2. 1. Small and medium-sized steel pipes with not too large wall thickness generally adopt high-frequency continuous welding to become high-frequency straight welded pipes. Secondly, the large-diameter or large-wall steel pipe is preliminarily formed by hydraulic press step by step, and then submerged arc welding is adopted, and then thermal expansion forming is carried out. The residual height of weld is small, which is beneficial to the coating of 3pe anticorrosive coating.