Continuous annealing of medium-Mn steels: Sensitivity of mechanical performance to minor changes in time-temperature heat treatment parameters

Studies on the intercritical annealing of medium-Mn steel sheets mostly focus on batch annealing conditions, a multi-hour process with low economic and technological efficiency. The literature on continuous annealing of these steels, which is crucial for their large-scale industrial production, is limited and primarily focuses on specific aspects of the process like annealing temperature or time. The following study comprehensively addresses all of the most important parameters of continuous annealing of Al-added medium-Mn steels. A broad range of soaking temperatures (700–760 °C) and times (2–120 s) was analyzed for heating (3 °C/s) and cooling (1 °C/s) conditions that are favorable for retaining austenite stability and feasible for industrial production. The effects of these conditions on the chemical and mechanical stability of retained austenite were examined. The resulting microstructures were correlated with the mechanical properties of the heat-treated steel. This allowed to determine the technological window for continuous annealing of medium-Mn steels. Moreover, the fractions and stability of retained austenite in a range from 18 to 34 %, for all applied thermal cycles, were linked to changes in the steel's mechanical properties. This confirmed that a high fraction of retained austenite alone is insufficient for achieving high plasticity. The key factor is the optimal mechanical stability of the austenite phase, as evidenced by the significantly better plasticity observed in a sample with 32 % RA compared to one containing 41 % RA. Furthermore, the relationship between global and local plasticity changes significantly as the stability of retained austenite decreases.