Bainitic press hardening of a silicon-boron steel – Tailoring strength and ductility by process-induced multiphase microstructures

Hot sheet metal forming is an integral part of the body construction in the automotive industry and enables the production of safety-relevant structural components with strength levels of around 1150 MPa–2000 MPa. However, due to its intrinsic stress states and high dislocation density, the martensitic microstructures of press-hardened steels exhibit only low ductility. Here, ductility is of highest importance for the energy absorption and crash performance of the body. A new bainitic press hardening process, which is realized by an austenitization step, an isothermal holding phase in the bainite range, and a final quenching step in the forming die, makes it possible to realize a multiphase press-hardened steel composed of bainite, martensite and retained austenite with increased strength and improved ductility. In addition, the new bainitic press hardening process requires only a single alloy to achieve customized strength classes. Conditions obtained are characterized by improved ductility at the same time, which can otherwise only be achieved by different alloy compositions. In the present work, a specifically developed silicon-boron steel 37SiB6 was used, and the influence on strength and ductility for different time-temperature treatments was investigated. In particular, the time of the isothermal bainitic holding step shows a major influence on the phase composition and mechanical properties. Accompanying X-ray diffraction analysis revealed an increased fraction of retained austenite for longer holding times.