On the Mechanical Stability of Austenite Matrix After Martensite Formation in a Medium Mn Steel
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INTRODUCTION
MARTENSITIC transformation is a first-order, solid-state phase transformation, which is critically important in fabricating the advanced high-strength steels (AHSS). The martensitic steels, which comprise the full martensitic phase, have an ultra-high strength due to their hierarchically fine microstructures.[1,2] The transformation-induced plasticity (TRIP) steels have an excellent combination of high strength and good ductility because of the martensitic transformation within the retained austenite grains, providing the TRIP effect to sustain the high work hardening rate.[3–5] The quenching and partitioning (Q&P) steels consist of martensite and retained austenite grains. The martensite phase can provide the high strength, while the retained austenite grains offer the TRIP effect to enhance the work-hardening behavior, resulting in a Q&P steel with high strength and good ductility.[6] The Q&P steels were fabricated by quenching to a temperature between the martensite start (Ms) and finish (Mf) temperature to obtain a certain amount of martensite that feeds the rest austenite matrix with C content at the elevated temperature.[7,8] This Q&P process has been applied to different alloy systems, such as medium C steel,[9] stainless steel,[10] and medium Mn steel.[11] Despite the various alloy systems, a proper amount of martensite is required to achieve the excellent mechanical properties of Q&P steels. This is because a proper amount of martensite
B.B. HE, Postdoctoral Fellow, and M.X. HUANG, Assistant Professor, are with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted December 21, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
can not only provide the high strength but also help to achieve the retained austenite grains with optimized mechanical stability. Therefore, it is important to predict the martensite volume fraction precisely during the quenching process. Generally, the first martensite nucleates at the austenite grain boundary and propagates across the whole grain until it halts at the opposite austenite grain boundary,[12] separating the prior austenite grain into a smaller grain size. The later transformed martensite is constrained within these small austenite grains as it is not able to transform across the prior martensite lath. Since the smaller austenite grain generally leads to a smaller martensite block,[13] the first transformed martensite block is larger than the later transformed martensite block.[14] However, the effect of former transformed martensite on later martensite formation within the retained austenite matrix is still not clear. The answer to this question is important because it will shed light on the prediction of the martensite volume fraction during the quenching process. Since the prior large austenite grain will be separated into different small austenite grains at micron scale after the intensive martensitic transformation, it is difficult to study the mechanical
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