FR2621840A1 - MOLDING ADDITIVE FOR CONTINUOUS CASTING OF STEEL AND METHOD FOR CONTINUOUS CASTING OF STEEL - Google Patents

Abstract

Disclosed is a molding additive for use in a mold for the continuous casting of steel. According to the invention, the additive comprises, as a base material, at least 50% by weight of synthetic calcium silicate which contains CaO and SiO2 in a total amount which is not less than 70% by weight, Al2 O3 in an amount not exceeding 8% by weight, Fe2 O3 in an amount not exceeding 1% by weight and F in an amount of 1 to 10% by weight, and the CaO / SiO2 ratio of which is not less than 1.20. The invention applies in particular to metallurgy.

Description

The present invention relates to a molding additive suitable for

  use in a mold for the continuous casting of steel, and more particularly, to a molding additive which contains synthetic calcium silicate as a base material and is suitable for use in a mold for casting

continuous steel.

  Such an additive for use in a mold for the continuous casting of steel consists of a primary material such as portland cement, yellow phosphorous slags or wollastonite and, if required, a SiO2-containing material. The molding additive further contains a flux material such as calcined sodium ash, borax, cryolite, sodium fluoride or flurorine, and a material

  carbon which serves as a melting rate adjusting agent.

  The additive is applied to the surface of a molten steel that has been poured into a mold, the additive being consumed while performing various functions. The important functions of the additive are (1) lubrication between the mold and a solidifying shell, (2) melting and absorption of any inclusions that rise to the surface of the molten steel and (3)

  thermal insulation compared to molten steel.

  The progressive development of continuous casting processes in Japan is remarkable, and active efforts are directed towards increasing the proportion of HCR (Hot Load Rolling) or HDR (Hot Rolling) in a complete casting process. continuous, the acceleration of the casting and so on. In this situation, it is strongly desired to employ even stricter criteria for molding additives which influence the quality of the castings and the stability of a casting process and furthermore there is a demand for the feed. in molding additives of various types having characteristics which

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  differ greatly from those of conventional molding additives. For these reasons, it has been proposed to provide a wide variety of chemical compositions which regulate various characteristics of these additives, such as the softening point, the melting point, the viscosity, the

  surface tension and crystallization temperature.

  In particular, it is very important to adjust the weight ratio of CaO to SiO2 (hereinafter called "CaO / SiO2 ratio") because this ratio has an influence

critical about these features.

  To achieve the functions (1) and (2) among the functions noted above of such a powder, it is very important to adjust the characteristics such as the softening point and the viscosity of the additive which indicates the importance of the choice of the chemical composition mentioned above. In order to obtain the function (3) regarding thermal insulation with respect to a molten steel, it is important to appropriately select powder characteristics such as apparent density and spreadability, as well as the melting powder that one can be

adjusted by a carbonaceous material.

  The base material of the conventional additive is selected from portland cement, yellow phosphor slags, synthetic slag, tyollastonite and the like. Each of these materials, however, has its merits and drawbacks, there is no material with characteristics that can

  meet all the conditions of a basic subject.

  For example, Portland cement is characterized by its relatively stable chemical composition. Moreover, since its CaO / SiO2 ratio is greater than that of the other base materials, by combining portland cement and a pearlite powder as a light material containing SiO 2, it is possible to obtain a low apparent density and a good thermal isolation and adjust the CaO / SiO2 ratio over a wide range. However, the portland cement contains 4CaO.A1203.Fe203 at 9 to 15% by weight and therefore the resulting additive contains Fe 2 O 3 in an amount of the order of 2% by weight. Fe 2 O 3 in the powder may react with a component (eg Al) of a molten steel to cause contamination of the molten steel and this will at the same time result in a change in the characteristics of the additive. As a result, it is impossible to obtain stable lubrication. In addition, since a powder which contains portland cement as a base material is moisture sensitive, it is difficult to granulate such a powder using a generally suitable granulation process which includes

  steps of hydration, mixing and extrusion.

  In contrast, yellow phosphor slags as well as synthetic slags having a composition similar to those of the former, are a substance melt-cooled in water and milled. As a result, such slags can be used as amorphous materials having excellent homogeneity in the distribution of the components, and granulation is easy. However, since the CaO / SiO ratio is relatively low (0.9-1.15), if an additive with a relatively high CaO / SiO 2 ratio is to be made, the amount of light matter containing SiO 2 added should be reduced to as a result, a problem in that the apparent density of the powder obtained is high. another disadvantage of this material lies in the fact that a powder composition having a CaO / SiO 2 ratio of 1.15 or more can not

to be obtained.

  Wollastonite has an even lower ratio of CaO / SiO2 and its use is therefore

  limited to an extremely narrow range of applications.

  In addition, wollastonite is inferior in terms of the stability of its components. Various other methods have been proposed, for example, a method for producing a powder having a high CaO / SiO 2 ratio by adding limestone or fluorine to a base material such as yellow phosphor slags or wollastonite having a relatively low CaO / SiO 2 ratio. However, all the conventional methods pose problems concerning the stability of the quality of the product. As a result, it was impossible to obtain a powder

of a desired quality.

  The present inventors have sought various types of materials in order to solve the above-described problems experienced in conventional types of powder-based materials and have found that for a base material for a molding additive of the present invention, a silicate synthetic calcium having CaO and SiO2 in a total amount not less than 70% by weight, Al 203 in an amount not exceeding 8% by weight, Fe203 in an amount not exceeding 1% by weight, and F in an amount of 1 to 10% by weight, and whose CaO / SiO 2 ratio is not lower than 1.20, was appropriate to achieve the object of the present invention. The present invention therefore relates to a molding additive suitable for use in a mold for the continuous casting of steel, which additive comprises, as a base material, at least 50% by weight of synthetic calcium silicate which contains CaO and SiO 2 in a total amount of not less than 70% by weight, A1 203 in an amount not exceeding 8% by weight, Fe 2 O 3 in an amount not exceeding 1% by weight and F in an amount of 1 at 10% by weight, and

  whose CaO / SiO2 ratio is not less than 1.20.

  The synthetic calcium silicate which is used as the base material of a molding additive according to the present invention has a relatively high ratio of CaO / SiO 2. Accordingly, if the additive is to be manufactured, a large amount of light material containing SiO 2 can be employed. Therefore, it is possible to produce a molding additive having low bulk density and good thermal insulation properties and also to choose the CaO / SiO 2 ratio over a wide range by changing the amount of SiO 2 -containing material to be added. Moreover, since the Fe 2 O 3 content of the synthetic calcium silicate is low, the amount of Al 2 O 3 produced as a result of the Fe 2 O 3 reaction with Al in a molten steel is small, and it is possible to prevent the contamination of the steel. . On the other hand, the extent of the change in the characteristics of the powder due to the Al 203 production is low and thus a stable lubrication is possible. Accordingly, the synthetic calcium silicate of the present invention has characteristics suitable for use as a base material of the molding additive for which a particularly strict criterion must be employed. The synthetic calcium silicate used in the present invention is easily obtained in the following manner. Materials such as CaCO 3, Ca (OH) 2, dolomite, siliceous sand, quartzite, bauxite, clay, chamotte, cullet, calcined sodium ash, lithium carbonate, cryolite, sodium fluoride, fluorine and coke powder are mixed to form a predetermined chemical composition, melted at a high temperature of not less than 1,400 C in a heating oven such as an electric oven, cooled to water and granules, dried at 100 C or more, and milled to a mesh size of less than 100 by a conventional pulverizer such as a

ball mill.

  Coke powder is added to reduce and eliminate Fe203 in a pig iron while the powder of

  glass is added to shorten the melting time.

  Since the synthetic calcium silicate thus obtained is an amorphous material melt-cooled in water and milled, its components are homogeneously distributed and it contains neither free CaO nor hydratable minerals such as 3CaO.SiO2. As a result, the present synthetic calcium silicate can be granulated by a granulation process which comprises the steps of hydration, kneading and extrusion or drying thereof

  by spraying in the form of a slurry.

  The composition of the synthetic calcium silicate material will now be described. Its CaO / SiO2 ratio is chosen to be not less than 1.20. this is because, if the CaO / SiO 2 ratio is not less than 1.20, the selection range of the CaO / SiO 2 composition of a molding additive can be enlarged and, if a large amount of light SiO 2, a powder having a low bulk density and good thermal insulation properties can be obtained. From this point of view, it is preferable that the CaO / SiO 2 ratio of the synthetic calcium silicate is as high as possible. However, as the CaO / SiO 2 ratio increases, the solidification point and the crystallization temperature of a melt become higher and thus the manufacture of the melt becomes remarkably difficult, with the result that the desired amorphous material is difficult to obtain. in a stable state. Accordingly, the CaO / SiO 2 ratio of the synthetic calcium silicate used in the present invention is preferably

  between 1.2 and 2.3, and more preferably between 1.2 and 1.9.

  The powder occasionally absorbs a large amount of Al 2 O 3 which rises to the surface of a molten steel in the mold. If the powdered slags contain A1203 in an amount of not less than 15% by weight, a high melting point mineral such as gehlenite (2CaO, Al 2 O 3 SiO 2) precipitates and the lubricating effect deteriorates. . As a result, the A1203 content of the synthetic calcium silicate used in the present invention does not exceed

  preferably not 8% by weight, and preferably not 5% by weight.

  Fe203 reacts with components contained in molten steel to cause contamination of the steel and also cause changes in the characteristics of the powdered slag. Consequently, it is necessary to limit the Fe 2 O 3 content of the synthetic calcium silicate to a little more than 1% by weight.

  weight and better not more than 0.3% by weight.

  F is added in order to adjust the viscosity of a melted product in its manufacture and to improve the efficiency of the working operation. If an excessive amount of F is added, a melting furnace is severely damaged and composition changes due to evaporation are found. If the amount of F added is excessively low, the effect of the drop in viscosity is small. Accordingly, the F content of the present synthetic calcium silicate is preferably

  from 1 to 10% by weight and more preferably from 2 to 7% by weight.

  On the other hand, flux components such as Na 2 O, Li 2 O and B 2 O 3 are added for purposes similar to those of F, so it is possible to adjust the melting point and the viscosity of a molten product during the

  manufacture of synthetic calcium silicate.

  However, given the degradation of a container of the melt or the amount of evaporation during manufacture, it is preferable that the total amount of

  added flux components does not exceed 15% by weight.

  The powder of the present invention consists of a base material, a material containing SiO 2, a flux material and a carbonaceous material, the list of which is given below: base material: silicate of synthetic calcium material containing SiO2: perlite, fly ash, siliceous sand, glass powder, diatomite or the like, flux material: calcined sodium ash, Li2CO3, NaF, Na3AlF6, fluorine, BaCO3, MgF2, borax or the like, and carbonaceous material : coke powder, carbon black,

natural graphite or the like.

  It is necessary to add the melting characteristics of the molding additive, such as the softening point, the melting point, the viscosity, the surface tension, the crystallization temperature and the melting rate according to the conditions. casting such as casting temperature, mold size, steel quality and casting speed. such melting characteristics are governed by the chemical composition of the additive and the materials described above must be mixed so that each of the materials can assume a composition

predetermined chemical

  According to the present invention, the chemical composition of the additive (or powder) to be used in the continuous casting of the steel is as follows: CaO = 20 to 45% by weight, SiO 2 = 25 to 50% by weight, weight ratio CaO / SiO 2 = 0.7 to 1.5, Al 2 O 3 = 0 to 10% by weight, Fe 2 O 3 = 0.1 to 2.0% by weight, MgO = 0 to 10% by weight, Na 2 O + K 2 O + Li 2 O = at 25% by weight, F = 2 to 15% by weight, B203 = 0 to 10% by weight, MnO = 0 to 5% by weight, BaO = 0 to 15% by weight, and

C = 0.5 to 10% by weight.

  The additive having the above chemical composition according to the present invention is obtained by mixing at least 50% by weight of a synthetic calcium silicate base material, 2 to 30% by weight of SiO 2 material, 3 to 30% by weight of SiO 2 material. % by weight of flux material and

  0.5 to 8% by weight of carbonaceous material.

  It is not desirable that the amount of the added synthetic calcium silicate be less than 50% by weight, since the homogeneous distribution and stability of the components which are characteristic of the

  synthetic calcium silicate are decreased.

  The SiO2-containing material is used to adjust the bulk density and the CaO / SiO 2 ratio of the powder. It is not desirable that the amount of SiO 2 containing material be less than 2% by weight since the bulk density can not be sufficiently lowered, even with the use of a light SiO 2 containing material such as perlite and in addition, the thermal insulation properties would deteriorate. Also, it is undesirable that the amount of SiO2-containing material added exceeds 30% by weight because the apparent density becomes too low and the

  amount of powder dust production increases.

  To adjust the merge characteristics, it is necessary to add the flow material to a

  amount not less than 3% by weight.

  However, if an excessive amount of flux material is added, its composition may change due to evaporation during melting and, in addition, the immersion nozzle used to pour a molten steel into a mold may be severely damaged. Therefore, it is preferable that the maximum amount of

  added flux of 30% by weight.

  The carbonaceous material is added to adjust the melting rate of the powder. If the amount of addition is less than 0.5% by weight, no substantial effect can be obtained. On the other hand, it is undesirable for the amount of addition to exceed 8% by weight because the melting rate decreases to an excessive degree. The additive according to the present invention is obtained by preparing each of the materials according to the chemical composition described above and mixing the materials prepared in a V-type mixer or a kneading mixer. In addition, granules having a columnar shape can be obtained by mixing the material mixture with water and granulating with an extrusion granulator. It is also possible to obtain an additive whose grains have a spherical shape, by converting the mixture of materials into a slurry form and carrying out spray drying.

EXAMPLE

  Preparation of a silicate-based material

synthetic calcium.

  A mixture of materials having the composition shown in Table 1 which follows was melted by heating at a temperature of 1650 to 1700 C in an electric furnace,

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  and the resulting melted product was cooled with water and ground. The granules were dried at 190 ° C. and finally milled to less than 100 mesh by a ball mill, thereby preparing base materials composed of synthetic calcium silicate 1, 2 and 3.

TABLE 1

  synthetic calcium silicate (% by weight)

1 2 3

  Limestone 50.5 52.0 56.5 Calcin 7.5 6.0 6.5

Chamotte 10,5 5,0 - -

  Quartztite 16.0 18.0 17.0 Dolomite 7.5 9.5 14.0 Fluorine 6.5 8.0 4.0 Calcined sodium ash 1.5 1.5 2.0 coke powder 3.0 3 3.0 (external percentate) Table 2 shows the compositions of synthetic calcium silicate obtained 1, 2 or 3. Table 2 further shows compositions of yellow phosphorous slag, synthetic slag and portland cement used to prepare some samples

of comparison powder.

TABLE 2

  SiO2 Al 2 O 3 Fe 2 O 3 Cao Mg Na 2 F weight ratio of CaO / SiO 2 synthetic calcium silicate 1 38.7 5.9 0.3 48.7 2.2 1.5 3.5 1.26 synthetic calcium silicate 2 36.0 2.7 0.3 52.2 3.4 1.7 4.8 1.45 synthetic calcium silicate 3 31.1 1.4 0.2 56.6 4.5 2.5 2.6 1.82 Portland cement 22.0 6.7 4.1 64.2 1.4 2.92 Yellow phosphorous slag 44.0 3.0 0.2 48.5 0.3 0.4 2.5 1.10 Synthetic slag 43 3.0 3.0 48.5 0.8 1.0 2.6 1.11 The base materials composed of synthetic calcium silicate 1, 2 and 3 were mixed with the mixtures of the materials which had the composition shown in Table 3 which follows by a V-type mixer to thereby prepare samples of molding additives of the present invention and samples of additives of

comparison molding.

  The results, when applied for actual casting processes as well as the compositions and typical characteristics of the respective samples

  in powder are shown in Table 3.

TABLE 3

  Samplers according to Samples of this invention comparison

1 2 3 4 5 6 7 8

  Synthetic calcium silicate 1 Synthetic calcium silicate 57 58 Thetic 2 Synthetic calcium silicate

Slag phos

  phorous 70 yellow i Synthetic slag 80): Portland cement 44

Content matter

  SiO2 SiO2 25 17 13 5 25 39 Fluorine 8 Flux material 14 13 13 10 10 13 16 18 D Material: carbonaceous 4 4 4 1 7 4 4 4

  _ 02 S39.2 33.5 31.9 36.3 39.7 38.3 34.8 31.0

  AI203 3.4 2.7 2.7 5.6 4.9 6.1 2.4 2.1

  o -_ Fe203 0.2 0.2 0.2 2.0 0.2 0.2 o CaO 31.0 37.9 39.8 40.9 30.5 30.1 38, 8 39.1

L ___23__1 3__1 9__0 __0 6_

  o MgO 2,3 3.1 3.2 1.9 2.0 1.1 0.6 0.6)> (cont'd) Na20 12.5 11.9 11.5 8.9 9.6 12.6 12.0 11.7

F 7.7 7.6 7.6 7.5 7.3 5.6 7.6 7.6 7.8

  free carbon 3.8 3.8 3.8 0.94 6.6 3.8 3.8 3.8 Weight ratio of CaO / SiO2 0.79 1.13 1.25 1.13 0.77 0.79 1,1 1,2

c Amolar point

  oe lissement (C) 1060 1110 1135 1130 1070 1030 111 114i Viscosity (poisons 3,6 2,4 1,2 2,9 4,0 3,5 2,3 1,1

1300 C)

  Bulk density 0.75 0.78 0.77 0.79 0.73 0.72 0.97 1.0 Property insulation good good good good good good mau-mau thermal vai-vai is

Default of Rarely Frequency

  m observed surface area m o observed Stability of lubricant oxooxo cation Quality stability oooooox Note) in Table 3, material containing SiO2: total percentages by weight of pearlite powder and glass powder, flux material: total weight percentage of ash calcined sodium and sodium fluoride, softening point: measured by a Seger cone method and at a temperature increase of 5 C / min, viscosity: measured by a platinum ball elevation method, apparent density: 1 liter container, naturally loading, property of thermal insulation: by the observation of the condition in a mold, surface defect: slag stain, pinhole, longitudinal crack, etc ..., lubrication stability: results of mold copper temperature measurements, and quality stability: frequency of occurrence of fractures

and surface defects.

  The synthetic calcium silicate material used for the mold additive for continuous casting of the present powder of the invention possesses the merits of both portland cement and yellow phosphorous slags. Accordingly, the mold additive of the present invention which contains at least% by weight of synthetic calcium silicate has the following characteristics: 1) As the light material containing SiO 2 is added, low bulk density and good thermal insulation properties, 2) if the amount of SiO 2 material to be added is adjusted, it is possible to manufacture various powders whose compositions are between a low ratio of CaO / SiO 2 and a high ratio of CaO / SiO 2, 3 ) as the Fe 2 O 3 content is low, a stable lubrication is obtained, 4) as it contains neither CaO nor 3CaO.SiO 2,

  granulation with water can be adopted.

  5) All components can be distributed

homogeneously.

Claims (5)

  A molding additive suitable for use in a mold for the continuous casting of steel, characterized in that it comprises, as a base material, at least 50% by weight of synthetic calcium silicate which contains CaO and SiO 2 in a total amount not less than 70% by weight, A1203 in an amount not exceeding 8% by weight, Fe203 in an amount not exceeding 1% by weight and F in an amount of 1 to 10% by weight; weight and whose CaO / SiO2 ratio is not less than 1.20.   2. Additive according to claim 1, characterized in that the CaO / SiO2 ratio of the silicate of   synthetic calcium is between 1.2 and 2.3.   3. Additive according to claim 1, characterized in that the CaO / SiO2 ratio of the silicate of   synthetic calcium is 1.2 to 1.9.   4. Additive according to claim 1, characterized in that the CaO / SiO2 ratio of the synthetic calcium silicate is 1.2 to 1.9 and said synthetic calcium silicate contains Al 203 in an amount not exceeding 5% by weight and Fe2O3 in an amount not not exceeding 0.3% by weight.   5. Process for the continuous casting of steel, characterized in that a molding additive is used   according to any one of the preceding claims.