Stability of retained austenite and work hardening behavior in ultra-fine medium carbon bainitic steel

The effect of retained austenite stability on plastic deformation was investigated by stepwise tensile test of medium carbon steel material at two different bainite isothermal temperatures below or above the martensite start (Ms) point. The results show that higher retained austenite transformation leads to higher elongation. Compared with the above-Ms bainitic steel (320 °C-12 h), the below-Ms bainitic steel of retained austenite (240 °C-48 h) has higher mechanical stability. This is because the transformation induced plasticity effect (i.e., the TRIP effect) of retained austenite plays a significant role in the process of tensile strain. The block retained austenite distributed between the bainite sheaf deforms rapidly in the initial stage, and the film retained austenite between the bainitic-ferrite lath transformation is slow in the later stage of tensile deformation. This is attributed to the low stability of block retained austenite, which easily transforms into hard and brittle martensite under external stress, while the film retained austenite has high stability and cannot easily undergo phase transformation under the action of external force. Meanwhile, the work hardening ability of the below-Ms bainitic steel is higher than that of the above-Ms bainitic steel at the initial stage of tension, but the opposite occurs in the later stage of tension.