Cold rolling and hot rolling are both processes for forming steel sections or steel plates. They have a great influence on the structure and properties of steel. Steel rolling is mainly hot rolling, and cold rolling is only used to produce small steel sections and metal sheets.
1. Cold rolling
It refers to the processing of steel plates or steel strips into various types of steel through cold processing such as cold drawing, cold bending, and cold drawing at room temperature.
Advantages: fast forming speed, high output, and no damage to the coating. It can be made into a variety of cross-sectional forms to meet the needs of use conditions; cold rolling can cause large plastic deformation of steel, thereby increasing the yield point of steel.
Disadvantages:
1. Although there is no hot plastic compression during the forming process, there are still residual stresses in the cross section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The cold-rolled steel section style is generally an open section, which makes the free torsional stiffness of the section low. It is easy to torsion when bent, and it is easy to bend-torsion buckling when compressed, and the torsion resistance is poor;
3. The wall thickness of cold-rolled steel is small, and there is no thickening at the corners where the plates are connected, so the ability to withstand local concentrated loads is weak.
2. Hot rolling
Advantages:
It can destroy the casting structure of the steel ingot, refine the grains of the steel, and eliminate the defects of the microstructure, so that the steel structure is dense and the mechanical properties are improved. 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 pouring can also be welded under high temperature and pressure.
Disadvantages:
1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and stratification occurs. Stratification greatly deteriorates the tensile performance of the steel along the thickness direction, and interlayer tearing may occur when the weld shrinks. The local strain induced by weld shrinkage 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-balanced stress without external force. Hot-rolled steel sections of various cross sections have this type of residual stress. Generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although residual stress is self-balanced, it still has a certain impact on the performance of steel components under external forces, such as deformation, stability, fatigue resistance, etc.
3. The main differences between hot rolling and cold rolling are:
1. Cold-rolled steel allows local buckling of the cross section, so that the bearing capacity after the bar buckling can be fully utilized; while hot-rolled steel does not allow local buckling of the cross section.
2. The causes of residual stress in hot-rolled steel and cold-rolled steel are different, so the distribution on the cross section is also very different. The residual stress distribution on the cross section of cold-bent thin-walled steel is bending type, while the residual stress distribution on the cross section of hot-rolled steel or welded steel is film type.
3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional performance of hot-rolled steel is better than that of cold-rolled steel.
