Continuous casting of billets is a widely used process in the steel industry. It is an important step in producing slabs and blooms that are used to make steel products such as I beams and other structural steel shapes. The process allows molten steel to be casted into a long strip which is then cut into individual billets. Despite its popularity, the process is prone to defects such as surface cracks.
Surface cracks, or longitudinal cracks, are a common defect when continuously casting billets. They are typically caused by the rapid cooling of the heated steel. When the steel cools too quickly, thermal stresses can be created within the material exceeding the yield strength of the steel and causing a surface crack.
Surface cracks can cause significant issues during the subsequent rolling and forming process. Excess localized stresses can cause further damage to the billet while in the mill. Additionally, the presence of a crack can increase the chances of a break or crack propagating further during the forming process. Surface cracks can also limit the amount of rolling and forming that can occur before the billet becomes too weak and must be scrapped.
The primary objective when preventing surface cracks during continuous casting is to control the rate of cooling of the steel. It is important to maintain a balance between cooling the steel quickly to create billets of the desired size and maintaining a rate slow enough to allow the steel to remain malleable.
The most common method for controlling the cooling rate of the steel is the use of coolant sprayed onto the molten steel from the water boxes. The coolant used depends largely on the type of steel being casted. The nozzle type and location also play a role in cooling the steel. Common nozzle types used for coolant include slot, showerhead, and spray. Additionally, coolant can also be injected directly into the steel as it passes through the mold.
Another key factor in controlling the rate of cooling is the mold or die used in the process. The size of the die and the type of surface finish on the inside of the die both have a large impact on the final cooling rate. The type of metal used for the die and the atmosphere inside the die are also important considerations.
Finally, the rate of the continuous casting process must remain uniform. If the speed of the machine fluctuates, it can affect the hot spots in the steel and cause the metal to cool too quickly and create a surface crack.
By controlling the rate of cooling through the use of coolant and the mold, maintaining a uniform casting speed, and using a high-quality die, the likelihood of surface cracks occurring can be significantly reduced. By doing this, more steel can be produced without impacting the quality of the billets.
