Development of New 390-440MPa Grade Bake-hardenable Steel Sheet with Low Yield Ratio

摘要

Automobile industry has been strongly required to improve fuel efficiency of automobile because of global environmental problems. In order to reduce the weight of automobile body, the application of high tensile strength steel sheets to the body parts has been progressing. However, thinner steel sheets should be used without deterioration in original structural performance. In addition to strength, automobile outer panel needs both of dent resistance and minute unevenness, that is surface quality. Dent resistance of panels is improved with increasing yield strength and the thickness of steel sheet. Meanwhile, surface quality is deteriorated with increasing yield strength. Thus, it was a problem to cope with dent resistance and surface quality using high strength steel sheets.