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otive steels are typically produced as a hotband product with a gage thickness between 1.8 and 3.3 mm.[1] Final properties are produced by cold rolling and subsequent annealing by either galvanizing the cold rolled sheet or batch annealing of the cold-worked coil. Recent work[2–4] on manganese steels has shown great promise for the automotive market, but the reported properties are often in a condition more typical of the hot-band product. A modified transformation-induced plasticity (TRIP) behavior is observed in these manganese steels where the retained c-austenite first transforms to e-martensite, which segments the austenite into smaller volumes. Subsequent transformation of e-martensite to a-martensite refines the grain structure further to produce both high tensile strength (>1200 MPa) and elongations to failure in excess of 25 pct. These tensile properties meet current goals that are considered breakthrough for the automotive industry: steel with 1000 MPa ultimate tensile strength and 30 pct total elongation or 1500 MPa ultimate tensile

DANIEL M. FIELD, Ph.D. Candidate, and DAVID C. VAN AKEN, Curators’ Professor, are with the Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409. Contact e-mail: [email protected] Manuscript submitted October 1, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

strength with 20 pct total elongation. However, these ‘‘hot band’’ properties also include very low yield strengths, on the order of 250 to 300 MPa. The work presented here shows that these manganese steels can be cold worked and annealed to increase the yield strength and still meet the target goals for the automotive market. Manganese steels studied in previous works,[2–4] were alloyed with greater than 14 wt pct Mn and varied in aluminum and silicon contents (FeMnAlSi-C). In this examination, an alloy with relatively low solute (Mn, Al, Si, C) content, 14.5 pct total, compared to the previously reported 18.5 pct solute content[2–4] was cast and processed to produce the two-stage or dual-TRIP behavior. The aim of the study was to reduce the manganese, and apply a traditional cold work and annealing process to the steel. In this regard, nucleation of e-martensite was thought to be of primary importance in formulating the new alloy. As shown by the work of Olson and Cohen[5,6] the e-martensite nucleation is facilitated by a low intrinsic stacking fault energy (ISFE), which encourages the nucleation of partial dislocations. Alloy formulation was conducted by balancing the chemical driving force for both the c-austenite (FCC) to e-martensite (HCP) transformation and c-austenite (FCC) to a-martensite (BCC) transformation while simultaneously reducing the intrinsic stacking fault energy. A thermodynamic model for the transformation of c-austenite (FCC) to e-martensite (HCP) reported by Pisarik and Van Aken[7] was used for this study. A steel composition by weight percentage of Fe-0.11C-2.46Si-11.5Mn-0.38Al-0.029N was melted using induction iron, ferrosilicon, electrolytic ma