FIELD OF THE INVENTION

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.

BACKGROUND TO THE INVENTION

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.

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. 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.

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 , W02015/158439-A1, US-2016/0082509-A1 , and W02019/101389-A1).

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.

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 of 2 to 30 wt.% carbon, 70 wt.% or more of one or more metal oxides, and with a total content of other components being 10 wt.% 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 combine with and thereby remove any oxygen from the molten steel. It is commonly believed that the resulting alumina 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. In time alumina build up may eventually completely block the flow of metal thereby rendering the nozzle unusable.

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 wt.% graphite, 65 wt.% or more of a spinel (MgO-AI ₂ O ₃ ), with a total content of other components being 10 wt.% or less; and wherein at least part of internal wall material within the nozzle is made of a graphite-less refractory material containing 90 wt.% or more of a spinel, with a total content of other components being 10 wt.% or less to avoid the formation of AI ₂ O ₃ 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 wt.%, and the content of AI ₂ O ₃ in the spinel is 55-80 wt.%. However, in the industrial continuous steel casting practice clogging may still occur and which has proven to be a very persistent problem.

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 wt.% or more; one or both of B ₂ O ₃ and R ₂ O (R is one selected from the group consisting of Na, K and Li) in an amount of 0.5 mass% or more; AI ₂ O ₃ in an amount of 50 wt.% or more; and free carbon in an amount of 8.0 to 34.5 wt.%, wherein a total amount of CaO, B ₂ O ₃ and R ₂ O is in the range of 1.0 to 15.0 wt.%, and a mass ratio of CaO/(B ₂ O ₃ +R ₂ O) 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 AI ₂ O ₃ so as to form a slag-based covering layer on the surface of the refractory material to prevent adhesion of AI ₂ O ₃ 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 AI ₂ O ₃ and 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 0.4x10" ³ to 4.0x10" ³ cm ² (cm H ₂ 0-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 AI ₂ O ₃ inclusion adhesion-preventing effect over a long period of time. Thus the disclosed submerged entry nozzle facilitates the formation of AI ₂ O ₃ inclusions originating from the refractory material use, and merely prevents sticking of the inclusions formed and thereby avoiding clogging of said nozzle.

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.

DESCRIPTION OF THE INVENTION

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 invention to provide a method of producing a submerged entry nozzle with improved resistance against alumina deposits in the nozzle passageway.

This and other objects and further advantages are met or exceeded by the present invention providing a submerged entry nozzle according to claim 1 and a method of manufacturing a submerged entry nozzle according to claim 11 , and with preferred embodiments in the dependent claims.

In order to achieve this object, the present invention proposes, in a first aspect, a submerged entry nozzle 1 through which molten steel can be poured from a tundish into a mould, said nozzle 1 comprising: a substantially tubular body 2 made from a carbon-bonded refractory material, extending from a first end 3 to a second end 4; an inner nozzle wall 9 surrounding a passageway 5 in use coming into contact with molten steel, extending through said tubular body 2 along a longitudinal axis (A) for an axial length (L) from said first end 3 towards said second end 4; one or more outlet ports 8 or outlet openings, opening into said passageway 5 in a region 7 adjacent to said second end 4, and wherein the refractory material is a carbon-bonded refractory material containing 2 to 40 wt.% carbon, 60 wt.% or more of one or more metal oxides and at least 60 wt.% thereof is alumina, with a total content of other components being 10 wt.% or less, the inner nozzle wall 9 from its outer-surface inwards is carbon-free over a distance of at least 1 mm and at least part of the inner nozzle wall 9 is provided with or has a solid melt assisted sintering layer comprising calcium-aluminate forming a gas impermeable layer.

As used herein, terms “calcium aluminate” and ‘calcium aluminates” are used interchangeably and intend mixtures of CaO and AI ₂ O ₃ and mixed-compound phases thereof.

In a preferred embodiment the calcium aluminate is made from a combination or a mixture of both CA2 and CA6 calcium aluminate, where C is CaO and A is AI ₂ O ₃ . 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 wt.%, and more preferably to maximum 2 wt.%. 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 SiO ₂ can be present. The SiO ₂ is a component not having any significant adverse effect on the mechanism described; however, it is preferred it keep its presence at a low level, and preferably to maximum 5 wt.%, and more preferably to maximum 3 wt.%.

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

In accordance with the invention it has been found that a solid melt assisted sintering layer based on a reaction between alumina (AI ₂ O ₃ ) and CaO forming a low viscous molten calcium-aluminate penetrating and filling the pores present in the refractory material from the outer-surface of inner nozzle wall 9 inwards into the tubular body results in a gas impermeable layer, and thereby preventing the formation of alumina based inclusions at the inner nozzle wall surface. This pore filing is in combination with a reaction between CaO and the AI ₂ O ₃ of the refractory material body and any AI ₂ O ₃ that might be present in for example a refractory slurry as described hereinafter, and results in the formation of calcium aluminate crystal growth that closes off the porosity of the refractory material in the area of the inner nozzle wall 9 surrounding the passageway 5 which in use comes into contact with molten steel. In this way in service a gas impermeable layer is present between the bulk of the refractory material of the submerged entry nozzle and the molten steel flowing through the passage way 5 of the submerged entry nozzle. The formed solid calcium-aluminate layer, and preferably being a mixture of CA2 and CA6, is thermally stable such that in service conditions of a steel casting operation the gas impermeable melt assisted sintering layer remains fully functional. The carbon-bonded refractory material of the inner nozzle wall 9 surrounding the passageway 5 from its outer-surface inwards is carbon-free, preferably over a depth of up to 10 mm, more preferably up to 5 mm, and preferably for at least 1 mm. A carbon-free oxidized zone from the inner nozzle wall 9 inwards, when formed, is porous and is thereafter impregnated with a carbon-free refractory material having a substantial amount of CaO and/or CaO precursor. The pore size of the carbon-free oxidized zone prior to impregnation is controlled, preferably controlled within the range of about 10 to 70 microns, and more preferably of about 10 to 50 microns. The completed submerged entry nozzle having the oxidized zone and carbon-free impregnated refractory material may be low temperature cured after impregnation or it may be used without such a cure if the submerged entry nozzle is subjected to a conventional preheat treatment prior to service. During high temperature service, the impregnated refractory material undergoes melt assisted sintering to further densify the layer or the liner region and forming the gas impermeable layer so as to prevent liquid steel infiltration and to prevent reverse carbon monoxide emissions from the carbon-bonded refractory body into the steel.

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 aluminium in the steel and carbon monoxide emitted from the carbon-bonded refractory, for example as known from E P-2441740-A1 , is the principal reaction mechanism in the formation and accumulation of harmful alumina deposits in the nozzle passageway 5.

Now the prevention of oxygen transfer from the refractory material to the molten steel means that no oxide grains are formed at the steel-refractory interface fixing the oxygen, and the refractory cast ware 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. Some earlier approaches in the prior art are based on solid state sintering using the addition of tiny grains of a sintering agent or other substance (e.g., alumina, silica, zirconia, and SiAION), creating a ceramic bond by firing at about 1000°C, thus below the melting temperature forming a porous network of grain-to-grain contacts. But the connected pores are still gas permeable and consequently the undesired build of oxides on the inner nozzle wall 9 of the submerged entry nozzle 1 still occurs and resulting quality problems in the cast steel may also still occur. Also, the clogging of a submerged entry nozzle may still occur.

In an embodiment of the submerged entry nozzle substantially the whole inner nozzle wall 9 is provided with a solid melt assisted sintering layer. In this way the whole inner nozzle wall 9 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.

In operational use, thus when in contact with molten steel in a continuous casting operation, the melt assisted sintering layer remains solid in order to maintain its gas impermeability. In an embodiment the melt assisted sintering layer has a solidus temperature point higher than 1650°C, preferably higher than 1700°C, and more preferably higher than 1730°C.

For the submerged entry nozzle 1 the solid melt assisted sintering layer is created at least part or in full on the inner nozzle wall 9 prior to the submerged entry nozzle having been in contact with molten steel.

For the invention described herein, CaO is used. Additionally or alternatively to CaO, a CaO precursor may be used. The CaO precursor may be a compound or substance susceptible of being transformed into CaO. The transformation may take place by heating the CaO precursor thereby providing CaO. Examples of CaO precursors that may be used in the invention include Ca(OH) ₂ , nitrate salts of calcium (e.g., calcium nitrate tetrahydrate), calcium-aluminate, and calcium-aluminate cement, the latter preferably in the form of a hydraulic active powder called CAC. Depending of the CaO precursor used it can be infiltrated into the carbon-free oxidized zone via a sol-gel method using water or an organic carrier for the CaO precursor or as a hydrous cement paste.

In a preferred embodiment the CaO precursor is formed by a calcium-aluminate cement, which can ideally be infiltrate into the carbon-free oxidized zone, preferably as a hydrous cement paste, being the liner infiltrate or refractory slurry. As known in the art for a cement paste the required water consistency can be assessed according to standard ASTM C-191. As pure CaO powder particles are very difficult to handle in an industrial scale of producing casting nozzles, considerable precautions have to be taken to avoid contact with moisture or CO ₂ , it is preferred that only a CaO precursor is being used.

In an embodiment the carbon-bonded refractory material has at least 70 wt.% of one or more metal oxides, and preferably of at least 80 wt.% of one or more metal oxides.

In an embodiment at least 70 wt.% of the one or more metal oxides in the carbon-bonded refractory material is formed by alumina, and preferably at least 85 wt.%. Other metal oxides may include small amounts of for example MgO, SiO ₂ , ZrO ₂ , and spinel.

In an embodiment the carbon-bonded refractory material has at most 25 wt.% carbon, more preferably at most about 20 wt.%, and most preferably at most 18 wt.%. Too high a carbon content may lead to a too high porosity level after oxidation and thereby adversely affecting the mechanical stability. Also obtaining a gas impermeable melt assisted sintering layer might become more difficult at too high porosity levels.

The carbon-bonded refractory material has at least 2 wt.% carbon. In an embodiment the carbon-bonded refractory material has at least about 8 wt.% carbon, and more preferably at least about 10 wt.%. The carbon is derived principally from graphite, carbon black or charcoal plus a lesser amount from any carbonaceous binder, such as pitch or resin.

In the carbon-bonded refractory material the balance is made by a total content of other components being 10 wt.% or less, and preferably of 5 wt.% or less, and refer to known components in the art of refractory materials to influence the conditions of manufacturing the SEN such as sinterability and filling formability. These other components should not interfere with the chemical reactions creating the gas impermeable layer.

A mixture of the various components forming the refractory material is commonly blended with a conventional carbonaceous binder, such as resin or pitch, and pressed (e.g. by means of cold isostatic pressing) into an appropriate refractory body such as the submerged entry nozzle of Figure 1. The pressed body is then fired in an appropriate reducing atmosphere in a conventional manner. Firing temperatures are usually between about 800°C and 1300°C.

In an embodiment of the submerged entry nozzle the melt assisted sintering layer has a thickness of at most 15 mm, and preferably of at most 10 mm. In an embodiment the thickness is at most 5 mm. In an embodiment the thickness is at least 1 mm.

In an embodiment of the submerged entry nozzle 1 it is made from one piece of refractory material.

In an embodiment of the submerged entry nozzle 1 said passageway 5 has a circular cross-section. In an embodiment of the submerged entry nozzle 1 said passageway 5 has a cylindrical contour.

The invention also relates to a method of producing a submerged entry nozzle according to the present invention, the method comprising the sequential steps of: providing a body of a submerged entry nozzle 1 made from a carbon-bonded refractory material containing 2 to 40 wt.% carbon, 60 wt.% or more of one or more metal oxides and at least 60 wt.% thereof is alumina, with a total content of other components being 10 wt.% or less; firing in an oxidizing furnace atmosphere said body to produce a porous, carbon-free, oxidized zone in the surface of the inner nozzle wall 9 wherein_the inner nozzle wall (9) from its outer-surface inwards is carbon-free over a distance of at least 1 mm, preferably over a distance of up to 5 mm, and more preferably up to 10 mm; infiltrating the oxidized zone of the surface of the inner nozzle wall 9 with a refractory comprising CaO and/or CaO precursor, and preferably infiltrating with a refractory slurry comprising the CaO and/or CaO precursor; optionally, in particular when a CaO precursor is being used, firing the refractory body with the infiltrated refractory, preferably the infiltrated refractory slurry, to transform the CaO precursor into CaO. Also any volatile components should be allowed to leave the SEN refractory body. heating the refractory body with the infiltrated refractory, preferably the infiltrated refractory slurry, in the oxidized zone of the surface of the inner nozzle wall 9 to a temperature sufficiently high to react the AI ₂ O ₃ in the refractory material with the infiltrated CaO and/or CaO precursor, where applicable also with any AI ₂ O ₃ in the refractory slurry, to form a solid melt assisted sintering layer comprising calcium aluminate on or in the inner nozzle wall 9.

The formed calcium-aluminate, and preferably being a mixture of CA2 and CA6, closes off the porosity; it has a melting point well above 1600°C, preferably well above 1650°C, and is thermally stable such that under service conditions of a steel casting operation the gas impermeable melt assisted sintering layer remains fully functional. The precursor melt has preferably a composition in the range of 40-50 wt.% AI ₂ O ₃ and 50-60 wt.% CaO.

The refractory body forming the submerged entry nozzle 1 is fired in an oxidizing furnace atmosphere to produce a porous, carbon free, oxidized zone in the interior of the inner nozzle way 9 surrounding the passageway 5. In order to protect the balance of the exterior surface portions of the refractory body against oxidation during the firing under these furnace conditions, the exterior body surfaces can be coated with a glaze; glaze forming frits are well- known in the art.

The firing schedule along with the particle size distribution of the refractory material are controlled to obtain the desired pore size in the carbon depleted, oxidized zone and to achieve the desired depth or thickness in the oxidized zone. The firing step in the oxidizing atmosphere is preferably conducted at a temperature of about 1000°C for several hours, e.g. for about 2 hours. This firing schedule produces an oxidized zone having a depth on the order of about 2 to 5 mm. An optimal depth for this zone is greater than about 1 mm, but preferably not greater than about 10 mm.

During the firing treatment, the surface of the unglazed inner nozzle wall 9 is exposed to the oxidizing furnace atmosphere. The oxygen containing atmosphere reacts with the carbon bond around the passageway 5 of the refractory body which develops a greater porosity within the oxidized zone by virtue of the depleted carbon exiting in the form of CO(g). An open pore size of a controlled dimension is obtained, and preferably is between about 10 to 70 microns is obtained in this zone. A more preferred pore size is between about 10 to 50 microns for the purpose of maximizing the infiltration of the carbon free refractory slurry into the oxidized zone, as will be explained in greater detail hereinafter. The infiltrated refractory then forms gas impermeable layer within the oxidized zone.

In order to increase adhesion of gas impermeable layer and reduce spalling, the refractory material is infiltrated into the porous, oxidized zone of the body 2. To enhance coating wettability and increase coating adhesion, the carbon is removed from the surface to be infiltrated, preferably through oxidation.

The refractory for infiltration into the oxidized zone is preferably material which contains no carbon and comprises at least CaO and/or CaO precursor as refractory material for the formation of the impermeable gas layer in the submerged entry nozzle. In a preferred embodiment a refractory slurry as liner infiltrate is prepared using a fine refractory powder.

In an embodiment the CaO and/or CaO precursor has an average particle diameter of between about 0.1-2.0 microns, preferably between about 0.1-1.5 microns, to facilitate a good infiltration into the pores. The CaO and/or CaO precursor particles preferably have a surface area of between about 1.5 and 9 m ² /gm.

The liner infiltrate is prepared as a slurry having at least 50 wt.% of CaO and/or CaO precursor with the balance predominantly water or an organic vehicle or carrier. In an embodiment the refractory slurry comprises at least 60 wt.% of CaO and/or CaO precursor. As CaO powder particles are difficult to handle in an industrial process of producing casting nozzles, it is preferred that only a suitable CaO precursor is being used.

The fired refractory nozzle 1 is preferably submerged into the refractory slurry and the system is evacuated to a pressure below atmospheric (less than 1 atm) for at least about 15 minutes. This vacuum treatment has the effect of removing entrapped oxygen from the pores of the oxidized refractory in the zone. The system is then re-pressurized, which forces the slurry into the evacuated pores of the substantially carbon free zone. The submerged entry nozzle 1 is then removed from the slurry and the organic/water is driven off through a low temperature cure such that the organic/water from the slurry volatizes. Since the nozzles are generally preheated prior to use, this curing step may be eliminated. As stated above, the vacuum/pressure infiltration technique is not necessary since the slurry will infiltrate the porous, oxidized zone without external pressurization, however, such external treatment is preferred since it reduces the time required to achieve proper infiltration.

The refractory body with the infiltrated refractory slurry is heated to a temperature of more than 1350°C such that the AI ₂ O ₃ and the CaO react with each other forming a low viscous molten calcium-aluminate penetrating and filling the pores present in the refractory material and thereby forming via melt assisted sintering a solid layer being gas impermeable layer. The heating is preferably in a controlled atmosphere to avoid carbon burn off in the other parts of the refractory body.

The invention relates also 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 steelmaking process, and preferably wherein molten steel is transferred or flows from a tundish into at least one casting mould.

In a further aspect of the invention it relates to refractory cast ware bodies other than a submerged entry nozzle and selected from the group comprising: a ladle slidegate plate, a ladle shroud, a stopper rod, a slagline sleeve, a pouring spout, and each being made of a carbon- bonded refractory material as herein described and claimed and wherein at least the refractory surface which in use comes into contact with molten steel is provided with a solid melt assisted sintering layer comprising calcium-aluminate forming a gas impermeable layer. Said solid gas impermeable layer is obtainable by a method as herein described and claimed.

DETAILED DESCRIPTION OF THE FIGURE

The invention will now be explained by means of the following, non-limiting figure. Fig. 1 shows schematically a one piece submerged entry nozzle 1 through which molten steel can be poured from a tundish into a mould, said nozzle comprising: a substantially tubular body 2 made from a carbon-bonded refractory material, extending from a first end 3 to a second end 4; an inner nozzle wall 9 surrounding a passageway 5 in use coming into contact with molten steel, extending through said tubular body 2 along a longitudinal axis A for an axial length L from said first end 3 towards said second end 4; and one or more outlet ports 8 or outlet openings, opening into said passageway 5 in a region 7 adjacent to said second end 4,

EXAMPLE

A suitable example of a carbon-bonded refractory material for producing a SEN in accordance with this invention has a composition of (in dry weight percent) about 77 wt.% AI ₂ O ₃ , 15 wt.% C, and up to 8 wt.% of components (other metal oxides and others) like TiO ₂ , Na ₂ O, SiO ₂ , ZrO2, and SiC. The bulk porosity is about 15%.

When this carbon-bonded refractory material is locally oxidized to create a carbon-free zone suitable for infiltration with CaO and/or CaO precursor, the porosity increases to about 40%. The amount of AI ₂ O ₃ is about 90 wt.% and total other components is about 10 wt.%.

The porosity can be infiltrated with a calcium aluminate cement comprising about 50 wt.% CaO and about 50 wt.% AI ₂ O ₃ . After firing to remove the carrier of the calcium aluminate cement the composition of this zone is about 78 wt.% AI ₂ O ₃ (the sum of the AI ₂ O ₃ in the refractory and from the cement), about 15 wt.% CaO and about 7 wt.% others.

Following heating to above 1450°C and assuming that all of the CaO is being consumed in the reaction with AI ₂ O ₃ , via liquid melt assisted sintering a dense pore filling layer is being created comprising of a favourable mixture of 46 wt.% CA6 and 54 wt.% CA2 (thereby neglecting the other components) having a melting point of about 1760°C, and is thermally stable such that under service conditions of a steel casting operation the gas impermeable melt assisted sintering layer remains fully functional.

However, not all the alumina from the refractory framework reacts with the CaO, such that there will be some residual alumina and thereby increasing the amount of CA2 compared to CA6.

It is preferred to keep the amount of SiO ₂ in the carbon-bonded refractory material low as the SiO ₂ -AI ₂ O ₃ system has a very high viscosity and adversely affecting the pore filling capacity of the CaO and/or CaO precursor. For that reason it is preferred to maintain the SiO ₂ in the bulk refractory material at a level of maximum 5 wt.%, and preferably at a maximum of 3 wt.%. The above-discussion is intended to be merely illustrative of the present submerged entry nozzle and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Accordingly, the specification and drawing are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawing, the disclosure, and the appended claims. The mere fact that certain measures are recited in different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the appended claims.