Characterization of Microstructures and Mechanical Properties of Cold-rolled Medium-Mn Steels with Different Annealing Processes

The cold rolled medium-Mn steel was processed by two different heat treatments (intercritical annealing and quenching+intercrictical annealing). The microstructure evolution was characterized by scanning electron microscopy, electron back scattered diffraction and transmission electron microscope. It was found that austenite reverted transformation took place during annealing processes. Blocky ultrafine ferrite/austenite duplex structure was formed after intercritical annealing, while lath ultrafine duplex structure was developed after quenching+intercritical annealing. Such ultrafine duplex structure has both high strength and high plasticity. The excellent properties and tensile behavior were related to the phase transformation from retained austenite to martensite, which was associated with the carbon content, the morphology of the retained austenite and the matrix microstructure in the steel with different processes.