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What are the differences and advantages of hot-rolled steel and cold-rolled steel?

Update:11-03-2021
Summary:

Both hot rolling and cold rolling are processes for for […]

Both hot rolling and cold rolling are processes for forming steel plates or profiles, and they have a great influence on the structure and properties of steel. The rolling of steel is mainly hot rolling, and cold rolling is usually only used for the production of small-sized steel and sheet steel with precision dimensions.

Common steel rolling process
Wire: coiled, all hot-rolled
Round steel: In addition to precision bright materials, they are generally hot-rolled, and forged materials are also available.
Strip steel: both hot-rolled and cold-rolled, cold-rolled products are generally thin
Steel plate: thinner steel plate, such as automotive plate
Angle steel: all hot rolled
Steel tube: Welded hot rolled and cold drawn both
Channel steel: hot rolled
H-beam: hot rolled
Rebar: Hot rolled
By definition, steel ingots or die steels are difficult to deform and process at room temperature. They are generally heated to 1100-1250°C for rolling. This rolling process is called hot rolling. The termination temperature of hot rolling is generally 800-900°C, and then it is generally cooled in air, so the hot rolling state is equivalent to normalizing treatment.

Most steels are rolled by hot rolling. The steel delivered in the hot-rolled state has a certain degree of corrosion resistance due to the high temperature, so it can be stored in the open air. However, this layer of oxide scale also makes the hot-rolled steel surface rough and the size fluctuates greatly. Therefore, it is required to use hot-rolled semi-finished products or finished products as raw materials and then cold-rolled production of steels with smooth surface, accurate size and good mechanical properties.

Advantages of hot rolling
It can destroy the casting structure of the steel 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. The bubbles, cracks and porosity formed during pouring can also be welded under the action of high temperature and pressure.

After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and delamination occurs. Delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and it is possible that 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.

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 such residual stresses. Generally, the larger the section size of the steel section, the larger the residual stress. Although the residual and stress are self-balanced, they still have a certain impact on the performance of steel components under external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.

Advantages of cold rolling
The molding speed is fast, the output is high, and the coating is not damaged, and it can be made into a variety of cross-sectional forms to meet the needs of the use conditions.

Cold rolling can cause large plastic deformation of steel, thereby increasing the yield point of steel.
Although there is no thermal 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;

The cold-rolled section steel style is generally an open section, so that the free torsional stiffness of the section is low. It is prone to torsion when under bending, and it is prone to bending and torsion buckling when under pressure, and its torsion resistance is poor;
The wall thickness of cold-rolled steel is small, and there is no thickening at the corners where the plates are joined, and the ability to withstand local concentrated loads is weak. Cold rolling Cold rolling refers to a rolling method that uses the pressure of rolls to squeeze steel to change the shape of the steel at room temperature. Although the processing process also heats up the steel plate, it is still called cold rolling. Specifically, hot-rolled steel coils for cold rolling are used as raw materials, which are subjected to pressure processing after pickling to remove oxide scale, and the finished products are hard-rolled coils.

Generally, cold-rolled steels such as galvanized and color steel plates must be annealed, so they have better plasticity and elongation, and are widely used in industries such as automobiles, home appliances, and hardware. The surface of the cold-rolled plate has a certain degree of smoothness and is relatively smooth to the touch, mainly due to pickling. Generally, the surface finish of hot-rolled plates cannot meet the requirements, so hot-rolled steel strips need to be cold-rolled, and the thinnest thickness of hot-rolled steel strips is generally 1.0mm, and cold-rolled 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 of cold rolling to the shape of the steel belongs to continuous cold deformation. The cold work hardening caused by this process increases the strength and hardness of the rolled hard coil and decreases the toughness index. For end use, cold rolling deteriorates the stamping performance, and the product is suitable for simple deformed parts.

The main difference between hot rolling and cold rolling
The difference between cold rolling and hot rolling is mainly the temperature of the rolling process. Cold is normal temperature, and hot is 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℃.

Appearance and surface quality
Since the cold plate is obtained by the hot plate after the cold rolling process, and cold rolling will also carry out some surface finishing, the surface quality of the cold plate (such as surface roughness) is better than that of the hot plate, so if the product is If there are higher requirements for coating quality such as subsequent painting, cold plates are generally selected, and hot plates are divided into pickled plates and unpickled plates. The surface of the pickled plates becomes a normal metallic color due to pickling, but not Therefore, the surface of the cold rolled plate is still not as high as the cold plate. The surface of the unpickled plate usually has an oxide layer, blackening, or a black iron tetroxide layer. In layman's terms, it's like it's been roasted on fire, and if the storage environment is not good, it will usually have some rust.
performance

Under normal circumstances, the mechanical properties of hot plates and cold plates are considered to be indistinguishable in engineering. Although cold plates have a certain degree of work hardening during the cold rolling process, (but do not rule out the strict requirements for mechanical properties, then it is necessary 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, depending on the degree of cold plate annealing. But no matter how annealed the cold plate is stronger than the hot plate.

Forming quality
Since the performance difference of the hot and cold plates is basically the same, the influencing factors of the formability depends on the difference of the surface quality. Since the surface quality is better from the cold plate, the steel plate of the same material is usually better than the hot plate. The shaping effect is better.More about:Pre-hardened 2738 Plastic Mould Steel

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