Hot rolling and cold rolling are both processes of forming steel plates or profiles, which have a great impact on the structure and properties of steel.
The rolling of steel is mainly hot rolling, and cold rolling is usually only used to produce small section steel and sheet steel.
Cold and hot rolling of common steel:
Wire rod: 5.5-40mm in diameter, coiled, all hot-rolled. After cold drawing, it belongs to cold drawn material.
Round steel: In addition to the size of bright steel, it is generally hot rolled, and there are also forged steel (with forging marks on the surface).
Strip steel: hot rolled and cold rolled, generally thin.
Steel plate: cold-rolled plate is generally thin, such as automobile plate; There are many hot rolled medium and thick plates with similar thickness as cold rolling, and the appearance is obviously different.
Angle steel: all hot-rolled.
Steel pipe: welded, hot-rolled and cold-drawn.
Channel steel and H-beam: hot rolled.
Rebar: hot-rolled steel.
hot-rolling
By definition, steel ingots or billets are difficult to deform and process at normal 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 the air, so the hot rolling state is equivalent to normalizing treatment.
Most steels are rolled by hot rolling. Due to the high temperature, the steel delivered in the hot rolling state has a layer of oxide scale on the surface, which has certain corrosion resistance and can be stored in the open air.
However, this layer of iron oxide also makes the surface of hot rolled steel rough and its size fluctuates greatly, so the 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:
The molding speed is fast, the output is high, and the coating is not damaged. It can be made into a variety of cross sections to meet the needs of the use conditions; Cold rolling can produce large plastic deformation of steel, thus improving 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 section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The cold-rolled section steel is generally of open section, which makes the free torsional rigidity of the section low. Torsion is easy to occur in bending, and flexural-torsional buckling is easy to occur in compression, and the torsional resistance is poor;
3. The wall thickness of cold-rolled formed steel is small, and there is no thickening at the corner of plate connection, so the ability to bear local concentrated load is weak.
cold rolling
Cold rolling refers to the rolling method of extruding steel with the pressure of roller to change the shape of steel at normal temperature. Although the processing process will also heat up the steel plate, it is still called cold rolling. To be specific, hot rolled steel coil is used as raw material for cold rolling, and pressure processing is carried out after oxide scale is removed by acid pickling. The finished product is rolled hard coil.
Generally, cold-rolled steel, such as galvanized steel and color steel, must be annealed, so the plasticity and elongation are also good, and are widely used in automobile, household appliances, hardware and other industries. The surface of cold-rolled sheet has a certain degree of smoothness, and it feels smooth, mainly due to pickling. Generally, the surface finish of hot rolled steel strip cannot meet the requirements, so the hot rolled steel strip needs to be cold rolled, and the thickness of 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 increases the strength and hardness of the rolled hard coil and decreases the toughness and plasticity index.
For end 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 steel ingot, refine the grain of steel, and eliminate the defects of microstructure, so as to make the steel structure compact and improve 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 porosity formed during pouring can also be welded under high temperature and pressure.
Disadvantages:
1. After hot rolling, the non-metallic inclusions (mainly sulfide and oxide, as well as silicate) in the steel are pressed into thin sheets, resulting in delamination. The delamination greatly worsens the tensile properties of the steel along the thickness direction, and may cause interlayer 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;
2. Residual stress caused by uneven cooling. Residual stress is the internal self-balanced stress without external force. Hot rolled section steel of various sections has such residual stress. The larger the section size of general section steel, the greater the residual stress. Although the residual stress is self-balanced, it still 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.
Summary:
The difference between cold rolling and hot rolling is mainly the temperature of rolling process. "Cold" refers to normal temperature, and "hot" refers to high temperature.
From the point of view of metallurgy, the boundary between cold rolling and hot rolling should be distinguished by recrystallization temperature. That is, the rolling below the recrystallization temperature is cold rolling, and the 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:
Because the cold plate is produced after the cold rolling process of the hot plate, and the cold rolling process also carries out some surface finishing, the cold plate is better than the hot plate in surface quality (such as surface roughness), so if there are high requirements for the coating quality of the product, such as the subsequent painting, the cold plate is generally selected, and the hot plate is divided into pickled plate and non-pickled plate. The surface of the pickled plate becomes normal metal color due to pickling, However, the surface is still not as high as that of the cold plate because it is not cold rolled. Generally, the surface of the non-pickled plate will have an oxide layer, a black layer, or a black layer of ferric oxide. Generally speaking, it looks like it has been baked by fire, and if the storage environment is not good, it will usually be rusty.
2. Performance: Generally, the mechanical properties of hot plate and cold plate are considered to be indistinguishable in engineering. Although cold plate has certain work hardening during cold rolling, (although the strict requirements for mechanical properties are not excluded, it needs 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 treatment depends on the degree of annealing of cold plate. However, the strength of annealed cold plate is higher than that of hot plate.
3. The formability of cold and hot plates is not much worse, so the factors affecting the formability depend on the difference of their surface quality. Because the surface quality is better than that of cold plates, the formability of cold plates is generally better than that of hot plates of the same material