The World Steel Association names more than 3,500 different brands, each of which has unique characteristics. This variety makes it possible to select the necessary material for a specific area of application — construction, manufacture of household appliances, transport, and so on.
At the same time, the chemical composition of steel plays an important but not the only role. Even if the grades and specifications are the same, production processing — cold or hot rolled steel — has a significant impact on the final products. Knowing what material to use will help you avoid overspending raw materials, as well as save time and money on additional processing.
The difference between these two materials lies in the processing technology in production. Hot-rolled steel is obtained during metal processing using high temperature. Cold rolling is carried out at a temperature close to room temperature. In fact, this is the same hot-rolled steel that has undergone additional processing to obtain more accurate dimensions and better surface quality. Instead of heat, mechanical stress is used to change the metal structure. Deformation hardening can increase metal strength by up to 20%, as well as improve surface quality.
Hot-rolled steel is carried out at a temperature above 926° C — this value is higher than the recrystallization temperature of steel. Under such conditions, the metal can be easily shaped, which makes it easier to work with the material.
The process is as follows. The workpiece — a large rectangular piece of metal — is heated and then sent for pre-treatment. There, the workpiece is flattened into a roll. It is kept at a high temperature, after which it is run through a series of rollers at high speeds to reach its final dimensions. For sheet metal, rolled steel is rolled into coils and cooled. For other shapes, such as rods or plates, materials are sectioned and packed.
This type of metal has its advantages and disadvantages. Hot-rolled steel is generally cheaper than cold rolled steel due to the shorter duration and simplicity of the manufacturing process. At the same time, when the material cools, the steel is slightly deformed: slightly rounded edges and slight distortions appear at the corners (ideally square ones will not work). Another disadvantage is the rough, “scaly” surface (to remove it, additional processing is required — grinding or sandblasting).
Hot-rolled steel is used where precise shapes and tolerances are not required, for example, in the manufacture of agricultural equipment, for the manufacture of railways, and so on.
Such products are usually harder, stronger than standard hot-rolled metals, and have greater corrosion resistance. Cold-rolled steel provides steel with lower dimensional tolerances and a wider range of surface treatments. The term “cold rolled” is mistakenly used for all products, when in fact this name refers to the rolling of flat sheets and coils.
Before starting cold rolling, the metal must first be “pickled” — removing the scale formed during the hot rolling process from the material surface in an acid bath. The steel is then cooled to ambient temperature, after which it is passed through cold rolls at a temperature below the recrystallization temperature. After that, annealing is carried out, a process that relieves steel stress and also increases hardness and yield strength.
At the exit from the rollers of the cold rolling mill, the metal is checked using thickness gauges. Modern cold rolling mills are able to use an X-ray thickness gauge to achieve high production speed of sheet steel that meets many requirements for thickness and uniformity. Thickness gauges detect and correct deviations in thickness in real time, which makes it possible to obtain a high-quality steel strip.
Machines for making this type of steel are designed to allow the steel to turn and push back through the rollers, which gradually approach each other until the appropriate thickness is reached. When three to six pairs of rollers are used sequentially, the multi-cell mill can reduce the thickness by a predetermined amount until the desired one is achieved.
Typically, the resulting metal has a thickness of 0.12 to 2.5 mm. On modern efficient cold-rolling mills, pickled sheets or strips can be rolled at speeds of up to 4 m/s, and permanently operating tandem machines can produce up to 2.5 million tons of cold-rolled steel per year.
Like any material, this type of rental has its pros and cons. The first ones include high-quality, well-processed, smooth surfaces with lower tolerances. Cold-rolled rods are of regular shape, have well-defined edges and angles, and pipes are concentric uniform and straight. Cold rolling also makes it very easy to bend finished materials. The ability of materials to resist fracture makes them suitable for various applications where the material needs to be bent and shaped.
Among the disadvantages of cold rolled steel is the higher cost of steel in comparison with hot-rolled steel, as well as the insufficient variety of cold-rolled forms (sheets, box shapes).
Cold-rolled stainless steel as a raw material is widely used in medical, aerospace, and automotive technology. It is used to make household appliances and furniture; in construction it is used to build garages, steel awnings and other industrial buildings.
