The plate in certain width after slitting line and stripping is unfolded and leveled. Then, they are sent into forming machine. Used the roller, the plate is bent into pipe state. The edge of the steel plate is melted by making use of the proximity effect and skin effect of high frequency current. Then, the plate is extruded and welded into the pipe by extruding roller. Straight seam welded pipe mill also is called ERW mill.