Hot rolling and cold rolling are both forming processes for steel sheets or profiles, and they have a great impact on the structure and properties of steel.
Steel rolling is mainly carried out hot, while cold rolling is generally only used to produce steel products of precise dimensions, such as small steel profiles and thin sheets.
Common cold and hot rolling steel situations:
Wire: 5.5 to 40 mm in diameter, wound, all hot-rolled. After cold drawing, it belongs to cold drawn material.
Round steel: Besides bright steel with precise dimensions, it is usually hot rolled, and there is also forged steel (with forging marks on the surface).
Steel strip: hot-rolled and cold-rolled products, and cold-rolled products are generally thinner.
Steel sheets: Cold-rolled sheets are generally thinner, like automotive sheets; There are more mid-thickness hot-rolled sheets, some of which are similar in thickness to cold-rolled sheets, and their appearance is obviously different.
Steel angle: all hot rolled.
Steel tubes: welded, hot rolled and cold drawn.
Profile 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 between 1,100 and 1,250°C for lamination. This rolling process is called hot rolling.
The end temperature of hot rolling is generally 800-900°C, and is usually air-cooled afterwards, so the hot rolling state is equivalent to normalizing processing.
Most steel products are hot rolled. Steel delivered hot rolled 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 iron oxide scale layer also makes the surface of hot-rolled steel rough, and the dimensions fluctuate greatly. Therefore, steel with smooth surface, accurate dimensions and good mechanical properties should be produced by cold rolling of hot rolled semi-finished products. products or finished products as raw materials.
advantage:
The forming speed is fast, the yield is high, and the coating is not damaged. It can be made into a variety of cross-section shapes to adapt to the needs of using conditions, which can cause significant plastic deformation of the steel; increasing the yield strength of the steel.
default:
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 rigidity of the section. It is prone to torsion when subjected to bending and torsional buckling when subjected to pressure, and its torsion resistance is low;
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 low.
cold rolled
Cold rolling refers to a rolling method that uses roller pressure to press steel at room temperature and change the shape of the steel. Although the process also heats the steel sheet, it is still called cold rolling. To be more specific, cold rolling uses hot-rolled steel coils as raw materials, which are pickled to remove scale and then pressure treated. The finished product is made of hard rolled coils.
Generally, cold rolled steel such as galvanized and colored steel sheets need to be annealed, so their plasticity and elongation are also good, and they are widely used in automobiles, household appliances, hardware and other industries. The surface of cold-rolled sheets has a certain softness and is smooth to the touch, mainly due to pickling. The surface finish of hot-rolled sheets generally does not meet the requirements, so hot-rolled steel strips must be cold-rolled. The thinnest thickness of hot-rolled steel strip is generally 1.0mm, and cold rolling can reach 0.1mm. Hot rolling involves rolling above the crystallization temperature point and cold rolling involves rolling below the crystallization temperature point.
The shape change of 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 single deformation parts.
advantage:
It can destroy the casting structure of steel ingot, refine the grains of steel, and eliminate microstructure defects, 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 gaps formed during casting can also be welded under the action of high temperatures and pressures.
default:
1. After hot rolling, non-metallic inclusions (mainly sulfides, oxides and silicates) inside the steel are pressed into thin sheets, causing delamination. Delamination significantly deteriorates the tensile properties of the steel in the through-thickness direction and can cause interlaminar tearing as the weld shrinks. The local deformation induced by weld shrinkage often reaches several times the strain at the yield point, which is much larger than the deformation caused by the load;
2. Residual stresses caused by uneven cooling. Residual stress is the self-balancing internal stress in the absence of external force. Hot-rolled steel sections of various sections have such residual stress. Generally, the larger the cross-sectional size of the section steel, the higher the residual stress. Although residual stresses are self-balancing, they still have a certain impact on the performance of steel components under the action of external forces. For example, it can have adverse effects on deformation, stability, fatigue resistance, etc.
To summarize:
The difference between cold rolling and hot rolling mainly lies in 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 must be distinguished by the recrystallization temperature. In other words, 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:
Since the cold plate is obtained from the hot plate after the cold rolling process and some surface finishing is also carried out at the same time as cold rolling, the surface quality of the cold plate ( such as surface roughness, etc.) is better. than that of the hot plate, so if the product is If there are higher requirements for the coating quality, such as subsequent painting, a Generally, cold plates are chosen and hot plates are divided into plates pickled and unpickled plates. The surface of pickled plates has normal metallic color because it has been pickled, but it has not been cold rolled, so the surface is still not the same. as high as cold plates. Unpickled plates. There is usually a black oxide layer on the surface, or a black Fe3O4 layer. Simply put, it looks like it has been burned by fire, and if the storage environment is not good, there will usually be a little rust.
2. Performance:
Generally speaking, there is no difference in the mechanical properties of hot plates and cold plates in engineering. Although cold plates exhibit some work hardening during the cold rolling process (this does not, however, exclude the case of strict mechanical performance requirements). which requires different treatment), the yield strength of the cold plate is generally slightly higher than that of the hot plate, and the surface hardness is also higher. The specific situation depends on the annealing degree of the cold plate. But regardless of the degree of annealing, 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 is basically not much different, the influencing factors of formability depend on the difference in surface quality. Since the surface quality of cold plates is generally better for steel plates of the same material, cold plates. are better than cold plates. The hot plate forming effect is better.
Daguang focuses on providing solutions such as precision CNC machining services (3-axis, 4-axis, 5-axis machining), CNC milling, 3D printing and rapid prototyping services.
