EP1677928A1

EP1677928A1 - Electromagnetic agitation method for continuous casting of metal products having an elongate section

Info

Publication number: EP1677928A1
Application number: EP04805290A
Authority: EP
Inventors: Siebo Kunstreich
Original assignee: Rotelec SA
Current assignee: Rotelec SA
Priority date: 2003-10-27
Filing date: 2004-10-22
Publication date: 2006-07-12
Anticipated expiration: 2024-10-22
Also published as: TWI324952B; BRPI0415903B1; JP4758903B2; RU2006118350A; CA2543368A1; RU2357833C2; AU2004286877A1; KR101089261B1; JP2007509752A; ES2285558T3; DE602004006010T2; EP1677928B1; US20070074845A1; FR2861324B1; CN100371108C; TW200533437A; BRPI0415903A; CN1863625A; AU2004286877B2; ZA200604177B

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

Electromagnetic stirring process for the continuous casting of metallic products of elongated section.

The present invention relates to the continuous casting of metals, in particular steel. It relates more particularly to the electromagnetic stirring of products with an elongated straight section during casting, and more precisely still the establishment, within the part still in the liquid state of the metal being cast, of a particular distribution of the flows at l using applied magnetic fields. It will be recalled that the term “product with an elongated cross section” means metallurgical products whose width is at least twice the thickness, in particular slabs, slabs, thin slabs, etc. Appeared in the field of the continuous casting of steel at the beginning of the seventies, the electromagnetic stirring quickly asserted itself there as an almost essential tool for the control of the flows in the well of liquid metal in the course of solidification. It is recalled that the principle most commonly implemented is that well known of Magneto-HydroDynamics (MHD) which, by means of a mobile magnetic field (rotating or sliding) generated by one or more generally several polyphase inductors arranged in the vicinity immediate flow of the product, drives the liquid metal in its movement. Suitably placed on the metallurgical height of the casting machine, these inductors, supplied with electric current at adjustable frequency, then allow various brewing modes, adaptable to the needs of the metallurgist. In addition, the constant progress made in understanding the mechanisms of metal solidification during continuous casting show the importance that the circulation movements of liquid metal have precisely on quality in general (ie internal health, surface cleanliness or inclusionary , solidification structure, etc.) of the solidified product obtained. In this regard, the movements imparted to molten metal during continuous casting can schematically be classified into two distinct categories, depending on whether one considers the ingot mold or, below, the stages of the secondary cooling of the casting machine. The movements imposed on the liquid metal within the ingot mold, where the liquid metal fraction is largely in the majority, aim essentially at controlling the flows at this sensitive location. This is in fact where the free surface of the cast metal is found, the internal cleanliness of which for example very much depends on the geometric shape of this surface. This is also where the first solidification skin is born, the major importance of which 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 stirring the metal in the liquid well under the ingot mold, in the secondary cooling zone consequently (we more commonly say "in the secondary"), we seek more an improvement of the metallurgical structure internal of the product via the development of the widest possible equiaxial type solidification which is known to be favorable both to the microsegregation of the alloying elements and to the absence of central porosities of the cast product, in particular. Thus, recourse to electromagnetic stirring in continuous casting of slabs is more and more frequent as soon as it is a question of producing products which require internal health free of porosity, such as heavy plates for boilermaking plates for example. or large welded tubes. It will simply be retained here, for a better understanding of the invention which will be explained below, that it is well known, as shown in the diagram in FIG. 3 attached from document FR 72.20546, to have, in the secondary cooling of a continuous slab casting machine, of linear inductors 41, 41 'arranged opposite one another, on either side of the large faces of the cast product and producing transverse sliding magnetic fields depending on the width of it. The aim is thus to establish flows within the liquid metal which, essentially, develop in two adjacent loops rotating in opposite directions. These loops 42, 43 are established parallel to the large faces and are staggered according to the height of the product poured on either side of a common transverse motor action area of the magnetic field, the currents of each loop going up along of a small face and descending along the small opposite face. Such a configuration of movements is conventionally referred to as a "butterfly wing configuration". It is possible, as shown in the attached figure 4 extracted from the document FR 82.10844, to multiply the transverse motor action zones 51, 52 ... of the magnetic fields according to the height of the casting machine. In this case, they are given opposite directions of rotation two by two between the nearest neighboring loops, in order for example to concern a volume stirred as large as possible for a given available stirring power. A topology of movements is thus produced, known as a "triple zero configuration" formed by three adjacent loops rotating in opposite directions two by two: a central loop 60 located between the two transverse motor zones 51 and 52 and two external loops 61 and 62, on either side of the central loop and rotating in the same direction. Whichever variant is chosen, it can be carried out both with inductors placed behind the support rollers of the secondary cooling zone of the casting machine, and between these rollers (FR 72.20547), or housed within even of these (FR 72.20546). It is also the same with regard to the means of implementing the invention which will be explained below. 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 long products, in continuous casting of flat products, the elongated shape of the straight section of the product does not easily lend itself to the establishment of a stable rotational movement around the casting axis. The main reason is probably due to the large 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 over the metallurgical height parallel to the large faces of the product does not involve such a handicap. It also has the advantage of ensuring better heat exchange between the top and bottom of the casting machine. ' The hotter molten metal from the top is brought into forced convection down by the downdrafts 42a and 43a, while the rising currents 42b and 43b come to seed the top in solidified metal crystallites collected at the bottom, thus promoting the early development of a large and regular equiaxial solidification from the periphery to the center of the cast product. However, one cannot develop these loops 42, 43 too vigorously upwards as one would like at the risk of disturbing the free surface of the metal in the mold. We now know in fact how much the preservation of the fragile hydrodynamic balance of 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 using a submerged nozzle with lateral outlet openings opening opposite the small walls of the mold has been almost generalized nowadays, supplanting the straight nozzle with a single axial outlet, therefore almost exclusively reserved for long products. A major advantage obtained in the mold flow lies in the fact that, as shown in the diagram in FIG. 1 attached, by a rebound phenomenon against the small end walls of the mold, the jet of hot liquid metal from each lateral opening 27, 27 ′ of the nozzle 26, is then naturally divided into two fractions. A main fraction 21 ^' is directed downward, in the direction of extraction of the cast product. The other, 22, - is reflected upwards so as to bring in the vicinity of the free surface 23 of the metal in the mold the enthalpy necessary to avoid freezing phenomena of the metal cast at the meniscus, which very often are the because of accidental stops of the casting. The aim is thus to produce in a mold a circulation mode known as a "double loop", as opposed to the "single loop" mode. The latter, shown in FIG. 7, results first of all in a phenomenon of the metal rising towards the meniscus as soon as it leaves the vents of the nozzle, very often resulting from an injection of anti-clogging argon in the nozzle from the flow distributor located above. This immediate ascent upwards is then extended by a surface current towards each small face and a descent along it. In this way, fairly quickly established in the mold, a map of the speeds generally directed downward in the direction of extraction of the product with the absence of the upper loop 22 for supplying "hot" metal to the meniscus. The "double loop" mode is however only acquired durably during casting if the casting conditions lend themselves to it (casting speed, slab width, depth of immersion of the pouring nozzle, argon flow rate anti-clogging, etc.). Random transitions in "single loop" mode can appear during the casting process if these conditions fluctuate, which in fact corresponds to the general case. In addition, an essential aspect, in terms of control. flows in "double loop" in the mold, lies in the conservation within the mold of a "left-right" symmetry of the recirculation movements at the meniscus on either side of the nozzle. We know 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 unacceptable roll of the surface which the operator knows well on the casting floor. . This means that care must be taken that the currents 22, 22 ′ of partial upward recirculation are above all stabilized over time to avoid the appearance of "left-right" asymmetries. These ascending currents, while being thermally efficient enough to supply the desired calories to the meniscus, must not however be too intense in terms of hydrodynamics in order to avoid over-agitation of the line of first solidification which forms at the edge of the meniscus against the cooled copper wall of the mold. The regularity of this line of first solidification is the pledge indeed of the homogeneity of the formation of the first skin in the top of the ingot mold without which risks are inevitably incurred from breakthroughs under the ingot mold by slag encrustations or by local weakening of the thickness of the solidified skin. Put more simply, in casting with submerged nozzle with lateral outlets, one can obtain, during the same casting in a random manner _, in any case, not necessarily desired, ingot mold flows which are either of the "double loop" type. , ^' either of the "single loop" type, or of unstable flows due to "left-right" asymmetries. It is in particular because of these difficulties in controlling flows in the upper part of continuous casting machines that we have seen more recently appear electromagnetic stirring systems already acting in the mold on the lateral outlet jets of the nozzle. As shown in the diagrams in FIGS. 2a and 2b attached, extracted from document JP 1,534,702, 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 path of the metal jets on either side of the nozzle 31. Depending on the setting of the direction of sliding of the fields, it is then possible to slow the current of the jet concerned (sliding of the fields against current, going from the small wall to the nozzle -fig 3b _j ) or, on the contrary, to accelerate it (sliding in co-current in the direction going from the nozzle to the small wall-fig 3b ₂ ). This makes it possible in principle to adjust the enthalpy contributions towards the surface of the cast metal as a function, for example, of the casting conditions, without disturbing too much the flow mode in the mold which, for its part, must be preserved as a priority. We can therefore clearly see, through the rapid reminder presented above of the prior art, the partitioning, see the antinomy, which exists in fact in the casting of products of elongated cross section between stirring of the metal in an ingot mold. one side and mixing in the secondary cooling on the other. The present invention specifically aims to overcome such a handicap.

In other words, applicable to the continuous casting of products with an elongated cross section, the slabs in particular, the invention aims, via an overall mixing movement studied of the molten metal according to the metallurgical height, to provide a good exchange of the metal still liquid in both directions between the secondary cooling zone and the mold. Therefore, both thermal and chemical homogeneity will be achieved between the top and bottom of the poured liquid metal well without disturbing the flow mode in the ingot mold and, if necessary, without depriving itself of the cumulative beneficial effects proper mixing in an ingot mold and mixing in secondary cooling respectively. A further aim of the invention is to contribute to the improvement of the metallurgical quality of steel grades for which good internal health is sought, such as grades for heavy plates or for large welded tubes, ferritic stainless steels, or electrical steels with silicon. Another additional aim is to be able to act on the flows in the secondary to use them at the level of the casting jets coming from the nozzle either as an accelerating agent, or on the contrary as a braking agent for the metal arriving in the ingot mold, or even as a means to counteract the inclinations of "left-right" asymmetry of the movements of the metal within the ingot mold. With these objectives in view, the invention relates to an electromagnetic stirring process in the secondary cooling zone of an installation for continuous casting of slabs, or other similar products with an elongated straight section, the ingot mold of which has a immersed pouring nozzle with lateral outlet openings directed towards the small faces, stirring process implemented using sliding magnetic fields generated by polyphase inductors placed near the cast metal, characterized in that one forces, in said secondary cooling zone, the establishment of a longitudinal flow of the liquid metal located in the middle region of the product poured according to two antagonistic collinear currents. In doing so, an overall circulation of liquid metal is naturally established in the secondary, which is organized into a figure of "four-leaf clover" having two lobes upper and two lower lobes and the two upper lobes of which extend in the mold up to the level of the jets coming from the outlet openings of the pouring nozzle. As a 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 up to the level of the jets coming from the outlet openings of the pouring nozzle merge with them in co-current to reinforce them. In accordance with 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 up to the level of the jets coming from the Outlets of the pouring nozzle are superimposed on them against the current to brake them. According to a particular embodiment of the method, the location of the longitudinal flow in the secondary is shifted laterally towards one or the other of the small walls of the cast product in order to counteract the tendencies to the "left-right" asymmetry of the movements of metal within the mold. In accordance with one implementation, the longitudinal metal flow is created in the middle region of the product poured according to two antagonistic collinear currents using collinear mobile magnetic fields sliding longitudinally in said median region, or by approaching the from each other, either by moving away. In accordance with a preferred implementation, the longitudinal metal flow is created in the middle region of the cast product according to two antagonistic collinear currents using mobile collinear magnetic fields sliding transversely along the width of the cast product, ie by approaching the 'from each other from the edge towards the center of the cast product, or by moving away from each other from the center towards the edge of the cast product. - In accordance with another preferred implementation, - the sliding magnetic fields are generated using polyphase linear inductors which are placed opposite the large faces of the cast product. As a variant, the inductors are supplied with electric currents of different intensities, in order to regulate differently the action on the two antagonistic collinear metallic currents created by the sliding magnetic fields which they generate. By "collinear" sliding of the metal fields or flows, it should be understood that the magnetic fields, respectively the currents of metal, slide not in parallel with each other but in the same line, like two collinear vectors with respect to two parallel vectors. As will be understood, the invention consists, in its main foundations, in creating in the secondary cooling zone a cross-stitch with two transverse branches and two longitudinal branches. The transverse branches (or horizontal if we assume the vertical pouring axis) develop according to the width of the cast product and the two longitudinal (or vertical) branches develop in the middle region (most often axial) of the cast product . And it is the formation in the secondary of such a stirring cross which, due to the recirculation flows in the four-lobed figure which results therefrom within the liquid well, will create a global configuration of the movements also concerning the region of the ingot mold such that the aforementioned aims of the invention will be achieved. The invention will be well understood and other aspects will appear more clearly in the light of the description which follows given with reference to the accompanying drawing plates in which: - Figures 1 to 4 are representative of the prior art already considered before. More precisely: * FIG. 1 is a usual diagram showing summarily in median vertical section parallel to the large walls of the ingot mold the known map of the movements of circulation of the liquid metal arriving in- an ingot mold for continuous casting of slabs by an immersed nozzle provided with 'side outlet openings facing the small side walls; * Figures 2a, 2b _! and 2b ₂ are diagrams, according to two views (in perspective on the left and in section on the right), of known modes of electromagnetic mixing in the mold for continuous casting of slabs with submerged nozzle with lateral outlets (cf. fig.l) using linear polyphase inductors housed on either side of the nozzle on each large wall and producing magnetic fields sliding horizontally in opposite directions two by two on the same large wall, either in the same direction as the jet of the metal to which it is applied (fig. 2b ₂ ), or in the opposite direction (fig. 2b _! and 2a); ^• * Figure 3 is a simplified diagram showing in perspective "a slab during continuous casting in the secondary cooling zone of the casting machine. This zone is provided with a pair of linear inductors arranged opposite one on the other on each side of the product along the width thereof and generating a horizontally sliding magnetic field so as to achieve an electromagnetic mixing mode in the form of "butterfly wings" known is for example from the document FR 72.20546 cited above * Figure 4 is a diagram similar to the previous one in Fig. 3, but showing a mode of electromagnetic mixing in "triple loop", as achieved for example by implementing the teaching of the document FR 82.10844 pre- - the other figures, numbered from 5 to 9 are specific to the invention. * Figure 5 is a general diagram, seen in axial vertical section parallel to the large walls of an ingot mold for continuous casting of slabs provided with an immersed nozzle with lateral outlet openings opening opposite the small side walls, showing the principle of global four-lobe clover mixing in the secondary cooling zone according to one of the two embodiments of the invention in which the longitudinal opposing 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; * Figure 6 is a diagram similar to that of Figure 5, but in the case where the flow mode in the mold is no longer in "double loop", but in "single loop"; * Figure 7a is a diagram which, on the basis of a reproduction of Figure 5, shows a means of carrying out stirring in four-lobe clover using linear inductors with a magnetic field sliding horizontally; * Figure 7b is a diagram similar to Figure 7a, but illustrating another embodiment of this implementation of the invention using this time linear inductors with magnetic field sliding vertically; * Figure 8 is also a diagram which on the basis of a reproduction of Figure 5, illustrates a preferred embodiment of the invention establishing a complementary circulation in "double loop" mode in the mold using inductors linear sliding field horizontally acting directly on the metal jets leaving the vents of the pouring nozzle; * Figure 9 illustrates the other variant implementation of the invention consisting of creating antagonistic longitudinal flows in the middle part of the cast product, no longer divergent, but convergent. It will be recalled that FIGS. 1 to 4 served as a support for the presentation of the prior art already made at the start of this thesis. We will not come back to this in the following. In FIGS. 5 to 9 representative of the mode of mixing in the secondary proper to 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 thick vertical or horizontal arrows. The convection movements produced are themselves represented by their main trajectories in the form of lines carrying arrowheads indicating the direction of movement movement on the carrier path. The solid lines represent active convection zones, therefore of circulation subjected to the action of sliding magnetic fields. The broken lines represent the zones of passive convection, in other words zones of re-circulation, which are necessarily complementary to the preceding ones to ensure the looping of the movements. In these figures, the same elements are designated with identical references. If necessary, not to unnecessarily overload certain figures, récureentes references have not been laid to allow more clarity ^'to the essential elements of the invention shown in these figures. On each of them there is shown an ingot mold 1 for continuous casting of slabs followed below by zone 2 of the secondary cooling of the casting machine, here deliberately stripped of the supporting rollers so as not to unnecessarily harm 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 the figures, they are not referenced in the figures. Furthermore, for greater clarity, the reference 6 will designate either the slab poured in itself or its still liquid core more generally called "solidification well". A submerged nozzle 4 centered on the casting axis A (merged here as is conventionally the case with the longitudinal axis of the cast product), feeds the ingot mold in molten metal from a distributor not shown located above. This nozzle is provided with gills of lateral outlets 5 and 5 ′ each turned opposite one and the other of the small walls 3 and 3 ′ respectively. The form of the cast product is determined by the internal dimensions of the mold defining the casting space into which the molten metal arrives in the form of jets 7, 7 ′ emerging from the openings of the nozzle 4 conventionally in a more or less average direction. horizontal, or slightly tilted downwards. The cast product thus progresses from the top, from the level of the meniscus 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 gradually solidifies from its periphery to the center by extracting its internal heat, first in a mold 1 in contact with the walls of cooled copper, then in the secondary cooling zone 2 under the effect water sprinklers. It will be 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 metal cast in an ingot mold (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 meet on each of the large faces of the cast product meet as the solidification progresses. About 3 or 4m below the meniscus 8, therefore within the secondary cooling zone 2, there is arbitrarily located on the longitudinal axis of the product (merged with the pouring axis A) a point P which will be qualified as center of the cross stitch 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) l ia, 11b forming a pair developing along the width of the cast product . In each of the two branches of the same pair, the liquid metal flows there in opposite directions two by two. Furthermore, the circulation of metal in one pair is opposite to that of the other pair. Due to the necessarily "finished" dimensional nature of the cast product, these branches, as can be seen, are connected in a way with each other by recirculation loops to form an overall flow developing in the plane of the large faces of the product cast in the shape of a four-leaf clover, the leaves constituting lobes L1, L2, L3, L4, the upper two of which, L1 and L4, extend to the mold at the outlet jets 7 and 7 '. Thus, according to the mode of mixing represented in FIGS. 5 to 8, the pair of vertical branches is in "divergent" type convection. The metal streams move away from each other from the center P. One, 10a, deflects towards the mold 1 located above, the other, 10b, deflates downward, in the direction of extraction of the poured product, towards the place of closure of the solidification well. In the horizontal pair l ia, 11b, the convection of the metal is then of the "convergent" type: the metallic currents converge towards each other in the direction of the center of confluence P by flowing from the small lateral faces of the product towards the longitudinal axis A. As already said, the metal currents which form these branches are created by sliding magnetic fields, themselves generated by linear inductors arranged in the immediate vicinity of the product cast opposite these large faces (preferably the two faces) . Of course, it is not necessary for the two pairs of branches to be simultaneously activated by the magnetic fields. Only one can be, for example the vertical branches 10a, 10b, the other, there ia, 11b, then naturally becoming the seat of a recirculation by reaction, because the center P functions as a node of passage of currents which keeps the mass flow rates and the momentum, and vice versa. On the other hand, according to this first brewing method of the invention, it is important that the vertical branches 10a and 10b be defluent, as shown in FIGS. 5 to 8. In the upper lobes L1 and L4 close to the ingot mold, the metal rises then in the center and down along the small faces, and vice versa in the lower lobes L2 and L3. It turns out that under these conditions, the implementation of the invention maximizes the exchanges of metallic material between the bottom and the top of the liquid well. On the one hand, in fact, the looping circulation of the metal in any lobe takes place in a direction of rotation opposite to that which is established in the two closest neighboring lobes. On the other hand, the strength of the casting jets 7 and 7 ′ then being systematically reinforced by the central flow 10a rising co-current, the recirculation loops L5 and L6 in the mold towards the meniscus 8 will be reinforced in their turn. Consequently, the "double loop" mode L5, L1, L4 and L6 present within the mold is thus moreover stabilized. In this way, it is easily understood that any element of liquid metal (which one will isolate by thought at an arbitrary place of the metallurgical height) will have a high probability of finding itself, by random borrowing of ascending or successive descending currents, at the at least once in the mold before going back down if it is initially in the secondary cooling zone and vice versa if it is initially chosen in the mold, it being understood that overall it will necessarily undergo an average downward movement 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 metallic material between the hot zones of the ingot mold and those colder of the secondary and this by reinforcing in the ingot mold the known means capable of stabilizing the "double" mode there. 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 metal of the early and ample equiaxial type, without risk of disturbing the flow mode in the ingot mold, on the contrary by reinforcing the stability. the "left-right" symmetry of the movements on either side of the nozzle, regardless of the local mode present: "double loop" -cf. Fig. 5-, or "simple loop -cf. Fig. 6- therefore by counteracting the natural random tendency of transition from one mode to the other. As already said, the branches 10 and 11 of the cross stitch 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. well known that such fields can be easily produced by conventional polyphase linear inductors Figure 7a illustrates a first implementation of the invention according to which two identical linear inductors 12 and 13 are placed horizontally at the same level in height on the machine of casting (collinear inductors) on either side of the casting axis and mounted in opposition so as to create transverse sliding collinear magnetic fields depending on 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 a little less than half the half width of the slab 6. In this case, the driving force for stirring is given by the converging transverse branches 11a, 11b of the stirring cross, and the longitudinal defluent flows 10a, 10b, are then obtained after passing from the point of confluence P. FIG. 7b illustrates a second implementation, equivalent to the previous one as regards 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 the longitudinal branches 10a and 10b are directly activated (the presence of which within the secondary is at the very basis of the invention), the upper inductor 14 then generating a magnetic field sliding upwards. casting machine in the direction of the mold, the lower inductor 15 producing a sliding field 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 pouring jets 7 and 7 ′ into zones of active convection. For this, we add to the pair of inductors already present in the secondary cooling to create the cross-connect 9, two additional linear inductors 16, 17 with horizontal sliding fields arranged collinearly from pasture and from other side of the nozzle 4 at the level metal jets 7 and 7 ′ emerging from the gills 5 and 5 ′ and sliding in co-current with said jets, from the nozzle to the small walls 3, 3 ′ of the ingot mold 1. The effect of convergence between the jets and the central flow rising from the bottom is thus further refined and, consequently, the local regime of the "double loop" type in the mold also. FIG. 9, similar to FIG. 5, is essentially distinguished from it by the fact 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 for implementing the invention which consists in creating longitudinal antagonistic collinear currents 20a, 20b in the middle part of the cast product 6 which this time converge towards each other in the direction of point P so as to provide circulation of the liquid metal which extends into the mold 1 by currents rising along the small faces 18, 18 'up to the level of the metal jets 7, 7' coming from the outlet gills 5, 5 'of the nozzle with which they oppose against the current to slow them down. There is generally a stirring configuration in the secondary with four lobes L1 to L4, and the loops of which therefore rotate 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 downward metal return currents 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 reality 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 dynamically modify the sliding directions of the acting magnetic fields, for example by reversing the polarities of the inductors which produce them, so as to be able to brake or accelerate the currents of the casting jets 7, 7 'on demand. from a brewing action located in the secondary far from these jets. We therefore see that a decisive interest of the invention is to ensure a good exchange up / down in the liquid well while being able to act remotely on the casting jets in an ingot mold, and this using an assembly simple and rustic electromagnetic brewing equipment whose components are widely available commercially. As will be clearly understood, the invention consists, in short, in judiciously using the electromagnetic stirring means currently available to carry out in the secondary a cutting in the long direction of the product into two strands juxtaposed and, in each strand, to install a butterfly wing type brewing configuration. In doing so, an overall flow system is created in the secondary with four lobes, the heart of which is the cross cross 9 with its center P. Preferably, for obvious reasons of symmetry, this division into two strands will be made half-width of the cast product, that is to say along the longitudinal axis thereof, since this axis generally merges with the casting axis. That said, it will suffice to unbalance the stirring forces between the two transverse branches 11 a, 11 b, for example via a differentiated adjustment of the intensities of the electric currents supplying the inductors 12, 13, to laterally move the middle position from the center P towards a small face 5 or towards the other 5 ′ and thus obtain a more selective effect on one side of the nozzle on the mold movements than on the other. Similarly, a similar imbalance on the longitudinal branches 10a, 10b will allow, with a given brewing equipment, a displacement up or down of the center P of the brewing cross without having to modify the location of this equipment on the casting machine. Admittedly, if we want to be able to intervene jointly on these two possibilities of adjusting the position of the center P of the cross-connect, it will be necessary to provide the secondary with equipment with four inductors so as to be able to activate electromagnetically each of the four branches 10a, 10b, lia and 11b. Whatever its mode of implementation, the invention provides one _. overall mixing of the metal over the metallurgical height capable of ensuring both thermal and chemical homogeneity between the top and the bottom of the liquid well without losing the beneficial effects specific to mixing in the mold and in 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 as long as its definition given in the claims which follow is respected. Thus, for example, if the linear inductors to be used have conventionally a planar structure, this arrangement is only preferential. May also be suitable, inducers of curved shape to best match the shape of the surface of the slab where they are placed on the metallurgical height.

Claims

1. Method of electromagnetic stirring in the secondary cooling zone of a continuous casting installation for metal products of elongated straight section whose ingot mold is provided with a submerged casting nozzle with lateral outlet openings directed towards the small faces, process stirring implemented using sliding magnetic fields generated by polyphase inductors arranged near the cast metal, characterized in that, in order to promote ^' the exchanges of liquid metal within the solidification well (6) between the secondary cooling zone (2) and the ingot mold (1), in said secondary cooling zone, the establishment of a longitudinal metal flow localized in the median region of the product is forced according to two antagonistic collinear currents (10a, 10b or 20a, 20b) and providing an overall circulation of the liquid metal in the form of a "four-leaf clover""having two upper lobes and two lower lobes, and whose upper lobes (L1, L4) extend in the mold up to the level of the jets (7, 7 ') coming from the outlet openings (5, 5') of the submerged pouring nozzle (4).

2. Brewing method according to claim 1, characterized in that one creates said antagonistic longitudinal collinear currents (10a, 10b) in the middle part of the cast product move away from each other, so that said two upper lobes (L1, L4) which extend in the mold up to the level of the jets (7, 7 ') coming from the outlet openings (5, 5') of the pouring nozzle merge with them in co- current to strengthen them.

3. Brewing method according to claim 1, characterized in that one creates said longitudinal antagonistic collinear currents (20a, 20b) in the middle part of the cast product converge towards each other so that the two lobes upper (L1, L4) which extend in the mold up to the level of the jets (7, 7 ') coming from the outlet openings (5, 5') of the pouring nozzle are superimposed against them for the curb.

4. Brewing method according to claim 1, characterized in that one locates laterally the location of the median longitudinal flow in the secondary towards one or the other of the small faces of the cast product. 0/3 16

5. Brewing method according to claims 2 or 3, characterized in that one creates the longitudinal metallic flow in the middle region of the product poured according to two antagonistic collinear currents using mobile collinear magnetic fields which slide longitudinally in the said median region, either by approaching one another, or by moving away.

6. Brewing method according to claims 2 or 3, characterized in that one creates the longitudinal metallic flow in the middle region of the product poured according to two antagonistic collinear currents using mobile collinear magnetic fields which slide transversely according the width of the cast product, either by approaching each other from the edge towards the center of the cast product, or by moving away from each other from the center towards the edge of the cast product.

7. Brewing method according to any one of the preceding claims, characterized in that the sliding magnetic fields are generated using polyphase linear inductors which are placed opposite the large faces of the cast product. .

8. Method according to claim 7, characterized in that the inductors are supplied with electric currents of different intensities.

9. Method according to any one of the preceding claims, characterized in that use is also made of: moving magnetic fields - sliding which act - directly in an ingot mold (1) on the metal jets (7, 7 ') leaving the vents (5, 5 ') of the nozzle (4).

10. Metal product of elongated cross section from a continuous casting installation, the secondary cooling zone of which is the seat of an electromagnetic stirring operation in accordance with that defined in claim 1.