JP2013018192A - Surface-treated steel plate and casing for home electric appliance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface-treated steel plate having a thin coating formed on the surface, stable colored appearance, and relatively favorable corrosion resistance; and a casing for a home electric appliance including the surface-treated steel plate.SOLUTION: A surface-treated steel plate 10 includes a base steel plate 1 and a coating 2 formed at least on one surface of the base steel plate, in which the coating includes an upper coating 3 and a lower coating 4, the lower coating includes an organic or inorganic-organic hybrid component as a binder, and contains a color pigment having a refractive index of ≥1.7 in an amount of ≥5 mass% and ≤25 mass% based on the mass of the solid content of the lower coating, the upper coating includes an organic component as a binder, and contains a coloring pigment having a refractive index of ≥1.7 in an amount of ≤3 mass% based on the mass of solid content of the upper coating. The lower coating has a thickness of ≥0.2 μm and ≤1 μm, the upper coating has a thickness of ≥0.5 μm, and the total film thickness of the lower coating and the upper coating is ≤3 μm.

Description

  TECHNICAL FIELD The present invention relates to a surface-treated steel sheet, specifically, a surface-treated steel sheet suitable for a housing for AV equipment typified by a thin display such as a liquid crystal television, an organic EL television, and a plasma television, and a home appliance using the same. It is related with the housing | casing.

  A coated steel plate (also referred to as pre-coated steel plate or PCM) is coated and baked on a base steel plate, wound up in a coil shape, and delivered to the user in that state. The user unwinds the coil and performs punching, bending, drawing, or a combination thereof to produce a product. Such a coated steel sheet is applied to many fields because it is not necessary for the user to perform a painting operation that deteriorates the working environment or makes it difficult to treat the waste liquid.

  For the production of coated steel sheets, a base steel sheet (typically a zinc-based plated steel sheet including zinc plating and zinc alloy plating) is subjected to chemical conversion treatment as a pretreatment, followed by application and baking of a primer coating (primer). Then, a two-coat two-bake method is generally used in which a top coat is applied and baked. However, on the “back surface” side opposite to the “front surface” which is the outer side of the final product, the paint developed for the back surface after the pre-treatment is used, and the coating is performed in a 1-coat 1-bake method. Sometimes.

  Painted steel sheet has many performances such as corrosion resistance (suppression of white rust and / or red rust at the edge of the coating), workability, coating hardness (scratch resistance), stain resistance, chemical resistance, weather resistance, etc. Must be achieved at a high level. However, in recent years, in addition to reducing costs, from the viewpoint of environmental measures and the rationalization of painting operations, the coating film that is a quality assurance aspect that requires the design of coated steel sheets, etc., has also been improved. It has been required to reduce the thickness (coating thickness). For example, in applications used on the back side of final products, such as back panel applications for thin displays, the coating thickness in the field of so-called chemical conversion steel sheets and fingerprint-resistant steel sheets, such as the first half of the total coating thickness of several microns in total. There is a demand for thinning the film.

When the coating film thickness is reduced, it is possible to reduce the burden on the insulator used to burn the organic solvent generated during paint baking, and to reduce the CO ₂ discharged therefrom. In addition, the amount of organic solvent discharged during coating baking depends on the coating thickness, so as the coating thickness decreases, the line speed can be increased within the range of the incinerator, and the painting work is streamlined. It becomes possible to do.

  However, when the coating thickness is reduced, for example, the brightness of the surface of the coated steel sheet is deteriorated, the coating film is uneven in color tone, or it is difficult to conceal the wrinkles of the base steel sheet, It is usually difficult to obtain a good appearance as a coated steel sheet. An invention aimed at improving this point is disclosed in Patent Document 1. In addition, Patent Document 2 discloses an invention aimed at improving the corrosion resistance reduction of the coated steel sheet accompanying the thinning of the coating film.

  In addition, as a method to reduce the burden on the insulator used to burn the organic solvent generated during paint baking, water-based paints that contain little or no solvent in the paint are also being investigated. Patent Document 3 discloses an invention aimed at achieving both corrosion resistance as a back panel application for a thin display.

  On the other hand, many surface-treated steel sheets (for example, so-called fingerprint-resistant steel sheets) in which a thin film of about several microns is formed on the surface of a base steel sheet using an aqueous chemical solution have been developed. For example, Patent Document 4 discloses a surface-treated steel sheet intended to achieve both fingerprint resistance and corrosion resistance.

JP 2008-55774 A JP 2010-115902 A JP 2010-247396 A JP 2000-263695 A

  The coated steel sheets described in Patent Documents 1, 2, and 3 all require a 2 μm or more (overcoat) coating film for ensuring design properties, and in addition, a chemical conversion coating film and / or an undercoat coating film are provided below the coating film. Is provided. Therefore, the coating film thickness is inevitably increased. On the other hand, in the conventional thin film type surface-treated steel sheet represented by Patent Document 4, the film formed on the steel sheet surface is basically a clear film, and the influence of the surface appearance (color tone and wrinkles) of the base steel sheet. It was a thing to receive greatly.

  Therefore, an object of the present invention is to provide a surface-treated steel sheet having a thin coating formed on the surface, a stable colored appearance, and relatively good corrosion resistance. Moreover, the housing | casing for household appliances provided with the said surface treatment steel plate is provided. In the present invention, “stable appearance” means that there is no visible difference in the surface on which the coating is formed in the appearance of one surface-treated steel sheet and between surface-treated steel sheets of the same type. Means that.

  In order to stabilize the appearance of the surface-treated steel sheet, it is conceivable to include a color pigment in the coating film formed on the surface of the base steel sheet as in the case of the coated steel sheet. Here, when the present inventors mainly use a color pigment having a refractive index of 1.7 or more, the surface appearance (color tone or wrinkles) of the base steel sheet is concealed with a film, and the surface-treated steel sheet having a stable appearance. It was found that can be obtained. Furthermore, it is possible to obtain a surface-treated steel sheet having a balance of various performances even if the coating film is thinned by making the coating film into multiple layers and mainly containing a color pigment having a refractive index of 1.7 or more in the layer on the base steel plate side. I found out that The present invention completed based on such knowledge is as follows.

  1st aspect of this invention is the surface treatment steel plate 10 provided with a base-material steel plate and the film formed in the at least one surface of this base-material steel plate, Comprising: A film is a base material from an upper-layer film and this upper-layer film. A lower layer coating formed on the steel plate side, the lower layer coating is a coating having an organic or organic-inorganic composite component as a binder, and a coloring pigment having a refractive index of 1.7 or more The upper layer film is a film containing an organic component as a binder, and a color pigment having a refractive index of 1.7 or more is contained in the upper layer film. 10% by mass or less with respect to the mass, the thickness of the lower layer film is 0.2 μm or more and 1 μm or less, the thickness of the upper layer film is 0.5 μm or more, and the film thickness of the lower layer film and the upper layer film A surface-treated steel sheet characterized by having a total of 3 μm or less. .

  In the surface-treated steel sheet according to the first aspect of the present invention, when carbon black is included as the color pigment contained in the lower layer coating, the content of the carbon black is 10% or less with respect to the solid content mass of the lower layer coating. It is preferable.

  In the surface-treated steel sheet according to the first aspect of the present invention, at least one of the lower layer film and the upper layer film contains an organic color pigment in an amount of 25% by mass or less based on the solid content mass of the lower layer film or the upper layer film. Is preferred.

  Here, “the organic color pigment is contained in an amount of 25% by mass or less with respect to the solid content mass of the lower layer film or the upper layer film” means that when the lower layer film contains an organic color pigment, the solid content mass of the lower layer film This means that the organic color pigment is contained in an amount of 25% by mass or less, and when the upper layer film contains an organic color pigment, the organic color pigment is contained in an amount of 25% by mass or less based on the solid content mass of the upper layer film. It means to contain.

  Furthermore, in the surface-treated steel sheet according to the first aspect of the present invention, it is preferable that one surface and the other surface of the base steel sheet have a color tone difference that is visually distinguishable.

  A second aspect of the present invention is a housing for home appliances using the surface-treated steel sheet according to the first aspect of the present invention.

  According to the first aspect of the present invention, a surface-treated steel sheet having a thin coating film formed on the surface, a stable colored appearance, relatively good corrosion resistance, and a good balance of various performances. Can do.

  When carbon black is included as the color pigment contained in the lower layer coating, the corrosion resistance of the surface-treated steel sheet is lowered by setting the carbon black content to 10% or less of the solid content mass of the lower layer coating. While preventing, the surface appearance (color tone and wrinkles) of the base steel sheet can be concealed by the coating.

  Moreover, it becomes easy to give a color tone to a surface-treated steel sheet when at least one of a lower layer film or an upper layer film contains an organic color pigment in an amount of 25% by mass or less based on the solid mass.

  Furthermore, the surface-treated steel sheet can be easily distinguished from the front surface and the back surface by adopting a form in which one surface and the other surface of the base steel sheet have a color tone difference that is visually distinguishable. It becomes easy to use.

  According to the second aspect of the present invention, there is provided a surface-treated steel sheet having a thin coating formed on the surface, a stable colored appearance, relatively good corrosion resistance, and a good balance of various performances. It can be set as a housing for home appliances.

1 is a view schematically showing a cross section of a surface-treated steel sheet 10. FIG.

  The above-described operation and gain of the present invention will be clarified from embodiments for carrying out the invention described below. Hereinafter, the present invention will be described based on embodiments shown in the drawings. However, the present invention is not limited to these embodiments. The drawings are simplified by changing the scale and the like as appropriate for the convenience of illustration and understanding.

<Surface treated steel plate>
FIG. 1 is a diagram schematically showing a cross section of the surface-treated steel sheet 10. As shown in FIG. 1, the surface-treated steel plate 10 includes a base steel plate 1 and a coating 2 formed on at least one surface of the base steel plate 1. The coating 2 includes an upper coating 3 and a lower coating 4 formed on the base steel plate 1 side from the upper coating 3. Moreover, the back surface coating 5 may be formed in the surface on the opposite side to the side in which the film 2 of the base-material steel plate 1 was formed. In the present specification, the surface of the surface-treated steel sheet 10 on which the coating 2 is formed is referred to as “front surface”, and the opposite surface is referred to as “back surface”. Hereinafter, the surface-treated steel sheet 10 will be described for each of these constituent requirements.

1. Base steel plate 1
The base steel plate 1 used for the surface-treated steel plate 10 is not particularly limited. For example, a zinc-based plated steel sheet with a zinc-containing plating layer used for general painted steel sheets, that is, a zinc-plated steel sheet or a zinc alloy-plated steel sheet, or alloyed plating obtained by alloying these platings with a substrate A steel plate can be used as the base steel plate 1.

  The zinc-based plated steel sheet may be produced by any of electroplating, hot dipping, and vapor phase plating. Examples of the galvanized steel sheet include hot dip galvanized steel sheet, electrogalvanized steel sheet, molten 5% Al—Zn alloy plated steel sheet, molten 55% Al—Zn alloy plated steel sheet, galvannealed steel sheet, electric Zn-Ni alloy plated steel sheet and the like can be mentioned.

The coating amount of the galvanized steel sheet is not particularly limited and may be within a general range. The plating adhesion amount of the zinc-based plated steel sheet is preferably 100 g / m ² or less in terms of one-side average adhesion amount. In the case of electroplating, this adhesion amount is more preferably 3 g / m ² or more and 50 g / m ² or less, and in the case of hot dipping, it is more preferably 30 g / m ² or more and 100 g / m ² or less. .

  In relation to the appearance of the surface-treated steel sheet 10, when the appearance of the surface-treated steel sheet 10 is relatively bright (whiter), the base steel sheet 1 has a relatively bright appearance, for example, electrogalvanizing. It is preferable to use a steel plate or a hot-dip galvanized steel plate. Conversely, when the appearance of the surface-treated steel sheet 10 is relatively dark, the base steel sheet 1 has a relatively dark appearance, such as a zinc alloy-based plated steel sheet, more specifically, an electrozinc-nickel alloy. It is preferable to use a plated steel sheet, an electrogalvanized-iron alloy plated steel sheet, an galvannealed steel sheet, or the like. In general, the electrogalvanized steel sheet has less color variation than the hot dip galvanized steel sheet. Therefore, when the electrogalvanized steel sheet is used as the base steel sheet 1, the appearance of the surface-treated steel sheet 10 is easily stabilized. .

2. Lower layer coating 4
The lower layer film 4 is a film having an organic or organic-inorganic composite component as a binder, and a color pigment having a refractive index of 1.7 or more is 5% by mass or more and 25% by mass with respect to the solid content mass of the lower layer film 4. % Or less. However, when part or all of the color pigment is carbon black, the content is preferably 10% by mass or less with respect to the solid content mass of the lower layer coating 4.

  For the formation of the lower layer film 4, for example, a known base chemical solution used for so-called chromium-free chemical conversion treatment can be used. Specifically, base chemicals such as E300 series manufactured by Nippon Parkerizing Co., Ltd. and Surfcoat series manufactured by Nippon Paint Co., Ltd. can be used. The lower layer coating 4 can be formed by adding a predetermined amount of the above-mentioned color pigment to such a base chemical solution to prepare a treatment solution, applying the treatment solution to a plated steel sheet, and baking it. Incidentally, these chromium-free commercial products are called organic-inorganic composite treatment (coating) in which a resin is combined with an inorganic additive such as silica or zirconia, a coupling agent, or the like.

  Examples of the color pigment having a refractive index of 1.7 or more contained in the lower layer coating 4 include carbon black as the black pigment and zinc white and titania as the white pigment.

  Of course, it is easier to stabilize the appearance of the surface-treated steel sheet 10 by concealing the surface appearance of the base steel sheet 1 if a large amount of such color pigments are contained. On the other hand, if the content of the color pigment is too large, the corrosion resistance of the surface-treated steel sheet 10 may be reduced. Among them, although carbon black has a large hiding power, it is a conductor, or when carbon black is used for a coating, the corrosion resistance of the surface-treated steel sheet is likely to be lowered. Accordingly, the appropriate content of the color pigment having a refractive index of 1.7 or more in the lower layer film 4 is 5% by mass or more and 25% by mass or less with respect to the solid content mass of the lower layer film 4, and carbon black is used as the color pigment. When it contains, content of this carbon black is 10 mass% or less.

Further, the lower layer film 4 may contain a color pigment having a refractive index of less than 1.7. Specific examples of the color pigment include various organic color pigments and barium sulfate. When the lower layer film 4 contains a color pigment having a refractive index of less than 1.7, the content thereof is preferably less than 25% by mass with respect to the solid content mass of the lower layer film 4. This is because when the amount exceeds 25% by mass, the ratio of the main component of the chemical solution responsible for the corrosion resistance and the like required for the coating decreases.
Note that the base chemical solution may contain silica or zirconia as described above. These are widely used to improve the performance of the coating as so-called extender pigments or rust inhibitors. These are also colored pigments having a refractive index of less than 1.7, but the coloring power is small. In the present invention, silica and zirconia are not included in the “color pigment having a refractive index of less than 1.7”.

  The thickness of the lower layer coating 4 is preferably thicker from the viewpoint of concealing the surface appearance of the base steel plate 1 and stabilizing the appearance of the surface-treated steel plate 10. In addition, the thicker the lower layer coating 4, the better the corrosion resistance of the surface-treated steel sheet 10. However, when the lower layer film 4 is too thick, the manufacturing cost of the surface-treated steel sheet 10 increases. Moreover, when the lower film 4 is too thick, the adhesiveness with respect to the base-material steel plate 1 of the lower film 4 will fall. This is presumably because the lower layer film 4 itself tends to cohesively break. Therefore, the appropriate thickness range of the lower layer coating 4 is 0.2 μm or more and 1.0 μm or less, and preferably 0.3 μm or more and 0.5 μm or less. The thickness of the lower layer coating 4 can be appropriately adjusted by adjusting the amount of the treatment liquid used for forming the lower layer coating 4 applied to the base steel sheet 1.

3. Upper layer coating 3
The upper film 3 is a film that is formed on the lower film 4 and is the outermost layer of the surface-treated steel sheet 10. The upper layer film 3 contains an organic component as a binder and contains an inorganic color pigment in an amount of 10% by mass or less based on the solid content mass of the upper layer film 3.

  For the formation of the upper layer film 3, for example, a known base chemical solution used for forming a surface film of a commercially available fingerprint-resistant steel sheet can be used. A fingerprint-resistant steel sheet is a steel sheet (usually a plated steel sheet) on which a thin film based on polyester resin, urethane resin, epoxy resin, or the like is formed. The upper layer film 3 can be formed by adding an appropriate amount of a color pigment to such a base chemical solution to prepare a treatment liquid, and applying and baking the treatment liquid on the lower layer film 4.

  The upper layer film 3 preferably contains as little inorganic pigment as possible. When the upper layer film 3 contains such a color pigment (particularly, the above-described color pigment having a refractive index of 1.7 or more), the appearance of the surface-treated steel sheet 10 is easily stabilized. However, if the upper layer film 3 contains a large amount of the color pigment, the chemical resistance of the surface-treated steel sheet 10 tends to be inferior mainly because the bonding of the film is hindered. Therefore, the upper layer film 3 does not contain a color pigment having a refractive index of 1.7 or more, or even if it contains it, it is set to 10% by mass or less with respect to the solid content mass of the upper layer film 3.

  The thicker the upper film 3 is, the better the corrosion resistance of the surface-treated steel sheet 10 is. From this point of view, the thickness of the upper layer film 3 is 0.5 μm or more and preferably 1.0 μm or more. On the other hand, when the upper layer film 3 is too thick, it leads to an increase in the manufacturing cost of the surface-treated steel sheet 10. On the other hand, if the upper layer film 3 is too thick, there is a possibility that a problem such as a tendency to form galling due to sliding with a mold occurs when the surface-treated steel sheet 10 is formed into any shape. Therefore, the thickness of the upper layer film 3 is preferably a total film thickness with the lower layer film 4 of 3.0 μm or less. The thickness of the upper layer film 3 can be adjusted as appropriate by adjusting the amount of the treatment liquid applied to form the upper layer film 3. In addition, from the viewpoint of suppressing the above-mentioned shape galling, a solid lubricant (typically wax) may be appropriately contained in the above-mentioned treatment liquid used for forming the upper layer film 3 within a range that does not affect the effect of the present invention. Good.

4). In addition, the coating on the front surface The lower coating 4 and / or the upper coating 3 may further contain an organic coloring pigment. In general, inorganic coloring pigments as described above are excellent in concealability because of their high refractive index. On the other hand, organic color pigments have an excellent coloring power although they have a low refractive index and thus are inferior in concealability. Therefore, the surface-treated steel sheet 10 is obtained by containing the inorganic color pigment in the film 2 to conceal the color variation and wrinkles of the base steel sheet 1 and further containing the organic color pigment in the film 2. The color tone can have a certain degree of freedom.

  Compared with the above-mentioned inorganic color pigment, the organic color pigment has a lower degree of decrease in the corrosion resistance of the surface-treated steel sheet 10 when it is contained in the coating 2. Therefore, the organic color pigment can also be contained in the upper film 3. The preferable content of the organic coloring pigment is 25% by mass or less based on the solid content mass, regardless of whether it is contained in either the lower layer film 4 or the upper layer film 3. That is, the preferable content of the organic color pigment is 25% by mass or less based on the solid content mass of the lower layer film 4 when the lower layer film 4 contains an organic color pigment, and the upper layer film 3 is organically colored. When it contains a pigment, it is 25 mass% or less with respect to the solid content mass of the upper film 3. FIG. The color tone of the surface-treated steel sheet 10 can be adjusted by appropriately adjusting the content of the organic coloring pigment within this range.

5. Back coating 5
In the surface-treated steel sheet of the present invention, the configuration of the back surface is not particularly limited. As shown in FIG. 1, when forming the back surface film 5, the back surface film 5 can be made the same as the structure of the lower layer film 4 mentioned above. If the back coating 5 and the lower coating 4 have the same configuration, it is possible to hardly consider contamination due to the mixing of the processing liquid used for forming the back coating 5 and the processing liquid used for forming the lower coating 4. Moreover, it is advantageous also in terms of manufacturing cost of the surface-treated steel sheet 10.

  On the other hand, the back surface of the surface-treated steel sheet 10 has little need to stabilize the appearance. Moreover, it is practically preferable that the front surface and the back surface of the surface-treated steel sheet 10 can be easily distinguished. That is, it is preferable that one surface and the other surface of the surface-treated steel sheet 10 have a color tone difference that can be visually discriminated. Therefore, the back surface coating 5 does not have to contain the above-mentioned inorganic color pigment or organic color pigment. By not containing the coloring pigment in the back surface coating 5, it becomes easy to make a color tone difference between the back surface coating 5 and the coating 2, and the front surface and the back surface of the surface-treated steel sheet 10 can be easily distinguished.

<Manufacturing method>
The manufacturing method of the surface-treated steel sheet 10 is not particularly limited. First, a base steel plate 1 is prepared, and the above-described coating 2 is formed on the surface of the base steel plate 1. The formation method of the said film 2 is not specifically limited, What is necessary is just to form by the method similar to a well-known painted steel plate or a fingerprint-resistant steel plate. For example, a roll coater method or a spray-ringer squeezing method can be used as a method for applying the treatment liquid for forming the coating film 2. Moreover, after application | coating of a process liquid, it dries or bakes at predetermined temperature with a dryer, oven, etc. FIG. The method for forming the back coat 5 is the same.

<Application>
The surface-treated steel sheet of the present invention can be used for a housing for home appliances. The surface-treated steel sheet of the present invention can be manufactured at low cost because the coating film is formed thin. Therefore, the surface-treated steel sheet of the present invention is suitable for a back panel of an AV device such as a thin display that is particularly required to be manufactured at a low cost.

  The following examples and comparative examples will specifically illustrate the effects of the present invention. The following examples are merely illustrative of the present invention and do not limit the present invention in any way. In the following, “%” represents “% by mass” unless otherwise specified.

1. Preparation of test plate (1) Base steel plate 0.5 mm thick electrogalvanized steel plate (zinc adhesion amount 20 g / m ² per side) to a predetermined size so that it can be easily processed after the pre-coating treatment described later. We cut out and used. The color tone of the base material had an E * value (= (L * ² + a * ² + b * ² ) ^1/2 ) of about 5 variations.

(2) Pretreatment for coating The base steel sheet was degreased and washed with a commercially available alkaline degreasing solution according to a conventional method to remove contaminants adhering to the surface. Then, hot air drying was performed and fully dried.

(3) Formation of coating (3) -1 Treatment solution adjustment The base pigment solution shown in Table 1 was blended with the color pigments shown in Table 2 to prepare a treatment solution. When the color pigment is powder, a predetermined amount of the color pigment is added to the base chemical solution in a container containing 20 g of glass beads with respect to 100 g of the base chemical solution, and this is stirred for 20 minutes with a hybrid mixer. The color pigment was uniformly dispersed. In the case where the color pigment was previously dispersed in a solvent, the color pigment was added to the base chemical solution and then dispersed by lightly stirring. The composition of the treatment liquid after blending in this manner is shown in Table 3 for the lower layer coating on the front surface and in Table 4 for the upper layer coating. As the treatment liquid for the back surface coating, a base chemical solution for the lower layer coating on the front surface (that is, the one before the color pigment blending) was used.

(3) -2 Application and drying of treatment liquid Using the above treatment liquid, a treatment liquid for a lower layer coating was first applied to the front surface, and baked at PMT 100 ° C. to form a lower layer coating. Next, a back surface coating treatment liquid was baked on the back surface at PMT 100 ° C. to form a back surface film. Finally, the upper layer coating treatment solution was applied onto the lower layer coating and baked at 170 ° C. (when the base chemical solution was U1) or 210 ° C. (when the base chemical solution was U2, U3, U4) to form an upper layer coating. In addition, when baking the upper layer film, water quench was performed immediately after baking.

  The treatment liquid is applied with a bar coater, and the thickness of each coating is 0.3 μm for the lower layer coating, 1.0 μm for the upper layer coating, and 0.5 μm for the back layer coating, unless otherwise specified in the table. did. In Table 5 or Table 6 to be described later, the thickness of a film having a description of the film thickness was taken as the thickness. Each film thickness value is preliminarily determined from the relationship between the count of the bar coater and the coating weight.

2. Performance Evaluation Performance evaluation was performed as follows using the test plate prepared by the above-described procedure. Tables 5 and 6 show the evaluation results of the test plates according to Examples and Comparative Examples.

<Color stability>
The front surfaces of about 3 to 5 test plates were compared with the naked eye, and “○” indicates that the appearance color variation is not recognized, and “X” indicates that it is recognized. “O” is accepted. Of those evaluated as “◯”, those that could be easily distinguished from the front surface and the back surface were evaluated as “「 ”.

<Corrosion resistance>
A salt spray test specified in JIS-Z2371 was performed for 120 hours on the sample with the front surface as the evaluation surface and the end surface of the test plate covered with polyester tape, and the white rust occurrence area ratio was visually evaluated. According to the following evaluation criteria, “○” or more was regarded as acceptable.

(Evaluation criteria)
◎: White rust occurrence area ratio less than 5% ○: White rust occurrence area ratio 5% or more, less than 20% △: White rust occurrence area ratio 20% or more, less than 50% ×: White rust occurrence area ratio 50% or more

<Hardness>
Using the front surface as an evaluation surface, the surface was measured according to the method of JIS K5600-5-4 (scratch hardness (pencil method)). A hardness of 4H or higher was evaluated as “◯”. “O” is accepted.

  The hardness is an index of the difficulty of galling. The measurement results in this experimental example do not necessarily represent the hardness of the coating itself, but are also affected by the hardness of the plated coating on the base steel sheet. In any case, however, the harder surface is less likely to cause galling.

<Adhesion>
Using the front surface as the evaluation surface, the test plate is subjected to the 0T bending test (23 ° C) so that the front surface is the outer surface, and the 180 ° contact bending test is performed. A cellophane tape was applied and a peel test was conducted. According to the following evaluation criteria, “◯” was accepted.

(Evaluation criteria)
○: No peeling at all-less than 10% peeled portion ×: 10% or more peeled portion-whole surface peeled

<Cost>
When the thickness of the lower layer coating exceeds 1 μm, or the total thickness of the lower layer coating and the upper layer coating exceeds 3 μm, the cost is “x”, and the thickness of the lower layer coating is 1 μm or less, A film having a total film thickness of 3 μm or less of the lower layer film and the upper layer film was evaluated as “◯” because it can be manufactured at low cost.

  As shown in Tables 5 and 6, the test plate according to the example provided various performances in a well-balanced manner, but the test plate according to the comparative example was inferior in any performance. Thereby, according to this invention, even if it thinned the film formed in the surface of a base-material steel plate, it turned out that the colored external appearance is stabilized and corrosion resistance etc. have comparatively favorable surface-treated steel plate. .

DESCRIPTION OF SYMBOLS 1 Substrate steel plate 2 Coating 3 Upper layer coating 4 Lower layer coating 5 Back coating 10 Surface-treated steel plate

Claims (5)

A surface-treated steel sheet comprising a base steel sheet and a coating formed on at least one surface of the base steel sheet,
The coating comprises an upper layer coating and a lower layer coating formed on the substrate steel plate side from the upper layer coating,
The lower layer coating is a coating using an organic or organic-inorganic composite component as a binder, and a color pigment having a refractive index of 1.7 or more is 5% by mass or more and 25% by mass with respect to the solid content mass of the lower layer coating. % Or less,
The upper layer film is a film having an organic component as a binder, and a color pigment having a refractive index of 1.7 or more is contained in an amount of 10% by mass or less based on the solid content mass of the upper layer film,
The thickness of the lower layer coating is 0.2 μm or more and 1 μm or less, the thickness of the upper layer coating is 0.5 μm or more, and the total thickness of the lower layer coating and the upper layer coating is 3 μm or less. A surface-treated steel sheet.   The surface-treated steel sheet according to claim 1, wherein the color pigment contained in the lower layer coating contains carbon black, and the content of the carbon black is 10% by mass or less based on the solid content mass of the lower layer coating.   The surface-treated steel sheet according to claim 1 or 2, wherein at least one of the lower layer coating or the upper layer coating contains an organic color pigment in an amount of 25% by mass or less based on the solid content mass of the lower layer coating or the upper layer coating.   The surface-treated steel sheet according to any one of claims 1 to 3, wherein the one surface and the other surface of the base steel sheet have a color difference that is visually distinguishable.   The housing | casing for household appliances using the surface-treated steel plate as described in any one of Claims 1-4.

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