Effect of paint baking on the halo ring and mechanical behavior of 30MnB5 hot-stamped steel resistance spot welding joints

The present work investigates the effect of paint baking (PB) on the resistance spot welding (RSW) joints of 30MnB5 (1.8 GPa grade) hot-stamped (HS) steel, with focus on the halo ring and fracture behavior under cross-tension loading condition. Microstructural and elemental distribution analyses reveal that severe segregation of alloying elements occurred in the fusion zone (FZ) and that a halo ring with a martensitic structure and a width of about 78 μm was formed in the as-welded joint. The PB process resulted in uniform distribution of alloying elements and recarburization in the halo ring region, thereby drastically decreasing the halo ring width to about 33 μm, which changed the fracture behavior of the joint. Therefore, the as-welded joint failed through the halo ring, while in the PB joint, the fracture propagated partly through the FZ and then redirected through the thickness direction. The PB joints exhibited superior mechanical performance to the as-welded joint, especially the energy absorption.