Constitutive Modeling of the Mechanical Properties of V-added Medium Manganese TRIP Steel
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THE considerable interest in ultra-fine grained (UFG) steels is due to their ability to achieve high strength without high alloying additions. The low ductility of UFG steels remains, however, a major obstacle to their industrial application. It was recently realized that cold-rolled steels with a medium (4 through 7 mass-pct) Mn content develop UFG microstructures after intercritical annealing because of the simultaneous recrystallization of ferrite and the reverse transformation of martensite to austenite.[1–7] Despite recent improvements in the combination of total elongation and ultimate tensile strength,[4,6] control of strain localization caused by a low work hardening rate is still a major issue with UFG medium Mn TRansformationInduced Plasticity (TRIP) steels. Many mechanisms have been proposed to improve the work hardening rate of UFG metals and alloys.[8–13] Ohmori et al.[10] suggested introducing cementite particles and showed that the work hardening increased with an increased volume fraction of cementite particles. Tsuji et al.[8] proposed a combination of fine carbide precipitates and a multiphase microstructure. Wang et al.[12] showed that nanotwinning resulted in a superior combination of ultimate tensile strength (UTS) and total elongation. Ma[15] recently reviewed several routes to improve the mechanical properties of UFG materials. The following microstructural features were considered SEAWOONG LEE, Graduate Student, and BRUNO C. DE COOMAN, Professor, Director, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Contact e-mail: [email protected] YURI ESTRIN, Professor, is with the Center for Advanced Hybrid Materials, Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia. Manuscript submitted September 10, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
essential to enhance ductility of UFG materials: a bimodal grain size distribution, a multiphase microstructure, the presence of nanoscale twins, the presence of nanosized precipitates, the occurrence of a deformation-induced phase transformation, and a low rate of dynamic recovery. Synergy is, however, essential, as none of these factors implemented individually yields the preferred level of improvement of the mechanical properties.[1,2,6,7,14] Suh et al.[2] showed that a bimodal grain structure can be achieved in medium Mn ferrous alloys with the additions of Al and Si when the amount of both alloying elements exceeds 3 mass pct. According to Lee et al.,[6] the martensitic transformation of retained austenite can raise the work hardening considerably in a 6 mass pct Mn TRIP steel. The austenite stability of the steel is mainly due to the combination of the UFG size of the retained austenite and its chemical composition.[5] Control of the grain size and the chemical composition of retained austenite are thus essential to obtain optimal conditions for the martensitic transformation kinetics during deformation. Finally, the comp
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