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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.

Kawasaki Steel Corporation has developed a new steel tube making process which includes warm-reducing after the electric resistance welding of steel strip. The tube properties such as bending, fatigue and torsion etc. can be improved by a warm-reducing process due to refining microstructure, spheroidizing cementite and improving texture. In this report, we present the metallurgy and the fundamental characterisitics of developed steel tubes. Furthermore, the practical characteristics and some application examples of the developed steel tubes to automotive parts are introduced.

Kawasaki Steel Corporation has developed a new steel tube making process which includes warm-reducing after the electric resistance welding of steel strip. The tube properties such as bending, fatigue, torsion and absorbed energy etc. can be improved by a warm-reducing process due to refining microstructure, spheroidizing cementite and improving texture.

Kawasaki Steel Corp. has developed new steel tube products with ultra fine microstructure termed History (High-speed tube welding and optimum reducing technology) tube which have the high strength, excellent formability and uniformity of seam hardness. These superior features can be achieved by newly developed thermo-mechanical control process (TMCP) technology in the stretch reducer. The History tube has been preferably applied to automotive parts due to these excellent properties for tube fabrications.