INVESTIGATION OF THE PROCESS WINDOW FOR DEFORMATION INDUCED FERRITE TO IMPROVE THE JOINABILITY OF PRESS-HARDENED COMPONENTS

Due to their high tensile strengths increasing the crashworthiness of the vehicles, ultra-high strength steels are increasingly used in the automotive industry, for example in components like B-pillars or tunnel. 22MnB5 is a premier candidate for this cause, since it can be press-hardened and phase-transformed into the martensitic phase, resulting in high hardness and tensile strength. However, complications can arise in the assembly of press-hardened components since conventional mechanical joining processes have their limitations due to high forces required for joining press-hardened steels, especially in multi-sheet layers. Therefore, this study focuses on the determination of an optimum process window to influence the 22MnB5 microstructure thermo-mechanically during press hardening, causing a local softening. This so-called deformation-induced ferrite improves ductility at the desired locations to ease the mechanical joining operation in the assembly. Investigations are performed on a forming dilatometer varying the cooling rate, the introduced amount of plastic strain and the forming temperature along with metallographic as well as microhardness measurements. Based on the laboratory tests, a process window of the deformation induced ferrite is derived for an application in a forming press.