PURPOSE: To obtain a special shaped tube whose dimensions of corner parts are smaller and which is excellent in flatness in planar parts.
CONSTITUTION: By rolling using a grooving roll and grinding, Vgrooves G1-G8 which are extended in the longitudinal direction are formed on a strip material S. The strip material S is bent in the width direction taking the V-grooves G1-G8 as valley bending lines, butt welding J or lap welding H is executed in both end parts in the width direction. Then, the V-grooves G1-G8 are preferentially crooked at the time of bending, so the dimensions of the corner parts are smaller and straight-line-like corner parts C1-C8 along the longitudinal direction of the strip material S is obtained. And, since bending stresses that act on the planar parts P1-P8 are mitigated, the planar parts P1-P8 after bending have higher flatness, groove shape or overlapping state is unified and welding condition is stabilized.
SAKAKI MASAHITO
NAKAMOTO KAZUNARI
