Development of microstructure and texture of medium carbon steel during heavy warm deformation

doi:10.1016/j.actamat.2004.01.024

Acta Materialia

Volume 52, Issue 8, 3 May 2004, Pages 2209-2220

https://doi.org/10.1016/j.actamat.2004.01.024 Get rights and content

Abstract

The microstructure and texture development of a medium-carbon steel (0.36% C) during heavy warm deformation (HWD) was studied using scanning electron microscopy and electron back scattering diffraction. The spheroidization of pearlite is accelerated due to the HWD, which leads to the formation of completely spheroidized cementite already after the deformation and coiling at 873 K (600 °C). The homogeneity of the cementite distribution depends on the cooling rate and the coiling temperature. The cooling rate of about 10 K/s (ferrite–pearlite prior to HWD) and deformation/coiling at 943–973 K (670–700 °C) lead to a homogeneous cementite distribution with a cementite particle size of less than 1 μm. The ferrite softening can be attributed to continuous recrystallization. Even up to fairly high deformation/coiling temperatures of 983 K (710 °C) the texture consists of typical deformation components. During the continuous recrystallization the amount of high angle grain boundaries can increase up to 70% with a ferrite grain size of 1–3 μm. An increase of the cooling rate up to 20 K/s (ferrite–pearlite–bainite prior to HWD) deteriorates the homogeneity of the cementite distribution and the softening of ferrite in the final microstructure.

Introduction

After conventional hot rolling of medium carbon steel, a lamellar pearlite is formed during γ–α transformation. The lamellar morphology of pearlite leads to mechanical properties unsuitable for a further cold treatment or for application in highly demanding components. The globular morphology of cementite provides some benefits such as high toughness, good cold formability and machinability. For such purposes the cold strip must either undergo a long annealing treatment to obtain higher cold formability or it must be quenched with a subsequent tempering for a good combination of strength and toughness.

The use of a heavy warm deformation (HWD), performed below the γ–α transformation-temperature accelerates essentially the spheroidization of pearlite. The rate of this process is accelerated by a factor of 10⁴ compared to annealing without deformation [1], [2]. But the spheroidized cementite itself cannot provide good mechanical properties. Other microstructural features like a homogeneous cementite distribution or ferrite condition, as well as the size of the cementite particles or the ferrite grains, influence the final mechanical properties. Therefore, a better understanding of the microstructure evolution during HWD is important for a successful introduction of such processing into the industrial production.

Section snippets

Material and experimental technique

A ferritic–pearlitic steel with a following composition in mass % was studied: 0.36% C, 0.53% Mn, 0.22% Si, 0.011% P and 0.002% S. The axially symmetric compression of samples with initial size 18 × 18 × 30 mm³ and plane strain compression of samples with an initial thickness of 60 mm, width of 50 mm and length of 45 mm, cut from an industrial slab, were carried out on the hot deformation simulator of the Max-Planck-Institute [3] with a strain rate of 10 s⁻¹. This servohydraulic press is capable of

CCT diagram

The continuous cooling transformation diagram (Fig. 1) was established after an austenite deformation at 1173 K (900 °C). This temperature was determined by pre-tests to provide fine recrystallized austenite grains as an initial microstructure before the transformation.

The main differences between the microstructures produced with different cooling rates (initial microstructures for the subsequent HWD) were the amount of proeutectoid ferrite, the presence of bainite and the thickness of

Conclusions

1.
Spheroidization of pearlite during the HWD is accelerated by the formation of cementite lamellae kinks, fracture of lamellae and cementite subboundaries. Higher equilibrium carbon concentration in ferrite (higher carbon solubility) near these lamellae defects provokes a quick local dissolution of the lamella that leads to its division with a subsequent or simultaneous spheroidization.
2.
A rather homogeneous distribution of cementite in an initially ferritic–pearlitic microstructure can be observed

Acknowledgments

The authors express their gratitude to the financial support of the European Coal and Steel Community (ECSC).

References (21)

S. Chattopadhyay et al.
Acta Met.
(1982)
D. Shin et al.
Scripta. Mater.
(2003)
R. Doherty et al.
J. Less-Common Metals
(1972)
E. Holmes et al.
Acta Met.
(1959)
J. Drolet et al.
Acta Met.
(1968)
R. Kaspar et al.
Steel Res.
(1988)
J. Robbins et al.
J. Iron. Steel Inst.
(1964)
O. Pawelski et al.
Materialprüfung
(1988)
T. Watanabe et al.
Physica Status Solidi
(1970)
B. Chalmers
Physical metallurgy
(1959)

There are more references available in the full text version of this article.

Cited by (166)

Improvement of magnetic properties via normalizing annealing for high-strength Fe-4.5wt.%Si electrical steel
2023, Journal of Magnetism and Magnetic Materials
Both excellent magnetic and mechanical properties are desired for electrical steels used in high-speed motors and engines of electric vehicles. Existing high-strength electrical steels (HSES) usually sacrifice magnetic properties for a trade-off of strengthening effect. In this work, Fe-4.5wt.%Si alloy was employed to fabricate HSES by dislocation and solution strengthening, and the magnetic properties were further improved by normalizing annealing after hot rolling. The effect of normalizing annealing (NA) on micro structure and magnetic/mechanical properties was investigated. The iron loss of the cold-rolled sheets can be significantly decreased with NA treatment. The iron loss P_1.0T/400Hz of 20.7 W/kg with a yield stress of 656 MPa, and P_1.0T/400Hz of 30.3 W/kg with a yield stress of 865 MPa can be achieved.
Effect of prior-austenite grain size on the microstructure and mechanical properties of ultrafine-grained dual-phase layer-structured steel produced by hot rolling of intercritical annealed bainite
2023, Journal of Materials Research and Technology
In this study, ultrafine-grained (UFG) ferrite/martensite (F/M) layer-structured (LS) steels were fabricated by hot rolling of intercritical annealed bainite steels with a rolling reduction of 75% to ease the strength-toughness trade-off dilemma, and the effect of prior-austenite grain (PAG) size of the bainitic steels on the microstructure and mechanical properties were systematically studied. The results showed that the layer thickness of martensite and ferrite was not affected by the PAG size, but the spacing of parallel lamellae bands (PLS), which evolved from the packet of bainite, decreased with the decrease of PAG size. The PLS had negligible influence on strength and ductility, but the decrease of the PLS led to the increase of delamination and both room temperature (RT) and cryogenic toughness enhancement. By reducing the PLS to introduce more delamination in this steel, the RT and cryogenic impact energy were increased 5.7 times (301 J) and 21 times (102 J at −196 °C), but without the sacrifice of strength (over 1.5 GPa) and ductility (total elongation of about 14%), compared with the initial bainite steel with the same composition respectively. The significantly improved toughness, especially the cryogenic toughness, was considered to result from the special UFG dual-phase layered structure as strain localization could be restrained and ductile fracture could be promoted by this special microstructure even at −196 °C. The cryogenic toughness improvement with the decrease of the spacing of PLS was confirmed to originate from the ductile crack path increase induced by delamination.
Effects of rapid heating on the phase transformation and grain refinement of a low-carbon mciroalloyed steel
2023, Journal of Materials Research and Technology
The present study aims to clarify the possibility of rapid heating in the preparation of low-carbon ultrafine (UFG) steel. The effects of heating rate on the phase transformation, microstructure and precipitation of a low-carbon microalloyed steel were elaborately investigated. The results manifest that rapid heating potentially provides a simple and innovative process for the preparation of UFG steels just by increasing the heating rate. The parent austenite grains were refined due to the more reserved (Nb, Ti, V)C precipitates and dislocations within a short heating process by rapid heating. However, an effective holding time was proposed in the rapid heating to ensure the refined austenite grains. Moreover, the synthetic influences of initial increased grain boundaries and redissolved (Nb, Ti, V)C carbides provoked the abnormal grains with weakened thermostability. Redissolution of (Nb, Ti, V)C carbides were apparent during austenite nucleation at a slow heating rate, and it was verified during the isothermal holding stage by rapid heating. In addition, the austenitic phase transition occurred at higher temperature with the increase of heating rate, by which the transformation rate was accelerated.
Effect of layered heterogeneous microstructure design on the mechanical behavior of medium carbon steel
2022, Materials and Design
Citation Excerpt :
Due to the lower rolling temperature of WR500, the proportion of UFG grains in ferrite is larger (31 %), making YS up to 1461 MPa, UTS up to 1511 MPa, and TE of 6 %. Increasing the rolling temperature provides a higher driving force for the DRX process of ferrite, which makes the UFG continuously transform to FG and CG, and makes the material soften, and the TE gradually increases [22]. The PSE showed a trend of first increasing and then decreasing, and this difference came from the different HDI mechanisms brought about by the uneven deformation of the multi-scale interface.
A heterogeneous microstructure composed of multiscale ferrite (spanning ultrafine, fine, and coarse grains) and granular or short rod cementite was obtained by warm rolling pseudoeutectoid microstructure. Combined with the short-time annealing process, the deformation tendency of ferrite grains and the dissolution and precipitation of cementite were adjusted. The mechanical properties of medium carbon steel are close to that of low alloy high strength steel. It was found that the GNDs are distributed in a gradient at the interface of the soft domain, and the closer to the boundary the higher the density, which contributes to the occurrence of the hetero-deformation induced (HDI) hardening mechanism and provides additional strength and plasticity. The increase in elongation of the annealed samples is due to the recrystallization of ferrite and the redistribution of cementite, which realizes the combination of HDI mechanism, Orowan strengthening and annealing softening mechanism. Additional strengthening mechanisms are calculated and a physical model of cementite ripening during annealing is proposed. The tensile fracture shows that the cracks mainly propagate along the grain boundaries of the layered microstructure, and the high density of dimples can make the propagation path of the cracks tortuous and increase the elongation.
How Sn addition influences texture development in single-phase Fe alloys: Correlation between local chemical information, microstructure and recrystallisation
2022, Materials Characterization
Sn addition is known to affect the recrystallisation texture development of α-Fe. To date, even though the texture effect of Sn has been experimentally verified, although indirectly, the underlying mechanism has not been studied. In the present study, we focus on the static recrystallisation behaviour of Fe-Si-Sn alloys to elucidate the texture preferences assessing for the effect of Sn solute addition. The experimental approach is based on sequential electron back-scatter diffraction (EBSD) texture analysis complemented with a site-specific chemical analysis of grain boundaries by atom probe tomography (APT). It is directly observed through our experiments that boundary segregation phenomena, which occur at the onset and later stages of recrystallisation, provide crucial understanding of texture development. Especially in the presence of solute, the orientation dependence of the energy stored by deformation largely determines the course of the subsequent recrystallisation. In the Fe-Si-Sn alloy, recrystallisation at the highest stored energy grains is hindered. As a consequence of segregation, {001}〈110〉 orientations recrystallise at later stages of the process, forming predominantly grains with orientations of the {h11}〈1/h12〉 fibre. These grains are suggested to form following an oriented nucleation mechanism, while their growth could be interpreted with Ibe and Lücke's theory of selective growth of 26.5 〈110〉 boundaries in bcc metals.
Microstructural evolution of a niobium-microalloyed steel during hot shear deformation and subsequent cooling
2022, Journal of Materials Processing Technology
Citation Excerpt :
According to Kalashami et al. (2017), cDRX caused the accumulation of dislocations at the LAGBs and resulted in HAGBs in ferrous alloys. Furthermore, Storojeva et al. (2004) pointed out that the merging of LAGBs during the growth of grains also resulted in LAGBs with a decreased number fraction. Thus, the fraction of HAGBs in the specimen sheared at 1073 K was higher than that of the specimen sheared at 973 K.
For the purpose of obtaining a niobium-microalloyed steel with a preferable ultrafine-grained ferrite microstructure, a thermomechanical controlled processing (TMCP) strategy that included a hot shear deformation conducted near the local phase transformation temperature and a subsequent cooling process is proposed. As a large plastic strain was enforced by the simple hot shear deformation, severe plastic deformation (SPD) of the niobium-microalloyed steel was realized. The proposed TMCP strategy was simulated physically on a Thermecmastor-Z compression machine combined with a multi-type cooling system. Various cooling rates of different cooling methods resulted in reconstructive and displacive phase transformations from γ-Fe to α-Fe and led to different microstructural morphologies. In addition to phase transformations, the precipitation of carbides, grain growth, plastic deformation, discontinuous dynamic recrystallization (dDRX) of retained austenite grains, and continuous dynamic recrystallization (cDRX) of ferrite grains occurred during hot shear and subsequent cooling. The effects of the strain rate and forming temperature on the microstructural and textural evolution of niobium-microalloyed steel during hot shear and subsequent cooling were investigated and discussed. A niobium-microalloyed steel with a homogenous ultrafine-grained ferrite microstructure and intense γ-fiber texture was fabricated when the forming temperature, strain rate of hot shear deformation, and cooling rate of subsequent mist cooling were 1073 K, 20 s^−1, and 10 K·s^-1, respectively.

View all citing articles on Scopus

View full text

Development of microstructure and texture of medium carbon steel during heavy warm deformation

Abstract

Introduction

Section snippets

Material and experimental technique

CCT diagram

Conclusions

Acknowledgments

Acta Met.

Scripta. Mater.

J. Less-Common Metals

Acta Met.

Acta Met.

Steel Res.

J. Iron. Steel Inst.

Materialprüfung

Physica Status Solidi

Physical metallurgy