Depending on the chemical composition of the steel, the state of the structure, the size of the product, the possible defects after cooling, and the conditions of the mill output, cooling site and cooling equipment, the hot rolled steel can be cooled to room temperature by different cooling methods.
1) Air Cooling This is a method of natural cooling in the air, which is widely used. After cooling in air, the metallographic structure is not martensite or semi-martensitic steel. After hot rolling, air cooling is used, such as low carbon steel, ordinary low alloy strength steel, most carbon structural steel and alloy structure. Steel, as well as austenitic stainless steel, etc., are cooled in this way.
2) Quick cooling This is a forced cooling method for blasting, spraying, and watering. The process is characterized in that the steel is naturally cooled after being cooled to a certain temperature for a certain period of time. For example, steels that require inspection of carbide mesh defects use this type of cooling. The carbon steel wire rod is also fast cooled after rolling to improve the comprehensive mechanical properties of the wire.
3) Slow cooling It is characterized by hot rolling of billets or steels stacked together to cool them slowly to prevent white point defects. The specific method of slow cooling depends on the production conditions, and can be carried out in a special slow cooling pit. It can be carried out in a special movable slow cooling box, or it can be stacked on the ground and covered with sand, asbestos residue and the like. Slow down. This cooling method is suitable for martensite, semi-martensitic and Leysite steels, such as high-speed tool steel, martensitic stainless steel, partially high-alloy tool steels and high-alloy structures, which are sensitive to stress during cooling. Very strong.
4) Heat treatment The heat treatment methods commonly used after hot rolling are annealing, high temperature tempering, normalizing, and the like.