A medium-Mn steel(0.2C5Mn)was processed by intercritical annealing at different temperatures(625 ℃ and 650 ℃).An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite laths was developed by austenite reverse transformation(ART)during intercritical annealing after forging and hot rolling.Ultrahigh ductility with a total elongation higher than 30% was achieved in the temperature range from-196 ℃ to 200 ℃,and high impact toughness no less than 200Jat-40 ℃ was obtained.Based on the analysis of microstructure and mechanical properties,it was found that the enhanced ductility was determined by the phase transformation effect of austenite(TRIP effect),while the delayed ductile to brittle transition was controlled by austenite stability. A medium-Mn steel (0.2C5Mn) was processed by intercritical annealing at different temperatures (625 ° C and 650 ° C). An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite laths was developed by austenite reverse transformation (ART) during intercritical annealing after forging and hot rolling. Ultrahigh ductility with a total elongation higher than 30% was achieved in the temperature range from -196 ° C to 200 ° C, and high impact toughness no less than 200 Jat-40 ° C was obtained. of microstructure and mechanical properties, it was found that the enhanced ductility was determined by the phase transformation effect of austenite (TRIP effect), while the delayed ductile to brittle transition was controlled by austenite stability.