Investigation of deoxidation of high-silicon austenitic sulfuric acid-resistant stainless steel

Abstract

The oxygen content in high-silicon austenitic sulfuric acid-resistant stainless steels is one of the most detrimental parameters to their corrosion resistance. Based on the ion-molecular coexistence theory (IMCT), a thermodynamic model of the slag-steel reaction of austenitic stainless steel containing 5.0 wt% Si with CaF2-CaO-Al2O3-MgO-SiO2 pentabasic slag was developed to investigate the deoxidation reaction and the oxygen control mechanism of the steel. The model was validated through experiments proposed in this study. The results of the slag-steel reaction indicated that the equilibrium oxygen content was determined by the greater of two factors: w[O]%, Si obtained from the [Si]-[O] equilibrium reaction controlled by the activity of SiO2 and w[O]%, Al obtained from the [Al]-[O] equilibrium reaction controlled by the activity of Al2O3. The system temperature and the basicity of slag are the most crucial among the multiple variables affecting the equilibrium oxygen content compared with Al2O3 and CaF2 in slag. However, achieving an ultra-low oxygen steel, both a basicity of slag greater than two and a low activity of Al2O3 in slag should be maintained. The total oxygen content in steel can reach a minimum value of 3.4 ppm when the slag composition encompasses w(CaF2)% = 29.38, w(CaO)% = 44.07, w(SiO2)% = 14.69, w(MgO)% = 9.89, w(Al2O3)% = 1.96. The high basicity of slag reduces the total oxygen content of stainless steel, whereas the influence of redox reactions between Si and Al results in a higher Al content in steel and the formation of more inclusions during solidification. Thus, the optimal Al2O3 content is less than 4% and the optimal basicity is 2.4 during the refining process.

Kinetic Investigation of the Deep Desulfurization of 5 wt% Si High-Silicon Austenitic Stainless Steel

“…Based on the sophisticated thermodynamic model elucidating the ultralow oxygen content equilibrium between [Si] and [O] within the steel's molten phase, an in-depth investigation into the reactive behavior of SS-5Si molten steel when interfaced with CaF 2 -CaO-Al 2 O 3 -MgO-SiO 2 slag was performed. The consequent findings, consisting of the equilibrium oxygen content within the molten steel under an assortment of reactive conditions (when [Si]-[O] reaction are at equilibrium), computed and verified via rigorous experimental results [35]. In the context of the interactions between the SS-5Si molten steel and the CaF 2 -CaO-Al 2 O 3 -MgO-SiO 2 slag under [Si]-[O] equilibrium, the specific conditions of the slag that dictate the balanced oxygen content in the molten steel were examined.…”

“…In practical manufacturing processes, to achieve an ultralow sulfur content (below 0.001%) in SS-5Si steel, the appropriate slag composition based on prior thermodynamic models is imperative to determine [35].…”

“…To address the prevailing challenges, our study examines the kinetics associated with desulfurization, using the CaF 2 -CaO-Al 2 O 3 -MgO-SiO 2 slag system. This study builds upon our seminal work investigating the thermodynamics and kinetics of deep deoxygenation in SS-5Si [35]. The study aims to determine the variables that can accommodate desulfurization to approximately 5 ppm.…”

“…Through these investigations, the corresponding slag compositions were determined. To explore the relationship between w[S] % in the molten steel, derived from the desulfurization reaction at (CaO)-[S] equilibrium, and its actual content, as well as the interconnections of deoxidation and desulfurization reactions in thermodynamic equilibria, we used the IMCT thermodynamic prediction model, analogous to our prior study [35]. Under conditions where w(MgO) % = 10 and w(CaF 2 ) % = 20, the influence of Al 2 O 3 content (ranging from 1-10%) on w[S] % in the molten steel at binary basicities of 1, 2, and 3 is shown in Figure 1.…”