Hot rolled and cold rolled medium manganese steel: Mechanical properties and microstructure

The current work aims at obtaining optimum amount of austenite after processing of a medium manganese steel in order to invoke partial twinning induced plasticity (TWIP) mechanism during deformation and to maximise elongation. Steel chemistry was optimised using thermodynamic calculations and the same was processed in different ways which includes continuous cooling from single phase austenitic region, quenching + austenite reverted transformation (ART) annealing and austenite reverted transformation (ART) annealing. DICTRA and Thermo-Calc simulation was done to understand the austenite to ferrite transformation kinetics during continuous cooling and the mode of equilibrium that is operative. The study suggests that quenching + austenite reverted transformation (ART) annealing is the ideal approach for processing this material in order to get the desired microstructure and properties. Thermo-Calc and Koistinen and Marburger's (KM) equation was coupled to estimate the fraction of austenite to be retained after isothermal holding at different intercritical temperatures and the optimum temperature found was used for annealing this steel. Microstructural characterization along with tensile test was done to ascertain the effect of processing route, initial microstructure on the mechanical properties. It was found that the morphology of austenite was greatly affected by the initial microstructure of the steel. Mechanical properties obtained after cold rolling (CR) and austenite reverted transformation (ART) annealing was found to be superior to that of hot rolling (HR) and austenite reverted transformation (ART) annealing. The difference in mechanical properties was correlated with the austenite morphology and the final microstructure.