Calcium metal is an addition agent used to treat molten steel. It reduces oxygen and oxide inclusions in steel, enables more uniform steel quality (including reduced continuous caster nozzle blockage), improves machinability, and increases through thickness ductility.
It is fed into the molten steel deep by a wire injection metallurgical technology called ladle feeding. This steelmaking additive is also used in iron and steel refining outside the furnace for deoxidation, desulfurization and other metallurgical effects.
The solubility of calcium is very low at steelmaking temperatures. The present invention provides for a method of increasing the concentration of soluble calcium in the iron-calcium system by adding calcium metal through cored wire injection to the liquid steel. The calcium is able to react with oxygen, sulphur and nitrogen at the steel/slag interface, thereby lowering the concentration of impurities (above all silica and alumina but also sulphur and phosphorus).
In a preferred embodiment, the recovery of soluble calcium from the slag is achieved by immersing the steelmaking slag, which has been eluted into an aqueous solution by a first step (Step S120), several times in water having carbon dioxide blown therein. Increasing the pH of the aqueous solution separated from the steelmaking slag can further increase the rate at which soluble calcium is recovered.
The addition of Ca to high alloy steels increases the fluidity and cleanliness of the liquid metal, making it less prone to damage from non-metallic inclusions (see Figure 3.5). The violent agitation that accompanies the injection can also control the morphology of these inclusions and prevent their deformation.
The evaporation point of calcium metal is lower than that of molten steel. Therefore, it is added in the form of a steel sheathed calcium wire to ensure that it is retained under the heat of the molten steel long enough to be pushed into the bottom of the ladle.
The sheath coating also protects the cored wire from the reoxidation of the surrounding steel during operation, making it possible to use the technology in high production rates. In addition, the coating does not degrade due to exposure to abrasive and acidic conditions in the steelworks.
The calcium added through the cored wires can also modify inclusion morphology, for example by breaking up interdendritic Al2O3 galaxies into fine Type III inclusions. This has the effect of reducing their tendency to clog continuous caster nozzles. The result of the treatment is a more stable steel with good casting properties. The calcium addition also reduces the amount of S in the final steel, thereby improving its machinability.
In its pure metallic form, calcium is a brittle white metal. However, in its compound forms it is an active redox catalyst and a key material for steelmaking.
In the steel industry, calcium is used as a desulphurizing agent in electric arc furnaces and AOD converters and as a deoxidizer in ladle refining. It is also used to modify the composition and shape of inclusions in liquid steel.
Calcium treatment modifies the morphology of oxide and sulphide inclusions in molten steel. It converts sulfide inclusions to a mixture of silicates and aluminates and makes them more globular. This reduces the directional anisotropy of the steel, improving its through thickness ductility.
It also prevents the formation of magnesium aluminate spinel (MAS) inclusions that are known to impact castability. In addition, it enables the modification of interdendritic Al2O3 galaxies into fine Type III inclusions. The latter can be cast without damaging the nozzles of continuous casting machines and are less likely to clog.
The addition of calcium to molten steel significantly reduces the oxygen content and arsenic concentration, resulting in higher fluidity, simplified continuous casting and greater machinability. However, it is important to note that the amount of elemental Ca added must be carefully controlled in order to achieve a successful dearsenic removal reaction.
This is because the redox reactions with O and S can only take place at the calcium vapour/liquid steel interface if the temperature is sufficiently low. The use of protective slag blankets and inert gas or refractory shrouded nozzles is therefore essential for protection.
In addition, Ca is highly effective at modifying the composition and shape of oxide and sulphide inclusions already present in the liquid steel. For example, the calcium treatment breaks up inter dendritic Al2O3 galaxies into fine Type III inclusions which will remain in the steel through solidification but are less prone to clogging the continuous cast nozzles. This also improves through-thickness ductility.
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