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To reduce the weight of automobile bodies, application of high-strength steel sheets is expanding. Furthermore, middle and high carbon steels are expected to be used to lower the environmental impact and cost in the automobile steel sheet industry. However, it is necessary to enhance the joint strength of the steel sheets. In this study, hat-shaped components were made using resistance spot (RS) welding or arc spot (AS) welding on S45C steel sheets (including 0.44% carbon), 1.4 mm thickness and strength of 1180 MPa grade. A dynamic three-point bending test was conducted on the components and their crashworthiness was compared. Some RS welds fractured (separated) during the three-point bending test even though the diameter of the weld metal was increased to 5√t (t means thickness of the sheet); however, AS welds did not fracture.

To achieve improvement of crashworthiness and weight reduction of car bodies, the application of high strength steel sheets to car bodies is expanding [1, 2]. However cross tension strength of spot welded joints decreases as material strength increases in the range of material strength exceeding 780MPa grade. Therefore arc spot welding was attempted to enhance the joint strength in a high strength steel sheet of 980MPa in tensile strength. Consequently, the following experimental results were obtained: 1) cross tension strength (CTS) of the arc spot welded joints doubled compared with CTS of spot welded joints when appropriate welding conditions such as heat input and hole diameter of the sheet were selected and 2) height of excess weld decay in addition to the diameter of weld metal for the arc spot welded joints played an important role in improving CTS.