Influence of the heat treatment parameters on the resulting mechanical properties of a hot formable medium manganese steel

Medium manganese steels belong to the 3rd generation of advanced high strength steels and offer potential for application in structural and safety-relevant automotive components. Compared to conventional boron-added steels, hot forming of these steels enables a higher combination of strength and ductility. Furthermore, they provide advantages in processing, such as lower forming temperatures and slower critical cooling rates. The properties of hot formed medium Mn steel components are defined by the process parameters of final heat treatment at the steel manufacturer and hot forming at the part manufacturer. The aim of this study is to identify the influences of these process parameters on the resulting mechanical properties of a 4.5 wt.% Mn-containing medium manganese steel. Heat treatments on strip specimens were carried out in a forming simulator to mimic the industrial process route consisting of continuous annealing and hot forming. The temperatures and times for annealing and reheating were varied. Tensile tests and hardness measurements were carried out. Finally, guided by regression analysis, optimal processing parameters were derived to achieve a high combination of strength and ductility. The retained austenite fractions and microscopical images were analysed for selected samples and correlated with the observed mechanical behaviour of the steel.