Abstract: Effect of intercritical annealing time on microstructure and mechanical properties of Fe-0.16C-0.2Si-4.78Mn-0.24Mo medium manganese steel was studied by means of scanning electron microscope, electron backscatter diffraction, X-ray diffraction and transmission electron microscope, room temperature tensile test and impact test at 0 ℃. The results show that the microstructure of the hot-rolled steel is mainly composed of martensite, while the microstructure after annealing for different time is composed of martensite and reverse austenite. With the increase of annealing time, the content of reverse austenite increases linearly, reaching a peak value of 12.74% after annealing for 30 h. At this time, in addition to film-like austenite and strip austenite, large sized blocky austenite also appears in the microstructure. The yield strength and tensile strength of the tested steel gradually decrease with the increase of annealing time, which are mainly attributed to the increase in reverse austenite content and the decrease in dislocation density. With the increase of annealing time, the impact absorbed energy at 0 ℃ gradually increases, and the elongation first increases and then decreases, which is due to the TRIP effect of reversing austenite improving the impact property and plasticity, but the stability of austenite decreases after annealing for 30 h, leading to a decrease in elongation. After annealing at 600 ℃ for 30 h, the tested steel exhibits the best TRIP effect.

Key words: medium manganese steel, reverse austenite, intercritical annealing, microstructure evolution, mechanical properties