Microstructure and Cu-precipitation Engineering in Warm-rolled Medium-Mn Steel Via Short-time Intercritical Annealing and Aging

Warm-rolled medium-Mn steels (MMnS) are attractive for automotive industry and energy applications, due to their balanced mechanical and corrosion properties. The present study investigated how newly-designed process routes with short post-warm-rolling heat treatments can further tune the microstructure and mechanical properties of an Fe-7Mn-0.05 C-1.5Al-1.5Si-1.5Ni-1.5Cu (wt%) steel. Warm-rolled MMnS (WarmR) was subjected to three heat treatment routes: intercritical annealing (IA) at 700 °C for 1 min (WRA), aging at 500 °C for 5 min (WRAg), and a combined IA + aging process (WRAA). The material under WarmR condition is characterized by a heavily deformed ferrite matrix and refined, mechanically stabilized austenite, providing high yield strength (YS: 926.2 ± 17.8 MPa) but low ductility (TE of 6 ± 1.2%) due to a suppressed transformation induced plasticity (TRIP) effect. In the material under WRAg condition, high density of nano-sized Cu precipitates form within the ferrite, further increasing the yield strength (YS: 1179 ± 17.6 MPa) but not ductility (TE 5.7 ± 1.5%). The WRA process promotes the reactivation of the TRIP effect and enhances ductility (TE of 14.5 ± 2.6%), albeit at the expense of strength (YS:743.7 ± 25.1 MPa). In contrast, the combined route (WRAA) allows to form nano-sized Cu precipitates with a moderate volume fraction while maintaining an active TRIP effect, thereby resulting in superior strength-ductility balance (YS: 938 ± 11.2 MPa and TE of 16.7 ± 0.9%). The present study provides a mechanistic understanding, connecting processing condition, micro/nanostructure evolution and mechanical properties, thereby, offering an effective pathway for tuning strength-ductility synergy in warm-rolled medium-Mn steels.