Steel with a hollow cross-section whose length is much greater than the diameter or circumference. Divided into circular, square, rectangular, and irregular steel pipes according to their cross-sectional shapes; Divided by material into carbon structural steel pipes, low alloy structural steel pipes, alloy steel pipes, and composite steel pipes; Classified by purpose, it is used for conveying pipelines, engineering structures, thermal equipment, petrochemical industry, mechanical manufacturing, geological drilling, and steel pipes for high-pressure equipment; According to production processes, seamless steel pipes are divided into seamless steel pipes and welded steel pipes. Seamless steel pipes are further divided into hot-rolled and cold-rolled (drawn) types, and welded steel pipes are further divided into straight seam welded steel pipes and spiral seam welded steel pipes.