What is Hot Rolled Stainless Steel?
Hot rolling uses a slab as raw material, after heating, it is made into strip steel by the rough rolling mill and finishing mill. The hot steel strip from the last rolling mill of the finishing rolling is cooled to a set temperature by laminar flow and then coiled into a steel coil by the coiler. The cooled steel coil undergoes different finishing operations according to the different needs of users.
Lines (flattening, straightening, cross-cutting or slitting, inspection, weighing, packaging and marking, etc.) are processed into steel plates, flat coils, and slit steel strip products. To put it simply, a piece of steel billet is heated (that is, the hot steel block that is burning red on the TV) after a few passes, and then trimmed to be straightened into a steel plate. This is called hot rolling.
Hot rolling is to soften the material at a temperature higher than the recrystallization temperature of the alloy and then use a pressing wheel to press the material into a cross-section of a thin sheet or billet to deform the material, but the physical properties of the material do not change.
What is Cold-Rolled Stainless Steel?
Cold rolling uses hot-rolled steel coils as raw materials and then undergoes cold tandem rolling after pickling to remove the oxide scale. The finished product is hard-rolled coils. Because the cold work hardening caused by continuous cold deformation increases the strength and hardness of the rolled hard coil, and the ductility index decreases, the stamping performance will deteriorate, and it can only be used for simple deformed parts.
Hard-rolled coils can be used as raw materials for hot-dip galvanizing plants because the hot-dip galvanizing units are equipped with annealing lines. To put it simply, cold rolling is processed and rolled based on hot-rolled stainless coils. The processing process is: hot rolling-pickling-cold rolling.
Cold rolling is a process in which materials that have been hot rolled, depicted, and deoxidized are further rolled with a pressure wheel at a temperature lower than the recrystallization temperature of the alloy to allow the material to recrystallize. After repeated cold pressing-recrystallization-annealing-cold pressing (repeated 2 to 3 times), the metal in the material undergoes a molecular level change (recrystallization), and the physical properties of the formed alloy change.
Therefore, the surface quality is good, the smoothness is high, and the product dimensional accuracy is high. The performance and organization of the product can meet some special requirements, such as electromagnetic performance, deep drawing performance, etc. Cold-rolled strips can be used in automobile manufacturing, electrical products, rolling stock, aviation, precision instruments, food cans, etc.
The Difference Between Hot Rolling and Cold Rolling
Cold rolled formed steel allows partial buckling of the section, which can make full use of the bearing capacity of the bar after buckling; while hot-rolled formed steel does not allow local buckling of the section.
Hot rolled stainless steel and cold rolled stainless steel have different causes of residual stress, so the distribution on the cross-section is also very different. The residual stress distribution on the cross-section of cold-formed thin-walled steel is curved, while the residual stress distribution on the cross-section of hot-rolled or welded steel is thin-film.
2b VS No.1 Finish Stainless Steel
Hot rolling and cold rolling are two common methods of processing metals, each with distinct characteristics and applications. Here’s a breakdown of the differences between them:
Hot Rolling
- Temperature: Hot rolling involves rolling metal at temperatures above its recrystallization temperature, which is typically over 1,000°C (1,832°F) for steel. This high temperature makes the metal more malleable and easier to shape.
- Process: In hot rolling, the metal is heated to a temperature where it becomes plastic and can be shaped. It is then passed through rollers to achieve the desired shape and dimensions. The metal can undergo significant deformation due to its high temperature.
- Surface Finish: The surface finish of hot-rolled metal is usually rough and may have scale or oxidation because of the high temperatures involved. It often requires additional processing to achieve a smooth surface.
- Properties: Hot rolling can induce some changes in the material’s microstructure, such as grain refinement. However, the final dimensions and surface quality may not be as precise as those achieved with cold rolling.
- Applications: Hot-rolled products are used in structural applications, such as beams, channels, and plates, as well as in the production of bars and pipes. The process is ideal for large-scale production.
Cold Rolling
- Temperature: Cold rolling is performed at or near room temperature, typically below 200°C (392°F). The metal is processed below its recrystallization temperature.
- Process: Cold rolling involves passing the metal through rollers at room temperature, which compresses and shapes it. This process increases the metal’s strength and hardness due to strain hardening.
- Surface Finish: Cold-rolled metal has a much smoother surface finish compared to hot-rolled metal. It often has a shiny, reflective surface and tighter tolerances.
- Properties: Cold rolling improves the metal’s mechanical properties, such as yield strength and tensile strength, due to the strain hardening effect. The final dimensions are more precise and uniform.
- Applications: Cold-rolled products are commonly used in applications requiring high precision and a good surface finish, such as in automotive parts, appliances, and electronics. It’s also used for producing thinner sheets and strips.
Comparison of the Advantages and Disadvantages of Hot Rolling and Cold Rolling
The free torsion stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsion resistance of hot-rolled steel is better than that of cold-rolled steel.
Hot rolling and cold rolling have a great influence on the structure and properties of steel. The rolling of steel is mainly hot rolling, and cold rolling is only used to produce small sections and thin plates.
The advantages of hot rolling: it can destroy the casting structure of the steel ingot, refine the grain 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; bubbles, cracks, and looseness formed during casting can also be welded under high temperatures and pressure.
Disadvantages of Hot Rolling:
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 delamination (interlayer) occurs. Delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and it is possible that 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 without external force. Hot-rolled steel sections of various cross-sections have such residual stresses. Generally, the larger the section size of the steel section, the larger the residual stress. Although the residual stress is self-balanced, 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.
The advantages of cold rolling: fast forming speed, high output, and without damaging the coating, it 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 improving The yield point of steel.
Disadvantages of Cold Rolling:
1. Although there is no thermal plastic compression during the forming process, residual stress still exists in the section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The cold-rolled section steel style is generally an open section, which makes the free torsional stiffness of the section lower. It is prone to torsion when under bending, and it is prone to bend torsion buckling when under pressure, and its torsion resistance is poor;
3. The wall thickness of cold-rolled steel is small, there is no thickening at the corners where the plates are joined, and the ability to withstand local concentrated loads is weak.