The invention relates to an apparatus and a method for the continuous manufacturing of steel strips coated with a varnish for electrical applications.

The terminology “electrical applications” relates to the use of steel strips to manufacture goods having specific magnetic properties, e.g. stator and rotor of electric motors, transformers and turbine of windmill. Steel for electrical applications are also known as electrical steel and generally comprises 0 to 6 weight percent of silicon. The electrical steels can be divided into two categories : the non-oriented steel and the oriented steel.

The electrical steels are usually coated with a varnish to insulate the steel in order to prevent the flow of electricity and to reduce the eddy current. This varnish is generally a water-based solution comprising 25 to 75 weight percent of resin and 5 to 15 weight percent of solvent (such as alcohol) and a balance consisting of water. The varnishing process comprises at least two steps : a first one where the varnish is coated in the form of a wet film and a second one where the varnish is cured to get a reticulated dry film. The thickness of the wet film is generally from 1.5 to 15 pm in order to have a dry film thickness from 0.8 to 5 pm.

In the manufacturing process, the varnishing takes place after an annealing. Depending on the line architecture, the varnishing section can be coupled with the annealing furnace or not. The fact that the varnish section is coupled or not with an annealing furnace impacts the line speed. Furthermore, the varnish needs to be reticulated in a drying step at the peak metal temperature (PMT) which also limits the maximal speed line. Consequently, the line speed can vary from 30 to 300 m.mim ¹ .

US 6 227 196 aims at coating a thin uniform layer of an insulating lacquer consistently at high speeds on an electrical steel strip. This apparatus comprises an engraved pick-up roll and an applicator roll having a substantially plain surface free of grooves or other indentations.

The purpose of this invention is to provide an apparatus and a method permitting to uniformly varnish an electrical steel strip for a wide range of line speed. Indeed, most of the coating apparatus for electrical steel are coupled with an annealing furnace leading to variation of the line speed.

This is achieved by providing an apparatus according to any one of the claims 1 to 5. This is also achieved by providing a method according to any one of the claims 6 to 13.

Other characteristics and advantages will become apparent from the following description of the invention.

Figure 1 illustrates an embodiment of the invention.

Figure 2 illustrates a second embodiment of the invention.

Figure 3 illustrates the influence of the hardness of the applicator roll and the speed line on the maximum varnish thickness .

The invention, as represented in Figure 1, relates to a coating apparatus 1 for the continuous manufacturing of steel strips S coated with a varnish for electrical applications comprising a tank 2, a coating roll 3 and an applicator roll 4 wherein:

- said tank 2 is able to contain a varnish solution and is configured such that said coating roll 3 dips into said varnish solution,

- said applicator roll 4 is configured to be in contact with said coating roll 3 and said steel strip S, and to homogeneously coat said steel strip in the width of said steel strip and the surface of said applicator roll has a hardness from 40 to 60 shores A.

Figure 2 shows a steel strip S advancing through a varnishing installation, along a direction D. The strip first runs through the coating apparatus 1, the purpose of which is to apply a varnish solution, then through a drying unit 5, the purpose of which is to eliminate the moisture of the varnish and then reticulate the varnish. A device 6 that measures the thickness of the coated varnish can be located between the coating apparatus 1 and the drying unit 5 and can be connected to a control unit 9 of the coating apparatus 1.

In the embodiment illustrated in Figure 2, the coating apparatus 1 is capable of coating the two faces of the strip simultaneously, but in an alternative embodiments the coating apparatus 1 can be made up only of the left part 1A or of the right part IB of the apparatus so that it only coats a single face of the strip.

One embodiment of the left part 1A of the coating apparatus 1 is illustrated in Figure 1. The right part IB is not described because it is symmetrical to the left part 1A with reference to the strip S. As illustrated in Figure 1, this device is made up of a first roll 4, which is the applicator roll 4. This applicator roll 4 is in contact on one hand with the strip S and on the other hand with a second roll 3, which is the coating roll 3. During the manufacturing process, the coating roll 3 is in rotation and is dipped in the tank 2 containing the varnish solution to be deposited on the strip S. The tank 2 is preferably equipped with a wiping device that squeezes the solution on the surface of the coating roll 3.

The wiping device makes it possible to ensure that the coating roll 3 is coated with the necessary quantity of solution to be deposited to obtain the required wet varnish thickness. This wiping device is preferably equipped with doctor blade. This device can be made up of two blades, for example, the first blade being located at the point where the coating roll 3 enters the tank 2 and makes it possible to remove residual varnish solution remaining from the preceding passage of the coating roll 3 through the tank 2, and the second blade being located at the point where the roll coating roll 3 exits the tank 2, and makes it possible to remove the excess solution. The blades can be made, for example of plastic, stainless steel or carbon fibre. The device can also be a single blade, for example, a blade made of stainless steel, located at the point where the coating roll 3 exits the tank 2.

This varnish solution has a viscosity from 2.5 x 10 ² Pa.s at 20°C to 4.0 x 10 Pa.s at 20°C. In a first example, the solution comprises dry extract between 30-50 weight percent composed of acrylic resin and phosphate pigments, co-solvent (alcohol) 5-10 weight percent and a balance consisting of water. In another example, the solution comprises dry extract of 40-60 weight percent being a mix of polyurethane resin and aluminium and silicon oxide, co-solvent (alcohol) 5-10 weight percent and a balance consisting of water.

The coating roll 3 is a roll coated by a ceramic whose surface is engraved. More precisely, the coating roll 3 is engraved with any patterns or cell structures. For example, the pattern can be a 60° or 30° or 70° hexagonal cells, line pattern, cobra pattern. The cells of the coating roll are filled with varnish solution. The surface of the coating roll can be made of ceramic. The cells of the coating roll can be etched with the aid of a laser or mechanical tooling.

In a first preferred embodiment, the coating roll 3 has a plurality of 60° hexagonally shaped cells, the line count of which is between 50 and 200 lines per centimetre, and the volume of which is from 5x10 ³ to 6xl0 ⁵ m ³ per square meter of roll surface, and preferably from IxlO ³⁵ to 5xl0 ³ m ³ per square meter of roll surface.

In second preferred embodiment, the coating roll 3 has a cobra pattern with a cell volume from 1x10 ^s to 8xl0 ⁵ m ³ per m ² of roll surface.

In a third preferred embodiment, the coating roll 3 has a quadrangular pattern with a cell volume from 1x10 ^s to 4xl0 ⁵ m ³ per m ² of roll surface.

After passage through the tank 2, the surface of the coating roll 3 coated with varnish solution comes into contact with the applicator roll 4. The coating roll 3 can rotate either in the clockwise direction or in the opposite direction. The applicator roll 4 is preferably itself driven in a direction of rotation that is opposite to the one of the coating roll 3.

The applicator roll 4, or at least its surface, is preferably made of a material that makes possible an optimum transfer of the solution, i.e. a transfer that minimizes the losses of solution due, for example, to absorption by the material of which the roll or its surface is made, or on the contrary by excessive slippage of the solution over the surface of the roll. This material can also exhibit resistance to chemical corrosion resistance as well as to mechanical wear. Preferably, the applicator roll 3 is made of elastomer. In a first preferred example, the applicator roll is made of chlorosulfonated polyethylene. In a second preferred embodiment, the applicator roll is made of polyurethane.

The applicator roll is configured to homogeneously coat said steel strip in the width direction. Consequently, there is no varnish pattern on the resulting coated strip. In order to achieve such a homogeneous coating, the applicator roll surface is a substantially plain surface free of grooves or other indentations.

Preferably, the applicator roll 3 has a roughness Ra from 0.5 pm to 3.5 pm. If the applicator roll roughness Ra is greater than 3.5 pm, inhomogeneity of the coated varnish can appear. Even more preferably, said applicator roll has a roughness Ra from 0.6 to 1.5 pm.

The applicator roll has a hardness from 40 to 60 shores A. The hardness can be measured following the ASTM D2240 standard.

If the hardness is below 40 shores A, then the lifespan of the applicator roll is severely reduced due to the coating process. Preferably, the applicator roll 3 has a hardness of at least 45 shores A, such a minimal hardness provides a better lifespan and still provide a good deformation of the roll leading to an uniform coating.

If the hardness is greater than 60 shores A, the maximum varnish wet thickness is too limited, especially for smaller line speed. Preferably, the applicator roll 3 has a hardness of at most 55 shores A.

Indeed, in Figure 3, the maximum varnish wet thickness that can be coated with a coating roll is plotted in function of the speed line. This is plotted for several hardness of the roll surface : 40 and 60 Shores A (as per the invention) and 80 shores A. All of the curves are plotted for a total force applied to the applicator roll of 3000 N per meter of width of the strip and a viscosity of 0.12 Pa.s.at 20°C.

It can be observed that for a coating roll according to the invention, the maximum wet thickness that can be coated is of at least 15 pm for a speed line of at least 30 m.min ⁴ .

Consequently, the claimed range of hardness for the applicator roll offers a wide range of varnish coating wet thickness while assuring a sufficient durability.

The invention also relates to a method for the continuous manufacturing of steel strips coated with a varnish, carried out by means of an apparatus as previously described, wherein a varnish layer is deposited on at least one of the faces of said strip, said varnish layer having a wet thickness from 1.5 pm to 15 pm and said varnish having a viscosity from 2.5 x 10 ² Pa.s at 20°C to 4.0 x 10 Pa.s at 20°C, and wherein said coating roll is supplied with varnish by dipping in said tank and said applicator roll being in contact with said coating roll and said steel strip and said steel strip running at a speed of at least 30 m.min .

Preferably, said steel strip is an electrical steel strip. Even more preferably, said steel strip is a non-oriented electrical steel strip. Said non-oriented electrical steel strip preferably comprises from 1.5 to 4 weight percent of silicon.

Preferably, the steel strip is running at a speed from 30 to 500 m.min . Even more preferably, the steel strip is running at a speed from 30 to 300 m.min ⁴ .

The contact pressure between the coating roll 3 and the applicator roll 4 must be sufficient to transfer the varnish solution from the coating roll to the applicator roll, although it must not be so great that it causes splashing which can result in losses of solution and therefore a non-optimal transfer of the solution. This pressure between said two rolls can be applied, for example, with the aid of a cylinder mounted on the coating roll 3. The total force FRR applied to the coating roll 3 is preferably from 500 to 3000 N per meter of width of the strip.

Following this contact, the applicator roll 4 is coated with the wet varnish solution which is to be deposited on the strip S.

The contact pressure between the applicator roll 4 and the strip must not be too great, at the risk of wiping off too much of the wet varnish solution film deposited, nor too low, at the risk of insufficiently wetting the strip with the solution. Moreover, a minimum pressure must be exerted by the roll on the strip to compensate for potential flatness defects in the latter. This pressure between the applicator roll 4 and the strip S can be applied, for example, with the aid of at least one cylinder mounted on the applicator roll. The total force F B applied to the applicator roll 4 is preferably from 2000 to 6000 N per meter of width of the strip.

The speed of rotation of the applicator roll 4 and of the coating roll 3 can be synchronized with the speed of displacement of the strip.

In one embodiment of the invention, the thickness of the wet varnish thickness deposited is measured with the aid of the thickness-measuring device 6.

In addition, or in an additional embodiment of the invention, the speed of rotation of the coating roll 3 can be controlled by the control unit 9 illustrated in Figure 1. This control unit 9 is connected to the device that measures the wet varnish thickness. If the thickness is less than a target thickness, the speed of the coating roll 3 is increased and vice-versa.

The thickness can be measured, for example, with the aid of an electromagnetic radiation gauge. These gauges are preferably installed as close as possible to the coating apparatus 1 because the moisture evaporates from the film very quickly, which can cause a significant variation in the measurements.

Following the step of the deposition of the wet varnish layer, the strip is subjected to a drying step. This drying step is carried out with the aid of a drying unit 5.

Preferably, following this drying phase, the thickness of the varnish layer deposited is from 0.8 pm to 5 pm after drying.

This drying unit 5 is, for example, a drying tunnel equipped with nozzles that spray air in the direction of the coated strip at a temperature between 120 and 350° C The drying unit can also be an inductor increasing the temperature of the strip to a PMT preferably between 220°C and 290°C.