The steel plate is first pressed into a U shape in the forming die, and then pressed into an O shape, and then the inner and outer submerged arc welding is carried out. After welding, the diameter is usually expanded at the end of the full length, which is called the UOE welded pipe, and the one that does not expand is called UO welded pipe. . Roll Bending the steel plate, and then carry out internal and external submerged arc welding. After welding, the diameter is expanded to RBE welded pipe or not to RB welded pipe. The steel plate is formed in the order of J type-C type-O type, and after welding, the diameter is expanded to JCOE welded pipe or non-expanded to JCO welded pipe. Among the above-mentioned LSAW welded pipes, UOE is commonly used. The expansion process of large-diameter steel pipes is the preliminary rounding stage. The fan-shaped block is opened until all the fan-shaped blocks touch the inner wall of the steel pipe. At this time, the radius of each point in the inner tube of the steel pipe within the step length is almost the same, and the steel pipe is initially round. The sector block starts to reduce the movement speed from the front position until it reaches the required position, which is the required inner circumferential position of the finished pipe. The expansion process stage of the large-diameter steel pipe is the rebound compensation stage. The fan-shaped block will further slow down at the position of stage 2 until it reaches the required position, which is the position of the inner circumference of the steel pipe before rebound as required by the process design. The sector block remains stationary for a while before rebounding on the inner circumference of the steel pipe. This is the pressure maintaining and stable stage required by the equipment and the diameter expansion process. After that is the unloading return stage of the large diameter steel pipe expansion process. The sector block rapidly retracts from the position of the inner circumference of the steel pipe before the rebound, until it reaches the initial expansion position, which is the small contraction diameter of the sector block required by the diameter expansion process.