Microstructure Regulation and Strengthening Mechanisms of a Hot-Rolled & Intercritical Annealed Medium-Mn Steel Containing Mn-Segregation Band
CHEN Xueshuang1, HUANG Xingmin1(), LIU Junjie1, LV Chao1, ZHANG Juan2
1.Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China 2.Applied Mechanics and Structure Safety Key Laboratory, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu 610031, China
Cite this article:
CHEN Xueshuang, HUANG Xingmin, LIU Junjie, LV Chao, ZHANG Juan. Microstructure Regulation and Strengthening Mechanisms of a Hot-Rolled & Intercritical Annealed Medium-Mn Steel Containing Mn-Segregation Band. Acta Metall Sin, 2023, 59(11): 1448-1456.
Recently, medium-Mn steel, used in the automotive industry, has attracted increasing attention as the one of the most promising candidates for the third generation of advanced high strength steels owing to its reasonable cost and excellent mechanical properties. In this study, the effect of intercritical annealing temperature on the microstructure and mechanical properties of a new composition steel was investigated, and its strengthening mechanism and related reasons were analyzed. In addition, a ultra-high product of strength and plasticity (> 70 GPa·%) of hot rolled medium manganese steel with a segregation band was eventually obtained. The results show that the grain size and orientation in the packet (defined by the original austenite grain boundary) significantly affect the mechanical properties and deformation microstructure of the material obtained under different temperatures. The obvious precipitation and dissolution processes of carbides occur at higher temperatures, and thus influence the mechanical stability of reversed austenite. During the tensile process, because it is easier to deform, the favorable packets in the non-segregation zone form an elongated-strip fine-grain zone along the loading direction, while the unfavorable packets form fragmentary grain regions. Moreover, martensite transformation preferentially occurs at the obvious orientation inside the austenite grain and the boundaries where large strain is accumulated. Through coordinated deformation, the adjacent packets eventually tend to form alternate distribution of the two kinds of micro-zone substructures, which is accompanied by the significant evolution of low-angle grain boundaries related to the dislocation activity. Due to the wide distribution of grain size in one packet, the reversed austenite in the non-segregation zone can withstand large deformation, which makes the austenite in the segregation zone undergo sufficient strain-induced martensitic transformation (SIMT), to obtain excellent combination of strength and toughness.
Fig.1 SEM image (a), electron backscattered diffraction (EBSD) phase map (b), and inverse pole figure (IPF) (c) of hot-rolled and quenched medium-Mn steel specimen (Inset in Fig.1a shows the high magnified image; M—martensite, HAGBs—high-angle grain boundaries, LAGBs—low-angle grain boundaries, PABs—prior-austenite grain boundaries, RD—rolling direction, TD—transverse direction)
Fig.2 XRD spectrum of hot-rolled and quenched medium-Mn steel specimen
Fig.3 XRD spectra (a) and retained austenite (RA) contents (b) of inter-critical annealed mendium-Mn steels before and after tensile deformation
Fig.4 SEM images (a-c), EBSD phase maps (d-f) and corresponding IPFs (g-i) of IA600 (a, d, g), IA650 (b, e, h), and IA700 (c, f, i) samples (Austenite has similar orientation in a single packet, which is roughly distinguished by packet boundaries (white dotted lines) with the adjacent one; insets in Figs.4a-c show enlarged images of corresponding non-segregation zones)
Fig.5 Engineering stress-strain curves (a) and corresponding work-hardening curves of IA600 (b), IA650 (c), and IA700 (d) samples (WHR—work hardening rate; S1-S3 show the different stages)
Fig.6 EBSD phase maps (a, c, e, g) and corresponding IPFs (b, d, f, h) of IA650 sample with different strains (Inset in Fig.6a is the kernel average misorientation (KAM) map of austenite grain in the selected area; the arrows in Fig.6b show that the internal orientation of some austenite grains changed significantly; the black solid circles in Figs.6e and f represent the orientation change inside a large α grain, while the dotted lines in Figs.6f and h represent the boundaries between bundle zone (I) and fragment zone (II)) (a, b) 10% strain (c, d) 20% strain (e, f) 40% strain (g, h) 60% strain
Fig.7 SEM image of bundle zone (I) and fragment zone (II) in IA650 sample after tensile deformation
Fig.8 EBSD phase maps (a, c) and IPFs (b, d) of IA600 (a, b) and IA700 (c, d) samples after tensile deformation
Fig.9 SEM images of segregation band in IA650 sample before (a) and after (b) tension (The arrows and circle represent the segregation area before and after tension, respectively)
Fig.10 Calculated martensite transformation ratios of segregation band in IA600, IA650, and IA700 samples
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