Silicon-calcium alloy is used extensively as a deoxidizer and desulfurizer in both converters and the ladle. It is more effective than aluminium, ferrosilicon or coke and does not require a balancing lime addition in the slag.
Ca treatment not only lowers oxygen and sulphur contents but also modifies the composition, size and shape of non-metallic inclusions. This improves cleanliness and reduces directional anisotropy.
The demand for steel with high strength and ductility in transverse direction has increased the importance of controlling the composition, size and shape of non-metallic inclusions. The use of calcium to modify the contents of oxide and sulfide inclusions and to control their distribution in the steel is a well-established process. The addition of Ca reduces the sulphur content, modifies the shape of inclusions and helps to eliminate directional anisotropy in through thickness ductility.
Inclusions containing more sulphide tend to aggregate strongly and can block or damage equipment such as continuous casting machines and nozzles. However, the actual sulphur concentration in molten steel is very low and the sulfide content can be controlled through chemical analysis adjustments, refractory shrouded nozzles and electromagnetic swirl stirring.
Calcium has a strong affinity for oxygen and can be used as an effective deoxidizer. Its boiling point is very low and its solubility in liquid steel approaches zero at ironmaking temperatures, however, the addition of calcium forms modified primary inclusions that have lower activities and melting temperatures than pure iron.
Inclusions, especially sulfide inclusions, can be a problem for steelmaking due to their formation of planes of weakness that can result in weld cracking problems or hydrogen accumulation at the inclusion-matrix interface (hydrogen pressure induced cracking). The shape of these inclusions is important since it can influence the strain that they place on the matrix during hot deformation. This can be controlled through a combination of chemical analysis adjustments, calcium treatment and electromagnetic swirl stirring.
The addition of calcium into molten steel changes the composition of sulfide inclusions, converting them from solid sulfides to liquid alumina. This has the added advantage of decreasing the sulfide content of the steel and can also help prevent clogging of continuous casting nozzles.
Furthermore, the modification of the morphology of MnS and Al2O3 inclusions by Ca has a beneficial effect during hot working because the globular shape of these inclusions, with their oxide core and sulfide layer, reduces directional anisotropy and improves through thickness ductility.
Steels produced with high sulphur and oxygen contents are susceptible to deformation problems during subsequent processing steps. The main problem is the formation of non-metallic inclusions that affect the ductility of the steel. The use of calcium in the steelmaking process helps to control the population of these inclusions and their shape.
During calcium treatment (known as Ca treatment), the volume fraction of oxide and sulphide inclusions is reduced through deoxidation and desulphurization. At the same time, the composition, size and morphology of the remaining inclusions is changed. Inclusions containing Al2O3 and SiO2 that are hard to machine are transformed into softer calcium aluminates and silicates with lower melting points. This improves free machining grades and increases the machinability of high quality structural and HSLA steels.
The elongated shape of sulphide inclusions is also changed. As a result, directional anisotropy of the toughness and ductility properties is reduced. In addition, sulfide inclusions are strengthened which makes them less prone to reoxidation during hot rolling.
One of the most important issues in steelmaking is control of the population of non-metallic inclusions. This can be achieved by calcium addition into the ladle, known as calcium treatment. However, the high vapor pressure of liquid calcium at steelmaking temperatures and the low solubility of calcium in the slag phase makes it difficult to perform calcium treatment efficiently and consistently with respect to different heats.
The kinetics of the modification of Al2O3 inclusions in hard wire steel has been studied. It was found that the modification time depends on the inclusion radii and the activity difference of Al between the interface layer between CA and C12A7 inclusions and the interface layer between molten steel and inclusions.
As a result of this, the inclusion shape can be controlled. For example, large inter dendritic Al2O3 galaxies can be transformed into fine Type III inclusions that will remain in the solidified steel but have less impact on mechanical properties and do not clog continuous caster nozzles. In addition, calcium treatment of inclusions increases ductility and toughness in free machining steel grades and improves hot workability even at low Mn levels.
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