Transformation and twinning induced plasticity in an advanced high Mn austenitic steel processed by martensite reversion treatment

The concept of strain-induced martensite and its reverse transformation (SIMRT) was employed to obtain fine grained (FG)/ ultra fine grained (UFG) structures in a Fe-Mn-Al-Si steel. The experimental steel was designed to show TRIP (transformation induced plasticity) and TWIP (twinning induced plasticity) effects as the characteristic deformation mechanisms. To this end, the cold rolled material (by 80% reduction) was annealed at predetermined temperatures in the range of 500-920 °C for different duration of 10-3600 s. The results indicated that UFG austenite structures were formed within a shorter soaking time at higher annealing temperatures (>680 °C); this was mainly attributed to activation of shear reversion mechanism. The reversion annealing at 920 °C for 20 s ended to a microstructure with more than 90% austenite holding the finest mean austenite grain size of 0.8 µm. One of the essential aims of the present study was to explore the effect of austenite grain refinement on TRIP/TWP mechanisms and mechanical properties of the experimental steel. The tensile test results revealed that despite the lower extent of TRIP and TWIP mechanisms by decreasing the grain size, the superior ductility preserved in grain refined structures. This was explained by shifting the γ→ά transformation to the higher strain level where the potential to strain hardening was more desirable. The proper meta-stability of austenite enabled the UFG structure to achieve the desired combination of yield strength (530 MPa) and ductility (70%).