Phase transformation and austenite stability during thermomechanical processing of high (∼5%) Al added low-density medium Mn steel

In this study, a high Al added low-density medium Mn steel has been developed by conventional melting casting route in an open-air induction furnace, followed by hot forging and hot rolling in the temperature range of 1050-800°C. Finally, it has been intercritically annealed at 750°C for 5, 30, 60, 120, and 180 minutes. The microstructural analysis shows the presence of dual-phase microstructure of delta ferrite and austenite in cast as well as hot forged specimens. While some of the austenite has been transformed to martensite (lenticular shape) in rolled specimen due to a higher cooling rate (water quenching) immediately after hot rolling. After 5 minutes of annealing, needle shape of reverted austenite and intercritical ferrite forms from martensite. As the annealing time increases to 30 minutes, the reverted austenite coalesces and undergoes further transformation into ferrite. This results in the formation of reverted austenite with needle and globular morphologies at 60 minutes of annealing. This annealing condition reveals the optimum mechanical stability due to its morphology and chemical composition, resulting in enhanced TRIP effect as compared to other annealing conditions. Further increase in annealing time to 120 and 180 minutes, volume fraction of reverted austenite decreases significantly due to more dissolution of reverted austenite to intercritical ferrite, leading to reduced TRIP effect. Specimen annealed for 60 minutes, possessing optimal mechanical stability of austenite, exhibits tensile properties with an ultimate tensile stress of 658.45±6 MPa and total elongation of 12±0.95 %, attributed to enhanced TRIP effect.