Abstract
KEYWORDS - Ultra-high-strength steel, Galvannealed steel, Absorbed energy, Bend radius, Crashworthiness
Increasing the strength of a steel sheet is essential for both reducing the weight of an automobile and improving its crashworthiness. Various kinds of high-strength steel sheets have been applied to date to automobile parts. The majority of steel sheets used for the main structural members have had tensile strength of up to 590 MPa, while ultra-high-strength steel sheets with tensile strength ranging 980 and 1470 MPa have been applied to members, such as bumper reinforcements and door beams, where high strength is required. The applications of ultra-high-strength steel sheets have expanded in recent years due to the demand for reduced weight vehicles. There are varieties of cold-rolled steel (CR) sheets, while few galvannealed steel (GA) sheets have been developed. Bending formability and hole-expansibility of conventional high-strength galvannealed steel sheets is poor.
In addition to good bending formability and hole-expansibility, wettability is important for ultra-high-strength galvannealed steel sheets. Surface quality is required for galvannealed steel sheets. Although ultra-high-strength steel sheets normally contain silicon for enhanced elongation, bending formability and hole-expansibility, silicon leads to poor wettability with molten zinc [1]. It is therefore difficult to manufacture ultra-high-strength galvannealed steel sheets with good formability and wettability.
In this study, ultra-high-strength galvannealed steel sheets with excellent bending formability, hole-expansibility and good wettability were developed by controlling the microstructures of steel sheets and reducing the silicon content. The minimum bend radius, which affects the higher absorbed energy, of the developed ultra-high-strength galvannealed steel sheets, is smaller than that of conventional ultra-high-strength galvannealed steel sheets, while the adhesion of the coating is equivalent to that of mild steels. It is possible that the ultra-high-strength galvannealed steel sheets developed will contribute to weight reduction of autobody and improved crashworthiness.