A novel medium-Mn steel with superior mechanical properties and marginal oxidization after press hardening

A novel Cr-alloyed medium-Mn steel was designed and manufactured for the hot forming application in automobile. Compared to the typical press hardening steel (PHS) grade of 22MnB5, it possesses more excellent mechanical combination and enormously reduced oxidization after press hardening. The synchrotron-based high-energy X-ray diffraction experiment indicated that the solute C atoms could be partitioned from martensite to austenite during the baking process and then segregated on geometry necessary dislocations that had been generated due to the austenite-to-martensite transformation during the previous cooling. This led to much hardened austenite grains, which could bear more load partitioned and accommodate the greater deformation until they transformed to martensite during deformation than the non-baked austenite grains, thus greatly improving both strength and ductility. Moreover, the thickness of oxidization layer on the invented steel is less than 3 μm after the hot forming at 750−810°C, much thinner than the 100 μm-thick oxidization layer on 22MnB5. Such a tremendously reduced oxidation results from both the lower soaking temperature employed for the hot forming and the formation of dense Cr/Al/Si oxide band at the bottom of the oxidation layer, the latter is because the designed steel contains much higher contents of Cr and Al than 22MnB5.