Steel pipe generally can be divided into two categories: one is welded steel pipe, the other is seamless steel pipe. 

Welded steel pipe, also known as welded pipe, it is made of steel sheet or strip that experience crimp forming welding. The production technology of welded steel pipe is simple, with high production efficiency, various specifications, less equipment investment, but the general strength is lower than the seamless steel pipe. Since the 1930s, with the rapid development of high quality strip steel rolling production and the progress of welding and inspection technology, the weld quality constantly improves. Welded steel pipe specifications increase day by day, and replace seamless steel pipe in more and more field. In terms of the form of welded steel pipe, it can be divided into the weld seam welded pipe and spiral welded pipe. 

The billet that welded steel pipe adopted is steel plate or strip steel. Because of the different welding process, welded steel pipe is divided into furnace welded pipe, welded (erw) pipe and automatic arc welding pipe. Due to its different welding form, it can be divided into straight seam welded pipe and spiral welded pipe. Because its shape of end is different, it can be divided into circular welded pipe and alien (square, flat, etc) welded pipe. Due to its different material and use, it can be divided into the following varieties: 

GB/T3091-1993 ( galvanized welded steel pipe for low pressure fluid conveying). It is mainly used for conveying water, gas and heating hot water or steam, air, oil and other general low pressure fluid and pipe for other purpose. It is represented by material level Q235A steel. 

GB/T3092-1993 ( galvanized welded steel pipe for low pressure fluid conveying). It is mainly used for conveying water, gas and heating hot water or steam, air, oil and other general low pressure fluid and pipe for other purposes. It is represented by material level Q235A steel. 

GB/T14291-1992 (welded steel pipe for mine fluid conveying). Straight seam welded steel pipe is mainly used for mine forced draught, drainage, wind shaft gas. It is represented by material level Q235A, B grade steel. 

GB/T14980-1994 (large diameter welded steel pipe for low pressure fluid conveying). It is mainly used for conveying water, sewage, gas, low pressure fluid such as air, steam heating and other uses. It is represented by material level Q235A steel. 

GB/T12770-1991 (stainless steel welded steel pipe for mechanical structure). It is mainly used in machinery, automobile, bicycle, furniture, hotel and restaurant decoration and other mechanical parts and components. Its representative materials are 0 cr13, 1 cr17, 00 cr18ni9 cr19ni11, 1, 0 cr18ni11nb, etc. 

GB/T12771-1991 (stainless steel welded steel pipe for fluid conveying). It is mainly used for low pressure corrosive medium. Its representative materials are 0 cr19ni9 cr13, 0, 00 cr19ni11, 00 cr18ni11nb cr17, 0, 0017 cr17ni14mo2, etc. 
 
The production technology of straight seam welded pipe is simple, with high production efficiency, low cost, rapid development. The strength of the spiral welded pipe is generally higher than that of seam welded pipe. It can use a narrow billet to produce the larger diameter welded pipe. It can also use the same width of billet to produce different-diameter welded pipe. But compared with straight seam pipe of the same length, weld length increased by 30 ~ 100%, and the production rate is lower. 

As a result, a small diameter welded pipe mostly adopts straight seam welding; Large diameter welded pipe mostly adopts spiral welded. (For example, the west-east gas pipeline adopts spiral welded pipe) 

There are a relatively new technology, called "heat reducing mill", that is to roll weld pipe again, for improving the surface quality and dimensional tolerance. This method can make its comprehensive performance close to that of seamless steel pipe.