Press-hardened steel alloys are being used for sheet-metal parts incorporated in vehicle body structures that may be assembled together with rivets or welding. One example of a press-hardened steel is boron steel sold under the designation Usibor® 22MnB5. Press-hardened steel can be water cooled or oil cooled to a desired level of hardness from 450 to 520 HV. Press-hardened steel may be annealed to reduce the hardness to 140 HV.
Press-hardened steel parts may be assembled to other steel parts by welding. But, new automotive assemblies may include combinations of parts made of different materials such as aluminum and composite parts. An Ultra High Strength Steel (UHSS) beam formed by press hardening and a composite part or an aluminum part cannot be efficiently joined together in a welding operation. The preferred technique for joining such part assemblies is to rivet or otherwise fasten the parts together. The hardness of such high strength parts poses significant challenges in high volume manufacturing operations because the rivets have difficulty penetrating the UHSS beam.