Steelmakers use calcium to modify slag and steel and non-metallic inclusions. The process is known as Ca treatment. The advantages directly attributable to Ca treatment include improved fluidity and cleanliness (reduction in nozzle blockage) as well as enhanced ductility, toughness and machinability.
Since calcium’s solubility is low at steelmaking temperatures, it can only be added to the liquid steel via cored wire. Cored wires inject calcium into the steel through a very intimate contact, even under high ferrostatic pressure.
Calcium is used to modify the composition, size and shape of oxide inclusions that form in steelmaking. This process is known as calcium treatment. It converts hard, solid (at steelmaking temperatures) alumina (Al2O3) and spinel (MgAl2O4) inclusions into liquid calcium aluminate inclusions. This improves the effectiveness of aluminium deoxidation and reduces oxygen ingression through the slag.
It is also used to lower the concentration of impurities in slag formation. This is achieved by reacting with sulphur and oxygen present in the molten steel. The reaction products are carried away in the slag leaving a low concentration of carbon, silica, alumina and phosphorus in the slag.
It is challenging to control calcium treatment due to the high reactivity of Ca with O and S as well as its very low solubility in liquid steel. It is therefore important to control the procedure for adding calcium wire into the steel. This is normally done using cored wires, such as PapCal or CaFe.
Inclusions are a vital part of the physics of steel making. They can be harmful, causing nozzle blockage and affecting castability, or they can benefit the final product by providing ductility and impact strength. In the last decades, there has been much improvement in the understanding of their origin, characterization and behavior. Tailoring inclusions for desirable properties has become a common practice in the field of inclusion engineering.
Non-metallic inclusions can be classified based on the type of oxide they contain, their chemical composition and stage in the process. Those formed before the start of solidification are called primary and those that form during solidification are secondary.
The addition of calcium can be used to modify the behavior of non-metallic inclusions, especially alumina. The spherical inclusions tend to cluster and the presence of calcium will inhibit this behavior and promote their flotation during ladle furnace treatment. This will significantly improve the machinability and cleanliness of the cast product.
During the steelmaking process, calcium is used as a flux in electric arc furnaces (EAF) and basic oxygen furnaces (BOF). It lowers the concentration of impurities like silica, phosphorus, and sulphur, which are damaging to the final product.
It prevents refractory erosion and also decreases the clogging of ceramic tubes that convey the steel between different vessels. The clogging of these tubes and the resulting wear reduces productivity, as well as affecting the quality of the finished product.
High calcium lime and dolomitic lime are key to clean steel production from the sintering stage through the steel melting stages of EAF and BOF. These limes are essential in controlling cleanliness factors including phosphorus removal, over oxidation in BOF, and modification of alumina inclusion morphology from dendrite shape to polyhedral spinel during Ca treatment. This is achieved through efficient addition practices. However, predicting the optimum calcium treatment requires an understanding of the effect of Ca on the inclusion morphology and sulphur level in the liquid steel.
Calcium can increase strength in steelmaking by decreasing the volume fraction of oxide and sulphide inclusions through deoxidation, desulphurization, and modification of their shape. In particular, calcium treatment transforms inter dendritic Al2O3 galaxies into fine Type III inclusions which are less detrimental to the mechanical properties of final products. Additionally, Ca treatment can prevent nozzle clogging during the continuous casting of liquid steel by changing the size and shape of sulphide inclusions from elongated to round.
Phosphorus present in the iron ore and scrap metal used to start the metal-making process can damage certain types of steel by lowering ductility. Quicklime removes phosphorus from the metal and lowers its percentage in the steel composition.
In addition, calcium changes the morphology of sulphide inclusions to make them more compatible with the matrix during hot deformation. This reduces directional anisotropy and increases through-thickness ductility of final products. It also improves the cleanliness of steel during rolling.
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