Abstract: A new heat treatment process was designed for low carbon medium manganese steel (0.2C-5Mn-0.5Si-1.5Al) to introduce preparatory microstructure. The effect of the preparatory microstructure on the microstructure and properties of the medium manganese steel was studied by means of SEM, EPMA, XRD and tensile test. The results show that by introducing pearlite phase into hot-rolled steel, a multiphase preparatory microstructure composed of ferrite, martensite and pearlite is successfully established. This achieves a differentiated enrichment of C and Mn elements within the preparatory microstructure. Further cold rolling and intercritical annealing treatments result in two types of retained austenite (blocky and lath) with differentiated morphology and size. These exhibit a synergistic transformation induced plasticity effect over a large strain range in the tensile test. The specimens with preparatory microstructure achieve optimal mechanical properties through intercritical annealing process at 700 ℃ for 5 min. The tensile strength exceeds 1000 MPa, the elongation after fracture reaches 48%, and the product of strength and elongation is close to 50 GPa·%. Compared with the annealed specimens without introduction of a preparatory microstructure, the heat treatment regime used in this study not only makes the distribution of retained austenite stability more reasonable, but also significantly increases the elongation considerably while maintaining the level of high strength, resulting in an excellent product of strength and elongation.

Key words: medium manganese steel, pearlite, retained austenite, microstructure evolution, mechanical properties