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　　Seamless steel pipe has hollow section, which is widely used to transport fluid, such as oil, natural gas, gas, water and some solid materials. Compared with solid steel such as round steel, steel pipe has the same bending and torsion strength and lighter weight. It is a kind of economic section steel. It is widely used in manufacturing structural parts and mechanical parts, such as petroleum drill pipe, automobile transmission shaft, bicycle frame and steel scaffold used in construction. It can improve the utilization rate of materials and simplify the manufacturing process, Saving materials and processing time, such as rolling bearing ring, jack sleeve, etc., has been widely used in steel pipe manufacturing. Steel tubes are also indispensable materials for all kinds of conventional weapons. Gun barrels and barrels are made of steel tubes. According to the shape of cross-sectional area, steel pipes can be divided into round pipe and special-shaped pipe. Because the area of the circle is the largest under the condition of equal perimeter, more fluid can be transported by the circular tube. In addition, when the ring section bears internal or external radial pressure, the force is more uniform, so most steel pipes are round.

　　However, the circular tube also has some limitations. For example, under the condition of plane bending, the bending strength of the circular tube is not as strong as that of the square or rectangular tube, and the square or rectangular tube is commonly used in some agricultural machinery skeletons, steel and wood furniture. Special shaped steel pipes with other cross-section shapes are needed according to different uses.

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