Effect of Thermomechanical Processing on the Microstructure and Mechanical Properties of a Medium Mn Steel Sheet with Nb Addition

Medium Mn steels have gained significant attention for their potential to achieve exceptional combinations of strength and ductility through controlled microstructural engineering, and intercritical annealing is recognized as a key processing route to optimize austenite formation and mechanical properties in these alloys. However, the precise relationship between annealing parameters and resulting microstructural evolution in medium Mn steels remains inadequately understood, particularly regarding the optimal conditions for achieving superior mechanical performance. Therefore, this study systematically investigated the microstructural development of 7 wt.% Mn steel during controlled hot-rolling and intercritical annealing to establish processing-structure-property relationships. Hot-rolling produced predominantly lath martensite microstructure with minimal austenite presence. Intercritical annealing was conducted at 700°C and 780°C for 1, 2, and 5 h, resulting in ferrite/martensite and austenite microstructures with austenite content ranging from 10 to 25 wt.%. NbC precipitates were identified after electrochemical extraction. On the other hand, samples annealed at 700°C demonstrated optimal mechanical properties in terms of ultimate tensile strength and elongation, while both hot-rolled samples and those treated at 780°C exhibited brittle behavior with low elongation. These findings establish that intermediate annealing temperatures provide the most favorable microstructural balance for achieving superior mechanical performance in medium Mn steels.