As a method of forming thick-walled steel pipe, the pressing method of top and bottom dies (hereinafter referred to as bending press) is usually used.


In this forming method, when a steel plate with a predetermined width and length is formed into a longitudinal steel pipe with a pipe axis direction, the width end of the steel plate is bent (hereinafter referred to as edge pressing). Next, a method of forming a drum by performing multiple bending operations in the width direction of the steel plate. In the main forming process of steel sheet after crimping, the width direction end of the steel sheet is formed in the center part (front half) of the width direction of the steel sheet, and in the center part of the front part in the width direction of the steel sheet. The end of one side opposite to the width direction of the steel plate faces the central part of the width direction of the steel plate (forming the latter part in turn), and is finally pressed (eventually) to the central part of the width direction. Steel plate. The tube body in such a state, in which the ends of the plates are cylindrical and the parts facing each other are not welded, that is, the slotted tube.


The slotted pipe is transported along the longitudinal direction of the steel pipe (vertical direction of the paper) and sent to the next process. In this state, the butt edge is welded by a welding machine to form a direct welded steel pipe. It should be noted that the cylindrical shape can be corrected by applying the expanding or reducing process to the welded steel pipe as required.