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 condition and selection of the welded seam position.