Effect of Retained Austenite on the Fracture Toughness of Quenching and Partitioning (Q&P)-Treated Sheet Steels
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NTRODUCTION
SAFETY and fuel reduction in the automotive industry require the development of new steels with combinations of higher strength and formability. These new advanced high-strength steels (AHSS) include twinning-induced plasticity (TWIP) steels[1] and steels with chemical compositions closer to conventional transformation-induced plasticity (TRIP) or dual-phase (DP) steels but processed by novel heat treatments, such as the nanobainitic steels[2,3] and the new quenching and partitioning (Q&P) steels.[4] In particular, the application of the Q&P process is reported with improved mechanical properties of tensile strength and elongation that cover the existing gap among DP, TRIP, and TWIP steels[5] as shown in Figure 1. Because of these promising results,[6–24] the Q&P process is attracting high interest from both industry and academia around the world. Steels with chemical compositions closer to conventional TRIP or DP steels but processed by novel Q&P treatments are always associated with the formation of bainite during the partitioning step.[25] However, the martensitic matrix and retained austenite in these Q&Ptreated steels ensure the high strength of the steels
RIMING WU, Ph.D. Student, WEI LI, Lecturer, and XUEJUN JIN, Professor, are with the Institute of Advanced Steels and Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] SHU ZHOU, Engineer, YONG ZHONG, Senior Engineer, and LI WANG, Prefessorate Senior Engineer, are with the Baosteel Research and Development Technology Center, Shanghai 201900, P.R. China. Manuscript submitted August 7, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
(ultimate tensile strength [UTS] > 1200 MPa) as well as good ductility. This differs from TRIP sheet steels (UTS < 1000 MPa) comprised of ferrite as matrix and carbide-free bainitic microstructures with interlathretained austenite obtained through austempering. The Q&P process initially involves austenite formation, followed by quenching below the martensite start temperature to obtain an appreciable fraction of martensite. A partitioning treatment follows the quenching step, with the aim to enrich the remaining untransformed austenite with carbon transporting from the supersaturated martensite. During the Q&P process, the remaining carbon-enriched austenite is retained in martensitic matrix at room temperature and carbides are suppressed. Research has shown that retained austenite is effective in improving the toughness of high-strength steels. Kim et al.[26] showed that in AISI M2 high-speed steel a high fraction (about 16 vol pct.) of retained austenite appeared to raise the fracture toughness. The toughness of a martensitic stainless steel was significantly improved[27] by raising an austenite fraction from 15 to 30 vol pct. The improvement can be attributed not only to the crack-arrest effect and stress-relief effect by retained austenite[28] but also to the martensite transformation (MT) ahead of the cracks.[29] Neverthele
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