Microstructure evolution and yield strength improvement of a low carbon medium manganese steel experienced intercritical annealing, pre-straining and tempering

The microstructure evolution and mechanical properties of a 0.15C1.2Si10Mn0.5 V medium manganese steel after intercritical annealing, pre-straining and tempering treatment are systematically analyzed. An intercritical annealing yields a microstructure containing ultrafine gamma + alpha coexisting structure while final tempering reduces martensitic supersaturation, which is beneficial to the ductility of the steel. The applied pre-strains between the two processes cause a part of unstable banded austenite in hot rolled state prematurely to transform into lath martensite that divides the coarse banded austenite into smaller ones. When the pre-strain is 0.02, there are about 21% strain-induced martensite produced in the microstructure and the steel exhibits the best comprehensive mechanical properties with the yield strength of 1264 MPa, tensile strength of 1506 MPa and total elongation of 19%. Actually, the increase in yield strength under pre-strains should be ascribed to dislocation strengthening in the strain-induced martensite and the precipitation strengthening of vanadium carbides during the deformation and subsequent tempering.