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Title:
METHOD FOR MANUFACTURING A COATED METAL STRIP WITH IMPROVED APPEARANCE AND WIPING DEVICE THEREFOR
Document Type and Number:
WIPO Patent Application WO/2023/088625
Kind Code:
A1
Abstract:
The present invention relates to a gas wiping device for controlling the thickness of a coating layer deposited on a running metal strip plated with molten metal (1) in an industrial hot-dip bath, wherein the gas wiping device comprises an upper confinement box (6A) and/or a lower confinement box (6B) being adjacent respectively to an upper external side (15A) of the wiping unit (5) and a lower external side (15B) of the wiping unit (5), said confinement boxes (6A, 6B) being each provided with a respective opening (13A, 13B) to blow an additional flow of non-oxidizing gas, so that, in use, the distance (17A) from an end of the upper confinement box (6A) to the running strip (1) and the distance (17B) from an end of the lower confinement box (6B) to the running strip (1) are separately adjustable, the flow rates of the additional flows being each also adjustable in relation to the respective distances (17A, 17B) between each confinement box and the running strip, based on a measured content of oxygen in the vicinity of the running strip (1) in front of the confinement boxes (6A, 6B), and wherein the cross-section of the respective openings (13A, 13B) of the confinement boxes (6A, 6B) is comprised between 1.5 to 5 times the cross-section of the nozzle slot (7) of the wiping unit (5).

Inventors:
DUBOIS MICHEL (BE)
PIZZUTO FABIAN (BE)
Application Number:
PCT/EP2022/078996
Publication Date:
May 25, 2023
Filing Date:
October 18, 2022
Export Citation:
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Assignee:
JOHN COCKERILL SA (BE)
International Classes:
B05C11/06; B05C3/12; C23C2/00; C23C2/04; C23C2/06; C23C2/12; C23C2/16; C23C2/18; C23C2/20; C23C2/26
Domestic Patent References:
WO2014199194A12014-12-18
WO2021105228A12021-06-03
WO2014199194A12014-12-18
WO2010130883A12010-11-18
WO2005010229A12005-02-03
WO2021105228A12021-06-03
Foreign References:
US20090159233A12009-06-25
US20180291493A12018-10-11
US20090159233A12009-06-25
Attorney, Agent or Firm:
AWA BENELUX (BE)
Download PDF:
Claims:
CLAIMS

1 . A gas wiping device for controlling the thickness of a coating layer deposited on a running metal strip plated with molten metal (1 ) in an industrial hot-dip bath, comprising a wiping unit (5) to blow a main wiping gas jet on the surface of the running strip emerging upwards from the molten metal bath, said wiping unit (5) being provided with a main chamber (6) fed by pressurized non-oxidizing gas and with at least an elongated nozzle slot (7) formed in the tip of the wiping unit (5), wherein the gas wiping device also comprises an upper confinement box (6A) and a lower confinement box (6B) being adjacent respectively to an upper external side (15A) of the wiping unit (5) and a lower external side (15B) of the wiping unit (5), said confinement boxes (6A, 6B) being each provided with a respective opening (13A, 13B) to blow an additional flow of non-oxidizing gas, so that, in use, the distance (17A) from an end of the upper confinement box (6A) to the running strip (1 ) and the distance (17B) from an end of the lower confinement box (6B) to the running strip (1 ) are separately adjustable, wherein the gas wiping device also comprises at least one oxygen sensor (18) provided in the confinement region to measure the concentration of oxygen in each of the confinement boxes (6A, 6B), the flow rates of the additional flows blown through the respective openings (13A, 13B) being each also adjustable depending on the measured content of oxygen in the vicinity of the running strip (1 ) in -front of- the confinement boxes (6A, 6B), in relation to the respective distances (17A, 17B) between each confinement box and the running strip, and wherein the cross-section of the respective openings (13A, 13B) of the confinement boxes (6A, 6B) is comprised between 1.5 to 5 times the crosssection of the nozzle slot (7) of the wiping unit (5).

2 . The device according to claim 1 , wherein the maximal height of each confinement box (6A, 6B) is ranging from 50 to 500 mm.

3. The device according to claim 1 , wherein the opening (13A, 13B) in each of the confinement boxes (6A, 6B) is located in use more rearward than the nozzle slot (7) of the wiping unit (5) with respect to the running strip (1 ), preferably, the difference of the distance between said opening (13A, 13B) and the running strip (1 ) and the distance between said nozzle slot (7) of the wiping unit (5) and the running strip (1 ) is at least 10 times the nozzle slot opening.

4 . The device according to claim 1 , wherein the distance (17A) from an end of the upper confinement box (6A) to the strip (1 ) and the distance (17B) from an end of the lower confinement box (6B) to the strip (1 ) are comprised between 5 and 40 mm, preferably between 5 and 30 mm, and more preferably between 10 and 15mm.

5. The device according to claim 1 , wherein the distance (17A) from an end of the upper confinement box (6A) to the running strip (1 ) or the distance (17B) from an end of the lower confinement box (6B) to the running strip (1 ) is smaller than the distance (17) from the wiping unit (5) to the strip (1 ).

6. The device according to claim 1 , wherein the upper confinement box (6A) and/or the lower confinement box (6B) are sealed to or delimited by, possibly in a flexible way, respectively the upper external side (15A) of the wiping unit (5) and the lower external side (15B) of the wiping unit (5).

7 . The device according to claim 1 , wherein the adjustable confinement boxes (6A, 6B) comprise a panel that can be adjusted and made of a mobile element pivotable around a fixed element, or made of a mobile element sliding with respect of a fixed element, said position of the mobile element being changed using a jack, so as to allow setting of the distance (14A, 14B) between an end of the confinement boxes (6A, 6B) and the running strip (1 ).

8 . A method for controlling the thickness of a coating layer deposited on a running metal strip in an industrial hot-dip installation, using the gas wiping device according to anyone of claims 1 to 7, wherein : the distance (17A) from an end of the upper confinement box (6A) to the running strip (1 ) and the distance (17B) from an end of the lower confinement box (6B) to the running strip (1 ) are separately adjusted ; a main pressurized non-oxidizing gas jet is blown through the main nozzle unit (5) on the metal strip plated with molten metal (1 ) coming out of a hot-dip pot (2) ; - at least one additional pressurized non-oxidizing gas jet is blown through the opening (13A, 13B) of one of the adjustable confinement box (6A, 6B) on the metal strip plated with molten metal (1 ) coming out of a hot-dip bath (2),

- an additional gas flow rate coming out of the opening (13A, 13B) of each confinement box (6A, 6B) is adjusted in relation to the distance (17A, 17B) between the confinement box and the running strip (1 ) and based on a measured content of oxygen in front of the respective confinement boxes (6A, 6B), said additional gas flow rate being controlled to be lower than 30% of the gas flow rate coming out of the wiping unit (5).

9 . The method according to claim 8, wherein the controlled gas flow rate coming out of the opening (13A, 13B) of the confinement boxes (6A, 6B) is comprised between 5 and 30% for each box of the gas flow rate coming out of the wiping unit (5).

10 . The method according to claim 8, wherein the controlled gas flow rate coming out of the opening (13A, 13B) of the confinement boxes (6A, 6B) is comprised between 10 and 20% of the gas flow rate coming out of the wiping unit (5).

11 . The method according to claim 8, wherein said pressurized non-oxidizing gas is nitrogen.

Description:
METHOD FOR MANUFACTURING A COATED METAL STRIP WITH IMPROVED APPEARANCE AND WIPING DEVICE THEREFOR

Field of the Invention

[0001] The present invention relates to a device and a corresponding method for improving the surface appearance of a hot-dip coated metal strip having a coating thickness adjusted by gas jet wiping. More particularly, improving of surface appearance is sought by decreasing the oxidation of the liquid film.

[0002] The solution prescribed in the present application applies more particularly to metal strips coated with magnesium in a mixture of zinc and aluminium.

Background and Prior Art

[0003] The coating process consisting in dipping a metal strip in a bath of molten metal is well known and used all over the world, especially in the case of coating a steel strip with zinc, aluminium, tin or alloys of those main metal elements to which others may be added such as magnesium, silicon, chromium, strontium, vanadium as well as impurities like Ti, Fe, Ca, etc.

[0004] As shown in FIG. 1 , a strip 1 is firstly dipped in the molten metal bath 2, then deflected by submerged rolls, usually a sink roll 3 and (a) deflecting roll(s) 4 to finally come out of the bath 2 upward. It is known that the thickness of liquid metal conveyed by the strip owing to viscosity of the liquid increases with the speed of the strip. Therefore, to reduce that thickness to a target value defined by the final customer, wiping of excess liquid is required. The most usual method used to perform that operation consists in utilizing the air knife principle. According to this method, a gas is blown at high speed through one or more nozzles 5 often called “air knives” (see FIG. 1 ) onto the running strip conveying the liquid metal. Usually there is at least one gas nozzle 5 on each side of the strip, an additional nozzle being possibly provided to control edge overcoating. The impingement of the high speed gas onto the strip creates pressure and shear stress profiles on the conveyed liquid film that force the excess of liquid to return to the coating bath.

[0005] The high speed gas nozzle that works like a knife on the liquid film is produced by the gas exhaust from a chamber under pressure 6 through a slot 7 having a length transverse to the running strip and a small thickness (FIG. 2). The gas used can be of any type including combustion gas and steam for example but the most usual method consists in using air for cost and availability reasons and nitrogen when a high surface quality is desired.

[0006] Typical values used in the zinc coating method for example are a steel strip running from 20 to 250 meter per minute with a coating thickness comprised between 2 and 60 microns, which requires a gas exiting from a chamber through a single slot opening which thickness is usually comprised between 0.7 to 2 mm at velocities comprised from 50 m/s to values up to sound velocity (close to 300 m/sec).

[0007] A drawback occurring when liquid metal is wiped by an oxidizing gas such a gas containing oxygen and/or water vapour, like air, is an oxidation of the liquid film. This implies that the physical properties of the coating liquid are thereby changed, as for example its viscosity and surface tension due to the effect of the small oxidized part of the film on the surface thereof but also internally. As it is also known, the gas jet is not totally stable after its exit in ambient environment, with the occurrence of high shear stress between the gas jet and the liquid film, and, as a result, waves can be formed in the coating. These are induced by oscillation of the wiping forces on the liquid film. The risk of significant oxidation of the wiped liquid has been observed to be much higher when the magnesium content in the coating is increased and/or when the line speed is low as the oxidation time increases. [0008] Those waves level off with time more or less depending on the liquid viscosity, its surface tension, the coating thickness and the residence time in liquid state. Therefore, reduction of the oxidation of the liquid film improves the surface quality and especially the undulation of the finished film.

[0009] Other defects such as tiny transversal oxide lines looking like angel hair may also be observed owing to oxidation but this mostly occurs when the Al+Mg content of the coating is high and the wiping jet strong.

[0010] This explains why, when high quality surfaces are desired, the use of a non-oxidizing gas is preferred instead of air. In addition, the dew point of the gas must be low to ensure that oxidation by the water vapour cannot happen as it would be the case when using combustion gas. If various gases might be used like the so-called noble gases (Xe, Ne, Ar, etc.), nitrogen is the preferred medium thanks to its availability and further its impact on manufacturing costs.

[0011] When a non-oxidizing gas is used to feed gas knives, the oxygen content of the ejected gas progressively increases however as soon as the gas jet enters into ambient air thanks to conveying of the latter. This means that the oxygen content of the injected gas progressively increases with the distance from the nozzle exit to the strip. It is therefore known that the higher the nozzle slot-to-strip distance the higher the oxygen content will become in the gas actually impinging onto the liquid metal. This justifies a former patented practice consisting in using a confinement box 8 around the nozzles 5, as very schematically shown in FIG. 3, to keep a low oxidizing atmosphere around the non-oxidizing gas jet.

[0012] A more complex example of confinement box is described in document WO 2014/199194 A1 which discloses an installation for hot dip coating of a metal strip comprising an adjustable confinement box. The installation comprises : means for moving said metal strip along a path, a pot for containing a melt bath, and a wiping system comprising at least two nozzles placed on either side of said path downstream the pot, the wiping system having a box with a lower confinement part for confining an atmosphere around the metal strip upstream of said nozzles and an upper confinement part for confining the atmosphere around the metal strip downstream of said nozzles, said wiping system having first moving means for vertically moving the lower confinement part with respect to the pot. The nozzles are vertically movable relative to the pot. The wiping system also comprises second moving means for vertically moving the upper confinement part with respect to both the pot and the lower confinement part.

[0013] A solution that has also been proposed is a confinement box located downstream just over the nozzle, fed with a non-oxidizing gas by a dedicated system consisting in pipes. Such a system is however quite complex as the box has lateral and top sides and one has to manage the edge baffle system that is used to control the edge over coating. In addition, it must be located quite close to the strip to be efficient and keep the oxidizer level low compared to ambient environment.

[0014] An example of such a confinement box is described in document WO 2010/130883 A1 , which relates to a method for producing a metal band with a metal coating that provides protection against corrosion, comprising a step of passing the band through a containment area defined :

(a) at the bottom by the wiping line and the upper outer faces of the wiping nozzles,

(b) at the top by the upper portion of two containment casings placed on either side of the band just above the nozzles and having a height of at least 10 cm in relation to the wiping line, and

(c) at the sides by the side portions of said containment casings.

[0015] The atmosphere in the containment area has an oxidising potential less than that of an atmosphere containing 4 vol.-% oxygen and 96 vol.% nitrogen and greater than that of an atmosphere containing 0.15 vol.-% oxygen and 99.85 vol.-% nitrogen.

[0016] WO 2014/199194 A1 discloses an installation for hot dip coating a metal strip comprising an adjustable confinement box, comprising: means for moving said metal strip along a path, a pot for containing a melt bath, and a wiping system comprising at least two nozzles placed on either side of said path downstream the pot, the wiping system having a box with a lower confinement part for confining an atmosphere around the metal strip upstream of said nozzles and an upper confinement part for confining the atmosphere around the metal strip downstream of said nozzles, said wiping system having first moving means for vertically moving the lower confinement part with respect to the pot. The nozzles are vertically movable relative to the pot. The wiping system comprises second moving means for vertically moving the upper confinement part with respect to both the pot and the lower confinement part.

[0017] The confinement boxes like that of WO 2014/199194 A1 , although being very efficient to avoid oxidant potential of the wiping gas on its way toward the strip, create operational problems like creation of skimming that needs to be removed, or dirt due to zinc dust generation and need of slot cleaning as the access to the bath and the nozzle slot are not possible anymore.

[0018] Finally, the solutions of the type “confinement box” which have been known now for 30 years have proven to be industrially impracticable especially at high line speed, such as 100 mpm and over, and seem to be more and more abandoned industrially.

[0019] Moreover, the inventors have identified that, when the line speed is higher than 60 mpm and the coating thickness is below 30pm, specific defects occur that are not due to a film oxidation located between the bath surface and the air knife but rather to a film oxidation located after the wiping gas impingement spot because at that location the relative velocity of the wiping gas and the top of the coating is high whereas the coating is close to its finished status.

[0020] FIG. 4 shows a typical theoretical film evolution under the gas knife. The physics of the process indicates that, in the after-wiping area 11 , the coating thickness 12 can still decrease due to the high shear stress induced by the gas flow moving in the same direction than the strip. A high relative velocity induces a strong oxidation of the liquid film when the wiping gas is oxidizing the coating metal and thus impacts the final surface quality.

[0021] Document WO 2005/010229 A1 relates to a method and device for hot-dip coating a metal strip. Once it has left the molten bath, the still molten metal coating which is present on a surface of the metal strip is blown off the metal strip by means of at least one gas flow emanating from a stripping nozzle to achieve a specific coating strength for the final remaining coating on the surface which is respectively impinged upon by the gas flow.

[0022] The gas flow flowing off the respective surface of the metal strip is sucked off by means of a suctioning device which is arranged in the vicinity of the stripping nozzle and the surface of the metal strip.

[0023] In this way, the formation of a gas stream flowing parallel to the strip surface is reliably prevented, which on the one hand promotes the oxidation of the coating metal applied to the strip surface and on the other hand would promote the formation of equally undesirable drainage structures. In the procedure according to this invention, the gas stream is instead removed in a controlled manner, and as soon as possible after the gas stream has impacted on the strip surface assigned to it. The occurrence of surface defects and the risk of excessive oxidation of the coating material are thus reduced to a minimum.

[0024] In document US 2009/159233 A1 , a gas wiping nozzle is used which includes a primary nozzle portion and at least one secondary nozzle portion provided either or both above and below the primary nozzle portion. The secondary nozzle portion jets a gas in a direction tilted from the direction in which the primary nozzle portion jets the gas, and the secondary nozzle portion jets the gas at a lower flow rate than the primary nozzle portion. The gas wiping nozzle has a tip whose lower surface forms an angle of 60° or more with the steel strip. By jetting a gas from the secondary nozzle portion at predetermined conditions, the gas jet can scrape molten metal effectively. By controlling the angle between the lower surface of the gas wiping nozzle and the steel strip, the plating can be scraped more effectively. Thus, the molten metal can be appropriately scraped without excessively increasing the gas pressure. Consequently, splashes can be reduced.

[0025] Document US 2018/0291493 A1 discloses an installation for continuous hot-dip coating of a metal strip. The installation includes a tank containing a bath of molten metal, a metal strip running through the bath and a confined wiping device. The confined wiping device includes at least two wiping nozzles placed on each side of a path of the strip after the strip has left the bath of molten metal. Each nozzle has at least one gas outlet orifice and an upper face. The confined wiping device also includes a confinement box adjacent each upper face. The confinement boxes are open on a face which faces the strip. Each box includes at least one upper part and two lateral parts.

[0026] Document WO 2021/105228 A1 discloses a gas wiping device for controlling the thickness of a coating layer deposited on a running metal strip plated with molten metal in an industrial hot-dip installation, comprising a main nozzle unit and a secondary nozzle unit, to blow a wiping jet on the surface of the running strip, said main nozzle unit and secondary nozzle unit being respectively provided with a main and secondary chamber fed by pressurized non-oxidizing gas and with at least a main and secondary elongated nozzle slot formed in the tip of the respective main and secondary nozzle units, said tips comprising each an external top side, facing in use the downstream side of the running strip, and making an angle with the running strip surface, wherein the secondary nozzle unit is adjacent the main nozzle unit over the external top side of the main nozzle unit tip, so that the upper external surface of the secondary nozzle unit is designed to form, in use, an angle with the running strip surface comprised between 5° and 45°, wherein the thickness of the second slot opening is comprised between 1 .5 and 3 times the thickness of the first slot opening, characterized in that the tip of the secondary nozzle unit has an external top side prolonged downstream by a first baffle plate making a first angle in use with respect to the running strip, so as to form a gas confinement region.

Aims of the Invention

[0027] The present invention aims to overcome the drawbacks of prior art.

[0028] In particular, the invention is intended to improve the appearance of a strip dip-coated with a metal liquid layer whose thickness is adjusted by gas jet wiping, owing to decrease of wiping non-oxidizing gas dilution in air.

[0029] An aim of the invention is to reduce oxidation of the liquid film, leading to an improved appearance of the coating. [0030] A goal of the invention is also to prevent or minimize the well- known defects of the method such as surface waviness after wiping, cloudy aspect and sag lines, pinhole defects used to appear at high pressure and with thin coatings, etc.

Summary of the Invention

[0031] A first aspect of the present invention relates to a gas wiping device for controlling the thickness of a coating layer deposited on a running metal strip plated with molten metal in an industrial hot-dip bath, comprising a wiping unit to blow a main wiping gas jet on the surface of the running strip emerging upwards from the molten metal bath, said wiping unit being provided with a main chamber fed by pressurized non-oxidizing gas and with at least an elongated nozzle slot formed in the tip of the wiping unit, wherein the gas wiping device also comprises an upper confinement box and a lower confinement box being adjacent respectively to an upper external side of the wiping unit and a lower external side of the wiping unit, said confinement boxes being each provided with an opening to blow an additional flow of nonoxidizing gas, so that, in use, the distance from an end of the upper confinement box to the running strip and the distance from an end of the lower confinement box to the running strip are separately adjustable, wherein the gas wiping device also comprises at least one oxygen sensor provided in the confinement region to measure the concentration of oxygen in each of the confinement boxes, the flow rates of the additional flows blown through the respective openings being each also adjustable depending on the measured content of oxygen in the vicinity of the running strip in front of (or within) the confinement boxes, in relation to the respective distances between each confinement box and the running strip, and wherein the cross-section of the respective openings of the confinement boxes is comprised between 1.5 to 5 times the cross-section of the nozzle slot of the wiping unit.

[0032] Considering that the metal strip is generally running upwards

(see figures), the term “downstream” means beyond, considering the upward direction of the strip (e.g. downstream/beyond the gas impingement point/spot on the strip). Similarly, “upstream” means below in the same context.

[0033] According to particular embodiments, the device further comprises at least one of the following characteristics or a suitable combination thereof :

- the maximal height of each confinement box is ranging from 50 to 500 mm ;

- the opening in each of the confinement boxes is located in use more rearward than the nozzle slot of the wiping unit with respect to the running strip, preferably, the difference of the distance between said opening and the running strip and the distance between said nozzle slot of the wiping unit and the running strip is at least 10 times the nozzle slot opening ;

- the distance from an end of the upper confinement box to the strip and the distance from an end of the lower confinement box to the strip are comprised between 5 and 40 mm, preferably between 10 and 30 mm, and more preferably between 10 and 15 mm ;

- the distance from an end of the upper confinement box to the running strip or the distance from an end of the lower confinement box to the running strip is smaller than the distance from the wiping unit to the strip ;

- the upper confinement box and/or the lower confinement box are sealed to or delimited by, possibly in a flexible way, respectively the upper external side of the wiping unit and the lower external side of the wiping unit ;

- the adjustable confinement boxes comprise a panel that can be adjusted and made of a mobile element pivotable around a fixed element, or made of a mobile element sliding with respect of a fixed element, said position of the mobile element being changed using a jack, so as to allow setting of the distance between an end of the confinement boxes and the running strip.

[0034] Another aspect of the invention relates to a method for controlling the thickness of a coating layer deposited on a running metal strip in an industrial hot-dip installation, using the gas wiping device as described above, wherein : the distance from an end of the upper confinement box to the running strip and the distance from an end of the lower confinement box to the running strip are separately adjusted ; a main pressurized non-oxidizing gas jet is blown through the main nozzle unit on the metal strip plated with molten metal coming out of a hot-dip pot ;

- at least one additional pressurized non-oxidizing gas jet is blown through the opening of one of the adjustable confinement box on the metal strip plated with molten metal coming out of a hot-dip bath,

- an additional gas flow rate coming out of the opening of each confinement box is adjusted in relation to the distance between the confinement box and the running strip and based on a measured content of oxygen in (front of) the respective confinement boxes, said additional gas flow rate being controlled to be lower than 30% of the gas flow rate coming out of the wiping unit.

[0035] According to particular embodiments, the method further comprises at least one of the following characteristics or a suitable combination thereof :

- the controlled gas flow rate coming out of the opening of the confinement boxes is comprised between 5 and 30% for each box of the gas flow rate coming out of the wiping unit ;

- the controlled gas flow rate coming out of the opening of the confinement boxes is comprised between 10 and 20% of the gas flow rate coming out of the wiping unit ;

- said pressurized non-oxidizing gas is nitrogen. Brief Description of the Drawings

[0036] FIG. 1 schematically represents a hot-dip coating installation according to prior art.

[0037] FIG. 2 schematically represents a high speed wiping gas nozzle unit used in hot-dip coating installations according to prior art.

[0038] FIG. 3 schematically represents a hot-dip coating installation provided with a confinement box according to prior art.

[0039] FIG. 4 depicts a typical coating film changes while passing under the air knife.

[0040] FIG. 5 schematically illustrates an embodiment of the present invention, with two confinement regions located on either side of the nozzle unit.

[0041] FIG. 6 represents a simulation diagram of oxygen’s distribution (expressed in mole fraction) obtained with the embodiment of FIG. 5.

[0042] FIG. 7 represents a simulation diagram of velocity vectors

(expressed in m/s) obtained with the embodiment of FIG. 5.

Description of a Preferred Embodiment of the Invention

[0043] As mentioned here above, an aim of the invention is to reduce oxidation of the deposited liquid film, below and/or beyond the impingement point of the wiping gas jet on the running strip. The lower the oxidation of the liquid film, the more the visual appearance of the strip dip-coated with a metal liquid layer is improved.

[0044] The present invention relates to a strip dip-coated with a metal liquid layer whose thickness is adjusted by gas jet wiping. In order to decrease the oxidation of the liquid film, the gas wiping device comprises one (or two) adjustable confinement box(es), respectively located on one side or on each side, i.e. upstream and/or downstream, of a wiping unit. These confinement boxes allow to keep a low oxidizing atmosphere around the non-oxidizing gas jet of the wiping unit. As explained above, when a non-oxidizing gas is used to feed gas knives, as it is the case in the context of the present invention, the oxygen content of the ejected gas progressively increases as soon as the gas jet enters into ambient air thanks to conveying of the latter. This means that the oxygen content of the injected gas progressively increases with the distance from the nozzle exit point to the strip. The confinement boxes of the present invention intend thus to keep a low oxidizing atmosphere around the wiping unit.

[0045] As very schematically shown in the embodiment of FIG. 5, the inventors have discovered that the most practical way to do that consists in adding a first adjustable confinement box 6A, just beyond the wiping unit 5, called “upper confinement box 6A”, and a second adjustable confinement box 6B, just below the wiping unit 5, called “lower confinement box 6B”. As illustrated in FIG. 5, the wiping unit 5 blows a main wiping gas jet on the surface of the running strip 1 . The wiping unit 5 is provided with a main chamber 6 fed by pressurized nonoxidizing gas, and with at least an elongated nozzle slot 7 formed in the tip of the wiping unit 5, in order to blow the main flow. The respective confinement boxes 6A, 6B are adjacent to this wiping unit 5 on each side thereof. More precisely, the wiping unit 5 comprises an upper external side 15A and a lower external side 15B, and the upper confinement box 6A and the lower confinement box 6B are adjacent to (or delimited by) respectively the upper external side 15A of the wiping unit 5 and the lower external side 15B of the wiping unit 5. In one embodiment, the confinement boxes 6A and 6B may also be connected or attached to the wiping unit 5 for sealing purpose, while the position of the external ends of the confinement boxes is not related to the movement of the nozzle. The purpose of the secondary gas sprays is to provide additional non-oxidizing gas in the viscous entrainment of the main wiping flow, which justifies the proposed design in terms of position and opening (see below). The inventors have found indeed that in case no additional fluid is added to the box(es), internal vortices are formed with the recirculation of the wiping gas and so have a negative impact on the surface quality and the wiping efficiency.

[0046] Still according to the invention, the confinement boxes 6A and

6B are each adjustable, that is to say, in use, the distance 17A from an end of the upper confinement box 6A to the running strip 1 and the distance 17B from an end of the lower confinement box 6B to the running strip 1 are separately adjustable. The higher the confinement box-to-strip distance, the higher the oxygen content will get in the gas actually impinging onto the liquid metal. Thenceforth, the aim of the present invention is to maintain the confinement boxes 6A and 6B as close as possible to the running strip 1. However, the distance should not to be too short in order to avoid defects on the strip. Practically, a distance below 10mm is known to be very unsafe due to possible occurrence of scratches. The minimum acceptable confinement box-to-strip distance will depends on different parameters like the thickness of the strip, constraints of the process, etc. The adjustable confinement boxes 6A and 6B of the present invention allow to offer an adjustment in situ of the distance to the strip 1 , in function of the behaviour of the strip 1 . For example, the adjustment of the box can be performed by means known of the one skilled in the art, such as for example a confinement box panel comprising a rotary mobile element around a fixed element, or a mobile element sliding with respect of a fixed element, said position of the mobile element being changed using a jack. The distance 17A from an end of the upper confinement box 6A to the strip 1 and the distance 17B from an end of the lower confinement box 6B to the strip 1 are advantageously comprised between 5 and 40 mm. The smaller the distance the most efficient will be the devices. The inventor have found however that industrially the distance should range between 10 and 20 mm if the strip shape allows to operate so close. These distances 17A, 17B are also smaller than the distance 17 from the wiping unit 5 to the strip 1 , as explained below.

[0047] According to some embodiments, the height of the confinement boxes is generally comprised between 50 and 300 mm, possibly maximum 500 mm. Upper and lower boxes heights can be different, especially for the lower confinement box owing to operation reasons.

[0048] According to some embodiments, the mechanical adjustment is performed on demand, in function of the strip type, shape and other process conditions. This mechanical adjustment can be performed visually or automatically, using a camera. In practice the operator will mostly decide for the best adjustment in operation.

[0049] The confinement boxes 6A and 6B are each provided with an opening 13A, 13B to blow an additional flow of non-oxidizing fluid, respectively beyond and below, and for example parallel to the main flow of the wiping unit 5 (all these flows may also be essentially perpendicular to the strip). These additional flows are not to be considered as wiping jets like the main flow, since they are not aimed at modifying the main flow efficiency or disturb the latter. The function of these secondary flows is mainly to avoid recirculation problems close to the strip 1 , and to compensate the gas that escape naturally from the confinement boxes 6A, 6B.

[0050] The openings 13A, 13B are designed to provide additional flow rates not higher than 30% of the main flow rate. Therefore, the size of the openings 13A, 13B of the confinement boxes 6A, 6B in the present invention will be higher than the one of the main slot 7 and preferably between 1 .5 and 5 times, more preferably between 1.5 and 3 times, the main slot opening size. The additional flow rate will also be preferably between 5 and 30%, and more preferably between 10 and 20% of the main flow rate when the distance 17A and/or 17B is between 10 and 15mm to avoid a significant impact on the wiping effect due to the main jet. In addition to the flow rate, the gas velocity of the secondary gas jets must be low to minimize its interaction on the knife efficiency. Finally, the openings 13A, 13B can be of any usual shape, such as nozzles, slots, holes, etc.

[0051] According to some embodiments, the flow rates of the additional jets are each separately adjustable. Indeed, in relation to the respective above-mentioned distance 17A, 17B between the confinement box 6A and 6B and the running strip 1 , the oxygen content leaking between each of the confinement boxes 6A, 6B and the strip 1 will be different. As the distance 17A, 17B is modified in function of the behaviour of the strip 1 , this oxygen content is also modified. Further the flow rate of the additional jets can also be adjusted in relation to this content of oxygen in the vicinity of the running strip 1 . To this end oxygen sensors 18 can be further provided in the confinement region, preferably one or more sensors 18 in each confinement box. They may advantageously be located at different positions across the width of the strip. These sensors 18 allow to measure the concentration of oxygen in the confinement box(es) more or less close to the strip and this measurement then allows to modify either the distances 17A and/or 17B or the flow rate of the additional jets provided in the respective confinement boxes 6A, 6B, in order to reduce, when needed, the oxygen content in the confinement region, or to keep the oxygen content below a predetermined threshold. In this way, the gas flow rate is adaptable, depending on the concentration of oxygen measured by the sensor(s) 18.

[0052] Preferably the secondary flows will be different for each side and will be adjusted to reach the oxygen content target that is measured separately on each side.

[0053] According to some embodiments only one confinement box is used with only one secondary flow.

[0054] In order not to modify or deteriorate the wiping effect, the additional non-oxidizing gas must be smoothly laid down beside the main jet. This means in practice that the openings 13A, 13B should not be too close to the exit of the main slot 7, and rather should be typically at a distance of at least 10 times the main nozzle opening, away and behind the main nozzle exit 7 (see Fig. 5). More precise values are difficult to be given due to a variety of possible designs available for the confinement boxes according to the invention but the inventors preferably prescribe designs able to get a laminar deposit for the additional flow. Said otherwise, the difference of distance in use between the opening 13A, 13B and the running strip 1 and the distance in use between the nozzle slot 7 of the wiping unit 5 and the running strip 1 is over 10 times the nozzle opening.

[0055] In addition to flow rate considerations, the general geometry of the confinement box configuration is important in order to keep an efficient confinement effect. The inventors have observed that the confinement effect is improved if the upper external side 14A of the lower adjustable confinement box 6A and the lower external side 14B of the upper adjustable confinement box 6B are designed to form, in use, a mean angle with respectively the upper external side 15A of the wiping unit 5 and the lower external side 15B of the wiping unit 5 greater than 30°, and preferably comprised between 45° and 75°.

[0056] Preferably, the gas velocity at the exit of the opening 13A, 13B of the confinement boxes 6A, 6B is lower than 50% of the gas velocity at the exit of the wiping unit 5.

[0057] Preferably, the non-oxidizing pressurized gas used for the main flow and the secondary flows is nitrogen. [0058] An important particularity of the instant invention is that the system made of the secondary gas flows sprayed in the confinement boxes and the main gas flow injected by the main wiping nozzle should not be confused with multi-orifice wiping nozzles found in prior art, whose behaviour is completely different. The system of the present invention is closer to wiping devices having a confinement box, but in the present case with much more versatility.

[0059] The secondary gas sources in the confinement boxes have a totally different function in comparison with that disclosed in US 2009/159233 for example. The purpose of the secondary gas sources is not to add some type of extra wiping function as explained in this Japanese patent and this explains why it is proposed here some values for the opening as well as its somewhat distal location with reference to the main nozzle. The key point is a relative mass flow of the added non-oxidizing fluid. In the invention this added flow must be adjusted so as not to disturb the main flow but it has a role of compensating the gas that escapes naturally from the box. This added nitrogen flow for example through a 2D slot has a “wide” opening but not too wide because otherwise it would be difficult industrially to have a reasonably uniform flow all across the whole width of the wiping nozzle.

EXAMPLE

[0060] As an example, if the main slot 7 is 1 mm thick and the gas flow rate is 0.2kg/sec/m of pure nitrogen, the opening 13A, 13B will be 2mm thick with a flow rate from 0.02 to 0.04 kg/sec/m.

[0061] Typical data for a tested embodiment in the configuration of

FIG. 6 and 7 are the following :

- Main nozzle : 1 mm thick, inlet pressure : 100mbar, N2 flow : 0.2kg/sec/m of nozzle length ;

- Secondary openings : 5mm thick ; N2 flow : same order of magnitude as main flow ;

- Wiping distance (17) : 30mm ;

- Confinement box distance (17A, 17B) : 20mm ; - Length of confined zone : 2 x 60mm.

[0062] It is seen on FIG. 6 that the highest concentrations of oxygen are repelled far from the main nozzle flow region. According to the requirement of minimizing the interaction of the secondary gas jets with the wiping efficiency, as shown on FIG. 7, the gas velocity vectors are very low in the two confinement regions (strip speed : 1m/sec).

List of reference symbols

1 Moving strip

2 Liquid metal pot

3 Sink roll

4 Deflecting roll(s)

5 Wiping unit

6 Pressurized chamber

6A Upper adjustable confinement box

6B Lower adjustable confinement box

7 Nozzle opening of the wiping unit

8 Confinement enclosure (prior art)

9 Wiping gas jet

10 Coating upstream of gas jet impingement

11 Coating downstream of gas jet impingement

12 Coating thickness

13A Opening of the upper additional flow

13B Opening of the lower additional flow

14A Upper external side of the upper confinement box

14B Lower external side of the lower confinement box

15A Upper external side of the wiping unit

15B Lower external side of the wiping unit Gas impingement spot Distance from the wiping unit to the stripA Distance from the upper confinement box to the stripB Distance from the lower confinement box to the strip Oxygen sensor