Tensile-shear properties of resistance spot welded joints of B1500HS hot forming steel

Three strength levels of DP steel (DP780, DP980,DP1180) and B1500HS resistance spot welded (RSW) joints were prepared and investigated in present paper. The influence of DP steel’s strength on tensile-shear properties of RSW joints were studied. The macroscopic morphology and microstructure of RSW joints were observed and analyzed by ultra-depth microscopy and scanning electron microscope (SEM). The hardness profile of welded joints were measured by Vickers microhardness tester. The results showed that strength of DP steel had negligible impact on tensile-shear strength of RSW joints but its fracture mode. The nugget was pulled out from DP steel side for B1500HS/DP780 RSW joint. The other two nuggets were pulled out from the B1500HS side. Their initial fracture locations were all located in subcritical HAZ (SCHAZ). The SCHAZ on the B1500HS side was seriously softened with 29%～36% hardness loss compared with its matrix. However, there was no obvious softening phenomenon for the SCHAZ on the DP780 side. The softening rates of the SCHAZ on the DP980 and DP1180 sides were 17% and 25%, respectively.Highlights: RSW joints composed of B1500HS and DP780, DP980, DP1180 were comparatively studied.