EP2768996B1 - Hot dip galvanizing method for coating a steel strip and apparatus therefor - Google Patents

Description

The invention relates to the iron and steel industry, and more particularly to the coating installations by dipping steel strips, whereby said strips are covered with a layer of zinc or zinc alloy (in the case of galvanizing), or another type of metal or metal alloy such as an aluminum-silicon alloy.

It is recalled that during the dip coating of a steel strip, the moving strip passes into a container containing the metal or the metal coating alloy, maintained in the liquid state. The coating is deposited on the strip that then emerges from the bath, and passes through a device controlling the thickness of the coating and contributing to its solidification, usually consisting of nozzles throwing a gas on the surface of the coating. Prior to entering the bath, the strip is heated by an annealing furnace and then cooled to a temperature close to the bath temperature to create optimal adhesion conditions between the strip and the coating.

During the passage through the bath, the bath is formed in the formation of oxides and intermetallic precipitates, essentially based on Zn and Fe in the case of a galvanizing bath, containing liquid zinc which will be considered in a preferred manner in the remainder of the description, without it constituting an exclusive application of the invention. These precipitates are called "mattes". Some mattes have a density higher than that of the bath, and decant at the bottom of the tank without hindering the galvanizing process. Others, on the other hand, have a density lower than that of the bath and float on its surface. They are likely to be incorporated in the coating of the strip, and thus to create defects therein. These low-density mattes, which will be the only ones considered in the rest of the text, must therefore be as far as possible away from the inlet zone of the strip in the bath (if this entry is made in the open air, this which is not always the case) and the exit zone of the band out of the bath, and be evacuated from the tank as and when they are formed.

For this purpose, the most conventionally, an operator standing in the vicinity of the tank pushes, with a tool, the matte towards a container located away from the entry and exit areas of the band, this container is then extracted from the tray and emptied using a robotic system or not. In other cases, the operator pushes the matte towards an area of the tank where a device such as a robot evacuates to a container outside the tray, in which they are collected.

This operation is uncomfortable and potentially dangerous for the operator, because it must be in the immediate vicinity of a bath of hot liquid metal, with the inconvenience and the risks related to the heat and the possibility of projections of the metal liquid. Furthermore, the coating thickness control system deposited on the strip consists of blowing nozzles, and can use inert gases such as nitrogen to limit the oxidation of the coating. The use of these inert gases is also a source of risk for the operator, because of the lack of oxygen in the atmosphere around the tank that it involves.

In addition, this operation of cleaning the mattes imposes a limitation of the running speed of the band, because a high speed favors the production of mattes, that the operator and the robot must have time to evacuate.

Also, the higher the speed of the web, the more the coating thickness control nozzles must project a large amount of gas to maintain the coating thickness constant. This has the effect of increasing the ambient temperature around the bath, because the blowing gas transports the heat of the strip and the bath to the working area of the operators.

Finally, in order to limit the thermal energy losses associated with the heating of the bath, it is envisaged that certain new coating installations are completely covered. It would therefore be necessary, in this case, to limit the external interventions, and in particular that of an operator for the removal, in order to avoid too frequent knockouts of the installation.

There is therefore a need to increase the safety, speed and efficiency of the evacuation of mattes compared to this conventional technique, without radically altering the galvanizing process itself and the overall design of the installation that implements it.

A solution devised by some steelmakers has been to replace, at least for the most part, the human intervention to bring matts into the robot's action zone by the action of electromagnetic devices. Using sliding fields generated by inductors such as linear motors, electromagnetic forces, to which the metal or liquid metal alloy is sensitive (so-called "magnetomotive" forces), move the metal or metal alloy liquid that drives the matte in an area of the tank where the robot is active, creating a recirculation path mattes leading them into said area. Such devices are described, for example, in the documents JP-A-10-053850 , JP-A-54-33234 , JP-A-2005-068545 , JP 11-006046 .

JP-A-54-33234 , for example, teaches to dispose of the field-sliding inductors all around the band in its exit zone of the tank, the sliding fields bringing the matts into the corner of the tank where there is a conveyor belt which evacuates the mattes out of the tank in a container that collects them. In his case, the entrance to the The strip in the galvanizing bath is, as is often the case, inside a tube immersed in the bath and connected upstream to the annealing furnace, and the matts which have decanted on the surface of the bath can not come into contact with the surface of the band in this area. It is therefore sufficient to place inductors in the environment of the exit zone of the strip.

JP-A-10-053850 teaches to have screens parallel to the strip in its area of entry into the tray, and sliding field inductors are arranged in the vicinity of the two ends of each screen. The magnetic fields thus generated make it possible to attract the mattes out of the zone between the screens and including the band.

In the case where there is no robot, such devices in any case facilitate the work of the operator who has more to act in a tray area whose surface is relatively limited.

Experience shows, however, that the effectiveness of these devices would benefit from further improvement. In particular, an evacuation as complete as possible of the mattes without human intervention should be possible, and with a minimum of inductors. Optimally, a single inductor could be sufficient if the tray is small.

The object of the invention is to provide a method and a device for removing supernatant low density matts at the surface of the galvanizing bath guaranteeing a better efficiency than known devices, by using a minimum of inductors.

For this purpose, the subject of the invention is a galvanization process by dipping a strip of steel running in a liquid metal bath, such as zinc, or a metal alloy contained in a tank, according to which away from the surface of the strip matts that are formed during galvanization and float on the surface of the bath, by means of at least one inductor, each inductor producing a sliding electromagnetic field oriented in a given direction and generating a force magnetomotive, all of said magnetomotive forces displacing said matte towards a receptacle responsible for collecting them and / or towards an area of the bath surface from which they are discharged, characterized in that, for at least one of said inductors, it intermittently reverses said direction of its sliding electromagnetic field so as to change the flow of matte inside the tray.

Among said inductors, at least two of them may be disposed along the exit zone of the bath strip, and the direction of their respective magnetic fields is intermittently reversed.

The invention also relates to a coating installation by dipping a steel strip, comprising a tray containing a liquid bath of metal or metal alloy in which the strip travels, and at least one inductor, each inductor creating an electromagnetic field and magnetomotive forces contributing to bring the mattes generated during the coating in the vicinity of a container intended to receive them and / or in the zone of action of a robot or an operator which brings them into said container, characterized in that at least one of said inductors comprises a device for reversing the direction of the electromagnetic field generated by said inductor.

It may comprise at least two inductors located on either side of the exit zone of the bath strip, and said inductors each comprise a device for reversing the direction of the electromagnetic field that it generates.

Said inductors can be mounted on brackets to adjust their location above the tray and their distance from the surface of the bath.

Said installation may comprise automated devices for controlling the distance between each of the inductors and the level of the surface of the bath.

According to one embodiment, two inductors frame the strip in its exit zone of the bath so as to move the matts away from the surfaces of the strip by moving them parallel to it, and two inductors are each arranged along a wall of the tank, substantially in the extension of the other two inductors.

In this case, the tray containing the bath has a generally rectangular shape, the container in which the mattes are collected, and / or the area of action of the robot or the operator from which they are removed, is placed in a corner the tray opposite one of the inductors, and in the corner of the tank opposite the other of the inductors is placed an inductor for directing the matte to said container.

The installation may comprise means for controlling the reversal of the direction of the electromagnetic field generated by at least one inductor which are themselves controlled by a device making it possible to evaluate the quantity of mattes accumulated in at least one zone of the tank. and to determine when such an inversion is desirable.

At least one of said inductors may be a three-phase linear motor.

Preferably, at least one of said three-phase linear motors is of the type in which the coils surround the magnetic core.

As will be understood, the invention is based on the use of sliding field inductors, at least one of which has the possibility of varying intermittently the direction of the sliding field during their use, so the direction of the magnetomotive force that causes the moving of the mattes. Eventually, if the tank containing the liquid coating metal is of small dimensions, the presence of a single inductor can be sufficient, if the direction of its sliding field can, according to the invention, be reversed intermittently.

This variation of the direction of the field makes it possible not to have a constant configuration of the privileged paths of circulation of the mattes on the surface of the bath.

Indeed, the inventors have found that such constancy of the circulation paths was detrimental to the effectiveness of the electromagnetic device for driving the mattes. It leads to the creation of dead zones and closed recirculation loops, located in certain areas of the tank. Mattes therefore tend to remain or accumulate there, and therefore can not be removed by the robot if the zone of action of this one does not concern the dead zones and the zones where the recirculation loops are located . And if they are, in addition, removed from the container collecting the mattes, it is necessary that an operator brings them into the container or the zone of action of the robot, with all the disadvantages mentioned above in terms of safety and working conditions.

Inversion (performed at regular intervals or not) of the direction of the field generated by at least one inductor, preferably at least by inductors flanking the two sides of the strip in its area of penetration into the tray, makes it possible to modify the circulation path of the mattes. In doing so, the dead zones and recirculation loops that could be established when the fields had a given direction are "broken" by the reversal of this direction, and the mattes that had eventually accumulated there are brought back into the circulation circuit that leads them to the robot's action area, or even directly to the container that collects them. Human intervention to perform this recirculation of mattes is no longer necessary. Also, the number of inductors that would be necessary to evacuate the mattes present on the entire surface of the bath can be reduced, knowing that it is not necessarily necessary that a given area of the tank, in particular those located relatively far from the belt, is permanently concerned by the traffic flows.

• la figure 1 représente un exemple de moteur linéaire qui est utilisable dans le cadre de l'invention ;
• la figure 2 représente le schéma électrique du moteur linéaire de la figure 1 ;
• les figures 3 à 5 représentent schématiquement les variations de l'orientation des forces magnétomotrices générées par le moteur linéaire de la figure 1 en fonction de la fréquence du courant qui le parcourt ;
• la figure 6 représente schématiquement en perspective un exemple d'installation de galvanisation à laquelle peut s'appliquer l'invention ;
• les figures 7 et 8 montrent schématiquement en vue de dessus l'installation de la figure 6 pour deux configurations possibles d'écoulement des mattes réalisables selon l'invention ;
• la figure 9 montre schématiquement en vue de dessus une variante de l'installation de la figure 6 dans laquelle un moteur linéaire supplémentaire est utilisé.

The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

• the figure 1 represents an example of a linear motor that can be used in the context of the invention;
• the figure 2 represents the circuit diagram of the linear motor of the figure 1 ;
• the Figures 3 to 5 schematically represent the variations in the orientation of the magnetomotive forces generated by the linear motor of the figure 1 depending on the frequency of the current flowing through it;
• the figure 6 schematically represents in perspective an example of galvanizing installation to which the invention can be applied;
• the Figures 7 and 8 schematically show in top view the installation of the figure 6 for two possible configurations of matte flow achievable according to the invention;
• the figure 9 schematically shows in top view a variant of the installation of the figure 6 in which an additional linear motor is used.

The general design of three-phase linear motors which, according to a preferred example of the invention, ensure the creation of sliding fields, is conventional, but their size and their characteristics must be suitable for the needs of the installation. A constraint is, in particular, to obtain a satisfactory efficiency of the sliding field when the motor is placed at a distance from the galvanizing bath optimally between 20 and 100 mm, distance which is generally avoided that the surface of the bath does not come in contact with the engine, or that projections of liquid zinc do not deteriorate.

• la vitesse de défilement de la bande et ses variations, qui créent des perturbations plus ou moins importantes à la surface du bain ;
• la vitesse de formation des mattes, qui dépend d'ailleurs entre autres de la vitesse de défilement de la bande, et qui, lorsqu'elle est importante parce que la bande défile rapidement, peut nécessiter une efficacité maximale des moteurs pour éloigner les mattes de la bande ; on aura alors intérêt à placer les moteurs au plus près de la surface du bain.

Theoretically, a motor-bath distance of 1 to 350 mm is possible (it is to be adjusted also according to the polar pitch and the engine power), knowing that the lower this distance, the higher the efficiency of the engine, all things being equal. But the exact geometry and operating conditions of the galvanizing system must be considered when choosing the optimal distance. The motors are also optimally mounted each on a bracket that allows to adjust their exact location above the bath, including height, according to the instant needs of the implementation of the invention which may vary according to various parameters such than :

• the running speed of the band and its variations, which create more or less significant disturbances on the surface of the bath;
• the rate of matte formation, which depends, moreover, on, among other things, the running speed of the strip, and which, when it is important because the strip is running rapidly, may require maximum efficiency of the engines in order to keep the matts away from the band ; it will be advantageous to place the motors as close to the surface of the bath.

The overall length and volume of each motor must be such that the engine can find its place in the production line, given the dimensions usual tray, tape and space available to implement the engines above the tray, especially when you want to implement on a pre-existing installation. In practice, the length of an engine is from 200 to 2000 mm, its width from 100 to 1000 mm and its height from 50 to 600 mm.

• soit de disposer de plusieurs jeux de moteurs, de largeurs différentes, et pouvant être changés rapidement entre deux opérations de galvanisation de bandes de largeurs différentes ;
• soit, comme on l'envisagera plus loin, d'utiliser plusieurs moteurs placés côte à côte et pouvant être mis en service ou hors service selon la largeur de la bande à revêtir.

The length and width of the motor define its active surface: the larger the active area, the greater the area swept by the engine, but also the larger the size of the engine, which can make its implementation difficult. Of course, all the engines of the same installation are not necessarily identical. The choice of the dimensions of the motor is adapted to the size of the zone which it must sweep. Optimally, the motors flanking the strip have a length of the order of the width of the strip to ensure that the mattes will be spaced from the entire penetration zone of the strip in the galvanizing bath. However, this condition is not always fulfilled on installations intended to treat strips of various widths (from 600 to 2000 mm for example). To remedy this, we can consider:

• either to have several sets of motors, of different widths, and can be changed quickly between two operations galvanizing bands of different widths;
• or, as will be discussed later, to use several motors placed side by side and can be put into service or out of service depending on the width of the band to be coated.

The polar pitch of the motor, that is to say the distance between two coils fed by the same phase, can vary from 50 to 700 mm. It corresponds to the zone of action of the magnetic field. The lower the polar pitch, the closer the motor should be to the bath surface for a given efficiency in matte training. Placement of the motor 100 mm from the surface of the bath is generally accompanied by the choice of a polar pitch of the order of 300 mm taking into account other preferred characteristics of the engines.

The operating frequency of the motors can range from 1 to 500 Hz. It influences the direction of the magnetomotive force in the liquid Zn, as has been seen above. The force is optimally as tangential as possible with respect to the surface of the bath, so as not to create agitation outside the immediate vicinity of the surface (in particular agitation which would tend to return to the heart of the bath matts having decanted at bottom of the tray or those supernatant on the surface) and ensure as efficient a displacement as possible supernatants supernatant on the surface. All things being equal, especially the polar pitch, the electromagnetic force is all the more tangential as the frequency is low.

The intensity of the current passing through each notch of the motors must be sufficient to create a magnetomotive force of 1000 to 20 000 ampere-turns, knowing that for a given winding, the higher the intensity of the current, the greater the magnetomotive force generated is .

The figure 1 schematically represents a three-phase linear motor of a type known in itself, used as an inductor in the context of the invention. It comprises, conventionally, a magnetic core 1 of length L and width l constituted by an assembly of sheets of soft iron. Soft iron is used to maximize the magnetic flux, and the sheet construction reduces the occurrence of eddy currents, hence Joule losses. The core comprises slots 2 in which are placed electrical conductors forming coils 3-8, these coils 3-8 are themselves connected to each other to form windings. In the example shown, it is a three-phase motor, comprising three windings of two coils arranged alternately. The coil 3 is thus connected to the coil 6, the coil 4 is connected to the coil 7 and the coil 5 is connected to the coil 8. Each coil 3-8 is supplied with a phase shift of 2π / 3 to create the magnetic field sliding that will create the magnetomotive force moving the mattes in the same direction as the field. The coils 3-8 can be cooled by an internal circulation of water.

The figure 2 shows the wiring diagram of the motor, with the star connection showing the alternation of the coil connections.

For the easy implementation of the invention, a phase inverter 30 is provided which makes it possible, in a single actuating operation, to modify the connections of the coils connected to the phases 1 and 2 (respectively, in the example represented , the coils 3, 5, 6, 8) so as to be able to instantly reverse the direction of the sliding field, knowing that the connections of the coils 4, 7 connected to the phase 3 remain unchanged. So, in the configuration shown in solid lines on the figure 2 , where the coils 3 and 6 are connected to the phase 1 and the coils 5 and 8 to the phase 2, the field slides from left to right according to the arrow 31. In the configuration shown in dotted line on the figure 2 where the coils 3 and 6 are connected to the phase 2 and the coils 5 and 8 are connected to the phase 1, the field slides from right to left according to the arrow 32.

• un pas polaire suffisamment long pour qu'il ne soit pas nécessaire de placer le moteur à une distance trop réduite du bain de galvanisation, ce qui pourrait l'endommager ;
• un pas polaire suffisamment réduit pour ne pas conduire à un moteur dont la longueur serait exagérément grande.

The polar pitch of the motor, that is to say the distance "p" between two coils fed by the same phase, for example coils 3 and 6 in the example shown, is, as has been said, 50 to 700 mm. A polar pitch of 300 mm for a motor length of 600 to 700 mm proves to be a good compromise between the various imperatives to reconcile:

• a polar pitch long enough that it is not necessary to place the motor at a too small distance from the galvanizing bath, which could damage it;
• a polar pitch small enough not to lead to a motor whose length would be exaggeratedly large.

The Figures 3 to 5 schematically the magnetomotive forces and their orientations in the galvanizing bath 9 for frequencies of current running through the motor of 10 Hz ( figure 3 ), 50 Hz ( figure 4 ) and 250 Hz ( figure 5 ). The arrows represent, depending on their orientations and their lengths, the preferred directions of said forces and their intensities. It can be seen that, as has been said, the lower the frequency, the more the magnetomotive force is exerted tangentially to the surface of the bath, and is therefore effective, at the same current intensity, for moving the matts in the desired direction. But a low frequency leads to a low intensity of the magnetomotive forces. The choice of the current frequency must also be made in combination with that of the polar pitch to obtain the geometry of the installation most favorable to its proper operation. Finally, it is judged preferable to have a relatively low frequency and a relatively high polar pitch so as not to be obliged to place the motor at a too small distance from the bath, in order to obtain a magnetomotive force of nevertheless suitable intensity, and which exercises mainly in an effective direction for the good circulation of the mattes. A current of frequency 10Hz, a polar pitch of 300 mm, a motor of total length of 600 to 700 mm comprising six coils of 96 turns, each traversed by a current of intensity 150 A, and thus providing a magnetomotive force of 15,000 Ampere-turns represents a good compromise if placed at a distance of 50 to 100 mm from the surface 10 of the bath 9.

The most conventional linear motors comprise a flat winding, with flat coils passing through the core (see for example the document EP-A-0 949 749 ). But for a greater compactness of the engine, particularly in width, it is preferable to give it the configuration shown schematically in the figures, where the coils 3-8 are arranged around the core 1. The document "Fluid flow in a continuous casting [0006] ISIJ International, 2001, vol.41 No. 8, pp851-858) describes in more detail such linear motors.

The figure 6 schematically represents a galvanizing installation equipped, in the example shown, four linear motors 11-14 of the type of that of the figure 1 and capable of implementing the invention. In a conventional manner, this installation comprises a bin 15 of generally rectangular shape, provided with means for maintaining the temperature of the liquid bath 9 of zinc or, more generally, of zinc alloy (or, remember, any other metal or metal alloy that can be used to coat the strip 16), it contains. The scrolling strip 16 to be galvanized penetrates the bath 9 in an oblique direction. Very often, as has been said, this penetration takes place, in fact, inside a protective tube, connected in its upstream part to the annealing line which has made it possible to adjust the temperature of the strip to a value close to that of the bath 9. For the sake of clarity, this tube has not been shown on the figure 6 , as well as Figures 7, 8 and 9 . The strip 16 passes around a roller located inside the tank 15, and leaves the bath 9 in the vertical, coated with its galvanizing layer, towards the other elements of the galvanizing installation known in themselves. same and having no influence on the design of the invention. As is known, the galvanized strip 16 passes, at its exit from the bath 9, between two gas blowing devices 17, 18 which adjust the thickness of the coating on each of the surfaces of the strip 16 and cool it, thus contributing to its good solidification. To collect the mattes, can be placed in a corner of the tray 15 a container in which the mattes can be collected after being pushed with the 11-14 engines, or, as shown, a robot 20 disposed in the vicinity of the tray 15 can be moved in all directions of space to extract the mattes bath 9 and send them in a container 19 placed next to the tray 15.

• la situation de la zone d'action de chaque moteur 11-14 ;
• et la distance verticale entre la surface 10 du bain 9 et chacun des moteurs 11-14.

The linear motors 11-14 are arranged on brackets 21-24 which make it possible to modify their respective positions above the bath 9 to optimize:

• the situation of the action zone of each engine 11-14;
• and the vertical distance between the bath surface 9 and each of the motors 11-14.

Indeed, due to the progressive consumption of zinc during galvanizing, the bath level 9 tends to drop during the operation, and if the distance between the motor 11-14 and the surface 10 increases, the force magnetomotive decreases. A progressive lowering of the engine 11-14 by its 21-24 stem makes it possible to keep this distance constant, and thus to keep the magnetomotive force constant in direction and in intensity, all other things being equal. Another way of acting on the magnetomotive force is to increase the intensity of the current flowing through the motor 11-14. Of course, it is possible to combine an adjustment of the distance between the motor 11-14 and the surface 10 of the bath 9 and a setting of the intensity of the current to control the magnetomotive force. Means may be provided for automatically controlling the distance between each motor 11-14 and the surface 10 of the bath 9 at the variation of the level of said surface 10.

The arrangement of the different main elements of the installation as shown on the figure 6 also appears on Figures 7 and 8 . Two motors 11, 12 frame the strip 16 in its outlet area of the bath 9 so as to move the matts away from the surfaces of the strip 16 by moving them parallel to it. Two motors 13, 14 are, in the non-limiting example shown, each disposed along a side wall of the tray 15 and parallel to it, substantially in the extension of the two other motors 11, 12, so as to follow said matte wall which penetrate into their respective areas of action, and send them to the zone of action 25 of the robot 20 which pushes them in the container 19 located in the immediate vicinity of the tank 15. In the example shown, the zone 25 of the robot 20 is opposite to one 14 of the motors arranged along a side wall of the tank 15.

The parallelism of the side walls of the tank 15 and the motors 13, 14 represented on the figures 6 , 7 and 8 is, as we have said, only an example of a non-limiting provision. The orientation of these motors 13, 14 is to be optimized according to the precise configuration of the tank 15 and the precise location of the action zone 25 of the robot 20. This optimization can lead to having at least one of these motors 13 14 obliquely with respect to the side wall of the tank 15 from which it is close.

The inventors have found that the effectiveness of such a system, operating in steady state with magnetometric forces substantially constant at least in direction, did not allow to achieve maximum efficiency of the evacuation of mattes.

Indeed, due to the stability of the flows on the surface of the bath 9, the creation of dead zones where the mattes accumulate and remain motionless without being captured by one of the motors 11-14, and also areas in which the matte circulate in loops, having little opportunity to escape to join the normal flow of circulation which must lead them in the zone of action 25 of the robot 20 (or directly in the container 19 if the one it is placed in the tray 15 itself). There is therefore an accumulation of mattes in certain areas, which may end up as a source of pollution for the entire bath 9 and deteriorate the quality of the galvanization.

The invention solves this problem by providing that at least one of the motors 11-14 has means for reversing the direction of the electromagnetic field it generates, so the direction of the magnetomotive force that causes the matte to move. This inversion can take place systematically at predetermined time intervals and be controlled manually or automatically, previous experiments having made it possible to determine with which optimal frequency this inversion must be carried out. depending on the conditions of the galvanization (including the running speed of the strip 16, the nature of the bath 9 ...). It can also take place irregularly, at times determined by the operator of the installation, or by any automated device operating, for example, by being slaved to means for evaluating the amount of mattes accumulated in a system. or specific areas of the bin 15.

This evaluation of the amount of mattes accumulated can be provided, for example, by an analysis of the images captured by cameras (infrared or otherwise) aimed at areas of potential accumulation of mattes. It makes it possible for an operator, or an automatic galvanization plant management device, to estimate that the accumulation of mattes in one or more areas of the bath surface 9 is about to become excessive or is already so, and it is therefore desirable to reverse the direction of the field of at least one of the motors 11-14.

The inversion of the direction of the magnetomotive force associated with the 11-14 engine (s) concerned causes a transient disturbance of the flow of the mattes, which thus makes it possible to shake previously stable zones (dead zones or recirculation loops). This agitation brings the matts that are in these areas within the new privileged path of circulation of the mattes which is thus created, and said mattes can be evacuated. This new recirculation path will, in turn, create new dead zones and recirculation loops, but they can be "broken" in the same way by a subsequent inversion of the direction of the field created by at least one of the inductors 11. 14.

These means of inverting the field of the inductor 11-14 can be constituted, very simply, by a switch that changes the power of the various coils 3-8. For that, as we have seen and represented on the figure 2 it is sufficient to provide a phase switch 30 which modifies the power supply of the motor coils. This switch 30 is installed in the electrical control cabinet of the installation and can be controlled remotely by an operator and / or by an automatic system. The change of direction of the sliding field is instantaneous.

In the case represented on the Figures 7 and 8 it is the motors 11, 12 which surround the strip 16 in its outlet zone of the bath 9 which are equipped with means for reversing the direction of the electromagnetic field they generate.

In the case of figure 7 a first state of operation of the motors 11-14 is shown in which the motors 11, 12 both drive the matts towards the left side wall of the tank 15. They are taken up by the field generated by the motor 14 located along the of this left side wall 26, and sent towards the container 19 if it is integrated with the tray 15, or, as shown, in the zone of action 25 of the robot 20. Simultaneously, the motor 13 located along the right side wall 27 of the tank 15 sends the matte that captures its electromagnetic field along the right side wall 27 to the zone of action 25 of the robot 20. These matts also tend to be deflected by the front wall 28 of the tank 15 towards the zone of action 25 of the robot 20. The various arrows represented on the figure 7 (as well as Figures 8 and 9 ) show the displacements of the mattes induced by the magnetomotive forces generated by the various motors 11-14.

The figure 8 represents a second state of operation of the motors 11-14, in which the directions of the fields generated by the motors 11, 12 flanking the strip 16, after a certain time of use of the configuration of the figure 7 have, according to the invention, been inverted with respect to the case of the figure 7 . This time, the matts in the vicinity of the strip 6 are oriented towards the motor 13 located along the right side wall 27 of the tank 15. The motors 13, 14 operate as in the case of the figure 7 . This inversion is already sufficient to create movements of the matts on the surface of the bath 9 which are able to "break" the dead zones and the recirculation zones created in the configuration of the figure 7 .

It will be passed manually or automatically in the configuration of the figure 7 when the accumulation of mattes in the new dead zones and recirculation loops created will be on the verge of becoming excessive, as previously described.

In the example shown, the two motors 11, 12 flanking the strip 16 both lead the matte in the same direction. But this configuration is not mandatory, it can be provided, if the location of the matte to move requires, the field directions of said motors 11, 12 are opposed, and permanently or temporarily.

Also, in the example shown, the two motors 11, 12 flanking the strip 16 have the same length and are exactly opposite. But this configuration is not mandatory and it can be expected that these motors 11, 12 have different lengths and / or are offset relative to each other, if it turns out that it is beneficial to the good evacuation of the mattes in the particular configuration of the tank 15 used.

The figure 9 schematically presents a variant of the case of Figures 6 to 8 , in which a fifth motor 29 arranged obliquely in the right front corner of the tank 15 has been added. It is thus located on the path of the matts pushed by the motor 13 situated along the right side wall 27 of the tank 15, and has in order to reinforce the effect of this motor 13 in the expedition of the mattes towards the action zone 25 of the robot 20. It is thus possible to reduce the size of the action zone 25 of the robot 20 and generally, increase the efficiency of the evacuation of mattes from the vicinity of Band 16 and direction of the zone of action 25 of the robot 20. The motors 11, 12 surrounding the band 16 have, as in the case of Figures 7 and 8 alternating their electromagnetic fields in one direction or the other.

It can also be envisaged that the various motors 11-14 or 11-14, 29, or at least some of them, are movable during operation in a direction that allows them to accompany the movement of the mattes, and thus to assist the movement of a given group of mattes for a longer duration than if the engine 11-14 or 11-14, 29 gave them only one pulse, when these matts are located below the zone initial action of the motor 11-14 or 11-14, 29.

Of course, the examples of Figures 6-9 are not limiting, both in terms of the number of engines and their disposition. It is also possible that other motors than motors 11, 12 flanking the strip 16 (in addition to them or in their place) can have their directions of action reversible. But the vicinity of the exit zone of the strip 16 being the most sensitive in terms of risks of pollution of the deposit of zinc, or coating metal alloy in general, by the mattes (if the zone of entry of the strip is protected by a tube connected to the annealing furnace as is often the case), it is clear that, preferably, motors of high efficiency must be arranged there. And especially if these motors 11, 12 are the most powerful device, it is preferably those which it will be most profitable to reverse the directions of action. It is also possible to replace one and / or the other of these two motors 11, 12, whose length is, if possible, of the same order as the width of the strip, by several smaller engines arranged next to each other and whose magnetic fields would have the same direction. This may be a way to solve a problem of size that could pose the implementation of a single large engine in the bath, particularly in the case of the motor 12 located between the input zone of the strip 16 in the bath 9 and the exit zone of the strip 16. This may also be a way to easily vary the size of the action zone of the motors flanking the strip 16 as a function of the width of the strip 16 if it can take several different values on the same coating plant. For this, it is enough to electrically out of order engines that overflow beyond the width of the band 16, or even to move them away from the tray 15.

Of course, the examples which have been described are not limiting and other provisions of the inductors are conceivable, in particular when the zone where the strip 16 enters the bath 9 must also be free of mattes if the strip 16 is It is found in the open air, or if the container 19 collecting the mattes and / or the zone of action 25 of the robot 20 are placed elsewhere than they are in the examples shown. The skilled person will be able to adapt the number and arrangement of the inductors to the particular geometry of its coating installation, the essential being the existence of the possibility of intermittently reversing the direction of action of at least one of the inductors to avoid perpetuation dead zones and recirculation loops on the surface of the bath 9, which is conducive to the accumulation of the mattes.

For containers 15 of small dimensions, it is possible to use only a single motor which is varied intermittently the direction of the sliding field it generates. In this case it may be wise to provide two containers 19 each located in the extension of said engine but opposite each other, to collect the matte displaced during the periods during which the engine field slides according to the one or the other direction.

By way of nonlimiting example, for an implementation of the invention on a galvanizing installation of steel strips 650 to 1350 mm in width normally moving at 60-120 m / min but being able to scroll to a greater than 200 m / min thanks to the use of the invention, it is possible to use a rectangular tank 4 x 3.20 m and four motors 11-14 arranged as on the Figures 6 to 8 These motors are powered by a 10Hz frequency current. They each have a pitch of 300 mm, a total length of 600 to 700 mm, and each comprise six coils of 96 turns, each traversed by a current of intensity 150 A, and thus providing a magnetomotive force of 15,000 amps. towers.

Claims (11)

1. A method for hot-dip coating of a steel strip (16) running in a bath (9) of liquid metal or of a metal alloy contained in a pan (15), according to which the dross which are formed during the coating and float at the surface (10) of the bath (9), are moved away from the surface of the strip (16), by means of at least one inductor (11-14, 29), each inductor (11-14, 29) producing a sliding electromagnetic field oriented along a given direction and generating a magnetomotive force, the whole of said magnetomotive forces displacing said dross towards a container (19) intended to collect them and/or towards an area (25) of the surface (10) of the bath (9) from which they are discharged, characterized in that, for at least one of said inductors (11-14, 29), said direction of its sliding electromagnetic field is reversed intermittently so as to modify the flows of the dross inside the pan (15).
2. The method according to claim 1, characterized in that among said inductors (11-14, 29), at least two (11, 12) of them are positioned along the area where the strip (16) exits the bath, and in that the direction of their respective magnetic fields are reversed intermittently.
3. A hot dip coating facility for a steel strip (16), including a pan (15) containing in the liquid state, a bath (9) of liquid metal or metal alloy, in which the strip runs (16), and at least one inductor (11-14, 29), each inductor (11-14, 29) generating an electromagnetic field and magnetomotive forces contributing to bringing the dross generated during the galvanization to the vicinity of a container (19) intended to receive them and/or into the action area (25) of a robot (20) or of an operator who brings them into said container (19), characterized in that at least one of said inductors (11-14, 29) includes a device allowing reversal of the direction of the electromagnetic field generated by said inductor (11-14).
4. The facility according to claim 3, characterized in that it at least includes two inductors (11, 12) located on either side of the exit area of the strip (16) from the bath (9), and in that said inductors (11, 12) each include a device allowing reversal of the direction of the electromagnetic field which it generates.
5. The facility according to one of claims 3 or 4, characterized in that said inductors (11-14, 29) are mounted on brackets (21-24) allowing adjustment of their location above the pan (15) and their distance to the surface (10) of the bath (9).
6. The facility according to one of claims 3 to 5, characterized in that it includes automated devices for servo-controlling the distance between each of the inductors (11-14, 29) and the level of the surface (10) of the bath (9).
7. The facility according to one of claims 3 to 6, characterized in that two inductors (11, 12) frame the strip (16) in the area where it exits the bath (9) so as to move the dross away from the surfaces of the strip (16) by having them move parallel therewith, and in that two inductors (13, 14) are each positioned along a wall (26, 27) of the pan (15), substantially in the extension of the two other inductors (11, 12).
8. The facility according to claim 7, characterized in that the pan (15) containing the bath (9) has a generally rectangular shape, in that the container (19) in which the dross are collected or the action area (25) of the robot (20) or of the operator is located in a corner of the pan (15) opposite to one of the inductors (13, 14), and in that in the corner of the pan (15) opposite to the other one of the inductors (13, 14), an inductor (29) is placed intended to orient the dross towards said container (19).
9. The facility according to one of claims 3 to 8, characterized in that it includes means for controlling the reversal of the direction of the electromagnetic field generated by at least one inductor (11-15) which are themselves subordinate to a device allowing to evaluate the amount of accumulated dross in at least one area of the pans (15).
10. The facility according to one of claims 3 to 9, characterized in that at least one of said inductors (11-14, 29) is a three-phase linear motor.
11. The facility according to claim 10, characterized in that at least one of said three-phase linear motors (11-14, 29) is of the type in which the coils (3-8) surround the core (1).

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