Phase Transformation and Texture Evolution During Cold Rolling and α'-M Reversion in High Manganese TRIP Steel
Lina WANG1,2, Ping YANG1(), Kai LI1, Feng'e CUI1, Weimin MAO1
1 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 2 Department of Materials, School of Tianjin, University of Science and Technology Beijing, Tianjin 301830, China
Cite this article:
Lina WANG, Ping YANG, Kai LI, Feng'e CUI, Weimin MAO. Phase Transformation and Texture Evolution During Cold Rolling and α'-M Reversion in High Manganese TRIP Steel. Acta Metall Sin, 2018, 54(12): 1756-1766.
To meet the requirement of environment, economy and safety, advanced high strength steels including dual phased (DP), complex phased (CP), transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels are widely used for automotive steel. Among them, high manganese TWIP and TRIP steels are particularly appealing due to their outstanding tensile strength and elongation. In contrast to high manganese TWIP steel, high manganese TRIP steel exhibits higher strength and work hardening rate due to strain induced martensitic transformation. The enhanced mechanical properties of high manganese TRIP steel are determined by both the stability of the retained austenite (γ ) and the initial microstructure. Strain induced martensitic transformation and subsequent reversion from deformed martensite to γ during annealing is often applied as one of the most effective methods for microstructure improvement. Microstructure and texture characteristics of high manganese TRIP steel during cold rolling together with the reversion of deformed bcc martensite (α'-M) at high temperature were investigated. It is shown that the γ was almost completely transformed into α'-M at medium cold rolling reduction. And a higher reduction after α'-M saturation resulted in dominantly the deformation of α'-M, hence thin laths paralleled to the rolling direction (RD) were obtained. The main components in α'-M were {113}<110>, {554}<225> and rotated cube ({001}<110>) textures at medium cold rolling reduction, which are the typical phase transformation textures. The {113}<110> texture rotated toward a more stable orientation {223}<110> and led to a strong cold rolling texture (<110>//RD) with increasing reduction. The reversion of martensite and recrystallization of γ proceeded at temperature ranging from 650 ℃ to 850 ℃. The reversion of α'-M proceeded in a diffusional mechanism, accompanying with the redistribution of Mn and Al between γ and α'-M. Deformed α'-M was merged by the adjacent γ , and columnar γ grains with a large amount of subgrains were obtained. The texture of reverted γ was approximately the same as that of the deformed γ , this phenomenon called texture inheritance was formed by the direct growth of γ . Subsequently, recrystallization of γ grains occurred by sub-grain coalescence and the columnar γ grains were instead by equiaxed γ grains.
Fig.1 Microstructures of cold rolled high manganese transformation-induced plasticity (TRIP) steel at different reductions (RD—rolling direction, ND—normal direction) (a~c) OM images at 0, 30% and 50% reductions, respectively(d~g) SEM images at 0, 30%, 50% and 70% reductions, respectively(h, i) ECC images at 70% and 90% reductions, respectively
Fig.2 XRD spectra of cold rolled high manganese TRIP steel at different reductions
Fig.3 Volume fractions of γ, ε-M and α′-M phases in cold rolled high manganese TRIP steel at different reductions
Fig.4 Orientation maps (a, d), band contrast maps (b, e) and pole figures of α′-M (c, f) in cold rolled high manganese TRIP steel (TD—transverse direction) (a~c) 50% reduction (d~f) 90% reduction
Fig.5 Orientation distribution function (ODF) figures in cold rolled high manganese TRIP steel (φ1, Φ, φ2—Euler angles) (a) γ phase, 30% reduction(b) α’-M phase, 50% reduction(c) α’-M phase, 90% reduction
Fig.6 SEM images showing microstructure evolution after annealing in 90% cold rolled high manganese TRIP steel at 850 ℃ for 20 s (a), 30 s (b), 60 s (c), 5 min (d), 10 min (e) and the volume fractions of γ after anneling at different annealing temperatures (f)
Fig.7 SEM image (a) and EDS results for Mn (b) and Al (c) of the 90% cold rolled high manganese TRIP steel after annealing at 850 ℃ for 5 min
Fig.8 ODF figures of γ phase in 90% cold rolled high manganese TRIP steel after annealing processes (a) 650 ℃, 1 h (b) 750 ℃, 5 min (c) 850 ℃, 60 s
Fig.9 Orientation maps of γ (a, e) and α′-M (b, f), pole figures of γ (c, g) and α′-M (d, h), misorientation angle distributions of γ (i, j) in two selections of the 90% cold rolled high manganese TRIP steel specimen after annealing at 850 ℃ for 30 s (Thin colored lines and thick red lines in Figs.9a, b, e and f indicate low angle boundaries and twin boundaries, respectively) (a~d, i) selection I (e~h, j) selection II
Fig.10 Orientation maps (a, d), band contrast maps (b, e), ODF figures of γ (c, f), misorientation angle distributions of γ (g, h) in 90% cold rolled high manganese TRIP steel after annealing at 850 ℃ for different times (Grey color and Euler angle color in Figs.10a and d indicate α’-M and γ, respectively) (a~c, g) 60 s (d~f, h) 10 min
Fig.11 Schematics of microstructure evolution in 90% cold rolled high manganese TRIP steel during annealing at 850 ℃ (a) columnar grains of γ, ε-M and α’-M (b) ε-M→γ transformation(c) α’-M→γ transformation (d) recrystallization of γ(e) accomplishment of α’-M reversion and growth of γ (f) growth of γ
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