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From the viewpoint of applying electric resistance welded (ERW) steel tubes for automobile structural parts, the formability degradation of steel sheets in a series of forming steps is investigated. Sheets are subjected several forming steps, such as a roll-forming into tubes, pre-forming, bending and bulge-forming. The formability degradation highly depends on the microstructure and aging property. Deterioration of the formability of sheets is diminished by the existence of the second phase in the ferrite matrix. The formability degradation of the steel sheet having an aging index (A.I.) ≤10MPa is well restrained so that the tube shows a much higher elongation and expanding ratio on bulge forming than the tube having A.I.>10MPa. These phenomena are applicable to all the tubes of tensile strength level form 290MPa to over 590MPa.

This paper presents the results of experimental analysis on the structural performance of steel, aluminum alloy and stampable sheets for bumper beams and the calculation results on the weight reduction of those bumper beams. Bending tests of straight beams, which were made of the three kinds of materials, were carried out in order to examine their structural performance. In consideration of failure modes, it was found that the maximum load and the absorbed energy of those beams can be evaluated by their material strength, their side-wall thickness and the height of their cross sections. Based on this evaluation method, the weight and the absorbed energy of the constant displacement were calculated using a common cross section. The weight reduction of aluminum alloy and stampable sheets is superior to 590MPa high tensile strength steel sheets.