実構造を想定した抵抗スポット溶接継手の接合強度に及ぼすナゲット軟化の影響

抄録

In recent years, the application of high strength steel sheets has been expanding in the automotive field in order to increase the strength of the automobile body. Moreover, the tensile shear strength and the cross tension strength are defined by JIS as the strength method of the resistance spot-welded joint. Recently, L-type tension strength which is the strength in the peeling direction assuming the actual structure is receiving increased attention. However, it is known that L-type tension strength decreases with increasing steel sheet strength. In this study, the effect of softened nugget on joint strength was investigated for the purpose of improving joint strength of L-type joints. Thus, nickel was added to the nugget and the nugget microstructure was controlled to achieve a nugget softening. As a result, nugget microstructure changed and softened by adding nickel. Then, the L-type tension strength was improved under the condition where the nugget hardness was greatly decreased. In order to investigate the mechanism of improving the joint strength, the test was stopped at an arbitrary load during the test, and the crack growth measurement at the nugget end was performed. It can be confirmed that cracks did not progress in the nugget as the nugget softens. It is considered that crack growth in the nugget is suppressed as the deformability by softening was improved. Therefore, it is considered that the L-type tension strength is improved by improving the deformability of the nugget.