Cold rolled seamless pipes
Cold rolled seamless pipes（GB3639-2000）are cold drawn or cold rolled precision
seamless steel pipes with high accuracy of the dimension and good finish of the surface used for the machinery and hydraulic equipment. The cold rolled process refers to the method adopted to process the steel plates or strips into various types of steel products by cold working processes such as the cold bend and cold drawn processes. Cold rolled seamless pipes not only include general steel pipes, low and medium pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking steel pipes, but also include carbon steel pipes with thin walls, alloy steel pipes with thin walls, stainless steel pipes with thin walls and special-shaped steel pipes. The hot rolled seamless pipes usually have outer diameter larger than 32mm and wall thickness between 2.5mm to 75mm; the cold rolled seamless pipes usually have the outer diameter less than 6mm and wall thickness less than 0.25 mm; the thin-wall pipes usually have the outer diameter less than 5mm and wall thickness less than 0.25mm. The cold rolled pipes generally have the high dimensional accuracy than the hot rolled pipes.
The differences in techniques
1. Buckling of parts on sections is allowed for cold rolled steel so as to make the best of the bearing capacity of bars after buckling, while buckling of parts on sections is not allowed for hot rolled steel.
2. There are different reasons for the residual stress of hot rolled steel and cold rolled steel produced. Therefore, distribution of the residual stress on sections has great differences too. The distribution of residual stress on the sections of cold bend thin-wall steel is the flexural type, while the distribution of residual stress on the sections of hot rolled steel or welded steel is the thin-film type.
3. The free torsion stiffness of hot rolled steel is higher than that of cold rolled steel, so the torsion resistance performance of hot rolled steel is better than that of cold rolled steel.
Advantages and disadvantages
Advantages
1. The speed of molding and the productivity are high. The coating will not be damaged. The sections can be made in various shapes so as to adapt to different conditions. Cold rolled processes can make steel have great plastic deformation so as to increase the yield point of steel.
2. Cold rolled processes can destroy the casting structure of steel ingots, refine the grain of steel products and eliminate the defects of microstructure so as to make the structure of steel products compact and improve the mechanical property of steel products. This kind of improvement reflects mainly along the rolling direction, which makes steel products not isotropic bodies any longer to some extent. Bubbles, cracks and shrinkage formed in casting can be repaired under the effects of high temperatures and pressure.
Disadvantages
1. The section of cold rolled steel is the open section generally, which has relatively low free torsion stiffness. The steel is twisted easily when it is bent; flexural-torsional buckling showed up easily when steel is pressed. The torsion resistance is relatively poor.
2. The wall of the cold rolled steel pipe is relatively thin. The corners connecting plates are not thickened. The bearing ability of the partial concentrated load is weak.
