hot rolled
Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains are deformed during processing, they recrystallize, which maintains an equiaxial microstructure and prevents the metal from hardening. The starting material is usually large pieces of metal, such as semi-finished casting products such as slabs, billets and billets. If these products come from a continuous caster in operation, the material is usually introduced directly into the rolling mills at the appropriate temperature. In smaller operations, the material starts at room temperature and must be heated. This is done in a gas or oil soaking pit for larger workpieces; for smaller workpieces, induction heating is used. As the material is worked, the temperature must be monitored to ensure that it remains above the recrystallization temperature. To maintain a safety factor, a finishing temperature is defined above the recrystallization temperature (between 50 and 100 °C above). If the temperature falls below this value, the material must be reheated before further hot rolling.[12].
Hot-rolled metals generally have little directionality in their mechanical properties, and lack deformation-induced residual stresses. However, in certain cases, non-metallic inclusions will impart some directionality and workpieces less than 20mm thick often have some directional properties. Non-uniform cooling will induce a large amount of residual stresses, which generally occurs in shapes that have a non-uniform cross section, such as I-beams. Although the finished product is of good quality, the surface is covered in scale, which is an oxide that forms at high temperatures. It is usually removed by stripping or a cleaning process, which reveals the smooth surface.[13] Dimensional tolerances are typically 2 to 5% of the corresponding design measurements.[14].
Hot rolled mild steel appears to have a wider tolerance for the level of carbon included than cold rolled steel and is therefore more difficult for a blacksmith to use. Also for similar metals, hot-rolled products appear to be less expensive than cold-rolled ones.[15].
Hot rolling is mainly used to produce sheet metal or simple cross sections, such as railway tracks. Other typical uses of hot rolled metal are:[16].
Lamination and forming
Rolling mills are often divided into roughing, intermediate and finishing sections. During form rolling, an initial billet (round or square) with an edge diameter usually ranging from 100 to 140 mm is continuously deformed to produce a certain finished product with a smaller cross-sectional dimension and geometry. Starting from a given billet, different sequences can be adopted to produce a given final product. However, since each rolling mill is significantly expensive (up to €2 million), a typical requirement is to reduce the number of rolling passes. Different approaches have been used to optimize the process, including empirical knowledge, the use of numerical models and artificial intelligence techniques. Lambiase and his collaborators[17][18] validated a finite element (FE) model to predict the final shape of a rolled bar when changing from a round to a flat section.
One of the main concerns when designing rolling mills is to reduce the number of passes. A possible solution to such requirements is the slit pass, also called split pass, which divides an incoming bar into two or more subparts, thus virtually increasing the cross-section reduction ratio per pass as reported by Lambiase.[19] Another solution to reduce the number of passes in rolling mills is the employment of automated systems such as the one proposed by Lambiase and Langella.[20] Subsequently, Lambiase further developed an automated system based on artificial intelligence and particularly an integrated system that includes an inferential engine based on Genetic Algorithms, a knowledge database based on an Artificial Neural Network trained by a parametric finite element model to automatically optimize and design rolling mills.[21].
cold rolled
Cold rolling is performed with the metal below its recrystallization temperature (usually at room temperature), increasing its strength through strain hardening by up to 20%. It also improves surface finish and keeps tolerances tighter. Commonly cold rolled products include sheets, strips, bars and rods; products that are usually smaller than the same hot rolled products. Due to the smaller size of the workpieces and their higher strength compared to hot-rolled material, four-stage or four-stage rolling machines are used.[2] Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.
Cold rolled sheets and strips are supplied in various forms: "fully hardened", "semi hardened", "quarter hardened" and "shallow rolled". Full laminate reduces thickness by 50%, while the others involve a smaller reduction. Cold-rolled steel is annealed to induce its ductility, which is simply known as "cold-rolled, closed annealing." Surface rolling involves the least amount of reduction, 0.5 to 1%. It is used to produce a smooth surface, uniform thickness and reduce the creep phenomenon by preventing Lüders bands from forming in subsequent processing, blocking dislocations on the surface. To prevent the formation of Lüders bands, it is necessary to create a substantial density of unpinned dislocations in the ferrite matrix. It is also used to remove roughness in galvanized steel. Surface rolled material is generally used in subsequent cold working processes when good ductility is required.
Other shapes can be cold rolled if the cross section is relatively uniform and the cross dimension is relatively small. Cold rolling forms require a series of different forming operations, for which sizing, decomposition, roughing, semi-roughing, semi-finishing and finishing lines are generally available.
If it is to be worked manually, softer, more consistent steels with lower levels of encapsulated carbon are the easiest to process, but at the cost of higher cost.[22].
Typical uses of cold rolled steel include metal furniture, desks, filing cabinets, tables, chairs, motorcycle exhaust pipes, computer cabinets and cases, appliances and components, shelving, lighting fixtures, hinges, pipes, steel drums, lawnmowers, electronic equipment cabinets, water heaters, metal containers, fan blades, pans, wall and ceiling mounting kits, and a wide variety of related products. construction.[23].