Abstract
In recent years, high-strength steel sheets are used in production of vehicle bodies to reduce weight. As a result, it is known that fuel efficiency can be improved. The resistance spot welding is a primary joining method used for the manufacturing of vehicle bodies. However, the fatigue strength of the high-strength steel sheets joints does not increase with the increase in the strength of the base material. Therefore, it is necessary to increase the fatigue strength and a post-heating treatment is used to do this. Tempering treatment is one post-heating method. The application of a postheating method improves the fatigue strength of resistance spot welded high-strength steel sheets. This is caused by controlling properties of spot welds such as residual stress and nugget hardness distribution. However, very few studies have been reported on the individual effects in the spot welds on fatigue strength. In this study, the effects of residual stress and hardness distribution on fatigue strength was investigated using 980 MPa class high-strength steel sheets was investigated by experiment and numerical simulation. First, in order to investigate the effect of tempering treatment on the spot welds Vickers hardness was confirmed. The results show that the nuggets of resistance spot welded joints were softened by tempering treatment. Second, Fatigue tests were conducted under shear load in order to investigate the effect of tempering treatment on fatigue life. The results show that the fatigue life was improved by tempering treatment. Finally, the stress was calculated in the spot welds using the numerical simulation by changing the hardness distribution and residual stress in the nugget. The results of numerical simulation showed that the stress decreased near sheet separation by tempering treatment. The amount of decrease in stress was larger when changing the hardness distribution rather than changing the residual stress.
