High-strength steel sheets (HSSs) have been increasingly used to reduce the weight of automobile bodies. It is necessary to enhance the reliability of their joint strength in order to realize optimal usage. Projection welding (PW) is one of the important welding methods for HSSs; however, research on PW is insufficient. In this research, therefore, the effect of welding conditions on maximum load in the press-peel test for joints, made by PW HSS of 590 MPa grade and square weld nuts, was investigated. It is found that the maximum load showed a tendency to increase with an increase in the current and a decrease in the electrode force. In several cases of the press-peel test, four welds of one joint fractured one by one in sequence and showed a peak load each time it broke. The first or second peak load became the maximum load of each joint. SEM (Scanning Electron Microscope) observation of the welds fractured at the early stage in the press-peel test revealed that the fractured surface can be classified into a good weld region and a bad one. The former was a dimple fracture surface or cleavage one and the latter was a relatively flat fracture surface with many oxides. The same analysis was conducted for joints with several welding conditions. It was consequently found that there is a good correlation between the area of good weld region and the maximum load. In addition, it was considered that the scattering of moderate expulsion during PW was effective for reducing oxides.
