Novel medium-Mn (austenite plus martensite) duplex hot-rolled steel achieving 1.6 GPa strength with 20 % ductility by Mn-segregation-induced TRIP mechanism

Ultra-high-strength (over 1 GPa) hot-rolled steel sheets have been actively developed to protect passengers in cases of vehicle crashes, and their applications have been expanded to cold-rolled steel sheets. A major alloying element for forming meta-stable austenite is Mn in (austenite + martensite) duplex microstructures, which is readily obtained at medium-Mn level ((3-10) wt.%). However, these medium-Mn hot-rolled duplex microstructures inevitably include Mn-segregated bands, which often lead to anisotropic mechanical properties and deteriorate the strength or uniform elongation. However, in this study, we show favorable effects of the Mn-segregated band, by carefully controlling the composition, size, and shape of austenite in Mn-rich and Mn-lean bands in medium-Mn duplex steels (composition; Fe-0.1C-10Mn-1Si-0.3Mo-0.5 V (wt.%)). The austenite grown coarsely in the Mn-rich band provoked transformation-induced plasticity (TRIP) more efficiently than the austenite finely transformed from the martensite in the Mn-lean band. The Mn composition acted more dominantly on the austenite stability than the austenite size, resulting in continuous TRIP in the austenite of the Mn-rich band. This austenite enables continuous strain hardening, thereby leading to high yield and tensile strengths of (1.0-1.6) GPa together with large ductility of 20%, which offers promise for new applications to ultra-high-strength automotive hot-rolled steel sheets. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.