Novel ultra-high-strength Cu-containing medium-Mn duplex lightweight steels

High strength and ductility and vehicle's weight reduction are key issues for improving fuel efficiency in automotive industries. In addition, structural reinforcement components require high yield strength because the prevention or minimization of deformation is more important than the absorption of impact energy. In this study, new ultra-high-strength duplex lightweight Fe-0.5C-12Mn-7Al-(0,3)Cu (wt%) steels have been developed by varying annealing temperature. Here, Cu, an austenite stabilizer, not only raises the austenite volume fraction but also delays the recrystallization due to a solute drag effect, while it promotes the formation of Cu-rich B2 particles and Cu-segregated interfacial layers. The steels show the planar slip and fine dislocation substructures (Taylor lattices) as desirable deformation mechanisms. Non-shearable Cu-rich B2 particles, solid solution hardening of Cu, and delayed recrystallization greatly improve the yield strength (similar to 1 GPa) and strain hardening. Through these unique and excellent tensile properties together with weight saving of 10.4%, the present work provides a desirable possibility for applications to automotive reinforcement components preferentially requiring an excellent yield-to-tensile ratio. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.