Hot rolled and cold rolled are both processes for forming steel plates or profiles, and they have a great impact on the organization and performance of steel.

Steel rolled is mainly hot-rolled, and cold rolled is usually only used to produce small-sized steel and thin plates with precise dimensions.

Common hot and cold rolled of steel:

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

2. Round steel: Except for bright materials with precise dimensions, it is generally hot-rolled, and there are also forged materials (there are forging marks on the surface).

3. Strip steel: There are both hot-rolled and cold-rolled, and cold-rolled materials are generally thinner.

4. Steel plate: Cold-rolled plates are generally thinner, such as plates for automobiles; there are more hot-rolled medium and thick plates, and some are similar in thickness to cold-rolled, but the appearance is obviously different.

5. Angle steel: All are hot-rolled.

6. Steel pipes: Welded, hot-rolled and cold-drawn.

7. Channel steel and H-beam: hot-rolled.

8. Rebar: hot-rolled.

What is hot rolling?

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

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

Most steel products are rolled by hot rolling. Due to the high temperature, a layer of iron oxide scale is formed on the surface of the steel delivered in the hot rolled state, so it has a certain corrosion resistance and can be stored in the open air.

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

Advantages: Fast forming speed, high output, no damage to the coating, can be made into a variety of cross-sectional forms to meet the needs of use conditions; cold rolled can cause great plastic deformation of steel, thereby increasing the yield point of steel.

Disadvantages:

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

2. The cold-rolled steel section style is generally an open section, which makes the free torsional stiffness of the cross section low. It is easy to torsion when bent, and easy to bend-torsion buckling when compressed, and the torsion resistance is poor;

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

What is cold rolling？

Cold rolled refers to a rolled method that changes the shape of steel by squeezing it with the pressure of rollers at room temperature. Although the steel plate will heat up during the process, it is still called cold rolling.

To be more specific, cold rolled uses hot-rolled steel coils as raw materials, and after pickling to remove the oxide scale, it is pressure-processed, and the finished product is hard-rolled coils.

Generally, cold-rolled steel, such as galvanized and color-coated steel plates, must be annealed, so the plasticity and elongation are also good, and they are widely used in the automotive, home appliances, hardware and other industries.

The surface of the cold-rolled plate has a certain degree of finish, and it feels smooth to the touch, which is mainly the function of pickling. The surface finish of the hot-rolled plate generally does not meet the requirements, so the hot-rolled steel strip needs to be cold-rolled. In addition, the thinnest thickness of the hot-rolled steel strip is generally 1.0mm, and the cold rolled can reach 0.1mm.

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

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

Advantages:

It can destroy the casting structure of the steel ingot, refine the grains 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 rolled direction, so that the steel is no longer isotropic to a certain extent; the bubbles, cracks and looseness formed during pouring 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) inside the steel are pressed into thin sheets, and stratification occurs. Stratification greatly deteriorates the tensile performance of the steel along the thickness direction, and 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;

2. 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 this type of residual stress. 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, such as deformation, stability, fatigue resistance, etc.

Conclusion:

The difference between cold rolled and hot rolled is mainly the temperature of the rolled process. "Cold" means room temperature, and "hot" means high temperature.

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

The main differences between hot rolled and cold rolled are:

1. Appearance and surface quality: Since cold rolled is the result of cold rolled of hot rolled, and some surface finishing is also performed during cold rolled, the surface quality (such as surface roughness) of cold rolled is better than that of hot rolled. Therefore, if there are high requirements for the quality of subsequent coating such as painting, cold rolled is generally selected.

In addition, hot rolled is divided into pickled and unpickled. The surface of pickled plate has a normal metallic color due to pickling, but it is not cold rolled, so the surface is still not as high as that of cold plate. Unpickled plate usually has an oxide layer on the surface, which is black, or has a black layer of ferroferric oxide. In layman's terms, it is like being roasted by fire, and if the storage environment is not good, it usually has a little rust.

2. Performance: Generally speaking, the mechanical properties of hot plates and cold plates are considered to be the same in engineering. Although cold plates undergo a certain degree of work hardening during the cold rolled process (but this does not rule out the situation where strict mechanical properties are required, which requires different treatment), cold plates usually have a slightly higher yield strength and surface hardness than hot plates. The specific situation depends on the degree of cold plate annealing. However, no matter how the annealing is done, the strength of cold plates is higher than that of hot plates.

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