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Increase of load capacity for trucks by weight reduction of the body has been required to cut down transportation costs. The weight reduction of truck body is feasible in terms of the application of high-strength steel sheet to body. Especially, the application to truck frame is important because it is heavy. The required properties of steel sheet for truck frame use are press formability and fatigue strength. Press formability can be optimized based on the previous investigation results. On the other hand, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Accordingly, the fatigue strength of high-strength steel sheet for truck frame use was investigated in this study. As for fatigue strength, the fatigue strength of steel sheet with scale is significant because the steel sheet is painted without pickling to remove the surface scale.

The effect of a concentration of stress at a welded joint on the fatigue limit was investigated from the point of view of the effective design of automotive parts made of high strength hot rolled steel sheets. The stress concentration at the welded joint was evaluated using FEM analysis and the effect of stress concentration on endurance limit was investigated by fatigue tests using specimens of practical welded joints and specimens with the simulated configuration of welded joints. It is pointed out that another effect besides stress concentration and residual stress should be considered to evaluate the endurance limits for high tensile strength materials of over 590 N/mm2.