Why is hot rolled steel different from cold rolled steel?

As we all know, hot rolling and cold rolling are processes of forming section steel or steel plate, which have a great impact on the structure and properties of steel. Steel rolling is mainly hot rolling, while cold rolling is only used to produce small section steel and thin plate. Now, let's introduce their differences and advantages and disadvantages!

Differences between cold rolled steel bars and hot rolled steel bars:

Hot rolled steel bars are directly processed on the steel mold in the steel furnace in the iron and steel processing plant. That is, the hot (so called "hot rolling") finished products come out of the furnace and can be used after cooling.
Cold rolled steel bars are made by cold processing hot rolled steel bars, such as cold drawing and drawing steel bars at room temperature.
The yield strength of hot-rolled steel bars is low and the plasticity is good.
Cold rolled steel bars have high yield strength and poor plasticity. Both have the same ultimate tensile and compressive strength.
1. The cold-rolled formed steel allows local buckling of the section, so that the bearing capacity of the bar after buckling can be fully utilized; However, local buckling is not allowed for the section of hot rolled steel.
2. The causes of residual stresses in hot rolled and cold rolled steel sections are different, so the distribution on the section is also very different. The residual stress distribution on the cold-formed thin-walled steel section is bending type, while that on the hot rolled steel section or welded steel section is membrane type.
3. The free torsion stiffness of hot rolled section steel is higher than that of cold rolled section steel, so the torsion resistance of hot rolled section steel is better than that of cold rolled section steel.

Advantages and disadvantages of hot rolling:

Advantages: It can destroy the casting structure of the ingot, refine the grain 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; Bubbles, cracks and looseness formed during pouring can also be welded under high temperature and pressure.
Disadvantages: 1. After hot rolling, non-metallic inclusions (mainly sulfide and oxide, as well as silicate) inside the steel are pressed into thin sheets, resulting in delamination (interlayer). Delamination greatly deteriorates the tensile properties of steel along the thickness direction, and may cause interlaminar tearing when the weld shrinks. The local strain induced by weld shrinkage often reaches several times of the yield point strain, which is much larger than the strain caused by 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 sections have this kind of residual stress. Generally, the larger the section size of steel section, the greater the residual stress. Although the residual stress is self phase equilibrium, it has some influence on the performance of steel members under external forces. For example, it may have adverse effects on deformation, stability and fatigue resistance.
Advantages and disadvantages of cold rolling:
Advantages: fast molding speed, high output, no damage to the coating, and can be made into a variety of cross section forms to meet the needs of the use conditions; Cold rolling can make steel produce great plastic deformation, thus improving the yield point of steel.

Disadvantages:

1. Although there is no thermal plastic compression in 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. Cold rolled section steel is generally open section, which makes the free torsional stiffness of the section low. Torsion is easy to occur in bending, and flexural torsional buckling is easy to occur in compression, with poor torsional resistance;
3. The wall thickness of cold-rolled formed steel is small, and there is no thickening at the corner of plate connection, so the ability to bear local concentrated load is weak.

Keywords: hot rolling mill, cold rolling mill