Hot rolling and cold rolling

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By definition, ingots or billets of steel are difficult to deform at room temperature and not easy to process; therefore, they are usually heated to 1100°C–1250°C before rolling, and this rolling process is known as hot rolling. The finishing temperature for hot rolling is generally 800°C to 900°C, after which it is usually cooled in air; thus, the hot-rolled state is equivalent to normalizing treatment. Most steel is rolled using the hot rolling method. Steel delivered in the hot-rolled state develops an oxide scale on its surface due to the high temperatures, which confers a certain degree of corrosion resistance, allowing it to be stored outdoors. However, this layer of iron oxide also makes the surface of hot-rolled steel rough and results in large size variations; therefore, for steel that requires a smooth surface, precise dimensions, and good mechanical properties, it is necessary to use hot-rolled semi-finished or finished products as raw materials and then proceed with cold rolling. Advantages: Fast forming speed, high production volume, and no damage to the coating; it can be made into a variety of cross-sectional shapes to meet the requirements of different usage conditions ； Cold rolling can cause significant plastic deformation in steel, thereby increasing its yield strength. Disadvantages: 1. Although no hot plastic compression occurs during the forming process, residual stresses remain within the cross-section, which inevitably affects the overall and local buckling behavior of the steel material ； 2. Cold-rolled section shapes generally have an open cross-section, resulting in a low free torsional stiffness of the cross-section. It is prone to torsion under bending, and to bending-torsional buckling under compression; its torsional resistance is poor ； 3. Cold-rolled formed steel has a thin wall thickness, and no thickening is applied at the corners where the plates meet, resulting in a weak capacity to withstand localized concentrated loads. The hot-rolled coil is uncoiled, and continuous welding begins, marking the start of the cold rolling process: after pickling, the steel goes through rolling in mills to produce hard-rolled coils; these cleaned hard-rolled coils then enter the heat treatment stage. Cold rolling refers to a rolling method in which steel is compressed under the pressure of rollers at room temperature, thereby changing its shape. Animation of the production process for cold rolling and galvanizing lines. Although the processing process also raises the temperature of the steel sheet, it is still called cold rolling. More specifically, cold-rolling uses hot-rolled steel coils as raw material; after pickling to remove the oxide scale, it undergoes pressure processing, and the resulting product is hard-coiled steel. Generally, cold-rolled steels such as galvanized and colored steel sheets require annealing, which gives them good plasticity and elongation rates; they are therefore widely used in industries such as automobiles, home appliances, and hardware. Cold-rolled sheets have a certain level of smoothness on their surface; they feel smooth to the touch, and this is mainly due to the pickling process. The surface finish of hot-rolled sheets generally does not meet the required standards; therefore, hot-rolled steel strips need to be cold-rolled. Additionally, the thinnest thickness of hot-rolled steel strips is usually 1.0 mm, while cold-rolling can achieve a thickness of 0.1 mm. Hot rolling is rolling above the crystallization temperature, while cold rolling is rolling below the crystallization temperature. The change in the shape of steel due to cold rolling is a form of continuous cold deformation; the cold working hardening resulting from this process increases the strength and hardness of the rolled coil, while reducing its toughness and ductility. For end-use purposes, cold rolling deteriorates the stamping properties, making the product suitable for parts that require only simple deformation. Advantages: It can disrupt the casting structure of steel ingots, refine the grain structure of the steel, and eliminate defects in the microstructure, thereby making the steel structure denser and improving its mechanical properties. This improvement is mainly evident in the direction of rolling, thereby making the steel less isotropic to a certain extent ； Bubbles, cracks, and porosity formed during casting can also be welded together under high temperature and pressure. Disadvantages: 1. After hot rolling, the non-metallic inclusions within the steel (mainly sulfides and oxides, as well as silicates) are compressed into thin layers, resulting in delamination. Stratification **degrades** the tensile properties of steel in the thickness direction, and interlayer tearing may occur during weld contraction. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much greater than the strain caused by loads ； 2. Residual stresses caused by uneven cooling. Residual stress is the stress that exists in internal self-equilibrium in the absence of external forces. All hot-rolled steel sections have such residual stress, and generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although residual stress is in self-equilibrium, it still has a certain impact on the performance of steel components under external forces. It can have adverse effects on deformation, stability, fatigue resistance, and other aspects. In summary, the main difference between cold rolling and hot rolling lies in the temperature during the rolling process. ““Cold” refers to normal temperature, while “hot” refers to high temperature. From a metallurgical perspective, the boundary between cold rolling and hot rolling should be defined by the recrystallization temperature. That is, rolling below the recrystallization temperature is cold rolling, while rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450°C to 600°C. The main differences between hot-rolled and cold-rolled sheets are as follows: 1. Appearance and surface quality: Since cold-rolled sheets are obtained from hot-rolled sheets through a cold rolling process, and surface finishing is also carried out during cold rolling, cold-rolled sheets have better surface quality (such as lower surface roughness) compared to hot-rolled sheets. Therefore, when high standards are required for subsequent coating processes such as painting, cold-rolled sheets are generally chosen. Hot-rolled sheets can be further divided into pickled and unpickled types; pickled sheets have a normal metallic color due to the pickling process, but their surface quality is still not as good as that of cold-rolled sheets. Unpickled sheets usually have an oxide layer on their surface, appear dull, or have a dark layer caused by iron tetroxide. In simple terms, it’s as if it’s been baked by fire, and if it’s stored in poor conditions, it usually develops some rust. 2. Performance: Under normal circumstances, the mechanical properties of hot plates and cold plates are considered to be identical in engineering applications. Although cold plates experience a certain degree of work hardening during the cold rolling process, (this does not rule out situations where strict requirements are placed on mechanical properties, in which case a distinction needs to be made.) Cold plates generally have a slightly higher yield strength than hot plates, as well as a higher surface hardness; the exact values depend on the degree of annealing of the cold plates. However, regardless of the annealing process, the strength of the cold plate is higher than that of the hot plate. 3. Forming properties: Since the performance of cold and hot plates is roughly similar, the factor that affects forming properties lies in the difference in surface quality. As the surface quality of cold plates is better, generally speaking, for steel plates of the same material, cold plates yield better forming results than hot plates.