Installation for continuously hot dip coating a steel strip and method therefore

Abstract

An installation for continuously hot dip coating a steel strip (1) comprising: - a liquid metal bath (2), - gas wiping nozzles equipped with lips (3) and with a lip cleaning tool (4) able to be moved along the nozzle lips, - confinement boxes (5) enclosing the atmosphere around the steel strip path above the wiping nozzles, - a lip cleaning space (6) being provided between said gas wiping nozzles and said confinement boxes, - means for feeding non-oxidizing gas into the confinement boxes (5), - means for feeding gas into said cleaning space (6).

Description

[0001] Installation for continuously hot dip coating a steel strip and method therefore

[0002] The present invention deals with an equipment for the continuous hot dip coating of a metal strip, in particular a steel strip.

[0003] In many industrial applications steel sheets are coated with a protective coating layer against corrosion. This type of sheet metal is used in a variety of industries to manufacture all kinds of parts, and notably parts exposed to the glance of the final user.

[0004] To obtain this type of sheet, continuous dip coating installations are 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 based on zinc or on aluminium. It may contain other chemical elements, iron coming from the dissolution of the steel strip, unavoidable impurities and possible additives such as lead, antimony.

[0005] In the recent years, aluminium, magnesium and even silicon are added to the zinc-based bath of molten metal to increase the corrosion performance of the coated steel sheets. The bath temperature depends on the nature of the metal: in the case of zinc-based baths, the bath temperature is set from 380 to 490°C.

[0006] The corrosion resistance of parts coated in this way is ensured by the coating, the thickness of which is usually achieved by gas wiping.

[0007] Before the steel strip passes through the molten metal bath, it first runs through an annealing furnace under a reducing atmosphere, in order to recrystallize after the significant work-hardening associated with the cold rolling operation, and to prepare its surface chemical state in order to promote the chemical reactions required for the quenching operation itself. The steel strip is heated to around 550 to 900°C, depending on the grade, 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 heat exchangers.

[0008] The steel strip is dipped by the tension of a sink roll immersed in the metal. After it emerges from the bath, the strip passes through wiping means used to regulate the thickness of the liquid metal coating on the steel strip. During the wiping operation, gas is blown through wiping nozzles towards the steel strip. The wiping nozzles are equipped with lips to control the nozzle gap, and so the wiping pressure and the coating thickness.

[0009] During production, the nozzle lips can be obstructed by droplets of solidified wiped metal, dust, or any particles. A local obstruction of the lips generates a wiping defect along the coated steel strip. This is detrimental for aspect quality.

[0010] To clean the lips without idling production, the lips are equipped with a lip cleaning tool that translate along the nozzle gap from one end of the lips to the other. Solidified droplets of wiped metal or any particles are loosened and mechanically removed from the lips by the passage of said cleaning tool. To access the tip of the lips over the whole width of the lips, the cleaning tool must be translated from one extremity to the other.

[0011] The cleaning tool can reach the tip of the lips from the inside of the wiping nozzles. However, the efficiency of the lip cleaning tool is increased if it meets the solidified droplets or any particles directly from the outside of the nozzle. When the cleaning tool is operated from the outside of the nozzle, a free space in front of the lips and including an operating device must be left free for translation.

[0012] After the wiping, the coating being liquid is sensitive to oxidation. When the coating contains elements, which oxidize easily like magnesium or aluminium, oxides are even more likely to appear on the surface of the metal bath. Oxides are known to generate surface defects on the coated steel strip and impart its aspect.

[0013] For this reason, it is known from the prior art to enclose the steel strip covered with liquid coating within an atmosphere during liquid coating solidification. This enclosure is usually named a confinement box. It usually retains an atmosphere with limited oxidizing power.

[0014] In a configuration where a free space must be kept in front of the lips of the wiping nozzles, the confinement box can’t reach the wiping nozzles. Consequently, ambient air comes into said free space and the oxidizing power in the confined atmosphere increases. Thus, the aspect quality of the coating is impaired. The patent WO2023 / 088625 discloses a gas wiping device comprising an upper and a lower confinement box being both adjacent to the respective upper and lower external sides of the wiping nozzles, said confinement boxes being each provided with a respective opening to blow a flow of non-oxidizing gas. According to this patent, said flow rate is adjustable depending on the measured content of oxygen in the vicinity of the running strip.

[0015] However, these disclosures are silent about the ambient air incoming in the confinement box through a free space for the lip cleaning tool.

[0016] The aim of the present invention is to provide a device able to produce a coated steel strip 1 , with very good aspect and surface quality.

[0017] This is achieved by an installation for continuously hot dip coating a steel strip 1 comprising:

[0018] - a liquid metal bath 2,

[0019] - gas wiping nozzles equipped with lips 3 and with a lip cleaning tool 4 able to be moved along the nozzle lips,

[0020] - confinement boxes 5 enclosing the atmosphere around the steel strip path above the wiping nozzles,

[0021] - a lip cleaning space 6 being provided between said gas wiping nozzles and said confinement boxes,

[0022] - means for feeding non-oxidizing gas into the confinement boxes 5,

[0023] - means for feeding gas into said cleaning space 6.

[0024] The invention is illustrated with the following figures, they are not limiting: Figure 1 shows an installation according to the invention.

[0025] Figure 2 is a perspective view showing the translation of the lip cleaning tool. For the sake of the figure understanding, the equipment behind the strip is not represented.

[0026] Figure 3 shows a preferred embodiment where non-oxidizing gas flows in the space for the cleaning tool through a perforated plate. For the sake of the figure understanding, the equipment is partially represented above the bath. Figure 4 is a perspective view of a preferred embodiment showing the translation of an anti-noise plates 9. For the sake of the figure understanding, the equipment behind the strip is not represented.

[0027] Figure 5 is a top view of the confinement boxes showing a preferred embodiment including lateral tightness.

[0028] Figure 6 shows a lateral side of the lip cleaning space.

[0029] The figures are labelled with the following references:

[0030] 1 : steel strip

[0031] 2: metal bath

[0032] 3: nozzle lips

[0033] 4: lip cleaning tool

[0034] 5: confinement box

[0035] 6: lip cleaning space

[0036] 7: duct supplying non-oxidizing gas

[0037] 8: perforated plate

[0038] 9: anti-noise plate

[0039] 10: flexible seal between the anti-noise plate and the confinement box

[0040] 1 1 : lateral side of the lip cleaning space.

[0041] The wiping nozzles according to the present invention are provided with a lip cleaning tool 4 that is operated from the outside of the nozzle.

[0042] The inventors have found that injecting non-oxidizing gas both in the confinement box 5 and in lip cleaning space 6 significantly improves the aspect quality of the coated steel strip 1 .

[0043] Preferably, both the confinement box and the lip cleaning space are fed by the same source of non-oxidizing gas. For example, they are fed by one duct supplying non-oxidizing gas 7.

[0044] Preferably, the non-oxidizing gas flow rate that is fed into the lip cleaning space can be adjusted. Advantageously, said non-oxidizing gas is fed into the lip cleaning space through a perforated plate 8. The holes in the perforated plate 8 are designed to size the gas flow rate fed into the lip cleaning space. The man skilled in the art will be able to design surface pierced by holes in the perforated plate in function of the oxidizing power atmosphere targeted in the confinement box and in the lip cleaning space.

[0045] In a preferred embodiment, the said perforated plate 8 is an integral part of the duct supplying non-oxidizing gas 7.

[0046] Preferably, the non-oxidizing gas flow rate fed into the confinement box and the non-oxidizing gas flow rate fed into the lip cleaning space satisfy the following equation:

[0047] 0.1 10.0

[0048] Where:

[0049] C is a non-oxidizing gas flow rate fed into the confinement box (5), and R is a non-oxidizing gas flow rate fed into the lip cleaning space (6).

[0050] The inventors have found that if the ratio C / R is from 0.1 to 10.0, the surface quality is improved.

[0051] Advantageously, the ratio C / R is from 0.2 to 5.0, and even from 0.5 to 2.0.

[0052] In a preferred embodiment, the equipment includes two metal plates called “anti-noise plates” 9, the function of which is to prevent the wiping gas streams emanating from each nozzle meeting one another in the lateral zones where the strip steel is not present. The interposition the of anti-noise plates 9 is useful, especially for preventing sound vibrations of very large amplitude from being generated by the frontal meeting of wiping gas streams.

[0053] The anti-noise plates 9 can be moved along the width direction, depending on the width of the strip, so that steel strips of different width can run through the installation.

[0054] In a preferred embodiment, the extremities of the confinement boxes 5 are fitted with flexible seals 10. Said seals can keep contact with the moveable anti- noise plates 9, whatever their width position. The inventors have found that the presence of seals improve even more the aspect quality to the coating.

[0055] This preferred embodiment is showed on figure 4 as a perspective view. It is also shown on figure 5 as a top view.

[0056] The flexible seals 10 ensure the lateral tightness of the confined area around the strip.

[0057] Preferably, the flexible seals can sustain an operating temperature above 200°C, and advantageously above 450°C.

[0058] Preferably additional sealing of the lip cleaning space is provided by any means ensuring its tightness.

[0059] The lip cleaning space must remain open in a region along the nozzle lips so that the lip cleaning tool is able to be moved. Leaks in said region in the front of the lip cleaning cannot be sealed.

[0060] In the other regions of the lip cleaning space, the tightness is ensured on both lateral sides 1 1. As well, leaks toward the back of the lip cleaning space or in the regions in contact with other elements like the wiping nozzles or the lips can be avoided by sealing. For example, the tightness of the lip cleaning space is provided by one or several seals. As seals, O-ring seals can be mounted in circular sections. Longitudinal and transversal seals can be used as well.

Claims

CLAIMS1 . An installation for continuously hot dip coating a steel strip (1 ) comprising:- a liquid metal bath (2),- gas wiping nozzles equipped with lips (3) and with a lip cleaning tool (4) able to be moved along the nozzle lips,- confinement boxes (5) enclosing the atmosphere around the steel strip path above the wiping nozzles,- a lip cleaning space (6) being provided between said gas wiping nozzles and said confinement boxes,- means for feeding non-oxidizing gas into the confinement boxes (5),- means for feeding gas into said cleaning space (6).

2. An installation according to claim 1 comprising means to set up a gas flow rate into the lip cleaning space.

3. An installation according to claims 1 or 2, wherein on each side of the steel strip (1 ) the gas fed into the confinement box (5), and the gas fed into the lip cleaning space (6) come from the same source through a duct (7).

4. An installation according to claim 2 to 3, wherein the means to set up the gas flow rate into the lip cleaning space comprise a plate (8), said plate being perforated with holes.

5. An installation according to claim 4, wherein said perforated plate (8) is an integral part of said duct (7).

6. An installation according to anyone of claims 1 to 5 equipped with anti-noise plates (9) in the lateral zones where the strip steel is not present, said antinoise plates preventing the frontal meeting of wiping gas streams.

7. An installation according to anyone of claims 1 to 6, wherein the confinement boxes are fitted with flexible seals (10) ensuring the lateral tightness of a confined area around the steel strip path.

8. An installation according to claim 7, wherein the flexible seals are able to sustain an operating temperature above 450°C.

9. An installation according to anyone of claims 1 to 8, wherein the lip cleaning space (6) is provided with tightening means ensuring the tightness of said space in the lateral sides (11 ).

10. A method for continuously hot dip coating a steel strip (1 ) in a hot dip coating installation according to anyone of claims 1 to 9, comprising the following steps:- immersing the strip in a liquid metal bath (2),- wiping the liquid metal on the steel strip by mean of said wiping nozzles equipped with lips (3), said lips being provided with an external lip cleaning tool (4) able to be translated along the nozzle lips (3) in said lip cleaning space (6),- driving said steel strip (1 ) covered with liquid coating in confinement boxes (5), and wherein both said confinement boxes (5) and said lip cleaning space (6) are respectively fed with a non-oxidizing gas.1 1 . A method according to claim 10 wherein the non-oxidizing gas flow rate fed into the lip cleaning space (6) can be adjusted, and wherein the following equation is satisfied:0.1Where:C is a non-oxidizing gas flow rate fed into the confinement box (5), andR is a non-oxidizing gas flow rate fed into the lip cleaning space (6).

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