Both hot rolling and cold rolling are steel plate or profile forming processes, they have a great impact on the structure and properties of steel.

Steel rolling is mainly hot rolling, cold rolling is usually only used to produce small steel and sheet steel and other precision size steel.

Common cold and hot rolling of steel:

Wire: 5.5-40 mm in diameter, coils, all hot rolled. After cold drawing, it belongs to cold drawing material.

Round steel: In addition to the precision of the size of the bright material is generally hot rolled, but also forged (surface traces of forging).

Strip steel: hot rolled cold rolled, cold rolled generally thin.

Steel plate: cold rolled plate is generally thin, such as automobile plate; Hot rolling medium thick plate more, and cold rolling similar thickness, appearance is obviously different.

Angle steel: all hot rolled.

Steel tube: welded hot rolled and cold drawn.

Channel and H beam: hot rolled.

Steel bar: hot rolled material.

Hot rolled

By definition, steel ingot or billet is difficult to deform and process at room temperature. It is generally heated to 1100 ~ 1250℃ for rolling. This rolling process is called hot rolling.

The termination temperature of hot rolling is generally 800 ~ 900℃, and then it is generally cooled in the air, so the hot rolling state is equivalent to normalizing treatment.

Most steel is rolled by hot rolling. Hot rolled steel, due to the high temperature, the surface of the formation of a layer of oxide sheet, thus has a certain corrosion resistance, can be stored in the open air.

However, this layer of oxide iron also makes the surface of hot rolled steel rough and the size fluctuates greatly, so the steel with smooth surface, accurate size and good mechanical properties should be used as raw material and then cold rolled.

Advantages:

Forming speed, high yield, and do not damage the coating, can be made into a variety of cross section forms, to meet the needs of the use conditions; Cold rolling can produce large plastic deformation of steel, thus raising the yield point of steel.

Disadvantages:

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 overall and local buckling properties of steel;

2. The cold-rolled section is generally open section, which makes the free torsion stiffness of the section lower. It is easy to twist when it is bent, and it is easy to bend and twist when it is pressed, and the torsion resistance is poor.

3. The wall thickness of cold-rolled shaped steel is small, and there is no thickening at the corner where the plate connects, so it has weak ability to bear local concentrated load.

Cold rolled

Cold rolling refers to the rolling method of changing the shape of the steel by squeezing the steel under the pressure of the roller at room temperature. It is called cold rolling, although the process also heats up the steel. To be more specific, cold rolling uses hot rolled steel coils as raw materials, which are processed under pressure after acid pickling to remove the oxide scale, and the finished products are rolled hard coils.

Generally cold rolled steel such as galvanized, color steel plate must be annealed, so the plasticity and elongation is also good, widely used in automobile, home appliances, hardware and other industries. The surface of cold-rolled plate has a certain degree of smoothness, and the hand feels smooth, mainly because of pickling. The surface finish of the hot rolled plate can not meet the requirements, so the hot rolled steel strip needs to be cold rolled, and the thickness of the hot rolled steel strip is generally 1.0mm, and the cold rolled steel strip can reach 0.1mm. Hot rolling is rolling above the crystallization temperature point, cold rolling is rolling below the crystallization temperature point.

The change of steel shape caused by cold rolling belongs to continuous cold deformation. The cold hardening caused by this process increases the strength and hardness of rolled hard coil and decreases the toughness and plastic index.

For end use, cold rolling deteriorates stamping performance and the product is suitable for parts that are simply deformed.

Advantages:

It can destroy the casting structure of steel ingot, refine the grain size of steel, and eliminate the defects of microstructure, so that the steel structure is compacted 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 casting 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) in the steel are laminated and layered. Delamination greatly deteriorates the tensile properties of steel along the thickness direction and may cause interlaminar tearing during weld shrinkage. The local strain induced by weld shrinkage is often several times of yield point strain, which is much larger than that caused by load.

2. Residual stress caused by uneven cooling. The residual stress is the internal self-phase equilibrium stress without external force. All kinds of hot rolled section steel have this kind of residual stress. The larger the section size of general section steel is, the greater the residual stress is. Although the residual stress is self-phase equilibrium, it has a certain effect on the performance of steel member under external force. Such as deformation, stability, fatigue resistance and other aspects may have adverse effects.

Conclusion:

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

From the metallic point of view, the boundary between cold rolling and hot rolling should be distinguished by recrystallization temperature. That is, the rolling below the recrystallization temperature is cold rolling, and the rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450 ~ 600℃.