Both hot rolling and cold rolling are processes for forming steel sheets or profiles, which have a great influence on the microstructure and properties of the steel.
The rolling of steel is mainly hot rolling, and the cold rolling is usually only used for the production of precision steel such as small steel and thin plates.
By definition, steel ingots or billets are difficult to deform at normal temperature and are difficult to process. Generally, they are heated to 1100 to 1250 ° C for rolling. This rolling process is called hot rolling.
The termination temperature of hot rolling is generally 800 to 900 ° C, and then generally cooled in air, so that the hot rolled state is equivalent to normalizing treatment.
Most steels are rolled by hot rolling. Steels delivered in hot-rolled state, due to high temperature, form a layer of iron oxide on the surface, so they have a certain degree of 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 has large fluctuations in size. Therefore, steels with smooth surface, precise size and good mechanical properties are required, and hot-rolled semi-finished products or finished products are used as raw materials for cold rolling production.
The molding speed is fast, the output is high, and the coating is not damaged, and various cross-section forms can be made to meet the requirements of the use conditions; cold rolling can cause a large plastic deformation of the steel, thereby increasing the yield point of the steel.
- Although there is no hot 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;
- The cold-rolled steel profile is generally an open section, so that the free torsional stiffness of the section is low. It is easy to twist when being bent, and it is prone to bending and torsion buckling when pressed, and the torsion resistance is poor;
- The cold-rolled steel has a small wall thickness, and there is no thickening at the corners where the plates are joined, and the ability to withstand localized concentrated loads is weak.
The hot coil is unrolled, the continuous welding begins, and the cold rolling process is officially entered: after pickling, it is rolled into a rolling mechanism to produce a rolled hard roll, and the rolled hard roll is washed into the heat treatment stage:
Cold rolling refers to a rolling method in which a steel material is pressed at a normal temperature by a pressure of a roll to change the shape of the steel. Although the process also heats up the steel sheet, it is still called cold rolling. Specifically, the hot-rolled steel coil for cold rolling is used as a raw material, and the scale is removed by pickling to remove the scale, and the finished product is a rolled hard roll.
Generally, cold-rolled steel, such as galvanized and color steel plates, must be annealed, so plasticity and elongation are also good, and are widely used in automobiles, home appliances, hardware and other industries. The surface of the cold-rolled sheet has a certain degree of smoothness, and the hand is relatively smooth to touch, mainly the work of pickling. The hot surface of the hot rolled sheet generally does not meet the requirements, so the hot rolled steel strip needs to be cold rolled, and the thickness of the hot rolled steel strip is generally 1.0 mm, and the cold rolling can reach 0.1 mm. Hot rolling is rolling at a crystallization temperature or higher, and cold rolling is rolling at a crystallization temperature or lower.
Cold rolling changes the shape of the steel into continuous cold deformation. The cold work hardening caused by this process causes the strength and hardness of the rolled hard roll to rise and the toughness index to decrease.
For end use, cold rolling deteriorates stamping performance and the product is suitable for parts that are simply deformed.
It can destroy the cast structure of the steel ingot, refine the grain of the steel, and eliminate the defects of the microstructure, so that the steel structure is compacted and the mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer an isotropic body to some extent; bubbles, cracks and looseness formed during casting can also be welded under high temperature and pressure.
- After hot rolling, non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets and delamination occurs. The delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and it is possible that interlaminar tearing occurs when the weld is shrunk. 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;
- Residual stress caused by uneven cooling. The residual stress is the internal self-phase equilibrium stress without external force. The hot-rolled steel of various sections has such residual stress. The larger the section size of the general section steel, the larger the residual stress. Although the residual stress is self-phase-balanced, it still has some influence on the performance of steel members under external force. Such as deformation, stability, fatigue and other aspects may have adverse effects.
The main differences between hot and cold rolling are:
- Appearance and surface quality:
Since the cold plate is obtained after the cold rolling process of the hot plate, and the cold rolling also performs some surface finishing, the cold plate is better than the hot plate in surface quality (such as surface roughness), so if the product is There are higher requirements for coating quality such as post-painting. Generally, cold plates are selected. The other hot plates are separated into pickled plates and unsick plates. The surface of the pickled plates is normal metal color due to pickling, but not Cold rolling, so the surface is still not cold plate high, the unsick plate usually has an oxide layer on the surface, hair, or the presence of iron tetraoxide. Generally speaking, it is fire-baked, and if the storage environment is not good, it usually has some rust.
2, performance: Under normal circumstances, the mechanical properties of hot and cold plates in the project is considered to be indistinguishable, although the cold plate has a certain degree of work hardening during the cold rolling process (but does not rule out the strict requirements on mechanical properties) , then it needs to be treated differently.) The cold plate is usually slightly higher than the yield strength of the hot plate, and the surface hardness is also higher. How to look at the degree of cold plate annealing. But no matter how the annealed cold plate strength is higher than the hot plate.
3, forming properties due to the performance of the hot and cold plate is basically not too much, so the influencing factors of the forming properties depends on the difference in surface quality, because the surface quality is better for the cold plate, so usually the same material steel plate The cold plate is better than the hot plate.