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 (gamma)-ferrite (alpha). 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 degrees C annealing. When the annealing temperature is further increased to 640 degrees 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., gamma ->epsilon martensite, gamma ->epsilon martensite -> alpha' martensite) at the same deformation stage, and the deformation mechanism of gamma is generally based on the cooperation of the TRIP effect and twinning. For the coarse metastable austenite grains annealed at 640 degrees C, the alpha' martensite transformation is assisted by the applied stress. An intergranular fracture occurs because of a high stress con-centration mismatch between the deformed martensite and ferrite, leading to premature fracture.