Calcium plays an essential role in the steel industry, being used as a deoxidizer in electric arc furnaces and AOD converters. It is also a desulfurizer.
Silicon-calcium alloy can be added to molten steel as an ideal composite deoxidizer and desulfurizer. It controls the shape and size of harmful inclusions and improves fluidity, machinability and ductility in the final product.
Ca treatment significantly reduces the sulphur content in molten steel by reacting with the dissolved oxygen and sulfur to form calcium sulfide (CaS). This reaction is very rapid and the concentration of sulphur and oxygen in the resulting slag is very low.
The addition of Ca also modifies the shape of non-metallic inclusions. Elongated sulphide inclusions reduce the toughness and ductility of rolled steel. When Ca is added to a steel, it flattens the inclusions and promotes their agglomeration, which results in more isotropic properties.
Ca addition is particularly important for Al killed steels because it decreases the volume fraction of oxide and sulphide inclusions by deoxidation and desulphurization and it controls the composition, morphology, and distribution of the remaining inclusions. Proper Ca treatment also eliminates nozzle clogging problems by transforming the solid Al2O3 inclusions into liquid calcium aluminates.
Aside from reducing inclusions, calcium treatment also modifies their shape to prevent damage during hot working. This is achieved by a combination of different mechanisms such as chemical analysis adjustments, calcium treatment and electromagnetic swirl stirring.
For example, calcium breaks up inter dendritic Al2O3 galaxies into fine Type III inclusions that do not clog continuous caster nozzles. In addition, the calcium in the treatment reacts with the sulphur in the steel to form sulphide-free Ca2S.
Moreover, when Ca is added to the melt after deoxidation with Al, it adsorbs on the surfaces of sulphide inclusions and forms a coating around them. As a result, the sulphide inclusions change their shape and composition from pure spinel to a composite formation consisting of a MAS core and an outer layer of calcium aluminate (xCaO-yAl2O3).
Adding calcium to high alloy steels increases fluidity and improves the cleanliness of the melt. It also reduces the formation of inclusions, which can cause rolled products to have poor mechanical properties, especially those produced using ERW or HFI welded processes.
Inclusions have a significant impact on steel purity, which is why they are one of the main targets for refinement. The shape of inclusions is particularly important, as they influence the final material structure and can clog continuous casting nozzles.
When magnesium is added to deoxidized liquid iron, it reacts with MAS and transforms it into calcium aluminate. This reaction encapsulates part of the inclusions, leaving them with an outer layer of CaS. This makes it hard for them to be consumed by oxygen or sulfur.
The kinetics of the modification of Al2O3 inclusions in steel has been investigated. It has been shown that the modification time is mainly determined by the diffusion of Al in the calcium aluminate layer. The interface between the inclusion and molten steel also influences the modification process.
The addition of Ca has a major influence on the microstructure of steels. It decreases the volume fraction of oxide and sulphide inclusions by deoxidation and desulphurization, and it affects the composition and morphology of the remaining inclusions.
In particular, it transforms the inter dendritic Al2O3 galaxies in an aluminum killed steel into fine Type III inclusions. This leads to a better equiaxed grain structure and improved through-thickness tensile properties, Charpy impact toughness and ductility. It also reduces nozzle clogging during continuous casting and enhances the machinability of the final product.
Compared with industrial pure iron, cast iron has higher carbon content and lower silicon content. Silica calcium alloy can deoxidize, desulfurize and enhance the silicon in cast iron as well as act as an effective inoculant to promote the formation of spheroidal graphite.
Adding calcium alloy in the form of cored wire is more efficient than adding it loosely in the steel melt. This is because the low melting point of calcium makes it evaporate rapidly if added into the steel melt directly.
The cored wire method is a new and innovative way of adding calcium to the steel melt. The calcium is contained within a metal sheath, which slows the melting of the cored wire during the injection process. This reduces the evaporation of calcium and increases the amount that is actually injected into the steel melt.
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