Apparatus for continuous hot-dip coating of steel strip and method therefor

Abstract

The invention relates to a device for the continuous hot-dip coating of a steel strip (1), comprising:-a liquid metal bath (2); -a gas wiping nozzle equipped with a lip (3) and a lip cleaning tool (4) movable along the nozzle lip; a closing box (5) closing the atmosphere around the steel strip path above the wiping nozzles; a lip cleaning space (6) arranged between the gas wiping nozzle and the closed box; -means for feeding a non-oxidizing gas into the closed tank (5); and a device for feeding gas into the cleaning space (6).

Description

Technical Field

[0001] This invention relates to equipment for continuous hot-dip galvanizing of metal strips, particularly steel strips. Background Technology

[0002] In many industrial applications, steel sheets are coated with a protective coating to prevent corrosion. This type of metal sheet is used in various industries to manufacture a wide variety of components, especially those exposed to the end user's view.

[0003] To obtain this type of sheet, a continuous dip-coating apparatus is used, in which a steel strip is immersed in a bath of molten metal. The molten metal is not limited. For example, it can be zinc-based or aluminum-based. It can contain other chemical elements, iron dissolved from the steel strip, unavoidable impurities, and possible additives such as lead and antimony.

[0004] In recent years, aluminum, magnesium, and even silicon have been added to zinc-based molten metal baths to improve the corrosion resistance of coated steel sheets. The bath temperature depends on the properties of the metal: in the case of zinc-based baths, the temperature is set between 380°C and 490°C.

[0005] The corrosion resistance of components coated in this way is ensured by the coating, the thickness of which is typically achieved through gas wiping.

[0006] Before passing through the molten metal bath, the steel strip first passes through an annealing furnace in a reducing atmosphere to recrystallize after the significant work hardening associated with the cold rolling operation and to prepare its surface chemical state to facilitate the chemical reactions required for the quenching operation itself. Depending on the steel grade, the steel strip is heated to approximately 550°C to 900°C for the time required for recrystallization and surface preparation. It is then cooled to a temperature close to that of the molten metal bath using a heat exchanger.

[0007] The steel strip is immersed in the bath under tension by submerged rollers. After emerging from the bath, the steel strip passes through a wiping device used to adjust the thickness of the liquid metal coating on the steel strip.

[0008] During the wiping operation, gas is blown toward the steel strip through wiping nozzles. The wiping nozzles are equipped with lips to control the nozzle gap, thereby controlling the wiping pressure and coating thickness.

[0009] During production, the spray lip may become clogged with solidified wiping metal droplets, dust, or any other particles. Localized clogging of the lip can create wiping defects along the coated steel strip. This is detrimental to the appearance quality.

[0010] To clean the lips without interrupting production, the lips are equipped with a lip cleaning tool that moves along the nozzle gap from one end of the lip to the other. Solidified wiping metal droplets or any particles are loosened and mechanically removed from the lip by the passage of this cleaning tool. To reach the tip of the lip across its entire width, the cleaning tool must move from one end to the other.

[0011] The cleaning tool can reach the tip of the lips from inside the wiping nozzle. However, its efficiency is increased if the lip cleaning tool encounters solidified droplets or any particles directly from the outside of the nozzle. When operating the cleaning tool from outside the nozzle, free space, including the operating mechanism, must be left in front of the lips for translation.

[0012] After wiping, the liquid coating is sensitive to oxidation. When the coating contains easily oxidized elements (such as magnesium or aluminum), oxides are more likely to appear on the surface of the metal bath. Oxides are known to cause surface defects and impair the appearance of coated steel strips.

[0013] For this reason, it is known in the prior art to enclose a steel strip covered with a liquid coating in an atmosphere during the solidification of the liquid coating. Such an enclosure is commonly referred to as a sealed box. It typically maintains an atmosphere with limited oxidizing capacity.

[0014] In configurations where free space must be maintained in front of the lip of the wiping nozzle, the enclosed chamber cannot reach the wiping nozzle. Therefore, ambient air enters this free space, and the oxidizing capacity of the enclosed atmosphere increases. Consequently, the appearance quality of the coating is compromised.

[0015] Patent WO2023 / 088625 discloses a gas wiping device comprising an upper enclosed chamber and a lower enclosed chamber, both adjacent to the corresponding upper and lower outer sides of a wiping nozzle. Each enclosed chamber has a corresponding opening for blowing a non-oxidizing gas stream. According to this patent, the flow rate can be adjusted based on the oxygen content measured near the running steel belt.

[0016] However, these public disclosures do not mention that ambient air enters the enclosed box through the free space used for lip cleaning tools. Summary of the Invention

[0017] The present invention aims to overcome the shortcomings of the prior art.

[0018] The object of this invention is to provide an apparatus capable of producing coated steel strips 1 with very good appearance and surface quality.

[0019] This is achieved using equipment for continuous hot-dip galvanizing of steel strip 1, which includes:

[0020] - Liquid metal bath 2,

[0021] - A gas wiping nozzle equipped with a lip part 3 and a lip cleaning tool 4 that can move along the lip part.

[0022] - Enclosed chamber 5, which seals off the atmosphere around the steel strip path above the wiping nozzle.

[0023] - Lip cleaning space 6, which is disposed between the gas wiping nozzle and the enclosed box.

[0024] - A device for introducing non-oxidizing gases into the sealed chamber 5.

[0025] - A device for introducing non-oxidizing gases into the clean space 6.

[0026] The non-oxidizing gas can be any gas with a lower oxidizing power than air. For example, it can be nitrogen or argon or a mixture thereof, containing less than 20% by volume, preferably less than 10% by volume, or even 5% by volume, of oxygen.

[0027] The enclosed chamber and clean space can be supplied with non-oxidizing gas by any means, such as through pipes or by a pump that blows the non-oxidizing gas. Attached Figure Description

[0028] The present invention is illustrated by the following figures, which are not limiting:

[0029] Figure 1 A device according to the present invention is shown.

[0030] Figure 2 This is a perspective view showing the translation of the lip cleaning tool. For ease of understanding, the equipment behind the steel belt is not shown.

[0031] Figure 3 A preferred embodiment is shown in which non-oxidizing gases flow through a perforated plate into a space used for cleaning tools. For ease of understanding, the portion of the device above the bath is shown in the accompanying drawings.

[0032] Figure 4 This is a perspective view showing a preferred embodiment of the translation of the noise reduction plate 9. For ease of understanding, the equipment behind the steel strip is not shown.

[0033] Figure 5 This is a top view of a sealed enclosure, showing a preferred embodiment including a lateral seal.

[0034] Figure 6 The side of the lip cleaning space is shown.

[0035] The attached figures are labeled with the following reference numerals:

[0036] 1: Steel strip

[0037] 2: Metal bath

[0038] 3: Spray on lips

[0039] 4: Lip cleaning tools

[0040] 5: Sealed box

[0041] 6: Lip Cleaning Space

[0042] 7: Pipelines for supplying non-oxidizing gases

[0043] 8: Perforated plate

[0044] 9: Noise Reduction Panel

[0045] 10: Flexible seal between the noise reduction panel and the enclosed enclosure

[0046] 11: The side of the lip cleaning space. Detailed Implementation

[0047] The wiping nozzle according to the present invention is provided with a lip cleaning tool 4 that is operated from outside the nozzle.

[0048] The inventors have discovered that injecting non-oxidizing gases into both the enclosed box 5 and the lip cleaning space 6 significantly improves the appearance quality of the coated steel strip 1.

[0049] Preferably, both the enclosed chamber and the lip cleaning space are supplied with the same non-oxidizing gas source. For example, they are supplied through a conduit 7 that supplies non-oxidizing gas.

[0050] Preferably, the flow rate of the non-oxidizing gas delivered into the lip cleaning space can be adjusted.

[0051] Preferably, the device is equipped with a means for adjusting the flow rate of the non-oxidizing gas delivered into the lip cleaning space.

[0052] Advantageously, the non-oxidizing gas is introduced into the lip cleaning space through a perforated plate 8. The perforated plate is a device for regulating the flow rate of the non-oxidizing gas introduced into the lip cleaning space.

[0053] The holes in the perforated plate 8 are designed to determine the magnitude of the gas flow rate into the lip cleaning space. Those skilled in the art will be able to design the surfaces of the perforated plate that are penetrated by the holes based on the oxidizing atmosphere targeted in the enclosed chamber and the lip cleaning space.

[0054] In a preferred embodiment, the perforated plate 8 is part of the conduit 7 for supplying non-oxidizing gases.

[0055] Preferably, the flow rates of the non-oxidizing gas supplied to the sealed chamber and the flow rates of the non-oxidizing gas supplied to the lip cleaning space satisfy the following equation:

[0056]

[0057] in:

[0058] C is the flow rate of the non-oxidizing gas fed into the closed box (5), and

[0059] R is the flow rate of non-oxidizing gas delivered into the lip cleaning space (6).

[0060] The inventors have discovered that surface quality is improved when the ratio C / R is between 0.1 and 10.0. In one preferred embodiment, such a ratio is set by adjusting the value of R, for example, by means of a device for adjusting the flow rate of non-oxidizing gas delivered into the lip cleaning space. In another preferred embodiment, such a ratio is set by adjusting the value of C. In yet another preferred embodiment, both the values ​​of R and C are adjusted.

[0061] Advantageously, the C / R ratio is 0.2 to 5.0, or even 0.5 to 2.0.

[0062] In a preferred embodiment, the device includes two metal plates referred to as "noise-damping plates" 9, which function to prevent the wiping airflows from each nozzle from converging in lateral areas where the steel belt is absent. The insertion of the noise-damping plates 9 is useful, particularly for preventing acoustic vibrations of very large amplitudes caused by the frontal convergence of the wiping airflows.

[0063] The noise reduction plate 9 can move along the width direction according to the width of the steel strip, so that steel strips of different widths can pass through the equipment.

[0064] In a preferred embodiment, a flexible seal 10 is mounted at the end of the enclosed box 5. This seal maintains contact with the movable noise-reducing plate 9 regardless of its width or position. The inventors have discovered that the presence of the seal improves the appearance quality of the coating.

[0065] This preferred embodiment is Figure 4 It is shown as a perspective view. It is also... Figure 5 It is shown as a top view.

[0066] The flexible seal 10 ensures lateral sealing of the enclosed area around the steel strip.

[0067] Preferably, the flexible seal is capable of withstanding operating temperatures above 200°C, advantageously above 450°C.

[0068] Preferably, an additional seal is provided for the lip cleaning space by any means that ensures its airtightness.

[0069] The lip cleaning space must remain open along the area where the lip cleaning tool is sprayed so that it can be moved. Leaks in the area in front of the lip cleaning tool cannot be sealed.

[0070] In other areas of the lip cleaning space, a seal is ensured on both sides 11. Similarly, sealing prevents leakage towards the rear of the lip cleaning space or in areas in contact with other components such as the wiping nozzle or lips. For example, the seal of the lip cleaning space is provided by one or more seals. As a seal, an O-ring can be installed in a circular cross-section. Both longitudinal and transverse seals can also be used.

Claims

1. An apparatus for continuous hot-dip galvanizing of steel strip (1), comprising: - Liquid metal bath (2); - A gas wiping nozzle equipped with a lip (3) and a lip cleaning tool (4) capable of moving along the lip of the nozzle. - A closed box (5) that seals off the atmosphere around the steel strip path above the wiping nozzle; - Lip cleaning space (6), which is disposed between the gas wiping nozzle and the enclosed box; - A device for introducing non-oxidizing gases into the enclosed box (5); - A device for introducing non-oxidizing gases into the clean space (6).

2. The device according to claim 1, further comprising means for setting the gas flow rate entering the lip cleaning space.

3. The device according to claim 1 or 2, wherein, On each side of the steel strip (1), the gas fed into the enclosed box (5) and the gas fed into the lip cleaning space (6) come from the same source through the pipe (7).

4. The device according to claim 2 or 3, wherein, The device for setting the gas flow rate into the lip cleaning space includes a plate (8) with holes.

5. The device according to claim 4, wherein, The perforated plate (8) is a component of the pipe (7).

6. The device according to any one of claims 1 to 5, wherein a noise-reducing plate (9) is provided in the lateral region where the steel belt is not present, the noise-reducing plate preventing the frontal convergence of wiping airflows.

7. The device according to any one of claims 1 to 6, wherein, The enclosure is equipped with a flexible seal (10) that ensures lateral sealing of the enclosed area around the steel strip path.

8. The device according to claim 7, wherein, The flexible seal can withstand operating temperatures above 450°C.

9. The device according to any one of claims 1 to 8, wherein, The lip cleaning space (6) is provided with a sealing device that ensures the sealing of the space at the side (11).

10. A method for continuously hot-dip galvanizing steel strip (1) in a hot-dip galvanizing apparatus according to any one of claims 1 to 9, comprising the following steps: - Immerse the steel strip in a liquid metal bath (2); - The liquid metal on the steel strip is wiped by the wiping nozzle equipped with the lip (3), the lip being provided with an external lip cleaning tool (4), the external lip cleaning tool (4) being able to translate along the spray lip (3) in the lip cleaning space (6); - Drive the steel belt (1) covered with a liquid coating into the enclosed box (5); and In this process, both the enclosed box (5) and the lip cleaning space (6) are filled with non-oxidizing gas.

11. The method according to claim 10, wherein, The flow rate of the non-oxidizing gas delivered into the lip cleaning space (6) can be adjusted, and the following equation is satisfied: in: C is the flow rate of the non-oxidizing gas fed into the sealed box (5), and R is the flow rate of the non-oxidizing gas delivered into the lip cleaning space (6).

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