Abstract: Effect of intercritical annealing time on the microstructure and mechanical properties of 0.21C-4.1Mn-1.85Si-0.05Nb-Fe cold rolled medium manganese steel was investigated by means of tensile testing machine, scanning electron microscope, and X-ray diffraction. The results show that with the increase of annealing time, the ferrite ratio decreases, the volume fraction of retained austenite first increases and then decreases, and the martensite size gradually increases. The yield strength of the steel first increases and then gradually decreases, while the tensile strength first decreases and then increases, the elongation as well as the strength-elongation product first increase and then gradually decrease. After annealing for 10 min, the engineering stress-engineering strain curve exhibits continuous yielding, but the insufficient work hardening ability leads to the worst plasticity. With annealing time increasing, the engineering stress-engineering strain curve exhibits a yield plateau, but the continuous TRIP effect occurring over larger strain range allows the tested steel to maintain lasting work hardening, contributing to a higher plasticity. After annealing at 690 ℃ for 60 min, the comprehensive mechanical properties of the tested steel are the best, with the tensile strength of 1036.9 MPa, elongation of 25.6%, and the product of strength and elongation of 26.5 GPa·%.

Key words: medium manganese steel, intercritical annealing time, retained austenite, mechanical properties