EP2249983B1 - Method and associated electromagnetic apparatus for rotating molten metal in a slab continuous-casting ingot mould - Google Patents

Description

The present invention relates to the continuous casting of metal slabs, in particular steel. It relates more particularly to the implementation of sliding magnetic fields in the mold, whose action on the cast liquid metal gives the latter a rotational movement about the casting axis.

It is known that the continuous casting of steel slabs is conventionally carried out in a vertical mold, or essentially vertical, composed of two large faces (or walls) facing each other, copper or copper alloy, energetically cooled by circulation of water, and two small side faces sealingly mounted to the right of the end of the large faces to define with them a casting space which will determine the format of the cast product. The molten metal is discharged by gravity into this space where it will gradually solidify in contact with the cooled metal walls of the mold as the solidified strand at the periphery is extracted downwards to complete its solidification in the stages of the secondary cooling of the casting machine. Thus, throughout the casting process, molten metal fills the casting space to a certain height level to form a meniscus (free surface of the liquid metal) covered with a slag and a steady flow of metal melt is continuously brought into the mold with the aid of a submerged nozzle (a few tens of centimeters below the meniscus) generally single and centered on the casting axis, and provided with lateral outlet openings which open in look at the small end faces.

The bases of the axial rotation of the molten metal at the meniscus in a continuous slab casting mold using sliding magnetic fields are already established and well known. Schematically, they consist in conferring on the metal an overall rotation around the casting axis in a single oblong movement by the implementation of driving forces produced by horizontally sliding magnetic fields generated by static polyphase inductors mounted on the large faces of the mold.

For example, the document EP 0151 648 proposes to implement four identical identical inductors mounted symmetrically on the large faces of the mold, at the rate of two inductors per large face, placed on either side of the nozzle, each partially covering a half width of the large face which receives them, between the nozzle and the small end faces. These inductors, three-phase, each generate a horizontal sliding magnetic field, whose direction of sliding is the same on both inductors of the same face and opposite to those produced by the two inductors vis-à-vis the other big face. It then results, from the interaction between the magnetic field generated by any inductor and the molten metal in the vicinity of this inductor, a thrust force on the metal according to the width of the mold. This interaction, repeated four times in the right section of the mold, namely one per inductor, installs a system with four driving forces, two of which, diagonally with respect to the casting axis, push the metal the nozzle to the small faces, so to the "outside", while the other two forces, facing the other diagonal, push the metal to "inside" small faces to the nozzle.

Another example: the Japanese patent application JP 57075268 it retains the principle of a single partial inductor by large face. Each inductor, in diagonal position with respect to the casting axis, occupies approximately ¾ of its large reception face. The remaining ¼ is thus left free of any action of the sliding field to allow the rotating metal stream to slow down before the frontal encounter with the small end face at right angles so that the impact energy is attenuated.

In the same spirit, the European patent EP No. 0096077 proposes him, a device configured on the basis of three inductors aligned by large face, jointly generating magnetic fields sliding horizontally in the same direction, but associated with means to make them act with differentiated thrust forces on the cast metal . Taken in the sliding direction of the field, the first inductor, in the vicinity of a small end face therefore, would ensure the speed of the mass of molten metal opposite, the second would ensure the maintenance speed in the middle part of the large face, while the third would be set to allow a deceleration of the flow of metal that passes before him before the frontal impact on the other small end face.

More recently, the European patent EP 0750958 seems to take another step further by proposing equipment for the rotation of the meniscus metal constituted by a single integral inductor by large face, therefore of the type described in JP 57075268 cited before, but served by a complex connectivity that connects it to its three-phase power supply. This sophistication of the electrical assembly, applied to an inductor of old design, aims to allow the implementation, here too, means for modulating the driving force according to the width of the mold. The goal is that the force is more intense in the end region of a large face to "push" the molten metal outward than that acting in the same end region facing each other. large face and facing in the opposite direction (thus pushing inwards). Thus, contrary to previous considerations on the desirable slowing down of the metal current before the frontal impact against the small end faces, this way of operating would make it possible, according to this document, to standardize both the axial rotation movement of the metal to the meniscus and the temperature of the metal in contact with the wall of the mold at this location. In fact, although this document is not explicit on this point, it appears to the analysis that the achievement of such an objective, with the means put in The work which has just been recalled is only really possible if the natural hydrodynamics of the metal bath in the mold have a configuration of the "double loop" type.

It will be recalled later what this expression covers in terms of a description of a mode of circulation of the molten metal within the mold, as opposed to the "single loop" mode in particular. In the immediate future, it will be seen that while the solution proposals for the oblong axial rotation of the metal to the meniscus are so prolix and for so many years in the literature, it is that an optimal solution has not yet been found. Now, it is precisely because the present invention takes into account at the first degree of importance the mode of circulation of the molten metal within the mold, which it claims as an optimal solution to ensure the meniscus movement. of stable and homogeneous oblong axial rotation of the liquid metal all along, or almost all along, the casting.

For this purpose, the invention firstly relates to a method for an oblong axial electromagnetic rotation of the molten metal in a continuous slab casting mold provided with a submerged casting nozzle centered on the casting axis. and having lateral outlet openings open opposite the small end faces of the mold, in which process has been mounted at least four polyphase magnetic field inductors sliding along the width of the mold on the large faces of the mold to reason of two inductors per large face, and the inductors arranged side by side on the same large face of the mold were set to create a system with four driving forces whose two forces attached to any pair of inductors located diagonally from each other with respect to the casting axis push the metal of the nozzle towards the small faces, thus "towards the outside", whereas the other two forces, attached to the other pair of inductors located diagonally from each other, push the metal of the small faces towards the nozzle, so "inward", the joint implementation of these four forces imparting generally to the molten metal an axial rotational movement oblong to the meniscus,
characterized in that, for the purpose of homogenizing said meniscus rotation movement during casting, the intensity of said driving forces is adjusted in a manner differentiated between them so that, in the vicinity of a large on the other hand, if the natural flow of the metal is stronger going "inwards" than "outwards", the highest intensities are applied to the two forces that push the metal "outwards", and conversely, if the flow is weaker in "inward" than "outward", the higher intensities are applied to the two forces that push the metal "inwards".

The term "natural flow of the metal" the flow which is established according to the casting conditions without the inductors are supplied with current.

In a preferred embodiment, the intensities of the driving forces of each pair of inductors located diagonally with respect to the casting axis are equalized to one another.

In another preferred embodiment, all the driving forces between them are equalized in intensity if and only if the natural flow mode of the ingot mold metal bath is of the "unstable flow" type.

According to a first main variant of this process, in which the circulation of the molten metal to the meniscus is directly taken into account, the meniscus velocity of the molten metal, measured in the vicinity of the same large face of the mold, is measured. flow progresses "inwards" and that whose flow progresses "outwards", a differential signal representative of the difference between said measured velocities, representative of amplitude and sign, is developed and the differentiation of said forces is regulated driving force, between forces pushing "inward" and those pushing "outwards", applying to them a difference in intensity that permanently makes said differential signal to zero.

According to a second main variant, in which the circulation of the metal to the meniscus is predicted to be taken into account, the natural flow mode of the molten metal within the ingot mold is predicted by taking into account parameters specific to the casting, and then differentiating the driving forces between them so as to further intensify the forces that push the metal "inwards" if the natural flow mode of the metal bath is of the "single loop" type, and conversely , so as to further intensify the forces that push the metal "outwards", if the natural flow mode of the metal bath is of the "double loop" type.

Preferably, not only the natural flow mode, but also the natural rate of circulation of the meniscus metal is predicted and the difference between the "outwardly" driving forces and those pushing toward the "interior" so that this difference is proportional to said natural speed predicted to the meniscus.

• une unité d'alimentation polyphasée des inducteurs en courant électrique dotée de moyens pour différencier les forces d'entraînement de chaque inducteur sur le métal en fusion coulé en lingotière;
• des moyens de mesure de vitesse pour mesurer, au voisinage de la même grande face de la lingotière, les vitesses au ménisque du métal en fusion dont l'écoulement progresse "vers l'intérieur" et celui dont l'écoulement progresse "vers l'extérieur" et pour élaborer un signal différentiel représentatif, en amplitude et en signe, de la différence entre lesdites vitesses mesurées;
• et des moyens de contrôle de l'unité d'alimentation électrique aptes, en réponse audit signal différentiel, à intervenir sur lesdits moyens de différenciation des forces d'entraînement pour faire tendre vers zéro ledit signal différentiel.

L'invention a encore pour objet un équipement électromagnétique pour l'exécution du procédé dans sa variante dans laquelle la circulation du métal en fusion au ménisque est prise en compte de manière prédictive pour obtenir une mise en rotation axiale oblongue d'un bain de métal en fusion dans une lingotière de coulée continue de brames pourvue d'une busette de coulée immergée centrée sur l'axe de coulée et présentant des ouïes de sortie latérales ouvertes en regard des petites faces d'extrémité de la lingotière, équipement comprenant au moins quatre inducteurs polyphasés à champ magnétique glissant montés sur les grandes faces de la lingotière à raison de deux inducteurs par grande face, les inducteurs disposés côte à côte sur une même grande face de la lingotière produisant,des forces d'entraînement qui poussent le métal en fusion selon la largeur de la lingotière dans le même sens entre elles et dans un sens opposé de celui des forces d'entraînement produites par les deux inducteurs en vis-à-vis sur l'autre grande face de sorte que se crée un système à quatre forces dont les deux forces attachées à une paire quelconque d'inducteurs situés en diagonale par rapport à l'axe de coulée poussent le métal de la busette vers les petites faces, donc "vers l'extérieur", alors que les autres deux forces, attachées elles à l'autre paire d'inducteurs en diagonale, poussent le métal "vers l'intérieur", des petites faces vers la busette, équipement caractérisé en ce qu'il comprend, dans le but de réaliser au ménisque le mouvement homogène de rotation axiale:

• une unité d'alimentation polyphasée des inducteurs en courant électrique dotée de moyens pour différencier les forces d'entraînement de chaque inducteur sur le métal en fusion coulé en lingotière;
• des moyens pour identifier le mode d'écoulement naturel du bain de métal en fusion au sein de la lingotière;
• et des moyens de contrôle de l'alimentation électrique aptes, en réponse aux moyens d'identification des modes d'écoulement, à intervenir sur lesdits moyens de différenciation des forces d'entraînement de manière à intensifier davantage les forces qui poussent le métal "vers l'intérieur" si le mode d'écoulement naturel du bain de métal est du type "simple boucle", et inversement à intensifier davantage les forces qui poussent le métal "vers l'extérieur" si le mode d'écoulement naturel du bain de métal est du type "double boucle".

The subject of the invention is also electromagnetic equipment for carrying out the method in its variant in which the speed of circulation of the molten metal at the meniscus is measured in order to make an oblong rotation of the molten metal in the upper part of the metal. a mold for continuous casting of slabs provided with a submerged casting nozzle centered on the casting axis and having open lateral outlet openings facing the small end faces of the mold, equipment comprising at least four distinct polyphase inductors sliding magnetic field mounted on the large faces of the mold with two inductors per large face, the inductors arranged side by side on a same large face of the mold producing driving forces which push the molten metal according to the width of the mold in the same direction with each other and in a direction opposite to that of the driving forces produced by the two inductors facing each other. on the other large face, so that creates a four-force system whose two forces attached to any pair of inductors located diagonally with respect to the casting axis push the metal of the nozzle towards the small faces, so "outwards", while the other two forces, attached them to the other pair of inductors diagonally push the metal "inward", small faces to the nozzle and characterized in that it comprises, for the purpose of achieving at the meniscus the homogeneous axial rotation movement:

• a polyphase power supply unit of the electric current inductors provided with means for differentiating the driving forces of each inductor on the molten metal cast in the mold;
• velocity measuring means for measuring, in the vicinity of the same large face of the mold, the meniscus velocities of the molten metal whose flow progresses "inwards" and that of which the flow progresses "towards the external "and to develop a differential signal representative, in amplitude and sign, of the difference between said measured speeds;
• and control means of the power supply unit adapted, in response to said differential signal, to intervene on said drive force differentiating means to cause said differential signal to zero.

The subject of the invention is also an electromagnetic equipment for carrying out the process in its variant in which the circulation of the molten metal to the meniscus is taken into account in a predictive manner to obtain an oblong axial rotation of a metal bath. melt in a continuous slab casting mold having a submerged casting nozzle centered on the casting axis and having open side outlet openings facing the small end faces of the mold, equipment comprising at least four polyphase inductors with sliding magnetic field mounted on the large faces of the mold with two inductors per large face, the inductors arranged side by side on the same large face of the mold producing, driving forces that push the molten metal according to the width of the mold in the same direction between them and in a direction opposite to that of the forces of entry. generated by the two inductors facing each other on the other large face so that a four-force system is created whose two forces attached to any pair of inductors located diagonally to the axis of cast push the metal of the nozzle towards the small faces, so "outwards", while the other two forces, attached them to the other pair of inductors diagonally, push the metal "inwards", small faces towards the nozzle, equipment characterized in that it comprises, in order to achieve the meniscus homogeneous axial rotation movement:

• a polyphase power supply unit of the electric current inductors provided with means for differentiating the driving forces of each inductor on the molten metal cast in the mold;
• means for identifying the natural flow mode of the molten metal bath within the mold;
• and power supply control means adapted, in response to the means for identifying the flow modes, to intervene on said differentiating means of the driving forces so as to further intensify the forces which push the metal "towards inside "if the natural flow mode of the metal bath is of the" single loop "type, and conversely to further intensify the forces that push the metal" outward "if the natural flow mode of the bath of metal is of the "double loop" type.

In a preferred embodiment, said power control means intervene on the differentiation means of the intensities of the driving forces in order to equalize the intensity of all the forces if and only if the natural flow mode of the Metal bath is of the "unstable flow" type.

In a preferred embodiment, completely automated, the means of identification of the flow mode of the metal bath within the casting mold are predictive in nature and constituted by a computer system comprising a programmed computer RAM in which are recorded identification charts (and / or their analytical form) constructed using a mathematical model of fluid mechanics describing the natural flows from the argon stream flow parameters, the cross-section casting slab, geometry and immersion depth of the nozzle, and casting speed.

At this point, it is worth recalling what is meant by "simple loop", "double loop" and "unstable flow" when we talk about the possible configurations taken by the natural hydrodynamics of a metal bath within a slab mold during casting. As will have been understood, in accordance with the very foundations of the invention, it is in fact these configurations, and they alone that will condition the topology of the field of electromagnetic driving forces to be applied in the mold to give the meniscus a rotational movement Axial oblong, homogeneous and well developed. Similarly, it seems appropriate to specify also what is meant by homogeneous rotation movement, as well as the benefits that one is entitled to expect on the quality of the cast metal.

• les figures 1a et 1b illustrent respectivement une configuration de type "simple boucle" et une configuration de type "double boucle", tel qu'elles se développent en cours de coulée au sein d'une lingotière de coulée continue de brames dans le plan principal médian B de la lingotière parallèle à ses grandes faces et passant par l'axe de coulée sur lequel est centrée la busette de coulée;
• les figures 2a et 2b illustrent, vu du dessus de la lingotière, les mouvements de circulation du métal au ménisque dans le cas d'un d'écoulement naturel de type "simple boucle"et respectivement "double boucle" du métal en fusion dans la lingotière;
• la figure 3a schématise la cartographie du champ de forces électromagnétiques d'entraînement selon l'invention au niveau du ménisque à appliquer à un écoulement naturel du métal en fusion de type "simple boucle" de la figure 2a;
• la figure 3b schématise l'autre cartographie du champ de forces électromagnétiques d'entraînement selon l'invention au niveau du ménisque à appliquer à écoulement naturel du métal en fusion de type "double boucle" de la figure 2b;
• la figure 4 représente, vu du dessus également, le mouvement circulatoire homogène du métal en fusion obtenu au ménisque par l'application du champ de forces d'entraînement selon la figure 3a à la topologie de mouvements en surface de type "simple boucle" de la figure 2a, ou par application du champ de forces selon la figure 3b à la topologie de mouvements en surface de type "double boucle" de la figure 2b;
• la figure 5 schématise la réalisation d'un équipement selon l'invention dans sa version de contrôle, par la mesure, du mode d'écoulement du métal en fusion au ménisque d'une lingotière de coulée continue de brames en acier pour y réaliser le mouvement homogène de rotation axiale du métal en fusion de la figure 4;
• la figure 6 schématise la réalisation d'un équipement selon l'invention dans sa version de contrôle, par prédiction, du mode d'écoulement naturel du métal en fusion au sein d'une lingotière de coulée continue de brames en acier pour réaliser au ménisque le mouvement homogène de rotation axiale du métal de la figure 4.

These clarifications will be made in the context of a more detailed description of the invention and the means of its implementation, given with reference to the accompanying drawings, given by way of example, and in which:

• the Figures 1a and 1b illustrate respectively a "simple loop" type configuration and a "double loop" type configuration, as they develop in casting course in a mold of continuous slab casting in the median main plane B of the ingot mold parallel to its large faces and passing through the casting axis on which is centered the casting nozzle;
• the Figures 2a and 2b illustrate, from above the mold, the circulation movements of the metal meniscus in the case of a natural flow type "single loop" and respectively "double loop" of the molten metal in the mold;
• the figure 3a schematizes the mapping of the field of electromagnetic driving forces according to the invention at the level of the meniscus to be applied to a natural flow of the molten metal of the "simple loop" type of the figure 2a ;
• the figure 3b schematizes the other mapping of the electromagnetic driving force field according to the invention at the level of the meniscus to be applied to the natural flow of the molten metal of the "double-loop" type of the figure 2b ;
• the figure 4 represents, from above also, the homogeneous circulatory movement of the molten metal obtained at the meniscus by the application of the driving force field according to the figure 3a the topology of surface movements of the "simple loop" type of the figure 2a , or by application of the force field according to the figure 3b the topology of surface movements of the "double loop" type of the figure 2b ;
• the figure 5 schematizes the realization of an equipment according to the invention in its control version, by the measurement of the flow mode of the molten metal to the meniscus of a continuous casting mold of steel slabs in order to carry out the homogeneous movement of axial rotation of the molten metal of the figure 4 ;
• the figure 6 schematizes the realization of an equipment according to the invention in its control version, by prediction, of the natural flow mode of the molten metal within a mold of continuous casting of steel slabs to achieve the meniscus homogeneous movement of axial rotation of the metal of the figure 4 .

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

Above all, it should be remembered that the electromagnetic force F, acting on an elementary volume of liquid metal to drive it in the direction of propagation of the magnetic field which creates this force, can be well approximated by the relation F = σ .V . B _eff ² , where σ is the electrical conductivity of the metal, B _eff is the effective intensity of the magnetic induction and V is the relative velocity of the field slip with respect to the metal. This relative velocity is given by the relation V = 2 τ. f, - v where τ is the polar pitch of the inductor, f the frequency of the electric current supplying this inductor and v the velocity of the metal subjected to the field, supposed to progress in the same direction as the field. It is also recalled that B _eff is directly derived, via Ampère's theorem, from the effective intensity I _eff of the electric current flowing in the conductors of the inductor.

Since, as a general rule, the polar pitch τ of the inductor is a fixed quantity imposed by construction, it can be seen that the intensity of the driving force F can be controlled by the intensity I _eff of the electric current supply, or by the frequency f of this current if it has for this purpose a variable frequency power supply. For the following, it will be assumed for simplicity that the driving force is controlled by the intensity of the electric supply current, the power supply being set so that the frequency of the latter is at a low value of 3 Hz. or even less, to obtain a sufficient depth of penetration of the magnetic induction in the molten metal in the vicinity of the inductor given the thickness of the wall of the ingot mold to be crossed and the composition of the metal of which it is formed

For the sake of clarity, an attempt has been made to explain the implementation of the invention in terms of driving forces of the liquid metal, rather than in terms of sliding magnetic fields, it being understood that these are the fields which produce these forces by interaction with the metal and that these fields are generated by the inductors whose operation is controlled by controlling the electric current (intensity or frequency) which feeds them.

Concerning the natural hydrodynamics of a bath of molten metal in a slab continuous casting mold supplied with molten metal by a central submerged nozzle with lateral outlet openings, it has been possible to demonstrate that this hydrodynamic could be manifested in three possible traffic patterns, two stable main modes, and one unstable mode.

A first stable mode is the "double loop" mode (better known by the English name of "double roll"). According to this mode, illustrated by Figures 1b and 2b each metal jet 1, which arrives in the mold by a side port 2 of the immersed nozzle 3 centered on the casting axis A, reaches a small end face 5 of the mold with an incidence and a momentum such that it divides after the impact into two opposite currents 7 and 8. A stream 8 descends in depth and a stream 7 rises, him, along the small face 5 to the meniscus 4 where, once reached this level, it develops into a blade 16 which progresses along the large faces 12, 12 'to the axis A of the mold to meet the paired blade 16' from the other small face 5 '.

A second stable mode is called "simple loop" (or "single roll"). In this mode, illustrated by FIGS. 1a and 2a, the above conditions as to the relative power of the incoming jets 1 are not satisfied. The thrust of Archimedes gas bubbles dispersed in the metal stream, from the injection of argon in the nozzle, is then preponderant: shortly after the exit of the ears 2 of the nozzle, a current 9, coming from the almost all of the metal jet 1, goes back to the meniscus 4, which thus becomes the seat of a circulation of molten metal progressing from the nozzle 3 to each of the small faces 5 and 5 ', where, once reached, the surface current dips down the mold.

If necessary, a detailed description of these two modes of metal flow can be found in the article of Pierre H. Dauby et al. presented at the 4th European Continuous Casting Congress held on 14, 15 and 16 October 2002 in Birmingham (GB) under the title "On the effect of liquid steel flow pattern and the need for dynamic electromagnetic control in the mold ".

These two main modes are complemented by a mode, not represented because fortunately less frequent, which expresses instabilities, generally transient, but not always, flows within the mold. It is known that a cause lies in the fact that during the casting, a casting parameter is modified, either intentionally (changes in formats during casting, for example) or fortuitously (flow of argon for example). ). This may be sufficient to force the flow of metal into the mold a transition between a "single loop" flow and a "double loop" flow, and vice versa, without anyone can do anything against it, or even know it. Another cause can be found in the occurrence of an asymmetry in the outgoing jets, following for example the appearance of a partial closure of a side nozzle of the nozzle. Yet another reason, perhaps the most common one, may be an "unfavorable" combination of values of the four essential parameters that govern casting (slab width, casting speed, argon flow, and immersion depth of gills of the nozzle) which then generates chaotic hydrodynamic phenomena using spatial distributions of complex and random energy, which can appear as a permanent oscillation between a "double loop" mode and a "single loop" mode, and vice versa. It is difficult in fact to simply describe this third mode other than by evoking phenomena of "left-right" swings of the molten metal mass in the mold on either side of the nozzle with repercussions on the meniscus in the form of rolling and pitching, which may call into question the very success of the casting if they persist for too long. It will be convenient to identify this mode of "unstable flow" if the speed measurement of the metal meniscus, taken for example at about mid-distance between the nozzle and a small face, fluctuates giving a zero result on average.

These reminders made, it should be also specified what is meant by "homogeneous" when this term is used to describe the axial rotation movement of the molten metal meniscus, and the metallurgical interest of such an axial rotation , as can be seen schematically on the figure 4 . A homogeneous axial rotation movement of the molten metal to the meniscus is formed when, in all points of the meniscus, the velocities along the walls are everywhere equal (or substantially equal). Otherwise, since the molten metal is an incompressible liquid, inevitably creates, sporadically and uncontrollably, recirculation mini-circuits that can degenerate into local vortex very harmful to the metallurgical cleanliness of the cast metal, as is known.

That said, the interest in itself of an axial rotation of the metal meniscus results in fact from the two main functions devolved to this gyratory movement.

A first function is that of the "stirring" of the metal bath which provides a thermal homogenization to the meniscus. Otherwise, local temperature gradients are installed which lead irremediably heterogeneities of solidification of the first skin in contact with the cooled copper wall of the mold, with the consequences that are known on the appearance of cracks on the product being solidified and the risks of breakthroughs related to it.

A second function is the "washing" of the solidification front. Gas bubbles or non-metallic particles, inevitably present in the molten metal, are often trapped by the infractuosities of a solidification front in dendritic growth, to become what are usually called inclusions. If the velocity of the sweep current exceeds a threshold value, specific to each case, these gas bubbles and particles are released and entrained with the metal stream until settling on the surface where they will be trapped by the supernatant cover slag. Thus, the skin of the solidified cast product is found to be free of inclusions and the quality of the product obtained is good.

It should be noted that this washing of the incipient front by a stream of metal sweeping it horizontally also contributes to the temperature homogeneity of the free surface of the molten metal by uniformizing the velocities. As already pointed out, since molten steel is a liquid, and therefore an incompressible state of matter, any heterogeneity of velocity at the surface can be the cause of sporadic occurrences of local vortices at the origin of metal pollution by entrainment of deep cover powder in the metal bath.

These reminders being made, we now place ourselves first on the assumption that the circulation of the metal bath is in "single loop" mode.

The image, seen from above of the mold, of the natural circulatory movements of the metal generated by the meniscus is illustrated by the figure 2a . As can be seen, we are dealing, in a way, with two opposing straw broom heads developing on both sides in the vicinity of the nozzle 3, and whose incipient strands 1, still grouped at the level of the nozzle (The outgoing jets 1), deviate rapidly to extend into a bundle of parallel strands 9 progressing to the vicinity of the small faces 5 where they then bend down to descend into the depths of the mold (cf. Fig. 1a ).

Referring now to the matched figure 3a which illustrates an implementation of the invention adapted to the case of "simple loop". The mold is of elongated rectangular cross-section defining the format of the slab to be cast. The nozzle immersed 3 is centered on the casting axis A. Four polyphase planar inductors (which will be assumed three-phase in this example) 10a, 10b, 10c and 10d magnetic field sliding along the width of the mold, are mounted opposite large faces 12 and 12 'of the mold with two inductors per large face. The inductors 10a and 10b are mounted aligned on the large face 12 on either side of the nozzle 3, and the inductors 10c and 10d are likewise on the large opposite face 12 '. These four inductors form a symmetrical set in the geometry of the mold, both of axial symmetry with respect to the casting axis A and plane symmetry with respect to the main median plane B of the ingot mold parallel to the large faces 12, 12 and passing through the casting axis A. Thus, for example, the inductor 10a is both the symmetrical inductor 10d disposed vis-à-vis relative to the main median plane B, the symmetrical l inductor 10b arranged side by side with respect to the secondary median plane (not shown) and the symmetrical inductor 10c placed diagonally with respect to the casting axis 3 (located at the intersection of the main median plane B and of the secondary median plane).

As can be seen, this architecture is such that each inductor covers about a half width of large face 12, 12 'on which it is centered. This overlap may be only partial, because it is not necessary for the magnetic field to act up to the level of the small end faces 5, 5 ', or indeed at the nozzle 3. On the contrary, it may be useful to provide a free space, a few centimeters, between two inductors juxtaposed to allow to accommodate a mechanical reinforcement of the structure of the mold.

• un premier couple de forces, en regard l'une de l'autre en diagonale sur chaque grande face 12 et 12' (les forces attachées aux inducteurs 10a et 10c), poussant le métal des petites faces 5 et 5' vers l'axe de coulée 3, et que l'on nommera, pour simplifier, forces poussant "vers l'intérieur";
• et un second couple de forces, en regard l'une de l'autre selon l'autre diagonale (les forces attachées aux inducteurs 10b et 10d), poussant le métal depuis l'axe de coulé 3 vers les petites faces 5, 5' et que l'on nommera forces poussant "vers l'extérieur".

The connection of the inductors to their power supply is such that the inductors arranged side by side on the same large face of the mold produce magnetic fields which slide in the same direction between them and in a direction opposite to that of the magnetic fields produced by the two inductors vis-à-vis the other large face. In the presence of molten metal in the mold, this results in a system of four driving forces, each attached to a separate inductor:

• a first pair of forces, facing one another diagonally on each large face 12 and 12 '(the forces attached to the inductors 10a and 10c), pushing the metal of the small faces 5 and 5' towards the axis 3, which will be called, for simplicity, forces pushing "inward";
• and a second pair of forces, facing each other along the other diagonal (the forces attached to the inductors 10b and 10d), pushing the metal from the casting axis 3 to the small faces 5, 5 ' and that will be called forces pushing "outward".

For the sake of clarity, these forces are represented by means of vectors placed inside the mold in the vicinity of the large walls, along the inductors concerned.

According to an essential characteristic of the invention, the driving forces of the liquid metal produced by two inductors side by side opposite a large face of the mold are of different intensity to each other.

Applied to the present case of a circulation of the metal of the "simple loop" type within the mold, this characteristic means, as shown in FIG. figure 3a , that the diagonal force torque pushing "inwards" (fat arrows) is of higher intensity than that of the other diagonal couple that pushes "outwards" (lean arrows)

Indeed, the inductors 10a and 10c acting "against the current" of the natural meniscus flow (cf. Fig. 2a ), they have to produce a driving force greater than that of their neighboring inducer, 10b and 10d respectively, which act in "co-current" of the natural meniscus flow. This, it will be understood, to aim to obtain a forced flow having a speed substantially identical in intensity at all points of the width of the moldwidth in the vicinity of large faces. If the forces of two inductors side by side on a large face were equal, the one pushing "inwards" and thus having to overcome the countercurrent of the natural flow on the half width considered, would produce a flow invariably less strong than on the other half width side, which would lead to a heterogeneous overall flow.

Therefore, it is understood that, according to the invention, the application of the set of couples of differentiated driving forces illustrated on the figure 3a (The forces 10a, 10c push more strongly than the forces 10b, 10d) leads to give the molten metal to the meniscus 4 an overall movement which passes from the natural configuration shown on the figure 2a , to an oblong gyratory configuration around the stable and well-formed casting axis A, as illustrated in FIG. figure 4 .

As already pointed out, it is the intensity of the electric currents supplying the inductors which is the privileged actuator to control the intensities of the forces and thus their differences between couples pushing "towards the inside" and couples pushing "towards outside" for the implementation of the invention. Thus, this gap will be all the greater as the circulation of the metal along the large walls is heterogeneous to better match the velocities of the metal moving with respect to each inductor plus the axial axial rotation movement of the metal to the meniscus will be homogeneous and well developed on the surface of the meniscus. The position of the optimum setting will obviously vary with the peculiarities of each casting. It can be reached, in any case close to it, for example by putting in place instruments for direct measurement of the local speed at the meniscus, on either side of the nozzle, for a regulation of the driving forces , or through predictive management, which will be described later with reference to Figures 5 and 6 .

A similar result as to the stable and homogeneous nature of the rotation movement of the metal to the meniscus will also be obtained in the case where the natural circulation mode of the metal bath in the mold is of the "double loop" type (cf. Fig. 2b )

Referring to this effect, at figure 3b it is found that the opposite provision to that of the figure 3a prevails: it is the force couple 10b, 10d pushing "outwards" which is energized this time relative to the torque 10a, 10c pushing "inwards". This respected disposition, the application of such a set of couples of differentiated forces leads to give the molten metal to the meniscus an overall movement which passes from the natural configuration of the figure 2b to an oblong gyratory configuration around the stable and homogeneous casting axis A, as illustrated in FIG. figure 4 .

On the other hand, in the case of an "unstable flow", the difference between the intensities of the forces "pushing inwards" and the forces pushing "inwards" will preferably be set to zero, and the intensities increased. until the meniscus has an axial rotational movement that is as homogeneous as possible.

We will now, with a joint reference first to Figures 5 and 6 , more concretely describe the constitution of an electromagnetic equipment according to the invention, according to two embodiments, and the electrical connections of the four inductors between them and with their polyphase power supply unit. The equipment is shown mounted in a functional position on a slab casting mold of which the single submerged nozzle 3 centered on the casting axis A has been represented, the large faces 12, 12 'and the small end faces 5 , so as not to overload the figure unnecessarily.

In the example, the two inductors placed diagonally to each other are connected to the same power supply. The inductors 10a and 10c are thus connected to the power supply 15a and the inductors 10b and 10d are connected to the power supply 15b.

Of course, the order of the polarities to be respected is that which will ensure the sliding of the magnetic fields in the desired directions. Thus, according to the exemplified assembly, the inductors produce respective magnetic fields that slide horizontally as shown in FIGS. 1c and 2c in order to achieve a gyratory movement of the meniscus metal which, seen from above, develops in the clockwise direction as shown in FIG. the figure 3 . It is understood that if for any reason we wanted a counterclockwise movement to the meniscus, it would be enough to reverse the polarities of the inductors.

The power unit is composed of two identical identical power supplies 15a and 15b each provided with means for differentiating the intensities of the driving forces by torque of inductors. Each pair of diagonally arranged inductors thus paired is connected to one and only one power supply: the torque 10a, 10c being supplied by the power supply 15a and the torque 10b, 10d by the power supply 15b. We Remember that these are polyphase feeds, preferably bi- or three-phase, so that the inductors can produce a sliding magnetic field. As already mentioned, it is preferable to opt for VFVF (Variable Voltage Frequency) type power supplies in order to be able to easily adjust the intensity of the electric current delivered to the inductors, thus the intensity of the magnetic field. and its frequency, so the speed of displacement of the sliding fields.

If the natural circulation of the metal bath is in "simple loop", the power supply is regulated so that it is the power supply 15a which, by the choice of the intensity of current (and also of its frequency, if necessary), makes to produce at the two inductors diagonally 10a and 10c that it feeds, a driving force of the metal stronger than that produced by the two other diagonally inductors 10b and 10d, connected to the supply 15b. And conversely, if the natural circulation of the bath is in "double loop". In the event of an "unstable flow", the two power supplies 15a and 15b are set to make the four inductors produce the same intensity of current.

The two embodiments of the equipment according to the invention are distinguished by the control mode of these power supplies.

According to a first variant, represented on the figure 5 , and based on a direct measurement of meniscus velocities, the control of the power supplies 15a and 15b according to the above criteria is effected by means of a regulator 13. Its function is to constantly adjust the difference between intensity of the currents to be applied between the pair of inductors which is to create the strongest force and the other pair, according to the information on the meniscus velocities that it receives from fluid velocity measuring means.

These measuring means consist of two velocity measuring probes 20 and 21. These probes dive weakly into the molten metal at distinct locations on the meniscus, on either side of the nozzle 3, preferably at an equal distance from the nozzle. it, and also at equal distance from the same large wall of the mold, here the large wall 12. It may be mechanical probes in which a torsion torque is formed under the impulse of the metal current, which depends therefore directly from the speed of the metal in flow. These speed sensors transmit their information to the regulator 13 in the form of signals carrying a sign indicating the direction of the measured speed.

The regulator 13, which receives these speed signals, makes the algebraic difference in order to develop a reference signal proportional to the difference in speeds and whose sign indicates which of the two metal currents in contact with the probes 20 and 21 on the one hand. and other of the nozzle, which is the strongest, and therefore which of the two pairs of inductors will generate the lowest thrust force. This instruction will allow the power supplies 15a, 15b to deliver the appropriate current intensities to the inductors, differentiated intensities so with a difference between they will result in differentiated thrust forces whose action on the metal will lead to zero towards the target signal, guaranteeing the desired homogeneity of the rotational movement of the metal meniscus.

If the speed signals of both probes become fluctuating around zero, the movement of the molten metal to the meniscus is unstable and the difference in current intensities to be applied between the two inductor pairs will be set to zero.

Of course, such a control loop of the driving forces on the meniscus speeds assumes a starting phase.
At the beginning of the casting, the four training forces will be equal in intensity. For the sake of clarity, the "inward" pushing forces (inductors 10a, 10c), as the "outward" pushing forces (inductors 10b, 10d) can be produced with an inductor current of 500 A for example.

Then, the regulator 13 proceeds to a first measurement of the velocities of the metal currents by the probes 20 and 21 placed in the vicinity of the wall 12 opposite the inductors 10b and 10a respectively and produces a signal representative of their difference. It is understood that this differential signal, in magnitude and sign, will depend on the natural flow mode of the metal in the mold. If necessary, please refer to Figures 2a and 2b to see that if the flow mode is in "simple loop" ( fig.2a ) the probe 20 will measure a speed much higher than that measured by the probe 21, and conversely if the flow is in "double loop" ( Fig. 2b ). The sign of this differential signal will thus teach the regulator 13 on the identity of the flow mode, and its amplitude will enable it to develop the intensity difference signal for the control of the power supplies 15a, 15b. Then, the loop regulation forces can settle and take over for most of the casting period, regardless of changes in the natural flow mode of the mold bath.

More generally speaking, the current intensity (and frequency) setpoint is preselected and applied to the four inductors at the start of the rotation of the metal, before the actual regulation phase. This preselection will be done manually or automatically according to recorded values, for example in a programmable controller, depending on the cast metal grades and / or quality objectives sought. Such a PLC (PLC type for example) could contain the regulator 13.

The second embodiment variant of the equipment, represented on the figure 6 , is based on a predictive approach to natural flows of molten metal. The control of the power supplies 15a and 15b according to the criteria previously stated is effected by means of control means 16. These are advantageously constituted by a programmable PLC type PLC known and commercially available (PLC = Programmable Logic Controller) and whose function is to calculate and impose setpoint values in current intensity (and if necessary in frequency also) to the power supplies 15a and 15b separately, the PLC 16 is here the body of the system that will determine which of the two pairs of inductors should create the most pushing force, but this time in response to the predictive identification of the mode natural flow of the metal bath in the mold, and no longer in "regulation" to cancel a difference signal from a direct measurement of meniscus velocities.

Also, the PLC 16 receives the information it needs for this task by means 17 for identifying the flow mode of the metal bath in the mold.

It should be noted that these identification means therefore replace the speed sensors of the first embodiment, because these, as will be explained later, are not easy to implement in a continuous casting mold.

These identification means 17 consist of a standard PC type computer (Personal Computer) with a RAM which contains the tools necessary for this identification.

It is useful to specify at this stage that the term "flow mode identification" is not only the qualitative prediction that it is a flow of the "simple" or "double loop" or "unstable" type of flow. ", but also the quantitative prediction of the flow velocity of the metal to the meniscus, it being understood that a velocity predicted at zero is assimilated to an unstable state.

In essence therefore, these tools will be constituted by appropriate software, built on a mathematical model of fluid mechanics able to predict the flow mode of the ingot mold from, on the one hand, two casting parameters fixed to This is the beginning of the mold thickness and the geometry of the nozzle and, on the other hand, of four variables that may vary during casting, such as the width of the slab, the casting speed, the depth of the slab immersion of the gills of the nozzle and the flow of argon injected. All these data, the fixed doublet as the variable quadruplet, are preferably introduced by automatic input from the general computer 19 of the casting installation and driving the casting operations. For a quick identification of the flow mode, the results produced by this software can be realized in the form of abacuses that the PLC can use in automatic reading or after their transcription in analytical form.

It may still be a database associating all the possible values of these sets "doublets-quadruplets" of quantities with the mode of flow that each of them underlies. During the casting, the regular comparison over time of a casting-specific "doublet-quadruplet" instantaneous set with those of this bank will then make it possible to retain the memorized element which will best match the data of the casting and identify thus, qualitatively and quantitatively, the natural flow mode of the molten metal within the mold.

Finally, for each flow mode thus calculated, the results given by the PC 17 will, of course, be able to provide a numerical value of the average natural speed of the metal to the meniscus, a value that will allow the control PLC 16 to determine a deviation value, for example 200 A (ie 600 A for the two most active and 400 A for the other two if the initial current at startup has been preselected at 500 A) and instruct in this sense the power supplies 15a and 15b to deliver the current intensities corresponding to the pairs of inductors concerned.

In summary, the identification means 17 will therefore provide the control means 16 with a signal whose amplitude is proportional to the natural flow velocity of the molten metal to the meniscus and whose sign (depending on whether the direction of this velocity is inwards or outwards), provides information on the identity of the type of flow in "single loop" or "double loop". The controller 16 then determines which of the two pairs of inductors should create the greatest force depending on the type of flow prevailing. It also calculates the difference in intensity of the supply currents between the two pairs of inductors concerned so that this difference is proportional to the average speed of the meniscus metal and transmits the corresponding instructions to the power supplies 15a and 15b.

If the PLC 16 receives from the PC 17 a signal of zero amplitude, it cancels the difference in intensity of the supply currents (and frequencies) and gives the same setpoint of supply current (and frequency) to the four inductors , setpoint corresponding to the preselected value or prerecorded according to cast metal grades and / or desired quality objectives.

It is understood that the invention provides homogenization "in line" of the axial rotation of the meniscus metal during casting. Thanks to the automatic acquisition of the quadruplet of variable parameters which conditions the flow mode, it will be possible at any moment, in response to the values of these quadruplets arriving at the PC 17 as the casting takes place, to apply the appropriate differentiation to the forces of thrust of the inductors which will permanently ensure the achievement of a meniscus in homogeneous rotation, whatever the flow patterns that could succeed in the ingot mold during casting. On the contrary, therefore, of prior known systems, adapted to a flow mode and only one, and therefore suitable for a casting sequence which is only a fraction of the total casting time, the invention provides an optimal active "cover" of the casting for the entirety of its duration, or its quasi-completeness with regard to the possible sequences of unstable flow.

The choice between a "regulating" system and a "predictive" system is left to the discretion of the user who will exercise it according to his wishes or needs. It will be noted here simply that the "predictive" variant of implementation appears a little more demanding in software equipment, but on the other hand advantages, often decisive, to leave the virgin meniscus of any diving instrument and not to have a limited life to a few hours at best of speed sensors.

It goes without saying that the invention can not be limited to the examples described above but that it extends to multiple variants and equivalents to the extent that its definition given by the appended claims is respected.

Thus, the inductors forming a pair connected to a given power supply, 15a or 15b, can be electrically connected to each other in parallel as shown in FIGS. Figures 5 and 6 , or in series.

Similarly, it is possible to provide as many power supplies as inductors. Each of them can then be powered by its own dedicated power supply, which will in particular add to the flexibility of the settings by allowing the need to create a slight imbalance in the intensities of the forces produced by the inductors diagonally.

Indeed, if it may appear more rational that the driving forces of two diagonally inductors are equal, this is not a mandatory provision of the invention. These forces may indeed themselves be different in intensity if it is considered desirable to ensure that the primary criterion for obtaining a homogeneous rotation meniscus is satisfied, namely the equality of the speeds of the metal melted in front of each inductor.

Similarly, the number of inductors may be greater than four, it being understood in this case that this number must remain even in order to provide each large face of the mold with the same number of inductors.

In addition, to the question that could arise to know how high on the mold must be mounted inductors, the answer is that in principle there is no obligation to go up to meniscus level. If the inductors are designed for sufficient electrical power, so for sufficient force, an installation of the inductors, even several tens of cm below the meniscus, will give it a very stable and homogeneous rotation.

Claims (3)

1. Process for imparting an electromagnetic axial oblong rotation movement to the molten metal in a continuous casting ingot mold for slabs equipped with a submerged nozzle centred on the casting axis and having lateral outlets that open towards the small end sides of the ingot mold, process in which, at least four polyphased inductors (10, 10b, 10c, 10d), each producing a magnetic field travelling along the width of the ingot mold, are mounted, as two inductors placed side by side per large side of said ingot mold, and said inductors are adjusted to create a system with four driving forces, of which the two forces associated to any pair of inductors (10b, 10d) located diagonally to one another with respect to the casting axis push the metal from the nozzle towards the small sides, i.e. "towards the exterior", while the other two forces, associated to the other pair of inductors (10a, 10c) located diagonally to one another, push the metal from the small sides towards the nozzle, i.e. "towards the interior", the combined application of these four forces imparting an axial oblong rotation movement to the molten metal at the meniscus, process characterized in that, with the aim of homogenising said rotational movement of the molten metal at the meniscus during the casting, the natural flow rate of the molten metal within the ingot mould being previously identified by taking into account parameters relating of the casting operation, said driving forces are differentiated amongst themselves in such a way as to further intensify the forces that push the metal "towards the interior" if the natural flow rate of the metal bath is of the "single loop" type, and conversely, in such a way as to further intensify the forces that push the metal "towards the exterior" if the natural flow rate of the metal bath is of the "double loop" type.
2. Process according to claim 1, characterized in that the intensities of the two driving forces associated to the two inductors of the same diagonally positioned pair are equalised.
3. Electromagnetic equipment for carrying out the process according to claim 1 for imparting an axial oblong rotation of molten metal in the upper part of a continuous casting ingot mold (1) for slabs equipped with a submerged nozzle (3) centred on the casting axis (A) and having lateral outlets (2) that open towards the small end sides of the ingot mould, equipment comprising

   - at least four distinct polyphased inductors (10a, 10b, 10c, 10d), producing each a travelling magnetic field, and mounted on the large sides (12, 12') of said ingot mould, as two inductors (10a, 10b) placed side by side on a same large side (12) of the ingot mould producing driving forces that push the molten metal, along the width of the ingot mould, both in the same direction, the later being opposite to that of the driving forces produced by the other two inductors (10c, 10d) placed on the other large side (12'),

   - a unit (15a, 15b) for polyphased supply of the inductors with electric current and equipped with means for differentiating the driving forces of each inductor on the molten metal cast in the ingot mold;

   and characterized in that it comprises:

   - means (17) to identify the natural flow rate of the bath of molten metal within the ingot mold as "single loop" or as "double loop" type;

   - and control means (16) of said electric supply unit, which are able, in response to the identification means (17), of acting on said means for differentiating the driving forces in such away as to further intensify the forces that push the metal "towards the interior" if the natural flow rate of the bath of metal is of the "single loop" type, and conversely, in such a way as to further intensify the forces that push the metal "towards the exterior" if the natural flow rate of the bath of metal is of the "double loop" type.

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