The Influence of Cooling Rate on Austenite Stability and Mechanical Properties in an Austenite–Ferrite Medium-Mn Steel

The impact of cooling rate on austenite stability and mechanical properties of a cold-rolled transformation-induced plasticity (TRIP) steel with nominal chemical composition of Fe-11Mn-4Al-0.2C (wt.%) was elucidated in the present study. Mechanical properties were obtained by tensile test and microstructure was analyzed by XRD, SEM, EMPA. Intercritical annealing in the range of 700~800 °C for 5min followed by either water-quenched or air-cooled, both led to a ferrite–austenite mixed microstructure. However, the austenite stability varied with the annealing temperature. The water-quenched and air-cooled samples annealed at 750 °C with medium austenite stability demonstrated significantly higher ductility than the samples annealed at 700 and 800 °C. The superior total elongation (TE) in 750 °C sample was not only contributed by TRIP effect of austenite, but also the cooperative deformation of ferrite. The air-cooled samples were characterized by higher austenite stability and ultimate tensile strength (UTS), but lower total elongation compared to the water-quenched samples, which is because the TRIP effect in the air-cooled samples without the cooperation of ferrite contributed primarily to work hardening and UTS, but less to TE. Moreover, the critical factor in controlling austenite stability of water-quenched sample was morphology, granular austenite showing the highest stability; while the dominant factor in austenite stability of the air-cooled sample was composition, enrichment of Mn and C improving austenite stability.