Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels
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THE austenite reversion treatment of medium Mn steels (3 to 10 mass pct Mn) has gained much attention recently, since the reverted austenite gives rise to both high strength and high elongation.[1] This is aligned with the development of the third-generation advanced high-strength steels with excellent mechanical properties and acceptable cost.[1] The austenite reversion treatment of medium Mn steels mainly consists of two processes: austenitization and quenching to form martensite, and subsequent intercritical annealing in the ferrite+austenite twophase region for the austenite reversion.[1–11] If growing from as-quenched lath martensite, the reverted austenite is mainly thin-film-like and primarily nucleated at lath boundaries,[2,6,7,11] and has almost identical orientations with the prior austenite.[12,13] During intercritical annealing, austenite is mainly stabilized by C and Mn partitioning from martensite, meanwhile, martensite gradually becomes ferrite due to C depletion and dislocation annihilation.[2,5,14] The width of reverted austenite and ferrite/martensite is in the order of a few FEI HUYAN, JIA-YI YAN, LARS HO¨GLUND, JOHN A˚GREN, ANNIKA BORGENSTAM are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted March 8, 2017.
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hundred nanometers. Therefore, after intercritical annealing, a fine microstructure of reverted austenite and ferrite/martensite is obtained. The elongation can be further enhanced, without compromising strength, by exploiting the transformation-induced plasticity (TRIP) effect brought about by the transformation of austenite to martensite under external stress. The TRIP effect is maximized by optimizing the phase stability of austenite.[15,16] The final microstructure and austenite stability of medium Mn steels are determined by the temperature and duration of intercritical annealing.[2,3,9,10,14] Therefore, it is important to understand and model the austenite growth during this process, since an accurate model benefits further steel design. Many simulations of the austenite reversion from martensite in the Fe-C-Mn and Fe-C-Mn-Si systems can be found in literature. In most of the simulations, a diffusion couple of austenite and martensite is used.[2–4,11,17–21] In this setup, the simulated temporal evolution of austenite volume fraction has three stages, i.e., a rapid increase under non-partitioning local equilibrium (NPLE) controlled by rapid carbon diffusion, a slow increase under partitioning local equilibrium (PLE) controlled by relatively slow diffusion of Mn in martensite, and a decrease to the equilibrium level under PLE due to homogenization of all alloying elements. When performed on medium Mn steels, such simulations predict much faster austenite formation than experimentally observed after short intercritical
annealing times, e.g., less than 1 hour,[7,9] primarily attributed to the NPLE stage. In addition, cementite
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