Effect of volume fraction and mechanical stability of austenite on ductility of medium Mn steel

A hot-rolled medium Mn (0.2C5Mn) steel is annealed at 650 °C to produce an ultrafine-grained duplex microstructure with different austenite volume fractions by austenite reverted transformation (ART) annealing, and the orientation relationship strictly obeys K–S orientation relationship before deformation. Tensile tests are carried out in a temperature range from − 196 to 400 °C to examine the effects of the austenite volume fraction and the deformation temperature on the tensile properties and the austenite stability. Microstructural observations reveal that the metastable austenite gradually transformed into α-martensite, which is controlled by the deformation strain, the temperature and the austenite volume fraction. Both strain hardening behavior and ductility of the studied steel are dependent on austenite volume fraction and deformation temperature significantly. The stress–strain curves of ART-annealed 0.2C5Mn steel assume an S shape and a very large work hardening rate of about 10 GPa is obtained at liquid nitrogen deformation temperature. Based on the experimental data, a quantitative relation is proposed to describe the ductility dependence on both the austenite volume fraction and its mechanical stability.