AU2015223734B2

AU2015223734B2 - Precoated metal sheet, method for manufacturing precoated metal sheet, and continuous coating device

Info

Publication number: AU2015223734B2
Application number: AU2015223734A
Authority: AU
Inventors: Kenji Inada; Fumiaki Nakamura; Manabu OTANI; Fumio Shibao
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2014-02-28
Filing date: 2015-03-02
Publication date: 2016-10-06
Anticipated expiration: 2035-03-02
Also published as: KR20160102542A; KR101810350B1; CN105939848B; MY183342A; JP2016112894A; WO2015129918A1; CN105939848A; JPWO2015129918A1; JP5935949B2; AU2015223734A1

Abstract

To purpose of the present invention is to obtain high levels of both the adhesiveness between coatings and the glossiness in a multilayer precoated metal sheet produced on a continuous coating line. A precoated metal sheet having a plurality of coating layers on one or both sides of a metal sheet (10), wherein the coating layers include: a coloring coating layer (111) disposed on the metal sheet (10) side, the coloring coating layer (111) containing a coloring pigment; a pigment-enriched layer (123) formed on the coloring coating layer (111), the pigment-enriched layer (123) having a coloring pigment concentration higher than the average coloring pigment concentration in the coloring coating layer (111), and the coloring pigment concentration increasing towards the obverse layer side; a diffusion layer (122) formed on the pigment-enriched layer (123), the diffusion layer (122) having a coloring pigment concentration that decreases towards the obverse surface side; and a clear layer (121) formed on the diffusion layer (122), the clear layer (121) containing no coloring pigment. The ratio d1/d2 between the thickness d1 of the clear layer (121) and the thickness d2 of the pigment-enriched layer (123) is 1.7≤d1/d2≤4.7.

Description

Description Title of Invention

PRE-COATED METAL SHEET, METHOD FOR MANUFACTURING PRECOATED METAL SHEET, AND CONTINUOUS COATING APPARATUS

Technical Field [0001]

The present invention relates to a pre-coated metal sheet, a method for manufacturing a pre-coated metal sheet, and a continuous coating apparatus that are capable of achieving both high glossiness and high adhesion between film layers.

Background Art [0002]

Pre-coated metal sheets, which are coated in advance at steelworks, for example, are used as coated metal sheets for various uses, such as home appliances, building materials, civil works, machines, automobiles, furniture, and containers. A pre-coated metal sheet is obtained by coating a surface (one surface or both surfaces) of a metal sheet, such as a steel sheet or a plated steel sheet, with one or a plurality of films. For example, a pre-coated steel sheet is obtained by coating a surface of a plated steel sheet with multiple films (e.g., a primer film, a colored film, and a clear film).

[0003]

Examples of a conventional continuous coating method for continuously coating a long metal sheet (metal strip) with multiple layers by using a continuous coating line include (1) successive coating and baking method, (2) wet-on-wet method, and (3) multilayer simultaneous coating method.

[0004] (1) Successive coating and baking method is a coating method that successively performs, film by film, application (coat) and baking (bake) of a coating material (e.g., see Patent Literature 1). For example, a coating method that performs the application and baking of an upper layer film after the application and baking of a lower layer film is called a two-coat two-bake (2C2B) method. (2) Wet-on-wet method is a coating method that applies an upper layer film before a lower layer film is dried, by utilizing a difference in surface tension between coating materials, for example, without performing baking, drying, and the like after the application of the lower layer film (e.g., see Patent Literature 2). This wet-on-wet method is also called a two-coat one-bake (2C1B) method or a three-coat one-bake (3C1B) method, depending on the number of lower layer films. (3) Multilayer simultaneous coating method is a coating method that simultaneously applies a plurality of films by using a multilayer coater, such as a slide curtain coater (e.g., see Patent Literature 2).

[0005]

Besides the above-described methods that continuously coat a long metal strip by using a continuous coating line, there is also a method that individually coats outer parts of an automobile body, for example, by using an air spray or the like. It is known that a wet-on-wet method of this individual coating method uses preheating. To prevent mixing of coating materials in upper and lower layers, the wet-on-wet method using pre-heating applies the upper layer after the lower layer is applied and pre-heated at a relatively low temperature to be semi-cured. For example, Patent Literatures 3 and 4 disclose applying a colored base coating material in a lower layer onto an outer part of an automobile body made of metal or plastic, by electrostatic coating or by using an air spray, for example, then pre-heating the lower layer (80 °C, ten minutes), applying a clear coating material in an upper layer onto the uncured coating surface, and then simultaneously baking the upper and lower layers (140 °C, 30 minutes).

Citation List

Patent Literature [0006]

Patent Literature 1: JP 2006-000806A

Patent Literature 2: JP 2009-248378A

Patent Literature 3: JP 2004-025046A

Patent Literature 4: JP 2004-008856A

Summary of Invention

Technical Problem [0007]

In general, to obtain a multilayer pre-coated metal sheet, a colored film is formed as a lower layer by applying a colored coating material containing a color pigment, and then a clear film is formed as an upper layer by applying a clear coating material not containing a color pigment. In this case, in the successive coating and baking method (2C2B) according to Patent Literature 1, the clear film in the upper layer is applied and baked after the colored film in the lower layer is baked to be completely dried; thus, coating steps are independent from each other. This is advantageous in that the clear film in the upper layer and the colored film in the lower layer do not mix with each other, and the clear film not containing a color pigment can have a sufficient thickness, so that the gloss of the clear film can be ensured. There is a problem, however, in that when the upper layer is applied, the clear coating material in the upper layer does not easily conform to the completely dried and cured colored film in the lower layer, and the adhesion between the upper and lower layers decreases, which makes peeling likely to occur at the interface between the films in the two layers.

[0008]

In the wet-on-wet method and the multilayer simultaneous coating method according to Patent Literature 2, the coating materials in the upper and lower layers easily conform to each other at the interface between the films, and high adhesion is obtained between the upper and lower layers. When the upper layer is applied, however, the colored film in a wet state in the lower layer and the clear coating material in the upper layer mix with each other to form a mixed layer. This causes a problem in that the clear film not containing a color pigment cannot have a sufficient thickness, so that the gloss of the pre-coated metal sheet significantly decreases.

[0009]

On this point, in the wet-on-wet method using pre-heating according to Patent Literatures 3 and 4, the colored film in the lower layer is semi-cured by preheating, which prevents mixing of the colored film and the clear film in the upper layer; thus, the gloss can be high to some extent.

[0010]

However, also in the wet-on-wet method using pre-heating, a color pigment contained in the colored film in the lower layer diffuses into the clear film in the upper layer, affecting the gloss of the pre-coated metal sheet after baking. The present inventors have found through extensive research that when heating conditions at the time of pre-heating are inappropriate, the color pigment concentration distribution at the boundary portion between the pre-heated colored film and the clear film becomes inappropriate, owing to diffusion and convection of the color pigment from the lower layer to the upper layer, thus causing a decrease in the gloss of the pre-coated metal sheet.

[0011]

Nevertheless, the effects of heating conditions at the time of pre-heating and the color pigment concentration distribution at the boundary portion between the colored film and the clear film on the gloss of the pre-coated metal sheet have not been clarified in detail. Hence, the glossiness of the pre-coated metal sheet has sufficient room for improvement if it is possible to appropriately control the heating conditions at the time of pre-heating and the color pigment concentration distribution at the boundary portion.

[0012]

Furthermore, the coating method using pre-heating according to Patent Literatures 3 and 4 is applied not to the case of continuously coating a metal strip using a continuous coating line, but to the case of individually coating outer parts of an automobile body, for example, and the time for pre-heating the lower layer is as long as several minutes to several tens of minutes. Therefore, with regard to a multilayer pre-coated metal sheet manufactured by continuously coating a metal strip using a continuous coating line, appropriate heating conditions at the time of preheating and appropriate color pigment concentration distribution at the boundary portion between the colored film and the clear film have not been clarified.

[0013]

Hence, the present invention, which has been made in view of the above problems, aims to achieve both the glossiness and the adhesion between films of a multilayer pre-coated metal sheet manufactured in a continuous coating line at high levels.

Solution to Problem [0014]

According to an aspect of the present invention in order to achieve the above-mentioned object, there is provided a pre-coated metal sheet including a plurality of film layers on one surface or both surfaces of a metal sheet, the film layers including a colored film layer that is placed on the metal sheet side and contains a color pigment, a pigment-concentrated layer that is formed on the colored film layer, and whose color pigment concentration is equal to or greater than an average value of color pigment concentration in the colored film layer and increases toward an outer layer side, a diffusion layer that is formed on the pigmentconcentrated layer, and whose color pigment concentration decreases toward the outer layer side, and a clear layer that is formed on the diffusion layer and does not contain the color pigment. A ratio dl/d2 between a thickness dl of the clear layer and a thickness d2 of the pigment-concentrated layer satisfies 1.7 < dl/d2 < 4.7.

[0015] A ratio cl/c2 between a maximum value cl and a minimum value c2 of a color pigment concentration ratio c in the pigment-concentrated layer based on the average value of the color pigment concentration in the colored film layer may satisfy 1.04 < cl/c2 < 2.0.

[0016]

The ratio dl/d2 may satisfy 2.0 < dl/d2 < 4.7.

[0017]

The thickness d2 of the pigment-concentrated layer may be equal to or greater than 2.2 pm and equal to or less than 8.0 pm.

[0018]

The thickness dl of the clear layer may be equal to or greater than 5.0 pm and equal to or less than 12.3 pm.

[0019]

According to another aspect of the present invention in order to achieve the above-mentioned object, there is provided a method for manufacturing a pre-coated metal sheet, in which the pre-coated metal sheet is manufactured by continuously coating a surface of a strip-shaped metal sheet, the method including: a first coating step of, by a first coating device, applying a colored coating material containing a color pigment onto the surface of the metal sheet or onto another film applied onto the surface of the metal sheet, to form a colored film; a pre-heating step of, by a heating device, heating the colored film at a heating temperature of 60 to 150 °C for a heating time of 1 to 10 seconds to bring the colored film into a half-dried state; a second coating step of, by a second coating device, applying a clear coating material not containing the color pigment onto the colored film in a half-dried state to form a clear film; and a baking step of, by a baking device, simultaneously baking the colored film and the clear film.

[0020]

According to another aspect of the present invention in order to achieve the above-mentioned object, there is provided a continuous coating apparatus, in which the pre-coated metal sheet is manufactured by continuously coating a surface of a strip-shaped metal sheet, the continuous coating apparatus including: a first coating device configured to apply a colored coating material containing a color pigment onto the surface of the metal sheet or onto another film applied onto the surface of the metal sheet, to form a colored film; a heating device configured to heat the colored film at a heating temperature of 60 to 150 °C for a heating time of 1 to 10 seconds to bring the colored film into a half-dried state; a second coating device configured to apply a clear coating material not containing the color pigment onto the colored film in a half-dried state to form a clear film; and a baking device configured to simultaneously bake the colored film and the clear film.

Advantageous Effects of Invention [0021]

According to the present invention, both the glossiness and the adhesion between films of a multilayer pre-coated metal sheet manufactured in a continuous coating line can be achieved at high levels.

Brief Description of Drawings [0022] [FIG. 1] FIG. 1 is a schematic diagram illustrating a line configuration of a continuous coating apparatus according to a first embodiment of the present invention.

[FIG. 2A] FIG. 2A is an explanatory diagram illustrating a stacked state of films and color pigment concentration distribution in a first coating step according to the embodiment.

[FIG. 2B] FIG. 2B is an explanatory diagram illustrating a stacked state of films and color pigment concentration distribution in a pre-heating step according to the embodiment.

[FIG. 2C] FIG. 2C is an explanatory diagram illustrating a stacked state of films and color pigment concentration distribution in a second coating step according to the embodiment.

[FIG. 2D] FIG. 2D is an explanatory diagram illustrating a stacked state of films and color pigment concentration distribution after the second coating step according to the embodiment.

[FIG. 2E] FIG. 2E is an explanatory diagram illustrating a stacked state of films and color pigment concentration distribution in a baking step according to the embodiment.

[FIG. 3A] FIG. 3A is a schematic diagram illustrating a step of applying an upper layer coating material onto a lower layer film by a conventional 2C2B method.

[FIG. 3B] FIG. 3B is a schematic diagram illustrating a step of applying an upper layer coating material onto a lower layer film by a conventional wet-on-wet method. [FIG. 3C] FIG. 3C is a schematic diagram illustrating a step of applying an upper layer coating material onto a lower layer film by a pre-heating method according to the present embodiment.

[FIG. 4] FIG. 4 is a graph showing an example of the color pigment concentration distribution in films of a pre-coated steel sheet manufactured by each of conventional coating methods.

[FIG. 5] FIG. 5 is a graph showing an example of the color pigment concentration distribution in film layers of a pre-coated steel sheet according to the embodiment. [FIG. 6] FIG. 6 is a graph showing a modification example of the color pigment concentration distribution in film layers of a pre-coated steel sheet according to the embodiment.

[FIG. 7] FIG. 7 is a schematic diagram illustrating a line configuration of a continuous coating apparatus according to a second embodiment of the present invention.

[FIG. 8] FIG. 8 is a schematic diagram illustrating a line configuration of a continuous coating apparatus according to a third embodiment of the present invention.

[FIG. 9] FIG. 9 is a schematic diagram illustrating a line configuration of a continuous coating apparatus according to a fourth embodiment of the present invention.

[FIG. 10] FIG. 10 is a schematic diagram illustrating a method for bending a precoated steel sheet.

[FIG. 11] FIG. 11 is a graph showing the Ti concentration ratio distribution in the film thickness direction of Comparative Examples 1 to 3.

[FIG. 12] FIG. 12 is a graph showing the Ti concentration ratio distribution in the film thickness direction of Examples 1 to 4 and Comparative Example 1.

[FIG. 13] FIG. 13 is a graph obtained by partially enlarging FIG. 12.

[FIG. 14] FIG. 14 is a graph showing the relation between dl/d2 and the test results of 60-degree gloss.

[FIG. 15] FIG. 15 is a graph showing the relation between heating temperature T, and dl, d2, and the test results of 60-degree gloss.

[FIG. 16A] FIG. 16A is an enlarged photograph of a bent portion of a pre-coated steel sheet according to Comparative Example.

[FIG. 16B] FIG. 16B is an enlarged photograph of a bent portion of a pre-coated steel sheet according to Example.

Description of Embodiments [0023]

Hereinafter, (a) preferred embodiment(s) of the present disclosure will be described in detail with reference to the appended drawings. In this specification and the drawings, elements that have substantially the same function and structure are denoted with the same reference signs, and repeated explanation is omitted.

[0024] [1. Overview of method for manufacturing pre-coated metal sheet]

First, an overview of a method for manufacturing a pre-coated metal sheet according to a first embodiment of the present invention will be described.

[0025]

As described above, examples of a conventional coating method for continuously coating a strip-shaped long metal sheet (metal strip) include a successive coating and baking method, a wet-on-wet method, and a multilayer simultaneous coating method. These conventional coating methods, however, have a problem in that high glossiness and high adhesion between films of a pre-coated metal sheet cannot both be achieved.

[0026]

To solve this problem, in the present embodiment, in a continuous coating line for coating a metal sheet with a plurality of films, a heating device for preheating (hereinafter called a pre-heating device) is placed to follow a first coating device that applies a colored coating material in a first layer (lower layer), and to precede a second coating device that applies a clear coating material in a second layer (upper layer). This pre-heating device is a simple heating device having lower output and heating temperature and smaller installation space than a conventional baking device.

[0027]

In the method for manufacturing a pre-coated metal sheet according to the present embodiment, in continuously coating a strip-shaped metal sheet by the continuous coating line, first, a colored film in the lower layer is applied onto the metal sheet. Then, the colored film is pre-heated under appropriate heating conditions to be brought into a half-dried state by the pre-heating device. Furthermore, without the colored film in a half-dried state being subjected to cooling and drying, a clear coating material in the upper layer is applied to form a clear film. After that, the colored film in the lower layer and the clear film in the upper layer are simultaneously baked.

[0028]

This continuous coating method by a wet-on-wet method using pre-heating can form four characteristic film layers (clear layer, diffusion layer, pigmentconcentrated layer, and colored film layer) having different color pigment concentration distribution characteristics in the colored film and the clear film of the pre-coated metal sheet. This makes it possible to manufacture a pre-coated metal sheet having high glossiness and having high adhesion between the colored film in the lower layer and the clear film in the upper layer.

[0029]

Hereinafter, detailed description will be given on a continuous coating apparatus for implementing the wet-on-wet method using pre-heating (hereinafter called a pre-heating method) and a method for manufacturing a pre-coated metal sheet using the apparatus. In the example described below, a pre-coated steel sheet in which a steel sheet is coated with upper and lower two films, a colored film and a clear film, is manufactured as a pre-coated metal sheet, but a pre-coated metal sheet of the present invention is not limited to this example. For example, a metal sheet made of any material other than a steel sheet may be used as a base material (metal sheet) to be coated. In addition, the present invention can also be applied to a case of manufacturing a pre-coated metal sheet in which a metal sheet is coated with three or more films.

[0030] [2. Configuration of continuous coating apparatus]

Next, an entire configuration of a continuous coating apparatus according to the first embodiment of the present invention will be described with reference to FIG. 1. FIG. 1 is a schematic diagram illustrating a line configuration of the continuous coating apparatus according to the present embodiment.

[0031]

As illustrated in FIG. 1, the continuous coating apparatus according to the present embodiment is a coating line for continuously coating a surface of a stripshaped steel sheet 10 (steel strip) with a predetermined width to manufacture a multilayer pre-coated metal sheet. While passing the steel sheet 10 at a constant line speed in the sheet-passing direction, the continuous coating apparatus continuously applies a plurality of films 11 and 12 onto at least one surface of the steel sheet 10, and further, bakes, cools, and dries the plurality of films 11 and 12 simultaneously. The line speed in passing the steel sheet 10 is, for example, 30 to 200 m/min.

[0032]

Such a continuous coating apparatus includes a roll coater 13 (corresponding to a first coating device of the present invention), a pre-heating device 14 (corresponding to a heating device of the present invention), a curtain coater 15 (corresponding to a second coating device of the present invention), a baking device 16 (corresponding to a baking device of the present invention), a cooling device 17, and a drying device 18.

[0033]

The roll coater 13 is an example of the first coating device that applies a colored coating material containing a color pigment onto the surface of the steel sheet 10. The colored coating material is a coating material for forming a colored film 11 in a lower layer, and contains the color pigment. The roll coater 13 causes a roll holding the colored coating material to come in contact with the surface of the steel sheet 10 while rotating the roll, thereby applying the colored coating material onto the surface of the steel sheet 10. Thus, the colored film 11 is formed as a first layer (lower layer) on the surface of the steel sheet 10.

[0034]

As the first coating device, either of a contact-type coating device (e.g., roll coater) and a non-contact-type coating device (e.g., curtain coater) may be used; as illustrated in FIG. 1, the roll coater 13, which is of a contact type, is preferably used. In general, it is difficult for a curtain coater to control thickness. In contrast, the roll coater 13 can control thickness relatively easily and highly precisely by adjusting a roll rotation speed and a gap between rolls, and thus can control the thickness of the colored film 11 in the lower layer highly precisely. In addition, the use of the roll coater 13 broadens a viscosity range of coating materials that can be applied.

[0035]

The pre-heating device 14 is an example of a heating device that heats the colored film 11 at a predetermined heating temperature T. The pre-heating device 14 is placed to follow the above-described roll coater 13 (the first coating device) and to precede the curtain coater 15 (the second coating device) on the coating line. As illustrated in FIG. 1, on the coating line between these two coating devices, only the pre-heating device 14 is installed, and a baking device, a cooling device, a drying device, or the like for the colored film 11 is not installed. On this point, the continuous coating line according to the present embodiment has a device configuration and an arrangement different from those of a continuous coating line of a conventional 2C2B method.

[0036]

The pre-heating device 14 is configured with, for example, a hot air drying furnace, an induction heating furnace, an infrared heating furnace, or a furnace using any of these in combination; any heating device capable of heating a coating material can be used. This pre-heating device 14 does not bake the colored film 11 on the steel sheet 10 by heating it for a long time at a high temperature, but heats the colored film 11 for a short time at the predetermined heating temperature T (e.g., 60 to 150 °C) lower than a baking temperature by a general baking device. This heating temperature T is, for example, equal to or lower than the boiling point of a volatile content of the colored coating material. Note that the heating temperature T is expressed by a peak metal temperature (PMT) of the steel sheet 10 heated by the pre-heating device 14.

[0037]

For the above reason, the pre-heating device 14 is configured with a heating device having lower output, lower heating temperature, smaller size, and less installation space than a baking device generally used in a conventional coating line. For example, the output (necessary electric energy per unit mass of the steel sheet 10) of the baking device 16, which is of a conventional general IH type, is 11 to 24 kWh/ton. In contrast, the output of the pre-heating device 14 can be 3 to 17 kWh/ton, which is as low as approximately 27 to 70 % of the output of the general baking device 16. In addition, the heating temperature of the baking device 16 is 200 °C or higher in many cases. In contrast, the heating temperature T of the preheating device 14 can be as low as approximately 60 to 150 °C. Furthermore, the total length of the baking device 16 in the sheet-passing direction is generally 30 m or greater. In contrast, the total length of the pre-heating device 14 can be approximately 1 to 5 m.

[0038]

This pre-heating device 14 is used to heat the colored film 11 to the low heating temperature T equal to or lower than the boiling point of the volatile content of the colored coating material. This brings the colored film 11 into a “half-dried state” in which it is not completely dried by volatilization of all of the volatile content of the coating material (i.e., not baked), but partially dried by volatilization of part of the volatile content of the coating material.

[0039]

The curtain coater 15 is an example of the second coating device that applies a clear coating material onto the colored film 11. The curtain coater 15 applies the clear coating material onto the colored film 11 that has been brought into a half-dried state by being heated by the above-described pre-heating device 14, to form a clear film 12.

[0040]

As the curtain coater 15, for example, a known curtain coater, such as a curtain flow coater or a roller curtain coater, can be used. This curtain coater 15 causes the clear coating material to flow down, from above the steel sheet 10 being passed, in the form of a curtain with a width larger than a sheet width of the steel sheet 10, thereby applying the clear coating material onto the colored film 11 without contact. Thus, the clear film 12 in an upper layer is formed on the colored film 11 in the lower layer.

[0041]

As the second coating device, either of a contact-type coating device (e.g., roll coater) and a non-contact-type coating device (e.g., curtain coater) may be used; as illustrated in FIG. 1, a non-contact-type coating device, such as the curtain coater 15, is preferably used. This is because when the clear coating material is applied onto the colored film 11 in a half-dried state by using a roll coater, which is of a contact type, there is a possibility of peeling of the colored film 11.

[0042]

The use of a non-contact-type coating device, such as the curtain coater 15, as the second coating device prevents peeling of the colored film 11 and enables stable formation of the films in the upper and lower layers with desired thicknesses. In addition, the use of the non-contact-type coating device provides the clear film 12 with a smooth and beautiful appearance, and further, enables application of a coating material that is likely to cause uneven transfer from a roll, and allows high-speed coating.

[0043]

In addition, as illustrated in FIG. 1, the curtain coater 15 is placed to follow the pre-heating device 14 on the coating line, and a cooling device and a drying device for forced cooling and drying of the colored film 11 are not provided between the pre-heating device 14 and the curtain coater 15. Therefore, the curtain coater 15 applies the clear coating material onto the colored film 11 that is not subjected to forced cooling and drying after being heated by the pre-heating device 14. Such a configuration of the coating line enables the curtain coater 15 to apply the clear coating material onto the colored film 11 before the temperature of the colored film 11 in a half-dried state ejected from the pre-heating device 14 decreases, to make the colored film 11 and the clear film 12 conform to each other appropriately.

[0044]

The baking device 16 is placed to follow the curtain coater 15, and simultaneously bakes the colored film 11 and the clear film 12 applied onto the steel sheet 10. This baking device 16 is configured with a general coating material baking furnace, for example, a hot air drying furnace, an induction heating furnace, an infrared heating furnace, or a furnace using any of these in combination.

[0045]

As described above, the baking device 16 illustrated in FIG. 1 is equivalent to a baking device generally used in a conventional coating line, and has significantly higher output, heating temperature, and the like and larger installation space than the pre-heating device 14. The heating temperature by the baking device 16 is higher than the boiling points of volatile contents of the colored coating material and the clear coating material (e.g., the heating temperature is 200 °C or higher), and the heating time by the baking device 16 is longer than that by the pre-heating device 14. Consequently, by passing the steel sheet 10 into the baking device 16, the colored film 11 and the clear film 12 are heated to a high heating temperature, and the two films 11 and 12 are baked onto the steel sheet 10 simultaneously.

[0046]

In addition, the cooling device 17 and the drying device 18 are placed to follow the baking device 16. The cooling device 17 water-cools the steel sheet 10 onto which the colored film 11 and the clear film 12 have been baked by the baking device 16. The drying device 18 is configured with a drier or the like, and dries the steel sheet 10 after the cooling by the cooling device 17.

[0047] [3. Specific examples of coating materials]

Next, definitions and specific examples of coating materials used for the colored film 11 (intermediate coat film) and the clear film 12 (topcoat film) will be described.

[0048] [3.1. Definitions of terms]

First, terms related to coating materials will be defined. A coating material includes a solid content and a volatile content. The solid content is a component that constitutes a film by baking, among components of the coating material, and is used to offer various functions of the coating material. This solid content includes a dissolved component that is dissolved in the volatile content and a suspended component that is suspended in a state of being dispersed in the form of fine particles without being dissolved in the volatile content. The volatile content of the coating material is a component that is volatilized by baking, among components of the coating material, and is used to impart flowability to the solid content to make it easy to apply. This volatile content is volatilized at the time of heating after the application, and does not constitute the film after the baking. Note that in this specification, volatilization of the volatile content of the coating material encompasses not only the volatilization of the volatile content at a temperature lower than its boiling point, but also vaporization of the volatile content at a temperature equal to or higher than its boiling point.

[0049]

An oil-based coating material is a coating material using an organic solvent as the volatile content. The solid content of the oil-based coating material includes various resins (dissolved component) and fine particles (suspended component) of wax, a pigment, a crosslinking agent, a matting agent, an extender pigment, an ultraviolet absorber, a curing agent, a corrosion inhibitor, or the like. In the volatile content (organic solvent) of the oil-based coating material, as the solid content, the various resins (dissolved component) are dissolved and the fine particles (suspended component) of wax, a pigment, or the like are dispersed. Examples of the oil-based coating material include an oil-based colored coating material and an oil-based clear coating material.

[0050] A water-based coating material is a coating material using a liquid mainly constituted by water as the volatile content. The solid content of the water-based coating material includes various resins (suspended component) and fine particles (suspended component) of wax, a pigment, a crosslinking agent, a matting agent, an extender pigment, an ultraviolet absorber, a curing agent, a corrosion inhibitor, or the like. In the volatile content (liquid mainly constituted by water) of the water-based coating material, as the solid content, fine particles of the various resins (suspended component) are dispersed without being dissolved and the fine particles (suspended component) of wax, a pigment, or the like are also dispersed. Examples of the water-based coating material include a water-based colored coating material and a water-based clear coating material.

[0051]

Here, the volatile content of the water-based coating material may be only water, or may be a mixture of an organic solvent and water (e.g., lower alcohol). In the latter case, a mass proportion of water in the volatile content of the water-based coating material is 95 mass% or greater, for example, but a mixture in which the mass proportion of water is less than 95 mass% may also serve as the volatile content of the water-based coating material.

[0052] A volatile content concentration is a proportion of a mass B of the volatile content in the total mass (the sum of a mass A of the solid content and the mass B of the volatile content) of a coating material (volatile content concentration = B/(A+B) [mass%]). The volatile content concentration of a coating material forming a film is highest at the time when the coating material is applied, and the volatile content concentration decreases as the applied film dries (i.e., as the volatile content in the film is volatilized).

[0053] [3.2. Specific examples of colored coating material]

The colored film 11 is an intermediate coat film, and is used to implement the design (e.g., color) of a pre-coated steel sheet and the functionality (e.g., improved hardness, chemical resistance, stain resistance, and corrosion prevention) of a film. The colored coating material is a coating material containing a color pigment, and is used to form the colored film 11. As the colored coating material, a generally known coating resin to which a color pigment and the like are added can be used, and for example, a resin commercially available as a coating material for precoating, such as a resin for anti-corrosive films, can be used.

[0054]

As a resin of the colored coating material, for example, a general-use resin used for a coating material for pre-coating, such as a polyester resin, a polyurethane resin, an epoxy resin, or a fluorocarbon polymer, can be used. As needed, two or more kinds of these resins may be mixed to be used, or a crosslinking agent, such as a melamine resin or an isocyanate resin, may be used. In particular, a resin of a type in which a polyester resin is cross-linked with a melamine resin or an isocyanate has excellent workability.

[0055]

As a color pigment contained in the colored coating material, for example, a general-use color pigment, such as titanium oxide, zinc oxide, iron oxide, aluminum oxide, barium sulfate, aluminum, or carbon black, can be used. It is particularly preferable to use a color pigment whose refractive index differs greatly from that of the above resin.

[0056]

In addition, as needed, a known leveling agent, pigment dispersant, wax, matting agent, or the like can be added to the colored coating material. The kinds and addition amounts of these additives are not particularly limited, and can be selected as needed. The colored coating material may be either of an oil-based coating material and a water-based coating material.

[0057]

An oil-based colored coating material is a colored coating material using an organic solvent as the volatile content. The solid content of the oil-based colored coating material is, for example, a color pigment, silica, or a lubricant. The volatile content (organic solvent) of the oil-based coating material is, for example, xylene, cyclohexanone, or methyl ethyl ketone. The volatile content concentration of the oil-based coating material (i.e., the concentration of the organic solvent) is, for example, 30 to 70 mass%, particularly 40 to 65 mass%. A specific example of the oil-based colored coating material is a colored coating material obtained by adding a white pigment (titanium oxide) or a black pigment (carbon black) to a clear coating material and stirring the mixture. The clear coating material is obtained by adding an acid catalyst to a mixed solution of a polyester resin and a melamine resin (a solution in which a polyester resin is cross-linked with a melamine resin) and stirring the mixture.

[0058] A water-based colored coating material is a colored coating material using a liquid mainly constituted by water as the volatile content. The proportion of water in the water-based colored coating material is, for example, 40 to 90 mass%. A specific example of the water-based colored coating material is a colored coating material obtained by adding a curing agent (e.g., a melamine resin or a polyisocyanate compound), a color pigment (e.g., carbon black), silica particles (e.g., spherical silica particles), and a lubricant (e.g., polyethylene resin particles) to an aqueous resin (e.g., a polyester resin, a urethane resin, or an acrylic resin).

[0059] [3.3. Clear coating material]

The clear film 12 is a topcoat film forming the outermost layer of a precoated steel sheet, and is used to implement the design (e.g., gloss) of the pre-coated steel sheet and the functionality (e.g., improved hardness, chemical resistance, stain resistance, and corrosion prevention) of a film. The clear coating material is a coating material not containing a color pigment, and is used to form the clear film 12. As the clear coating material, a generally known coating resin to which a lubricant and the like are added can be used, and for example, a resin commercially available as a coating material for pre-coating, such as a resin for anti-corrosive films, can be used.

[0060]

As a resin of the clear coating material, for example, a general-use resin used for a coating material for pre-coating, such as a polyester resin, a polyurethane resin, an epoxy resin, or a fluorocarbon polymer, can be used. As needed, two or more kinds of these resins may be mixed to be used, or a crosslinking agent, such as a melamine resin or an isocyanate resin, may be used. In particular, a resin of a type in which a polyester resin is cross-linked with a melamine resin or an isocyanate has excellent workability.

[0061] A lubricant can be added to the clear coating material as needed.

Components of the lubricant are not particularly limited, and polytetrafluoroethylene (PTFE), polyethylene, microcrystalline, or the like can be used.

[0062]

In addition, as needed, a known leveling agent, wax, matting agent, or the like can be added to the clear coating material. The kinds and addition amounts of these additives are not particularly limited, and can be selected as needed. The clear coating material may be either of an oil-based coating material and a water-based coating material.

[0063]

An oil-based clear coating material is a clear coating material using an organic solvent as the volatile content. The solid content of the oil-based clear coating material is, for example, silica or a lubricant. The volatile content (organic solvent) of the oil-based clear coating material is, for example, xylene, cyclohexanone, or methyl ethyl ketone. The volatile content concentration of the oil-based coating material (i.e., the concentration of the organic solvent) is, for example, 30 to 70 mass%, particularly 40 to 65 mass%. A specific example of the oil-based clear coating material is a clear coating material obtained by adding an acid catalyst to a mixed solution of a polyester resin and a melamine resin (a solution in which a polyester resin is cross-linked with a melamine resin) and stirring the mixture.

[0064] A water-based clear coating material is a clear coating material using a liquid mainly constituted by water as the volatile content. The proportion of water in the water-based clear coating material is, for example, 40 to 90 mass%. A specific example of the water-based clear coating material is a clear coating material obtained by adding a curing agent (e.g., a melamine resin or a polyisocyanate compound), silica particles (e.g., spherical silica particles), and a lubricant (e.g., polyethylene resin particles) to an aqueous resin (e.g., a polyester resin, a urethane resin, or an acrylic resin).

[0065] [4. Method for manufacturing pre-coated metal sheet]

Next, the method for manufacturing a pre-coated metal sheet according to the present embodiment will be described. Described below is a method for manufacturing a pre-coated steel sheet by continuously coating a steel sheet with multiple layers by using the continuous coating apparatus (coating line) illustrated in FIG. 1. Description is also given as appropriate on a stacked state of films in each step of the manufacturing method and the distribution of color pigment concentration in films, with reference to FIG. 2.

[0066]

The method for manufacturing a pre-coated metal sheet according to the present embodiment includes (1) First coating step, (2) Pre-heating step, (3) Second coating step, (4) Baking step, (5) Cooling step, and (6) Drying step. Each step is described below.

[0067] (1) First coating step

The steel sheet 10 is a hot dip galvanized steel sheet, an electro galvanized steel sheet, a cold-rolled steel sheet, or the like, and has a sheet thickness of 0.3 to 3.2 mm, for example, and a sheet width of 600 to 1828 mm, for example.

[0068]

As illustrated in FIG. 1, first, the strip-shaped steel sheet 10 passed on the coating line is introduced into the roll coater 13 (the first coating device). This roll coater 13 applies a colored coating material onto a surface of the steel sheet 10 to form the colored film 11 (lower layer film) on the surface of the steel sheet 10. A thickness D1 of the colored film 11 at the time of this application may be 5 to 50 pm, for example; described below is an example in which D1 = 24 pm. Note that it is possible to form a chemical conversion treatment layer on the surface of the steel sheet 10, in advance, by a chemical conversion treatment device (not illustrated) before applying the colored coating material, and form the colored film 11 on the chemical conversion treatment layer. It is also possible to form a primer film, in advance, by applying a primer coating material onto the steel sheet 10 or a chemical conversion treatment layer, and form the colored film 11 on the primer film.

[0069]

The colored film 11 applied in the above manner contains a color pigment at a substantially uniform concentration in the thickness direction. Therefore, as illustrated in FIG. 2 A, within the colored film 11, the color pigment concentration is substantially constant from the base material (the steel sheet 10) side to the outer layer side.

[0070] (2) Pre-heating step

Then, the steel sheet 10 on which the colored film 11 is formed is introduced into the pre-heating device 14 (heating device). This pre-heating device 14 pre-heats the colored film 11 on the steel sheet 10. On this occasion, the preheating device 14 does not bake the colored film 11 by heating it for a long time to a high heating temperature, but heats the colored film 11 at the low heating temperature T (e.g., 60 to 150 °C) equal to or lower than the boiling point of the volatile content of the colored coating material. A heating time t of the steel sheet 10 by the pre-heating device 14 is very short, and is preferably 1.0 to 10 seconds, for example. For example, in the case where the pre-heating device 14 has a length of installation of 5 m and the steel sheet 10 is passed at a line speed of 100 m/min, the heating time of the steel sheet 10 by the pre-heating device 14 is three seconds.

[0071]

Such heating (pre-heating) at low temperature for a short time by the preheating device 14 volatilizes part of the volatile content (e.g., an organic solvent or water) of the colored coating material from the colored film 11, causing the volatile content concentration in the colored film 11 to decrease, which brings the colored film 11 into a“half-dried state”.

[0072]

In the case where the colored coating material is an oil-based colored coating material, its boiling point is 150 to 160 °C; hence, the heating temperature T by the pre-heating device 14 is preferably PMT 60 to 150 °C, which is equal to or lower than the boiling point, further preferably PMT 90 to 140 °C . In the case where the colored coating material is a water-based colored coating material, its boiling point is 90 to 120 °C; hence, the heating temperature T by the pre-heating device 14 is preferably PMT 50 to 80 °C, which is equal to or lower than the boiling point. In this manner, the pre-heating device 14 heats the colored film 11 to the appropriate heating temperature T equal to or lower than the boiling point of the volatile content of the colored coating material, thereby drying the colored film 11 to an appropriate level to bring it into a half-dried state in which mixing of an upper layer coating material can be prevented. Note that this pre-heating slightly reduces the thickness D1 of the colored film 11 (e.g., from 24 pm to 22 pm).

[0073]

In addition, since the colored film 11 is only heated for a short time at low temperature in the pre-heating step, the color pigment contained in the colored film 11 hardly moves inside the colored film 11. Therefore, as illustrated in FIG. 2B, within the pre-heated colored film 11, the color pigment concentration is still substantially constant in the thickness direction.

[0074] (3) Second coating step

Then, the steel sheet 10 where the colored film 11 in a half-dried state is applied is introduced into the curtain coater 15 (the second coating device). This curtain coater 15 applies a clear coating material onto the colored film 11 in a halfdried state to form the clear film 12 (upper layer film) on the colored film 11 (lower layer film). That is, a clear film is a “film formed by applying a clear coating material”, and partly contains a color pigment as will be described later. A thickness D2 of the clear film 12 at the time of this application may be 10 to 100 pm, for example; described below is an example in which D2 = 40 pm.

[0075]

As illustrated in FIG. 1, the curtain coater 15 is placed to immediately follow the pre-heating device 14 on the coating line. Therefore, the second coating step by the curtain coater 15 is performed right after the heating step by the preheating device 14, and between the two steps, the colored film 11 is not subjected to forced cooling and drying using a cooling device and a drying device. Consequently, in the second coating step, the clear film 12 is applied onto the colored film 11 still in a half-dried state.

[0076]

In the above manner, the clear film 12 is applied onto the colored film 11 in a half-dried state. At the time when the clear film 12 is applied, as illustrated in FIG. 2C, the color pigment concentration of the colored film 11 is constant as in the case of FIG. 2B, and the color pigment concentration of the clear film 12 is zero. However, as time elapses after the application of the clear film 12, mass transfer occurs at the interface between the colored film 11 and the clear film 12, and thus a pigment-concentrated layer 123 and a diffusion layer 122 are formed in the clear film 12. That is, the color pigment contained in the colored film 11 in the lower layer moves to the upper layer side by convection and concentration diffusion, and enters the clear film 12 in the upper layer to be diffused. As a result, as illustrated in FIG. 2D, the color pigment concentration in the colored film 11 decreases, and the pigment-concentrated layer 123 and the diffusion layer 122 containing the color pigment and a clear layer 121 not containing the color pigment are formed in the clear film 12.

[0077]

The pigment-concentrated layer 123 is formed on the base material side in the clear film 12, right above the colored film 11. This pigment-concentrated layer 123 is a portion where the color pigment concentration continuously increases toward the outer layer side of the clear film 12. The color pigment concentration reaches its highest peak at the outer layer side of the pigment-concentrated layer 123.

[0078]

The diffusion layer 122 is formed right above the pigment-concentrated layer 123 (on the outer layer side) in the clear film 12. This diffusion layer 122 is a portion of the clear film 12 where the color pigment concentration continuously decreases toward the outer layer side of the clear film 12.

[0079]

Furthermore, the clear layer 121 containing substantially no color pigment is formed on the outermost layer side in the clear film 12. In the present embodiment, the colored film 11 is pre-heated before the application of the clear film 12 to be brought into an appropriate half-dried state. Thus, the color pigment moves and diffuses from the colored film 11 to the clear film 12, but its diffusion range is limited, and the color pigment does not reach the outermost layer portion of the clear film 12. Consequently, the clear layer 121 containing substantially no color pigment is formed in the outermost layer portion in the clear film 12.

[0080]

In addition, since the color pigment moves from the colored film 11 to the pigment-concentrated layer 123 and the diffusion layer 122 of the clear film 12 as described above, the color pigment concentration in the colored film 11 decreases as a whole. In particular, in the outer-layer-side portion of the colored film 11, which is adjacent to the pigment-concentrated layer 123, the color pigment concentration greatly decreases, and there may occur a valley where the color pigment concentration is lower than in the central portion in the thickness direction of the colored film 11. In this manner, the applying of the clear film 12 in the upper layer causes a decrease in the color pigment concentration of the colored film 11 in the lower layer; the colored film 11 whose color pigment concentration has decreased after the application of the clear film 12 will be called a colored film layer 111.

[0081]

As described above, by convection and diffusion actions of substances (e.g., color pigment) between the clear film 12 in the upper layer and the colored film 11 in the lower layer, three layers (the pigment-concentrated layer 123, the diffusion layer 122, and the clear layer 121) are formed in the clear film 12. In addition, the colored film 11 becomes the colored film layer 111 whose color pigment concentration has decreased after the application of the clear film 12. As a result, film layers of a two-layer structure of the colored film 11 and the clear film 12 changes to film layers of a four-layer structure (the colored film layer 111, the pigment-concentrated layer 123, the diffusion layer 122, and the clear layer 121 in that order from the lower layer side).

[0082] (4) Baking step

After that, the steel sheet 10 on which the colored film 11 and the clear film 12 are formed is introduced into the baking device 16. This baking device 16 heats the colored film 11 and the clear film 12 on the steel sheet 10 to a heating temperature (e.g., 200 °C or higher) significantly higher than the boiling points of volatile contents of the coating materials, to simultaneously bake the colored film 11 and the clear film 12.

[0083]

Such high-temperature heating (baking) by the baking device 16 completely volatilizes the volatile content of the colored coating material from the colored film 11, and completely volatilizes the volatile content of the clear coating material from the clear film 12. This causes the volatile content concentration in the two films 11 and 12 to decrease to substantially zero, bringing the two films 11 and 12 into a completely dried, cured state.

[0084]

As a result, as illustrated in FIG. 2E, the colored film 11 and the clear film 12 are reduced in thickness. The thickness D1 of the colored film 11 is reduced from 22 pm to 15 pm, for example, and the thickness D2 of the clear film 12 is reduced from 40 pm to 15 pm, for example. This results in a reduction in the thickness of each of the clear layer 121, the diffusion layer 122, the pigmentconcentrated layer 123, and the colored film layer 111. Note that the color pigment concentration distribution of each layer does not change.

[0085] (5) Cooling step

Then, the high-temperature steel sheet 10 onto which the colored film 11 and the clear film 12 have been baked in the above manner is introduced into the cooling device 17. This cooling device 17 sprays cooling water on the steel sheet 10 to cool it to a temperature close to ordinary temperature.

[0086] (6) Drying step

After that, the steel sheet 10 cooled in the above manner is introduced into the drying device 18. This drying device 18 vaporizes moisture attached to a surface of the clear film 12 of the steel sheet 10 to dry the steel sheet 10.

[0087]

Through the above steps, the long steel sheet 10 is continuously coated with the colored film 11 and the clear film 12, so that a two-layer pre-coated metal sheet is manufactured. Described above is an example in which a front surface of the steel sheet 10 is coated with two layers, but a rear surface of the steel sheet 10 may also be coated with one layer or multiple layers by a known coating method.

[0088]

Next, description is given on the reuse of the clear coating material in the second coating step and on the adhesion between the colored film 11 in the lower layer and the clear film 12 in the upper layer, with reference to FIGS. 3A to 3C. FIGS. 3A to 3C are schematic diagrams each illustrating a step of applying a clear coating material onto the colored film 11 by using the curtain coater 15.

[0089] FIG. 3A illustrates a step of applying a clear coating material by a conventional 2C2B method (successive coating and baking method). In the case of applying the clear coating material in the upper layer by using a curtain coater after completely baking the colored film 11 in the lower layer, as illustrated in FIG. 3 A, the coating material of the colored film 11 and the applied clear coating material do not mix with each other. That is, the clear coating material in a flowing state flows down in the form of a curtain from the curtain coater, to be attached onto the colored film 11 of the steel sheet 10 passing under the curtain coater, so that the clear film 12 is formed. Concurrently, an excess clear coating material 12a flows down downward from both sides in the width direction of the steel sheet 10. On this occasion, since the colored film 11 is in a completely dried state, the clear coating material does not mix with the coating material of the colored film 11. Consequently, the clear coating material 12a flowing down from the both sides in the width direction of the steel sheet 10 can be collected to be reused.

[0090]

However, in the 2C2B method illustrated in FIG. 3A, the clear coating material in the upper layer does not conform to the colored film 11 in the lower layer, which has been baked to be completely dried and cured. This causes a problem in that, after the clear film 12 is baked, the adhesion between the colored film 11 and the clear film 12 decreases, making the clear film 12 likely to peel off from the colored film 11 at the interface between the two films.

[0091] FIG. 3B illustrates a step of applying a clear coating material by a conventional multilayer simultaneous coating method. In the case of simultaneously applying a colored coating material in the lower layer and the clear coating material in the upper layer by using a multilayer curtain coater, as illustrated in FIG. 3B, the two coating materials mix with each other because they are in a wet state. Consequently, even if an excess colored coating material 11a and the clear coating material 12a flowing down downward from both sides in the width direction of the steel sheet 10 are collected, they cannot be reused and must be disposed of because they are mixed with each other. As described above, the multilayer simultaneous coating method illustrated in FIG. 3B has a problem in that, since the coating materials cannot be collected to be reused, disposal volume of the coating materials increases and causes an increase in coating cost.

[0092]

In contrast, FIG. 3C illustrates a step of applying a clear coating material by a wet-on-wet method using pre-heating (hereinafter called a pre-heating method) according to the present embodiment. In the present embodiment, the curtain coater 15 applies the clear coating material in the upper layer onto the colored film 11 in a half-dried state in the lower layer. As illustrated in FIG. 3C, the clear coating material flowing down in the form of a curtain from the curtain coater 15 is attached onto the colored film 11 to form the clear film 12, and the excess clear coating material 12a flows down downward from both sides in the width direction of the steel sheet 10 to be collected. On this occasion, since the colored film 11 is in a half-dried state, the clear coating material does not mix with the colored film 11. Consequently, the collected excess clear coating material 12a can be reused because it is not mixed with the colored coating material. This produces an effect of reducing disposal volume of the coating materials and coating cost.

[0093]

Furthermore, the applying of the clear coating material in a wet state onto the colored film 11 in a half-dried state makes the two coating materials conform to each other at the interface between the colored film 11 and the clear film 12, and makes the roughness of the interface higher than that in the case of FIG. 3 A. Consequently, after the colored film 11 and the clear film 12 are baked simultaneously, the adhesion between the colored film 11 and the clear film 12 is improved. This produces an effect of making the clear film 12 unlikely to peel off from the colored film 11 at the interface between the two films.

[0094]

As described above, according to the method for manufacturing a pre-coated metal sheet according to the present embodiment, the pre-heating device 14 brings the colored film 11 in the lower layer into a half-dried state, and the clear coating material in the upper layer is applied onto the colored film 11 without the colored film 11 being subjected to cooling and drying. This improves the adhesion between the colored film 11 in the lower layer and the clear film 12 in the upper layer, and improves the reusability of the clear coating material in the lower layer, thus reducing disposal volume of the coating materials.

[0095] [5. Appropriate range of volatile content concentration of film in half-dried state]

Next, description is given on an appropriate range of the volatile content concentration of the colored film 11 that is brought into a half-dried state by the preheating device 14.

[0096]

As described above, in the case where the colored film 11 in the lower layer has a high volatile content concentration and its state is close to a wet state, when the clear coating material in the upper layer is applied, the two films 11 and 12 easily conform to each other and their adhesion is improved. On the other hand, the colored film 11 in a state close to a wet state and the clear film 12 mix with each other to form a mixed layer, making it difficult to form the clear layer 121 not containing the color pigment.

[0097]

In the case where the colored film 11 in the lower layer has a low volatile content concentration and its state is close to a dried state, the two films 11 and 12 do not easily conform to each other and their adhesion decreases. Furthermore, since it is difficult for the color pigment to move from the colored film 11 in a state close to a dried state to the clear film 12, the clear layer 121 is easily formed, but the pigmentconcentrated layer 123 and the diffusion layer 122 are difficult to form in the clear film 12.

[0098]

As described above, the adhesion between the films and the formability of the clear layer 121 have a trade-off relationship. The formability of the clear layer 121 and the formability of the pigment-concentrated layer 123 and the diffusion layer 122 also have a trade-off relationship. Hence, to appropriately form the three layers of the clear layer 121, the diffusion layer 122, and the pigment-concentrated layer 123 in the clear film 12 and to ensure the adhesion between the films, it is preferable to control heating conditions at the time of pre-heating so that the volatile content concentration of the colored film 11 in a half-dried state is adjusted to fall within an appropriate range.

[0099]

Hence, in the present embodiment, when the colored film 11 is heated by the pre-heating device 14 to be brought into a half-dried state, heating conditions are controlled so that the volatile content concentration of the colored film 11 becomes 22 to 64 mass%. Note that the volatile content concentration of the colored film 11 is the volatile content concentration of the colored coating material forming the colored film 11.

[0100]

In the case where the colored coating material in the lower layer is an oil-based colored coating material, the volatile content concentration (the concentration of the organic solvent) of the colored film 11 in a half-dried state is preferably 22 to 55 mass%. When the volatile content concentration of the colored film 11 in a halfdried state is 55 mass% or less, the oil-based colored coating material of the colored film 11 and the clear coating material in the upper layer do not mix with each other. This allows the clear layer 121 to be formed appropriately in the clear film 12.

When the volatile content concentration of the colored film 11 is 22 mass% or greater, the oil-based colored coating material of the colored film 11 in a half-dried state and the clear coating material easily conform to each other, which improves the adhesion between the films in the two layers, and the pigment-concentrated layer 123 and the diffusion layer 122 can be formed appropriately in the clear film 12. Furthermore, as compared with a conventional successive coating and baking method, the output specification and installation space of the pre-heating device 14 according to the present embodiment can be reduced by approximately 40 to 50 %, which makes it possible to save the space and energy of the continuous coating apparatus.

[0101]

In the case where the colored coating material in the lower layer is a water-based colored coating material, the volatile content concentration (the concentration of water, or the concentration of the mixture of water and the organic solvent) of the colored film 11 in a half-dried state is preferably 25 to 64 mass%. When the volatile content concentration of the colored film 11 in a half-dried state is 64 mass% or less, the water-based colored coating material of the colored film 11 and the clear coating material in the upper layer do not mix with each other. This allows the clear layer 121 to be formed appropriately in the clear film 12. When the volatile content concentration of the colored film 11 in a half-dried state is 25 mass% or greater, the water-based colored coating material of the colored film 11 and the clear coating material easily conform to each other, which improves the adhesion between the films in the two layers, and the pigment-concentrated layer 123 and the diffusion layer 122 can be formed appropriately in the clear film 12. Furthermore, as in the case of the oil-based colored coating material, the output specification and installation space of the pre-heating device 14 can be reduced by approximately 40 to 50 %, which makes it possible to save the space and energy of the continuous coating apparatus.

[0102] [6. Characteristics of pre-coated steel sheet]

Next, description is given on characteristics of a pre-coated steel sheet manufactured by the above-described method for manufacturing a pre-coated steel sheet according to the present embodiment.

[0103]

The pre-coated steel sheet is coated by the pre-heating method (wet-on-wet method using pre-coating) according to the present embodiment, and is manufactured by coating one surface or both surfaces of the steel sheet 10 serving as a base material with at least two films. The pre-coated steel sheet according to the present embodiment is, as described above, obtained by coating one surface of the steel sheet 10 with the colored film 11 in the lower layer and the clear film 12 in the upper layer.

[0104]

Furthermore, as illustrated in FIG. 2, the pre-coated steel sheet according to the present embodiment includes the pigment-concentrated layer 123 and the diffusion layer 122 between the colored film layer 111 and the clear layer 121. As described with reference to FIG. 2, the clear film 12 applied onto the colored film 11 is divided into three layers of the clear layer 121, the diffusion layer 122, and the pigment-concentrated layer 123, in accordance with the color pigment concentration distribution. In the clear film 12, the outermost layer portion not containing the color pigment is the clear layer 121, the base-material-side portion containing the color pigment at a high concentration is the pigment-concentrated layer 123, and the portion between the clear layer 121 and the pigment-concentrated layer 123 is the diffusion layer 122. In addition, the colored film 11 whose color pigment concentration has decreased owing to the application of the clear film 12 becomes the colored film layer 111.

[0105]

In this manner, in the pre-coated steel sheet according to the present embodiment, the films in the upper and lower two layers (the colored film 11 and the clear film 12) formed by the two coating steps are divided into four layers (the colored film layer 111, the pigment-concentrated layer 123, the diffusion layer 122, and the clear layer 121 in that order from the lower layer side), in accordance with the color pigment concentration distribution.

[0106]

The colored film layer 111 is a layer containing the color pigment at a substantially constant concentration in the thickness direction. A thickness d4 of the colored film layer 111 is, for example, 8 to 13 pm. The pigment-concentrated layer 123 is a layer containing the color pigment that has moved from the colored film 11 at a high concentration, and a thickness d2 of the pigment-concentrated layer 123 is, for example, 2.2 to 8 pm. The diffusion layer 122 is a layer containing the color pigment that has diffused from the colored film 11 into the clear film 12, and a thickness d3 of the diffusion layer 122 is, for example, 3 to 12 pm. The clear layer 121 is a transparent layer not containing the color pigment, and a thickness dl of the clear layer 121 is, for example, 5 to 12.3 pm.

[0107]

Here, detailed description is given on the color pigment concentration distribution in the film layers of the pre-coated steel sheet according to the present embodiment, with reference to FIGS. 4 to 6. FIG. 4 is a graph showing an example of the color pigment concentration distribution in film layers of a pre-coated steel sheet manufactured by each of conventional methods, a 2C2B method and a wet-on-wet method. FIGS. 5 and 6 are graphs showing examples of the color pigment concentration distribution in the film layers of the pre-coated steel sheet according to the present embodiment. Note that the graphs of FIGS. 5 and 6 show the color pigment concentration ratio of each layer when the average value of the color pigment concentration in the colored film layer 111 is defined as 1.0.

[0108]

First, description is given on the color pigment concentration distribution of the pre-coated steel sheet according to each of the conventional coating methods, and problems thereof, with reference to FIG. 4.

[0109]

In a conventional 2C2B method, since the clear film in the upper layer is applied after the colored film in the lower layer is baked, the color pigment does not move between the two films. Therefore, as illustrated in FIG. 4, the color pigment concentration of the colored film in the lower layer (thickness: Dl) is constant in the thickness direction, and the color pigment concentration of the clear film in the upper layer (thickness: D2) is zero. In the case of this 2C2B method, the clear film not containing the color pigment can have a large thickness D2, but unlike in the present embodiment, the pigment-concentrated layer 123 having a high color pigment concentration cannot be formed. Consequently, the colored film cannot diffusely reflect light efficiently, which leaves room for improvement in the gloss of the precoated steel sheet. In addition, in the 2C2B method, the adhesion between the colored film and the clear film is very low.

[0110]

In a conventional wet-on-wet method, since the clear film in a wet state in the upper layer is applied onto the colored film in a wet state in the lower layer, the two films mix with each other to form a mixed layer. Therefore, as illustrated in FIG. 4, the color pigment is contained across the entire thickness direction from the colored film in the lower layer (thickness: Dl) to the clear film in the upper layer (thickness: D2), and the color pigment concentration gradually decreases toward the outer layer side. In this manner, in the wet-on-wet method, a clear layer not containing the color pigment is not formed. Consequently, incident light cannot appropriately pass through the outer layer portion of the film, and further, the colored film cannot specularly and diffusely reflect the incident light appropriately, which result in a significant decrease in the gloss of the pre-coated steel sheet.

[0111]

In contrast, description is given on the color pigment concentration distribution of the pre-coated steel sheet according to the present embodiment, with reference to FIGS. 5 and 6.

[0112]

As illustrated in FIG. 5, in the colored film layer 111 (thickness: d4) of the pre-coated steel sheet according to the present embodiment, the color pigment concentration is substantially constant in the central portion in the thickness direction of the colored film layer 111, but decreases from the central portion toward the outer layer side. On the basis of an average value Cave of the color pigment concentration in this colored film layer 111, the distribution of a color pigment concentration C of each of the other layers is evaluated relatively. A color pigment concentration ratio c used below is the ratio between C and Cave (c = C/CAVE).

[0113]

The pigment-concentrated layer 123 (thickness: d2) has a color pigment concentration equal to or greater than the average value Cave of the color pigment concentration in the colored film layer 111. In this pigment-concentrated layer 123, the color pigment concentration continuously increases from the base material side toward the outer layer side. The color pigment concentration ratio c becomes a minimum value c2 at the interface between the pigment-concentrated layer 123 and the colored film layer 111, and becomes a maximum value cl at the interface between the pigment-concentrated layer 123 and the diffusion layer 122. The color pigment concentration ratio c of the pigment-concentrated layer 123 is, for example, within a range of 1.0 to 1.2. The boundary portion between the pigmentconcentrated layer 123 and the colored film layer 111 has a color pigment concentration lower than that around the boundary portion. This is because, at the time of pre-heating, the color pigment that had existed in a portion of the colored film 11 corresponding to the boundary portion has moved to the outer layer side (the clear film 12 side) by convection and concentration diffusion, and the pigmentconcentrated layer 123 has been formed.

[0114]

As illustrated in FIG. 5, in the diffusion layer 122 (thickness: d3), the color pigment concentration continuously decreases from the base material side toward the outer layer side, and the color pigment concentration is zero at the boundary between the diffusion layer 122 and the clear layer 121. The color pigment concentration ratio c of the diffusion layer 122 is, for example, within a range of 0 to 1.2. Not containing the color pigment, the clear layer 121 (thickness: dl) has a color pigment concentration of zero.

[0115]

As described above, the pre-coated steel sheet according to the present embodiment has a feature in the color pigment concentration distribution of the four-layer structure in the film layers. In particular, the clear layer 121 containing substantially no color pigment is formed on the outermost layer side of the clear film 12, and the pigment-concentrated layer 123 having a markedly high color pigment concentration is formed on the base material side of the clear film 12. Such combination of the clear layer 121 and the pigment-concentrated layer 123 significantly improves the glossiness of the pre-coated steel sheet for the following reasons.

[0116]

In general, when light falls on a pre-coated steel sheet, light that has passed through a clear film in the outermost layer is reflected by a colored film in the lower layer. Here, the amount of reflected light increases with an increase in the color pigment concentration of the colored film. However, an increase in color pigment concentration of a colored coating material causes problems of a decrease in the storage stability of the colored coating material, and deterioration of the adhesion of the colored film at the time of steel sheet working.

[0117]

In contrast, in the present embodiment, in manufacturing the pre-coated steel sheet, the colored film 11 in the lower layer is pre-heated for a short time, as described above, unlike in a conventional technology. Thus, the clear layer 121 with an appropriate thickness dl not containing the color pigment can be formed on the outermost layer side of the clear film 12 more stably than in a conventional wet-on-wet method. In addition, the pigment-concentrated layer 123 having a markedly high color pigment concentration can be formed on the base material side of the clear film 12 (in the neighborhood of the boundary with the colored film 11). Consequently, the pigment-concentrated layer 123 can diffusely reflect light that has passed through the clear layer 121 and the diffusion layer 122 efficiently to the outer layer side. Therefore, according to the present embodiment, the gloss of the precoated steel sheet as an end-product can be significantly improved.

[0118]

Note that the technical scope of the present invention encompasses the case of a concentration distribution in which, as illustrated in FIG. 6, the boundary portion between the pigment-concentrated layer 123 and the colored film layer 111 does not have a color pigment concentration lower than that around the boundary portion. Also in this case, the pigment-concentrated layer 123 has a color pigment concentration equal to or greater than the average value Cave of the color pigment concentration in the colored film layer 111, and the color pigment concentration continuously increases toward the outer layer side. Consequently, the pigmentconcentrated layer 123 of FIG. 6 can diffusely reflect incident light as efficiently as the pigment-concentrated layer 123 of FIG. 5 can, which provides a pre-coated steel sheet with high glossiness.

[0119]

In addition, in manufacturing the pre-coated steel sheet according to the present embodiment, the clear film 12 in the upper layer is applied in a state where the colored film 11 in the lower layer is brought into a half-dried state by the abovedescribed pre-heating; therefore, the two films conform to each other favorably at the interface between the colored film 11 and the clear film 12. Consequently, after the colored film 11 and the clear film 12 are baked simultaneously, the adhesion between the colored film 11 and the clear film 12 is significantly improved. Therefore, the pre-coated steel sheet according to the present embodiment has sufficiently high adhesion (working adhesion) between films subjected to working (e.g., bending or press working).

[0120]

As described above, the pre-coated steel sheet according to the present embodiment has advantages of high adhesion between the films in the upper and lower layers, and a high gloss of the surface of the steel sheet. Furthermore, in manufacturing the pre-coated steel sheet, the thickness dl of the clear layer 121 not containing the color pigment and the thickness d2 of the pigment-concentrated layer 123 can be controlled by adjusting the heating conditions (e.g., heating time t and heating temperature T) of the pre-heating of the colored film 11. This control makes it possible to achieve both the adhesion between the upper and lower layers and the glossiness of the pre-coated steel sheet at high levels. As a result, the precoated steel sheet manufactured by the pre-heating method according to the present embodiment can have a higher gloss than that in the case of a conventional wet-on-wet method. Furthermore, when the heating conditions of the pre-heating and dl and d2 are controlled favorably, the pre-coated steel sheet according to the present embodiment can have a higher gloss than that in the case of a successive coating and baking method (e.g., 2C2B).

[0121]

Here, in view of enhancing the gloss of the pre-coated steel sheet, more detailed description is given on the interrelation between the thicknesses of the clear layer 121 (thickness: dl), the diffusion layer 122 (thickness: d3), the pigmentconcentrated layer 123 (thickness: d2), and the colored film layer 111 (thickness: d4) according to the present embodiment.

[0122] (1) Appropriate range of dl/d2

First, description is given on an appropriate range of a ratio dl/d2 between the thicknesses of the clear layer 121 and the pigment-concentrated layer 123 according to the present embodiment. When dl/d2 is controlled to be equal to or greater than 1.7 and equal to or less than 4.7 as expressed by the following formula (1), both the adhesion between the upper and lower layers and the gloss of the precoated steel sheet can be achieved at high levels.

[0123]

If dl/d2 is less than 1.7, the clear layer 121 does not have a sufficient thickness dl, and thus a target gloss (e.g., a gloss that is 1.7 times or more higher than that in the case of a wet-on-wet method) is not obtained. On the other hand, if dl/d2 is greater than 4.7, the pigment-concentrated layer 123 does not have a sufficient thickness d2, and thus the adhesion between the films deteriorate, and a gloss higher than that in the case of a successive coating and baking method (e.g., 2C2B) is difficult to obtain. Hence, to ensure high adhesion between the films and obtain a gloss sufficiently higher than that in the case of a wet-on-wet method, dl/d2 is preferably controlled to be 1.7 to 4.7.

[0124]

Furthermore, as expressed by the following formula (2), dl/d2 is further preferably controlled to be 2.0 or greater. In this case, the 60-degree gloss of the pre-coated steel sheet according to the present embodiment can be improved to be 100 or higher, which means that a gloss higher than a gloss (60-degree gloss: approximately 90) in the case of a successive coating and baking method (e.g., 2C2B) can be obtained.

[0125] (2) Appropriate range of cl/c2

Next, description is given on an appropriate range of a ratio cl/c2 between the maximum value cl and the minimum value c2 of the color pigment concentration ratio c in the pigment-concentrated layer 123. As expressed by the following formula (3), cl/c2 is preferably equal to or greater than 1.04 and equal to or less than 2.0.

[0126]

When cl/c2 is 1.04 or greater, diffuse reflection of incident light easily occurs at a high-concentration portion (peak portion) of the color pigment extending across the pigment-concentrated layer 123 and the diffusion layer 122, which increases the amount of light incident on an optical receiver of a glossmeter. Consequently, the 60-degree gloss of the pre-coated steel sheet according to the present embodiment can be improved to be 100 or higher, which means that a gloss higher than a gloss (60-degree gloss: approximately 90) in the case of a successive coating and baking method (e.g., 2C2B) can be obtained. If cl/c2 is less than 1.04, a gloss higher than that in the case of a wet-on-wet method can be obtained, but the high-concentration portion has an insufficient peak value and a gloss higher than that in a successive coating and baking method (e.g., 2C2B) cannot be obtained in some cases.

[0127]

If cl/c2 is greater than 2.0, the enrichment degree of the color pigment in the pigment-concentrated layer 123 gets too intense. This may cause embrittlement of the pigment-concentrated layer 123 and deterioration of the working adhesion between the films. Hence, to maintain working adhesion, cl/c2 is preferably 2.0 or less.

[0128] (3) Appropriate range of d2

Next, description is given on an appropriate range of the thickness d2 of the pigment-concentrated layer 123. As expressed by the following formula (4), d2 is preferably equal to or greater than 2.2 pm and equal to or less than 8.0 pm.

[0129]

When d2 is 2.2 pm or greater, the pigment-concentrated layer 123 can diffusely reflect incident light appropriately, which provides a pre-coated steel sheet having a higher gloss than a conventional one. If d2 is less than 2.2 pm, diffuse reflection by the pigment-concentrated layer 123 is insufficient, which causes a decrease in the gloss of the pre-coated steel sheet.

[0130]

If d2 is greater than 8.0 pm, a gloss improvement effect by the pigmentconcentrated layer 123 is saturated, bringing the gloss of the pre-coated steel sheet to an upper limit value, and the working adhesion between the film layers decreases. Hence, to ensure the working adhesion between the film layers while obtaining a high gloss of the pre-coated steel sheet, d2 is preferably 2.2 to 8.0 pm.

[0131] (4) Appropriate range of dl

Next, description is given on an appropriate range of the thickness dl of the clear layer 121. As expressed by the following formula (5), dl is preferably equal to or greater than 5.0 pm and equal to or less than 12.3 pm.

[0132]

In order for the pre-coated steel sheet to have a high gloss, the clear layer 121 not containing the color pigment needs to exist on the outermost layer side of the film layer. If the thickness dl of this clear layer 121 is less than 5.0 pm, the clear layer 121 cannot have a thickness needed to diffusely reflect incident light, which makes the gloss of the pre-coated steel sheet insufficient. On the other hand, if dl is greater than 12.3 pm, the amount of light that reaches the pigment-concentrated layer 123 below the clear layer 121 decreases, so that a sufficient gloss cannot be obtained. Hence, to obtain a high gloss of the pre-coated steel sheet, dl is preferably 5.0 to 12.3 pm.

[0133]

Described above using the formulas (1) to (5) are appropriate ranges of parameters, such as dl/d2, cl/c2, d2, and dl, according to the present embodiment. In the method for manufacturing a pre-coated steel sheet according to the present embodiment, these parameters can be controlled by controlling the heating conditions (e.g., heating temperature T and heating time t) of the colored film 11 in the pre-heating step.

[0134]

According to the method for manufacturing a pre-coated steel sheet according to the present embodiment, when the heating temperature T and the heating time t are set to 60 to 150 °C and 1 to 10 seconds, respectively, in the preheating step, a pre-coated steel sheet satisfying the above formulas (1) and (3) to (5) can be manufactured. Furthermore, when the heating temperature T and the heating time t are set to 90 to 150 °C and 1.9 to 10 seconds, respectively, a pre-coated steel sheet satisfying all of the above formulas (1) to (5) can be manufactured.

[0135] [7. Conclusion]

Detailed description has been given above on the pre-coated metal sheet according to the first embodiment of the present invention, the method for manufacturing the pre-coated metal sheet, and the continuous coating apparatus. According to the present embodiment, the colored film 11 in the lower layer is heated for a short time at low temperature by the pre-heating device 14 to be brought into a half-dried state, and then the clear coating material in the upper layer is applied to form the clear film 12 in the upper layer on the colored film 11 in a half-dried state.

[0136]

In this manner, both the adhesion between the films in the upper and lower layers and the gloss of the pre-coated steel sheet can be achieved at high levels. That is, by appropriately controlling heating conditions at the time of pre-heating, the clear layer 121 not containing the color pigment (thickness: dl) can be stably formed in the outermost layer of the clear film 12, and the pigment-concentrated layer 123 having a markedly high color pigment concentration (thickness: d2) can be formed below the clear layer 121. Furthermore, the ratio dl/d2 between the thicknesses of the clear layer 121 and the pigment-concentrated layer 123 can be controlled to be within a favorable range (1.7 < dl/d2 < 4.7). Consequently, the pigmentconcentrated layer 123 can diffusely reflect incident light that has passed through the clear layer 121 efficiently to the outer layer side, so that the gloss of the pre-coated steel sheet can be significantly improved. Therefore, the pre-coated steel sheet according to the present embodiment achieves significantly higher glossiness than a pre-coated steel sheet manufactured by a conventional wet-on-wet method.

[0137]

Furthermore, by further appropriately controlling heating conditions at the time of pre-heating, the ratio cl/c2 between the maximum value cl and the minimum value c2 of the color pigment concentration ratio c in the pigment-concentrated layer 123 is controlled to be within a favorable range (cl/c2 > 1.04). This further improves the function of the pigment-concentrated layer 123 diffusely reflecting incident light. Consequently, the pre-coated steel sheet according to the present embodiment can have higher glossiness than a pre-coated steel sheet manufactured by a conventional successive coating and baking method (2C2B method).

[0138]

In addition, the above manufacturing method makes the films in the upper and lower layers easily conform to each other, and increases the roughness of the interface between the films in the two layers; thus, the adhesion between the two layers is improved after the two layers are completely dried in final baking. Consequently, the adhesion between the films in the upper and lower layers can be significantly improved as compared with a conventional successive coating and baking method (2C2B method), in which a coating material in the upper layer is applied and baked after a film in the lower layer is completely dried. This makes peeling unlikely to occur at the interface between the films in the two layers, and facilitates layer mixing control.

[0139]

In addition, when the clear film 12 in the upper layer is applied by using the curtain coater 15, the colored film 11 in the lower layer (half-dried state) and the clear coating material in the upper layer (wet state) do not mix with each other; thus, only the clear coating material can be collected to be reused. This enables recycled use of the clear coating material, thus reducing disposal volume and coating cost of the clear coating material and offering excellent compatibility with the environment.

[0140]

Furthermore, as compared with a baking device of a conventional successive coating and baking method, the pre-heating device 14 can be reduced in output and installation space, and there is no need to install a plurality of sets of a cooling device and a drying device in the coating line. Consequently, as compared with a conventional successive coating and baking method, facilities of the coating line can be simplified and their space and energy can be saved. This facilitates multilayer coating on the steel sheet 10 using a simple continuous coating line, in a plating step of the steel sheet 10.

[0141] [8. Other embodiments]

Next, other embodiments of the present invention will be described. Described in the first embodiment is an example of two-layer coating, but the present invention can be applied to a case of multilayer coating with three or more films.

[0142] [8.1. Second embodiment] FIG. 7 is a schematic diagram illustrating a line configuration of a continuous coating apparatus according to a second embodiment of the present invention. As illustrated in FIG. 7, the continuous coating apparatus according to the second embodiment has a line configuration in which coating facilities for coating with a film in a third layer (a curtain coater 21, a baking device 22, a cooling device 23, and a drying device 24) are additionally installed to follow the coating line (see FIG. 1) of the continuous coating apparatus according to the first embodiment.

[0143]

In the second embodiment, first, the roll coater 13 (the first coating device) applies a colored coating material onto the steel sheet 10 to form the colored film 11 in a first layer, and the pre-heating device 14 heats the colored film 11 to bring it into a half-dried state.

[0144]

Then, the curtain coater 15 (the second coating device) applies a clear coating material onto the colored film 11 in a half-dried state to form the clear film 12 in a second layer. Furthermore, the baking device 16 simultaneously bakes the film 11 in the first layer and the film 12 in the second layer, then the cooling device 17 water-cools the steel sheet 10 onto which the films 11 and 12 have been baked, and the drying device 18 dries the steel sheet 10.

[0145]

After that, the curtain coater 21 (the third coating device) applies a clear coating material onto the clear film 12 in a dried state to form a clear film 20 in the third layer. Note that as the third coating device, a roll coater or another coating device may be used instead of the curtain coater 21. Furthermore, the baking device 22 bakes the film 20 in the third layer, then the cooling device 23 water-cools the steel sheet 10 onto which the film 20 has been baked, and the drying device 24 dries the steel sheet 10.

[0146]

In the above manner, in the second embodiment, a pre-coated steel sheet coated with the films 11, 12, and 20 in the three layers is manufactured. According to the second embodiment, the clear layer 121, the diffusion layer 122, the pigmentconcentrated layer 123, and the colored film layer 111 are formed in the colored film 11 in the first layer and the clear film 12 in the second layer. A total thickness dl of a clear layer (= dl’ + D2’), which is the sum of a thickness dl’ of the clear layer 121 and a thickness D2’ of the clear film 20 in the third layer, is preferably controlled to satisfy 1.7 < dl/d2 < 4.7. In this case, like the two-layer pre-coated steel sheet in the first embodiment, the three-layer pre-coated steel sheet can also have high glossiness owing to the above four-layer structure.

[0147]

Like the first embodiment, the second embodiment provides excellent adhesion between the colored film 11 in the first layer and the clear film 12 in the second layer. In addition, in the second coating step using the curtain coater 15, the clear coating material in the second layer can be collected to be reused. Furthermore, in the second embodiment, in the third coating step using the curtain coater 21, the clear coating material in the third layer can be collected to be reused.

[0148]

In the second embodiment, after the application of the clear film 12 in the second layer, the third layer is applied after the first and second layers are baked by the baking device 16. Without being limited to this example, it is possible not to install the baking device 16, the cooling device 17, and the drying device 18 illustrated in FIG. 7 and to perform coating with the clear films 12 and 20 in the second and third layers by a wet-on-wet method by using the two curtain coaters 15 and 21, for example.

[0149] [8.2. Third embodiment] FIG. 8 is a schematic diagram illustrating a line configuration of a continuous coating apparatus according to a third embodiment of the present invention. As illustrated in FIG. 8, the continuous coating apparatus according to the third embodiment has a line configuration in which coating facilities for coating with a film in a lower layer (a curtain coater 34, a baking device 35, a cooling device 36, and a drying device 37) are additionally installed to precede the coating line (see FIG. 1) of the continuous coating apparatus according to the first embodiment.

[0150]

In the third embodiment, first, the curtain coater 34 (the third coating device) applies a primer coating material onto the steel sheet 10 to form a primer film 31 in a first layer. Note that as the third coating device, a roll coater or another coating device may be used instead of the curtain coater 34. Furthermore, the baking device 35 bakes the primer film 31 onto the steel sheet 10, then the cooling device 36 water-cools the steel sheet 10, and the drying device 37 dries the steel sheet 10.

[0151]

Then, the roll coater 13 (the first coating device) applies a colored coating material onto the primer film 31 in a dried state to form a colored film 32 in a second layer. Then, the pre-heating device 14 heats the colored film 32 to bring it into a half-dried state.

[0152]

Then, the curtain coater 15 (the second coating device) applies a clear coating material onto the colored film 32 in a half-dried state to form a clear film 33 in a third layer. Furthermore, the baking device 16 simultaneously bakes the films 32 and 33 in the second and third layers. After that, the cooling device 17 water-cools the steel sheet 10 onto which the films 32 and 33 have been baked, and the drying device 18 dries the steel sheet 10.

[0153]

In the above manner, in the third embodiment, a pre-coated steel sheet coated with the films 31, 32, and 33 in the three layers is manufactured. According to the third embodiment, the four-layer structure (the clear layer 121, the diffusion layer 122, the pigment-concentrated layer 123, and the colored film layer 111) can be formed in the colored film 32 in the second layer and the clear film 33 in the third layer. This four-layer structure allows a pre-coated steel sheet with high glossiness to be manufactured. Furthermore, as in the first embodiment, the adhesion between the colored film 32 in the second layer and the clear film 33 in the third layer is excellent, and the clear coating material in the third layer can be collected to be reused. The primer coating material in the first layer can also be collected to be reused.

[0154]

In the third embodiment, the second layer is applied after the primer film 31 in the first layer is baked by the baking device 35. Without being limited to this example, it is possible not to install the baking device 35, the cooling device 36, and the drying device 37 illustrated in FIG. 8 and to perform coating with the primer film 31 in the first layer and the colored film 32 in the second layer by a wet-on-wet method by using the curtain coater 34 and the roll coater 13, for example.

[0155] [8.3. Fourth embodiment] FIG. 9 is a schematic diagram illustrating a line configuration of a continuous coating apparatus according to a fourth embodiment of the present invention. As illustrated in FIG. 9, the continuous coating apparatus according to the fourth embodiment has a line configuration in which coating facilities for coating with a film in a second layer (a curtain coater 44 and a pre-heating device 45) are additionally installed in the middle of the coating line (see FIG. 1) of the continuous coating apparatus according to the first embodiment.

[0156]

In the fourth embodiment, first, the roll coater 13 (the third coating device) applies a primer coating material onto the steel sheet 10 to form a primer film 41 in a first layer, and the pre-heating device 14 heats the primer film 41 to bring it into a half-dried state.

[0157]

Then, the curtain coater 44 (the first coating device) applies a colored coating material onto the primer film 41 in a half-dried state to form a colored film 42 in a second layer. Furthermore, the pre-heating device 45 heats the colored film 42 in the second layer to bring it into a half-dried state.

[0158]

After that, the curtain coater 15 (the second coating device) applies a clear coating material onto the colored film 42 in a half-dried state to form a clear film 43 in a third layer. Furthermore, the baking device 16 simultaneously bakes the films 41, 42, and 43 in the first to third layers, then the cooling device 17 water-cools the steel sheet 10 onto which the films 41, 42, and 43 have been baked, and the drying device 18 dries the steel sheet 10.

[0159]

In the above manner, in the fourth embodiment, a pre-coated steel sheet coated with the films 41, 42, and 43 in the three layers is manufactured. According to the fourth embodiment, the four-layer structure (the clear layer 121, the diffusion layer 122, the pigment-concentrated layer 123, and the colored film layer 111) can be formed in the colored film 32 in the second layer and the clear film 33 in the third layer. This four-layer structure allows a pre-coated steel sheet with high glossiness to be manufactured.

[0160]

Furthermore, excellent adhesion between the primer film 41 in the first layer and the colored film 42 in the second layer is achieved as well as excellent adhesion between the colored film 42 in the second layer and the clear film 43 in the third layer. In addition to the clear coating material in the third layer, the colored coating material in the second layer can be collected to be reused. Moreover, since only one baking device 16, which performs final baking, one cooling device 17, and one drying device 18 need to be installed, facilities of the coating line for three-layer coating can be significantly simplified and space can be saved, as compared with a coating line for a conventional three-coat three-bake method.

[0161] [8.4. Additional embodiments]

Described in the above first to fourth embodiments are examples in which one surface of the steel sheet 10 is coated, but the present invention is not limited to these examples. A coating method of the present invention can be applied to coating of either of one surface and both surfaces of the steel sheet 10.

[0162]

For example, in the continuous coating apparatus according to the first embodiment (see FIG. 1), in addition to the first coating device (the roll coater 13) and the second coating device (the curtain coater 15) for coating a surface (one surface) of the steel sheet 10, there may be installed a third coating device for coating a surface on the other side (rear surface) of the steel sheet 10. Concurrently with the first coating step of applying a first film (the colored film 11) by using the first coating device, the heating step of bringing the colored film 11 into a half-dried state by using the pre-heating device 14, and the second coating step of applying a second film (the clear film 12) by using the second coating device, which are performed on the one surface of the steel sheet 10, a third coating step of forming a third film by using a third coating device may be performed on the surface on the other side of the steel sheet 10. After that, a baking step of simultaneously baking the first to third films on the both surfaces of the steel sheet 10 may be performed. Note that also in the second to fourth embodiments (see FIGS. 7 to 9), the surface on the other side (rear surface) of the steel sheet 10 may be coated by using an additional coating device concurrently with the coating of the one surface of the steel sheet 10 described above.

[Examples] [0163]

Next, Examples of the present invention will be described. Note that Examples described below are only condition examples employed to assess the feasibility and effect of the present invention, and the present invention is not limited to the conditions of Examples described below.

[0164] <1. Overview of test>

Chemical conversion treatment liquids, a colored coating material, and a clear coating material were prepared and two-layer coating was performed on steel sheets, whereby samples of pre-coated steel sheets were fabricated. Here, samples obtained by coating a steel sheet with a colored film and a clear film by the preheating method according to an embodiment of the present invention are Examples of the present invention. Samples obtained by coating a steel sheet with a colored film and a clear film by conventional methods, a 2C2B method and a wet-on-wet method, are Comparative Examples. Samples obtained by the pre-heating method but subjected to pre-heating under heating conditions (heating temperature T and heating time t) falling outside an appropriate range are also Comparative Examples.

[0165]

With regard to the samples of Examples and Comparative Examples, elementary analysis in the film thickness direction was performed, and thicknesses dl, d3, d2, and d4 of a clear layer, a diffusion layer, a pigment-concentrated layer, and a colored film layer and a maximum value cl and a minimum value c2 of a color pigment concentration ratio c of the pigment-concentrated layer were measured.

[0166]

Furthermore, the 60-degree gloss of each sample was measured by using a glossmeter, and glossiness was evaluated. In addition, the adhesion between the films in the upper and lower layers of each sample was evaluated by a 180-degree adhesion bending (0T bending) test.

[0167]

Table 1 shows the test conditions and evaluation results of this test.

[0168] [Table 1]

[0169] <2. Preparation of samples> <2.1. Preparation of coating materials to be used> (1) Chemical conversion treatment liquid 1

Chemical conversion treatment liquids used for the samples were prepared as follows.

An aqueous solution containing 5g/L of “3-Glycidoxypropyltrimethoxysilane” available from Shin-Etsu Chemical Co., Ltd., as a silane coupling agent, l.Og/L of “Snowtex-N” available from Nissan Chemical Industries, Ltd., as water-dispersed silica, 0.5g/L (zirconium ion basis) of zirconyl ammonium carbonate, as a zirconium compound, and 25g/L of polyacrylic acid, as an aqueous acrylic resin, was prepared as Chemical conversion treatment liquid 1.

[0170] (2) Chemical conversion treatment liquid 2

After blending 80 parts by mass of “HUX-320” available from ADEKA CORPORATION, as a urethane resin, 15 parts by mass of “Snowtex-N” available from Nissan Chemical Industries, Ltd., as silica sol, and 5 parts by mass of CHEMIPEARL (W500) available from Mitsui Chemicals, Inc., as aqueous wax, ion-exchanged water was added and the solid content of the blend was adjusted to be 20 %; thus, Chemical conversion treatment liquid 2 for a rear surface was prepared.

[0171] (3) Clear coating material (upper-layer coating material)

First, “VYLON (registered trademark) 270” (hereinafter called PES), which is an amorphous polyester resin available from Toyoboseki Co., Ltd., was dissolved in an organic solvent (cyclohexanone and Solvesso 150 (product name) were mixed at a mass ratio of 1:1 to be used). Next, a melamine resin “CYMEL (registered trademark) 303” available from Mitsui Cytec Ltd., as a curing agent, was added to the organic solvent in which the polyester resin was dissolved. On this occasion, the melamine resin was added in a manner that the resin solid contents satisfy the following mass ratio: the polyester resin solid content: the melamine resin solid content = 100: 35. Furthermore, 0.5 mass% of an acid catalyst “Catalyst 600” available from Mitsui Cytec Ltd. was added to the mixed solution of the polyester resin the melamine resin. In this manner, a polyester/melamine-based coating material was prepared as a clear coating material not containing a color pigment.

[0172] (4) Colored coating material (lower-layer coating material)

Polytetrafluoroethylene (hereinafter called PTFE), which is a commercially available lubricant, was added to the clear coating material obtained in (3) above so as to account for 2 mass% in a dried film. Next, titanium oxide “Multi-rack 106 white” available from TOYOCOLOR CO., LTD. was added so as to account for 60 mass% in a dried film. After that, titanium oxide was dispersed in the clear coating material by using a disperser; thus, a white coating material was obtained. In this manner, a polyester/melamine-based coating material (containing titanium oxide as a color pigment) was prepared as a colored coating material.

[0173] <2.2. Preparation of samples of pre-coated steel sheets> (1) Base material

An electroplated steel sheet was used as a base material (metal sheet) to be coated.

[0174] (2) Coating and drying/curing

Chemical conversion treatment liquids 1 and 2 were applied respectively onto a front surface and a rear surface of the base material to form chemical conversion treatment layers. Then, the colored coating material in the lower layer was applied onto the front surface of the base material by bar coating under conditions such that a dried film thickness D1 would be 15 pm, and the colored film was pre-heated at a predetermined heating temperature T (PMT) for a heating time t. Furthermore, the steel sheet after the pre-heating of the colored film was placed on a hot plate held at the same temperature as the heating temperature T of the lower layer, and the clear coating material in the upper layer was applied onto the colored film by blade coating under conditions such that a dried film thickness D2 would be 15 pm. After that, the colored coating material and the clear coating material applied onto the steel sheet were baked at PMT 230 °C (drying and curing). In this manner, samples were prepared.

[0175] (3) Heating conditions for lower layer film

As shown in Table 1, for Examples 1 to 7 of the pre-heating method, the heating temperature T (PMT) at the time of pre-heating the colored film in the lower layer was set to 60 °C (Example 1), 90 °C (Example 2), 120 °C (Example 3), 150 °C (Example 4), 120 °C (Example 5 to 7), and 150 °C (Example 8). For Comparative Examples 3 to 8 of the pre-heating method, the heating temperature T (PMT) at the time of pre-heating the colored film in the lower layer was set to 40 °C (Comparative Example 3), 50 °C (Comparative Example 4), 160 °C (Comparative Example 5), 150 °C (Comparative Examples 6 and 7), and 60 °C (Comparative Example 8).

[0176]

Samples of a 2C2B method (Comparative Example 1) and a wet-on-wet method (Comparative Example 2) were also fabricated. In the 2C2B method (Comparative Example 1), the heating temperature of the colored film in the lower layer was set to 230 °C, and the colored film was completely dried and cured. In the wet-on-wet method (Comparative Example 2), after the colored film was applied, the clear coating material was applied onto the colored film in a wet state without heating the colored film.

[0177] <3. Test method> <3.1. Titanium concentration ratio measurement test> (1) Measurement method

Elementary analysis in the film thickness direction was performed, while etching the films and a galvanized layer, from the outermost layer of each sample coated with the colored film and the clear film, by argon sputtering of glow discharge emission spectrometry (GDS). The measured elements were C, N, O, Si, Ti, Zn, and Fe. As a glow discharge emission spectrometer, “GD-PROFILER2” available from HORIBA, Ltd. was used.

[0178] (2) Ti concentration ratio measurement

The thicknesses dl, d3, d2, and d4 of the respective layers (clear layer, diffusion layer, pigment-concentrated layer, and colored film layer) of the four-layer structure were determined, on the basis of the sample of Comparative Example 1 (2C2B) prepared with a thickness configuration similar to those of the samples of Examples (pre-heating method). On this occasion, the distribution of Ti concentration ratio (see the following formula) when the Ti molar concentration of the colored film layer is defined as 1 was measured for each sample.

Ti concentration ratio = (Ti molar concentration)/(maximum Ti molar concentration of the colored film layer) [0179] (3) Thickness measurement

Since titanium oxide is a white color pigment, measurement of the Ti concentration ratio in the thickness direction allows measurement of the color pigment concentration distribution and the thickness of each layer. Hence, the thickness of a layer not containing the color pigment (Ti) from the outermost surface of the film was determined, as the thickness dl of the clear layer. In addition, the thickness of a layer from a local maximum value (when a plurality of local maximum values exist, the local maximum value of the concentration distribution that is closest to the outermost layer) of the Ti concentration distribution in the clear film to a local minimum value on the base material side was determined, as the thickness d2 of the pigment-concentrated layer. The thickness of a layer between the clear layer and the pigment-concentrated layer was determined, as the thickness d3 of the diffusion layer. The thickness of a layer between the pigment-concentrated layer and the base material was determined, as the thickness d4 of the colored film layer.

[0180] <3.2. Gloss measurement test>

As an indicator of the glossiness of a sample surface, 60-degree gloss according to ”JIS Z 8741” was measured. As a glossmeter of 60-degree gloss, a glossmeter “UGV-6P” available from Suga Test Instruments Co., Ltd. was used. In the measurement, an incidence angle and an acceptance angle were each adjusted to be 60 degrees. That is, out of the total reflected light reflected by the sample (total reflection), only the intensity of a specular reflection component, which is generally determined as the gloss of a pre-coated steel sheet, was measured. Note that diffuse reflectivity and total reflectivity are known as ways to measure light reflectivity; in general, the relation of “total reflectivity = specular reflectivity + diffuse reflectivity” is satisfied. These values are different from 60-degree gloss measured in the present application.

[0181] <3.3. Working adhesion evaluation test>

As illustrated in FIG. 10, each of the samples of pre-coated steel sheets was worked into a 50 mm χ 100 mm rectangular sheet and was subjected to bending in a manner that an evaluation surface 10a faces outward. This bending, carried out in a 20 °C atmosphere, was “adhesion bending (0T)”, in which a spacer is not placed between portions of the steel sheet 10. After that, “Cellotape (registered trademark)”, available from Nichiban Co., Ltd., with a width of 24 mm was attached onto the film at a bent portion and then peeled off; thus, the film at the bent portion was attempted to be peeled off, and the residual state of the film was visually observed. The residual degree of the film was evaluated in five levels, and when the score was 5 or more, indicating no occurrence of the peeling of the film, it was determined that the working adhesion between the films in the upper and lower layers was favorable. (“Good” in Table 1). When the score was 4 or less, it was determined that the working adhesion was unfavorable (“No Good” in Table 1).

[0182] <3.4. Cross-sectional SEM observation>

Gold evaporation was performed on each sample, then the sample was embedded in a resin, and a cross section of the sample was polished together with the resin. After that, the state of the cross section (polished surface) of the sample was observed with a FE-SEM.

[0183] <4. Evaluation results>

Next, the results of the above tests will be described referring to Table 1 as appropriate.

[0184] <4.1. Measurement results of titanium concentration ratio> FIG. 11 is a graph showing the Ti concentration ratio distribution in the film thickness direction of Comparative Examples 1 to 3 of Table 1. FIG. 12 is a graph showing the Ti concentration ratio distribution in the film thickness direction of Examples 1 to 4 and Comparative Example 1 of Table 1. FIG. 13 is a graph obtained by partially enlarging FIG. 12.

[0185]

As shown in FIG. 11, in Comparative Example 1 (2C2B method), Ti does not exist on the outer layer side, and a layer with high Ti concentration exists on the base material side. In other words, in Comparative Example 1, a clear layer with zero Ti concentration and a colored film layer with high Ti concentration exist, but a pigment-concentrated layer and a diffusion layer do not exist.

[0186]

In Comparative Example 2 (wet-on-wet method), Ti exists in a large amount on the outer layer side of the film, and Ti reaches the outermost layer of the film. This is presumably because the clear film and the colored film in a wet state mixed with each other to form a mixed layer, and thus Ti of the colored film moved to the clear film. As described above, in Comparative Example 2, a clear layer with zero Ti concentration does not exist and the Ti concentration distribution is ununiform.

[0187]

In Comparative Example 3 (pre-heating method), as compared with Comparative Example 2, a large amount of Ti is distributed in the central portion in the film thickness direction, but Ti has moved to the neighborhood of the outermost layer of the film, and a definite clear layer and pigment-concentrated layer do not exist, presumably for the following reason. That is, in Comparative Example 3, the heating temperature T at the time of pre-heating the colored film in the lower layer was 40 °C, which is too low, and the heating time t was 0.4 seconds, which is too short; therefore, the colored film in the lower layer was not brought into an appropriate half-dried state by the pre-heating. Consequently, when the clear coating material in the upper layer was applied, the colored film and the clear film mixed with each other, and a definite clear layer and pigment-concentrated layer were not formed.

[0188]

In contrast, in Examples 1 to 4 (pre-heating method), as shown in FIGS. 12 and 13, the amount of movement of Ti to the outer layer side is smaller than those of Comparative Examples 2 and 3, and Ti does not exist, i.e., a clear layer not containing Ti is formed, in a range of at least 5 pm from the outermost layer. A peak of Ti concentration exists around film thicknesses of 14 to 17 pm. On the outer layer side of the peak, a diffusion layer in which Ti concentration continuously decreases from the peak toward the outer layer side is formed. In addition, on the base material side of the peak, a pigment-concentrated layer in which Ti concentration continuously increases toward the outer layer side to reach the peak is formed. On the base material side of the pigment-concentrated layer, a colored film layer having a substantially constant Ti concentration is formed.

[0189]

As described above, in Examples 1 to 4, a definite four-layer structure of the clear layer, the diffusion layer, the pigment-concentrated layer, and the colored film layer exists. Particularly in Example 4 (T = 150 °C), as shown in FIG. 13, a peak with a markedly high Ti concentration ratio exists around a film thickness of 16 pm, and a valley with a markedly low Ti concentration ratio exists around a film thickness of 21 pm. This proves the existence of a marked pigment-concentrated layer in Example 4.

[0190]

As shown in Table 1, in Examples 1 to 8, the ratio dl/d2 between the thickness dl of the clear layer and the thickness d2 of the pigment-concentrated layer is equal to or greater than 1.7 and equal to or less than 4.7, and falls within the aforementioned appropriate range of dl/d2 (1.7 < dl/d2 < 4.7) for obtaining a high gloss. This is presumably because in Examples 1 to 8, the pre-heating of the colored film in the lower layer was performed under appropriate heating conditions (i.e., heating temperature T = 60 to 150 °C, heating time t = 1.0 to 10 seconds), which allowed dl/d2 to fall within the appropriate range. Furthermore, in Examples 2 to 8, in which T = 90 to 150 °C, dl/d2 falls within a more favorable appropriate range of dl/d2 (2.0 < dl/d2 < 4.7).

[0191]

In contrast, dl/d2 of Comparative Examples 4 and 6 to 8 fall outside the above appropriate range of dl/d2. In Comparative Example 4, the short heating time t of 0.7 seconds and the too low heating temperature T of 50 °C presumably made the pre-heating of the colored film insufficient. In Comparative Examples 6 and 7, T was 150 °C, which is within the appropriate range, but the too long heating times t of 11 seconds and 20 seconds presumably caused the colored film to be excessively pre-heated to be dried more than necessary. In Comparative Example 8, despite T being a low temperature of 60 °C, the markedly too long heating time t of 600 seconds presumably caused the colored film to be excessively pre-heated to be dried more than necessary. Furthermore, in Comparative Example 5, the too high heating temperature T of 160 °C made the amount of the solvent volatilized from the colored coating material at the time of pre-heating excessively large, so that the tests were not able to be performed.

[0192]

The above results prove that, in the method for manufacturing a pre-coated steel sheet according to the embodiment, by heating the colored film in the lower layer under appropriate heating conditions (T = 60 to 150 °C, t = 1.0 to 10 seconds) at the time of pre-heating, the definite four-layer structure can be formed in the film layers of the pre-coated steel sheet after final baking, and dl/d2 can be controlled to fall within an appropriate range.

[0193] <4.2. Evaluation results of gloss> FIG. 14 is a graph showing the relation between dl/d2 and the test results of 60-degree gloss of Table 1. FIG. 15 is a graph showing the relation between the heating temperature T, and dl, d2, and the test results of 60-degree gloss of Table 1. Note that FIG. 15 shows the results when a rate of temperature rise “= (PMT [°C] -25 [°C])/heating time t [s]” at the time of pre-heating the lower layer film was approximately 35 °C/s.

[0194]

As shown in Table 1 and FIG. 14, in Examples 1 to 8, which satisfy 1.7 < dl/d2 < 4.7, the 60-degree gloss is 85 or higher, which means that a gloss significantly higher (1.7 times or more higher) than the 60-degree gloss (= 50) in the case of Comparative Example 2 (wet-on-wet method) is obtained. Furthermore, in Examples 2 to 8, which satisfy 2.0 < dl/d2 < 4.7, the 60-degree gloss is 103 or higher, which means that a gloss higher than the 60-degree gloss (= 90) in the case of Comparative Example 1 (2C2B method) is obtained.

[0195]

In addition, the results of Comparative Examples 3 and 4 (pre-heating method) show that as the heating temperature T of the colored film in the lower layer decreases from 60 °C, the 60-degree gloss decreases to approach the 60-degree gloss in the case of Comparative Example 2 (wet-on-wet method). In Examples 2 to 8, in which T > 90 °C, the 60-degree gloss is substantially constant at 100 or higher. In Comparative Examples 6 to 8 (pre-heating method), which do not satisfy appropriate conditions (1 second < t < 10 seconds) of the heating time t at the time of pre-heating, a gloss higher than that of Comparative Example 2 (wet-on-wet method) is obtained, but the gloss is lower than that of Comparative Example 1 (2C2B method). Note that these Comparative Examples 6 to 8 have a problem of low adhesion between the films, as will be described later (see Table 1).

[0196]

The above results prove that satisfying 1.7 < dl/d2 < 4.7 results in at least a gloss significantly higher than that in the case of a conventional wet-on-wet method, and further, satisfying 2.0 < dl/d2 < 4.7 results in a gloss higher than that in the case of a conventional 2C2B method.

[0197]

As shown in Table 1 and FIG. 15, in Examples 1 to 8 (pre-heating method), when “60 °C < T < 150 °C”, “d2 > 2.2 pm”, and “5.0 pm < dl < 12.3 pm” are satisfied, the 60-degree gloss is 85 or higher, which means that a gloss significantly higher than that of Comparative Example 2 (wet-on-wet method) is obtained.

Furthermore, in Examples 2 to 8 (pre-heating method), when “90 °C < T < 150 °C”, “d2 > 2.8 pm”, and “8.0 pm < dl < 12.3 pm” are satisfied, the 60-degree gloss is 100 or higher, which means that a gloss higher than that of Comparative Example 1 (2C2B method) is obtained.

[0198] <4.3. Evaluation results of working adhesion>

Next, description is given on the results of evaluating the working adhesion between the films in the upper and lower layers by the “adhesion bending (0T)” test illustrated in FIG. 10. FIG. 16A is an enlarged photograph of the bent portion of the pre-coated steel sheet according to Comparative Example 1 (2C2B). FIG. 16B is an enlarged photograph of the bent portion of the pre-coated steel sheet according to Example 1 (heating temperature T = 60 °C).

[0199]

As shown in Table 1, in Comparative Example 1 (2C2B), the clear film in the upper layer peeled off from the colored film in the lower layer as shown in FIG. 15A. Peeling occurred similarly in Comparative Examples 6 to 8 (pre-heating method) as well. This is presumably because in Comparative Examples 1 and 6 to 8, the lower layer film was dried and cured at the time of heating the lower layer film, which made the adhesion low between the upper and lower layer films.

[0200]

In contrast, in Examples 1 to 8 (pre-heating method), Comparative Example 2 (wet-on-wet method), and Comparative Examples 3 and 4 (pre-heating method), the clear film in the upper layer did not peel off from the colored film in the lower layer as shown in FIG. 15B. This is presumably because in Comparative Example 2, the lower layer film was not pre-heated, which made the adhesion high between the upper and lower films in a wet state, and because in Examples 1 to 8 (pre-heating method) and Comparative Examples 3 and 4 (pre-heating method), the upper layer film was applied after the lower layer film was brought into an appropriate half-dried state by pre-heating, which resulted in a significant improvement in the adhesion between the films in the upper and lower layers, as compared with Comparative Examples 1 and 6 to 8.

[0201]

In addition, according to the evaluation test results of 60-degree gloss and the test results of working adhesion, which are described above, in Examples 1 to 8 (pre-heating method), which satisfy appropriate conditions of dl/d2 (1.7 < dl/d2 < 4.7), a high 60-degree gloss of 85 or higher and high working adhesion are both achieved. In contrast, in Comparative Example 2 (wet-on-wet method), and in Comparative Examples 3 and 4 (pre-heating method), which do not satisfy 1.7 < dl/d2 < 4.7, working adhesion is high, but 60-degree gloss is as low as approximately 50 to 70. In Comparative Examples 6 to 8 (pre-heating method), 60-degree gloss is high to some extent, but the adhesion between the films is low.

[0202]

The above results prove that the pre-heating method of the method for manufacturing a pre-coated steel sheet according to the embodiment makes it possible to form the above four-layer structure in the film layers of the pre-coated steel sheet after final baking, and satisfying 1.7 < dl/d2 < 4.7 makes it possible to achieve both high glossiness and high working adhesion between the films.

[0203] <4.4. Evaluation results of cross-sectional SEM observation>

Next, description is given on the results of observing and evaluating a crosssectional SEM image of each sample in the above cross-sectional SEM observation test.

[0204]

In the sample of Comparative Example 2 (wet-on-wet method), the colored film in the lower layer and the clear film in the upper layer mixed with each other to form a mixed layer. In Comparative Example 3 (pre-heating method, T = 40°), the interface between the colored film in the lower layer and the clear film in the upper layer was greatly curved. Hence, it is presumed that in these Comparative Examples 2 and 3, the clear layer and the pigment-concentrated layer are not formed. Consequently, incident light cannot appropriately pass through the outer layer portion of the film having no clear layer, and is not diffusely reflected by a pigmentconcentrated layer, which is presumably the reason for the low 60-degree gloss of the samples of Comparative Examples 2 and 3.

[0205]

In contrast, in Examples 1 to 4 (pre-heating method), the interface between the colored film in the layer and the clear film in the upper layer was smoother than those of Comparative Examples 2 and 3. Hence, it is presumed that in Examples 1 to 4, the clear layer and the pigment-concentrated layer are appropriately formed. Consequently, incident light appropriately passes through the clear layer and is diffusely reflected efficiently by the pigment-concentrated layer, which is presumably the reason for the 60-degree gloss of the samples of Examples 1 to 4 being significantly higher than those of Comparative Examples 2 and 3.

[0206]

The preferred embodiment(s) of the present disclosure has/have been described above with reference to the accompanying drawings, whilst the present disclosure is not limited to the above examples. A person skilled in the art may find various alterations and modifications within the scope of the appended claims, and it should be understood that they will naturally come under the technical scope of the present disclosure.

Reference Signs List [0207] 10 steel sheet, metal sheet 11, 32, 42 colored film 12, 20, 33, 43 clear film 13 roll coater 14, 45 pre-heating device 15, 21, 34, 44 curtain coater 16, 22, 35 baking device 17, 23, 36 cooling device 18,24,37 drying device 31,41 primer film 111 colored film layer 121 clear layer 122 diffusion layer 123 pigment-concentrated layer

Claims

CLAIMS Claim 1 A pre-coated metal sheet comprising a plurality of film layers on one surface or both surfaces of a metal sheet, the film layers including a colored film layer that is placed on the metal sheet side and contains a color pigment, a pigment-concentrated layer that is formed on the colored film layer, and whose color pigment concentration is equal to or greater than an average value of color pigment concentration in the colored film layer and increases toward an outer layer side, a diffusion layer that is formed on the pigment-concentrated layer, and whose color pigment concentration decreases toward the outer layer side, and a clear layer that is formed on the diffusion layer and does not contain the color pigment, wherein a ratio dl/d2 between a thickness dl of the clear layer and a thickness d2 of the pigment-concentrated layer satisfies 1.7 < dl/d2 < 4.7.
Claim 2 The pre-coated metal sheet according to claim 1, wherein a ratio cl/c2 between a maximum value cl and a minimum value c2 of a color pigment concentration ratio c in the pigment-concentrated layer based on the average value of the color pigment concentration in the colored film layer satisfies 1.04 < cl/c2 < 2.0.
Claim 3 The pre-coated metal sheet according to claim 1 or 2, wherein the ratio dl/d2 satisfies 2.0 < dl/d2 < 4.7.
Claim 4 The pre-coated metal sheet according to claim 1 or 2, wherein the thickness d2 of the pigment-concentrated layer is equal to or greater than 2.2 pm and equal to or less than 8.0 pm.
Claim 5 The pre-coated metal sheet according to claim 1 or claim 2, wherein the thickness dl of the clear layer is equal to or greater than 5.0 pm and equal to or less than 12.3 pm.
Claim 6 The precoated metal sheet according to claim 1 or claim 2, wherein the coloured film layer includes a polyester resin and a melamine resin and the clear layer includes the polyester resin and the melamine resin.
Claim 7 A method for manufacturing a pre-coated metal sheet, wherein the pre-coated metal sheet according to any one of claims 1 to 6 is manufactured by continuously coating a surface of a strip-shaped metal sheet, the method comprising: a first coating step of, by a first coating device, applying a colored coating material containing a color pigment onto the surface of the metal sheet or onto another film applied onto the surface of the metal sheet, to form a colored film; a pre-heating step of, by a heating device, heating the colored film at a heating temperature of 60 to 150°C for a heating time of 1 to 10 seconds to bring the colored film into a half-dried state; a second coating step of, by a second coating device, applying a clear coating material not containing the color pigment onto the colored film in a half-dried state to form a clear film; and a baking step of, by a baking device, simultaneously baking the colored film and the clear film.
Claim 8 A continuous coating apparatus, wherein the pre-coated metal sheet according to any one of claims 1 to 6 is manufactured by continuously coating a surface of a strip-shaped metal sheet, the continuous coating apparatus comprising: a first coating device configured to apply a colored coating material containing a color pigment onto the surface of the metal sheet or onto another film applied onto the surface of the metal sheet, to form a colored film; a heating device configured to heat the colored film at a heating temperature of 60 to 150°C for a heating time of 1 to 10 seconds to bring the colored film into a half-dried state; a second coating device configured to apply a clear coating material not containing the color pigment onto the colored film in a half-dried state to form a clear film; and a baking device configured to simultaneously bake the colored film and the clear film.