In the steelmaking process, calcium refining agents play a crucial role in sintering and steel melting as well as in secondary metallurgy processes. Calcined dolomitic lime is added in EAF and ladle refining where it plays a dual role: it eliminates sulphur and phosphorus and saturates the slag in magnesium oxide thereby extending the life of the basic refractory lining.
The results showed that the calcium addition lowered the sulfur content in liquid steel. The content of dissolved oxygen in the molten steel decreased as well. This was because the calcium participated in deoxidation and reduced the content of dissolved oxygen in molten steel. It also reacted with elements Mg, Al and Si in the molten steel, forming small-size inclusions.
The number and size of the inclusions in the casting slab increased with increasing calcium addition. It is because the calcium mainly reacts with inclusions in the molten steel. The content of MgO and Al2O3 in the inclusions decreased with increasing wire feeding amount.
In order to obtain a high strength, it is necessary to reduce the size of nonmetallic inclusions in molten steel. It is essential to keep the oxygen and sulfur concentrations in molten steel in a very low range, so that the arsenic removal reaction can take place. A low sulfur level is also required to avoid hard alumina and MgAl2O4 spinel inclusions. In addition, the calcium treatment process is used to minimize nozzle blockage and give inclusion shape control.
Arsenic removal in steelmaking is a complex process that requires a large amount of deoxidation. The oxygen and sulfur in the molten steel react with calcium and consume it. Thus, it is crucial to control the concentration of oxygen and sulfur in the molten steel within a narrow range.
When calcium is added in LF refining, it reacts with silicate inclusions to modify them. This reduces the size and number of inclusions, and it can also prevent nozzle blockage in the submerged entry nozzle.
The addition of calcium can also decrease the reoxidation rate in LF, and it can help to achieve a high level of quality and productivity in the production of steel. This is because the calcium can increase the fluidity of the liquid metal and reduce the gas content in it. This can improve the quality of the steel and prevent a variety of defects such as sensitization. Moreover, the calcium can also help to improve the quality of the steel by decreasing the content of soluble gases and reducing the sulphur content in the slag.
Although there are many ways to reduce damage during rolling, including steel chemical analysis adjustments and calcium treatment, elongated inclusions remain a problem. These inclusions cause planes of weakness which can lead to weld shrinkage strains (lamellar tearing) or hydrogen accumulation at the interfaces between the inclusion and the matrix (hydrogen pressure induced cracking).
Calcium additions improve the formation of nonmetallic inclusions, which are more stable during welding. They also stabilize the morphology of sulphide inclusions and give more isotropic strength to the structure.
In addition, calcium is able to remove arsenic from molten steel by using the inverse oxidation process. As the concentrations of oxygen and sulfur in molten steel decrease, the activity of arsenic decreases and it becomes easier to remove by inverse oxidation. The results of three experiments with different types of calcium alloy dearsenicifier show that the content of arsenic in molten steel can be reduced to below 0.2%. This is the optimum level for arsenic removal.
Ca addition in steelmaking is a common practice to control the formation of non-metallic inclusions and improve the quality of steel. It reduces the oxygen and sulphur content of the liquid steel, and modifies the shape of the remaining sulphide inclusions. Inclusion morphology control is very important during steelmaking because elongated inclusions cause poor toughness and ductility properties. The violent agitation that accompanies the calcium injection in the ladle furnace (LF) also reduces the gas content of the liquid steel, which prevents nozzle clogging during casting.
The calcium alloy dearsenication reaction can also remove arsenic from molten steel by decreasing the content of soluble aluminum in the liquid steel and lowering the MgO and Al2O3 content of the slag in the AOD. Industrial application shows that when LF refining is performed by using low basicity and high calcium slag, the inclusion rating of the resulting steel is fine C-type with a small amount of class-A inclusions and surface polishing qualification rate is higher than 88.7%.
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