Effect and mechanism of Al content on the microstructure and low-temperature impact toughness of warm-rolled and tempered medium-Mn steels

The insufficient low temperature toughness of medium-Mn steels remains a critical bottleneck restricting their application in low temperature environments. This study systematically investigates the effect of aluminum (Al) content (1.25 vs. 2.50 wt.%) on the microstructural evolution and low temperature toughness of warm-rolled and tempered Fe-5Mn-0.3C medium-Mn steels. The results demonstrate that increasing the Al content from 1.25 wt.% to 2.50 wt.% promotes precipitate coarsening and local aggregation, weakens effective boundary subdivision, and accelerates microstructural recovery. Crystallographically, the higher Al content strengthens the cleavage-sensitive //RD texture orientation while reducing the fraction of the slip-favorable //RD orientation. Moreover, the //ND component conducive to delamination toughening is significantly degraded. Consequently, the 1.25Al steel exhibits superior low temperature toughness, with an upper shelf energy (USE) of 152 J and a ductile-to-brittle transition temperature (DBTT) of −89 °C. In contrast, the 2.50Al steel undergoes severe embrittlement, showing a lower USE of 108 J and a significantly higher DBTT of −38 °C. The deterioration in toughness is attributed to the synergistic effect of microstructural coarsening, unfavorable texture evolution, and the consequent suppression of stable crack propagation energy. The findings clarify the mechanism by which Al content affects low-temperature toughness and provide guidance for designing high-toughness medium-Mn steels for temperature service.