Hot rolled and cold rolled are the process of steel plate or profile forming. They have a great influence on the structure and performance of steel.
The rolling of steel is mainly hot rolling, and cold rolling is usually used only for the production of steel with precise sizes such as small-sized section steel and thin plates.
The diameter of 5.5-40 mm, coil, all is hot rolled material.
After cold drawing, it is cold drawing material.
Generally hot rolled in addition to fine bright materials with precise size, also has forging materials (the surface has forged marks).
Both hot rolled and cold rolled, cold rolled materials are generally thinner.
Cold rolled plates are generally thinner, such as car boards;
Hot rolled medium thick plate, have the similar thickness of cold rolled, the appearance is obviously different.
All is hot rolled.
Including welding hot rolled and cold drawing.
U-steel & H-steel
Hot Rolled Definition
Ingot and billet at room temperature are very difficult to deformation, not easy processing, general rolling when heating to 1100 ~ 1250 ℃, the rolling technology called hot rolled.
Hot rolled end temperature is 800 ~ 900 ℃, generally cooling in the air, so hot rolling state is equivalent to normalizing treatment.
Most of the steel is rolled in hot rolled.
Due to the high temperature, the surface of hot rolled steel produces a layer of oxidation sheet, thus has certain corrosion resistance, can be stored in the open air.
But the oxidation layer also makes hot-rolled steel has a rough surface and large size volatile, so the steel require a bright clean surface, precise size and good mechanical properties need to be produced into semi-finished by hot rolling or finished products as raw materials to produce by cold rolling.
Rapid forming speed, high yield and undamaged coating, can be made into a variety of cross-section forms to meet the requirements of the use conditions;
Cold rolling can make steel produce a lot of plastic deformation, thus improving the yield point of steel.
- Although there is no thermoplastic compression in the forming process, residual stresses still exist in the cross-section, which must have an impact on the whole and local characteristics of the steel.
- The cold-rolled steel style is the generally open section, which makes the free torsion stiffness of the section lower. It is easy to turn torsion when bending, and it is easy to crankle when under pressure. Its anti-twisting performance is poor.
- The wall thickness of cold rolled steel is small, and the corners are not thickened at the corners of the plate, and the capacity of the local concentrated load is weak.
Cold Rolled Definition
Cold rolling means that under normal temperature, press steel with the pressure of roller to change the steel shape.
Although the process also heats up the plate, it is still called cold rolling.
To be specific, cold rolling take hot rolled steel coil as raw material and do press fabricating after removing oxidized skin by acid wash, its finished product is hard to roll.
Generally cold rolled steel, such as galvanizing and color steel, must be annealed, so the plastic and elongation rate is better, widely used in automobile, home appliance, hardware industries.
The surface of the cold rolled sheet has a certain degree of finish, the hand feels smooth, it is the work of acid wash basically.
Generally, hot rolled plate surface smoothness cannot meet the requirements, so hot rolled steel strip needs cold rolling, and the minimum thickness of hot-rolled steel is general at 1.0 mm, while cold rolling can reach 0.1mm.
Hot rolling is the rolling process beyond crystallization temperature while cold rolling is the rolling process under the crystallization temperature.
The change of cold rolled steel shape is a continuous cold deformation, and the cold hardening of this process causes the strength, hardness, and toughness of hard rolled coil to decrease.
For end users, cold rolling deteriorates the stamping performance, and the product is suitable for simple deformed parts.
It can destroy the cast 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 direction of rolling so that the steel is no longer isotropic to some extent.
The bubbles, cracks and loose bubbles formed during pouring can also be soldered under high temperature and pressure.
- After hot rolling, non-metallic inclusions (mainly sulphides and oxides, and silicates) inside the steel are pressed into thin slices and stratified.
The layering of steel in the direction of the thickness of the steel is greatly deteriorated, and there may be a laminar tear at the weld shrinkage.
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 stresses caused by uneven cooling.
The residual stress is the stress of internal self – equilibrium under the action of no external force, and the hot-rolled profile steel of various sections have such residual stresses, and the larger the section size of the normal section steel, the greater the residual stress.
Although the residual stresses are self-balanced, still has some influence on the performance of steel components under external forces. Such as deformation, stability, fatigue can have adverse effects.
The difference between cold rolling and hot rolling are mainly the temperature of the rolling process.
“Cold Rolled” is a normal temperature, “Hot Rolled” is a high temperature.
From the point of view of metal, the boundary between cold rolling and hot rolling should be distinguished by recrystallization temperature.
Lower than the temperature of recrystallization is cold rolled, while higher than the rolling temperature of recrystallization is hot rolled.
Steel recrystallization temperature is 450 ~ 600 ℃.
Hot rolled steel vs. cold rolled steel
Appearance and surface quality
Since the cold-rolled plate is obtained after the cold rolling process, and the cold rolling also makes some surface finishing, the cold rolled plate is better than the hot rolled plate on the surface quality (such as surface roughness).
So if there is a high demand for the coating quality of the finished product, the cold plate is generally chosen.
In addition, the hot-rolled plate is divided into the pickled and the unpickled plates.
The surface of the pickling board is a normal metallic color because of the acid washing, but it is not cold rolled.
The unpickling plate usually has an oxide layer, dark color, or exist a layer of trioxide.
In layman’s terms, it’s just like a roast with fire, and it’s usually rusted if it’s not placed under good environmental condition.
In general, the mechanical properties of hot rolled plates and cold rolled plates are considered to be indistinguishable in engineering, though cold rolled plate in the cold rolling process has a certain process of hardening. But if you don’t rule out strict mechanical performance, you need to treat it differently.
The yield strength of the cold plate is usually a little higher than the hot plate, and the surface hardness is higher too. In specific, it depends on the degree of cold plate annealing.
But anyway, the annealing strength cold rolled plate is higher than hot rolled plate.
Since the performance of cold rolled and the hot rolled plate is similar, the influence factor of forming performance depends on the difference of surface quality.
Because the surface quality of cold rolled plate is better, so generally speaking to the steel plate of the same material, the forming effect of the cold rolled plate is better than the hot plate.
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