A Feasible Thermomechanical Process of a Duplex-Phase Fe–Mn–Al–C Steel for Significantly Increasing Ductility Without Loss of Strength

The controlled thermomechanical treatment was conducted for the mechanical property improvement of Fe–Mn–Al–C duplex low-density steels. The tensile ductility experienced a significant increase from 37 to 66 pct by decreasing the reduction of cold rolling from 30 to 20 pct during the thermomechanical process, and the ultimate tensile strength was also slightly improved. The enhanced strength and markedly improved ductility can be primarily attributed to a much higher work hardening ability resulting from a more efficient transformation-induced plasticity (TRIP) effect. The higher work hardening ability is mainly manifested by the occurrence of a remarkable recovery of work hardening rate, which can even cause an anomalous secondary work hardening occurring in the tensile deformation of the thermomechanically treated duplex steel. Decreasing cold rolling reduction in the thermomechanical process can introduce less dislocations, and then larger austenite grain size is obtained after recovery and recrystallization during annealing process, so that the austenite with an appropriate stability can ensure the TRIP effect to work continuously up to much higher strain levels. As a result, a strength-ductility combination of 58 GPa pct was achieved in this investigated duplex steel.