Thermodynamics of Dephosphorization, Desulfurization and Deoxidation in High Cr Containing Liquid Iron

Recently, there has been an increased demand for the ferritic stainless steels due to their lower cost of production and high performance to replace austenitic stainless steel in various applications. This situation prompted studies on quality improvement of ferritic stainless steels. This includes refining processes for high Cr containing liquid iron such as dephosphorization, desulfurization and inclusion control during deoxidation process. Phosphorus and sulfur increase the brittleness of steel and degrade the weldability and corrosion resistance in steel products. In order to control these impurities in stainless steelmaking process, the thermodynamic behavior of phosphorus and sulfur in liquid Fe-Cr alloy should be known. Aluminum deoxidation and calcium treatment in steelmaking process are very important to reduce oxygen content and control the desulfurization reaction as well as inclusion modification in liquid steels. In the present study, thermodynamics of phosphorus and sulfur in liquid Fe-Cr alloys was studied using the slag/metal equilibration technique to measure the equilibrium distribution of these elements between liquid slags and carbon saturated Fe���Cr alloys under a CO atmosphere. In case of phosphorus, carbon saturated Fe���Cr-P alloys containing chromium up to 20wt% was equilibrated with CaO-CaF2 and CaO-CaF2-Al2O3 slags in the temperature range from 1573 K to 1723 K at 1 atm CO atmosphere. The phosphate capacities of CaO-CaF2 and CaO-CaF2-Al2O3 slags were separately determined by equilibrating these slags with carbon saturated Fe-P melts under a CO atmosphere. Using the Wagner���s interaction parameter formalism, the experimental results were thermodynamically analyzed to determine the specific effects of phosphorus and chromium on phosphorus as their interaction parameters in liquid iron. Thermodynamic of sulfur in liquid Fe-Cr alloys was also studied using the similar experimental method to measure the effect of chromium on the equilibrium sulfur distribution between CaO-Al2O3 slags and carbon saturated Fe���Cr���S alloys containing chromium up to 30wt% in the temperature range from 1823 K to 1923 K. The sulfide capacity of CaO-Al2O3 slags was separately determined by equilibrating the slag with carbon saturated Fe-S melts under a CO atmosphere. The experimental results were thermodynamically analyzed to determine the specific effect of chromium on sulfur as an interaction parameter in liquid iron. In order to check the accuracy of the interaction parameter of chromium on sulfur in liquid iron determined using carbon saturated Fe-Cr alloys, a MgO saturated CaO���Al2O3 slag of known sulfide capacity was equilibrated with liquid Fe-Cr-S alloys under Ar atmosphere at 1873K. In order to improve the desulfurization efficiency in a special grade high Cr ferritic stainless steel, the slag composition and oxygen potential control are very important. In the present study, the effect of CaF2 content in a CaO-Al2O3-SiO2-MgO-CaF2 slag system on the sulfide capacity was determined at 1873K. The effect of oxygen potentials controlled by H2 blowing, Si or Al deoxidation on desulfurization efficiency was also measured to find an optimum desulfurization condition for an Fe-22wt%Cr alloy at 1873 K. Alumina inclusions in steels are usually the result of deoxidation reaction by Al in liquid steels. Calcium in liquid iron has a very strong affinity with oxygen and sulfur. Therefore, the equilibrium oxide or sulfide inclusion composition can be determined by the interactions among Al, Ca, O and S in liquid steels. In the present study, a pure CaS phase was equilibrated with Fe-Al-O-S and Fe-Cr-Al-O-S alloys in the presence of CaO or CaO.Al2O3 saturated slag in the temperature range from 1873 K to 1923 K. Thermodynamic equilibrium among Al-O-S in Fe and Fe-Cr melts was well attained with activities of CaS, CaO and Al2O3 in the system. Based on the thermodynamic relations determined in the present study, a stability diagram for CaO-Al2O3 oxide inclusion formation in liquid Fe-Cr alloy could be constructed. In order to verify the inclusion stability diagram, inclusion evolution experiments were also carried out by adding deoxidizing agents of Al and Ca in Fe-Cr-S alloys together with a CaO-Al2O3 slag. The variation of inclusion size, morphology and composition of inclusions in the melt was measured with time. The observed final inclusion compositions agreed well with calculated ones in the stability diagram.Docto