Reduce the temperature of hot - processed steel from the high temperature at the end of processing to room temperature. Different microstructure can be obtained during the cooling process of rolled steel, which affects the performance of the product. Therefore, the cooling method after hot processing should adopt natural cooling, forced cooling, slow cooling and other methods respectively according to the technical requirements of the product and the characteristics of steel.
Natural cooling is divided into air cooling and reactor cooling.
The hot rolled steel is cooled by air cooling. Air cooling is usually carried out on special cold beds. There are many forms of cold bed, such as step type, disc roller type, skip type, pendulum type, inclined roll type, reciprocating pawl type, chain type, etc. The rolling material moves forward on the cooling side of the cold bed, and it is required to minimize the scratches caused by the bending and the relative friction between the cold bed surface. Rolling final cooling temperature of the cooling bed under the (temperature) should be less than 200 ℃, in order to prevent the metal rolling bent in hang hang, piled up or directly after cold straightening due to affect the quality of straightening straightening temperature was too high. The cooling speed of the rolled material on the cold bed is related to the size of the rolled material, the density of the rolled material on the cold bed and climatic conditions. Hot rolled strip and wire are cooled in the air as coils, although it is also air cooled, but the cooling speed is slow, especially the cooling speed of steel coil core is only equal to the cooling speed of general rolling stock. Air cooling is the most commonly used cooling method. It can be used when there is enough cold bed area and there is no special requirement for steel properties and when there is no thermal stress crack or martensite or semi-martensite.
After stacking the cold steel in the cold bed for a short time, it is piled up and cooled naturally in the air. The cooling rate of the rolled products is slowed down by reducing the contact area between the steel and air. Dismantle the heap heap center temperature should not be greater than 200 ℃. Reactor cooling is applied to billet and steel which is sensitive to white spot and stress.
Faster cooling than natural cooling, also known as accelerated cooling. It can be cooled by air blower, water mist cooling, air and water mixing cooling, water cooling, etc. Accelerated cooling was first used to shorten cooling time to reduce cold bed area.
From the 1960s, accelerated cooling was not only used to improve the productivity of the cold bed, but also used to :(1) refine the grains of the hypoeutectic steel and improve the comprehensive performance of the material; (2) to reduce or eliminate the mesh structure of hypereutectic steel; (3) replace the quenching process or quenching self-tempering process of the tempered steel with the high-temperature post-rolling and the processed grain structure to save energy and improve material properties; (4) replace the heat treatment process before further processing of some steels, such as replacing the lead bath quenching before steel wire drawing (see soxinization treatment), etc.; (5) use the rapid cooling to remove or reduce the iron oxide on the surface of rolled materials, improve the surface quality of rolled materials and metal yield, and reduce the pickling time and acid consumption before secondary processing. The purpose and cooling method of accelerating cooling after rolling of rolled steel of different varieties and varieties, such as billet cooling, wire cooling, wire cooling, profile control cooling, steel plate control cooling and tube control cooling are different.
A cooling that is slower than natural cooling. The purpose of slow cooling is: (1) to prevent white spots on certain steels (such as alloy structural steels and alloy tool steels); (2) avoid cracks in some steel with strong stress sensitivity (such as high-speed tool steel, martensitic stainless steel, etc.) during air cooling due to thermal stress and tissue stress; (3) avoid excess martensite and surface hardness on steel (such as spring steel) during air cooling.