Effect of a two-step annealing process on deformation-induced transformation mechanisms in cold-rolled medium manganese steel

In this work, the relationship between the mechanical properties and deformation-induced transformation mechanisms in transformation-induced plasticity (TRIP) medium manganese steel subjected to a two-step annealing process is comprehensively examined. Two-step annealed samples have bimodal microstructures, which are reversely transformed from partially recrystallized thermal martensite into lath- and globular-shaped grains of austenite (γ)–ferrite (α). This process widens the distribution of austenite stability, increasing the product of strength and elongation (PSE) from 35.5 GPa% to 46.9 GPa% at 600 °C annealing. When the annealing temperature is further increased to 640 °C, the difference of partitioning Mn content in austenite transforms the strain-induced TRIP effect into the stress-assisted TRIP effect. The yield strength and PSE are significantly reduced to 596 MPa and 30.5 GPa%, respectively. In addition, different degrees of stability in austenite occur in different martensite transformations (i.e., γ → ε martensite, γ → ε martensite → α′ martensite) at the same deformation stage, and the deformation mechanism of γ is generally based on the cooperation of the TRIP effect and twinning. For the coarse metastable austenite grains annealed at 640 °C, the α’ martensite transformation is assisted by the applied stress. An intergranular fracture occurs because of a high stress concentration mismatch between the deformed martensite and ferrite, leading to premature fracture.