Longitudinal Submerged Arc Welded Pipe or simply LSAW steel pipe and Straight Seam Submerged Arc Welded Pipe or simply SSAW steel pipe belong to submerged-arc welding (SAW). But LSAW steel pipe adopts single steel sheet as raw material, its advantages include: the thickness of the steel plate can reach more than 100mm, and there is no crescent bends; since it is straight seam, forming is more convenient, but also it is easy to weld tracking, which is suitable for two-step pre-precision welding production; and there is the whole body of mechanical expansion process, which can basically eliminate the residual stress. All of advantages above make LSAW can be produced big diameter steel pipe (the maximum can reach 1422mm), high-grade pipe (50mm), high steel grade (X80/X100/X120), and large structure tube that used in two, three, four types of regional and marine engineering. LSAW steel pipe and high-frequency Longitudinal Welded Pipe (HFW steel pipe) all belong to Longitudinal Welded Pipe, but the high-frequency welded pipe can be made of medium and small welded pipe (400mm), and there is intermediate frequency heat treatment process. When the diameter, steel grade is the same, high-frequency welded pipe is more superior, for example, foreign pulp pipe often use high-frequency welded pipe (HFW).