Why are we divided into hot-rolled steel and cold-rolled steel? What is the difference?

Hot rolling and cold rolling are both processes for forming steel plates or profiles, and they have a great impact on the structure and properties of steel.

The rolling of steel is mainly hot rolling, while cold rolling is usually only used to produce steel products with precise dimensions such as small steel sections and thin plates.

Keywords: hot rolling, cold rolling

Why are we divided into hot-rolled steel and cold-rolled steel What is the difference

Common cold and hot rolling situations of steel:

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

Round steel: In addition to bright steel with precise dimensions, it is generally hot-rolled, and there are also forged steel (with forging marks on the surface).

Strip steel: both hot-rolled and cold-rolled, and cold-rolled products are generally thinner.

Steel plates: Cold-rolled plates are generally thinner, such as automotive plates; there are more hot-rolled medium-thick plates, some with similar thickness to cold-rolled plates, and their appearance is obviously different. ​

Angle steel: all hot rolled. ​

Steel pipe: both welded, hot rolled and cold drawn. ​

Channel steel and H-shaped steel: hot rolled. ​

Rebar: hot rolled steel.

hot rolled

By definition, steel ingots or billets are difficult to deform and process at room temperature. They are generally heated to 1100 to 1250°C for rolling. This rolling process is called hot rolling.

The termination temperature of hot rolling is generally 800 to 900°C, and is generally cooled in the air afterwards, so the hot rolling state is equivalent to normalizing treatment.

Most steel products are rolled using hot rolling methods. Steel delivered in a hot-rolled state has a layer of iron oxide scale formed on its surface due to the high temperature, so it has a certain degree of corrosion resistance and can be stored in the open air.

However, this layer of iron oxide scale also makes the surface of hot-rolled steel rough and the dimensions fluctuate greatly. Therefore, steel with smooth surface, precise dimensions and good mechanical properties must be produced with hot-rolled semi-finished products or finished products as raw materials and then cold-rolled.

Advantages: fast molding speed, high output, and no damage to the coating. It can be made into a variety of cross-section forms to adapt to the needs of use conditions; cold rolling can cause large plastic deformation of steel. Thereby increasing the yield point of steel.

shortcoming:

1. Although there is no hot plastic compression during the forming process, there are still residual stresses in the section, which will inevitably affect the overall and local buckling characteristics of the steel;

2. Cold-rolled steel sections generally have open sections, which results in lower free torsional stiffness of the section. It is prone to torsion when subjected to bending and torsional buckling when subjected to pressure, and its torsional resistance is poor;

3. The wall thickness of cold-rolled formed steel is small, and there is no thickening at the corners where the plates are connected, so the ability to withstand localized concentrated loads is weak.

Cold rolled

Cold rolling refers to a rolling method that uses the pressure of rollers to squeeze steel at room temperature and change the shape of the steel. Although the process also heats up the steel plate, it is still called cold rolling. Specifically, cold rolling uses hot-rolled steel coils as raw materials, which are pickled to remove scale and then pressure processed. The finished product is hard rolled coils.

Generally, cold-rolled steel such as galvanized and color steel plates must be annealed, so their plasticity and elongation are also good, and they are widely used in automobiles, home appliances, hardware and other industries. The surface of cold-rolled plates has a certain smoothness and feels smooth to the touch, mainly due to pickling. The surface finish of hot-rolled plates generally does not meet the requirements, so hot-rolled steel strips need to be cold-rolled. The thinnest thickness of hot-rolled steel strips is generally 1.0mm, and cold rolling can reach 0.1mm. Hot rolling is rolling above the crystallization temperature point, and cold rolling is rolling below the crystallization temperature point.

The change in the shape of the steel material caused by cold rolling is continuous cold deformation. The cold work hardening caused by this process increases the strength and hardness of the hard rolled coil and decreases the toughness and plasticity index.

For end use, cold rolling deteriorates the stamping performance and the product is suitable for parts with simple deformation.

advantage:

It can destroy the casting structure of the steel ingot, refine the grains of the steel, and eliminate defects in the microstructure, thereby making the steel structure dense and improving the mechanical properties. 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 the action of high temperature and pressure.

shortcoming:

1. After hot rolling, the non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are pressed into thin sheets, causing delamination. Delamination greatly deteriorates the tensile properties of the steel along the thickness direction and may cause interlaminar tearing as 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 load;

2. Residual stress caused by uneven cooling. Residual stress is the internal self-balanced stress in the absence of external force. Hot-rolled steel sections of various sections have such residual stress. Generally, the larger the cross-section size of the section steel, the greater the residual stress. Although residual stress is self-balanced, it still has a certain impact on the performance of steel components under the action of external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.

Summarize:

The difference between cold rolling and hot rolling is mainly the temperature of the rolling process. “Cold” means normal temperature, and “hot” means high temperature. From a metallurgical point of view, the boundary between cold rolling and hot rolling should be distinguished by the recrystallization temperature. That is, rolling below the recrystallization temperature is cold rolling, and rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450~600℃.

The main differences between hot rolling and cold rolling are:

1. Appearance and surface quality:

Because the cold plate is obtained after the cold rolling process of the hot plate. Moreover, cold rolling also undergoes some surface finishing, so the surface quality of the cold plate (such as surface roughness, etc.) is better than that of the hot plate. Therefore, if there are higher requirements for the coating quality such as subsequent painting of the product, Generally choose cold plate. In addition, the hot plate is divided into pickling plate and unpickled plate. The surface of the pickled plate has a normal metallic color because it has been pickled, but it has not been cold rolled, so the surface is still not as high as the cold plate. The surface of the unpickled plate usually has an oxide layer, becomes black, or there is a black layer of Fe3O4. In layman’s terms, it looks like it has been roasted by fire, and if the storage environment is not good, it will usually have a little rust.

2. Performance: Generally speaking, there is no difference in the mechanical properties of hot plates and cold plates in engineering. Although the cold plates have certain work hardening during the cold rolling process, (but this does not rule out situations where strict mechanical performance requirements are required) , then they need to be treated differently). The yield strength of the cold plate is usually slightly higher than that of the hot plate, and the surface hardness is also higher. The specific situation depends on the degree of annealing of the cold plate. But no matter how annealed, the strength of the cold plate is higher than that of the hot plate.

3. Forming performance: Since the performance of hot and cold plates are basically similar, the influencing factors of formability depend on the difference in surface quality. Since the surface quality of cold plates is better, generally speaking, steel plates of the same material , the forming effect of the cold plate is better than that of the hot plate.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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