EP0589762A1 - Casting tube for metal and process for manufacturing such a tube - Google Patents

Abstract

Casting tube for metal, including a body (22) made from refractory material, in which a flow channel (24) for the molten metal is provided. The tube includes an annular chamber (32) provided in the body (22), arranged around the channel (24) in the vicinity of the periphery of this channel, extending virtually over the entire length of the channel, connected to means (34) for creating a reduced pressure (vacuum) with respect to the environment of the tube. The chamber (32) forms a screen to the migration of gaseous products, such as carbon monoxide, towards the channel. The tube may also include a sleeve (30) made from refractory material interposed between the periphery of the channel (24) and the chamber (32). The tube is used preferably for the continuous casting of steel. <IMAGE>

Description

The present invention relates to a metal casting nozzle and methods of manufacturing this nozzle.

It applies in particular to the continuous casting nozzles of steel, in particular of steel calmed with aluminum.

Conventionally, a continuous steel casting installation comprises at least one ingot mold fed by a distributor of molten steel.

The molten steel flows into the ingot mold through the channel of at least one nozzle fixed to the bottom of the distributor, in line with the upper opening of the ingot mold.

To resist thermal stresses in contact with molten metal, the nozzles are usually made of a refractory material. The nozzles are also covered with layers of internal and external enamel to prevent oxidation of the refractory material during the usual preheating of the nozzles and also to make the porous refractory material impermeable during the use of the nozzles.

Depending on the grade of the cast steel or the type of material constituting the nozzles, there are problems of degradation of the nozzles by corrosion, or problems of clogging of the nozzles with deposits of oxides on the surface of the flow channel. steel. These oxide deposits include, for example, alumina, lime aluminate or alumina titanate.

It is known to manufacture nozzles in a refractory material of vitreous silica (SiO₂). The nozzles made from this material are resistant to thermal shock and clogging by oxide deposits, but have little resistance to corrosion by certain grades of steel containing, for example, manganese.

It is also known to manufacture nozzles in a composite material mainly comprising alumina (Al₂O₃) and graphite associated by a carbon bond. These nozzles usually comprise between 20 and 30% by mass of graphite. The high graphite content promotes the resistance of these nozzles to thermal shock. These also resist wear well. On the other hand, the alumina-graphite nozzles clog relatively quickly.

The closure of the nozzles is particularly rapid when the cast steel is calmed with aluminum. In this case, oxide deposits essentially containing alumina are formed in the nozzle channel. This limits the sequence lengths and can lead to stopping the casting in the event of complete clogging of the nozzles.

Furthermore, a certain amount of the deposited oxides is likely to be entrained by the cast steel and therefore to form inclusions degrading the metallurgical properties of the steel.

In order to understand the blocking phenomenon, three tests were carried out.

On the one hand, an alumina-carbon rod was immersed for two hours in a bath of XC 38 steel at 1550 ° C. to which 0.25% aluminum was added, in an enclosure under controlled atmosphere.

After two hours, there is the presence of an alumina deposit on the walls of the bar.

On the other hand, a bar of pure, carbon-free alumina was immersed in the same bath for two hours.

After two hours, no deposit is on the bar.

Finally, an alumina-carbon rod coated with a thick layer of pure alumina was immersed in the steel bath for two hours.

After two hours, there is a deposit of alumina on the bar.

Taking into account these tests, the idea accepted until then that the plugging of the nozzles was due solely to the alumina contained as inclusions in the cast steel which sticks to the refractory wall of the nozzle, is false.

Indeed, if this were the case, the pure alumina bar should have a deposit after two hours like the alumina-carbon bar coated with a thick layer of pure alumina.

• l'alumine contenue en tant qu'inclusions dans l'acier se collant sur les parois réfractaires de la busette,
• l'aluminium contenu dans l'acier qui lorsqu'il entre en contact avec les parois de la busette s'oxyde pour former de l'alumine Al₂O₃ et germer contre la paroi.

In fact, the clogging of the nozzles is due to two phenomena:

• the alumina contained as inclusions in the steel which sticks to the refractory walls of the nozzle,
• the aluminum contained in the steel which when it comes into contact with the walls of the nozzle oxidizes to form Al formerO₃ alumina and germinate against the wall.

Indeed, by immersing a sample consisting of an alumina-graphite bar in a bath of molten steel containing elements eager for oxygen such as aluminum, the chemical equilibria described below are observed.

The alumina-graphite sample contains impurities in the form of oxides (SiO₂, Na₂O, K₂O, ...) which are reduced by the carbon C of the sample to form carbon monoxide CO gas.

CO is released on the surface of the sample and dissociates into intermediate elements [C] and [O].

This dissociation on the surface of the sample locally disturbs the balance between the elements [C] and [O] contained in the steel bath. Oxidation of the oxygen-hungry elements of the bath is then oxidized by the oxygen released. and therefore precipitation and growth of oxides on the surface of the sample.

• dans une région éloignée de la surface de l'échantillon, les équilibres chimiques stables suivants:
  
• dans une région voisine de la surface de l'échantillon, une évolution des équilibres chimiques dans le sens suivants : $$\text{CO (gazeux) --> [C] + [O]}$$
  
  $$\text{2[Al] + 3[O] --> Al₂O₃ (précipité)}$$
  

In the case of a bath of steel calmed with aluminum, we have in particular:

• in a region distant from the sample surface, the following stable chemical equilibria:
  
• in a region close to the surface of the sample, changes in the chemical equilibria in the following directions: $$\text{CO (gas) -> [C] + [O]}$$
  
  $$\text{2 [Al] + 3 [O] -> Al₂O₃ (precipitated)}$$
  

In the case of a nozzle manufactured with the material Al₂O₃-C, the carbon monoxide dissociates on the surface of the steel flow channel and causes on this surface the precipitation of oxides, in particular of alumina, produced from elements contained in cast steel, in particular aluminum. Oxide deposits gradually block the nozzle channel.

In addition, carbon monoxide is formed not only from oxygen from impurities in the form of oxides contained in the nozzle, but also from oxygen in the air surrounding the nozzle. The oxygen in the air infiltrates through the junction of the nozzle and the distributor and through the walls of the nozzle made of porous refractory material. The oxygen in the air combines with the carbon in the nozzle to form gaseous carbon monoxide which migrates to the surface of the channel. This migration is favored by the depression created in the channel by the flow of liquid steel.

The object of the invention is to remedy the blockage of the nozzles made of refractory material comprising in particular alumina-carbon, avoiding the formation of oxide deposits on the surface of the molten metal flow channel, this with simple and easy to implement means.

To this end, the subject of the invention is a metal casting nozzle, in particular of continuous steel casting, comprising a body of refractory material in which is formed a liquid metal flow channel, characterized in that it comprises an annular chamber formed in the body, disposed around the channel near the periphery of this channel, extending approximately over the entire length of the channel, connected to means for placing under vacuum in relation to the environment of the nozzle or insufflation of a neutral gas, so that the chamber forms a screen for the migration of gaseous products such as carbon monoxide towards the channel, and in that it further comprises a jacket of refractory material without carbon, interposed between the periphery of the channel and the annular chamber.

• la chemise est fabriquée dans un matériau réfractaire sans carbone comprenant par exemple de l'alumine, de la zircone, des nitrures d'aluminium (AlN) ou de bore (BN), des spinelles, de la magnésie, des borures notamment de zirconium (ZrB₂) ;
• le corps de busette est fabriqué dans un même matériau réfractaire que la chemise ;
• le corps est fabriqué dans un matériau composite comprenant de l'alumine et du graphite.

According to other characteristics of the invention:

• the jacket is made of a carbon-free refractory material comprising, for example, alumina, zirconia, aluminum nitrides (AlN) or boron (BN), spinels, magnesia, borides, in particular zirconium ( ZrB₂);
• the nozzle body is made of the same refractory material as the jacket;
• the body is made of a composite material comprising alumina and graphite.

The invention also relates to methods of manufacturing a nozzle as defined above.

• on réalise la busette par pressage isostatique d'un matériau réfractaire dans lequel est incorporé un élément fusible occupant le volume d'une chambre annulaire que l'on veut ménager autour d'un canal d'écoulement du métal en fusion,
• on cuit l'ensemble de la busette à une température élevée faisant fondre l'élément fusible de façon à libérer le volume formant la chambre annulaire.

According to a first manufacturing process:

• the nozzle is produced by isostatic pressing of a refractory material in which a fusible element occupying the volume of an annular chamber which it is desired to provide around a flow channel for the molten metal,
• the entire nozzle is cooked at a high temperature melting the fusible element so as to release the volume forming the annular chamber.

• on fabrique séparément un insert tubulaire et un corps tubulaire de busette, par compression isostatique et cuisson de matériaux réfractaires, l'insert étant destiné à délimiter un canal d'écoulement du métal en fusion;
• on fixe l'insert dans le corps coaxialement à ce dernier de façon que l'espace libre entre l'insert et le corps forme une chambre annulaire autour du canal.

According to a second manufacturing process:

• a tubular insert and a tubular nozzle body are produced separately, by isostatic compression and curing of refractory materials, the insert being intended to delimit a flow channel for the molten metal;
• the insert is fixed in the body coaxially to the latter so that the free space between the insert and the body forms an annular chamber around the channel.

• la Fig.1 est une vue schématique d'une partie d'une installation de coulée continue d'acier comportant une busette selon l'invention;
• la Fig.2 est une vue en coupe longitudinale, à grande échelle, de la busette selon l'invention;
• les Fig.3 à 5 sont des vues en coupe longitudinale représentant des éléments utilisés pour la fabrication de busettes selon l'invention.

The invention will be better understood with the aid of the description which follows, given solely by way of example and made with reference to the appended drawings, in which:

• Fig.1 is a schematic view of part of a continuous steel casting installation comprising a nozzle according to the invention;
• Fig.2 is a longitudinal sectional view, on a large scale, of the nozzle according to the invention;
• Figs. 3 to 5 are views in longitudinal section showing elements used for the manufacture of nozzles according to the invention.

FIG. 1 shows a part of a continuous steel casting installation designated by the general reference 10, intended for example for the manufacture of metallic semi-finished products such as blooms or slabs.

Conventionally, the installation comprises a distributor 12 of molten steel supplying an ingot mold 14 comprising an upper mold 16 and rollers 18 drive and guide the semi-finished product during solidification.

The distributor 12 is itself supplied with molten steel by a ladle not shown in the figure.

The molten steel flows by gravity into the ingot mold 14, passing through at least one nozzle 20 according to the invention, fixed, by known means, to the bottom of the distributor 12 in line with the upper opening of the ingot mold 14.

The nozzle 20 will now be described in more detail with reference to FIG. 2.

The nozzle 20 comprises a nozzle body 22, of generally tubular shape, made of a refractory material, preferably a composite material mainly comprising alumina (Al₂O₃) and graphite (C).

The body 22 includes a channel 24 for the flow of molten steel. The channel has an upper end 26 intended to be connected, by known means, to a steel flow orifice of the distributor, and a lower end 28 forming two branches 28A, 28B opening out to the outside of the nozzle by diametrically opposite orifices.

The periphery of the channel 24 is delimited over almost the entire length of the latter by the internal surface of a cylindrical jacket 30, made of refractory material without carbon, fixed in the body 22 of the nozzle by means and methods which will be described later.

An annular chamber 32, coaxial with the channel 24, surrounds the external surface of the jacket 30, extending almost over the entire length of the latter. The wall of the chamber 32 has a small thickness compared to the thickness of the wall of the nozzle body 22.

The end branches 28A, 28B of the channel are also surrounded by corresponding parts of the jacket 30 and of the chamber 32.

The chamber 32 is arranged in the vicinity of the periphery of the channel 24, being separated from this channel by the wall of the jacket 30 interposed between the periphery of the channel and the chamber.

Conventionally, the internal and external surfaces of the nozzle are covered with enamel layers, not shown in the figures, so as to preserve the nozzle against oxidation and to waterproof the porous surfaces of the refractory materials.

The chamber 32 is connected via an orifice 34 to a vacuum source of known type, not shown in the figures, for the depression of the chamber with respect to the environment of the nozzle or a source of insufflation of a neutral gas, for example argon.

Also shown in Fig.2, a ring 36, conventionally arranged around the nozzle body 22, consisting of a refractory product, generally of graphite zirconia, intended to resist corrosion by the powder of the mold.

In the example described, the nozzle 20 has an external diameter of 100 mm, the channel 24 has a diameter of 70 mm, the wall of the jacket 30 has a thickness of 4 mm and the wall of the annular chamber 32 has a thickness 2 mm.

The chamber 32 makes it possible, on the one hand, to collect the gaseous products, in particular carbon monoxide, which forms in the refractory material of the nozzle body 22 and migrate towards the channel 24, and, on the other hand, evacuate these gaseous products through orifice 34.

According to a first embodiment of the invention, the jacket 30 is made of a carbon-free refractory material comprising, for example, alumina, zirconia, aluminum nitrides (AlN) or boron (BN) , spinels, magnesia, borides, in particular zirconium (ZrB₂).

Thus, in the absence of carbon, there is no formation of carbon monoxide in the wall of the jacket 30 separating the chamber 32 from the channel 24.

According to a second embodiment of the invention, the body 22 is made of a refractory material, identical to that of the jacket 30 of the nozzle, comprising carbon-free alumina. In this case, the jacket 30 may have come integrally with the body 22 and form a single block with the latter.

We will now describe two methods of manufacturing a nozzle according to the invention, with reference to FIGS. 3 to 5.

The first manufacturing process makes it possible to produce a nozzle in which the jacket 30 is integrally formed with the body 22 and forms a single block with the latter. This process includes the following steps.

Firstly, the nozzle is produced by isostatic pressing of a refractory material in which is incorporated a fusible or volatile element 32A occupying the volume of the annular chamber 32 which one wishes to provide in the nozzle (see Fig. 3) .

The fuse element 32A is manufactured for example from a polymeric material or from wax.

The whole is then baked at a high temperature of about 1000 ° causing the polymer to melt. The molten polymer vaporizes through the wall of porous refractory material and releases the volume it initially occupied. This volume forms an annular chamber around the channel of the nozzle.

A nozzle 20 is then obtained as shown substantially in FIG. 2, the jacket 30 having come integrally with the body 22.

The second method of manufacturing the nozzle comprises the following steps.

A tubular insert 30 and a tubular body 22 of nozzle, as shown in FIGS. 4 and 5 respectively, are manufactured separately by isostatic pressing and curing of refractory materials, according to a known method.

The insert 30 is then fixed in the body 22 coaxially with the latter, for example with a cement of known type, so that the free space between the insert 30 and the nozzle body 22 forms an annular chamber around the channel. of the nozzle.

As can be seen in FIG. 4, the insert and manufactured in three parts, a first part 30A, coaxial with the body being introduced by the upper end of the body and the other two parts 30B, 30C, forming the branches of the insert, being introduced through the lower orifices of the body.

The invention is not limited to the embodiments described.

The nozzle body and the inner liner can be made from a variety of refractory materials.

The nozzle can be used in continuous or discontinuous casting installations and can supply molten molds of various types with molten metal.

The invention has many advantages.

The plugging of a conventional nozzle being done by oxidation of the carbon contained in the refractory material, the annular vacuum chamber of a nozzle according to the invention makes it possible to avoid the migration of carbon monoxide gas in the channel of the nozzle.

By manufacturing the inner jacket of the nozzle in a carbon-free refractory material, the formation of carbon monoxide gas is avoided at the periphery of the channel and, in the case of a continuous casting of very low carbon steel, prevents an unwanted transfer of carbon into the steel passing through the nozzle channel.

Claims (6)

1. Metal pouring nozzle, in particular of continuous steel casting, comprising a body (22) of refractory material in which is formed a liquid metal flow channel (24), characterized in that it comprises a chamber annular (32) formed in the body (22), disposed around the channel (24) close to the periphery of this channel, extending approximately over the entire length of the channel (24), connected to means (34 ) vacuuming the environment of the nozzle or insufflating a neutral gas, so that the chamber (32) forms a screen for the migration of gaseous products such as carbon monoxide towards the channel ( 24), and in that it further comprises a jacket (30), made of a carbon-free refractory material, interposed between the periphery of the channel (24) and the annular chamber (32). 2.- nozzle according to claim 1, characterized in that the jacket (30) is made of a carbon-free refractory material comprising for example alumina, zirconia, aluminum nitrides (AlN) or boron ( BN), spinels, magnesia, borides, in particular zirconium (ZrB₂). 3. Nozzle according to claim 1 or 2, characterized in that the nozzle body (22) is made of the same refractory material as the jacket (30). 4.- nozzle according to claim 1 or 2, characterized in that the body (22) is made of a composite material comprising alumina and graphite. 5. Method for manufacturing a nozzle according to any one of claims 1 to 4, characterized in that: - the nozzle is produced by isostatic pressing of a refractory material in which a fusible element (32A) is incorporated occupying the volume of a chamber annular (32) which it is desired to provide around a flow channel for the molten metal, - The entire nozzle (22) is cooked at a high temperature melting the fusible element so as to release a volume forming the annular chamber (32). 6. A method of manufacturing a nozzle according to any one of claims 1 to 4, characterized in that: - A tubular insert (30) and a tubular body (22) of a nozzle are made separately, by isostatic compression and baking of refractory materials, the insert (30) being intended to delimit a channel (24) for the flow of metal in fusion; - the insert (30) is fixed in the body (22) coaxially with the latter so that the free space between the insert (30) and the body (22) forms an annular chamber (32) around the channel ( 24).

Images