WO2012019777A1

WO2012019777A1 - Method for applying a paint system on a moving steel strip to form a coating, and steel strip thus produced

Info

Publication number: WO2012019777A1
Application number: PCT/EP2011/004066
Authority: WO
Inventors: Neil Mathieson Craik; Martin Simon Brunnock; Robert Gordon Ireson; Joel Michel Robert Rousseau; Sreedhara Saema
Original assignee: Tata Steel Uk Limited
Priority date: 2010-08-12
Filing date: 2011-08-12
Publication date: 2012-02-16
Also published as: WO2012019777A8

Abstract

The invention relates to a method for applying a paint system on a moving metal strip to form a coating. According to the invention, the paint system is cured or dried using radiation, which paint system is applied on the moving steel strip passing through or leaving a Continuous Annealing Section at a strip speed of at least 160 m/min using a roller coating technique, and in that the paint system is cured or dried on the steel strip. The invention also relates to a steel strip having a coating layer of such a cured or dried paint system.

Description

METHOD FOR APPLYING A PAINT SYSTEM ON A MOVING STEEL STRIP TO FORM A COATING, AND STEEL STRIP THUS PRODUCED

The invention relates to a method for applying a paint system on a moving steel strip to form a coating and a coated steel strip produced thereby.

All kinds of paint systems are industrially applied on moving steel strips, such as solvent based paint systems, water based paint systems, polymer paint systems or organic paint systems, for instance as a coating for corrosion protection. Organic paint systems are applied in a continuous coil-coating process, and this process is conventionally carried out using roller-coating techniques. Roller-coating is used throughout the flat strip coating industries, whether on metal, paper or plastic strip.

Organic paint systems are usually applied to provide pre-painted steel strip, and such paint systems are typically applied on stand alone coil-coating lines, the fastest having a maximum operating speed of around 140 m/min. Before the paint system is applied the strip has already been processed, for instance on an annealing or galvanising line. Often the strip has been oiled, and has to be degreased before the paint is applied.

A disadvantage of the known organic paint systems used in coil-coating steel strip is that the used application speeds are low. Another disadvantage is that these known organic coatings contain organic solvents, which have a large impact on the environment during production of the coated strip, or need high investments to reduce the impact on the environment. A further disadvantage is that the known organic coatings must be cured or baked at temperatures above room temperature.

It is an object of the invention to provide a method for applying a paint system on a moving steel strip to form a coating that is faster than the coil-coating of the known paint systems.

It is another object of the invention to provide a method for applying a paint system on a moving steel strip to form a coating which method is more environmentally friendly than the conventional paint systems used in the steel coil-coating industry without the need for additional investment to remove solvents from the exhaust gases.

It is a further object of the invention to provide a method for applying a paint system on a moving steel strip to form a coating which method is more economical, for instance because the coating does not need heating to cure. According to the invention there is provided a method for applying a paint system on a moving steel strip to form a coating, wherein the paint system is cured or dried using radiation, which paint system is applied on the moving steel strip passing through or leaving a Continuous Annealing Section at a strip speed of at least 160 m/min using a roller coating technique, and wherein the paint system is cured or dried on the steel strip.

The inventors have found that it is possible to apply a paint system that is dried or cured using radiation on a moving steel strip at high speeds, e.g. speeds of 160 m/min or more. This is mainly due to the fact that paint systems that are dried or cured using radiation do not need a high curing or drying temperature, which is needed for conventional organic paint systems. The limitation in application speed when using conventional organic paint systems is usually a consequence of the infrastructure of the coating line and the need for long heating times in a convection oven to heat up the steel strip to temperatures at which the water or solvent evaporates, or to temperatures above 200° C to reach the curing or baking temperature. Since paint systems that are dried or cured using radiation can be dried or cured at temperatures below 100° C, the heating of the strip is no longer a limiting factor, and higher application speeds are possible. The amount of energy that is needed to cure or dry the paint system is supplied in the form of radiation, which makes it possible to dry or cure the paint system fast.

The application of the paint system that is dried or cured using radiation on the moving steel strip passing through or leaving a Continuous Annealing Section has the advantage that a number of process related steps are eliminated, such as numerous handling steps, oiling and subsequent cleaning, et cetera. The application of the paint system that is dried or cured using radiation and the curing or drying thereof in or after the Continuous Annealing Section thus is a very economical method to provide a coating on a steel strip.

Preferably the speed of the steel strip is between 200 and 800 m/min, more preferably between 250 and 600 m/min, and even more preferably between 300 and 500 m/min. At higher speeds the application of the paint systems that are dried or cured using radiation is economically more viable, and at the moment it appears optimal to use a speed between 300 and 500 m/min. Presently most Continuous Annealing Sections run at a speed between 180 and 350 m/min, but the maximum speed could be higher in the future.

Preferably the coating thickness of the coating after radiation curing or drying is between 1 and 30 micrometres. On the one hand, this makes it possible to provide a relatively thin pre-paint coating having a thickness between 1 and 15 micrometres, which can be post-painted either by the supplier of the pre-painted strip or by the purchaser of the pre-painted strip. On the other hand, this makes it possible to provide thicker layers having a thickness between 10 and 30 micrometres which can also be used as a finish coating. Usually coating thicknesses of cured paint systems have a thickness between 5 and 25 micrometres, and often even a maximum thickness of 20 micrometres is used.

According to a preferred embodiment the radiation curable paint system is an electron beam radiation curable paint system that is cured using electron beam radiation curing. For application on a fast moving steel strip, the paint system will usually have a viscosity of at most 200 mPa.s at the temperature at which it is applied. Electron beam radiation curable paint systems are usually heated up to approximately 40 - 60° C before they are applied. Electron beam curable paint systems can be cured fast without the need to heat the substrate, and the curing of the paint system thus is very energy efficient since the substrate needs not to be heated in a convection oven.

According to another preferred embodiment the radiation curable paint system is an ultraviolet radiation curable paint system that is cured using ultraviolet radiation curing, wherein preferably the paint system is a hybrid organic/inorganic paint system. Also ultraviolet radiation curable paint systems can be cured fast without the need to heat the substrate in a convection oven, and thus here the curing is energy efficient as well. The viscosity of the paint system will be the same as for electron beam curable paint systems.

Preferably, when using ultraviolet radiation curing the paint system is a hybrid organic/inorganic paint system of which the thickness of the coating after curing is between 1 and 15 micrometres. Ultraviolet radiation curing is a preferred curing method since apparatus for this type of radiation are commercially available.

According to a further preferred embodiment the paint system that is dried by radiating is a paint system that is dried using infrared radiation, wherein preferably the paint system is a water-based paint system. Paint systems that are dried using infrared radiation can be dried fast and at low temperatures compared to conventional solvent- based paint systems. The viscosity of the paint system will be the same as for electron beam curable paint systems.

According to a still further preferred embodiment the paint system that is dried using radiation is a paint system that is dried using induction heating of the steel strip, wherein preferably the paint system is a water-based paint system. Induction heating is very suitable to heat a metal substrate such as steel fast, thereby making it possible to dry for instance a water-based paint system in a short time. Apparatus for induction heating is commercially available.

According to a preferred embodiment the paint system that is dried or cured using radiation is partly cured or dried using retained heat from the Continuous Annealing Section in the steel strip before and/or during electron beam or ultraviolet radiation curing or infrared radiation drying or drying using induction heating of the steel strip. The retained heat from the Continuous Annealing Section in the steel strip provides an additional partly curing or drying of the coating layer of the paint system, thus lowering the energy level needed for the curing by electron beam or ultraviolet radiation curing or the drying by infrared radiation drying or induction heating. This use of retained heat is only possible when temper rolling is not needed.

It is possible to apply the paint system that is dried or cured using radiation directly on the steel strip. In this way a single layer topcoat is provided on the steel substrate that can be used as a pre-paint (for thin coatings) or as a finish coating (for thicker coatings).

According to a preferred embodiment a pre-treatment layer, a pre-treatment- primer layer or a primer layer is applied on the steel strip before the paint system that is dried or cured using radiation is applied, preferably the pre-treatment, pre-treatment- primer or primer layer has a thickness of 0.1 - 10 micrometres, more preferably has a thickness of 0.5 - 5 micrometres. The pre-treatment layer, pre-treatment-primer layer or primer layer can be applied as a separate paint layer in the Continuous Annealing Section using a roller coating technique, or can be applied using a different coating technique. The two-layer products that are formed in this way have the advantage that the pre-treatment/primer improves the adherence of the radiation cured or dried coating to the metal substrate. As a pre-treatment/primer layer a water-based paint system can be used; this pre-treatment/primer layer should be applied in the Continuous Annealing Section before the paint system that is dried or cured using radiation is applied, and the water or solvent, if present, must be removed before the paint system that is dried or cured using radiation is applied.

According to a preferred embodiment the paint system that is dried or cured using radiation is applied using gravure roller coating techniques or deformable roller coating techniques, which are the most often used techniques for applying paint systems on a moving strip. Deformable roller coating techniques are especially suitable for high speed coating of radiation curable coatings due to the higher viscosity of the paint. Gravure roller coating is the preferred technique for water-based paint systems at high speeds due to the lower coating viscosities and the thinner coating layers needed to be applied. Gravure rollers are used for this coating technique, as known in the art for other substrates than steel strip.

According to a preferred embodiment, the paint system that is dried or cured using radiation is applied on both sides of the steel strip. Usually both sides of a steel strip have to be coated. It is possible to use radiation curing or drying for paint systems that have a different thickness at the opposite sides.

It is also possible to apply the paint system on a metallic coated steel strip such as aluminium coated steel strip or zinc coated steel strip. The paint system in this case is mainly used for decorative purposes.

According to a preferred embodiment, the steel strip is suitable for hot forming operations, such as a boron steel strip, or wherein the steel strip is suitable for high temperature use up to 600° C.

It will be clear from the above that both water-based paint systems and water-free paint systems can be used using radiation. Water-free paint systems can be used for electron beam radiation curing and ultraviolet radiation curing, as follows from the above.

According to a second aspect of the invention there is provided a steel strip having a coating consisting of a layer of an electron beam or ultraviolet or infrared radiation cured or dried paint system or a paint system dried using induction heating, or consisting of a pre-treatment layer, pre-treatment-primer layer or primer layer on top of which a layer of an electron beam or ultraviolet or infrared radiation cured or dried paint system or a paint system dried using induction heating has been applied. This steel strip can be provided in accordance with the method according to the first aspect of the invention, at a high speed.

According to a preferred embodiment, the layer of an electron beam or ultraviolet or infrared radiation cured or dried paint system or a paint system dried using induction heating has a thickness between 1 and 30 micrometres. Such thicknesses are commercially used to prevent oxidation of the steel strip.

Preferably, the pre-treatment layer, pre-treatment-primer layer or primer layer has a thickness between 0.1 and 10 micrometres, preferably between 0.5 and 5 micrometres. Such layers provide a good adhesion for the electron beam or ultraviolet or infrared radiation cured or dried paint system or a paint system dried using induction heating.

Preferably, the steel strip has been provided with a metallic coating. To obtain a very good corrosion resistance steel strips need a metallic coating. However, the metallic coated steel strip is advantageously provided with a paint layer in accordance with the method of the invention for aesthetic purposes.

The invention will be elucidated referring to a number of examples and to the attached drawing.

Figure 1 shows the lay-out of a Continuous Annealing Section followed by a temper mill and equipment for applying and curing a radiation curable coating.

Figure 1 shows schematically that after hot rolling and cold rolling of a steel strip (not shown) the steel strip will enter a Continuous Annealing Section where the strip will be heated in a furnace to anneal the strip and, after passing an accumulator, the strip is temper rolled in a temper mill an passes through a tension leveller. After another accumulator, a coating section is provided which has an application part for applying the paint and a curing part for curing or drying the paint. After the paint is cured or dried, the strip passes through a last accumulator and is cut and wound into rolls.

Though a temper mill and tension leveller are shown in Figure 1, these are not always used.

In the coating section the paint is applied onto the steel strip and directly thereafter cured or dried using radiation. The steel strip can run at a speed between 180 and 350 m/min, and during normal operation the speed will for instance be 200 m/min or 250 m/min, but lower speeds are also possible. Such high speeds are possible due to the radiation curing or drying of the paint, which does not need high temperatures, contrary to conventional organic paints. Radiation curing or drying of the paint can be performed below 100° C.

The radiation curable/dryable paints are applied using roller coating techniques, such as a gravure roller coating technique or a deformable roller coating technique.

Such roller coating techniques are known in the art as such.

The paint can be applied with a thickness such that the coating after drying has a thickness between 1 and 30 micrometers. For a pre-paint coating, the thickness will be between 1 and 15 micrometers. For a finish coating, the thickness will usually be between 15 and 30 micrometers.

Usually electron beam radiation curable paint is heated to approximately 40 - 60°

C before it is applied. This type of paint is water-free and can be cured very fast without the need to heat the substrate, so no convection oven is needed.

Ultraviolet radiation curable paint can be handles in the same way as electron beam radiation curable paint. Preferably ultraviolet radiation curable paint is a hybrid organic/inorganic paint, of which the thickness is 1 to 15 micrometers after curing.

Infrared radiation is preferably used for water-based paints, but can also be used for water-free paints. Infrared radiation provides a fast drying at low temperatures.

Induction heating can also be used for curing or drying paints. Here the steel strip is heated by induction, which is known as such. The heated substrate will dry for instance a water-based system in a short time.

When the temper mill and tension leveller are not used, the steel strip will have retained heat from passing through the annealing section. This retained heat will provide an additional curing or drying of the coating. The retained heat will lower the energy level needed for curing or drying the paint.

The coating can be applied as a two-layer product, with first a pre-treatment or primer or pre-treatment-primer layer, having a thickness of preferably 0.5 - 5 micrometers. This layer can be applied as a separate layer using a roller coating technique or a different coating technique. On top of this layer a radiation curable or dryable paint is applied. The pre-treatment/primer layer improves the adherence of the paint to the metal substrate. If water or solvent is present in the pre-treatment/primer layer, it must be removed before the paint is applied.

A number of tests have been performed to evaluate the coatings on the steel substrate. These tests have been performed at different line speeds and with different paints. In all cases, the substrate used was DC04 steel, a strip having a width of 0.3 metre, the paint being applied with a 3 -roll deformable coater.

Table 1 shows the results for a ultraviolet curable pre-treatment primer paint. The paint temperature in the delivery section was 50 °C. For the ultraviolet curing section 6 rows of lamps are used. The coating thickness is calculated using the weight-strip- weight method assuming a density of the coating of 1.3 g/cm³.

Example 1 is a comparative example at a line speed of 100 m/min. Examples 2 - 5 are examples according to the invention, which at all line speeds give a good surface finish.

Table 1

Table 2 show the results for a. electron beam curable topcoat The paint temperature in the delivery section was 50 °C. The coating thickness is calculated using the weight-strip-weight method assuming a density of the coating of 1.27 g/cm³.

Example 1 and example 3 are comparative examples at a line speed of 100 m/min. Examples 2, 4, 5 and 6 are examples according to the invention. For these examples, different dilution percentages of TBCH are used. The surface finish varies, but all surfaces are good from a practical point of view. Line Power of EB Coating

Example speed Paint curing unit Thickness Surface Finish

(m/min) (kGy) (μηι)

Feint ribs

EB curable

visible but

1 100 topcoat, no 40 18.7

surface dilution

acceptable

EB curable

Heavy wood-

2 200 topcoat, no 40 14.4

grain effect dilution

Slight wood-

EB curable

grain effect but

3 100 topcoat, 3% 40 12.9

surface dilution TBCH

acceptable

EB curable Heavy wood-

4 200 topcoat, 3% 40 17.3 grain effect dilution TBCH due to ribbing

EB curable Heavy wood-

5 300 topcoat, 3% 33 17.4 grain effect dilution TBCH due to ribbing

Feint ribs

EB curable

visible but

6 200 topcoat, 5% 30 16.3

surface dilution TBCH

acceptable

Table 2

Table 3 shows the results for a near infrared curable water-based thin organic coating, combined with convection. The coating thickness is calculated using the weight-strip-weight method assuming a density of the coating of 1.27 g/cm³.

Example 1 is a comparative example at a line speed of 100 m min. Examples 2 and 3 are examples according to the invention, which at all line speeds give a good surface finish. Coating

Line speed Drying Post heat Surface

Example Thickness

(m/min) technique (°C) Finish

(μηι)

NIR +

100 200 1.9 Good

1 Convection

2 200 Convection 280 2.3 Good

3 300 Convection 280 2.1 Good

Table 3

Claims

1. Method for applying a paint system on a moving steel strip to form a coating, characterised in that the paint system is cured or dried using radiation, which paint system is applied on the moving steel strip passing through or leaving a Continuous Annealing Section at a strip speed of at least 160m/min using a roller coating technique, and in that the paint system is cured or dried on the steel strip.

2. Method according to claim 1, wherein the speed of the steel strip is between 200 and 800 m/min, preferably between 250 and 600 m/min, more preferably between 300 and 500 m/min.

3. Method according to claim 1 or 2, wherein the coating thickness of the coating after curing or drying is between 1 and 30 micrometres.

4. Method according to any one of the preceding claims 1 - 3, wherein the radiation curable paint system is an electron beam radiation curable paint system that is cured using electron beam radiation curing.

5. Method according to any one of the preceding claims 1 - 3, wherein the radiation curable paint system is an ultraviolet radiation curable paint system that is cured using ultraviolet radiation curing, wherein preferably the paint system is a hybrid organic/inorganic paint system.

6. Method according to any one of the preceding claims 1 - 3, wherein the paint system that is dried using radiation is a paint system that is dried using infrared radiation, wherein preferably the paint system is a water-based paint system.

7. Method according to any one of the preceding claims 1 - 3, wherein the paint system that is dried using radiation is a paint system that is dried using induction heating of the steel strip, wherein preferably the paint system is a water-based paint system.

8. Method according to claims 4, 5, 6 or 7 wherein the system that is dried or cured using radiation is partly cured or dried using retained heat from the Continuous Annealing Section in the steel strip before and/or during electron beam or ultraviolet radiation curing or infrared radiation drying.

9. Method according to any one of the preceding claims, wherein a pre-treatment layer, a pre-treatment-primer layer or a primer layer is applied on the steel strip before the paint system that is dried or cured using radiation is applied, preferably the pre-treatment, pre-treatment-primer or primer layer has a thickness of 0.1 - 10 micrometres, more preferably has a thickness of 0.5 - 5 micrometres.

10. Method according to any one of the preceding claims, wherein the roller coating technique is a gravure roller coating technique or deformable roller coating technique.

1 1. Method according to any one of the preceding claims, wherein the paint system that is dried or cured using radiation is applied on a metallic coated steel strip such as aluminium coated steel strip or zinc coated steel strip.

12. Method according to any one of the preceding claims, wherein the paint system that is dried or cured using radiation is applied on both sides of the steel strip.

13. Method according to any one of the preceding claims, wherein the steel strip is suitable for hot forming operations, such as a boron steel strip, or wherein the steel strip is suitable for high temperature use up to 600° C.

14. Steel strip having a coating consisting of a layer of an electron beam or ultraviolet or infrared radiation cured or dried paint system or a paint system dried using induction heating, or consisting of a pre-treatment layer, pre-treatment-primer layer or primer layer on top of which a layer of an electron beam or ultraviolet or infrared radiation cured or dried paint system or a layer of a paint system dried using induction heating has been applied.

15. Steel strip according to claim 14, wherein the layer of an electron beam or ultraviolet or infrared radiation cured or dried paint system or the layer of a paint system dried using induction heating has a thickness between 1 and 30 micrometres.

Steel strip according to claim 14 or 15, wherein the pre- treatment layer, pre- treatment-primer layer or primer layer has a thickness between 0.1 and 10 micrometres, preferably between 0.5 and 5 micrometres.

17. Steel strip according to claim 14, 15 or 16, wherein the steel strip has been provided with a metallic coating.