Abstract: To address the high costs of achieving lightweight automobiles while simultaneously obtaining high strength and high plasticity, a low-density medium manganese steel with the composition 0.36C-4.5Al-7.6Mn-0.31V-0.31Si-0.07Ti was developed. The effect of various reverse transformation annealing temperature on the microstructure, retained austenite (content, stability and dislocation density) and mechanical properties of the low-density medium manganese steel was explored by means of OM, SEM, EBSD, XRD and tensile testing machine. The results indicate that the reverse transformation annealed experimental steel comprises a mixed structure of elongated δ ferrite, lath martensite and blocky retained austenite along the rolling direction. As the reverse transformation annealing temperature increases from 650 ℃ to 780 ℃, the martensite content decreases, the retained austenite content rises from 50.14% to 58.97%, the average austenite grain size increases first and then decreases, while the austenite KAM value (proportional to dislocation density) shows the opposite trend. Optimal mechanical properties of the tested steel are achieved when reverse transformation annealed at 780 ℃ for 1 h, with yield strength of 739.1 MPa, tensile strength of 884.2 MPa, elongation of 37.80% and the product of strength and elongation of 33.42 GPa·%.

Key words: low-density medium manganese steel, TRIP effect, reverse transformation annealing, retained austenite