Deep desulfurization and deoxidation are both essential steps in steelmaking. However, the kinetics and thermodynamics differ. The magnesium co-injection process is faster than the traditional lime-only method, and allows for lower sulphur contents in the ladle slag.
The nonmetallic inclusions in the tested steel samples changed over time, from manganese silicates to a mixture of liquid forsterite and forsterite-based solid solutions. This resulted in a better performance of the evaluation criteria with regard to castability.
High sulfur content in steel is associated with low corrosion resistance and promotes stress corrosion cracking or hydrogen induced cracking. This is why the American Petroleum Institute (API) specifies a maximum sulfur content of 0.0024% for oil and gas pipe production.
In order to achieve the API target, deep desulfurization is a key factor for the steelmaking process. This involves a combination of chemical reactions to form suitable refining ladle slag with advantageous properties for the assimilation of sulfur-based inclusions in the steel and for the subsequent removal during secondary metallurgy.
Various mixtures were investigated, including those with fluorspar and those without, in order to find the best slag composition for deep desulfurization. The results are shown in Table 2 and in Fig. 2, which shows the initial and final sulfur content as well as the efficiency of the different mixtures used. It can be seen that the most efficient mixture was CA8F, which reached a final sulfur content of 0.0005% in less than 20 minutes.
Steelmaking processes are typically very oxidizing, making it difficult to decrease the sulphur content of liquid steel. This is why desulphurization takes place in a separate step, usually during steel refining in the ladle furnace.
During this process, slag is mixed with hot steel to remove sulphur and to form calcium-rich slag. It is also used to modify oxide and sulfide inclusions in the steel, which improves castability.
The slag-liquid metal reaction is controlled by kinetics and thermodynamics. The thermodynamics of this reaction indicate that a high temperature is required to achieve good results.
This is why the deep desulfurization of pipeline grades is performed using magnesium or lime co-injection at the beginning of the heating period at the stirring station. In this case, the sulphur in the slag is converted to carbon and the reoxidation of inclusions in the steel melt can be avoided. The sulphur content is then reduced to specified levels. This is a strong process and enables the pipe to meet its specific requirements.
The steelmaking process involves a lot of water, from the cooling of blast furnace slag to the transport of the steel-slag mix through the ladle. This water is exposed to high temperatures and can react with the slag to form magnesium compounds. Depending on the slag composition, this can lead to an increase in the calcium content in the steel.
Reduction of MgO from fluoride-bearing slag has been studied, and thermodynamic models for these reactions have been developed. Commercial thermodynamic software packages such as Thermo-Calc and FactSage have recently been updated with these models, making it possible to model the reactions in detail.
The most commonly used reagent for hot metal desulfurization is lime, which reacts with dissolved sulphur to form CaS. However, this reaction is less efficient than the reaction with magnesium. The MMI process uses a combination of magnesium and lime co-injection, which provides a more effective and faster desulfurization. This allows a lower sulfur concentration to be achieved during tapping, which is important from the point of view of quality.
In steelmaking, calcium exists only in vapour form at the steelmaking temperatures. Therefore, the interaction between the Ca vapour and oxygen (O) or sulfur(S) is very limited. Calcium is added to the liquid steel through cored wire injection which makes the contact between the gaseous Ca and the solid steel as intimate as possible.
The aim of this calcium treatment operation is to make the inclusion population in the top slag less detrimental to steel processing. This goal is achieved by modifying the shape of the included particles. For example, Ca breaks up inter dendritic Al2O3 galaxies into fine Type III inclusions which are less likely to clog the continuous caster nozzle.
In practice, the critical calcium content required for the transformation of the inclusions into low melting point 12CaO*7Al2O3 and CaS is determined by thermodynamic calculations. This defines the "liquid zone" range of inclusion calcium treatment. This range is narrow and depends on several important production parameters.
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