A Scale-up Study on Chemical Segregation and the Effects on Tensile Properties in Two Medium Mn Steel Castings

ORIGINAL RESEARCH ARTICLE A Scale-up Study on Chemical Segregation and the Effects on Tensile Properties in Two Medium Mn Steel Castings T.W.J. KWOK, C. SLATER, X. XU, C. DAVIS, and D. DYE Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250  C for various times to study the degree of chemical homogeneity and d-ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some d-ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed d-ferrite in the 5 kg ingot turned into coarse d-ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed >900 MPa yield strength, >1100 MPa tensile strength, and >40 pct elongation with <10 pct difference in strength and no change in ductility when compared to a fully homogenized sample. https://doi.org/10.1007/s11661-021-06533-w Ó The Author(s) 2021 I. INTRODUCTION MEDIUM Mn steels are an emerging class of steel which has shown great potential in energy absorbing applications. A medium Mn steel of composition Fe–10Mn–1.5Al–0.2Si–0.15C was developed as part of a 3rd-Generation Advanced High-Strength Steel (3GAHSS) Development Project by the U.S. Department of Energy. Termed as the ‘‘High Strength  Exceptional Ductility’’ steel, it had a tensile strength of 1200 MPa and elongation of 37 pct and could be used in the front or rear pillars of an automotive body in white (BIW) to protect from front or rear impact.[1] T.W.J. KWOK is with the Department of Materials, Imperial College London, Prince Consort Road, London, UK. Contact e-mail: C. SLATER is with the Warwick Manufacturing Group, University of Warwick, Coventry CV4 7AL, UK. X. Xu is with the Department of Materials, Imperial College London and also with the School of Materials, Sun Yat-Sen University, Shenzhen 519082, China. C. DAVIS is with the Warwick Manufacturing Group, University of Warwick. D. DYE is with the Department of Materials, Imperial College London. Manuscript submitted May 17, 2021; accepted October 18, 2021. METALLURGICAL AND MATERIALS TRANSACTIONS A The Mn content in these steels (4 to 12 wt pct) is significantly lower than high Mn twinning-induced plasticity (TWIP) steels (16 to 30 wt pct) and are, therefore, more attractive from an industrial perspective in terms of cost and ease of production. The high Mn content in TWIP steels posed many challenges to steelmakers during industrialization efforts over the past two decades.[2,3] Feasibility studies showed that Mn segregation in cast ingots led to edge cracking during hot rolling.[3,4] It had been theorized that these problems may be avoided in medium Mn steels due to the lower Mn content. However, medium Mn steels are still relatively heavily alloyed compared to more lean steel grades such as dual-phase (DP) steels and similar problems faced by TWIP steels may persist. The first major problem is chemical microsegregation during casting.[5,6] While the extent of segregation may not be as severe as in TWIP steels, the Mn content is still sufficiently high to be a concern. In a study on a quenching and partitioning (QP) steel of composition Fe–4.5Mn–1.5Si–0.3C, Hidalgo et al.[7] showed that a segregation range of 2 wt pct Mn resulted in different martensite fractions across the steel. This resulted in inhomogeneous strain gradients during tensile testing and premature failure. Liang et al.[8] showed in another QP steel of composition Fe–3Mn–1.5Si–0.25C that Mn segregation led to banding of alternate equiaxed (Mn-rich) and lath (Mn-depleted) type microstructures. The difference in austenite stability between grains with the two microstructures also led to strain inhomogeniety and poor ductility. Tight composition control in medium Mn steels is important as slight variations in austenite composition may lead to different active deformation mechanisms, i.e., transformation induced plasticity (TRIP), TWIP, or TWIP þ TRIP.[9] The TWIP + TRIP mechanism is usually sought after in medium Mn steels as it provides the optimal balance between strength and ductility. However, the composition window where the TWIPþTRIP mechanism is active is usually very narrow.[10] Nevertheless, Wang et al.[11] showed that microsegregation in a fairly lean medium Mn steel could be avoided by twin roll casting. Their steel of composition Fe–4Mn–1.8Al–0.6Si–0.3C with a cast thickness of 2.5 mm did not show significant microsegregation and was also able to demonstrate the TWIP þ TRIP effect after final processing. The second problem is the retention of d-ferrite to room temperature. d-ferrite is the first phase to form during the solidification of medium Mn steels but is expected to transform to austenite at typical hot rolling and slab reheat temperatures (<1280  C).[12,13] However, d-ferrite can be stabilized to room temperature when there are excess Al or Si additions.[14,15] The effect of delta ferrite varies depending on the alloy composition and desired mechanical properties. Some researchers do not consider d-ferrite to be a problem as it does not appear to have adverse effects on tensile properties in certain alloys and may even be beneficial to ductility.[16,17] However, d-ferrite is usually not beneficial from a strength perspective as it is difficult to refine the grain size of d-ferrite and is, therefore, typically weaker than the much finer austenite and a-ferrite matrix.[15,18] When deformed to large strains, the plasticity mismatch between d-ferrite grains and the matrix may cause interface cracking which might result in premature failure.[19] Mn segregation at the d-ferrite interface has also been shown to reduce impact properties in medium Mn steels.[20] It is, therefore, important that alloy development efforts consider the size, morphology, and distribution of d-ferrite in medium Mn steels and how these factors may change during a transition from lab-scale to full-scale production material. Building upon previous work on an 8 wt pct Mn medium Mn steel,[21] this study aims to elucidate some of the problems which may be encountered during scale-up of medium Mn steels from (...truncated)