PURPOSE: To form a large diameter square steel tube provided with sharp corner parts which consists of a homogeneous and stable material by removing residual stress in the corner parts R of the square steel tube, improving toughness and correcting distortion, bend and twist without being accompanied with work hardening.
CONSTITUTION: A thick steel plate is uniformly heated, that steel plate is bending formed by hot press working and a cross section orthogonal to the direction of the longitudinal axis is approximately formed into an approximate square- shaped steel plate. After the major part of working quantity for forming a steel tube is executed by hot working, this material is gradually cooled and an approximate square-shaped steel tube with one seam or two seams is constituted by mutually butt welding open edges in the direction of the longitudinal axis. The cross section of steel tube is formed as a standard by executing the rest of working quantity by cold plastic working and, on the other hand, distortion, twist, etc., of the formed steel tube due to hot working or the like are corrected and the mechanical strength of steel tube is improved.
NAKAJIMA HIROSHI
NAKAJIMA SHIN
