RU2613804C1 - Slag-forming mix for continuous steel casting with high aluminium content - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: slag-forming mix comprising (wt %): carbon (5-8), fluorine (6-9), calcium oxide (30-40), aluminium (10-18), silicon (5-9), sodium (9-12), lithium (3-5), boron (6-10), manganese (1-2) and inevitable impurities (the rest). Steel is provided with an aluminium content of up to 2 wt % at a slag melting temperature 350-400°C below the liquidus temperature of steel and slag viscosity of 0.06-0.25 Pa⋅.

EFFECT: chemical inertness of molten slag-forming mix to liquid steel.

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Description

The invention relates to the field of ferrous metallurgy, to the compositions of slag-forming mixtures used to protect metal in the mold during the continuous casting of steel, in particular steel with a high aluminum content (1-2%).

A slag-forming mixture (SCO) is used during continuous casting in a mold of a continuous steel casting unit (UNRS) to prevent heat loss through a metal mirror, protect steel from secondary oxidation, reduce friction between the workpiece and the walls of the mold, and control heat removal.

Known slag-forming mixture for continuous casting of rail steel, including amorphous graphite, pulverized waste production of lime, ferrosilicon, aluminum, soda ash and cryolite with a ratio of components, wt.%:

providing basicity (CaO / SiO ₂ ) - 0.6-0.9 and the following chemical composition of the slag-forming mixture, wt.%:

The technical result is an increase in the quality of steel by improving the macrostructure, reducing the contamination of steel by non-metallic inclusions and reducing the rejection of continuously cast billets on surface defects due to improved lubrication of the molten SCO mold (Patent RU 2260494, IPC B22D 11/108, published September 20, 2005).

A disadvantage of the known SCO is the high content of SiO ₂ . At such a content of silicon dioxide, intense chemical interaction with a liquid metal having a high aluminum concentration (1-2 wt.%) Occurs, which leads to a significant change in the chemical composition and physicochemical properties of the initial slag-forming mixture.

Known slag-forming mixture for continuous casting of steel, obtained by mixing amorphous graphite, fluorine-containing substances and materials containing oxides of silicon, aluminum and alkali metals, which has a basicity (CaO / SiO ₂ ) of 0.5-1.1 and the following chemical composition, wt. %:

C 14.0-20.0 Cao 19.0-31.0 SiO ₂ 28.0-38.0 Al ₂ O ₃ 3.0-8.0 F 4.0-9.0 (Na ⁺ + K ⁺ ) 4.0-8.0

while as materials containing oxides of silicon, aluminum and alkali metals, the mixture contains feldspar mixture, soda ash, finely ground quartzite and cement in the following ratio, wt.%:

Amorphous graphite 19-26 Fluorine-containing substance 13-17 Feldspar mixture 15-20 Soda ash 3-8 Fine quartzite 11-15 Cement Rest

The technical results of the analogue invention are: improving the quality of steel by improving the macrostructure, reducing the pollution of steel by non-metallic inclusions and reducing the rejection of continuously cast billets on surface defects due to improved lubrication of the mold of the molten SCO (Patent RU 2430808, IPC B22D 11/111, published 10.10. 2011).

A disadvantage of the known SCO is also a high content of SiO ₂ . The use of this SCO is not suitable for steels with a high concentration of aluminum, since there is a multiple decrease in the concentration of SiO ₂ (up to 3-5%) and its growth in Al ₂ O ₃ (up to 35-40%). Thus, the initial physicochemical properties, including the melting temperature and viscosity, which were achieved by selection, in a certain ratio, of various fluxing fluorine-containing components, as well as the necessary concentration of alkaline oxides, will vary significantly.

Known slag-forming mixture for continuous casting of steel, obtained by mixing fluorspar, graphite, silica fume, characterized in that it additionally contains technical soda, syenite concentrate, coke dust, lithium carbonate, technical chalk, magnesium oxide, slag from ferrochrome production, silicate block, colemanite, microcalcite and cement, in the following ratio of components, wt.%:

Fluorspar 10.0-25.0 Graphite 0.1-6.0 Dust Quartz 10.0-24.0 Technical soda 1,0-17,0 Syenite concentrate 2.0-24.0 Coke dust 1.0-6.0 Lithium carbonate 1.0-5.0 Chalk technical 3.0-23.0 Magnesium oxide 0.5-5.0

one or more components from the group:

Ferrochrome slag 30.0-45.0 Silicate block 4.0-12.0 Colemanite 2.0-15.0 Microcalcite no more 18.0 Cement Rest,

however, it has the following chemical composition, wt.%:

C free 0.5-8.0 SiO ₂ 21.0-42.0 Al ₂ O ₃ 0.5-12.0 Cao 23.0-52.0 MgO 0.5-10.0 R ₂ O 1.0-15.0 F 1.0-13.0 Li ₂ O no more than 3.0 B ₂ O ₃ no more than 5,0 Inevitable impurities Rest

The basicity of the slag-forming mixture corresponds to the range of 0.85-1.25.

The technical result is an increase in the quality of steel as a result of improving its macrostructure, reducing contamination with non-metallic inclusions, reducing the number of surface defects due to improved lubrication and heat removal in the mold, as applied to various groups of steel grades and types of UNRS (Patent RU 2555277, IPC B22D 11/111, published July 10, 2015).

A disadvantage of the known slag-forming mixture is also the high chemical activity of SiO ₂ . The use of this mixture in the continuous casting of steel with high aluminum concentrations leads to a serious change in the chemical composition of the mixture and its physicochemical properties. An increase in the melting temperature of the SCO, as well as the precipitation of crystalline phases at higher temperatures (above 1300 ° C), change the viscosity and lubricating properties of the mixture, which increases the likelihood of surface cracks and an emergency stop of the continuous casting process due to metal breakthrough.

The closest analogue of the claimed invention is a slag-forming mixture for continuous casting of steel, containing, wt.%: CaO 10-35, SiO ₂ 10-40, Al ₂ O ₃ to 12, MgO to 4, MnO to 4, B ₂ O ₃ to 6, Na ₂ O + K ₂ O + Li ₂ O 4-15, C _free 2-12, FeO up to 3, F 6-10. Part of CaO is introduced in the form of CaCO ₃ additive _of 1-10%, which leads to improved protection of the poured metal from interaction with air due to the oncoming CO stream, conducting a smooth melting of the slag-forming mixture without foaming slag and mixing with the solid slag-forming mixture. The cleaning level is reduced by 10-15 kg / t, the yield of the highest surface quality group is increased by 10-15% (Patent RU 2245756, IPC B22D 11/08, B22D 11/111, published on 02/10/2005 - prototype).

A disadvantage of the known SCO is that a too wide range of SiO ₂ contents is given from 10 to 40%. Moreover, the lower limit of the concentration of SiO ₂ (10% wt.) Exceeds the equilibrium value with the chemical potential of oxygen in the metal-slag system, determined by the aluminum content (1-2%). The total content of alkali metal oxides is given, although they affect not only physicochemical properties such as viscosity and surface tension, but also the nature of the SCO crystallization, which determines the lubricating and thermophysical properties, as well as the thickness of the slag skull on the walls of the mold. Their influence is different and has an extreme look. Therefore, exceeding the optimal concentration leads to the opposite effect.

The technical result of the claimed invention is to ensure the chemical inertness of the molten SCO to liquid steel with a high aluminum content, while ensuring the necessary physicochemical properties of the molten slag. Provided: viscosity values in the recommended range (from 0.06 to 0.25 Pa 0,2s), a melting point 350-400 ° C lower than the liquidus temperature of steels with a high aluminum content (> 1%).

The specified technical result is achieved in that a slag-forming mixture based on CaO-Al ₂ O ₃ for continuous casting of steel with a high concentration of aluminum up to 2 wt.%, Containing carbon, fluorine, oxides of calcium, aluminum, silicon, sodium, lithium, boron, manganese , according to the invention, has the following chemical composition, wt.%:

C 5-8 Cao 30-40 Al ₂ O ₃ 10-18 SiO ₂ 5-9 Na ₂ O 9-12 Li ₂ O 3-5 F 6-9 B ₂ O ₃ 6-10 MnO 1-2 Inevitable impurities Rest

To ensure the necessary chemical inertness of the slag-forming mixture, first of all, the content of easily reducible silicon dioxide (SiO ₂ ) was maximally limited. Since silica (SiO ₂ ) reduces the temperature of separation, the fraction of crystalline phases and improves the lubricating properties of the SCO, its minimum content is limited to 5%. The content of Al ₂ O ₃ , SiO ₂ and CaO is selected in the required ratio to provide the necessary melting point of the SCO, taking into account the maximum possible limitation of the content of Al ₂ O ₃ , since an increase in the concentration of corundum (Al ₂ O ₃ ) increases the tendency of SCO to precipitate crystalline compounds . In addition, it must be taken into account that an increase in the Al ₂ O ₃ content in the molten SCO layer can occur as a result of the assimilation of non-metallic inclusions containing Al ₂ O ₃ , including those resulting from the secondary oxidation of steel. The reduced silica content in the SCO leads to a high tendency of the molten layer to crystallize, which leads to the appearance of a rough surface of the slag skull, which significantly reduces its lubricating properties, as well as the heat removal from the surface of the crystallizing ingot to the crystallizer wall. To reduce the degree of development of this phenomenon, lower the susceptibility of the SCO to crystallization, boron oxide was introduced into its composition. Since with an increase in the content of boron oxide its reduction is facilitated by aluminum contained in the steel (1-2%), its content is limited to 10%. Sodium oxide and even more lithium oxide lower the viscosity of the slag melt, ensuring its infiltration into the resulting gap between the surface of the solidified ingot and the wall of the mold. However, an excessive decrease in viscosity can lead to complete leakage of slag into the resulting gap, which leads to a violation of the continuous casting process. Therefore, the content of Na ₂ O and Li ₂ O is limited to the ranges of 9-12% and 3-5%, respectively. The minimum fluorine content of 6% is due to the need to lower the melting temperature and provide the required viscosity values. The fluorine concentration is limited to 9%, since with an increase in the fluoride content, the predisposition of liquid slag to crystallization increases, as well as the negative impact on the environment due to the formation of a number of volatile adverse fluorides, including hydrogen fluoride. The carbon content in the mixture is determined by the need to obtain the necessary rate of its melting and compensation of heat losses on the SCO melting. At a carbon concentration of less than 5%, the melting rate is too high, and at a content of more than 8%, the likelihood of carburization of the metal, as well as boiling of the mixture with the formation of CO and CO ₂ , increases, which can lead to the formation of surface defects. The composition of the developed SCO includes MnO (1-2%) to increase the oxidation efficiency of the contained carbon. At a lower MnO content, carbon can remain and pass into the cast metal, leading to undesired carburization. On the contrary, at a higher MnO content in liquid slag, its rather high content remains, which leads to the oxidation of aluminum contained in steel.

Examples of carrying out the invention

The mixtures, the compositions of which are shown in table 1, were placed in a nickel crucible, heated in an inert atmosphere of argon in a resistance furnace to temperatures of 1300-1350 ° C and kept for 20 minutes until complete melting and homogenization. After melting and cooling, the slag sample was crushed, mixed with pure carbon, and the chemical composition was determined by X-ray diffraction method. After determining the composition, a cylindrical sample was prepared and the melting temperature was determined using a high-temperature microscope. Each pre-molten mixture was heated to a temperature of 1300 ° C and the viscosity was determined on a rotational viscometer. The results are shown in table 1.

From table 1 it is seen that each of the proposed compositions (2-4), including the prototype composition (1), has a low melting point (1088-1105 ° C) and viscosity values in the range (0.125-0.151).

To determine the change in the melting temperature and viscosity after interaction with the liquid metal during continuous casting, we studied the changes in the SCO parameters after contact with the liquid metal. The composition of the steel is presented in table 2.

A metal sample weighing 1.5 kg was placed in a periclase crucible, and in an inert atmosphere of argon was heated in an induction furnace to a temperature of 1530 ° C. Then, a slag mass of 0.15 kg was planted on the meniscus of the metal. After the mixture was transferred to the surface of the molten metal, the temperature was maintained at 1530 ° C for 15 minutes, since the estimated contact time of the molten SCO with liquid metal during continuous casting of steel at a speed of 0.7-0.9 m / min is approximately 10 minutes. After 15 minutes, the inductor was turned off, and after cooling the crucible, a slag sample was taken. Then, the melting temperature and viscosity of the slag samples were determined. The results are presented in table 3.

From the data of table 3 it follows that there was a change in the melting temperature and viscosity in all compositions. However, the melting temperature in composition 1 (prototype) has changed to a value that significantly goes beyond the required range (1090-1150 ° C). Also, due to a significant change in the concentrations of components in composition 1, the viscosity value increased from 0.151 Pa⋅s to 0.415 Pa⋅s, which also exceeds the recommended values (0.06-0.25 Pa⋅s). Since compositions 2-4 have a satisfactory chemical inertness to liquid metal, changes in the melting temperature and viscosity occurred to a lesser extent and their final values are in the recommended range.

Thus, the inventive mixtures of compositions 2-4 have the necessary physico-chemical properties, and chemical inertness allows you to maintain their values in the required interval during the continuous casting of steel with a high aluminum content (1-2%).

The use of the developed SCO for continuous casting of steel with a high aluminum content (1-2%) should provide the necessary surface quality of continuously cast metal as a result of maintaining the initial physicochemical properties in the required interval.

Claims (2)

Slag-forming mixture based on CaO-Al ₂ O ₃ for continuous casting of steel with an aluminum concentration of up to 2 wt.%, Containing carbon, fluorine, oxides of calcium, aluminum, silicon, sodium, lithium, boron and manganese, characterized in that it has the following chemical composition, wt.%: C 5-8 Cao 30-40 Al ₂ O ₃ 10-18 SiO ₂ 5-9 Na ₂ O 9-12 Li ₂ O 3-5 F 6-9 B ₂ O ₃ 6-10 MnO 1-2 Inevitable impurities Rest