1. The method of using roller cold bending to push and deform to process round welded tubes or seamless steel tubes into square, rectangular, and other special-shaped tubes, that is, square steel tubes, is a cold bending forming process in the field of metal pressure processing, just like the deformation methods such as stamping, bending, and drawing. According to the power transmission mode of the roller, roller cold bending forming is divided into two types: active and passive. At present, active cold bending units driven by electric motors are widely used at home and abroad, and there are more than 2,000 units in China alone. The annual production capacity of cold-bent steel reaches more than 20 million tons. Most of the units use steel plates as raw materials and are rolled and bent into round tubes or special-shaped open plates. There are relatively few units used to produce special-shaped tubes such as square and rectangular, and there are only more than ten large and medium-sized units in the country. The special-shaped tube forming units using roller passive pushing and round-to-square cold bending technology are even rarer in China. At present, in the wave of increasingly personalized development of domestic and foreign economic and social development, special-shaped steel, as an economic cross-section profile that can more appropriately meet different stress conditions, has attracted more and more attention. To adapt to the development of industrial modernization such as building steel structures, bridges, shipbuilding, locomotives and vehicles, and mechanical processing, the demand for various special-shaped steel pipes, especially large-sized, thick-walled square and rectangular steel pipes, is increasing. Since "911" in the United States in 2000, material experts from all over the world have been working hard to develop building steel structure profiles with small surface area, large specifications, thick walls, high rigidity, high strength, and strong fire resistance. There is a general focus on the research on the performance and production process technology of square and rectangular steel pipes. With an ultra-wide product specification range of 50-500 mm side length and a large adaptability of 4-30 mm wall thickness, it not only fills the gap in this professional field in my country but also becomes the world's largest roller cold-bent round-to-square forming production unit with the largest side length and the thickest wall thickness. Not only does it achieve multi-purpose use of one machine, but the sizing accuracy is higher than the national standard. With the continuous expansion of the application market of square and rectangular pipes,

2. Several theoretical bases for cold bending of closed profiles Closed section profiles are also called hollow profiles. The most commonly used closed profiles are symmetrical cross-section profiles such as round tubes and square steel tubes. From a technical point of view, the cold bending of closed profile sections is a complex deformation process, and there is no complete theory to interpret and analyze it. Often, the deformation conditions and basic parameters can only be determined based on the accumulation of experience. Some experts in the industry have used concepts such as reduction, width, elongation coefficient, and hole type system in the theory of steel rolling deformation to analyze the process of closed profiles such as round to square, but they are not used in actual application. The unrestricted wall thickness direction of closed profiles is the main reason for the difficulty of deformation and force analysis. For this reason, this article can only make a brief analysis from several basic related points: with the introduction of process practice as the main content section.

2.1. The law of constant circumference of the neutral line
The law of constant circumference of the neutral line of section is the starting point and calculation basis of process parameters for deformation analysis, raw material selection, roll design, and finished product quality control in the process of cold bending forming such as circle to square. The cold-bending forming of metal is essentially a process of controlling and changing the curvature of the relevant parts of the metal deformation section to meet the requirements. This process must be accompanied by tensile or compressive deformation of the inner and outer layers of the cross-section. In the part with increased curvature, the outer layer material is under tension and the inner layer is under compression. There must be a layer between the inner and outer layers of the material with a radial stress of 0. These points with 0 radial stress in the section are connected to form the neutral layer of the section. It is a constant circumference layer. The neutral layer of flat steel plates and round tubes is at 1/2 plate thickness. When the tensile and compressive stresses of the deformed part of the section exceed the yield limit of the material and are less than the fracture limit, permanent plastic deformation occurs, achieving the purpose of cold bending. Cold bending practice has proved that as the material is bent and deformed, the neutral layer will shift toward the center of the curved surface.

2.2. Longitudinal micro-extension law
Bending deformation during cold bending is subject to the material's ultimate deformation rate, i.e., the ultimate elongation rate. Otherwise, cracks or breaks will appear at the bend. When the material is bent and deformed, the outermost metal fiber at the corner is subjected to the greatest tensile stress, which is also the most vulnerable layer. The change in this tensile stress increases with the decrease in the bending radius. When setting the round-to-square process, the above formula can be used as the basis for selecting the bending radius of the square tube section corner. In the cold bending industry, the square tube standards of various countries in the world not only restrict the conditions such as the mechanical properties of the material composition but also set the limit on the outer arc radius of the bend according to the material and wall thickness of the pipe. According to the provisions of the national standard GB/T6728-2002 for cold-bent hollow steel for structures in my country.