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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.

Weldable vibration damping steel sheet consists of two skin steel sheets and viscoelastic resin film that contains filler metal powder to get electric conductivity. In this study, it is investigated the effect of filler powder (type; size and content) on weld defects. The defects are observed when the resin layer is thick comparing with the powder diameter or the content of metal powder is insufficient. The defect with bare skin sheets which is spherical hole is caused by joule overheating. Another defect is observed when Zn-Fe coated skin sheets are used. According to the observation of welding current and voltage wave forms. It is formed by an arc electric discharge between metal powder and skin sheet, which is caused by cutting off current paths due to the evaporation of Zn-Fe coated layer. Based on these results, it is determined the suitable filler powder to produce VDSS with good weldability.

To improve the performance of zinc-rich painted steel sheets, such as Zincrometal®, we at first developed Zn-Ni plated steel sheet with a thin (about 1μm) organic-silicate composite film entirely different from a zinc-rich paint film. This product (EZN-UC) is superior to zinc-rich painted steel sheet in corrosion resistance, anti-powdering property and weldability. This product (EZN-UC), however, is not able to provide bake hardenability due to the high curing temperature of the film. In response to increasing demand for the product with bake hardenability, we have developed new organic-silicate composite coated steel sheet (EZN-UCII), which allows the coating to be cured in the temperature range, where bake hardenability can be maintained.

A new vibration-damping sheet (viscoelastic material sandwich type) for automobiles has been developed jointly by NKK and Japanese plastic maker. This product has high damping capacity and good press formability eqaul to normal steel sheets, because this product uses the hard viscoelastic material which has good adhesion to steel sheets. The sound level shall be reduced by order of 2∼3 dB, if this product is applied to oil pans. Fatigue limit is improved as compared with early vibration-damping sheet for which the soft viscoelastic material was used. Spot welding can be done in case that an electric short circuit is used.