Pipe formed by bending rolls

Abstract

In a pipe forming process using bending rolls, load applied to an upper roll is reduced to suppress the deflection of the upper roll, thereby suppressing creation of large end gap at the longitudinal center of the pipe, thus enabling production of high-strength, thick-walled elongated pipes. This is achieved either by (1) setting the roll spacing L between lower rolls to a value greater than the sum of the diameter Dwu of the upper roll and the diameter Dwl of each of the lower rolls , and setting the amount S of tightening of the upper roll with respect to the lower rolls to a value greater than the radius Rwl of each of the plural rolls; or by (2) effecting pipe forming work on the sheet material such that the spacing L of the lower rolls satisfies the following condition: (Dp+Dwl)>L≧0.85(Dp+Dwl), where Dp represents the outside diameter of the product pipe and Dwl represents the diameter of one of the plural rolls; or by (3) preparing the sheet material having leading and trailing bent end regions bent beforehand over a length not smaller than ⅕ of the entire circumference of the pipe to be produced; and effecting bending by the bending rolls such that the length of the regions bent by the bending rolls is less than ⅗ the entire circumference of the pipe.