Enhanced tensile properties of a reversion annealed 6.5Mn-TRIP alloy via tailoring initial microstructure and cold rolling reduction

doi:10.1016/j.jmst.2017.12.008

Abstract

Abstract:

The feasibility of improving the overall performance of medium Mn steels was demonstrated via tailoring the initial microstructure and cold rolling reduction. The combined effects of cooling patterns after hot rolling (HR) and cold rolling (CR) reductions show: (1) as the cooling pattern varied from furnace cooling (FC) to oil quenching (OQ), the intercritically annealed microstructure was dramatically refined and the fraction of recrystallized ferrite dropped, regardless of CR reductions. This resulted in both high yield/ultimate tensile strengths (YS/UTS) but low total elongation to fracture (El); (2) as the CR reduction increased from 50% to 75%, the OQ-samples after annealing exhibited a more refined microstructure with relatively higher fractions of retained austenite and sub-structure, leading to higher YS and UTS but lower El; whereas the FC samples appeared to exhibit little difference in overall tensile properties in both cases. The differences in microstructural evolution with cooling patterns and CR reductions were explained by the calculated accumulated effective strain (ε_AES), which was considered to be related to degrees of recovery and recrystallization of the deformed martensite (α’). The optimal tensile properties of ～1?GPa YS and ～40?GPa·% UTS×El were achieved in the OQ-50%CR annealed samples at 650?°C for 1?h. This was quite beneficial to large-scale production of ultra-high strength steels, owing to its serious springback during heavy cold working.

Key words: Medium Mn steel, Reversion annealing, Microstructural evolution, Mechanical properties, Initial microstructure, Cold rolling

Minghui Cai, Hongshou Huang, Junhua Su, Hua Ding, Hodgson Peter D.. Enhanced tensile properties of a reversion annealed 6.5Mn-TRIP alloy via tailoring initial microstructure and cold rolling reduction[J]. J. Mater. Sci. Technol., 2018, 34(8): 1428-1435.

Figures/Tables 11

Fig 1. Schematic illustration of thermo-mehcanical process of a Nb-Mo microalloyed 6.5?Mn alloy. HR, CR, OQ, FC and IA represent hot rolling, cold rolling, oil quenching, furnace cooling and intercritical annealing, respectively.

Fig. 2. SEM micrographs of Nb-Mo microalloyed 6.5?Mn alloy after hot rolling and different cooling patterns: (a) oil-quenching (OQ) and (b) furnace-cooling (FC).

Fig. 3. TEM micrographs of a Nb-Mo microalloyed 6.5?Mn alloy after hot rolling and cold rolling: (a) OQ?+?50%CR; (b) OQ?+?75%CR; (c) FC?+?50%CR; (d) FC?+?75%CR. αB, α′d and γR represent the bainitic ferrite, deformed martensite and retained austenite, respectively.

Fig. 4. SEM micrographs of a reversion annealed Nb-Mo microalloyed 6.5?Mn alloy at 650?°C for 1?h after hot and cold rolling: (a) OQ?+?50%CR?+?IA; (b) OQ?+?75%CR?+?IA; (c) FC?+?50%CR?+?IA; (d) FC?+?75%CR?+?IA. SG, SF, α and γR represent the sub-grain, stacking fault, ferrite and retained austenite, respectively.

Fig. 6. EBSD mappings of a reversion annealed Nb-Mo microalloyed 6.5?Mn alloy at 650?°C for 1?h after hot and cold rolling: (a) OQ?+?75%CR?+?IA; (b) FC?+?75%CR?+?IA; (c) the corresponding relative fractions of recrystallized, substructured and deformed ferrite.

Table 1 The statistical grain size distributions of γR and α of OQ/FC?+?75%CR?+?IA steels from EBSD.

Process	Phases	Grain size distribution, d (μm)					Grain sizes, d (μm)
		＜0.15	＜0.35	＜0.55	＜1.15	≥1.15	Min	Max	Ave.
OQ-75%	α	41.2%	41.9%	9.1%	6.5%	1.3%	0.06	2.47	0.20
OQ-75%	γR	29.0%	50.1%	16.6%	4.5%	0	0.06	0.91	0.20
FC-75%	α	26.4%	37.4%	12.9%	20.2%	3.1%	0.08	2.01	0.32
FC-75%	γR	25.5%	51.2%	17.4%	5.9%	0	0.08	0.86	0.22

Fig. 7. XRD patterns of a reversion annealed Nb-Mo microalloyed 6.5?Mn alloy at 650?°C for 1?h after hot and cold rolling: (a) prior to tensile testing; (b) after tensile fracture.

Fig. 8. Engineering stress (σ)-strain (ε) curves and the corresponding overall tensile properties of a reversion annealed Nb-Mo microalloyed 6.5?Mn alloy at 650?°C for 1?h after hot and cold rolling. A, B, C and D indicate OQ?+?50%CR?+?IA, OQ?+?75%CR?+?IA, FC?+?50%CR?+?IA and FC?+?75%CR?+?IA, respectively.

Fig. 9. Schematic illustration of microstructural evolution of (a) OQ and (b) FC samples processed by IA at 650?°C for 1?h. α′d, SG, α and γR represent the deformed martensite, sub-grain, ferrite and retained austenite, respectively.

Fig. 10. True stress (σ)-strain (ε) curves and the statistical Lüders band propagation (LBP) strain of a reversion annealed Nb-Mo microalloyed 6.5?Mn alloy after hot and cold rolling. The Portevin-Le Chatelier phenomenon-related serration curves are magnified.

Fig. 11. EBSD kernel average misorientation (KAM) maps of the ferrite phase in a reversion annealed Nb-Mo microalloyed 6.5?Mn alloy after hot and cold rolling: (a) OQ?+?75%CR?+?IA; (b) FC?+?75%CR?+?IA; (c) the KAM distribution corresponding to (a) and (b).

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