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The difference between hot rolled and cold rolled seamless steel pipes

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The difference between hot rolled and cold rolled seamless steel pipes

October 26
12:52 2020
What is the difference between hot-rolled seamless steel pipe and cold-rolled seamless steel pipe? Are ordinary seamless steel pipes hot-rolled seamless steel pipes?

Cold-rolled seamless steel pipes are usually smaller in diameter, while hot-rolled seamless steel pipes are usually larger in diameter. The accuracy 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.

Due to different manufacturing processes, seamless steel tubes are divided into hot-rolled (extruded) seamless steel tubes and cold-drawn (rolled) seamless steel tubes. Cold drawn (rolled) tubes are divided into round tubes and special-shaped tubes.

1) Different hot-rolled seamless pipes are divided into ordinary steel pipes, medium and low 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 (turntable) seamless steel pipes are divided into ordinary steel pipes, medium and low pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes and other steel pipes, as well as carbon thin-wall steel pipes, alloy thin-wall steel pipes, and stainless steel thin-wall steel pipes. 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 maximum outer diameter of the thin-walled tube is 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 steel sections allow local buckling of the sections, which can make full use of the bearing capacity of the steel bars after buckling; while hot-rolled steels do not allow local buckling of the sections.

2. Hot-rolled steel and cold-rolled steel have different causes of residual stress, so the distribution on the section is also very different. The residual stress distribution on the cross-section of cold-formed thin-walled steel is curved, while the residual stress distribution on the cross-section of hot-rolled steel or welded steel is a thin film.

3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional strength of hot-rolled steel is better than that of cold-rolled steel.

4) Different advantages and disadvantages Cold-rolled seamless pipe refers to steel plates or strips processed into various types of steel plates by cold drawing, cold bending, and cold drawing at room temperature.
Advantages: fast forming speed, high output, without damaging the coating, can be made into various cross-sectional forms to meet the needs of use conditions; cold rolling will cause large plastic deformation of steel, thereby increasing the yield rate of steel points.

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 style of cold-rolled steel is usually an open section, which will Reduce the free torsional stiffness of the profile. It is prone to torsion during bending, and it is prone to bending and torsional buckling during compression, resulting in poor torsion resistance. 3. The wall thickness of cold-rolled steel is small, the corners of the plate joints are not thickened, and the ability to withstand local concentrated loads is very weak.

Hot-rolled seamless pipe is opposite to cold-rolled seamless pipe. The rolling temperature of cold-rolled seamless pipe is lower than the recrystallization temperature, while the rolling temperature of hot-rolled seamless pipe is higher than the recrystallization temperature.

Advantages: It can destroy the cast structure of the steel 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 the steel is no longer isotropic to a certain extent. It can also weld bubbles, cracks and looseness formed during the casting process under 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 (intermediate layer) occurs. Delamination will greatly reduce the tensile properties of the steel in the thickness direction, and interlaminar tearing may occur when the weld shrinks. The local strain caused by weld shrinkage usually 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 internal self-balancing stress without external force. Hot-rolled steel sections of various cross-sections have this residual stress. Generally, the larger the size of the steel section, the greater the residual stress. Although the residual stress is self-balancing, it still has a certain effect on the performance of steel parts under the action of external force. For example, it may adversely affect deformation, stability and fatigue resistance.

3. The thickness and edge width of hot rolled steel products are not easy to control. We are familiar with thermal expansion and contraction. Because at the beginning, even if the length and thickness meet the standard, there will be a certain negative difference after the final cooling. The greater the negative difference, the thicker the thickness and the more obvious the performance.

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