Effect of cold deformation extent and ART annealing duration on the microstructure and mechanical properties of a medium manganese steel

The effect of cold rolling reduction extent and austenite reverted transformation (ART) annealing duration on the microstructure evolution and the ensuing mechanical properties of a medium Mn steel was investigated. Cold rolling reduction was varied from 18.18% to 72.72% followed by ART annealing for different time duration (2-6 h) at a pre-determined temperature of 650 degrees C. The retained austenite fraction was found to increase with cold deformation extent as well as with the ART annealing duration. With increase in the extent of cold reduction the morphology of microstructure gradually shifted from a lamellar structure to globular, owing to recrystallization of the heavily deformed sample. Cold reduction level of similar to 36.36% was found to be the threshold limit for morphology transition from lamellar to globular. A two-fold increase in the dislocation density of the retained austenite phase was observed after 6 h of ART annealing of the 18.18% cold deformed specimen in comparison to the 72.72% cold deformed specimen. The higher dislocation density in the less deformed specimen was associated with the recrystallization and a higher austenite to martensite transformation during the quenching post ART annealing process. A greater yield point elongation in samples deformed to a higher extent was revealed, which was completely absent in samples deformed to a lesser extent. Variation in strength-ductility combination was correlated with the retained austenite content and its stability, both of which varied with cold deformation and the duration of ART annealing.