Effects of Hot Band Annealing on the Mechanical Properties of Cold-Rolled and Intercritically Annealed Medium Manganese
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dium manganese steels are candidate materials for 3rd generation advanced high-strength steels (3GAHSS) containing 5 to 12 wt pct Mn, together with low C and other optional alloying elements.[1–10] These steels have recently received significant research interest particularly because of their attractive mechanical properties while being leaner in alloy content in comparison to exceptionally ductile steels such as twinning-induced plasticity (TWIP) steels or austenitic stainless steels. Medium Mn steels can achieve ultimate tensile strengths (UTS) in excess of 800 MPa and high total elongations (TE) of more than 30 pct. Processing of medium Mn steels inevitably includes a final intercritical annealing step to facilitate Mn partitioning into austenite and to generate an ultrafine (< 2 lm) microstructure with high amounts of retained austenite (up to 60 vol pct).
RADHAKANTA RANA is with Tata Steel, Wenckebachstraat 1, 1970 CA IJmuiden, The Netherlands. EMMANUEL DE MOOR, JOHN G. SPEER, and DAVID K. MATLOCK are with the Advanced Steel Processing & Products Research Center (ASPPRC), Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] Manuscript submitted March 15, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Intercritical annealing of medium Mn steels is typically applied to cold-rolled steels. Before cold rolling, the steel is hot-rolled to an intermediate gauge. Due to the relatively high Mn content leading to low-temperature transformation products after coil cooling, a high cold rolling force is expected, limiting the cold rollability of medium Mn steels. Therefore, a hot band annealing (HBA) step prior to cold rolling is often employed to soften the hard as-hot-rolled (HR) microstructure.[10] HBA also serves to partition Mn into austenite since it is typically completed at temperatures in the intercritical range. The majority of the processing-related literature on medium Mn steels concentrates on the effects of cold rolling and intercritical annealing,[1–5,7,8] while the effects of pre-processing, i.e., HBA, on the final mechanical properties have not been reported systematically. Therefore, here some early results on the effects of HBA on the quasi-static tensile properties of subsequently cold-rolled and intercritically annealed medium Mn steels are reported. The results show that the thermal history during HBA is important in defining the final mechanical behavior, and without a proper HBA treatment, it may not be possible to achieve the high strength–ductility combinations characteristic of 3GAHSS medium Mn steels. A medium Mn steel containing 7.39Mn-0.14C1.55Al-0.2Si (wt pct) (denoted 7MnAl steel) was vacuum induction melted, laboratory cast, hot rolled to 4-mm-thick sheet, and air cooled to room temperature. Subsequently, sheet samples were subjected to batch-type hot band annealing at 650 C for 1, 2, and 4 days (24, 48, and 96 hours), air cooled to room temperature, and cold rolled to 1.5 mm thickness giving 62.5 pct thickness reduction. For final processing, the cold-rolled sheets wer
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