Microstructure Evolution and Mechanical Behavior of a CMnSiAl TRIP Steel Subjected to Partial Austenitization Along with
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INTRODUCTION
THE requirements of vehicle weight reduction and improved crash performance have promoted the development of new generations of advanced high strength steels (AHSS) with a good combination of strength and ductility. As one of the promising path towards these properties, quenching and partitioning (Q&P) heat treatment[1,2] has attracted considerable interest in the last several years. This treatment aims to retain some austenite at room temperature that will transform into martensite upon straining, delaying the onset of necking through the transformation-induced plasticity (TRIP) effect,[3] which ultimately offers excellent work-hardening behavior.
H. KONG, Q. CHAO, P.D. HODGSON, and H. BELADI are with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. Contact e-mail: [email protected] M.H. CAI is with the School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China. E.J. PAVLINA is with the AK Steel Research and Innovation Center, Middletown, OH 45005. B. ROLFE is with the Institute for Frontier Materials, Deakin University and also with the School of Engineering, Deakin University, Geelong, VIC 3216, Australia. Manuscript submitted November 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The concept of the Q&P process is to develop a multi-phase microstructure in steels, mainly consisting of martensite, retained austenite, and/or ferrite.[1,2,4,5] In a typical Q&P process, the steel is initially subjected to a full or partial austenitization treatment, which is followed by rapid cooling to a temperature between the martensitic transformation starting temperature (Ms) and finishing temperature (Mf) to form a certain fraction of martensite. The steel is subsequently held either at the same temperature (i.e., a one-step Q&P process) or at a higher temperature (i.e., a two-step Q&P process) to allow carbon partitioning from the initial supersaturated martensite to the untransformed austenite. Finally, the steel is quenched to room temperature, where the austenite with insufficient carbon enrichment is likely to transform into fresh martensite while the sufficiently stabilized austenite may be retained at ambient temperature. The extent of the carbon enrichment in retained austenite is dependent on the steel composition and heat treatment schedules. Most of the current research has been focused on the Q&P processing of low carbon steels after full austenitization, which normally produces a duplex microstructure composed of martensite and retained austenite.[6–10] Here, the carbon enrichment in the retained austenite is mostly ascribed to the carbon rejection from the initial supersaturated martensite. In comparison, if the Q&P processing is performed
following partial austenitization, one can expect higher extent of carbon and manganese diffusion from ferrite into the intercritical austenite due to the different solubilities in these two phases, which would further stabilize the austenite.[11] Moreover, proeutectoid ferrite mi
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