Observation on Formation of Fresh Martensite from the Reversed Austenite During Water-Quenching Process in Fe-0.2C-5Mn S
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e phase transformation mechanism in intercritical annealing process is very important[1–4] for low alloy steels. It determines the final microstructure of steels which is related to the mechanical properties. The mechanism is complicated as it can occur from various initial microstructures,[3–5] such as as-quenched martensite, cold-rolled ferrite, and so on. In this study, we focus on the phase transformation behavior in medium Mn steel (new generation automobile steel with good strength and ductility[6,7]) with initial microstructure of as-quenched martensite. In some previous work,[4–8] it was usually thought that the final microstructure only consisted of ferrite (tempered CHUAN ZHAO, Ph.D. Student, is with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, P.R. China, and also with the Institute for Structural Materials, Central Iron and Steel Research Institute, Beijing 100081, P.R. China. CHI ZHANG, Associate Professor, and ZHI-GANG YANG, Professor, are with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University. Contact e-mail: [email protected] WEN-QUAN CAO, Research Faculty, and YU-QING WENG, Professor, are with the Institute for Structural Materials, Central Iron and Steel Research Institute. Manuscript submitted February 2, 2015. Article published online July 17, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
martensite) and retained austenite. The ferrite (tempered martensite) and the fresh martensite (newly transformed from reversed austenite) were not carefully distinguished. The width of retained austenite lath was considered as the width of reversed austenite lath and some calculation about growth of reversed austenite has been done based on this consideration.[4] It is not so accurate. So it is necessary to study the phase transformation behavior during the cooling which determines the final microstructure and the mechanical properties of steel. To study the phase transformation behavior during cooling in medium Mn steel, a high-purity ternary Fe-CMn alloy was prepared by vacuum-induction melting; the composition of the alloy is demonstrated in Table I. After homogenization, the ingot was forged into rods with diameter of 15 mm. Before further heat treatments, the rods were cut into specimens with thickness of 4 mm and diameter of 15 mm. During following step, all specimens were austenitized at 1123 K (850 C) for 1800 seconds and water-quenched firstly. The specimens were then intercritically annealed by salt bath at 953 K (680 C) with different time (120, 600, 21,600, 129,600 seconds). The intercritical annealing temperature was selected based on the calculation of Thermo-Calc: the Ae1 [(a + c)/(a + c + h)] and Ae3 [c/(a + c)] temperatures for Fe-0.2C-5Mn steel in this study are 903 K (630 C) and 993 K (720 C), respectively. After intercritical annealing, the specimens were quenched into water again. The volume fraction
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