| 摘要 |
PROBLEM TO BE SOLVED: To provide the production method of the triangular tube and the forming die used for its production capable of forming a triangular cross sectional shape with changing a cross sectional shape in taper, reducing a production cost and improving external appearance quality. SOLUTION: The triangular tube production method consists of a first process in which the neighborhood of both end parts of a steel plate 1 cut in a prescribed shape beforehand is bent with a prescribed curvature, a second process in which the center part of the steel plate 1 bent in the first process is bent with a prescribed curvature, a restricted forming process, in which, while pressurizing the center part of the steel plate 1 formed in the second bending process from left/right in the longitudinal direction, both ends are pushed down from above to be restricted/butted for forming, and a welding process to weld the butted parts of both end parts after the restricted forming process without unloading.
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