Microstructure and low-temperature toughness of intercritically annealed Fe–9Mn-0.2C Medium-Mn steels containing Al, Cu, and Ni

The correlation between the microstructure and low-temperature toughness of intercritically annealed medium-Mn steels with different Al, Cu, and Ni contents is discussed in terms of the fracture mode and brittle crack propagation. Al-free medium-Mn steel showed coarse austenite grains with an equiaxed shape, while Al-added medium-Mn steels exhibited lath-shaped austenite grains with a nano-laminated structure at the boundaries. The Al-free medium-Mn steel showed the lowest absorbed energy at all test temperatures owing to intergranular fractures caused by the segregation of C and Mn along the equiaxed austenite grain boundaries. The addition of Al to medium-Mn steel improved the impact toughness by effectively suppressing intergranular cracking by forming a nano-laminated structure. Thus, it is suggested that the formation of a nano-laminated structure has a beneficial effect on low-temperature toughness because it alleviates the initiation of brittle cracks by eliminating Mn segregation at the grain boundaries. In particular, the combined addition of Al and Ni increased the impact toughness slightly, whereas adding Al and Cu decreased the impact toughness due to the segregation of Cu at the grain boundaries.