EP1677928B1 - Electromagnetic agitation method for continuous casting of metal products having an elongate section - Google Patents

Abstract

The electromagnetic stirring in a continuous casting installation for slabs, in which the mould (1) is equipped with an immersed casting nozzle (4) with lateral outlet holes (5, 5') directed towards the small faces, is carried out with the aid of sliding magnetic fields generated by polyphase induction coils arranged in the proximity of the cast metal. With the primary aim of favoring liquid metal exchanges at the heart of the solidification well (6) between the secondary zone (2) and the mould, one forces the establishment of a longitudinal metal flow in the central region of the cast product along two opposed collinear currents (10a, 10b). An independent claim is also included for a metal product with a elongated straight section from a continuous casting installation using this method of electromagnetic stirring.

Description

The present invention relates to the continuous casting of metals, especially steel. It relates more particularly to the electromagnetic stirring of products with an elongate cross-section during casting, and even more specifically the establishment, within the still-in-liquid portion of the cast metal, of a particular distribution of the flows in the casting. magnetic fields applied.

It is recalled that the term "product with elongated straight section" metallurgical products whose width is at least twice the thickness, including slabs, bramettes, thin slabs, etc ....

Appeared in the field of continuous casting of steel in the early seventies, electromagnetic stirring quickly became a tool almost inevitable for the control of the flows in the liquid metal well being solidified. It is recalled that the principle most commonly used is that well known Magneto-HydroDynamic (MHD) which, by means of a movable magnetic field (rotating or sliding) generated by one or more generally several polyphase inductors arranged in the vicinity immediate flow of the cast product, causes the liquid metal in its displacement. Suitably placed on the metallurgical height of the casting machine, these inductors, supplied with adjustable frequency electric current, then allow varied mixing modes, adaptable to the needs of the metallurgist.

Moreover, the constant progress made in understanding the solidification mechanisms of the metal during the continuous casting show the importance that precisely the circulation movements of the liquid metal have on the quality in general (ie internal health, surface or inclusion cleanliness). solidification structure, etc.) of the solidified product obtained.

In this regard, the movements printed to the molten metal during the continuous casting can schematically be classified into two distinct categories, depending on whether one considers the mold or, below, the stages of the secondary cooling of the casting machine.

The movements imposed on the liquid metal within the mold, where the liquid metal fraction is largely in the majority, are essentially aimed at controlling the flow at this sensitive location. This is indeed the free surface of the cast metal whose internal cleanliness for example depends a lot on the geometric shape of this surface. It is here also that the first solidifying skin is born, whose major importance is known both for the surface quality of the cast product obtained and for the control of the casting process itself.

On the other hand, by a stirring of the metal in the liquid well under the mold, in the zone of the secondary cooling consequently (it is said more commonly "in the secondary"), one seeks more an improvement of the metallurgical structure. internal of the product via the development of a solidification of the largest possible equiaxe type that is known to be favorable both to the microsegregation of the alloying elements and the absence of central porosity of the cast product, in particular. Thus, recourse to electromagnetic stirring in continuous slab casting is more and more common when it comes to producing products that require internal health free from porosity, such as heavy plate for boiler plates, for example or large welded tubes.

We will simply retain here, for a better understanding of the invention which will be exposed later, that it is well known, as shown in the diagram of Figure 3 attached from document FR 72.20546, to have, in the secondary cooling of a continuous casting machine slabs, linear inductors 41, 41 'disposed opposite one another, on both sides of the large faces of the cast product and producing transversely sliding magnetic fields according to the width of it. It is thus intended to establish within the liquid metal flows which, for the most part, develop into two adjacent loops rotating in opposite directions. These loops 42, 43 are established parallel to the large faces and are arranged according to the height of the product cast on either side of a common transverse driving action zone of the magnetic field, the currents of each loop going up along a small face and descending along the small opposite face. Such a movement pattern is conventionally referred to as a "butterfly wing configuration".

It is possible, as shown in Figure 4 attached extracted it, the document FR 2528739, to increase the height of the casting machine the transverse driving action areas 51, 52 ... magnetic fields. In this case, they are given opposing directions of rotation in pairs between adjacent nearest loops, in order, for example, to relate to the largest possible brew volume for a given available brewing power. A so-called "triple-zero configuration" of three adjacent loops rotating in opposite two-to-two directions is thus produced: a central loop-60 located between the two transverse drive zones 51 and 52 and two outer loops, and 62, on both sides of the central loop and rotating in the same direction.

Whatever the variant adopted, this can be achieved both with inductors placed behind the support rollers of the secondary cooling zone of the casting machine, and between these rollers (FR 2187468), or housed within even of these (FR 2187467). It is also the same with regard to the means of implementation of the invention which will be explained later.

It seems that, historically, the discovery of this type of movement, based on a recirculation of the metal in loops developing in a plane parallel to the large faces of the slab, comes from the fact that, unlike the long products, in continuous casting of flat products, the elongated shape of the cross section of the product does not lend itself easily to the establishment of a stable rotational movement around the casting axis. The main reason is probably the strong speed gradients that this requires in the thickness of a product, which hardly exceeds twenty cm for the thickest.

On the other hand, a configuration in stepped loops, of the type shown in FIGS. 3 and 4, developing on the metallurgical height parallel to the large faces of the product does not have such a handicap. It also has the advantage of ensuring a better heat exchange between the top and bottom of the casting machine. The hotter molten metal from the top is forced convection downward by the downward currents 42a and 43a, while the ascending streams 42b and 43b seed the top solidified crystallites of solidified metal collected at the bottom, thereby promoting the development early uniform and broad equiaxed solidification of the periphery to the center of the cast product. However, these loops 42, 43 can not be developed too vigorously upwards as one would like, at the risk of disturbing the free surface of the metal in the mold. It is now known to what extent the preservation of the fragile hydrodynamic equilibrium of ingot mold flows prevailing at this level is necessary to obtain the surface, subcutaneous and internal quality of the cast product.

However, it is precisely that the introduction of the metal to be poured from the top of the mold with the aid of a submerged nozzle with lateral outlet openings opening opposite the small walls of the mold has become almost universal. nowadays, supplanting the straight nozzle with single axial outlet, therefore almost exclusively for long products. A major advantage obtained on the flows in the mold is the fact that, as shown in the diagram of Figure 1 attached, by a phenomenon of rebound against the small end walls of the mold, the jet of hot liquid metal from each side lip 27, 27 'of the nozzle 26, is then naturally divided into two fractions. A main fraction 21 is directed downwards in the direction of extraction of the cast product. The other, 22, is reflected upwardly so as to bring to the vicinity of the free surface 23 of the ingot mold metal the enthalpy required to avoid freezing phenomena of the cast metal at the meniscus, which are often the cause accidental stops of the casting. It is thus intended to produce in the mold a circulation mode called "double loop", as opposed to the "single loop" mode.

The latter, represented in FIG. 7, first results in a phenomenon of rise of the metal towards the meniscus as soon as it leaves the openings of the nozzle, very often resulting from an injection of argon of anti-capping in the nozzle from the tundish located above. This immediate upward rise then continues by a surface current to each small face and a descent along it. In this way, a cartography of the speeds generally directed downwards in the extraction direction of the product is established relatively quickly in the mold, with the absence of the upper loop 22 for supplying "hot" metal to the meniscus.

The "double loop" mode, however, is acquired durably during casting only if the casting conditions are suitable (casting speed, slab width, depth of immersion of the casting nozzle, argon flow anti-capping, etc ...). Random transitions in "single-loop" mode may occur during casting even if these conditions fluctuate, which is in fact the general case.

Moreover, an essential aspect, in terms of mastering "double-loop" flows in the mold, lies in the conservation within the mold of a "left-right" symmetry of the meniscus recirculation movements of both sides. other of the nozzle. It is known in fact that the appearance of asymmetries at this level is at the origin of phenomena of oscillations of the metal bath which can lead to a phenomenon of prohibitive roll of the surface that the operator knows well on the floor of casting . This means that one must ensure that the currents 22, 22 'of partial recirculation upwards are stabilized above all in time to avoid the appearance of asymmetries "left-right". These updrafts, while being sufficiently thermally efficient to provide the desired calories to the meniscus, must not, however, be too hydrodynamically intense to avoid over-shaking the line of first solidification which forms at the edge of the stream. meniscus against the cooled copper wall of the mold. The regularity of this line of first solidification is the guarantee of the homogeneity of the formation of the first skin in the top of the mold without which risks are inevitably incurred breakthroughs under the ingot mold in slag incrustations or local weakenings the thickness of the solidified skin.

More simply, in casting with immersed nozzle with side openings, it is possible to obtain during the same casting in a random manner or, in any case, not necessarily desired, flows in the mold which are either of the "double loop" type, either of the "simple loop" type, or unstable flows due to "left-right" asymmetries.

It is in particular because of these difficulties in controlling the flows in the upper part of the continuous casting machines that more electro-stirring systems have already appeared to appear operating in the mold already on the lateral outlet jets of the nozzle. As shown in the diagrams of FIGS. 2a and 2b, taken from JP 57075270, horizontally movable magnetic fields are produced by polyphase linear inductors 30a, 30b and 30a ', 30b' arranged on the large walls of the mold 32. opposite the exit trajectory of the metal jets on either side of the nozzle 31. According to the setting of the direction of sliding of the fields, it is then possible to slow down the current of the jet concerned (sliding fields against current, ranging from the small wall to the nozzle -fig 3b ₁ ) or, on the contrary, to accelerate (sliding co-current in the direction from the nozzle to the small wall-fig 3b ₂ ). This makes it possible to adjust, in principle, the enthalpy contributions towards the surface of the cast metal as a function, for example, of the casting conditions, without unduly disturbing the ingot mold flow mode, which itself is to be preserved as a priority.

It is therefore clear, through the rapid recall presented above of the prior art, the partitioning, see the antinomy, which exists in fact in casting of products of elongated cross section between mixing of the metal in the mold of one side and brewing in the secondary cooling of the other.

The present invention is precisely to overcome such a handicap. In other words, applicable to the continuous casting of products with an elongate cross-section, slabs in particular, the invention aims, by means of a studied overall stirring movement of the molten metal according to the metallurgical height, to provide a good exchange of the metal still liquid in both directions between the zone of the secondary cooling and the mold. As a result, both thermal and chemical homogeneity between the top and the bottom of the poured liquid metal well will be achieved without disturbing the flow mode in the mold and, if necessary, without depriving itself of the cumulative beneficial effects thereof. mixing in the mold and stirring in the secondary cooling respectively.

An additional object of the invention is to contribute to the improvement of the metallurgical quality of steel grades which are in search of good internal health, such as grades for heavy plate or for large welded tubes, ferritic stainless steels, or electrical silicon steels.

Another complementary aim is to be able to act on the flows in the secondary to use them at the pouring jets from the nozzle either as an accelerating agent or, on the contrary, as a braking agent for the metal arriving in the mold, or else as a means of counteracting the "left-right" asymmetry of movements of the metal within the mold.

With these objectives in view, the subject of the invention is an electromagnetic stirring process in the zone of secondary cooling of a continuous casting plant for slabs, or other similar products with an elongate cross-section, the mold of which is provided with a submerged casting nozzle with lateral outlet openings directed towards the small faces, brewing method implemented using sliding magnetic fields generated by polyphase inductors arranged close to the cast metal, characterized in that one forces, in said zone of the secondary cooling, the establishment of a longitudinal flow of the liquid metal located in the median region of the cast product according to two collinear currents antagonists.

In doing so, an overall circulation of the liquid metal in the secondary is established, which is organized into a "four-leaf clover" with two lobes upper and two lower lobes and the two upper lobes extend into the mold to the level of the jets from the outlets of the nozzle casting.

According to one variant, these two longitudinal antagonistic collinear currents are created in the middle part of the product which move away from each other, so that the two upper lobes which extend in the mold to the level of the jets from the outlets of the outlet of the casting nozzle merge with them in co-current to strengthen them.

According to another variant, these two longitudinal antagonistic collinear currents are created in the middle part of the product which converge towards one another so that the two upper lobes which extend in the mold to the level of the jets coming from the Outlets of the casting nozzle are superimposed on them against the current to slow them down.

According to a particular embodiment of the process, the location of the longitudinal flow in the secondary towards the one or the other of the small walls of the cast product is shifted laterally in order to counter the tendencies to the "left-right" asymmetry of the movements. metal within the mold.

According to one embodiment, the longitudinal metal flow is created in the median region of the cast product in two collinear currents antagonistic using collinear movable magnetic fields sliding longitudinally in said middle region, or approaching one on the other, either by going away.

According to a preferred embodiment, the longitudinal metal flow is created in the median region of the cast product in two collinear currents antagonist using collinear moving magnetic fields sliding transversely along the width of the cast product, or by approaching the one from the other from the edge to the center of the cast product, either moving away from each other from the center towards the edge of the cast product.

According to another preferred implementation, the sliding magnetic fields are generated using polyphase linear inductors that are available opposite the large faces of the cast product.

Alternatively, the inductors are supplied with electric currents of different intensities, in order to regulate differently the action on the two antagonistic collinear metal currents created by the sliding magnetic fields they generate.

By "collinear" sliding of fields or metallic flows, it must be understood that the magnetic fields, respectively the metal currents, slide not parallel to each other but in the same line, as in the case of two vectors colinear with respect to two parallel vectors.

As will have been understood, the invention consists, in its main foundations, to create in the secondary cooling zone a stirring cross with two transverse branches and two longitudinal branches. The transverse branches (or horizontal if one assumes the vertical axis of casting) develop according to the width of the cast product and the two longitudinal (or vertical) branches develop in the median (most often axial) region of the cast product .

And it is the formation in the secondary of such a mixing cross which, because of the four-lobe recirculation flows resulting in the liquid well, will create a global configuration of the movements also concerning the region of the mold as the abovementioned goals of the invention will be achieved.

• les figures de 1 à 4 sont représentatives de l'art antérieur déjà considéré auparavant.
  Plus précisément:
  • * la figure 1 est un schéma habituel montrant sommairement en coupe verticale médiane parallèle au grandes parois de la lingotière la cartographie connue des mouvements de circulation du métal liquide arrivant dans une lingotière de coulée continue de brames par une busette immergée dotée d'ouïes de sortie latérales s'ouvrant en regard des petites parois latérales;
  • * les figures 2a, 2b₁ et 2b₂ sont des schémas, selon deux vues (en perspective à gauche et en section à droite), de modes connus de brassage électromagnétique en lingotière de coulée continue de brames avec busette immergée à ouïes de sortie latérales (cf. fig.1) à l'aide d'inducteurs polyphasés linéaires logés de part et d'autre de la busette sur chaque grande paroi et produisant des champs magnétiques glissant horizontalement en sens opposés deux à deux sur la même grande paroi, soit dans le même sens que le jet de sortie du métal auquel il est appliqué (fig. 2b₂), soit en sens contraire (fig. 2b₁ et 2a);
  • * la figure 3 est un schéma simplifié montrant en perspective une brame en cours de coulée continue dans la zone du refroidissement secondaire de la machine de coulée. Cette zone est dotée d'une paire d'inducteurs linéaires disposés en regard l'un de l'autre de chaque coté du produit selon la largeur de celui-ci et générant un champ magnétique glissant horizontalement de manière à réaliser un mode de brassage électromagnétique en forme "d'ailes de papillon" connu est par exemple du document FR 2187467 précité;
  • * la figure 4 est un schéma analogue au précédent de la fig. 3, mais montrant un mode de brassage électromagnétique en "triple boucle", tel que réalisé par exemple par la mise en oeuvre de l'enseignement du document FR 2528739 pré-mentionné;
• les autres figures, numérotées de 5 à 9 sont propres à l'invention. Plus précisément:
  • * la figure 5 est un schéma général, vu en coupe verticale axiale parallèle aux grandes parois d'une lingotière de coulée continue de brames dotée d'une busette immergée à ouïes de sortie latérales s'ouvrant en regard des petites parois latérales, montrant le principe du brassage global en trèfle à quatre lobes dans la zone du refroidissement secondaire selon l'une des deux mises en oeuvre de l'invention dans laquelle les courants antagonistes longitudinaux s'éloignent l'un de l'autre, et la cartographie des mouvements de circulation du métal liquide qui en résulte au sein de cette zone juste en dessous de la lingotière;
  • * la figure 6 est un schéma analogue à celui de la figure 5, mais dans le cas où le mode d'écoulement en lingotière est, non plus en "double boucle", mais en "simple boucle";
  • * la figure 7a est un schéma qui, sur la base d'une reproduction de la figure 5, montre un moyen de réalisation du brassage en trèfle à quatre lobes à l'aide d'inducteurs linaires à champ magnétique glissant horizontalement;
  • * la figure 7b est un schéma analogue à la figure 7a, mais illustrant un autre mode de réalisation de cette mise en ouvre de l'invention à l'aide cette fois d'inducteurs linéaires à champ magnétique glissant verticalement;
  • * la figure 8 est aussi un schéma qui sur la base d'une reproduction de la figure 5, illustre un mode préféré de réalisation de l'invention instaurant une circulation complémentaire en mode "double boucle" en lingotière à l'aide d'inducteurs linéaires à champ glissant horizontalement agissant directement sur les jets de métal sortant des ouïes de la busette de coulée;
  • * la figure 9 illustre l'autre variante de mise en oeuvre de l'invention consistant à créer des écoulements longitudinaux antagonistes dans la partie médiane du produit coulé, non plus divergents, mais convergents.

The invention will be better understood and other aspects will appear more clearly in view of the following description given with reference to the attached drawing plates in which:

• Figures 1 to 4 are representative of the prior art already considered before.
  More precisely:
  • FIG. 1 is a conventional diagram showing summarily in median vertical section parallel to the large walls of the mold the known cartography of the circulation movements of the liquid metal arriving in a mold of continuous casting of slabs by a submerged nozzle with outlet openings. lateral opening opposite the small side walls;
  • FIGS. 2a, 2b ₁ and 2b ₂ are diagrams, in two views (in perspective on the left and in section on the right), of known electromagnetic stirring modes in continuous slab casting mold with immersed nozzle with side outlet openings. (see fig.1) using polyphase linear inductors housed on both sides of the nozzle on each large wall and producing magnetic fields sliding horizontally in opposite directions in pairs on the same large wall, or in the same direction as the exit jet of the metal to which it is applied (Fig. 2b ₂ ), or in the opposite direction (Fig. 2b ₁ and 2a);
  • FIG. 3 is a simplified diagram showing in perspective a slab during continuous casting in the zone of the secondary cooling of the casting machine. This zone is provided with a pair of linear inductors disposed opposite one another on each side of the product according to the width thereof and generating a magnetic field sliding horizontally so as to achieve an electromagnetic stirring mode in the form of "butterfly wings" known is for example FR 2187467 cited above;
  • FIG. 4 is a diagram similar to the previous one of FIG. 3, but showing a method of electromagnetic stirring in "triple loop", as realized for example by the implementation of the teaching of the document FR 2528739 mentioned above;
• the other figures, numbered from 5 to 9 are specific to the invention. More precisely:
  • FIG. 5 is a general diagram, seen in axial vertical section parallel to the large walls of a continuous slab casting mold with a submerged nozzle with lateral outlet openings opening opposite the small side walls, showing the principle of global stirring in four-lobed clover in the zone of the secondary cooling according to one of the two implementations of the invention in which the longitudinal antagonistic currents move away from each other, and the mapping of the movements circulation of the resulting liquid metal within this zone just below the mold;
  • FIG. 6 is a diagram similar to that of FIG. 5, but in the case where the ingot mold flow mode is no longer a "double loop" but a "single loop";
  • Fig. 7a is a diagram which, on the basis of a reproduction of Fig. 5, shows a means for making four-lobed clover brewing using horizontally sliding linear magnetic field inductors;
  • FIG. 7b is a diagram similar to FIG. 7a, but illustrating another embodiment of this implementation of the invention, this time using linear inductors with a vertical sliding magnetic field;
  • FIG. 8 is also a diagram which, on the basis of a reproduction of FIG. 5, illustrates a preferred embodiment of the invention introducing complementary circulation in "double-loop" mode in an ingot mold using inductors linear sliding field horizontally acting directly on the metal jets exiting gills of the casting nozzle;
  • FIG. 9 illustrates the other alternative embodiment of the invention consisting in creating antagonistic longitudinal flows in the median part of the cast product, no longer divergent, but convergent.

It will be recalled that FIGS. 1 to 4 served to support the presentation of the prior art already made at the beginning of this memoir. We will not come back to this in the following.

In FIGS. 5 to 9 representative of the mode of mixing in the secondary of the invention in these two variants (divergent or convergent in the center), the sliding magnetic fields, just like the linear inductors which produce them, are represented by vertical or horizontal arrows thick. The convection movements produced are represented by their main trajectories in the form of arrows carrying arrowheads indicating the direction of movement of the movement on the carrier trajectory. The solid lines represent active convection zones, therefore of circulations subjected to the action of sliding magnetic fields. The discontinuous lines represent the zones of passive convection, otherwise called recirculation zones, which are necessarily complementary to the previous ones to ensure the closure of the movements.

In these figures, the same elements are designated under identical references. If necessary, in order not to unnecessarily overburden certain figures, recurrent references have not been worn in order to allow more clarity to the essential elements of the invention shown in these figures.

On each of them there is shown a mold 1 of continuous casting slabs followed below by the zone 2 of the secondary cooling of the casting machine, here voluntarily stripped support rollers to not unnecessarily impair the clarity of the drawing. The views being in a plane parallel to the large walls of the mold, only the small side walls are visible at 3 and 3 ', which will determine the small faces 18, 18' of the cast product 6. The large faces being in the plane of figures, they are not referenced in the figures. For the sake of clarity, the reference will be indifferently referred to as the slab cast in itself or its still liquid core more generally called "solidification well".

A submerged nozzle 4 centered on the casting axis A (here confused as is conventionally the case with the longitudinal axis of the cast product) feeds the molten metal mold from a not shown distributor above. This nozzle is provided with lateral outlet openings 5 and 5 'each facing each other of the small walls 3 and 3' respectively. The format of the cast product is determined by the internal dimensions of the ingot mold defining the casting space in which the molten metal arrives in the form of jets 7, 7 'coming out of the openings of the nozzle 4 conventionally in a more or less average direction. horizontal, or slightly inclined downward. The cast product thus progresses from the top, from the meniscus level 8, downwards, in the direction of extraction of the casting machine, vertically or along a curved path in a plane orthogonal to that of the figure, with an extraction speed (casting speed) usually of the order of one meter per minute. During its progression, it progressively solidifies from its periphery to the center by extraction of its internal heat, first in mold 1 in contact with the cooled copper walls, then in the zone of secondary cooling 2 under the effect watering ramps.

It is recalled that the metallurgical height (or depth of the solidification well) is conventionally defined as the difference in dimensions on the vertical between the level of the free surface of the molded metal (or meniscus) and that of the bottom of the solidification well. at the bottom of the secondary cooling zone, where the finishing solidification fronts which develop on each of the large faces of the cast product meet as the solidification progresses.

At about 3 or 4m below the meniscus 8, therefore within the secondary cooling zone 2, a point P is arbitrarily marked on the longitudinal axis of the product (merging with the casting axis A) which will be described as center of the brewing cross 9 specific to the invention. This cross 9 is a cross with four branches, collinear two by two: two longitudinal branches (here vertical) 10a, 10b, forming a pair aligned on the casting axis A, and two transverse branches (here horizontal) 11a, 11b forming a pair developing according to the width of the cast product. In each of the two branches of the same pair, the liquid metal flows in opposite directions two by two. Moreover, the circulation of the metal in one pair is the opposite of that of the other pair.

Due to the necessarily "finite" dimensional nature of the cast product, these branches, as can be seen, are connected in some way to each other by recirculation loops to form an overall flow developing in the plane of the large faces of the product. poured into a four-leaf clover figure, the leaves constituting lobes L1, L2, L3, L4, the two upper ones, L1 and L4, extending to the mold at the outlet jets 7 and 7 '.

Thus, according to the stirring mode shown in FIGS. 5 to 8, the pair of vertical branches is in "divergent" type convection. The metal currents move away from each other from the center P. One, 10a, deflects towards the mold 1 above, the other, 10b, deflects downwards, in the direction of extraction of the poured product, towards the closing point of the solidification well. In the horizontal pair 11a, 11b, the convection of the metal is then of the "convergent" type: the metal currents confluence towards each other towards the confluence center P by flowing the small lateral faces of the product towards the axis longitudinal A.

As already stated, the metal currents forming these branches are created by sliding magnetic fields, themselves generated by linear inductors arranged in close proximity to the cast product facing these large faces (preferably both sides). Of course, it is not necessary that the two pairs of branches are simultaneously activated by the magnetic fields. Only one can be, for example the vertical branches 10a, 10b, the other, 11a, 11b, then naturally becoming the seat of a recirculation reaction, because the center P functions as a current passage node that keeps mass flow rates and momentum, and vice versa.

On the other hand, according to this first stirring mode of the invention, it is important for the vertical branches 10a and 10b to be defluent, as shown in FIGS. 5 to 8. In the upper lobes L1 and L4 close to the mold, the metal goes back then in the center and down along the small faces, and conversely in the lower lobes L2 and L3.

It is found that under these conditions, the implementation of the invention maximizes the exchange of metallic material between the bottom and the top of the liquid well. On the one hand, in fact, the loop circulation of the metal in any lobe takes place in a direction of rotation opposite to that which is established in the two nearest nearest lobes. On the other hand, the force of the casting jets 7 and 7 'is then systematically reinforced by the central flow 10a back to co-current, recycle loops L5 and L6 in the mold to the meniscus 8 will be strengthened in turn. Therefore, the "double loop" mode L5, L1, L4 and L6 present within the mold is thus furthermore stabilized.

In this way, it is easy to understand that any element of liquid metal (which will be isolated by thought at an arbitrary point of the metallurgical height) will have a high probability of finding itself, by random borrowing of successive ascending or descending currents, at the same time. at least once in the mold before descending if it is initially in the zone of the secondary cooling and vice versa if it is initially chosen in the mold, it being understood that globally it will necessarily undergo a mean displacement downwards in the direction of extraction with an average speed equal to the casting speed. In other words, this implementation of the invention maximizes the exchange of molten metal material between the hot zones of the mold and the colder ones of the secondary, and by reinforcing the known means for stabilizing the "double" mode in the mold. loop".

Such an exchange contributes in particular to a better evacuation of the overheating as well as to the advent of a solidification of the early and ample equiaxed type metal, without the risk of disturbing the ingot mold flow mode, on the contrary by reinforcing the stability symmetry "left-right" movements on either side of the nozzle, and whatever the local mode present: "double loop" -cf. Fig. 5-, or "simple loop -fc Fig. 6- So counteracting the natural random tendency of transition from one mode to another.

As already said, the branches 10 and 11 of the stirring cross 9 are generated by the action applied to these places of sliding magnetic fields. The lines of force of these are orthogonal to the surface of the cast product, or at least have an orthogonal main component to maximize the electromagnetic coupling with the liquid metal.

It is well known that such fields can easily be produced by conventional polyphase linear inductors.

FIG. 7a illustrates a first implementation of the invention according to which two identical linear inductors 12 and 13 are placed horizontally at the same height level on the casting machine (collinear inductors) on either side of the axis of rotation. casting and mounted in opposition so as to create collinear magnetic fields sliding transversely along the width of the cast product, small faces 18, 18 'towards the center. These inductors are advantageously dimensioned so as to each generate a sliding magnetic field, along an active convection branch (11a or 11b), of length equal to slightly less than half of the half width of the cast slab 6.

In this case, the driving force of stirring is given by the convergent transverse branches 11a, 11b of the stirring cross, and the longitudinal flow streams 10a, 10b, are then obtained after passage of the confluence point P.

Figure 7b illustrates a second implementation, equivalent to the previous one as to the effects obtained. According to this second variant, the collinear linear inductors 14 and 15 mounted in opposition are arranged vertically on the casting axis. In this way, this time activates directly the longitudinal branches 10a and 10b (whose presence within the secondary is at the very base of the invention), the upper inductor 14 then generating a magnetic field sliding up the casting machine towards the mold, the lower inductor 15 producing a field sliding down towards the bottom of the well.

Figure 8 illustrates a preferred embodiment of the invention. It consists in transforming the upper edge of the upper recirculation lobes L1 and L4 reinforcing the casting jets 7 and 7 'in active convection zones. For this, the pair of inductors already present in the secondary cooling are added to create the stirring cross 9, two additional linear inductors 16, 17 with horizontal sliding fields arranged collinearly on either side of the nozzle 4 at the level of metal jets 7 and 7 'emerging from the gills 5 and 5' and co-running with said jets, from the nozzle to the small walls 3, 3 'of the mold 1. The effect of convergence between the jets and the central flow up from the bottom is thus further strengthened and, consequently, the local regime type "double loop" in the mold as well.

FIG. 9, similar to FIG. 5, however, differs essentially from this in that the directions of circulation of the metal in each of the four branches of the cross 9 are reversed. This FIG. 9 thus illustrates the second main variant of implementation of the invention which consists in creating longitudinal antagonistic collinear currents 20a, 20b in the median part of the cast product 6 which this time converge towards each other in the direction of the point P so as to provide an overall circulation of the liquid metal which extends in the mold 1 by currents rising along the small faces 18, 18 'to the level of the metal jets 7, 7' from the exit ports 5, 5 'of the nozzle with which they oppose against the current to slow them down.

There is generally a brewing configuration in the secondary four lobes L1 to L4, and whose loops turn in opposite directions compared to the first variant. However, due to the antagonistic effect of the upper lobes L1 and L4 on the jets 7 and 7 ', the return currents of the metal downwards in the central part of the liquid well are less channeled and confined, but much more diffuse and dispersed in the product section than in said first variant.

It is understood that these two main variants are in fact only two different and complementary facets of the same invention and which may be present jointly during the implementation of the brewing process. It will indeed be easy to modify dynamically the sliding directions of the magnetic fields acting, for example by inverting the polarity of the inductors that produces them, so as to be able to brake or accelerate the currents of the casting streams 7, 7 ' from a brewing action located in the secondary away from these streams.

It can therefore be seen that a decisive advantage of the invention is to ensure a good up / down exchange in the liquid well while being able to act remotely on the casting jets in the mold, and this by means of a mounting simple and rustic electromagnetic stirring equipment whose components are widely available commercially.

As will be understood, the invention consists, in short, judiciously use electromagnetic stirring means currently available to achieve in the secondary a cutting in the long direction of the product in two strands juxtaposed and in each strand, install a butterfly wing type brewing configuration. In doing so, we create an overall flow system in the secondary four-lobe whose heart is the stirring cross 9 with its center P.

Preferably, for obvious reasons of symmetry, this division into two strands will be half width of the cast product, that is to say along the longitudinal axis thereof, because this axis is generally confused with the casting axis.

That said, it will suffice to unbalance the stirring forces between the two transverse branches 11a, 11b, for example via a differentiated adjustment of the intensities of the electric currents supplying the inductors 12, 13, to move laterally the center position of the center P towards a small face 5 or 5 'to the other and thus obtain on the movements in the mold a more selective effect on one side of the nozzle than on the other.

Likewise, a similar imbalance on the longitudinal branches 10a, 10b will make it possible, with a given brewing equipment, to move up or down the center P of the brewing cross without having to modify the location of this equipment on the casting machine.

Certainly, if one wants to be able to intervene jointly on these two possibilities of adjustment of the position of the center P of the stirring cross, it will be necessary to equip the secondary one of equipment with four inductors so as to be able to activate electromagnetically each of the four branches 10a, 10b, 11a and 11b.

Whatever its mode of implementation, the invention provides a total mixing of the metal on the metallurgical height capable of ensuring both thermal and chemical homogeneity between the top and bottom of the liquid well without depriving itself of the beneficial effects specific to ingot brewing and in the secondary cooling respectively, and without disturbing or even stabilizing the local flow mode in the mold.

It goes without saying that the invention is not limited to the examples described above, but that it extends to multiple variants or equivalents insofar as its definition given in the following claims is respected.

Thus, for example, if the linear inductors to be used conventionally have a planar structure, this provision is only preferential. Curved inductors can also be used to better match the shape of the slab surface where they are placed on the metallurgical height.

Claims (10)

1. Method of electromagnetic stirring in the secondary cooling zone of a plant for the continuous casting of metal products of elongate cross section, the mould of which is provided with a submerged casting nozzle having lateral discharge outlets directed towards the small walls of the mould, which stirring method is implemented by means of travelling magnetic fields generated by multiphase inductors placed near the cast metal, characterized in that, for the purpose of promoting liquid metal exchange within the solidification pool (6) between the secondary cooling zone (2) and the mould (1), a longitudinal metal flow is forcibly established in the said secondary cooling zone, said flow being localized in the middle region of the cast product as two opposing collinear streams (10a, 10b, or 20a, 20b) and providing circulation of the liquid metal as a "four-leaf clover" configuration design having two upper lobes and two lower lobes, said upper lobes (L1, L4) extending into the mould right up to the level of the jets (7, 7') coming out from the discharge outlets (5, 5') of the submerged casting nozzle (4).
2. Stirring method according to claim 1, characterized in that the said longitudinal opposing collinear streams (10a, 10b) in the middle region of the cast product, which move away from each other, are created in such a way that the said two upper lobes (L1, L4) which extend into the mould right up to the level of the jets (7, 7') coming out from the discharge outlets (5, 5') of the casting nozzle (4) merge cocurrently with the said jets in order to reinforce them.
3. Stirring method according to claim 1, characterized in that the said longitudinal opposing collinear streams (20a, 20b) in the middle region of the cast product, which converge on each other, are created in such a way that the two upper lobes (L1, L4) that extend into the mould up to the level of the jets (7, 7') emanating from the discharge outlets (5, 5') of the casting nozzle (4) are superposed counter-currently on the said jets in order to brake them.
4. Stirring method according to claim 1, characterized in that the location of said longitudinal flow in the secondary is shifted laterally towards one or other of the small sides of the cast product.
5. Stirring method according to claims 2 and 3, characterized in that said longitudinal metal flow is created as two opposing collinear streams by means of collinear moving magnetic fields that travel longitudinally in the said central region, either coming closer together, or further apart.
6. Stirring method according to claims 2 and 3, characterized in that said longitudinal metal flow is created as two opposing collinear streams by means of collinear moving magnetic fields that travel transversely over the width of the cast product, either coming closer together from the edge towards the centre of the cast product, or moving further apart from the centre towards the edge of the cast product.
7. Stirring method according to any one of the preceding claims, characterized in that said travelling magnetic fields are generated by means of multiphase linear inductors that are placed facing the large walls of the cast product.
8. Method according to claim 7, characterized in that said inductors are supplied with electric currents of different intensities.
9. Method according to one of the preceding claims, characterized in that other travelling magnetic fields are also used that act directly in the mould (1) on the jets (7, 7') of metal discharging from the outlets (5, 5') of the nozzle (4).
10. Flat metal product obtained from a continuous casting plant, the secondary cooling zone of which being the location of an electromagnetic stirring operation according to that defined in claim 1.

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