Cr-alloyed novel press-hardening steel with superior combination of strength and ductility

A novel press-hardening steel (PHS) with an ultra-high tensile strength of 1680 MPa and an excellent total elongation of 9.2% has been developed and produced at an industrial scale. In addition to high strength, the new PHS showed significantly improved ductility as compared to the baseline PHS grade 22MnB5 (similar to 7.0%) mainly because of its nano-sized retained austenite (RA), which was formed as a result of dynamic carbon partitioning after the martensitic transformation during the hot forming process. Dynamic carbon partitioning from the martensite to austenite during cooling and RA stabilization was achieved by adding optimum amounts of Si and Cr to the novel PHS without alternating the industrial hot forming process. To investigate the effect of the RA on the mechanical performance of the new PHS, microstructure without RA was prepared for comparison by direct water quenching after the austenitization to suppress the carbon migration. Detailed characterization revealed that the RA was beneficial for improving both the uniform and post-uniform elongation of the novel PHS. The effects of Si and Cr on the formation of the RA were investigated with the aid of thermo-kinetic calculations. The excellent combination of strength and ductility of the novel PHS will enable a significant weight reduction when it replaces the current PHS grade 22MnB5 for a variety of vehicle body structure components.

Automated summary

The newly developed PHS steel combines high strength and ductility, attributed to the nano-sized retained austenite formed through dynamic carbon partitioning. The presence of retained austenite improves the uniform and post-uniform elongation of the steel. Additionally, the addition of Si and Cr affects the formation of retained austenite.