A submerged entry nozzle and a method of producing a submerged entry nozzle

Abstract

This invention relates to a nozzle for guiding molten metal, for example molten steel. More particularly, the invention relates to a so-called submerged entry nozzle (also called SEN or casting nozzle) used in a continuous casting process for producing steel. The invention relates also to a method of manufacturing a submerged entry nozzle.

Description

[0001] A submerged entry nozzle and a method of producing a submerged entry nozzle

[0002] D e s c r i p t i o n

[0003] This invention relates to a nozzle for guiding molten metal, for example molten steel. More particularly, the invention relates to a so-called submerged entry nozzle (also called SEN or casting nozzle) used in a continuous casting process for producing steel. The invention relates also to a method of manufacturing a submerged entry nozzle.

[0004] In a continuous casting steel-making process, molten steel is poured from a ladle into a large vessel known as a tundish. The tundish has one or more outlets through which the molten steel flows into one or more respective moulds. The molten steel cools and solidifies in the moulds to form continuously cast solid lengths of metal. A submerged entry nozzle (also called SEN or casting nozzle) is located between the tundish and each mould, and guides molten steel flowing through it from the tundish to the mould. The melt transfer from the tundish into each mould is achieved by a submerged entry nozzle which is arranged in a vertical use position and which typically provides the following features: a generally rigid tube-like or pipe-like shape, defining a central longitudinal nozzle axis, and comprising an inner nozzle wall, surrounding a flow-through channel, which extends along an axial length between an inlet opening at a first nozzle end, being an upper end in a use position of the nozzle, and at least one outlet opening at a second nozzle end, being a lower end in the use position, to allow a continuous flow stream of a molten metal from its inlet opening along said flow-through channel via said outlet opening(s) into an associated molten metal bath in said mould by force of gravity.

[0005] An ideal submerged entry nozzle has the following main functions. Firstly, the nozzle serves to prevent the molten steel flowing from the tundish into the mould from coming into contact with air since exposure to air would cause oxidation of the steel, which adversely affects its quality. Secondly, it is highly desirable for the nozzle to introduce the molten steel into the mould in a as smooth and non-turbulent manner as possible. This is because turbulence in the mould causes the flux on the surface of the molten steel to be dragged down into the mould (known as ‘entrainment’) and thereby generating impurities in the cast steel. A third main function of a submerged entry nozzle is to introduce the molten steel into the mould in a controlled manner in order to achieve even solidified shell formation and even quality and composition of the cast steel, despite the fact that the steel solidifies most quickly in the regions closest to the mould walls.

[0006] It will be appreciated that designing and manufacturing a submerged entry nozzle which performs all of the above functions to an acceptable degree is an extremely challenging task. Not only must the nozzle be designed and manufactured to withstand the forces and temperatures associated with fast flowing molten steel, but the need for turbulence suppression combined with the need for even distribution of the molten steel in the mould create extremely complex problems in fluid dynamics.

[0007] According to the prior art a generic submerged entry nozzle has at least one, often two lateral outlet openings (e.g., patent document EP-2226141 -A2) and sometimes two lateral and one bottom outlet openings (e.g., patent document US-3,991 ,815). Most designs are based on the idea to influence the flow of the melt stream on its way leaving the submerged entry nozzle. Many modified submerged entry nozzle designs have been developed to influence the flow of the outflowing metal melt into the mould (e.g., patent documents US- 2014 / 0103079-A1 , WO2015 / 158439-A1 , US-2016 / 0082509-A1 , and W02019 / 101389-A1).

[0008] Thus in broad terms a submerged entry nozzle through which molten steel can be poured from a tundish into a mould, has a structure comprising: a substantially tubular body, extending from a first end to a second end; an inner nozzle wall surrounding a passageway which in use comes into contact with the molten steel, and extending through said tubular body along a longitudinal axis from said first end towards said second end; and one or more outlet ports or outlet openings, opening into said passageway in a region adjacent to said second end.

[0009] Commonly the tubular body of the SEN is made from a refractory material, and in practice is most often made from a carbon-bonded refractory material. Such a carbon-bonded refractory material typically comprises 2 to 30 % by mass carbon, 70 % by mass or more of one or more metal oxides, with a total content of other components being 10 % by mass or less. Since oxygen may form undesirable bubbles or voids within the cast metal during a casting process, it is common to introduce aluminium during the secondary steelmaking process in order to react with and thereby remove any oxygen from the molten steel. It is commonly believed that the resulting alumina (AI2O3) tends to accumulate on the inner surface of submerged entry nozzles employed during the casting process. This build-up restricts the flow of metal through the nozzle, which, in turn, affects the quality and flow of metal exiting the submerged entry nozzle. Over time, alumina build-up may eventually completely block the flow of metal thereby rendering the nozzle unusable.

[0010] Patent document EP-1036614-A1 discloses a co-pressed submerged entry nozzle for use in a continuous casting process, said nozzle being employed to introduce a molten steel from a tundish into a mould: wherein at least part of portions surrounding discharge openings in said nozzle, are made of a graphite-containing refractory material containing 5-35 % by mass graphite, 65 % by mass or more of a spinel (MgO-AI2O3), with a total content of other components being 10 % by mass or less; and wherein at least part of internal wall material within the nozzle is made of a graphite-less refractory material containing 90 % by mass or more of a spinel, with a total content of other components being 10 % by mass or less to avoid the formation of AI2O3 layers forming on the working surfaces thereof, thus avoiding clogging of the discharge openings of the nozzles. Preferably the content of MgO in the spinel is 20-45 % by mass, and the content of AI2O3in the spinel is 55-80 % by mass. However, in the industrial continuous steel casting practice clogging may still occur and which has proven to be a very persistent problem.

[0011] European patent document EP-2441740-A1 discloses a submerged entry nozzle for use in a continuous casting process, the submerged entry nozzle being build up from two components of different refractory materials co-pressed together into a shaped body having an integral structure. The component forming the inner wall or inner lining of the nozzle is made from a refractory material containing: a CaO component in an amount of 0.5 % by mass or more; one or both of B2O3and R2O (R is one selected from the group consisting of Na, K and Li) in an amount of 0.5 mass% or more; AI2O3in an amount of 50 % by mass or more; and free carbon in an amount of 8.0 to 34.5 % by mass, wherein a total amount of CaO, B2O3and R2O is in the range of 1.0 to 15.0 % by mass, and a mass ratio of CaO / (B2O3+R2O) is in the range of 0.1 to 3.0. A substantial amount of free carbon is present to improve thermal shock resistance of the refractory material. The components are subjected to a reaction with a refractory aggregate consisting primarily of AI2O3to form a slag-based covering layer on the surface of the refractory material to prevent adhesion of AI2O3 and other inclusions. The film-like slag-based covering layer, which is a slag phase including a molten phase formed on the working surface while maintaining an adequate viscosity at a temperature around a molten steel temperature, has a function of smoothening the working surface and a function similar to a protective film for the working surface, to allow particles of AI2O3and other inclusions from molten steel to flow toward the molten steel without fixedly adhering onto the refractory material. It is an important feature that the refractory material has a permeability of 0.4*10~3to 4.0x10~3cm2(cm H2O sec) as measured at room temperature after firing under a non-oxidizing atmosphere at 1000°C. This gas permeability is required for the migration and enrichment of the volatile component towards the working surface and formation of the slag-based covering layer, continuously during a casting operation. The continuous formation of the slag-based covering layer allows to maintain the AI2O3inclusion adhesion-preventing effect over a long period of time. Thus the disclosed submerged entry nozzle facilitates the formation of AI2O3inclusions originating from the refractory material use, and merely prevents sticking of the inclusions formed and thereby avoiding clogging of said nozzle.

[0012] Another approach to limit clogging has been the development of an argon injected nozzle, which allows argon to permeate the porous interior diameter of the nozzle during casting, thereby forming a protective layer of inert gas which hinders the bonding of the dispersed alumina to the refractory material. The argon also reduces the CO partial pressure at the refractory-molten metal interface, again decreasing the possibility for adherence of alumina deposits. The argon-injection technology has extended nozzle life a step further at an ever- increasing cost, the expense of large volumes of argon required during casting and the increased manufacturing costs of the more complex SEN-argon nozzles. And the argon introduces an inherent mould level instability increasing the risk of defect entrainment.

[0013] JP-2010131634-A1 discloses a SEN made of a refractory, comprising vitrification components on the inside of the refractory. As a result, a molten glass phase is formed at the interface between the molten steel and the refractory. The non-metallic inclusions (AI2O3) present in the molten steel react with the glass phase, resulting in a dense and viscous formation at the interface between the refractory and the molten steel. At the outer peripheral side, said refractory is made of alumina-graphite. However, said vitrification layer on the one hand and the surrounding alumina-graphite layer on the other hand have very different physical properties, especially very different expansion behavior at increased temperatures, which can lead to stresses between these layers and thus to damage to the SEN. To avoid such damage, a space or a special mortar layer is necessary between these two layers for the purpose of relieving stress, which makes the production of such a SEN very complex.

[0014] It is therefore an object of the present invention to provide a submerged entry nozzle with improved resistance against alumina deposits in the nozzle passageway. It is another object of the present invention to provide a submerged entry nozzle with improved resistance against alumina deposits in the nozzle passageway which does not tend to build up mechanical stresses during use, i.e., when exposed to temperature, which could lead to damage to the SEN. It is another object of the present invention to provide a submerged entry nozzle with improved resistance against alumina deposits in the nozzle passageway which is easy to produce.

[0015] It is another object of the invention to provide a method of producing such a submerged entry nozzle.

[0016] In order to achieve this object, the present invention proposes a submerged entry nozzle through which molten steel can be poured from a tundish into a mould, said nozzle comprising: a substantially tubular body made from a refractory material, extending from a first end to a second end; an inner nozzle wall surrounding a passageway, in use coming into contact with molten steel, extending through said tubular body along a longitudinal axis from said first end towards said second end; one or more outlet ports or outlet openings, opening into said passageway in a region adjacent to said second end; wherein said refractory material comprises a first refractory material and a second refractory material; wherein said first refractory material is made from a first refractory mixture; wherein said second refractory material is made from a second refractory mixture; wherein said first refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said first refractory mixture: at least one metal oxide: 60 to 90% by mass;

[0017] Carbon: 10 to 40% by mass; wherein said second refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said second refractory mixture: AI2O3: 70 to 95% by mass;

[0018] CaO: 5 to 30% by mass;

[0019] Carbon: below 8% by mass; and wherein said second refractory material forms at least a part of said inner nozzle wall.

[0020] The invention is based on the surprising finding that a submerged entry nozzle with improved resistance against alumina deposits in the nozzle passageway can be provided if the tubular body of the submerged entry nozzle is made from a refractory material, wherein said refractory material comprises a first refractory material and a second refractory material, wherein said first refractory material is made from a first refractory mixture, wherein said second refractory material is made from a second refractory mixture, and wherein said first and second refractory mixture has the chemical composition according to the invention.

[0021] In this respect, the invention is based firstly on the finding that, in order to solve the underlying objects, the refractory material of the tubular body of the submerged entry nozzle must be comprised of at least two refractory materials, whereby the two refractory materials must each be made from a different refractory mixture. By producing the refractory material from at least two different mixtures, it is possible to provide a tubular body which satisfactorily solves all of the aforementioned objects. In this respect, it has been recognized according to the invention that producing the refractory material from at least two different mixtures allows providing the at least two different mixtures such that mechanical stresses between the two refractory materials can be avoided. Since this allows mechanical stresses between the first and second refractory material to be suppressed, the first and second refractory material can also be present directly adjacent to one another, which allows simple manufacture of the submerged entry nozzle according to the invention. At the same time, however, the first and second mixture each has further components in addition to aluminum oxide, which give the first and second refractory material, made from the first and second refractory mixture, different, specific properties. At the same time, it was recognized in accordance with the invention that it is only possible on the basis of the different first and second refractory mixtures to give the first and second refractory material specific structural properties which are optimal for their respective purpose. In this respect, only on the basis of two different mixtures can the first and second refractory material each be given different optimum properties required for their specific purpose, in particular also with regard to their respective porosity, grain size and grain size distribution.

[0022] The invention is further based on the finding that one of the refractory materials, comprised by the refractory material of the tubular body, and which is referred to herein as "second refractory material" and which forms at least a part of the inner nozzle wall, is advantageously made from a refractory mixture, which is referred to herein as "second refractory mixture" and which is a low-carbon or carbon-free mixture based on alumina and calcium oxide, these being present in the proportions according to the invention. As a result, the second refractory material forms low-melting phases of calcium aluminates by subjecting the submerged entry nozzle to temperature. These low-melting phases of calcium aluminates thus form a largely gas-tight layer during use of the submerged entry nozzle, which leads to improved resistance against alumina deposits in the nozzle passageway. The purpose of the second refractory material is therefore essentially to provide such a gas-tight layer during use of the submerged entry nozzle.

[0023] The invention is further based on the finding that one of the refractory materials, comprised by the refractory material of the tubular body, and which is referred to herein as "first refractory material", is advantageously made from a refractory mixture, which is referred to herein as "first refractory mixture", and which is a high-carbon mixture based on alumina and carbon, these being present in the proportions according to the invention. As a result, the first refractory material forms a mechanically and thermally high-strength alumina-carbon material during use of the submerged entry nozzle, which provides high strength to the tubular body of the submerged entry nozzle according to the invention. The purpose of the first refractory material is therefore essentially to give the tubular body of the submerged entry nozzle according to the invention a high mechanical and thermal strength. According to the invention, the second refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said second refractory mixture: AI2O3: 70 to 95% by mass;

[0024] CaO: 5 to 30% by mass;

[0025] Carbon: below 8% by mass.

[0026] According to the invention, it has been found that the properties of the second refractory material are particularly advantageous, i.e., in particular low-melting calcium aluminate phases are formed, which form a gas-tight layer during the application of the submerged entry nozzle, which at the same time does not melt on contact with the molten steel, if the second refractory mixture has a chemical composition in which the proportion of AI2O3is increasingly approaching a proportion of 85% by mass and the proportion of CaO is increasingly approaching a proportion of 15% by mass. In this respect, according to a preferred embodiment, it may be provided that the second refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said second refractory mixture: AI2O3: 75 to 90% by mass;

[0027] CaO: 10 to 25% by mass;

[0028] Carbon: below 8% by mass.

[0029] According to an even more preferred embodiment, it may be provided that the second refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said second refractory mixture:

[0030] AI2O3: 80 to 90% by mass;

[0031] CaO: 10 to 20% by mass;

[0032] Carbon: below 8% by mass.

[0033] The proportion of "carbon" in the first and second refractory mixture is free carbon.

[0034] According to the invention, the proportion of carbon in the second refractory mixture is less than 8% by mass. According to the invention, it was found that the lowest possible proportion of carbon has an increasingly advantageous effect on the formation of a low-melting calcium aluminate phase. Preferably, therefore, it may be provided that the second refractory mixture has a chemical composition, comprising carbon in a proportion below 5% by mass, even more preferably in a proportion below 3% by mass and even more preferably in a proportion of 0% by mass, in each case in relation to the mass of said second refractory mixture.

[0035] According to the invention, it was found that a chemical proportion of SiO2in the second refractory mixture can have an advantageous effect on the formation of low-melting phases from the second refractory material. In this respect, low-melting phases in particular can form in the calcium-aluminate-silica system so-called CAS phases. In this respect, according to one embodiment, it may be provided that the second refractory mixture has a chemical composition, comprising SiO2in a proportion up to 10% by mass, in relation to the mass of said second refractory mixture. More preferably it may be provided that the second refractory mixture has a chemical composition, comprising SiO2in a proportion in the range from 2 to 10% by mass, in relation to the mass of said second refractory mixture. Accordingly, it may be provided that the second refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said second refractory mixture: AI2C>3: 69,5 to 93% by mass;

[0036] CaO: 5 to 30% by mass;

[0037] SiO2: 0.5 to 10% by mass;

[0038] Carbon: below 8% by mass; more preferably

[0039] AI2O3: 74 to 88% by mass;

[0040] CaO: 10 to 25% by mass;

[0041] SiO2: 1 to 10% by mass;

[0042] Carbon: below 8% by mass; and even more preferably

[0043] AI2O3: 78 to 88% by mass;

[0044] CaO: 10 to 20% by mass;

[0045] SiO2: 2 to 10% by mass;

[0046] Carbon: below 8% by mass. According to one embodiment, it may be provided that the second refractory mixture has a chemical composition, comprising MgO and ZrO2in a proportion up to 10% by mass, in relation to the mass of said second refractory mixture. According to the invention, it was found that the wear resistance of the low-melting phase can be improved by the presence of these oxides in proportions up to 10% by mass.

[0047] According to one embodiment, it may be provided that the second refractory mixture has a chemical composition, comprising further oxides, i.e., oxides other than AI2O3, CaO, SiO2, MgO and ZrO2, in a proportion below 4% by mass, more preferably in a proportion below 3% by mass, in each case relation to the mass of said second refractory mixture.

[0048] According to one embodiment, it may be provided that the oxides and carbon are homogenously distributed over the volume of the second refractory mixture. In particular, this has the advantage that the second refractory material has uniform properties over the volume and in this respect also is able to form a gas-tight layer over the entire volume of the second refractory material during the application of the submerged entry nozzle.

[0049] The information on chemical compositions given herein is determined in accordance with the standard ISO 12677:2011-10.

[0050] A "mixture" in the sense of the invention is, according to the usual nomenclature in the field of refractory technology, a formulation, i.e., a blend or a batch of one or more components, in particular, components in the form of refractory raw materials and binders.

[0051] In order to obtain the second refractory mixture with the chemical composition according to the invention, known refractory raw materials can be used. In this respect, known raw materials based on these oxides can be used in particular for the chemical proportions of AI2O3and CaO. In this respect, in order to provide the proportions of AI2O3in the second refractory mixture, at least one of the following refractory raw materials may preferably be provided in the second refractory mixture: fused alumina, sintered alumina, calcined alumina, tabular alumina or calcium-aluminate cement. Furthermore, in order to provide the proportions of CaO in the second refractory mixture, at least one of the following refractory raw materials may preferably be provided in the second refractory mixture: calciumaluminate cement or lime.

[0052] In addition to the aforementioned raw materials, the second refractory mixture may comprise one or more of the conventional binders for refractory mixtures. According to a preferred embodiment, the second refractory mixture comprises at least one organic binder, for example, polyvinyl alcohol (PVA).

[0053] According to the invention, it has been found that it is advantageous for the formation of low- melting phases from the second refractory material if the raw materials of the second refractory mixture, in particular the alumina-based raw materials, comprise a fine fraction. In this respect, according to a preferred embodiment, it is provided that said second refractory mixture comprises at least one raw material in a fine particle size. A "fine particle size" in the sense of the invention is in particular a particle size below 100pm and particularly preferably a particle size below 5pm.

[0054] According to a preferred embodiment, it may be provided that the second refractory mixture has a particle size, wherein at least 20% by mass, more preferably at least 30% by mass and even more preferably at least 40% by mass are present in a particle size below 100 pm, in each case in relation to the total mass of the second refractory mixture.

[0055] According to a preferred embodiment, it may be provided that the second refractory mixture has a particle size, wherein at least 2% by mass, more preferably at least 3% by mass and even more preferably at least 4% by mass are present in a particle size below 5pm, in each case in relation to the total mass of the second refractory mixture.

[0056] The information on grain sizes given herein is determined by sieving in accordance with the standard DIN EN 1402-3:2003 for grain sizes of 63pm and above and by laser diffraction in accordance with the standard ISO 13320:2020 for grain sizes below 63pm.

[0057] According to a preferred embodiment, it may be provided that the second refractory mixture has a smaller grain size than the first refractory mixture. In this respect, it has been found according to the invention that the mechanical and thermal properties of the first refractory material can be adjusted in a particularly advantageous manner if it is prepared from a refractory mixture having a coarser grain size than that of the second refractory mixture.

[0058] According to a preferred embodiment, it may therefore be provided that the D90 value of the first refractory mixture is higher than the D90 value of the second refractory mixture. As is known, the D90 value indicates the grain size at which 90% by mass of the particles are smaller than this value.

[0059] According to the invention, said second refractory material, made from the second refractory mixture, forms at least a part of said inner nozzle wall. According to a preferred embodiment, said second refractory material forms at least 70%, more preferably at least 80% and even more preferably at least 90% of the surface of said inner nozzle wall. If the second refractory material forms the surface of said inner nozzle wall to the extent as set forth above, the resistance of the submerged entry nozzle according to the invention against alumina deposits in the nozzle passageway is particularly effective. In this way the whole inner nozzle wall is gas impermeable, thereby avoiding all problems associated by the build-up of oxides, in particular alumina, on the inner nozzle wall during a steel casting process.

[0060] According to a preferred embodiment, said second refractory material has a thickness in the range from 1 to 10 mm. The thickness of the second refractory material at a position of the inner nozzle wall is the shortest distance through the second refractory material, starting from the respective position on the surface of the inner nozzle wall. According to the invention, it has been found that when the thickness of the second refractory material is less than 1 mm, the gas-tight layer resulting from the second refractory material may be too thin to prevent the formation of alumina-based inclusions at the inner nozzle wall surface. On the other hand, if the thickness of the second refractory material exceeds 10 mm, the mechanical and thermal properties of the submerged entry nozzle may deteriorate. In particular, this can lead to mechanical stresses between the first and second refractory material. A thickness of the second refractory material of 5 mm has been found to be optimal, so that according to a preferred embodiment it can be provided that said second refractory material has a thickness in the range from 3 to 8 mm.

[0061] As explained above, the second refractory mixture is composed in such a way that it can form a low-melting layer of calcium aluminates when exposed to temperature. In accordance with the invention, it has been found that said layer, based on a reaction between alumina (AI2O3) and CaO, forms a high viscous molten calcium-aluminate layer resulting in a gas impermeable layer, and thereby preventing the formation of alumina-based inclusions at the inner nozzle wall surface.

[0062] When due to its inherent porosity the refractory material is gas permeable, in use the atmosphere of the refractory is in contact with the molten steel, the cast steel is strongly undersaturated in CO and consumes the suboxide species, under formation of new oxides from the cast steel. The consumption of the refractory atmosphere by the steel continually drives replenishment in the refractory, where the oxides sustaining the carbothermic reaction continually get exhausted. On balance, the carbothermic exchange between the refractory and the steel transfers oxygen from the refractory to the steel, where it creates new alumina inclusions locally and may lead to clogging formation. Thus, according to the inventors it is believed that the reaction between the dissolved aluminum in the steel and carbon monoxide emitted from the carbon-bonded refractory, for example as known from EP-2441740-A1 , is the principal reaction mechanism in the formation and accumulation of harmful alumina deposits in the nozzle passageway.

[0063] Now the prevention of oxygen transfer from the refractory material to the molten steel means that no oxides are formed at the steel-refractory interface capturing the oxygen, and the refractory can function without deterioration of the molten cast stream. The significant reduction or even the elimination of clogging also improves the liquid-metallurgical quality of the cast through effects such as lessening the need for argon shrouding into the submerged entry nozzle, and improvement of the mould level stability by reduced argon escape, and thereby improving the direct cleanness and defect freeness of the casts.

[0064] As used herein, terms "calcium aluminate" and "calcium aluminates" are used interchangeably and intend mixtures of CaO and AI2O3and mixed-compound phases thereof.

[0065] In a preferred embodiment the calcium aluminate, which can form from the second refractory material when using the submerged entry nozzle according to the invention, is made from a combination of different CA-Phases, for example a mixture of both CA2 and CA6 calcium aluminate, where C is CaO and A is AI2O3. The CA2 is also known as calcium dialuminate and CA6 is also known as calcium hexaluminate. In calcium aluminate also a spinel phase might be formed, but in accordance with the invention its presence is preferably limited to maximum 5% by mass, and more preferably to maximum 2% by mass. Spinel is inert and would not undergo the reactions with the CaO, but it would make it more difficult to achieve full porosity close off. In calcium aluminate also some SiO2can be present.

[0066] In an embodiment the ratio, in % by mass, between CA2 / CA6 is >1 , and more preferably >1.1. The CA6 is predominantly platelet shaped and a too high a content of CA6 may have an adverse effect on the pore filing capacity of the calcium aluminate.

[0067] In operational use, thus when in contact with molten steel in a continuous casting operation, the low-melting layer of calcium aluminates formed from the second refractory material remains solid in order to maintain its gas impermeability. In an embodiment the second refractory material has a solidus temperature point higher than 1650°C, preferably higher than 1700°C, and more preferably higher than 1730°C.

[0068] For the submerged entry nozzle, low-melting layer of calcium aluminates preferably is created at least part or in full on the inner nozzle wall prior to the submerged entry nozzle having been in contact with molten steel.

[0069] According to the invention, the said first refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said first refractory mixture:

[0070] At least one metal oxide: 60 to 90% by mass;

[0071] Carbon: 10 to 40% by mass

[0072] According to the invention, it has been found that the properties of the first refractory material are particularly advantageous, i.e., it can be given high mechanical and thermal strength in particular, if the first refractory mixture has a chemical composition in which the proportion of AI2O3is increasingly approaching a proportion of 75% by mass and the proportion of carbon is increasingly approaching a proportion of 25% by mass. In this respect, according to a preferred embodiment, it may be provided that the first refractory mixture has a chemical composition, comprising the following oxide and carbon in the following proportions, in each case in relation to the mass of said first refractory mixture:

[0073] At least one metal oxide: 65 to 85% by mass; Carbon: 15 to 35% by mass.

[0074] According to the invention, it has been recognized that in order to avoid mechanical stresses, it is advantageous if the first and second refractory material are be based on the same metal oxide, whereby aluminum oxide, on which both the first and the second refractory mixture are based, has proven to be advantageous in this respect. Accordingly, according to a preferred embodiment, said at least one metal oxide of said chemical composition of said first refractory mixture is AI2O3.

[0075] In order to prevent the formation of low-melting phases in the first refractory material, it can preferably be provided that the proportion of CaO in the first refractory mixture is as low as possible. According to a preferred embodiment, it may therefore be provided that said first refractory mixture has a chemical composition, comprising CaO in a proportion below 5% by mass, more preferably in a proportion below 3% by mass and even more preferably in a proportion below 1% by mass, in each case in relation to the mass of the said first refractory mixture.

[0076] According to one embodiment, it may be provided that the first refractory mixture has a chemical composition, comprising further oxides, i.e., oxides other than AI2O3, in a proportion below 10% by mass, more preferably in a proportion below 5% by mass, and even more preferably in a proportion below 3% by mass, in each case in relation to the mass of said second refractory mixture.

[0077] In order to obtain the first refractory mixture with the chemical composition according to the invention, the known refractory raw materials can be used. In this respect, in case of the preferred embodiment according to which the at least one metal oxide of said chemical composition of said first refractory mixture is AI2O3, the known raw materials based on this oxide can be used in particular for the chemical proportions of AI2O3. In this respect, in order to provide the proportions of AI2O3in the first refractory mixture, at least one of the following refractory raw materials can preferably be provided in the first refractory mixture: fused alumina, sintered alumina, tabular alumina or calcined alumina.

[0078] In particular, the known carbon-based raw materials can be used for the chemical proportions of carbon in the first refractory mixture. In this respect, in order to provide the proportions of carbon in the first refractory mixture, preferably at least one of the following refractory raw materials can be provided in the first refractory mixture: graphite, carbon black or charcoal. Preferably, graphite is provided as carbonaceous raw material in the first refractory mixture.

[0079] In addition to the aforementioned raw materials, the first refractory mixture may further comprise one or more of the antioxidants for carbon containing mixtures, preferably metallic silicon, in particular silicon powder, or boron oxide.

[0080] In addition to the aforementioned raw materials, the first refractory mixture may comprise one or more of the usual binders for refractory mixtures for producing a alumina-carbon refractory material. According to a preferred embodiment, the first refractory mixture comprises at least one carbonaceous binder, particularly preferably at least one of the following binders: pitch or resin.

[0081] According to a preferred embodiment, it is provided that said first refractory material does not form at least a part of said inner nozzle wall since the first refractory material is not designed to form a gas-tight layer. Accordingly, if first refractory material would form at least a part of said inner nozzle wall, alumina deposits could build up on these parts of the nozzle passageway.

[0082] According to a preferred embodiment, said first refractory material is arranged in a direction radially outwardly from said second refractory material in relation to the passageway. In other words, the first refractory material is arranged immediately behind the second refractory material from a direction away from the passageway, or rather it traps or embraces the second refractory material. This results in a thermally and mechanically particularly stable structure of the tubular body, which is also particularly easy to manufacture.

[0083] According to an alternative embodiment, the first refractory material and the second refractory material are separated by an intermediate layer. This intermediate layer can, for example, serve to relieve stresses between the first refractory material and the second refractory material. In this respect, according to a preferred embodiment, it may be provided that the chemical composition of the intermediate layer is "between" the chemical composition of the first and second refractory mixture. In this respect, the intermediate layer may, for example, have a chemical composition, in relation to the mass of the intermediate layer, with a proportion of AI2O3, CaO and carbon in each case between that of the first and second refractory mixture.

[0084] In an embodiment of the submerged entry nozzle, it is made from one piece of refractory material.

[0085] In an embodiment of the submerged entry nozzle said passageway has a circular crosssection.

[0086] In an embodiment of the submerged entry nozzle said passageway has a cylindrical contour.

[0087] In one embodiment, said refractory material is non-fired. In this case, in order to provide the submerged entry nozzle ready for use, i.e., for use in a continuous casting process, the refractory material is preferably still to be fired beforehand, in particular as set forth herein.

[0088] In one embodiment, said refractory material is fired, and wherein said first refractory material is carbon-bonded. In this case, the refractory material was fired as described herein such that the first refractory mixture is in the form of a carbon-bonded alumina-carbon matrix.

[0089] The invention also relates to a method of producing a submerged entry nozzle according to the invention, the method comprising the steps of: providing said first refractory mixture; providing said second refractory mixture; forming said first refractory mixture to said first refractory material; forming said second refractory mixture to said second refractory material.

[0090] The first refractory mixture and the second refractory mixture may have the features as described herein.

[0091] According to the method, the first refractory material is formed or molded, respectively, from the first refractory mixture, and the second refractory material is formed or molded, respectively, from the second refractory mixture. The forming can be carried out according to the technologies known from the prior art for forming refractory mixtures. According to a preferred embodiment, said steps of forming are realized by pressing, particularly by isostatic pressing.

[0092] According to a preferred embodiment, said steps of forming are realized by co-pressing said first refractory mixture to said first refractory material and said second refractory mixture to said second refractory material. In this respect, the first refractory mixture and the second refractory mixture can be pressed together, in particular pressed together isostatically. In this case, said first refractory mixture is simultaneously pressed to said first refractory material and said second refractory mixture is pressed to said second refractory material in a single, common pressing step.

[0093] Following the steps of forming said first refractory mixture to said first refractory material and of forming said second refractory mixture to said second refractory material, it may be provided that the first and second refractory material is fired, particularly in a reducing atmosphere.

[0094] The firing is preferably carried out in such a way that a carbon-bonded alumina-carbon refractory material is formed from the first refractory material and the second refractory material forms a low-melting calcium-aluminate phase.

[0095] The firing temperatures are preferably between about 800 C and 1 ,300°C.

[0096] The invention also relates to the use of the submerged entry nozzle according to this invention or obtainable by the method according to this invention in a continuous casting steel-making process, and preferably wherein molten steel is transferred or flows from a tundish into at least one casting mould.

[0097] Further features of the invention follow from the claims, the exemplary embodiment as illustrated in the figure and the associated description of the exemplary embodiment. All features of the invention can be combined with each other, individually or in combination, as desired.

[0098] An exemplary embodiment of the invention is described below with reference to the figure. Schematized and not to scale, it is illustrated in

[0099] Figure 1 a lateral sectional view of an exemplary embodiment of a submerged entry nozzle according to the invention.

[0100] The submerged entry nozzle according to the exemplary embodiment is designated in its entirety by the reference sign 1 . The outer geometry of the submerged entry nozzle 1 corresponds to conventional submerged entry nozzles according to the state of the art and is shown in the Figure in its vertical use position.

[0101] Submerged entry nozzle 1 comprises a substantially tubular body 2 made from a refractory material, and which extends from a first end 3 to a second end 4. In the vertical use position according to the Figure, the first end 3 is the upper end and the second end 4 is the lower end of the tubular body 2. The refractory material from which said tubular body 2 is made, is comprised of a first refractory material 5 and a second refractory material 6.

[0102] The submerged entry nozzle 1 further comprises an inner nozzle wall 7 surrounding a passageway 8, in use coming into contact with molten steel, extending through said tubular body 2 along a longitudinal axis 9 from said first end 3 towards said second end 4. In the use position according to the Figure, the longitudinal axis 9 extends vertically through the tubular body 2. The passageway 8 has a circular cross-section and a cylindrical contour.

[0103] At the upper end of the passageway 8, an inlet opening 10 opens into the passageway 8. At the lower end of the passageway 8, it ends with a distance to the second end 4 of the tubular body 2. Two opposite outlet ports 11 , 12, each extending horizontally in the use position according to the Figure, are opening into the passageway 8 in a region adjacent to the second end 4. The outlet ports 11 , 12 are opening to the outside of the tubular body 2 through respective outlet openings 13, 14.

[0104] When used in a continuous casting steel-making process, a continuous flow of molten steel enters into the submerged entry nozzle 1 through its inlet opening 10, flows through and along the passageway 8 and the outlet ports 11 , 12 and finally leaves the submerged entry nozzle 1 through the outlet openings 13, 14. The second refractory material 6 forms the entire surface of the inner nozzle wall 7 and the entire surface of the walls of the outlet ports 11 , 12. Hence, when flowing through the submerged entry nozzle 1 , the molten steel only comes into contact with the second refractory material 6. The second refractory material 6, forming the surface of the inner nozzle wall 7, has a uniform thickness of 5 mm at any position on the inner nozzle wall 7, whereby the thickness of the second refractory material 6 at a position of the inner nozzle wall 7 is the shortest distance through the second refractory material 6, starting from the respective position on the surface of the inner nozzle wall 7. In the same way, the second refractory material 6, which forms the surface of the inner wall of the outlet ports 11 , 12, also has a uniform thickness of 5 mm.

[0105] The first refractory material 5, on the other hand, is arranged in a direction radially outwardly from said second refractory material 6 and thus is arranged immediately behind the second refractory material 6 from a direction away from the passageway 8 and the outlet ports 11 , 12. The first refractory material 5 thus completely embraces or surrounds the second refractory material 6 on the side of the second refractory material 6 facing away from the passageway 8 and the outlet ports 11 , 12.

[0106] The first refractory material 5 was made from a first refractory mixture according to Table 1.

[0107] Table 1

[0108] The first refractory mixture further comprised an amount of 10% by mass, in relation to the mass of the first refractory mixture without the binder, of a carbonaceous binder in the form of phenolic resin (Novolak).

[0109] The D90 value of the first refractory mixture was 0.2mm. The first refractory mixture had the chemical composition according to Table 2.

[0110] Table 2

[0111] The second refractory material 6 was made from a second refractory mixture according to

[0112] Table 3.

[0113] Table 3

[0114] The second refractory mixture further comprised an amount of 10% by mass, in relation to the mass of the second refractory mixture without the binder, of a binder in the form of polyvinyl alcohol (PVA).

[0115] The D90 value of the second refractory mixture was 0.1 mm.

[0116] The second refractory mixture had the chemical composition according to Table 4.

[0117] Table 4

[0118] The proportions of carbon and oxides in the first and second refractory mixture are determined by a combination of the standards ISO 12677 and ISO 21068-2. For this purpose, the proportion of oxides was determined to 100% by mass according to ISO 12677. Secondly, the proportion of carbon was determined according to ISO 21068-2. Then, the determined 100% by mass of oxides and the determined % by mass of carbon were added. The resulting total mass (100% by mass oxides plus % by mass carbon) was then normalized to 100% by mass

[0119] For producing the submerged entry nozzle 1 from the first and second refractory material, the first and second refractory material were provided and co-pressed by isostatic pressing. By this isostatic co-pressing, the first refractory material 5 and the second refractory material 6 were formed.

[0120] After said co-pressing, a green body of the submerged entry nozzle 1 was provided.

[0121] The green body of the submerged entry nozzle 1 , i.e., with non-fired refractory material, defined a first exemplary embodiment of the invention.

[0122] Said green body was subsequently fired in a reducing atmosphere at a maximum temperature of about 1 ,000°C for 6 hours.

[0123] During firing, a carbon-bonded alumina-carbon refractory material was formed from the first refractory material 5 and the second refractory material 6 formed low-melting phases of calcium aluminates.

[0124] Finally, the fired submerged entry nozzle was cooled down.

[0125] After cooling down, a submerged entry nozzle 1 with fired refractory material, according to a second exemplary embodiment of the invention, was obtained. In the fired submerged entry nozzle 1 , thus obtained, the first refractory material 5 is a carbon-bonded alumina-carbon refractory material whereas the second refractory material 6 forms a gas-tight layer comprising low-melting calcium aluminate phases.

[0126] The liquidus temperature of the second refractory material 6 was determined by putting the submerged entry nozzle into a pre-heat station and melding phase were observed at temperatures lower than 1 ,730°C, namely, at about 1 ,100°C.

[0127] In order to examine the properties of the obtained submerged entry nozzle 1 , the submerged entry nozzle 1 was used in a continuous casting steel-making process, in order to guide and pour molten steel from a tundish to a mold.

[0128] After use, the condition of the submerged entry nozzle 1 was investigated. The submerged entry nozzle 1 proved to be thermally and mechanically stable. The tubular body 2 of the submerged entry nozzle 1 did not show any stress cracks, especially not in the interface between the first refractory material 5 and the second refractory material 6.

[0129] Further, the inner nozzle wall 7 which came into contact with the molten steel during the casting process was investigated. Accordingly, no accumulations of alumina could be detected on the nozzle wall 7, which could have formed during the casting process. The submerged entry nozzle 1 thus showed complete resistance to alumina deposits in the nozzle passageway 8 during the casting process.

Claims

A submerged entry nozzle and a method of producing a submerged entry nozzleC l a i m s1 . A submerged entry (1) nozzle through which molten steel can be poured from a tundish into a mould, said nozzle (1) comprising:1.1 a substantially tubular body (2) made from a refractory material, extending from a first end (3) to a second end (4);1 .2 an inner nozzle wall (7) surrounding a passageway (8), in use coming into contact with molten steel, extending through said tubular body (2) along a longitudinal axis (9) from said first end (3) towards said second end (4);1 .3 one or more outlet ports (11 , 12) or outlet openings, opening into said passageway (8) in a region adjacent to said second end (4); wherein1 .4 said refractory material comprises a first refractory material (5) and a second refractory material (6); wherein1 .5 said first refractory material (5) is made from a first refractory mixture; wherein1 .6 said second refractory material (6) is made from a second refractory mixture; wherein1 .7 said first refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said first refractory mixture: at least one metal oxide: 60 to 90% by mass;Carbon: 10 to 40% by mass; wherein1 .8 said second refractory mixture has a chemical composition, comprising the following oxides and carbon in the following proportions, in each case in relation to the mass of said second refractory mixture:AI2O3: 70 to 95% by mass;CaO: 5 to 30% by mass;Carbon: below 8% by mass; and wherein1 .9 said second refractory material (6) forms at least a part of said inner nozzle wall (7).

2. The submerged entry nozzle (1) according to claim 1 , wherein said at least one metal oxide of said chemical composition of said first refractory mixture is AI2O3.

3. The submerged entry nozzle (1) according to at least one of the preceding claims, wherein said second refractory material (6) forms at least 70% of the surface of said inner nozzle wall (7).

4. The submerged entry nozzle (1) according to at least one of the preceding claims, wherein said first refractory material (5) does not form at least a part of said inner nozzle wall (7).

5. The submerged entry nozzle (1) according to at least one of the preceding claims, wherein said second refractory material (6) has a thickness in the range from 1 to 10 mm.

6. The submerged entry nozzle (1) according to at least one of the preceding claims, wherein said first refractory material (5) is arranged in a direction radially outwardly from said second refractory material (6), in relation to the passageway (9).

7. The submerged entry nozzle (1) according to at least one of the preceding claims, wherein said second refractory mixture has a chemical composition, comprising SiO2in a proportion up to 10% by mass, in relation to the mass of said second refractory mixture.

8. The submerged entry nozzle (1) according to at least one of the preceding claims, wherein said first refractory mixture has a chemical composition, comprising CaO in a proportion below 5% by mass, in relation to the mass of the said first refractory mixture.

9. The submerged entry nozzle (1) according to at least one of the preceding claims, wherein said refractory material is non-fired.

10. The submerged entry nozzle according to at least one of claims 1 to 8, wherein said refractory material is fired, and wherein said first refractory material (6) is carbon- bonded.

11. A method of producing a submerged entry nozzle (1) according to at least one of claims 1 to 10, the method comprising the steps of:A. providing said first refractory mixture;B. providing said second refractory mixture;C. forming said first refractory mixture to said first refractory material (5);D. forming said second refractory mixture to said second refractory material (6).

12. The method according to claim 11 , wherein said steps of forming are realized by pressing.

13. The method according to claim 12, wherein said pressing is isostatic pressing.

14. The method according to at least one of claims 11 to 13, wherein said steps of forming are realized by co-pressing said first refractory mixture to said first refractory material (5) and said second refractory mixture to said second refractory material (6).

15. The method according to at least one of claims 11 to 14, wherein, following the steps C. and D., said first refractory material (5) and said second refractory material (6) are fired in a reducing atmosphere.

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