WO2017125649A1 - Method for rotary electromagnetic stirring of a molten metal during casting of a product having a wide cross-section and apparatus for implementing same - Google Patents

Abstract

During the casting of XXL metal products of round, square or rectangular cross-section, an electromagnetic stirring apparatus is used that is specific to the casting machine receiving same and that includes polyphase, linear or circular inductors having a moving (sliding or rotating) magnetic field and, in order in particular to correctly homogenize the temperature of the molten metal in planes perpendicular to the casting axis, said apparatus is implemented such that the cast molten metal is stirred in a plurality of adjacent axial rotary flow loops, distributed side by side around the casting axis in order to best occupy the entire cross-section of the cast product.

Description

 METHOD FOR ROTATIONAL ELECTROMAGNETIC BREWING OF A MELTING METAL DURING THE CASTING OF A BROAD SECTION PRODUCT

 AND EQUIPMENT FOR ITS IMPLEMENTATION.

The present invention relates to the casting of metals, especially steel. It is particularly aimed at the casting of metal products with a large square cross-section, rectangular or round-shaped, thick oars and large round or square blooms. More precisely, it relates to electromagnetic stirring which generates a movement. of axial rotation of the molten metal during casting.

 It is recalled that the electromagnetic stirring of molten metal during the continuous casting of steel has been used for a long time to improve the productivity of continuous casting machines and the quality of cast products obtained.

 One of the main uses of electromagnetic stirring, and in which the present invention is incorporated, is to create rotating motions of the molten metal, either in the form of a circulation loop rotating around the casting axis, so in the plane of the cross-section of the cast product (BF 7520225), or in a vertical plane parallel to the large faces of the cast product, in the form then of multiple loops with axes of rotation perpendicular to the casting axis (EP 0550785, EP 0151648).

 As is known, these movements are generated by moving magnetic fields produced by polyphase static electromagnetic inductors mounted on the casting machine around and in the immediate vicinity of the cast product, already at or below the mold, in the stages. secondary or final cooling of the machine.

 It is recalled that an inductor of this type consists essentially of a succession of electric windings wound on a ferromagnetic body, coils that produce magnetic poles when traversed by an electric current supplied by a two-phase or three-phase power supply. Like the stator of an asynchronous motor, the windings of the inductor are electrically connected to the phases of the power supply to form a system with a pair of magnetic poles of opposite signs per phase (two-phase biphase or three-phase motor). A moving magnetic field is thus generated, which moves along the inductor. If it penetrates into an electrically conductive medium, as is the cast molten metal, this moving field then generates a volume force (Lorentz force) which sets in motion (we say "stirring") the molten metal according to the well-known principles of rotary or linear asynchronous induction motor.

In the case of the continuous casting of long products, ie of round or square format (blooms or billets), as shown in FIG. 1 attached, the electromagnetic stirring equipment used is conventionally constituted by an annular-shaped inductor (circular). Fig. la or square-Fig. lb) surrounding the cast product. This inductor produces a magnetic field which passes through the product cast from side to side perpendicular to the casting axis and which rotates about this axis. This creates an axial rotation movement of the molten metal in the manner of an asynchronous electric motor whose rotor would be the molten metal.

In the case of the continuous casting of an elongated rectangular formai, therefore a slab, the usual practice is this time to use a brewing equipment formed by inductors of rectilinear shape (so-called "linear" as opposed to circular) which produce a traveling magnetic field (traveling magnetic field), such an engine ^{"straight electric"} bïpoiaire 'biphase or -triphasé. Inductive genes, used alone or in pairs. are mounted parallel to a large face of the cast slab.

 It is specified that the term "pair of inductors" means a pair of electrically paired linear inductors, that is to say powered by synchronous polyphase electric currents.

 Used alone (see Figures 2a and 2b), a sliding field linear inductor always works in tangential magnetic flux (also called "longitudinal"). This means that it generates in the molten metal a thrust that decreases exponentially when one moves away from it. As shown in document BF 8210844, if this thrust takes place in the secondary cooling stage of the casting machine and is oriented perpendicularly to the casting axis, and therefore according to the width of the slab, the metal in brewed fusion forms a double loop of circulation known as "loop in wings of butterfly"

 Used in pairs (or double pairs), they are then mounted opposite each other, thus defining between them a rectangular gap that is occupied by the cast slab.

 In this case, as shown in FIG. 3a illustrating the teaching of the document EP 0151648 (KSC), one equipment with a pair of linear inductors is mounted at the level of the mold with one inductor per face. Each inductor operates in tangential magnetic flux with opposite sliding directions of the two magnetic fields. This results in individual thrusts in the opposite direction on the molten metal in the immediate vicinity of the large walls of the mold, thus a zero thrust in the central zone of the slab, which leads to create an axial rotary movement of the molten metal in all its right section.

 By "axial" is meant "rotation around the axis of casting or parallel to it.This meaning will be valid for the rest of the presentation.

 Another known document, illustrated in FIG. 3b, shows a pair of linear inductors used in the flux flow in the same sliding direction of the magnetic fields, which generates a substantially constant thrust throughout the thickness of the cast slab. .

In the category of electromagnetic stirring equipment acting, this time, at the stage of the secondary cooling stage, are in particular well-known and widespread devices in the world of steel which are the stirring rollers or stirring rolls. -roll EMS). They are rollers for supporting and guiding the slab in the secondary cooling made hollow to receive a linear inductor (USP 3,882,923), or two inductors aligned and coupled in the case of very wide slabs (WO 201 1 / 1 17479A1). These brewers, generally used in pairs, are installed, or in parallel one above the other, and operate in tangential magnetic flux, or face to face on either side of the slab, and then operate in magnetic flux through. But, each time, they have the effect of pushing the molten metal horizontally along the width of the slab to create the double circulation loops in "butterfly wings" mentioned above. They can also be used in double pairs to create quadruple circulation loops, by reversing the sliding direction ^'' of ^' ' fields '' on two mounted inductors aligned on each major face of the cast slab (WO 201 1/1 17479).

 In contrast, in the mold, as shown for example in EP 0550785 (NKK), it is more commonly used a brewing equipment formed by a battery of two pairs of polyphase linear inductors, or four identical inductors. The inductors are mounted on the two large walls of the mold, with two inductors aligned per wall and placed on either side of the casting nozzle. They are electrically connected in a magnetic flow through to generate a substantially constant thrust through the thickness of the slab, because we want to act on the "fresh" metal jets out of the gills of the nozzle, or to accelerate them (sliding fields magnetic in the direction of the jets towards the outside of the mold), or to slow them down (sliding of the fields against the direction of the jets towards the interior of the mold).

 Such operating processes make it possible to install or stabilize selected modes of circulation of molten metal known as "double-roll" or "single-roll" in the vertical plane within the mold ( see EP 0550785 and EP 0151648 already cited).

 It is now acquired, and the work of the applicant has largely contributed, that not only the productivity of a continuous casting machine, but also the metallurgical quality of the finished cast product, both in terms of microstructure and in terms of cleanliness Inclusive, porous or segregated axial, are closely related to the circulation movements of the molten metal during the course of its casting. Control of these movements is the key to the success of the quality and productivity of continuous casting and it is precisely the practice of electromagnetic stirring of steel that allows such control.

 However, if the current good practice of electromagnetic stirring, as it has just been briefly mentioned, is quite capable of controlling the movement of molten steel during casting, it is in fact only for cast products of common formats, and not for very large formats, ie those, to fix the ideas, whose thickness (or the diameter for the circles) is of the order of the meter, even more.

Now, it turns out that a demand is now emerging from a certain world steel industry for casting, not only in ingot, but also in continuous or semi-continuous casting of such formats, whether round or square, or rectangular, and that we will later designate XXL formats for convenience of language. _: I! It turns out that such formats are not apprehendable by extrapolation, to such large dimensions, current calculation methods and existing knowledge. When studying these projects with CFD (computer fluid dynamics) simulations, the following problems actually appeared:

- Plus the cast format increases; the more the distribution of the temperature within the liquid metal becomes inhomogeneous, and this thermal heterogeneity then becomes the main problem for casting XXI formats. ^■■ .- - ·· ■ -

 - Mixing in multiple loops in the vertical plane parallel to the casting axis seems a priori appropriate. However, the known conventional cross-flow magnetic stirring methods are no longer applicable to XXL formats because of the large dimension of the cross-section of the cast product. The reason is that the magnetic field, which must cross the large distance between the two inductors, then strongly decreases in the center of the product and that the thrust (Lorentz force) in the middle of the slab then becomes too weak to generate a stirring movement. efficient over the entire thickness of the slab.

 - The same phenomenon occurs in annular inductors for round and square products. The distance between the paired poles then becoming so great for sections XXL, that the torque (Lorentz forces) then becomes too low towards the center of the cast product. The rotational movement of the liquid metal then remains concentrated at the periphery and no longer develops towards the center of the product.

 Finally, and even more importantly, the rotational movement of the molten metal around the casting axis, which remains essentially limited to the peripheral region of the XXL product, is no longer able to sufficiently standardize the temperature within the metal. melting, neither at the solidification front, nor at the temperature gradient between the inside and the outside of the cast product.

 The Applicant has discovered that these difficulties inherent in XXL formats could nevertheless be overcome by introducing axial rotational merging into "adjacent multi-loops", namely an organized plurality of axial rotary recirculation loops distributed side by side on the circumference of the casting axis to achieve, above all, to homogenize well the temperature in planes perpendicular to the casting axis.

 Thus, and with this main objective of thermal homogenization in view, the subject of the invention is primarily a method of electromagnetic stirring of molten metal during the casting of metal products in XXL format, of round, square, or rectangular, in which a brewing equipment is used comprising linear multiphase inductors with a sliding magnetic field, characterized in that the molten metal is stirred in the form of a plurality of adjacent axial circulation loops, distributed around the casting axis and occupying at best the entire cross section of the cast product.

According to a preferred implementation, said stirring is carried out by disposing said polyphase linear inductors sliding magnetic field. on the axis of casting to form an air gap, in that the magnetic fields sliding in air gap so that they slide in opposite directions on any two adjacent inductors, and in that the same polarity magnetic poles couples of poles connected to the same electrical phase and located opposite one another on two separate inductors paired. .

The invention also relates to an electromagnetic stirring equipment of metal products XXL format. round, square or rectangular equipment for implementing the method according to claim 1 or 2 and comprising at least one pair ^of linear inductors, magnetic field sliding mounted symmetrically, opposite one another relative at a plane of symmetry (P) of the cast product, around the casting axis so as to define an air gap in which the cast product passes, at least one polyphase power supply provided with electrical connection means with said inductors, and means for adjusting the direction of sliding of the magnetic fields on each inductor, equipment characterized in that said connecting means impose the same polarity on each pair of magnetic poles belonging to the same phase and placed symmetrically with respect to said plane (P) facing each other on two paired inductors.

 In an alternative embodiment, the brewing equipment comprises at least two pairs of sliding magnetic field linear inductors, forming an assembly of at least four inductors arranged around the casting axis so as to define a air gap in which the cast product passes, equipment characterized in that said means for adjusting the direction of sliding of the magnetic fields are configured so that the magnetic fields of any two adjacent inductors slide in opposite directions.

 Before going further, it is important to clarify and clarify some terms and expressions used above: (a) what is meant by "neighboring inducers" (b) how to conveniently define "the direction of slip" d a magnetic field of the inductor, and (c) how to understand unambiguously what in practice means sliding "in the same direction" or "in opposite directions" on two neighboring inductors.

 (a) Thus, are neighbors, in the sense of the invention, two inductors which follow each other directly, ie without intermediate analog inductor or spacer, on the circumference of the casting axis , whatever the direction of rotation chosen around this axis to pass from one inductor to another.

 (b) To answer this second interrogation, it is necessary first of all to take into consideration the convention according to which the electric phases are named in an immutable order and determined in the time, because given thus by the network of distribution of the energy. electrical: U then V (delayed by 90 °) for a two-phase power supply or U then V (behind 120 °) then W (lagging 240 °) for a three-phase power supply. Therefore, the direction of sliding of the magnetic field along the inductor which produces it will depend on the sequencing chosen for the order of connection of the phases on the coils of the inductor.

Thus, for example in the case of a rotary inductor, the magnetic field will rotate according to the U.V sequencing for a two-phase inductor (Fig. 1b) and U, V, W for a -phase inductor (Fig. 1a), that is, clockwise. For the case of a linear inductor three-phase of Figure 2a. the magnetic field will also slide according to U, V. W, i.e. from left to right in the figure .. And it will slide from right to left, as shown in Figure 2b, if the sequencing was changed by permutation of two phases between them (V and W in the example) without changing the magnetic polarity North or South of each pole.

Also, is it agreed, in the context of the invention, always place the U phase on the ^"pole" of the inductor located ^"at one of ^'his' TwO e ^émités." Therefore, if the sequence of phases ^" in time is in a sequential order U, V, W, the direction of the magnetic field slip is always given by the sequencing of the magnetic poles U, V, W, this sequencing given by the electrical connections between the windings of the inductor and the power supply.

 (c) In order to compare the sliding directions of the magnetic fields between several inductors installed on a casting machine, it suffices to consider the sliding direction of the field of each one by traversing them one after the other around the axis of casting. As a result, the fields of the two inductors shown in FIG. 3a slide in the same direction, according to the definition of the invention, while, seen from above in the figure, it looks like they slide in the opposite direction. It also results that the fields of the two inductors shown in FIG. 3b slide in the opposite direction according to the definition of the invention, whereas, seen from a top in the figure, it seems that they slide in the same direction.

 We will now present, in a more geometric language, another comparative definition of the sliding directions of magnetic fields, more general than the previous one, ie independently of the casting machine to which the "brewing equipment is intended The reader will choose the one that suits him best.

 1) If they are arranged in parallel, one facing each other, two linear inductors (ie whose magnetic poles are aligned with each other), produce magnetic fields sliding in opposite directions if one can be transformed in the other by an operation of symmetry with respect to a plane parallel to the line of the poles of the inductor. Otherwise, they produce fields that slide in the same direction.

 2) If they are arranged aligned one after the other, two linear inductors produce magnetic fields sliding in the same direction, if one can be transformed into the other by a translation operation parallel to its line of poles. Otherwise, they produce fields that slide in opposite directions.

 In the description of the prior art of electromagnetic stirring in continuous casting of steel products of standard format which was made at the beginning of this memoir, it was seen that, except for the case shown in FIG. 3a, the inductors are used in coupling magnetic flux-flow, both in circular configuration (rotating field applied to blooms round or square section) in linear configuration by pair of inductors (sliding fields applied to rectangular products such as slabs).

This means that the same-phase poles facing each other on two inductors in opposite directions are of opposite polarity. It εη is quite different in the case of ï-n ^ n - nn ti m ⁱ r. ' ^Pc T o the even- nnlarité ₁ ui is imposed on the poles of the same phase which face two paired inductors. This, in simple terms, is what a decisive feature of a device that produces a tangential magnetic flux, whatever the size of its air gap, must be.

 The invention will be better understood and other aspects and advantages will appear more clearly in view of the following description given with reference to the accompanying figures.

 In these figures, the letters U, V and W represent the three electrical phases of the three-phase supply phase shifted by 120 °. They also designate, as a consequence, the phase windings on the inductors connected to this power supply, and the complementary letters U *, V *, W * denote the phase windings on the inductors wound in the opposite direction of the windings. U, V, W so that the electric current flows in the opposite direction.

 The letters N and S denote the north or south polarity of the poles when the electrical phase which supplies them is at the maximum of the intensity of current. As it is the phase windings which generate the magnetic poles on an inductor, it will be seen in the figures that, by convention, a phase winding U, V, or W is always associated with a pole of polarity N, whereas in winding U *, V * or W * will always be associated with a pole of polarity S.

 - Figures 1, 2 and 3 are electromagnetic principle diagrams that relate to the prior art cited in the introduction of this memo. For this reason, they have all been grouped together on a single board;

 FIGS. 4a and 4b are two variants of the same functional schematic diagram of an elementary stirring equipment according to the invention, ie to a pair of tangential flow linear inductors, of rectilinear shape, in a work situation on a XXL slab casting;

 - Figure 5 is a block diagram similar to those of Figures 4, but in the case of arcuate linear inductors for casting a bloom XXL circular shape;

 FIGS. 6a and 6b show two variants of the same operating principle diagram of a standard brewing equipment according to the invention, ie two pairs of tangential flow linear inductors (ie a set of four inductors), of rectilinear shape, in working situation on a XXL slab casting

 - Figure 7 is a block diagram of a standard stirring equipment according to the invention, i.e. two pairs of arcuate tangential flow linear inductors, in working situation on a round XXL blooms casting;

- Figure 8 is a diagram similar to that of Figure 1 but with straight linear inductors used for casting square XXL blooms; _. . >^' _:. ^■ _. - _. - and Figure 9 shows schematically a complete mixing equipment as used at a slab continuous casting mold XXL with a submerged nozzle with four side openings.

 In all these figures, the same elements are represented by identical numerical references.

As has already been seen, FIG. 1a is a block diagram of a known three-phase annular inductor with a rotating magnetic field: This inductive type, with a pair of poles per phase, of annular shape with a circular base, is usually used. for the continuous casting of round long products (blooms, billets), either at the ingot mold or, below, in the secondary cooling zone of the casting machine. It presents here an additional educational interest, useful for the disclosure of the invention, which is to serve as a circle of phases for a three-phase system. On this circle, U, V and W denote the three phases of a balanced three-phase power supply. As a result, over time, the maximum intensity of electric current passes, with a regular speed which depends on its frequency of current, from phase U to phase V then to phase W, and so on. On a rotating field inductor, as shown schematically here, the pairs of paired magnetic poles of each phase are excited by windings wound in the opposite direction so as to have magnetic polarities of opposite signs (N or S). Thus, the marking U, V, W, U *, V * and W * points the physical positions of the poles _. magnetic on the inductor when they are each, in turn, in full phase (ie with a maximum intensity of the electric current in the associated winding).

 The sequence, taken in this order, namely U, W *, V, U *, W, V * (and again U ....), - represents, in space, the ordered physical succession of the poles on the inductor thus configured. The same sequence, but taken in time, thus shows that the displacement of the magnetic field of one pole from the inductor to the next develops in the clockwise direction. This results in a field rotating around the longitudinal axis of the inductor, which will be confused here with the axis of the casting. We see that if we switch two phases between them, for example V and W, without changing the magnetic polarities, we carry out the sequence U, V *, W, U *, V, W *, which then expresses the fact that the magnetic field reverses its direction of movement on the inductor to turn counterclockwise.

 FIG. 1b illustrates the functional diagram of an inductor with two pairs of poles per phase, but two-phase this time and of annular form with a square base for casting long products of square format. If necessary, we will be able to use it, in the same way as with figure la, to find on such a circle of phases the attributes proper to a two-phase system. It will be easily realized that, in this case, it will be possible to invert the direction of rotation of the magnetic field indifferently, by switching the phases U and V (ie to put V in advance of 90 ° on U), or by reversing the direction of the current in the windings of a phase.

In any case, the same provisions prevail if the moving field inductor is linear instead of annular. Indeed, if by thought we unwind the three-phase inductor of the figure, 1a after having cut for example between. the poles. U. and. V *, ri gets the schema of a three-phase linear inductor to. sliding field, such as that illustrated in FIGS. . . . ^■

In view of the phase circle of the three-phase system, it is immediately understood that the magnetic field produced by this linear inductor 1 slides, in Figure 2a, from its left end to its right end (arrow v). Thus, as any linear inductor in isolation in space, the inductor 1 naturally generates in the cast metal product, here a slab 2, ^"" a: flow ^'' of inductibn ^{"" magneti:} ne ^"of deviation tangential ire. a flux-which-has for main property to generate a thrust of Lorentz whose intensity decreases rapidly when one moves away from the inductor.

 Always by the thought, if, without changing anything at the polar signs of the poles, we simply switch two phases between them, for example V and W (U remaining on the pole at the left end of the inductor), the direction of slip the field is inverted on the inductor thus reconfigured.

 This is verified for a three-phase power supply. In the case of a two-phase power supply, the four-pole phase circle U, V, U * and V * will easily verify that the magnetic field changes direction of slip by simply inverting the polar sign, ie the polarity, of the two poles coupled on the same phase.

 . FIGS. 3 and 3 illustrate what happens when the inductor 1 is brought closer to an identical identical inductor 3 in order to place them in parallel, on either side of a plane of symmetry P which is the principal plane of symmetry of the rectangular casting slab 2.

 The case of FIG. 3b is that of the usual conventional configuration: the magnetic poles facing each side of the plane of symmetry P are at the same phase, but of opposite polarities. Everything then happens as if such a couple of inductors 1, 3 produced a magnetic field "piston" sliding from their left end to their right end (arrow v or v '), sort of like the bars of a mobile ladder whose fixed slides would be the inductors themselves. Thus, like any pair of inductors paired and configured for use on a casting machine, the brewing equipment of Figure 3b generates a "through" type sliding induction flow. This results in particular that the Lorenz thrust produced on the molten metal is, this time, almost constant intensity through the entire thickness of the cast slab, as shown by the sticker associated with the bottom of the figure.

The case of Figure 3a is also known, although less common than that seen above: the magnetic poles facing each side of the main plane of symmetry P of the slab 2 are also of inverse magnetic polarities, but the correspondence of phases on both inductors 1 and 3 opposite is random, so it is no longer respected. Despite appearances, the two magnetic fields slide in the same direction (see above the comparative definition according to the invention). The magnetic flux produced are of the "tangential" and outbreaks of Lorentz, which then exert only sensitive to the ^'periphery of the cast slab 2, are opposite and therefore cooperate to perform a global axial rotational melt of the molten metal, as shown in the associated vignette at the bottom of the figure.

 The following figures are all relating to the invention.

 Figures 4 and 5 relate to a brewing equipment that can be described as elemental in that it comprises a single pair. linear inductors, while the following figures relate to a brewing equipment with four linear inductors, which can be called standard key or majority because this configuration-two-pair of inductors is largely the most common.

 The elementary stirring equipment illustrated in FIG. 4a thus comprises a pair of rectilinear linear three-phase inductors 1 and 3. The two inductors are placed parallel to each other, face to face and at a distance so as to define between them an air gap for the passage of a cast product 2 XXL rectangular format, so a slab (or a square bloom). In other words, the two inductors 1 and 3 are symmetrical to one another with respect to the main plane of symmetry P of the product XXL cast.

 The wiring of this equipment to a three-phase power supply is not visible in the figure, but the equipment is presented in its functional electrical configuration ready to perform a stirring action from magnetic fields sliding in opposite directions (cf. the two arrows v oriented from left to right in the figure). This configuration, of the "tangential magnetic flux" (or longitudinal) type, differs from those of the prior art, with a through-flow shown in FIG. 3b, or even with longitudinal flow shown in FIG. 3a, by the fact that this time, the magnetic poles that face each other on the two paired inductors are constantly not only connected to the same phase of the three-phase power supply, but kept at the same magnetic polarity S or N. In other words, a polarity inverter magnetic poles (which will be admitted integrated in the power supply not shown) has been biased for each of the three phases on the inductor 3 relative to the inductor 1 in its configuration of Figure 3b.

We recall here the tracking convention, very convenient in practice, which arbitrarily consists in always applying, in three-phase as in two-phase, the phase U on the pole located at one of the ends on a sliding field inductor, rather than on a pole having two neighbors. Thus, for the pair of linear inductors exempliflée in Figure 4a, we see that the U phase is applied to the pole at the left end of each of the two inductors. It is thus easy to note that the field then slides from this end towards the other end, thus from left to right in the figure, in the respect of the sequencing of the three phases of the supply recalled by the clockwise direction on the circle. of three-phase phases of Figure la. · _^".. ....

It is the same in the case of FIG. 4b, where, after permutation of the V and W phases, it can be seen that, starting from the end pole U, the sequencing of the three phases being of U, V *, W, U *, V, W *, on each inductor 1 and 3, their magnetic field thus slides from -.-, right to the left in the figure (anti-clockwise on - the circle, phases), generating each in the molten metal cast a tangential magnetic induction flux. As we see on the associated vignettes, whether in a direction of sliding of the magnetic fields or in the other, the action of such a mode of brewing results in Lorentz surges on the molten metal located in slab edge and spread across the entire width of it. This results in a setting in motion of the molten metal at these points along two parallel active branches in the same direction 4 and 5, complemented by a middle common branch 6, which itself is passive because it corresponds to of metal flow ^"back in an area where the Lorentz surges are zero (or almost zero), which results in the formation of two adjacent axial circulation loops in mutual counter-rotation 7 and 8 _{, which are} identical, well formed and stable and which concern the entire cross-section of the cast slab XXL occupied by the molten metal.

 A similar situation is found in the case of the casting of a XXL bloom of round format as shown in Figure 5. In this figure, a bloom XXL 9 of round format during casting is surrounded by a brewing equipment 10 formed this time by two paired identical linear inductors 1 1 and 12, semicircular arcuate form which abut at their ends 15, each then being symmetrical to the other relative to the plane of symmetry P of the bloom 9 passing by the casting axis A. Their protruding magnetic poles 13, 14 are turned inwards to define a closed air gap 16 of size adjusted to the round bloom jig 9. As can be seen, from their end pole U, the sequencing of the phases on each inductor 1 1, 12 is of the type U, W *, V, U *, W, V *, This expresses that the sliding magnetic field they produce each progresses from one pole to the next in this order. In other words, the two magnetic fields, slide in opposite directions, that of the inductor 11 progressing in the clockwise direction shown by the arrow VI 1, that of the matched inductor 12 progressing in the counterclockwise direction shown by the arrow V12.

 It is important to note, as clearly shown in FIG. 5, that the symmetry by the axial plane P is also respected for the magnetic polarity N or S of each pair of poles 13-14 located opposite each other each on separate inductors. It is under such a characteristic of electrical wiring of matched inductors 1 1 and 12 that the production of a tangential magnetic flux is guaranteed on each inductor.

 It is for this reason that the stirring action is then expressed by Lorentz surges on the molten metal located at the periphery of the XXL bloom 9. These thrusts develop identically into two active branches 17, 18 half circle each according to the sequencing of the phases on the poles of their respective inductor and which cooperate with each other to form a median common branch 19 of passive return of the metal flux in the plane P, where the Lorentz flares are zero.

 The assembly results in the formation of two mutually counter-rotating adjacent axial circulation loops 20 and 21 which are identical, well formed and stable, and whose addition concerns the entire cross-section occupied by the. cast molten metal.

We will now consider the mixing mode in multi-axial loops adjacent to the invention taken to a higher rank than that described so far, namely a mode realizing, not two, but four loops, or more. Previously, it is appropriate to refer to FIG. 9, since it makes it possible in the first place to specify the technological means of the invention.

 This figure relates to an ingot mold for the continuous casting of steel XXL slabs, with a submerged casting nozzle with four lateral outlet openings to ensure a sufficient incoming flow of molten metal compatible with the requirements imposed by the production of cast products from such imposing wings - - - -

As can be seen, the electromagnetic stirring equipment comprises a polyphase power supply 22 followed by a phase switch 23 which manages the sliding directions of the magnetic fields on each of the four linear inductors 24a, 24b, 25a. 25b connected downstream. As shown in the wiring lines 26 and 27, the four inductors are grouped into two pairs (24 and 25). Each pair of paired inductors 24a, 24b and 25a, 25b is connected to the phase switch 23 separately from the other, but they are both synchronously connected to the common polyphase supply 22.

 Polyphase power supplies of this type are commercially available. They make it possible to transform the current, supplied by the electrical energy distribution network (50 or 60 Hz), into a low frequency current (say between 2 and 10 Hz) suitable for the electromagnetic stirring of molten metals. Here, furthermore, the power supply 22 will incorporate current inverters to enable the N / S polarity of the magnetic poles generated by the windings of the inductors to be reversed.

 The four linear inductors exemplified herein are of rectilinear shape, because they are dedicated to the stirring of an XXL 2 slab. For this purpose, they are mounted, with the aid of suitable fasteners not shown, on the two large walls. 28 and 29 of a continuous casting mold 30 provided with a submerged nozzle 31 on the casting axis A. The mounting is carried out so that two paired inductors are placed symmetrically one opposite the other, On either side of the vertical main plane of symmetry P of the slab 2, it follows that each large wall 28 and 29 of the mold is thus equipped with two unpaired aligned inductors (eg 24a and 25a on the wall 28). ) and arranged symmetrically with each other on either side of the secondary plane of symmetry Q of the slab, orthogonal to the main plane P and secant on the casting axis A.

 Note that according to an alternative embodiment of the stirring equipment, the fasteners can be adapted to a mounting of the inductors, not at the level of the mold, but below, in the secondary cooling of the casting machine, without what has just been said must be changed.

For the sake of simplicity, as has already been done for thumbnail figures, the physical position of the magnetic poles is no longer shown, but only with the help of arrows the direction of sliding of the magnetic fields produced by the inductors. Remember that it is important that the magnetic poles of the same phase face each other on two inductors of the same pair and that they constantly have the same polarity N or S. It is with this condition that the magnetic induction fluxes, which act on the molten metal, will be tangential flows, thus respecting an essential functional characteristic of the brewing equipment of the invention.

 Of course, as long as the inductors are not supplied with electric current, the natural configuration of the hydrodynamics of the molten metal remains prevalent, namely an unstable meniscus, weakly agitated with disparate and random wavelets, except in the vicinity of the gills of the metal. the nozzle 31 through which the jets of hot metal supplying the mold. It is different as soon as the invention is implemented. This is now seen in this same figure 9, considering the four activated inductors to each produce a sliding magnetic field.

 According to the invention, and the convention adopted to define the sliding directions of the magnetic fields between them, the fields slide in opposite directions on any two adjacent inductors. This is the case, obviously, of the two unpaired inductors 24a and 25a (or 24b and 25b) mounted on the same large wall 28 (respectively 29) of the mold 30, on either side of the secondary plane of symmetry Q This is also the case of the two adjacent inductors of the same pair (24a and 24b, or 25a and 25b) mounted symmetrically opposite one another, on either side of the main plane of symmetry P.

 The inductors of the pair 24 reproduce the electromagnetic configuration of the paired inductors shown in FIG. 4a. The magnetic fields produced slide from left to right in the figure, ie in opposite directions on their respective inductors. We see that it is the same for the two paired inductors forming the pair 25 and which, they reproduce the configuration of two paired inductors of Figure 4b. In this case, therefore, the magnetic fields slide opposite those of the first pair of inductors, that is to say from the right to the left in this figure 4b.

 Thus, the magnetic fields slip two by two in opposition to the two jets of "fresh" molten metal coming out of the nozzle 31 in the direction of the small faces 32 and 33 of the mold. The "tangential" induction flows, which thus progress on the half-widths of each large wall 28 and 29 of the mold, significantly modify the configuration of the motions of the molten cast metal. In essence, the unstable natural agitation of the meniscus in the absence of mixing gives way to a configuration of circulatory movements in four adjacent loops 34, 35, 36 and 37, in axial rotation parallel to the casting axis A. These loops are organized side by side, in mutual counter-rotation, around the casting axis A to occupy each a quarter of the cross section of the cast product so that all the brewing movements occupies the best surface of the meniscus of the cast metal in its entirety, or almost all of it.

In fact, everything happens as if the two jets of the nozzle parallel to the large walls 28 and 29 of the mold are reinforced by a thrust action of magnetic flux, while the other two jets, perpendicular to the plane of these large walls are, themselves, thwarted, ^or even fade almost completely, by a braking action of these magnetic fluxes. We understand that if the sliding directions of the four Magnetic fields are reversed, the hydrodynamic pattern of mixing into four adjacent circulatory loops 34-37 is retained. Simply, the direction of axial rotation of each loop would be reversed so that, this time, are the jet of the nozzle directed to the small walls 32 and 33 of the mold which will be thwarted, and the other two directed to the large walls 28 and 29 which will be strengthened.

 We will now be able to continue the presentation of the invention in a manner similar to FIG. 6, which shows, in detail, the electromagnetic configurations of the four polyphase linear sliding-field magnetic inductors for slats XXL of FIG. here again acts of three-phase inductors.

 FIG. 6a shows the four identical rectilinear linear inductors grouped into two electrically synchronous pairs. A first pair is formed by two paired inductors 24a and 24b mounted facing one another, on either side of the main plane of symmetry P of the slab 2, over a half width thereof ( the left on the figure). The second pair is formed by the pair of paired inductors 25a and 25b mounted in a similar manner to those of the previous pair, but on the other half width of the slab (the right part in the figure). Thus, each large face of the slab 2 is provided with two inductors aligned, but not matched, namely the two inductors 24a, 25a for one of the large faces and the two inductors 24b, 25b for the other. It is recalled that these pairs of unpaired aligned inductors are therefore symmetrical to one another with respect to the plane. of secondary symmetry Q of the slab, secant at right angles to the main plane P on the casting axis A.

As shown, the four adjacent elongated loops axial movement 34, ^'35, 36 and 37, stirring the molten metal of the slab 2, are distributed regularly round about the casting axis A (and in quoted order) mutual counter-rotation between two neighboring loops and defining each time between them a common branch of passive return of the molten metal.

 These four circulatory loops, which are stable and well formed, are produced using the tangential flow electromagnetic configuration shown in FIG. 6a and according to which the magnetic fields on two adjacent inductors, aligned 24a and 25a, or paired 24a and 24b, slide in opposite directions. In this case, the magnetic fields on two aligned inductors slide "getting closer" to each other (we say, they converge), of. so that the large passive branches of the metal of the oblong buckles of the brew transport this molten metal of the axis A towards the small laces, of, the slab, then. that the small branches bring back the metal of the large faces towards the axis of casting.

This electromagnetic configuration is obtained by wiring _: inductors to their three-phase supply so that _... . . . ,. .

on the inductor 24a, the sequence of the phases is the following: U, W *, V, U *, W, V *. ^"" from the U-phase poise 38 (conventionally chosen at the end of the inductor), located here at the slab edge so that the magnetic field produced slides from this edge to the center of the face; on its matched inductor 24b, the sequence of the phases is also U, W *, V, U *, W. V * from the end phase pole U 39 selected in the terminal position on the same slab edge so that the field magnetic product slides from this edge to the center of the face, so in opposite direction to that of the paired neighboring inductor 24a;

 on the inductor 25a, aligned with the inductor 24a on the same large face of the slab, but not matched to it, the sequence of the phases is also the following: U, W *, V, U *, W,

^" V * from the ^" end pole ^" phase U 40 also chooses terminally at the edge of the slab, but at the other end of the so that the magnetic field produced slides from this edge to the center face, so in a direction opposite to that of his neighbor 24a;

^■ - and its paired inductor 25b, the phase sequence is U again, * W, V, U *, W, V * for the U phase pole end 41 selected end position at the same edge of slab to that the moving magnetic field slides from this edge towards the center of the face, thus in the opposite direction to that of the paired neighboring inductor 25a;

 Figure 6b illustrates a situation quite similar in substance to that of Figure 6a. Simply, the phase switch 23 has been activated to swap the V and W phases on each of the four inductors, all things being equal. It follows that the magnetic fields on two aligned inductors slip this time "away" from each other (we say they diverge), out that the direction of axial rotation of each of the four loops of metal circulation is reversed with respect to the electromagnetic configuration of the brewing equipment of Figure 6a above.

 Regardless of the electromagnetic configuration adopted, it is important, as shown in the two FIGS. 6, that not only the magnetic fields slide in opposite directions on any two adjacent inductors (for example, on the paired inductors facing each other 24a, 24b, and on the unpaired aligned inductors 24a, 25a), but that, in order to work in a longitudinal magnetic induction flux, the magnetic poles at the same phase opposite two adjacent inductors, paired or not, have the same polarity N or S.

 This characteristic relating to the magnetic polarity of the poles immediately appears in FIG. 6 with regard to the paired inductors, for example 24a and 24b. It is less immediate with respect to unpaired inductors, for example 24a and 25a. This is due to the fact that these rectilinear inductors are aligned, the example considered relating to the casting of a slab, so to. n cast product. of rectangular cross section. But it will also appear in FIGS. 8 and 8, which relate to the casting of long products XXL, the four linear field-sliding inductors constituting the equipment according to the invention, show an air gap, this once closed, which completely envelops the product poured on its periphery.

Figure 7 shows a. Circular brewing equipment mounted around a round bloom XXL 71 in the secondary cooling stage of a machine. continuous casting. It consists of four identical three-phase bipolar linear inductors 72a. 72b. 73a 73b of araped form in guart ^ ^p o ^p rr.le assembled continuously one after the other, each styling an angular sector of 90 ° of the cast product.

 The inductor pairs 72a, 72b and 73a, 73b, arranged symmetrically with respect to the main plane of symmetry P of the bloom, each form a pair. The two pairs (72 and 73) are symmetrical with respect to the secondary plane of symmetry Q, which in this case, because of the high order of symmetry of the cast XXL bloom, is equivalent to the plane P.

For the sake of clarity, the ^'oles magnetic links are not shown on the arcuate flèches- each inductor show the sliding direction of the magnetic fields produced by each of them. As can be seen, the fields slide in opposite directions on any two adjacent inductors. If necessary, the reader will be able to reconstitute the sequencing of the electrical phases on each inductor for this purpose, using the circle of phases. He will take care, however, to keep the same magnetic polarity on the poles connected to the same phase and placed opposite each other on two paired inductors, thus ensuring opposite directions of sliding of the two magnetic fields. It will also take care that the pairs of poles at the same phase closest on two unpaired inductors (and therefore between the inductors 72a and 73b on the one hand, and between the inductors 72b and 73a on the other hand) are of same magnetic polarity. It is this second characteristic that is the guarantor of a tangential flow electromagnetic configuration for sliding magnetic fields.

 By such an electromagnetic configuration of the equipment of the invention, a stirring configuration of the molten metal is created in the form of four adjacent loops 74, 75, 76, 77 in axial rotation, stable and regularly distributed over the around the axis, of casting A, mutual counter-rotation between two neighbors and occupying the best the entire cross section of the product XXL cast.

An entirely equivalent situation is found in the case of the casting of a square XXL bloom, as can be seen in FIG. 8. In this case, the linear inductors 82a, 82b, 83a and 83b, of rectilinear shape. , are mounted around the square bloom 81 at the rate of one inductor per side. Paired pairs of inductors (82a, 82b, and 83a, 83b) are symmetrical with respect to any diagonal plane P of symmetry of the cast product. The two pairs of inductors 82, 83 are symmetrical with each other with respect to the other plane of diagonal symmetry Q. On any two adjacent inductors, the magnetic fields slide in opposite directions. The brewing loops which are then formed, 84, 85, 86 and 87, are generally triangular in shape, stable and well formed, regularly distributed over the circumference of this casting A, in mutual counter rotation and occupying the best . the entire cross section of the XXL product poured. ^"

Having reached the end of the presentation, it may be useful to re-emphasize all of the essential features of the invention. know. :. ·. -: · _- _: -

1) opposite directions of sliding magnetic fields on. neighboring inductors - - -

2) the same polarity N or S poles, put under the same. electric hase and placed symmetrically on two paired inductors facing each other with respect to. plan The principle of symmetry of the product flows in a tangentic magnetic flux, that is to say a high Lorentz force at the periphery of the cast product and zero at the center, and

 3) XXL dimensions of the cast product, which causes the molten metal to be driven to form a plurality of adjacent multiple circulation loops, in opposite axial axial rotation in a plane perpendicular to the flow rate (i.e. a cross section of the cast product).

 The main reason for which the present invention consists in generating multiple loops of axial circulation of the molten metal in a plane perpendicular to the casting axis, is the homogenization of the temperature in this plane. In fact, unlike traditionally sized cast formats, the long distances specific to the XXL format sections cause differences in the molten metal. considerable temperature, both radially and in the tangential direction of the cast product. This is true for the liquid metal in the ingot mold and the meniscus, as for the lower metal, under the mold, in the secondary cooling of the casting machine.

 However, there is another reason for brewing in multiple adjacent axial rotation loops, namely stabilizing the convective movements of the molten metal at the meniscus in the mold. Indeed, these large XXL sizes are usually poured into the mold with a submerged nozzle with four outlet openings oriented, not down as for the. traditional formats, but horizontally, or even upwards, to bring more enthalpy to the meniscus. The four liquid metal jets entering the mold could ideally create a natural flow in eight meniscus recirculation loops. However, in reality, the flow of the liquid metal is never symmetrical nor stable. The loops are sort of competing with each other, overlapping, canceling, reversing and creating fluctuations in the meniscus, which contribute to deteriorate the good lubrication at this point of the wall of the mold, and lead to the low in the molten metal the covering powder which covers it in the mold.

 The use of two pairs of inductors as illustrated above, for the casting of an XXL format, whether rectangular, square or round, will transform the naturally unstable movement into four controlled and stable loops.

In addition, in casting XXL slabs depending on the choice of the direction of sliding magnetic fields, we can: ¹ - - -r: - ^· · ^■

- or increase ("suck") the flow of the jets of the nozzle that is. moving towards the small walls of the mold and therefore curb the jets which are directed towards the large ^faces,: ■

 - either increase the flow towards the large faces and slow down the flow towards the small faces.

This possibility offered by the implementation of the invention can somehow "adjust" the flow of "fresh" metal entering the ingot, ie the speed of the jets of the nozzle, is still applicable, of course, with equipment with three pairs of inductors. When it comes, for example, to the casting of a round XXL bloom, six axial recirculation loops, in a lingoiière provided with a submerged nozzle with six outlets offset by 60 ° when these six ears are located in the plane of symmetry ^• main P and the two planes secondary symmetry Q.

 It goes without saying that the invention can not be limited to the examples described, but that it extends to multiples or equivalents to the extent that is respected its definition given by the claims attached below.

 Especially the power supply of the inductors can be chosen of two-phase type, instead of three-phase. It is then easily realized, simply by building the phase circle only for the U and V phases shifted at 90 °, that permuting these two phases together, or inverting the magnetic polarities on two poles of the same phase, produces an inversion the direction of displacement of the magnetic field in an equivalent manner. Additioning these two operations, whatever the order, would therefore, if necessary, retain the direction of displacement of the magnetic field, while assigning a given phase to the pole of choice on the inductor.

 Similarly, it may be advantageous to make the brewing equipment of the invention with a magnetic core common to several inductors.

Claims

1. A method of rotating electromagnetic stirring of molten metal during the casting of metal products XXL, round, square or rectangular, wherein dii ^"uses a brewing equipment comprising linear multiphase inductors with sliding magnetic field characterized in that the molten metal is stirred in the form of a plurality of adjacent axial rotation loops, distributed around the casting axis and occupying at best the entire cross section of the casting axis. cast product.

2. Method according to claim 1 characterized in that one carries out said stirring by arranging said polyphase linear inductors sliding magnetic field around the axis of casting to form an air gap, in that one settles the magnetic fields sliding in this air gap so that they slide in opposite directions on any two adjacent inductors, and in that the same pole polarity couples pairs of poles connected to the same electrical phase and located one facing each other on the other two separate matched inductors.

.

3. Equipment for electromagnetic stirring of metal products in XXL format, of round, square or rectangular section, equipment for implementing the method according to claim 1 or 2 and comprising at least one pair of linear field inductors.

^Magnetic sliding, mounted symmetrically facing each other, relative to a plane of symmetry (P) of the cast product, on round about the casting axis so as to define a gap through which the cast product, at least one polyphase power supply provided with electrical connection means with said inductors, and means for adjusting the direction of sliding of the magnetic fields on each inductor, characterized in that said connection means impose the same polarity at each pair of magnetic poles belonging to the same phase and placed symmetrically with respect to said plane (P) facing each other over two paired inductors.

4) Electromagnetic stirring equipment according to claim 3 comprising at least two pairs of sliding magnetic field linear inductors forming an assembly of at least four inductors arranged on the circumference of the co-axial axis so as to define a air gap in which the cast product passes, equipment characterized in that said means for adjusting the direction of sliding of the magnetic fields are configured ^" so that the magnetic fields of any two adjacent inductors slip in opposite directions.