The difference between hot rolled and cold rolled steel pipe
The difference between hot rolled and cold rolled steel pipe mainly depends on the temperature of rolling process. If it is over the recrystallization temperature, this process is called hot rolled; while if it is below the recrystallization temperature, this process is called cold rolled.
Hot rolled (extruded) seamless steel tube : round solid billet → heating → perforation → three-high cross rolling, continuous rolling or extrusion → tube stripping → sizing (or reducing) → cooling →billet tube→ straightening → hydraulic test (or flaw detection) → marking → storage.
Cold rolled (drawn) seamless steel tube: round solid billet → heating → perforation → heading → annealing → pickling → oiling (copper plating) → multi-pass cold drawn (cold rolled) → billet tube→ heat treatment → straightening → hydrostatic test (flaw detection) → marking → storage.
Products under different processes have different characteristics.
Hot Rolled Seamless Steel Pipe
Advantages: it can destroy the casting structure of ingot, refine the grain of steel, and eliminate the defects of microstructure, so that the steel structure is compact and mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to some extent; Bubbles, cracks and porosity formed during pouring can also be welded together under high temperature and pressure.
Disadvantages: After hot rolling, the non-metallic inclusions (mainly sulfide, oxide, as well as silicate) inside the steel are pressed into thin sheets, resulting in lamination (interlayer). Lamination greatly degrades the tensile properties of the steel along the thickness direction and may lead to interlayer tearing during weld shrinkage.The local strain induced by weld shrinkage often reaches several times of the yield point strain, which is much larger than the strain induced by load. Residual stress caused by uneven cooling, is the internal self-phase equilibrium stress under the action of no external force. The hot rolled steel section of all sections have this kind of residual stress, the larger the section size of the general section, the larger the residual stress. Although the residual stress is self-phase equilibrium, it has a certain effect on the performance of steel members under the action of external forces. For example, it may produce adverse effects on deformation, stability, anti- fatigue and other aspects. For hot-rolled steel products, it is difficult to control the thickness and side width.We are familiar with thermal expansion and cold contraction. Even if the length and thickness of the hot rolling reach the standard at the beginning, there will still be a certain negative difference after cooling. The wider the negative difference is, the thicker the thickness will be. So for large steel, it can not be too accurate in steel side width, thickness, length, angle, and edge line.
Cold Rolled Seamless Steel Pipe
Advantages: Fast molding speed, high yield, and no damage to the coating. It can be made into a variety of cross section forms, to meet the needs of the usage conditions. Cold rolling can produce great plastic deformation of steel, thus raising the yield point of steel.
Disadvantages: Although there is no thermal plastic compression during the forming process, residual stress still exists in the section, which will inevitably affect the overall and local buckling characteristics of the steel. Cold rolled steel section is generally open section, so that the section of the free torsional stiffness is low. Torsion is easy to occur in bending, bending and torsional buckling is easy to occur in compression, and the torsional resistance is poor. The cold rolled section steel wall thickness is smaller, and there is no thickening at the corner of the plate connection, so the ability to bear local concentrated load is weak.
- Dimension accuracy: cold rolled steel pipe has high accuracy in dimension;
- Appearance: The surface of cold-rolled steel pipe is bright, while the surface of hot-rolled steel tube has obvious oxidation skin or red rust;
- Diameter: The diameter of cold-rolled steel pipe is smaller than that of hot-rolled steel pipe (Diameter of hot rolled steel pipe is larger than 32mm, and wall thickness is between 2.5-75mm; while diameter of cold rolled steel pipe can be 5mm, and wall thickness can be less than 0.25mm);
- Price: cold rolled steel tube is 1000-2000 more expensive than hot rolled steel pipe per ton;
- Use: Hot rolled steel pipe is used in the field in which dimension is not so precise, such as fluid transport and mechanical structure; while cold rolled steel pipe is used in precision instruments, such as hydraulic systems, pneumatic…
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