Hot rolled steel VS cold rolled steel
Hot rolled steel VS cold rolled steel
Table of Contents
Both hot rolling and cold rolling are processes for forming steel plates or sections, and they have a great influence on the organization and properties of steel.
Steel is mainly rolled by hot rolling, cold rolling is usually used only for the production of small sections and thin plates and other steel of precise dimensions.
Common steel cold and hot rolling situations
- Wire: 5.5-40 mm diameter, coiled, all hot-rolled material. After cold drawing, it is cold drawn steel.
- Round steel: In addition to the size of precision bright material is generally hot-rolled, there are also forged material (surface forging traces).
- Strip: Hot-rolled and cold-rolled are both available, and cold-rolled material is generally thinner.
- Steel plate: cold-rolled plate is generally thinner, such as automotive plate; hot-rolled medium-thick plate is more, there are similar thicknesses with cold-rolled, the appearance is obviously different.
- Angle: all hot-rolled.
- Steel pipes: both welded hot-rolled and cold-drawn.
- Channels and H-beams: hot-rolled.
- Rebar: Hot rolled steel.
What is hot rolled?
By definition, ingots or billets are difficult to deform at room temperature and are not easily processed, and are generally heated to 1100-1250°C for rolling, a rolling process called hot rolling.
The termination temperature of hot rolling is generally 800-900°C, after which it is generally cooled in air, and thus the hot rolling state is equivalent to normalizing treatment.
Most of the steel is rolled by hot rolling method. Hot-rolled steel, due to the high temperature, the surface generation of a layer of iron oxide, and thus has a certain degree of corrosion resistance, can be stored in the open.
However, this layer of iron oxide also makes the surface of hot-rolled steel rough, size fluctuations, so the requirement for a smooth surface, accurate size, good mechanical properties of steel, to use hot-rolled semi-finished or finished products as raw materials and then cold-rolled production.
Advantages of hot-rolled
Forming speed, high yield, and do not damage the coating, can be made into a variety of cross-sectional forms to adapt to the needs of the conditions of use; cold rolling can make a large plastic deformation of steel, thereby increasing the yield point of steel.
Disadvantages of hot rolling
- 1. Although the forming process does not undergo thermal plastic compression, but there are still residual stresses in the cross-section, which must have an impact on the overall and local yielding characteristics of steel;
- 2. Cold-rolled steel style is generally open section, making the free torsional stiffness of the section is low. In the bending prone to twisting, bending and torsional buckling when pressed, poor torsional performance;
- 3. The wall thickness of cold rolled formed steel is smaller and not thickened at the corners where the plates are articulated, so the ability to withstand localized concentrated loads is weak.
What is cold-rolled
Cold rolling is a rolling method in which the steel is extruded by the pressure of rolls at room temperature to change the shape of the steel. Although the process also heats up the steel sheet, it is still called cold rolling. To be more specific, cold rolling uses hot rolled steel coils as raw material, and after pickling to remove oxidation, pressure processing is carried out, and the finished product is rolled hard coils.
General cold-rolled steel, such as galvanized, color steel sheet must be annealed, so plasticity and elongation is also better, widely used in automotive, home appliances, hardware and other industries. The surface of cold-rolled sheet has a certain degree of finish and is smooth to the touch, mainly due to the work of pickling. Hot-rolled sheet generally does not meet the surface finish requirements, so hot-rolled steel strip needs to be cold-rolled, and hot-rolled steel strip thickness is generally the thinnest in 1.0mm, cold-rolled can reach 0.1mm. Hot-rolled is the crystallization temperature point above the rolling, cold-rolled is the crystallization temperature point below the rolling.
The change of steel shape by cold rolling is continuous cold deformation, and the cold hardening caused by this process makes the strength and hardness of the rolled hard coil rise and the toughness and plastic index fall.
For end use, cold rolling deteriorates the stamping properties and the product is suitable for simple deformation of parts.
Advantages of cold rolling
It can destroy the casting organization of the ingot, refine the grain of steel and eliminate microstructural defects, thus making the steel organization dense, 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 casting, but also at high temperatures and pressure can be welded together.
Disadvantages of cold rolling
- 1. After hot rolling, the non-metallic inclusions inside the steel (mainly sulfides and oxides, and silicates) are pressed into thin sheets and delamination occurs. Delamination greatly deteriorates the properties of the steel in tension along the thickness direction, and there is a risk of interlayer tearing during weld shrinkage. Local strains induced by weld shrinkage often reach several times the yield point strain, which is much larger than the load-induced strain;
- 2. Residual stresses caused by uneven cooling. Residual stress is the internal self-equilibrium stress in the absence of external forces, a variety of cross-sectional hot-rolled steel sections have such residual stresses, generally the larger the section size of the steel, the greater the residual stress. Although the residual stress is self-balancing, but the performance of steel members under the action of external forces still has a certain impact. Such as deformation, stability, fatigue resistance and other aspects may have a negative effect.
1. Appearance and surface quality
The cold plate is obtained from the hot plate after the cold rolling process, and the cold rolling process will also carry out some surface finishing, so the cold plate is better than the hot plate in terms of surface quality (such as surface roughness), so if there are high requirements for the quality of coating such as post-sequence painting, generally choose the cold plate, and the hot plate is divided into pickled plate and unpickled plate, pickled plate surface due to pickled so into the normal metal color, but not cold rolled So the surface is still not as high as the cold plate, not pickled plate will usually have an oxide layer on the surface, ooze, or the presence of iron tetraoxide ooze layer. In layman’s terms, it is similar to fire baked, and if the storage environment is not good, usually with some rust.
In general, the hot plate and cold plate in the engineering of its mechanical properties are considered no difference, although the cold plate in the cold rolling process there is a certain degree of hardening, (but does not exclude the strict requirements of mechanical properties, that need to be treated differently), cold plate is usually slightly higher than the yield strength of the hot plate, the surface hardness is also higher, how specific need to see the degree of annealing cold plate. But no matter how annealed cold plate strength is higher than the hot plate.
3. Forming performance
Due to the basic performance of hot and cold plate is not too much, so the forming performance of the factors affecting the difference in its surface quality, due to the surface quality of the cold plate to be good, so generally speaking, the same material steel plate, cold plate than the hot plate forming effect to come to some good.
To sum up
The main difference between cold rolling and hot rolling is the temperature of the rolling process. “cold” for room temperature, “hot” for high temperature.
From a metallurgical point of view, the boundaries of cold rolling and hot rolling should be distinguished by the recrystallization temperature. That is, below the recrystallization temperature of rolling for cold rolling, above the recrystallization temperature of rolling for hot rolling. The recrystallization temperature of steel is 450-600 ℃.
Source: Stainless Steel Plate Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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