Influence of Temperature and Grain Size on Austenite Stability in Medium Manganese Steels
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RECENTLY, based on the pioneering work by Miller[1] and Merwin,[2,3] medium Mn steels have attracted considerable attention as a strong candidate for third generation advanced high strength steel (3G-AHSS) in the automotive industry because of their excellent mechanical properties.[4–12] Medium Mn steels typically contain 4~10 wt pct Mn with relatively low carbon, and have excellent products of strength and elongation over 30 GPa pct. For example, Luo et al.[13] achieved a tensile strength of 850 to 950 MPa combined with a ductility of 20 to 30 pct in a Fe-5Mn-0.2C steel. Shi et al.[14] reported that a Fe-7Mn-0.2C steel exhibited a high tensile strength of 1420 MPa with 31 pct total elongation. Cai et al.[15] developed a Fe-11Mn-0.18C-3.8Al steel having a tensile strength of 1007 MPa and a total elongation of 65 pct. Cold-rolled and hot-dip galvanized trial coils with two compositions,
YULONG ZHANG is with the Research Institute, Baoshan Iron & Steel Co. Ltd., Shanghai, 201900, P.R. China, and also with the State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel), Shanghai, 201900, P.R. China, and also with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] LI WANG is with the Research Institute, Baoshan Iron & Steel Co. Ltd., and also with the State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel). KIP O. FINDLEY, and JOHN G. SPEER are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Manuscript submitted October 11, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fe-7Mn-0.14C and Fe-10Mn-0.15C-1.5Al, have been manufactured through batch annealing (BA) and continuous annealing (CA) mass production lines in 2014.[16] All of the coils exhibited tensile strengths of ~1000 MPa and ductilities of ~40 pct, including the 7Mn steel selected for the current investigation. The properties of medium Mn steels are dependent on the alloying levels and intercritical annealing processing steps, which control the volume fraction and stability of retained austenite.[17,18] During intercritical annealing, C and Mn partition from ferrite into austenite, enhancing the stability of the austenite. Optimized stability results in austenite retention and a transformation-induced plasticity (TRIP) effect, providing superior mechanical properties. Therefore, many studies on the relationship between intercritical annealing and austenite volume fraction have been performed using a variety of compositions.[19–21] In principle, with increasing intercritical annealing temperature, the volume fraction of retained austenite gradually increases to a maximum and then drops sharply because a portion of austenite transforms to martensite during cooling, which can be predicted by the model derived by De Moor et al.[22] Austenite stability is known to be a critical aspect related to the TRIP effect and the understanding
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