Steel pipes have hollow cross-sections, and their length is much greater than the diameter or circumference of the steel. According to the cross-sectional shape, they are divided into round, square, rectangular and special-shaped steel pipes; according to the material, they are divided into carbon structural steel pipes, low alloy structural steel pipes, alloy steel pipes and composite steel pipes; according to the purpose, they are divided into steel pipes for pipelines, engineering structures, thermal equipment, petrochemical industry, machinery manufacturing, geological drilling, high-pressure equipment, etc.; according to the production process, they are divided into seamless steel pipes and welded steel pipes.