Effect of Intercritical Annealing Temperature on Austenite Formation in Medium-Mn Steels: A Thermodynamic and Experimental Study

The effects of intercritical annealing (IA) temperature and manganese content on austenite formation and stability in medium-Mn steels were investigated using combined computational and experimental approaches. Three steels containing 3, 4, and 5 wt.% Mn were annealed at 700–760 °C for 60 min, and their microstructures were analysed to assess the influence of Mn content on phase transformation behaviour. Thermodynamic (JMatPro, Thermo-Calc) and kinetic (DICTRA) simulations were used to model phase stability, austenite growth, and elemental partitioning under both equilibrium and non-equilibrium conditions. The modelling results were validated using dilatometry, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results showed that increasing Mn content promotes higher austenite fractions during IA but reduces its carbon enrichment, which adversely affects thermal stability due to higher martensite start (Ms) temperatures. DICTRA simulations also revealed that Mn and Al develop distinct concentration gradients across the ferrite/austenite interface, especially in higher-Mn steels, and that these gradients correlate with the measured EDS values in retained austenite laths. The revealed microstructures were composed of ferrite, retained austenite, and small amounts of cementite and martensite-austenite constituents. Overall, the study demonstrates that Mn content strongly affects both the amount and stability of retained austenite, and that IA parameters must be carefully optimized to tailor microstructure and mechanical behaviour in medium-Mn advanced high-strength steels.