RU2605410C1 - Slag forming mixture for steel refining - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy and can be used for steel refining in "ladle-furnace" aggregates and vacuum vessels. Slag-forming mixture contains as flux secondary aluminium production wastes and a slag component and additionally sodium bicarbonate at following ratio of components, wt%: sodium bicarbonate 1.0-2.0, wastes from production of secondary aluminium 10.0-30.0, slag component makes up balance. Secondary aluminium production wastes, having following chemical composition, wt%: fine Al metal 5.0-20.0, Al₂O₃ 50.0-75.0, MgO 5.0-12.0, SiO₂ 1.0-10.0, (NaCl+KCl+NaF+KF+Na₂O+K₂O) 5.0-20.0.

EFFECT: achieving low viscosity of slag, high degree of refinement and desulphurisation of steel, resistance of lining.

1 cl, 1 dwg, 3 tbl

Description

The invention relates to the metallurgical industry and can be used for refining various grades of steel, guidance and liquefaction of slag in the units "ladle furnace" (ACP) and vacuum cleaners for out-of-furnace steel processing.

Known solid slag-forming mixture for refining steel in a ladle furnace unit (Dyudkin D.A. Steel production on a ladle furnace unit: monograph / D.A. Dyudkin, S.Yu. Bat, S.E. Grinberg, S.N. Marintsev. - Donetsk, Southeast Ltd., 2003. - 300 p.), To ensure high refining ability, a solid slag-forming mixture for refining steel in a ladle furnace should provide:

- removal of sulfur from steel;

- deoxidation of steel;

- cleaning steel from non-metallic inclusions;

- ensuring the proper chemical composition of the finished steel;

- protection of steel from the effects of the atmosphere;

- thermal insulation of the metal surface;

- high speed guidance of liquid slag melt on the metal mirror;

- low melting point and heat capacity of the slag;

- low aggressiveness to the lining of the automatic gearbox;

- minimum gas permeability;

- a stable slag structure not prone to self-decay.

Most fully meet these requirements solid slag-forming mixtures, consisting of flux and slag component. To ensure high desulfurization ability, the slag component contains from 50 to 70% CaO, from 20 to 35% Al ₂ O ₃ and 10% SiO ₂ . To ensure a high rate of slag guidance, to reduce its viscosity and melting point, the flux consists of fluorite CaF ₂ , and to ensure high deoxidizing ability, a “cross-section” of aluminum wire is introduced into the flux.

A slag-forming mixture of this composition provides the best conditions for desulfurization and refining of steel. A slag-forming mixture of this composition achieves a distribution coefficient of sulfur (S) / [S] of more than 600.

The disadvantage of such slag-forming compositions is the high toxicity of volatile fluorine compounds, the low resistance of the lining due to corrosion by fluorine, the high cost of fluorspar (fluorite source) and aluminum wire for deoxidation. The main disadvantage of the known slag-forming mixture is self-scattering of slag after cooling.

Known solid slag-forming mixture for the refining of metal in the ACP (RF patent No. 2322512, op. 04/20/2008), consisting of a slag component and flux in the following ratio, mass. %: slag component - lime 77-89; flux - aluminum concentrate AVK-50F 11-23. The slag component - lime contained in the slag-forming mixture - lime provides a high degree of desulfurization of steel, and flux - metal aluminum provides a high degree of deoxidation of metal and slag, reducing the cost of aluminum for deoxidizing steel, reducing harmful emissions into the atmosphere, increasing the level of physicomechanical properties of steel.

A disadvantage of the known slag-forming mixture is the absence of slag stabilizers in the composition of stabilizers preventing it from spilling upon cooling. In addition, AVK-50F, in fact, is secondary fractionated aluminum with a high content of metallic aluminum with a particle size of 5-50 mm. The introduction of such material into the ladle leads to an increase in the aluminum content in steel; therefore, it is unsuitable for processing a number of steel grades in which there is a restriction on the aluminum content. First of all, these are steels cast on bloom caster with an open jet, rail, wheel grades, steels for power engineering products. In these steels, it is not allowed to exceed the aluminum content above 0.004-0.005%, and the introduction of crushed aluminum into the ladle for the purpose of deoxidizing the steel will lead to an excess of the aluminum content in steel above the permissible level of 0.004-0.005%.

In addition, the minimum melting point in the CaO-Al ₂ O ₃ binary system is 1390 ° C and corresponds to the eutectic composition of 12CaO · 7Al ₂ O ₃ mayenite, and in the CaO-Al ₂ O ₃ -SiO ₂ ternary system, the minimum melting point is 1190 ° C, which can significantly reduce the viscosity of the slag and increase its refining properties, i.e. the slag component of the mixture must necessarily contain silicon oxide SiO ₂ .

The closest analogue in technical essence to the claimed solution is a slag-forming mixture containing waste from the production of secondary aluminum (OPVA) and a slag component in the form of slag from the production of silicocalcium and lime (copyright certificate No. 1089146, IPC C21C 5/54, published April 30, 84. Bull. No. 16).

A disadvantage of the known slag-forming mixture is the presence of two hygroscopic constituents in the mixture — lime and silicocalcium production slag, which can lead to hydrogen entering the metal and the absence of components in the mixture ensuring stabilization of the slag after cooling.

For these reasons, the use of this mixture in modern units of out-of-furnace steel processing will not be technologically advanced and will not allow solving the problems facing out-of-furnace steel processing.

The technical result of the present invention is the creation of a complex slag-forming mixture, providing a low slag viscosity, a high degree of steel refining and desulfurization, an increase in the durability of the AKP lining and stabilization of the refining slag after cooling.

The specified technical result is achieved in that the slag-forming mixture for refining steel, containing as a flux waste from the production of secondary aluminum and the slag component, according to the invention it additionally contains sodium bicarbonate in the following ratio of components, wt. %:

sodium bicarbonate 1.0-2.0 secondary production waste aluminum 10.0-30.0 slag component rest,

moreover, the waste production of secondary aluminum have the following chemical composition, mass. %:

Al metal fine 5.0-20.0 Al ₂ O ₃ 50.0-75.0 MgO 5.0-12.0 SiO ₂ 1.0-10.0 (NaCl + KCl + NaF + KF + Na ₂ O + K ₂ O) 5.0-20.0.

When using the claimed slag-forming mixture in the automatic gearbox during out-of-furnace processing, a refining slag is produced on the metal mirror that meets the requirements given in (Dyudkin D.A. Steel production on a ladle furnace unit: monograph / D.A. Dyudkin, S.Yu. Bat , S.E. Grinberg, S.N. Marintsev. - Donetsk, Southeast Ltd., 2003-300 p.). The proposed composition of the slag-forming mixture has a complex effect on the slag, increasing its technological properties and stabilizing after cooling. The increased content of alumina in OPVA in the amount of 50.0-75.0 mass. % liquefies it, increases the surface tension at the metal-slag boundary. As a result, slag inclusions are less entangled in the steel melt, and metal kings in the slag. The metallic aluminum contained in the OPVA composition (5.0–20.0 wt.%) In the form of a fine powder increases the degree of slag deoxidation and, accordingly, is not absorbed by the metal.

In the experiments, the steel located in a 120-t ladle was treated with the declared slag-forming mixture containing OPVA and sodium bicarbonate as a flux, while the aluminum content in the obtained steel did not exceed a dangerous level of 0.004-0.005% (restriction on casting conditions for bloom caster) . There was no need to use fluorspar. Steel was successfully cast on the bloom caster with an open stream.

Fine metallic aluminum in an amount of 5.0 to 20.0 mass. % deoxidizes ladle slag, increasing its desulfurizing ability and the ability to absorb non-metallic inclusions from steel. In addition, ladle slag with a reduced content of iron and manganese oxides has a lower density, which also contributes to a better separation of slag from steel.

The presence of magnesium oxide in the flux in the amount of 5.0-12.0 mass. % increases the concentration of periclase in the slag and contributes to its crystallization on the walls of the bucket, forming a protective skull, reducing the aggressive effect on the magnesia lining of the bucket.

Compounds of alkali metals K and Na in the form of oxides and salts in an amount of 5.0 to 20.0 mass. % (total) contained in OPVA have a stabilizing effect on the slag, preventing it from scattering upon cooling.

By the methods of X-ray phase analysis it was found that when flux is introduced into the slag-forming mixture in the form of OPVA, after cooling of the slag, the whitewash does not fully stabilize. It is presented as a mixture of stable high-temperature modification of larnite - β-C ₂ S and unstable - shannonite - γC ₂ S. To increase the stabilizing effect, an additional stabilizing additive is introduced into the composition of the slag-forming mixture, which increases the stability of belite over the entire range of flux introduction in the form of sodium bicarbonate in the amount of 1-2 mass. %

When introducing into the composition of the slag-forming mixture together with OPVA sodium bicarbonate less than 1.0 mass. % it does not have a stabilizing effect. The introduction of sodium bicarbonate in the composition of the slag-forming mixture of more than 2.0 mass. % impractical, since it increases the cost of the mixture.

An example embodiment of the invention

To assess the effect of the ratio of the slag component and the flux in the slag-forming mixture on the properties of refining slags in a wide range of their phase and chemical compositions, covering all cases of steel refining, from oxides (ppm) CaO, SiO ₂ , Al ₂ O ₃ , MgO, MnO, and FeO, a slag component of a slag-forming mixture was prepared. The chemical composition of the slag component was installed on experimental steel melts, with a limitation of the aluminum content in the steel. The chemical composition of the slag component is shown in table 1.

The experiments were carried out using simplex-lattice planning methods with restriction of the area of the simplex. As factors of variation, the content in the proposed slag-forming mixture of the slag component, flux and sodium bicarbonate is taken. Secondary aluminum production waste (OPA) was used as a flux: dust from the production of secondary aluminum subjected to briquetting on a hydraulic briquetting press. The chemical composition of OPVA is shown in table 2.

As sodium bicarbonate, technical soda ash was used according to GOST 5100-85.

The experiment was carried out in full accordance with the industrial technology of the work. In accordance with the experimental design, the slag component was prepared by co-grinding the oxides (analytical grade) to a particle size of not more than 100 μm. The milled mixture was placed in a periclase-graphite crucible and heated in a Tamman furnace to a melting temperature of 1400-1500 ° C. The flux (OPVA) together with sodium bicarbonate was mixed before being introduced into the melt, pressed at a pressure of 50 MPa and introduced into the molten slag component in the form of briquettes crushed to a size of 5-10 mm in an amount corresponding to the experimental design. After the introduction of OPVA and sodium bicarbonate into the slag melt, the melt was heated to a temperature of 1550 ° C and its viscosity was measured. After determining the viscosity, the crucibles were removed from the furnace and placed in a heat-insulating unit for slow cooling.

Petrographic studies were carried out on chilled slag samples, and the chemical and mineralogical composition was determined.

Slag viscosity at a temperature of 1550 ° C and the content of mineral phases in the slag were considered as response functions. Table 3 shows the experimental design and its results.

The test results show that when the OPVA content is from 10.0 to 30.0%, a low slag viscosity is observed, which indicates high refining properties of the slag. When introduced into the slag-forming mixture of OPVA less than 10.0 mass. % in the slag, a significant amount of γ-C ₂ S shannonite is formed (93.0%) and the slag becomes unstable and begins to decompose upon cooling. When introduced into the slag-forming mixture of OPVA more than 30.0 mass. % in the slag, a significant amount of spinel MA (MgO · Al ₂ O ₃ ) is formed, which has a high melting point, which leads to an increase in the homogeneity of the slag and its viscosity.

The desulfurizing properties of the slag were evaluated using the Mannesmann index, which reflects both the thermodynamics and kinetics of the desulfurization process, and is determined by the following ratio:

The best desulfurizing properties of refining slag are achieved with a value of this index from 0.35 to 0.45% ^-1 (Wcisio, Z. Role of slag in the steel refining process in the ladle / Z. Wcisio, A. Michaliszyn, A. Baka / / Journal of Achievement in Materials and Manufacturing Engineering - 2012. - No. 12. - Vol. 55, issue 2. - P. 390-395.). The test results showed that when the content in the slag-forming mixture of the flux-forming component based on OPVA in an amount of from 10.0 to 30.0 mass. % Mannessman index in refining slags is in the range of 0.1-0.3% ^-1 (less than 0.35). Based on the formula (1), the presence in the composition of OPVA SiO ₂ in an amount of from 1.0 to 10.0 mass. % and Al ₂ O ₃ in an amount of 50.0-75.0 mass. % contributes to a decrease in the Mannesman index, i.e. in the range of flux content in an amount of from 10.0 to 30.0 mass. % slag-forming mixture has high desulfurization properties. When the content in the slag-forming mixture of flux less than 10.0 mass. % and more than 30.0 mass. % Mannessman index becomes larger, which indicates a decrease in desulfurizing properties of the slag-forming mixture as a whole.

Data on the interfacial tension between the metal and slag also indicate that, with the optimal composition of the slag-forming mixture, the resulting refining slags have a high interfacial tension, which indicates their high refining ability.

The content in the optimal region of the stable phase of βC ₂ S indicates the stabilizing effect of flux based on OPA on slag white. The stabilizing effect on belite is exerted by alkali metal compounds K and Na contained in OPA.

When the OPVA content in the slag-forming mixture is from 10.0 to 30.0%, the content of periclase (MgO) is observed in the slag composition, which indicates that the slag does not corrode the lining, but, on the contrary, protects it.

Additionally, to determine the more accurate boundaries of the optimal region, a flux-based OPA was introduced into the slag component heated to a temperature of 1350 ° C in an amount of 10.0 to 30.0 mass. %, in increments of 2.0%. After complete melting of the flux, the slag was kept at this temperature for 15 minutes and cooled with the furnace.

Slag was subjected to x-ray phase, x-ray structural and chemical analyzes. The test results showed that when OPVA is introduced into the slag-forming mixture as a flux, slag bleach is stabilized in two ways: stabilization of the high-temperature modification of βC ₂ S belite with alkali metal ions that make up the OPVA and chemical stabilization due to the conversion of belite C ₂ S to C ₂ gelenite AS. Figure 1 shows the effect of the amount of OPVA flux in the slag-forming mixture on the phase composition of the slag.

According to the test results, when fully stabilized, γC ₂ S disappears due to the conversion of a part of belite to gelenite according to the scheme

and stabilization of the remaining belite in βC ₂ S, while the total amount of belite decreases from 100 to 55%, and the content of gelenite rises to 45%. The test results also indicate that although the slag-forming mixture has good refining properties, starting with a flux content in the slag-forming mixture of 10 mass. %, however, in the claimed range, the flux in the form of OPVA does not have a long stabilizing effect on hardened slag. The duration of the stable existence of chilled slag pieces does not exceed several hours. This, most often, is enough to reduce the negative impact of slag on refractories, but not enough to use hardened slag as a raw material in the production of crushed stone.

To enhance the stabilizing effect, sodium bicarbonate was additionally introduced into the composition of the slag-forming mixture. An additional additive was introduced into the OPA-based flux; the mixture was mixed and pressed at a pressure of 50 MPa. The flux (OPVA) together with the stabilizer was introduced into the slag-forming component, heated to a temperature of 1350 ° C, in an amount corresponding to the design of the experiment. After the mixture was completely melted, the slag was kept at this temperature for 15 minutes and cooled together with the furnace.

After cooling, the degree of slag dispersion was determined, equal to the ratio of the amount of mass of the sample passing through a 063 sieve to the mass of the initial sample multiplied by 100%. The experimental design and stabilization results of slag with sodium bicarbonate are also shown in table 3.

The test results indicate that the additional stabilizer acts throughout the claimed range of ingredients, providing a stabilizing effect on the slag.

Moreover, it was established on experimental swimming trunks that when processing steel on a 120-ton ladle, attempts to enter more than 30 masses. % flux into the slag-forming mixture led to cases of slag spills from the ladle. This was due to the fact that the oxidation of aluminum contained in the waste from the production of secondary aluminum is exothermic and the introduction of a large amount of aluminum into the ladle as a part of the flux causes too violent reaction, there is a danger of foaming of slag and its splashing out of the ladle. To ensure the safety of production, it is not recommended to increase the flux consumption of more than 30 mass. %

In addition, the need to limit the consumption of flux is determined by its influence on the increase in the aluminum content in the treated steel. Therefore, the inventive slag-forming mixture containing OPVA as a flux with an additional stabilizer in the form of sodium bicarbonate is recommended for the treatment of steels for which even a small increase in the aluminum content up to 0.004-0.005% (steels cast on bloom caster with an open stream, steels for power engineering is critical) , rails, etc.). This possibility is ensured by the fact that the stabilizing effect of the compounds of alkali metals of sodium and potassium complements the stabilizing effect of alumina, which reduces the amount of flux in the proposed slag-forming mixture. At the same time, the stabilization effect of ladle slag is achieved, and an increase in the aluminum content in steel, as shown by the experience of processing steels at AKP-120, will be limited to a maximum of 0.002-0.003%.

Claims (1)

Slag-forming mixture for steel refining, containing secondary aluminum waste and a slag component as a flux, characterized in that it additionally contains sodium bicarbonate in the following ratio, wt.%:
sodium bicarbonate 1.0-2.0 secondary production waste aluminum 10.0-30.0 slag component rest,
moreover, the waste from the production of secondary aluminum have the following chemical composition, wt.%:
Al metal fine 5.0-20.0 Al ₂ O ₃ 50.0-75.0 MgO 5.0-12.0 SiO ₂ 1.0-10.0 (NaCl + KCl + NaF + KF + Na ₂ O + K ₂ O) 5.0-20.0

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