paper presents, trends, and discusses data for these metrics with a focus on tensile response and CVN energy. The implications of process-induced directionality and orientation are then quantified relative to requirements to arrest propagating shear failures.

This paper reports on the experience with the production of 27/33 mm X80 heavy wall thicknesses, large OD (48”) in Shouqin Steel Co., Ltd. (SQS). Considering the technology capability of the plate mill in SQS, a optimized rolling and cooling process was developed to achieve stable heavy gauge X80 mechanical properties. The importance of the slab reheating process and rolling schedule will be discussed in the paper. In addition, the per pass reductions logic used during recrystallized rough rolling, and special emphasis on the reduction of the final roughing pass prior to the intermediate holding resulting in a fine uniform prior austenite microstructure will be discussed. The optimized cooling process application after finish rolling guarantees the steady control of the final bainitic microstructure with optimum M/A phase for heavy gauge X80 plates. The plates produced by this process achieved good flatness and excellent mechanical properties. SQS has produced 10000 tons 27mm X80 for the Middle Asia C Line Project and 1000 tons 33mm X80 for the 3rd West-to-East Natural Gas Transmission Pipeline Project in 2013–2014. The products utilizing optimized rolling and cooling process showed extremely excellent low temperature toughness.

With the increase of wall thickness of pipeline steel, DWTT fracture toughness has become more and more sensitive to production process, including alloy design and processing, so it need more tight and more systematic control from the alloy design, reheating, rolling and finally cooling to improve upon DWTT property of heavy gauge X80 strip. In this paper, Through hot simulation and subsequent analysis, three points are significant to ensure DWTT property of 22mm X80 strip, including improvement Tnr by adding high Nb addition, increasing pass reduction in the rough rolling stage, and as well as low coiling temperature for obtaining dispersed, uniform MA constituents. As shown from test results, through reasonable alloy design and processing optimization, austenite grain size of 22mm X80 strip can be controlled to less than 9µm, volume fraction of MA constituents less than 10 percent with a size less than 2µm, which is significant to fulfill fracture toughness requirements of hot rolled X80 strip. Based on solid theoretical foundation and tight control, the test results of DWTT of 22mm X80 strip fulfill 85 percent in shear area at −15°C.

Aging of microalloyed steel pipe can occur at relatively low temperatures associated with the pipe coating process and/or during long term storage or use. The aging phenomenon is primarily attributed to C diffusion to dislocations and subsequent pinning of these dislocations. Important factors in the aging process include time, temperature, chemical composition and plastic deformation (arising from the pipe forming process). The work presented in this paper uses a Box-Behnken experimental design to determine the effect of time, temperature, location in the UOE pipe (90° or 180° to the weld), position through the pipe wall thickness (ID, CL or OD) and the steel’s C/Nb ratio (0.60, 1.25 and 1.80) on the change in yield strength of three (uncoated) X70 UOE pipes. Quantitative microstructure analysis is undertaken to determine the grain size and microconstituent fractions of the as-received pipe material. Quadratic equations and response surface(s) correlating the significant aging variables with changes in the longitudinal yield stress of the pipe are developed. Both through thickness position and the C/Nb ratio, followed by aging temperature, had the largest effect on the change in longitudinal yield strength.

This study is focused on the effect of vanadium addition to an API X100 steel. Laboratory heats were melted and control rolled based on a coil plate production process. Compared to non vanadium steel, it was found that 0.06% vanadium added steel had 8 to 14% higher yield and tensile strengths in all directions, while the toughness and ductility were similar for both steels. It is believed that the higher strength of the vanadium steel was partially due to its smaller grain size, larger fraction of sub grain boundaries and a higher number of uniformly distributed carbonitride precipitates in the vanadium steel.

Over the past twenty years, significant advances have been made in the field of microalloying and associated application, among which one of the most successful application cases is HTP practice for heavy gauge, high strength pipeline steels. Combined the strengthening effects of TMCP and retardation effects of austenite recrystallization with increasing Nb in austenite region, HTP conception with low carbon and high niobium alloy design has been successfully applied to develop X80 coil with a thickness of 18.4mm used for China’s Second West-East pipeline. During this process, big efforts were made to further develop and enrich the application of microalloying technology, and at the same time the strengthening effects of Nb have been completely unfolded and fully utilized with improved metallurgical quality and quantitative analysis of microstructure. In this paper, the existing status and strengthening effect of Nb during reheating, rolling, cooling and welding have been analyzed and characterized based on mass production samples and laboratory analysis. As confirmed, grain refinement remains the most basic strengthening measure to reduce the microstructure gradient along the thickness, which in turn enlarges the processing window to improve upon low temperature toughness, and finally make it possible to develop heavy gauge, high strength pipeline steels with more challenging fracture toughness requirements.As stated by a good saying that practice makes perfection. Based on application practice and theoretical analysis, HTP has been extended to develop heavy gauge and high strength pipeline steels with increasing requirements, including X80 SSAW pipe with a thickness of 22.0mm and above, X80 LSAW pipe combining heavy gauge and large diameter, heavy gauge X80 LSAW pipe with low temperature requirements, as well as X90 steels. In this paper, alloy design, production processing, as well as mechanical properties and microstructure used for these products would be illustrated.

Repeated deformation with optimum reductions in roughing mill exceeds the critical amount of strain required for recrystallization to occur. Recrystallization during roughing mill rolling refines austenite grain size. The uniform recrystallized grain as a starting microstructure, further pancake grains during finishing mill rolling and accelerated cooling ensures fine grain microstructure in strip.Gleeble 3500 has also been used for physical simulation of two rolling schedules to study the strain accumulation in 7 & 9 passes during rolling process. Actual plant trials were also conducted with 7 and 9 passes schedule in roughing mill. To match the total reduction of 9 passes schedule, additional reduction per pass was applied during 7 pass schedule rolling, which has resulted in improved toughness and uniformity in mechanical properties.The evolution of microstructure and texture of API 5L X70 line pipe steels produced using two different roughing rolling schedules with 7 and 9 passes, having fixed total reduction, was studied using the electron back scattered diffraction (EBSD) technique. This paper covers detailed study on strength & toughness, microstructures, including dislocation density, grain boundary angle analysis and Texture measurement, to compare changes in texture with 7 and 9 passes roughing mill schedules.This study has not only benefited in improving productivity for suitable of thickness-width combination but also saved cost of ferro-alloys and improved the quality.

This paper analyzes precipitation and dislocation strengthening behaviors of a 27mm thick Niobium-bearing Grade X80 steel plate for strain based design line pipe manufacture. The steel is produced by thermal-mechanical processing (TMCP) and is characterized with granular bainite and polygonal ferrite microstructure. Mechanical properties of both the steel and the UOE pipe are briefly introduced. Transmission electron microscope (TEM) is used to investigate the fine grain structure, distribution of the precipitates and dislocations in the steel. Precipitate morphologies, volume fractions of M(C,N), M₃C, CaS, AlN and Cu are extensively studied respectively by Electrolytic Chemical Phase Analysis (ECPA) and X-ray Small Angle Diffraction (X-ray SAD). Dislocations in the steel are characterized with Positron Annihilation analysis. The results prove that precipitation hardening reveal a 58.1MPa strengthening contribution by the precipitates less than 20nm in size. Dislocation hardening is approximately 176MPa to the present studied steel and 198MPa to the pipe.

The paper provides a brief introduction of Niobium microalloy technology, which could refine the as-rolled microstructure, inhibit the grain growth during heating and soaking, strength steel due to fine precipitates after tempering, increase hardenability, improve steel toughness and weldability. The research and application of Niobium-bearing steels for low temperature service pressure vessels, such as Al-killed C-Mn steel and Ni steels have been introduced in detail.

With the exploitation of gas mixed with sulfur, the requirement of hydrogen induced cracking (HIC) resistance for medium carbon vessels serving at environmental with hydrogen sulfide solution become more rigorous. Through the research on HIC, the formation reasons of those are: MnS inclusions, Al₂O₃ inclusions, P segregation, Bainites and Macro segregation of slab. Thus if these parameters are controlled appropriately, the qualified medium carbon vessels for resistance to HIC steel will be produced. The conclusions are following: the controlled measurements to cleanliness on liquid steel, macro segregation controlled on slab, large reduction at rolling and normalizing are used to produce the HIC resistance vessels. The properties of tensile strength and low temperature impact meet the standard properly. Meanwhile there aren’t hydrogen induced cracking found in any samples.

Extensive investigations of metallurgical roles played by Nb microalloying in advanced products of seamless steel tube have been carried out. The results show that with Nb microalloyed , the recrystallized austenite grain (RAG) and final ferrite grain of tubular steel are evidently refined even experiencing a piercing and a continuous rolling at very high temperature, and a certain quantity of (Nb,V)(C,N) and (Ti,Nb,V)(C,N) particles form on air cooling. Moreover, for quenching (Q) & tempering (T) treated tubular steels, the nanoscale particles of (Nb,V) (C,N) further precipitate on heating stage of Q at 900–1000°C, leading to a significant refinement of prior austenite grain (PAG) and final martensitic or bainitic packet/block structures, and during subsequent T at 600–700°C, producing an improved resistance to softening.The microalloying with 0.02–0.04wt%Nb in Q&T martensitic OCTGs of 155ksi or higher grades, have produced an adequate combination of ultrahigh strength and good toughness in casings for ultra-deep well and high-collapse services, perforation gun tube and drill pipe, etc. The SSCC resistance of Q&T martensitic OCTGs including C110 casings and SS105 drill pipe has been also evidently improved due to the 0.02–0.04wt%Nb microalloyed. With 0.03–0.05wt%Nb microalloyed and carbon, C_eq and P_cm reduced, an excellent strength/toughness/weldability combination has been achieved in Q&T bainitic line pipe and structural tube of X60–X100. In addition, the N80_1 class OCTGs with 0.24wt%C-0.02wt%Nb in hot rolling delivery has an improved and qualified mechanical property.

The article describes the production process and actual product properties of Cr-Mo steel for gasifier developed by Wugang. By taking key technological measures such as optimized composition design, grain refining, microstructure homogenization, simulated post welding heat treatment process , Wugang has successfully produced SA387Gr11Cl2 steel plate under hydrogen environment ,which satisfies the performance requirements of gasifier. Comparing with the same product produced by foreign advanced steel works, Wugang produced steel plate for gasifier is of good actual product properties with equivalent properties of those from foreign advanced steel works.

The effect of austenite deformation and continuous cooling on the evolution of microstructure in a high temperature processing (HTP) concept pipeline steel was investigated in this research. It was found that without austenite deformation, the transformed microstructure consists of blocky quasi-polygonal ferrite (QF) grains and parallel bainitic ferrite (BF) laths at a cooling rate of 0.5 °C/s. With increased cooling rates, the fraction of BF laths is raised and the microstructure reaches full BF at cooling rates of 5°C/s and higher. After austenite deformation, BF laths disappear at low cooling rates of 0.5~1°C/s and QF is the dominant phase. At a higher cooling rate of 5°C/s, the fraction of QF is reduced and acicualr ferrite (AF) becomes the main phase surrounded by QF grains. Increasing the cooling rate further, QF disappears and fthe raction of BF rises, finally leading to a BF dominant microstructure at a cooling rate of 50°C/s. Factors influencing these microstructure evolution characteristics were discussed, including segregation of niobium atoms at austenite grain boundary and the introduction of intragranular nucleation sites by deformation.

The microstructural development during roughing and early finishing of thick, high Nb-Ti steels containing high and low Mn contents has been investigated for conventional cold charging (CCR) and quasi compact strip production (CSP) conditions. Multi-pass rolling simulations were performed, followed by interrupted accelerated cooling, to study the “as-coiled” ferrite/pearlite microstructure. Local heterogeneous regions, LHR, were often found after CSP simulations, but not after CCR where the microstructure was generally uniform. Coarse, local heterogeneous regions can be prevented in HTP steels though complete recrystallization of the as-cast austenite during and after roughing by i) applying sufficient strain at roughing temperatures above the TiNb(C,N) precipitation region, ii) slow intermediate cooling and iii) using high Mn contents to suppress precipitation. For thick HTP skelp, the initiation of dynamic recrystallization is of little use in avoiding LHR due to limited strain available for completing the necklacing process.

The hot deformation behavior of nickel base superalloy 617B has been studied using hot compression experiments. Specimens were deformed in the temperature range of 1120–1210 °C with strain rates of 0.1 to 20 s-1 and total strain of above 0.85. True stress-true strain curves and deformation microstructures were studied. The results show that dynamic recrystallization is the mainly softening mechanism during the high temperature deformation of Inconel617B, and it can be effectively promoted by increasing deformation temperature. When deformation temperature is higher than 1180 °C, completely recrystallized microstructures can be obtained. Dependence of peak stress on the temperature and strain rate is modeled using the hyperbolic-sine Arrhenius-type equation. The average activation energy is determined to be 549kJ/mol.

Three kinds of Nb-Ti-Mo ferrite matrix micro-alloyed steels with three different Ti contents were designed. With optical microcopy, transmission electron microcopy, energy spectrum analysis and tensile test, the microstructures, precipitate particles as well as mechanical properties of three steels were studied. The strength and toughness of steel with0.092% Ti content were verified to adequate for Q600 grade refuge alternative plates. The results show that there were two kinds of particles with obvious different size in ferrite micro-alloy steels. The dominating precipitates are compound carbonitrides of Nb, Ti and Mo, which are less than 10nm and precipitated in chains or dispersed in the interior of grain. Another kind of particles are titanium carbonitrides with size about 200–300 nm. By the method of tensile tests at room temperature and at 500°C, the Nb-Ti-Mo ferrite matrix micro-alloyed steels have excellent mechanical properties. In addition, the samples with 0.092% Ti content have yield strength at 500°C exceeding400MPa, which are suitable as refuge alternation-plates.

The corrosion of pipe is most common problem for oil and gas industry. In this article, three kinds of tubes will be analyzed in terms of their resistance against stress corrosion. They are respectively N80 / 1, N80/ Q and P110. The loading method chosen in this test is constant tensile stress loading. In the test, samples will be separated in different groups, gradually loaded under specific levels and then soaked in H2S saturated solution. What can get from this test is threshold value of stress corrosion and stress-life curve, which can be used for evaluating the stress corrosion property of materials, as well as giving guidance for practical engineering.

Designing a competitive free-machining steel composition enabling less expensive steelmaking, manufacturing and recycling has long been a desirable objective. Foremost amongst a new approach has been simply to promote graphite formation to act as an internal lubricant, thus reducing or eliminating the need for special alloying additions (e.g. Pb, S, P, Bi, Se, Te) which can make the steels difficult to process or re-cycle, and, as more stringent health and safety legislation is introduced might eventually lead to restrictions or total prohibition from certain manufactured products. However, the sluggish kinetics of graphite formation in steel does not lend itself to the normal requirement of rapid manufacturing in the high tonnage steel industry. In consequence, this paper reports the machining characteristics of experimental carbon steel with a composition which accelerates graphite formation during a high temperature anneal. Three starting microstructures prior to annealing have been considered; martensite, bainite and ferrite/pearlite. These influence the eventual graphite dispersion and hence the machinability characteristics. The machining characteristics have been measured and also compared with commercial free-cutting steel grades.

Coal, as the most major energy in China, accounted for about 70% of China’s primary energy production and consumption. While the percentage of coal as the primary energy mix would drop in the future due to serious smog pollution partly resulted from coal-burning, the market demand of coal will maintain because the progressive process of urbanization. In order to improve productivity and simultaneously decrease safety accidents, fully-mechanized underground mining technology based on complete equipment of powered support, armored face conveyor, shearer, belt conveyor and road-header have obtained quick development in recent years, of which powered support made of high strength steel plate accounts for 65 percent of total equipment investment, so, the integrated mechanical properties, in particular strength level and weldability, have a significant effects on working service life and productivity. Take hydraulic powered supports as example, this paper places priority to introduce the latest development of high strength steel plates of Q550, Q690 and Q890, as well as metallurgical design conception and production cost-benefits analysis between QT plate and TMCP plate. Through production and application practice, TMCP or DQ plate demonstrate great economic advantages compared with traditional QT plate.

The phase transformation rule, microstructures and properties of 500HBW wear-resistant steel produced in Jinan Company of Shandong Iron and Steel Co. ltd. have been investigated in this paper. When the chemical composition of steel is given, the cooling rates after austeniting affect on the properties of steel greatly. The hardness and tensile strength increasing as increasing of cooling rates. The main cause is due to appearance and increasing of Bainite and Martensite other than Ferrite and Pearlite at room temperature. The cooling rate has distinct effects upon the mechanical properties of steel plates. With the 3–55°C/s cooling rate, the tensile strength vary in 979–1690MPa, the hardness changed from 341 to 596HV10. In the procedure of manufacture of 500HBW wear-resistant steels, the quickly cooling rate should be got to keep suitable microstructure and good hardness and toughness.

The CCT (continuous cooling transformation) curve of the test steel was obtained on Gleeble-1500 thermal mechanical simulator based on the thermal dilation measurement. The test steels were the deformation with 40% under the temperature of 900°C and then were cooled to room temperature at different cooling rates. The phase transformation law and hardness change rule were emphatically investigated at the cooling rate from 0.05°C/s to 30°C/s by optical microscopy (OM), scanning electron microscope (SEM) and Vickers hardness tester. The regression model of phase transformation was calculated. The results show that the transformation of ferrite and pearlite are inhibited, which cause the dynamic CCT curve shift to the right. The microstructure of the test steel consists of ferrite and pearlite after phase transformation at the cooling rate of 0.05°C/s; the granular bainite appears at the cooling rate of 0.5°C/s due to the interaction of Mo and B; as the cooling rate increases to 2°C/s, the microstructure mainly consists of lath bainite; the martensite is observed at the cooling rate of 2°C/s and there would be total martensite if the cooling rate is higher than 5°C/s and the grain growth rate is faster at the same time. The hardness of test steels increases with the increase of cooling rate, and increases obviously when the cooling rate is lower than 5 °C/s. A theoretical basis is provided to obtain the best bainite/martensite composite microstructure. The martensitic transformation start temperature is 354.6°C and the critical cooling rate is 5°C/s. The test value and the regression model can fit well and reflect the test value trend, showing a high precision of regression.

In this paper, different morphology and size of nano epsilon carbides (ε-carbides) were obtained by low temperature tempering for a low alloy abrasion resistance steel, the variation of mechanical properties and three-body impact wear resistance as the variation of carbides were investigated. The results shown that as the formation of needle like type ε-carbides, the yield strength, low temperature impact toughness and three-body impact wear resistance were all increased. While when the ε-carbides grow up to the bigger rodlike type, the hardness and toughness decreased sharply, as well as the three body impact abrasion wear resistance. The wear mechanisms of the steel obtained different carbides were also analyzed.

The chemistry composition, process parameters and the test results of Q890D free boron high strength steel plate used for engineering machinery was studied. The 16 ~ 40 mm thickness steel plates with good mechanical properties that was yield strength of 930 ~ 970 MPa, tensile strength of 978 ~ 1017 MPa, elongation of 13.5 ~ 15%, the average impact energy value of more than 100 J were developed by improving steel purity, adopting the reasonable controlled rolling and cooling process, using reasonable off-line quenching and tempering process. The test plates have good crack resistance in 60 °C preheat temperature condition because of that there are no any cracks in the surfaces, cross-section and roots of welding joints.

High Mn steels have great plasticity when deformed due to twinning, known as TWIP effect (Twinning Induced Plasticity) or due to martensitic transformation, called as TRIP effect (Transformation Induced Plasticity). The stacking fault energy (SFE) controls the deformation mechanism. So calculation of SFE value and Ms temperature is essential for designing of high manganese steels. For twinning to occur, it is usually necessary for the steel stacking fault energy (SFE) to be in the range of 18–35 mJ/m2. If the SFE is <18 mJ/m2, twinning is replaced by martensitic transformation. However, if it is >35 mJ/m2, then the slipping processing will be the only mechanism that contributes to the plastic deformation of steel. For alloys with Mn content between 15% and 25% the SFE is intermediate and then TRIP and TWIP effects coexist. In this work stacking fault energy of some compositions of High manganese steel has been calculated through thermodynamic model and X-ray line profile analysis and these values are verified with the SFE values available in literature. It was found that value of stacking fault energy calculated through thermodynamic model is found to be close with the value calculated through TEM node method and have large difference in values with comparison to X — ray diffraction method. Different empirical equations are used to calculate the Stacking fault energy of high manganese compositions and Martensite start temperature (Ms). Empirical equations are found to be more suitable for SFE calculation for austenitic stainless steels. Ms Temperature calculation from empirical equations is found to be more suitable for austenitic and high manganese steels.

As an important deoxidizer in steelmaking, the Aluminum in low carbon Aluminum-killed steel has a significant influence on the steel inclusion number and finished product property. In this paper, the influence of soluble (free) Aluminum content (in the range between 50ppm and 550ppm) on the property of final product of batch annealed Ti-IF steel is investigated on an industrial scale. Uniaxial tensile testing and optical microstructure observation were conducted. It was found that apart from the yield strength, the variation of Al content within the above range does not have any remarkable influence on other mechanical property. The microstructure shows equiaxed morphology regardless of the soluble Aluminum content. And the grain size was all graded within the range between 9.0 and 9.5. The final product shows no variation of mechanical property and formability according to the soluble Al content.

The flow stress, microstructure and softening mechanism of NM550 wear-resistant steel were investigated via hot compression test by using Gleeble-1500 thermal simulator in a temperature range from 800 to 1150 °C and strain rate range from 0.1 to 10s−1. With optical microscopy (OM), the effects of temperature, strain rate and true strain on the flow behavior and microstructure of the steel were discussed. The results show that the dynamic recrystallization occurs at higher temperature and lower strain rate. Hot deformation behavior of the steel specimens is sensitive to temperature and strain rate, while the strain rate affects the hot deformation behavior greatly. The apparent stress exponent and the apparent activation energy of the specimens are 7.395 and 389.611 kJ/mol, respectively. With the experimental data, the hot deformation constitutive equation and processing map of the NM550 wear-resistant steel are given. Finally, a combination of the constitutive equation and processing map provides a method for comprehensively investigating the hot deformation behavior of the steel under different conditions, and provides theoretical basis for the determination of rolling procedure in industrial production.

The amount of reinforcing bar (or “rebar”) currently produced globally is estimated at over 400 million tons per year. With global infra-structure demands increasing, this production level is forecasted for continued expansion. Product requirements along with production levels are ever increasing, but as an industry there is minimal to no effort being put forth to produce a better product. Rather, rebar is being commoditized to a point where price is the only real consideration taken into account when purchasing rebar for major projects. This “pricing only” mentality results in a product that barely meets minimum standards.The benefits of an enhanced quality rebar are many. Metallurgically intelligent and properly processed micro-alloyed rebar goes beyond simply compensating for a lack of tensile strength in concrete. A metallurgically intelligent/properly processed micro-alloyed rebar product has optimized strength and ductility, improved bendability and toughness, with good weldability, increased performance in seismic conditions and improved fatigue properties.The basis for these performance improvements comes from producing steel with as fine as possible uniform cross sectional grain size in the “as-rolled” condition. By designing a plant specific tailor-made alloy and then using intelligent metallurgy via proper process control and taking into account the existing equipment capabilities of a given mill, one can develop an optimum product that will not just meet the minimum current requirements of standards and designers, but will comfortably exceed current minimum requirements. In addition, this properly designed and produced rebar will result in benefits during fabrication and more demanding construction environments. When it is properly designed, this new intelligently engineered product will be cost effective and superior to the current carbon-manganese steel designs being commonly used.This paper is designed to give a solid understanding of the proper production processing and intelligently engineered metallurgical requirements needed to achieve a cross sectional grain size in “as-rolled” rebar that is as fine as possible within the limitations of the process and production parameters of the individual plant. The product of this improved process will be superior to the current rebar being produced to simply shore up the poor tensile properties of concrete. This product is currently being produced commercially in South East Asia

Ferritic stainless steel is being paid attention by steel industry and downstream users relying on its good over-all properties and low cost. With improvement of metallurgical technology and researching on fundamental theory, further development is obtained in properties optimization and quality stabilization of modern ferritic stainless steel, its marked feature are super purification and Nb/Nb-Ti stabilization. Nowadays, modern ferritic stainless steel is used widely in auto industry, home appliances, solar water heater, construction and decoration fields. The paper focuses on the effect mechanism of Nb technology on modern ferritic stainless steel and summarizes its application progress in typical downstream industries in China.Nb in modern ferritic stainless steel improves mainly in formability, corrosion resistance, weldability, high temperature and surface qualities. Studies show Nb is one of most effective element to improve drawability of ferritic stainless steel, Nb(CN) precipitation in high temperature bring the result of no solid solution C during hot rolling, which is extremely beneficial for the formation of positive texture and improve the formability consequently. The effect of Nb and Ti on improving intergranular corrosion by strongly stabilization C and N. Nb show the positive effect on weldability of ferritic stainless steel by Nb (C, N) hindering grain growing and formation of fine equiaxed crystal grain in welding HAZ, which make the possibility of Nb-bearing stainless steel application in higher requirement on weldability.

Niobium Bearing Low Carbon Low Alloy (LCLA) value-added S355 structural steels reduces the overall material and construction costs for many high strength construction steel and heavy equipment applications. The recent development of the Nb-LCLA Approach is a value-added low cost approach for many structural steel applications including windtower supports, beams and other structural plate applications. Case examples are presented from Brazil, China and the USA. These Nb-bearing steels at lower carbon content compared to the traditional higher carbon normalize heat treated grades are more cost effective and reduce structural fabrication time through improved weldability as well.

It is well known that the strengthening mechanisms of Nb Microalloying on low carbon flat products have been widely studied and recognized since Microalloying 75 in Washington, DC, but the application research of Nb Microalloying on higher carbon long products in excess of 0.020% carbon has been incomplete and full of debate for a long time due to limited solubility of Nb(CN) in austenite, dominant application of V Microalloying in long products, as well as uncertainty about the effects of Nb. In this paper, some new phenomena have been presented and clarified based on the application cases of Nb Microalloying in long products, including high strength rebars microalloyed with small amount Nb and high-carbon wire rod microalloyed with Nb addition up to 0.02% used for prestressed steel strand. Contrary to the traditional opinions, Nb-bearing rebars manifest higher tensile-to-yield ratio compared with V-bearing rebars, especially for high strength rebars of HRB500 and HRB600, which would be discussed based on production processing of rebar products. In addition, Nb Microalloying for high carbon wire rod used for perstressed steel strand also demonstrates encouraging application prospect by reducing net carbide of entectoid steel with carbon content up to 0.087%, as shown from test results of Nb-bearing 87B product. As for these abnormal phenomena, industrial production and subsequent microstructure analysis have been carried out to further validate and clarify the strengthening mechanisms of Nb Microalloying, which will facilitate the understanding of Nb Microalloying technology.

The combined addition of molybdenum and niobium is usually applied in fire-resistant steels with appropriate controlled rolling and controlled cooling. In order to develop new alloying strategy based on the advantage of vanadium resource at Panzhihua Iron& steel (group) Co., research on the Vanadium-containing fire-resistant weathering steels are conducted in this paper. Methods such as electron microscope, phase analyzing, tensile test, three dimension atom probe and so on were used to investigate the effect of vanadium on microstructure and tensile properties at both room temperature and high temperature up to 600°C for different processes. The relationship between the microstructure and properties was analyzed. It has been found that the amount of vanadium precipitates would be raised as vanadium content rose. Adding vanadium into the steel was effective to increase the room temperature and elevated temperature yield strength, especially increased the content of nitrogen. Vanadium mainly exists in the pro-eutectoid ferrite of the test steel, the “hidden” vanadium will precipitate from base microstructure when the temperature elevated to 600°C, simultaneously improve the elevated temperature properties. The alloying strategy of fire-resistant weathering steels at Pangang Steel is ascertained through the experimental work. The study offers the essential technical support for the further development of products.

This paper reviews the recent development of V-N microalloying technologies and its applications in HSLA steels in China. Enhanced-nitrogen in V-containing steels promotes precipitation of fine V(C,N) particles, and improves markedly precipitation strengthening effectiveness of V(C,N), therefore, there is a significant saving of V addition in a given strength requirement. V-N microalloying can be used effectively for ferrite grain refinement as well by the nucleation of intra-granular ferrite promoted by VN precipitates in Austenite in V-N steels. V-N microalloying process is a cost-effective way which has been widely used for high strength rebars, section steels, forging steels, seamless pipes, and CSP strip steels in China.

This paper presents an actual case of a new industrial building at CBMM’s plant in Araxá, Brazil as an example of lean design using microalloyed steels. The structure consists mostly of microalloyed ASTM A572 steel grades 65 and 50 instead of the conventional carbon manganese ASTM A36 steel. The application of grade 65 with more than 450 MPa of yield strength is an innovative solution for this type of construction in South America. A complete welding evaluation performed on the low carbon, niobium microalloyed grade 65 steel showed the welding properties and benefits. Niobium’s effect of increasing strength and toughness simultaneously resulted in relevant savings in total steel consumption for the project. The paper also quantifies the expected savings in costs, energy and carbon dioxide emissions.

The separate or combined addition of micro-alloying elements to the conventional weathering steel is usually applied in producing higher yield strength weathering steels. The target-oriented researches on the Vanadium-containing weathering steels were conducted in this paper in order to combine with the advantage of vanadium resource at Panzhihua Iron & steel (group) Co… A Glbeele-3500 thermal simulator was used to investigate the effect of the deformation amount and the nitrogen content on the precipitation temperature-time (PTT) curves of the vanadium-containing weathering steel and relationship between the nitrogen content or the deformation amount, temperature and precipitation time was analyzed. It has been found that the PTT curves of test samples are C curve shape. The temperature corresponding to the shortest time of precipitation is also found. Increasing the nitrogen content will make PTT curve move to the left significantly. Additionally, increasing deformation amount can also accelerate the precipitation rate of V (C, N) in the austenite and makes PTT curve move to the left obviously. It has also been found that the more the deformation amount, the shorter the start time of the V (C, N) precipitation. The study offers the essential technical support for the further development of products.

A high-performance Q370qE-HPS bridge steel with low carbon content (≤0.10wt%), Nb microalloying (0.025–0.050wt%) and low carbon equivalent (CEV) (≤0.38%) has been produced using TMCP procedure. The microstructure and mechanical properties of typical steel plates and layers parallel to rolling surface were characterized. The results show that the microstructure consists of fine-grained quasi-polygonal ferrite (QPF), less pearlite and a large number of fine dispersed Nb-rich precipitates. Consequently, an excellent combination of high strength, high toughness and low yield-to-tensile strength ratio (YR) is obtained. In addition, the QPF grain refinement and pearlite reduction take place simultaneously with increasing distance from central to surface of an identical plate or decreasing thickness of plate, leading to increase in yield strength (Rp0.2), impact energy (EKV2, -40°C) and YR, and approximately the same tensile strength (Rm). The effect of QPF grain size on Rp0.2 and effects of QPF grain size and pearlite area fraction on EKV2 (-40°C) can be described by Hall-Petch formula and Boltzmann model, respectively, while the YR is in a good linear relation to QPF grain size and pearlite area fraction.

For the purpose of reducing alloys additions and then its cost of conventionally combined Nb-V microalloyed Q460 grade steel plates for construction, a HSLA steel alloying design with only V-N microalloying of 0.06–0.08%V and free of niobium, and corresponding thermal mechanical controlled process were facilitated by better understanding the third generation TMCP concept and V-N microalloying technology. These resulted in successfully commercializing gauge 50mm V-N microalloyed Q460GJC steel plates with good combination of high strength >470MPa and improved toughness of Akv@0°C>150 J, and low YS/UST ratio of 80% as well. Theoretical calculation indicated that the contribution by fined-grain and precipitate strengthening amount to ~66% of yield strength of the newly developed plate steels.

Recently, with the rapid upgrading of the equipment in the steel Corp, the rolling technology of TMCP has been rapidly developed and widely applied. A large amount of steel plate has been produced by using the TMCP technology. The TMCP processes have been used more and more widely and replaced the heat treatment technology of normalizing, quenching and tempering heat process. In this paper, low financial input is considered in steel plate production and the composition of the steel has been designed with low C component, a limited alloy element of the Nb, and certain amounts of Mn element. During the continuous casting process, the size of the continuous casting slab section is 300 mm × 2400 mm. The rolling technology of TMCP is controlled at a lower rolling and red temperature to control the transformation of the microstructure. Four different rolling treatments are chosen to test its effects on the 390MPa grade low carbon steel of bainitic microstructure and properties. This test manages to produce a proper steel plate fulfilling the standard mechanical properties. Specifically, low carbon bainite is observed in the microstructure of the steel plate and the maximum thickness of steel plate under this TMCP technology is up to 80mm. The mechanical property of the steel plate is excellent and the KV2 at -40 °C performs more than 200 J. Moreover, the production costs are greatly reduced when the steel plate is produced by this TMCP technology when replacing the current production process of quenching and tempering. The low cost steel plate could well meet the requirements of producing engineering machinery in the steel market.

The current trend of 500MPa high strength ribbed bars was studied. The influence factors of mechanical properties were investigated by analyzing the final control of converter, optimization of chemical composition, deoxidation alloying, soft blowing system, stable continuous casting and heating system. Based on using different overseas and domestic vanadium-nitrogen alloy, the principle of vanadium micro-alloying for 400MPa and 500MPa steel bars were discussed. The mechanical properties were analyzed when using different micro-alloying methods, the 90-day aging experiment was carried out to study the change of mechanical properties. It was showed that the content of V, C, Mn had obviously effect on mechanical properties, of which content of vanadium was the main factor. Meanwhile, it was beneficial to improve the mechanical performance by increasing the content of free nitrogen.

As a kind of impurity and displaying with FeS and MnS form in steel, Sulfur can make the disadvantage effect on the performance of hot-working, welding and corrosion resistance. The high content sulfur in steel can cause the hot brittle phenomenon for the steel. For the welding steel, when the sulfur content is higher, the drawing performance of wire rod become worst and the yield of wire rod decrease. When the sulfur is lower, the automatic wire feeding performance for the gas shielded welding become worst and the weld seam is not smooth. According to the results of welding expert research, 0.010%≤ S≤ 0.020% in CO2-shielded welding wire steel is reasonable.The productive process of CO2-shielded welding wire steel in Long Product Department of Tangshan Iron and Steel Company is Combined Blowing Converter, LF Refining and Continuous Casting. On the basis of further study for this process characteristic, it is obviously that LF refining was the key to control sulfur content in CO2-shielded welding wire steel, and the sulfur content precision control technology was formulated including Converter process and LF refining process. In Converter process, The charging hot metal sulfur content is 0.015%≤ S≤ 0.035%, and the S>0.040% hot metal are prohibited to smelt welding steel. The cool material was priority to the tin ball in the melting process. The proportion of 0.015%≤ S≤ 0.030% in the ladle liquid steel before LF is 93.65%. In LF refining process, when the Ls≤ 30, R≤ 2.0, w(FeO+MnO)≥ 1.0%, the purpose of appropriate desulphurization in LF can be achieved. The proportion of 0.010%≤ S≤ 0.015% in the liquid steel after LF is 84.29%. The sulfur content of welding wire steel after LF varies from 0.008% to 0.019%, and the average sulfur content is 0.013%. The welding wire steel clearness and performance meet user requirements.

The technological development of value-added applications for niobium (Nb) microalloyed structural steels has expanded into the high carbon long products sector. Micro additions of 0.005 to 0.020%Nb in various high carbon steel compositions have exhibited improved processing, such as wire drawability or formability during manufacturing, higher productivity at reduced operational cost as well as improved mechanical properties compared to traditional non-Nb bearing high carbon steels. Although the Nb solubility is limited in high carbon steels compared to low carbon steels, the optimized Nb content has been defined based on experimental results and now transferred into industrial operations. The Nb delays the pearlite transformation, hence resulting in a finer interlammelar spacing compared to traditional high carbon pearlitic steels. Consequently, the optimized Nb content leads to improved mechanical properties due to this finer interlamellar pearlite spacing. Opportunities exist to transfer this new Nb-technology into wire rods, bolts, rails and tire cord.

The results of experiments show that Nb can significantly lower phase transformation temperature and refined both lamellar space of pearlite and pearlite colony. The refinement on microstructure leads to improvement in steels’ mechanical properties. However, there is more ferrite formed on grain boundary because of a small amount of Nb addition. To add Cr together with Nb can effectively avoid the formation of ferrite on the grain boundary while microstructure is refined. The results are successfully applied into the development of rail steel U68CuCr which has much higher corrosion resistance as well as better mechanical properties in comparison to conventional rail steel U75V.

Structural steels with yield strength requirements greater or equal to 690 MPa can be produced through controlled recrystallization hot rolling coupled with precipitation strengthening or purposeful heat treatment through quench and tempering (Q&T). High strength structural steel and wear/abrasion resistant requirements greater or equal to 360 Brinell hardness (BHN) are produced by the development of microstructures of tempered lower bainite and/or martensite through the Q&T process. While these Q&T microstructures can produce very high strengths and hardness levels making them ideal for 690 MPa plus yield strength or wear/abrasion resistant applications, they lack toughness/ductility and hence are very brittle and prone to cracking. While tempering the microstructures helps in improving the toughness/ductility and reducing the brittleness, strength and hardness can be sacrificed. In addition, these steels typically consist of alloy designs containing boron with carbon equivalents (CE) greater than 0.50 to achieve the desired microstructures. The higher CE has a negative influence on weldability.To achieve optimum mechanical properties, microstructure through either controlled recrystallization hot rolling coupled with precipitation strengthening or the Q&T process with the lowest CE and cost it is important that the metallurgical design engineer understands how to properly design the alloy (C, Mn, Si, Cr, Mo, Cu, Ni), the proper use of microalloy metallurgy, especially niobium and titanium, the proper rolling/cooling/QT process design and proper use of boron metallurgy. By having the proper understanding of an optimized metallurgical/alloy/process design will result in the proper cross sectional microstructure, optimum balance of strength, hardness, toughness, ductility, weldability and final shape achieved at the lowest cost.This paper will explain the proper alloy, microalloy design, boron metallurgy and processing to produce optimized high strength/wear resistant structural steels at the lowest cost. This paper is the metallurgical basis of the implementation of optimized high strength/wear resistant steels used recently in China for the successful design of improved performance lighter weight heavy duty 150 ton haul truck buckets and 40 ton dump truck buckets.

The dynamic recrystallization behaviors of the microalloyed non-quenching and tempering steels of 38MnVS and C38N2 were studied by the simulation of hot deformation in the Gleeble3800. The stress versus strain curves of both steels were measured at different deformation temperature and deformation strain rate. The relationship among deformation temperature, deformation strain rate and the peak stress was analysis to calculate the activation energy of the hot deformation. The dynamic recrystallization behaviors and the grain size measurements were identified based on the microstructural examination. The hot processing map and the hot deformation instability map were drawn based on the relationship among deformation strain rate, deformation temperature and the peak stress to optimize the hot deformation parameters. Both 38MnVS and C38N2 was processed by the controlled forging and controlling cooling process (CFCC-process), it was pointed out that Nb plays an important role in the dynamic recrystallization and the processing of the non-quenching and tempering steels.

The demand for more sustainable development promotes the need for components and steel structures with a longer useful life and better performance. Upgrade of wear steel plate used in key industry segments such as mining, recycling and road building results in the stable growth of global market with high quality grade Q&T wear plates (Hardness HBW≥400, and Yield strength ≥690 Mpa). SSAB has now expanded its wear steel product range by both thicker and thinner Q&T plate to meet the needs of the market, and can offer wear plates from 0.7 mm to 160 mm. The continuous research and development is being done to offer even thicker plates. This article introduces the performance and advantages of high quality grade Q&T wear resistant steel products (plate, strip, tube and round bars) produced in SSAB, and also describes typical applications in some industrial segments such as material handling and construction machinery.

Hot compression tests were performed on gleeble-3800 thermal simulation machine to study the effect of different deformation parameters on microstructure of non-quenched and tempered steel with ferrite and pearlite. The results show that low deformation temperature can increase the content of ferrite. Increasing the degree of deformation has significant effect on grain refinement. Because large deformation can enhance the degree of fragmentation of grains, promote nucleation of eutectoid ferrite precipitate and increase the ferrite content, and non-quenched and tempered steel toughness is increased. High cooling speed can refine ferrite size and decrease the pearlite spacing significantly. In the temperature range of phase transition, slow cooling after rapid cooling can reduce the average size of pearlite and improve the toughness of the microstructure remarkably.

Improving the material of axles is not only used to enhance the reliability of the use of railway wagon axle, but also an effective means to raise railway transportation capacity. The axle steel heat-treated by “double normalizing and tempering” is applied as a sample in the current research. Not only the influence of V micro-alloying elements on the microstructure of the sample is studied, their existing morphology, quantity and distribution feature, as well as changes of the mechanical properties are also analyzed. The analysis shows that the main strengthening mechanisms of the V micro-alloying axle steel is the fine grain strengthening and dispersion strengthening, the grain refinement can improve the strength while improving the toughness, it is the most important way of strengthening and toughening.

Strengthen-mechanism and corrosion resistance of niobium addition high strength low alloy weathering H-beams produced by ultra-fast cooling technology, had been investigated in this research. The results show that the microstructure of the ultra-fast cooling weathering H-beams sample was consisted of polygonal ferrite and pearlite as well as fine, dispersive and uniform Nb(C,N) precipitates. Niobium element plays its role of grain refinement and precipitation strengthening by refining ferrite grains and obtaining (Nb,Ti)(C,N) precipitation, which significantly improved the strength and toughness. Furthermore, the corrosion resistance performance of weathering H-beams was also studied by simulated atmosphere-salt spray test. It was found that the corrosion rate and thickness reduction of the weathering H-beams with homogeneous rust layer were lower than that of conventional carbon steel Q345. The results also revealed that the corrosion process can be divided into initial stage in which corrosion rate increased with accumulation of corrosion products and the second stage in which homogenous, adherent and compact rust layer started to protect the steel substrate out of corrosion solution.

Microstructure evolution and impact thoughness of V-N-Ti and Nb-V-Ti microalloyed bulb flat steel in rolling process have been investigated. The results indicate that the V-N-Ti steel has superior impact toughness than Nb-V-Ti steel. Compared with Nb-V-Ti steel, the relatively higher N content increase the amount of undissolved TiN in V-N-Ti steel at 1220 °C, which can effectively inhibit the austenite grain coarsening. Meanwhile, N can also promote the precipitation of V(C,N) in V-N-Ti steel during the rolling process, V(C,N) can be able to promote the intragranular ferrite formation, inhibit the grain boundary ferrite grain coarsen and refine the ferrite grain, which lead to a favourable toughness; While the microstructure of Nb-V-Ti steel is coarsen ferrite and bainite which are detrimental to impact toughness. Compared with Nb-V-Ti steel, the V-N-Ti steel has a more appropriate composition design for bulb flat steel.

Two kinds of Cr-Ni-Cu low alloyed steels were designed, 0.1%C-0.7%Cr-1.2%Ni-0.7Cu and 0.1%C-0.7%Cr-0.3%Ni-0.5Cu. With the method of SEM, XRD and electrochemical analysis and testing technology, periodic immersion accelerated corrosion test was carried out to investigate the corrosion resistance of the designed steels in simulated marine environment. The steel with best corrosion resistance was selected, and then focused on the variation of its corrosion rate with time. The results indicated that the designed Cu-Cr-Ni low alloyed steels showed better corrosion resistance than 20MnSi, the ratio of their corrosion rates was 0.44. The corrosion rate of designed steels decreased gradually to 3~4 g/(mm2·h) with the elongation of test period, while the corrosion rate of 20MnSi kept downward trend, not reach stability, and the corrosion rate gap between them became smaller. The Cr element banding enriched in the inner rust can withstand the diffusion of Cl−. Besides, the addition of Ni raised the self-corrosion potential of the bare steels and promoted the transformation of γ-FeOOH to α-FeOOH, and consequently, improved the stability of the rust and the corrosion resistance of steels.

The paper is subjected to take some heat treatment process to improve the plasticity and toughness in low carbon microalloyed submarine pipeline steel. The results showed that the combination of furnace cooling after normalizing, annealing at AC1+60–100°C and two-step intercritical tempering above AC1’ can obtained the optimum plasticity. The uniform elongation and toughness was 23.82 % and 230 J at −40°C respectively with NLTT process. However, the uniform elongation was up to 26.26% with 16.49% (volume fraction) obtained austenite when the cooling mode was furnace cooling after normalizing in the N’LTT process.

Thermal simulator and transmission electron microscope (TEM) were adopted to investigate the precipitation behavior in different stage of controlled rolling, controlled cooling and the effect of cooling route on precipitation. It is found that the undissolved cuboid precipitates TiN in reheating process was covered by Nb precipitates during rolling process and the content of Nb increased relatively. The strain-induced NbC precipitates were found to be spherical shaped when waiting for a few seconds after rough rolling. The size of precipitates obtained from two-stage cooling process, in which fast and slow cooling were employed respectively, was much smaller and sparse distributed than that obtained from one stage cooling process with slow cooling rate and that is beneficial for precipitation strengthening. The results supplies the theoretical basis for practical application.

The present paper involves a fundamental research on microdomain yield behavior of an ultrahigh strength low alloy steel with high temperature tempered bainite. The smooth cylinder specimen was took from deep water mooring chain links from the steel with the chemical composition of 0.23C–0.25Si −0.70Mn–3.55 (Cr+Ni+Mo) −0.13 (V+Nb+Ti) (mass %) ,which was quenched from 1253K and then tempered at 873K Its macroscopic yield strength is 1120MPa and the tensile strength is 1250MPa In-situ neutron diffraction measurements of loading tension have suggested that a good linear elastic deformation can be kept up to 500MPa stress, and then (200) priority non-linear elastic strain, that is the yield of crystal lattice occur at 700MPa and the (110) non-linear elastic strain was found at 800MPa. The (200) and (110) nonlinear elastic strain increases gradually when the stress was further increased, however, the (211) kept its linear elastic deformation stage as before. The sub-microstructural analysis carried out using TEM and additional determine the nature and quantitative analysis has revealed that there are three kinds of alloy carbides: (1) θ-M3C cementites with an average particle size of less than 50 nm which inside laths and lath boundaries; (2) ε-M2C formed uniformly within the ferrites with a length of less than 200 nm and width of less than 20 nm; (3) ultra-fine high density MC cohered with matrix α-Fe and its particle size is about 2 nm. The whole microdomain yield behaviour of the material was possibly influenced by the fcc-MC with high density. The results of CLT (constant load), SSRT (slow strain rate) and KIscc test of the present chain in seawater solution indicate, that threshold value of SCC (stress corrosion cracking) stress exceed 0.8 tensile strength and the chain’s KIscc value is double of KIscc value of 4340 steel type parts. MC not only form strong hydrogen trap, but also slow down microdomain yield likely by means of increasing yield strength of crystal lattice, thus reduce SCC sensibility of the steel.

The effects of heat treatment on microstructure and mechanical properties of V microalloyed bulb flat steel were studied. The results showed that a large number of granular bainite was formed in the period of rolling or normalizing continuous cooling. The low-temperature toughness of the steel was decreased obviously with the formation of granular bainite. Akv₋₄₀ could be up to 46J by tempering treatment with the granular decomposed. Furthermore Akv₋₄₀ could be up to 144J by normalizing and tempering treatment since that the granular bainite decomposed sufficiently.

The development of physically-based models for the microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where the transformation of austenite to a variety of transformation products eradicates the hot deformed microstructure. This paper reports on a model Fe-30wt%Ni based alloy, which retains a stable austenitic structure at room temperature, and has therefore been used to model the development of austenite microstructure during hot deformation of conventional low carbon-manganese steels, thus providing for an understanding of the role of austenite microstructure on the subsequent transformation behaviour. It also provides an excellent model alloy system for microalloy additions, providing insight relating to the precipitation location and kinetics in the deformed austenite. This research will discuss the results from a microalloyed Fe-30% Ni-Nb alloy in which the strain induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed. Additional results will be shown for the effect of strain path reversal on the microstructural development of a model Fe-30wt%Ni alloy, compared with a C-Mn steel processed in a similar manner in order to establish a clearer understanding of the role of the austenite microstructure on subsequent transformation behaviour.

This paper investigated two kinds of corrosion resistant low alloy steels depending on the environment of the North China see (Steel S) and South China sea (Steel N), respectively. The mechanical and corrosion properties of the two steels were analyzed in this paper. Tin was added into both steels to improve the corrosion resistance. Structure and mechanical properties of the two steels were detected, and the results revealed that the microstructures of both steels were ferrite and little divorced pearlite. The yield strength and impact toughness at −40°C of the steel S are 423MPa and 98 J, respectively. The yield strength and impact toughness at −40°C of the steel N are 437 MPa and 70 J, respectively. The properties mentioned above met or even exceeded the requirement (yield strength 355 MPa, toughness 34 J) in these areas. The corrosion resistant properties of the two steels were also investigated via the means of immersion test and electrochemical experiment. The immersion test indicated that the corrosion rate of steel S and steel N was 0.00938 mg/h·cm2 and 0.00838 mg/h·cm2, respectively, when completely immersed for 168 hours, and the corrosion rate was much lower than that of E36. The Electrochemical experiments showed that the corrosion potential (E_corr) of both steels was higher in contrast to E36, which indicated a lower corrosion trend.

A continuous demand for thick plates for structural applications with yield strength of 450–485 MPa, high toughness and cold resistance requires the development of new methods of additional grain refinement at all stages of thermo-mechanical control process (TMCP). The effect of Ti-Nb precipitates morphology on grain growth during reheating was investigated. It has been shown that Nb is a key element for grain growth control. Submicron Nb carbonitrides suppress austenite grain growth at lower slab reheating temperatures. The effect of Ti additions on suppression of grain growth during low-temperature reheating is negligible. Parameters of reheating such as duration and temperature were adjusted in order to maximize the effect of Nb microalloying and to receive finer austenite grain before hot rolling. Grain refinement in the course of hot deformation was studied through physical modeling. Using the recrystallization model, new rolling schedules were introduced. Industrial trials were conducted; the effect of reheating and deformation parameters on cold resistance of steel was investigated. Implementation of the results of given study into production practice made it possible to produce plates with excellent set of properties, including strength, toughness, and cold resistance.

The E690 grade offshore platform steel was obtained by top and bottom blowing converter, curved spray continuous casting machine and thermo-mechanical control processing (TMCP). The static continuous cooling transformation curves (CCT curves) of the rolled plate were drawn by adopting thermal expansion method and metallographic analysis. The microstructures at different cooling rates were observed by optical microscope (OM) and scanning electron microscope (SEM). Results showed that the super-cooled austenite in tested steel was relatively stable and there was little high temperature microstructure transformation at the cooling rate of 0.1°C/s. The regression model of phase transformation temperature and cooling rates were obtained. According to the results of orthogonal experiment, the most optimal heat treatment process was firstly heated at 890 °C, wind cooling and then tempered at 630 °C for 60 min.

The corrosion behavior of four kinds of low alloyed steels in acid chloride solution (10%NaCl, pH=0.85) was studied on the basis of International Maritime Organization(IMO) standard. Results indicated that, with the increase of Cu and Ni content in steels, the corrosion rate exhibited a law of linear decline, polarization curve moved to positive direction, corrosion potential raised, current density significantly decreased and the interface electric resistance obviously increased. The mechanism of improvement the corrosion resistance of Cu bearing steel can be mainly attributed to the Cu enrichment on the surface of rust layer by means of re-deposition particles(100~500 nm) and keeping high stability. However, Ni was not found in the rust layer but improved the corrosion potential of matrix.

Hyundai Steel Company had developed the inverted angles as second only to the Japan steel maker in 1992. Current market share of Hyundai Steel Company is second position after JFE Steel Corporation. Main application of the inverted angles is the reinforcement of shipbuilding as the stiffener of hull. Hyundai Steel Company has also been developing the low temperature steel grade applied for LPG carrier’s cargo tank and secondary barrier using slab. This steel grade require the guarantee of -60°C impact toughness and weldability.

Based on the technological requirements and market demand, Nb micro-alloying D36 grade high strength shipbuilding plate has been successfully developed in HBIS. In this papers, the rational chemical compositions design, smelting and rolling process of Nb micro-alloying D36 grade high strength shipbuilding plate were introduced. Its various performance figures not only comply with the rules of nine classification societies of CCS, LR, ABS NK, DNV, BV, GL, KR and RINA but meet users’ requirements. It indicates that HBIS have capacity producing Nb micro-alloying D36 grade high strength shipbuilding plate.

Tensile tests and fracture toughness tests supported by detailed fractographic examination have been carried-out on two sets of C-Mn weld metal samples with identical microstructure but different inclusion surface features. The tests showed that the tensile and fracture toughness properties were not significantly changed by the presence or absence of sulphide “patches” formed on inclusions. Analysis at a detailed level suggests that cleavage fracture initiated from inclusions with “sulphide” patches tends to be associated with higher fracture toughness than initiated from inclusions without “patches”.The fractographic investigation revealed that inclusions initiated cleavage in three ways: (1) inclusion cracking followed by crack propagation into matrix, (2) inclusion decohesion from the matrix accompanied by cracking adjacent particle or brittle phase to trigger cleavage, and (3) cleavage starting from an inclusion with local inclusion/matrix interface separated. No sulphide “patches” were found on inclusions associated with the first mode of cleavage initiation, but sulphide “patches” were always associated with the second and third modes. Relationship between the cleavage initiation modes, nature of a sulphide coating, and fracture toughness are discussed.

The influence of nitrogen addition on the microstructure and mechanical properties of vanadium microalloyed steels was studied by thermal simulation and bench scale rolling experiment . The results show that addition of nitrogen enhances the starting temperature of transformation, and promotes the formation of intragranular ferrite. Increasing nitrogen promotes acicular ferrite formation, and refines MA constituent. Increasing vanadium promotes the precipitation strengthening, but has little effect on toughness. For steel rich of nitrogen, vanadium mainly precipitates in austenite in the form of VN. For low nitrogen steels, however, vanadium precipitates as VC in ferrite. The lattice misfit degree of austenite-ferrite, VC-ferrite and VN-ferrite are 6.72%, 3.89% and 1.55% respectively. It indicates that VN precipitated in austenite acts as preferential nucleation sites and promotes the intragranular ferrite transformation.

The corrosion performance of high strength weathering steel with Cu, Cr micro-alloying under as-rolled, annealed, normalized, and TMCP states was studied. Results showed that the microstructures of steels under as-rolled, annealed and normalized states were composed of pearlite and ferrite. Average grain size of as-rolled steel was the largest, followed by annealed and normalized. The microstructure of TMCP was mainly consisted of martensite. The corrosion potentials of naked steels in TMCP, as-rolled, annealed and normalized states had little difference, but the corresponding corrosion currents were 0.1287× 10−5A•cm−2, 0.4418× 10-5A•cm−2, 0.7364× 10−5A•cm−2 and 1.064× 10−5A•cm−2, respectively. However, the variation tendency of polarization resistance was contrary to the corrosion currents. After two months, the polarization resistance of the TMCP steel decreased to 1304Ω•cm2, but the annealed, as-rolled and normalized steel increased to 1881Ω•cm2, 1561Ω•cm2 and 1068Ω•cm2, respectively. The arrangement of annual corrosion rate from high to low was: normalized, as-rolled, annealed and TMCP steel, and the rule of rust layer from thick to thin was similar.

The effect of ultra-fast cooling(UFC) and conventional accelerated cooling(AcC) on the mechanical properties and microstructure of controlled rolled AH32 grade steel plates on industrial scale were compared using tensile test, Charpy impact test, welding thermal simulation, and microscopic analysis. The results show that the properties of the plate produced by UFC are improved considerably comparing to that by AcC. The yield strength is increased with 54 MPa without deterioration in the ductility and the impact energy is improved to more than 260 J at -60 °C with much lower ductile-to-brittle transition temperature(DBTT). The ferrite grain size is refined to ASTM No. 11.5 in the UFC steel with uniform microstructure throughout the thickness direction, while that of the AcC steel is ASTM No. 9.5. The analysis of nucleation kinetics of α-ferrite indicates that the microstructure is refined due to the increased nucleation rate of α-ferrite by much lower γ→α transition temperature through the UFC process. The Hall-Petch effect is quantified for the improvement of the strength and toughness of the UFC steel attributed to the grain refinement.

In recent years, there has been development of several significant pipeline projects for the transmission of oil and gas from deep water environments. The production of gas transmission pipelines for application demands heavy wall, high strength, good lower temperature toughness and good weldability. To overcome the difficulty of producing consistent mechanical property in heavy wall pipe Shougang Steel Research in cooperation with the Shougang Steel Qinhuangdao China (Shouqin) 4.3m heavy wide plate mill research was conducted.This paper reports on the experience with the research and development of pipeline up to 40.5mm in wall thickness grade 485FDU intended for applications in the Middle East-India deep water environments. The plate was produced at Shougang Steel’s 4.3m heavy wide plate mill in Qinhuangdao China (Shouqin). Alloy design and processing strategy along with production parameters are introduced. The achieved mechanical properties in both plate and pipe are reported. The pipe tensile test results in transverse and longitudinal directions achieved yield strength and ultimate tensile strength of 525~555MPa and 625~650MPa respectively, which are higher than plate yield and ultimate tensile strengths of 475~515MPa and 615~640MPa respectively. The increase of strength during pipe making is due to the creation of a fine uniform grain bainite and polygonal ferrite microstructure from the alloy/processing design. Even with this DNV485FDU strength level and heavy wall the low temperature fracture toughness achieved was excellent. Drop weight tear testing (DWTT) of pipe body (19mm thickness samples, two sides average thinning) achieved an average of 95% shear at -27°C. And crack tip opening displacement (CTOD) of pipe body achieved an average of 1.84mm at -10°C. All of these results demonstrated that the heavy wall 40.5mm DNV485FDU production at Shougang Steel’s 4.3m wide heavy plate mill (Shouqin) can meet the technical specifications of “3500m Deep Water Pipeline Development” and DNV-OS-F101.

Ni-Cu-P coating by chemical plating has excellent corrosion resistance,wear resistance, thermal stability and electrical conductivity. In this paper, Ni-Cu-P was prepared by chemical deposition methods, the optimum process of chemical plating Ni-Cu-P. By using scanning electron microscopy(SEM), spectrum analyzer, Autolab workstation on the corrosion resistance of the coating. The results showed that: (1)after the specimen surface chemical plating nickelcopper phosphorus treatment, in the same corrosion potential, the corrosion current density was lower than that of Ni-P coating and substrate materials; (2)had a significant effect on corrosion resistance of quantity of citric acid sodium on nickel copper phosphorus alloy plating, coating corrosion resistance with increasing citric acid sodium content first increased and then decreased, and inaddition level of sodium citrate is 40g/L, the corrosion current density reaches a minimum, 14.51×10–6A/cm2; (3)under the same conditions, the Ni-Cu-P alloy coating pH impact is the biggest,the maximum impedance 1268.05Ω; (4)in the 3.5%NaCl solution, with the change of copper content, the main trend of the corrosion current is decreased first and then increased, and the content of copper in Energy spectrum analysis within 5.18Wt% corrosion current density of a minimum of 14.51×10–6A/cm2, the corrosion resistance. With the increase of Cu content in the coatings, the P content first increased and then decreased in the coating, the content of Ni decreased first and then increased; (5)the best technology:NiSO₄6H₂O, CuSO₄ 25g/L, 5H₂O 0.15g/L, C₆H₅Na₃O₇•2H₂O 40g/L, NaH2PO2H2O 25g/L, CH3COONa 15g/L, KIO3 0.03g/L, C12H25NaO4SO3 0.01g/L, pH4.75 ± 0.01,temperature 80 ± 1 °C, deposition time of 2h.

The effect of copper on the corrosion properties of mooring chain steel in synthetic seawater at room temperature were investigated by weight loss tests, electrochemical methods and corrosion product analysis. The results indicated that the mooring chain steel exhibited active dissolution behavior in synthetic seawater. Corrosion potential shifted to noble direction and corrosion current decreased with copper contents. Therefore, the weight loss reduced and the polarization resistance was increased with increasing copper addition. The improvement of corrosion with copper was attributed to the change of corrosion rust that became smaller granules with higher adhesion strength as well as thicker layer when the steel was added more copper content. Meanwhile, the microcrack of inner rust was prevented to grow.

The susceptibility to hydrogen embrittlement in high strength mooring chain steel with different boron content (0, 0.003 %, 0.008 %) were investigated by electrochemical hydrogen charging technique and tensile test. The results revealed that appropriate boron content can effectively depress hydrogen induced embrittlement. Precharged with a low current density, this effect seemed to be unobvious. It gradually became clearly with the increasing current density. The increase of resistance to the hydrogen embrittlement for 3B and 8B after adding appropriate boron was attributed to three facts. The first was that the segregation of boron atoms along grain boundaries reduced the grain boundary segregation of phosphorus, which prohibited hydrogen concentration at the grain boundaries, depressing the possibility of the intergranular fracture due to H. The second was that the segregation of boron increased intergranular cohesion, enhanced grain boundary strength, and refined the final microstructure. The third was that the addition of boron changed the state of hydrogen traps, leading to the small amount of diffusible hydrogen. That is to say, hydrogen transferred to these defects by dislocations was accordingly decreased, which led to the low sensitive of hydrogen induced cracking.