Hot-formed steels (here called “Boron steels”) offer a great weight saving potential versus conventional cold-formed steels used for crash relevant structural parts.
Boron steels allow complex shaped parts due to the hot-forming process, which can be a direct or indirect process. In the direct hot forming process first the sheet metal with an initial yield strength of around σy=400 MPa is blanked and then heated in an oven up to some 950° Celsius. In the next step the “hot” sheet metal is stamped and at the same time rapidly cooled down (quench hardening process) in the stamping die.
During this process the yield strength increases up to approximately σy > 1100 MPa in the final stamped part. Due to the enormous strength and the very good dimensional control (nearly no springback), more and more hot-formed parts are used in vehicle design. Especially in the body structure hot-formed steels are used for crash relevant parts.
However the quench hardening process causes a significant reduction of the material ductility. The designer has to ensure via crash simulation that the crash impact can be handled without fracture of the Boron steel component. The prediction of the deformation and fracture behavior therefore is one of the major interests during the vehicle development process.
The paper will give an overview on
material properties of Boron steels,
a failure modeling approach for crash simulation,
the validation of the failure modeling approach,
application examples
that demonstrate the lightweight potential of Boron steels for structural applications and the benefit of the chosen simulation approach.