Effects of soaking temperature on the microstructure and mechanical properties of a medium Mn steel for hot/warm forming

The increasing demands for better crashworthiness of vehicles require structural components with excellent energy absorption performance and anti-intrusion resistance. Here, we explored the potential of using a medium Mn steel with a 10 wt.% Mn content for hot/warm forming with the aim to produce automotive components with improved strength–ductility combination. The soaking temperature was varied from 670 to 870 °C for tuning the microstructure and the resultant mechanical properties. It is found that a duplex microstructure consisting of ferrite with a high dislocation density and retained austenite with a volume fraction of 53.5% is formed at 720 °C, leading to an exceptional strength–ductility combination (yield strength: 1016 MPa; ultimate tensile strength: 1407 MPa; total elongation: 32.7%). The energy absorbed before fracture is 39.8 GPa·%, which is four times more than that of the widely-used 22MnB5 boron steel. On basis of the experimental results, the effects of soaking temperature on austenite reverse transformation, ferrite recrystallization and martensitic transformation are discussed in depth to provide guidance for optimizing the strength–ductility combination of medium Mn steels processed by hot/warm forming.