Abstract: The phase transformation, microstructure and mechanical properties of the 0.2C-5Mn-0.5Si-2.5Al medium manganese TRIP steel under different annealing processes were studied by means of CALPHAD (calculation of phase diagram) method, SEM (scanning electron microscopy), XRD (X-ray diffractometry) and tensile test. The influence of Al on the law of phase transformation, process, microstructure and properties were studied. The research results show that after the addition of 2.5 %(mass fraction)Al, the two-phase temperature region is significantly expanded, and the A₃ temperature is obviously increased, which is beneficial to increase the intercritical annealing temperature, so the austenite reverse transformation process is accelerated, the retained austenite content after intercritical annealing for short time(1, 3 min) is effectively increased. Due to the addition of 2.5 %Al, δ ferrite appears in the microstructure. In range of 760-880 ℃, with the increase of intercritical annealing temperature, the elongation shows a trend of increasing first and then decreasing, while the yield strength decreases slightly, the tensile strength continues to increase. For 1 min annealing, with the increase of intercritical annealing temperature, the elongation and the product of strength and ductility shows a trend of increasing first and then decreasing. While for 3 min annealing, with the increase of intercritical annealing temperature, the elongation and the product of strength and ductility all show a trend of continue to decrease. The optimum mechanical properties are obtained by annealing at 760 ℃ for 3 min, where the tensile strength is 927.69 MPa, the elongation is 50.12%, and the product of strength and ductility is 46 503.00 MPa·%.

Key words: medium manganese TRIP steel, annealing process, phase transformation, microstructure, mechanical properties