EP2430208A1

EP2430208A1 - Method for producing a coated metal strip with an improved appearance

Info

Publication number: EP2430208A1
Application number: EP10726150A
Authority: EP
Inventors: Luc Diez; Jean-Michel Mataigne; Bertrand Orsal; Hubert Saint Raymond
Original assignee: ArcelorMittal Investigacion y Desarrollo SL
Current assignee: ArcelorMittal Investigacion y Desarrollo SL
Priority date: 2009-05-14
Filing date: 2010-05-11
Publication date: 2012-03-21
Anticipated expiration: 2030-05-11
Also published as: CA2761776C; CA2761776A1; CN102459684B; KR20120026548A; US11597990B2; ZA201108195B; UA108611C2; KR101528116B1; ES2443312T3; US20180291493A1; WO2010130895A1; EP2430208B1; RU2509822C2; US20230183847A1; JP2012526916A; PL2430208T3; KR20140053417A; AU2010247251A1; US20120052206A1; MA33273B1

Abstract

The invention relates to a method for producing a metal strip having an anti-corrosion metal coating. Said method comprises the following steps: the metal strip is passed through a melt; the coated metal strip is then dried by means of nozzles projecting a gas on either side of the strip, the gas having an oxidising power lower than that of an atmosphere consisting of 4% vol. of oxygen and 96% vol. of nitrogen; and the strip is then passed through a confinement region defined, in the bottom part, by the drying line and upper faces of the drying nozzles, in the top part, by the upper part of two confinement boxes placed either side of the strip, just above the nozzles, and at least 10 cm high in relation to the drying line, and on the sides, by lateral parts of the confinement boxes, the atmosphere in the confinement region having an oxidising power lower than that of an atmosphere consisting of 4% vol. of oxygen and 96% vol. of nitrogen, and higher than that of an atmosphere consisting of 0.15% vol. of oxygen and 99.85% vol. of nitrogen. The invention also relates to a coating installation and a confined drying device (10; 20; 30) for implementing said method.

Description

Process for producing a coated metal strip having an improved appearance

The invention relates to a method for manufacturing a metal strip having an improved appearance, more particularly intended to be used for the manufacture of skin parts for motorized land vehicles, without being limited thereto.

Steel sheets for the manufacture of parts for land motor vehicles are generally coated with a zinc-based, corrosion-resistant metallic layer deposited either by hot dipping in a zinc-based liquid bath or by electroplating in an electrolytic bath comprising zinc ions.

^: The galvanized sheets for the manufacture of skin parts are then shaped and assembled to form a box in white, which is then coated with at least one layer of paint, which provides increased protection against corrosion as well as a good surface appearance. For this purpose, conventionally, automakers first apply on the white box a layer of cataphoresis, then a layer of primer paint, a base coat of paint, and possibly a layer of varnish. To obtain a satisfactory painted surface appearance, it is generally applied a total paint thickness of between 90 and 120 μm, consisting of a layer of cataphoresis 20 to 30 μm thick, a layer of primer paint from 40 to 50 μm, and a base coat of 30 to 40 μm, for example.

In order to limit the thickness of paint systems to less than 90 μm, some car manufacturers have proposed either to avoid the cataphoresis step, or to limit the number of paint layers to increase productivity. However, to date, this reduction in thickness of the paint system is always done at the expense of the final painted surface appearance of the part and is not implemented industrially. Indeed, the base-based zinc-based coatings have what is called a corrugation of their surface, which can currently be compensated for only by significant layers of paint on pain of having an appearance called "skin" orange "unacceptable for body parts.

The corrugation W (waviness in English) of the surface is a geometric irregular softness, pseudoperiodic, of rather long wavelength (0.8 to 10 mm) that one distinguishes from the roughness R which corresponds to the geometrical irregularities low wavelengths (<0.8mm). In the present invention, the arithmetic average Wa of the corrugation profile, expressed in μm, has been retained to characterize the corrugation of the surface of the sheet, and the corrugation measurements have been carried out with a cutoff threshold of 0 , 8 mm and designated by Wa _o , 8-

The object of the invention is therefore to provide a method for manufacturing a metal strip coated with an anticorrosive coating, the waviness Wa _o , _{8 of which} is reduced compared with the strips of the prior art, thus allowing to produce painted metal parts requiring a reduced total paint thickness compared to the parts of the prior art. Another object of the invention is to provide an installation for implementing such a method.

For this purpose, a first object of the invention is constituted by a method of manufacturing a metal strip having a corrosion protection metal coating, comprising the steps of:

- pass the metal strip through a bath of molten metal, then

- Squeezing the coated metal strip by means of nozzles throwing a gas on either side of the strip, said gas having an oxidizing power lower than that of an atmosphere consisting of 4% by volume of oxygen and 96% by volume of nitrogen then

- to pass the band in a defined containment zone: In the lower part by the spin line and the upper faces of said spin nozzles,

• in the upper part by the upper part of two containment boxes placed on either side of the strip, just above said nozzles, and having a height of at least

10 cm from the spin line, and

On the sides, by the lateral parts of said confinement caissons, the atmosphere prevailing in said confinement zone having an oxidizing power lower than that of an atmosphere consisting of 4% by volume of oxygen and 96% by volume of nitrogen and greater than that of an atmosphere consisting of 0.15% by volume of oxygen and 99.85% by volume of nitrogen.

In preferred embodiments, the method according to the invention may further comprise the following features, taken alone or in combination:

- The containment boxes have a height of at least 15 cm, preferably 20 cm or 30 cm, with respect to the spin line; the confinement caissons are supplied with gas having an oxidizing power lower than that of an atmosphere consisting of 4% by volume of oxygen and 96% by volume of nitrogen, and preferably greater than that of an atmosphere consisting of 0.15% by volume of oxygen and 99.85% by volume of nitrogen. the dewatering gas consists of nitrogen,

the metal strip is a steel strip.

The subject of the invention is also an installation for continuous coating of metal strips by hot dipping, comprising: means for moving a metal strip,

a crucible containing a bath of molten metal, and

- A confined spinning device consisting of at least two spin nozzles placed on either side of the path of the strip after its outlet of the bath of molten metal, each nozzle being provided with at least one gas outlet orifice and comprising an upper face, which is surmounted by a containment box open on a face facing the strip, each box comprising at least an upper part and two lateral parts.

In preferred embodiments, the installation according to the invention may further comprise the following features, taken alone or in combination: the upper parts of the containment boxes consist of a bottom plate and an upper plate ;

each of the containment caissons is compartmentalized by a series of vertical blades extending from the upper face of the nozzle to the upper part of the containment caissons; the distance D between the end of the lateral parts of the containment boxes and the band is between 5 and 100 mm and preferably between 10 and 100 mm;

the height H of the containment boxes with respect to the spin line is greater than or equal to 10 cm; the confined wiping devices furthermore comprise soundproofing plates on each side of the strip, facing a part of the outlet orifice of the wiping nozzles;

the containment caissons further comprise bank confinement pieces placed between the confinement caissons, above the noise protection plates, facing the banks of the strip;

- the bank containment rooms can be moved horizontally and vertically;

each of the bank confinement pieces consists of two rectangular plates parallel to the strip, connected by a lateral plate placed facing the banks of the strip;

each of the bank confinement pieces consists of two rectangular plates inclined with respect to the scroll plane of the band and connected along their vertical edge placed facing the banks of the band;

- The bank containment parts further comprise a return means connecting the rectangular plates, the rectangular plates being sufficiently inclined relative to the strip travel plane to be in contact with the side portions of the containment boxes;

the installation comprises edge containment pieces placed between the containment boxes, facing the edges of the strip and extending facing a part of the outlet orifice of the spinning nozzles;

the spin nozzles are provided with a single outlet orifice in the form of a longitudinal slot whose width is at least equal to that of the band to be coated.

The invention further relates to a confined wiper device as defined above.

The features and advantages of the present invention will appear better in the following description given by way of non-limiting example.

With reference to FIG. 1, the first step of the method according to the invention consists in continuously passing a metal strip B, such as a steel strip, in a coating bath 1 containing molten metal contained in FIG. A crucible 2. Before being immersed in this bath 1, the band B is generally annealed in an oven which makes it possible in particular to prepare the surface.

The running speed of the strip on the industrial lines is generally between 40m / min and 200m / min, for example, and is preferably greater than 120 m / min, or even greater than 150 m / min.

The composition of the coating bath to be used in the process according to the invention may in particular be based on zinc or zinc alloy, but also based on aluminum or aluminum alloy. These elements both protect the band against corrosion.

The composition of the bath may also contain up to 0.3% by weight of optional addition elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi . These various elements may allow, among other things, to improve the corrosion resistance of the coating or its fragility or adhesion, for example. Those skilled in the art who know their effects on the characteristics of the coating will know how to use them according to the complementary aim sought. It has also been verified that these elements do not interfere with the control of the corrugation obtained by the process according to the invention. In certain circumstances, however, it will be preferred to limit the titanium content to less than 0.01%, or even less than 0.005% because this element can generate pollution problems of degreasing and phosphating baths of automobile manufacturers. Finally, the bath may contain unavoidable impurities from the crucible feed ingots or even the passage of the strip in the bath. It will be possible to mention, in particular, iron, etc.,

The bath is maintained at a temperature between liquidus +10 ^° C. and 750 ^° C., the temperature of the liquidus varying according to its composition. For the range of coatings used in the present invention, this temperature will therefore be between 350 and 750 ° C. It will be recalled that the liquidus is the temperature beyond which an alloy is in the fully fused state.

After passing through the crucible 2, the metal strip B coated on its two faces is then subjected to spinning by means of nozzles 3 placed on either side of the band B, nozzles which project a spin gas towards the surface This conventional operation, well known to those skilled in the art, allows to adjust the thickness of the coating accurately, while it is not yet solidified. One of the essential characteristics of the process according to the invention consists in choosing a dewatering gas having an oxidizing power lower than that of an atmosphere consisting of 4% by volume of oxygen and 96% by volume of nitrogen. In particular, it will be possible to use pure nitrogen or argon or still more mixtures of nitrogen or argon and oxidizing gases such as, for example, oxygen, mixtures of CO and CO ₂ or mixtures of H ₂ and H ₂ O. It is also possible to use mixtures of CO and CO ₂ or mixtures of H ₂ and H ₂ O without addition of inert gas. At the end of the spinning, the other essential characteristic of the process according to the invention is the passage in a delimited confinement zone:

in the lower part by the spin line L and the upper outer faces of the spin nozzles 3,

- in the upper part by the upper part of two containment boxes C placed on either side of the band, just above the nozzles 3, and having a height of at least 10 cm from the line of spin I, and

on the sides, by the lateral parts of the confinement caissons C, the atmosphere prevailing in the confinement zone having an oxidizing power lower than that of an atmosphere consisting of 4% by volume of oxygen and 96% by volume of nitrogen and greater than that of an atmosphere consisting of 0.15% by volume of oxygen and 99.85% by volume of nitrogen.

To determine the oxidizing power of the atmosphere surrounding the band, one will proceed to the evaluation of its oxygen equivalent partial pressure at equilibrium.

When the only oxidizing gas present is O ₂ , mixed with an inert gas (nitrogen, argon), this pressure is then equal to the volume content of O ₂ that can be measured in real time by means of a suitable sensor.

When other oxidizing gases, such as H ₂ O or CO _2, are present in a mixture with a reducing gas such as H ₂ or CO, for example, the equivalent oxygen partial pressure is calculated by the mass action law. at the gas temperature considered.

For example, for the couple H ₂ / H ₂ O, the reaction is written as follows:

H ₂ +1/2 O ₂ <- H ₂ OA thermodynamic equilibrium, the partial pressures of gases obey the following relation: AG with R the constant of the perfect gases, T the temperature of the gases in Kelvin, and ΔG the variation of free energy associated with the reaction, which is found in the thermodynamic tables, in calories per mole or in Joules per mole according to the value taken for the constant R.

From this relationship, the pCb value, an equilibrium partial oxygen pressure at equilibrium, is extracted for the gaseous mixture under consideration.

In the context of the invention, pθ _{2 must} be between 0.0015 and 0.04 in the confinement atmosphere. The present inventors have indeed found that by using a dewatering gas according to the invention and by passing the strip in such a confinement zone, surprisingly a coating having a smaller corrugation than that of the coated strips was obtained. of the prior art.

In the context of the present application, the term "spinning line" is understood to mean the shortest segment connecting the nozzle and the sheet, corresponding to the minimum path carried out by the dewatering gas, as designated by the letter L in FIG. .

The containment boxes used in the process according to the invention may be fed with low oxidizing gas, see inert or may simply be fed by the flow of spin gas escaping from the nozzles.

The oxidizing power of the dewatering gas is limited to that presented by a mixture consisting of 4% by volume of oxygen in 96% by volume of nitrogen, since, beyond this degree of oxidation, the undulation of the coating is not improved over the prior art.

On the other hand, a low limit is imposed on the oxidizing power of the confinement atmosphere, which is fixed to the oxidizing power of a mixture consisting of 0.15% by volume of oxygen in 99.85% by volume of nitrogen, since if this confining atmosphere is not sufficiently oxidizing, its use will favor the vaporization of zinc from the not yet solidified coating, vapors which can then come foul the containment box and / or redeposit on the band thus creating defects of crippling aspects.

Although all kinds of spin nozzles can be used for the implementation of the method according to the invention, it is more particularly preferred to choose nozzles whose outlet orifice is in the form of a blade whose width exceeds that of the tape to be coated. This type of nozzle makes it possible to achieve good confinement of the lower part of the spinning zone. In particular, triangular section nozzles can be advantageously used, as shown diagrammatically in FIG. 1, in particular. These nozzles are generally 30 or 40 cm from the surface of the bath.

In compliance with these instructions, there is indeed a surprising and significant improvement in the undulation of the coatings in question, as shown by the tests presented below.

When the coated sheet is completely cooled, it can undergo a skin-pass operation that gives it a texture to facilitate its subsequent shaping. Indeed, the operation of the skin-pass makes it possible to transfer to the surface of the sheet a sufficient roughness so that its shaping is carried out under good conditions, by favoring a good retention of the oil applied on the front plate its formatting.

This skin-pass operation is generally performed for metal sheets intended for the manufacture of body parts for land motor vehicles. When the metal sheets according to the invention are intended for the manufacture of household appliances, for example, this additional operation is not carried out.

The skin-shaped sheet or not is then shaped, for example by stamping, bending or profiling, and preferably by stamping, to form a part that can then be painted. In the case of parts for household appliances, it may also optionally subject this layer of paint to an annealing by physical and / or chemical means, known in themselves. For this purpose, the painted piece can be through a hot air or induction furnace, or under UV lamps or under a device beam scattering electron.

For the production of parts for the automobile, it is soaked in a cataphoresis bath, and applied successively, a primer coat, a base coat of paint, and optionally a layer of finishing varnish.

Before applying the cataphoresis layer on the part, it is previously degreased and phosphated to ensure the adhesion of cataphoresis. The cataphoresis layer provides the part with additional protection against corrosion. The primer coat, usually applied by spray, prepares the final appearance of the part and protects it from gravel and UV. The base coat of paint gives the piece its color and its final appearance. The varnish layer gives the surface of the part a good mechanical resistance, a resistance against aggressive chemical agents and a good surface appearance.

The layer of paint (or paint system) used to protect and guarantee an optimal surface appearance to galvanized parts, for example, has a cataphoresis layer 10 to 20 μm thick, a primer coat of less than 30 μm, and a base coat of less than 40 microns.

In cases where the paint system further comprises a layer of varnish, the thicknesses of the different layers of paint are generally as follows:

- cataphoresis layer: less than 10 to 20 μm, - primer coat: less than 20 μm,

base paint layer: less than 20 μm and advantageously less than 10 μm, and

- Varnish layer: preferably less than less than 30 microns.

The paint system may also not include a cataphoresis layer, and include only one layer of primer paint and a base coat of paint and possibly a layer of varnish. Trial-

The tests are carried out from a cold-rolled steel sheet of the IF-Ti type, which is passed through a crucible containing a bath of variable composition. It is maintained at a temperature of 70 ° C. beyond the liquidus of the composition.

At the outlet of the bath, the coating obtained is wrung with nitrogen, by means of two conventional nozzles so as to obtain a coating thickness of about 7 .mu.m.

The path of the steel strip between the coating bath outlet and the post-spin zone is subdivided into four zones:

a zone 1 going from the bath outlet to a distance of 10 cm below the spin line,

a zone 2 going from the end of zone 1 to the spin line,

a zone 3 going from the end of zone 2 to a distance of 10 cm above the spinning line and

- A zone 4 from the end of zone 3 until the solidification of the metal coating.

At each of these zones, containment boxes are set up with various nitrogen-based atmospheres comprising a volume fraction of oxygen as indicated in the following table, or constituted by air. Specific sensors make it possible to check the oxygen content in the caissons.

Once the sheet is coated, three sets of samples are taken. The first series does not undergo other modifications, the second series is stamped according to a mode of equibiaxial deformation to 3.5% (Marciniak), the third series is first subjected to a skin-pass operation with a rate lengthening of 1, 5%, then stamped as the second.

As the test progresses, the ripple values Wa ₀ , _{8 are} measured _. This measurement consists in acquiring by mechanical palpation, without pad, a profile of the sheet of length 50 mm, measured at 45 ° from the rolling direction. We deduct from the signal obtained the approximation of its general form by a polynomial of degree at least 5. The wavulation Wa is then isolated from the roughness Ra by a Gaussian filter with a cut-off of 0.8mm. The results obtained are summarized in the following table:

READ not: not evaluated, *: according to the invention

On reading the test results, it is clear that the method is applicable to many types of coatings.

Moreover, its influence on the level of waviness of the coatings obtained is also noted. In particular, tests 1, 3, 5, 7, 11, 13 and 17 show that by not controlling the spin atmosphere, the corrugation is not at a satisfactory level.

Tests 8 and 14 show that a spinning atmosphere with too high oxygen content and thus with too much oxidizing power also does not achieve satisfactory levels, although they are slightly improved compared to prior art.

Tests 10 and 16 further demonstrate the need to maintain a minimum of oxidizing power in the containment atmosphere and the need not to confine the strip from the coating bath to avoid zinc vaporization which generates defects in the atmosphere. unacceptable aspects.

In order to implement the process according to the invention, the present inventors have developed various confined wiping devices which will be described as indicative and nonlimiting with reference to the appended figures 2 to 10 which represent: FIG. 2: a perspective view of an embodiment of a confined wiper device according to the invention,

- Fig. 3: a perspective view of an embodiment of a confined wiper device according to the invention,

- Fig. 4: a sectional view of the device of FIG. 3, FIG. 5: a perspective view of an embodiment of a confined wiper device according to the invention,

- Fig. 6: a perspective view of an embodiment of a confined wiper device according to the invention,

- Fig. 7: a sectional view of the device of FIG. 6, FIG. 8: a top view of the device of FIG. 6,

- Fig. 9: a top view of an embodiment of a confined wiper device according to the invention, - Fig. 10: a top view of an embodiment of a confined wiper device according to the invention.

Referring firstly to FIG. 3, there can be seen a first embodiment of a confined wiper device 20 according to the invention which comprises two identical spin-off nozzles 3, placed at the same level, of B. These wiper nozzles 3 have a generally triangular shape and each consist of two longitudinal metal plates 4 and 4 '(not visible) fixed together by means of two lateral triangular plates 5 and 5 '(not shown). The longitudinal metal plates 4 and 4 'are assembled so that a thin slot remains between them to allow the passage under pressure of the spin gas, conveyed by means not shown.

The confined wiper device 20 also comprises two containment boxes 21 and 22 each placed on the upper outer faces of each nozzle 3, formed of the upper metal plates 4 and welded thereto. The box 22 consists of the assembly of two side plates 24 and an upper part consisting of a horizontal plate 25 and a vertical plate 23. The plates 24 and 25 are preferably of the same width, which width may be equal to or less than the depth of the nozzle 3.

The box 21 is in all respects identical to the box 22. The confined spinning device 20 finally comprises two metal plates 6 called "noise baffles" whose function is to prevent the flow of spin gas emanating from each nozzle 3 do not meet in the lateral zones where band B is not present. Indeed, the same coating plant can see scrolling strips having various width formats and the interposition of such plates 6 is useful, in particular to avoid generating very significant sound vibrations. If we now consider Figure 4, we can see a sectional view of the device of Figure 3 on which are shown the two spray nozzles 3, an arrow materializing the flow of spin gas on both sides. other of the band. The height of the containment boxes 21 and 22, represented by the letter H is measured between the spin line and the upper part of the boxes. In the process according to the invention, it has been seen that this height must be at least 10 cm to obtain satisfactory results in terms of undulation. Depending on the value of the width of the side and top plates 24 and 25, the distance D separating the boxes 21 and 22 of the band B is greater or smaller. As a result of various tests, the present inventors have been able to observe that a distance D of between 5 and 100 mm makes it possible to extract the spin gas satisfactorily while remaining sufficiently far from the path of the strip B to avoid any contact.

The distance Z between the end of the nozzles 3 and the band B is preferably between 3 and 25 mm, in conventional manner.

If we now consider Figure 2, we can see another embodiment of a confined spinning device 10 according to the invention. As before, this device comprises spin nozzles 3 identical to those described for FIG. 3 and noise-reducing plates 6.

It further comprises two containment boxes 11 and 12 placed and fixed on the upper face 4 of the spin nozzles 3. The box 12 here comprises an inclined top plate 13, connected to two triangular side plates 14. The box 11 is identical at the caisson 12.

As for the boxes of FIG. 3, the boxes 11 and 12 have a width that can be at most equal to the depth of the nozzles 3.

In this embodiment, the height H of the containment boxes 11 and 12 is measured between the spin line and the upper edge of the plates 13.

This embodiment has the particular advantage of including a smaller volume than that of FIG. 3, which facilitates the management of the confinement atmosphere and makes it possible to consume a smaller quantity of inerting gas when the feeding in this one is necessary. If we next consider Figure 5, we can see another embodiment of a confined spin device 30 according to the invention. This is generally identical to the device 20 of FIG. 3 and comprises in particular two containment boxes 31 and 32 comprising an upper part consisting of vertical plates 33 connected to horizontal plates 35, and side portions 34. Each of the boxes 31 and 32 is further compartmentalised by a series of vertical blades 36 extending from the upper face of the wiper nozzle 3 until at the upper part 35 of the containment boxes 31 and 32.

This particular arrangement has the advantage of limiting the oxygen inputs in the containment boxes 31 and 32.

FIG. 6 shows another embodiment of a confinement device according to the invention similar to that represented in FIG. 3, but furthermore comprising bank confinement pieces 26, placed between the containment caissons 21 and 22, at the above the noise plates 6 and in front of the banks of the band B. As their name suggests, these parts have the function of further confining the atmosphere surrounding the band B on its banks. In a preferred embodiment, these bank confinement pieces can be moved horizontally and vertically to accommodate different tape formats to be coated.

In the embodiment shown in FIG. 6, the edge containment piece 26 consists of two rectangular plates parallel to the band B connected by a lateral plate placed facing the banks of the band B.

FIG. 7 shows the relative positioning of the confinement part 26 above the noise protection plate 6.

As illustrated in Figure 8, the side plate has a width C greater or smaller, depending on whether one wants to achieve a significant confinement or not in banks.

Figure 9 shows another embodiment of the containment parts according to the invention. The part 27 consists of two rectangular plates inclined with respect to the band B running plane and connected along their vertical edge placed facing the banks of the band B.

This embodiment has the advantage of limiting the oxygen inputs even more than the design of FIG. 6. The inclined positioning of the two rectangular plates favors the flow of gas from inside the box outwards and disadvantages the flow of gas from the outside to the inside of the box.

FIG. 10 shows another embodiment of the confinement pieces according to the invention in which the confinement piece 28 further comprises a return means 29 connecting the inclined rectangular plates and here taking the form of a spring. These plates are inclined relative to the running plane of the band B in order to be in contact with the lateral parts of the containment boxes 21 and 22.

The edge containment pieces just described are placed above the noise plates 6. However, it is possible to extend them to the outlets of the spinning nozzles to give them a noise-canceling function, making them the use of such plates without object.

Claims

A method of manufacturing a metal strip having a corrosion protective metal coating, comprising the steps of:

- pass the metal strip through a bath of molten metal, then

- to pass the band in a defined containment zone:

^B in the lower part by the spin line and the upper faces of said spin nozzles,

• in the upper part by the upper part of two containment boxes placed on either side of the band, just above said nozzles, and having a height of at least 10 cm from the spin line, and on the sides, by the lateral parts of said confinement caissons, the atmosphere prevailing in said confinement zone having an oxidizing power lower than that of an atmosphere composed of 4% by volume of oxygen and 96% by volume of oxygen. nitrogen and greater than that of an atmosphere consisting of 0.15% by volume of oxygen and

99.85% by volume of nitrogen.

2. Method according to claim 1, wherein said containment boxes have a height of at least 15 cm with respect to the spin line.

3. Method according to claim 1 or 2, wherein said containment boxes are supplied with gas having an oxidizing power lower than that of an atmosphere consisting of 4% by volume of oxygen and 96% by volume of nitrogen.

4. Method according to any one of claims 1 to 3, wherein the dewatering gas consists of nitrogen.

5. Method according to any one of claims 1 to 4, wherein the metal strip is a steel strip.

6. Installation of continuous coating of metal strips by hot dipping comprising:

means for scrolling a metal strip,

a crucible (2) containing a bath of molten metal (1), and

a confined wringing device (10; 20; 30) consisting of at least two wringing nozzles (3) placed on either side of the path of the strip after its exit from the molten metal bath (1); ), each nozzle (3) being provided with at least one gas outlet and comprising an upper face (4), which is surmounted by a containment box (11, 12; 21, 22; 31, 32) open on a face facing the strip, each box (11, 12; 21, 22; 31, 32) comprising at least one upper portion (13; 23, 25; 33, 35) and two side portions (14; 24; 34).

7. Installation according to claim 6, wherein said upper parts of the containment boxes (21, 22; 31, 32) consist of a bottom plate (23; 33) and a top plate.

(25; 35).

8. Installation according to claim 6 or 7, wherein each of said containment boxes (31, 32) is compartmentalized by a series of vertical blades (36) extending from the upper face of the nozzle (3) to the upper part (35) of said containment boxes (31,

32).

9. Installation according to any one of claims 6 to 8, wherein the distance D between the end of the side portions (14; 24; 34) of said containment boxes (11, 12; 21, 22; 31, 32). and the band is between 5 and 100 mm.

10. Installation according to any one of claims 6 to 9, wherein the height H of said containment boxes (11, 12; 21, 22; 31, 32) relative to the spin line is greater than or equal to 10 cm.

11. Installation according to any one of claims 6 to 10, wherein said confined wiping devices (10; 20; 30) further comprise noise-reducing plates (6) on each side of the strip, facing a part of the outlet of said wringing nozzles (3). 12. The apparatus according to claim 11, wherein said containment boxes (11, 12; 21, 22; 31, 32) further comprise edge containment pieces (26; 27; 28) placed between said containment boxes. (11,

12; 21, 22; 31, 32), above said noise plates (6), opposite the banks of the strip. o

13. Installation according to claim 12, wherein said bank confinement pieces (26; 27; 28) can be moved horizontally and vertically.

14. An installation according to claim 12 or 13, wherein each of said bank confinement pieces (26) consists of two rectangular plates parallel to the strip, connected by a side plate placed facing the edges of the strip.

15. Installation according to any one of claims 12 to 14, wherein each of said bank confinement pieces (27; 28) consists of two rectangular plates inclined relative to the plane of travel of the strip and connected along their length. Vertical edge placed facing the banks of the strip.

16. Installation according to claim 15, wherein said bank confinement rooms (28) further comprising a return means (29) connecting said rectangular plates, said rectangular plates5 being sufficiently inclined relative to the strip of the scroll plane for being in contact with the lateral parts of said containment boxes (21, 22).

17. Installation according to any one of claims 6 to 10, comprising edge containment pieces placed between said containment boxes (11, 12; 21, 22; 31, 32), opposite the banks of the strip and extending opposite a portion of the outlet orifice of said wringing nozzles (3).

18. Installation according to any one of claims 6 to 17, wherein said wiping nozzles (3) are provided with a single outlet orifice in the form of a longitudinal slot whose width is at least equal to that of the strip. to put on.

Confined wringing device (10; 20; 30) as defined in any one of claims 6 to 18.