EP0122856A1 - Method and apparatus for the continuous coating of a strip with an oxidizable coating - Google Patents

Abstract

Method and installation for the continuous coating of a strip using an oxidizable coating.

The object of the invention is to provide, in an installation where the gas from the protective enclosure of the wringing means for regulating the coating is recycled, an improvement in recycling rates and a saving of additional inert gas . For this result, provision is made for a continuous purification of this gas by contacting with a reducing substance. If the protective enclosure also contains the minimized flowering nozzles, zinc vapor can be introduced into the gas. This reacts on oxygen and, by condensation, provides germs.

Description

The present invention relates to a method and an installation for the continuous deposition of a coating on a strip, this deposition being carried out by passing the strip through a bath of coating material heated to deLadesonoointdefusion. apply in carticu-Lier to the coating of a steel sheet with a layer of metaL such as zinc.

Processes have been known for a long time in which a thin strip of metal is driven continuously first in surface preparation and preheating stations, then, using immersed rollers, breaks in a bath of molten coating material, zinc for example, after which it leaves the bath following an upward vertical path. IT drives at its exit from the bath a layer of liquid coating material, the thickness of which depends, in particular, on the running speed, the temperature of the bath and the surface condition of the strip. This liquid layer solidifies during rescue. The strip cools while it is above the bath.

In order to obtain a coating layer of uniform thickness, free of impurities and with regular crystallization, it is necessary to precisely control all the factors involved in the operation.

In the oldest technique, rollers were used to equalize the thickness of the coating layer while it was still liquid. Patent FR 1,563,457 has described a more efficient method which consists in sending a layer of gas, preferably air in the case of a coating of lead and vapor of vapor, onto this layer of liquid coating material. dry water in the case of zinc, this jet of gas being supplied by a nozzle in the form of a slot whose shape, position and orientation are precisely defined, as well as the gas pressure, in order to provide a jet in Blade shape which removes the external fraction of the liquid thickness and causes it to fall back towards the bath, causing with it the grime and oxides which could come from the surface of the bath.

In current practice, the gas used most frequently is air.

Furthermore, to obtain a coating layer with fine and regular crystallization, a process known as "minimized flowering" is known in which a gas laden with crystallization seeds is projected onto the strip.

Usually this gas is compressed air in which fine solid zinc particles have been introduced. To prevent these particles which do not bind to the band from spreading in the workshop, a suction mouth is placed in the immediate vicinity of the blowing nozzle and the sucked air is recycled after filtration.

The combination of accelerated cooling under the effect of the gas jet and the presence of a large number of germs leads to fine and regular crystallization.

These methods have on the whole satisfactory results; However, the requirements of the users are always in the direction of increasing severity, and it has been found that the consistency of the finished product is not entirely perfect with regard to the presence of oxides on the surface and the Crystallization of the coating material, in particular with regard to thin sheets with thin coating thickness.

IT is known (GB-A-777.353) to preserve the coating from oxidation at the time of its exit from the bath by causing the strip to pass through an enclosure whose walls plunge into the bath and which has at its upper part a narrow opening where the band comes out. An inert gas remains in this enclosure, which contains rollers intended to equalize the thickness of the coating. (FR-A-2.454.470)

It has been proposed to further improve the quality, to use an inert gas of high purity, for example nitrogen with very low oxygen content. Such a gas is expensive, and plans have been made to recycle it, but, despite the precautions taken: circuits as tight as possible, cooling, filtration, it is difficult to achieve a recycling rate of more than 50%.

The high price of very low oxygen nitrogen and other high purity gases is a cause of the high cost price.

The main object of the present invention is to reduce the consumption of high purity inert gas without exaggerated increase in the complication of the installation, so as to achieve a total significant reduction in cost prices.

Another object of the invention is an improvement in the quality of the product by better regularity of the crystallization.

The invention therefore provides a process for continuously coating a. strip using an oxidizable coating material, method according to which the strip is passed through a bath containing the coating material in the liquid state; the strip is brought out of this bath in an ascending direction; it is subjected to a control operation of the thickness of the layer of liquid coating material entrained by the strip, this control operation being carried out in an enclosure substantially isolated from the atmosphere and containing a first non-oxidizing or weakly oxidizing gas. who undergoes at least partially a recycLage; The regularization operation being optionally followed by a germ projection operation during spraying, a second gas charged with crystallization seeds of said coating material is sprayed onto the coating material still liquid, this gas then being at least partially recycled, which has the particularity that one purifies at least a part of the first gas and / or at least a part of the second gas by bringing it into contact with a reducing substance to reduce its oxygen content below a value chosen to The advance.

The combination of known recycling with purification carried out during this recycling allows very precise control of the oxygen content of the gas, and great flexibility in adapting this content to needs. The quantity of reducing substance required is low, since it only corresponds, in normal operation, to compensating for the influx of oxygen, and it adjusts to requirements.

Preferably, the coating thickness control and germ blowing operations take place in a common enclosure, in which the first and second gases are mixed. In this way, the quality is further improved by virtue of the fact that the strip can be protected from the atmosphere until crystallization, and the installation is simplified due to the existence of a single enclosure and, possibly, of a single purification device, placed on one or the other of the recycling circuits or at a point common to these two circuits.

According to an interesting modality, When the purification of the second gas is carried out, the purification and the introduction of crystallization seeds are carried out simultaneously in said second gas before being used for the projection of germs.

In this case, if the substance which gives rise to the crystallization germs is reducing, it is advantageous to provide that said substance is introduced into said second gas, then the second gas is brought to a temperature high enough to lower its oxygen content. à The value chosen, by reaction of oxygen with said substance.

This reduction reaction can be improved by injecting a hydrocarbon, in small quantity, The substance which gives rise to germs, zinc for example, then playing more than the rattle of catalyst in the hydrocarbon-oxygen reaction, in addition to its possible clean reducE role.

Then the second gas is brought, which contains the result of the oxidation of said substance and optionally a part of the unreacted substance, at the temperature conditions suitable for said germ projection operation. Preferably, the reducing substance is introduced into the second gas in the vapor state and after the oxidation of part of this vapor, the second gas is cooled to cause the said substance to form the germs by condensation in the state. solid.

The fact of combining the introduction of germs and the purification, allows an obvious simplification of the installation, therefore a reduction of investments.

Introducing the reducing substance in the vapor state leads to a further improvement in the quality thanks to a better dispersion of this substance in the second gas, therefore a better consistency of the oxygen content and also of the germ content. crystallization.

According to another interesting modality, which can be added or substituted for the previous one, the gas to be purified is brought into contact with a hot surface, in the presence of the reducing substance. This is advantageously a hydrocarbon (methane for example) introduced in small quantities. This hot surface can be formed by plates heated by an appropriate means, but also by the sheet itself leaving the bath, in the case where the metallic coating bath is at high temperature (manufacture of aluminized sheet for example). This modality is particularly suitable in the case where one wishes to be able to use at will or to leave at a standstill the minimized flowering equipment. Indeed, this modality can be implemented by acting as well on the first gas as on the second gas, the seeds of crystallization being introduced into the latter for example in a classical manner. It can also be implemented inside the enclosure common to the two circuits if there is one.

The invention also provides, for the implementation of the process which has just been described, an installation comprising means for successively continuously passing a strip through a molten bath of material. coating, to bring this strip out of the bath in an upward direction, means for regulating the thickness of the layer of coating material Licuide entrained by the strip, these means being able to comprise at least one wiping nozzle arranged to blow a jet of gas in the form of a blade in the direction of the band, these means being arranged inside an enclosure open downwards and comprising lateral walls which plunge into the bath and an upper wall having a narrow slot intended for the outlet of the strip, the enclosure being associated with a circuit for recycling the gas which it contains and for sending this gas to the spinning nozzle (s), the installation further comprising at least one blowing nozzle for cooling the strip below the solidification point of the coating material and optionally projecting on it crystallization seeds, this blowing nozzle being associated with a circuit for recycling the second gas which comp orte means for introducing seeds of crystallization in said second gas upstream of the blowing nozzle (s), this installation further comprising means for introducing into the circuit for the recycling of the gas from the enclosure and / or into the circuit for the recycling of said second gas a reducing substance, and means for bringing the gas corresponding to a temperature where said substance reacts with the oxygen contained in the gas to bring its concentration below the chosen value.

Preferably, the means for regulating the coating thickness and the germ blowing nozzle (s) are arranged in a common enclosure and said means for introducing the reducing substance (s) and for bringing the gas to the reaction temperature are arranged either in the enclosures, or in only one of the circuits for recycling.

According to an interesting realization The means for introducing the reducing substance into the gas and bringing it to reaction temperature, consist of an enclosure traversed by the gas and containing a bath of reducing substance in the Liquid State and a plasma torch disposed above this bath for vaporizing said substance. The purification reaction can be improved by adding a small amount of hydrocarbon.

According to another embodiment, simpler to install, the means for introducing the reducing substance into the gas consist of an enclosure containing a bath of reducing substance in the liquid state, and means for forcing the gas to lick the surface of this bath or to splash around. The quantity of reducing substance introduced into the gas is a function of the temperature of the metallic bath of the reducing substance, and / or of the flow rate of gas bubbling through this enclosure. As in the previous embodiment, the purification reaction can be improved by the injection of a hydrocarbon, in small quantity.

Preferably, with one or the other of the embodiments which have just been mentioned, said means for introducing the reducing substance into the gas are arranged on the circuit for recycling the second gas, and between said means and the or germ blowing nozzles iL is provided means for cooling the second gas to the formation of germs by solid state condensation of the reducing substance.

According to another interesting embodiment, the means for deoxidizing the gas are obtained by placing, in said enclosure, in the immediate vicinity of the narrow slot provided in the upper wall of the enclosure, plates heated to high temperature and by introducing into The enclosure, near these plates, a small amount of hydrocarbon in order to obtain the deoxidation of the gases contained in the enclosure. This device can be installed at another location on either circuit, but the advantage of the arrangement just described is better control of the oxygen content. Indeed, the tape exit slot is the main passage for the entry of oxygen into the enclosure, by circulation against the current of the strip.

• Fig. 1 est une vue schématique, en coupe verticaLe de L'instaLLation.
• Fig. 2 est une vue schématique parteil-Le en coupe verticale d'un appareiLLage pour introduire La substance réductrice dans Le gaz.
• Fig. 3 est une vue analogue à La figure 2 d'un autre appareiLLsge pour introduire La substance réductrice dans Le gaz.
• Fig. 4 est une vue schématique de L'appareiLLage permettant La réaction hydrocarbure-oxygène avec pLaques chauffées disposées à La sortie de L'enceinte.
• Fig. 5 est une vue schématique d'un appareiLLage fonctionnant suivant Le même principe que ceLui de La figure 4, mais pLacé dans un circuit de recircuLation.

The invention will now be explained in more detail using non-limiting examples of practical implementation, relating to the obtaining of a zone coating on a steel sheet and illustrated by the figures among which

• Fig. 1 is a schematic view, in vertical section of the installation.
• Fig. 2 is a schematic partial view in vertical section of an apparatus for introducing the reducing substance into the gas.
• Fig. 3 is a view similar to FIG. 2 of another device for introducing the reducing substance into the gas.
• Fig. 4 is a schematic view of the apparatus allowing the hydrocarbon-oxygen reaction with heated plates placed at the outlet of the enclosure.
• Fig. 5 is a schematic view of an apparatus operating according to the same principle as that of FIG. 4, but placed in a recirculation circuit.

The strip to be coated arrives through the left on Figure 1, it first passes through an oven 2, controlled reducing atmosphere, which ensures both the cleaned e ^g and the surface preparation, optionally a heat treatment, and L ' adjustment of the sheet temperature to a temperature close to that of the bath.

The strip 1 guided by rollers 3, 4, 5 then descends into the bath of molten zinc 7, then rises vertically below the bath and is sent after a roll 6 to a winding station, not shown. A sheath 8 which plunges into the bath and communicates with the oven 2, surrounds the strip on its path between the oven and the bath 7, so as to prevent any formation of oxide on the cleaned and hot metal before its contact with the zinc of the bath.

On leaving the bath, the strip is surrounded by a bottomless box 9, the side walls of which plunge into molten zinc. The roof of the box has a very narrow slot 10 for the exit of the strip ₁ upwards.

Inside the box are arranged two nozzles 11, in the form of an elongated slot, intended to regulate the thickness of the coating to the desired thickness, and, above the nozzles 11, two other nozzles 12, for cooling and / or minimized flowering.

The spinning nozzles 11 are supplied with nitrogen by a recycling circuit which comprises an extraction pipe 13 by which the gas is extracted from the box 9, a cold water cooler 14, which lowers the temperature of the gas to improve the operation of Pump 15 which follows Him. A filter 16 is interposed between the refrigerant and the pump. A supply line 17 connects the pump 15 to the dewatering nozzles 11. To this line 17 is connected a line 18 of nitrogen nitrogen provided with a valve and connected to a source of high purity nitrogen 19.

The minimized flowering nozzles are supplied by an analogous circuit, comprising an extraction line 20, a coolant 21, a pump 22 and a supply line 23, but without an auxiliary line.

Three possible positions of the purification device have been shown in dashes:

In Z4, a purification device connected to the circuit of minimized flowering gas is shown, and yes is then combined with the device for supplying seeds of crystallization.

In 25 there is shown a purification device connected to the circuit of the wiping gas and in this case it can be constituted by means of injecting a gaseous or liquid hydrocarbon, or a similar substance and a hot surface which comes hit The gas.

In 26, there is shown a purification device placed in the box 9, near the slot 10. This device may include one or more hot surfaces. The injection of hydrocarbon can be placed at another location in the circuits.

FIG. 2 shows an apparatus for introducing a reducing substance which is preferably placed at the spacing designated by 24 in FIG. 1.

This apparatus comprises a closed enclosure 30 which contains a bath of liquid z-inc 31 and, in addition, from this bath a plasma torch 32 arranged to vaporize the zinc of the bath. The enclosure 30 is connected to the pipes 20, 23 by two pipes 33, 34, on either side of the pump 22, so as to constitute a circuit parallel to the circuit which comprises the nozzles 12 for the projection of germs. An adjustment valve is provided on the gas inlet pipe 33 to the enclosure.

FIG. 3 shows a self-apparatus for introducing a reducing substance which can be substituted for that of FIG. 2. It comprises an enclosure 40 in which a bath of liquid zinc 41 is maintained at the temperature chosen to introduce the desired amount of zinc vapor in the gas. The free surface of the bath 41 is also defined accordingly. The gas inlet 43 and outlet 42 conduits are arranged in the same manner as in the case of FIG. 2. If it is desired to increase the quantity of zinc vapor introduced into the gas, it is also possible to provide for bubbling a little gas by a cane 44 immersed in the bath, this cane being connected to a source of high purity nitrogen 46. On the other hand, a tube 45, connected to the pipe 43, makes it possible to introduce a very small amount of 'hydrocarbon; the latter in the presence of zinc powder improves the deoxidation of the recirculating gas.

In the case of these two devices, the formation of germs, that is to say of zinc particles takes place mainly in the pipe 23 where the gas cools naturally or forcibly before reaching the nozzles 12.

FIG. 5 represents another embodiment of purification equipment. In an enclosure 50 are arranged two concentric nozzles supplied 51, 52, the first of which is supplied with gas to be purified through an inlet pipe 53, provided with a valve 54, and the second is supplied with methane, or another hydrocarbon, via a supply line 55 provided with a valve 56. The mixture of gas to be purified and methane is sprayed onto a plate 57 heated for example by resistors up to a temperature sufficient for the Free oxygen to disappear . The purified gas is returned in circuit by a return line 58. Such equipment can be arranged both in the position provided in 24 in FIG. 1, as in the position identified in 25. If it is in position 24, a conventional device for introducing germs must be provided.

According to the variant shown in Figure 4, two plates 60 are arranged on either side of the slot 10 through which the strip 1 leaves the enclosure 9, which corresponds to position 26 of Figure 1. These plates 60 are heated by resistors 61 to a temperature such that the oxygen entering through the slot 10 against the current of the strip 1 immediately reacts on the methane introduced into the gas near the hot surfaces.

This more effective arrangement is only to be recommended if the methane contents are low enough not to entail the risk of explosion.

Some figures are given below relating to walks carried out in the installation just described, with a strip of 1 m wide deflating at 35 meters / minute: for a coating thickness corresponding to 100 g / m ² on each side, The flow rate of nitrogen blown at the nozzles was 2800 _N m ³ / hour under an overpressure of 0.1 bar at the inlet of the nozzles, the pressure in the box being substantially in equilibrium with atmospheric pressure. The temperature of the atmosphere in the chamber 9 was 150 ° C., that of the strip, at the level of the slot 10 was 430 ° C. while the solidification temperature was 420 ° C.

In a first step, the zinc vaporization device and the methane injection were out of service. For a quantity of additional nitrogen injected into the recirculation circuit, of 200 m ³ / h, the oxygen content of the nitrogen in circulation was 2%.

• - tension 100 V
• - intensité 70 à 100 A
• - argon 45 1 /ain
• - hydrogène 10 1/min
• - température de La bande à La sortie fente 10 380°C.

In a second step, the zinc vaporization device described in FIG. 2 was in action, the temperature of the zinc in the bath 31 was 460-500 ° C., the employment characteristics of the plasma torch were as follows:

• - voltage 100 V
• - intensity 70 to 100 A
• - argon 45 1 / ain
• - hydrogen 10 1 / min
• - temperature of the strip at the slit outlet 10 380 ° C.

The oxygen content in the circulating nitrogen was less than 200 ppm.

In another test, the other conditions were the same, the device of FIG. 3 was used. The temperature of the zinc in the annexed crucible was 500 ° C. and the flow rate of nitrogen washing the surface of this crucible was 25 m ³ / h, The nitrogen flow bubbling through the crucible was 2 m ³ / h and The quantity of methane injected was 1 m ³ / h.

An oxygen content in nitrogen was also obtained of less than 200 ppm.

In a fourth test, the device of FIG. 4 was used, the methane flow rate injected was 2 m ³ / h, and the temperature of the hot surface 60 was 700 ° C. An oxygen content in nitrogen of 10 to 20 ppm was obtained.

The device of FIG. 4 therefore makes it possible to obtain very high purities of oxygen, it must however be observed that it does not provide germs for the minimized flowering nozzles. If this is necessary, a separate diet should be provided for these germs. It is for example possible to operate in parallel a zinc vaporization device according to FIG. 2 or 3.

Claims (16)

1. Process for continuously coating a strip using an oxidizable coating material, method according to which the strip is passed through a bath containing the coating material in the liquid state; the strip is brought out of this bath in an ascending direction; it is subjected to an operation to regulate the thickness of the layer of liquid coating material entrained by the strip, this operation of regulation being carried out in an enclosure substantially isolated from the atmosphere and containing a first non-oxidizing or weakly oxidizing gas. who undergoes at least partial recycling; The coating thickness regularization operation being optionally followed by a germ projection operation during the spraying, a second gas charged with crystallization germs of said coating material is sprayed onto the coating material still liquid. gas then being at least partially recycled, characterized in that at least part of the first gas and / or at least part of the second gas is purified by bringing it into contact with a reducing substance in order to reduce its oxygen content below of a value chosen in advance. 2. Method according to claim 1, characterized in that the operations for regulating the thickness of the coating and for blowing germs take place in a common enclosure, in which the first and second gases are in contact. 3. Method according to one of claims 1 or 2 and in which the said second gas is purified, characterized in that the purification and the introduction of crystallization germs are carried out simultaneously in the said second gas before being used for The projection of germs. 4. Method according to claim 3 and in which the substance which gives rise to the crystallization germs is reducing, characterized in that said substance is introduced into said second gas, then the second gas is brought to a temperature high enough to lower its oxygen content to the value chosen by reaction of the oxygen on the said substance, and then the second gas is brought, which contains the result of the oxidation of the said substance and optionally a part of the unreacted substance, at the temperature conditions suitable for the said germ projection operation. 5. Method according to claim 4, characterized in that the said substance in the vapor state is introduced into the second gas, and in that after the oxidation of part of this vapor, the second gas is cooled. to cause said substance to form germs by condensation in the solid state. 6. Method according to claim 5, characterized in that a small quantity of hydrocarbon is also introduced into the second gas, in order to improve the purification operation of the second gas by reduction of the oxygen, the substance which gives rise to germs playing a catalytic role in the reaction of the hydrocarbon with oxygen, in addition to its possible own reducing role. 7. Method according to one of claims 1 to 6 characterized in that Said substance which gives rise to germs is Zinc. 8. Method according to one of claims 1 to 7, characterized in that the gas to be purified is reacted with a reducing substance of the hydrocarbon type introduced in small quantity, on a hot surface (heated plate or coated sheet eLLe even in The case of aluminized sheet). 9. Process according to claim 8, characterized in that the hydrocarbon is methane and in that the coating material is zinc or one of its alloys, or aluminum or one of its alloys. 10. InstaLLation for the implementation of the method according to one of claims 1 to 9, and comprising means for successively continuously passing a strip (1) through a molten bath (7) of coating material, and for bring this strip out of the bath in an upward direction, means (11) for regulating the thickness of the layer of liquid coating material entrained by the strip, these means may comprise at least one wringing nozzle arranged to blow a jet of gas in the form of a blade in the direction of the strip, these means being arranged inside an enclosure (9) open downwards and comprising lateral walls which plunge into the bath and an upper wall having a narrow slot ( 10) intended for leaving the strip, the enclosure being associated with a circuit (13, 17) for recycling the gas which it contains and for sending this gas to the spinning nozzle (s), the installation comprising in addition to a blowing nozzle (12) for cooling ir The strip below the solidification point of the coating material and possibly projecting seeds of crystallization, this blowing nozzle being associated with a circuit (20, 23) for recycling the second gas which optionally includes means for introducing seeds of crystallization in said second gas upstream of the blowing nozzle or nozzles, characterized in that it further comprises means for introducing into the circuit for the recycling of the gas from the enclosure and / or into the circuit for the recycling said second gas a reducing substance, and means for bringing the gas corresponding to a temperature where said reducing substance reacts with the oxygen contained in the gas to bring its concentration below the chosen value. 11. Installation according to claim 10, characterized in that the means for regulating the coating thickness and the or the blowing nozzles are arranged in a common enclosure and in that said means for introducing the reducing substance and for bringing the gas to the reaction temperature are arranged either in the enclosure or in only one of the circuits for recycling. 12. InstaLLation according to one of claims 10 or 11, characterized in that the means for introducing the reducing substance into the gas and bringing the latter to the reaction temperature consist of an enclosure traversed by the gas and containing a bath of reducing substance in the Liquid State and a plasma torch disposed above this bath to vaporize the said substance. 13. Installation according to one of claims 10 or 11, characterized in that the means for introducing the reducing substance into the gas consist of an enclosure containing a bath of reducing substance in the liquid state, and means for forcing the gas to Lick The surface of this bath or to bubble. 14. Installation according to one of claims 12 or 13, characterized in that said means for introducing the reducing substance into the gas are arranged on the circuit for the recycling of the second gas and in that between said means and the or the blowing nozzles iL is provided means for cooling the second gas until the formation of germs by solid state condensation of the reducing substance. 15.InstaLLation according to one of claims 10 to ¹⁴ , characterized in that it further comprises means for introducing into the gas a hydrocarbon. 16. InstaLLation according to claim 15, characterized in that the means for bringing the gas into contact with a hot wall where the reducing substance of the hydrocarbon type reacts on oxygen, are arranged in said enclosure near the narrow slot provided in the upper wall of the enclosure.

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