Microstructural Characteristics and Tensile Behavior of a Hot-Rolled Medium-Mn Steel (0.25C-8.5Mn-0.5Si-2.5Al) Processed by Intercritical Annealing Treatment

Microstructural characteristics and tensile behavior of a novel hot-rolled medium-Mn steel, 0.25C-8.5Mn-0.5Si-2.5Al (wt.%), under different intercritical annealing temperatures (TIA) were evaluated. The results show that an ultrafine duplex lamellar microstructure with a maximum volume fraction of retained austenite (RA) of 65 vol.% was obtained at TIA of 750 °C. It was found that both the stability of the RA and its stability range play critical role in controlling the sustainable TRIP effect. The product of ultimate tensile strength and total elongation as high as ~ 66 GPa% was obtained at TIA of 725 °C, which is much higher than those of previously investigated medium-Mn steels with similar Mn contents, indicating a great potential for the tested steel. The analysis of the fracture surface revealed that the steel exhibited a kind of micro-delamination fracture, which was found to be primarily nucleated at the interfaces of the transformation-induced martensite and ferrite, and the micro-delamination fracture became more significant with increasing TIA.