Austenite transformation and deformation behavior of a cold-rolled medium-Mn steel under different annealing temperatures

In this study, the austenite transformation in a cold-rolled medium-Mn steel (MMnS; 7 wt% Mn) was adjusted by inter-critical annealing (IA) at 680 °C (above the Ac1 temperature) and by partition annealing (PA) at 650 °C (below the Ac1 temperature). The deformation behavior associated with the microstructural evolution, and crystallographic changes were investigated using in situ synchrotron X-ray diffraction during tensile deformation. Electron backscatter diffraction was used to characterize the microstructure. A considerable amount of austenite (approximately 30 and 20 vol%) was promoted by reversion transformation during the IA and PA treatments, respectively. The difference in deformation behaviors between the IA and PA specimens was attributed to the different mechanical stabilities of the reverted austenite. The relatively low mechanical stability of retained austenite (RA), due to less Mn enrichment during IA, led to a pronounced activation of the transformation-induced plasticity effect, which improved the strain hardening capacity, the ultimate tensile strength, and total elongation. However, the low recovered/recrystallized fraction of the ferrite phase resulting from PA contributed to a significant increase in the yield strength. The current understanding of the characteristics and mechanical stability of RA induced by annealing at different temperatures below and above the Ac1 temperature will help in further optimizing annealing parameters to achieve better mechanical properties for MMnS.