Effect of intercritical annealing on microstructure and toughness of medium-Mn steel with elongated prior-austenite grains formed via two-step hot rolling process

Fe-9 mass%Ni alloy is widely used as a cryogenic steel owing to its excellent low-temperature strength and toughness. However, Ni is an expensive element, with medium-Mn steel considered an inexpensive alternative. Considering the Fe-10%Mn-0.1%C alloy is brittle at low temperatures, the application of intercritical annealing with two-step hot rolling could lead to toughening. Herein, the effect of intercritical annealing on the toughness of a Fe-10%Mn-0.1%C alloy with elongated prior-austenite grains (PAGs) formed via a two-step hot-rolling process was investigated. Intercritical annealing was performed on the specimens with and without two-step hot rolling. For both specimens, intercritical annealing resulted in softening of α'-martensite and an increase in the amount of retained austenite. In the specimen not subjected to the two-step hot rolling process, the fracture morphology transitioned from ductile to intergranular with a decrease in the temperature. Intercritical annealing improved the toughness when ductile fracture occurred. In the case of intergranular fracture, the effect of intercritical annealing on the toughness was negligible. In the two-step hot-rolled specimen with elongated PAGs, the fracture morphology transitioned from ductile to separation fracture with ductile fracture, and intercritical annealing improved the toughness at all temperature ranges. The improvement in toughness during separation fracture is attributed to the expansion of the plastic zone owing to ductile crack progression and the formation of sub-cracks, which promote the strain-induced transformation of retained austenite and ε-martensite.