The calcium used in the deoxidizing alloy is absorbed into the steel melt, forming a strong oxide that absorbs oxygen much more effectively. This reduces the content of dissolved oxygen in liquid steel and thereby helps to prevent inclusion formation.
The graph shows the number density of inclusions in liquid steel at the beginning, before calcium addition and end of LF refining and in casting slab for different Si-Ca wire feeding amount.
The thermodynamics of the oxide systems B2O3-Al2O3-MgO, B2O3-Al2O3-CaO, and B2O3-MgO-CaO were studied at 1873 K. The coordinates of the liquidus surfaces of these oxide systems were determined. The energy parameters were also determined. The results showed that the dissolved oxygen in the steel melt is more likely to react with calcium than sulfur or arsenic. This fact makes it important to control the content of oxygen in the molten steel before deoxidation.
In order to reduce the oxide inclusions in the molten steel, it is necessary to use a high-quality deoxidizer. In addition, it is necessary to stir the molten steel vigorously to make the inclusions collide, agglomerate, and surface to the meniscus. These phenomena will lead to the formation of composite inclusions. These inclusions are mainly composed of calcium-arsenic compounds and calcium aluminates. They can be removed by alumina-based slag treatment. However, the slag system is not suitable for removing phosphorus and sulfur. This is because it does not form a soluble CaO-CaF2 compound with these elements.
The deoxidizing ability of calcium depends on the oxygen super-saturation state of liquid steel. In steelmaking practice calcium is normally used to modify primary inclusions rather than as a deoxidizer, since it has a low boiling point and limited solubility in the liquid steel, and the addition of calcium causes the formation of a range of primary inclusions with different morphologies, surface areas and compositions.
Aluminum is one of the most effective deoxidizers, but it produces alumina inclusions which are hard and can cause nozzle clogging in continuous casting of liquid steel. These alumina inclusions are difficult to remove by chemical treatment, because they have high melting temperature and are not easily disintegrated in the rolling process.
The use of a complex deoxidizing mixture of alumino-silico manganese with calcium allows the removal of alumina inclusions by reducing their size, number and distribution. In addition, calcium also has a high affinity for sulphur. This makes it possible to control the sulphur activity in the steel by modifying sulphide inclusions, which have almost unlimited solubility in the liquid steel but have very small quantities of soluble oxides.
The temperature dependences of the stability region of solid and liquid calcium aluminate phases have been determined by thermodynamic calculations. Moreover, the liquidus surfaces of the oxide systems B2O3-Al2O3-CaO-MgO and B2O3-Al2O3-Ca-MgO were calculated.
In addition, the kinetic model of alumina inclusions in calcium-modified steel has been derived. The model takes into account the interaction of Al gas with CaO, oxygen concentrations at the interface between the CA inclusion and molten steel as well as the unreacted core.
A comparison between the calculated and experimental results reveals that the model is very accurate. This is especially true for the radii of the CA inclusions, which match closely with experimental data. The kinetic model also predicts the modification time for the CA inclusions to change into C12A7, which is in good agreement with the experimental data. The kinetic model predicts that the modification time is proportional to the size of the CA inclusion, and that the modification time increases as the radius of the inclusion decreases.
It has been shown experimentally that calcium alloy is an effective deoxidizer. It eliminates oxygen and sulfur from molten steel by forming melting slags. It also reduces inclusions and allows them to be floated from the molten steel. In this way, the resulting steel has a better mechanical performance.
It is possible to keep the concentration of free oxygen at 0.002% by adding 0.05% of calcium alloy to molten steel that has been preliminarily deoxidized with Si, Mn or Al. This is significantly lower than the concentration of oxygen in molten steel that can be kept by using other deoxidizers.
In addition, the addition of calcium alloy decreases the size of inclusion particles in molten steel. This makes them more plastically deformable during hot rolling, allowing them to break into fine independent pieces. As a result, the Young’s modulus of inclusions decreases. This results in a reduction of the amount of arsenic contained in the inclusions.
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