Abstract: In the manufacture of spiral-welded steel pipe, material strip or plate is fed in its longitudinal direction and continuously formed into spiral pipe form. One side-edge of the strip and the other side-edge of the strip that has already been spirally formed are welded together by the electric-resistance method where they meet. The edges of the strip and formed pipe are held together simultaneously with or immediately after the electric-resistance welding from both inside and outside the pipe using a pair of pressure rolls. The bead formed as a result of the electric-resistance welding is reformed simultaneously with or immediately after the holding by the pressure rolls is accomplished. Then, the electric-resistance welded side-ends are welded again by the arc method.

Abstract: In the manufacturing of a spiral pipe, there is first determined a maximum curvature which should be imparted to a strip of pipe forming material in order that a desired residual moment will be imparted to the finished pipe after the strip bent to the maximum curvature is allowed to spring back from the bent condition, which maximum curvature is determined based on the thickness, Young's modulus and yield stress of the strip, the desired curvature of the finished pipe, and the residual moment to be imparted to the finished pipe. By adjusting the position of three rows of forming rolls positioned at the apexes of a triangle, the flat strip is continuously bent into a spiral that has the thus determined maximum curvature.