WO2024110603A1

WO2024110603A1 - A hot dip coating device and a method of operating thereof

Info

Publication number: WO2024110603A1
Application number: PCT/EP2023/082895
Authority: WO
Inventors: Roel Marinus Maria MALLENS; Ernesto Montagna
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2022-11-24
Filing date: 2023-11-23
Publication date: 2024-05-30

Abstract

The present invention relates to a method of operating a hot dip coating device. The hot dip coating device comprises a snout, air knives, a container comprising a liquid metal bath. The method comprises positioning a first gutter comprising a first rim inside the snout. The method further comprises positioning a second gutter comprising a second rim facing the air knives, outside the snout; wherein the second gutter is positioned between the snout and the air knives to remove surface dross between the snout and the air knives using the second gutter such that the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane.

Description

A HOT DIP COATING DEVICE AND A METHOD OF OPERATING THEREOF

Field of the invention

The present invention relates to a method for operating a hot dip coating device. In a further aspect the present invention relates to a hot dip coating device comprising a snout, air knives and a container comprising a liquid metal bath. The present invention also relates to a method of coating a steel strip using a hot dip coating device.

Background of the invention

Hot dip coating is a well-known process for protecting a steel product against corrosion. A hot dip coating device is used to provide a metal coating such as zinc or aluminum on a moving metal sheet. In the context of the present invention, the metal sheet is also called a metal strip. The hot dip coating device typically comprises a container for a liquid metal bath having the coating material in use. An ingot is placed as partially immersed within the liquid metal bath of the container. At the operating temperature of the liquid metal bath, dissolution of iron from the steel strip into the metal bath can occur that forms intermetallic particles. Apart from the intermetallic particles, liquid metal oxidised by the oxygen in the surrounding air are formed as oxide particles. These oxide particles stay in the top layer of the liquid metal. These intermetallic particles as well as the oxide particles are known as dross.

An example of a hot dip coating device is a hot dip galvanizing device. It is known that in a continuous galvanizing line, the molten zinc bath is mostly saturated with iron, which dissolves from the steel strip that is continuously passed through the bath. The amount of iron above solubility at the prevailing bath temperature is present in dross. Dross present on the surface of the liquid metal bath is known as surface dross. The surface dross is also called floating dross or top dross. Surface dross originated from oxidised liquid metal are present at and around the air knives, where fresh liquid metal is in contact with surrounding air that contains oxygen. This surface dross is generated continuously and it accumulates at an area that is in between the snout and the air knives. Dross is known as a bottom dross when it stays within the liquid metal bath or present as submerged in the liquid metal bath. Both surface dross and bottom dross will have detrimental effects upon the final products that are manufactured from the steel strips. For example, small dross particles may be incorporated in the coated alloy layer and affect the appearance of the coated steel parts. This is particularly disadvantageous for steel strips, from which exterior automotive parts are shaped by pressing. The included particles may cause inhomogeneous deformation resulting in surface defects and irregularities such as tiny projections and bulges, even when present at the nonexposed side of the automotive parts. The unevenness in the surface of the steel parts results in undesired reflections that affect the appearance in an unacceptable way. Since the metallic coating is applied after the hot dip coating step, the applied coating should fulfil requirements such as the metal layer has to be without any dross or debris. This is important not only to be able to subject the coated steel strip to forming operations but also for the final appearance of the final steel product formed from the coated steel strip. Removing dross from the liquid metal bath of a hot dip coating device is therefore desired.

There are different methods used to remove dross from the metal bath of a hot dip coating device. A method that is used for removing the bottom dross may not be an efficient method for removing the surface dross. An existing solution of removing the surface dross or surface debris is to manually move it to the sides of the liquid metal bath with a rake and then scoop it out. This is typically operated by personnel or by a robot. This is not a continuous process and hence surface dross will accumulate in between the times of its removal. Moreover, it is a time consuming process. Further, these processes cannot be used to access areas that are difficult to be reached manually within the liquid metal bath. Korean patent application KR20140081453A describes a dross removal device that installs a suction unit at the end of a plating bath snout to remove the dross by suction. The suction unit is first bent in the outer horizontal direction at the end of the snout and is again bent in the upper direction to form a suction space. From the suction unit, molten zinc and dross are sucked by a transfer pipe and are transferred to a bucket. Other methods include moving the surface dross with electromagnetic devices. The electromagnetic device requires increased electrical power that can trouble the movement of the dross layer as the dross layer can have considerable mechanical strength. This also moves a substantial portion of the liquid metal together with the dross, which is not desirable. These existing solutions also easily disturb the surface layer of the dross and the liquid metal bath. This leads to submerging of the surface dross into the liquid metal bath making it difficult to further remove the surface dross from the liquid metal bath.

Another problem of most existing solutions of dross removal is that they are not operating continuously. This is disadvantageous as it will result in more surface dross accumulation within the metal bath, which is difficult at a later stage to remove in large quantities. On a commercial scale in the steel industry, for example for a hot dip galvanizing line, a single galvanizing bath is continuously operated for time periods of some weeks up to several months, before any shut down occurs in view of a necessary maintenance and replacement. In practice such a galvanizing line is used to produce galvanized steel strips of different qualities in consecutive runs. In particular a switch from producing steel strips that meet less stringent quality requirements to steel strips for high demanding end purposes is complicated and critical. After such a switch of production, if the freshly produced steel strip does not meet the required higher standards, it will likely be rejected. Hence, there is a requirement for a continuous removal of surface dross from the liquid metal bath of a hot dip coating device that will not affect the quality of steel production if switch of production occurs. Moreover, existing methods are troublesome to remove surface dross from places that are difficult to access within the liquid metal bath. Especially aforementioned existing methods cannot be operated without shutting down of the system in order to remove surface dross from non-accessible places. Hence, there is a need to find a reliable solution to efficiently remove dross especially surface dross from a liquid metal bath in a hot dip coating device.

Objectives of the invention

It is an object of the present invention to provide an efficient method for removing surface dross in a hot dip coating device from places that are difficult to access within the liquid metal bath of the hot dip coating device.

It is an object of the present invention to provide a continuous method for removing surface dross in a hot dip coating device.

It is also an objective of the invention to provide a method for removing surface dross that can be used in a commercial galvanizing line.

It is also an object of the invention to provide a hot dip coating device where surface dross is removed from places that are difficult to access within the liquid metal bath of the hot dip coating device.

It is also an object of the invention to provide a hot dip coating device where surface dross is continuously removed from places that are difficult to access within the liquid metal bath of the hot dip coating device.

It is another object of the invention to provide a method of coating a steel strip using a hot dip coating device in which surface dross is removed from places that are difficult to access within the liquid metal bath of the hot dip coating device. Description of the invention

One or more of these objectives are reached with a method of operating a hot dip coating device, comprising:

- a snout;

- air knives;

- a container comprising a liquid metal bath; the method comprising positioning a first gutter comprising a first rim inside the snout; wherein the method further comprises positioning a second gutter comprising a second rim facing the air knives, outside the snout; wherein the second gutter is positioned between the snout and the air knives to remove surface dross between the snout and the air knives using the second gutter such that the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane.

A hot dip coating device provides a metal coating on a moving metal sheet, comprising a liquid bath of metal coating material in use, wherein the metal coating material is to be provided on the moving metal sheet in use. During the hot dip coating process, surface dross accumulates on the surface of the liquid metal bath of the hot dip coating device. The metal strip leaves the liquid metal bath in a near-vertical direction after which the excess of the applied metallic coating is blown off with a high pressure air/gas wiping device which is known as air knives or gas knives. The air knives or gas knives use air or gas such as nitrogen for blowing the excess of the metallic coating. In the context of this invention, the air knives and gas knives can be used interchangeably. One of the critical areas where surface dross accumulates in the liquid metal bath is the area between the air knives and the snout or the area between where the strip exits the bath and the snout outer wall. Dross formed in the back flow from the air knives accumulates against the snout wall as the surface flow is typically in that direction. At some point, the surface dross will be pushed into the bulk of the metal bath leading to dross contamination. Also, this place is difficult to reach and to clean as the room between the snout and air knives is relatively small. So it is a critical area to be cleaned so that surface dross does not accumulate in large quantities over there. The present invention provides a solution to this problem. In a first aspect of the present invention, there is provided a method of operating a hot dip coating device. The hot dip coating device comprises a snout, air knives, a container comprising a liquid metal bath. The method comprises positioning a first gutter comprising a first rim inside the snout. The method further comprises positioning a second gutter comprising a second rim facing the air knives, outside the snout. The second gutter is positioned between the snout and the air knives to remove surface dross between the snout and the air knives using the second gutter such that the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. A gutter can be manufactured from the same material as the snout or can be made from a different material. The gutter typically comprises a bottom part and a front part that are connected to each other to form a ‘ l_ ’ shape. The bottom part of the gutter can be placed connecting to the outside or to the inside wall of the snout. The first gutter, in use, is positioned at an inner wall of the snout. The second gutter, in use, is positioned at an outside wall of the snout. The front part of the gutter comprises an edge portion at open end which is known as a rim. The front part of the first gutter comprises an edge portion at open end, which is known as a first rim and the front part of the second gutter comprises an edge portion at open end, which is known as a second rim. The second gutter is placed between the snout and the air knives in the hot dip coating device so that the second gutter can collect the surface dross from that area. The bottom part of the second gutter is placed connecting to the outside wall of the snout such that the second rim faces the air knives. The area between the front part and the bottom portion of the second gutter is a collector area that is used for collecting the surface dross from where it can be removed. The second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. This horizontal plane is the same plane where the surface bath level of the liquid metal bath also lies. Thus, the second rim, the first rim and the surface bath level of the liquid metal bath are in the same horizontal plane.

The dross removal device described by the Korean patent application KR20140081453A requires to bend the end of the snout in an outer horizontal direction and is again bent in the upper direction to form a ' I— I ' shape, in which the upper part is opened to form a space inside. The dross removal device of KR20140081453A can only be attached to the end of the snout. According to this prior art document, the end of the snout can only be bent either to the inside or to outside but it cannot be bent to both inside and outside. This prior art is silent on removing the dross from inside the snout when the snout has a bent formed to its outside.

The collector area of the second gutter of the present invention, collects the surface dross from where it can be removed by a removal device such as a pump that is connected to the second gutter. Typically, together with the surface dross, an amount of liquid metal also flows into the second gutter. To avoid too much flowing of the liquid metal into the second gutter, the second gutter is placed just below the surface bath level of the hot dip coating device. For the surface dross to freely flow from the liquid bath into the second gutter, the second rim of the second gutter is positioned just below the surface bath level of the hot dip coating device. If the second rim of the second gutter is placed above the surface bath level, no surface dross and/or liquid metal will flow into the second gutter. If the second rim of the second gutter is kept too deep from the surface bath level, then too much liquid metal will flow in and at some depth the surface dross will remain within the liquid metal bath. Also, if this distance is too large or the second rim is kept too deep, then if a pump is connected to the second gutter, the pumping effort will be increased in order to remove the surface dross from the second gutter to outside. Hence an optimal distance between the second rim of the second gutter and the surface bath level of the liquid metal bath is preferred. In one embodiment of the present invention, the method comprises placing the second rim of the second gutter below the surface bath level of the liquid metal bath. This allows a natural free flow of the surface dross from the liquid metal bath to the second gutter due to gravity. Typically, the second rim of the second gutter is placed at least 5 mm below the surface bath level of the liquid metal bath of the hot dip coating device. In one embodiment of the present invention, the method comprises placing the second rim of the second gutter at a distance of 5-20 mm below the surface bath level of the liquid metal bath, preferably at a distance of 5-15 mm below the surface bath level of the liquid metal bath, more preferably at a distance of 5-10 mm below the surface bath level of the liquid metal bath. This ensures that the second rim of the second gutter is at an optimal depth so that the surface dross freely flows from the liquid metal bath into the second gutter.

The modular design of the second gutter according to the invention, results in a device that can be easily assembled and disassembled to the outer snout wall and can be used in a commercial hot dip coating line such a hot dip galvanizing line. One embodiment of the present invention comprises a method of attaching the second gutter to the snout of the hot dip coating device. Preferably, the second gutter is made in one piece, for ease of manufacturing. This also allows easy assembling of the second gutter to the snout. The second gutter can be detachably connected to the outer snout wall, for example with fasteners through the supports. This allows to detach the second gutter from the snout for any purpose such as for repairing or for intermediate cleaning or for emptying. The present invention can be used for all types of coating using hot dip techniques and is particularly useful for coating a metal sheet with zinc or a zinc alloy, preferably a zinc aluminium alloy, a zinc magnesium alloy or a zinc aluminium magnesium alloy, or with aluminium or an aluminium alloy, preferably an aluminium silicon alloy. In an embodiment, the second gutter is removably attached to the snout. This allows for a detachable connection of the second gutter to the snout. In another embodiment of the snout, the vertical position of the second gutter can be adjusted with respect to the surface bath level of the liquid metal bath. Further, this allows for an easy adjustment of the second gutter from the snout which has the advantage that the vertical distance between the surface bath level outside the second gutter and the liquid level of the fluid within the second gutter can easily be adjusted according to the required specifications. In a possible embodiment, the method comprises maintaining the liquid level of the second gutter, lower than the surface bath level of the liquid metal bath. Typically, the surface bath level of the liquid metal bath in the container can be adjusted by controlling the depth of an ingot in the liquid metal bath. The liquid level of the second gutter can be lowered by pumping the liquid from the second gutter. This ensures that the amount of the liquid metal which is collected together with the surface dross in the second gutter can be minimised. Another embodiment of the present invention comprises controlling the distance between the second rim and the first rim such that the second rim and the first rim are arranged in the same horizontal plane. The distance can be controlled using a controller attached to the second gutter and/or to the first gutter. The controller allows to change the distance between the surface bath level and the second rim of the second gutter and/or the surface bath level and the first rim of the first gutter so that it is ensured that the second rim, first rim and the surface bath level of the liquid metal bath lie in the same horizontal plane.

As explained before, the place between the air knives and the outer wall of the snout is difficult to be reached and to be cleaned, this critical area needs to be cleaned regularly and also continuously. This makes sure that the surface dross does not accumulate in large quantities over there and does not prohibit the operation of the hot dip coating line. In a possible embodiment, the method comprises continuously removing surface dross from the surface bath level of the hot dip coating device. For continuous removal, the second gutter can be connected to a pump that can continuously remove surface dross from the second gutter. A continuous operation also prohibits any requirement for shutting down of the system for specific periods. The pumping system can remove the surface dross into a separate dross collector container through transfer tubes. The first gutter can also be connected to a pump that can continuously remove surface dross from the first gutter. Thus, separate pumps can be connected to the second gutter and to the first gutter. In one embodiment of the present invention the method comprises pumping out the surface dross from the second gutter and/or from the first gutter. The pumping can be continuously performed so that the collected surface dross is continuously removed from the second gutter. This also allows to maintain the surface bath level outside the second gutter to be above the liquid level of the fluid within the second gutter. In a possible embodiment, the flow rate of the surface dross into the second gutter and/or into the first gutter is in the range of 0.03- 0.24 kg/cm s. The flow rate of the surface dross is expressed as dross per cm of gutter length, per second.

In a further aspect, the present invention relates to a hot dip coating device. The hot dip coating device comprises a snout, air knives, a container comprising a liquid metal bath and a first gutter comprising a first rim inside the snout. The hot dip coating device further comprises a second gutter comprising a second rim facing the air knives, outside the snout. The second gutter is positioned between the snout and the air knives to remove surface dross between the snout and the air knives using the second gutter such that the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. The first gutter is placed at the inside wall of the snout. The second gutter is placed at the outside wall of the snout. The second gutter is placed between the snout and the air knives in the hot dip coating device. As explained before, this position is important because a lot of surface dross is generated by the air knives in this area and this surface dross flows towards the snout. On installation, the second rim of the second gutter is configured to align with the surface bath level of the hot dip coating device. The second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. This also ensures that the second rim and the first rim are configured to align with the surface bath level of the hot dip coating device in the same horizontal plane. However, the liquid level inside the second gutter and the liquid level inside the first gutter may not be on the same horizontal plane. The second rim of the second gutter is placed below the surface bath level of the liquid metal bath. This position of the second gutter ensures that the surface dross that is present in the area between the snout and the air knives can easily flow into the second gutter. In another embodiment of the invention, the second rim of the second gutter is placed at a distance of 5-20 mm below the surface bath level of the liquid metal bath, during use, preferably at a distance of 5-15 mm below the surface bath level of the liquid metal bath, during use, more preferably at a distance of 5-10 mm below the surface bath level of the liquid metal bath, during use. Since the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane, this also results in having the first rim of the first gutter placed at a distance of 5-20 mm below the surface bath level of the liquid metal bath, during use, preferably at a distance of 5-15 mm below the surface bath level of the liquid metal bath, during use, more preferably at a distance of 5-10 mm below the surface bath level of the liquid metal bath, during use. The present invention provides a device of great simplicity, which can be easily implemented in a commercial hot dip coating device. In one embodiment, the second gutter is attached to the snout of the hot dip coating device. The second gutter can be removably attached to the snout. In such a case, the bottom part of the second gutter is removably attached to the outside wall of the snout. When it is removably attached, it can be removed from the system, as per requirement. In such a case, the hot dip coating device can still function as a normal hot dip coating device without having a second gutter. The second gutter can also be permanently integrated to the outside wall of the snout. In one embodiment the second gutter is part of the snout. In one embodiment of the present invention, a control device for controlling the distance between the second rim and the first rim is attached to the second gutter. Thus, during use, the second rim and the first rim are arranged in the same horizontal plane. This ensures a proper aligning of the second gutter vertically with the surface bath level of the liquid metal bath; where the second gutter is attached to the snout. The controllers ensure that the liquid level of the fluid in the second gutter is always lower than the surface bath level of the liquid metal bath. An alignment control means is used for this purpose. If the second gutter is full, no more surface dross will flow into the second gutter. This will make the surface dross to start accumulating on the top surface of the liquid metal. A removal system such as a pump may optionally be attached to remove the collected surface dross and/or the liquid metal from the second gutter. This will ensure that the second gutter is not full during operation. The removal system connected to the second gutter for removing the collected surface dross will ensure that the hot dip coating device can operate continuously, if required. Typically, a small amount of liquid metal may also be removed together with the surface dross. In one embodiment, the hot dip coating device comprises a pump to remove surface dross and/or liquid metal from the second gutter. This ensures that the level of the surface dross in the second gutter is lower than the level of surface dross in the liquid metal bath so that newly formed surface dross can easily flow into the second gutter in a continuous manner. The pump can also be operated on specific intervals of time to pump out the surface dross.

In one embodiment the hot dip coating device is a hot dip galvanizing device. In a hot dip galvanizing device, a steel strip runs through a molten zinc bath comprising of zinc, aluminum, iron, and unavoidable impurities, and optionally a small amount of one or more alloying elements. For coating a steel metal sheet, it is often preferred if the liquid bath of metal is a liquid bath of zinc or a zinc alloy, preferably a zinc aluminium alloy, a zinc magnesium alloy or a zinc aluminium magnesium alloy, or wherein the liquid bath of metal is a liquid bath of aluminium or an aluminium alloy, preferably an aluminium silicon alloy. In the context of the invention, the metal sheet or the metal strip usually has a length of at least a few hundred meters, a width of up to approximately 2 meters and a thickness of at most a few millimeters. The second gutter in the hot dip galvanizing device is placed at an outside wall of the snout such that it is placed between the snout and the air knives. The second rim of the second gutter is configured to align below the zinc bath level of the hot dip galvanizing device. In a hot dip galvanizing device, the second gutter collects surface dross or surface debris and removes it by for example, pumping it from the second gutter to the outside.

One or more of the objectives of the invention are realized by providing a method of coating a metal sheet using the abovementioned hot dip coating device comprising passing the steel strip through the snout into the container comprising the liquid metal bath, passing the steel strip out of the liquid metal bath with a sink roll, after the steel strip leaves the liquid metal bath, adjusting the coating amount of molten metal of the steel strip with air knives, collecting the surface dross inside the snout by using the first gutter to remove the surface dross from the liquid metal bath, collecting the surface dross between the snout and the air knives by using the second gutter to remove the surface dross from the liquid metal bath. The sink roll is provided in the container below the surface level of the liquid metal bath. The coating amount of molten metal of the steel strip is adjusted by blowing off the excess of applied metal coating with air knives. As mentioned above, the hot dip coating device comprises the first gutter comprising a first rim inside the snout. The hot dip coating device comprises the second gutter comprising a second rim facing the air knives outside the snout. In a possible embodiment of the invention, the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. In a possible embodiment of the invention, the second gutter is positioned between the snout and the air knives to remove surface dross between the snout and the air knives using the second gutter such that the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. In a possible embodiment of the invention, the second rim of the second gutter is placed below the surface bath level of the liquid metal bath. In this embodiment, as the second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane, the first rim of the first gutter is also placed below the surface bath level of the liquid metal bath. In a possible embodiment of the invention, the second rim of the second gutter is placed at a distance of 5-20 mm below the surface bath level of the liquid metal bath, preferably at a distance of 5-15 mm below the surface bath level of the liquid metal bath, more preferably at a distance of 5-10 mm below the surface bath level of the liquid metal bath. In a possible embodiment, the second gutter is attached to the snout of the hot dip coating device. In another possible embodiment, the second gutter is part of the snout. In a possible embodiment of the invention, a second gutter is attached to the outside wall of the snout and a first gutter is attached to the inside wall of the snout.

Brief description of the drawings

The invention is further explained by the following figures.

FIG. 1 shows a schematic of a production line comprising an annealing section and a hot dip coating device, according to the present invention;

FIG. 2 shows an embodiment of the present invention comprising a snout and a hot dip coating device;

FIG. 3 shows another embodiment of the present invention comprising a snout and a hot dip coating device;

FIG. 4 shows a schematic view of gas flows in a snout according to the present invention; and

FIG. 5 shows a schematic view of the second gutter and first gutter attached to a snout, according to the present invention.

Detailed description of the drawings

Coated steel strips are used for manufacturing parts for example in the automotive industry, where the applied coating is required to fulfil requirements like the metal layer should not be with any dross as that can contaminate a subsequently applied coating. The present invention embodiments reduce the chance of any contamination in a coating by removing surface dross from the liquid metal bath of a hot dip coating device.

Figure 1 relates to a schematic of a production line comprising an annealing section 1 and a hot dip coating device 19. The annealing section 1 for a steel strip 2 which, as such, is well known to a person skilled in the art. The annealing section 1 comprises a first heating part 3, for example a direct flame furnace, a connecting chamber 4 connecting the first heating part 3 and a second heating part 5, for example a radiant tube furnace. In the connecting chamber 4 one or more devices 6 are located, arranged on one or both sides of the steel strip 2 for oxidising the steel strip 2 using an oxidising gas mixture. The steel strip 2 is processed or annealed, in the direction of (A) and optionally the annealing section comprises a cooling part 22. The production line further comprises a hot dip coating device 19 which for example can be a hot dip galvanising device. The metallic coating is applied by means of the hot dip coating wherein in a continuous or semi-continuous process, the metal steel strip 2 passes through a bath of molten metal of for instance Zn, Zn + Fe alloy, Zn + Al or Zn + Mg + Al. The removal of excess metallic coating from the moving metal strip 2 controls the thickness of the applied metallic coating. The steel strip 2 is passed from the annealing section 1 into the hot dip coating device 19 through the introduction point 31 , using hot bridles 16. The hot dip coating device 19 further comprises sink rolls 28 to pass the steel strip 2 out of the metal bath 18.

Figure 2 shows a schematic of the snout 50 where the steel strip 2 is passed from the annealing section 1 through the introduction point 31 into the hot dip coating device 19. The steel strip 2 is passed from the annealing section 1 into a hot dip coating device 19 using hot bridles 16. The hot dip coating device 19 comprises a liquid metal bath 18 having the surface bath level 20. An ingot 15 is placed as partly immersed within the liquid metal bath 18. The hot dip coating device 19 also comprises sink rolls 28 to pass the steel strip 2 out of the liquid metal bath 18. The steel strip 2 passes through the sink rolls 28 into the air knives 14. Air knives 14 are used for adjusting the coating amount of molten metal of the steel sheet.

Figure 3 shows a schematic of a snout 50. A second gutter 60 is attached to the outside wall of the snout and a first gutter 160 is attached to the inside wall of the snout. The second rim of the second gutter is placed below the surface bath level 20 of the liquid metal bath 18. The second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. Thus, the first rim of the first gutter is also placed below the surface bath level 20 of the liquid metal bath 18. Figure 3 shows the first gutter that is attached to the inside wall of the snout. The steel strip 2 is passed through the snout 50 into the liquid metal bath 18 of the hot dip coating device 19 and it leaves the liquid metal bath 18 in a near-vertical direction into the air knives 14. The surface dross 80 that is present in between the snout 50 and the air knives 14 flows into the second gutter 60 where it is collected and removed for example using a pump.

Figure 4 shows a schematic view of a snout 50. The arrows 11 , 12 denote the flow of various gases within the snout 50. A gas having a gas flow 11 is passed from the hot bridle 16 into the snout 50. At its one end, the snout 50 comprises a first opening through which the steel strip is introduced from a preceding annealing section 1. At its other end, the snout 50 comprises a second opening wherein the second opening is immersed within the liquid metal bath. Optionally a different gas having a gas flow 12 can be introduced into the snout 50 through a plurality of injection points or inlets 9. Figure 4 shows two of these injection points or inlets that are placed in opposing directions on the wall of the snout 50. The second gutter 60 is attached to the outside wall of the snout 50. The first gutter 160 is attached to the inside wall of the snout 50. The second gutter 60 comprises an edge portion at its open end, which is known as the second rim 63. The second rim 63 of the second gutter 60 is aligned to be placed just below the surface bath level 18 of the liquid metal bath 20 so that the surface dross 80 from outside the snout can easily flow into the second gutter 60. Similarly, the first rim 163 of the first gutter 160 is aligned to be placed just below the surface bath level 18 of the liquid metal bath 20 so that the surface dross 80 from inside the snout can easily flow into the first gutter 160. The second rim of the second gutter and the first rim of the first gutter are arranged in the same horizontal plane. The level of the surface dross in the second gutter and the first gutter may not be on the same level. By keeping the second rim of the second gutter 60 lower than the level of surface dross in the liquid metal bath, the surface dross 80 can freely flow into the second gutter 60 by the aid of gravitational force.

Figure 5 shows a zoomed in view of an embodiment of the present invention. The second gutter 60 in use, is placed at an outside wall of the snout 50. The second gutter 60 comprises a bottom part 61 and a front part 62 that are connected to each other to form a collector area in between. The bottom part 61 of the second gutter is attached to the outside wall of the snout 50. The second rim 63 of the second gutter is aligned to be placed just below the surface bath level 20 of the liquid metal bath so that the surface dross 80 can easily flow into the second gutter 60. The second rim of the second gutter is placed, for example, 7 mm below the surface bath level of the liquid metal bath. The second gutter 60 is placed to the outside wall of the snout 50 such that the second rim 63 of the second gutter faces the air knives 14. By keeping the liquid level 66 of the second gutter 60 lower than the surface level in the liquid metal bath, the surface dross 80 freely flows into the second gutter 60, aided by the gravitational force. The surface dross 80 collected in the collector area that is present in between the front part and the bottom part of the second gutter, can be removed, for example, by pumping out. The first gutter 160 is placed at an inside wall of the snout 50. The bottom part of the first gutter is attached to the inside wall of the snout 50. The second rim 63 of the second gutter 60 and the first rim 163 of the first gutter 160 are arranged in the same horizontal plane. The first rim 163 of the first gutter is aligned to be placed just below the surface bath level 20 of the liquid metal bath so that the surface dross 80 from inside the snout can easily flow into the first gutter 160. By keeping the liquid level of the first gutter lower than the surface level in the liquid metal bath, the surface dross 80 can freely flow into the first gutter 160. Although the invention has been discussed in the foregoing with reference to an exemplary embodiment of the hot dip coating device of the invention, the invention is not restricted to these particular embodiments which can be varied in many ways without departing from the invention. The discussed exemplary embodiments shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiments are merely intended to explain the wording of the appended claims without intent to limit the claims to these exemplary embodiments. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using these exemplary embodiments.

Claims

Claims A hot dip coating device (19) comprising:

- a snout (50);

- air knives (14);

- a container (22) comprising a liquid metal bath (18);

- a first gutter (160) comprising a first rim (163) inside the snout (50); wherein the hot dip coating device (19) further comprises a second gutter (60) comprising a second rim (63) facing the air knives (14) outside the snout (50); wherein the second gutter (60) is positioned between the snout (50) and the air knives (14); to remove surface dross (80) between the snout (50) and the air knives using the second gutter (60) such that the second rim (63) of the second gutter (60) and the first rim (163) of the first gutter (160) are arranged in the same horizontal plane. The hot dip coating device (19) according to claim 1 , wherein the second rim (63) of the second gutter is placed at a distance of 5-20 mm below the surface bath level (20) of the liquid metal bath (18), during use, preferably at a distance of 5-15 mm below the surface bath level (20) of the liquid metal bath (18), during use, more preferably at a distance of 5-10 mm below the surface bath level (20) of the liquid metal bath (18), during use. The hot dip coating device (19) according to any one of claims 1-2, wherein the second gutter (60) is attached to the snout (50) of the hot dip coating device (19). The hot dip coating device (19) according to any one of claims 1-2, wherein the second gutter (60) is part of the snout (50). The hot dip coating device (19) according to any one of claims 1-4, wherein a control device for controlling the distance between the second rim (63) and the first rim (163) is attached to the second gutter (60) . The hot dip coating device (19) according to any one of claims 1-5, wherein the hot dip coating device (19) comprises a pump (15) to remove surface dross (80) and/or liquid metal from the second gutter (60). The hot dip coating device (19) according to any one of claims 1-6, wherein the hot dip coating device (19) is a hot dip galvanizing device. 8. A method of coating a steel strip (2) using a hot dip coating device (19) according to the any one of the claims 1-7, comprising passing the steel strip (2) through the snout (50) into the container (22) comprising the liquid metal bath (18), passing the steel strip (2) out of the liquid metal bath (18) with a sink roll (28), after the steel strip (2) leaves the liquid metal bath (18), adjusting the coating amount of molten metal of the steel strip (2) with air knives (14), collecting the surface dross inside the snout (50) by using the first gutter (160) to remove the surface dross from the liquid metal bath (18), collecting the surface dross between the snout (50) and the air knives (14) by using the second gutter (60) to remove the surface dross from the liquid metal bath (18).

9. The method according to claim 8, wherein the second rim (63) of the second gutter (60) is placed at a distance of 5-20 mm below the surface bath level (20) of the liquid metal bath (18), preferably at a distance of 5-15 mm below the surface bath level (20) of the liquid metal bath (18), more preferably at a distance of 5-10 mm below the surface bath level (20) of the liquid metal bath (18).

10. The method according to any one of the claims 8-9, comprising maintaining the liquid level (66) of the second gutter (60), lower than the surface bath level (20) of the liquid metal bath (18).

11. The method according to any one of the claims 8-10, comprising controlling the distance between the second rim (63) and the first rim (163) such that the second rim (63) and the first rim (163) are arranged in the same horizontal plane.

12. The method according to any one of the claims 8-11 , comprising continuously removing surface dross (80) from the surface bath level (20) of the hot dip coating device (19).

13. The method according to any one of the claims 8-12, comprising pumping out the surface dross (80) from the second gutter (60) and/or from the first gutter (160).

14. The method according to any one of the claims 8-13, wherein the flow rate of the surface dross into the second gutter (60) and/or into the first gutter (160) is in the range of 0.03-0.24 kg/cm s. Use of the hot dip coating device (19) according to any one of the claims 1-7 in a hot dip coating method for producing a coated steel strip.