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Abstract
High-strength steels (HSSs) have been increasingly used in car bodies to simultaneously achieve the weight reduction and high collision safety of vehicles. In resistance spot welding, which is widely used for joining car bodies, low cross tension strength (CTS) of joints using HSSs is a problem. In this study, we focused on friction element welding (FEW) to enhance the CTS. To investigate the joint strength of steel sheets jointed by FEW, a pre-hole was provided in the upper sheets, and the element was passed through the pre-hole to joint with a lower sheet, and then cross tension tests of the joints were conducted. In addition, the microstructures of the joints were observed to investigate the fracture positions. In the cross tension test, three types of fracture modes were observed. When the carbon content of the lower sheet was as low as 0.07–0.15 mass%, the joint broke at the head of the element. However, when the carbon content was increased to 0.20 mass%, the joints were fractured at the softened area of heat affected zone in the thickness direction of lower sheets. While the carbon content was further increased to 0.30 mass% (C30), the cracks propagated inside the area quenched from the two-phase temperature region (inter-critically quenched area) of the lower sheets and then fractured in the lower sheet thickness direction. Consequently, the CTS was decreased. To clarify the mechanism of the decrease in CTS, the fracture surface was observed together with the hardness difference measurement between ferrite and martensite by the nanoindentation method. As a result, it was clarified that the hardness difference between the two phases was significantly large, leading to the ductile fracture. Based on these findings, the low CTS of C30 was inferred to be caused by the poor local ductility in the inter-critically quenched area.