According to the different shape of pipe cross-sectional area, the seamless steel tube can be divided into circular pipes and special pipes. Because when the circumference is same, area of round is the largest, so using circular pipes can convey more fluid. In addition, when bearing internal or external radial stress, the circular section will bear uniform stress; therefore, most of the steel tube is pipe. However, the circular tube also has some limitations, such as under the conditions of plane bending, the circular tube has smaller bending strength than that of rectangular tube. So according to different uses, there are also special pipes with special section shape.

Special pipe is the general name of tubes except for circular tube. According to the steel tube cross-section shape and size, they can be divided into seamless steel tube with equal wall thickness, seamless steel tube with unequal wall thickness and seamless steel tube with changeable diameter. Special seamless steel tubes are widely used in various structural parts, tools and machinery parts. Compared with the circular pipes, special seamless steel tubes generally have the larger moment of inertia and modulus of section, which can greatly reduce the structural weight and save steel.