Synergistic Deformation Mechanisms with Austenite, Ferrite and κ-Carbide During Flow Behavior in a Ferrite-Based Lightweight Steel

The high-temperature flow characteristics and dynamic microstructural evolution of a Fe−3.3Mn–9.6Al–0.3C ferritic-based duplex lightweight steel containing δ-ferrite, austenite and κ-carbide were systematically examined at strain rates and temperatures in the range of 0.01-10 s⁻¹ and 800-1100 °C, respectively. Correlation of true stress-strain curves, constitutive equations with a processing map in the present steel was investigated in-depth. The results show that the most favorable hot working domains were achieved within the conditions of 950-1100 °C/0.01-1 s⁻¹. These optimum domains were associated with distinct underlying mechanisms resulting from different microstructural responses. Among them, the lamellar structures comprising intra-granular κ-carbides and austenite exhibited an evident strain gradient with the adjacent δ-ferrite. This gradient resulted in less lattice restriction, thereby promoting strain hardening instead of dynamic recrystallization. At a strain rate of 1 s⁻¹, the formed inter-granular κ-carbides were increased, which facilitated dynamic recrystallization and acted as an obstruction at grain boundaries to hinder the dynamic recrystallization grain enlargement. At higher strain rates (> 1 s⁻¹) and lower temperature (< 950 °C) unstable domain, the inhomogeneity in microstructure size and crystallographic orientation served as an indication of deformation instability. Throughout the plastic deformation, the deformation mechanisms were determined by κ-carbide hardening, continuous dynamic recrystallization marked by the gradual misorientation development of subgrains towards high-angle boundaries, and discontinuous dynamic recrystallization observed at the interfaces between δ-ferrite and the lamellar structures.