Nozzle for continuous casting of steel

Abstract: The invention is related to a continuous casting nozzle for casting of aluminum killed steel without clogging of the bore of the nozzle. The surface layer of the bore of the continuous casting nozzle contacting with the molten steel is composed of a refractory comprising graphite from 10 to 35 wt%, a neutral or basic aggregate from 30 to 50 wt% like Al 2 O 3 and pottery stone as the rest part of the above mentioned refractory.

Description: FIELD OF THE INVENTION The present invention relates to a continuous casting nozzle for permitting effective prevention of narrowing or clogging of the nozzle bore through which molten steel passes from tundish to mold in performing continuous casting of the molten steel containing aluminum such as aluminum-killed steel. THE RELATED ART A continuous casting nozzle for casting molten steel is used for the following purposes. A continuous casting nozzle, which is a means feeding molten steel from a tundish to a mold, is used for such purpose of preventing the molten steel from being oxidized by contacting with the open air and from splashing when the molten steel is poured from a tundish to a mold, and rectifying the flow of the molten steel poured for preventing non-metallic inclusion and slag present near or on the mold surface from being entrapped in the cast steel strand. Material of a conventional continuous casting nozzle for casting molten steel comprises such material as graphite, alumina, silica, silicon carbide and recently zirconia. However, there are following problems in the case of casting aluminum-killed steel and the like. As for the aluminum-killed steel and the like, aluminum, which is added as a de-oxidizer to molten steel, reacts with oxygen existing in the molten steel to produce non-metallic inclusion such as alpha-alumina. Therefore, in casting the aluminum-killed steel and the like, the non-metallic inclusion such as alpha-alumina adheres and accumulates onto the surface of the bore of the continuous casting nozzle, so that the bore is narrowed or clogged up in the worst case, which makes stable casting to be difficult. Furthermore, the non-metallic inclusion such as alpha-alumina, adhered or accumulated onto the surface of the bore, peels off or falls down, and is entrapped in the cast steel strand, thus degrading the quality of the cast steel strand. For the purpose of preventing the above-mentioned narrowing or clogging of the bore caused by the non-metallic inclusion such as alpha-alumina, there is a commonly used method for preventing the non-metallic inclusion such as alpha-alumina existing in the molten steel from adhering or accumulating on the surface of the bore of the nozzle by ejecting inert gas from the inner surface of the nozzle bore toward the molten steel flowing through the bore (for example, Japanese Patent Publication No. Hei 6-59533/1994). However, there are problems as described below for the above-mentioned method wherein inert gas is ejected from the inner surface of the nozzle. A large amount of the ejected inert gas causes entrapment of bubbles produced by the inert gas into the cast steel strand, resulting in defects based on pinholes. On the other hand, a small amount of the ejected inert gas causes adhesion and accumulation of the non-metallic inclusion such as alpha-alumina onto the surface of the bore of the nozzle, thus causing narrowing or clogging, in the worst case, of the bore. Additionally, it is constructionally difficult to uniformly eject the inert gas from the inner surface of the nozzle bore toward the molten steel flowing through the bore. And in the case that the casting is performed in a long period of time, a stable control of the amount of ejected inert gas becomes gradually more difficult as the composition and the structure of the material consisting of the continuous casting nozzle degrades. And moreover, it becomes difficult to eject inert gas uniformly from the inner surface to the nozzle bore. As a result, the non-metallic inclusion such as alpha-alumina adhere and accumulate onto the surface of the bore of the nozzle so that the bore is narrowed or clogged up in the end. It is thought that the clogging of the nozzle by the non-metallic inclusion, specially by the alumina inclusion, is caused as described below. (1) Alumina inclusion is produced from aluminum in the steel by secondary oxidation, such as oxidation by entrapped air passing through a refractory junction and refractory structure. (2) In addition silica contained in conventional alumina-graphite material of nozzle reacts with the graphite to produce silicon-monooxide(SiO), which oxidizes aluminum in steel to produce alumina. (3) Alumina inclusion is produced by diffusion and cohesion of the alumina particles produced in the above process. (4) Graphite on the surface of the nozzle bore vanishes and the surface of the bore becomes rough and thus the alumina inclusion is apt to accumulate on the rough surface of the bore. On the other hand, as a counterplan in view of nozzle material, a alumina-graphite nozzle containing a non-oxide raw material (SiC, Si 3 N 4 , BN, ZrB 2 , SIALON etc.) that has low reactivity with aluminum oxide, or a nozzle consisting of the non-oxide material itself is proposed (for example, Japanese Patent Publication No. Sho 61-38152/1986). However, this counterplan is not practical in the case of the alumina-graphite nozzle because the adhesion preventing effect is not recognized and further corrosion resistance is decreased unless much of the non-oxide material is added. Also, the nozzle consists of only the non-oxide material is not suitable for practical use in view of material cost and manufacturing cost, although a substantial effect is expected. A nozzle consisting of graphite-oxide raw material containing CaO is proposed. A low-melting-point material is produced by a reaction of CaO in the oxide raw material containing CaO (CaO (CaO ZrO 2 , CaO SiO 2 ,2CaO SiO 2 etc.) with Al 2 O 3 , which is easily separated from the steel (for example, Japanese Patent Laid-Open Publication No. Sho 62-56101/1987). However, the reactivity of CaO with Al 2 O 3 is apt to be influenced by a temperature condition of the molten steel in casting, and there is a case that amount of CaO is not sufficiently secured for satisfying spalling resistance and corrosion resistance when plenty of Al 2 O 3 inclusion is contained in the steel. A graphite-containing casting nozzle as disclosed in JP-A-56014061 comprises alumina: 30 to 80 wt %, graphite: 3 to 55 wt %, fused silica: 0 to 50 wt %, at least one compound of feldspar or pottery stone containing at least 2,5 wt % alkaline: 1.5 to 30, ferro-silicon: 0.5 to 20 wt % and an organic binder. The casting nozzle is preferably formed by mixing of said compounds, drying, formed by a rubber press and sintered. The nozzle contains preferably at least one compound of aluminum titanate, cordierite, boron carbide, boric oxide and boric acid. However the above nozzle is intended to enhace the resistance to oxidation and to lower a heat transfer to prevent metal freezing in the bore. And it can not prevent the nozzle clogging due to accumulation of oxide inclusion as it can not prevent air penetration because of a high porosity. Therefore an object of the present invention is to provide a new nozzle provided with a layer having a least permeability to prevent air penetration through wall of the nozzle bore. SUMMARY OF THE INVENTION The object of the present invention is to provide a continuous casting nozzle having following features. The invented nozzle forms a glass layer at the surface of the bore of the nozzle when the nozzle is used, thereby preventing air from being entrapped through refractory structure, smoothing the bore surface of the nozzle and preventing the accumulation of alumina. Also, the object of the present invention is to provide a continuous casting nozzle which prevents erosion by products having a low-melting produced by a reaction between the aggregate in a refractory and alumina in the steel, and to provide the nozzle which is not influenced by a temperature of the molten steel in casting, and which is able to prevent the bore from narrowing or clogging economically, comparatively easy and stable. Thus the invented nozzle eliminates the gas injection into the nozzle bore. In the first embodiment of the present invention, the surface layer of the bore of a continuous casting nozzle contacting with molten steel is formed of a refractory comprising graphite from 10 to 35 wt%, a neutral or a basic aggregate from 30 to 50 wt% and pottery stone containing the sericite (K 2 O·3Al 2 O 3 ·6SiO 2 ·2H 2 O) as the main component making up the balance up to 100% the above mentioned refractory. In another embodiment of the present invention, the surface layer of the bore of a continuous casting nozzle contacting with molten steel is formed of a refractory comprising graphite from 10 to 35 wt%, a neutral or a basic aggregate from 30 to 50 wt% and pottery stone containing the sericite (K 2 O·3Al 2 O 3 ·6SiO 2 ·2H 2 O) making up the balance up to 100 % of the above mentioned refractory, the said refractory being added binder, kneaded, formed, and sintered in the anti-oxidizing atmosphere. As for a neutral or a basic aggregate, one or more than one component from the group of Al 2 O 3 , ZrO 2 , or MgO can be selected. It is preferable that the pottery stone containing the sericite as the main component is calcinated at a temperature equal to or more than 800°C so as to vanish crystal water and contain alkaline component from 1 to 5 wt%. As for the pottery stone having above mentioned component, it is preferable that a mixing weight ratio of pottery stone with an average grain diameter equal to or less than 250µm is equal to or less than 60% relative to the whole of the pottery stone content. And as for the binder a thermosetting resin, for example, phenol resin is preferably selected. With respect to forming process of the refractory to the nozzle, CIP (Cold isostatic pressing) should be preferably selected. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a longitudinal cross section of a nozzle according to the present invention provided with the surface layer of the bore of the nozzle composed of the invented refractory. FIG. 2 shows a longitudinal cross section of a nozzle according to the present invention provided with the surface layer of the bore of the nozzle and the lower part (a part immersed in the molten steel) of the nozzle, both of them composed of the invented refractory. EMBODIMENTS OF THE INVENTION A major characteristic of a continuous casting nozzle of the present invention is that the main component of a refractory of the surface layer of the bore of the nozzle is pottery stone. During usage, when silica in the above mentioned refractory coexisting with graphite or carbon , the following reactions are usually caused. SiO 2 (S) + C(S) = SiO(g) + CO(g) 3SiO(g) + 2Al = Al 2 O 3 (S) + 3Si 3CO(g) + 2Al = Al 2 O 3 (S) + 3C As shown in the above reactions, decomposition of the silica produces SiO(g) and CO(g), which react with aluminum in the steel to form Al 2 O 3 and it becomes the source of oxygen to the steel. However, as for the pottery stone, the pottery stone particles do not decompose even if it is coexisting with graphite, namely SiO 2 in sericite (K 2 O·3Al 2 O 3 ·6SiO 2 ·2H 2 O), which is the main mineral of the pottery stone, is stable. This fact is found from an experiment which verified that the particles do not decay and bubbles are not produced, which is confirmed by means of a microscope observation, in a briquette consisting of the pottery stone, resin powders and carbon powders which underwent a heat-treatment at a temperature of 1500°C for 24 hours with burying it in a coke breeze. The half-melting temperature of the pottery stone is about 1400°C, so that it melts at the bore surface contacting with the molten steel to form a glass coat for smoothing the structure of the surface of the bore and for preventing air from being entrapped through a refractory structure. This is found from the fact that the permeability is decreased such that the permeability after performing heat-treatment at a temperature of 1500°C for 1 hours is as small as about 9.5x10 -5 darcy, in contrast the permeability after performing heat-treatment at a temperature of 1000°C for 1 hours is about 9.5x10 -4 darcy. As for kinds of pottery stone, it is possible to use the following kinds of pottery stone, that is sericite, kaolin, feldspar and pyrophyllite. The sericite with refractoriness from SK20 to SK27 (SK(Seger cone) is a Japanese Standard for refractoriness ) is suitable, considering formation of a glass layer and erosion resistance against the molten steel, as the surface of the bore contacting with the molten steel is half-molten in use. Although the mixing amount of the pottery stone is making up the balance up to 100% of the mixing amount of other components, a mixing weight ratio of the pottery stone is equal to or more than 30 wt% in order to actively form the glass coat on the surface of the bore in use as continuous casting nozzle, preferably. Also, it is preferably that the mixing weight ratio of the pottery stone is equal to or less than 60 wt% because degree of softening deformation is large in a range of over 60 wt%. Therefore, the most preferable mixing weight ratio of the pottery stone is from 30 wt% to 60 wt%. In this case the aggregate of pottery stone particles does not decompose even coexisting with graphite. The reason for preferably using the pottery stone calcinated at a temperature equal to or more than 800°C to vanish crystal water is that the crystal water is released from the pottery stone at a temperature in a range of from 500 to 800°C in calcination and the refractory cracks is caused to occur by virtue of an unusually large coefficient of thermal expansion in this range. The alkaline component of the pottery stone from 1 to 5 wt% is preferable to control the melting point of pottery stone adequdately It is preferable that a mixing weight ratio of pottery stone with an average grain diameter equal to or less than 250µm is equal to or less than 60% relative to the whole of the pottery stone content because, in the range of over 60%, structural defects such as lamination are apt to be produced in molding, and softening deformation of pottery stone particles is apt to happen when used in a continuous casting nozzle. As for the a neutral or a basic aggregate to be mixed, one or more than one component from the group of Al 2 O 3, ZrO 2, or MgO can be selected, which enhance the corrosion resistance of the nozzle. With regard to graphite, to prevent the softening deformation and to maintain heat-impact resistance of the pottery stone, preferably, a mixing weight ratio of the graphite is equal to or more than 10 wt%. Also, it is preferably that the mixing weight ratio of the graphite is equal to or less than 35 wt% from the view point of manufacturing of the nozzle. If the volume ratio of the graphite relative to the pottery stone is too large, structural defects such as lamination are apt to be produced in the range of over 35 wt%. And considering thermal conductivity and oxidation resistance, natural graphite is suitable as the graphite to be mixed. And the most preferable process of the mixed material to nozzle shape is CIP(cold isostatic pressing) to produce the nozzle having a high heat resistance. As for the binder for forming the nozzle body a thermosetting resin, for example phenol resin or epoxy resin, is preferably used and the mixing ratio is preferably 5 to 15 wt% of the mixed material. Sintering of the formed body is preferably performed in the nonoxidizing atmosphere to minimize the burning loss of the graphite mixed in the material. The graphite is mixed to enhance the erosion resistance and oxidation resistance and the sintering temperature is preferably 1000 to 1200 °C to obtain a sufficient strength of the nozzle. The continuous casting nozzle for steel according to the present invention will be described in detail with reference to the accompanying drawings of nozzle for continuous casting. As shown in FIG. 1, a surface layer 2 of the bore 1, through which the molten steel flows, of the immersion nozzle 10 consists of a refractory having the chemical composition as described above. The rest part of the nozzle 3 is composed of regular refractory, for example, of alumina-graphite which is already known in public. The dimensions of the nozzle are about 1,000mm in total length, about 60mm in diameter of the bore, 160mm in outer diameter, and about 50mm in thickness. FIG. 2 shows another embodiment of the invention, a nozzle comprising a refractory according to the present invention at the surface layer of the bore of the nozzle and the lower part (a part immersed in the molten steel) of the nozzle. In the bore 1 of the nozzle for continuous casting, the adherence and accumulation of non-metallic inclusion such as the alpha-alumina are depressed. EXAMPLES The present invention is explained with examples as described below. The samples Nos. 1 to 5 (hereinafter referred to as the "sample of the present invention") having the chemical composition within the scope of the present invention, and the samples Nos. 6 to 8 (hereinafter referred to as "sample for comparison") having chemical composition out of the scope of the present invention were prepared as shown in Table 1, and phenol resin in the state of powder and liquid were added in an amount within a range of from 5 to 10 wt% to each of the mixed materials. From the mixed materials above, the following formed bodies were prepared. A first formed body (hereinafter referred to as the "formed body 1") with dimensions of 30mm by 30mm by 230mm for examining an amount of adhesion of non-metallic inclusion such as alumina and corrosion resistance against the molten steel, a second formed body (hereinafter referred to as the "formed body 2") with dimensions of 50mm in diameter by 20mm for examining permeability, and a third formed body (hereinafter referred to as the "formed body 3") with dimensions of 100mm in outer diameter, 60mm in inner diameter and 250mm in length for examining spalling resistance, were respectively prepared, and then the bodies were sintered in reducing atmosphere at a temperature in a range from 1000 to 1200°C and the samples 1 to 8 were prepared. Physical properties (porosity and bulk density) for each of the above-mentioned samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 8 are shown in Table 1. The spalling resistance of each of the sintered formed bodies 3 of the samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 8 were examined after heating at a temperature of 1500°C for 80 minutes in an electric furnace and then rapidly cooling in water. The results are shown in Table 1. An erosion ratio (%) and an amount of adhesion of non-metallic inclusion such as alumina onto the sintered formed bodies 1 of the samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 8 were examined after immersing in molten steel, which contains aluminum in a range from 0.02 to 0.05 wt% at a temperature of 1550°C for 180 minutes. The results are shown in Table 1. The permeability for each of the sintered formed bodies 2 of the samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 8 were examined after heating at a temperature of 1500°C for 60 minutes in an electric furnace and then cooling. The results are shown in Table 1. It is easily understood from Table 1 that the samples of the present invention are superior in the spalling resistance. Also, the non-metallic inclusion such as alumina does not adhere in spite of the low erosion ration, thereby effectively preventing narrowing or clogging of the continuous casting nozzle of the molten steel. And also, the samples of the present invention prevented air from being entrapped through the refractory in practical use because of small permeability. On the other hand, it is obvious that the sample for comparison No. 6 is remarkably inferior in the spalling resistance and the corrosion resistance against the molten steel, although a small amount of alumina adheres due to much pottery stone content. As for the sample for comparison No. 7, the amount of adhesion of alumina is remarkably large, because it contains Al 2 O 3 and SiO 2 , which decomposes to supply oxygen in the steel, instead of the pottery stone. As for the sample for comparison No. 8, it does not contain SiO 2 and contains only Al 2 O 3 instead of pottery stone. It has a high permeability and the amount of adhesion of alumina is remarkably large although it contains no mineral source of oxygen to the steel. Therefore, with the use of the continuous casting nozzle for casting steel according to the present invention, it is possible to perform stable casting with preventing narrowing or clogging of the bore caused by the non-metallic inclusion such as alumina without deterioration of the refractory structure. According to the present invention, 5 to 7 charges of approximately 300 ton of low carbon aluminum killed steel is continuously cast with one nozzle without clogging by 2 strand slab caster in real operation, though, with conventional nozzle, clogging up of one nozzle occurred within 2 to 4 charges under same condition.

Claims: A continuous casting nozzle for casting molten steel from tundish to mould, wherein the surface layer of the bore of said nozzle contacting with the molten steel is formed of a refractory comprising graphite from 10 to 35 wt %, a neutral or a basic aggregate from 30 to 50 wt % and the balance up to 100 % being pottery stone containing sericite (K 2 O • 3Al 2 O 3 • 6SiO 2 • 2H 2 O) and said refractory is formed to said nozzle by being added a thermosetting resin and being sintered in an anti-oxidizing atmosphere. A continuous casting nozzle for casting molten steel from tundish to mould according to claim 1, wherein said aggregate is selected from one or more than one component from the group of Al 2 O 3, ZrO 2 and MgO. A continuous casting nozzle for casting molten steel from tundish to mould according to claim 1 or 2, wherein said pottery stone containing sericite is calcinated at a temperature equal to or more than 800 degree C so as to vanish crystal water and contains alkaline component from 1 to 5 wt %. A continuous casting nozzle for molten steel from tundish to mould according to claim 1, 2 or 3, wherein a mixing weight ratio of said pottery stone, whose average grain diameter equal or less than 250 µm, is equal to or less than 60% relative to the whole of said pottery stone content.

Inventors: MUROI TOSHIYUKI, TAKASU TOSIKAZU, MUROI, TOSHIYUKI, TAKASU, TOSIKAZU
Applicants: AKECHI CERAMICS KK, TOKYO YOGYO KK, AKECHI CERAMICS KABUSHIKI KAISHA, TOKYO YOGYO KABUSHIKI KAISHA
Application year: 1997
Main classification: B22D41
IPC codes: B22D11, B22D41, C04B35