F2008-04-023
Development of Ultra-High-Strength Galvannealed Steel Sheets with Excellent Bending Formability and Hole-Expansibility
Increasing the strength of a steel sheet is essential both for reducing the weight of an automobile and improving its crashworthness. Various kinds of high-strength steel sheets have so far been applied to automobile parts. Most of the steel sheets used for main structural members have had a tensile strength up to 590MPa. The ultra-high-strength steel sheets whose tensile strength ranges between 980 and 1470MPa have been applied to members, such as bumper reinforcements and door beams, where high strength is required. Applications of the ultra-high-strength steel sheets have expanded over the last years because of demands for weight reduction of vehicles. There are varieties of cold-rolled steel (CR) sheets, while few galvannealed steel (GA) sheets have been developed so far. In recent years, the ultra-high-strength galvannealed steel sheets have been demanded to satisfy both high strength and corrosion resistance. The parts used with ultra-high-strength steel sheets are reinforcements. When these steel sheets are applied to reinforcements. In some cases, they are usually formed by bend-forming. The formability of a steel sheet becomes poor as its strength increases. It is therefore difficult to manufacture ultra-high-strength galvannealed steel sheets with good bending formability. Furthermore surface quality is also required to GA steel sheets. In this study, we developed ultra-high-strength galvannealed steel sheets with excellent bending formability and hole-expansibility by controlling microstructure of steel sheets. Minimum bend radius of the developed ultra-high-strength galvannealed steel sheets is smaller than that of the conventional GA ultra-high-strength steel sheets. The smaller minimum bend radius at the corner of parts is, the higher absorbed energy is. Other properties, such as spot weldability and adhesion of coatings, are the same those of mild steels or those of conventional ultra-high-strength steels. The applications of the developed ultra-high-strength galvannealed steel sheets can contribute to weight reduction and improvement of crashworthness.
Session: Body Design