JP2010115902A - Coated steel sheet and case for electronic device manufactured by using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coated steel sheet having a coating film which can maintain corrosion resistance of a plane part and stability of color tone and has a thickness of 10 μm or less, and to provide a case for an electronic device manufactured by using the coated steel sheet. <P>SOLUTION: The coated steel sheet, including a coating layer comprising two or more coating films on the surface subjected to a chemical conversion treatment, has the thickness of the coating layer is 10 μm or less, and the outermost coating film, which is the outermost layer among the two or more coating films, containing 5-50 mass% of color pigment based on the total of a binder component and a pigment component, and having the film thickness of 2 μm or more and the Young's modulus of the outermost surface of the coating film of 5 GPa or less. A lower coating film, which is a coating film other than the outermost coating film, contains 5-30 mass% of pigment based on a solid content of the coating material, and has a coating film of 2 μm or more thickness, wherein a pigment contained in the coating film contains, based on a solid content of the coating material, 5-30 mass% of porous silica satisfying an oil absorption of 100-1,000 ml/100 g and a specific surface area of 100-1,000 m<SP>2</SP>/g as a rust-preventive pigment. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a coated steel sheet provided with a coating film layer comprising two or more coating films and an electronic device casing using the same, and more specifically, comprises two or more coating films and has a total film thickness of 10 μm or less. A coated steel sheet with improved coating surface corrosion resistance, and an electronic device casing having the coated surface on the side with the coating layer with improved corrosion resistance as the outside of the device, specifically, The present invention relates to a housing for AV equipment typified by a thin TV panel such as a liquid crystal TV, an organic EL TV and a plasma TV.

  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. The application of coated steel sheets is widespread in many fields because the user does not need to perform painting work that makes the working environment worse and waste liquid treatment is troublesome.

  The production of the coated steel sheet is performed by subjecting a base steel sheet (typically a zinc-based plated steel sheet including zinc plating and zinc alloy plating) to a chemical conversion treatment as a pretreatment, and then applying and baking an undercoat paint (primer). Next, a two-coat two-bake method is generally performed in which a top coat is applied and baked in order. However, on the “back side” opposite to the “front side”, which is the outside of the final product, coating is performed in a 1-coat 1-bake method using the paint developed for the back side after the pretreatment. There is also.

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. However, in recent years, in addition to cost reduction, environmental design and rationalization of painting work, the design of coated steel sheets is required. Regarding the coating film formed on the quality assurance side, it has been required to reduce the film thickness (coating film thickness) of the entire coating film, and specifically, one target is to make it 10 μm or less. It is coming. 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, as described above, when the coating film thickness is reduced, it is difficult to achieve the above-described many performances at a high level and in a well-balanced manner. For example, as the coating thickness is reduced, it is usually difficult to ensure the lightness stability and the background color and the surface hiding property. Therefore, an invention for improving this point is disclosed in Patent Document 1. Has been.
JP 2008-55774 A

  When the inventor evaluated the coating film according to the prior art, when the coating film thickness is 10 μm or less, not only is it difficult to ensure the above-mentioned coating film performance, but the coating film itself has moisture or the like. It has become clear that the phenomenon of transmission may be noticeable. Specifically, a zinc-based plated steel sheet (a general term for steel sheets on which a plated layer containing zinc provided for sacrificial corrosion protection is formed, regardless of the plating method, its composition, and the presence or absence of alloying treatment). When a salt spray test is performed on a coated steel sheet in which a coating film according to the prior art is formed on a base substrate made of a material having a thickness of 10 μm or less, the coating after the salt spray test for 240 hours is performed without any wrinkles. It became clear that white rust may occur on the surface.

  Therefore, the present invention has such a coating surface corrosion resistance (hereinafter referred to as “planar portion corrosion resistance” in order to distinguish it from the corrosion resistance at the end of the coating film) even for a thin film having a coating thickness of 10 μm or less. It is possible to provide a coated steel sheet that can maintain workability without causing deterioration, and preferably achieves high color tone stability, and a casing for an electronic device using such a coated steel sheet. Objective.

As a result of studies conducted by the present inventors in order to solve the above problems, the following knowledge was obtained.
(A) The surface portion is particularly required to have corrosion resistance on the outside surface during use, that is, the coating film on the front surface, and the coating film on the front surface is generally formed by laminating a plurality of coating films. As a countermeasure for improving the flat surface corrosion resistance of a coated steel sheet having such a coating film layer, there are mainly two ways of thinking.

  That is, (1) increasing the hardness of the outermost coating film (hereinafter referred to as “outermost coating film”) by increasing the hardness of the outermost coating film, and (2) the coating film. The antirust function of the anticorrosive pigment used for at least one of the coating films constituting the coating film other than the outermost coating film (hereinafter, this coating film is collectively referred to as “lower coating film”) among the layers. Especially to increase. In the present invention, the “outermost layer coating film” is defined as not including a coating film only for protective use, so-called “clear coating film” or “overcoat”. It is further formed on the outermost layer coating film as necessary. In addition, the “lower coating film” is defined as not including a coating ground treatment such as a chemical conversion film formed by chemical conversion treatment.

  (B) Among these countermeasures, the countermeasure for increasing the barrier property of the outermost layer coating film is likely to generate cracks because the hardness of the outermost layer coating film is increased, and the workability as a coating film (press formability, etc.) Will fall.

  (C) Therefore, at least one of the coatings constituting the lower layer coating should contain an anticorrosive pigment having an especially excellent antirust function, while the outermost layer coating does not increase the hardness. Rather, it is preferable to suppress the Young's modulus of the outermost surface and ensure the workability as a measure that can improve both the corrosion resistance of the flat surface and the workability.

  (D) In this way, it is possible to reduce the thickness of the lower coating film by including at least one of the coating films constituting the lower coating film with a rust-preventing pigment having an especially excellent anti-rust function. Become. As a result, the outermost layer coating film can be made relatively thick. As the film thickness of the outermost layer coating film increases, the change in film thickness of the lower layer coating film is less likely to appear as a change in color tone, so that the color tone as the coating layer becomes more stable.

The present invention completed based on the above knowledge is as follows.
(1) A coated steel sheet provided with a coating layer comprising two or more coating layers on a surface subjected to chemical conversion treatment, wherein the coating layer has a thickness of 10 μm or less, and the two or more coating layers The outermost coating film, which is the coating film forming the outermost layer, contains a coloring pigment of 5% by mass or more and 50% by mass or less with respect to the solid content of the paint, and has a film thickness of 2 μm or more and the Young's modulus of the outermost surface of the coating film 5 GPa or less, and the lower layer coating film other than the outermost layer coating film contains a pigment in an amount of 5% by mass to 30% by mass with respect to the solid content of the paint and has a film thickness of 2 μm or more. A porous silica as a rust preventive pigment provided with a film, wherein the pigment contained in the coating film satisfies the oil absorption amount: 100 ml / 100 g or more and 1000 ml / 100 g or less and the specific surface area: 100 m ² / g or more and 1000 m ² / g or less. , 5 mass% or more with respect to the solid content of the paint 30 Coated steel sheet, which comprises an amount percent.

  Here, “paint solid content” refers to the solid content (coating film) when the paint is baked, and its mass is measured as follows. That is, a predetermined amount of paint or paint raw material (main resin or the like) is placed in an oven, and the inside of the oven is heated while measuring its mass to solidify the paint or paint raw material. The mass measurement value of the solidified product when solidified until there is no mass change in the oven is defined as the mass of the solid content of the paint.

  (2) The coated steel sheet according to the above (1), wherein the pigment contained in one of the lower layer coating films contains one or two color pigments selected from titania and carbon black.

  (3) The coated steel sheet according to (1) or (2), wherein a film thickness of the outermost layer coating film is equal to or greater than a film thickness of the lower layer coating film.

  (4) The coating substrate is an alloyed hot-dip galvanized steel sheet, and the thermal emissivity from the back surface of the surface on which the coating film layer is formed is 40% or more. The coated steel sheet described.

  (5) The coated steel sheet according to any one of (1) to (4), wherein the back surface is provided with a coating layer having one or more selected from inorganic coating and inorganic-organic composite coating.

(6) The coated steel sheet as described in (5) above, wherein the total amount of the coating layer adhered is 10 mg / m ² or more and 1000 mg / m ² or less.

  (7) A casing for electronic equipment using the coated steel sheet according to any one of (1) to (6) above, wherein the surface on which the coating film layer is formed forms the outside of the casing A housing for electronic equipment, characterized by being

  Although the coated steel sheet according to the present invention has a film thickness of 10 μm or less, it can be excellent in flat surface corrosion resistance and workability, and can be excellent in color tone stability. Therefore, by using such a coated steel sheet, it is possible to provide a casing for an electronic device that is environmentally friendly and has high productivity while having excellent flat surface corrosion resistance and designability. Become.

Hereinafter, a coated steel sheet according to the present invention and a casing for an electronic device using the same will be described.
1. Base Steel Sheet The base steel sheet used in the coated steel sheet of the present invention is not particularly limited, and is generally a zinc-based plated steel sheet having a plating layer containing zinc, which is generally used in a coated steel sheet, that is, a galvanized steel sheet or a zinc alloy. A plated steel sheet or an alloyed plated steel sheet obtained by alloying these platings with a substrate may be used.

  The zinc-based plated steel sheet may be produced by any of electroplating, hot dipping, and vapor phase plating. Examples of galvanized steel sheets include hot dip galvanized steel sheets, electrogalvanized steel sheets, molten 5% Al—Zn alloy plated steel sheets, molten 55% Al—Zn alloy plated steel sheets, alloyed hot dip galvanized steel sheets, electrical Zn—Ni. Examples include alloy plated steel sheets. However, from the viewpoint of increasing the thermal emissivity as described later, it is preferable to use an alloyed hot-dip galvanized steel sheet. Moreover, it is preferable to set it as an galvannealed steel plate also from a viewpoint of ensuring electroconductivity.

The coating amount of the galvanized steel sheet is not particularly limited and may be within a general range. Preferably, the average adhesion amount on one side is 100 g / m ² or less. More specifically, the adhesion amount is more preferably 3 g / m ² or more and 50 g / m ² or less in the case of electroplating, and 30 g / m ² or more and 100 g / m ² or less in the case of hot dipping. .
The thickness of the steel sheet is determined depending on the application, but if it is excessively thick, there is a concern that the workability is lowered.

2. Paint base treatment In the production of a coated steel sheet, it is common to pre-process the base steel sheet (paint base treatment) before coating in order to ensure adhesion and corrosion resistance of the coating film. The coated steel sheet according to the present invention is also subjected to a chemical conversion treatment as a coating ground treatment to minimize a decrease in corrosion resistance in a general sense due to a decrease in coating film thickness.

  The chemical conversion treatment is not particularly limited, but from the viewpoint of environmental protection in recent years, it is preferable to use a chromium-free coated steel sheet, and therefore, it is preferable to use a treatment liquid that does not substantially contain chromium, not a chromate treatment. . A typical example of such a chemical conversion treatment liquid is a silica-based treatment liquid in which a silicon compound such as liquid phase silica, vapor phase silica and / or silicate is used as a main film component and optionally an organic resin is allowed to coexist.

The chemical conversion treatment is not limited to silica chemical conversion treatment. In addition to silica, various chromium-free treatment solutions for use in chemical conversion treatment have been proposed and are expected to be proposed in the future. Such a chromium-free processing solution can also be used. What is necessary is just to select the adhesion amount of the chemical conversion treatment film formed according to the chemical conversion treatment to be used. In the case of a silica-based chemical conversion treatment liquid, the normal adhesion amount will be in the range of 1 mg / m ² or more and 20 mg / m ² or less in terms of Si.

  Prior to the chemical conversion treatment, surface conditioning treatment with an acidic or alkaline aqueous solution containing an iron group metal ion such as Ni is often performed. Prior to that, alkaline degreasing or the like is usually performed to clean the base steel sheet.

3. Front surface The coated steel sheet according to the present invention has the above-mentioned chemical conversion formed on a surface (hereinafter referred to as “front surface”) that forms the outside of the housing in a product having a housing made of the coated steel plate. A treatment is performed, and a coating layer composed of two or more coating layers is provided thereon. The total thickness of the coating layer according to the present invention is 10 μm or less.

  The coating layer composed of two or more coating layers can be broadly classified into a coating film forming the outermost layer (outermost layer coating film) and a coating film other than the coating film forming the outermost layer (lower layer coating film). it can. Among these, the outermost layer coating film contains a coloring pigment of 5% by mass or more and 50% by mass or less with respect to the solid content of the coating material, the film thickness is 2 μm or more, and the Young's modulus of the coating film outermost surface is 5 GPa or less.

On the other hand, the lower layer coating film includes a coating film containing 5% by mass or more and 30% by mass or less of pigment with respect to the solid content of the coating material and has a film thickness of 2 μm or more. : 100 ml / 100 g or more and 1000 ml / 100 g or less and specific surface area: 100 m ² / g or more and 1000 m ² / g or less of porous silica as a rust preventive pigment is contained in an amount of 5% by mass to 30% by mass with respect to the solid content of the paint. . In addition, the lower layer coating film may be formed from a plurality of coating films, and in that case, at least one of the lower layer coating films (layers) composed of the plurality of coating films may have the above-described configuration.

  Here, the “paint solid content” refers to a solid content (coating film) when the paint is baked, and this “paint” does not include a treatment liquid for coating base treatment. The mass of the paint solids is measured as follows. That is, a predetermined amount of paint or paint raw material (main resin or the like) is placed in an oven, and the inside of the oven is heated while measuring its mass to solidify the paint or paint raw material. The mass measurement value of the solidified product when solidified until there is no mass change in the oven is defined as the mass of the solid content of the paint. Therefore, “mass% with respect to the solid content of the paint” refers to a mass ratio when the weight measurement value is 100%.

(1) Outermost layer coating film First, the outermost layer coating film will be described in detail.
A) Binder component The binder component constituting the outermost layer coating film is composed of a binder resin, a curing agent, and other components, which are the main components of the binder, and the outermost layer coating can realize the Young's modulus of the outermost surface described later. If there are, these types are not particularly limited. As the binder resin, for example, one or more selected from polyester resins, acrylic resins, epoxy resins, polyurethane resins, silicone polyester resins, polyvinyl chloride resins, fluororesins, and the like can be used. Further, examples of the curing agent include melamine resin and polyisocyanate compound, and the curing agent may be appropriately set according to the purpose, including whether or not a crosslinking catalyst is blended. From the viewpoint of setting the Young's modulus of the outermost surface to 5 GPa or less as described later, the Tg of the binder resin is preferably in the range of 10 ° C. or more and 40 ° C. or less for the polyester resin.

  The content of the binder component is preferably 40% by mass to 95% by mass with respect to the solid content of the paint. When the content of the binder component is excessively large, the content of the pigment is relatively lowered and the corrosion resistance is lowered. On the other hand, when the content is excessively low, the strength of the coating film is lowered, the scratch resistance is lowered, the workability is lowered, or the coloring of a desired color tone is difficult. A particularly preferable range of the content is 50% by mass or more and 90% by mass or less based on the solid content of the paint.

B) Pigment component “Pigment” refers to the entire pigment for the purpose of rust prevention and coloring, and the outermost coating film according to the present invention contains a colored pigment among the pigments. The specific type is not particularly limited. What is necessary is just to use suitably according to desired color tone, such as inorganic pigments, such as a zinc oxide, a titanium oxide, a calcium carbonate, a kaolin, organic pigments, such as copper phthalocyanine and toluidine red, and also carbon black. Further, in order to satisfy a predetermined design, bright pigments such as aluminum flakes and mica may be used as necessary as part or all of the colored pigments.

  The content of the color pigment is set to 5% by mass or more and 50% by mass or less based on the solid content of the paint. If the content is too small outside this range, it will be difficult to stably develop a predetermined color tone, and if it is too large, the coating film strength may be reduced, or the flat surface corrosion resistance may be significantly reduced. To do.

The outermost layer coating film may contain a rust preventive pigment in addition to the color pigment. Examples of the antirust pigment include aluminum tripolyphosphate, phosphoric acid and phosphorous acid Zn, Mg, Al, Ti, Zr, and Ce salts, Ca ion exchange silica, and oil absorption of 100 ml / 100 g to 1000 ml / 100 g and Examples thereof include porous silica particles satisfying a specific surface area of 100 m ² / g or more and 1000 m ² / g or less (hereinafter abbreviated as “porous silica pigment”). Even when the rust preventive pigment is contained, it is particularly preferable that the content of the entire pigment including the rust preventive pigment and the color pigment is 5% by mass or more and 50% by mass or less based on the solid content of the paint.

When the oil absorption amount and the specific surface area of the porous silica pigment are less than 100 ml / 100 g or less than 100 m ² / g, it is difficult to obtain sufficient corrosion resistance. Conversely, if the oil absorption exceeds 1000 ml / 100 g or exceeds a specific surface area of 1000 m ² / g, it is not suitable because it impairs the stability and paintability of the paint when added to the paint.

  The shape of the pigment is not particularly limited, but when the shape is excessively large (as a specific example, the average primary particle size of the pigment may be much larger than 10 μm which is the maximum value of the entire thickness of the coating layer according to the present invention). The possibility of causing inconveniences in the coating process is increased. For example, when applying using a roll coater, if the shape of the pigment is excessively large, it becomes difficult to pass through the roll gap, and the pigment content in the paint at the time of preparation and the pigment content in the applied paint And will be very different. As a result, there is a concern that the coating film cannot obtain predetermined characteristics. Moreover, when the shape of the pigment is excessively large, the thickness of the coating layer obtained by drying is not uniform, and there is a concern that cracking may occur in the secondary processing. Accordingly, the shape of the pigment is preferably within twice the thickness of the entire coating layer as the average primary particle size. Further, from the viewpoint of stably performing the coating process of the outermost layer coating film, the average primary particle diameter of the pigment is preferably within 3 times the thickness of the outermost layer coating film, and particularly within 1 times preferable.

  The “average primary particle size” varies to some extent depending on the measurement method, but any method can be used to prevent the pigment from falling off the coating layer and to provide stable coating properties and good coating film processability. From the viewpoint of realizing the above, it is preferable to be within about twice the thickness of the entire coating layer, and from the viewpoint of production stability, it is preferable to be within about three times the thickness of the outermost coating layer. The basic trend remains the same.

  Here, when the coated steel sheet according to the present invention is used for outdoor use or the like, in order to suppress the occurrence of red rust from the cut end face, at least one kind of non-chrome that satisfies the following requirements (i) and (ii): A compound may be contained.

(i) The electric conductivity of water when dissolved in ion exchange water (electric conductivity: 4 μS / cm or less) at a concentration of 0.1% by mass is 500 μS / cm or more,
(ii) No thermal decomposition occurs up to 200 ° C.

As for the non-chromium compound, an alkali metal phosphate is preferable, and examples thereof include potassium dihydrogen phosphate, dipotassium hydrogen phosphate, and sodium tripolyphosphate.
The addition amount of the red rust inhibiting additive compound is preferably 1% by mass or more and 10% by mass or less based on the solid content of the paint.

C) Other components In addition to the binder component and pigment described above, the components contained in the outermost layer coating film include leveling agents, resin beads such as acrylic beads and PTFE for improving the appearance irregularities and pressability, Examples thereof include conductive powder for improving weldability and electromagnetic wave shielding properties, wax for improving lubricity, and the like, which may be appropriately contained as necessary.

  However, since it is feared that the processability is lowered when the content of these components is excessively increased, the total content thereof is preferably 30% by mass or less, and 20% by mass or less with respect to the solid content of the paint. It is particularly preferable that

D) Film thickness The thickness of the outermost coating film is 2 μm or more. As will be described later, since the lower layer coating film contains a rust preventive pigment that is particularly excellent in rust prevention function, the coating layer has sufficient flat surface corrosion resistance within a practical range if the thickness of the outermost layer coating film is 2 μm. It is realized to obtain a coating film having a stable color tone. Specifically, if it is less than 2 μm, the color tone of the coating film layer is easily affected by the color tone of the lower layer coating film. For example, when the film thickness of the lower layer coating film changes due to a process reason, Variations in the color tone of the film tend to appear as variations in the color tone of the coating layer.

  From the viewpoint of highly realizing the stability of the color tone, the thickness of the outermost layer coating film is preferably 3 μm or more. In this case, for example, even if the film thickness of the outermost layer coating film fluctuates due to process reasons and the film thickness is reduced by 1 μm, the outermost layer coating film thickness of 2 μm can be secured. The influence of the color tone of the lower layer coating film hardly appears in the color tone of the coating layer. Therefore, the color tone of the coating film layer is hardly changed due to the change in the film thickness of the outermost layer coating film.

  On the other hand, since the total coating thickness of the outermost layer coating film and the lower layer coating film is 10 μm or less and the minimum film thickness of the lower layer coating film is 2 μm as will be described later, the upper limit of the outermost layer coating film is 8 μm. . From the viewpoint of color stability, it is preferable that the film thickness of the outermost layer coating film is equal to or greater than the film thickness of the lower layer coating film.

E) Young's modulus The Young's modulus of the outermost surface of the outermost layer coating film is 5 GPa or less. When the Young's modulus of the outermost surface exceeds 5 GPa, the workability deteriorates because the hardness of the coating film surface is high. The range of the preferred Young's modulus is 0.3 GPa or more and 5 GPa or less, and particularly preferably 0.5 GPa or more and 2 GPa or less.

In addition, the measuring method of the Young's modulus of the outermost surface of an outermost layer coating film is performed as follows.
Using a commercially available surface film property testing machine (for example, Fisherscope HM-2000 manufactured by Fisher Instruments Co., Ltd.), a 20 mm × 20 mm coated sample cut by any method such as shearing is placed on the measurement stage of the testing machine. Install with the paint side facing up.

  A Vickers indenter consisting of a quadrangular pyramid is pressed against this fixed coating sample, the maximum load is 2 mN, the application time when a load is applied is 20 s, the creep time is 5 s, the application time when the load is reduced is 20 s, after the load is removed Push in with a creep time of 5s. The measurement temperature is room temperature.

  Measure the Young's modulus by pushing indenters 30 points for each paint sample, and remove 5 points from the maximum value in descending order of numerical values, 5 points from the minimum value in ascending order of numerical values, and the remaining 20 points. Is the Young's modulus of the outermost surface of the coating film.

F) Manufacturing method The manufacturing method of an outermost layer coating film is not specifically limited. According to a conventional method, the above-described constituent components are dissolved and / or dispersed in an appropriate medium to form a liquid composition for forming a coating film. It is sufficient to apply the coating and heat to volatilize the medium and cure the binder component to form a solid coating.

(2) Lower layer coating film Subsequently, the lower layer coating film will be described in detail. A lower layer coating film means layers other than the coating film which makes the outermost layer among the coating film layers formed on the surface where the chemical conversion treatment of the steel plate was made. Therefore, the lower layer coating film may be a single layer or may be two or more layers. In the following description, in each item, the case of one layer will be described first, and the case of two or more layers will be described as appropriate.

A) Binder component The binder component that constitutes the lower layer coating film is also composed of a binder resin, a curing agent, and other components, which are the main components of the binder, and these types are not particularly limited, but both corrosion resistance and workability are compatible. From this viewpoint, a system containing a melamine resin and a polyisocyanate compound as a curing agent in one or more binder resins made of a polyester resin, an epoxy resin, and a polyurethane resin is preferable. When a lower layer coating film is comprised from several coating films, the binder component of each coating film may differ, and one part or all part may be the same. From the viewpoint of ensuring adhesion between the coating films, the same component is preferred.

B) Pigment component In the present invention, the pigment contained in the lower layer coating film contains 5% by mass or more and 30% by mass or less of pigment with respect to the solid content of the paint, and the oil absorption: 100 ml / 100 g or more and 1000 ml / 100 g or less, In addition, a porous silica pigment composed of porous silica particles satisfying a specific surface area of 100 m ² / g or more and 1000 m ² / g or less is included as a rust preventive pigment. Since the lower layer coating film containing such a porous silica pigment is particularly excellent in the corrosion resistance of the flat portion, the thickness of the lower layer coating film can be about 2 μm.

The reason why the range of the oil absorption amount and the specific surface area of the porous silica pigment is preferable is the same as that when added to the outermost layer coating film.
Moreover, also about the suitable range about the shape of a porous silica pigment, the average primary particle diameter of a porous silica pigment is 2 of the thickness of the whole coating layer similarly to the case of the pigment contained in an outermost layer coating film. It is preferable to satisfy at least one of within 3 times and within 3 times the thickness of the lower layer coating film containing the porous silica pigment, and it is more preferable to satisfy both. When the lower layer coating film is formed from a plurality of layers, the average primary particle diameter of the porous silica pigment is preferably within 3 times the coating film thickness for each coating film.

  The content of the porous silica pigment is set to 5% by mass or more and 30% by mass or less with respect to the solid content of the paint. When the content is less than 5% by mass, it becomes difficult to realize sufficient planar portion corrosion resistance when the thickness of the lower layer coating film is 2 μm. On the other hand, when the content exceeds 30% by mass, workability is lowered, and cracks are likely to occur in the coating film when it is molded.

  Here, in addition to the porous silica pigment, other rust preventive pigments such as aluminum tripolyphosphate and calcium exchange silica may be contained. However, the content of the porous silica pigment is set to 5% by mass or more based on the solid content of the paint from the viewpoint of ensuring the corrosion resistance of the flat portion, and the porous silica pigment and other rust preventive pigments from the viewpoint of maintaining the workability. The total content of is 30% by mass or less based on the solid content of the paint.

  In addition, pigments other than the rust preventive pigment, and specific examples may include one or two color pigments selected from titania and carbon black. By doing in this way, the concealability with respect to a base steel plate is provided also to a lower layer coating film. For this reason, the stability of the color tone as the whole coating film further increases, which is preferable.

  However, it should be avoided that the content of the above-mentioned porous silica pigment relative to the total of the binder component and the pigment component is relatively decreased and the planar portion corrosion resistance is decreased due to the inclusion of the color pigment. Accordingly, the content of the porous silica pigment is maintained in the range of 5% by mass or more and 30% by mass or less with respect to the solid content of the paint as in the case where the color pigment is not contained, and the content of the color pigment is porous. The total content with other pigments containing porous silica is 30% by mass or less based on the solid content of the paint. If this content is maintained, even when a coloring pigment or the like is contained, it is realized that the minimum film thickness of the lower layer coating film is 2 μm as described below.

  When the lower coating film is composed of two or more coating films, only one of the coating films constituting the lower coating film contains the porous silica pigment in a content of 5% by mass or more and 30% by mass or less. However, it is preferable that all the coating films individually contain the porous silica pigment in the above content.

C) Other components Although the lower layer coating film may contain components other than the binder component and the pigment component in the same manner as the outermost layer coating film, the total content of the other components is from the viewpoint of ensuring the corrosion resistance of the plane portion. It is preferable to set it as 10 mass% or less with respect to coating-material solid content, and it is especially preferable to set it as 5 mass% or less.

D) Thickness As described above, the coating layer other than the outermost coating layer among the coating layers having an upper limit of 10 μm is the “lower coating layer”, and the thickness of the outermost coating layer is 2 μm or more. Therefore, when the lower coating film is composed of one layer, the upper limit value of the film thickness is 8 μm. On the other hand, the lower limit of the thickness of the lower layer coating film in this case is 2 μm. When the thickness of the lower layer coating film is less than 2 μm, it becomes difficult to obtain desired plane portion corrosion resistance.

  When the lower layer coating film is composed of two or more layers, the upper limit value of the thickness of the entire lower layer coating film is 8 μm. Therefore, the individual film thickness of the coating film constituting the lower layer coating film is less than 8 μm. On the other hand, about the minimum of the thickness of the coating film which comprises a lower layer coating film, among the coating films which comprise a lower layer coating film, the coating film which contains a porous silica pigment by 5 to 30 mass% content At least one is 2 μm or more. When the thickness of this coating film is less than 2 μm, it becomes difficult to obtain desired flat surface corrosion resistance. In addition, when there are a plurality of coating films containing a porous silica pigment in a content of 5% by mass or more and 30% by mass or less, and these are continuously laminated, the cumulative film thickness of those coatings should be 2 μm or more. That's fine.

E) Manufacturing method The manufacturing method of a coating film is not specifically limited, either. As in the case of the outermost layer coating film, it is dissolved and / or dispersed in an appropriate medium to form a liquid composition for coating film formation, and this is applied to the base steel sheet by a suitable method at a predetermined thickness, and heated. Is appropriately performed to form a coating film. In addition, it is preferable to add a dispersing agent from a viewpoint of making it easy to disperse | distribute said porous silica pigment.

4). Back surface The coated steel plate according to the present invention is not particularly limited with respect to the coating film on the surface (hereinafter referred to as “back surface”) that forms the inside of the housing in a product having a housing made of the coated steel plate. The coating layer formed in the said front surface may be provided, and the coating film different from this coating layer may be formed.

  However, when the coated steel sheet according to the present invention is applied to a housing for an electronic device, the back surface preferably has a predetermined physical property, and a specific coating layer is provided from the viewpoint of realizing this physical property. It is preferable to provide. These points will be described below.

(1) Thermal emissivity It is preferable that the thermal emissivity from the back surface of the coated steel plate according to the present invention is 40% or more. By having such a high thermal emissivity, the efficiency of absorbing (absorbing heat) generated from the inside of the electronic device is increased, and the failure occurrence rate of components inside the electronic device is reduced when applied as a casing of the electronic device. It becomes possible to make it.

(2) Coating layer It is preferable that the back surface is provided with a coating layer having at least one kind selected from inorganic coating and inorganic-organic composite coating.
By having such a coating layer, corrosion resistance and electrical conductivity are realized.

  Inorganic coating refers to chromate treatment, phosphate treatment, treatment containing silicon and silicate, etc. "Inorganic organic composite coating" refers to organic coating such as water-soluble phenolic resin in the inorganic coating. Refers to the processing that was performed. From the viewpoint of environmental protection, a compound containing no Cr is preferable, and examples include compounds containing one or more elements selected from Si, P, V, Al, Mg, Mo, Zr and Ti.

The adhesion amount of the coating layer is preferably 10 mg / m ² or more and 1000 mg / m ² or less. When the amount of adhesion is less than 10 mg / m ² , there is a concern that the corrosion resistance and wrinkle resistance will decrease. On the other hand, when the adhesion amount is more than 1000 mg / m ² , there is a concern that the workability is lowered or the surface resistance described below becomes too high.

  The method for attaching the coating layer is not particularly limited, and a liquid composition for forming the coating layer is prepared by dissolving and / or dispersing the inorganic material and / or the organic-inorganic composite material and / or their precursors constituting the coating layer in an appropriate medium. This may be applied to the back surface of the base steel plate by a known method, and the medium may be volatilized appropriately by heating or the like and cured to a coating layer made of an inorganic material and / or an organic-inorganic composite material.

(3) Surface resistance It is preferable that the surface resistance value of the back surface of the coated steel plate which concerns on this invention is 5 ohms or less. By having such a low surface resistance value, when it is applied as a casing of an electronic device, it is possible to reduce the defect occurrence rate of components inside the electronic device by installing the device.

5). Forming From the coated steel sheet of the present invention, for example, a member forming a casing in a product can be formed by a conventional method such as punching or press forming. This casing has excellent flat surface corrosion resistance despite its thin coating thickness of 10 μm or less. Therefore, AV equipment typified by thin panels for electronic devices such as liquid crystal televisions, organic EL televisions, and plasma televisions. It is preferable to be used as a housing (housing) for equipment.

Example 1
A commercially available chemical conversion liquid (EC2330 manufactured by Nippon Paint Co., Ltd.) was applied to a hot-dip galvanized steel sheet (single-side zinc adhesion amount: 45 g / m ² ) so as to be 5 mg / m ^{2 in} terms of Si. Was heated to 80 ° C. in 10 seconds to form a chemical conversion coating on the front surface.

  Base paint: Byron 63CS (manufactured by Toyobo Co., Ltd.) as a main resin, a curing agent (melamine resin: Cymel 303, manufactured by Mitsui Chemicals) and a curing catalyst (catalyst 4050, Mitsui Chemicals, Inc.) (Made by company) was added so that it might become a mass ratio of 30: 6: 0.03 in conversion of solid content, this was mixed, and the base coating material was produced.

A) Preparation of white paint Using the above base paint, 25% titania (CR-90, manufactured by Ishihara Sangyo Co., Ltd.), which is a pigment component, and porous silica (H-31, AGC Corporation S) 5% of ITEC) was added, and the pigment was sufficiently dispersed to prepare a white paint.

B) Preparation of black paint Using the above base paint, 5% of carbon black (M-100, manufactured by Mitsubishi Chemical Corporation), which is a pigment component, and porous silica (H-31, AGC S) 5% of ITEC) was added, and the pigment was sufficiently dispersed to prepare a black paint.

  For both white and black paints, the pigment is dispersed by mixing 20 g of glass beads with 100 g of paint and stirring for 20 minutes with a hybrid mixer to uniformly disperse the pigment added to the paint in the paint. I let you. Thereafter, only the glass beads were filtered.

Details of the pigments used in the white and black paints are shown in Table 6.
Next, the above-mentioned white and black paints are applied onto the chemical conversion coating on the front surface using a bar coater and heated so that the temperature reached by the plate is 210 ° C. in 40 seconds. An undercoat film was formed. Adjust the film thickness by changing the amount of thinner (a solvent in which cyclohexanone and Solvesso 150 are mixed 1: 1) and the number of the bar coater. Undercoat films of three different thicknesses of 1, 2, and 3 μm Got.

  Subsequently, the above black paint was applied onto the undercoat film on the front surface, and the plate was heated to reach 230 ° C. in 50 seconds to form an upper coat film on the front surface. . The film thickness was adjusted by adding thinner and changing the number of the bar coater to obtain top coat films of four types of film thicknesses of 1, 2, 3, and 4 μm.

With respect to the 24 types of test pieces thus obtained (2 types of undercoat paint types × 3 types of undercoat film thickness × 4 types of topcoat film thickness), the color tone was measured by the following method.
The color tone (L *, a *, b *) of each coating sample was measured using a color difference meter CR-300 manufactured by Minolta Co., Ltd. The measurement points were measured at 5 points for each sample, and the average value was calculated.

  The measurement results are shown in Tables 1 to 4.

  Tables 1 and 2 are for showing the variation of the L * value when the film thickness of the lower layer coating film is changed while fixing the film thickness of the outermost layer coating film. Table 1 shows that the lower layer coating film is black. Table 2 shows the results when the lower layer coating is white.

  As shown in Tables 1 and 2, particularly in Table 2 where the color of the outermost layer coating film and the color of the lower layer coating film are different, by setting the outermost layer film thickness to 2 μm or more, It is possible to suppress the variation of the L * value based on the variation of the film thickness.

  Tables 3 and 4 are for showing the variation of the L * value when the film thickness of the outermost layer coating film is changed while fixing the film thickness of the lower layer coating film. Table 3 shows that the lower layer coating film is black. Table 4 shows the results when the lower layer coating is white.

  As shown in Tables 3 and 4, when the outermost layer coating film thickness is 3 μm, even if the outermost layer coating film thickness varies by ± 1 μm, the fluctuation range of the L * value at that time is relatively small. Therefore, it can be confirmed from these tables that the variation of the L * value due to the variation in the thickness of the outermost layer coating film is particularly suppressed by setting the outermost layer coating film thickness to 3 μm or more.

(Example 2)
(1) Preparation of test piece A) Preparation of base material As the base material, three types of alloyed hot dip galvanized steel sheet (GA), electrogalvanized steel sheet (EG) and hot dip galvanized steel sheet (GI) shown in Table 5 were used. used. All the steel plates were 250 × 300 mm in size and 0.5 mm in thickness.

  After performing alkali degreasing and water washing on both surfaces of each substrate-plated steel sheet according to a conventional method, the following steps were performed to prepare a coated steel sheet.

B) Front surface chemical conversion treatment A commercially available chemical conversion treatment solution (EC2330 manufactured by Nippon Paint Co., Ltd.) is applied so as to be 5 mg / m ^{2 in} terms of Si, so that the ultimate temperature of the plate is 80 ° C. in 10 seconds. Heating was performed to form a chemical conversion coating on the front surface.

C) Back surface chemical conversion treatment A commercially available chemical conversion treatment solution (CT-E300 manufactured by Nihon Parkerizing Co., Ltd.) was applied so that the coating amount was 500 mg / m ² , so that the ultimate temperature of the plate reached 80 ° C. in 10 seconds. To form a back-surface chemical conversion coating.

D) Undercoating film As a front surface undercoating material, a polyester-based coating material manufactured by Nippon Fine Coatings Co., Ltd. (FLC3900 primer clear type, main resin molecular weight of 10,000 or more, cross-linking agent: melamine, main resin Tg of about 10 ° C.) The various pigments shown in Table 6 were blended to obtain the undercoat paint shown in Table 7. In dispersing the pigment, a container containing 20 g of glass beads with respect to 100 g of the paint weight was stirred for 20 minutes with a hybrid mixer, so that the pigment added to the paint was uniformly dispersed in the paint. Thereafter, only the glass beads were filtered. Here, porous silicas 1, 2, and 3 shown in Table 6 are porous silicas as antirust pigments. In Table 6, the oil absorption amount, specific surface area, and average primary particle size of each pigment are values described in the catalog of each pigment.

  The undercoat paint shown in Table 7 was applied onto the chemical conversion coating on the front surface and heated so that the ultimate temperature of the plate was 210 ° C. in 40 seconds to form an undercoat coating on the front surface. The film thickness was adjusted by changing the amount of thinner (a solvent in which cyclohexanone and Solvesso 150 were mixed 1: 1) and the bar coater count, and a lower layer thickness of 1 μm or more and 2 μm or less was obtained. In the table, PWC (Pigment Weight Content) means the pigment concentration (unit: mass%) relative to the solid content of the paint.

E) Top coat film The top coat film shown below is applied onto the undercoat film on the front surface and heated so that the temperature reached by the plate is 230 ° C in 50 seconds. Formed. The film thickness was adjusted by changing the amount of thinner (solvent in which cyclohexanone and Solvesso 150 were mixed 1: 1) and the bar coater number, and an outermost coating film of 2 μm or more and 10 μm or less was obtained.

  Topcoat paint α: A paint in which 5% of carbon black is added to the solid content of the paint using a polymer polyester paint FLC5000 clear type (hardening agent melamine) manufactured by Nippon Fine Coatings Co., Ltd.

  Top coating paint β: A paint in which 5% carbon black and 5% aluminum flakes are added to the solid content of the coating using a polymer polyester paint FLC5000 clear type (hardening agent melamine) manufactured by Nippon Fine Coatings Co., Ltd.

  Top coating γ: Low molecular weight polyester resin P645 manufactured by Mitsui Chemicals Co., Ltd. 10 g of Cymel 303 manufactured by Mitsui Chemicals, Ltd. as a curing agent for 100 g of polyester resin, and Catalyst 4050 manufactured by Mitsui Chemicals, Inc. as a curing catalyst A clear paint in which 0.1 g is added to 100 g, and 5% of carbon black is added to the solid content of the clear paint.

  Top coating δ: Mitsubishi Rayon Co., Ltd. acrylic resin Dianal HR-2011 with 40 g of Mitsui Chemicals Co., Ltd. as a curing agent for 100 g of acrylic resin and Mitsui Chemicals Co., Ltd., Catalyst 4050 as a curing catalyst. A clear paint in which 1 g is added to 100 g of the paint, and 5% of carbon black is added to the solid content of the clear paint.

  Regarding the dispersion of the paint to which the pigment was added, 20 g of glass beads were mixed with 100 g of the paint and stirred for 20 minutes with a hybrid mixer to uniformly disperse the pigment added to the paint in the paint. Thereafter, only the glass beads were filtered.

F) Configuration of the coated steel plate Table 8 shows the configuration of the coated steel plate produced by the above procedure.

(2) Evaluation of front surface A) Plane portion corrosion resistance Conditions for compliant with JIS Z2371 after cutting out a test piece of 70 mm × 150 mm size from each test piece of a coated steel plate by shearing and sealing the cut end face And subjected to a salt spray test for 240 hours. Thereafter, the occurrence state (area ratio) of white rust and red rust from the flat surface portion excluding the sealed end surface portion was measured. The evaluation criteria are as follows, with a pass of ○ or higher:
◎: No white rust occurrence ○: White rust, red rust area ratio less than 5%,
Δ: White rust, red rust area ratio 5% or more, less than 20% ×: White rust, red rust area ratio 20% or more.

B) Coating film processability A 0T bending test (23 ° C.) was performed on the test piece, and the presence or absence of cracks in the 180 ° adhesion bending coating film was investigated using a 10-fold loupe. The evaluation criteria are as follows, with a pass of ○ or higher:
A: No crack at all,
○: There are very few cracks (a level that can be confirmed with a magnifying glass but cannot be visually confirmed)
Δ: Slightly cracked (within 5 visually)
X: There are quite many cracks (more than 5 visually).

C) Hardness test Measured according to the method of JIS K5600-5-4 (scratch hardness (pencil method)). The case where the hardness was H or higher was considered good.

D) Measurement of Young's modulus of outermost layer of coating film Using a Fisherscope HM-2000 manufactured by Fischer Instruments Co., Ltd., Young's modulus of the outermost surface of the coating film was determined by the following method.

  Sample installation method: A painted sample cut to a size of 20 mm × 20 mm by shearing was placed on the measurement stage of the Fisherscope HM-2000 with the coating surface up.

Measurement conditions: The measurement conditions are as follows.
Push-in maximum load: 2mN
Application time when load is applied: 20s
Creep time: 5s
Application time when load decreases: 20s
Indenter type: Square pyramid (Vickers indenter)
Measurement temperature: 23 ° C
Number of measurements: Measure 30 points for each sample, determine Young's modulus, remove 5 points from largest value to largest numerical value, remove 5 points from smallest value to smallest numerical value, and average the remaining 20 points on the outermost surface of the coating Young's modulus was determined.
As an evaluation standard, a sample having a Young's modulus exceeding 5 GPa was evaluated as x and a sample having a modulus within 5 GPa was evaluated as ◯.

E) Measurement of thermal emissivity The thermal emissivity was determined as follows. The spectral reflection spectrum (R (λ)) in the wavelength region of 2.5 μm to 25 μm was measured using an infrared absorption spectrum measuring apparatus manufactured by Bio-Rad Laboratories. This spectral reflection spectrum (R (λ)) was substituted into the following formula (1), and the thermal emissivity was determined by setting the integration range to 2.5 to 25 μm.

  Where α: thermal emissivity (%), G (λ): relative value when the absolute temperature is 293K in the Planck's thermal radiation spectrum distribution, R (λ): spectral reflectance measured by a spectrophotometer is there.

As evaluation criteria, those having a thermal emissivity of 40% or more were evaluated as ◯ (good), and those having a thermal emissivity of less than 40% were evaluated as x (defective).
Table 8 shows the results of the above evaluation. In Table 4, No. The state after the corrosion resistance evaluation of 38, 39, 40, 41 and 42 is shown in FIGS.

No. in Table 8 It is a photographic image which shows the state after 38 corrosion-resistance evaluation. No. in Table 8 It is a photographic image which shows the state after 39 corrosion resistance evaluation. No. in Table 8 It is a photographic image which shows the state after 40 corrosion resistance evaluation. No. in Table 8 It is a photographic image which shows the state after 41 corrosion resistance evaluation. No. in Table 8 It is a photographic image which shows the state after 42 corrosion-resistance evaluation.

Claims (7)

A coated steel sheet provided with a coating layer comprising two or more coating layers on the surface subjected to chemical conversion treatment,
The coating layer has a thickness of 10 μm or less,
The outermost coating film which is the coating film forming the outermost layer among the two or more coating films contains a coloring pigment of 5% by mass or more and 50% by mass or less with respect to the solid content of the paint, and has a film thickness of 2 μm or more and The Young's modulus of the outermost surface of the coating is 5 GPa or less,
A lower layer coating film which is a coating film other than the outermost layer coating film includes a coating film having a film thickness of 2 μm or more and containing a pigment of 5% by mass to 30% by mass with respect to the solid content of the coating, The porous silica as a rust-preventing pigment satisfying the paint solid content is the oil absorption amount: 100 ml / 100 g or more and 1000 ml / 100 g or less and the specific surface area: 100 m ² / g or more and 1000 m ² / g or less. A coated steel sheet comprising 5% by mass or more and 30% by mass or less.   The coated steel sheet according to claim 1, wherein the pigment contained in one of the lower coating films contains a color pigment composed of one or two kinds selected from titania and carbon black.   The coated steel sheet according to claim 1 or 2, wherein a film thickness of the outermost layer coating film is equal to or greater than a film thickness of the lower layer coating film.   The coated steel sheet according to any one of claims 1 to 3, wherein the coated base material is an alloyed hot-dip galvanized steel sheet, and the thermal emissivity from the back surface of the surface on which the coating layer is formed is 40% or more.   The coated steel sheet according to any one of claims 1 to 4, further comprising a coating layer having at least one kind or two kinds selected from an inorganic coating and an inorganic-organic composite coating on the back surface. The coated steel sheet according to claim 5, wherein the total amount of the coating layer is 10 mg / m ² or more and 1000 mg / m ² or less.   It is the housing | casing for electronic devices which uses the coated steel plate in any one of Claim 1 to 6, Comprising: The surface in which the said coating-film layer was formed is a surface which makes the outer side of the said housing | casing, It is characterized by the above-mentioned. Enclosure for electronic equipment.