The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel

The effects of prior austenite (γ) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (γ_R) and ferrite (α) after intercritical annealing at 640 °C for 30 min. The phase fractions and the chemical composition of γ_R were almost identical in both types of specimens. However, their microstructural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to −196 °C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior γ grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine α and α′ martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior γ grains through the recrystallization of αʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel.