Advanced Steels

It has been generally believed that steel is a kind of advanced materials, presenting characteristics to meet a variety of requirements. They could be applied to the circumstances subject to the elevated temperature up to 650°C and cryogenic temperature down to −196°C, to the applied stresses from 100 up to 5,000 MPa, to the corrosion of atmosphere, acid, alkali, salt, etc. Steels has been widely used for construction, automobile, rails, shipbuilding, petrochemistry, machinery, weaponry, daily life, etc.

As one of the vital structural materials, steel has played an important role in national economic development. Under the background of global warming, holding back carbon footprint has become the main task of our mankind. As a giant source of CO

2

emission, it is rather a severe challenge for steel industry to develop further under

Energy Saving and Emission Reduction

Policy (ESER). This article has reviewed and envisioned such practice on steel production, and analyzed how to make innovation on steel material based on Baosteel’s own practice so as to provide material solution for down-stream sectors. High strength, high toughness, long service life and versatility of steel material are the trend for material innovation.

The structure steel industry has experienced a revolution during 4 decades. Faced the challenge of global change in climate and environment, the higher strength ductility steels and the environment friendly steels are needed. The R&D for high strength steel production and application in Ansteel has made impressive progress. However, more attention had been paid on the development of new-type high strength steels with higher strength and better properties. The multi-phase microstructure, lower y/s, and corrosion resistance performance structure steel result in new generation high strength steel, which have properties that are often much superior to those exhibited by the older steels. This chapter presents a general review of new generation high strength steel research and development in Ansteel and predicts the development for advanced high strength steel in the foreseeable future.

TISCO is the earliest and the largest enterprise of stainless steel production in China. After continuous technical reconstruction, especially 500,000t stainless steel enlarging capacity reconstruction and 1.5 million tons of new stainless steel revamp project, stainless steel output reached 3 million tons. In recent years, a group of proprietary technology with independent intellectual property rights was formed through the independent innovation. In aspects of the stainless steel development, TISCO has optimized product structure and the products are widely applied in the high-end market.

India is the fifth largest steel-producing nation in the world. The Indian steel industry accounts for over 7% of the world’s total steel production. The domestic crude steel production grew at a compounded annual growth rate of 8.6% during 2004–2005 to 2008–2009. The National Steel Policy of the Government of India has a target for taking steel production up to 110 MT by 2019–2020. While 2007 was an exciting period in the history of Indian steel industry, corresponding to 7% growth over 2006; 2008 witnessed an unprecedented global economic meltdown with only a marginal growth of 3.7%. Consumption declined, in fact, from July 2008 onwards. However, 2009 was a year of great resilience and recovery for the Indian steel industry. For April–December 2009, the provisional data released by Joint Plant Committee indicates a 7.8% rise in consumption of total finished steel. World Steel Association forecasts India’s Apparent Steel Usage (ASU) to increase at 13.9% during 2009–2010, over 2008–2009, to reach 63 MT compared to the forecast of 10.7% for The world and 6.7% for China for the same period. Similar figure for 2010–2011 over 2009–2010 stand at 13.7% for India, 2.8% for China and 5.3% for World. The prospective increase in the ASU figures are substantiated through a scrutiny of the consumption patterns in India. While the per capita demand in India, at around 50 kg, is nowhere near to the world average of around 150 kg, or, about 400 kg for developed countries; the rural India, at around 5 kg per person, lags even farther behind in comparison to urban India. However, in consideration of the extensive infrastructure development planned by the government in both rural and urban areas, these consumption figures have a strong scope to increase. Accordingly, a number of major Indian and Global steel players are into a massive capacity expansion mode in India, either through Brownfield or Greenfield route. On a conservative estimate, the steel demand in India is expected to touch around 90 MTPA by 2015 and around 150 MTPA by 2020. Steel supply is, however, expected to reach only around 88 MTPA by 2015 and around 145 MTPA by 2020.While the demand for steel will continue to grow in traditional sectors, specialized steel is also increasingly being employed in various hi-tech engineering industries. Globally, a relation can be observed between steel consumption and the GDP growth rate. Overall, India, being in a high growth phase with huge planned infrastructure development, is bound to witness sustained growth in the steel requirement in the years to come.

Steels are now considered to be a category of new materials, which have progressed year by year to meet the market requirements for high performance. Although it is well known that the performance of steel is dependent upon microstructure, the potential of microstructure evolution is really unknown by people at present due to the steadily developing steel metallurgy. The study is needed to investigate the potential microstructure evolution phenomena in steels and to develop new technology to improve the properties through microstructure control, by which the safety and reliability of steels in service could be eventually improved remarkably. The issues such as steel metallurgy for ultra cleanliness and ultra homogeneity, phase transformation mechanism of meta-stable austenite subject to temperature and load changes, carbon diffusion and partitioning during transformation, multi-scale characterization of structures, and microstructure stability subject to temperature and load changes are thought to be the key points to clarify the microstructure evolution at the existing circumferences. As a result, it can be expected that the fundamentals of microstructure control featured by multi-phase, meta-stable and multi-scale (M

3

) could be established, and then the technologies to process high performance steels could be developed: the third generation HSLA steels with improved toughness and/or ductility (

A

KV(−40°C)

≥ 200 J and/or

A

≥ 20% as

R

p0.2 in 800–1,000 MPa), the third generation advanced high strength steels (Rm ×

A

≥ 30 GPa% as Rm from 1,000 to 1,500 MPa) for automobiles with improved ductility and low cost, and the third generation heat resistant martensitic steels with improved creep strength (σ

10,000

650

≥ 90 MPa). It can be expected that the new technology developed will improve the safety and reliability of steel products in service remarkably for infrastructures, automobiles and fossil power station in the future.

A high level of requirements for structural cold-resistant steels is provided by controlling their structure and properties at every process stage. Development of oil and gas deposits in Siberia, in the Far East, in the arctic shelf of the northern seas and establishment of infrastructure of Northern areas calls for the necessity of applying cold-resistant steels with various strength characteristics levels for fabricated structures, which include: marine oil producing and exploration drilling platforms, arctic service vessels, ice-breakers, tankers, deep-water equipment for platform service, hoisting equipment, reservoirs, cisterns, maritime terminals, pontoons, etc.

Quenching–Partitioning–Tempering (Q–P–T) process is developed to yield ultra-high strength and high toughness steel based on Q–P process suggested by Speer et al. The chemical composition of Q–P–T steel is designed as <0.5C, 1.5Si (or Al), 1.5Mn, 0.02Nb, 0.2Mo (mass%) in which complex carbide precipitation during tempering may contribute further hardening; or <0.5C, 1.5Si, 1.5Mn (mass%) in which η (θ) carbide precipitation may offer strengthening beside the martensite formation. Ultimate tensile strength and total elongation of >2,000 MPa and >10%, ~1,500 MPa and ~15%, and ~900 MPa and ~20% are obtained with Q–T–P processed steels containing 0.4, 0.2 and 0.1C, respectively. Steels processed by Q–P–T generally contain ~5% retained austenite. Several examples are given in this article. The thermodynamics and kinetics as well as strengthening and toughing mechanisms for Q–P–T process are briefly discussed.

Japan faces severe and complicated resources problems. The population reached its peak and decreases gradually. Now the domestic market has been saturated. The raw material prices have risen suddenly and the product values have been deflated. The present paper discusses the change in new boundary conditions for new paradigm of economic growth based on the large material stock and high material technology of steels.

Our recent study has revealed that the Hall–Petch coefficient in ferritic iron has clear carbon concentration dependence in the range up to 60 ppm-C and it has been proposed that such a behavior deeply relates to the segregation of carbon at grain boundary. On the estimation of carbon concentration at grain boundary, two kinds of model could be applied: McLean model and Seah-Hondros model. In this study, the carbon concentration at grain boundary in ferritic iron was estimated by using these models for the temperature of 973 K. Under the presumption that the saturation atomic carbon concentration at grain boundary is 0.25 (equivalent to Fe

3

C), 6 ppm-C and 57 ppm-C were obtained from McLean model and Seah-Hondros model, respectively, for the critical matrix concentration that causes the grain boundary saturation. Hence, it was concluded that Seah-Hendros model is reasonable to explain the carbon concentration dependence of the Hall–Petch coefficient in ferritic iron.

Nano-precipitates of alloy carbides TiC, NbC and VC which have the same NaCl-type crystal structure in tempered martensite have been characterized by means of high-resolution transmission electron microscope (HRTEM) and correlated to the hydrogen trapping property. The examination of whether the amount of hydrogen absorbed by the TiC particles depends on their surface area or volume indicate that the coherent and semi-coherent TiC particles trap hydrogen at the precipitate/matrix interface at ambient temperature while the incoherent TiC particles trap hydrogen inside themselves only at high temperatures. Coherent and semi-coherent NbC and VC particles also demonstrate a surface area dependence of hydrogen trapping capacity with NbC > TiC ≫ VC. Contrary to TiC, incoherent NbC and VC particles are unable to trap hydrogen.

Inclusions are unavoidable even in super-clean advanced steels because of the necessary melting process. The effect of two kinds of typical inclusions such as TiN and AlN have been studied by means of specially designed SEM in situ tensile and fatigue tests for two advanced ultra-high-strength steels MA250 and GE1014 respectively. These unique experiments of SEM in situ tensile and fatigue test directly trace the entire process of crack initiation and propagation till fracture from tested specimens. TiN inclusion in MA250 often characterizes with large blocky cubic morphology. Crack easily initiate at the sharp corners of TiN or TiN/matrix interfaces or very often directly initiate inside TiN particles because of its brittleness. These cracks easily propagate to the matrix and to cause early failure. For elimination the harmful effect of TiN on this kind ultra-high-strength steel. An advanced ultra-high-strength steel without Ti designated as GE1014 was developed by GE Power, USA. Very small inclusion AlN particles (only several microns) can exist in GE1014. If these AlN small particles distribute in steel as inclusion chains, cracks often directly initiate at the inclusion chains among AlN small particles and line up to develop voids, which rapidly propagate to the matrix till early failure. These valuable experimental results reveal the harmful effect of inclusions in micro-scale and can be connected with tensile and fatigue loading processes for understanding early failure mechanisms, which are helpful for practical life prediction of these advanced steel components.

This study examined the phase transformation of ordered intermetallic precipitates to thermodynamically stable austenite in iron alloys assisted by a dislocation glide or climb. In Fe-Mn-Ni alloys, ordered fct θ-MnNi precipitates formed in the martensite grains and at the lath and grain boundaries during aging after quenching. With further aging, stacking faults and twins formed in the θ-MnNi particles by the glide of Shockley-type 1/6<112> partial dislocations. The crystal structure of the twins was similar to the face centered cubic structure, and these twins transformed to austenite as a result of iron diffusion into the twins. In Fe–Ni–Ti alloys, austenite nucleated first at the interface of the ordered hexagonal η-Ni

3

Ti intermetallic precipitates and martensite matrix. However, with further aging, an ordered face centered cubic (fcc) γ′-Ni

3

Ti phase formed from the η-Ni

3

Ti phase by the dislocation climb of 1/4<0001> type edge dislocations. The final transformation to austenite occurred by the diffusion of iron into the γ′-Ni

3

Ti phase.

The secondary phases exert strong influence on the mechanical properties, processing properties and performance of steels. The volume fraction, size, shape and distribution of the secondary phases must be effectively controlled in order to obtain the excellent properties and performance. On the basis of equilibrium solubility product formula, precipitation thermodynamics and kinetics, and classical nucleation and growth theory, the theoretical calculation method and procedure for the PTT (precipitation fraction–time–temperature) curve and NrT (nucleation rate–temperature) curve for main secondary phases precipitated in austenite and in ferrite have been proposed. The calculation results are well in agreement with practical experimental results. By analyzing the calculation results, some important deductions for the practical control of secondary phases in steels have been made.

Nanostructured steels composed of ultrafine grains (UFG) with sizes smaller than 1 μm perform surprisingly high strength but sometimes show limited tensile ductility. In the present paper, systematic experimental results on mechanical properties of the nanostructured steels with ferrite single phase are firstly shown. The limited tensile ductility of the nanostructured ferritic steels was due to very small uniform elongation, which was attributed to the early plastic instability in the UFG microstructures. This basic understanding suggests one of the ways to overcome the low tensile ductility: if the strain-hardening of the matrix is enhanced by any means, such as dispersing fine second phase in the matrix, both high strength and adequate ductility can be managed even in nanostructures. Actual examples of the nanostructured steels that could achieve good strength–ductility balance were also introduced. Dispersing fine carbides within the UFG ferrite matrix was actually effective to manage both strength and ductility. Also ultrafine dual-phase structure composed of ferrite and martensite resulted in both high strength and large uniform elongation. It was also shown that transformation induced plasticity caused by deformation induced martensite transformation of metastable austenite could work in nanostructured steels. The present results clearly indicate that using multi-phases is the promising direction for managing both high strength and adequate ductility in nanostructured steels.

We are coming into the data exploration age of science, so many things cannot be done without support from databases and data analysis. The same is true to steels. With some examples of trend analysis and material property chart analysis, this article attempts to discuss the importance and necessity of data exploration and data-sharing.

In recent years, economic and environmental considerations have increased the need to safely extend the service life of components and structures beyond their original design life. It well known that fatigue and delayed fracture are the two important mechanisms for the failure of steel components and structures in service. Based on our systematic studies of the fatigue failure and delayed fracture behaviours of high strength steels in the last 10 years, we proposed a new kind of concept to improve both fatigue failure resistance and delayed fracture resistance of high strength steels, which comprises the combination of inclusion modification, structure controlling, hydrogen trap controlling and grain boundary strengthening, and it is called IST & GST technology in this paper. Firstly, basic considerations for the improvement of service life of high strength steels are introduced. Secondly, methods such as inclusion modification, structure controlling and hydrogen trapping controlling are discussed for improve the fatigue failure and delayed fracture resistance of high strength steels. Finally, based on IST & GST technology, examples of development of long life high strength steels such as a newly developed 2,000 MPa grade high strength spring steel with excellent fatigue failure resistance and 1,500 MPa grade high strength bolt steel with superior delayed fracture resistance are introduced.

Two major drivers for the use of newer steels in the automotive industry are 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 US 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 alloying elements such as Ti or Nb. AHSS is a general term used to describe various families of steels. The most common AHSS is dual-phase steel which consists of a ferrite–martensite microstructure. These steels are characterized by high strength, good ductility, low yield to tensile strength ratio and high bake-hardenability. Another class of AHSS is the 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 the latest class of AHSS steels finding interest among the US automakers. These steels consist of a ferrite–bainite microstructure with significant amount of retained austenite phase and exhibit the highest combination of strength and elongation, so far, among the AHSS in use. High level of energy absorbing capacity combined with a sustained level of high n value up to the limit of uniform elongation as well as high bake-hardenability make these steels particularly attractive for safety critical parts and parts requiring complex forming. Finally, martensitic steels with very high strengths are also in use for certain parts. All of the above kinds of steels will be discussed in this paper.

Dual phase steels for production on CGL, having tensile strength range from 590 to 1200 MPa, was developed in Nb-bearing Cr–Mo steels with carbon contents 0.06 and 0.15 mass%. The role of Nb in these steels, as well as the formation and transformation characteristics of austenite as a function of intercritical annealing temperature, cooling rate from intercritical annealing temperature to 460°C were investigated. The mechanical property tests of the steels treated by CGL were performed. The OIM and EBSD image quality analyses were employed in this investigation for analyzing microstructures and the phase transformation products of austenite. The results show each strength grade dual phase steel has the excellent combination of tensile strength and ductility properties. The addition of Nb can further improve the tensile strength of the Cr–Mo dual phase steel without ductility reduction. This relates to microstructure refined by the Nb in steels and the proper combination of volume fraction of recrystallized ferrite and martensite.

Improvement of safety, reduction of energy consumption, and reduction of emission become one of the most highlighted issues for automotive industry in recent years. One of the most significant solutions, i.e. lightweight car body has been described in this paper. Design, implementation, and characterization of parameters of lightweight car body are reviewed as well. In the later part of this paper, the development of high strength steels (HSS) and advanced high strength steels (AHSS) and their typical properties and microstructures are introduced. The application of these high strength steels for lightweight car body can largely improve safety and performance of vehicles.

We present a design strategy for a new type of age hardenable ultrahigh strength TRIP-assisted steels with a good tensile elongation. The alloys have a low carbon content (below 0.02 mass% C), 9–15 mass% Mn and, in order to reduce costs, only minor additions of Ni, Al, Ti, and Mo (of the order of 1–2 mass%). After quenching the microstructure of these steels comprises martensite and different amounts of retained austenite. During age hardening the strength and ductility can be increased simultaneously: The strength is increased by the formation of nanosized intermetallic precipitations in the martensite. At the same time new austenite is forming by a partitioning of Mn and Ni from martensite to austenite. The increase of the uniform elongation during aging is a result of a tempering of the quenched martensite and a TRIP effect due to the austenite. The precipitation state is also of importance for the ductility. Atom probe results reveal high contents of Ni, Mn and Al in the homogeneously distributed particles. The nano-sized precipitates have a high dispersion owing to the good nucleation conditions in the heavily strained martensite matrix in which they form. The approach of combining a moderate precipitation hardening with a TRIP effect enables to produce steels with good combinations of strength, ductility and low costs.

In this study, research and development on the 3rd generation automobile steel, with targets of

R

m

×

A

no less than 30 GPa% at

R

m

level of 1–1.5 GPa, was carried out to fabricate high strength and high ductility steel by two methodologies, one is the medium manganese steels fabricated by intercritical annealing through austenite reverted transformation (ART-annealing) and another is the conventional carbon steels processed by quenching and partitioning(Q&P). The ultrafine grain sized austenite-ferrite duplex microstructure and the tempered martensite-fresh martensite-austenite multiphase microstructure were demonstrated based on the microstructure characterization in ART-annealed medium manganese steels and Q&P processed conventional carbon steels. In both heat treatment conditions, substantially enhanced ductility (30–40%) at ultrahigh tensile strength level (1–1.5 GPa) was obtained, which results in a significant improvement of the product of tensile strength to total elongation about 30–40 GPa%. Analysis on the work hardening behaviors and the relationship between microstructures and mechanical properties of the studied steels indicates that the greatly improved ductility results from the aid of the phase transformation induced plasticity (TRIP effects) and the ultrahigh strength stems from the hard matrix, such as the ultrafine grained duplex structure in ART-annealed steels and the martensite matrix in Q&P processed carbon steels. It is interesting to find that a strong dependence of the product of tensile strength to total elongation on the fraction of retained austenite phase of steels produced by both ART-annealing and Q&P processing techniques. It was proved that both ART-annealing and Q&P processes can be applied to fabricate the third generation automobile sheet steels offering ultrahigh strength and high ductility.

Sheet steel is a fundamental material used for various applications, such as automobiles, electrical appliances, building construction, and so forth. As such, the further strengthening of sheet steel is considered very important, as it has strong potential to help solve current global environmental issues through weight reduction and improvements in the efficiency of final products. However, at present, the capability of such steel has not been fully exploited. For further strengthening, the exploitation of material science together with application technologies is essential to overcome the abundance of inherent problems. In this paper, first of all, the recent progress concerning sheet steel used in automobiles is reviewed, taking into account history and technological problems, such as formability, fatigue properties, and hydrogen embrittlement. Since deformation and fracture behaviors are common subjects, focus is placed not only on strength, but also on the ductility (i.e., yielding, work hardening, and necking) of single- and complex-phase steels, on the fatigue properties of both base and welded steels, and on hydrogen embrittlement. Recent progress regarding the science involved and future scientific problems related to dislocation behavior in sheet steel are also discussed, together with prospects for the future.

Attention has been given to the development of new higher strength steel grades by employing duplex microstructures of bainitic or martensitic ferrite with retained austenite. Such microstructures rely first upon appropriate alloying to stabilise austenite against thermal decomposition, either isothermally to bainite or athermally to martensite. The alloying, typically by involving Si additions, promotes chemical stabilisation indirectly through suppression of carbide precipitation and hence increase of austenite carbon concentration. The most recent novel concept, aimed at higher strength martensitic grades, combines enhanced Si alloying with an interrupted quench, followed by an anneal to allow carbon migration from supersaturated martensite regions to untransformed austenite, a process which has become known as quenching and partitioning, or Q&P. Recent progress in understanding the detailed evolution of microstructure during the Q&P heat treatment, and the potential for exploration of the resulting microstructures in various steel grades, will be described.

Two important objectives of the automotive industry are the decrease in car weight and improvement in safety. High strength steels (HSS) especially advanced high strength steels (AHSS) and the third-generation HSS are the main measures to reduce automotive weight and improve safety in steel industry. Quenching and partitioning (Q&P) has been recently proposed by Speer as a fundamentally new way of producing martensite steels containing a considerable amount of retained austenite, it possesses good balance properties between tensile strength and elongation. In order to produce Q&P steel, a special thermal treatment is required, it consists of a two step thermal treatment. A special line designed by Baosteel especially for UHSS production was launched in March 2009, it is easy to implement the annealing cycle of Q&P. The industry trails with conventional C–Si–Mn TRIP780 composition by Q&P concept were carried out in this line. The results show that a good balance property is achieved, the microstructure and other properties are measured and discussed. A component of B pillar is made successfully by Q&P1000.

Many studies about high Mn austenitic steels have been done due to their excellent mechanical properties such as tensile strength, uniform ductility, and wear resistance. In this paper, we tried to summarize the effects of Al addition on carbide precipitation, stacking fault energy (SFE), dynamic strain aging (DSA), tensile properties, and hydrogen delayed fracture in high Mn austenitic steels such as Hadfield and TWIP steels. The addition of Al suppressed the cementite precipitation due to the decreases in both activity and diffusivity of C in austenite. However, the κ′-carbide precipitation with a chemical formula (Fe, Mn)

3

AlC

x

may occur in the high Mn and Al austenitic steel. The addition of Al linearly increased the stacking fault energy of austenite. The Al addition increased the strain hardening rate in the Hadfield steels owing to the increase in the DSA effect caused by the increased solute C content, while it decreased the strain hardening rate in the high Mn TWIP steel because of the decreases in mechanical twinning caused by high SFE and in DSA effect caused by slow diffusivity of C atoms. The addition of Al improved the resistant property to the hydrogen delayed fracture in the high Mn TWIP steels.

Martensite phase transformation and microstructure features in DP, TRIP and TWIP steels are discussed. For the obtaining of better properties, details in controlling the production of DP and TRIP steels are described based on the understanding of the stability of microstructures. Substitution of P for Al and/or Si in TRIP steel is evaluated from the aspects of thermodynamic and kinetic estimation. The minimum fast cooling rate and optimal over-aging temperature and time has been calculated for TRIP steel. Stack fault and its effect on phase transformation in high Mn steel are evaluated in the relationship with composition, strain, and heat treatment.

A TRIP steel containing 7 mass% Mn was annealed at various temperature for 10 min and changes in microstructure and mechanical behavior due to different annealing temperature were investigated. Excellent mechanical properties of high tensile strength over 1,200 MPa with high elongation over 20% could be obtained for proper annealing temperature range. Volume fraction and mechanical stability of austenite were highly dependent on annealing temperature, which shows a transition from stable to metastable behavior of austenite. Effects of austenite grain size and solute element contents in austenite grains on stability were discussed.

Advanced TMCP products, such as X80/X100 linepipe steels and YS 460MPa grade shipbuilding steel, have been developed by POSCO. In order to apply X80/X100 linepipe to strain-based design (SBD) concept, the main issue is to prevent strain aging after thermal coating. In case of YS 460MPa shipbuilding steel, challenging study is necessary to increase plate thickness up to 80 mm without deterioration of strength and toughness. It was found that solute carbon started to segregate at dislocation tangles in coarse ferrite during and after UOE pipe forming, resulting in strain aging. And it was shown that the optimum microstructure of X80 and X100 steels was the mixture of acicular ferrite, bainitic ferrite and fine ferrite. Those X80 and X100 steels revealed good strain hardening resistance as well as reasonable compressive strain capacity. The developed YS 460MPa grade steel plate for shipbuilding showed a good mechanical property at quarter and mid-thickness and also an excellent weldability. It was confirmed that the developed steel had a high CTOD value both HAZ and base metal to prevent an initiation of brittle crack and a good crack arrestablity to suppress long distance propagation of brittle crack.

Low-carbon bainitic steels offer a solution to produce strip and plate products with unique properties in terms of strength and toughness. MoNb-based alloying concepts have a great potential to further develop and optimize such steels. Mo can promote the transformation of acicular ferrite and bainite through its retardation effect on the proeutectoid ferrite transformation, and therefore improve the strength and toughness of low alloy steels. An optional alloying element is boron. Special attention is focused on the cross effects of Mo with the microalloying elements with regard to hardenability, grain size control and precipitation strengthening. Important metallurgical effects like recrystallization, microstructure and precipitation behavior are analyzed and related to mechanical properties. Different processing options for strip and plate mills are being discussed in that respect. Finally, some examples of applications of Mo in high grade pipeline steel, high strength engineering machinery steel, fire-resistant steel and hot-rolled strip for automobile are briefly introduced.

Recent works at KIMAB and Arcelor Research indicate that the mechanical properties of bainitic steels can be improved with V-microalloying. This is potentially important since there is expected to be a shift from existing precipitation hardened HSLA steels towards bainitic types as demand arises for higher strength. Although the bainitic transformation temperature is too low to promote precipitation strengthening, it seems that fine V(C, N) particles can effectively prevent the recovery of the dislocation structure of the bainite during coiling of hot-rolled strip product, giving an increase in strength. There are also indications that vanadium combined with nitrogen in bainitic steel promotes the formation of lower-bainite, referring to a bainitic microstructure with smaller crystallographic units delineated by high-angled boundaries, with associated increase in strength and good toughness. Recent investigations on bainitic rail steels in Japan and Europe demonstrate that vanadium is also useful to give fine laths microstructures and improve wear resistance.

This paper reviews current research that aims to understand and manipulate the properties of advanced steels through control of the nanostructure, with an emphasis on thermomechanically produced steels. While the concepts of such strengthening mechanisms have been understood and used for many years, it is now possible through advanced characterisation methods to gain detailed insights. Similarly, our knowledge of phase transformations supported by improved modelling also allows us to design microstructures that have length scales in the range of 10–100 s of nanometres. Examples are provided that relate to the application of this concept in Advanced High Strength Steels of precipitation hardening, bake hardening, the development of ultrafine ferrite microstructures, nanoscale dispersed multiphase steels and nanoscale bainite.

The effects of different compositional alternatives of Nb-Ti-Mo, Nb-Ti-Mo-B, and Nb-Ti-B with the basic composition of 0.08C-1.8Mn were studied aiming at optimization of steel composition and hot rolling and cooling technology of a microalloyed high strength hot strip steels with YS ≥700 MPa. In the work, four experimental steels with different combinations of micro-amounts of Nb, Ti, and B are compared. Moreover, two experimental steels, i.e. the steel of Nb-Ti and the steel of Nb-Ti-B were subjected to CCT diagram experiment in order to find the effects of sub micro-amount B in this type of steels. It has been found that sub micro-amount B addition brings about significant influence on the tensile properties, microstructures, and phase transformation characters of this kind of steels. It has been also found that as coiling temperature after hot rolling, 600°C is better than 570°C when strip steel is applied under as hot rolled condition. If strip steel is applied under as annealed condition, 570°C is the preferable coiling temperature leading to larger strength increment and higher elongation after annealing. Boron microalloying is more suitable for as annealed strip steels. The CCT diagram study shows that sub micro-amount B addition strongly lowers the transformation start temperature and transformation stop temperature and obviously increases hardness at fast cooling rates (44–1°C/s). B suppresses ferrite transformation during continuous cooling at comparatively fast cooling rates promoting the granular bainite formation and increasing steel hardness. It is common that for this type of steels that in both B free and B containing steels, at certain cooling rates martensite islands is prone to occur. B shifts the martensite islands phase presence from faster cooling rates to slower cooling rates. In the case of as hot rolled high strength strip steel, martensite islands are to be avoid; while for the as annealed high strength hot strip steel, martensite islands are beneficial leading to further increase of strength level and to the improvement of elongation value after annealing.

Increasing the steel grade and gas pressure in pipeline project means the crack arrest toughness for pipe needs to enhance greatly. In this paper, effect factors of X80 toughness were studied. Through commercial production of X80 steel, it was found that the content of carbon or sulphur influenced impact energy obviously. Large numbers of statistical data showed that impact energy would well satisfy the requirement of X80 hot-rolled steel strip for the Second West -East Gas Pipeline if [C] was between 0.035 and 0.065 wt.% and [S] below 30 ppm. Through EBSD analysis, it was concluded that the effective grain size was smaller, toughness would be better. Fine and equably dispersed M-A structure would improve the strength, while structure with strip or sharp angel would deteriorate the toughness of X80 steel. Development of X80 strips for Chinese first full-scale burst test was also introduced in the paper.

A series of fundamental and applied investigations were carried out to develop high grade pipeline steel with high Mn high Nb design, and it mostly focused on that the static and dynamic recrystallization behaviors of high Mn high Nb pipeline steel. Various experimental methods were adopted, which include stress relaxation tests, physical metallurgical modeling analysis, the etching of prior austenite grains and TEM observation of precipitates. According to the results, new control rolling technology is bring forward under high Mn high Nb approach, as a consequence, fine and homogeneous prior austenite grains can be generated by complete static recrystallization prior to finish rolling, and the coarsening can be suppressed largely by drag effect, through proper finish rolling process, prior austenite grain will be flatten fully and uniformly to higher flow stress or S

v

, and the mixed grains structure caused by partial dynamic recrystallization may be avoided. The physical metallurgy principle for refinement of prior austenite grain through rough rolling and pancake through finish rolling can be adopted for refinement the austenite grain in high Mn high Nb structural steel.

It is well known that nitrogen in solid solution can strongly enhance both, solid solution hardening and grain boundary hardening (fine grain hardening) of steels with a face centered cubic crystal lattice. The present work shows a quantitative relation between the critical resolved shear stress of single crystals and the yield strength of polycrystals of such solid solutions. This represents one more step towards understanding the strength of this excellent class of materials.

In contrast with highly nitrogen alloyed stainless steels such as classical HNS, an optimized, partial or total replacement of carbon by nitrogen does not seem to have been frequently studied. Many arguments based on: (1) usual concepts of physical metallurgy, (2) the observed impact of interstitial alloying on steel properties, (3) cost efficiency and the demand of sustainable development, (4) an overview of steel sorts likely to become nitrogen upgraded are examined. Probable scenario of the evolution of steel-making including nitrogen alloying concern is discussed.

Production expansion of high nitrogen steels, both with low nickel content and without nickel, in various structural classes will be the main trend.

A review of the structure–property–performance relationship in a technologically important cold-rolled and annealed metastable austenitic stainless steel (AISI 301LN SS) is presented. AISI 301LN SS is cold-rolled to 63% reduction and subsequently annealed at 600–1,000°C from 1 to 100 s. Cold-rolled and annealed samples are studied through X-Ray Diffraction (XRD), Superconducting Quantum Interference Device (SQUID), transmission electron microscopy (TEM) and tensile testing to understand the morphology of the cold-rolled AISI 301LN and the annealed martensite to austenite reversion, the formation of nano/submicron grain sizes and the mechanical properties achieved. Tests show that cold-rolled samples annealed at 600 and 700°C exhibit partial α′→ γ reversion, while for the case of 800–1,000°C annealing treatments, the reversion from α′-martensite to γ-austenite is almost complete, along with rapid austenite grain growth. Tensile tests performed on AISI 301LN nano/submicron grained SS reveal a high yield strength of ~700 MPa, which is twice the typical yield strength of conventional fully annealed AISI 301LN SS. An analysis of the relationship between yield strength and grain size in these nano/submicron grained SS indicates a classical Hall–Petch behavior, despite the temperature dependence observed due to an interplay between fine grained austenite, solid solution strengthening, precipitate hardening and strain hardening.

UNS S31035 is a newly developed austenitic stainless steel with the highest creep strength among the commercial available heat resistant grades for the next generation of coal fired power plants. Alloy 800HT is a well developed material that has been recommended as a candidate material for generation IV nuclear power plants. In this chapter, several advanced heat resistant austenitic stainless steels are reviewed, but the focus will be on these two materials. The influences of composition on the structural stability and on the creep behavior are discussed. The creep mechanisms at different temperatures and loading conditions have been identified. The interaction between dislocations and precipitates and their contribution to the creep rupture strength are discussed. Different models have been used to evaluate the long-term creep behavior of the grades. Finally, highly alloyed composite tube products for different corrosive steam boiler applications are introduced.

Laboratory research, pilot trials, and industrial implementation of T/P92, S30432 and S31042 boiler steel tubes/pipes used for 600°C steam parameter fossil fuel fired ultra super critical (USC) power plants in China in the past decade are summarized with the emphasis on the technical breakthrough of compositional optimization and the best fit heat treatment. The microstructural stability of the steel tubes/pipes during long-term service is discussed. The advancement of industrial production of T/P92, S30432 and S31042 steel tubes/pipes and their expected application in China is also introduced in the present chapter. The obtained results show that the steel producers and boiler manufacturers of China have been able to successfully produce T/P92, S30432 and S31042 boiler steel tubes/pipes used for 600°C steam parameter USC power plants, while the supplying capability will be gradually increased. The authors briefed the on-going national research projects in the field of advanced boiler steels in China and discussed the potential national research projects in the sector.

Strengthening mechanisms in creep of 9% Cr steel are examined in terms of solute hardening, precipitation or dispersion hardening, dislocation hardening and boundary or sub-boundary hardening at 550–650°C. The creep strengthening is mainly caused by the retardation of onset of acceleration creep, which effectively decreases the minimum creep rate and increases the creep life. The sub-boundary hardening enhanced by fine distributions of precipitates along boundaries gives the most important strengthening way in creep of tempered martensitic 9Cr steel. A dispersion of nanometer size MX nitrides along boundaries and the addition of boron significantly improve long-term creep strength. Excess addition of boron and nitrogen causes the formation of large particles of boron nitrides during normalizing heat treatment at high temperature, which offsets the benefits due to boron and nitrogen. Newly alloy-designed 9Cr-3W-3Co-0.2V-0.05Nb steel with 130–160 ppm boron and 70–90 ppm nitrogen exhibits excellent creep strength of base metal and no degradation in welded joints at 650°C.

In this paper, the production current status of Chinese mould steels is briefly introduced. In our present works, the research and design of new mould steels, microstructure controlling and new surface treatment technologies in mould steels are developed to meet the requirements of the manufacturing and to reduce the waste of resources. The main contents are as follows: (1) Developing the new plastic mould steels for different service conditions, such as the prehardened plastic mould steel, the corrosion resistant plastic mould steel and the age hardening plastic mould steel; (2) Designing the new high strength and high toughness cold work mould steels for the deformation of high strength plate steels; Meantime, a new retained austenite controlling techniques is attempted to improve the dimension stability of moulds; (3) Developing the new hot work mould steels (e.g. SDH3, SDHA and DM) and discovering its mechanism of alloy design; (4) Applying a series of improved surface technologies to enhance the service life of moulds, such as plasma nitriding, plasma boriding and synthesizing VC coating. Finally, some suggestions about how to develop new mould steels and improve its service performance are proposed.

Plastic mould steels are the most versatile alloy steels whose consumption amount is the largest among mould and die steels. Pre-hardened plastic mould blocks are mainly used for manufacturing large moulds of automobile interior trimming products and shells of large-size household appliance, etc. The metallurgical processes for large-size pre-hardened mould blocks of steel 718 was developed by Central Iron & Steel Research Institute (CISRI) and Dongbei Special Steel Group Co., Ltd. (DSSC) together and the effects of relative processes, such as LF + VD melting process, ingot mold design, argon protective ingot casting process, high temperature homogenizing process and forging process for heavy ingots, quenching process and high temperature tempering process for mould blocks on material purity and hardness uniformity were systematically tested. Results show that sulfur content can be controlled as low as 0.005%, and total oxygen content as low as 12 ppm if slag basicity of LF and VD were kept within 3.5–4.0 and 3.0–3.5 respectively. Rejection rate caused by inclusions at ingot bottom was reduced from 6.81 to 1.55% by optimizing the design of the 28 t ingot mold bottom taper shape and ingate chamfering. It was confirmed by argon protective casting test that argon flow rate should be controlled at 4–8 m

3

/h. The macro-segregation and hardness deviation were improved from ≤grade 3.0 and ≤5.0 HRC to ≤grade 2.0 and ≤3.5 HRC respectively by adopting high temperature homogenizing process. The macro-porosity and the ultrasonic testing result was improved from ≤grade 3.0 and C/d to ≤grade 2.0 and D/d respectively by applying FM forging method instead of drawing by flat anvil. The hardness deviation on the cross section of 650 mm × 1,080 mm mould blocks was ≤3.5 HRC with pre-hardening treatment i.e. water–air alternatively timed quenching + high temperature tempering by using electrical heating furnace.

The production, application and development trend of China’s high-speed steel is introduced, the status of China’s high-speed steel grades, product structure, technical standards, production technology and major quality issues are analysed, and the development opportunities and challenges of China’s high-speed steel in future are discussed also in this article. Data shows that professional manufacturers of high-speed steel are the main production body. Variety and quality of China’s high-speed steel basically meet domestic demand, the general high-speed steel partly exports, the proportion of low-alloy high-speed steel are higher, and powder metallurgy high-speed steel and some high performance high-speed steel rely on import. As the rapid development of China’s manufacturing industry, the union model of high-speed steel industry union, and increasing export capacity of high-speed steel and high-speed steel tools, the Chinese high-speed steel has a good opportunity for development; while, as the development of carbide cutting tools, the change of the scale degree and the change of die and tool industry, China must face the challenges of high-speed steel.

The microstructure and mechanical properties of weld joints of high nitrogen stainless steel are studied. Thermal simulation, gas tungsten arc welding, gas metal arc welding and laser welding were conducted. The main results are summarized as follows: (1) thermal simulation results indicate that the microstructure of the heat-affected zone consists of austenite and a small amount of δ-ferrite. Cr

23

C

6

occurs on the grain boundaries, and Cr

2

N does not exist in the HAZ. The hardness of HAZ is higher than that of the base metal, indicating no softening in the HAZ under appropriate welding conditions. The impact toughness of coarse-grained heat-affected zone is improved at first and then decreased with the increase of the cooling rate, whereas two brittle zones exist in the HAZ. (2) For gas metal arc welding and laser welding, the N-content of the weld metal increases as the N

2

fraction in the Ar + N

2

shielding gas is increased. The nitrogen pore can be avoided when the N

2

fraction of the shielding gas is lower than a critical amount. For laser welding, higher heat input and more N

2

in shielding gas decrease the porosity in the weld metal. (3) The microstructure in the weld metal of laser welding is austenite and δ-ferrite. The size of δ-ferrite increases with increasing heat input. The hardness of weld metal increases with decreasing heat input and increasing N

2

amount in the shielding gas. The toughness increases when the heat input decreases, whereas the composition of shielding gas shows no influence. (4) Cr-Mn-Ni-N welding wire is suitable for the welding of high nitrogen stainless steel. The microstructure in the weld metal is austenite and δ-ferrite. The strength of the weld metal obviously increases with the addition of some N

2

into the shielding gas. The weld metal has good toughness. But the HAZ of weld joint with multi-pass welding shows low toughness.

Three different strengthening mechanisms are usually selected to achieve the mechanical requirements in eutectoid steels: refinement of the interlamellar spacing of pearlite, solid solution and precipitation hardening. In the case of precipitation hardening, traditionally vanadium microaddition has been the one considered although other possibilities, such as Cu, have been evaluated recently. These mechanisms provide a proper microstructure for a wide range of industrial eutectoid steel applications. In other cases, together with a minimum strength level, toughness is also required. This implies that additional microstructural parameters need to be controlled. Among them, one of the most relevant features affecting toughness behavior in pearlitic steels is the “ferrite unit”, the crystallographic region with similar ferrite orientation. This paper analyses the application of microalloying and thermomechanical processes combined with continuous cooling schedules during transformation, in order to obtain optimized strength–toughness combinations. The results show that in addition to the classical role of vanadium as providing an additional increase in strength through precipitation hardening, a pancake austenite microstructure before transformation can be obtained if proper schedule rolling passes are selected. This microstructure provides finer and more homogeneous “ferrite units” than those obtained by equiaxed austenites. The refinement of the “ferrite unit” in the final microstructure has been confirmed both by EBSD measurements. In order to predict the “ferrite unit” size, an empirical equation has been proposed as a function of the austenite characteristics prior to transformation and the cooling rate during transformation.

Laboratory and industrial experiments were carried out to investigate non-metallic inclusions in high strength alloy steel refined by using high basicity and high Al

2

O

3

content slag. It was found that the steel/slag reaction time largely affected non-metallic inclusions in steel. With the reaction time increased from 30 to 90 min in laboratory study, MgO-Al

2

O

3

spinels were gradually changed into CaO-MgO-Al

2

O

3

system inclusions surrounded by a softer CaO-Al

2

O

3

outer layer. By using high basicity slag which contained as much as 41 mass% Al

2

O

3

in the laboratory study, the fraction of low melting temperature CaO-MgO-Al

2

O

3

system inclusions were remarkably increased to above 80%. In the industrial experiment, during the secondary refining, the inclusions changed in the order of Al

2

O

3

→ MgO-Al

2

O

3

→ CaO-MgO-Al

2

O

3

. Through the LF and RH refining, most inclusions can be transferred to CaO-Al

2

O

3

and CaO-MgO-Al

2

O

3

system inclusions of lower melting temperature.

The microstructure change by warm deformation in low alloy steels with different initial ferrite (α) + cementite (θ) duplex structures is discussed in the present paper. In high carbon steels, heterogeneous deformation introduced in pearlite containing lamellar θ promotes dynamic recrystallization (DRX) of α for mild deformation of less than 1.2 in true strain. On the other hand, the original α grains become elongated and only subgrains are formed by dynamic recovery in the case of (α + θ) duplex structure containing equiaxed spheroidized θ. Equiaxed fine α grains, approximately 2 μm in diameter and mostly bounded by high-angle boundaries, are formed with spheroidized θ by DRX during compression of the pearlite by 75%. When the (α + θ) duplex structure containing spheroidized θ was deformed, the original α grains become elongated and only subgrains are formed by dynamic recovery. For the tempered martensite, equiaxed α grains similar to those in the deformed pearlite were obtained after 50% compression. This indicates that the critical strain needed for the completion of DRX is smaller for the tempered martensite than for the other structures. Lath martensite in a higher carbon alloy is more suitable for DRX because of its finer initial grain size. DRX α grain size is finer in a higher carbon alloy because of stronger pinning effect by θ particle.

Mainly introduced new products and technologies developments of Pangang rails. Since 1970s of last century, Pangang always applied itself to rail products and technologies developments. After 2004, Pangang started technical renovations with the representation of continuous casting and universal rolling, rail production equipments have meet the world advanced level, and successively developed series of rail products including 350 km/h passenger dedicated rail, 1300 MPa heavy haul rail, on-line heat treatment rail and exported rail. Not only satisfied the domestic requirements, but also expanded international markets. Realizing rail production system innovation and new product development.

The effect of 12 T strong magnetic field on molybdenum carbide precipitation in a Fe-C-Mo alloy was investigated by means of transmission electron microscopy and selected area electron diffraction technique. The sequence of molybdenum carbide precipitation in Fe-C-Mo alloy during tempering changes due to the effect of 12 T strong magnetic field. The precipitation of (Fe, Mo)

6

C was greatly promoted when a strong magnetic field was applied.