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The strength of steel sheets used in tailored blanks for automotive bodies has been rising. In order to obtain guidelines for the choice of appropriate welding processes and materials, the formability of welded steel sheets were investigated. The purposes of this work are 1) to determine an upper steel strength limit that can be welded and 2) to obtain the requirements of materials from a viewpoint of formability. Laser, mash seam, and plasma arc welding were employed up to 980MPa high strength steels. The results suggested that laser welding was the best welding process because of its small heat input. It could be applied to 980MPa steels. 590MPa was the maximum steel strength grade to which mash seam and plasma arc welding could be applied, because the mash seam and the plasma arc welding thermal cycles softened the HAZ of high strength steel. The requirement of materials for the formability of laser welded steels is high elongation of base metals.

Gas shielded metal arc welding is generally applied to automobile chassis parts. However, the weld parts with the E-coat show poor corrosion resistance. Therefore, the corrosion mechanism of the weld parts was investigated. The results found two reasons why the weld parts corroded faster than the non weld parts:(1)inadequate phosphating (2)defects in the E-coat. After detailed investigation, it was clarified that the major cause of poor corrosion resistance was the defects in the E-coat caused by slags formed on the surface of the weld bead. Therefore the amount of slag has to be decreased to improve the corrosion resistance. The effect of shielding gas composition on the amount of slag was then investigated. In the case of Ar and oxidizing gas mixture, the corrosion resistance improved as the oxidizing gas content decreased. This was due to the reduction of slags.