Steel having a hollow section whose length is much greater than its diameter or circumference. According to the section shape is divided into round, square, rectangular and shaped steel pipe; According to the material is divided into carbon structural steel pipe, low alloy structural steel pipe, alloy steel pipe and composite steel pipe; According to the use, it is divided into pipeline, engineering structure, thermal equipment, petrochemical industry, machinery manufacturing, geological drilling, high pressure equipment with steel pipes, etc. According to the production process, it is divided into seamless steel pipe and welded steel pipe, of which seamless steel pipe is divided into hot rolled and cold rolled (drawn) two kinds, welded steel pipe is divided into straight welded steel pipe and spiral welded steel pipe. Steel pipes are not only used to transport fluids and powdery solids