FR2797276A1 - DEVICE AND METHOD FOR MONITORING THE CONTINUOUS COATING BY IMMERSION OF A METAL STRIP IN VERTICAL RECTILINE PATTERN - Google Patents

Abstract

The invention concerns a method for controlling a metal coating process on a moving strip (2) which consists in causing the strip (2) to pass vertically in a metal coating bath (1) contained in a container (10), causing it to pass through a slot (13) provided at the base of the container, said slot being extended downwards by a channel (14) for passing through the strip, and maintaining the coating metal in the channel with electromagnetic means (15); placing on one side of the channel a ionising source (20) and on the other detectors (31, 32, 33, 34); measuring the intensity of the radiation flux received by the detectors; and deducing therefrom a value representing the amount of coating metal between the source and the detection means on the path of the radiation (R), and an indication of the level of the bottom meniscus (3).

Description

<U> Device and method for controlling continuous </ U> <U> coating by immersing a metal strip in </ U> <U> vertical rectilinear scroll. </ U>

The present invention relates to the continuous coating, by immersion in a bath of molten metal, of a metal strip, in particular a steel strip running vertically straight up and down. More specifically, the invention relates to the detection of the position of the meniscus of molten metal to be established in the lower opening of the immersion container between which between the tape to be coated.

The invention is particularly directed to the galvanization of sheet metal strips, typically of a thickness of the order of a few tenths of millimeters to a few millimeters. Conventionally, in a galvanizing operation, the deposited coating metal is zinc; still other metals or alloys may be suitable.

The current method of galvanizing from molten zinc is known as dip galvanizing. The strip to be coated is driven in scrolling and circulates from top to bottom then from bottom to top while passing in the low position around a roller of horizontal axis immersed in a bath of molten zinc. The strip on which the zinc is deposited spring galvanizing bath upwards, substantially vertically. Various means, known in themselves, can be used to ensure the solidification of the zinc and the regularity of the deposited layer (for example, gas blow-off nozzles).

Another technique, in the field of which the present invention is situated, and known in particular by WO 94/13850 and WO-93/18198, consists in passing the strip only from bottom to top through the zinc bath. The band then passes through a slot, formed in the bottom of the container, containing the zinc. To contain the molten zinc and prevent its flow by the space between the band and the edges of the slot, is used or sliding field inductors placed under the container, around a vertical channel opening in the bottom crucible, the walls of the channel being sealingly connected to the bottom of the crucible. This channel extends over a certain height, typically of the order of one meter, and its section is determined so as to provide a space of the order of 1 to 2 cm on each side of the strip, sufficient to prevent the band come rubbing against the walls of the channel. The set formed by the channel and the container which surmounts it is called crucible. It will be noted that the strip is conventionally guided by return rollers, arranged respectively one at the bottom, under the crucible, and the other at the top of the installation, at a distance of the order of 10 to 20 meters above the lower roller. The large distance separating the two holding rollers is necessary to allow the coating metal to solidify before the tape passes over the top roll. For the same reason, no guide roller can be used near the crucible, since it could strongly mark the still insufficiently solidified coating.

The band passes from bottom to top in this channel before crossing the container containing the molten zinc bath, a height of the order of 50 cm for example. Above the bath, spinning nozzles are arranged on each side of the strip, to regulate the thickness of the coating by blowing an air knife and simultaneously participate in its cooling and solidification.

The liquid zinc is maintained in suspension in the channel, substantially at mid-height thereof, on the one hand by the upward drive effect caused by the high-speed running of the strip, typically the an order of several m / s, and secondly, predominantly, by the electromagnetic forces developed in the molten conductive metal by the sliding field inductors, also called linear motors, placed on either side of the channel.

Relatively large electromagnetic forces are required to keep the liquid zinc levitated, counteracting its hydrostatic pressure. The magnetic field generating these forces is therefore a relatively low frequency field, greater than 30 Hz, typically of the order of 50 to 200 Hz.

Maintaining the molten zinc in the channel is of course a necessity. There should be no downward flow of this metal as this would be a detrimental leak to the process. The level of the lower meniscus, ie the lower surface of the liquid zinc, inside the said channel, must not be too high either, otherwise the passage time of the band in the zinc bath.

Neither should there be too much fluctuations in the level of the lower meniscus, nor irregularities in its shape, as this could lead to unevenness of the lining longitudinal irregularities if the level of the lower meniscus varies, transverse irregularities if the shape of the meniscus deviates substantially from a horizontal plane.

Insofar as the retention or lift effect results from the interaction of the variable magnetic fields with the electrically conductive coating metal, the effect therefore only occurs if there is liquid zinc in the channel at the level of the inductors, which guarantees the presence of this metal in the channel. In addition, the maintenance of the lower meniscus in the channel is automatically ensured, since the levitation forces are sufficient to prevent the liquid zinc from flowing. Inductors or linear motors thus provide their primary function which is to seal the bottom of the molten coating metal crucible.

However, there is currently no sufficiently reliable and accurate operational method for determining the position and shape of the lower meniscus, and using it as a process control parameter, as well as a safety signal in the event of a failure. possible failure to maintain the level within acceptable limits.

Given the relatively small section of the channel, the presence of molten metal, and the fact that the channel is also traversed by the high speed band, of the order of 100 m / min for example, the Determining the position and shape of the lower meniscus through the bottom of the channel is almost impossible with sufficient reliability and accuracy.

The present invention aims to solve the problems indicated above. It is particularly intended to allow rapid and reliable detection of the position of the lower meniscus, in particular to prevent possible leakage of coating metal through the channel.

It also aims to provide a method and an installation of the type defined above, for producing a metal coating on a moving strip, which make it possible to achieve the most regular coating possible.

With these objectives in view, the subject of the invention is a device for controlling the production of a metal coating on a moving strip, comprising a container intended to contain a liquid bath of coating metal, the bottom of the container comprising a slot for passage of the strip, extending downwardly through a passage channel of the strip, and electromagnetic sealing means placed outside said channel, being provided to contain the coating metal and maintain the lower meniscus said metal inside said channel, control device characterized in that it comprises, in the vicinity of a desired level of maintenance of the lower meniscus, at least one source of radioactive radiation and means for detecting said radiation , placed substantially opposite, on both sides of said channel.

The invention is based on the use of the principle of absorption of ionizing radiation by the metal. In the present case, since the walls of the channel are relatively thin and where the radiation source and the detecting means can be placed fairly close to the inner surface of said walls, it is the coating metal which is the main absorber of radiation. As soon as said coating metal is in the path of the radiation between the source and the detection means, there is a weakening of the radiation flux received by the detection means, with respect to the radiation flux received in the absence of the coating metal. In a simplified manner, it can be said that a decrease in flux captured by the detection means signals a lowering of the level of the lower meniscus.

It will be noted that the radiation necessarily passes through the strip to be coated, although this does not significantly disturb the measurement because of the small thickness of the strip, of the order of a few tenths of a millimeter, whose absorbency is therefore low by compared to that of the coating metal, which is likely to occupy all the space of the channel, and whose thickness is typically close to 3 cm, or about fifty to one hundred times that of the strip. In addition, the material of the coating baths, such as for example zinc or tin, is particularly more absorbent for the radiation used than the steel constituting the strip.

In a manner known per se, and as indicated in the introductory part of this memo, the electromagnetic sealing means used to maintain the bath of liquid metal in levitation are inductors, arranged as linear motors, formed of coiled son. In order to be able to place the source sufficiently close to the channel, the device preferably comprises a tubular passage arranged between the wires of the windings in order to move the radioactive source there and bring it to the direct vicinity of the channel wall during the use of the system, and remove it outside these periods in a protective container located outside of said linear motors. The establishment of the source, and to a lesser extent that of the detection means, faces clutter problems because they must necessarily be placed at the height of the linear motors, and therefore practically through these engines, passing through the relatively restricted spaces between the electrical conductors forming the coils. This is why a gamma radiation source is preferentially used rather than a more bulky X-ray tube. In the case where one wishes to make measurements over a large extent, for example over a large part of the width of the band, or the height of the channel, and since an extended source would be difficult to implant, it will preferentially use several sources put in place individually as indicated above.

The detection means may consist of sensors also placed in the direct vicinity of the channel, and connected by bores made in the linear lift motor to a housing outside said linear motor and containing the measurement electronics associated with the sensors. The sensors may be scintillators associated with photomultipliers.

According to a preferred arrangement, the detection means comprise at least one detector offset vertically with respect to the source, and located substantially in the vertical plane orthogonal to the band and passing through the source. The path of the radiation between the source and the detector is then oblique with respect to the running direction of the strip. If, to simplify the explanation, we assimilate the lower meniscus to a horizontal plane, and assuming that the source is placed lower than the detector, it will be understood that, depending on the position of the meniscus between the level of the source and the level of the detector, the absorption of radiation by the metal of the bath will vary between a minimum when the meniscus is at the detector, the radiation then passing through the gaseous atmosphere of the channel, and a maximum when the meniscus is at the level of the source, since then all the radiation passes into the metal of the bath, as will be better understood later in the description of the example.

According to a complementary arrangement, several detectors are distributed substantially vertically and are associated with the same source. the detectors will preferably be placed at the source and above this level. The detector or sensors located above the source will operate as indicated above, and will evaluate the position of the lower meniscus, between the limits formed by the position of the sensor and that of the source. The detector placed at the source, so almost right in front of it, on the other side of the channel, is the closest to the source and therefore the most sensitive since receiving a maximum flow. By cons, remaining in the above approximation of a meniscus plane, we see that the detector placed at the source operates virtually all or nothing to report the level of the meniscus. either the meniscus is higher than the source and the radiation perceived by the detector is then maximum, the meniscus is below the source and then the absorption is maximum, the flow variation measured between the two positions being abrupt.

In all the cases of use of several detectors, calculation means will also be used to determine, as a function of the position of each detector and of the intensity of the radiation flux received by each detector, a value representative of the quantity of the detector. metal coating in the gap between the source and each detector, and deduce the position and shape of the lower meniscus.

A more accurate assessment of the meniscus position can only be made in the case of a single source-level sensor by using several sources placed at different heights themselves, to situate the meniscus relative to the levels of the meniscus. these different sources. It will be understood that the latter solution is more difficult to implement since it requires the manipulation of several radioactive sources.

In the preferred case of the use of several detectors associated with the same source, the detector situated at the source therefore serves essentially as a low level alarm of the meniscus, whereas the other detector (s) situated higher will be used for more precise measurement of the meniscus level and the regulation of the process.

Note also that it would be possible, similarly, to place at least one detector at a lower level than the source, and to use this detector for both a continuous position measurement as indicated above. and as a low level alert. However, in this case, the low level alert would be by detection of a maximum of absorbed flow, ie a minimum of flow perceived by the detector, and this in the context of a gradual variation of the flow. Such a detection would be less frank, and therefore less reliable than the previously described all-or-nothing detection, which also has the advantage of being performed at maximum captured flux intensity, whereas a detector placed further from the source necessarily a smaller flow.

According to yet another arrangement, at the same source are associated several detectors distributed substantially horizontally. This arrangement makes it possible to determine the level of the lower meniscus at different points in the horizontal direction transverse to the band. In combination with the preceding characteristic relating to the use of several vertically distributed detectors, a kind of detector matrix is obtained, for example four horizontal rows of five detectors each, from which it is possible not only to evaluate the altitude position. meniscus but also its shape, and variations in altitude and shape over time, which makes it possible to act in regulation in particular on the intensity of the lift forces generated by the linear motors, but also possibly on others. process parameters that may affect the shape of the lower meniscus, such as web speed, bath temperature, curved or strip tile, etc.

The invention also relates to a method for controlling the production of a metal coating on a moving ferromagnetic strip, in which the strip is passed substantially vertically in a bath of the coating metal contained in a crucible, making it passing through a slot in the bottom of said crucible, said slot extending downwardly through a passage channel of the strip, and the coating metal is contained in said channel by electromagnetic sealing means placed in the channel; outside said channel, the method being characterized in that a radioactive source is placed on one side of the channel and on the other side means for detecting radiation emitted by the source, the intensity of the flow of the channel is measured. radiation received by the detection means, a value representative of the amount of coating metal between the source and the detection means on the tr additive radiation, and an indication on the presence of said metal in the channel at the height of the source and means of detection.

Preferably, the detection means comprise at least one detector offset vertically with respect to the source, and the height position of the lower meniscus is determined from the measurement of the intensity of the radiation flux received by said detector, the intensity of this flux being representative of a thickness of the coating metal traversed by the radiation between the source and said detector, the ratio between said thickness and the distance between the source and said detector being representative of the level of the lower meniscus.

Preferably, the intensity of the supply current of the electromagnetic sealing means is regulated as a function of the results of the measurements provided by the detection means.

Other features and advantages will appear in the description which will be made of a steel strip galvanizing installation according to the invention.

FIG. 1 is a partial schematic perspective view of such an installation; FIG. 2 is a plan view of the installation; FIG. 3 is a sectional view according to FIG. Fig. 4 is a sectional view along line IV-IV of Fig. 2; Fig. 5 is an enlarged detail view of the adjacent area of the lower meniscus.

The continuous galvanizing installation illustrated in the figures comprises a container 10 containing a molten zinc bath 1, traversed from bottom to top by the steel strip 2 to be galvanized. The strip 2 is driven in a manner known per se in the direction of the arrow F, and is guided by a lower return roller 11 and an upper return roller 12. It is held in slight tension between these rollers. The container 1 is located between the two rollers 11, 12, closer to the lower roll, so that a sufficient distance exists between the zinc bath 1 and the upper roll 12, so that the zinc is solidified before the band passes on the upper roll.

A slot 13 for passage of the strip is formed in the bottom of the container which is extended downwardly by a channel 14, of section corresponding to the slot of the bottom of the container. Electromagnetic sealing means such as inductors 15 are placed on either side of the channel to generate on the molten zinc which tends to flow in said channel electromagnetic forces directed upwards, opposing this flow. These inductors, shown diagrammatically in the drawings in which some of the wound conductors 151 of which they have been formed, are symbolically represented are typically linear motors powered by a fairly low frequency current, for example from 150 to 200 Hz, producing a field sliding up.

It is pointed out that in the drawings the actual relationship between the different dimensions has not been deliberately ignored, for the sake of clarity, to better illustrate the problems which the invention remedies and the solutions it provides. It is recalled that in practice.

the thickness of the strip 2 is of the order of a few tenths of a millimeter, and its width of the order of several tens of centimeters, or the meter, the width of the slot 13 and the passage channel 14 of the band is of the order of a few centimeters, typically about 3 cm, the height of the channel 14 is of the order of one meter and the height of the coating metal 1 in the container 10 of about 50 cm, whereas the distance between the lower rollers 11 and upper guide 12 of the strip is of the order of 10 to 20 m, all these dimensions being given by way of example.

Above the container 10 are arranged spinning nozzles 16 in the form of ramps extending transversely on either side of the path of the strip, to blow towards the strip an air jet ensuring the cooling zinc deposited and participating in the regularity of its thickness.

According to the invention, a radioactive source 20, for example cobalt <B> 60 </ B> Co, having a low activity, 200 MB for example, is placed in service at the bottom of a hole 152 drilled in one of the linear motors, in the direct vicinity of the channel wall 141 14. Inoperative, the source is removed, by means of a rod 21 sliding in the hole 152, in a safety container 22 placed outside the linear motor 15 .

Detectors 31, 32, 33, etc. are placed on the other side of the channel 14, against the wall thereof. In the example shown, these detectors are arranged in the form of a matrix of three horizontal lines of five detectors each. In the horizontal direction, the detectors 31, 32 are best distributed in the transverse direction of the strip. Although for the sake of clarity the figure shows the detectors distributed over the entire width of the channel, in practice, the spacing between two detectors will be of the order of 20 to 50 mm. beyond which the radiation from the single source would be too offset from the detector to obtain a usable measurement. If we therefore want to make effective measurements over the entire width of a band several decimetres wide, we will use several sources spread over the width. However, it will be understood from FIG. 4 that several horizontally distributed detectors make it possible to take measurements of absorption of the radiation in various directions in a horizontal plane, and thus make it possible to obtain an indication of the position of the lower meniscus 3 at various points. across the width of the channel.

The detector 31 in the middle of the lower line is located directly opposite the source 20. Two other detectors 33 and 34 are arranged above the central detector. The other detectors are similarly arranged above each detector in the lower line.

All the detectors are connected, by optical fibers, to housings 40 situated outside the linear motors 15, containing the photomultipliers and the electronic circuits associated with the detectors. The calculation means necessary for determining the level of the lower meniscus 3 and evaluating its shape from the measurement results of each detector are deported, for example at a general driving computer (not shown).

In the drawing of FIG. 5 it is illustrated how a detector, for example the detector 33, offset with respect to the source 20, makes it possible to evaluate the position of the lower meniscus 3 as a function of the measured intensity of the flow reaching the detector.

It can be seen that the radiation R coming from the source 20 and joining the detector 33 is oblique across the channel 14.

When the meniscus is in the position marked by the pattern 3, the radiation R passes over a major part of its trajectory through the molten zinc 1, which therefore absorbs a large part of the radiation. The detector 33 then provides a relatively weak signal corresponding to the radiation not absorbed by the zinc.

When the meniscus is higher, in the position represented by the dotted line 3 ', most of the path of the radiation R is in the air and only a small part is absorbed by the molten zinc. The signal provided by the detector 33 is then much larger. Note that there is not a proportionality between the amount of radiation absorbed and the amount of zinc passed through the radiation.

Moreover, as long as the meniscus is above the level of the source 20 and the detector 31, the latter captures a maximum radiation flux coming directly from the source. As soon as the meniscus passes below this level, the absorption by the zinc is maximum, the abrupt transition allowing a rapid detection of the passage, particularly well adapted to generate for example a low alarm signal, signaling an imminent risk of leakage .

Tests carried out by the inventors have shown that the response time in such a situation with the source of low <U> activity </ U> mentioned above could decrease to less than 0.1 seconds, whereas for a measurement finer the level according to the method explained above, the response time was typically 0.1 to 1 second.

Claims (5)

A device for controlling the production of a metallic coating on a moving strip (2), in which a container (10) for containing a liquid coating metal bath (1) has a bottom having a slot ( 13) of the strip, said slot extending downwardly by a channel (14) for passage of the strip, and electromagnetic sealing means (15) placed outside said channel being provided to contain the coating metal and maintaining the lower meniscus (3) of said metal within said channel, characterized in that it comprises, in the vicinity of a desired level of maintenance of the lower meniscus, at least one source (20) ) of ionizing radiation and means (31, 32, 33) for detecting said radiation, placed substantially opposite, on either side of said channel. 2. Device according to claim 1, characterized in that the detection means comprise at least one detector (33, 34) offset vertically with respect to the source. 3. Device according to claim 2, characterized in that a same source (20) are associated several detectors (31, 33, 34) distributed substantially vertically. 4. Device according to claim 2 or 3, characterized in that a same source (20) are associated several detectors (31, 32) distributed substantially horizontally. 5. Device according to claim 1, characterized in that the electromagnetic sealing means (15) are inductors formed of son (151) wound and that it comprises a tubular passage (152) arranged between the winding son to move the radioactive source (20) and bring it to the direct vicinity of the wall (141) of the channel (14). 5. Device according to claim 1, characterized in that the radiation source (20) is a source of gamma radiation. 7. Device according to claim 3 or 4, characterized in that it comprises calculation means (40) for determining, as a function of the position of each detector (31, 32, 33, 34) and the intensity of the radiation flux received by each detector, a value representative of the amount of coating metal (1) in the gap between the source and each detector, and deduce the position and shape of the lower meniscus (3). 8. A method of controlling the production of a metallic coating on a moving ferromagnetic strip, in which the strip (2) is passed substantially vertically in a bath of the coating metal (1) contained in a container (10), passing it through a slot (13) formed in the bottom of said container, said slot extending downwardly through a channel (14) for passage of the strip, and the coating metal is contained in said channel by electromagnetic sealing means (15) placed outside said channel, characterized in that a radioactive source (20) is placed on one side of the channel and on the other side means (31, 32 , 33, 34) for detecting the radiation emitted by the source, the intensity of the radiation flux (R) received by the detection means is measured, a value representative of the amount of coating metal between the source is deduced therefrom. and the means of d detection in the path of radiation, and an indication of the presence of said metal in the channel up to the source and detection means. 9. Method according to claim 8, characterized in that the detection means comprise at least one detector (33) offset vertically relative to the source (20), and the height position of the lower meniscus (3) is determined from measuring the intensity of the radiation flux (R) received by said detector, the intensity of this flux being representative of a thickness of the coating metal traversed by the radiation between the source (20) and said detector (33), the ratio between said thickness and the distance between the source and said detector being representative of the level of the lower meniscus. 10. The method of claim 8, characterized in that regulates the intensity of the supply current of the electromagnetic sealing means (15) according to the results of the measurements provided by the detection means.