hot rolled steel
Why Distinguish Between Hot Rolled Steel And Cold Rolled Steel
Hot rolled and cold rolled are both processes for forming steel plates or profiles, and they have a great impact on the structure and performance of steel. What is the difference between them?
Steel rolling is mainly hot-rolled, and cold-rolled steel is usually only used to produce small-sized steel and thin plates with precise dimensions.
Common hot and cold rolling of steel:
Wire rod: 5.5~40 mm in diameter, coiled, all hot-rolled. After cold drawing, it belongs to cold-drawn material;
Round steel: except for bright materials with precise dimensions, it is generally hot-rolled, and there are also forged materials (with forging marks on the surface);
Strip steel: both hot-rolled and cold-rolled, cold-rolled materials are generally thinner;
Steel plate: cold-rolled plates are generally thinner, such as plates for automobiles. There are more hot-rolled medium and thick plates, and the appearance is obviously different;
Angle steel: all hot-rolled;
Steel pipe: both welded hot-rolled and cold-drawn;
Channel steel and H-shaped steel: hot-rolled;
Rebar: hot-rolled material
Hot rolling
By definition, steel ingots or billets are difficult to deform and process at room temperature. They are generally 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 they are generally cooled in air, so the hot rolling state is equivalent to normalizing.
Most steel products are rolled by hot rolling. Steel delivered in hot rolling state has a layer of iron oxide on the surface due to high temperature, so it 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 the size fluctuates greatly. Therefore, steel products that require smooth surface, precise size and good mechanical properties should be cold rolled with hot rolled semi-finished products or finished products as raw materials.
Advantages:
Fast forming speed, high output, no damage to coating, can be made into a variety of cross-sectional forms to meet the needs of use conditions. Cold rolling can cause large plastic deformation of steel, thereby increasing the yield point of steel.
Disadvantages:
1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, resulting in stratification. Stratification greatly deteriorates the tensile properties of the steel along the thickness direction, and interlayer tearing may occur when the weld shrinks. 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;
2. Residual stress caused by uneven cooling. Residual stress is the internal self-balanced stress in the absence of external force. Hot-rolled steel sections of various cross-sections have this type of residual stress. Generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although the residual stress is self-balanced, it still has a certain impact on the performance of steel components under external forces. For example, it may have an adverse effect on deformation, stability, and fatigue resistance.
Cold rolling
Cold rolling refers to a rolling method that changes the shape of steel by squeezing steel with the pressure of rollers at room temperature. Although the processing process will also cause the steel plate to heat up, it is still called cold rolling. Specifically, cold rolling uses hot-rolled steel coils as raw materials, and after pickling to remove the oxide scale, it is pressure processed, and the finished product is a hard rolled coil.
Generally, cold-rolled steel such as galvanized and color-coated steel plates must be annealed, so the plasticity and elongation are also good, and they are widely used in industries such as automobiles, home appliances and hardware. The surface of cold-rolled plates has a certain degree of finish, and it feels smooth to the touch, which is mainly due to pickling. The surface finish of hot-rolled plates generally does not meet the requirements, so hot-rolled steel strips need to be cold-rolled. In addition, the thinnest thickness of hot-rolled steel strips is generally 1.0mm, and cold rolling 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 in the shape of steel 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 reduces the toughness and plasticity index.
For terminal use, cold rolling deteriorates the stamping performance, and the product is suitable for parts with simple deformation.
Advantages:
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.
Disadvantages:
1. Although there is no hot plastic compression during the forming process, there are still residual stresses in the cross section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The cold-rolled steel section is generally an open section, which makes the free torsional stiffness of the section low. It is easy to torsion when bent and bending-torsion buckling when compressed, and the torsional performance is poor;
3. The wall thickness of the cold-rolled 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.
The main differences between hot rolling and cold rolling are:
1. Appearance and surface quality
Since cold rolling is the result of cold rolling of hot rolling, and some surface finishing is also performed during cold rolling, the surface quality (such as surface roughness, etc.) of cold rolling is better than that of hot rolling. Therefore, if there are high requirements for the quality of subsequent coating such as painting, cold rolling is generally selected. Hot rolling is divided into pickled and unpickled. The surface of pickled plate has a normal metallic color due to pickling, but it is not cold rolled, so the surface is still not as high as that of cold plate. Unpickled plate usually has an oxide layer on the surface, which is black or has a black layer of ferroferric oxide. In layman’s terms, it is like being roasted by fire, and if the storage environment is not good, it usually has a little rust.
2. Performance
Generally speaking, the mechanical properties of hot plates and cold plates are considered to be the same in engineering. Although cold plates undergo a certain degree of work hardening during the cold rolling process (but this does not rule out the situation where strict mechanical properties are required, which requires different treatment), cold plates usually have a slightly higher yield strength and surface hardness than hot plates. The specific situation depends on the degree of cold plate annealing. However, no matter how the annealing is done, the strength of cold plates is higher than that of hot plates.
3. Forming performance
Since the performance of hot and cold plates is basically the same, the factors affecting the forming performance depend on the difference in their surface quality. Since the surface quality of cold plates is better, generally speaking, for steel plates of the same material, the forming effect of cold plates is better than that of hot plates.
Hot rolling and cold rolling are two common steel processing technologies. GloryRail can process products according to customer needs.
