WO2011020602A1

WO2011020602A1 - Method for applying a coating on a metal strip, apparatus therefor, and coated strip

Info

Publication number: WO2011020602A1
Application number: PCT/EP2010/005066
Authority: WO
Inventors: Martin Simon Brunnock; Neil Mathieson Craik; Robert Gordon Ireson; Corinne Lucienne Odette Howse; Paul Anthony Stirling
Original assignee: Corus Uk Limited
Priority date: 2009-08-20
Filing date: 2010-08-18
Publication date: 2011-02-24

Abstract

The invention relates to a method for applying an organic coating on a moving metal strip. According to the invention the organic coating is applied on the moving metal strip in a continuous annealing process line (CAPL) after the annealing section of the CAPL, using a spray disc technology to apply the organic coating. The invention also relates to an apparatus for applying an organic coating on a moving metal strip.

Description

METHOD FOR APPLYING A COATING ON A METAL STRIP, APPARATUS THEREFOR, AND COATED STRIP

The invention relates to a method for applying an organic coating on a moving metal strip.

It is known in the art to apply an organic coating on a moving metal strip in a continuous way at constant speed, for instance using roller-coating operations. This technique is used in the steel industry to coat cold-rolled and optionally temper-rolled strip material, including metallic coated strip material such as zinc or aluminium or magnesium based alloy coated strip material, so as to provide the steel strip with a good corrosion protection during handling, transport and storage of the material. The coating is usually removed by the customer and after that the strip can be painted at the customer's factory. Other known technologies for applying an organic coating are the dipping process and the spray squeegee process, but these techniques are considered to be less advantageous for metal strips.

However, roller-coating is considered relatively slow and furthermore expensive for the production of thin organic coatings on cold reduced steel, as seen from the point of view of the steel industry. The typical speeds are 75 to 120 m/min, whereas the steel industry would appreciate the capability to apply at speeds of 300 m/min and higher. Moreover, with roller-coating it is difficult to control the thickness of the thin organic coating, especially when the speed of the steel strip is varied.

It is an object of the invention to provide a method for applying an organic coating on a moving metal strip which is suitable for high speeds, preferably above 200 m/min.

It is another object of the invention to provide a method for applying an organic coating on a moving metal strip, in which the organic coating is a thin organic coating, including thin organic coatings having a dry film thickness below 5 microns.

It is a further object of the invention to provide a method for applying an organic coating on a moving metal strip which is energy efficient.

It is moreover an object of the invention to provide an apparatus for applying an organic coating on a moving metal strip. According to the invention there is provided a method for applying an organic coating on a moving metal strip, wherein the organic coating is applied on the moving metal strip having a speed between 30 metres per minute and 800 metres per minute, in a continuous annealing process line (CAPL) after the annealing section of the CAPL, using spray disc technology to apply the organic coating.

Using spray disc technology in a CAPL it is possible to apply an organic coating on the moving metal strip at high speeds of the metal strip. The organic coating can be thin while the thickness can be accurately controlled, also when the speed of the strip is changing.

The spray disc technology is known as such and is mainly used in the printing industry and for the wetting of textiles and the like at strip speeds up to 1000 m/min. The spray disc technology is also known to be used for the coating of individual objects such as housings of refrigerators. However, the coating of a metal strip using the spray disc technology in a CAPL is not known in the art.

The inventors of the present invention have found that it is possible to apply a thin organic coating on a moving steel strip at speeds up to 200 m/min, and with modification to the CAPL will be able to apply a coating at speeds up to 600 m/min or higher. The adherence of the coating to the strip is acceptable at such high speeds, as well as the surface quality of the coating. This speed is far above the maximum speed of 180 m/min that is reached by other steel strip manufacturers when applying thin organic coatings on a steel strip in continuous production lines using other coating techniques. This invention thus constitutes a significant improvement in the art.

Preferably, the metal strip on which the organic coating is applied is a steel strip. Especially steel strips that are annealed in a CAPL at high speeds. The metal strip can also be an aluminium strip.

The steel strip is usually a cold rolled strip, preferably having a thickness between 0.2 mm and 2.0 mm. This type of steel strip is usually annealed after cold rolling to provide a good workability during further use. Cold rolled steel strip can have a width up to 2000 mm or more.

Preferably, the organic coating is applied using a water based paint system, the water based paint system optionally containing co-solvents, or the organic coating is applied using a solvent based paint system. Water based paint is very suitable for coating of a metal strip using the spray disc technology, since it has a low viscosity and can be easily dried, contrary to conventional thermally cured solvent based paint systems that are used in metal coil-coating. The water based paint system can contain co-solvents, as is usual in certain paint systems.

According to a preferred embodiment, the organic coating is a primer or pre- treatment primer or monolayer. Such coatings are especially used to provide the metal strip with a coating having a good corrosion protection, which, if required, can be post- painted at the customer's factory.

When applying an organic coating it is preferred to use a thin organic coating having a thickness of up to 8 microns, preferably a thin organic coating having a thickness of up to 6 microns, and more preferably a thin organic coating having a thickness between 1 and 5 microns. Thin organic coatings provide a corrosion resistance that is sufficient to protect the metal strip during handling, transport and storage of the strip. It is advantageous to apply an organic coating that is as thin as possible to achieve the correct balance of finished product requirements, that is the cost of the coating system, corrosion requirements and the processing of the coated metal strip, such as weldability and direct post paintability. It may be necessary to apply smoothing or levelling of the coating after the coating has been applied, for instance using a roller or air knife, but this depends on the viscosity of the paint system. Smoothing or levelling is known when using the spray disc technology on other substrates, such as paper.

Preferably the moving metal strip has a speed between 30 metres per minute and 600 metres per minute, still more preferably a speed between 200 and 500 metres per minute. A speed of up to 800 m/min covers the speeds used in the annealing lines that are nowadays used, and thus provides a coating technique in which the spray disc system is functioning optimally. Lower speeds are preferred when such is imposed by other limiting conditions in the CAPL.

According to a preferred embodiment, the moving metal strip has a variable speed between 30 and 600 metres per minute and the strip is accelerated or decelerated with an acceleration or deceleration of up to 30 metres per minute per second. The spray disc technology makes a control possible such that variable speeds of the metal strip can still be used when applying the organic coating. Most coating technologies, such as roller- coating, do not provide the possibility to vary the speed during coating and need considerable time to control the thickness after a change in speed of the metal strip.

Preferably the moving metal strip has a temperature between 25° C and 100° C when the organic coating is applied on the moving metal strip, preferably a temperature between 30° C and 50° C. After the annealing step the metal strip is cooled and the temperature of the strip is lowered, but the strip still has a temperature that is higher than the ambient temperature when leaving the annealing section. This means that heat is retained in the strip and this heat will partially dry the organic coating. By choosing the location in the CAPL where the organic coating is applied on the metal strip, a' suitable temperature can be used to dry the coating. The temperature of the strip should not be above 100° C, for otherwise the applied coating would start boiling; a strip temperature between 30° C and 50° C is suitable to dry the coating fast enough and to provide the dried coating with a good surface quality.

According to a preferred embodiment a heating step to dry the organic coating is provided after the organic coating has been applied on the moving metal strip, preferably using infrared radiation, more preferably using medium wavelength infrared radiation. Since it will not always be possible to choose a location in the CAPL where the metal strip still has a relatively high temperature and the heat retained in the strip will not always be sufficient to dry the coating before the coating on the strip enters a further part or section of the CAPL, an additional drying step is then needed before the coating on the strip enters such a further part or section. Infrared radiation is a suitable drying technique since it can adapt to changing speeds of the metal strip. Medium wavelength infrared radiation is preferred because it mainly heats a few top microns of the coating, and not the metal strip. However, other drying techniques can be used, such as forced air convection, catalytic infrared radiation and gas powered infrared radiation.

According to a first preferred embodiment the organic coating is applied on the moving metal strip between an exit accumulator which is located after the annealing section of the CAPL and a temper mill in the CAPL which is located after the exit accumulator of the CAPL. Here the retained heat is still relatively high and in most CAPLs a substantially vertical section is present where the coating can be applied on the strip. Usually, at this location the speed of the strip will be constant.

According to a second preferred embodiment the organic coating is applied on the moving metal strip between a temper mill in the CAPL which is located after the exit accumulator of the CAPL and a further accumulator located after the temper mill. Here, the metal strip has already passed the temper mill, meaning that there are more locations in the CAPL where the coating can be applied on the moving metal strip. However this also means that it may not always be possible to apply the organic coating on the metal strip moving at a constant speed.

According to a second aspect of the invention there is provided an apparatus for applying an organic coating on a moving metal strip, wherein the apparatus comprises a spray disc device which is placed in a continuous annealing process line (CAPL) after the annealing section of the CAPL.

This apparatus is suitable to perform the method as described above to coat a moving metal strip with a thin organic coating at high speeds. A spray disc device in a CAPL for applying an organic coating on a moving metal strip is not known in the art.

Preferably the spray disc device is positioned either between the exit accumulator and a temper mill in the CAPL which is located after the exit accumulator of the CAPL or between the temper mill and a further accumulator positioned after the temper mill. A heating device for heating the organic coating can be positioned directly after the spray disc device. Usually, a temper mill is installed in a CAPL to provide the metal strip with a good surface quality (appearance, texture and shape) and to optimise metallurgical properties. The spray disc device can be located either before or after the temper mill, each location having consequences for the heat retained in the metal strip and thus the drying properties of the metal strip, and the constant speed or not of the metal strip at that location having consequences for the control of the spray disc device.

According to a third aspect of the invention there is provided a cold rolled metal strip provided with an organic coating having a thickness between 1 and 5 microns, using the method and/or the apparatus as described above. Cold rolled metal strip having a thin organic layer with a thickness between 1 and 5 microns is not known so far, but is feasible when produced with the method and/or apparatus according to the invention.

The invention will be elucidated referring to the accompanying drawings and the example hereinbelow.

Figure 1 schematically shows the end part of a continuous annealing process line with a location for the sprays discs and drying equipment according to the invention directly after the annealing section but prior to the temper mill.

Figures 2a, 2b and 3 schematically show the end part of the CAPL with different locations for the spray discs and drying equipment according to the invention after the temper mill.

Continuous annealing process lines (CAPLs) can have various lay-outs, especially after the exit accumulator of the CAPL. Usually a temper mill is part of the CAPL and positioned directly after the exit accumulator. This opens a number of possibilities for the location of the spray discs and the drying equipment in the CAPL.

Figures 1 and 3 show the exit accumulator 1 which is positioned after the annealing furnace of the CAPL. All Figures 1 - 3 show the temper mill 2 and a number of turn rolls 3; in Figure 3 not all the turn rolls have been indicated. In all Figures 1 - 3 tensioning equipment (bridles) 4 is present before and after the temper mill 2. Figures 2a and 2b show a horizontal accumulator 5 positioned after the temper mill and the tensioning equipment. Figure 3 shows a configuration utilising an additional vertical accumulator 6. In Figures 1 and 3 the horizontal accumulator after the temper mill 2 is not shown. The strip of annealed steel S runs through the exit accumulator 1 and the temper mill 2 with tensioning equipment 4, but can follow different routes between the temper mill 2 and the horizontal accumulator 5 and potentially the vertical accumulator 6.

Figure 1 shows that according to the invention the spray discs 10 can be positioned directly after the exit accumulator 1. Here the strip will still be relatively warm, a factor that will help the application and minimise energy requirements to subsequently dry the coating. Drying equipment 11 is installed directly after the spray discs to fully dry the organic coating on the steel strip S before it has to contact a turn roll 3. Figure 2a shows another embodiment of the invention, in which the spray discs

10 and the drying equipment 11 are positioned between the (tensioning equipment 4 after the) temper mill 2 and the horizontal accumulator 5. Here the temperature of the steel strip S will be somewhat lower then the temperature of the strip directly after the exit accumulator (not shown in this Figure 2a).

Figure 2b shows that an additional loop has been made in the lay-out of the CAPL between the temper mill 2 and the horizontal accumulator 5. The spray discs 10a and the drying equipment 11a can be positioned as in Figure 2a directly after the tensioning equipment 4 after the temper mill 2, or the spray discs 10b and drying equipment 1 Ib can be positioned further down the line.

Figure 3 shows a vertical accumulator 6 situated after the temper mill in the layout of the CAPL. This drawing shows how the configuration can be altered to facilitate coating application at constant speed.

The drying equipment 11 used infrared heating to partially and/or fully dry the organic coating applied by the spray discs 10, preferably medium wavelength infrared heating is used to essentially heat the coating and not the strip. However, other drying techniques are also possible. It is important that the coating is fully dry at this stage otherwise residual moisture could cause rusting of the steel strip S or result in a soft coating that is easily damaged.

To apply the coating using the spray discs 10 the steel strip S can be run vertically, semi-vertically or horizontally. The spray discs offer the possibility to apply the coating at a variety of thicknesses. For coating steel the coating should preferably have a dry film thickness up to 12 microns, and more preferably a thickness between 1 and 5 microns.

The use of spray discs makes it possible to apply the coating at very high speeds, and so the normal speeds of the CAPL can be followed. The maximum speed of a CAPL nowadays is 800 metres a minute.

Spray discs also make it possible to apply a coating having a constant thickness while the steel strip S is running at a non-constant speed. In Figures 2a and 2b the strip S is running at a non-constant speed at the location of the spray discs and drying equipment. In Figure 1 the strip S is running either at constant speed or at non-constant speed at the location of the spray discs and drying equipment. In Figure 3 the strip S is running at a constant speed at the location of the spray discs and drying equipment. The spray discs can be controlled such that the coating thickness remains constant when the velocity of the strip S changes.

It will be clear to the person skilled in the art that other configurations of the

CAPL are also possible.

Example

After the exit accumulator (1) the strip S is pre-heated to 50°C using medium wavelength infrared radiation to ensure the strip has a temperature of at least 35° C when it reaches the spray disc(s). The temperatures used in the pre-heat and post-heat treatments are shown in Table 1. Table 1 also shows parameters relating to the line speed, spray volume, smoothing roll roughness and dry film coating thickness.

The spray discs (Ahlbrandt Rotorspray) having a diameter of 80 mm are arranged at 120 mm intervals across the width of the strip. Each disc is installed at an angle between 40 and 90°, at a distance between 20 and 100 mm above the strip surface. To ensure an even coating distribution the spray discs operate in pairs, wherein the discs of each pair rotate in opposite directions.

As the strip passes below the spray disc it is coated with an organic coating such as Granocoat ^® _, the viscosity of the coating should not exceed 150 mPa.s. The organic coating can be aqueous or in certain instances can contain water and up to 1% of an organic solvent. A wet film coating thickness of 0.5 or less to 10 μm is obtainable when using one row of spray disc applicators, whereas a wet coating thickness of 0.5 or less to 20 μm is achievable when two rows of spray disc applicators are installed.

After application of the organic coating, smoothing rollers or air knives are used to obtain a substantially homogeneous coating thickness. In this example, 6.5 ml/m² of the organic coating is applied on the moving strip having a speed of 250 m/min. To obtain a coating thickness of 2 μm, the coating is levelled using a smoothing roller 120, of which the number is equivalent to the surface roughness of the roller. For example, smoothing roller 120 has a reduced surface roughness relative to smoothing roller 80.

Following application and levelling, the coating is subjected to a post-heat treatment at 100°C. The coating is then dried and cured in a convection oven or by using infrared radiation. The drying time required is at least one second and is dependent on the speed of the moving strip. To cure the coating, a peak metal temperature (PMT) of between 60 - 200⁰C is used. If the coating is applied using one row of spray disc applicators, the dry film thickness of the coating is 0.5 or less to 5 μm, whereas if two rows of spray disc applicators are used, the dry film thickness is 0.5 or less to 10 μm.

Table 1

*: Not measured for all variants

The Ti content in the coating is indicative of coating thickness, wherein 9 mg/m² of Ti corresponds to a coating thickness of 1 μm.

Claims

1. Method for applying an organic coating on a moving metal strip, characterised in that the organic coating is applied on the moving metal strip having a speed between 30 metres per minute and 800 metres per minute, in a continuous annealing process line (CAPL) after the annealing section of the CAPL, using a spray disc technology to apply the organic coating.

2. Method according to claim 1, wherein the metal strip on which the organic coating is applied is a steel strip.

3. Method according to claim 2, wherein the steel strip is a cold rolled strip, preferably having a thickness between 0.2 mm and 2.0 mm.

4. Method according to any one of the preceding claims, wherein the organic coating is applied using a water based paint system, the water based paint system optionally containing co-solvents.

5. Method according to any one of the preceding claims, wherein the organic coating is a primer or pre-treatment primer or monolayer.

6. Method according to any one of the preceding claims, wherein the organic coating is a thin organic coating having a thickness of up to 8 microns, preferably a thin organic coating having a thickness of up to 6 microns, and more preferably a thin organic coating having a thickness between 1 and 5 microns.

7. Method according to any one of the preceding claims, wherein the moving metal strip has a speed between 30 metres per minute and 600 metres per minute, more preferably a speed between 200 and 500 metres per minute.

8. Method according to claim 7, wherein the moving metal strip has a variable speed between 30 and 600 metres per minute and the strip is accelerated or decelerated with an acceleration or deceleration of up to 30 metres per minute per second.

9. Method according to any one of the preceding claims, wherein the moving metal strip has a temperature between 25⁰C and 100⁰C when the organic coating is applied on the moving metal strip, preferably a temperature between 3O⁰C and 5O⁰C.

10. Method according to any one of the preceding claims, wherein a heating step to dry the organic coating is provided after the organic coating has been applied on the moving metal strip, preferably using infrared radiation, more preferably using medium wavelength infrared radiation.

11. Method according to any one of the preceding claims 1 - 10, wherein the organic coating is applied on the moving metal strip between an exit accumulator which is located after the annealing section of the CAPL and a temper mill in the CAPL which is located after the exit accumulator of the CAPL.

12. Method according to any one of the preceding claims 1 - 10, wherein the organic coating is applied on the moving metal strip between a temper mill in the CAPL which is located after the exit accumulator of the CAPL and a further accumulator located after the temper mill.

13. Apparatus for applying an organic coating on a moving metal strip, characterised in that the apparatus comprises a spray disc device which is located in a continuous annealing process line (CAPL) after the annealing section of the CAPL.

14. Apparatus according to claim 13, wherein the spray disc device is located either between the exit accumulator and a temper mill in the CAPL which is located after the exit accumulator of the CAPL or between the temper mill and a further accumulator located after the temper mill, a heating device for heating the organic coating being located directly after the spray disc device.

15. Cold rolled metal strip provided with an organic coating having a thickness between 1 and 5 microns, using the method and/or the apparatus according to any one of the preceding claims.