ES2242008T3 - PROCEDURE AND DEVICE FOR THE COATING OF A METAL BAND BY DIP. - Google Patents

Abstract

Procedure for coating a metal band (1) by immersion by means of a liquid metal (4), in which: - said band is circulated through a narrow channel (10, 12, 15, 33) in which the liquid metal is injected at least on both sides of the band; - a movement of circulation of said liquid metal is created inside the narrow channel; characterized in that a difference in level of the pressures (13, 14; 16, 17; 19, 20) that are exerted respectively on each of the two sides of the band is also created, to control the position of the band in the channel narrow.

Description

Procedure and device for coating of a metal band by immersion.

Technical field

The present invention relates to a procedure for coating a metal band by immersion, that is by continuous passage through a liquid metal. The invention also relates to a device for Performing this procedure.

State of the art

The invention will be described with reference to Dip galvanization of a steel band. But nevertheless, It is necessary to consider that this application is only mentioned in title example, and that the invention can advantageously be applied also to other types of metal substrates in the form of a band or wire, as well as with other metals and alloy alloys.

The document JP-A-2 8356 describes a procedure and a device for coating a band of steel by immersion by means of a liquid metal. Is posing the band through a narrow channel traveled by the liquid metal to both sides of the band. The channel is fed by a tank separated comprising the liquid metal, coupled to a pump that creates a movement of circulation in closed circuit. The deposit It has a capacity greater than that channel. It is liquid metal is filtered by a purification device before introducing it in that channel.

Immersion galvanization of a band of steel generally comprises two main operations, namely annealing of the band and the coating deposit. The corresponding installation then comprises two large parts, to Know an annealing oven and a galvanizing tank.

The annealing is intended to prepare the web, in particular its surface, with a view to the coating operation that follows. In particular, it ensures the recrystallization of the hardened structure in the case of a cold rolled strip; it also performs, in all cases, a cleaning and fanning of the surface by means of an appropriate, generally reducing atmosphere, comprising hydrogen and nitro-
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Schematically, conventional galvanization it consists of passing a steel band through a bath of molten zinc and then, at the exit of this bath, regulate the zinc layer thickness by draining by means of sheets of air or nitrogen under pressure.

In the course of its journey in the zinc bath, the band is guided by several rollers, in particular a first roller, called bottom roller, completely submerged in the zinc, under which it passes before tracing vertically towards the bath surface, and other rollers, called rollers of stabilization, deflection or guidance, often partially or even fully submerged in the proximity of the surface upper bath of liquid metal, intended to ensure the stabilization, eventually the flatness of the coated band, before drained.

These rollers, motorized or free, enter contact with the surface of the moving band and can thus generate surface defects. They must therefore be frequently disassembled for inspection, repair and even replaced, since their status greatly influences Zinc coating quality. They will result from it then frequent stoppages of the production line and a loss of non-negligible productivity. Moreover, the expert in the matter knows well the difficulty of regulating and maintaining the coating metal composition. This one is practically not never a pure metal, but rather an alloy that contains different elements in certain but variable quantities of one type to the other, aimed at improving the characteristics of coatings, such as aluminum additions in galvanization baths.

Finally, you kill them, that is some aggregates of particles of intermetallic compounds, formed among others by the dissolution of the surface end of the steel bands by the liquid metal bath, also cause good appearance defects known, and must therefore be regularly eliminated. Result of it some losses of time and of metal, that harm the productivity and profitability of procedures conventional.

It has already been proposed to replace the bottom roller by a fixed semicircular tube, also submerged in the bath of liquid metal, and provided with grooves through which injects liquid metal coating. Thus an area of pressurized metal that keeps the band at a distance from the surface of the submerged tube, without this distance can however be regulated and controlled precisely. In practice too, This solution does not prevent the stability problems of the band, nor surface defects caused by contact with guide rollers, nor the difficulty of regulating the composition of the liquid metal, nor finally the problems linked to You kill them to eliminate.

Presentation of the invention

The invention aims to propose a procedure for coating a metal band by immersion, which allows avoiding the aforementioned inconveniences, in particular suppressing all submerged rollers and ensuring a constant composition of the liquid metal in the area of coating. According to another aspect, the invention proposes a device for performing this procedure.

According to the invention, a procedure for coating a metal strip by immersion of a liquid metal, in which said band is passed through a volume of said liquid metal, is characterized in that it is made circulating said band through a narrow channel, because the liquid metal in this narrow channel at least on both sides of said band, and because a movement of said movement is created liquid metal with respect to said band, inside said narrow channel It is then verified that the contact coating of the metal band surface with a flow of liquid metal rather than a coating of said surface by immersion in a liquid metal bath.

According to the invention, said channel is fed narrow with liquid metal from a separate source, constituted for example by a separate deposit; said deposit separated is advantageously arranged higher than said channel narrow, so as to ensure food at least partially due to a gravimetric effect. A part or all of the power supply, however, can also be ensured by One or several bombs.

According to an interesting feature, it is submitted the liquid metal at least one treatment before introduce it into said narrow channel. In particular said treatment may consist of filtering the liquid metal before Enter it in the narrow channel. This reduces the risk of deposit floating particles, such as compounds intermetallic, on the surface of the band.

According to another characteristic, it is measured at intervals determined, preferably continuously, the chemical composition and / or the temperature of the liquid metal and, if necessary, adjust this chemical composition and / or this temperature before introducing the liquid metal into said narrow channel. Result possible so keep at least noticeably constant these conditions governing the immersion coating operation of the metal band.

According to another advantageous feature, it is performed the feeding of said narrow channel in closed circuit. Be thus avoid any contact of the liquid metal with the air environment, and therefore any risk of metal alteration liquid, in particular by oxidation.

According to one characteristic particularly interestingly, said narrow channel is fed with liquid metal to an independently adjustable pressure on each face of the band. According to a well known procedure, the pressure of liquid metal feed, as the case may be, by varying the level difference between the narrow channel and the separate tank, or by modifying the pressure provided by the pumps feeding.

According to yet another feature particularly advantageous, the flow rate of the metal stream liquid with respect to the band, inside said channel Narrow, it is between 0.1 m / s and 5 m / s.

It has been advantageous to apply a pressure identical to both sides of the band in the narrow channel. Without however you can imagine applying different pressures to both sides of the band, in particular in order to influence the surface reactivity of said metal band.

A device for the realization of this procedure essentially comprises a narrow channel, about means for circulating the web to be coated through said narrow channel, as well as the means to feed said channel narrow with liquid metal and to create a movement of circulation of the liquid metal with respect to the band, in the inside the narrow channel.

The narrow channel section according to the invention is determined in a way that allows the passage of the widest band which must be coated in it, without needing a flow Excessive liquid metal.

Taking into account the width of the bands current, the narrow channel width according to the invention is generally between 1 m and 2 m. It is evident that this width could be increased or decreased depending on particular circumstances

The height of the narrow channel defines the thickness of the liquid metal layer in circulation, and therefore influences also on the flow of liquid metal in the narrow channel. This height is preferably less than 6 cm, and preferably still less than 3 cm, so that you can limit the flow of metal liquid in the narrow channel.

The narrow channel according to the invention can present a flat or curved shape, in particular partially cylindrical In the latter case, the curvature must be limited so as not to unnecessarily complicate the trajectory and the Guided band.

The band to be coated circulates through said channel under the action of the traction exerted by the systems of drag of the coating line. His career is determined by the guiding means, generally rollers, arranged outside the channel, upstream and current below it, so that they do not damage the surface of the band during the coating operation.

Inside the narrow channel, the band must since then be kept a minimum distance from the walls of the channel, in order to avoid any risk of contact with these. As a rule, this distance is between 1 mm and 15 mm, depending on various parameters such as speed and the width of the band, the nature of the liquid metal, the length and / or shape of the narrow channel path.

The narrow channel according to the invention comprises on the one hand liquid metal feeding means, and by other means means to create a movement of movement of the liquid metal with respect to the band, inside the channel narrow.

Said feeding means include less a separate deposit, with a capacity clearly greater than the narrow channel; this tank is connected to the narrow channel by a power circuit that leads to a means of introduction of liquid metal into the narrow channel, on both sides of the band that circulates through it. These means of introduction they may consist of injectors arranged in at least one end of the channel, or for some injectors arranged in the channel walls parallel to the plane of the band, or also for a combination of these two types of injectors.

To ensure the circulation of liquid metal, even in the narrow channel, the separate tank can be raised above the narrow channel, or the circuit of Feeding can comprise at least one pump.

Also, advantageously, the device It also includes means for controlling and regulating the temperature and / or composition of the liquid metal, means to add fresh liquid metal as well as means to filter the liquid before its introduction into the narrow channel.

In addition, the channel is provided with means of tightness, such as locks at their ends, which allow the band to pass but prevent metal leaks liquid.

Finally, the channel can advantageously be integrated in a closed circuit, which also includes minus a feed tank, eventually raised, a narrow channel power circuit, a circuit of recovery of excess liquid metal, means of liquid metal treatment, and eventually at least one Liquid metal circulation pump.

The method and device of the invention they allow coating a band by immersion in a metal stream animated liquid with high speeds with respect to the band. These speeds are higher than those resulting from the passage of band for a resting liquid metal bath.

With respect to the prior art, the Procedure of the invention offers numerous advantages, which will be explained later, during the description of the plans annexes

Brief description of the drawings

Examples of embodiment of the device the invention are schematically represented in the drawings annexes, in which

Fig. 1 recalls the principle of a conventional immersion coating installation of the prior art;

Fig. 2 shows the beginning of the procedure of the present invention;

Fig. 3a illustrates a first application case of the process of the invention, with a narrow rectilinear channel practically horizontal;

Fig. 3b illustrates a second application case of the process of the invention, with a narrow curved channel in parable shape;

Fig. 3c illustrates a third application case method of the invention, with a narrow channel that approximates the usual trajectory of the band in a metal bath liquid;

Fig. 4 shows different arrangements of the liquid metal feed holes, practiced in the narrow canal walls (Fig. 4a, b, c), and

Fig. 5 illustrates different configurations possible narrow channel with its power circuit liquid metal (Figs. 5a, b, c, d).

These figures constitute only some simplified schematic representations, in which only they have reproduced the necessary elements for the compression of the invention. The direction of the liquid metal circulation is indicated by some arrows. Identical elements are designated by the same numerical references in all figures.

Embodiments of the invention

Figure 1 represents a device conventional galvanization by immersion of a steel band, integrated into a treatment line. In this device, a belt to be coated 1 is treated in an annealing oven continuous 2, from which it comes out of a tube 3 to dive into a melting zinc bath 4. Band 1 passes under a bottom roller 5, completely submerged in liquid zinc 4, and guided to then by stabilization rollers 6 submerged in the proximity of the upper surface of the zinc bath 4, before leave the zinc bath following a vertical path. The excess of zinc is drained by means of gas sheets, usually of air or nitrogen, under pressure 7. Band 1 then rises vertically to an upper deflection roller 8, which is located high enough for the zinc coating to solidify before reaching it. Band 1 continues next Your way through a treatment line.

In this classic device, the band that should coating circulates through a liquid zinc bath at rest 4, and is in direct contact with several submerged rollers 5, 6.

The principle of the process of the invention is illustrated in Figure 2. In this case, the strip to be coated 1, which comes from a continuous annealing furnace not shown, circulates through a narrow channel 10 in which a liquid metal stream 9. In this figure, channel 10 is rectilinear and slightly tilted up, and circulation of the liquid metal is secured by a pump 11. In this illustration, band 1 and liquid metal stream 9 circulate upward, indicated by arrows. A few systems of tightness, not shown, are provided at the ends of input and output of band 1 on channel 10.

Other possible configurations are illustrated and described in figure 3. This figure 3 shows how it is possible control, at least in part, the position of the band in a narrow channel, according to the process of the invention.

As indicated above, a liquid metal circulation movement, for example zinc in fusion, on channel 10, on both sides of band 1. This current circulates preferably at high speed, and is accompanied inevitably of pressure gradients between band 1 and 10 channel walls. Hydraulic paths that liquid metal still on channel 10 can be devised so that they create some pressure differences between the faces of the band and that influence thus in the position of the band in the channel. Indeed, even if they are of small amplitude, these pressure differences they act on important surfaces, namely the surface of both sides of the band in the channel, and can thus generate some considerable forces.

As an example, a pressure difference between the two sides of the band can be obtained from several ways:

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creating ones different liquid metal flow rates on both sides, while maintaining circulation speeds enough;

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modifying the distance between the band and the channel wall: at equal flow metal, a decrease in this distance causes an increase in the pressure; Y

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 differentiating the liquid metal injection between the two sides of the band, in which concerns both the injection pressure and the number and / or the arrangement of the injectors.

Figure 3a illustrates the simple case of a channel narrow 12 rectilinear, practically horizontal, in which the Band 1 circulates equally between the two walls. The pressures 13 exerted on the underside of the band are higher than the 14 applied on the upper face, in order to support the weight of band 1 while keeping it in a centered position on channel 12.

In Figure 3b, channel 15 has the form of a parable, corresponding to the form called "in chain" which naturally takes the band 1 suspended freely between two upper rollers not shown.

A parabolic trajectory of this type of band 1 can be maintained by permanent control of the band speed upstream and downstream of the channel 15. It is also maintained thanks to the pressures that the metal liquid exerts on the band; if pressures 16 on the face internal are higher than the 17 exercised on the face external, they are even able to compensate for traction in the band. In said channel configuration, it is particularly easy to ensure the tightness of the channel, collecting the metal liquid that overflows at the two ends of the channel to enter it again at the bottom, for example with the help of pumps 18.

Finally, Figure 3c shows a channel 15 of a form appreciated to the usual path of a band 1 in a dip coating installation of the type illustrated in figure 1. In this configuration, band 1, which suffers the traction exerted by the treatment line, is supported by the application of higher pressures 19 on the inner face than the 20 exercised on the outer face. In addition, band 1 can be guided in a controlled way to both the entrance and the exit of the channel, by differential pressures 21, which replace advantageously, that is to say without contact, the guide rollers 6 of Figure 1

Also in this case, the tightness of the channel it is easily secured by collecting excess liquid metal at the two ends of the channel to reintroduce it in the part lower, for example with the help of a common pump 22.

The feeding of the narrow channel with liquid metal is preferably ensured by the holes made in the walls of the channel, so that the pressures acting on the faces of the surface are distributed as uniformly as possible.
band.

Figure 4 shows various arrangements possible of these feed holes in a portion of a channel wall 23. In the examples shown, the holes they can be either grooves 24 parallel and / or perpendicular to the direction 25 of movement of the band (fig. 4a), or about holes, for example circular 26, arranged along lines also parallel to the direction 25 of band travel (fig. 4b), or also holes, for example circular 27,  distributed regularly, for example to the tresbolillo, on the wall of the channel (fig. 4c).

These provisions are given as simple examples and do not exclude in any way any other provision of holes that allow to obtain a uniform distribution of the channel pressures

The possibility has already been mentioned above, in the process of the invention, of performing different operations in the circulating liquid metal, in particular of filter the particles of intermetallic compounds and control and / or to adjust the temperature and / or chemical composition of the metal liquid, before introducing it into the lining channel.

On the other hand, the interest in providing an external deposit, that is, different from the channel narrow proper but that communicates with this one, and put the liquid metal in circulation between the narrow channel - small capacity- and the outer tank of greater capacity.

Figure 5 illustrates different configurations possible of the liquid metal circuit, with a unit of treatment, possibly combined with a deposit raised.

In the arrangement of Figure 5a, a channel horizontal rectilinear 12, traveled by a band 1, is fed live by means of a pump 28. The liquid metal is recovered at the ends of channel 12 and sent again by pump 28, in beginning to a treatment unit 29 and then directly to channel 12.

Figure 5b also represents a channel horizontal rectilinear 12 traversed by a band to be coated 1. This channel 12 is gravity fed from a reservoir raised 30 of large capacity, while the liquid metal in excess is recovered at the ends of channel 12 and sent from new, always by gravity, to a collection tank 32. From of this, the liquid metal is then sent back to the tank raised 30 by means of a pump 31. A treatment unit 29 is disposed between the tank 30 and the channel 12 and / or between the pump 31 and reservoir 30.

The configuration of Figure 5c is analogous to the of Figure 5a, with the difference that channel 33 fits into this case with the path of band 1 on a line of conventional immersion coating. Excess liquid metal is recovered at the raised ends of channel 33 and sent back to the low point of this channel, through a pump 28 and of a treatment unit 29.

Finally, Figure 5d illustrates an arrangement, analogous to that of figure 5b, but in which channel 33 fits with the path of the band 1 in a coating line by conventional immersion Channel 33 is fed directly into its low by gravity, from an elevated tank 30 of great capacity; excess liquid metal is recovered in the raised ends of channel 33 and sent again, also by gravity, to a collection tank 32. From this, the metal liquid is forwarded to the raised tank 30 by means of a pump 31. A treatment unit 29 is provided between the reservoir 30 and channel 33, and / or between pump 31 and reservoir 30

With respect to the previous immersion technique of a moving band in a liquid metal bath, of great volume, at rest, the present invention offers numerous advantages as regards both the actual procedure and The quality of the coated products.

In the matter of procedure, an important advantage is the great flexibility conferred on the treatment line in terms of the nature of the deposited metal. It is indeed enough to change the feed tank to move from a liner to
other.

Moreover, the continuous circulation of the metal liquid between the lining channel and a separate reservoir allows to treat the liquid metal continuously. It can be controlled in effect, and if necessary adjust, both the temperature and the Chemical composition of liquid metal.

It is also possible to eliminate the bushes, that is the particles of intermetallic compounds that float on the metal liquid and that run the risk of depositing on the band. This removal can be done by any appropriate procedure,  such as filtering, decanting, centrifugation, etc., in particular in the treatment unit mentioned above.

Finally, it is worth emphasizing the ease of maintenance of the device of the invention. Having missing submerged rollers, maintenance refers essentially to the pumps, which can otherwise be duplicated and then kept in alternation without the line of coating must be stopped.

The quality of the coating depends on particular of the reactivity between the coating metal and the surface of the band to be coated. As an example, steels heavy duty intended to be galvanized for industry automobile, have a weakly reactive surface with with respect to zinc; This results in a lack of wettability that It can lead to serious coating defects.

In this regard, it has been found that the high relative velocity between the band and the liquid metal in the lining channel caused an improvement in reactivity between the liquid metal and the band, and contributed to suppress the defects of coating mentioned.

Also, the establishment of a speed significant relative, for example from 0.5 m / s to 1 m / s, between the band and liquid metal in the channel, helps to reduce the risk of sticking on the band, of suspended particles in the Liquid metal. This effect is added to that of the operation of removal of the bushes mentioned above.

Finally, the process of the invention resets the symmetry between the two sides of the band, due to that neither side of the band is now in contact with rollers. The actual duration and conditions of exposure of both sides of the band to liquid metal are identical.

The device of the invention is clearly more simple than conventional galvanizing devices by immersion. You need less liquid metal, it offers a greater flexibility of operation and allows to obtain products Coated in better quality. Finally, it can be integrated without great difficulty, in an existing treatment line, as shown Figure 3c

Claims (22)

1. Procedure for coating a metal band (1) by immersion by means of a liquid metal (4), in which:

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be circulates said band through a narrow channel (10, 12, 15, 33) in that the liquid metal is injected at least on both sides of the band;

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be creates a movement of said liquid metal in the inside the narrow channel;

characterized in that a difference in level of the pressures (13, 14; 16, 17; 19, 20) that are exerted respectively on each of the two sides of the band is also created, to control the position of the band in the channel narrow. 2. Method according to claim 1, characterized in that said difference in the pressures exerted on each of the two faces of the band is obtained by creating different flows of liquid metal on the two faces. 3. Method according to claim 1, characterized in that said difference in the pressures exerted on each of the two faces of the band is obtained by modifying the distance between the band and the wall of the narrow channel, causing a decrease in this distance by pressure increase at equal flow. Method according to claim 1, characterized in that said difference in the pressures exerted on each of the two faces of the band is obtained by differentiating the injection of liquid metal at the level of the two faces of the band. 5. Method according to any of the preceding claims, characterized in that the narrow channel (10, 12, 15, 33) and therefore the path of the band have a curved shape, preferably a parabolic shape, in "chain" or partially cylindrical. Method according to any of the preceding claims, characterized in that said narrow channel is fed by gravity from a separate liquid metal source (30). 7. Method according to any of the preceding claims, characterized in that the liquid metal is filtered before introducing it into the narrow channel. Method according to any of the preceding claims, characterized in that the temperature of the liquid metal is preferably measured continuously and, if necessary, adjusted before being introduced into the narrow channel. 9. Method according to any of the preceding claims, characterized in that the chemical composition of the liquid metal is preferably measured continuously and, if necessary, before being introduced into the narrow channel. Method according to any one of claims 6 to 9, characterized in that the narrow channel is fed in a closed circuit. Method according to any one of the preceding claims, characterized in that the respective pressures on each of the two sides of the band inside the narrow channel are independently regulated from each other. 12. Method according to any of claims 1 to 11, characterized in that the flow rate of the liquid metal stream with respect to the band inside the narrow channel is between 0.5 and 1 m / s. Method according to any one of claims 1 to 12, characterized in that the band to be coated circulates through the narrow channel under the action of the traction exerted by dragging systems of the coating line, the path of the band being determined by guiding means, preferably rollers, arranged outside the narrow channel, upstream and downstream thereof. 14. Method according to any one of claims 1 to 13, characterized in that, within the narrow channel, the band is maintained at a minimum distance from the walls of the channel, between 1 and 15 mm, depending on parameters such as speed of the width of the band, the nature of the liquid metal, the length and / or the shape of the narrow channel. 15. Device for coating a metal strip by immersion according to the method of any of claims 1 to 14, characterized in that it comprises a narrow channel (10, 12, 15, 33), means for circulating the band to be coated through the narrow channel as well as means, preferably comprising at least one pump (11, 18, 22, 28, 31), to feed said narrow channel with liquid metal and to create a movement of liquid metal circulation with with respect to the inside of the narrow channel. 16. Device according to claim 15, characterized in that said feeding means comprise at least one separate tank (30), with a capacity greater than the narrow channel and that said separate tank is connected to the narrow channel by a feeding circuit that conducts to means (24, 26, 27) for introducing the liquid metal into said narrow channel, on both sides of the band (1) that circulates therein. 17. Device according to claim 16, characterized in that said separate tank (30) is raised with respect to the narrow channel, in order to create a circulation of the liquid metal by gravity on one side between the separate tank and the narrow channel, and on the other part, inside the narrow channel. 18. Device according to any of claims 15 to 17, characterized in that they comprise means (29) for filtering the liquid metal before its introduction into said narrow channel. 19. Device according to any of claims 15 to 18, characterized in that it comprises means (29) for measuring and, if necessary, adjusting the temperature of the liquid metal before its introduction into said narrow channel. 20. Device according to any of claims 15 to 19, characterized in that it comprises means (29) for measuring and, if necessary, adjusting the chemical composition of the liquid metal before its introduction into said narrow channel. 21. Device according to any of claims 15 to 20, characterized in that the narrow channel is integrated in a closed circuit, which also comprises at least one power tank, possibly raised, a narrow channel power circuit, a recovery circuit of excess liquid metal, liquid metal treatment means and possibly at least one metal circulation pump
liquid. 22. Device according to any of claims 15 to 21, characterized in that the height of the narrow channel, which defines the thickness of the circulating liquid metal layer, is less than 6 cm, and preferably less than 3 cm, so as to limit the flow of liquid metal in the narrow channel.