JP4992500B2 - Chrome-free painted steel plate with excellent red rust resistance - Google Patents

Description

  The present invention relates to a chromium-free painted steel sheet that is useful for manufacturing home appliances, building materials, automobile parts, and the like that may be used outdoors or in an environment where water is applied. The coated steel sheet of the present invention can more effectively suppress the occurrence of red rust from the end face as compared with a conventional chromium-free coated steel sheet.

  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. Thereafter, the user unwinds the coil and performs punching, bending, drawing, or a combination of these 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.

  For the production of coated steel sheets, a steel sheet (typically a zinc-based plated steel sheet having a zinc-containing plating layer containing zinc plating and zinc alloy plating) is applied as a pretreatment, followed by undercoating. A two-coat two-bake system is generally used in which coating (primer) is applied and baked, and then top-coating is applied and baked in order. However, the back side opposite to the “front side” may be coated by a 1-coat 1-bake method using a paint (back side paint) developed for the back side after pretreatment. .

  Coated steel sheets vary in order of required performance such as corrosion resistance, workability, coating film hardness (scratch resistance), stain resistance, chemical resistance, weather resistance, and the like depending on applications. In coated steel sheets for products used outdoors such as air conditioner outdoor units and building materials, corrosion resistance is very important, and weather resistance is also necessary. Particularly in an air conditioner outdoor unit, many end surfaces cut by punching are exposed without being exposed. Therefore, in addition to the corrosion resistance of the flat plate part and the corrosion resistance of the cross-cut part (saddle part) where a cross is crooked up to the steel plate with a cutter knife, which is a general evaluation method for corrosion resistance, investigate the corrosion resistance of the cut end face. Is required. Each of these corrosion resistance evaluations has typically been performed by a salt spray test or a cycle test incorporating a salt spray cycle, and in part by a wet test.

  Conventionally, corrosion resistance has been improved mainly by rust preventive pigments contained in the undercoat film (primer layer or undercoat). The top coating film (top coat) contains a lot of added components to improve the coating film hardness, appearance, stain resistance, weather resistance, etc., especially in the case of the front side. One reason is that there is no room for adding rust pigments. The weather resistance mainly depends on the type of resin and curing agent of the top coat film, but can be improved by containing an ultraviolet absorber or a light stabilizer.

  Conventional coated steel sheet with excellent end face corrosion resistance is made of zinc-plated steel sheet as a coating base, and after applying coating chromate treatment as a pretreatment on both sides, at least the front side is strontium chromate An undercoat paint film is formed from an undercoat paint (primer) containing a chromate rust preventive pigment such as, and then an overcoat paint film is formed.

  However, both the chromate treatment and the undercoating paint containing strontium chromate are treatments using hexavalent chromium compounds that are harmful to the human body. A chromium-free painted steel sheet that does not use any chromium compound has come to be directed. In particular, since the recent RoHS directive, it has become an urgent task to make the coated steel plates chrome-free.

  Regarding the chromate treatment, a number of chromium-free chemical conversion treatments based on silica based on liquid phase silica and gas phase silica and containing a rust preventive component have been proposed, and many commercially available products are on the market. On the other hand, various types of phosphate-based, molybdate-based, boric acid-based, and silica-based compounds have been proposed for anti-rust pigments to be included in the undercoat paint.

  In Patent Document 1 described below, a coating film formed from a coating composition containing a rust preventive pigment composed of a mixture of a phosphate-based rust preventive pigment such as aluminum tripolyphosphate and ion-exchange silica is excellent in end face corrosion resistance. Are listed. On the other hand, even if this coating film is applied to a primer, it is described in Non-Patent Document 1 below that the corrosion resistance of both the end face and the crosscut portion is insufficient in the salt spray test.

In the following Patent Document 2, a coated steel sheet in which a primer layer containing magnesium oxide or calcium hydroxide having a particle size of 10 μm or less is formed on the surface of a galvanized steel sheet, and a top layer (top coat film) is formed thereon is an end face. It is described that it is excellent in corrosion resistance. The end face corrosion resistance is evaluated by the white rust occurrence rate by the salt spray test.
Japanese Patent Laid-Open No. 9-12931 JP 2005-225052 A Kobe Steel Engineering Reports Vol. 54, No. 1, pp. 62-65 (2004)

  Many chrome-free coated steel sheets have been proposed so far, but the end face corrosion resistance is low compared to the conventional coated steel sheets with a chromate treatment and an undercoat film containing chromate-based anticorrosive pigments. It was found that the corrosion resistance (end surface red rust resistance) against red rust generation from the end surface when exposed to long rain was insufficient. The occurrence of red rust from the end face not only corrodes the steel sheet, but also greatly deteriorates (soils) the appearance of the product and its installation location, so that there are many complaints from users and improvement has been desired.

  The present invention prevents the occurrence of red rust from the cut end face even when exposed to long rain, exhibits excellent end face red rust resistance, and at the same time, there is no coating swelling, and the balance between corrosion resistance, workability and hardness An object of the present invention is to provide a chromium-free coated steel sheet that has excellent performance equivalent to that of a conventional chromium-free painted steel sheet and can be manufactured at low cost.

  The occurrence of red rust in the coated steel sheet is basically prevented by the sacrificial anticorrosive ability of zinc contained in the plated layer of the zinc-based plated steel sheet that is the coated base material. That is, when zinc is preferentially dissolved and ionized over iron when it comes into contact with water, red rust caused by iron ionization (oxidation) is prevented.

  As a cause of the occurrence of red rust on the end face of the coated steel sheet when exposed to long rain, when the amount of rain increases, the rain itself does not contain much impurities and the electrical conductivity of the rain itself decreases, resulting in rainwater on the end face. When attached, it is presumed that the sacrificial anticorrosive function of zinc does not work, iron is oxidized, and red rust is generated.

Chromate-based rust preventive pigments typified by strontium chromate are known to be excellent in end face red rust resistance, but the following points are considered as the end face red rust suppression mechanism.
(1) Strontium chromate has a higher solubility in water and is relatively soluble in water compared to typical anticorrosive pigments (eg, aluminum tripolyphosphate and silica) contained in chromium-free coated steel sheets. When it comes into contact with water, it exudes relatively easily from the end face of the coating, and by increasing the electrical conductivity of the water in the vicinity of the end face, it promotes the sacrificial anticorrosive ability of zinc and suppresses the iron oxidation reaction, that is, the occurrence of red rust.

(2) Since the dissolved strontium chromate is weakly alkaline in the aqueous solution state, it forms a passive film on the iron surface.
(3) For zinc, the progress of corrosion of zinc is suppressed by the self-repairing action of Cr ⁶⁺ similar to the chromate film.

  The present inventors pay attention to the fact that (1) and (2) can be achieved with compounds other than chromate among the above mechanisms, and rust prevent compounds having the properties (1) and (2). It aimed at solving the said subject by making it contain as a pigment.

  As described above, in the conventional coated steel sheet, the rust preventive pigment is usually contained in the undercoat film (primer layer) regardless of whether the chromate-based or chromium-free coated steel sheet. However, when the primer contains a property described in (1) above, that is, a compound having a relatively high water solubility, the amount of the compound that dissolves in a wet environment is large, resulting in blistering of the coated steel sheet. It is easy and the corrosion resistance of the buttocks deteriorates remarkably. That is, as shown in the above (1), when the rust preventive pigment in the undercoat film has the property of being easily soluble in water, water penetrates into the undercoat film due to osmotic pressure, and the surface free energy is large. It is considered that water accumulates at the interface between the steel sheet surface and the undercoat coating film, and the coating film bulges.

  However, aluminum tripolyphosphate and silica used in Patent Document 1 and calcium hydroxide, magnesium oxide and magnesium hydroxide used in Patent Document 2 are all very low in water solubility, and at least the above-mentioned It does not have water solubility necessary for the mechanism of (1). Therefore, with such anti-corrosive pigments with insufficient water solubility, although it can improve the corrosion resistance of the steel sheet surface, it hardly contributes to elution into water, so it contributes little to increasing the electrical conductivity of the water at the end face of the steel sheet. The effect of preventing red rust on the end face is insufficient.

  In addition, Non-Patent Document 1 mentions that there is a possibility that the corrosion resistance may be improved by adding a pigment having a high elution property in a corrosive environment. This causes the swelling of the coating film. In addition, the corrosion resistance that has been studied in the past is also often aimed at preventing white rust from being generated by salt spray, and is not effective for generating red rust from the end face due to water with low electrical conductivity. It was.

  The inventors of the present invention include a relatively small amount of a compound having the characteristics required for the mechanisms (1) and (2) as a rust preventive pigment in the top coat (top coat), that is, the outermost layer of the coat. Thus, the present inventors have found that it is possible to effectively prevent the occurrence of red rust on the end face caused by the influence of water with low electrical conductivity, without using a chromate-based rust preventive pigment, and without causing blistering of the coating film. did.

Here, the present invention is a coated steel sheet in which one or more coating films are formed on at least one surface of a coated base plate made of a plated steel sheet having a Zn-containing plating layer, and the outermost coating film of the coating film is A chromium-free coated steel sheet containing 0.5 to 30% by mass of a non-chromium compound satisfying the following (A) to (C) as a rust preventive pigment:
(A) pH of 7 or more and 12 or less when dissolved in ion exchange water at a concentration of 0.1% by mass;
(B) electric conductivity when dissolved in ion exchange water at a concentration of 0.1% by mass is 500 μS / cm or more, preferably 700 μS / cm or more;
(C) No thermal decomposition occurs up to 200 ° C.

  In the present invention, the “chromium-free coated steel sheet” means that portions other than the coated substrate, that is, the coating film and the chemical conversion treatment film of the base are substantially free of chromium. The steel plate that is the coating substrate may be a steel plate containing chromium. Moreover, it is allowed that a coating film and a chemical conversion treatment film contain a trace amount chromium as an impurity. When a coating film consists of one layer of coating film which may be employ | adopted by the back surface side, the coating film becomes an outermost layer coating film.

When the coating film is a coating film composed of two or more layers, at least one coating film other than the outermost layer, preferably an undercoat coating film (primer layer) when the coating film is two layers, contains chromium. It is preferable to contain 10-60 mass% of 1 type or 2 types or more of anti-rust pigments (in the case of 2 types or more, which may contain the said compound). Thereby, the corrosion resistance other than the end face red rust resistance, that is, the corrosion resistance of the flat plate portion and the flange portion, and further the corrosion resistance against the occurrence of white rust on the end face are improved. As such a rust preventive pigment, various rust preventive pigments that have been conventionally added to undercoat films of chromium-free coated steel sheets can be used. The rust preventive pigment contained in at least one layer of the coating film other than the outermost layer is selected from aluminum tripolyphosphate and Ca-exchanged silica, and the coating film has the following (A), (B ′), and (C). It is preferably substantially free of rust preventive pigments that meet the requirements:
(A) pH of 7 or more and 12 or less when dissolved in ion exchange water at a concentration of 0.1% by mass;
(B ′) electric conductivity of 700 μS / cm or more when dissolved in ion exchange water at a concentration of 0.1% by mass;
(C) No thermal decomposition occurs up to 200 ° C.

  The compound capable of effectively preventing the occurrence of red rust on the end face caused by the influence of water having low electrical conductivity is preferably sodium tripolyphosphate, and the thickness of the outermost coating film is preferably 3 μm or more and 30 μm or less. The coating film of the outermost layer may be formed on both surfaces of the coated steel plate, or may be formed only on one surface of the coated steel plate directed to the front surface side or the back surface side of the product. It is preferable to have a chemical conversion treatment film which does not contain chromium between the said coating base material and the said coating film.

According to the present invention, it is also a chromium-free solvent-based paint for forming an outermost layer coating film on the steel sheet surface, and a non-chromium compound satisfying the following (A) to (C) is used as an anticorrosive pigment, There is also provided a paint effective for preventing the occurrence of red rust on the end face, characterized by containing 0.5 to 30% by mass based on the solid content:
(A) pH of 7 or more and 12 or less when dissolved in ion exchange water at a concentration of 0.1% by mass;
(B) electric conductivity when dissolved in ion exchange water at a concentration of 0.1% by mass is 500 μS / cm or more, preferably 700 μS / cm or more;
(C) No thermal decomposition occurs up to 200 ° C.

  In the coated steel sheet according to the present invention, the outermost-layer coating film (in the case of a two-layer coating film) has the characteristics (A) to (C) that are highly water-soluble and show basicity in an aqueous solution. By containing a relatively small amount of the compound as a rust preventive pigment, the occurrence of red rust can be suppressed without causing blistering of the coating film even when exposed to long rain. In addition, there is no significant adverse effect on processability and coating film hardness. Furthermore, if a relatively large amount of anticorrosive pigment is contained in the inner layer coating film, for example, the undercoat film, chromate using conventional chromate-based anticorrosion pigments, including flat plate corrosion resistance and buttock corrosion resistance. This makes it possible to obtain a chromium-free coated steel sheet having excellent corrosion resistance comparable to that of a coated steel sheet.

  The coated steel sheet according to the present invention, which is excellent in end face red rust resistance, is particularly suitable for application to products used outdoors, such as an air conditioner outdoor unit, where many exposed end faces are exposed by punching. Is not limited to that.

  Below, the coated steel plate which concerns on this invention is demonstrated for every element for the example of the coated steel plate which has a two-layer coating film of an undercoat and a top coat film on at least one side which is a preferable embodiment of this invention.

(1) Coated base material In the coated steel sheet of the present invention, the sacrificial anticorrosive ability of zinc is used for the occurrence of red rust from the end face, so the coated base steel sheet is a zinc-based plated steel sheet having a zinc-containing plated layer, A galvanized steel sheet or a zinc alloy plated steel sheet. 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, and electrical Zn—Ni. Examples include alloy plated steel sheets. The plating adhesion amount is not particularly limited, and may be within a general range. If the coating amount is too small, the corrosion resistance is lowered, and if too much, the workability is deteriorated.

(2) Pre-coating treatment In the production of coated steel plates, it is common to pre-treat (base treatment) the base steel plate before painting in order to ensure coating film adhesion and corrosion resistance. This pretreatment is generally performed by a chemical conversion treatment, and before that, a 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 and the like are performed to clean the base steel sheet.

  In order to obtain a chromium-free coated steel sheet, the chemical conversion treatment is performed using a chemical conversion treatment solution that does not substantially contain chromium, not a chromate treatment. A typical example of such a chemical conversion treatment liquid is a silica chemical conversion 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 a resin is sometimes allowed to coexist.

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

(3) Undercoat film The base resin constituting the undercoat film is not particularly limited, and can be selected from various resins that have been used for undercoating of coated steel sheets, such as epoxy resins, polyurethane resins, and polyester resins. Examples of the crosslinking agent combined with the base resin include melamine resin and polyisocyanate compound. If necessary, a crosslinking catalyst is blended.

The rust preventive pigment to be contained in the undercoat coating is not particularly limited, but in the present invention, a chromate rust preventive pigment represented by strontium chromate is not used in order to make it chromium-free.
The rust preventive pigment preferably used for the undercoat film in the present invention has a relatively low water solubility, specifically, an electric conductivity when dissolved in ion exchange water at a concentration of 0.1% by mass. It is less than 700 μS / cm, particularly 500 μS / cm or less. Examples of such rust preventive pigments 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 to 1000 ml / 100 g, Specific examples include amorphous silica particles having a specific surface area of 200 to 1000 m ² / g and an average particle size of 2 to 30 μm. Preferred rust preventive pigments are phosphate rust preventive pigments such as aluminum tripolyphosphate, silica rust preventive pigments, or a combination of both. Particularly preferred are aluminum tripolyphosphate and / or Ca-exchanged silica. The undercoating film satisfies the requirements (A) and (C) used for the overcoating film in the present invention, and (B ') is dissolved in ion exchange water at a concentration of 0.1% by mass. It is preferable not to contain a rust preventive pigment having an electric conductivity of 700 μS / cm or more in a substantial amount (1% by mass or more).

  If the undercoating film contains a substantial amount of anti-corrosion pigment that satisfies the above requirements (A), (B '), (C), the anti-corrosion pigment will melt when contacted with water, Water enters the undercoat from the surface. The invading water accumulates at the steel plate interface having a large surface free energy, thereby causing a swelling of the coating film. As a result, in addition to adversely affecting the appearance, the corrosion resistance is significantly deteriorated.

  On the other hand, when the top coat film contains a substantial amount of the anti-corrosion pigment that satisfies the above requirements (A) to (C), the anti-corrosion pigment starts to dissolve when it comes into contact with water. When an undercoating film that does not contain a rust preventive pigment that satisfies the above requirements (A), (B ′), and (C) exists in the lower layer, the undercoating film serves as a water barrier layer due to osmotic pressure. For this reason, water does not easily penetrate to the interface between the steel plate and the undercoat coating film, so that no large film swelling occurs.

  The content of the anticorrosive pigment in the undercoat coating film is preferably 5 to 60% by mass, and more preferably 10 to 30% by mass. When the amount of the rust preventive pigment is too small, not only the effect of improving the corrosion resistance of the flat plate portion and the heel portion and the white rust resistance of the end face, but also the effect of improving the coating film hardness is not sufficiently exhibited. On the other hand, if the amount of the rust preventive pigment exceeds 60%, the workability of the coated steel sheet is lowered.

  In addition to the resin (and the crosslinking agent) and the rust preventive pigment, the undercoat coating film may further contain one or two or more of color pigments, leveling agents, beads, and the like. However, as the pigment content increases, the processability may decrease. Therefore, even when a pigment other than the rust preventive pigment is further contained, the total amount of the pigment is 60% by mass or less, and particularly workability is emphasized. In such a case, the content is preferably 35% by mass or less.

  The thickness of the undercoat coating film (dry coating film thickness) is desirably 1 to 20 μm. More desirably, the thickness is 2 to 15 μm. If the coating film thickness is too thin, the corrosion resistance is lowered. If the coating thickness is too thick, the cost and the workability and adhesion may be lowered as the internal stress of the coating film increases.

(4) Top coating film The top coating film satisfies all of the following (A) to (C) as a rust preventive pigment (hereinafter, also referred to as an end face red rust resistant rust pigment) for improving end face red rust resistance. Containing the compound in an amount of 0.5 to 30% by weight:
(A) pH of 7 or more and 12 or less when dissolved in ion exchange water at a concentration of 0.1% by mass;
(B) electric conductivity when dissolved in ion exchange water at a concentration of 0.1% by mass is 500 μS / cm or more, preferably 700 μS / cm or more;
(C) No thermal decomposition occurs up to 200 ° C.

  Examples of commercially available pigments with this property are sodium tripolyphosphate (pentasodium triphosphate), potassium hydrogen phosphate, magnesium acetate tetrahydrate, sodium carbonate monohydrate, and hydrocalumite treated phosphorus. Zinc oxide (EXPERT NP-530 N20 manufactured by Toho Pigment) and calcium phosphite (EXPERT NP-1020C N20 manufactured by the company). The fact that these pigments satisfy the above (A) and (B) is shown in Table 1 described later. The requirement (C) is necessary so that the compound does not decompose during baking after painting. Of these, sodium tripolyphosphate is particularly preferred at present.

  Since it is used as a rust preventive pigment, it is preferable that any compound is used in the form of a powder and the average particle diameter thereof is in the range of 0.1 to 20 μm. As shown in Table 1, strontium chromate, which is a typical representative antirust pigment, also satisfies the above three requirements. However, in the present invention, no chromium compound is used in order to make chromium free. Since a non-chromium compound is used, it is preferable to use a compound having an electric conductivity of (B) of 700 μS / cm or more.

  A compound satisfying the above requirements (A) to (C) has a relatively high water solubility as defined in (B). Therefore, when a large amount is added as an anticorrosive pigment to the undercoat, The film swells. However, when this type of compound is added to the top coat film, if the amount is 30% by mass or less, the occurrence of swelling of the paint film is unlikely to occur, and other properties, particularly the appearance and workability, are not adversely affected. And, when it is contained in the top coating film in an amount of 0.5% by mass or more, it has been found that the end face red rust resistance of the coated steel sheet is remarkably improved, and the occurrence of red rust from the end face can be suppressed even when exposed to long rain. did. It has not been known so far that this type of rust preventive pigment is effective in resistance to red rust on the end face. In the case of a top coat, as described above, even if the rust-preventive pigment elutes slightly from this coat, it is difficult for the paint to swell. It can be blended in a larger amount than the membrane.

  From the above, in the present invention, the end face red rust resistant rust preventive pigment composed of one or more compounds satisfying the above (A) to (C) is added in an amount of 0.5 to 30% by mass in the top coat film. Contain. When the content of the pigment is less than 0.5% by mass, the improvement of the end face red rust resistance is insufficient, and when it exceeds 30% by mass, the coating film swells frequently and the physical properties of the coating film deteriorate.

  The base resin of the top coat film is not particularly limited, and for example, one or more selected from polyester resin, acrylic resin, epoxy resin, polyurethane resin, silicone polyester resin, polyvinyl chloride resin, fluororesin and the like are used. be able to. The crosslinking agent may be the same as that described for the undercoat film, and a crosslinking catalyst is blended as necessary.

  The top coat film may further contain a color pigment in addition to the above-mentioned end face-resistant red rust-proof rust pigment. Examples of the color pigment include inorganic pigments such as zinc oxide, titanium oxide, calcium carbonate, and kaolin, organic pigments such as copper phthalocyanine and toluidine red, and carbon black. The color pigment is preferably 30% by mass or less, and the total amount of the pigment combined with the rust preventive pigment is preferably 50% by mass or less.

  The top coat film may contain additives other than the above components. Examples of such additives are ultraviolet absorbers and light stabilizers effective for improving weather resistance, waxes effective for improving workability (press moldability), and the like.

  The film thickness of the top coat film is desirably 2 to 50 μm, and more desirably 5 to 30 μm. If the coating film thickness is too thin, the corrosion resistance and the concealing property are insufficient, and if it is too thick, the cost and the workability and adhesion due to the increase in coating film internal stress may be reduced.

  The above-mentioned undercoat and topcoat are both formed by applying and baking (heating and drying) a paint in which necessary components are dissolved or dispersed in a solvent (that is, the undercoat and the topcoat). Each paint may be either a solvent-based paint whose solvent is an organic solvent or a water-based paint whose solvent is water or a mixed solvent of water and a water-miscible organic solvent. However, the top coating is preferably a solvent-based coating because the water-resistance of the end face red rust resistant rust preventive pigment is relatively high. The coating is usually a two-coat two-bake method, but a two-coat one-bake method in which baking is performed only after application of the top coat is possible in some cases.

  The method for applying the undercoat and topcoat is not particularly limited, and may be performed by a roll coater, a curtain flow coater, a spray gun, a bar coater, or the like. Baking after application is performed so that the PMT (steel plate maximum temperature) set according to the resin and the cross-linking agent constituting the coating film is obtained. This temperature is generally in the range of 100-280 ° C. The heating method may be hot air drying or oven drying.

  The above-mentioned top coat film may be provided on both sides of the coated steel plate, or may be provided only on one side. Even with a coated steel sheet containing only the above-mentioned end-face red rust-resistant anti-rust pigment on one side only, the exposed end face touches water, and this rust-preventive pigment quickly dissolves into the water covering the end face. This is because the occurrence of red rust can be suppressed.

When the top coat film according to the present invention is formed only on one surface of the coated steel sheet, the surface may be either the front surface side or the back surface side.
For example, the paint on the front side contains a large amount of pigments and other additives (wax, etc.) in addition to the resin for film formation. What is necessary is just to make 0.5-30 mass% of end face anti-rust red rust preventive pigments in the coating film of the outermost layer on the side. In the case of a coated steel sheet with the back side coated with a single layer coating of the back side paint, if there is room for the end face red rust resistant rust pigment to be contained in the top coat on the front side, this may be done. Or you may make a 1 layer coating film of the back surface side contain an end face red rust-proof antirust pigment. In the case of a one-layer coating film, it becomes the outermost layer coating film, so even if the coating film contains an end face red rust-proof rust pigment having a relatively high water solubility, the amount is 30% by mass or less. If it exists, coating film swelling hardly occurs.

  When the end face red rust resistant rust pigment is contained only in the outermost coating film on one side of the front or back side, the content of the rust preventing pigment in the coating film is preferably 1 to 30% by mass. Yes, more preferably 2 to 20% by mass. When the end face red rust resistant rust preventive pigment is contained in the top coat film on both sides and / or contained in the undercoat coat on one side or both sides, the lower limit of the content can be made lower.

  The present invention can also be applied when the coating film has three or more layers. Also in that case, the above-mentioned end face red rust resistant rust preventive pigment is contained in the outermost coating film in an amount of 0.5 to 30% by mass. At that time, the pigment can be contained in an inner layer other than the innermost layer (for example, the middle layer) in an amount of 30% by mass or less. Here, the condition that the intermediate layer can contain the end face red rust-resistant rust preventive pigment is that the end face red rust preventive rust pigment is not included in the coating layer on the inner layer side of the intermediate layer. In this case, the reason why the end surface red rust preventive pigment can be contained in the middle layer is considered as follows.

  The case where the coating film is a lower layer, an intermediate layer, and an upper layer will be described as an example. Similar to the blister suppression effect, when the rust preventive pigment satisfying the requirements (A) to (C) is contained in the middle layer in a substantial amount, and the layers (A), (B ′), (C ) If there is an undercoating film that does not substantially contain a rust-preventive pigment, the undercoating film serves as a water barrier layer, so that the steel plate and the undercoating film can be used as a reservoir for water. Since it becomes difficult for water to advance to the film interface, large coating swelling does not occur. Here, it is considered that the reason why water does not collect at the interface between the middle layer and the upper and lower coating films is that there is no place for water accumulation at the interface between the coating film and the coating film.

After applying the alkaline degreasing and water washing to a hot-dip galvanized steel sheet with a coating thickness of 0.8 mm (zinc adhesion amount: 45 g / m ² per side, dimensions: 300 × 250 mm) according to conventional methods, Chemical conversion treatment was performed on both sides using a silica-based chromium-free chemical conversion treatment solution (Surfcoat EC2330). The chemical conversion treatment was performed so that the adhesion amount was 4 to 8 mg / m ² in terms of the Si adhesion amount.

  Add anti-rust pigments selected from the various anti-corrosion pigments shown in Table 1 to commercial undercoats and topcoats that do not contain antirust pigments. And a top coat for the back side. Of the anticorrosive pigments shown in Table 1, zinc phosphate hydrocalumite, calcium phosphite hydrocalumite, and magnesium phosphite are all NP-530 N20, NP-1020C N20, and NP manufactured by Toho Pigment, respectively. -1802. The aluminum tripolyphosphate was K-WHITE # 82 manufactured by Teika, the silica was H-31 manufactured by Dokai Chemical Industry, and the calcium ion exchanged silica was Shieldex manufactured by Fuji Silysia Chemical. All other pigments were general reagents.

  The base paint used was a polyester paint (IP512 primer clear type, main resin molecular weight 10000 or more, crosslinker: melamine) made by Nippon Fine Coatings Co., Ltd. The top surface coating paint contains titania as a white pigment and is baked polyester resin paint (IPT206-CF585, main resin molecular weight of about 8000, crosslinker: melamine) manufactured by Nippon Fine Coatings Co., Ltd. This was a baked-type polyester resin paint (IPT236-CF756, main resin molecular weight of about 3000, cross-linking agent: melamine).

  The types of anticorrosive pigments used are shown in Table 2 for the undercoat paint on both sides, in Table 3 for the top coat on the front side, and in Table 4 on the top coat on the back side. The added rust preventive pigment was uniformly dispersed in the paint by stirring a container containing 20 g of glass beads for 100 g of the paint with a hybrid mixer for 20 minutes.

  Using these paints, coating was performed at any one of the coating orders and baking temperatures shown in Table 5 to obtain samples of coated steel sheets having the coating composition shown in Table 6. The coating was carried out with a bar coater, and the coating thickness was unified to 11 μm on the front surface side, 5 μm on the back surface side, and 16 μm on the front surface side, and 10 μm on the back surface side.

The coated steel sheet thus obtained was evaluated for end face red rust resistance, presence or absence of coating film swelling during a wet test, and corrosion resistance by the following tests. The test results are also shown in Table 6.
(1) End face red rust test 1
For each sample of coated steel plate, 10 test pieces of 1 cm × 4 cm rectangular size (sum of end face length is 10 cm) are cut out by shearing (total end face length in each sample is 1 m), 10 tests The pieces are separately immersed in 20 ml of ion exchange water in a beaker. After leaving the beaker in a constant temperature bath at 40 ° C. for 96 hours, the test piece is taken out and the state of occurrence of red rust from the end face of each test piece (observation of the color tone of the solution discolored by the red rust generated from the end face) is measured. The evaluation criteria are as follows:
A: No occurrence of red rust (solution is transparent and not colored),
○: Some red rust is generated from the end face (solution is transparent, lightly colored in red),
Δ: Red rust is generated (the solution has a little solid red rust and colored reddish brown),
X: A large amount of red rust is generated (the solution contains a large amount of solid solid red rust and is colored dark red brown).

(2) End face red rust test 2
A test piece schematically shown in FIG. 1 was prepared, and red rust was generated from the end face of each test piece after being left for 96 hours under wet test conditions (temperature: 49 ° C., relative humidity 95% or more) (described in FIG. 1). The occurrence of red rust at the evaluation site of red rust on the end surface of the surface is visually measured. The evaluation criteria are as follows:
A: No occurrence of red rust (no red rust is seen from the end face),
○: Some red rust is generated from the end face (red rust is generated on the end face of less than 10 mm in length),
Δ: occurrence of red rust (length 10 mm or more, occurrence of red rust on end face of less than 30 mm),
X: A large amount of red rust occurred (red rust occurred on almost the entire end face (30 mm or more)).

  The test piece of FIG. 1 was produced by bonding three plates produced by stamping from a coated steel plate to a support plate in the arrangement as shown in the figure using a spacer. Both the spacer and the support plate were made of polyvinyl chloride. The enlarged portion of the dotted line portion in FIG. 1 schematically shows a burr when punched. In punching, burrs remain at the end portions in this way, and in the contact portions of adjacent plates arranged as shown in FIG. 1, a minute gap is actually formed between the burrs, and water can enter from this gap. In an air conditioner outdoor unit, there is a case where a processed part connected with a burr is formed so that it can be removed afterwards so that piping can be joined later. This test is a test simulating such intrusion of water from the burr part.

(3) Coating film swelling test A test piece having a size of 70 mm x 150 mm was cut out from each sample of the coated steel sheet by shearing, and the test piece was left for 96 hours under the same wet test conditions as described above. Measure the occurrence of blisters (blister width). The evaluation criteria are as follows:
○: There is almost no occurrence of coating film swelling (maximum swelling width is less than 2 mm),
X: Many blisters with a width of 2 mm or more occurred.

(4) Corrosion resistance test A cross-cut having a depth reaching the base steel sheet is put into a test piece cut out from each sample of the coated steel sheet to the same size as the film swelling test, and then subjected to a 96-hour salt spray test. Thereafter, the swollen width (maximum value) of the coating film from the cross-cut part (saddle part) of the test piece is measured. The evaluation criteria are as follows:
○: Coating film swelling width is less than 2 mm,
Δ: 2 mm or more and less than 4 mm,
X: 4 mm or more.

  As can be seen from the test results shown in Table 6, the coated steel sheet of No. 33 in which the top coating film contains conventional strontium chromate as a rust-preventive pigment showed good performance of ◯ or more in all test items. But it is not chrome free. Further, the end face red rust resistance 2 was lower than the result 1, and the corrosion resistance of the burr portion was apt to be lowered.

  On the other hand, even if it is a completely chrome-free coated steel sheet that does not use such chromic anticorrosive pigments, the end face red rust resistance is applied to at least one of the top coat and the back coat according to the present invention. The coated steel sheets of No. 1-30 to which rust preventive pigments are added, in addition to the good end face red rust resistance of the evaluation of ◯ or higher, are also the coating bulge and rust corrosion resistance of No. 33 chromate rust preventive pigments. It was confirmed that the corrosion resistance was good in addition to the end face red rust resistance. In addition, in the top coat film on the front surface, the symbols A to U, and in the top coat film on the back surface, the symbols A to C, G and H are coating films containing such a rust preventive pigment. In addition, with regard to end face red rust resistance, the test results of red rust 1 and red rust 2 show the same results in all examples, and even if the connection process is performed at the burr part, the occurrence of red rust can be effectively prevented. Recognize.

  On the other hand, in the comparative examples of No. 31, 32, 34, and 35 in which neither the top coat film nor the undercoat film contains an end face red rust-resistant rust preventive pigment, the undercoat film or the undercoat film and the topcoat film are coated. Since the only anti-corrosion pigment contained in the film was aluminum tripolyphosphate with low water solubility, the film swelling under wet test conditions and the rust corrosion resistance in the salt spray test were good, but the end face red rust resistance Was not good. On the other hand, in the present invention, No. 2 to 13, 15 to 29 of No. 36 to 47 in Table 2 in which the end coat red rust resistant anti-rust pigment contained in the top coat was contained in the undercoat. In Comparative Examples Nos. 52 to 66, it was confirmed that a large number of coating film bulges from the end face occurred on the flat surface under the wet test conditions because the top coating film did not contain such a rust preventive pigment. In this case, although the end face red rust resistant rust preventive pigment was contained in the undercoat film, the end face red rust resistance was also greatly reduced. This is presumably because the rust preventive pigment was eluted from the undercoat coating film and the coating film was swollen, so that a sufficient amount of the end face red rust resistant rust preventive pigment was not supplied to the end face.

It is explanatory drawing which shows the shape and arrangement | positioning of the test piece used for the end face red rust resistance test in the Example.

Claims (7)

A coated steel sheet in which two or more coating films are formed on at least one surface of a coated base plate made of a plated steel sheet having a Zn-containing plating layer, and the coating film of the outermost layer of the coating film has the following (A) to (C A chromium-free coated steel sheet containing 0.5 to 30% by mass of a non-chromium compound satisfying (1) as a rust preventive pigment.
(A) pH of 7 or more and 12 or less when dissolved in ion exchange water at a concentration of 0.1% by mass;
(B) The electric conductivity when dissolved in ion exchange water at a concentration of 0.1% by mass is 500 μS / cm.
And above;
(C) No thermal decomposition occurs up to 200 ° C. One or two or more kinds of anticorrosive pigments containing at least one layer other than the outermost layer of the two or more layers and containing no chromium (in the case of two or more types, may contain the non-chromium compound). The coated steel sheet according to claim 1, containing 10 to 60% by mass.   The coated steel sheet according to claim 1, wherein the non-chromium compound is sodium tripolyphosphate, and the outermost-layer coating film is a coating film formed from a solvent-based paint.   The coated steel sheet according to any one of claims 1 to 3, wherein a thickness of the coating film of the outermost layer is 3 µm or more and 30 µm or less.   The coated steel sheet according to any one of claims 1 to 4, further comprising a chemical conversion treatment film containing no chromium between the coating substrate and the coating film. The rust-preventive pigment coating contains at least one layer other than the outermost layer is selected from tripolyphosphate aluminum and Ca exchange silica and that this coating must not have a rust preventive pigment to satisfy the following requirements in I containing The coated steel sheet according to any one of claims 2 to 5, which is characterized.
(A) pH of 7 or more and 12 or less when dissolved in ion exchange water at a concentration of 0.1% by mass;
(B ′) electric conductivity of 700 μS / cm or more when dissolved in ion exchange water at a concentration of 0.1% by mass;
(C) No thermal decomposition occurs up to 200 ° C. A chromium-free solvent-based paint for forming an outermost layer coating film in a coated steel sheet in which two or more coating films are formed on at least one surface of a coated substrate, and satisfying the following (A) to (C) A paint comprising 0.5 to 30% by mass of a compound as an antirust pigment based on the total solid content in the paint.
(A) pH of 7 or more and 12 or less when dissolved in ion exchange water at a concentration of 0.1% by mass;
(B) an electric conductivity of 500 μS / cm or more when dissolved in ion exchange water at a concentration of 0.1% by mass;
(C) No thermal decomposition occurs up to 200 ° C.

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