The Advantages and Disadvantages of Hot Rolling and Cold Rolling

Advantages of hot rolling:

Hot rolling steel can be made into a variety of cross-section forms to meet the needs of service conditions; cold rolling can cause great plastic deformation of the steel, thereby improving the yield point of the steel.

Disadvantages of hot rolling:

1. Although there is no hot plastic compression in the forming process, there is still residual stress in the section, which will inevitably affect the global and local buckling characteristics of the steel;
2. Generally, the cold-rolled section is open section, which makes the free torsion stiffness of the section low. Torsion is easy to occur in bending, bending and torsion buckling are easy to occur in compression, and the torsional performance is poor;
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.

Advantages of cold rolling:

It can destroy the casting structure of ingot, refine the grain size of steel, and eliminate the defects of 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 porosity formed during pouring can also be welded under the action of high temperature and pressure.

Disadvantages of cold rolling:

1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, resulting in delamination. Delamination greatly deteriorates the tensile properties of the 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 that caused by the load.
2. Residual stress caused by uneven cooling. The residual stress is the internal self-equilibrium stress without external force. All kinds of hot-rolled sections have this kind of residual stress. The larger the section size of general section steel, the greater the residual stress. Although the residual stress is self-balanced, it still has some influence on the performance of steel members under external force. Such as deformation, stability, anti-fatigue and other aspects may have adverse effects.

Summary:

The difference between cold rolling and hot rolling is mainly the temperature of rolling process. “Cold” means normal temperature and “hot” means high temperature.

From the viewpoint of metallography, the boundary between cold rolling and hot rolling should be distinguished by recrystallization temperature. That is to say, rolling below recrystallization temperature is cold rolling, and rolling higher than recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450 ~ 600 ℃.

The main differences between hot rolling and cold rolling are as follows:

1. Appearance and surface quality: since the cold plate is obtained after the cold rolling process of the hot plate, and some surface finishing is also carried out during cold rolling, the surface quality (such as surface roughness) of the cold plate is better than that of the hot plate. Therefore, if the coating quality of the product’s subsequent painting is higher, the cold plate is generally selected, and the hot plate is divided into acid pickling plate, pickling plate and pickling plate The surface is normal metal color due to acid pickling, but it is not cold-rolled, so the surface is not as high as the cold plate. The surface of the non-pickling plate usually has an oxide layer, blackening, or ferric oxide black layer. Generally speaking, it looks like the fire has been roasted, and if the storage environment is not good, it usually brings a little rust.
2. Performance: Generally speaking, the mechanical properties of hot plate and cold plate are considered to be no difference in engineering. Although the cold plate has certain work hardening in the cold rolling process (but it does not exclude the strict requirements on mechanical properties, it needs to be treated differently). The yield strength and surface hardness of the cold plate are slightly higher than those of hot plate. The specific way depends on the annealing degree of the cold plate. However, the strength of annealed cold plate is higher than that of hot plate.
3. Formability because the performance of cold and hot plate is not too bad, so the influencing factors of formability depend on the difference of surface quality. Because the surface quality is better from the cold plate, generally speaking, the forming effect of cold plate is better than that of hot plate.