Abstract: The initial structure of lath martensite of 0.22C-3.97Mn-1.57Si-0.1Al-0.043Nb medium manganese steel was obtained by pre-quenching, and then reverse phase transformation annealing was performed at different temperatures. The microstructure evolution and mechanical properties of the steel were studied by means of scanning electron microscopy (SEM), X-ray diffractometer (XRD) and tensile test machine. The results show that the microstructure of the experimental steel after reverse phase transformation annealing is ferrite, lath martensite and retained austenite. With the increase of annealing temperature, the tensile strength of the experimental steel gradually increases, the yield strength gradually decreases, the elongation and product of strength and elongation first increases and then decreases. When annealed at 670-690 ℃, the coordinated deformation of the multi-phase structure and the TRIP effect of retained austenite are obvious, and the experimental steel shows excellent mechanical properties. When annealed at 690 ℃, the best mechanical properties can be obtained with the tensile strength of 1062 MPa, the elongation after fracture of 27.0%, and the product of strength and elongation of 28.7 GPa·%. When the annealing temperature is higher than 690 ℃, the hard phase martensite content increases, the mechanical stability of retained austenite is poor, and the tensile strength of the experimental steel increases but the plasticity decreases. The work hardening ability of ferrite increases and the mechanical stability of retained austenite decreases, which inbibite the occurrence of Lüders and sawtooth rheology.

Key words: reverse phase transformation annealing, retained austenite, medium manganese steel, mechanical properties