Hot rolling and cold rolling are processes of forming steel plates or profiles, which have a great impact on the structure and properties of steel.
Hot-Rolling
By definition, ingots or billets are difficult to deform and process at room temperature. Generally, they are heated to 1100~1250 ℃ for rolling. This rolling process is called hot rolling.
The end temperature of hot rolling is generally 800~900 ℃, and then it is generally cooled in air, so the hot rolling state is equivalent to normalizing treatment.
Most steel is rolled by hot rolling. The steel delivered in hot rolling state has a layer of oxide scale on its surface due to high temperature, so it has certain corrosion resistance and can be stored in the open air.
However, this layer of iron oxide scale also makes the surface of hot rolled steel rough and the size fluctuates greatly. Therefore, steel with smooth surface, accurate size and good mechanical properties should be produced by cold rolling with hot rolled semi-finished products or finished products as raw materials.
Advantage:
With fast molding speed, high output, and no damage to the coating, it can be made into a variety of cross section forms to meet the needs of the use conditions; Cold rolling can make steel produce great plastic deformation, thus improving the yield point of steel.
Disadvantages:
- Although there is no thermal plastic compression in the forming process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel.
- Cold rolled section steel is generally open section, which makes the free torsional stiffness of the section low. Torsion is easy to occur in bending, and flexural torsional buckling is easy to occur in compression, with poor torsional resistance.
- The wall thickness of cold-rolled formed steel is small, and there is no thickening at the corner of the plate connection, so the ability to bear local concentrated load is weak.
Cold Rolling
Cold rolling refers to the rolling method of extruding steel and changing the shape of steel with the pressure of rolls at room temperature. Although the processing process will also make the steel plate warm, it is still called cold rolling. Specifically, the hot rolled steel coil used for cold rolling is used as the raw material, and the pressure processing is carried out after the scale is removed by pickling, and the finished product is the rolled and hardened coil.
Generally, cold rolled steel, such as galvanized steel and color steel plate, must be annealed, so it has good plasticity and elongation, and is widely used in automobile, household appliances, hardware and other industries. The surface of the cold-rolled sheet has a certain degree of finish, which is smooth to the touch, mainly due to pickling. Generally, the surface finish of the hot rolled plate cannot meet the requirements, so the hot rolled steel strip needs to be cold rolled. The thickness of the hot rolled steel strip is generally 1.0mm, and the cold rolled steel strip 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 steel shape caused by cold rolling belongs to continuous cold deformation. The cold work hardening caused by this process makes the strength and hardness of rolled hard coil increase and the toughness and plasticity index decrease.
For terminal use, cold rolling deteriorates the stamping performance, and the product is suitable for parts with simple deformation.
Advantage:
It can destroy the casting structure of the 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; Bubbles, cracks and looseness formed during pouring can also be welded under high temperature and pressure.
Disadvantages:
- After hot rolling, non-metallic inclusions (mainly sulfides, oxides and silicates) inside the steel are pressed into thin sheets and delamination occurs. Delamination greatly deteriorates the tensile properties of steel along the thickness direction, and may cause interlaminar tearing when the weld shrinks. The local strain induced by weld shrinkage often reaches several times of the yield point strain, which is much larger than the strain caused by load.
- Residual stress caused by uneven cooling. Residual stress is the internal self balanced stress without external force. Hot rolled steel sections of various sections have this kind of residual stress. Generally, the larger the section size of steel section, the greater the residual stress. Although the residual stress is self phase equilibrium, it has some influence on the performance of steel members under external forces. For example, it may have adverse effects on deformation, stability and fatigue resistance.
The Main Differences Between Hot Rolling And Cold Rolling:
1. Appearance And Surface Quality:
As the cold plate is obtained from the hot plate after the cold rolling process, and some surface finishing is also carried out during the cold rolling process, the cold plate is better than the hot plate in terms of surface quality (such as surface roughness). Therefore, if there is a high requirement for the coating quality of the product after painting, the cold plate is generally selected, and the hot plate is divided into acid washed plate and non acid washed plate. The surface of the acid washed plate becomes normal metal color because of the acid washing, However, the surface is not as high as that of the cold plate because it is not cold rolled. The surface of the plate that is not pickled usually has an oxide layer, black hair, or a black layer of ferric oxide. Generally speaking, it is like the fire has been baked, and if the storage environment is not good, it will usually bring a little rust.
2. Performance:
Generally, the mechanical properties of hot plate and cold plate are considered to be indistinguishable in engineering. Although there is some work hardening of cold plate during cold rolling, (although it does not rule out that strict requirements for mechanical properties need to be treated differently), the yield strength of cold plate is generally slightly higher than that of hot plate, and the surface hardness is also higher. The specific way depends on the degree of annealing of cold plate. However, the strength of annealed cold plate is higher than that of hot plate.
3. Formability
As the performance of cold and hot plates is not much worse, the influencing factors of formability depend on the difference of their surface quality. Because the surface quality is better from cold plates, generally speaking, the forming effect of cold plates is better than that of hot plates for steel plates of the same material.