Application of Quenching and Partitioning Processing to Medium Mn Steel
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Increasing requirements related to passenger safety and weight reduction in the automotive industry have led to the development of a first generation of advanced high strength steels (AHSS) such as dual phase and transformation-induced plasticity (TRIP) steels. Second generation austenitic high-Mn twinning-induced plasticity (TWIP) steel with a Mn content in the range of 15 to 25 wt pct exhibits an excellent combination of tensile strength (~1 GPa) and ductility (~60 pct) due to a dynamic Hall–Petch effect resulting from the gradual increase of the density of mechanical twins during deformation.[1,2] The higher alloying costs and the lower productivity associated with high-Mn TWIP steel are currently the main drivers behind the development of the intermediate and medium Mn steel. Adequate combinations of mechanical properties have been reported for intercritically annealed intermediate and medium Mn steels. Their high work-hardening rate is achieved by the TRIP effect or a combination of two plasticity-enhancing mechanisms, the TWIP effect and the TRIP effect.[3–5] The Mn content in these materials is typically one third of the Mn content of the high-Mn TWIP steels, but despite their lower Mn content, these steels achieve excellent mechanical properties with a strength-ductility balance in the range of 35,000 to 45,000 MPa pct. Quenching and partitioning (Q&P) processing was proposed by Speer et al.[6] as a new approach to produce
EUN JUNG SEO and LAWRENCE CHO, Graduate Students, and BRUNO C. DE COOMAN, Professor and Director, are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Contact e-mail:[email protected] Manuscript submitted July 21, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
steel microstructures consisting of a low-C martensitic matrix containing a considerable volume fraction of retained austenite. Earlier studies have shown that the Q&P processing of various AHSS enables the use of the TRIP effect to achieve a pronounced improvement of mechanical strength and ductility.[7–9] The Q&P processing consists of three stages: an initial quenching stage, a partitioning stage, and a final quenching stage. The austenitized steel is initially quenched to a temperature, TQ, in the Ms Mf temperature range, and partially transformed to primary martensite. It is then partition treated at the partitioning temperature, TP. During the partitioning stage, C diffuses from the supersaturated primary martensite into the untransformed austenite. As a result, the Ms temperature of the C-enriched austenite is lowered. This leads to the stabilization of the untransformed austenite upon cooling to room temperature. If C does not partition enough to austenite, some of the austenite will transform to secondary martensite in the final quenching stage. The final microstructure consists of C-enriched austenite islands in a low-C lath martensite matrix. The martensite contributes a high strength level to the material and the C-e
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