後通電を施した高強度鋼板抵抗スポット溶接継手の疲労強度に及ぼす鋼板強度の影響に関する検討

抄録

In this paper, fatigue tests were carried out on resistance spot welded joints with post-heating treatment of 590 MPa grade high-strength steel sheets(hereinafter 590 MPa steel) and 980 MPa grade high-strength steel sheets (hereinafter 980 MPa steel) in order to investigate the influence of strength level of base steels on fatigue strength. Welding conditions were decided such that nugget diameter would be about 4√t. Post-heating parameters were decided so that inside of the nugget or edge of the nugget could be softened. The fatigue tests were conducted under shear loading and peel loading. As a result, hardness at edge of the nugget was harder in 980 MPa steel than in 590 MPa steel in all tempering parameter. However, the results of fatigue tests under shear loading show that fatigue life was independent of strength level of steels in all tempering parameter. Moreover, fatigue life under shear loading has relationship with the degree of softening at edge of the nugget. The degree of softening at edge of the nugget is subtraction hardness at edge of the nugget with tempering treatment from hardness at edge of the nugget without tempering treatment. Therefore, fatigue life under shear loading is inferred to be affected by not only hardness at edge of the nugget, but also hardness of hardened area in the vicinity of the nugget edge like HAZ. The results of fatigue tests under peel loading indicate that fatigue life of the joints with tempering treatment was longer in 980 MPa steel than in 590 MPa steel. This is because the compressive residual stress (z-direction) at edge of the nugget is higher in 980 MPa steel than in 590 MPa steel.