Steel Billet Cooling
Steel billets are the basic form of steel before it is processed into a variety of shapes and grades. The cooling process following hot rolling is critical to ensure that the microstructures of the billets create the desired properties such as strength, ductility and machinability. Billets need to be cooled in order to reduce the high temperature of hot rolling and to prevent thermal damage or cracking.
The foundation of the cooling process is defined by the cooling rate from one homogeneous temperature to another. Once the cooling rate and conditions are established, the steel can then be cooled in various ways such as spraying with water or salt, fan cooling, and direct gas quenching.
Although the cooling rate and conditions primarily determine the properties of steel, various factors can influence the cooling rate. These factors include the size and shape of the billets, the type and concentration of the coolant, the duration and intensity of the cooling cycle, and the cooling medium delivery method. The shape, size, and type of billet all play a role in how quickly the billet cools. Larger and thicker billets take longer to cool and therefore require a longer cooling time or time period. The type and concentration of coolant are also critical, as salt and water cooling rates vary.
Once a cooling rate is established, the temperature at which a billet cools is also critical for the development of the microstructures. Temperature designation for steel starts at Ar3 and goes up to A100. This temperature represents the temperature which the steel cools from. For example, steel cooled from an Ar3 temperature will always form a ferrite-cementite microstructure. The temperature designation variance allows for components cooled from different temperatures to have different mechanical properties due to varying microstructures.
The steel can be cooled in many different ways, but most of the time water cooling is used because it is the most efficient. Water cooling takes advantage of the large latent heat of water, which is a much higher value than any of the other main cooling media. It is also relatively easy to control, is cost-effective, and is a safe way to cool steel. Other methods include air cooling, fans, salt baths and direct gas quenching.
Air cooling is removed once the temperature of the billet reaches a set temperature, usually 100 degrees Celsius. Once the temperature of the billet has reached an acceptable level, it is then further cooled by a fan, usually by rotating it in a large cooling chamber. Another method, for instance cooling with a salt bath, allows the controlling of the cooling rate by the concentration of salt in the bath and by controlling the temperature of the bath. Finally, direct gas quenching is a method which cools steel very quickly, usually to below Ar3 temperatures, and is used for steel that is intended for high strength applications.
The goal of the steel cooling process is to cool it quickly yet still maintain the optimal properties that the steel will possess when processed into its final form. The proper cooling method and rate must be established up front and the cooling process must be monitored and controlled during the entire process. Taking the time to ensure that the billet cools to the desired temperature can help prevent problems during subsequent processing and save time, energy, and money.
