The paper presents the results of a thermodynamic modeling of the chromium and boron reduction from slags of reduction period of argon-oxygen decarburization (AOD) by a complex reducing agent containing silicon and aluminum. Using the simplex lattice method, an experiment planning matrix is constructed containing 16 compositions of the oxide system СаО – SiO2 – (3 – 6 %) В2О3 – 12 % Cr2O3 – 3 % Al2O3 – 8 % MgO of variable basicity 1.0 – 2.5. The results of thermodynamic modeling are graphically presented in form of dependence of equilibrium distribution of chromium and boron on the slag composition at temperatures of 1600 and 1700 °C. The constructed diagrams make it possible to quantify the influence of the temperature, basicity and B2O3 in the slag on equilibrium interphase distribution of chromium and boron. It is established that increasing the slag basicity from 1.0 to 2.5 improves the process of chromium reduction, but restores the boron stability. With an increase in B2O3 content in the slag, a slight deterioration of chromium reduction process occurs, while the boron content in the metal increases. With a simultaneous increase in basicity up to 2.5 and a decrease in boron oxide in the slag from 5 to 3 %, the interphase distribution coefficient of chromium is reduced to 1.5·10–3. Changing the process temperature from 1600 to 1700 °C does not have a negative effect on the process of chromium reduction, but worsens the boron reduction conditions. Based on analysis of the formed slag phases and thermodynamics of the reactions of their formation, it is established that chromium is mainly reduced by aliminum with only partial development of silicothermy. The residual silicon content reduces boron, thereby limiting its concentration in the metal. The results of high-temperature experiments showed high correspondence with the results of thermodynamic studies.

Influence of boron in a wide content range (0.0004–2.0

With the aim of studying the occurrence of chromium reduction during argon-oxygen refining using boron oxide as a flux, thermodynamic modelling of the equilibrium interfacial distribution of chromium between slag of the oxide system CaO–SiO2–(3–6

Now the main industrial method for producing stainless steel is smelting in an argon-oxygen decarburization (AOD) furnace, therefore the paper presents the results of thermodynamic modeling of the desulfurization process of low-carbon semi-finished stainless steel during the reduction period of AOD process by treating it with boron-containing slags. The use of boron oxide as a fluxing material instead of fluorspar reduces the environmental harm and decrease the viscosity of the formed slags. Using the simplex lattice method of experiment planning, a matrix was constructed containing 16 compositions of the oxide system СаО–SiO2–(3-6%)В2О3–12%Cr2O3–3%Al2O3–8%MgO with variable basicity of 1.0–2.5. Based on the generalization of the thermodynamic modeling results, approximating mathematical models in the form of a reduced third-degree polynomial were constructed. The adequacy of the models is verified by three control points not included in the experimental design matrix using the t-criterion at a significance level of 0.01. The results of mathematical modeling are presented graphically in the form of diagrams of the dependence of the equilibrium sulfur distribution on the slag composition at temperatures of 1600 and 1700°C. The constructed diagrams made it possible to quantitatively estimate the effect of temperature, basicity and boron oxide content on the equilibrium interphase distribution coefficient of sulfur. It is found that an increase in slag basicity from 1.0 to 2.5 in the considered range of boron oxide content (3.0–6.0%) improves the metal desulfurization process, ensuring an increase in the equilibrium interphase distribution coefficient of sulfur from 0.1 to 5.0–7.0 at temperatures of 1700 and 1600°C. It’s shown that the process of metal desulfurization in slags with low basicity of 1.05–1.15 is accompanied by a slight decrease in the sulfur content in the metal. At the same time, the concentration of boron oxide has virtually no negative effect on the process of metal desulfurization. Slags with increased basicity up to 2.0–2.5 have more favorable refining properties. The sulfur concentration in the metal during their formation decreases from 0.015 to 0.007–0.008%.

The rapid growth of demand for stainless steel and, accordingly, its production, which occurred in the second half of the 20th century and continues till today, makes it necessary to conduct studies of the properties of oxide systems that will contribute to the improvement of metallurgical production technologies for such steel. Therefore, in this paper, using the method of simplex grids for experimental planning and vibration viscometry, a study was conducted of the effect of basicity and boron oxide content on the viscosity and crystallization onset temperature of slags of the СаО–SiO2–B2O3–12%Cr2O3–3%Аl2O3–8%МgO oxide system formed during the reduction period of the production of low-carbon stainless steel by the argon-oxygen decarbonization (AOD) process, which is currently the main method for producing corrosion-resistant steel. The introduction of boron oxide into AOD-slags is a possible solution to the problem of instability of the physical properties of slags during smelting, caused by the volatility of fluorspar fluorides, traditionally used as a flux, and compliance with increasingly stringent environmental requirements by eliminating the formation of toxic fluorine compounds. Based on the results of experimental studies of the viscosity of slags of the studied oxide system depending on the chemical composition and temperature, approximating mathematical models in the form of a reduced third-degree polynomial are constructed. Graphically, the results of mathematical modeling are presented in the form of “composition – property” diagrams, which allow quantitatively determining the effect of temperature and chemical composition of the slags under study on viscosity and their composition on the crystallization onset temperature. It is noted that at 1600 and 1650°C, an increase in the boron oxide content in the slag from 3.0 to 6.0% has a favorable effect on the fluidity of the formed slags in the basicity range of 1.0-2.5. For example, an increase in the boron oxide concentration from 3.0 to 6.0% ensures a decrease in the viscosity of the slags from 2.0 to 0.5 Pa s at a temperature of 1600°C and from 0.4 to 0.3 Pa s at a temperature of 1650°C in the region of increased basicity up to 2.0-2.5.

Using the method of simplex lattices for experiment planning, the viscosity of slags of the CaO-SiO2-Cr2O3-3%Al2O3-8%MgO-6%B2O3 system was studied in a wide range of chemical composition. For each viscosity value, adequate mathematical models are obtained in the form of a reduced III degree polynomial. The results of mathematical modeling are presented graphically in the form of composition-viscosity diagrams. The range of compositions of the studied slags with a basicity of 1.5-2.0 has a high liquid mobility in the temperature range of 1600 -1700°C and their viscosity does not exceed 0.3 Pa·s.

Using the simplex lattice method of experiment planning lattices, the physicochemical properties of slags of the СаО–SiO2–B2O3–2 % Cr2O3–3 % Аl2O3–8 % МgO oxide system are studied in a wide range of chemical composition (CaO/SiO2 = 1.0–2.5; 0–6 % B2O3), i. e. viscosity, crystallization onset temperature, equilibrium distribution of sulfur between slag and metal. Mathematical models of slag properties dependence on its composition are constructed in the form of a reduced polynomial of the third degree. The results of mathematical modeling of the sulfur removal reaction and experimental studies of the viscosity of slags and their crystallization onset temperatures are presented graphically in the form of “composition–property” diagrams. It’s found that the addition of up to 6% boron oxide significantly liquefies slags and reduces the temperature of the onset of crystallization, while maintaining the possibility of deep desulphurization of the metal. For effective metal processing, the composition range of slags with a basicity of 2.0–2.5 and a content of 4.0–6.0 % boron oxide is optimal. These slags have high liquid mobility (viscosity of the formed slags does not exceed 0.3 Pa•s) and provide the sulfur content in the metal not more than 0.003–0.004 %. The slags of this area are homogeneous, since their crystallization onset temperature doesn’t exceed 1500 °C. Experimental studies of metal desulfurization under the basic slags of the studied local simplex confirmed the data characterizing the influence of the boron oxide content and the basicity of the formed slags on their viscosity and its role in the efficiency of the desulfurization process.

The effect of basicity on the phase composition, structure, viscosity and crystallization temperature of CaO–SiO2–Al2O3–MgO–B2O3 fluorine-free slags was studied using vibrational viscometry and thermodynamic phase composition modeling in combination with Raman spectroscopy. The viscosity of the studied slags was established to depend mainly on the phase composition. Despite the complex reticular structure, consisting mainly of [SiO4] and [BO4] structural units with a large quantity of bridging oxygen, long slags (having a wide temperature range of viscosity variations) with low basicity possess high flowability in a liquid state as well as a low temperature of crystallization. This can be explained by the fact that the introduction of boron oxide is accompanied by the formation of a significant quantity of low-melting phases (CaO–B2O3, 2CaO–B2O3, and CaO–MgO–2SiO2) in the slag, which reduces the slag crystallization temperature, as well as increases its overheating and decreases viscosity. At an increase in basicity, long slags smoothly transform into short ones (with a narrow temperature range of viscosity variations); moreover, their structure is simplified along with an increase in the quantity of high-melting phases. In this case, despite the structural simplification, the slag crystallization temperature increases, which leads to the reduction in the degree of overheating and an increase in the viscosity.

Influence of basicity on viscosity, crystallization onset temperature, phase composition, and structure of slags of the СаО – SiO2 – 18 % Cr2O3 – 6 % B2O3 – 3 % Аl2O3 – 8 % МgO system in the basicity range (B = CaO/SiO2 ) from 1.0 up to 2.5 was studied using vibrational viscometry, thermodynamic modeling, and Raman spectroscopy. It was established that the physical properties of slags depend on the balance of polymerization degree and phase composition. Acid slags with a basicity of 1.0 belong to the category of “long” slags and are characterized by an increased proportion of high-temperature phases up to 34.1 %. However, despite the fact that the proportion of high-temperature phases is 1.6 times higher compared to the proportion of low-temperature ones, they are characterized by a simpler silicate structure, providing a viscosity of no more than 0.25 Pa·s at a crystallization onset temperature of 1530 °C. An increase in basicity of slags of the studied oxide system (up to 2.5), along with an increase in the proportion of high-temperature phases (by almost 5.9 times), is accompanied by formation of a more complex silicate structure. The resulting four-coordination structural elements [CrO4] and [AlO4] are embedded in the silicate structure and complicate it, which increases the polymerization degree. Thus, at basicity of 2.5, due to a high proportion of high-temperature phases in the slag and development of polymerization process, slag crystallization onset temperature increases to 1700 °C and its viscosity reaches 1.0 Pa·s at a temperature of 1670 °C.

The effect of basicity on the phase composition, structure, viscosity and crystallization temperature of CaO–SiO 2 –Al 2 O 3 –MgO–B 2 O 3 fluorine-free slags was studied using vibrational viscometry and thermodynamic phase composition modeling in combination with Raman spectroscopy. The viscosity of the studied slags was established to depend mainly on the phase composition. Despite the complex reticular structure, consisting mainly of [SiO 4 ] and [BO 4 ] structural units with a large quantity of bridging oxygen, long slags (having a wide temperature range of viscosity variations) with low basicity possess high flowability in a liquid state as well as a low temperature of crystallization. This can be explained by the fact that the introduction of boron oxide is accompanied by the formation of a significant quantity of low-melting phases (CaO–B 2 O 3 , 2CaO–B 2 O 3 , and CaO–MgO–2SiO 2 ) in the slag, which reduces the slag crystallization temperature, as well as increases its overheating and decreases viscosity. At an increase in basicity, long slags smoothly transform into short ones (with a narrow temperature range of viscosity variations); moreover, their structure is simplified along with an increase in the quantity of high-melting phases. In this case, despite the structural simplification, the slag crystallization temperature increases, which leads to the reduction in the degree of overheating and an increase in the viscosity.

The effect of basicity and content of boron oxide on viscosity, crystallization temperature, phase composition, and structure of the СаО – SiO2 – B 2 O 3 – 12 % Cr 2 O 3 – 3 % Аl 2 O 3 – 8 % МgO fluorine-free slag system in the range of boron oxide content 3 – 6 % and basicity 1.0 – 2.5 is studied by vibrational viscometry, thermodynamic phase composition modeling (HSC Chemistry 6.12 (Outokumpu)), and Raman spectroscopy. It was found that physical properties of the studied slags mainly depend on the balance between the degree of structure polymerization, nature of the bond with it, and phase composition. With a low basicity of 1.0, slags are “long” and an increase in the content of boron oxide from 3 to 6 % makes them more fusible, reducing the crystallization temperature of the slag from 1340 to 1224 °C, and its viscosity from 1.0 – 0.8 to ~0.25 Pa·s at 1600 – 1660 °C, despite the significant complication of the structure, reflected in the growth of the bridging oxygen index BO from 1.10 to 1.49. With an increase in basicity, slags transfer from “long” to “short” and the content of calcium oxide increases, which, being a donor of free oxygen ions (O 2– ), acts as a modifier of the slag structure. Thus, with a basicity of B = (CaO/SiO 2 ) = 2.5, slags have a simpler structure (BO = 0.50 – 0.53) relative to slags with a basicity of 1.0, while the addition of boron oxide complicates it only slightly (an increase in BO from 0.5 up to 0.53). Increasing the concentration of B 2 O 3 lowers the crystallization temperature from 1674 to 1605 °C and the viscosity from 1.0 to 0.3 Pa·s at 1660 °C as a result of the formation of low-melting compounds (mostly 2CaO·B 2 O 3 ).

The influence of B2O3 on viscosity, crystallization temperature, phase composition, and structure of fluorine-free slags of the СаО–SiO2–B2O3–2 % Cr2O3–3 % Аl2O3–8 % МgO system in the range of boron oxide content from 0 to 6% and basicity (B = CaO/SiO2 ) equal to 1.0 is studied by vibrational viscometry, phase composition thermodynamic modeling in combination with raman spectroscopy. It is found that boron oxide-free slag has the simplest structure, as indicated by the average amount of bridging oxygen BO equal to 0.73, and has a higher viscosity compared to boron-containing slags of 0.85‒0.60 Pa•s in the temperature range of 1400‒1450°C, and crystallization temperature of 1402°C. This is due to the high proportion of high-temperature compounds in the formed slag. The introduction of 2 to 6% boron oxide complicates the structure of the slag, which is accompanied by an increase in the average amount of bridging oxygen BO to 0.98 and 1.28, respectively. The resulting four-coordination structural elements [BO4], [CrO4] and [AlO4] are embedded in the silicon-oxygen lattice of the slag and complicate it, which increases the degree of polymerization. However, the formation of low-melting eutectics CaO•B2O3 and 2CaO•B2O3 after introduction of 2 and 6% boron oxide, despite the complexity of the structure of the formed slags, leads to a decrease in crystallization temperature to 1346 and 1265°C, an increase in the degree of slag superheating and a decrease in its viscosity to 0.9 and 0.55 Pa•s already at 1350°C. Thus, the addition of boron oxide to slag, despite the structure complication of the formed oxide system, leads to an increase in the liquid mobility of the slag due to the formation of a high proportion of low-temperature phases and a decrease in the proportion of medium- and high-temperature phases and a lower binding energy of boron with oxygen compared to silicon

The viscosities of the slags of the CaO–SiO2–Cr2O3–3

An assessment of the possibility of steel direct microalloying with cerium was performed using thermodynamic modeling of cerium reduction from slags of CaO– SiO2– Ce2O3 system containing 15 % Al2O3 and 8 % МgO, additional additives of reducing agents (aluminum or ferrosilicoaluminium), at temperatures of 1550 and 1650 °C using the HSC 6.1 Chemistry (Outokumpu) software package. Depending on the additional additives of aluminum or ferroglycoaluminium, metal temperature, slag basicity and content of cerium oxide, 0.228 to 40.5 ppm of cerium transfers into the metal. With an additional additive of aluminum from slag (Y1) containing 1.0 % of cerium oxide, 0.228 ppm of cerium is transferred to the metal at 1550 °C. An increase in the system temperature to 1650 °C is accompanied by a slight increase in cerium content, reaching no more than 0.323 ppm. When added to ferrosilicoaluminium metal, cerium content in the metal is higher and amounts to 0.402 and 0.566 ppm at 1550 and 1650 °C, respectively. When concentration of cerium oxide in the slag (Y2) increases to 7.0 %, more signifcant increase in cerium content in the metal is observed, reaching in temperature range of 1550 – 1650 °C, 1.65 – 2.31 ppm with aluminum additives and 2.90 – 4.05 ppm with ferrosilicoaluminium additives. The most noticeable increase in cerium content in the metal is observed with an increase in slag basicity. During formation of slags with basicity of 2 – 3, containing 1 – 7 % Ce2O3, the equilibrium concentration of cerium in the metal varies from 0.5 to 4 ppm with aluminum additives and 1 – 7 ppm with ferro­silicoaluminium additives at 1550 °C. Slags transfer to the increased (up to 3 – 5) basicity is accompanied by an increase in the equilibrium content of cerium in the metal to 4 – 12 ppm with aluminum additives and 7 – 20 ppm with ferrosilicoaluminium additives at Ce2O3 content of 3 – 7 % and, as a result, an increase in efciency of cerium reduction process.

The article describes theoretical and experimental studies of dependence of viscosity, coefficients of sulfur and boron distribution between slag and metal, and wear degree of periclase-carbon refractories on basicity and boron oxide content in slag. It is shown that formed slags have basicity of 2.0 – 5.0 and rather high liquid mobility. These slags are characterized by an equilibrium interfacial distribution coefficient of sulfur increased to 5 – 20, which provides equilibrium sulfur content in the metal reduced to 0.001 – 0.005 %. The results of fundamental studies of the physicochemical properties of refining slags of СаО – SiO2 – В2O3 – Al2O3 – MgO system formed the basis for development of the composition of environmentally friendly fluorine-free ladle slags and technological methods for their formation in ladle-furnace unit. The recommended composition of such slags of low viscosity, which allows deep metal desulfurization, direct steel microalloying with boron and low aggressive effect on periclase-carbon refractories, provides formation of slags with a basicity of 3.0 – 4.0, containing 1 – 4 % B2O3 , 15 % Al2O3 and 8 % MgO. The formation of environmentally friendly ladle slags of the recommended composition was carried out in a ladle-furnace by loading lime, boron-containing material – colemanite (Turkey) containing 39 – 41 % B2O3 , 26 – 28 % CaO, not more than 5 % SiO2 and 3 % MgO, and pyramidal aluminum into the steel-teeming ladle for slag deoxidation and boron recovery. Introduction of the developed technology for the formation of ladle slags of recommended composition ensured the production of economically alloyed low-carbon structural boron-containing steels with a low sulfur content, incl. for large diameter pipes with high strength properties.

The study of the physicochemical properties of the CaO-SiO2-B2O3-MgO-Al2O3 slags system was carried out using the method of simplex lattices for experimental planning, the HSC 6.1 Chemistry software (Outokumpu) and the method of electro-vibration viscometry. It is shown that the studied oxide system containing 25% Al2O3, 8% MgO and 1–10% B2O3 has a low viscosity in the range of 0.05–0.4 Pa·s with a basicity of 2–5 at temperature 1500–1600 °C. It was found that an increase in the basicity of the slag from 2 to 5 in the boron oxide concentration range of 1–10% leads to an increase in the sulfur distribution coefficient from 1 to 20 and a decrease in the sulfur content in the metal from 0.017 to 0.0015%. Consequently, an increase in the basicity of the slag favourably affects the development of the metal desulfurization process. In the area of basicity 2–3, the boron interphase distribution coefficient decreases from 450 to 100 with an increase in the B2O3 content from 1.0 to 10.0% in slags, and the boron concentration in the metal increases from 0.005 to 0.02%. An increase of the basicity to 5.0 shows a high degree of boron reduction from slag to metal. The results of the laboratory experiment confirmed the results of thermodynamic modelling.

Thermodynamic modeling of the reduction of cerium from slags of the CaO–SiO2–Ce2O3 system containing 15% Al2O4 and 8% MgO, with aluminum dissolved in the metal together with calcium carbide additives at temperatures of 1550 and 1650°C was performed using the HSC 8.03 Chemistry software package (Outokumpu) based on the simplex lattice planning method used to minimize Gibbs energy. The results of thermodynamic modeling are presented in the form of composition-property diagrams (equilibrium content of cerium in the metal) for temperatures of 1550 and 1650°C. It is shown that the formation of slags within a basicity range of 2–3 containing 1–7% Ce2O3 provides an equilibrium concentration of cerium in the metal varying from 1 to 7 ppm at a temperature of 1550°C. The displacement of slags in the area of increased basicity (up to 3–5) is accompanied by an increase in the equilibrium concentration of cerium in the metal up to 7–23 ppm with a content of 3–7% Ce2O3 and as a consequence an increase in the efficiency of the process of cerium reduction. At a temperature of 1650°C, the equilibrium concentration of cerium in the metal within the basicity range of 2–3 and having a Ce2O3 content of 1–7% varies from 2 to 12 ppm. The displacement of slags in the area of increased basicity (up to 3–5) is accompanied by an increase in the equilibrium concentration of cerium in the metal to 7–33 ppm with a Ce2O3 content of 3–7%. The positive influence of the temperature factor basicity of slags and the content of cerium oxide on the process of its reduction is qualitatively explained from the standpoint of the formation of the phase composition of the slags of the studied oxide system and the thermodynamics of chemical reactions of reduction of cerium with aluminum dissolved in the metal, as well as with aluminum dissolved in the metal together with calcium carbide additives.

Thermodynamic modeling results of lanthanum equilibrium content in metal under the slag of CaO–SiO2–La2О3–Al2O3–MgO system corresponding to the chemical composition of 16 points of local simplex plan are presented using the HSC 8.03 Chemistry (Outokumpu) software package in combination with the simplex planning lattice method. In the work, slag is represented by CaO–SiO2–La2O3–15% Al2O3–8% MgO oxide system in a wide range of chemical composition for temperatures of 1550 and 1650°C, and metal contains 0.06% C, 0.25% Si, 0.05% Al (in this expression and hereinafter in mas. %). The mathematical modeling results are shown graphically in the form of composition–equilibrium content diagrams of lanthanum. There is a significant effect of slag basicity on the lanthanum equilibrium content in metal. An increase in slag basicity from 2 to 5 at a temperature of 1550°C leads to an increase in the lanthanum equilibrium content from 0.2 ppm in the region of lanthanum oxide concentration of 1–5% to 7 ppm in the region of increased concentration of lanthanum oxide to 4–7%. Hence, the increase in slag basicity favorably affects the development of lanthanum reduction. Increase in metal temperature also has positive effect on lanthanum reduction process. As temperature rises to 1650°C, the lanthanum equilibrium content in metal increases from 0.2 ppm in the region of lanthanum oxide concentration of 1–3% to 12 ppm in the region of increased concentration of lanthanum oxide to 4–7%. In diagrams of chemical composition of slag containing 56–61% CaO, 12–14% SiO2 and 4–7% La2O3 the lanthanum content in metal at level of 7–12 ppm is ensured in temperature range from 1550 to 1650°C. Therefore, a decisive role of slag basicity is confirmed, as well as the concentration of lanthanum oxide and temperature factor into the development of lanthanum reduction from slags of the studied oxide system by aluminum dissolved in metal.

Thermodynamic modeling results of lanthanum equilibrium content in metal under the slag of CaO – SiO 2 – La 2 О 3 – Al 2 O 3 – MgO system corresponding to chemical composition of 16 points of local simplex plan are presented using the HSC 8.03 Chemistry (Outokumpu) software package in combination with the simplex planning lattice method. In the work, slag is represented by CaO – SiO 2 – La 2 O 3 – – 15 % Al 2 O 3 – 8 % MgO oxide system in a wide range of chemical composition for temperatures of 1550 and 1650 °C, and metal contains 0.06 % C, 0.25 % Si, 0.05 % Al (in this expression and hereinafter in mass.%). The results of mathematical modeling are shown graphically in the form of composition - equilibrium content diagrams of lanthanum. There is significant effect of slag basicity on the lanthanum equilibrium content in metal. An increase in slag basicity from 2 to 5 at temperature of 1550 °C leads to an increase in the lanthanum equilibrium content from 0.2 ppm in the region of lanthanum oxide concentration of 1 – 5 % to 7 ppm in the region of increased concentration of lanthanum oxide to 4 – 7 %, hence the increase in slag basicity favorably affects development of lanthanum reduction. Increase in metal temperature also has positive effect on lanthanum reduction process. As temperature rises to 1650 °C, the lanthanum equilibrium content in metal increases from 0.2 ppm in the region of lanthanum oxide concentration of 1 – 3 % to 12 ppm in the region of increased concentration of lanthanum oxide to 4 – 7 %. In diagrams of chemical composition of slag containing 56 – 61 % CaO, 12 – 14 % SiO 2 and 4 – 7 % La 2 O 3 , the lanthanum content in metal at level of 7 – 12 ppm is ensured in temperature range from 1550 to 1650 °C. Therefore, there can be confirmed a decisive role of slag basicity, concentration of lanthanum oxide and temperature factor in development of lanthanum reduction from slags of the studied oxide system by aluminum dissolved in metal.