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Duplex stainless steels (DSSs) consisting of ferrite and austenite show better mechanical properties and corrosion resistance than single-phase (ferritic, austenitic, and martensitic) counterparts,[1] and therefore, their structural applications have prevailed in recent years. In addition to many efforts improving mechanical properties like other structural materials, intensive research on less or lean alloying in DSSs have been carried out because (duplex and single-phase) stainless steels are basically heavy alloyed with Cr, Ni, Mn, Mo, etc. As a result, near Ni-Mo free DSSs in which Ni-Mo are replaced with Mn-N were recently developed.[2–6] Unlike conventional DSSs with stable austenite, a lack of Ni, a strong austenite stabilizer makes austenite of near Ni-Mo free DSSs metastable. This results in transformation-induced plasticity (TRIP). So, near Ni-Mo free DSSs exhibit impressive strength–ductility combinations over 1 GPa 60 pct. Deformation of austenite strongly depends on the stacking fault energy (SFE): TRIP at low SFE below 20 mJ/m2, twinning-induced plasticity (TWIP) at intermediate SFE of 20 to 50 mJ/m2, and dislocation gliding (or microband-induced plasticity) at

JEOM YONG CHOI, Researcher, is with the STS Product Group, POSCO Technical Research Laboratory, Pohang 790-785, Korea. SI WOO HWANG, Researcher, and KYUNG-TAE PARK, Professor, are with the Department of Material Science and Engineering, Hanbat National University, Daejeon 305-719, Korea. Contact e-mail: [email protected] Manuscript submitted July 25, 2012. Article published online December 20, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

high SFE over 50 mJ/m2.[7–9] TWIP enhances strain hardenability and so it increases both strength and ductility of austenite as similar to TRIP. Basically, TWIP is more efficient on enhancing ductility than TRIP because TWIP does not involve hard martensitic transformation. The SFE depends on chemical composition at a fixed temperature. Accordingly, high ductile DSSs can be developed by controlling austenite chemistry for the purpose of inducing TWIP: Despite coexistence of a considerable amount of ferrite (~50 pct), deformation of DSSs is reported to be dominated by austenite deformation.[5,6] In the current investigation, the deformation characteristics and microstructures of a TWIP-aided high-ductile DSS developed by the above concept are described. A DSS with a nominal composition of Fe-20Cr-3Mn2Cu-2Si-2Ni-0.2N was prepared by induction melting at N2 atmosphere and hot rolling. The current composition was made by adding Cu, Si, and Ni, which are the substitutional elements increasing the SFE of austenite,[10] into the base composition of the previous TRIP aided near Ni-Mo free DSSs.[5,6] under considerations of (1) Ni content as low as possible for raw material cost savings, (2) higher SFE than TRIP aided near Ni-Mo free DSSs enough to show TWIP, and (3) austenite fraction less than 0.6 to ensure the duplex advantages. The hot-rolled plates were annealed (1373 K [1100 C] for 30 minutes) and then cold roll