A continuous casting nozzle for casting molten 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 formed of a refractory comprising graphite from 10 to 35 wt% and roseki as the rest part of the graphite, and the main component of the roseki is pyrophyllite (Al 2 O 3 ·4SiO 2 ·H 2 O) as mineral component. Further, silicon carbide from 1 to 10 wt% is contained in the 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 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 purpose as following. As for continuous casting molten steel, a continuous casting nozzle is used for such the 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 of molten steel comprises such material as graphite, alumina, silica, and silicon carbide. 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, reacts with oxygen existing in the molten steel to produce non-metallic inclusion such as α-alumina. Therefore, in casting the aluminum-killed steel and the like, the non-metallic inclusion such as the α-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 the α-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 reduction or clogging of the bore caused by the non-metallic inclusion such as α-alumina, there is a commonly used method for preventing the non-metallic inclusion such as α-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 the inert gas is ejected from the inner surface of the nozzle forming the bore. 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 the α-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, according as the structure and the structure of the material consisting of the continuous casting nozzle degrades. As a result, the non-metallic inclusion such as the α-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 existing in the steel by secondary oxidation, such as oxidation by entrapped air passing through a refractory junction and refractory structure or oxidation by supplying oxygen obtained from reduction of silica in a carbon-containing refractory. (2) Alumina inclusion is produced by diffusion and cohesion of the alumina produced in the above process. (3) Carbon 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 nozzle in which a non-oxide raw material (SiC, Si 3 N 4 , BN, ZrB 2 , SIALON, etc.) that has low reactivity with aluminum oxide is added to alumina-graphite nozzle consisting of the non-oxide material itself is proposed (for example, Japanese Patent Publication No. Sho 61-38158/1986). 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 for producing low-melting-point material by a reaction of CaO in an oxide raw material containing CaO (CaO·ZrO 2 , CaO·SiO 2 , 2CaO·SiO 2 , etc.) with Al 2 O 3 and forming the low-melting-point material in the steel (for example, Japanese Patent Laid-Open Publication No. Sho 62-56101). However, 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. OBJECT OF THE INVENTION The object of the present invention is to provide a continuous casting nozzle having following features. (1) A glass layer is formed at the surface of the bore of the nozzle when the nozzle is used, thereby preventing air from being entrapped through refractory structure, which prevents alumina from being produced. (2) To prevent erosion by products having a low-melting-point on account of a reaction between an aggregate in a refractory and alumina in the steel, by smoothing the bore surface of the nozzle without the use of a mechanical means such as the ejecting of an inert gas. (3) To provide a continuous casting nozzle which is able to prevent the bore from narrowing or clogging economically, comparatively easy and stably. SUMMARY OF THE INVENTION In 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% and roseki as the rest part of the graphite, and the main component of the roseki is pyrophyllite (Al 2 O 3 •4SiO 2 •H 2 O) as mineral composition Concretely, 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% and roseki as the rest part of the graphite, and the main component of the roseki is pyrophyllite (Al 2 O 3 •4SiO 2 •H 2 O) as mineral composition, said refractory being added binder, kneaded, formed, and baked in the anti-oxidizing atmosphere. It is preferable that the roseki containing the pyrophyllite 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%. It is preferable that, in the roseki containing the pyrophyllite as the main component, a mixing weight ratio of roseki of an average grain diameter equal to or less than 250µm is equal to or less than 60% relative to the whole of the roseki content so as to form a glass layer at the surface contacting with the molten steel. In the present invention, it is also a preferable embodiment that the surface layer of the bore of a continuous casting nozzle contacting with the molten steel is formed of a material comprising graphite from 10 to 35 wt%, silicon carbide from 1 to 10 wt% and roseki as the rest part of the graphite and the silicon carbide, and the main component of the roseki is pyrophyllite, said refractory being added binder, kneaded, formed, and baked in the anti-oxidizing atmosphere. It is preferable that the roseki containing the pyrophyllite 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%. It is preferable that, in the roseki containing the pyrophyllite as the main component, a mixing weight ratio of roseki with an average grain diameter equal to or less than 250µm is equal to or less than 60% relative to the whole of the roseki content. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross section of a nozzle according to the present invention comprising a refractory at the surface layer of the bore of the nozzle contacting with molten steel. FIG. 2 shows cross section of a nozzle according to the present invention comprising a refractory at the surface layer of the bore of the nozzle and the lower part (a part immersed in the molten steel) of the nozzle. EMBODIMENTS OF THE INVENTION A major characteristic of a continuous casting nozzle of the present invention is that the main component of the surface layer of the bore of a refractory is roseki. When silica coexisting with graphite contacts with molten steel containing aluminum, 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 , thereby providing an oxygen supply source for the steel. However, as for the roseki, the roseki particles do not decompose even if it is coexisting with graphite, namely SiO 2 in pyrophyllite (Al 2 O 3 •4SiO 2 •H 2 O) which is the main mineral of the roseki is stable. This fact is found, from the facts that the particles do not decay and bubbles are not produced, by means of a microscope observation after forming a briquette consisting of the roseki, resin powders and carbon powders and performing heat-treatment at a temperature of 1500°C for 24 hours with burying it in a coke breeze. The half-melting temperature of the roseki is about 1500°C, so that it melts at the working surface contacting with the molten steel to form a glass coat for smoothing the structure of the surface of the bore and 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 95x10 -5 darcy, in contrast the permeability after performing heat-treatment at a temperature of 1000°C for 1 hours is 95x10 -4 darcy. To actively form the glass coat on the surface of the bore in use as continuous casting nozzle, preferably, a mixing weight ratio of the roseki is equal to or more than 65 wt%. Also, it is preferably that the mixing weight ratio of the roseki is equal to or less than 90 wt%, because degree of softening deformation is large within a range of over 90 wt%. The mixing amount of the roseki is the rest part of the graphite or the rest part of the mixing amount of the graphite and silicon carbide. To prevent the softening deformation and to maintain heat-impact resistance of the roseki, 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, because the volume ratio of the graphite relative to the roseki is too large so that structure defects such as lamination are apt to be produced in the range of over 35 wt%. Considering thermal conductivity and oxidation resistance, natural graphite is suitable as the graphite to be mixed. To actively cause a bloating phenomenon in sintering of the roseki, it is preferable that a mixing ratio of the silicon carbide is equal to or more than 1 wt%. And the mixing ratio of the silicon carbide should be equal to or less than 10 wt%, because erosion is too large in the range of over 10 wt%. The reason for using the roseki calcinated at a temperature equal to or more than 800°C to vanish crystal water is that the crystal water is released from the roseki at a temperature in a range of from 500 to 800°C in sintering and the refractory cracks by virtue of an unusually large coefficient of thermal expansion in this range. It is preferable that a mixing weight ratio of roseki with an average grain diameter equal to or less than 250µm is equal to or less than 60% relative to the whole of the roseki content, because, in the range of over 60%, structure defects such as lamination are apt to be produced in molding and softening deformation of roseki particles is apt to happen when used in a continuous casting nozzle. As for kinds of roseki, it is possible to use three kinds of roseki, that is pyrophyllite matter roseki, kaolin matter roseki, and sericite matter roseki. The pyrophyllite matter roseki with refractoriness from SK29 to SK32 (SK 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. Both of the kaolin matter roseki and the sericite matter roseki is not preferable, because the kaolin matter roseki has a greater refractoriness from SK33 to SK36, and the sericite matter roseki has a smaller refractoriness from SK26 to SK29. With regard to a refractory structure comprising graphite from 10 to 35 wt% and roseki from 65 to 90 wt% as the rest part of the graphite, the main component of which being pyrophyllite (Al 2 O 3 •4SiO 2 •H 2 O), particles of the roseki are not decomposed and it does not becomes an oxygen supplying source into the steel not as same as SiO 2 even if it coexists with the graphite. Also, it has an effect that adhesion of Al 2 O 3 and metal is prevented, because a half-melting temperature of the roseki is about 1500°C near a casting temperature of the molten steel, allowing a glass coat layer to form at a working surface contacting with the molten steel, which smoothes the working surface structure and prevents air from being entrapped and diffused through the refractory structure. The continuous casting nozzle for steel according to the present invention will be described in detail with reference to the accompanying drawings. Fig. 1 shows an embodiment of a vertical sectional view of the immersion nozzle according to the present invention. This nozzle 10 is placed between a tundish and a mold, and used as an immersed nozzle for pouring the molten steel from the tundish to the mold. As shown in Fig. 1, a surface layer 2 of the bore 1, through which the molten steel flows, of the continuous casting nozzle 10 consists of a refractory having the chemical composition as above described. The rest part of the nozzle 3 is composed of regular refractory, for example, of alumina-graphite. The dimensions of the nozzle are about 1m in total length, about 6cm in diameter of the bore, 16cm in outer diameter, and about 5cm in thickness. And, the thickness of the surface layer of the bore made of the refractory in connection with the present invention is from about 2 to about 15mm. Fig. 2 shows another embodiment of a nozzle, in which the whole part immersed in the molten steel in at the mold is formed of a refractory according to the present invention. In both of embodiments, alumina usually aggregates at the lower part of the nozzle bore and makes the stable flow of molten steel difficult. The immersed nozzle according to the present invention prevents adhesion or accumulation of non-metallic inclusion such as the alumina in the molten steel onto the surface layer 2. The present invention is now described by means of examples. EXAMPLES [EXAMPLE 1] 8 mixed materials with different composition were prepared and phenol resin in the state of powder and liquid was added in an amount within a range of from 5 to 10 wt% to each of 8 mixed materials. From 8 materials 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 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 prepared, and then the bodies were sintered in reducing atmosphere at a temperature in a range from 1000 to 1200°C. Thus, the samples Nos. 1 to 5 (hereinafter referred to as the "sample of the present invention") shown in Table 1 having the chemical compositions within the scope of the present invention and the samples Nos. 6 to 8 (hereinafter referred to as "sample for comparison") having chemical compositions out of the scope of the present invention 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 was examined after heating at a temperature of 1500°C for 30 minutes in an electric furnace and then rapidly cooling by water. The results are shown in Table 1. An erosion ratio (%) and an amount of adhesion of non-metallic inclusion such as alumina of each of 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 was 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 and the non-metallic inclusion such as alumina does not adhere in spite of the low erosion ration, thereby effectively preventing reduction or clogging of the continuous casting nozzle of the molten steel. Also, it is possible for the samples of the present invention to prevent 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 roseki 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 roseki. As for the sample for comparison No. 8, a large amount of non-metallic inclusion such as alumina adheres and the permeability is large, although a mineral for supplying oxygen into the steel is eliminated, in other words it does not contain SiO 2 instead of the roseki and only contains Al 2 O 3 . [EXAMPLE 2] This example is related to the nozzle made up of refractory including silicon carbide in the first example of the present invention. Samples were prepared in the same process as in the first example. Phenol resin in the state of powder and liquid was added in an amount within a range of from 5 to 10 wt% to each of 8 mixed materials, and then the resultant raw materials obtained by mixing and kneading the above materials were sintered . From 8 materials 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 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 prepared, and then the bodies were sintered in reducing atmosphere at a temperature in a range from 1000 to 1200°C. Thus, the samples Nos. 1 to 5 (hereinafter referred to as the "sample of the present invention") shown in Table 1 having the chemical compositions within the scope of the present invention and the samples Nos. 6 to 8 (hereinafter referred to as "sample for comparison") having chemical compositions out of the scope of the present invention 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 2. 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 was examined after heating at a temperature of 1500°C for 30 minutes in an electric furnace and then rapidly cooling by water. The results are shown in Table 2. An erosion ratio (%) and an amount of adhesion of non-metallic inclusion such as alumina of each of 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 2. The permeability for each of the formed bodies B of the samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 8 was examined after heating at a temperature of 1500°C for 60 minutes in an electric furnace and then cooling. It is easily understood from Table 2 that the samples of the present invention are superior in the spalling resistance and the non-metallic inclusion such as alumina do not adhere in spite of the low erosion ratio, thereby effectively preventing reduction or clogging of the nozzle of the molten steel. Also, it is possible for the samples of the present invention to prevent 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 roseki content. As for the sample for comparison No. 7, the amount of adhesion of alumina and the permeability is large, because silicon carbide is not added. As for the sample for comparison No. 8, it is obvious that the corrosion resistance against the molten is remarkably inferior, because a large amount of silicon carbide is added. Therefore, according to the continuous casting nozzle of molten steel of the present invention, it is possible to perform stable casting with preventing reduction 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, approximately 600 to 800 ton of a low carbon aluminum killed steel (C:0.04%, Mn:0.33%, Al:0.051%) is continuously cast with one nozzle without clogging by 2 strand slab caster. Meanwhile, 360 ∼ 480 ton of the same low carbon aluminum killed steel was continuously cast with one nozzle of conventional alumina-graphite without clogging by the same caster.

Claims: A continuous casting nozzle for casting molten steel to be placed between a tundish and a mould as an immersed nozzle, wherein the surface layer of the bore of said continuous casting nozzle contacting with the molten steel is formed of a refractory comprising graphite from 10 to 35 wt% and roseki from 65 to 90 wt%, and the main component of the roseki is pyrophyllite (Al 2 O 3 4SiO 2 H 2 O) as mineral component. A continuous casting nozzle for casting molten steel to be placed between a tundish and a mould as an immersed nozzle according to claim 1, wherein the surface layer of the bore of the continuous casting nozzle contacting with the molten steel is formed of a refractory comprising graphite from 10 to 35 wt% and roseki from 65 to 90 wt%, and the main component of the roseki is pyrophyllite (Al 2 O 3 4SiO 2 H 2 O) as mineral composition, said refractory being added binder, kneaded, formed, and sintered in the reducing atmosphere. A continuous casting nozzle steel to be placed between a tundish and a mould and to be used as an immersed nozzle according to claim 1 or 2, wherein said roseki containing the pyrophyllite 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%. A continuous casting nozzle to be placed between a tundish and a mould as an immersed nozzle according to claim 2 or 3, wherein a mixing weight ratio of roseki of an average grain diameter equal to or less than 250 µm in the roseki containing the pyrophyllite as the main component is equal to or less than 60 % relative to the whole of the roseki content. A continuous casting nozzle of molten steel to be placed between a tundish and a mould as an immersed nozzle according to claim 2, wherein, in addition to the graphite and the roseki, silicon carbide from 1 to 10 wt% is contained in the refractory, said refractory being added binder, kneaded, formed, and sintered in the reducing atmosphere. A continuous casting nozzle to be placed between a tundish and a mould as an immersed nozzle according to claim 5, wherein said roseki containing the pyrophyllite 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%. A continuous casting nozzle to be placed between a tundish and a mould as an immersed nozzle according to claim 5 or 6, wherein a mixing weight ratio of roseki of an average grain diameter equal to or less than 250 µm in the roseki containing the pyrophyllite as the main component is equal to or less than 60 % relative to the whole content of the roseki.

Inventors: MUROI TOSHIYUKI, OGURI KAZUMI, MUROI, TOSHIYUKI, OGURI, KAZUMI
Applicants: AKECHI CERAMICS KK, AKECHI CERAMICS KABUSHIKI KAISHA
Application year: 1997
Main classification: B22D41
IPC codes: B22D41