Evolution of microstructure and mechanical properties of medium Mn steels during double annealing

► Ultrafine microstructure of ferrite, retained austenite and/or martensite. ► Double morphology of microstructural features: lath-like and polygonal. ► Mn redistribution seems to be controlled by diffusivity similar to those in ferrite. ► Austenite stabilization controlled by two factors: chemical enrichment and its size. ► Improved TRIP effect due to the presence of important retained austenite fraction.