Technical calcium carbide, as well as metallic and silicocalcium, is an effective oxygen deoxidizer in steel production. It has been found that calcium treatment significantly modifies the composition and size of nonmetallic inclusions present in continuously cast slabs, billets, and like shapes. This gives a significant influence on the stress-governed regime of fatigue.
When steel is being melted, oxygen from the air can react with it and reduce its strength and quality. Slag, a byproduct of the smelting process, helps absorb this unwanted oxygen and protect the metal from further oxidation. But sometimes, even slag can't completely prevent oxidation. This is where calcium deoxidation comes in.
The injection of calcium into the molten steel can help reduce oxidation and improve its quality. A large number of studies have been carried out on the thermodynamics of this reaction, and many methods of deoxidation with calcium have been proposed.
Aluminum is the most common deoxidizer added in the teeming ladle, but a number of other strong metals are also used. These include ferroalloys of manganese and silicon as well as calcium silicide and niobium. These powerful deoxidizers have the advantage of avoiding the formation of high-melting calcium aluminates (CT).
As the name indicates, this method involves addition of calcium (Ca) to the molten steel in the ladle. Ca is the most effective deoxidizing element. Its density is much lower than that of iron and it exists in a gaseous state at the steelmaking temperature. Hence it can react directly with the oxygen and sulfur (S) molecules.
The addition of Ca results in formation of slag which absorbs the impurities produced in the smelting process. It also protects the liquid steel from further oxidation by reducing its reactivity with oxygen.
Besides deoxidation, calcium is also used for desulfurization. This is achieved by the addition of calcium carbide (CaC). The use of this deoxidizer reduces unfavorable factors such as nozzle nodulation, eliminates waste material, saves labor and material, improves the steelmaking environment and saves energy. It is a popular way for steelmakers to meet the demanding requirements of the stress-controlled regime. In addition, it provides inclusion morphology control.
Adding calcium to the steel during smelting is usually done in the form of a stable Ca silicon alloy (CaSi), Ca manganese silicide, Ca aluminium silicate or pure calcium. It is injected into the ladle by cored wire to achieve intimate contact between calcium and liquid steel at a high ferrostatic pressure. This prevents nozzle clogging during continuous casting and improves machinability, ductility and impact strength in the final product.
Since the density of Ca is much lower than that of steel, it is not as effective a deoxidizer as Si and Al. Its low density also makes it difficult to inject Ca into the steel using standard injection methods. Consequently, it is injected deep in the steel where it reacts with inclusions and oxygen and sulfur. This process modifies the composition, size and structure of oxide inclusions in the liquid steel to form a solid calcium aluminate. This eliminates sulfide stringers and reduces directional anisotropy during hot working operations.
This method involves reheating the steel to above its recrystallization temperature, and then rolling it into flat sheets. It’s a popular option for producing things like sheet metal, railroad rails and bars.
During this secondary metallurgy phase, calcium is added to the liquid steel to modify its inclusions (e.g., oxides and sulfides). Ca addition not only deoxidizes the inclusions but also modifies their morphology. This gives the inclusions a more globular shape, which eliminates directional anisotropy in the resulting through-thickness ductility.
Quicklime is also used to extract phosphorus from the liquid steel. Phosphorus can damage the ductility of steel, which is why it’s important to keep its percentage in the steel composition as low as possible. Adding calcium during the desulphuring process allows it to further lower this amount. This makes the steel more ductile and stronger overall. This is especially useful for high-strength and corrosion resistant grades of steel. Moreover, calcium treatment also allows for inclusion shape control which increases fatigue resistance.
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