Continuous casting is a process which liquid steel is poured into an open-ended, water-cooled mould, which is then slowly extracted to form solidified shapes of a fixed cross-section. The process is carried out continuously and includes a variety of measures taken to assure quality and minimize wastes.
The cooling of the liquid steel is critical in the continuous casting process. A uniform cooling rate is essential in order to form a homogeneous and fine-grained structure which is necessary for the desired product properties. The cooling process is further complicated by the phenomenon of solidification shrinkage. This occurs as the strand of solidified metal progressively cools, resulting in a decreases cross-section and length of the cast product.
The initial formation of a shell along the strand surface is known as ‘skin effect’. This shell is relatively thick compared to the interior and is due to a lower cooling rate on the exterior surface compared to the interior. The result of the skin effect is a non-homogeneous structure and reduces the amount of useful product.
The skin effect and solidification shrinkage need to be overcome in order to efficiently form the desired product shape. This is accomplished by different techniques such as casting sprays, stoppers and fall-off devices, combined with the precise control of casting and process parameters for optimum cooling conditions. Various types of mould lubricants and exterior coatings of the strand can also be used to enhance the release of the strand from the mould.
The cooling rate of the liquid steel has to be sufficient to eliminate the formation of hot tears and laminations in the solidified strand. Hot tears and laminations occur as a result of an insufficient cooling rate, resulting in an interaction between several areas undergoing solidification at different rates, forming discontinuities.
Overall, the most important consideration during continuous castingis achieving the right balance between cooling rate and the possibility of forming hot tears and laminations. Different installations may require different solutions depending on the size of the casting and product quality. Careful consideration must be given to the mould design and heating/cooling system for reliable chemical and mechanical properties of the product.
Generally, the continuous casting process is carried out in a continuous and automated manner. The process involves the coordination of various parameters, such as the composition and temperature of the liquid steel, to ensure that each casting meets the desired specifications. As the strand exits the mould, it is cut into individual pieces by means of a guillotine cutter. The size and number of pieces depends on the desired product properties. The cast products are then cleaned, cooled and transferred to further processing stages, such as hot rolling or forging.
In conclusion, the continuous casting process is a complicated process that requires careful measurement and control of numerous parameters in order to achieve the desired product quality. The most important considerations are the uniform cooling rate and the elimination of hot tears and laminations. Various techniques, such as the use of casting sprays, stoppers and fall-off devices, can be used to attain the desired product shape and the use of lubricants, coatings and cooling systems can assist in ensuring a successful result.
