Device for cleaning a hot dip coating bath and method to perform hot dip coating of a steel sheet

Abstract

A hot dip coating equipment able to coat a steel strip (1) running along a given path, comprising a liquid metal bath, having a bath level (2), provided with one sink roll (3) and at least one stabilizing roll (4), a pair of gas knives (5), wherein said equipment is fitted with at least one drain channel (6) forming a chute starting at the bath level (2), pumping means (7) being connected to the drain channel (6), and wherein said drain channel does not include any portion crossing an extraction line (12) formed by an intersection of the bath level (2) with the steel strip (1) at the point where the strip leaves the bath.

Description

[0001] Device for cleaning a hot dip coating bath and method to perform hot dip coating of a steel sheet

[0002] The present invention relates to the equipment of a plant 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 in particular appearance parts.

[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 450 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 650 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] After passing through the annealing furnace, the steel strip runs through a duct, also called a "snout", under a protective atmosphere with respect to the steel, and is immersed in the molten metal bath. The lower part of the snout is immersed in the metal bath to form a liquid seal with the surface of the bath and inside this snout, through which the steel strip passes as it runs through said snout.

[0009] 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. One or more additional immersed rolls are added for stabilizing the movement strip during the wiping operation.

[0010] At the point where the steel strip leaves the bath, the strip crosses the surface of the metal bath. The metal bath can be covered with oxide particles. Particles may come from the elements comprised in the metal bath. Secondly, particles may come from the steel dissolution in the metal bath that forms iron containing intermetallic compound particles. Thirdly, particles may come from impurities inherent to the manufacturing process.

[0011] When the bath contains chemical elements, which oxidize easily like magnesium or aluminium, oxides particles are even more present on the surface of the bath than when the bath is only composed of zinc.

[0012] In the wiping area, liquid metal flows back down into the bath. This backflow contains oxides coming from the reaction with the oxygen of the atmosphere.

[0013] The way the particles will spread on the surface of the bath depends on the liquid streams in the bath. The stabilizing rolls moving in the bath influence the liquid streams.

[0014] The particles carried by the strip are revealed, or even magnified during the wiping operation. Therefore, the bath must be continuously cleaned by removing the particles to ensure a final product free from visual defects.

[0015] The patent FR2816638 discloses an enclosure for isolating the liquid metal in the region where the strip leaves the liquid metal bath, and for recovering the metal oxide particles and intermetallic compound particles by the liquid flowing from this region into said enclosure. However, to perform maintenance operations on the immersed equipment, it is mandatory to put it off-line to ensure the safety of the operators. As the strip is enclosed, a line stop is mandatory. Furthermore, the strip must be cut to free the enclosing equipment, and after maintenance of the equipment, the strip must be reengaged into said enclosure before joining the strip again inside the enclosure. This makes lost production time even longer.

[0016] Another drawback of this invention is that the particles are removed only at the point where the strip leaves the bath and nowhere else from the bath.

[0017] The aim of the present invention is to provide a cleaning bath equipment that doesn’t require to stop the production nor to cut the steel strip for maintenance. Another objective of the present invention is to provide a cleaning equipment adaptable to the region where the particles concentrate.

[0018] This is achieved by a hot dip coating equipment able to coat a steel strip running along a given path, comprising a liquid metal bath, having a bath level, provided with one sink roll and at least one stabilizing roll, a pair of gas knives, wherein said equipment is fitted with at least one drain channel forming a chute starting at the bath level, pumping means being connected to the drain channel, and wherein said drain channel does not include any portion crossing an extraction line formed by an intersection of the bath level with the steel strip at the point where the strip leaves the bath.

[0019] The invention is illustrated with the following figures, they are not limiting: Figure 1 shows an installation from the prior art with one stabilizing roll.

[0020] Figure 2 shows an installation from the prior art with two stabilizing rolls.

[0021] Figure 3 show the extraction line where the strip leaves the bath.

[0022] Figure 4 shows a preferred embodiment of the invention with one drain channel.

[0023] Figure 5a shows a setting according to the invention where the drain channel is rotated towards the region where the strip leaves the bath.

[0024] Figure 5b shows a setting according to the invention where the drain channel is rotated towards the edge of the bath. Figure 6 shows another embodiment with two drain channels.

[0025] Figure 7 shows a drain channel with an opening. It also shows means for setting the chute height.

[0026] Figure 8 shows a preferred embodiment with one stabilizing roll and with the drain channel mounted on the support of the stabilizing roll.

[0027] The figures are labelled with the following references:

[0028] 1 : steel strip

[0029] 2: bath level

[0030] 3: sink roll

[0031] 4: stabilizing roll(s)

[0032] 5: wiping nozzles

[0033] 6: drain channel

[0034] 7: pumping means

[0035] 8: clamping flange

[0036] 9: round portion with circular section at the end of the drain channel

[0037] 10: spring washers

[0038] 1 1 : opening in the drain channel

[0039] 12: extraction line

[0040] 13: support of the stabilizing roll

[0041] H: chute height

[0042] The drain channel 6 according to the invention does not include any portion crossing the extraction line 12. It can be removed for maintenance during production. It is different from the prior art that encloses the steel strip in the region where the strip leaves the liquid metal bath.

[0043] Preferably, the drain channel is placed at the vicinity of a point where the strip leaves the bath.

[0044] Preferably, at least one drain channel 6 is placed along the extraction line 12 on at least one side of the path of the steel strip.

[0045] Preferably, the device comprising the drain channel 6 is mounted on a support connected to the stabilizing roll 4. This can be realized by mounting the drain channel 6 on the support 13 of the stabilizing roll 4. In another preferred embodiment, another drain channel 6 is placed along the extraction line 12, on the other side of the path of the steel strip.

[0046] The inventors have observed that the immersed roll configuration influences the concentration of surface particles. In a configuration where there is only one stabilizing roll in the bath, the particles concentrate on one side of the path of the steel strip. In other configurations, the particles can concentrate in different regions, for example along the path of the steel strip, on one or on both sides, or along the edge of the bath.

[0047] The inventors have found that introducing the drain channel and the immersed roll mounted together in the bath is very efficient in terms of maintenance time and operator safety. This way, the settings can be performed out of the bath. Furthermore, the handling of the equipment is simplified by maneuvering, immersing, and attaching the stabilizing roll 4 and the drain channel 6 together in one piece of equipment.

[0048] The particles on the surface of the bath are removed from the bath surface by the flow generated by the chute of the drain channel. Entrapped particles are brought into the drain channel. Pumping means allows to empty the drain channel.

[0049] Preferably, the pumping means comprise a motor, a shaft and an impeller connected to the drain channel. The motor can be placed above the level of the bath for frictionless operation. The motor drives the propeller via a shaft dipping into the bath. The impeller is connected by a pipe to the drain channel. For example, the impeller can be placed below the level of the drain channel to ease the flow out of said channel.

[0050] The motor can be pneumatic to sustain high working temperatures. Preferably, the flow rate generated by the pumping means can be set from 1 to 10 m3per hour.

[0051] The evacuation pipe can convey the entrapped particles contained in the liquid metal to a region of the bath where they can be removed by skimming. For example, skimming may occur manually or may be performed by a robot. The drain channel 6 has a cylindrical shape and comprises an opening 1 1 allowing the liquid metal from the bath to flow into said channel. Once the particles to be removed from the bath surface are entrapped in the flow generated by the chute, the pumping means can keep said flow in a steady state. The depth of a layer of the metal bath that flows into the drain channel through its opening is defined by the chute height.

[0052] Preferably, a chute height H from the bath level 2 to the opening in the drain channel 1 1 is set by the angular position of the drain channel 6.

[0053] Preferably, the drain channel is equipped with means for adjusting its angular position. Said means can comprise two round portions having a circular section 9 at both ends of the drain channel. The means can also comprise a pair of clamping flanges 8. The flanges 8 are tightened by any means, for example by screws and bolts. The clamping flanges press on the round portions with circular section.

[0054] The round portions 9 allow the drain channel 6 to be rotated around the axis of their circular section. Once the aimed angular position is achieved by rotation, the round portions 9 are clamped by the flanges 8.

[0055] The drain channel can be rotated towards the path of the strip. Such a setting shown on figure 5a is relevant in a configuration where particles concentrate in the region where the strip leaves the bath.

[0056] The drain channel can also be rotated towards the exterior of the bath containment. Such a setting shown on figure 5b is relevant in a configuration where particles concentrate on the edge of the bath.

[0057] In a preferred embodiment, the clamping force of the flanges on the round portions 9 at both ends of the drain channel is exerted by spring washers 10 included in the tightening means. The man skilled in the art will be able to design the spring washers needed to compensate the thermal expansion caused by the heat of the molten metal bath.

[0058] Thanks to the spring washers 10, the man skilled in the art can also design the clamping force that pushes onto the round portions, which are in proportion to the friction force between the flange and round portion. The friction force must be sufficient to keep the drain channel in position during production. Based on the knowledge of the man skilled in the art, friction forces can be gentle enough so that the angular position can be modified during production, for example by mean of a lever arm connected to the drain channel and protruding out of the bath. By actuating the lever arm, the angular position of the drain channel can be adjusted.

[0059] Preferably, the chute height is set from 1 to 30 mm. If the chute height is smaller than 1 mm, the surface tension of the liquid may not be sufficient for the first tear to drop into the drain channel. If the chute height is higher than 30 mm, the flow rate generated by the pumping means may not be sufficient to empty the channel and it would be submerged.

[0060] In an advantageous embodiment, the chute height is from 4 to 15 mm.

[0061] The man skilled in the art will be able to adjust the value of the flow in function of the number of particles to be removed from the bath, and in function of the way they spread on the bath.

[0062] Any coating metal can be used in the invention. For example, the coating metal bath is an aluminium-based metal bath, meaning that it contains more than 50 weight % of aluminium.

[0063] In another embodiment, the metal bath is a zinc-based metal bath, meaning that it contains more than 50 weight % of zinc.

[0064] Preferably, the metal bath comprises by weight from 1 ,0 to 10,0 % of Al and 1 ,0 to 10,0 % of Mg, impurities from the manufacturing process up to 0,1 %, the remainder being Zn.

[0065] Advantageously, the metal bath comprises by weight from 1 ,0 to 5,0 % of Al and 1 ,0 to 5,0 % of Mg, impurities from the manufacturing process up to 0,1%, the remainder being Zn.

Claims

CLAIMS1. A hot dip coating equipment able to coat a steel strip (1 ) running along a given path, comprising a liquid metal bath, having a bath level (2), provided with one sink roll (3) and at least one stabilizing roll (4), a pair of gas knives(5), wherein said equipment is fitted with at least one drain channel (6) forming a chute starting at the bath level (2), pumping means (7) being connected to the drain channel (6), and wherein said drain channel does not include any portion crossing an extraction line (12) formed by an intersection of the bath level (2) with the steel strip (1 ) at the point where the strip leaves the bath, and wherein the drain channel (6) has a cylindrical shape and comprises an opening (1 1 ) allowing the liquid metal from the bath to flow into said drain channel (6).

2. A hot dip coating equipment according to claim 1 , wherein the at least one drain channel (6) is placed along the extraction line (12), on at least one side of the path of the steel strip.

3. A hot dip coating equipment according to claims 1 or 2, wherein said device comprising a drain channel (6) is mounted on a support connected to the stabilizing roll (4).

4. A hot dip coating equipment according to claims 2 or 3, wherein another drain channel (6) is placed along the extraction line (12), on the other side of the path of the steel strip.

5. A hot dip coating equipment according to anyone of claims 1 to 4, wherein the pumping means (7) comprise a motor, a shaft and an impeller connected to the drain channel by a pipe.

6. A hot dip coating equipment according to anyone of claims 1 to 5, wherein means are provided for adjusting the angular position of the drain channel(6).

7. A hot dip coating equipment according to claim 6, wherein the drain channel (6) has round portions with circular sections (9) at both ends, wherein said portions are clamped in position by clamping flanges (8) mounted with tightening means.

8. A hot dip coating equipment according to claim 7, wherein the means tightening the clamping flanges include spring washers (10).

9. A method for continuously hot dip coating a steel strip (1 ) in a hot dip coating equipment according to anyone of claims 1 to 8, comprising the following steps:- immersing the steel strip (1 ) into the liquid metal bath,- extraction the strip from the bath through the extraction line (12) formed by the intersection of the bath level (2) with extracted the steel strip (1 ),- positioning at least one drain channel (6), wherein said drain channel does not include any portion crossing the extraction line (12),- pumping the liquid metal out of the drain channel (6),- wiping the liquid metal on the steel strip.

10. A method according to claim 9 performed in an equipment according to claim 6, wherein a chute height H from the bath level (2) to the opening (1 1 ) in the drain channel (6) is set by adjusting the angular position of the drain channel (6).

11. A method according to anyone of claims 9 or 10, wherein the liquid metal pumped out of drain channel is conveyed through an evacuation pipe to a region of the bath where particles contained in said pumped liquid can be removed by skimming.

12. A method according to anyone of claims 9 to 1 1 , wherein the metal of the bath is based on zinc.

13. A method according to anyone of claims 9 to 12, wherein the chute H has a height from 1 to 30 mm.