FR2894167A1 - EQUIPMENT FOR CONTINUOUS METAL CASTING DISTRIBUTION CASTING - Google Patents

Abstract

This equipment comprises: - a cooling circuit putting a coolant in circulation - a polyphase power supply - a pouring tube (4) fixed on the bottom of the ladle around the drain opening of the latter - Mountable around said pocket tube, an annular electromagnetic inductor (1) rotating magnetic field, polyphase type provided with a pair of poles per phase of said power supply to which it is connected, and each pole is formed by an electrical winding (10) wound in layers (20) around an inwardly projecting pole tooth (3) having a lateral narrowing (11) at its terminal active face (9), the pole teeth being connected between them at their other end by an outer magnetic peripheral yoke (12) closing the magnetic flux and each electrical winding (10) incorporates a heat extractor, co it is advantageously arranged by a flattened annular tight tube (14) connected to the cooling circuit for an internal circulation of cooling liquid and, interposed between the pouring tube and the inductor, a hollow annular non-magnetic screen (15) for thermal protection of the inductor, cooled by an internal circulation of coolant being connected to said cooling circuit. The implementation of this equipment imparts an axial rotation movement to the molten metal vein passing from the ladle to the distributor, with the aim of effect a tundish in conditions favorable to obtaining a satisfactory metallurgical quality of the solid metal after casting.

Description

Equipment for continuous casting casting of metals.

  The invention relates to the casting of metals using a ladle which empties from the bottom by means of a dip tube opening below the level of molten metal cast in a tundish placed under this pocket . More specifically, the invention relates to reducing the turbulence of the liquid metal entering the distributor from the ladle. It applies more particularly to the continuous casting of steel.

  It is recalled that the metal in the liquid state, conventionally derived from the melting furnace or a secondary metallurgy reactor following the furnace, is conveyed via a ladle to a distributor. The latter, with drain holes calibrated in its bottom, will then distribute by gravity the molten metal to one or more continuous casting molds below and in which initiates the solidification of the metal by its periphery in contact with the walls of copper or copper alloy energetically cooled which are formed these ingot molds. The centripetal solidification then progresses downstream of the mold under the effect of watering ramps arranged below, as the cast product is pulled down in the stages of the secondary cooling of the casting machine. Once the solidification is complete, and after straightening the product cast horizontally, the latter is cut to length to form slabs, blooms or billets, for subsequent rolling shaping. The distributor thus acts as a reservoir placed upstream of the casting chain while providing the steelmaker an additional opportunity to still act on the molten metal, in particular to refine it further and / or to get rid of it just before its solidification of non-metallic impurities using a supernatant slag that is formed for this purpose and prevent them from being then entrained in the ingot mold with the liquid metal streams. It is understood that these metal-dairy exchanges, the efficiency of which of course requires some agitation of the bath, except that in order to constantly renew the metal-dairy interface, they risk being unable to develop under good conditions if the steel bath shows too much agitation. However, because of the high and narrow shape of the ladles, the distributor is fed violently liquid metal each opening of a new pocket and the steel is poured by dip tube under a large flow and with a speed large jet, at least in the first part of pocket emptying, which causes considerable turbulence in the bath during these periods. These turbulences are known to be at the origin of a possible deterioration of the final quality of the solidified steel obtained. They can cause, for example, entrainment in the bath of whole slag fragments, or lead to its frank emulsion. Argon, conventionally used in jets to fight against clogging of the pocket tubes, is then also driven en masse with the steel in the mold. In addition, these turbulences cause premature wear of the refractory lining of the distributor and generate heterogeneities in the flows of the metal in the mold. Various solutions have already been proposed to date to try to overcome these difficulties. Among them, there are, for example, two types of approach. The first is the use of impact tiles directly above the pocket jet. This is usually shallow boxes open on top. The base, very hard refractory material, rectangular, trapezoidal or circular, is bordered by a low vertical wall. The jet of molten steel released by the pocket above directly impacts the base which, thanks to its geometry and in cooperation with the side wall which serves as a rebound surface, will disperse it by redirecting it globally towards the bottom of the distributor . In this way, turbulence in the vicinity of the cover slag is reduced and the risk of particle entrainment is reduced. In addition, particularly when the distributor is partitioned into two adjacent chambers communicating by threshold at their upper part, one receiving steel pocket, the other pouring into the mold, the steel has an average residence time more important in the dispatcher, thus favorable to a better settling of the inclusions. An example of this technology, marketed inter alia under the name "Turbo-stop", is described in WO 03/082499. However, such a solution is often expensive, because the refractory material of the slab erodes quickly, and difficult to optimize, particularly as regards the maintenance of a stable geometry of the impact surfaces. In addition, it acts little or no on the release of gas bubbles. The second approach, described for example in WO 92/17295, uses the electromagnetic stirring techniques of steel in the liquid state. The splitter is divided into two adjoining chambers communicating with each other this time through an opening at the bottom of the partition wall. A first chamber, approximately square in cross section, receives the molten steel from the pocket and delivered by the pouring tube. In this chamber, the liquid metal is rotated axially under the action of a sliding field electromagnetic inductor mounted externally against the partition wall. The rotational movement will promote coalescence, thus the decantation of non-metallic inclusions and gas bubbles. The steel, freed of these non-metallic impurities and its gaseous ballast, then passes into the second chamber, where it is cast in the mold. It is understood that this type of solution uses heavy equipment, bulky, expensive, and a significant energy expenditure.

  The invention provides an alternative response to current solutions by a vigorous electromagnetic rotation of the steel around the casting axis directly into the pocket tube before arriving in the distributor. Centrifugation of the flows directly into the pocket tube in fact induces a strong expansion of the metal jet at its exit from the tube, which contributes to significantly reducing the speeds at the impact of the jet on the bottom of the distributor and the turbulent intensity in the liquid metal. It also improves the settling of inclusions and gas bubbles which, being centrifuged inside the jet, enlarge and settle more easily.

  For this purpose, the subject of the invention is primarily equipment for casting molten metal in a continuous casting distributor from a pocket located above and provided with a pouring tube fixed on the bottom around the opening of draining the latter and intended to lead under the molten metal level in the distributor, equipment characterized in that it comprises: - a cooling circuit containing a circulating cooling liquid (water preferably); - a polyphase power supply; - Mountable around said casting tube, an annular rotating magnetic field inductor, polyphase type provided with a pair of poles per phase of said power supply to which it is connected to generate said rotating magnetic field, and each pole is formed by a wound electrical winding, advantageously in layers, around an inwardly projecting pole tooth having a lateral narrowing at its polar end face, the pole teeth being interconnected at their other end by a magnetic yoke external device for closing the magnetic flux, and in which each electrical winding incorporates a heat extractor, advantageously constituted by an annular sealed flattened tube inserted between two successive layers and whose free ends protruding from the winding are connected to said cooling circuit for be cooled by circ internal ulation of a coolant; and, interposed between the casting tube and the inductor, a hollow heat shielding annular shield of the non-magnetic metal inductor (such as copper) also cooled by an internal circulation of coolant being connected to said circuit of cooling.

  Advantageously, this heat shield is advantageously, in a manner known per se, segmented according to its periphery into portions having electrical discontinuities between them in order to counteract the fugitive currents developing under the effect of the variable magnetic field of the inductor. Preferably, the inductor is formed, in a known manner, by two complementary parts, carried by cradles in pivoting articulated half-shells.

  Advantageously, the power supply of the inductor comprises a resonant electrical circuit on which the inductor is mounted in series with an adjustable capacitance. the invention also relates to a method of continuous casting of the molten metal, in particular steel, from a ladle equipped with a pocket tube at its bottom to a distributor located under said pocket through said tube, such that, during the passage of the molten metal in said tube, a traversing magnetic field directed perpendicularly to the axis of said tube and generated by a polyphase annular electromagnetic inductor with built-in internal heat extractor as defined herein is used; beforehand, surrounding the pocket tube remotely with the interposition of a cooled thermal shield, and in that said magnetic field is rotated around the casting axis so as to carry with it the molten metal in a rotational movement within said casting tube It will be noted that the principle, as well as the implementation technology, at the base of the invention, namely the electromagnetic rotation ue of the liquid metal during its transfer into a pouring tube which channels it between two containers, are close to those described in WO 2005/002763. The latter, in fact, describes a continuous metal casting plant equipped with a polyphase ring magnetic inductor rotating for an electromagnetic rotation of the molten metal around the casting axis in a submerged nozzle of continuous casting in the mold, nozzle around which the inductor is mounted closer, that is to say with a minimum free air gap to maximize the coupling.

  However, a transfer of this technology from the ingot mold casting to the pocket tube, even if it is conceivable in itself, actually involves the resolution of problems inherent in differences, not only in size, but in use and reliability. metallurgical target. As for the goal first. Spinning in a nozzle is primarily aimed at faster settling in the ingot mold of the anti-corking argon which is injected upstream, in particular during the casting of steel shades soaked with aluminum. On the other hand, nothing of the kind for the rotation in the pouring tube at the outlet of the pocket. First, because the argon injection does not take place at this level of the casting, but downstream. Secondly, and above all because the metallurgical goal aimed here is a "softening" of the metal jet in the distributor in order to reduce its mechanical impact on the bottom of the container, thus offering an alternative solution to the existing sacrificial bowl called "turbo" -stop "It will then be remembered that a pocket tube has a diameter approximately three times greater than that of a mold casting nozzle (of the order of 150 mm to fix idea, instead of the conventional 50 mm for a nozzle ). In addition, the metallurgical height in a pocket is significantly higher than that in a tundish. Again the ratio is 1 to 3, even beyond. In short, it all contributes to the fact that the flow of liquid metal and the velocity of the metal stream within such a casting tube are considerably greater than what happens with a nozzle. This implies at least the implementation of magnetic fields much more intense, while for reasons of space, the inductor is necessarily as compact as possible. In addition, the molten steel, when it passes through the pocket tube to feed a distributor, necessarily displays overheating, so a temperature, higher than when it will flow through the nozzle in the mold where it will solidify. In summary, while the nozzle inductor will have to work a long time, continuously, the entire time of the casting in fact, delivering a power, certainly sustained under a current intensity of about fifty amperes pulsating to 50hz, so all in all compatible with existing compact technologies, the pocket tube inductor on the contrary will be solicited over shorter periods (the emptying time of the pocket), but to deliver, in particular conditions of use thermal, much more severe, a much higher power, which the existing compact technologies do not yet know how to answer. The invention will be better understood and other aspects and advantages will appear more clearly in view of the following description given by way of example of embodiment and with reference to the attached drawing plates in which: - Figure 1 shows schematically, seen in cross-section, the inductor remotely surrounding a pocket tube with the interposition of a thermal protection screen, in accordance with the invention; FIG. 2 is a diagram similar to the previous one, but intended to show clearly the propagation of the magnetic field of force through the inductor gap at any given instant of the operation of the inductor; FIG. 3 is a block diagram showing the articulation of the two half-shells constituting the inductor; - Figure 4 shows, in partial section, the internal structure of an electrical phase winding of the inductor, according to the radial longitudinal sectional plane A-A of Figure 1; FIG. 5 shows the evolution of the intensity B of the magnetic field in the center of the air gap as a function of the change in the intensity I of the excitation current of the inductor; In the figures, the same elements are designated by identical references.

  It will be recalled first of all that an annular inductor with a pair of poles per phase, as retained for the implementation of the invention, makes it possible to create a magnetic field perpendicular to the casting axis traversing the metal almost uniformly. liquid through (said field through), this field being rotatable about this axis when the windings of the inductor are properly connected to the phases of the associated power supply. The air gap of the inductor has a circular section in order to best fit the outer shape of the pocket tube and thus maximize the electromagnetic coupling with the molten metal passing through the tube. Advantageously use a three-phase power supply associated with an inductor with six protruding poles coiled, diametrically opposite two by two so as to form three pairs of poles (one per phase of the supply) regularly distributed around the tube. In a preferred embodiment of the invention, in order to facilitate its placement around the pocket tube 4 (FIG. 1), the annular inductor 1 is in the form of two half-inductors 2a and 2b, on the one hand and else of a separation plane S passing through the casting axis 16, and each comprising three poles 3. Each pole of a half-inductor mates with its corresponding located on the other half-inductor, in axial symmetry with respect to the casting axis 16. When the two half-inductors are closed on themselves and locked in the service position, that is to say once contiguous, they completely surround the casting line and are then substantially equivalent both functionally and mechanically to a single monolithic annular inductor. Each pole forms a polar tooth 3 laminated magnetic sheet ending in an active face 9 facing inwardly, all of these six active faces defining the air gap of the inductor. In use, each face will therefore be arranged opposite and in the immediate vicinity of the pocket tube 4. Each polar tooth 3 is surrounded by an electrical phase winding 10 which leaves at least the portion 17 of the tooth closest to the gap . It is essential that this protruding portion 17 of the tooth has at the active face 9 a lateral longitudinal narrowing 11 whose primary function is to increase the distance between the polar faces therebetween. This narrowing can advantageously be made from a beveling of the magnetic metal sheets constituting the polar tooth before assembly. The angle of the bevel is to be adjusted according to the diameter of the air gap, but it should be remembered that, preferably, the area of the polar face 9 will not, in the projected area, be less than half that of the section 3. In addition, care shall be taken to ensure that the narrowing bevel on the body of the tooth starts at only two-thirds of the length of the tooth and even beyond it if possible in order to maximize mass. Magnetic inductor.

  It will be noted that it is thanks to this narrowed truncated tip shape of the pole teeth 3 at their free end that it is possible to retain a "through" character in the magnetic field despite the compactness of the inductor. The separation distance thus doubled between contiguous active faces 9 counteracts indeed a parasitic phenomenon bridging the field lines between them, otherwise. It can be seen in FIG. 2 that, at any moment, the magnetic field force lines in the air gap essentially connect two diametrically opposed poles whose intensity of the phase current is maximum and that only a residual portion of the loop between neighboring poles.

  The operational height of the poles is between 50 (minimum value) and 500 mm, depending on the space available between the tundish and the bottom of the pocket. Their internal diameter (diameter of the air gap) is close to the external diameter of the pocket tube to within a few mm of functional clearance, so as to ensure the best possible electromagnetic coupling between the stator that forms the inductor and the rotor that constitutes the column of liquid metal within the casting tube 4. To fix the ideas, this diameter can be of the order of 25 to 35 cm. The pole teeth are interconnected at their other end by an outer circumferential yoke closing the magnetic flux 12 (12a and 12b when the inductor is in two parts). The poles 3 and the yoke 12 are laminated in grain-oriented Fe-Si plates with an initial thickness of 0.3 mm so as to minimize the hysteresis losses. FIG. 3 shows that the two half-inductors 2a and 2b are placed in two semi-tubular cradles 5a and 5b carried by articulated arms 6 about a pivot axis 7, so as to allow a separation movement on the part and the arms are animated by hydraulic cylinders 8 which ensure their closing-opening and allow to exert a sufficient contact force (greater than 200 kgf) between the yokes 12a and 12b of the two half inductors once the inductor closed. Close contact between yokes is indeed desirable to the correct looping of the magnetic field lines between the two half-inductors, therefore to a high magnetic performance of the inductor. In addition, a high clamping force is favorable to prevent vibrations that would otherwise inevitably be generated by the oscillating electro-magnetic forces involved. The electric windings 10 consist of turns of copper wires 13 supporting current densities greater than 10 A / mm 2 and wound in longitudinal single-layer webs extending throughout the winding (winding said "in long layers"). The winding of each pole is connected to the three-phase power supply (not shown) so that a pair of poles (two diametrically opposite poles) connected to the same phase is provided with windings wound in opposite directions so that the current of phase there flows in opposite directions so that at each moment their active faces 9 is of opposite polarity. The coils 10 are supplied with three-phase currents at medium frequency ranging from about 50 to 600 Hz. The fact of opting for high frequencies in this range makes it possible, at constant intensity of the currents, to increase the electromagnetic force torque on the metal column in the tube 4. However, this choice imposes in return the use of a frequency converter, unlike the operation at the mains frequency (50 or 60 Hz). According to an essential characteristic of the invention, clearly visible in FIG. 4, the electric windings of the inductor incorporate heat extractors. Thus, one (or more) cooling tube 14 is inserted between the "long layers" 20 of the electrical winding, about mid-thickness. This narrow tube, of elongated section to cover the entire height of the coil which receives it, is preferably copper or other metal good conductor of heat. It is therefore wound in at least one layer and is traversed by a cooling liquid through the inlet-outlet tips 18 and 19 which protrude laterally from the winding to allow its connection to a water cooling circuit, not shown here. In the exemplary embodiment shown in the figure, the thickness of the tube 14 is roughly equivalent, as we see, to that added with two long layers. In addition, in this example, not only one cooling tube, but three, 14, 14 ', 14 ", regularly distributed over the thickness of the coil, for a better cooling efficiency, is provided. The inductor is able to generate a high intensity electromagnetic field (several thousand gauss) in its air gap for low values of the inductive currents (a few tens of amperes.) By virtue of the longitudinal bevelling 11 of the edges of the active faces 9, this field is moreover almost uniformly transverse in the gap, thus making it possible to generate a uniformly decreasing field of forces from the wall to the liquid metal. the center of the pocket tube 4, thus developing a torque that is not only concentrated at the periphery of the flowable metal vein Thus, the liquid metal in the tube is rotated axially with a significant speed even in the central part of the tube, which makes it possible to avoid excessive depression in the central part of the tube where the metal would tend to "leak "and to undergo a strong downward vertical acceleration, thus canceling some of the beneficial effect of spinning. It is also emphasized that by supplying the inductor with a resonant circuit, the intensity of the excitation currents can be greatly increased. As can be seen in FIG. 5, the proposed technique makes it possible, in a wide range of intensity of the inductor currents, to very greatly increase the intensity of the electromagnetic field in the gap, by increasing the intensity of the inductive currents. at values well beyond the threshold intensity corresponding to the magnetic saturation of the yoke 12 of the inductor. Apart from its function of mechanical maintenance of the assembly, the primary role of this cylinder head 12 is to channel the magnetic field lines and increase, in the air gap of the motor, the intensity of this magnetic field up to to reach its saturation value in it (represented on the curve by the point SM in the figure). Beyond this threshold value, it is the magnetic field generated by the inductors, directly in the air, which contributes to the increase of the intensity of the field in the air gap of the motor. In operation, the inductor 1 is very close (between 2 and 5 mm) of the pocket tube 4 whose external temperature is of the order of 1100 1200 C. According to another essential provision of the invention, a thermal protection of the inductor, with respect to the strong thermal radiation emitted by the pocket tube 4 (according to the fourth power of the absolute temperature) is ensured. This protection is achieved by an annular heat shield 15 interposed between the two and preferably developing over the entire height of the inductor, which it also constitutes a structural element. This thin screen, non-magnetic metal preferably to be permeable to the incident magnetic field (copper, aluminum or Cu or Al alloys), is hollow to allow internal circulation of coolant. For this purpose, it has at its upper and lower ends input-output tips (not shown) for its connection to a cooling circuit, which, for practical reasons, will be advantageously the same as that ensuring the cooling of the extractors This screen preferably has an orange slice structure (well known) which makes it possible to present local electrical discontinuities distributed around its entire periphery. In this way, the tendency to form eddy currents on the metal screen 15 under the effect of the incident mobile magnetic field (thus locally variable) is counteracted. Of course, this heat shield 15, which is body by construction with the inductor, like him will be designed in two symmetrical semi-annular parts 15a and 15b abutted to each other when the inductor is in the closed position.

  In this respect, the inductor handling device (Fig. 3) can be mounted on the distributor without any particular difficulties. When a ladle is placed above the distributor to be emptied, the articulated arms 6 will close the inductor around the pouring tube 4. Like the electrical connection of the inductor, the circuit The cooling of the heat shield 15 and the in-situ extractors 14 can remain permanently connected. In any case, it must be already a little before the opening of the drain hole of the bottom of the pocket, while simultaneously with this event, the inductor 1 is energized so that the rotating magnetic field that it generates is applied immediately to the vein of molten metal which will go through the pouring tube 4 at more than 4m / sec.

  According to another alternative embodiment of the invention, the annular inductor (in one or two parts) can be mounted directly and permanently on the receiving stand of the ladle. The positioning of the latter above the splitter will then allow the connection of the inductor to its power supply and its connection to the cooling water circuit. This variant certainly requires to be equipped with more inductors, but limits their handling. To finish the presentation, we will encrypt some data specific to the implementation of the invention. Conventionally, a steel plant pocket delivers the molten metal which contains a flow rate ranging from 5 to 9t / min depending on the capacity of the facilities. This metal flows into a protective pocket tube that is about 120 to 150 mm inside diameter. Depending on the guard height between the bottom pocket and the distributor, it is possible to house an inductor around the pocket tube, the active portion of which is between about 200 and 500 mm in length. Under these conditions, and if we assume for simplicity that all the inner passage section of the tube is used by the metal jet, the residence time of the latter in the air gap of the inductor is of the order of 0.3 sec, this in all cases in the case of steel (density of 7.8). To achieve rotating the steel jet in such a short electromagnetic action time, the inductor will deliver a power around 40 kW (13 kW per phase of the power supply) to produce a magnetic field of the order of 0.3-0.5 Tesla, a field ten times more intense than in the case of an electromagnetic stirrer in the mold, while the size allowed for the latter is significantly greater than that allowed for the inductor of the mold. the invention because of congestion. In summary, the implementation of the equipment according to the invention imparts an axial rotational movement to the molten metal vein passing from the ladle to the distributor through the pocket tube, with the effect of a splitter casting in favorable conditions for obtaining a satisfactory metallurgical quality of the solid metal after casting while reducing or even eliminating some consumables reported (the special refractory pieces placed in the bottom under the jet) which increase the cost of running the castings.

  It goes without saying that the invention can not be limited to the embodiment described but that it extends to multiple variants and equivalents to the extent that is respected its definition given by the appended claims. In particular, with regard to the internal cooling of the inductor, it is certainly preferable, especially for better mechanical strength of the assembly, to wind the windings in long layers with insertion at selected locations in the thickness of 14 flattened thin metal tubes, it is possible nevertheless to provide alternatively pancake coils this time which stack along and around the polar tooth with interposition between some of them hollow metal discs cooled by internal circulation.

Claims (10)

  1- Equipment for casting molten metal in a continuous casting distributor from a pocket placed above and provided with a pouring tube (4) fixed on the bottom around the opening and intended to open under the level of molten metal in the distributor, equipment characterized in that it comprises: - a cooling circuit containing a circulating cooling liquid; a polyphase power supply; - Mountable around said pocket tube, an annular electromagnetic inductor (1) rotating magnetic field, polyphase type provided with a pair of poles per phase of said power supply to which it is connected, and each pole is formed by a electric winding (10) wound around a polar tooth (3) protruding inwardly with a lateral narrowing (11) at its active end face (9), the pole teeth being interconnected at their other end by an outer magnetic peripheral yoke (12) for closing the magnetic flux and having each electrical winding (10) incorporating a heat extractor (14); - And, interposed between the casting tube and the inductor, a hollow annular screen (15) for thermal protection of the inductor and cooled by an internal circulation of coolant being connected to said cooling circuit.   2- Equipment according to claim 1 characterized in that, said electrical winding (10) being wound in layers (20), said heat extractor is constituted by at least one annular flattened tube (14) connected to said cooling circuit in order to be cooled by an internal circulation of a coolant.   3- Equipment according to claim 1 characterized in that said cooling circuit is a water circuit.   4- Equipment according to claim 1 characterized in that said thermal protection screen (15) is non-magnetic metal. 35 40   5- Equipment according to claim 1 or 4, characterized in that said heat shield (15) is of segmented structure to present electrical discontinuities along its periphery.   6. Equipment according to claim 2, characterized in that the annular flattened tube (14) is located mid-thickness of the electric winding (10). 10   7- Equipment according to claim 1, characterized in that said inductor (1) has an air gap having a diameter of 25 to 30 cm.   8. Equipment according to claim 1, characterized in that said inductor (1) is formed in two parts (2a, 2b) carried by a structure with two half-shells (5a, 5b) articulated pivoting.   Equipment according to claim 1, characterized in that said polyphase power supply comprises a resonant electrical circuit on which said inductor is connected in series with an adjustable capacitance.   10-Method of continuous casting of molten metal, especially steel, from a ladle equipped with a pocket tube at its bottom to a distributor located under said pocket through said tube, characterized in that during the passage of the molten metal in said tube, a traversing magnetic field, directed perpendicular to the axis of said tube and generated by a polyphase annular electromagnetic inductor cooled internally and surrounding the pocket tube at a distance with interposition of a cooled thermal shield, and in that said magnetic field is rotated around the casting axis so as to carry with it the molten metal in a rotational movement within said casting tube . 35