To improve the productivity of the press bending process, that is the method of forming a longitudinally welded large-diameter pipe from heavier thickness and higher strength plate, decreasing the number of press steps is required. However, with decreasing the number of press steps, the unevenness of pipe roundness becomes greater. Therefore, to clarify the influence of the number of press steps on the expansion process with 12 dies, the expansion process of a pipe with the shape of a cyclic arc with a straight region is investigated. With increasing number of press steps, unevenness of the press-bend pipe decreases, but the crossing force during expansion and unevenness of the expanded pipe are less in the case of the pipe formed with 11 steps. In this case, all dies are in contact with the straight region equally at the beginning of expansion, and the crossing force is the less. Furthermore, the entire die surface is in contact with the pipe at the end of expansion, and unevenness of the expanded pipe is less. To implement this surface contact, the ratio of the radius of the arc to the pipe inner diameter should be larger than 0.8. From these results, the number of press steps on press bending process can be decreased to 11, which means the productivity of the press bending process should increase dramatically.
