Osamu Sonobe

Development of UTS 980 MPa Grade Steel Tube with Excellent Formability for Automotive Body Parts

The enhanced performance of an ultra-high strength steel (UHSS) tube with tensile strength of 980 MPa grade is reported. Recently, strong consideration has been given to the use of steel tubes for automotive components because of the associated weight saving, which leads eventually to a reduction of carbon dioxide emissions. Furthermore, the application of steel tubes offers the advantages of increased stiffness and reduced welding processes due to their closed cross-sectional structure. The newly-developed UHSS tubes have an excellent combination of high strength and ductility (ultimate tensile strength: 1016 MPa, elongation to fracture: 14 %) and show remarkable formability such as rotary-bending workability and hydro formability. The key technologies to obtain these superior properties are the application of a cold rolled sheet steel with a DP (Dual Phase) microstructure having an optimized ferrite/martensite ratio and a CBR (Chance-free Bulge Roll) tube forming mill, which minimizes additional strain in the tube making process. The developed UTS 980 MPa grade steel tube has been applied to an actual automotive body part and contributed to not only weight saving but also improved safety.

Hydroformability of 980MPa and 1180MPa ultra-high strength ERW steel tubes

High strength steel tubes have attracted attention as materials for reducing auto body weight. However, there have been few reports on hydroforming using materials with nominal tensile strengths exceeding 980MPa. Therefore, free bulge forming tests and rectangular section bulge forming tests were carried out with electric resistance welded (ERW) tubes having nominal tensile strengths of 980MPa and 1180MPa. These steels are dual-phase steels consisting of martensite and ferrite. In the free bulge forming tests, the limiting bulging ratio (LBR) under axial feeding was 17% for the 980MPa material and 5% for the 1180MPa material. In the rectangular section bulge tests with a bulging ratio of 4%, it was possible to avoid rupture of the 1180MPa material at the heat-affected zone (HAZ) by applying axial feed loading or selecting the proper welded seam position. Under the same rectangular section bulging test conditions, forming of the 980MPa material without defects was possible regardless of the axial feeding c...