Both hot rolling and cold rolling are processes for forming steel plates or profiles, and they have a significant impact on the structure and properties of the steel.
Steel rolling mainly uses hot rolling, while cold rolling is usually only used to produce small-sized steel and thin plates, and other steel materials requiring precise dimensions.
I. Hot Rolling
By definition, steel ingots or billets are difficult to deform at room temperature and are not easy to process. They are generally heated to 1100-1250°C for rolling; this rolling process is called hot rolling.
The finishing temperature of hot rolling is generally 800-900°C, after which it is usually cooled in air. Therefore, the hot-rolled state is equivalent to normalizing treatment.
Most steel materials are rolled using the hot rolling method. Steel delivered in the hot-rolled state has a layer of iron oxide scale on its surface due to the high temperature, thus providing a certain degree of corrosion resistance and allowing for outdoor storage.
However, this layer of iron oxide scale also makes the surface of hot-rolled steel rough and the dimensions fluctuate considerably. Therefore, steel requiring a smooth surface, precise dimensions, and good mechanical properties needs to be produced by cold rolling using hot-rolled semi-finished or finished products as raw materials.
Advantages:
Fast forming speed, high output, and no damage to the coating. It can be made into a variety of cross-sectional shapes to meet the needs of different applications; cold rolling can cause significant plastic deformation in the steel, thereby increasing the yield point of the steel.
Disadvantages:
1. Although there is no hot plastic compression during the forming process, residual stress still exists within the cross-section, which inevitably affects the overall and local buckling characteristics of the steel;
2. Cold-rolled steel sections are generally open sections, resulting in low free torsional rigidity of the cross-section. They are prone to twisting under bending and prone to flexural-torsional buckling under compression, resulting in poor torsional resistance;
3. Cold-rolled steel has a smaller wall thickness, and there is no thickening at the corners where the plates are joined, resulting in weak resistance to localized concentrated loads.
II. Cold Rolling
Cold rolling refers to a rolling method that uses the pressure of rollers to compress steel at room temperature, changing the shape of the steel. Although the processing process also causes the steel plate to heat up, it is still called cold rolling. More specifically, cold rolling uses hot-rolled steel coils as raw material. After pickling to remove scale, it undergoes pressure processing, resulting in a finished product called a cold-rolled coil.
Generally, cold-rolled steel, such as galvanized and color-coated steel sheets, requires annealing, so it has better plasticity and elongation, and is widely used in the automotive, home appliance, and hardware industries. Cold-rolled sheets have a certain degree of surface smoothness and feel relatively smooth to the touch, mainly due to the pickling process. Hot-rolled sheets generally do not meet the required surface smoothness, so hot-rolled steel strips need to be cold-rolled. Also, the thinnest hot-rolled steel strip is generally 1.0mm thick, while cold rolling can achieve a thickness of 0.1mm. Hot rolling is rolling above the crystallization temperature, while cold rolling is rolling below the crystallization temperature.
Cold rolling changes the shape of the steel through continuous cold deformation. The cold work hardening caused by this process increases the strength and hardness of the cold-rolled coil, while reducing its toughness and ductility.
For end-use applications, cold rolling worsens stamping performance, making the product suitable for parts with simple deformation.
Advantages:
It can destroy the casting structure of the steel ingot, refine the grain size of the steel, and eliminate microstructural defects, thus making the steel structure denser and improving its mechanical properties. This improvement is mainly reflected along the rolling direction, so that the steel is no longer 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 (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, resulting in delamination. Delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and may cause interlayer tearing during weld shrinkage. 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 an internal self-balancing stress without external force. All hot-rolled steel sections of various cross-sections have this type of residual stress. Generally, the larger the cross-sectional size of the steel, the greater the residual stress. Residual stress, although self-balancing, still has a certain impact on the performance of steel components under external forces. For example, it can have adverse effects on deformation, stability, and fatigue resistance.
III. Summary:
The main difference between cold rolling and hot rolling is the temperature of the rolling process. "Cold" refers to room temperature, and "hot" refers to high temperature.
From a metallurgical 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℃.
The main differences between hot-rolled and cold-rolled steel are:
1. Appearance and surface quality:
Since cold-rolled sheets are obtained from hot-rolled sheets after the cold rolling process, and cold rolling also involves some surface finishing, cold-rolled sheets have better surface quality (such as surface roughness) than hot-rolled sheets. Therefore, if there are high requirements for the quality of subsequent painting and coating of the product, cold-rolled sheets are generally chosen. Hot-rolled sheets are further divided into pickled and unpickled sheets. Pickled sheets have a normal metallic color due to pickling, but since they are not cold-rolled, their surface quality is not as high as cold-rolled sheets. Unpickled sheets usually have an oxide layer on the surface, appearing dull, or having a black layer of iron oxide. Simply put, they look like they've been heated, and if stored in a poor environment, they will usually have some rust.
2. Performance: Generally, the mechanical properties of hot-rolled and cold-rolled sheets are considered to be the same in engineering applications, although cold-rolled sheets undergo some work hardening during the cold rolling process (however, this does not exclude situations where strict requirements on mechanical properties exist, in which case they need to be treated differently). Cold-rolled sheets usually have a slightly higher yield strength and surface hardness than hot-rolled sheets. The specific values depend on the degree of annealing of the cold-rolled sheet. However, regardless of the annealing process, the strength of cold-rolled sheets is higher than that of hot-rolled sheets.
3. Forming performance: Since the performance of cold-rolled and hot-rolled sheets is basically similar, the influencing factors on forming performance depend on the differences in surface quality. Because the surface quality of cold-rolled sheets is better, generally speaking, for steel sheets of the same material, cold-rolled sheets have better forming performance than hot-rolled sheets.
