What Is Cold Rolling And Hot Rolling In Steel

In the steel industry, we often hear about the concepts of hot rolling and cold rolling, so what are they?

In fact, the steel billets produced from the steelmaking plant are only semi-finished products and must be rolled at the rolling mill to become qualified steel products. Hot rolling and cold rolling are two common rolling processes.

The rolling of steel is mainly carried out by hot rolling, while cold rolling is mainly used to produce small-sized steel sections and thin plates.

The following are common cold and hot rolling situations of steel:

Wire: with a diameter of 5.5-40 millimeters, coiled in coils, all made of hot-rolled material. After cold drawing, it belongs to cold drawn materials.

Round steel: In addition to precision sized bright materials, 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 are available, and the cold-rolled material is generally thinner.

Steel plate: Cold rolled plate is generally thinner, such as automotive plate; There are many hot-rolled medium and thick plates, some of which have a similar thickness as cold-rolled ones, but their appearance is significantly different. Angle steel: all hot-rolled.

Steel pipes: both welded, hot-rolled, and cold drawn.

Channel steel and H-shaped steel: hot-rolled

Steel bars: hot-rolled materials.

Hot rolling and cold rolling are both processes for forming steel plates or profiles, which have a significant impact on the microstructure and properties of the steel.

The rolling of steel is mainly based on hot rolling, while cold rolling is usually only used to produce precision sized steel such as small section steel and thin plates.

The termination temperature of hot rolling is generally 800-900 ℃, and then it is generally cooled in air, so the hot rolling state is equivalent to normalizing treatment. Most steel is rolled using the hot rolling method. Steel delivered in hot-rolled condition, due to high temperature, forms a layer of iron oxide on the surface, which has a certain degree of corrosion resistance and can be stored outdoors. But this layer of iron oxide also makes the surface of hot-rolled steel rough, with significant size fluctuations. Therefore, steel that requires a smooth surface, precise size, and good mechanical properties should be produced using hot-rolled semi-finished or finished products as raw materials and then cold-rolled.

Advantages: Fast molding speed, high yield, and no damage to the coating. It can be made into various cross-sectional forms to meet the needs of usage conditions; Cold rolling can cause significant plastic deformation of steel, thereby increasing its yield point.

Disadvantages: 1. Although there is no thermal plastic compression during the forming process, residual stress still exists in the section, which inevitably affects the overall and local buckling characteristics of the steel;

2. The style of cold-rolled steel is generally an open section, which reduces the free torsional stiffness of the section. Torsion is prone to occur when subjected to bending, and bending torsion buckling is prone to occur when subjected to compression, resulting in poor torsional performance;

3. The cold-rolled formed steel has a smaller wall thickness and no thickening at the corners of the plate connection, resulting in weak ability to withstand localized concentrated loads.

Cold rolling refers to the rolling method of changing the shape of steel by squeezing it with the pressure of a rolling roller at room temperature. Although the processing process can 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, undergoes acid washing to remove oxide scales, and then undergoes pressure processing to produce rolled hard coils. Generally, cold-rolled steel such as galvanized and color steel plates need to be annealed, so their plasticity and elongation are also good, and they are widely used in industries such as automobiles, household appliances, and hardware.

The surface of cold-rolled sheet has a certain degree of smoothness, and it feels relatively smooth to the touch, mainly due to acid washing. The surface smoothness of hot-rolled plates generally does not meet the requirements, so hot-rolled steel strips need to be cold-rolled. The thinnest thickness of hot-rolled steel strips is generally 1.0mm, and cold-rolled steel strips can reach 0.1mm.

Hot rolling is rolling above the crystallization temperature point, while cold rolling is rolling below the crystallization temperature point. The change in steel shape caused by cold rolling belongs to continuous cold deformation, and the cold work hardening caused by this process increases the strength and hardness of the rolled hard coil, while the toughness and plasticity index decrease.

For end use, cold rolling deteriorates stamping performance and products are suitable for simple deformed parts.

Advantages: It can destroy the casting structure of steel ingots, refine the grain size of steel, and eliminate defects in microstructure, thereby making the steel structure dense and improving its mechanical properties. This improvement is mainly reflected in the direction of rolling, 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. After hot rolling, non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, resulting in delamination. Layering greatly deteriorates the tensile performance of steel along the thickness direction, and there is a possibility of interlayer tearing during weld shrinkage. The local strain induced by weld seam shrinkage often reaches several times the yield point strain, which is much larger than the strain caused by load;

2. Residual stress caused by uneven cooling. Residual stress refers to the stress that self equilibrates internally without external forces, and is present in various hot-rolled steel sections. Generally, the larger the section size of the steel, the greater the residual stress. Although residual stress is self equilibrium, it still has a certain impact on the performance of steel components under external forces. It may have adverse effects on deformation, stability, fatigue resistance, and other aspects.