What is the difference between hot rolled seamless steel pipe and cold rolled seamless steel pipe?Is the usual seamless steel pipe a hot-rolled seamless steel pipe?
Cold-rolled seamless steel pipes are usually small diameter, and hot-rolled seamless steel pipes are usually large diameter.The precision of cold-rolled seamless steel pipe is higher than that of hot-rolled seamless steel pipe, and the price is also higher than that of hot-rolled seamless steel pipe.
Seamless steel pipes are divided into hot-rolled (extruded) seamless steel pipes and cold-drawn (rolled) seamless steel pipes due to their different manufacturing processes.Cold drawn (rolled) tubes are divided into round tubes and special-shaped tubes.
1) Hot-rolled seamless pipes for different purposes are divided into ordinary steel pipes, low and medium pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological steel pipes and other steel pipes.Cold-rolled (dial) seamless steel pipes are divided into ordinary steel pipes, low and medium pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, oil cracking pipes and other steel pipes, as well as carbon thin-walled steel pipes, alloy thin-walled steel pipes, and stainless steel thin-walled steel pipes. Steel pipe, special-shaped steel pipe.
2) The outer diameter of hot-rolled seamless pipes of different sizes is generally greater than 32mm, and the wall thickness is 2.5-75mm.The diameter of cold-rolled seamless pipe can reach 6mm, and the wall thickness can reach 0.25mm.The outer diameter of the thin-walled tube can reach 5mm, and the wall thickness is less than 0.25mm.Cold rolling has higher dimensional accuracy than hot rolling.
3) Process differences 1. Cold-rolled forming steel allows local buckling of the section, which can make full use of the bearing capacity of the bar after buckling; while hot-rolled steel does not allow local buckling of the section.
2. The reasons for the residual stress of hot-rolled steel and cold-rolled steel are different, so the distribution on the cross-section is also very different.The residual stress distribution of cold-formed thin-walled steel sections is curved, while the residual stress distribution of hot-rolled or welded steel sections is film-like.
3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional resistance of hot-rolled steel is better than that of cold-rolled steel.
4) Different advantages and disadvantages Cold-rolled seamless pipes refer to steel sheets or steel strips that are processed into various types of steel by cold-drawing, cold-bending, and cold-drawing at room temperature.
Advantages: The forming speed is fast, the output is high, and the coating is damaged, and it can be made into a variety of cross-sectional forms to meet the needs of the use conditions; cold rolling can cause large plastic deformation of the steel, thereby increasing the yield strength of the steel point.
Disadvantages: 1. Although there is no thermoplastic compression during the forming process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel; 2. The cold-rolled section steel is generally an open section, which makes the free torsional stiffness of the section low. .It is easy to twist when bending, easy to bend and twist when it is compressed, and has poor torsional resistance; 3. The wall thickness of cold-rolled steel is small, and the connecting corners of the plates are not thickened, so the ability to bear local concentrated loads is weak.
Hot-rolled seamless pipes are relative to cold-rolled seamless pipes.Cold-rolled seamless pipes are rolled below the recrystallization temperature, and hot-rolled seamless pipes are rolled above the recrystallization temperature.
Advantages: It can destroy the casting structure of the ingot, refine the grain of the steel, eliminate the defects of the structure, make the steel structure dense, and improve the mechanical properties.This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; the bubbles, cracks and looseness formed during the casting process can also be welded at high temperature and high pressure.
Disadvantages: 1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, and silicates) inside the steel are pressed into thin sheets, and delamination (interlayer) occurs.Delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and interlaminar tearing may occur when the weld shrinks.The local strain caused by the shrinkage of the weld often reaches several times the yield point strain, which is much larger than the strain caused by the load;
2. Residual stress caused by uneven cooling.Residual stress is the internal self-equilibrium stress without external force.Hot-rolled sections of various cross-sections have such residual stresses.Generally, the larger the section size of the steel profile, the greater the residual stress.Although the residual stress is self-balancing, it still has a certain influence on the performance of the steel member under the action of external force.For example, it can adversely affect deformation, stability and fatigue resistance.
3. Hot-rolled steel products are not easy to control in terms of thickness and side width.We are familiar with thermal expansion and contraction.Because at the beginning, even if the length and thickness are up to the standard, there will be a certain negative difference after the final cooling.The larger the negative difference, the thicker the thickness and the more obvious the performance.