EP0242347A2 - Apparatus for metal slurry casting - Google Patents

Abstract

Apparatus which comprises, between a casting vessel (1) equipped with a spout (3) and a continuous-casting ingot mould (11), a cooled conduit (6) the upstream end of which relative to the direction of flow of the metal, is connected to the outlet of the spout (3) and the downstream end of which is provided with means (8) for introducing the liquid metal into the ingot mould (11) in which the introduction means (8) comprise a pipe whose lower end ends in a nozzle the diameter of the outlet cross-section of which is smaller than the diameter of the spout (3). The conduit (6) may consist of a tubular pipe having an internal cross-section greater than the outlet cross-section of the said spout, or of a duct provided with a lining in refractory material and a gastight roof equipped, over at least part of its length, with air cooling means. <IMAGE>

Description

The present invention relates to a device for the casting of a metal, in particular steel, in the pasty phase in a continuous casting ingot mold.

In the description which follows, particular reference will be made to steel. This is obviously only an example intended to illustrate the invention, because it is also applicable to any liquid metal meeting the conditions set out in this patent application.

The continuous casting of steel essentially consists in supplying steel, continuously, an ingot mold of suitable shape intended to form an ingot called slab, bloom or billet, according to the dimensions which it presents. The steel begins to solidify on contact with the cooled walls of the ingot mold, and it is extracted from the bottom of the latter in the form of an ingot the skin of which is solidified to a certain thickness and the core of which is still liquid. solidifies later gradually.

The steel which is thus continuously cast comes from a container such as a ladle or, more often, a distribution basket, which is provided, at its lower part, with a casting device generally called a "nozzle". of casting ". Usually, this device essentially consists of a refractory brick in which the tap hole is formed and which is equipped with an appropriate system for adjusting or interrupting the pouring jet.

The continuous casting described above has been known and implemented for a long time.

Also known are the specific defects to which it can give rise, such as various types of segregation, which are strongly influenced by the casting temperature of the steel.

Usually, the temperature of the steel in the ladle or in the distribution basket is quite clearly higher than its temperature at the start of solidification, that is to say its liquidus temperature. This temperature difference, called overheating, is generally of the order of 20 ° C to 40 ° C, and frequently close to 35 ° C.

This overheating of the steel in a pocket or in a distribution basket is subject to two contradictory conditions which are also well known to specialists. On the one hand, it is desirable to operate with as little overheating as possible, in order to reduce the risks of heterogeneities linked to segregation during solidification. On the other hand, too great a reduction in overheating leads to an increase in the quantity of steel lost in the form of a pocket bottom or a distributing basket, as well as a reduction in the settling speed of the inclusions.

We also know the advantages of casting steel in the pasty phase. This technique makes it possible to obtain fine and homogeneous structures which clearly reduce the segregations, and in particular the axial segregation in the cast products. It also increases the casting speed by reducing the risk of breakthroughs.

By the expression "casting in the pasty phase", it is to be understood, within the meaning of the present invention, that the steel contained in the ladle or the tundish is at its usual temperature of overheating, and that it is cooled during its passage from this pocket or from this basket to the mold. This cooling is such that upon entering the mold, the steel is at a temperature within its solidification interval, that is to say between its liquidus and solidus temperatures. As a result, steel contains at this mo ment a certain solid fraction consisting of solidified particles entrained by the rest of the steel, which is liquid, and that the steel is then generally in the pasty phase. This casting technique is described in particular in application EP-A-0089196.

The objective of the present invention is to propose a particular device making it possible to create conditions which lead to the appearance of a pasty phase in the metal, in particular in the steel, during its path between the pocket or the tundish on the one hand and the casting mold continues on the other.

The device for the casting of a metal in the pasty phase, which is the subject of the present invention, which comprises between a casting container provided with a nozzle and a continuous casting mold, a cooled channel whose upstream end , relative to the direction of flow of the metal, is connected to the outlet of said nozzle and the downstream end of which is provided with means for introducing the molten metal into said ingot mold, is essentially characterized in that said means for introducing comprise a conduit whose lower end ends in a nozzle whose diameter of the outlet section is less than the diameter of the nozzle. This arrangement makes it possible to create, at the outlet of this nozzle, a pressure drop which causes the vertical duct to be filled with liquid steel.

Correlatively, this vertical duct has a length sufficient to constitute a column of liquid steel applying at the outlet of the nozzle a ferrostatic pressure capable of ensuring at the nozzle a flow rate equal to the flow rate of the nozzle.

In addition, the geometric characteristics of said means for introducing the liquid steel into the ingot mold are such that the liquid steel does not entirely fill the interior space of the said channel.

In a manner known per se, said channel is inclined relative to the horizontal by an angle between 5 ° and 80 °, its outlet section being located at a level lower than that of its inlet section.

This angle of inclination must be at least 5 °, in order to prevent the liquid steel from freezing in the channel; however, it does not exceed 80 °, so as to allow the desired cooling of the steel.

Within the meaning of the present application, it should be understood that the channel inclined to the horizontal is preferably rectilinear and that the mold is horizontally spaced from the nozzle by a distance corresponding to the length of this channel. It would not, however, depart from the scope of the invention to give this channel any other configuration, for example an annular shape, allowing the ingot mold to be brought closer to the nozzle and possibly to bring the ingot mold under the nozzle, in alignment along a vertical axis. common. Such a configuration would have the additional advantage of reducing the transverse size of the device.

Also according to the invention, the casting device comprises means for introducing a protective gas inside said channel.

In particular, these means consist of at least one opening made in the vicinity of the downstream end of said channel; this opening is connected, by means known per se, to a source of protective gas such as argon.

The shielding gas introduced through said opening sweeps the part of the interior of the channel which is not occupied by the liquid steel. Liquid steel is thus protected from contact with ambient air as it travels through the channel and there is therefore practically no risk of oxidation.

In addition, the device of the invention advantageously comprises means for varying the pressure of said gas inside said channel, in order to regulate the flow of metal during casting.

According to a first variant of the invention, said channel is constituted by a tubular conduit having an inner section greater than the outlet section of said nozzle. The interior section of said channel is advantageously at least double the outlet section of the nozzle. This difference in section makes it possible to reduce the risks of clogging of the channel by solidified metal.

This tubular conduit can be a steel tube energetically cooled with water. It must then be replaced frequently to avoid the risk of explosion in the event of the tube breaking through and contact between the liquid steel and the cooling water. This tubular conduit can also be made of another material, resistant to '' erosion by liquid steel, such as a sintered material; in this case, the tubular conduit is more expensive than a steel tube, but its replacement will be less frequent.

According to a second variant of the invention, said channel has an open section and it is provided internally with a lining of refractory material.

According to an interesting implementation of this variant, the channel consists of a channel provided with a lining of refractory material and a gas-tight vault equipped with means for introducing and discharging a gaseous agent, such as a protective gas.

This vault is intended to prevent any contact of the liquid steel with the ambient air, and to delimit, above the liquid steel, a space in which a protective gas such as argon can circulate.

Also according to the invention, this vault is provided externally, over at least part of its length, with air cooling means, in particular cooling fins.

These fins and, more generally, these air cooling means are advantageously surrounded by an envelope in which is circulated, in particular between the fins, an air current with adjustable flow rate making it possible to vary the intensity of said cooling.

The cross section of the channel can have any shape without departing from the scope of the present invention.

In particular, it has been found to be advantageous for the channel to consist of two walls, preferably planar, intersecting along a longitudinal edge.

In this embodiment of the invention, the walls of the channel can advantageously form an angle between 45 ° and 135 °, and preferably close to 90 °. In the case where said walls are not planar, this angle would be that which the planes tangent to the walls would form along said longitudinal edge.

The channel is usually arranged so that its bisector plane is vertical; it can however be rotated about an axis parallel to the aforementioned longitudinal edge so as to vary the surface of liquid steel subject to cooling by radiation.

According to a particular embodiment of this variant, said channel has an additional longitudinal wall, substantially vertical, disposed inside the channel where it is connected to the bottom and where it can be moved transversely so as to vary the passage section of said channel. .

Thanks to this movable wall, it is possible either to vary the surface of liquid steel subject to radiation cooling or to cast the steel with different flow rates while retaining the same height of liquid steel in the channel.

According to another particularly interesting embodiment of this variant, at least one region of the bottom of said channel is made of a material having a high heat conductivity, said region of the bottom of the channel is subjected to intense cooling and at least part of said region is not lined with refractory material.

This region is advantageously located near the upstream end of said canal. It is preferably constituted by a plate of a material having a high heat conductivity, in particular copper, said plate being provided with water cooling means. the interruption of the refractory lining in said region normally leaves it without protection. It may nevertheless prove advantageous to deposit a thin film of a refractory material, in particular ZrO₂, in order to lower the working temperature of the copper while only reducing the efficiency and intensity of the cooling very slightly.

In the context of this particular embodiment, the part of said region which does not have a refractory lining forms a zone preferably extending over the entire width of the bottom of the channel.

It would not, however, depart from the scope of the present invention to limit the transverse extent of this zone to a fraction of the width of the bottom of the channel, or to split this zone into a plurality of partial zones distributed along the width of the bottom of the channel. and separated by walkways of refractory material.

The device of the invention may also include means for adjusting the inclination of the channel, making it possible to take into account factors such as the composition and the temperature of the metal to be cast in order to obtain the desired pasty phase.

According to yet another variant, the casting device according to the invention may include means for rotating said channel around its longitudinal axis.

The speed of rotation of the channel around the longitudinal axis can obviously vary depending on various factors such as the composition or the initial temperature of the metal to be cast. In the context of steel casting, this speed of rotation is preferably between 0.1 t / s and 5 t / s.

In this variant, the channel is preferably constituted by a tubular conduit having a section large enough not to be filled by the liquid metal which circulates there. The tubular conduit preferably has a circular section, in order to ensure as uniform lateral drive of the metal as possible during the rotational movement. This rotational movement increases the heat exchange surface between the tubular conduit and the liquid metal; it also causes agitation of the liquid metal, which increases the heat exchange coefficient inside the tubular conduit.

• figure 1 représente, en coupe, un dispositif conforme à l'invention comprenant un conduit tubulaire refroidi à l'eau; la
• figure 2 montre une vue générale d'un canal à section ouverte, con­forme à une variante de la présente invention; la
• figure 3 représente une coupe transversale d'un canal, suivant la fi­gure 2, pourvu d'une voûte de refroidissement par air; la
• figure 4 illustre une section d'un canal conforme à la variante de la figure 2; la
• figure 5 illustre une autre section d'un canal conforme à la variante de la figure 2; la
• figure 6 montre une vue schématique en élévation, en coupe longitu­dinale, d'un canal conforme à la variante de la figure 2, dans lequel une partie du fond est dépourvue de garnissage réfractaire; et la
• figure 7 présente divers modes de réalisation de la région refroidie du canal de la figure 6, chaque fois à l'aide d'une vue en plan de cette région et d'une coupe transversale à travers celle-ci, suivant les lignes A-A, B-B et C-C respectivement.

The object of the invention will be better understood by referring to the description below of particular embodiments illustrated by the accompanying drawings, in which the

• Figure 1 shows, in section, a device according to the invention comprising a tubular conduit cooled with water; the
• Figure 2 shows a general view of an open section channel, according to a variant of the present invention; the
• 3 shows a cross section of a channel, according to Figure 2, provided with an air cooling vault; the
• Figure 4 illustrates a section of a channel according to the variant of Figure 2; the
• Figure 5 illustrates another section of a channel according to the variant of Figure 2; the
• Figure 6 shows a schematic elevational view, in longitudinal section, of a channel according to the variant of Figure 2, wherein a portion of the bottom is devoid of refractory lining; and the
• FIG. 7 shows various embodiments of the cooled region of the channel of FIG. 6, each time using a plan view of this region and a cross section through it, along the lines AA, BB and CC respectively.

In all the figures, identical elements are designated by the same reference numerals. the directions of movement of the materials are indicated by arrows. Finally, elements which are not essential for understanding the invention have not been shown, so as not to overload the drawings.

In Figure 1, the ladle 1 containing the liquid steel 2 is provided with a nozzle 3 which has a diameter D₁. It is a nozzle of known type, made of refractory material, which is not part of the present invention. The nozzle 3 opens into an elbow 4 formed in a refractory brick 5, the nozzle fitting tightly to the inlet of this elbow 4. At the outlet of the elbow 4, the refractory brick 5 is connected to the upstream end of a channel 6, slightly inclined on the horizontal, equipped with a cooling circuit 7. The channel 6 can be made of steel; it must then be replaced after each pour; it can also be made of a sintered material based on (ZrO₂) and it can then be used for several casting operations. This channel 6 has a larger internal section than that of the nozzle 3, in order to reduce the risks of clogging of the channel 6 with solidified steel. The cooling agent, for example water or a water mist, preferably enters this circuit at the downstream end of the channel 6 and leaves it at the upstream end. At the downstream end of the channel 6 is adapted a vertical conduit 8, made of refractory material, which is connected to the channel 6 by means of an elbow 9. The lower end of the conduit 8 comprises a nozzle 10 whose internal diameter D₂ is less than the diameter D₁ of the nozzle. The vertical duct 8 opens into the continuous casting mold 11. The connections of the channel 6 on the one hand with the refractory brick 5 and on the other hand with the vertical duct 8 are, in a manner known per se, gas and liquid steel. In the vicinity of the downstream end of the channel 6, an opening 12 is provided which, by means known per se and not shown, places the interior space of the channel 6 in communication with a source of argon (not shown).

This device operates as follows: the liquid steel 2 flows from the ladle 1 through the nozzle 3 then successively, through the firebrick 5, the channel 6 and the conduit 8 to reach the ingot mold 11 To facilitate the flow of liquid steel, the channel 6 has a slight inclination, for example of the order of 5 °. In the example illustrated, the channel 6 has a length L = 800 mm and an internal diameter D = 60 mm, while the nozzle had a diameter D₁ = 15 mm. We determine the diameter D₂ which, associated with a given length of the vertical duct 8, will ensure the nozzle a steel flow equal to the flow of the nozzle while permanently maintaining in the channel 6 a steel layer having a constant thickness d, by example d = 30 mm. Conversely, if one sets a diameter D₂, less than D₁, one will determine the height H required to fulfill the aforementioned flow conditions. The intensity of the cooling provided by the circuit 7 will then be adjusted so that the liquid steel undergoes in this channel 6 the desired temperature drop from the temperature, with overheating, which it exhibits at the outlet of the nozzle 3.

The intensity of the cooling can be modified by varying the flow rate and / or the temperature of the cooling agent, generally the water, which flows through the circuit 7.

Stabilization of the steel level in the tubular conduit can be ensured by adjusting the argon pressure. It is thus possible to adjust the size of the heat exchange surface, whatever the pressure drops in the circuit.

Figure 2 shows a general view of a continuous casting installation comprising an open section channel. This installation consists of a ladle 1 fitted with a pouring nozzle 3, a casting device generally designated by 13 and a continuous casting mold 11. The ladle 1, the nozzle 3 and the mold 11 are conventional; they are not part of the invention and will not be described further here. The casting device 13, shown in section along line II of FIG. 3, is made up of a channel 6, inclined at an angle of about 18 ° relative to the horizontal, of a curved portion 4 connecting the nozzle 3 at the entrance to channel 6 and a submerged nozzle 14 opening into the mold 11. The inclined channel 6 is provided with a vault 15, gas-tight, which extends over its entire length. The vault 15 carries cooling fins 16 which are in turn surrounded by an envelope 17. The vault 15 is provided with an inlet and an outlet for protective gas, for example argon, marked respectively 18 and 19 , and envelope 17 has orifices 20, respectively 21, for the inlet and outlet of the cooling air, respectively. The orifices 18 and 19, respectively 20 and 21, are connected to a source of argon, respectively of air, not shown. The portion 4 is curved so as to be connected tangentially on the one hand to the nozzle 3 and on the other hand to the channel 6. This curved portion 4 consists of a metal carcass internally lined with refractory material. Its curvature is determined so as to avoid untimely rebounds of the steel in the channel, when it leaves the nozzle, so that the lining undergoes minimum erosion under the effect of the flow of liquid steel.

The nozzle 14 consists, in a manner known per se, of a portion of tube of refractory material intended to immerse in the liquid steel present in the ingot mold 11.

In FIG. 3 is shown a cross section, along line II-II of FIG. 2, of the casting device, showing the constitution of the channel 6 and of the arch 15.

The channel 6 consists of a U-shaped metal channel, comprising a bottom 22 and side walls 22ʹ and lined with refractory material 23. The roof 15 is fixed in a gas-tight manner to the metal channel 22; it carries cooling fins 16 surrounded by a closed envelope 17.

Figure 4 shows a variant of the device of the invention, in which the cross section of the channel 6 consists of two walls 24, 25 intersecting at an angle of about 90 °. This channel can be rotated around its longitudinal edge represented by the point 0, which causes a variation in the width and consequently in the area of the upper surface of the liquid steel (FIG. 4, a and b).

Another variant of the invention is illustrated in FIG. 5. Here, a vertical longitudinal wall 26 can take several positions by transverse displacement inside the channel 6. This movable wall makes it possible to vary, according to the conditions of the casting. , the height h of steel in the channel and / or the surface area of the upper surface of the liquid steel.

The mode of operation of this device is easily understood from the description which has just been made.

By way of example, it will be indicated that with a particularly simple device of the type shown in FIGS. 2 and 3, it was possible to eliminate an overheating of 25 C during the casting of steel under a flow rate of 5 kg / s, with a channel having a length L = 3 m and a width l = 0.075 m.

The device of the invention therefore makes it possible to flow the steel with little or no overheating without incurring unacceptable losses of metal by solidification in the pocket or in the distribution basket.

FIG. 6 schematically illustrates a particularly advantageous variant of the device of the invention, mounted between a casting ladle 1 containing superheated metal and a continuous casting ingot mold 11. This device comprises a channel, generally designated by the reference numeral 13, which consists of a metal bottom 22 and side walls 22ʹ also metallic. The metal used for this purpose is preferably steel. The bottom and the walls of the channel are lined with a refractory lining 23, 23ʹ. In order not to overload the drawing, we have not shown certain elements already described, such as the cooling vault 15 and the protective enclosure 18 (see FIG. 2).

In the bottom 22 of the channel is inserted a plate 27 of a material having a high heat conductivity, in particular copper, which is subjected to intense cooling. This refroi This is preferably provided by a water circulation circuit, symbolized by the arrows 28 for entering and leaving 29 for the water.

The refractory lining 23, 23ʹ is interrupted on a part of the upper surface of the plate 27, which is thus exposed directly to contact with the liquid metal. As shown in FIG. 7, this uncovered part of the plate 27 can be (a) an area extending over the entire width of the channel, (b) an area extending over only a fraction of the width, or (c) a plurality of partial zones distributed along the width of the channel and separated by walkways of refractory material.

The mode of operation of this device is easily understood by referring to FIG. 6.

The superheated metal, in particular steel, coming from the ladle 1, reaches the upstream portion of the channel 13 where it flows over the refractory lining 23ʹ without undergoing significant cooling. It then arrives on the uncoated part of the copper plate 27, where it undergoes intense cooling for a limited time due to the relatively small dimensions of the contact surface. The tests show that, on such a very localized contact surface, the steel instantly loses its overheating, which causes the appearance of solidified particles within the stream of liquid steel. This carries away the solidified particles, which therefore cannot be deposited on the plate 27. During its flow over the refractory lining 23, the steel continues to be cooled and becomes increasingly rich in solidified particles. It thus acquires the pasty state sought upon entry into the mold 11.

This particular device makes it possible to replace the conventional unidirectional flow of heat, that is to say the flow along the thickness of the copper plate, by a bi- or even three-way flow which is clearly more efficient.

When the entire upper surface of the cooled plate is brought into contact with a liquid metal, the heat can only flow according to the thickness of the plate. On the other hand, when only a part of this surface is exposed to liquid metal and the rest of the surface is protected, in particular by a refractory lining, the flow of heat in the plate takes place at once according to the thickness and parallel to the plane of the surface. This results in an increase in the cross-section offered to the heat flow, and therefore an improvement in the cooling efficiency.

It goes without saying that the invention is not limited to the embodiments which have just been described and illustrated.

In particular, it would not be departing from the scope of the present invention to choose the dimensions of the device in such a way that an argon pressure below atmospheric pressure is used in regime, which could allow degassing of the steel. liquid if necessary.

It can be seen that the device of the invention offers greater driving flexibility than conventional nozzles, because it allows regulation of the steel flow rate and completely independent cooling regulation.

In addition, system security is increased in two ways. On the one hand, the quantity of steel which risks being brought into contact with water in the event of a system failure is greatly reduced, since such a failure can only occur in channel 6. Furthermore, it is it is possible to completely eliminate the use of water and go beyond air cooling, by adapting the length and inclination of the channel as appropriate.

Claims (6)

1. Device for the casting of a metal in the pasty phase, which comprises between a casting container (1) provided with a nozzle (3) and a continuous casting mold (11), a cooled channel (6), of which the upstream end, relative to the direction of flow of the metal, is connected to the outlet of said nozzle and the downstream end of which is provided with means (8) for introducing the molten metal into said ingot mold, characterized in that said introduction means comprise a conduit whose lower end ends in a nozzle whose diameter of the outlet section is less than the diameter of the nozzle. 2. Device according to claim 1, characterized in that said channel is constituted by a tubular conduit having an inner section greater than the outlet section of said nozzle. 3. Device according to either of claims 1 and 2, characterized in that said channel consists of a channel provided with a lining of refractory material and a gas-tight vault equipped, on at least part of its length, air cooling means, in particular cooling fins. 4. Device according to either of claims 1 to 3, characterized in that at least one region of the bottom of said channel is made of a material having a high heat conductivity, in that said region of the bottom of channel is subjected to intense cooling and in that at least part of said region is not lined with refractory material. 5. Device according to either of claims 1 to 4, characterized in that it comprises means for the introduction of a gas into the free space situated above the surface of the metal in said channel and means for varying the pressure of said gas inside said channel in order to regulate the flow of metal during casting. 6. Device according to either of claims 1 to 5, characterized in that it comprises means for rotating said channel around an axis parallel to its longitudinal axis, respectively around its longitudinal axis.

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