EP1239981B1 - Method for vertical continuous casting of metals using electromagnetic fields and casting installation therefor - Google Patents

Abstract

The invention concerns a method which consists in: simultaneously subjecting the meniscus of the molten metal present in the ingot mould to the action of an axial alternating electromagnetic field tending to provide it with a general dome-like shape and to the action of a transverse direct electromagnetic field designed to attenuate the surface agitation of the meniscus. The implementing installation comprises an ingot mould (1) with cooled assembled plates (2, 3 and 4, 5) for casting metal slabs, an alternating current coil (17) enclosing the ingot mould at the meniscus (12) of the molten metal to produce an axial magnetic field, collinear with the casting axis (11), and a direct magnetic field winding passing through the large plates of the ingot mould at the meniscus (12) perpendicular to the casting axis.

Description

The invention relates to the continuous casting of metals. Specifically, it concerns the electromagnetic devices installed in the casting molds continuous acting on the liquid metal present in said ingot molds.

The use of electromagnetic fields to influence the movements of liquid steel in continuous casting ingots of any size is today classic. The imposition of rotating electromagnetic fields (case of pouring blooms and billets of square or slightly rectangular section) or sliding (case of pouring slabs of rectangular section whose width is much greater than the thickness) a main objectives of homogenizing solidification structures across the whole the section of the product, and to improve the surface condition of the product, as well as its cleanliness inclusionary in particular in the vicinity of its surface. In continuous slab casting, it is also known to impose static electromagnetic fields in the mold to obtain a stabilization of the meniscus (free surface of the molten metal at the top of the mold). This stabilization makes it possible to increase the speed of casting of the products, therefore the productivity of the casting machine continues. Electromagnetic devices to achieve this effect are known as "electromagnetic brakes".

• l'amélioration de la qualité de surface des produits bruts de coulée, qui passe par la réduction du nombre de criques superficielles et de la profondeur des rides d'oscillation ;
• l'amélioration de la propreté inclusionnaire sous-cutanée du produit coulé, qui passe par une réduction de la taille des "cornes de solidification" qui se forment au cours des oscillations de la lingotière, ces cornes étant des sites potentiels de piégeage des inclusions et des bulles de gaz présentes dans le métal liquide dans la lingotière, et aussi par une suppression du captage des inclusions par le front de solidification, en profitant de l'effet de "lavage" de ce front par le métal liquide entraíné par le brassage électromagnétique (les mécanismes relatifs à ces problèmes seront décrits en détail plus loin) ;
• l'obtention d'une stabilité suffisante du ménisque pour garantir une lubrification optimale de l'interface lingotière-métal solide par le laitier de couverture qui s'y infiltre à l'état liquide, pour que cette lubrification améliorée donne accès à des vitesses de coulée significativement supérieures aux vitesses usuelles.

The known uses of electromagnetic fields in continuous casting molds have not, for the moment, been sufficient to solve in a completely satisfactory manner all the quality problems of the cast products. Among these persistent problems we can cite:

• improving the surface quality of the raw casting products, which involves reducing the number of surface cracks and the depth of oscillation wrinkles;
• improving the subcutaneous inclusion cleanliness of the cast product, which involves reducing the size of the "solidification horns" which form during the oscillations of the mold, these horns being potential sites for trapping inclusions and gas bubbles present in the liquid metal in the ingot mold, and also by eliminating the capture of inclusions by the solidification front, taking advantage of the effect of "washing" of this front by the liquid metal entrained by electromagnetic stirring (the mechanisms relating to these problems will be described in detail below);
• obtaining sufficient meniscus stability to guarantee optimal lubrication of the ingot mold-solid metal interface by the cover slag which infiltrates therein in the liquid state, so that this improved lubrication gives access to casting significantly higher than usual speeds.

Satisfactory resolution of these problems would lead to an increase in the productivity of the casting machine and of the whole steelworks. In addition to the increase in the casting speed already mentioned, it would decrease the frequency of shredding operations (grinding the product surface to remove defects) and would therefore increase the proportion of products of sufficient quality to be shipped directly to hot rolling. However, no currently known technique allows to achieve all of the above qualitative objectives simultaneously. Moreover, the techniques known to make it possible to satisfy one or the other of these objectives are either expensive, or require a delicate development because they are very sensitive to others casting conditions. Among them, in addition to the aforementioned methods implementing magnetic fields, we can cite the systems printing in the ingot mold non-sinusoidal oscillations, the embossed molds with roughness of controlled hot face, cover slags with optimized composition, etc.

The object of the invention is to propose a method and an installation for casting continuous metals to meet productivity and quality objectives expected by users of continuous metal casting machines, especially steel.

With these objectives in view, the invention relates to a continuous casting process vertical of metal products in a mold with cooled assembled plates as defined in appended claims 1 and 2.

The invention also relates to an installation for vertical continuous casting of metals comprising an ingot mold with cooled assembled flat plates, two of which large facing each other to define a casting space as defined in claim 3 and attached appended.

As will be understood, the invention consists in creating in the liquid metal present at the interior of the casting mold continues at least two electromagnetic fields acting simultaneously on said metal in the meniscus area. One of these fields is an axial alternating field, the other is a transverse continuous field, both being exerted at meniscus level. They are produced using inductors installed or producing their effect in the vicinity of the meniscus.

Schematically speaking, the alternating field collinear with the casting axis is used to "dominate" the meniscus, that is to say to show the domed domed shape it takes slightly naturally in contact with the wall of the mold, while the continuous field transverse acts as an electromagnetic brake to reduce irregularities local geometries on the surface of this meniscus resulting from convection movements underlying generated by this alternative field.

• une composante surfacique de confinement qui tend à repousser la périphérie du ménisque loin de la paroi de la lingotière, donc à le "creuser" en bordure en le lissant en surface. Cette force est surtout active à haute fréquence.
• et une composante volumique de brassage qui, en raison de la configuration des mouvements de convection du métal liquide qu'elle procure (brassage en anneau avec remontée du métal au centre de la lingotière) "enfle" la partie centrale du ménisque. Cette force est en revanche surtout active à basse ou moyenne fréquence. C'est d'ailleurs pour cette raison qu'elle est à l'origine d'instabilités de surface. L'effet maximum de cette force de brassage est obtenu à moyenne fréquence, à savoir autour de 200 Hz pour fixer les idées, mais en tous cas inférieure à 500 Hz, quelque soit la nature ou l'épaisseur de la lingotière ou le format du produit métallurgique coulé.

Theoretically, the application of a single alternating magnetic field could be sufficient on its own to obtain a rounded and smooth meniscus. Indeed, the electromagnetic force generated on the liquid metal has, at the same time,

• a surface confinement component which tends to push the periphery of the meniscus away from the wall of the mold, therefore to "dig" it at the edge by smoothing it on the surface. This force is especially active at high frequency.
• and a volume component of stirring which, due to the configuration of the convection movements of the liquid metal which it provides (ring stirring with metal rising in the center of the mold) "swells" the central part of the meniscus. However, this force is mainly active at low or medium frequency. It is for this reason that it is the source of surface instabilities. The maximum effect of this stirring force is obtained at medium frequency, namely around 200 Hz to fix ideas, but in any case less than 500 Hz, whatever the nature or thickness of the mold or the format of the cast metallurgical product.

These are the two combined actions - peripheral repulsion and upward mixing in the center (which could be obtained from the same magnetic field pulsating) which give the meniscus a sought-after pronounced domed form.

In the same vein, but in order to solidify the metal in electromagnetic confinement, that is to say out of any material contact with the cooled wall of a mold, it has already been proposed to create an environment magnetic at the level of the ingot mold constituted by the superposition of two axial fields, that is to say both directed along the pouring axis, one being periodic (the confinement field), the other being constant for produce radial vibrational forces in the confined liquid metal. These fields are generated by individual coils encircling the upper part of the mold, one being supplied with alternating current at a frequency between 500 and 5000 Hz, the other being supplied with direct current. To limit the stirring effect of the alternating field, it has even been proposed to add a third encircling coil to create where the previous two already act an additional periodic axial magnetic field at industrial frequency (EP-A 0100 289, or the article by Ch. Vivès "Effects of forced electromagnetic vibrations during the solidification of aluminum alloys: Part II. Solidification in the presence of colinear variable and stationary magnetic fields published in the journal Metallurgical and materials transactions B, Vol. 27B, n ° 3, June 1, 1996, pages 457 to 464 "). This type of teaching is found, for example, although very succinctly in this case, in document DE 35 17 733 (1986), which moreover proposes to implement, alongside a variable axial magnetic field high frequency confinement, a continuous field which can be either axial or transverse, but which must act over the entire height of the ingot mold, which inevitably leads to electromagnetic assemblies of extreme technological complexity.

That said, whatever the intended application, solidification under confinement or, like of the present invention, geometric control of the meniscus, the problem that arises is to manage to transfer sufficient electromagnetic energy to the cast metal through the copper ingot mold. At the frequency levels selected (above 500 Hz), it would be necessary to effect, due to the magnetic screen effect that opposes the metal wall of the mold, segment it vertically to allow it to behave like a "cold crucible" electromagnetic".

Such a measure is complex to implement both in terms electromagnetic due to the inevitable electrodynamic instabilities linked to nature liquid of the final armature (the liquid metal within the ingot mold) on which one acts by the intermediate susceptor that is the mold itself. It is also complex by the makes the ingot mold above all a bottomless vertical crystallizer whose tightness lateral must always be perfectly secured, the format of which must be geometrically stable (avoid the phenomena of swelling of the large faces) and whose circuit of cooling is rigorously optimized. Such segmentation of the mold, large side faces in particular, would mean having to reconsider deeply a already proven design of the mold on the technological and on the plan functional.

In fact, due to its construction in four copper or copper alloy plates assembled in the corners (two large planar facing faces and two small faces ends), a slab ingot mold acts naturally like a "cold crucible", but for medium frequencies. At 200 Hz, most of the electromagnetic power delivered by an inductor is easily transferable to the molten metal through the walls whose thickness rarely exceeds 40 or 45 mm. But, at this frequency, the deformation of the meniscus resulting, as explained above, from the combination of the metal containment and convection force, leads to strong fluctuations in the time of the "average" deformity of the meniscus. Therefore, in accordance with a essential characteristic of the invention, a directed continuous magnetic field is applied perpendicular to the casting axis which, also used at the meniscus, will act as an electromagnetic brake on metal convection movements underlying liquid generated by the centripetal force at 200 Hz of the meniscus bulge and thereby lead to a smoothing effect of the meniscus on the surface.

• la figure 1 qui montre schématiquement, vue en coupe longitudinale, une lingotière de coulée continue de brames d'acier selon l'art antérieur ;
• la figure 2 qui montre schématiquement en perspective une lingotière de coulée continue de brames d'acier selon l'invention ;
• la figure 3 qui montre schématiquement cette même lingotière selon l'invention vue en coupe longitudinale ;
• la figure 4 qui montre schématiquement en perspective une première variante de la lingotière précédente ;
• la figure 5 qui montre une configuration de la lingotière la rendant très perméable aux champs électromagnétiques.

The invention will be clearly understood, and other aspects and advantages will appear better on reading the description which follows, given by way of example of embodiment of the invention and with reference to the plates of attached drawings in which:

• Figure 1 which schematically shows, in longitudinal section, a mold for continuous casting of steel slabs according to the prior art;
• Figure 2 which shows schematically in perspective a mold for continuous casting of steel slabs according to the invention;
• Figure 3 which schematically shows the same mold according to the invention seen in longitudinal section;
• Figure 4 which shows schematically in perspective a first variant of the previous ingot mold;
• Figure 5 which shows a configuration of the mold making it very permeable to electromagnetic fields.

In the figures, the same elements are designated by identical references.

A conventional ingot mold 1 for continuous slab casting according to the prior art shown diagrammatically in FIG. 1 has four flat walls, made of copper or copper alloy, energetically cooled by internal water circulation, namely two large walls in look 2, 3 - of which only one 2 is visible in Figure 1- and two small walls 4, 5 of closure at the end. For the sake of simplification, the internal cooling means walls 2, 3, 4, 5 of the mold 1 (generally a lining defining channels verticals inside which water is circulated) have not been shown.

The ingot mold 1 is oriented vertically, thus defining a casting axis 11. In during casting, it oscillates vertically at low amplitude as indicated by the arrow 6. The ingot mold is supplied with liquid steel 7 by a nozzle 8 made of refractory material mounted in the bottom of a distributor not shown constituting a reserve of liquid steel. The liquid steel 7 introduced into the mold 1 solidifies against the faces of the large cooled metal walls 2, 3, (and incidentally against the small end faces 4, 5) to form a solidified skin 9. The thickness of the skin 9 increases progressively measure that the slab 10 being solidified is extracted by the open bottom of the ingot mold 1, in the direction of arrow 31, by means of extraction not known represented.

The free surface 12 of the liquid steel 7 (usually called "meniscus") is covered by a covering slag essentially based on metal oxides, of which the functions, all useful for the casting operation, are multiple. First, it stops the thermal radiation emitted by the surface 12 of the liquid steel 7, and thus attenuates its cooling. Above all, it provides lubrication of the interface between the solidified skin 9 and the walls 2, 3, 4, 5 of the mold 1, by the following mechanism. The cover slag is deposited on the surface 12 of the liquid steel 7 in powder form. It forms a layer upper 13 which remains in the solid state, while its lower layer 14, brought into contact of molten steel 7, is in the liquid state, which allows it to infiltrate between the skin solidified 9 and the walls of the mold. This is where it plays its role as a lubricant. We note, however, the presence of a slag cord 15, i.e. a slag band of cover which solidified on contact with the cooled metal walls 2, 3, 4, 5. This slag cord 15 covers the entire perimeter of the mold and may have a significant maximum thickness, of the order of 10 to 20 mm.

The presence of the slag cord 15, combined with the vertical oscillation movements 6 of the mold, causes the appearance of surface defects on the slab 10 during its solidification. The solidified skin 9 strikes the slag bead 15 during the phases of raising the ingot mold 1. This forms what is called a "solidification horn" 16, namely a curvature of the upper end of the solidified skin 9 in the direction of inside the mold 1, as well as more or less deep oscillating wrinkles at the surface of the solidified cast product. This solidification horn 16, and the oscillation wrinkle associated, are privileged sites for the formation of segregations and cracks which degrade the quality of the final product, as well as for trapping non-metallic inclusions and gas bubbles rising along the front of solidification of the lower regions of the liquid steel 7.

A known remedy for these problems (see the article entitled "Improvement of surface quality of steel by electromagnetic mold "by H. Nakata, M. Kokita, M. Morisita and K. Ayata, in Proceedings of the International Symposium on Electromagnetic Processing of Materials, 1994, Nagoya) could consist of the imposition of an electromagnetic field AC at a frequency between 100 and 100,000 Hz, preferably between 200 and 20,000 Hz, by means of a multispire coil surrounding the ingot mold 1 on all sound perimeter at the meniscus and therefore generating an alternating magnetic field according to the axis of casting.

The device according to the invention, shown diagrammatically in FIGS. 2 and 3, includes such a coil 17 connected to an alternating current generator (not shown) operating at a frequency belonging to the above-mentioned range. The electromagnetic field of coil 17 generates induced currents in liquid steel 7, especially at the meniscus 12. As already indicated, the interactions between field and currents then generate an electromagnetic force whose effect at the level of wall of the mold is a centripetal effect 18 which digs the periphery of the meniscus and whose the effect within the liquid metal 7 is a stirring effect which causes swelling at the center of the meniscus 12. The higher the frequency of the electromagnetic field, all things being equal, the greater the penetration of the field inside the liquid steel 7 is weak, so more the electromagnetic forces (whose intensity does not depend on the current frequency) are concentrated in a small peripheral volume. So in the above-mentioned frequency range, confinement forces 18 of an intensity are obtained sufficient to obtain a repulsion of the liquid steel 7 which is hollowed out there and by consequently ceases to be in contact with the slag cord 15.

A pronounced dome-shaped surface 12 is thus obtained for liquid steel 7 in the mold 1. As a result, as shown in FIG. 3, we succeed in reducing, or even remove the solidification horns 16, and also reduce the thickness of the slag bead 15 since the temperature of its immediate environment is higher. Another consequence is that the liquid cover slag 14 has much better possibilities of infiltrating between the solidified skin 9 and the walls 2, 3, 4, 5 of the mold, this which improves lubrication and therefore allows higher casting speeds than in the conventional practice. The level at which solidification of the liquid steel 7 begins in the ingot mold is also better controlled and stable, which contributes to improving the surface condition of the slab 10. Finally, the effect of the pressure variations induced in the slag is attenuated liquid cover 14 by the oscillations of the mold 1 on the upper part of the solidified skin 7. The formation of the solidification horns is thus greatly reduced, which results in marked attenuation, or even disappearance of the oscillation wrinkles at the slab surface 10.

The characteristics of the coil 17 (its geometry, its number of turns, its height total, its position relative to the meniscus) and the intensity of the current flowing therein chosen so as to generate an electromagnetic field with an intensity of 500 to 3000 Gauss near the walls of the mold in the meniscus area.

However, the imposition of an alternating electromagnetic field, as we have just to describe it, also has shortcomings and drawbacks. This alternative field, due to its repulsion and metal mixing effects in the meniscus area, generates disturbances of the meniscus surface whose frequency spectrum can be extended (from 0.05 Hz to several Hz). Local agitation of liquid steel by the component rotation of the alternating electromagnetic field can also contribute to it. In this case, it cover slag entrainments occur within the liquid steel 7 which deteriorate the inclusiveness of the slab 10. The flow conditions of the slab 10 are also deteriorated, since lubrication is carried out irregular. There may also be fluctuations in the location line of the first solidification in an ingot mold resulting in irregularities in the solidified thickness according to the inside perimeter of the mold.

To remedy these problems, according to the invention, the field is superimposed electromagnetic alternating collinear with the axis of casting a continuous magnetic field oriented transversely to the direction of pouring of the slab 10, going from a large wall 2 from the mold to the other 3, and also applied to the meniscus. This field continuous magnetic effect stabilizes the surface of the liquid steel 7 present in the ingot mold 1, in this case the meniscus 12, by damping its vibrations. It allows also to stabilize the position of the first solidification line on the perimeter inside the mold and thereby reduce the risk of slag tearing off due to electromagnetic stirring while generating sufficient stirring intensity to wash the solidification front. On the other hand, it slows the circulation of metal liquid in the underlying area of the meniscus, whether this circulation is due to forces electromagnetic generated by the alternating field or coming from the jets of liquid metal leaving the nozzle 8.

As shown in Figures 2 and 3, this transverse continuous magnetic field can be created by an electromagnet supplied with direct current by a generator (not represented). It consists of two coils 19, 20, of common horizontal axis, opposite one on the other on both sides of the large faces 2, 3 of the mold, and surrounding each a pole piece 21, 22 made of a soft ferromagnetic material or iron-silicon alloy sheets. The active face of the pole pieces 21, 22 turned opposite a large wall of the mold is left free and positioned as close as possible to it. These active faces are formed by bolted stacking of sheets of iron-silicon alloy, according to the usual embodiment of the magnetic poles of machines induction, then rigidly attached to the body of the pole pieces. The rear part of these are integral with a magnetic circuit, forming a yoke 23, which surrounds the ingot mold and which may even be constituted by the frame of the casting machine, if necessary. The coils are wound in the same direction so that the pole pieces 21, 22 have active magnetic faces having polarities of opposite signs. We will note that, in FIG. 2, the part of the cylinder head 23 surrounding the small wall 4 of the mold 1, the closer to the observer, has been severed, so as to make visible the coil 17. This design reduces magnetic field losses by channeling the lines of force and concentrating them at the pole pieces 21, 22, where the field electromagnetic continuous, mainly horizontal direction, cross the mold 1 and the liquid metal 7. The intensity of the magnetic field in the center of the mold will be preferably between 0.2 and 1 Tesla over a height of the order of 100 to 200 mm in the meniscus area.

This magnetic yoke 23 can be made of solid material so as to ensure the rigidity and mechanical strength of the assembly, sufficient to allow the support of pole pieces 21, 22. It will moreover be advantageous to provide modular elements and interchangeable, also of laminated structure, intended to extend the active faces pole pieces 21 and 22. Such an arrangement will make it possible, on the basis of an electromagnet standard size, to be able to systematically minimize the air gap separating it from walls 2 and 3 of the mold whatever the format to be cast.

The continuous magnetic field thus created interacts with the velocity field in liquid steel 7. Induced currents appear in the liquid metal 7, determined by the vector product of speed and magnetic induction. In turn, these currents induced interact with the magnetic field that gave birth to them to create a electromagnetic force, of Laplace, which here is a force of braking of the flows of liquid steel 7. In this way, the movements of liquid steel 7 are greatly attenuated in the vicinity of the meniscus generated by the alternating electromagnetic field used for give its domed shape to the surface 12 of the liquid steel 7 which contributes to stabilizing the meniscus level fluctuations. In fact, the recirculations of liquid metal due to electromagnetic stirring, and located near the walls of the mold in the part meniscus 12 convex, velocity components perpendicular to the field continuous magnetic, which makes it possible to brake them effectively. In addition, as shown in FIG. 3, the nozzles 8 usually used in continuous casting of steel slabs have side vents 24, 24 'through which the molten steel enters the mold 1, which are oriented towards the small walls 4, 5 of the mold. When he entered the ingot mold, liquid steel 7 therefore has the main component of its perpendicular speed to the transverse continuous magnetic field. This also produces a braking effect of this component, with the advantageous consequence that the jets steel supply outlet from nozzle 8 descend less deeply into the well liquid. A better homogeneity of the solidification structure of the slab 10, and also better inclusion cleanliness, since non-metallic inclusions are driven to a shallower depth than in the absence of a field electromagnetic continuous and therefore have an easier time to settle on the surface and be there trapped by the cover slag 13. The washing effect of the solidification front by rising liquid metal recirculation currents 7 is also reinforced. The absence solidification horns is also favorable for good subcutaneous inclusion cleanliness. As for the movements associated with deformations of the liquid steel 7-slag interface coverage 12, 13 such as standing or traveling waves that affect the meniscus stability, they are also considerably reduced.

As already said, the polar endings of the parts 21, 22 are preferably formed by an assembly of vertically oriented metal sheets separated by sheets of insulating material, in a manner comparable to what is done to constitute the electrical transformer cores. If these poles are massive, the magnetic field alternating axial generated by the coil 17 can develop induced currents there which heat by Joule effect, which could make it necessary to cool them. A laminated structure, on the contrary, naturally ensures their thermal maintenance at low temperature without the need for a forced cooling circuit. Of more, these induced currents can disturb the operation of the current generator continuous supplying the coils 19, 20. It may however be sufficient to limit this laminated construction at the poles 21, 22 and to keep a cylinder head 23 made of solid material which, as already said, provides the whole solidity and rigidity required.

The spatial distribution of the magnetic field depends on the geometry of the parts polar 21, 22 and the electrical connection mode of the coils 19, 20. Figure 4 represents a variant of the invention, in which intensity gradients of the continuous magnetic field at the meniscus. Such a configuration can sometimes be advantageous for eliminating certain progressive waves at the free surface 12 of the steel liquid 7. To obtain such gradients, it is possible, as shown, to give a shape crenellated to the pole pieces 21, 22 surrounded by the coils 19, 20. Thus, the pole piece 21 has two salient north poles 25, 26 and the pole piece 22 has two south poles projections 27, 28 arranged facing the two north poles 25, 26. Like the arrows 29, 30 the symbolize, it is between these salient poles 25, 27 and 26, 28 that the magnetic field continuous at the highest intensity. The location and geometry of these salient poles 25, 26, 27, 28 are determined by the nature of the hydrodynamic disturbances at eliminate, which themselves depend on the geometry of the cast product 10 and the conditions supply of liquid metal 7 to the mold 1.

In continuous slab casting, the distance between the large walls 2, 3 of the ingot mold is most often in the range of 200 - 300 mm, or even less on installations pouring thin slabs. It is therefore possible to create without particular difficulties a magnetic field whose effects are felt from a large wall 2, 3 to another, and which acts also in the vicinity of the small walls 4, 5 if, as shown, the pole pieces 21, 22 extend over the entire width of the mold 1. On the other hand, create a field magnetic which would cross the mold 1 from a small wall 4, 5 to the other would be more difficult and generally ineffective because these small walls 4, 5 are 1 to 2 m apart or more, so very distant from each other. But in the case of pouring products from square or slightly rectangular section (blooms or billets), especially if they are of large dimensions (300 to 400 mm side for example), it may be desirable to create two horizontal continuous magnetic fields, perpendicular each to two sides opposite of the mold, by means of electromagnets similar, for example, to those which have just been described. These two fields do not interact with each other, because each acts on a component of the speed of the liquid steel 7 of different orientation.

As shown in FIG. 5, in a known manner already mentioned at the beginning, it is possible to divide the walls of the mold 1 vertically, over at least part of its height subject to said field, in a plurality of sectors 43 separated by a material of insulating jointing 44, in order to counteract the self-induction effect of the mold itself with regard to the axial alternating magnetic field generated by the encircling coil 1 7 and thus improve the electrical efficiency of the installation.

As has been said, the frequency of the alternating current supplying the coil 17 for create the axial alternating magnetic field is normally between 100 and 100,000 Hz. In the low frequency range (100 to 2000 Hz, it is possible to use "pulsed" alternating currents, ie whose maximum intensity varies periodically between a phase with a maximum value and another with a minimum value which can reach zero. The phases in which the maximum current intensity has a minimum value can absorb very low frequency disturbances affecting the stability of the surface 12 of the liquid steel 7 and the line of first solidification of the metal poured into the mold. Generally, pulsed current cycles follow one another at a frequency (called "pulse frequency") from 1 to 15 Hz, preferably 5 to 10 Hz.

• une action de freinage sur les écoulements de brassage générés par la partie rotationnelle des forces électromagnétiques dues au champ alternatif ;
• une action directe de freinage sur la vitesse de pulsation des ondes de surface sur le ménisque.

The damping effect of meniscus level disturbances by the axial continuous magnetic field is attributed to the combination of two actions:

• a braking action on the stirring flows generated by the rotational part of the electromagnetic forces due to the alternating field;
• direct braking action on the pulsation speed of surface waves on the meniscus.

The numerical data which have been indicated are valid for the application of the invention in the continuous casting of steel. However, the invention is of course applicable to the continuous casting of metals other than steel, when this casting is carried out on installations similar to those which have been described.

Claims (9)

1. Process for the vertical continuous casting of metal products in an oscillating mould having cooled plates joined together, in which process the molten metal to be cast is cast by keeping it in contact with the said oscillating cooled plates and the region of the meniscus of the liquid metal present in the mould is subjected to the action of an axial alternating magnetic field, collinear with the direction of casting, tending to impose on the said meniscus a domed overall shape, characterized in that a magnetic field generated by an AC current pulsed at a frequency of less than 500 Hz is used and the said region of the meniscus (12) is also subjected to a continuous magnetic field directed transversely to the direction of casting (11) in order to allow the shape of the said meniscus (12) to be stabilized.
2. Process according to Claim 1, characterized in that the said axial alternating electrical magnetic field is generated by a pulsed AC current with a pulse frequency of between 1 and 15 Hz, preferably between 5 and 10 Hz.
3. Plant for the vertical continuous casting of metals, comprising an oscillating mould (1) having cooled plates (2, 3 and 4, 5) joined together, of which two (2, 3) are long, facing one another in order to define a casting space, in which the cast metal is brought into contact with the said cooled plates which plant is of the type having an electromagnetic coil (17) supplied with AC current at a frequency of less than 500 Hz and surrounding the mould in the region of the meniscus (12) of the liquid metal which is present therein so as to produce therein an alternating magnetic field directed along the casting axis (11), characterized in that it also includes an electromagnetic inductor (19 to 23) which produces a continuous magnetic field passing through the long plates (2, 3) of the mould in the region of the meniscus (12) perpendicular to the casting axis.
4. Plant according to Claim 3, characterized in that the said electromagnetic inductor is formed by at least one electromagnet supplied with DC current, consisting of two coils (19, 20), having a common horizontal axis, which are placed on either side of the mould (1), each coil being wound around a pole piece (21, 22) placed in the region of the meniscus (12) and forming an integral part of a yoke-forming magnetic circuit (23).
5. Plant according to Claim 4, characterized in that the said pole pieces (21, 22) have a crenellated shape creating magnetic field intensity gradients.
6. Plant according to Claim 4 or 5, characterized in that the said magnetic yoke (23) surrounds the mould (1).
7. Plant according to one of Claims 3 to 6, characterized in that it is divided, at least in its upper portion, into several vertical sectors (43) separated by an insulating material (44).
8. Plant according to Claim 4, characterized in that the pole pieces (21, 22) are made of laminations.
9. Plant according to Claim 4 or 8, characterized in that the pole pieces (21, 22) comprise interchangeable attached modular elements.

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