RU2378085C1 - Slag-forming mixture for steel continuous casting - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to ferrous metallurgy. Mixture contains following components in wt %: carbon-bearing ingredient 5-10, fluorine-containing ingredient 10-20, synthetic fused slag-forming ingredient is the rest. In the capacity of slag-forming oxide basis it is used synthetic fused slag-forming ingredient of system CaO-SiO₂-Na₂O, containing CaO 30-45%, SiO₂ 40-65% and Na₂O 5-15% at ratio CaO/SiO₂ 0.45-0.95.

EFFECT: there is achieved reduction of total content of moisture in mixture, increasing of stream assimilation relative to floating from steel aluminium oxides.

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Description

The present invention relates to ferrous metallurgy, and in particular to compositions of slag-forming mixtures (SCO) for continuous casting of steel.

A three-component SCO is known, containing 30-60% graphite, 10-20% fluorspar, and 30-50% blast furnace slag, the fused slag-forming base of the CaO-SiO ₂ -Al ₂ O ₃ -MgO system (Ed. St. USSR No. 582053, C21C 5/54, 1977). The mixture is simple in composition. But due to the high graphite content, significant carburization of, for example, low carbon steel occurs. The aluminum oxides contained in blast furnace slag (up to 12-15%) reduce the assimilative ability of the slag melt of the mixture with respect to these oxides floating from the steel.

A four-component SCO is known, containing 2-12% graphite, 10-25% fluorspar, 2-35% nepheline and a slag-forming fused base of the CaO-SiO ₂ -Al ₂ O _{3 system} in the form of dump slag produced by carbon-free ferrochrome (Aut. St. USSR No. 692682, B22D 7/00, 1979).

The disadvantage of this mixture is the reduced assimilative ability of the slag melt of the mixture with respect to the aluminum oxides floating from the steel due to the high content of these oxides in the mixture introduced by nepheline and dump slag.

The closest analogue of the inventive mixture is a three-component SCO containing 2-15% carbon-containing material (graphite), a slag-forming base consisting of 30-60% Portland cement of the CaO-SiO ₂ -Al ₂ O _{3 system} and fused synthetic slag powder of the CaO-SiO _{2 system} Al ₂ O ₃ -Na ₂ O + K ₂ O-CaF ₂ (Aut. St. USSR No. 570645, C21C 5/54, 1977)

The disadvantage of this mixture is the unstable content of fluorine in the synthetic slag due to the difficulty of regulating the process of volatilization of fluorine during the joint melting of the components of the mixture and calcium fluoride. Hence the unstable fluorine content in the mixture and in its slag melt, and hence the unstable composition of the mixture and the physicochemical properties of its slag melt.

A significant disadvantage of the known mixture is the high initial moisture content in it and its high moisture absorption capacity (hygroscopicity) during storage.

So, during prolonged storage of mixtures in open containers, the moisture content increases several times due to its absorption from the atmosphere, which can lead to the formation of gas bubbles in the body of the ingot and then to the suspension of the ingot crust or its breakthrough.

Another disadvantage of this mixture is the reduced assimilative ability of its slag melt with respect to Al oxides floating up from steel during continuous casting, which is a consequence of the high (up to 8-10%) initial content of aluminum oxides entering the mixture from Portland cement and synthetic fused slag . Upon receipt of 8-10% additional aluminum oxides from steel into slag, the technological properties of the slag melt of the mixture sharply deteriorate - the melting temperature and slag viscosity sharply increase. In the mold, the mixture with slag crumbles, slag-metal cakes are formed, a coarse welt up to 5 mm thick is formed around the mold perimeter, which can lead to disruption of the normal technological regime of steel casting - to “hang” of the ingot crust or to “break” its crust.

The technical effect when using the proposed SCO composition is to increase its quality due to the extremely low total moisture content in it, the practically absence of moisture absorption capacity (hygroscopicity) during storage of mixtures in open containers and the absence of aluminum oxides in the mixture, which significantly increases its assimilative capacity (capacity ) with respect to aluminum oxides floating up from steel without a noticeable change in the physicochemical properties of the slag melt of the mixture.

All this leads to an increase in the quality of the continuously cast ingot.

The specified technical effect is achieved by the fact that the slag-forming mixture including the carbon-containing ingredient and the slag-forming oxide base, as the slag-forming oxide base, it contains a synthetic slag-forming ingredient in fused form from the oxide system CaO-SiO ₂ -Na ₂ O and additionally a fluorine-containing ingredient in the following ratio ingredients, wt.%:

carbon containing ingredient 5-10 fluoride ingredient 10-20 synthetic fused slag-forming ingredient rest.

The synthetic fused ingredient contains 30-45% CaO, 40-65% SiO ₂ and 5-15%

Na ₂ O with a CaO / SiO ₂ ratio of 0.45-0.95.

As a carbon-containing ingredient, cryptocrystalline (amorphous) graphite of the GLS-2 and 3 grades and carbon-containing dust of a dry coke quenching unit (USST dust) are used.

The fluorine-containing ingredient is used in the form of fluorspar fluorite powder concentrate with a content of 80-92% CaF ₂ and a flotation concentrate with a content of 90-98% CaF ₂ .

All ingredients used are virtually free of aluminum oxides.

The analysis of scientific, technical and patent literature shows the absence of coincidence of the distinguishing features and their combination of the proposed SCO composition with the features of known technical solutions.

Based on this analysis, we can conclude that the claimed technical solution meets the criteria of "inventive step" and "novelty."

When the content of the carbon-containing ingredient in the mixture is less than 5%, the conditions for warming the metal mirror are worsened, and if it contains more than 10%, carburization of the metal is possible, which is undesirable, for example, for low-carbon steels.

When the fluorine-containing ingredient in the mixture is less than 10% and in the fused base less than 5% sodium oxides, the melting temperature and viscosity of the slag melt of the mixture sharply increase, its assimilative ability decreases.

When the fluorine-containing ingredient contains more than 20% in the mixture and more than 15% of sodium oxides in the fused base, the melting temperature and viscosity of the slag melt of the mixture decrease so much that the liquid slag flows onto the mold tray, and the “traces” of the mold swing on the surface of the cast ingots become so deep. that transverse cracks appear along these tracks. The surface quality of the ingots is reduced.

The content limits in the mixtures of the fluorine-containing ingredient and the fused slag-forming base are selected taking into account the necessary physicochemical properties of the mixtures and the basicity of the slag melt of the mixtures in the range 0.7-1.3.

The SCO of the optimal composition for the crystallizer contains 7.5% graphite, 15% fluorspar concentrate and 77.5% fused synthetic slag-forming base. The SCO for the bucket instead of graphite contains dust USSTK.

Specific examples with boundary (No. 1 and No. 2) and average (No. 3 and No. 4 - mixtures of optimal composition) values of the contents of the ingredients of the new mixture and average (No. 5) of the contents of the ingredients of the known mixture (Aut. St. USSR No. 570645 - prototype ) are presented in table 1.

Table 1 Component compositions of the claimed (No. 1-4) and known (No. 5) mixtures. Mixture number The content of ingredients, wt.% Graphite Dust dust Fluorspar Concentrate Slag-forming base Fused base Cement one 5 - 10 85 - 2 10 - twenty 70 - 3 7.5 - fifteen 77.5 - four - 7.5 fifteen 77.5 - 5 8.5 - - 46.5 45

All mixtures were tested for moisture absorption when stored in open containers for one month and an ambient temperature of 18-20 ° C.

The moisture content was determined at a temperature of 105 ° C and 600 ° C. Test data of the average compositions of the two mixtures (No. 3 and No. 5) are presented in table 2.

table 2 The moisture content in mixtures No. 2 and No. 5, wt.% (Moisture determination temperature: in the numerator - 105 ° C, in the denominator - 600 ° C). Mixture number Shelf life, day 0 5 fifteen thirty 3

5

From the data of table 2 it can be seen that the total moisture content in the new mixture is significantly lower, both in freshly prepared and after prolonged storage.

The increase in moisture content in the known mixture No. 5 occurs during its storage due to the absorption of moisture by cement.

From table 2 it is seen that after storage of the known mixture No. 5 for 5 days, it becomes unsuitable for use in the continuous casting machine because of the danger of gas bubbles forming in the continuously cast ingot (the generally accepted safe total moisture content in the mixtures should not exceed 0.8% )

In the new mixture No. 3, when stored for 30 days, the moisture content remained practically unchanged.

Testing of mixture No. 3 of medium composition (melting point of the mixture 1150 ° C, viscosity at 1300 ° C - 0.6 P · s) after storage for 30 days was carried out when casting low-carbon and low alloy steels at a speed of 0.7-0.9 m / min In the mold, mixture No. 3 was used; in the mold bucket, mixture No. 4 was used.

No coarse slag-metal cakes were observed in the scoop on the surface of the slag coating.

The mixture formed a three-layer coating in the mold: the thickness of the liquid layer was 10-12 mm, the half-sintered layer was 3-5 mm and the upper powdery dark layer was 20-25 mm thick. No rough welt formation was observed along the perimeter of the crystallizer. The total consumption of the mixture was 1.1 kg / t of cast steel.

Assessment of the surface quality of the investigated ingots (on 7 heats) showed the absence of longitudinal and transverse cracks. Rough non-metallic inclusions larger than 1 mm and gas bubbles were also absent. There were no freezes and breakthroughs of the ingot crust on the studied swimming trunks.

Thus, the positive results of the tests give reason to recommend a mixture of a new composition for use in continuous casting of steel.

The technical and economic effect of using the SCO of a new composition consists in the possibility of a significant increase in its shelf life with virtually no change in the total moisture content, as well as in the absence of hangs and breakthroughs of the ingot crust and in improving the surface quality of continuously cast ingots.

Claims (2)

1. Slag-forming mixture for continuous casting of steel, containing a carbon-containing ingredient and a slag-forming oxide base, characterized in that it further comprises a fluorine-containing ingredient, and as a slag-forming oxide base contains a synthetic fused slag-forming ingredient of the CaO-SiO ₂ -Na ₂ O system in the following ratio ingredients, wt.%:
carbon containing ingredient 5-10 fluoride ingredient 10-20 synthetic fused slag forming ingredient rest 2. Slag-forming mixture for continuous casting of steel according to claim 1, characterized in that the synthetic fused slag-forming ingredient contains CaO 30-45%, SiO ₂ 40-65% and Na ₂ O 5-15% with a ratio of CaO / SiO ₂ 0, 45-0.95.