Continuous casting billets solidification and cooling is a very important process in steel production. The billets is solidified and cooled down in the mould to meet the physical and chemical property requirements of the product. In order to ensure the product quality, casting billets solidification and cooling must meet the metallurgical requirements, i.e. to guarantee the billets with the desired property and inner totally or partially homogeneity in this process.
For the continuous casting billets solidification and cooling, reasonable principles and techniques should be followed to ensure reliable and stable product quality. There are impact factors such as mould size and shape, mould material, inner/external structure of billet, loss-on-ignition of billet, cooling rate and metallotropic transformation of billet which affect casting billets solidification and cooling.
1. Mould Size and Shape
Mould size and shape have important impacts on the solidification and cooling of the casting billets because the mould confines the cooling rate and the dimensional distortion during solidification and cooling. Large moulds allow more cooling time while the billets have a higher probability to deform in large moulds. On the contrary, the billets have less deformation when they shrink in small moulds.
2. Mould Material
Mould materials have a great impact on the cooling rate of casting billets. Mould materials used mainly are copper and copper alloys, aluminum and aluminum alloys. Cooper and copper alloys have good low-temperature conductivity, high stability and heat transfer capacity, so copper moulds are used for most continuous casting billets solidification and cooling process.
3. Inner/External Structure of Billet
Inner/external structure of the billet is crucial for the continuous casting billets solidification and cooling process. Core layer of the billet absorbs more cooling down energy which determines its microstructure and mechanical property. The thicker the cooling layer is, the faster the cooling rate will take. In addition, the cooling rate is firstly affected by the cooling rate of the surface layer of billet toner the core layer.
4. Loss-on-ignition of Billet
During continuous casting, the billet causes the oxidation of some part of the residual carbon inside itself. This oxidation is measured as LOI (Loss on Ignition). Higher LOI indicates more loss-on-ignition and higher oxidation rate during continuous casting billets solidification and cooling.
5. Cooling Rate
Cooling rate has an impact on billet structure and mechanical property. The cooling rate should be fast and uniform to meet the required quality of the product. For a certain billet, low cooling rate leads to small grains in casting billets which causes the aggregation of impurities and metallurgical defects. However, high cooling rate gives small grains which results in long billet and low elongation ratio.
6. Metallotropic Transformation
Metallotropic transformation is the process of changing from one to another crystal structure. The most common metallotropic transformation during continuous casting billets solidification and cooling is ferrite to austenite. Different cooling rate can lead to different crystal phase transformation. Low cooling rate will cause slow phase changes and higher cooling rate will cause faster phase changes.
Therefore, in order to ensure reliable and stable product quality, reasonable principles and techniques should be applied to the continuous casting billets solidification and cooling process. Appropriate mould size and shape, mould material, inner/external structure of the billet, loss-on-ignition of the billet, cooling rate and metallotropic transformation must be considered and optimized according to product requirements.
