The welded square pipe, also known as welded pipe, is a square tube welded by steel plate or steel strip after curling. Welded square pipe production process is simple, high production efficiency, variety specifications, less equipment, but the general strength is lower than seamless square pipe. Since the 1930s, with the rapid development of high-quality strip continuous rolling production and the progress of welding and inspection technology, the quality of weld seam has been continuously improved, the variety and specifications of welded square pipe have been increasing day by day, and seamless square pipe has been replaced in more and more fields. Welded square tubes are divided into longitudinal welded pipe and spiral welded pipe according to weld form.

　　The longitudinal welded pipe has the advantages of simple production process, high production efficiency, low cost and rapid development. The strength of spiral welded pipe is generally higher than that of straight seam welded pipe. It can be produced with narrow blank with larger pipe diameter. It can also be produced with the same width of blank with different pipe diameter. However, compared with the straight seam pipe of the same length, the weld length is increased by 30~100% and the production speed is low.

　　Therefore, the smaller diameter of the welded pipe is mostly used in the straight seam welding, the larger diameter welded pipe is mostly used in spiral welding. Carbon steel flange