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Aluminum-Free Steelmaking: Desulfurization and Nonmetallic Inclusion Evolution of Si-Killed Steel in Contact with Cao-Sio2-Caf2-Mgo Slag

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Aluminum-Free Steelmaking: Desulfurization and Nonmetallic Inclusion Evolution of Si-Killed Steel in Contact with Cao-Sio2-Caf2-Mgo Slag processes Article Aluminum-Free Steelmaking: Desulfurization and Nonmetallic Inclusion Evolution of Si-Killed Steel in Contact with CaO-SiO2-CaF2-MgO Slag Stephano P. T. Piva and Petrus Christiaan Pistorius * Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA; [email protected] * Correspondence: [email protected] Abstract: In some applications, deep desulfurization and deoxidation of steels without the use of aluminum are required, using Si as a deoxidant instead, with double-saturated slags in the CaO-SiO2-CaF2-MgO system. This work studied the desulfurization and nonmetallic inclusion evolution for the system using an induction furnace and compared the results with FactSage kinetic simulations. Steel samples were taken from the steel melt and analyzed with ICP-MS and combustion analysis for chemistry, and SEM/EDS for nonmetallic inclusion quantity, size, and composition. The results indicate that the steel was deeply desulfurized, with a final sulfur partition coefficient of 580; MgO was reduced from the slag, yielding dissolved [Mg] that transformed liquid Mn–silicate inclusions into forsterite and MgO. Intentional reoxidation of the melt with oxidized electrolytic iron demonstrated a significant concentration of dissolved [Mg] in the steel, by the formation of additional forsterite and MgO upon reoxidation. Citation: Piva, S.P.T.; Pistorius, P.C. Keywords: Aluminum-Free Steelmaking: steelmaking; slag; kinetics; clean steel; nonmetallic inclusions; ladle treatment Desulfurization and Nonmetallic Inclusion Evolution of Si-Killed Steel in Contact with CaO-SiO2-CaF2-MgO Slag. Processes 2021, 9, 1258. 1. Introduction https://doi.org/10.3390/pr9081258 For certain steel products, alumina-containing nonmetallic inclusions are undesirable, whether because of their effect on steel castability (the tendency of solid alumina and spinel Academic Editors: Chenn Q. Zhou inclusions to clog nozzles) or on product quality (due to the hardness and high melting and Tyamo Okosun point of alumina-based inclusions). In certain applications, deformable, liquid inclusions such as manganese silicates are more desirable [1]. Manganese sulfides can impair product Received: 15 June 2021 toughness, and their formation is limited by decreasing the concentration of sulfur in the Accepted: 16 July 2021 liquid steel. To avoid aluminum pickup and enable deep desulfurization, slags in the Published: 21 July 2021 CaO-SiO2-CaF2-MgO system are typically used in such cases: fluorspar (CaF2) significantly increases the solubility of CaO [2], while the absence of Al2O3 avoids aluminum pickup. Publisher’s Note: MDPI stays neutral To obtain the deep levels of deoxidation and desulfurization (often obtained by the with regard to jurisdictional claims in addition of aluminum as a deoxidant), many processes instead rely on silicon as a primary published maps and institutional affil- deoxidant. To obtain a low oxygen activity at the steel–slag interface, to enable deep iations. desulfurization, the silica activity in the slag must be low when using Si as deoxidant. The basic nature of CaO-rich, CaF2-bearing slags and the high solubility of CaO allow liquid slags with low silica activity to be used. The low silica activity (deeply reducing conditions) also drives the reduction of other oxides such as MgO and CaO from the slag, yielding Copyright: © 2021 by the authors. dissolved magnesium and calcium [Mg] and [Ca] in the steel. Reduction of MgO from slag, Licensee MDPI, Basel, Switzerland. refractory, or both has been demonstrated for both Si-killed and Al-killed steel types [3–5]. This article is an open access article The resulting [Mg] can react with nonmetallic inclusions in steel, for example, transforming distributed under the terms and alumina inclusions into spinels and periclase (in Al-killed steel). The reduction of CaO does conditions of the Creative Commons Attribution (CC BY) license (https:// not seem to significantly affect nonmetallic inclusions in most cases, because the near-zero creativecommons.org/licenses/by/ solubility of Ca in steel (at typical secondary-metallurgy oxygen activities) limits the rate 4.0/). of Ca transfer from slag to steel [6,7]. Processes 2021, 9, 1258. https://doi.org/10.3390/pr9081258 https://www.mdpi.com/journal/processes Processes 2021, 9, 1258 2 of 11 If Al2O3 were present in such slags, the alumina would likewise be reduced readily; the resulting [Al] would react with liquid manganese silicates (deoxidation products) in the steel, forming potentially solid alumina-containing phases. Although the transient inclusion pathway for steels in contact with Al2O3-bearing slag is well known [4,8,9], the inclusion pathway for Si-killed steels in contact with slags in the CaO-SiO2-CaF2-MgO system has not been described in the literature. The industrial approach to using fluoride-based slag to desulfurize Si-killed steel has been well described [10,11]. However, fundamental laboratory studies of this approach have not been reported, and until recently, thermodynamic modeling of the reactions was not feasible because of the paucity of relevant thermodynamic data. Recent updates to commercial thermodynamic packages such as Thermo-Calc [12] and FactSage [13] have added solution models for slags containing significant concentrations of CaF2, enabling thermodynamic and kinetic modeling of the reaction of steel with fluoride-containing slag. In the work reported here, changes in oxide inclusion composition during desul- furization and concomitant MgO reduction from Al2O3-free slags were experimentally characterized and compared with thermodynamic and kinetic modeling. 2. Materials and Methods 2.1. Experimental Setup Secondary steelmaking refining was simulated on laboratory scale by using a high- frequency induction furnace, with a graphite crucible as a susceptor and a MgO crucible as the refractory container. The selected steel and slag compositions are shown in Tables1 and2, and Figure1. A mass of 600 g of electrolytic iron and 0.66 g of iron(II)sulfide were melted and heated to 1600 ◦C, under a high-purity Ar atmosphere. Details of the experimental configuration were reported previously; previous experiments using the same configuration showed repeatability in characterizing steel–slag reaction kinetics and nonmetallic inclusion evolution [5,14–20]. From previous experiments, the concentration of dissolved oxygen in molten iron produced from the electrolytic iron is approximately 200 ppm (parts per million by mass). The molten iron was deoxidized using a laboratory-produced silicomanganese alloy (SiMn) with an approximate composition of 69%Mn-19%Si-10%Fe-2%C, added as 5–10 mm lumps. After deoxidation, 100 g of a pre-fused mixture of CaO, MgO, Al2O3, and SiO2 (composition shown in Table2 and Figure1) was added to the top of the steel melt. The steel and slag were held for 3 h, during which time steel samples were taken using cylindrical fused-quartz sampling tubes (4 mm inside diameter). After 3 h, the steel was intentionally reoxidized to precipitate out dissolved Mg and Ca as oxide inclusions. For reoxidation, 3 g of electrolytic iron was used; the electrolytic iron had been oxidized beforehand in the air at 800 ◦C, causing an increase in mass (oxygen pickup) of 0.02 g (measured with a 0.0001 g-resolution balance). An additional steel sample was taken after reoxidation. Table 1. Aimed steel composition at deoxidation, for laboratory trial. wt%C wt%Si wt%Mn ppm Otot ppm S Aim 0.03 0.33 1.15 200 400 Table 2. Aimed slag composition for laboratory trial. wt%CaO Wt%SiO2 Wt%MgO Wt%CaF2 Aim 56 24 5 15 Processes 2021, 9, x FOR PEER REVIEW 3 of 11 2.2. Modeling Setup A steel–slag–inclusion kinetic simulation was set up, using FactSage macro pro- cessing interfacing with Microsoft Excel to calculate the expected chemical interaction be- tween the slag, deoxidized steel, and inclusions within the steel. The calculation proce- dure and assumptions were as described previously [14,22,23]. In these calculations, no fitting parameters are needed; only the mass transfer coefficient in the steel needs to be calibrated. This mass transfer coefficient was estimated from the analyzed [S] in the steel. Table 1. Aimed steel composition at deoxidation, for laboratory trial. wt%C wt%Si wt%Mn ppm Otot ppm S Aim 0.03 0.33 1.15 200 400 Table 2. Aimed slag composition for laboratory trial. Processes 2021, 9, 1258 3 of 11 wt%CaO Wt%SiO2 Wt%MgO Wt%CaF2 Aim 56 24 5 15 ◦ FigureFigure 1.1. Pseudo-ternaryPseudo-ternary CaO-SiOCaO-SiO22-MgO-15%CaF-MgO-15%CaF22 phase diagram (isothermal(isothermal sectionsection atat 16001600 °C)C) asas plottedplotted withwith FactSageFactSage 8.18.1 usingusing the the FToxid FToxid database database (SLAGH (SLAGH liquid liquid slag slag model). model). The The red red dot dot is theis the aimed aimed composition composition to ensureto ensure double double saturation satura- withtion bothwith (MgO)both (MgO)ss (periclase)ss (periclase) and (CaO)and (CaO)ss (lime).ss (lime). 3. ResultsCross sections of the steel samples were mounted and metallographically polished to a one-micron finish for nonmetallic inclusion analysis by scanning electron microscopy with 3.1. Experimental Measurements energy-dispersive X-ray microanalysis (SEM/EDS). An FEI ASPEX Explorer instrument was used,The evolution at an accelerating over time voltageof the steel of 10 comp kV,osition, an approximate and the oxide spot sizeinclusion of 45%, composi- and a magnificationtion and size are of plotted 1200×. in For Figures these conditions,2–4. The sizes the shown spatial in resolution Figure 5 are was the approximately apparent two- 0.4dimensional micron; the sizes pixel (equivalent size on scanning circle diameters). electron micrographsThe [S] decayed was from 0.25 micronits initial [21 value]. The of samples400 ppm were to 4 ppm also chemicallyafter 125 min analyzed (Figure for 2). bulkOver composition, the same period, by combustion the [Si] levels analysis dropped for [S] (performed at the US Steel Research Center), and ICP-MS for [Si], [Mn], and [Mg] (performed at R.J.
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