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Why is there a distinction between hot-rolled steel and cold-rolled steel？

Hot rolling and cold rolling are both processes of forming steel plates or profiles, which have a significant impact on the microstructure and properties of steel. What is the difference between them? 

1The rolling of steel is mainly based on hot rolling, and cold rolling is usually only used to produce small-sized steel and thin plates with precise dimensions.

Common situations of cold and hot rolling of steel:

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

Round steel: In addition to precision sized bright materials, it is generally hot-rolled, and there are also forged materials (with forging marks on the surface);  

Strip steel: both hot-rolled and cold-rolled, cold-rolled materials are generally thinner;  

Steel plate: Cold rolled plates are generally thinner, such as automotive plates. There are many hot-rolled medium thick plates with significantly different appearances;

Angle steel: all hot-rolled;

Steel pipes: both welding, hot rolling, and cold drawing are available;

Channel steel and H-beam: hot-rolled;

Steel bars: hot-rolled materials.

hot-rolling

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

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

Most steel is rolled by hot rolling method. Steel delivered in hot-rolled state has a certain corrosion resistance due to the formation of a layer of iron oxide scale on the surface due to high temperature, and can be stored outdoors.

But this layer of iron oxide also makes the surface of hot-rolled steel rough and the size fluctuates greatly. Therefore, steel with smooth surface, precise size, and good mechanical properties should be produced using hot-rolled semi-finished or finished products as raw materials and then cold-rolled.

advantage:

Fast molding speed, high output, and no damage to the coating, can be made into a variety of cross-sectional forms to meet the needs of usage conditions. Cold rolling can cause significant plastic deformation in steel, thereby increasing its yield point.

Disadvantages:

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

2. The style of cold-rolled steel is generally an open section, which reduces the free torsional stiffness of the section. It is prone to torsion when subjected to bending and bending and buckling when subjected to compression, resulting in poor torsional performance;

3. Cold rolled formed steel has a relatively small wall thickness and is not thickened at the corners where the plates are connected, resulting in weak ability to withstand localized concentrated loads.

cold rolling

Cold rolling refers to a rolling method that uses the pressure of a rolling mill to extrude steel at room temperature and change the shape of the steel. Although the processing also causes the steel plate to heat up, it is still called cold rolling. Specifically, hot-rolled steel coils are used as raw materials for cold rolling, and after acid washing to remove oxide scales, they are subjected to pressure processing to produce rolled hard coils.

Generally, cold-rolled steel such as galvanized and color coated steel plates require annealing, so their plasticity and elongation are also good, and they are widely used in industries such as automobiles, home appliances, and hardware. The surface of cold-rolled sheet has a certain degree of smoothness, which feels relatively smooth to the touch, mainly due to acid pickling. Hot rolled plates generally do not meet the required surface smoothness, so hot-rolled steel strips need to be cold-rolled. In addition, the thinnest thickness of hot-rolled steel strips is generally 1.0mm, while cold-rolled steel strips can reach 0.1mm. Hot rolling is rolling above the crystallization temperature point, while cold rolling is rolling below the crystallization temperature point.

The change in shape of steel by cold rolling belongs to continuous cold deformation, and the cold work hardening caused by this process increases the strength and hardness of the rolled hard coil, while the toughness and plasticity indicators decrease.

For end use, cold rolling deteriorates stamping performance, making the product suitable for simple deformed parts.

advantage:

It can destroy the casting structure of steel ingots, refine the grain size of steel, and eliminate defects in the microstructure, thereby making the steel structure dense and improving its 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 high temperature and pressure.

Disadvantages:

1. After hot rolling, non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are compressed into thin sheets, resulting in delamination. Layering greatly deteriorates the tensile properties of steel along the thickness direction, and there is a possibility of interlayer tearing during weld shrinkage. The local strain induced by weld shrinkage often reaches several times the yield point strain, much larger than the strain caused by load;

2. Residual stress caused by uneven cooling. Residual stress is the stress that self balances internally in the absence of external forces, and various hot-rolled steel sections have such residual stresses. Generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although residual stress is self balanced, it still has a certain impact on the performance of steel components under external forces. It may have adverse effects on deformation, stability, and fatigue resistance.

The main difference between hot rolling and cold rolling is:

1. Appearance and surface quality

Due to the fact that cold plate is obtained after the cold rolling process of hot plate, and some surface finishing is also carried out during cold rolling, cold plate has better surface quality (such as surface roughness, etc.) than hot plate. Therefore, if there are high requirements for the quality of coating such as subsequent painting of the product, cold plate is generally chosen. Hot plates are divided into acid washed plates and non acid washed plates. The surface of acid washed plates has a normal metallic color due to acid washing, but it is not cold-rolled, so the surface is still not as high as that of cold plates. Non acid washed plates usually have an oxide layer, blackening or iron oxide blackening layer on the surface. Simply put, it looks like it has been roasted over fire, and if stored in a poor environment, it usually has some rust.

2. Performance

In general, the mechanical properties of hot and cold plates are considered to be indistinguishable in engineering. Although cold plates undergo some work hardening during the cold rolling process (although strict requirements for mechanical properties cannot be ruled out, so they need to be treated differently), cold plates usually have slightly higher yield strength and surface hardness than hot plates, depending on the degree of annealing of the cold plate. But no matter what, the strength of annealed cold plates is higher than that of hot plates.

3. Forming performance

Due to the fact that the performance of cold and hot plates is not significantly different, the factors affecting their formability depend on the difference in surface quality. As the surface quality is better for cold plates, generally speaking, for steel plates of the same material, the formability of cold plates is better than that of hot plates

Summary:

The main difference between cold rolling and hot rolling is the temperature during the rolling process. Cold "refers to room temperature, while" hot "refers to high temperature.

From the perspective of metallurgy, the boundary between cold rolling and hot rolling should be distinguished by the recrystallization temperature. Rolling below the recrystallization temperature is called cold rolling, while rolling above the recrystallization temperature is called hot rolling. The recrystallization temperature of steel is between 450 ℃ and 600 ℃.