| 摘要 |
PURPOSE:To improve the toughness of a weld zone by preforming both end edges of a belt-like steel product curved to a tubular shape and butted to each other in such a manner that the cross section has a C or V shape, then subjecting the edges to resistance welding. CONSTITUTION:The belt-like steel sheet 10 is formed to the tubular shape by fin rolls 20 and both end edges 10a, 10a are compressed by fin parts 20a. A recessed V shape is preformed to the end edge 10a and a projecting V shape to the other. Both the end edges 10a, 10a are butted and resistance-welded to each other by rolling in the end period of a forming stage. The V-shaped cross sections are formed in the weld butt parts where the degree of deteriora tion in toughness is large by the above-mentioned method and therefore, the course of breakdown increases as compared to the square butt parts even if said course passes the brittle parts. The breakdown is thereby prevented. The progression of the brittle fracture of the weld zone is thus prevented by working the butt parts to the vee shape in the above-mentioned manner, by which the toughness is improved.
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