The effect of laser welding on microstructure and mechanical properties in heavy-gage press hardening steel alloys

Abstract

The hot stamping steel grade 22MnB5 steel producing a target tensile strength of 1500 MPa is standardly used for press hardened components in passenger cars. The thickness of such components is usually limited to 2 mm or less. More recently, the application of press hardened components has been also considered for components of heavy commercial vehicles requiring much thicker sheet gages. The heavier gage poses increased challenges towards the press hardening process as well as to assembly technology. This study, being part of a larger development program, evaluates laser welding of heavy gage press hardened steel. The steel used is hot rolled standard 22MnB5 as well as a variant modified by niobium and molybdenum microalloying for optimizing hardenability and toughness. The press hardened steel samples were subjected to double-pass laser welding. The welded samples of the modified steel variant were found to have a more favorable microstructure in the heat affected zone resulting in improved mechanical properties and more ductile fracture behavior. The improvements relate to the inhibition of austenite grain coarsening in the heat affected zone. The mechanisms of grain growth limitation during laser welding of the modified steel will be outlined.