CN-122029302-A - Apparatus for continuous hot dip coating of steel strip and method thereof

Abstract

An apparatus for continuous hot dip coated steel strip (1) comprising-a liquid metal bath (2), -a gas wiping nozzle provided with a lip (3) and lip cleaning means (4) movable along the nozzle lip, -a closing box (5) closing the atmosphere around the strip path above the wiping nozzle, -a lip cleaning space (6) arranged between the gas wiping nozzle and the closing box, -means for feeding non-oxidising gas into the closing box (5), -means for feeding gas into the cleaning space (6).

Inventors

• Paul Delicio
• Remy Valentine
• Patrice VisANT Hernandez

Assignees

• 安赛乐米塔尔公司

Dates

Publication Date

20260512

Application Date

20241014

Priority Date

20231026

Claims (11)

1. 1. An apparatus for continuous hot dip coated steel strip (1) comprising: -a bath (2) of liquid metal; -a gas wiping nozzle provided with a lip (3) and a lip cleaning means (4) movable along the nozzle lip; -a closing box (5) closing the atmosphere around the steel strip path above the wiping nozzle; -a lip cleaning space (6) arranged between the gas wiping nozzle and the closed box; -means for feeding a non-oxidising gas into said closed box (5); -means for feeding a non-oxidizing gas into said cleaning space (6).
2. 2. The apparatus of claim 1, comprising means for setting a flow of gas into the lip cleaning space.
3. 3. The apparatus according to claim 1 or 2, wherein on each side of the steel strip (1) the gas fed into the closed box (5) and the gas fed into the lip cleaning space (6) come from the same source through a pipe (7).
4. 4. A device according to claim 2 or 3, wherein the means for setting the flow of gas into the lip cleaning space comprises a plate (8) provided with holes.
5. 5. The apparatus according to claim 4, wherein the perforated plate (8) is an integral part of the pipe (7).
6. 6. The device according to any one of claims 1 to 5, provided with a noise-protection plate (9) in the lateral areas where no steel strip is present, said noise-protection plate preventing frontal intersection of the wiping air flows.
7. 7. The apparatus according to any one of claims 1 to 6, wherein the enclosure is fitted with a flexible seal (10), the flexible seal (10) ensuring lateral tightness of the enclosed area around the steel strip path.
8. 8. The apparatus of claim 7, wherein the flexible seal is capable of withstanding operating temperatures above 450 ℃.
9. 9. The device according to any one of claims 1 to 8, wherein the lip cleaning space (6) is provided with sealing means which ensure tightness of the space at the side (11).
10. 10. A method for continuously hot dip coating steel strip (1) in a hot dip coating plant according to any one of claims 1 to 9, comprising the steps of: -immersing the steel strip in a bath of liquid metal (2); -wiping the liquid metal on the steel strip through the wiping nozzle provided with a lip (3) provided with an outer lip cleaning tool (4), the outer lip cleaning tool (4) being translatable along the nozzle lip (3) in the lip cleaning space (6); -driving said steel strip (1) covered with liquid coating into a closed box (5), and Wherein both the closed box (5) and the lip cleaning space (6) are fed with a non-oxidizing gas, respectively.
11. 11. Method according to claim 10, wherein the flow of non-oxidizing gas into the lip cleaning space (6) can be adjusted, and wherein the following equation is fulfilled: Wherein: C is the flow rate of the non-oxidizing gas fed into the closed box (5), and R is the flow of non-oxidizing gas into the lip cleaning space (6).

Description

Apparatus for continuous hot dip coating of steel strip and method thereof Technical Field The invention relates to a device for the continuous hot dip coating of metal strip, in particular steel strip. Background In many industrial applications, steel sheets are coated with a protective coating that resists corrosion. This type of sheet metal is used in various industries to manufacture a wide variety of components, particularly components that are exposed to the end user's line of sight. In order to obtain this type of sheet material, a continuous dip coating plant is used in which the steel strip is immersed in a bath of molten metal. The molten metal is not limited. For example, it may be zinc-based or aluminum-based. It may contain other chemical elements, iron from the dissolution of the steel strip, unavoidable impurities and possible additives such as lead, antimony. 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 nature of the metal, and in the case of zinc-based baths, the bath temperature is set at 380 ℃ to 490 ℃. The corrosion resistance of components coated in this way is ensured by the coating, the thickness of which is generally achieved by gas wiping. Before the strip passes through the bath of molten metal, it is first passed through an annealing furnace under a reducing atmosphere in order to recrystallise after significant work hardening associated with the cold rolling operation and to prepare its surface chemical state in order to promote the chemical reactions required by the quenching operation itself. Depending on the steel grade, the strip is heated to about 550 ℃ to 900 ℃ for the time required for recrystallization and surface preparation. Then, it is cooled to a temperature close to the temperature of the molten metal bath using a heat exchanger. The steel strip is immersed by the tension of the immersed rolls in the metal. After it emerges from the bath, the strip passes through wiping means for adjusting the thickness of the liquid metal coating on the strip. During the wiping operation, gas is blown through the wiping nozzle toward the steel strip. The wiping nozzle is equipped with a lip to control the nozzle gap, thereby controlling the wiping pressure and the coating thickness. During production, the nozzle lip may become clogged with solidified wiping metal droplets, dust or any particles. Partial blockage of the lips can create wiping defects along the coated steel strip. This is detrimental to the quality of the appearance. In order to clean the lips without stopping production, the lips are equipped with lip cleaning means which translate along the nozzle gap from one end of the lips to the other. The solidified wiping metal droplets or any particles are loosened and mechanically removed from the lip via passage of the cleaning tool. In order to contact the tip of the lip over the entire width of the lip, the cleaning tool must translate from one end to the other. The cleaning tool may reach the tip of the lip from the interior of the wiping nozzle. However, if the lip cleaning tool encounters a coagulated droplet or any particle directly from outside the nozzle, its efficiency will increase. When the cleaning tool is operated from outside the nozzle, free space including the operating means must be left in front of the lip for translation. After wiping, the coating in liquid form is sensitive to oxidation. When the coating contains an element that is susceptible to oxidation (such as magnesium or aluminum), the oxide is more likely to appear on the surface of the metal bath. Oxides are known to produce surface defects on coated steel strip and to impair its appearance. For this reason, it is known from the prior art to enclose a steel strip covered with a liquid coating in an atmosphere during solidification of the liquid coating. Such a closure device is commonly referred to as a closure box. It generally maintains an atmosphere with limited oxidizing power. In a configuration in which free space must be maintained in front of the lip of the wiping nozzle, the enclosure cannot reach the wiping nozzle. Thus, ambient air enters the free space, and the oxidizing ability in the closed atmosphere increases. As a result, the appearance quality of the coating is impaired. Patent WO2023/088625 discloses a gas wiping apparatus comprising an upper closed tank and a lower closed tank, both adjacent to respective upper and lower outer sides of a wiping nozzle, said closed tanks each being provided with a respective opening to blow a flow of non-oxidising gas. According to this patent, the flow rate is regulated in response to the oxygen content measured in the vicinity of the running steel strip. However, these disclosures do not mention that ambient air enters the enclosure through free space for the lip cleaning tool. Disclosure