JP7655772B2 - Painting method for coated steel plate, coated steel plate and coating evaluation method - Google Patents

Description

The present invention relates to a coating method for coated steel sheets that can accurately estimate and evaluate the state of the coating film formed on the coated steel sheet during coating, and can form a coating film with excellent appearance.

Conventionally, various methods have been used to coat the surface of a metal strip, such as a galvanized steel strip, which is generally continuously running, with a coating liquid such as a conversion agent or paint, such as immersing the metal strip in the coating liquid, spraying the coating liquid onto the surface of the metal strip, or applying the coating liquid using a roll coater or brush.

Of these various means, roll coating is generally performed using a roll coater that has two (two-roll coater) or three (three-roll coater) coating rolls arranged on the surface of the steel sheet as it travels continuously.

Figure 1 shows an example of a two-roll coater. It is equipped with a pickup roll 4 and an applicator roll 5. The pickup roll 4 picks up the coating liquid 2 in the coater pan 1, which is then transferred to the applicator roll 5, which then transfers the transferred coating liquid 2 to a steel strip (steel plate) 6, thereby coating the steel plate with the coating liquid. There are two methods for coating a steel plate with the coating liquid: natural coat and reverse coat. The roll coater shown in Figure 1 is a reverse coater, and the solid arrow in the figure indicates the direction of rotation of the rolls.

In the coating method for coated steel sheets using a roll coater as described above, the state of the coating film formed is usually checked after coating, and if any defects are found, the coating conditions are adjusted thereafter.
Therefore, from the viewpoint of eliminating waste of materials such as steel strips and coating liquid, there has been a need for the development of technology that can estimate the state of the coating film formed on a steel sheet when coating with a coating liquid and that can control the coating conditions of the coating liquid based on the estimated coating film state.

In view of the above circumstances, the present invention aims to provide a coating method for coated steel plates that can accurately estimate and evaluate the state of the coating film formed on the coated steel plate during coating and can form a coating film that is free of defects and has excellent appearance, a coated steel plate on which a coating film that is free of defects and has excellent appearance is formed, and a coating evaluation method that can accurately estimate and evaluate the state of the coating film formed on the coated steel plate during coating.

The inventors conducted research to solve the above problems regarding a coating method for coated steel sheets, in which a coating liquid is applied to a continuously running steel sheet using a roll coater equipped with a pick-up roll and an applicator roll. As a result, they discovered that by evaluating the state of the pick-up roll, the applicator roll, and the excess coating liquid scraped off by the gap or nip between the pick-up roll and the applicator roll during coating, it is possible to accurately estimate and evaluate the state of the coating film formed on the steel sheet during coating, and further, by adjusting the conditions of the pick-up roll and the applicator roll based on the obtained evaluation results, it is possible to form a coating film without defects and with excellent appearance.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A method for coating a steel plate, in which a coating liquid is applied to a continuously running steel plate by a roll coater equipped with a pickup roll and an applicator roll, comprising the steps of:
Photographing the state of the coating film of the coating liquid formed on the pickup roll, the state of the coating film of the coating liquid formed on the applicator roll, and the state of the excess coating liquid scraped off by the gap or nip between the pickup roll and the applicator roll (state of the curtain) during the coating;
A coating method for coated steel plate, characterized in that the coating liquid is applied while adjusting the roll circumferential speed and gap or nip pressure of the pickup roll and the applicator roll based on the evaluation results of the photographed image.

2. The coating state of the coating solution formed on the pickup roll and the coating state of the coating solution formed on the applicator roll are the uniformity, width and pitch of the roping formed on each roll,
2. The method for coating a coated steel plate according to claim 1, wherein the state of the curtain is determined by the uniformity of the excess coating liquid that has been scraped off and the presence or absence of irregular lifting defects of the excess coating liquid.

3. The method for coating a coated steel sheet according to 1 or 2 above, characterized in that the Ford cup viscosity (Ford cup No. 4) of the coating liquid is 45 to 80 seconds.

4. The coating method for coated steel sheets described in 1 or 2 above, characterized in that the roll coater is a two-roll coater.

5. A coated steel sheet having a coating film formed by the coating method according to any one of 1 to 4 above,
The coating film is characterized in that, when the height (H) and roughness (R) of the surface of the coating film are measured using a 3D microscope, the average height (H _ave ), the average roughness (R _ave ) and the film thickness (T) (μm) satisfy the following (1) and (2):
0.30×T≦H _ave ≦0.50×T...(1)
R _ave ≦0.09×T...(2)

6. A coating evaluation method for evaluating the state of a coating of a coating liquid formed on a steel sheet by a roll coater equipped with a pickup roll and an applicator roll, comprising the steps of:
A coating evaluation method, characterized in that the coating state of the coating liquid formed on the pickup roll, the coating state of the coating liquid formed on the applicator roll, and the state of excess coating liquid scraped off by the gap or nip between the pickup roll and the applicator roll (state of the curtain) are photographed during the coating, and the coating state of the coating liquid is evaluated from the photographed images.

The present invention provides a coating method for coated steel sheets that can accurately estimate and evaluate the state of the coating film formed on the coated steel sheet during coating, and can form a coating film that is free of defects and has excellent appearance, a coated steel sheet on which a coating film that is free of defects and has excellent appearance is formed, and a coating evaluation method that can accurately estimate and evaluate the state of the coating film formed on the coated steel sheet during coating.

FIG. 1 is a perspective view showing a roll coater including a pickup roll and an applicator roll. 1 is a cross-sectional view showing a schematic diagram of one embodiment of a roll coater used in the coating method for coated steel plate of the present invention. FIG. FIG. 2 is a cross-sectional view showing a schematic diagram of roping formed on a roll. The graph shows examples of evaluation points that serve as the criteria for evaluating the state of the coating film of the coating liquid formed on the pickup roll, the state of the coating film of the coating liquid formed on the applicator roll, and the state of the curtain on a five-point scale. The best evaluation score (5 points) and the worst evaluation score (1 point) were compared for each of the coating state of the coating liquid formed on the pickup roll, the coating state of the coating liquid formed on the applicator roll, and the state of the curtain.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
(Painting method for coated steel sheets)
The coating method for coated steel plate of the present invention (hereinafter sometimes simply referred to as "the coating method of the present invention") is a coating method in which a coating liquid 2 is coated onto a continuously running steel plate 6 by a roll coater equipped with a pickup roll 4 and an applicator roll 5 (hereinafter sometimes collectively referred to as "roll 3") as shown in Figure 1.

As shown in FIG. 2, the pickup roll 4 is installed so that the lower part of the roll is immersed in the coating liquid 2 in the coater pan 1. As the pickup roll 4 rotates, it draws up the coating liquid 2 in the coater pan 1. FIG. 2 illustrates an example in which the pickup roll 4 rotates counterclockwise.

The applicator roll 5 contacts the peripheral surface of the pickup roll 4 with a predetermined nip pressure or gap, and the coating liquid 2 on the pickup roll 4 that has moved to the transfer section is transferred to the peripheral surface of the applicator roll 5 while being squeezed to a predetermined film thickness by the nip pressure or gap. The coating liquid 2 transferred to the applicator roll 5 moves toward the steel plate 6 as the applicator roll 5 rotates, and is continuously applied to the surface of the steel plate 6. In the example shown in FIG. 2, the applicator roll 5 rotates clockwise, and the coating liquid 2 transferred from the pickup roll 4 moves toward the steel plate 6 as the applicator roll 5 rotates, coating the steel plate 6 with the coating liquid 2.

The material and dimensions of the pickup roll 4 are not particularly limited, and can be appropriately selected depending on the type of the coating liquid 2 and the required performance.
The material and dimensions of the applicator roll 5 are not particularly limited, and can be appropriately selected depending on the type of coating liquid 2 and the required performance. For example, when the applicator roll 5 is a metal roll, it does not come into contact with the pickup roll 4, so a gap is formed on the contact surface with the pickup roll 4. On the other hand, when the applicator roll 5 is a rubber roll, it comes into contact with the pickup roll 4 at a predetermined pressure, so a predetermined nip pressure is formed on the contact surface with the pickup roll 4.

In FIG. 1, the coating liquid 2 is transferred from the applicator roll 5 to the steel sheet 6 while the steel sheet (steel strip) 6 is wound around a backup roll 8. However, it is also possible to eliminate the backup roll by placing roll coaters on both sides of the steel sheet 6, or by placing a support roll on the side opposite the coating surface.
The pickup roll 4 and the applicator roll 5 may each be provided with a scraping device (not shown) for removing the coating liquid 2 that has not been transferred and remains.

The type of the coating liquid 2 is not particularly limited, and can be appropriately selected depending on the type of coating film to be formed on the steel sheet 6 (a chemical conversion coating, an undercoat coating, a topcoat coating, etc.).
Furthermore, from the viewpoint of more significantly exerting the effect of the present invention of being able to form a coating film free of defects and excellent in appearance, it is preferable that the viscosity of the coating liquid 2 is high, and specifically, the Ford cup viscosity (Ford cup No. 4) is preferably about 45 to 80 seconds, and more preferably about 50 to 60 seconds. If the Ford cup viscosity (Ford cup No. 4) of the coating liquid 2 is 45 seconds or more, the appearance of the coating film is likely to deteriorate due to defects such as roping, and therefore the effect of the present invention can be more significantly exerted.

The roll coater may be a two-roll coater equipped with the pickup roll 4 and the applicator roll 5 as shown in FIGS. 1 and 2, or may be a multi-roll coater further equipped with other rolls (not shown) such as an intermediate roll or an auxiliary roll.
Among these, the roll coater is preferably a two-roll coater, since this makes it possible to more significantly exhibit the effect of the present invention of being able to form a coating film free of defects and excellent in appearance.

In the coating method of the present invention, as shown in FIG. 2, when coating the coating liquid 2, photographs are taken of the coating state of the coating liquid 2 formed on the pickup roll 4, the coating state of the coating liquid 2 formed on the applicator roll 5, and the state of the excess coating liquid 7 scraped off by the gap or nip between the pickup roll 4 and the applicator roll 5 (state of the curtain 7),
The coating of the coating liquid 2 is performed while adjusting the roll circumferential speed and the gap or nip pressure of the pickup roll 4 and the applicator roll 5 based on the evaluation results of the photographed image.

The coating film state, such as roping of the coating liquid 2 formed on the pickup roll 4 and the applicator roll 5, and the state of the curtain 7 greatly affect the surface properties of the coating film 6A formed on the steel plate 6. Therefore, by accurately grasping and evaluating these states, the state of the coating film 6A formed on the steel plate 6 can be accurately estimated and evaluated during coating, without actually observing it.
Then, based on the results of the evaluation, the roll circumferential speed and the gap or nip pressure of the pick-up roll 4 and the applicator roll 5 are adjusted, thereby bringing the state of the coating liquid 2 formed on the pick-up roll 4, the coating liquid 2 formed on the applicator roll 5, and the curtain 7 into a good state, thereby forming a coating film 6A that is free of defects and has excellent appearance.

The state of the coating film formed on the pickup roll 4 and the applicator roll 5 refers to the state of the coating film formed by the coating liquid 2 transferred onto each roll 3, such as the presence and shape. The state of the coating film of the coating liquid 2 formed on the roll 3 is not particularly limited as long as it allows the state of the coating film 6A formed on the steel plate 6 to be grasped, and examples of such state include the presence or absence, uniformity, size, width or pitch of the ropings 2', 2" formed on each roll 4, 5, or unevenness or thickness of the coating film. Among these, from the viewpoint of being able to more reliably estimate and evaluate the state of the coating film 6A, it is preferable that the state of the coating film formed on the pickup roll 4 and the applicator roll 5 is the state of uniformity, width and pitch of the ropings 2', 2" formed on each roll 4, 5.

The roping 2', 2" refers to protuberances formed in the coating film of the coating liquid 2 on the roll 3, as shown in Figures 1 and 3. The occurrence of this roping 2', 2" is mainly due to the peripheral speed of the roll 3, the nip pressure or gap, and the physical properties of the coating liquid (viscosity, surface tension, etc.), and is more likely to occur as the viscosity of the coating liquid 2 is higher and the difference in peripheral speed between the pick-up roll 4 and the applicator roll 5 is smaller. However, since this roping is gradually leveled and smoothed after the coating liquid 2 is coated on the steel plate 6, its occurrence does not pose a problem if it is of a certain size, and since it is uniformly smoothed after the coating liquid 2 is coated, it is important that the roping 2', 2" formed on the pick-up roll 4 and the applicator roll 5 is formed uniformly.
The width of the ropings 2', 2" is the width of the convex portions of the ropings 2', 2", as shown in FIG. 3, and the pitch of the ropings 2', 2" is the distance between the ropings 2', 2", as shown in FIG. 3.

The state of the curtain 7 refers to the presence, shape, and other conditions of the excess coating liquid scraped off by the gap or nip between the pickup roll 4 and the applicator roll 5. The state of the curtain 7 is not particularly limited as long as it allows the state of the coating film 6A formed on the steel sheet 6 to be grasped, and examples of the state of the curtain 7 include the presence or absence, uniformity, size, and the presence or absence of irregular lifting defects of the excess coating liquid. Among these, from the viewpoint of more reliably estimating and evaluating the state of the coating film 6A, the uniformity of the excess coating liquid and the presence or absence of irregular lifting defects of the excess coating liquid are preferable.
The uniformity of the excess coating liquid refers to the uniformity of the shape of the scraped off excess coating liquid when it falls (uniformity of the shape of the curtain 7), and is uniform when the excess coating liquid falls in the same state (when the shape of the curtain 7 continues in the same state), but is not uniform when there are places where the way it falls is not uniform or there is fluctuation in the way it falls (when there is no uniformity or continuity in the shape of the curtain 7).Furthermore, the failure to lift the excess coating liquid refers to a state in which there is unevenness in the way the excess coating liquid falls.

Here, the film state of the coating liquid 2 formed on the pickup roll 4 and the applicator roll 5, and the state of the curtain 7 can all be photographed by a photographing device 10. For example, in Fig. 2, photographing devices 11, 12, and 13 are provided at positions where the film state of the coating liquid 2 formed on the pickup roll 4, the film state of the coating liquid 2 formed on the applicator roll 5, and the state of the curtain 7 can be observed, respectively, and photographs of each state can be taken.
The type of the photographing device 10 is not particularly limited as long as it can accurately photograph the state of the coating film formed on the pickup roll 4 and the applicator roll 5, and the state of the curtain 7; for example, a high-speed camera can be used.
It is preferable to observe the state of the coating film formed on the pickup roll 4 and the applicator roll 5, and the state of the curtain 7, using still images and video.

The method for evaluating the captured still images and video images is not particularly limited as long as the method can accurately estimate and evaluate the state of the coating film 6A formed on the steel plate 6.
As one of the evaluation methods, for example, a method in which the state of the coating film formed on the pickup roll 4 and the applicator roll 5 and the state of the curtain 7 are each evaluated on a 5-point scale using photographed images is given.
4 shows images (examples of evaluation points) of the standard states when performing a 5-point evaluation on the coating state of the coating liquid 2 formed on the pickup roll 4, the coating state of the coating liquid 2 formed on the applicator roll 5, and the state of the curtain 7. For example, by comparing images of the coating state of the coating liquid 2 formed on the pickup roll 4, the coating state of the coating liquid 2 formed on the applicator roll 5, and the state of the curtain 7 with the examples of evaluation points shown in FIG.

Figure 5 compares the best evaluation score (5 points) and the worst evaluation score (1 point) for the photographed images of the coating state of the coating liquid 2 formed on the pickup roll 4, the coating state of the coating liquid 2 formed on the applicator roll 5, and the state of the curtain 7. As can be seen from Figure 5, the photographed image with an evaluation score of 1 shows poor uniformity and clarity of the coating ropings 2', 2", and the state of the curtain 7 is also uneven.

If the roping state of the coating film formed on the pickup roll 4, the roping state of the coating film formed on the applicator roll 5, and the state of the curtain 7 are all good, there is no need to adjust the roll circumferential speed, gap, nip pressure, etc. of the roll 3 described below; however, if they are not good, the roll circumferential speed and the gap or nip pressure of the roll 3 are adjusted.
The criteria for determining whether the roping state of the coating film formed on the pickup roll 4 and the roping state of the coating film formed on the applicator roll 5 are good or not can be appropriately set according to the required performance. For example, when the above-mentioned five-level evaluation is performed, a threshold value for the total evaluation value is set, and depending on whether the total value falls below the threshold value, it is possible to determine whether or not to adjust the roll circumferential speed of the roll 3 and the gap or nip pressure.

The method for adjusting the roll circumferential speed and the gap or nip pressure of the pickup roll 4 and the applicator roll 5 is not particularly limited, and may be performed according to commonly used procedures and conditions.
For example, when the roping of the coating film formed on the pickup rolls 4 and 5 is non-uniform, the roping can be made uniform by adjusting the roll circumferential speed difference between the pickup rolls 4 and 5. When the curtain 7 is non-uniform or when there is an irregular lifting failure of the excess coating liquid, the state of the curtain 7 can be adjusted by adjusting the roll circumferential speed difference between the pickup rolls 4 and 5.

The roll circumferential speeds of the pickup roll 4 and the applicator roll 5 are not particularly limited and can be set appropriately according to the conditions of the coating device. For example, the roll circumferential speeds of the pickup roll 4 and the applicator roll 5 are preferably set in the range of 10 to 200 mpm, and it is more preferable to set the roll circumferential speed of the pickup roll 4 to 20 to 30 mpm in order to adjust the difference in roll circumferential speed between the pickup roll 4 and the pickup roll 5.

(painted steel plate)
The coated steel sheet of the present invention is a coated steel sheet having a coating film formed by the above-mentioned coating method of the present invention.
The coating film is characterized in that the surface irregularity height (H) and roughness (R) are measured using a 3D microscope, and the average irregularity height (H _ave ), average roughness (R _ave ) and film thickness (T) (μm) satisfy the following (1) and (2):
0.30×T≦H _ave ≦0.50×T...(1)
R _ave ≦0.09×T...(2)

When the coating film satisfies the above relationships (1) and (2), a coating film that is free of defects and has excellent appearance can be obtained.
From the same viewpoint, the above formula (1) is preferably the following formula (1)', and the above formula (2) is preferably the following formula (2)'.
0.35×T≦H _ave ≦0.45×T...(1)'
R _ave ≦0.07×T...(2)'

The height (H) and the roughness (R) are the height and roughness of the unevenness formed on the coating surface, and can be measured by a 3D microscope. The average height (H _ave ) and the average roughness (R _ave ) are the average values of these, and are calculated by averaging the height (H) and the roughness (R) of three arbitrarily selected points.

The coating film mentioned above refers to all coatings formed on steel sheets, and includes not only undercoat coatings (primers) and topcoat coatings, but also chemical conversion coatings and other protective films.

(Painting evaluation method)
As shown in Figs. 1 and 2, the coating evaluation method of the present invention is a coating evaluation method for evaluating the state of a coating of a coating liquid 2 formed on a steel plate 6 by a roll coater equipped with a pickup roll 4 and an applicator roll 5,
During the coating, the coating state of the coating liquid 2 formed on the pickup roll 4, the coating state of the coating liquid 2 formed on the applicator roll 5, and the state of the excess coating liquid scraped off by the gap or nip between the pickup roll 4 and the applicator roll 5 (state of the curtain 7) are photographed, and the coating state of the coating liquid 2 is evaluated from the photographed images.
The coating film state, such as roping of the coating liquid 2 formed on the pickup roll 4 and the applicator roll 5, and the state of the curtain 7 greatly affect the surface properties of the coating film 6A formed on the steel plate 6. Therefore, by accurately grasping and evaluating these states, the state of the coating film 6A formed on the steel plate 6 can be accurately estimated and evaluated during coating, without actually observing it.

The configuration of the coating evaluation method of the present invention is the same as that described above in the coating method of the present invention.

The present invention provides a coating method for coated steel sheets that can accurately estimate and evaluate the state of the coating film formed on the coated steel sheet during coating, and can form a coating film that is free of defects and has excellent appearance, a coated steel sheet on which a coating film that is free of defects and has excellent appearance is formed, and a coating evaluation method that can accurately estimate and evaluate the state of the coating film formed on the coated steel sheet during coating.

Claims (5)

A coating method for a coated steel plate, comprising the steps of: coating a coating liquid on a continuously running steel plate by a roll coater equipped with a pickup roll and an applicator roll,
During the coating, the state of the coating film of the coating liquid formed on the pickup roll, the state of the coating film of the coating liquid formed on the applicator roll, and the state of the excess coating liquid scraped off by the nip (state of the curtain) are photographed by cameras provided separately,
A coating method for coated steel plate, characterized in that the coating liquid is applied while adjusting the roll circumferential speed and gap or nip pressure of the pickup roll and the applicator roll based on the evaluation results of the photographed image. the state of the coating film of the coating liquid formed on the pickup roll and the state of the coating film of the coating liquid formed on the applicator roll are the states of uniformity, width and pitch of the roping formed on each roll,
2. The method for coating a coated steel plate according to claim 1, wherein the state of the curtain is determined by the uniformity of the excess coating liquid that has been scraped off and the presence or absence of irregular lifting defects of the excess coating liquid. The coating method for coated steel sheets according to claim 1 or 2, characterized in that the coating liquid has a Ford cup viscosity (Ford cup No. 4) of 45 to 80 seconds. The coating method for coated steel sheets according to claim 1 or 2, characterized in that the roll coater is a two-roll coater. A coating evaluation method for evaluating a state of a coating of a coating liquid formed on a steel sheet by a roll coater equipped with a pickup roll and an applicator roll, comprising the steps of:
The coating evaluation method is characterized in that, during the coating, the coating state of the coating liquid formed on the pickup roll, the coating state of the coating liquid formed on the applicator roll, and the state of the excess coating liquid scraped off by the nip (state of the curtain) are photographed with cameras provided separately, and the coating state of the coating liquid is evaluated from the photographed images.

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