Difference Between Hot Rolled and Cold Rolled Steel Compare the Difference Between Similar Terms

Both hot rolling and cold rolling are processes for forming Steel Plates or profiles, and they have a great influence on the structure and properties of steel.
The rolling of steel is mainly based on hot rolling, and cold rolling is usually only used to produce steel products with precise dimensions such as small sections and thin plates.
1. Common steel cold and hot rolling conditions
Wire: 5.5-40 mm in diameter, coiled, all hot-rolled. After cold drawing, it is a cold drawn material.
Round steel: In addition to bright materials with precise dimensions, they are generally hot-rolled, and there are also forged materials (with forging marks on the surface).
Strip steel: both hot-rolled and cold-rolled, and cold-rolled products are generally thinner.
Steel Plate: Cold-rolled plates are generally thinner, such as automotive plates; there are many hot-rolled medium-thick plates, some of which are similar in thickness to cold-rolled plates, and their appearance is obviously different.
Angle steel: all hot rolled.
Steel pipe: Both welded hot rolled and cold drawn.
Channel steel and H-beam: hot rolled.
Steel bar: hot rolled material.
2. Hot rolling
By definition, steel ingots or billets are difficult to deform and process at room temperature, and are generally heated to 1100-1250°C for rolling. This rolling process is called hot rolling.
The termination temperature of hot rolling is generally 800-900°C, and then it is generally cooled in the air, so the hot rolling state is equivalent to normalizing treatment.
Most steel products are rolled by hot rolling method. Due to the high temperature, the steel delivered in the hot-rolled state has a layer of oxide scale on the surface, 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 the hot-rolled steel rough and the size fluctuates greatly. Therefore, the steel with smooth surface, accurate size and good mechanical properties is required to use hot-rolled semi-finished products or finished products as raw materials and then cold-rolled for production.
advantage:
The forming speed is fast, the output is high, and the coating is not damaged. It can be made into a variety of cross-sectional forms to meet the needs of the use conditions; cold rolling can cause large plastic deformation of the steel, thereby increasing the yield point of the steel.
shortcoming:
1. Although there is no thermal plastic compression during the forming process, there are still residual stresses in the section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The style of cold-rolled section steel is generally an open section, which makes the free torsional stiffness of the section low. It is easy to twist when it is bent, and it is easy to buckle when it is compressed, and its torsional performance is poor;
3. The wall thickness of cold-rolled formed steel is small, and there is no thickening at the corners where the plates are connected, so the ability to withstand local concentrated loads is weak.
3. Cold rolling
Cold rolling refers to the rolling method of extruding steel with the pressure of rolls at room temperature to change the shape of steel. Although the process also heats up the steel plate, it is still called cold rolling.
Specifically, hot-rolled Steel Coils are used as raw materials for cold rolling, and pressure processing is carried out after pickling to remove scale, and the finished product is hard-rolled coils.
Generally, cold-rolled steel such as Galvanized Steel and color steel plate must be annealed, so the plasticity and elongation are also good, and it is widely used in automobiles, home appliances, hardware and other industries. The surface of the cold-rolled sheet has a certain degree of smoothness, and it feels smoother to the touch, mainly due to pickling. Generally, the surface finish of the hot-rolled sheet cannot meet the requirements, so the hot-rolled steel strip needs to be cold-rolled, and the thinnest 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 by cold rolling belongs to continuous cold deformation. The cold hardening caused by this process increases the strength and hardness of hard-rolled coils, and decreases the ductility and plasticity indicators.
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 the steel ingot, refine the grains 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; the bubbles, cracks and looseness formed during pouring can also be welded under high temperature and pressure.
shortcoming:
1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and stratification occurs. Delamination greatly deteriorates the properties of the steel in tension through the thickness, and there is a possibility of interlaminar tearing as the weld shrinks. The local strain induced by the shrinkage of the weld often reaches several times the strain at the yield point, which is much larger than the strain caused by the load;
2. Residual stress caused by uneven cooling. Residual stress is the internal self-equilibrium stress without external force. Hot-rolled steel sections of various sections have this kind of residual stress. Generally, the larger the section size of the section steel, the greater the residual stress. Although the residual stress is self-equilibrium, it still has a certain influence on the performance of the steel member under the action of external force. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.
Four. Summary
The difference between cold rolling and hot rolling is mainly the temperature of the rolling process. "Cold" means normal temperature, and "hot" means high temperature.
From a metallological point of view, the boundary between cold rolling and hot rolling should be distinguished by the recrystallization temperature. That is, rolling below the recrystallization temperature is cold rolling, and rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450-600°C.
The main differences between hot rolling and cold rolling are:
1. Appearance and surface quality
Since the cold plate is obtained by the hot plate after the cold rolling process, and some surface finishing will be carried out at the same time, the surface quality (such as surface roughness) of the cold plate is better than that of the hot plate, so if the product If there is a higher requirement for coating quality such as subsequent painting, cold plates are generally selected, and hot plates are divided into pickled plates and un-picked plates. The surface of the pickled plate has a normal metallic color due to pickling, but it is not The surface is cold-rolled, so the surface is still not as high as the cold plate, and the surface of the un-pickled plate usually has an oxide layer, blackening, or a black layer of ferric iron tetroxide. In layman's terms, it looks like it has been baked by fire, and if the storage environment is not good, it will usually have a little rust.
2. Performance
In general, the mechanical properties of hot plates and cold plates are considered to be indistinguishable in engineering, although there is a certain work hardening of cold plates in the cold rolling process, (but it does not rule out the strict requirements on mechanical properties, then need Treat them differently), the cold plate usually has a slightly higher yield strength than the hot plate, and the surface hardness is also higher, depending on the degree of annealing of the cold plate. But in any case, the strength of the annealed cold plate is higher than that of the hot plate.
3. Formability
Since the performance of the hot and cold plates is basically not much different, the influencing factors of the forming performance depend on the difference in the surface quality. Since the surface quality is better than that of the cold plate, generally speaking, for steel plates of the same material, the cold plate is better than the cold plate. The forming effect of the hot plate is better.