Hot and cold rolling are procedures to form steel plates or profiles and significantly affect the structure and properties of steel.
The main steel rolling method is hot rolling, while cold rolling is generally only used for producing small-sized steel and thin sheets that require precise dimensions.
Hot lamination
Cold rolling refers to a rolling method that manipulates the shape of steel by applying pressure with rollers at room temperature.
Despite the increase in temperature of the steel during the process, it is still known as cold rolling.
More specifically, cold rolling involves the use of hot-rolled steel coils as raw materials, which, after acid washing to remove oxide scale, undergo pressure processing, resulting in hard-rolled coils as the finished product.
Cold-rolled steels, such as galvanized and color-coated sheets, generally undergo annealing, which results in good plasticity and elongation rates.
They are widely used in industries such as automotive, home appliances and hardware. The surface of cold-rolled sheets has a certain degree of smoothness and is quite shiny, mainly due to acid washing.
Hot-rolled sheets generally do not meet the required surface smoothness, so hot-rolled steel strips need to be cold-rolled.
Furthermore, the minimum thickness of hot rolled steel strips is normally 1.0mm, while cold rolling can reach 0.1mm. Hot rolling is carried out above the crystallization temperature, while cold rolling takes place below the crystallization temperature.
Shape changes in steel due to cold rolling refer to continuous cold deformation. The cold hardening that occurs during this process increases the resistance and hardness of the laminated hard coil, while decreasing its toughness and plasticity indices.
From an end-use perspective, cold rolling deteriorates stamping performance, making the product suitable for parts that require simple deformation.
Benefits:
Cold rolling can disrupt the casting structure of the steel ingot, refine the steel grains and eliminate microscopic structural defects, making the steel structure compact and improving its mechanical properties. This improvement is seen mainly in the direction of rolling, making the steel to some extent anisotropic rather than isotropic. Bubbles, cracks and gaps formed during casting can also be welded under high temperature and pressure.
Disadvantages:
1. After hot rolling, non-metallic inclusions (mainly sulfides and oxides, as well as silicates) within the steel are pressed into thin layers, resulting in a phenomenon known as delamination.
Delamination significantly deteriorates the tensile properties of the steel along the thickness direction and can cause interlaminar rupture during weld retraction. The local deformation induced by weld shrinkage often reaches several times the yield point deformation, much greater than the deformation caused by the load.
2. Residual stress caused by uneven cooling. Residual stress is an internally balanced stress in the absence of external forces. All types of hot-rolled steel sections have this type of residual stress, and the larger the section size of the overall steel, the greater the residual stress.
Although residual stress is self-balancing, it still has a certain impact on the performance of steel components under external forces. For example, it can have adverse effects on deformation, stability and fatigue resistance.
Hot Rolling vs Cold Rolling
The main distinction between cold rolling and hot rolling is the temperature during the rolling process; “cold” indicates room temperature, while “hot” means high temperature.
From a metallurgical point of view, the demarcation between cold rolling and hot rolling should be based on the recrystallization temperature.
That is, rolling conducted below the recrystallization temperature is considered cold rolling, and rolling above this temperature is considered hot rolling. The recrystallization temperature of steel varies from 450 to 600°C.
The main differences between hot rolling and cold rolling are:
1. Appearance and surface quality:
Cold-rolled sheets, being derived from hot-rolled sheets subjected to cold rolling processes, are often subjected to various surface finishing techniques. As a result, they exhibit superior surface quality (e.g. reduced surface roughness) compared to hot-rolled sheets.
Therefore, if high-quality paint or similar coatings are required to be applied in subsequent product steps, cold-rolled sheets are generally preferred.
Hot rolled sheets can be further classified into acid-washed and non-acid-washed. Acid washed sheets, after being subjected to acid washing, have a regular metallic color, but their surface quality is not as high as cold rolled sheets since they are not cold rolled.
Leaves not washed with acid often have an oxidized layer, appearing blackened or having a layer of black iron (III) oxide. In layman's terms, they look charred by fire and often rust if stored in poor conditions.
2. Mechanical Properties:
Generally, in engineering applications, the mechanical properties of hot-rolled sheets and cold-rolled sheets are considered to be identical, although cold-rolled sheets undergo a certain degree of hardening during the cold rolling process. (However, if strict mechanical property requirements are required, specific consideration will be required.)
Cold-rolled sheets typically have slightly higher yield strength and surface hardness than hot-rolled sheets, although the exact values depend on the degree of annealing of the cold-rolled sheets. Regardless of the annealing process, the strength of cold-rolled sheets exceeds that of hot-rolled sheets.
3. Formability:
Since the properties of cold-rolled and hot-rolled sheets are quite similar, their formability depends mainly on differences in surface quality.
As cold-rolled sheets have better surface quality, they generally provide better forming results than hot-rolled sheets of the same material.
