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 34 and the diameter Dwl of each of the lower rolls 32, and setting the amount S of tightening of the upper roll 34 with respect to the lower rolls 32 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 32 satisfies the following condition: (Dp+Dwl)>L≧0.

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 34 and the diameter Dwl of each of the lower rolls 32, and setting the amount S of tightening of the upper roll 34 with respect to the lower rolls 32 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 32 satisfies the following condition: (Dp+Dwl)>L≧0.

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 34 and the diameter Dwl of each of the lower rolls 32, and setting the amount S of tightening of the upper roll 34 with respect to the lower rolls 32 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 32 satisfies the following condition: (Dp+Dwl)>L≧0.