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Abstract
High-strength steels (HSSs) have been increasingly used in car bodies in order 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 our previous study, we focused on friction element welding (FEW) of HSSs to improve CTS and reported that the poor local ductility of the area quenched from two-phase temperature region (inter-critically annealed and quenched area) due to the presence of soft ferrite (α) and hard martensite (M) causes a decrease in CTS of FEW. The local ductility of steels with dual phase structure is known to be affected by the microstructural morphology. Therefore, in this study, we prepared joints of FEW with both the equiaxial α (C30) and the acicular α (C30L) in dual phase structure by controlling parent structures prior to the inter-critical annealing and quenching during FEW. C30L exhibited the higher CTS than C30. C30L fractured along the thickness direction of the lower sheet from the inter-critically annealed and quenched area near the edge of the joint to the softened area of heat affected zone, while C30 fractured along the inter-critically annealed and quenched area; however, the dimples were observed on the fracture surfaces irrespective of materials. Taking into account the fact that C30 and C30L showed no difference in the hardness distribution of the joints and the hardness difference between α and M hardness, it was suggested that morphology of α plays an important role in determining CTS. It is inferred that the local strain concentration in the inter-critically annealed and quenched area is relaxed by the presence of fine acicular α, which leads to the improvement of the local ductility, i.e. CTS.