JP5299532B2 - Chrome-free painted steel plate with excellent red rust resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent occurrence of red rust from an end face to be observed in chromium-free coated steel sheet not utilizing chromate treatment or chromate based rust prevention pigments without deteriorating corrosion resistance. <P>SOLUTION: In a chromium-free coated steel sheet, one or more layers of coating films are formed on both surfaces, respectively, of a coating base material comprising a plated steel sheet having a plated layer containing zinc. When one hundred samples of the coated steel sheet cut into a rectangle of 0.5 cm&times;4.5 cm are immersed in ion exchange water (&le;4&mu;S/cm) of 200 ml at 50&deg;C for 30 min under applying ultrasonic vibration with a frequency of 40 kHz, electric conductivity of immersion water is &ge;30&mu;S/cm. A coating film of outermost layer contains a compound (preferably an alkali metal phosphate) of 1 to 30% by mass with the proviso that (A) electric conductivity of the water is &ge;500&mu;S/cm when the compound is dissolved in ion exchange water (&le;4&mu;S/cm) in 0.1% by mass and (B) the compound is not thermally decomposed by 200&deg;C. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

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 while maintaining equivalent corrosion resistance as compared with the 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. 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 surface” side opposite to the “front surface”, painting may be performed by a 1-coat 1-bake method using a paint developed for the back surface after the pretreatment.

  Painted steel sheets are required to have many performances such as corrosion resistance, workability, coating film hardness (scratch resistance), stain resistance, chemical resistance, weather resistance, etc., but the order of the required performance varies depending on the application. Corrosion resistance is very important in coated steel sheets for products used mainly outdoors such as air conditioner outdoor units and water heaters. In such a product, the end face of the steel plate when the coated steel plate is cut and punched may be exposed at a place where the final shape can be visually recognized. Particularly in the product having such a shape, corrosion resistance of the steel plate end face generated by cutting (hereinafter referred to as end face corrosion resistance) is required.

  The conventional coated steel sheet with excellent end face corrosion resistance uses a zinc-based plated steel sheet as the coating base, and after first applying a ground treatment with an effect of improving corrosion resistance such as chromate treatment, at least on the front side, An undercoat film (primer) is formed using an undercoat paint (primer) containing a chromate-based rust preventive pigment such as strontium chromate, and then an overcoat film is formed so as to satisfy desired properties and appearance.

  However, with increasing environmental awareness, not only hexavalent chromium compounds such as strontium chromate but also chromium-free coated steel sheets that do not use any chromium compounds including trivalents have 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. Therefore, alternative technologies have been proposed to replace chromate treatment and chromate rust preventive pigments.

  Patent Document 1 describes that 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. Has been. On the other hand, even if this coating film is applied to a primer, it is described in the following non-patent document 1 that the corrosion resistance of both the end face and the crosscut portion is insufficient in the salt spray test.

  In Patent Document 2, a coated steel sheet in which a primer layer containing magnesium oxide or calcium hydroxide having a particle diameter of 10 μm or less is formed on the surface of a galvanized steel sheet, and a top layer (overcoat film) is formed thereon, is end face corrosion resistance. It is described that it is excellent. The end face corrosion resistance is evaluated by the white rust occurrence rate by the salt spray test.

  Patent Documents 3 and 4 disclose chromium-free coated steel sheets provided with a coating film containing aluminum tripolyphosphate and a hydrocalumite compound as anticorrosive pigments.

Japanese Patent Laid-Open No. 9-12931 JP 2005-225052 A JP 2004-237498 A JP 2006-97023 A Kobe Steel Engineering Reports Vol. 54, No. 1, pp. 62-65 (2004)

  Several techniques of chrome-free coated steel sheets including Patent Documents 1 to 4 have been proposed so far, and all of them are described as having predetermined corrosion resistance. However, the actual chromium-free coated steel sheet is inferior to the end face corrosion resistance as compared with the conventional coated steel sheet containing a chromate-based anticorrosive pigment, causing a problem in practical use.

  As a specific example, an air conditioner outdoor unit whose housing (housing) is made of chrome-free coated steel sheets is installed and installed outdoors, such as on the rooftop, and then sold and used after the building is completed. In the meantime, red rust was generated from the cut end face of the coated steel sheet used in the air conditioner outdoor unit, and the rust was attached to the surrounding area and discolored. The occurrence of red rust on the end face significantly deteriorates the appearance of the product and the place where it is installed.

  The present invention provides a chromium-free coated steel sheet that is excellent in end face corrosion resistance, suppresses occurrence of red rust from the end face, and at the same time has excellent corrosion resistance equivalent to that of a conventional chromium-free painted steel sheet, and can be manufactured at low cost. The purpose is to provide.

  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 as the base material. That is, when the steel plate end face portion comes into contact with moisture, zinc is preferentially dissolved and ionized over iron, thereby preventing red rust caused by iron ionization (oxidation).

  However, in an actual environment (for example, an environment that is exposed to the outdoors for a long period of time as described above), the water adhering to the end surface of the coated steel plate that has been exposed to rainfall or condensation does not contain much impurities, It is thought that electric conductivity is low. As a result, it is presumed that the sacrificial anticorrosive function of zinc is difficult to work and red rust is generated.

  It is known that coated steel sheets containing chromate-based rust preventive pigments typified by strontium chromate in the coating are excellent in end face red rust resistance, but the end face red rust suppression mechanism is as follows. Can be considered.

  (1) Although strontium chromate is considered to be sparingly soluble, its solubility in water is high compared to typical anticorrosive pigments (eg, aluminum tripolyphosphate, silica) contained in chromium-free coated steel sheets. Since it is easily dissolved in water, it oozes relatively easily from the end face of the coating when it comes into contact with water, increasing the electrical conductivity of water near the end face. Thereby, the sacrificial anticorrosive ability of zinc is promoted, and iron oxidation reaction, that is, red rust generation is suppressed.

(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 Cr6 + similar to the chromate film.

  Among the above mechanisms, (1) is considered to be achievable with compounds other than chromate. That is, when water comes into contact with the steel sheet (end face), the coated steel sheet that raises the electrical conductivity of the water on the end face of the steel sheet by dissolving the substance in the water coating into the water will cause red rust on the end face. It is considered difficult to occur.

  In this respect, the aluminum tripolyphosphate and silica used in Patent Document 1 and the calcium hydroxide, magnesium oxide, and magnesium hydroxide used in Patent Document 2 are all very low in water solubility. Therefore, it does not have water solubility necessary for at least the mechanism (1). Therefore, the effect of preventing red rust on the end face is insufficient.

  Here, Non-Patent Document 1 mentions that there is a possibility that the corrosion resistance may be improved by blending a pigment having a high elution property in a corrosive environment. However, in Non-Patent Document 1, there is no mention of a specific degree of elution and a substance name, and the swollen width of the coating film on the end face is evaluated by a salt spray test. It cannot be said that effective disclosure for preventing red rust on the end face is made.

  However, if only the effect of (1) is obtained, an effect of suppressing red rust on the end face can be obtained, but depending on the type of pigment to be added, the corrosion resistance is significantly impaired in a conventional corrosion resistance evaluation method such as a salt spray test. Sometimes. That is, it is preferable to use a chromium-free pigment that can also exhibit the performances (2) and (3) of strontium chromate.

  In the coated steel sheet using zinc-based plated steel sheet as the coating base, the present inventors ensured the sacrificial anticorrosive ability of zinc in an environment where it comes into contact with rainwater to ensure end surface corrosion resistance and implement countermeasures against end surface red rust. The present invention has been achieved by specifying conditions having corrosion resistance equivalent to that of a conventional chromium-free painted steel plate that has not been made.

The present invention relates to a chromium-free coated steel sheet in which two or more coating films are formed on both surfaces of a coated substrate made of a plated steel sheet having a Zn-containing plated layer.
1st side surface WHEREIN: The coated steel plate which concerns on this invention contains the at least 1 sort (s) of non-chromium compound with which the coating film of outermost layer satisfy | fills the requirements of following (A) and (B) in the at least single side | surface of the said coating base material. The coating thickness of the outermost layer is 5 to 30 μm, the content of the non-chromium compound in the coating layer of the innermost layer is 1% by mass or less, and this coated steel sheet is 0.5 cm × 4.5 cm. The electrical conductivity of the immersion water is 30 μS / cm or more when 100 samples of 100 mm cut into a rectangular shape are immersed in 200 ml of ion-exchanged water at 50 ° C. for 30 minutes under application of ultrasonic vibration at a frequency of 40 kHz. The P concentration of water is 0.5 μg / ml or more.
(A) 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,
(B) No thermal decomposition occurs up to 200 ° C.

  In the present invention, “ion-exchanged water” means water that has been deionized by ion exchange until the electric conductivity is 4 μS / cm or less. The electrical conductivity of rainwater and condensed water containing almost no impurities such as long rain during the rainy season is at this level.

2nd side surface WHEREIN: The coated steel plate which concerns on this invention is at least 1 sort (s) of at least 1 type of coating films other than an innermost layer satisfy | filling the requirements of following (A) and (B) in the at least single side | surface of the said coating substrate. Contains a non-chromium compound, the film thickness of the outermost coating film is 5 to 30 μm, the content of the non-chromium compound in the innermost coating film is 1% by mass or less, Electricity of immersion water when 100 samples of 0.5cm x 4.5cm rectangle coated with polyester film were immersed in 200ml of ion exchange water at 50 ° C for 30 minutes under application of ultrasonic vibration of frequency 40kHz The conductivity is 10 μS / cm or more, and preferably, the P concentration of the immersion water is 0.5 μg / ml or more.
(A) 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,
(B) No thermal decomposition occurs up to 200 ° C.

In at least one side of the chromium-free coated steel sheet, the outermost layer coating film is the first side surface of the present invention, and the second side surface of the present invention is a coating film having two or more layers, other than the innermost layer. The at least one layer of coating film contains at least one non-chromium compound that satisfies the following requirements (A) and (B) (hereinafter, this non-chromium compound is also referred to as a compound that imparts an end face red rust preventing effect). Preferably:
(A) The electric conductivity of water when dissolved in ion exchange water at a concentration of 0.1% by mass is 500 μS / cm or more,
(B) No thermal decomposition occurs up to 200 ° C.

From yet another aspect, the present invention is a chromium-free coated steel sheet in which two or more coating films are formed on both sides of a coated base material made of a plated steel sheet having a Zn-containing plated layer,
(1) At least on one side, the outermost layer coating film contains at least one non-chromium compound satisfying the above (A) and (B) in an amount of 0.5 to 30% by mass, thickness of the membrane Ri 5~30μm der, der content of less than 1 wt% of the chromium-free compound of the innermost layer of the coating Luke, or
(2) On at least one surface, at least one coating film other than the innermost layer contains at least one non-chromium compound satisfying the above (A) and (B) in an amount of 5 to 30% by mass, thickness of the coating film at least one layer other than the inner layer 5μm or more and 50μm or less, and the thickness of the outermost layer of the coating film is Ri 5~30μm der, containing the non-chromium compound of the innermost layer of the coating amount Ru der than 1 mass%,
A chrome-free coated steel sheet is also provided.

  Preferably, the non-chromium compound satisfying the above (A) and (B) is selected from alkali metal phosphates and chlorides and alkaline earth metal hypophosphites, more preferably alkali metal phosphoric acid. Selected from salt.

In a preferred embodiment, the chromium-free coated steel sheet of the present invention satisfies one or more of the following:
The one side is preferably the back side as a coated steel plate;
-Having a paint ground treatment film containing no chromium between the coating substrate and the one or more coating films;
-The coated substrate has at least two coatings on one side, and the innermost coating of the two or more coatings contains a non-chromium compound that satisfies the requirements (A) and (B). Not done.

The present invention also relates to a housing obtained by molding the chromium-free painted steel sheet .

  Conventionally, the corrosion resistance of a coated steel sheet has been ensured mainly by containing a relatively large amount of an anticorrosive pigment such as strontium chromate in the undercoat film (primer layer or undercoat), that is, the innermost film. . 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.

  According to the present invention, when there are two or more outermost coating films or coating films, a non-chromium compound having a relatively high water solubility such as an alkali metal phosphate is applied to the coating film other than the innermost layer. By containing a relatively small amount, the end surface corrosion resistance of the coated steel plate using the zinc-based plated steel plate as a coating base is remarkably improved, and the occurrence of red rust on the end surface is prevented.

  The coated steel sheet according to the present invention can reliably exhibit the sacrificial anticorrosive ability of zinc exhibited by the galvanized steel sheet of the base material even in an outdoor environment exposed to rainwater, suppress the occurrence of red rust on the end face, and generate red rust. It is possible to prevent deterioration of the appearance of the product and its installation location due to the above. In addition to the effect of suppressing red rust, by appropriately selecting a non-chromium compound, the same level of corrosion resistance as that of a conventional chromium-free painted steel sheet can be achieved with respect to the swelling of the coating film due to the corrosion of the exposed end face.

  The coated steel sheet according to the present invention with excellent end surface red rust property is suitable for application to products used outdoors, such as air conditioner outdoor units and water heaters, where many exposed end surfaces are exposed by punching. . However, the use of the coated steel sheet according to the present invention is not limited thereto.

It is explanatory drawing of the coating blister (blister) generation | occurrence | production mechanism seen when a relatively high water-soluble compound is contained in an undercoat coating film. It is explanatory drawing which shows the shape and arrangement | positioning of the test piece used for the end face red rust test in the Example, (A) is a front view, (B) is a side view.

  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) Substrate steel plate In the coated steel plate of the present invention, since the sacrificial anticorrosive ability of zinc is used for the occurrence of red rust from the end face, the base steel plate is a zinc-based plated steel plate having a plated layer containing zinc, that is, galvanized. A steel plate or a zinc alloy plated steel plate is 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, and electrical Zn—Ni. Examples include alloy plated steel sheets.

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 to 50 g / m ^{2 in} the case of electroplating and ^{20 to} 100 g / m ² in the case of hot dipping. If the coating amount is too small, the corrosion resistance is lowered, and if too much, the workability is deteriorated.

  The thickness of the steel sheet is determined depending on the application, but it is considered that red rust is likely to occur when the steel sheet is too thick. There is no problem if the thickness is about 2.0 mm or less, which is usually used as a coated steel sheet.

(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 chemical conversion treatment, and before that, surface conditioning treatment is often performed using an acidic or alkaline aqueous solution containing an iron group metal ion such as Ni. Prior to that, alkaline degreasing or the like is usually 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-based 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 an organic resin is allowed to coexist in some cases.

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) Coating film (undercoat and topcoat)
In the present embodiment, the case where both the front surface and the back surface have two layers of an undercoat coating film (innermost layer coating film) and a top coating film (outermost layer coating film) will be described. However, it is also possible for one or both surfaces to have only one layer of coating or to have three or more layers. The overcoating film is sometimes referred to as a top coat, and the undercoating film is sometimes referred to as a primer.

(3-1) Top coat (outermost layer)
The coated steel sheet according to the present invention contains a non-chromium compound having a relatively high water solubility in the coating film, and sufficiently utilizes the elution into the rainwater to sufficiently provide the sacrificial anticorrosive ability of the base zinc-based plated steel sheet. It is characterized by preventing the occurrence of end surface red rust.

The extent of this elution can be embodied by the following two methods (a) and (b):
(a) Immersion test (hereinafter referred to as surface immersion) in which 100 samples of a coated steel plate cut into a rectangle of 0.5 cm × 4.5 cm are immersed in 200 ml of ion-exchanged water at 50 ° C. for 30 minutes under application of ultrasonic vibration at a frequency of 40 kHz. The electrical conductivity of the immersion water obtained in the test) is 30 μS / cm or more, preferably the P concentration of the immersion water is 0.5 μg / ml or more;
(b) After coating the front and back surfaces of the coated steel plate with a polyester film, 100 samples cut into a rectangle of 0.5 cm × 4.5 cm were applied to 200 ml of ion-exchanged water at 50 ° C. with ultrasonic vibration at a frequency of 40 kHz. The electrical conductivity of the immersion water obtained in the immersion test (hereinafter referred to as end face immersion test) for 30 minutes is 10 μS / cm or more, preferably the P concentration of this immersion water is 0.5 μg / ml or more. .

  In the surface immersion test of (a), the end surface and all the front and back surfaces of the coated steel sheet sample are in contact with ion-exchanged water, whereas in the (b) end surface immersion test, only the end surface of the coated steel sheet sample is ionized. Contact with exchange water. The preferable electrical conductivity of the immersion water is 45 μs / cm or more in the surface immersion test (a), and 15 μs / cm or more in the end surface immersion test (b). The upper limit of the electric conductivity of the immersion water is not particularly limited, but is preferably 700 μs / cm or less in the surface immersion test (a) and 140 μs / cm or less in the end surface immersion test (b).

  In many cases, rainwater can contact not only the end surface of the coated steel sheet but also at least one side of the front and back surfaces. When the product is used in such a situation, ions can be eluted not only from the end face of the paint film but also from the surface of the paint film. Is preferred. However, in an actual corrosive environment, a situation where elution from the surface of the coating film cannot be expected is also assumed. In such a case, it is preferable that the electrical conductivity of immersion water rises only by the elution from an end surface. Assuming that case, as another method, the end surface corrosion resistance of the coated steel sheet is defined by an end surface immersion test in which only elution from the end surface is evaluated.

  The sample for the end face immersion test can be prepared by attaching a commercially available polyester adhesive film to the front and back surfaces of a coated steel sheet of an appropriate size and bringing it into close contact, and then cutting it to a predetermined size. As the polyester adhesive film, for example, Nitto Denko's polyester adhesive tape No. 31 series can be used.

  The coated steel sheet according to the present invention only needs to satisfy either one of the above-mentioned characteristics (a) and (b) for end face red rust resistance (that is, only one of the surface immersion test and the end surface immersion test may be performed). ) Preferably satisfy both properties (a) and (b).

In the surface immersion test, in order for the immersion water after immersion to have the high electrical conductivity specified in (a) above, ions should be eluted not only from the end face of the coating film but also from the coating film surface. Is advantageous. Therefore, the top coat film which is the outermost layer paint film contains at least one non-chromium compound having a relatively high solubility satisfying the following (A) and (B) in an amount of 0.5 to 30% by mass. Is preferred:
(A) 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, and
(B) No thermal decomposition occurs up to 200 ° C.

The condition (B) is for preventing the compound from decomposing during baking after coating.
Examples of non-chromium compounds that meet the above requirements (A) and (B) are alkali metal phosphates or chlorides or alkaline earth metal hypophosphites. Specific examples include sodium tripolyphosphate (pentasodium triphosphate), potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium dihydrogen phosphate, sodium chloride, calcium hypophosphite and the like.

  In view of the balance with the corrosion resistance other than the end face, it is preferable to select an alkali metal phosphate as the non-chromium compound. Here, “phosphate” is not limited to orthophosphate, but polyphosphate (condensed phosphate) such as tripolyphosphate, phosphite, metaphosphate, pyrophosphate, etc. Including.

  Preferred alkali metal phosphates include sodium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate, and these 1 type (s) or 2 or more types can be used. Moreover, potassium dihydrogen phosphate is mentioned as a preferable alkali metal phosphate.

  However, even non-chromium compounds satisfying (A) and (B), for example, phosphates, do not necessarily suppress the occurrence of red rust. For example, hydrocalumite-treated zinc phosphate (for example, EXPERT NP-530 N20 manufactured by Toho Pigment) and certain types of magnesium phosphite (for example, Toho Pigment NP-1802) are both chromium-free pigments. Although it is commercially available and satisfies both of the above (A) and (B), even if these are added to the top coat in an amount of 1 to 30% by mass, the immersion in the surface immersion test described above The electric conductivity of water is less than 30 μS / cm, and the effect of suppressing red rust on the end face of the coated steel sheet is not sufficient. The compound satisfying the above (A) and (B) is selected from those whose electric conductivity of immersion water in the immersion test satisfies the above (a) or (b), preferably an alkali metal phosphate.

  The content of the non-chromium compound satisfying the above (A) and (B), preferably the alkali metal phosphate, in the coating film is such that the front surface or When it is contained only in the outermost layer coating film (eg, top coating film) on the back side, it is preferably 1 to 30% by mass, more preferably 2 to 20% by mass. When a non-chromium compound such as alkali metal phosphate is contained in the top coating film on both sides, the lower limit may be about half of the above. That is, the content in this case is preferably 0.5 to 30% by mass, more preferably 1 to 20% by mass. When this non-chromium compound is also contained in a coating film other than the top coating film (outermost layer) (an undercoating film or an intermediate coating film in the case of having three or more layers), the lower limit of the content is slightly more May be lower.

  The film thickness of the outermost layer coating film (eg, top coating film) in this case is not particularly limited, but is usually 0.5 μm or more and 50 μm or less. When the thickness is 0.5 μm or less, the performance required for the coating including corrosion resistance becomes insufficient. When it exceeds 50 micrometers, the workability of a coated steel plate will fall remarkably. A preferred film thickness is 1 to 30 μm.

  On the other hand, in the end face immersion test, only the end face comes into contact with ion exchange water. Since the contact area of ion-exchanged water is limited to a narrow area such as an end face, in order to make the immersion water have high electrical conductivity as defined in (b) in the end face immersion test, the above (A ) And (B), and a relatively high solubility of at least one non-chromium compound is preferably included in a fairly high proportion, that is, in an amount in the range of 5-30% by weight.

  In the end face immersion test, the front and back surfaces do not come into contact with ion-exchanged water, and therefore it is not necessary to include a non-chromium compound satisfying the above (A) and (B) in the outermost layer coating, and any coating other than the innermost layer is required. What is necessary is just to make it contain in a film | membrane. For example, when the coating film has three layers, the non-chromium compound may be contained in one or both of the intermediate layer (intermediate coating film) and the outermost layer (top coating film). Since the non-chromium compound satisfying the above (A) and (B) has a relatively high water solubility, when it is contained in the innermost layer (undercoat coating film), it tends to cause swelling of the coating film.

  In this case, the content of the non-chromium compound satisfying the above (A) and (B), preferably the alkali metal phosphate, in the coating film is 5 to 30% by mass, preferably 10 to 10%, as described above. 30 mass%, More preferably, it is 10-20 mass%. Even when this non-chromium compound is contained in two or more coating films (for example, a top coating film on both surfaces), the content per coating film is preferably within this range.

  In order to elute a sufficient amount of the above-mentioned non-chromium compound (preferably alkali metal phosphate) into water from a small area called an end face, a coating film other than the innermost layer containing this non-chromium compound (for example, a top coating film or It is preferable to increase the thickness of the three or more intermediate coating films within a range that does not impair other performances. A preferable film thickness is 5 μm or more and 50 μm or less, and a more preferable film thickness is 8 to 30 μm.

  The base resin of the coating film (in this embodiment, the top coating film) containing a non-chromium compound such as alkali metal phosphate is not particularly limited. For example, polyester resin, acrylic resin, epoxy resin, polyurethane resin, silicone polyester resin One or more selected from polyvinyl chloride resin, fluororesin, and the like can be used. The crosslinking agent may be the same as that described for the undercoat film described later, and a crosslinking catalyst is blended as necessary.

The top coat film may contain a commonly used chromium-free rust preventive pigment in addition to the non-chromium compound described above. For example, the previously exemplified hydrocalumite compound, magnesium phosphite, aluminum tripolyphosphate, porous silica (oil absorption: 100 to 1000 ml / 100 g, specific surface area: 200 to 1000 m ² / g, average particle size: 2 to 30 μm ) And the like. Those other than hydrocalumite compounds are preferred.

  The top coat film usually contains a color 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 content of the color pigment in the coating film is preferably 30% by mass or less, and the total amount combined with the non-chromium compound and the rust preventive pigment is 60% by mass or less, particularly 50% by mass or less of the coating film. It is preferable.

  As illustrated later in Example 7, in addition to the relatively high water-soluble non-chromium compound used in accordance with the present invention, a relatively large amount of other pigments such as rust preventive pigments and colored pigments with low water solubility are used. (For example, 20% by weight or more) coexistence improves the end face red rust resistance in the end face red rust test performed by sealing both surfaces of the coated steel sheet. The reason for this is that a large amount of water-resistant rust preventive pigments and colored pigments are present in the coating film (20% by weight or more), so that the amount of water penetrating from the coating surface to the inside of the coating film increases (water-soluble Low rust preventive pigment, water permeates the interface between the color pigment and the coating resin). As a result, in addition to the elution of non-chromium compounds with relatively high water solubility present at the extreme surface, in the vicinity of the end surface and inside the coating It is presumed that non-chromium compounds having a relatively high water solubility are also eluted.

  The top coat film may contain other additives in addition to the above components. For example, UV absorbers and light stabilizers effective for improving weather resistance, and waxes effective for improving processability (press moldability).

  As described above, the top coat film is required to have various performances other than the corrosion resistance. Particularly, the front surface of the coated steel sheet is required to have a high level of designability, workability, weather resistance, and the like. Therefore, it is preferable that the top coat film containing the non-chromium compound satisfying the above (A) and (B) of the present invention is formed on the back side of the coated steel sheet. In that case, the structure of the top coat film on the front side is not particularly limited. For example, it can be set as the coating-film structure containing the additive mainly intended to improve the designability, workability, and weather resistance.

  The film thickness of the top coat film is desirably 0.5 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.

(3-2) Undercoat coating (innermost layer coating)
In conventional coated steel sheets using chromate-based rust preventive pigments such as strontium chromate in order to improve the corrosion resistance, the rust preventive pigments are often included in the undercoat. One reason for this is that, as described above, the top coat film requires various performances and the degree of freedom in designing the coat film is limited, while the undercoat film has fewer restrictions. .

  Also in the coated steel sheet of the present invention, various undercoating rust preventive pigments and additives may be included in the undercoat coating film. However, when a non-chromium compound satisfying the above (A) and (B) is contained in a substantial amount in the undercoat coating film, blistering (blister) tends to occur. An estimation mechanism of this phenomenon is shown in FIG. Since the compound satisfying (A) has a relatively high water solubility, the water invading from the surface of the coated steel sheet is concentrated near the non-chromium compound in the undercoat film, and the water content is on the surface of the coated steel sheet having a large surface free energy. It is thought that the film bulges when concentrated.

  In order to prevent the swelling of the coating film, the non-chromium compound that satisfies the above (A) and (B) and has a relatively high water solubility is not contained in a substantial amount (for example, contained) in the undercoat coating film. Is preferably 1% by mass or less).

  The base resin constituting the undercoat coating 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 anticorrosive pigment to be contained in the undercoat film is one having a relatively low water solubility, specifically, when dissolved in ion exchange water (electric conductivity: 4 μS / cm or less) at a concentration of 0.1% by mass. Has an electric conductivity of 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, Ca-exchanged silica, porous silica particles having an oil absorption of 100 to 1000 ml / 100 g, a specific surface area of 200 to 1000 m ² / g, and an average particle size of 2 to 30 μm.

  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. If 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, when the amount of the rust preventive pigment exceeds 60% by mass, 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.

(3-3) Coating method The above-described undercoat and topcoat are both applied and baked (heat-dried) with a paint in which necessary components are dissolved or dispersed in a solvent (ie, undercoat and topcoat). It is formed. Each paint may be either a solvent-based paint whose solvent is an organic solvent or a water-based paint whose solvent is a mixed solvent of water or water and a water-miscible organic solvent. However, when the non-chromium compound satisfying the above (A) and (B) is contained in the top coat film, since this non-chromium compound has a relatively high water solubility, the paint for forming this top coat film is a solvent system. A paint is preferred.

  When such a non-chromium compound is contained in the coating material at a relatively high ratio, the viscosity of the coating material may increase. In that case, the viscosity of the paint may be adjusted by adding an appropriate amount of a dehydrating agent such as alcohol, if necessary.

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 coating (drying of the coating film) is performed so as to have a PMT (maximum temperature reached by the steel sheet) set in accordance with the resin and the crosslinking agent constituting the coating film. This temperature is generally in the range of 100-280 ° C. The heating method may be hot air drying or oven drying.

(3-4) When one side has a coating film of three or more layers When it has a coating film of three or more layers on one side, in a so-called intermediate coating film (coating film that is neither an upper coat (outermost layer) nor an undercoat (innermost layer)) In addition, non-chromium compounds (such as alkali metal phosphates) that satisfy the above-mentioned (A) and (B) may be contained. In this case, the non-chromium compound satisfying the requirements (A) and (B) in which the alkali metal phosphate or the like is contained in the intermediate coating film is expected to be eluted mainly only from the end face.

  The intermediate coating film in such a case may be tested by the end face immersion test described above so that the electric conductivity of the immersion water is 10 μS / cm or more. In that case, it was mentioned above that it is preferable to increase the content of the non-chromium compound in the coating film and / or increase the thickness of the coating film.

(4) Molding The casing can be formed from the coated steel sheet of the present invention by a conventional method such as punching or press molding. This housing has excellent resistance to red rust on the end face, so it is useful as a housing for equipment that is often used outdoors, such as air conditioner outdoor units and water heaters. Red rust is generated even when exposed to rainwater. It is suppressed.

(5) Test Method A test method for simulating and evaluating the occurrence of red rust on the end face of the coated steel sheet will be described in relation to the present invention.

  As an accelerated test for evaluating the corrosion resistance of a conventional coated steel sheet, a salt spray test or a cyclic corrosion test in which salt spray and wet, dry, etc. are repeated has been often used. Each of these test methods is an excellent method characterized by each, but regarding the occurrence of red rust on the end face of the coated steel sheet, even if the test results are good, it cannot be said that they are good in the actual environment.

  This is considered to be due to the extremely low salinity in the water adhering to the steel plate end face due to rainwater and condensation unless the environment is a coastal area. In such an environment (ie, an environment with low salinity) This is thought to be because the corrosion mechanism is different from the environment of high salinity, such as the formation of stable corrosion products, and the order of corrosion resistance can change.

  On the other hand, as described in the examples below, the test method in which the coated steel sheet is immersed in ion-exchanged water and the wet test are closer to the actual environment when exposed to rainwater or condensation, and the end surface of the coated steel sheet It is suitable as an accelerated test for evaluating the red rust situation.

Paint test:
A chromium-free solvent-based coating composition for forming the outermost layer coating film on the steel sheet surface, as with surface immersion test described above, the coating material and the obtained dried film into a rectangular 0.5 cm × 4.5 cm Using 100 cut samples, these samples were immersed in 200 ml of ion-exchanged water at 50 ° C. for 30 minutes under application of ultrasonic vibration at a frequency of 40 kHz, and the electric conductivity of the immersion water was 50 μS / cm or more, A chromium-free solvent-based paint having a P concentration of the immersion water of preferably 0.5 μg / ml or more is also provided. This steel plate is preferably a galvanized steel plate.

  That is, without producing a coated steel sheet having this paint as a top coat, the paint itself is made into a suitable water-insoluble, peelable and heat-resistant substrate (for example, a polytetrafluoroethylene sheet or a substrate coated with it). ), And the coating film is dried by baking to form a film. After the formed film is peeled off from the substrate, 100 samples of the above dimensions are prepared from the obtained film, and then into ion-exchanged water. The coating is used for the formation of a top coat on a coated steel sheet whose base material is a galvanized steel sheet by measuring the electrical conductivity of the immersion water during immersion and preferably measuring the P content. The end face red rust resistance can be predicted. By using a paint satisfying a predetermined condition in this test, a coated steel sheet having excellent end surface red rust resistance can be produced.

  As in the end face immersion test described above, it is also possible to evaluate the paint only by elution from the end face. That is, 100 samples which were cut into a rectangle of 0.5 cm × 4.5 cm after covering the front and back surfaces of the paint film obtained in the same manner as described above with a polyester film were subjected to ion exchange water (electric conductivity: 4 μS) at 50 ° C. / Cm or less) If the paint has an electric conductivity of 10 μS / cm or more when immersed in 200 ml for 30 minutes under application of ultrasonic vibration at a frequency of 40 kHz, a coated steel sheet having excellent resistance to red rust on the end face is manufactured. can do.

In either case, the thickness of the test paint film is 10 μm in terms of dry thickness.
In the surface immersion test using the sample of the coating film, both sides of the film come into contact with water, and thus the amount of elution of the non-chromium compound is larger than when the coated steel plate sample is immersed. Therefore, the lower limit of the electric conductivity of the immersion water is set to 50 μs / cm. In this case, the electric conductivity of the immersion water is preferably 80 μs / cm or more and 1000 μs / cm or less.

The electric conductivity of the immersion water in the end face immersion test is preferably 15 μs / cm or more and 300 μs / cm or less.
In the case where the coating test is performed while the coating film is not peeled off while being adhered to the coating substrate, the same standard may be adopted by performing the testing in the same manner as in the case of the coated steel sheet described above.

The following examples illustrate the effectiveness of the present invention. The examples are for purposes of illustration and are not intended to limit the invention. In Examples,% is% by mass unless otherwise specified.
The type of the non-chromium compound (or chromium-free pigment) used in each example was dissolved in the liquid conductivity [ion exchange water (electric conductivity: 4 μS / cm or less, the same applies below) at a concentration of 0.1% by mass. Table 1 together with the electrical conductivity of water when the Table 1 also shows strontium chromate as an example of a conventional chromium pigment.

  Of the non-chromium compounds and chromium pigments (chromium compounds) shown in Table 1, zinc phosphate hydrocalumite, calcium phosphite hydrocalumite, and magnesium phosphite are all NP-530 N20 manufactured by Toho Pigment Co., Ltd. NP-1020C N20, NP-1902. Aluminum tripolyphosphate is K-WHITE # 82 manufactured by Teika, porous silica is H-31 manufactured by Dokai Chemical Industry, calcium ion-exchange silica (Ca-exchange silica) is Shieldex manufactured by Fuji Silysia Chemical, and strontium chromate is Kikuchi Color Stokuro C and Titania manufactured by Ishihara Sangyo Co., Ltd. were Type CR CR90. For other non-chromium compounds, general chemical reagents were used.

  In addition, in the table | surface which displays the coating-film structure of the top coat film and undercoat film in each Example, both the addition amount (%) of a non-chromium compound and chromium system pigment (chromium compound) is the mass of the compound with respect to paint solid content. % (Hereinafter abbreviated as PWC).

Example 1
After applying alkali 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), a silica system manufactured by Nippon Paint Co., Ltd. Using a chromium-free chemical conversion treatment solution (Surfcoat EC2330), chemical conversion treatment was performed on both sides as instructed by this product. The chemical conversion treatment was performed so that the adhesion amount was 4 to 8 mg / m ² in terms of Si adhesion amount.

  One or two or more non-chromium compounds selected from Table 1 are added to a commercially available undercoat and topcoat that does not contain these compounds, and the undercoat for both sides A top coating for the front side and a top coating for the back side were prepared.

  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 coating is a baking type polyester resin coating (IPT206-CF585, main resin molecular weight of about 8000, melamine) made by Nippon Fine Coatings, which contains titania as a white pigment, and the back coating is manufactured by Nippon Fine Coatings. Baked polyester resin paint 1 (IPT236-CF756 PWC 30% type, main resin molecular weight about 3000, crosslinking agent: melamine) and Nihon Fine Coatings baked polyester resin paint 2 (NFC880 beige (PWC 50%) type : Resin molecular weight of about 4000, it was crosslinkers melamine). The types of non-chromium compounds added to each paint are shown in Table 2 for the undercoat paint for both sides and in Table 3 for the top coat for the back face. No non-chromium compound was added to the top coating for the front side. The added non-chromium compound 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, first, the undercoat film on the front surface (PMT 170 ° C.), the second undercoat film on the back surface (PMT 220 ° C.), and the third, the top coat film on the front surface (PMT 170 ° C.) Finally, coating was performed at the order of the top coat film (PMT 230 ° C.) and the baking temperature (PMT = maximum reached plate temperature) on the back surface to obtain a sample of the coated steel sheet having the coating film composition shown in Table 4. The coating was performed with a bar coater, and the coating thickness was unified to 11 μm on the front side for the undercoat and 5 μm on the back side, and 16 μm for the top side and 10 μm on the back side.

  In addition, as a comparative example containing conventional hexavalent chromium, a similar experiment and a performance evaluation test described below were carried out on an undercoating paint and an overcoating paint using strontium chromate as an antirust pigment.

  The coated steel sheet thus obtained was tested for end face red rust resistance, electrical conductivity measurement after immersion in ion-exchanged water, solution analysis, and the presence or absence of film swelling during a wet test. The test results are also shown in Table 4.

(1) End face red rust test 1
For each sample of the coated steel sheet, 10 test pieces each having a rectangular size of 1 cm × 4 cm (the sum of the end face lengths is 10 cm) are cut out by shearing (total end face length of the test pieces in each sample is 1 m), 10 pieces Are separately immersed in 20 ml of ion-exchanged water in a beaker. After leaving the beaker in a constant temperature bath at 40 ° C. for 72 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.

◎: Almost no red rust (solution is transparent and almost 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
Test specimens schematically shown in FIG. 2 were prepared, and after the samples were left for 96 hours under wet test conditions (temperature: 49 ° C., relative humidity of 95% or higher), red rust was generated from the end faces of the test specimens (see FIG. 2). The state of occurrence of red rust at the end face red rust evaluation site is measured visually. The evaluation criteria are as follows.

◎: Almost no red rust generated (almost no red rust generated from the end face),
○: Some red rust is generated from the end face (red rust is generated on the end face with a total length of less than 10 mm),
Δ: Red rust is generated (red rust is generated on the end face having a length of 10 mm or more and less than 30 mm), 1
X: A large amount of red rust occurred (red rust occurred on almost the entire end face (30 mm or more)).

  In addition, the test piece schematically shown in FIG. 2 was prepared by adhering three plates produced by punching 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. 2 (B) 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. 2, a minute gap is actually formed between the burrs, and water can enter from this gap. In the outdoor unit of an air conditioner, a processed part connected with a burr may be formed so that the pipe can be joined later so that the pipe can be removed. This test is a test simulating such intrusion of water from the burr part.

(3) Ion exchange water immersion test (electric conductivity of immersion water)
(3-1) For each sample of the coated steel plate, 100 pieces of 0.5 cm × 4.5 cm strip size pieces (the sum of the end face lengths is 10 cm) are cut out by shearing (each of the test pieces in each sample). Total end face length is 10m).
(3-2) These 100 test pieces are immersed together in 200 ml of 50 ° C. ion exchange water in a beaker placed on an ultrasonic vibration device.
(3-3) While maintaining a temperature of 50 ° C., a 40 kHz ultrasonic vibration is applied to the beaker for 30 minutes (device used: US CLEANER manufactured by ASONE).
(3-4) After the application of ultrasonic vibration is completed, the test piece is immediately removed, and the resulting aqueous solution (immersion water) is used to conduct electricity with an electric conductivity meter (D-54SE manufactured by Horiba, Ltd.). Measure the degree.

(4) Swelling of paint film after wet test A test piece having a size of 70 mm × 150 mm was cut out from each sample of the coated steel sheet by shearing, and this test piece was subjected to the same wet test conditions as described in (2) above. After being allowed to stand for a period of time, the state of occurrence of the blister on the flat surface (blister width) is measured. The evaluation criteria are as follows (○ passes):
○: There is almost no occurrence of film swelling from the flat surface (maximum swelling width is less than 2 mm),
X: Many coating film swelling of width 2mm or more generate | occur | produces from a plane part.

  As shown in Table 4, in No. 28 and 29 in which the back side top coat contains strontium chromate and No. 30 in which the undercoat on both sides contains strontium chromate, ion-exchanged water for the coated steel sheet Even though the electric conductivity of the immersion water in the immersion test was relatively low, the end surface was excellent in corrosion resistance and the coating film swelled well. As described above, strontium chromate has a large passivation effect on zinc and iron, and this passivation effect can suppress the occurrence of red rust on the end face.

On the other hand, in the painted steel sheet of the chromium-free, the Nanba1～1 1 according to the present invention that the electric conductivity of the immersion water in the deionized water immersion test is a 30 [mu] S / cm or more, none of the top coating and primer coating Although the chromium compound was not contained, the results of the end face red rust test 1 and 2 were good, and the end face corrosion resistance was excellent. In particular, No. 1 containing a non-chromium compound with high water solubility such as potassium hydrogen phosphate in the top coat film. 1 to 11, the result of the swelling of the coating film was good and the corrosion resistance was also good, and depending on the conditions, a result comparable to that obtained when strontium chromate was contained was obtained. However, in No. 12 in which the non-chromium compound was contained in the undercoat film, the end face corrosion resistance was good, but the result of the film swelling was unsatisfactory. Therefore, it can be seen that a non-chromium compound capable of suppressing end face red rust is preferably contained in the top coat film.

  On the other hand, the top coat film and the undercoat film do not contain a chromium compound, and even if an alternative non-chromium compound is added as in the present invention, the electrical conductivity of the immersion water in the ion exchange water immersion test is 30 μS. In Nos. 13 to 27 below / cm, red rust was likely to occur, and the red rust resistance was poor. Further, in Nos. 13 to 16 in which the undercoat coating film contains a non-chromium compound having a relatively high water solubility, similarly to No. 12, the swelling of the coating film was unsatisfactory.

  As shown in Table 1, magnesium phosphite and calcium phosphite hydridocalumite exhibit liquid conductivity satisfying the above requirement (A) and are relatively water-soluble. However, Nos. 18, 22 and 27, each of which is contained alone in the top coat film, have low electrical conductivity in the ion-exchange water immersion test of the coated steel sheet, and are aluminum tripolyphosphate, calcium ion-exchange silica, or Similar to Nos. 13 to 17, 19 to 21, and 23 to 26, in which the top coating film contains a rust preventive pigment having low water solubility such as titania, the results were inferior to the end face red rust resistance.

  From these results, in order to improve the red rust resistance of the coated steel sheet, ion exchange that evaluates the elution of the compound from the coated steel sheet, not the water-solubility of the anticorrosive compound itself contained in the top coat film. It can be seen that the electrical conductivity of the immersion water in the water immersion test is a guide.

(Example 2)
After applying alkali 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), a silica system manufactured by Nippon Paint Co., Ltd. Using a chromium-free chemical conversion treatment solution (Surfcoat EC2330), chemical conversion treatment was performed on both sides as instructed by this product. The chemical conversion treatment was performed so that the adhesion amount was 4 to 8 mg / m ² in terms of Si adhesion amount.

  One or more non-chromium compounds selected from Table 1 are added to commercially available undercoats and topcoats that do not contain them, and undercoats for the front side and undercoats and topcoats for the back side Was prepared. This addition was not made to the top coating for the front side. As will be described later, the undercoat paint for the back surface was prepared by adding 35% aluminum tripolyphosphate and 15% titania listed in Table 1 to the commercially available undercoat paint as anticorrosive pigments. The added anticorrosive pigment was uniformly dispersed by the method described in Example 1.

  The base paint used was a front surface undercoat paint made by Nippon Fine Coatings, a polyester paint (CLC type of FLC3900 primer, main resin molecular weight 10000 or more, cross-linking agent: melamine, Tg of about 10 ° C.), back surface undercoat paint Made by Nippon Fine Coatings Co., Ltd. (PB10P primer clear type, main resin molecular weight 10000 or more, cross-linking agent: melamine, Tg approx. 40 ° C PWC with 35% aluminum tripolyphosphate and 15% titania added) ) The front surface top coat paint contains titania as a white pigment and is a baking type polyester resin paint (SRF05, main resin molecular weight of about 8000, crosslinker: melamine) manufactured by Nippon Fine Coatings Co., Ltd. Baking type made by Su Riesuteru resin paint (NSC880 dark beige (PWC30%) of a type.

The type and amount of the added non-chromium compound are shown in Table 5 for the undercoat paint for the front surface and in Table 6 for the top coat paint for the back surface.
Using these paints, first, the undercoat film on the front surface (PMT 170 ° C.), the second undercoat film on the back surface (PMT 220 ° C.), and the third, the top coat film on the front surface (PMT 170 ° C.) Finally, coating was performed at the order of the top coat film (PMT 230 ° C.) and the baking temperature (PMT = maximum reached plate temperature) on the back surface to obtain a sample of the coated steel sheet having the coating film composition shown in Table 7. The coating was performed with a bar coater, and the coating thickness was unified to 10.5 μm for the front side of the undercoat and 5 μm for the back side, 14.5 μm for the front side and 10 μm for the back side.

  In addition, as a comparative example containing conventional hexavalent chromium, a similar experiment and a performance evaluation test described below were carried out on an undercoating paint and an overcoating paint using strontium chromate as an antirust pigment.

  About the coated steel sheet thus obtained, the following points regarding end face red rust resistance, electrical conductivity measurement and solution analysis after immersion in ion-exchanged water, presence or absence of coating film swelling during wet test, coating film swelling width after salt spray test The test was conducted at The test results are also shown in Table 7.

(1) End face red rust test 1
(2) End face red rust test 2
End face red rust tests 1 and 2 were both conducted and evaluated as described in Example 1.

(3) End face red rust test 3
This test investigates the elution status from the end face only.
A polyester adhesive tape (manufactured by Nitto Denko) was previously applied to both surfaces of each sample of the coated steel plate and sealed. Thereafter, 10 test pieces each having a rectangular size of 1 cm × 4 cm (the sum of the end face lengths is 10 cm) are cut out by shearing (the total end face length of the test pieces in each sample is 1 m), and the 10 test pieces are separated. The sample is immersed in 20 ml of ion exchange water in a beaker. After leaving the beaker in a constant temperature bath at 40 ° C. for 72 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: Almost no red rust is generated (the solution is transparent and hardly 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).

(3) Ion-exchange water immersion test (electric conductivity of immersion water, solution analysis)
(3-1) For each sample of coated steel plate, 100 test pieces each having a 0.5 cm × 4.5 cm strip size (the sum of end face lengths is 10 cm) are cut out by shearing (total end face length in each sample) Is 10m).
(3-2) These 100 test pieces are immersed together in 200 ml of 50 ° C. ion exchange water in a beaker placed on an ultrasonic vibration device.
(3-3) While maintaining a temperature of 50 ° C., a 40 kHz ultrasonic vibration is applied to the beaker for 30 minutes (device used: US CLEANER manufactured by ASONE).
(3-4-1) After the application of ultrasonic vibration was completed, the test piece was immediately removed, and the obtained aqueous solution (immersion water) was used to measure the conductivity with an electric conductivity meter (D-54SE manufactured by Horiba, Ltd.). Measure electrical conductivity.
(3-4-2) After the application of ultrasonic vibration is completed, the test piece is immediately removed, and the P concentration in the solution is measured by ICP-AES using the obtained aqueous solution (immersion water).

(4) Swelling of paint film after wet test A test piece having a size of 70 mm × 150 mm was cut out from each sample of the coated steel sheet by shearing, and this test piece was subjected to the same wet test conditions as described in (2) above. After being allowed to stand for a period of time, the state of occurrence of the blister on the flat surface (blister width) is measured. The evaluation criteria are as follows (○ passes):
○: There is almost no occurrence of film swelling from the flat surface (maximum swelling width is less than 2 mm),
X: Many coating film swelling of width 2mm or more generate | occur | produces from a plane part.

(5) Swelling of paint film after salt spray test (corrosion resistance)
A test piece obtained by cutting a 70 mm × 150 mm size test piece from each sample of the coated steel sheet by shearing was placed in a salt spray tester in accordance with JIS K5600 7-1 for 240 hours, and then the film swelling width from the end face was measured. Then, the corrosion resistance is evaluated. Judgment criteria are as follows (○ is acceptable):
◯: Maximum equivalent film swell width (less than +1.0 mm) compared to the reference plate,
X: Increase of the maximum paint film swelling width (1.0 mm or more) compared to the reference plate,
[Reference plate is N5 sample shown in Table 6].

  As shown in Table 7, in N10 and O5 in which the undercoat paint on the front surface or the top coat paint on the back surface contains strontium chromate, the electrical conductivity of the immersion water in the ion exchange water immersion test of the coated steel sheet is Although relatively low, it was excellent in end face corrosion resistance, the coating film swelling after the wet test was good, and the coating film swelling width in the salt spray test was also good. As described above, strontium chromate has a large passivating effect on zinc and iron, and it is presumed that the passivating effect can suppress generation of red rust on the end face and have excellent corrosion resistance.

On the other hand, according to the present invention, the electrical conductivity of immersion water in the ion exchange water immersion test of the coated steel sheet is 30 μS / cm or more A5, B5, C5, C6, C7, D5, D6, D7, E5, E6, E7, F5, G5, H5, H6, H7, I5, I6, in each sample of I 7, none of the top coating and primer coating not containing chromium compounds Nevertheless, the end face rust resistance tests 1 and 2 results Was excellent and the end face was resistant to red rust. In particular, B5, C5, C6, C7, E5, E6, E7, G5, H5, H6, in which a non-chromium compound having high water solubility such as potassium dihydrogen phosphate and dipotassium hydrogen phosphate is contained in the top coat film, In the H7 sample, the result of the film swelling from the flat surface after the wet test is good, the end face red rust resistance is also good, and depending on the conditions, a result comparable to the case of containing strontium chromate is obtained. It was. However, in the N1 and N3 samples in which this kind of compound was contained in the undercoat film, although the end face red rust resistance was relatively good, the result of the film swell on the film surface after the wet test was unsatisfactory. .

  In addition to A5, B5, C5, C6, C7, D5, D6, D7, E5, E6, E7, F5, G5, H5, H6, H7, N1, N3 samples, salt spray The swollen width of the coating film after the test was also good, and it was confirmed that even if a pigment having a high dissolution property was added, the conventional corrosion resistance test was not adversely affected. This is because if the pigment having a high elution property is an alkali metal phosphate containing P, when these pigments are eluted, some action (formation of hopite, phosphophyllite, etc.) It seems to have done.

On the other hand, when I5, I6, I7, and N4 using pigments such as sodium chloride as the highly soluble pigment, the end face red rust resistance is excellent, but the corrosion resistance evaluated in the salt spray test is low.
On the other hand, in the comparative example outside the scope of the present invention, the top coat film and the undercoat film did not contain a chromium compound, and the electrical conductivity of the immersion water in the ion exchange water immersion test was less than 30 μS / cm. In certain samples of J5, K5, K6, L5, L7, M5, N5, N6, N7, N8, and N9, red rust was likely to occur and the end face red rust resistance was poor. Further, in the N2 and N4 samples in which the undercoat coating film contains a non-chromium compound having a relatively high water solubility, the swelling of the coating film from the flat surface after the wet test is unsatisfactory as in the N1 and N3 samples. It was.

  As described in Example 1, magnesium phosphite exhibits liquid conductivity that satisfies the requirement (A) and is relatively water-soluble. However, the M5 sample containing it alone in the top coat film has low electrical conductivity in the ion exchange water immersion test of the coated steel sheet, and is water-soluble such as aluminum tripolyphosphate, calcium ion exchange silica, porous silica or titania. In the same manner as the J5, K5, K6, L5, L7, and N5 samples in which the top coating film contains a low rust-preventive pigment, the results were inferior to the end face red rust resistance.

  From this result, in order to improve the red rust resistance of the coated steel sheet, ion-exchanged water that evaluates the elution of the compound from the coated steel sheet, not the water-solubility of the rust-preventing compound contained in the top coat film itself. It can be seen that the electrical conductivity of the immersion water in the immersion test is a guide.

Example 3
After applying alkali 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), a silica system manufactured by Nippon Paint Co., Ltd. Using a chromium-free chemical conversion treatment solution (Surfcoat EC2330), chemical conversion treatment was performed on both sides as instructed by this product. The chemical conversion treatment was performed so that the adhesion amount was 4 to 8 mg / m ² in terms of Si adhesion amount.

  One or more non-chromium compounds selected from Table 1 are added to commercially available undercoats and topcoats that do not contain these, and undercoats and topcoats for the front side and undercoats for the back side Was prepared. This addition was not made to the top coat for the back side. As will be described later, the undercoat paint for the front surface is prepared by adding 21% aluminum tripolyphosphate and 9% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The paint was prepared by adding 35% aluminum tripolyphosphate and 15% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The added anticorrosive pigment was uniformly dispersed by the method described in Example 1.

  As the base paint, the front surface undercoat paint was made by Nihon Fine Coatings polyester-based paint (FLC3900 primer clear type, main resin molecular weight 10000 or more, cross-linking agent: melamine, Tg about 10 ° C with PWC (Paint with 21% aluminum tripolyphosphate and 9% titania added), backside undercoat is a polyester-based paint made by Nippon Fine Coatings (PB10P primer clear type, main resin molecular weight 10000 or more, crosslinking agent: melamine, Tg approx. Bake type polyester resin paint (SRF05, made by Nippon Fine Coatings Co., Ltd.) containing 35% tripolyaluminum triphosphate and 15% titania added by PWC at 40 ° C. Main resin molecular weight about 800 , The cross-linking agent: melamine), back top coat paint was Japan Fine Coatings Co., Ltd. of baking type polyester resin paint (NFC880 beige (PWC30%) type).

Table 8 shows the types and amounts of the non-chromium compounds added to the top coating for the front surface.
Using these paints, first the undercoat film on the back surface (PMT 170 ° C.), the second undercoat film on the front surface (PMT 220 ° C.), the third the top coat film on the back surface (PMT 170 ° C.), and finally Coating was performed in the order of the top coating film (PMT 230 ° C.) and baking temperature of the front surface using the coating materials shown in Table 8, and a sample of a coated steel sheet having the coating film composition shown in Table 9 was obtained. The coating was performed with a bar coater, and the coating thickness was unified to 10.5 μm for the front side of the undercoat and 5 μm for the back side, 14.5 μm for the front side and 10 μm for the back side.

  About the coated steel sheet obtained in this way, resistance to red rust resistance, electrical conductivity measurement after immersion in ion-exchanged water and solution analysis, presence or absence of coating film swelling during a wet test, coating film swelling width after a salt spray test, The test was carried out as outlined. The test results are also shown in Table 9.

(1) End face red rust test 1
(2) End face red rust test 2
(3) Ion-exchange water immersion test (electric conductivity of immersion water, solution analysis)
(4) Coating swelling after wetting test Each of the above tests was carried out by the same method and evaluation criteria as described in Example 1 or Example 2.

(5) Swelling of paint film after salt spray test (corrosion resistance)
Tests were performed and evaluated as described in Example 2. However, T samples listed in Table 9 were used as the reference plate.

(6) Bending Workability A 0T bending test (23 ° C.) was performed on the test piece, and the presence or absence of cracks in the 180 ° adhesive bending coating film was examined with a 10-fold magnifier and visually. The evaluation criteria are as follows, and the case of ◎ and ○ is good:
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).

(7) Pencil hardness test It measured according to the method of JIS K5600-5-4 (scratch hardness (pencil method)). The evaluation criteria are as follows, and the case of ○ mark is good:
○: F or more,
Δ: HB,
X: B or less.

(8) Coating film adhesion For each sample, immerse it in boiling water for 2 hours, take it out, put squares at 2 mm intervals in the grid (25 squares), and then use the Eriksen test machine Overhanging 7 mm, a tape was applied to the portion, and the coating film residual ratio was measured. The evaluation criteria are as follows, and a score of ○ or higher was accepted:
A: 25/25,
○: 23-24 / 25,
Δ: 15-23 / 25,
X: 15 or less / 25.

(9) Chemical resistance Each sample was evaluated for the appearance of the coating surface when immersed in a 5% HCl aqueous solution (acid) or 5% NaOH aqueous solution (base) for 24 hours in a normal temperature environment (23 ° C.). It was. Judgment criteria are as follows.
○: No blister occurred, blister less than 0.5 mm occurred,
Δ: Blister generation of 0.5 mm or more and less than 3.0 mm,
X: Blister generation of 3.0 mm or more occurred.

  In the P, Q, and R samples according to the present invention in which the electric conductivity of immersion water in the ion exchange water immersion test of the coated steel sheet is 30 μS / cm or more, both the top coat and the undercoat paint contain a chromium compound. Despite the absence, the results of end face red rust test 1 and 2 were good, and the end face red rust resistance was excellent. In this system, since a non-chromium compound was added to the top coating film on the front surface, the results of coating swelling from the flat surface after the wet test were good in all samples.

  In addition, for P and Q samples whose non-chromium compound is an alkali metal phosphate, in addition to the end face red rust resistance, the film swell width after the salt spray test is not inferior to that of the comparative sample. It was confirmed that the addition of a highly soluble pigment did not adversely affect the conventional corrosion resistance test. The reason is as described in the second embodiment.

  On the other hand, in the R sample in which the non-chromium compound having high elution property is sodium chloride, the object of the present invention of improving the end face red rust resistance can be achieved, but the corrosion resistance evaluated by the salt spray test is lowered.

  On the other hand, in the S and T samples of the comparative examples, the top coat film and the undercoat film do not contain a chromium compound, and the electrical conductivity of the immersion water in the ion exchange water immersion test is less than 30 μS / cm. Red rust was likely to occur and the end face red rust resistance was unsatisfactory.

  In addition, even if a non-chromium compound is added to the top coating on the front surface where various performances are required, unlike the back surface, all the P to T samples have processability, hardness, adhesion, and chemical resistance. It was confirmed that it was sufficiently satisfied.

  From this result, in order to improve the red rust resistance of the coated steel sheet, ion-exchanged water that evaluates the elution of the compound from the coated steel sheet, not the water-solubility of the rust-preventing compound contained in the top coat film itself. It can be seen that the electrical conductivity of the immersion water in the immersion test is a guide.

Example 4
After various alkali-degreasing and water washing were performed on various Zn-based plated steel sheets shown in Table 10 of the coated substrate, this product was prepared using a silica-based chromium-free chemical conversion treatment liquid (Surfcoat European Community 2330) manufactured by Nippon Paint Co., Ltd. Chemical conversion treatment was applied to both sides as instructed. 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.

  A non-chromium compound selected from Table 1 was added to a commercially available undercoat and topcoat that did not contain them to prepare a frontcoat undercoat and a backcoat undercoat and topcoat. This addition was not made to the top coating for the front side. As will be described later, the undercoat paint for the front surface is prepared by adding 21% aluminum tripolyphosphate and 9% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The paint was prepared by adding 35% aluminum tripolyphosphate and 15% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The added anticorrosive pigment was uniformly dispersed by the method described in Example 1.

  The base paint used is a front surface undercoat polyester-based paint manufactured by Nippon Fine Coatings (clear type FLC3900 primer, main resin molecular weight 10000 or more, crosslinker: melamine, main resin Tg at about 10 ° C with PWC (Paint containing 21% aluminum tripolyphosphate and 9% titania), and backside undercoat is a polyester-based paint made by Nihon Fine Coatings (PB10P primer clear type, main resin molecular weight 10000 or more, crosslinker: melamine, main resin Tg approx. 40 ° C. with PWC and 35% tripolyaluminum phosphate and 15% titania added), and the top surface paint is titania as a white pigment. SRF05, main resin molecular weight about 8 00, the cross-linking agent: melamine), back top coat paint was Japan Fine Coatings Co., Ltd. of baking type polyester resin paint (NFC880 beige (PWC30%) type).

Table 11 shows the types and addition amounts of the non-chromium compounds added to the top coating for the back surface.
Using these paints, first, the undercoat film on the front surface (PMT 170 ° C.), the second undercoat film on the back surface (PMT 220 ° C.), and the third, the top coat film on the front surface (PMT 170 ° C.) Finally, coating was carried out at the order of the top coating film (PMT 230 ° C.) and baking temperature of the back surface using the coating materials shown in Table 12, and a sample of a coated steel sheet having the coating film composition shown in Table 13 was obtained. The coating was performed with a bar coater, and the coating thickness was unified to 10.5 μm for the front side of the undercoat and 5 μm for the back side, 14.5 μm for the front side and 10 μm for the back side.

  About the coated steel sheet obtained in this way, resistance to red rust resistance, electrical conductivity measurement after immersion in ion-exchanged water and solution analysis, presence or absence of coating film swelling during a wet test, coating film swelling width after a salt spray test, The test was carried out as outlined. The test results are also shown in Table 12.

(1) End face red rust test 1
(2) Ion exchange water immersion test (electric conductivity of immersion water, solution analysis)
(3) Coating swelling after wetting test Each of the above tests was carried out by the same method and evaluation criteria as described in Example 1 or Example 2.

(4) Swelling of coating film after salt spray test (corrosion resistance)
Tests were performed and evaluated as described in Example 2. However, the J sample in Table 12 was used as the reference plate.

  As can be seen from Table 12, in the sample according to the present invention in which the electric conductivity of the immersion water in the ion exchange water immersion test of the coated steel sheet is 30 μS / cm or more, both the top coat and the undercoat paint contain a chromium compound. In spite of not having done, and regardless of the type of the plating base material, the end face red rust test result was good and the end face red rust resistance was excellent. In all the samples, since a non-chromium compound having a relatively high solubility was added to the back coating on the back surface, the result of the film swelling from the flat surface after the wet test was also good.

  On the other hand, in the comparative sample in which the top coat film and the undercoat film do not contain a chromium compound and the electrical conductivity of the immersion water in the ion exchange water immersion test is less than 30 μS / cm, any plating substrate However, red rust was easily generated, and the red rust resistance of the end face was poor.

(Example 5)
After applying alkali 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), a silica system manufactured by Nippon Paint Co., Ltd. Using a chromium-free chemical conversion treatment solution (Surfcoat EC2330), chemical conversion treatment was performed on both sides as instructed by this product. The chemical conversion treatment was performed so that the adhesion amount was 4 to 8 mg / m ² in terms of Si adhesion amount.

  A non-chromium compound selected from Table 1 is added to a commercially available undercoat and topcoat that does not contain these compounds, so that an undercoat for the front surface, an undercoat for the back surface, and an intermediate paint for the front surface A top coat for the front side and a top coat for the back side were prepared. In this example, the front surface was a three-layer coating film of top coat, intermediate coat and undercoat. As will be described later, the undercoat paint for the front surface is prepared by adding 21% aluminum tripolyphosphate and 9% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The paint was prepared by adding 35% aluminum tripolyphosphate and 15% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The added anticorrosive pigment was uniformly dispersed by the method described in Example 1.

  The base paint used is a polyester-based paint made by Nippon Fine Coatings Co., Ltd. (clear type of FLC3900 primer, main resin molecular weight 10000 or more, cross-linking agent: melamine, Tg approx. (Paint with 21% aluminum oxide and 9% titania), and backside undercoat paint is a polyester paint manufactured by Nippon Fine Coatings Co., Ltd. (PB10P primer clear type, main resin molecular weight 10000 or more, crosslinking agent: melamine, Tg approx. (Coating with 35% tripolyaluminum phosphate and 15% titania added by PWC at 40 ° C), the intermediate coating on the front surface is a baked polyester resin paint (NFC880 beige color (PWC 30%) by Nippon Fine Coatings) Type), top coating of front side Is a baking type polyester resin paint (SRF05, main resin molecular weight of about 8000, crosslinker: melamine) manufactured by Nippon Fine Coatings, which contains titania as a white pigment, and the back coating is a baking type polyester manufactured by Nippon Fine Coatings. It was a resin paint (NFC880 beige color (PWC 30%) type).

  The paint used to form the top coat on the front surface and the top coat on the back (paint symbol B, J, Y), and the paint used to form the top coat on the front surface (paint symbol P , Z) Table 13 shows the types and amounts of the non-chromium compound added and the base paint.

  Using these paints, first the undercoat film on the back surface (PMT 170 ° C.), the second undercoat film on the front surface (PMT 220 ° C.), and then the intermediate coat film on the front surface (PMT 170 ° C.), The back coating film (PMT 170 ° C.) and finally the top coating film (PMT 230 ° C.) were applied to obtain a sample of a coated steel sheet having the coating film composition shown in Table 14.

  The coating is performed with a bar coater. The thickness of the undercoat is 10.5 μm for the front side, 5 μm for the back side, 10 μm for the intermediate coat on the front side, and 14.5 μm for the top coat side. The back side was unified to 10 μm.

  About the coated steel sheet obtained in this way, resistance to red rust resistance, electrical conductivity measurement after immersion in ion-exchanged water and solution analysis, presence or absence of coating film swelling during a wet test, coating film swelling width after a salt spray test, The test was carried out as outlined. The test results are also shown in Table 14.

(1) End face red rust test 1
(2) Ion exchange water immersion test (electric conductivity of immersion water, solution analysis)
(3) Coating swelling after wetting test Each of the above tests was carried out by the same method and evaluation criteria as described in Example 1 or Example 2.

(4) Swelling of coating film after salt spray test (corrosion resistance)
Tests were performed and evaluated as described in Example 2. However, the YYZ sample in Table 14 was used as the reference plate.

  As can be seen from Table 14, in the sample according to the present invention in which the electric conductivity of the immersion water in the ion-exchange water immersion test of the coated steel sheet is 30 μS / cm or more, any of the top coating, intermediate coating, and undercoating coating is used. Despite containing no chromium compound, the results of the end face red rust test were good and the end face red rust resistance was excellent. In addition, for the sample in which the non-chromium compound was added only to the intermediate coating on the front surface, the result of the end face red rust test was good, and the result of the film swelling from the plane after the wet test was also good. .

  On the other hand, in the sample of the comparative example in which the top coat film and the undercoat film do not contain a chromium compound and the electrical conductivity of the immersion water in the ion exchange water immersion test is less than 30 μS / cm, the occurrence of red rust occurs. It was easy to occur and the end face red rust resistance was unsatisfactory.

  From this result, in order to improve the end face red rust resistance of the coated steel sheet, by adding a non-chromium compound with high elution property to the upper layer coating film than the undercoat film, the conventional performance is maintained and the end surface red rust resistance is maintained. It can be seen that it is possible to improve.

(Example 6)
As shown in Table 15, hot-dip galvanized steel sheets with different coating thicknesses and zinc adhesion amounts per side (Table 1) were used as the coating base, and this coating base was subjected to alkaline degreasing and water washing. Using a silica-based chromium-free chemical conversion treatment solution (Surfcoat EC2330), chemical conversion treatment was performed on both sides as instructed by this product. The chemical conversion treatment was performed so that the adhesion amount was 4 to 8 mg / m ² in terms of Si adhesion amount.

  A non-chromium compound selected from Table 1 was added to a commercially available undercoat and topcoat that did not contain them to prepare a frontcoat undercoat and a backcoat undercoat and topcoat. This addition was not made to the top coating for the front side. As will be described later, the undercoat paint for the front surface is prepared by adding 21% aluminum tripolyphosphate and 9% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The paint was prepared by adding 35% aluminum tripolyphosphate and 15% titania listed in Table 1 as a rust preventive pigment to a commercially available undercoat paint. The added non-chromium compound was uniformly dispersed by the method described in Example 1.

  The base paint used is a front surface undercoat polyester-based paint manufactured by Nippon Fine Coatings (clear type FLC3900 primer, main resin molecular weight 10000 or more, crosslinker: melamine, main resin Tg at about 10 ° C with PWC (Paint containing 21% aluminum tripolyphosphate and 9% titania), and backside undercoat is a polyester-based paint made by Nihon Fine Coatings (PB10P primer clear type, main resin molecular weight 10000 or more, crosslinker: melamine, main resin Tg approx. 40 ° C. with PWC and 35% tripolyaluminum phosphate and 15% titania added), and the top surface paint is titania as a white pigment. SRF05, main resin molecular weight about 8 00, the cross-linking agent: melamine), back top coat paint was Japan Fine Coatings Co., Ltd. of baking type polyester resin paint (NFC880 beige (PWC30%) type).

Table 16 shows the types and addition amounts of the non-chromium compounds added to the top coating for the back surface.
Using these paints, first, the undercoat film on the front surface (PMT 170 ° C.), the second undercoat film on the back surface (PMT 220 ° C.), and the third, the top coat film on the front surface (PMT 170 ° C.) Finally, coating was carried out in the order of the top coating film (PMT 230 ° C.) and baking temperature of the back surface using the coating materials shown in Table 16, and a sample of the coated steel sheet having the coating film composition shown in Table 17 was obtained. The coating was performed with a bar coater, and the coating thickness was unified to 10.5 μm for the front side of the undercoat and 5 μm for the back side, 14.5 μm for the front side and 10 μm for the back side.

About the coated steel plate obtained in this way, the end face red rust property test was implemented in the following way. The test results are also shown in Table 17.
(1) End face red rust test 4
For each sample of the coated steel sheet, 10 test pieces each having a rectangular size of 1 cm × 4 cm (the sum of the end face lengths is 10 cm) are cut out by shearing (total end face length of the test pieces in each sample is 1 m), 10 pieces The test pieces are separately immersed in 20 ml of ion exchange water in a beaker or in a solution in which sodium chloride is added to the ion exchange water and the electric conductivity of the solution is adjusted to 100 μS / cm. After leaving the beaker in a constant temperature bath at 40 ° C. for 120 hours, the test piece is taken out and the occurrence of red rust and white rust from the end face of each test piece is measured. The evaluation criteria are as follows.

○: No red rust or white rust △: Only white rust occurs ×: Red rust occurs (white rust occurs)

  As can be seen from Table 17, in systems where an alkali metal phosphate compound (potassium dihydrogen phosphate) is contained in the top coat of the coated steel sheet, both ion-exchanged water and aqueous sodium chloride solution can be applied outdoors. It was confirmed that it had excellent end face red rust resistance in both the range of the thickness of the hot dip plated steel sheet and the amount of coating adhesion.

  On the other hand, for systems that do not contain the above-mentioned alkali metal phosphate compound, which has high elution, in the top coat of the coated steel sheet, in the ion-exchanged water immersion test, melting that is estimated to be applicable in outdoor applications It was confirmed that the end face red rust resistance was unsatisfactory within the range of the thickness of the plated steel sheet and the amount of coating.

  In the 100 μS / cm sodium chloride aqueous solution immersion, the occurrence of only white rust was confirmed in G1N, G2N with thin plate thickness, and G2N, G7N, and G9N with a large zinc basis weight. This is because the abundance ratio of the zinc end face to the end face of the coated steel sheet is higher than that of other plated steel sheets, and the electric conductivity is higher than that of ion-exchanged water, so that the sacrificial anticorrosive function of zinc is likely to work It is estimated that

(Example 7)
After applying alkali degreasing and water washing to a hot-dip galvanized steel sheet with a coating thickness of 0.8 mm (attachment amount: 45 g / m ² per side, dimensions: 300 × 250 mm), Nippon Paint Co., Ltd. silica-based chromium Using a free chemical conversion treatment solution (Surfcoat EC2330), chemical conversion treatment was performed on both sides as instructed by this product. 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.

  A non-chromium compound selected from Table 1 was added to a commercially available undercoat and topcoat that did not contain them to prepare a frontcoat undercoat and a backcoat undercoat and topcoat. This addition was not made to the top coating for the front side.

  The base coating used is a polyester base coating made by Nippon Fine Coatings Co., Ltd. (Clear type of FLC3900 primer, main resin molecular weight of 10,000 or more, crosslinking agent: melamine, main resin Tg of about 10 ° C.), back surface Undercoat paint made by Nippon Fine Coatings Co., Ltd. (PB10P primer clear type, main resin molecular weight 10000 or more, cross-linking agent: melamine, main resin Tg about 40 ° C.), front top coat is titania as white pigment Baked polyester resin paint (SRF05, main resin molecular weight of about 8000, cross-linking agent: melamine) made by Nippon Fine Coatings Co., Ltd., and baked polyester resin paint (NSC880 clear paint made by Nippon Fine Coatings Co., Ltd.) )Met.

  The undercoat paint for the front surface was prepared by adding 21% aluminum tripolyphosphate and 9% titania listed in Table 1 as anticorrosive pigments to the above base paint (clear type of FLC3900 primer). The undercoat paint for the back surface was prepared by adding 35% aluminum tripolyphosphate and 15% titania listed in Table 1 of anticorrosive pigments to the above base paint (clear type of PB10P primer). For these undercoat paints, the added rust preventive pigment was uniformly dispersed by the method described in Example 1.

  In the top coating for the back surface, the PWC is adjusted by adding the indicated amount of the color pigment described in Table 18 to the above base coating (NSC880 clear coating) in some cases, and the non-chromium compound described in Table 19 is further added. After adding in the indicated amount, a container containing 20 g of glass beads with respect to 100 g of paint weight was stirred by a hybrid mixer to uniformly disperse the compound and pigment. Table 19 shows the stirring time of the pigment at this time.

  Using these paints, first, the undercoat film on the front surface (PMT 170 ° C.), the second undercoat film on the back surface (PMT 220 ° C.), and the third, the top coat film on the front surface (PMT 170 ° C.) Finally, coating was performed in the order of the top coat film (PMT 230 ° C.) and the baking temperature using the coating materials described in Table 19 (also described in Table 20) to obtain samples of coated steel sheets. The coating is carried out with a bar coater, and the coating thickness is unified to 10.5 μm for the front side of the undercoat and 5 μm for the back side, and 14.5 μm for the top side of the top coat. The thickness was changed to 5 μm, 10 μm, and 15 μm (only 10 μm for some paints).

  About the coated steel sheet thus obtained, end face red rust resistance, electrical conductivity measurement and solution analysis after immersion in ion-exchanged water, pigment particle size measurement of the back coating film on the back surface, presence or absence of film swelling during the wet test, salt spray test The test was carried out in the following manner for the subsequent swollen width of the coating film. The test results are shown in Table 20 together with the type and amount of the non-chromium compound in the back coating used.

(1) End face red rust test (with seal)
Tests and evaluations were performed in the same manner as the end face red rust test 3 described in Example 2. That is, the test piece was prepared by pasting a polyester tape on both surfaces of each sample of the coated steel sheet and then cutting.

(2) End face red rust test (no seal)
Tests and evaluations were performed in the same manner as the end face red rust test 1 described in Example 1.

(3) Ion-exchange water immersion test 2 (with seal, electrical conductivity of immersion water)
(3-1) A polyester tape (manufactured by Nitto Denko) was previously applied to both surfaces of each sample of the coated steel plate and sealed. Thereafter, 100 test pieces each having a 0.5 cm × 4.5 cm strip size (the sum of end face lengths is 10 cm) are cut out by shearing (the total end face length in each sample is 10 m).
(3-2) These 100 test pieces are immersed together in 200 ml of 50 ° C. ion exchange water (4 μS / cm or less) in a beaker placed on an ultrasonic vibration device.
(3-3) While maintaining a temperature of 50 ° C., a 40 kHz ultrasonic vibration is applied to the beaker for 30 minutes (device used: US CLEANER manufactured by ASONE).
(3-4) After the application of ultrasonic vibration is completed, the test piece is immediately removed, and the resulting aqueous solution (immersion water) is used to conduct electricity with an electric conductivity meter (D-54SE manufactured by Horiba, Ltd.). Measure the degree.

(4) Swelling of coating film after wet test The same method and evaluation criteria as described in Example 1 were used.
(5) Measurement of particle size of non-chromium compound in back coating film on top surface The coating of the non-chromium compound after stirring using a grindometer (crushed gauge) manufactured by TP Giken Co., Ltd. The particle size was measured. The grindometer groove depth used was 0-100 μm (minimum scale 10 μm), and the particle size of the non-chromium compound in the paint was determined.

(6) Swelling of coating film after salt spray test (corrosion resistance)
Tests were performed as described in Example 2 to evaluate corrosion resistance. However, as the reference plate, a sample having a film thickness of 10 μm using 46 paint of Table 20 was used.

  As can be seen from the results shown in Table 20, in the results of carrying out the end face red rust test without a seal, in a system containing an alkali metal phosphate having a relatively high dissolution property as a non-chromium compound, PWC, film thickness, The end face red rust resistance was good regardless of the added amount and the content particle size, while the system containing other non-chromium compounds had low end face red rust resistance.

  When the front and back surfaces of the coated steel sheet are sealed so that the non-chromium compound is eluted only from the end face, and the end face red rust property test is performed, even in a system containing an alkali metal phosphate that was good in the non-seal test, It was confirmed that end face red rust resistance may be insufficient depending on the combination of pigment concentration, addition amount, and film thickness. For example, the pigment concentration in the coating film is low (eg, less than 20%), the addition amount of alkali metal phosphate is small (eg, less than 5%), and the coating thickness is thin (eg, 5 μm). This was the case when at least one of the conditions was met.

  As a result of measuring the electrical conductivity of immersion water in order to evaluate the elution amount of non-chromium compounds from only the end surface of the coated steel plate by the immersion test with the front and back surfaces of the coated steel plate sealed, It was confirmed that the effect of improving the end face red rust resistance was confirmed when the value was 10 μS / cm or more, which was a very small value, and generally good when the value was 20 μS / cm or more.

The film swelling was not affected because a non-chromium compound was added to the top coating film, and no blistering occurred in any coated steel sheet.
As for the corrosion resistance, when the particle size was made fine, an improvement effect was confirmed although it was slightly. Similarly, it has been confirmed that the corrosion resistance is improved when a conventional chromium-free rust preventive pigment is added. It was confirmed that sufficient corrosion resistance can be obtained even when a non-chromium compound having a relatively high water solubility is used.

(Example 8)
As shown in Table 21, a Teflon (registered trademark) plate (thickness: 2 mm) was coated with a coating containing a non-chromium compound in a base coating with a bar coater to a thickness of 10 μm and baked at PMT 230 ° C. A coating film was prepared.

  The base paint used was a baking type polyester resin paint (NFC880 beige (PWC 50%) type: main resin molecular weight of about 4000, melamine crosslinking agent) manufactured by Nippon Fine Coatings. After adding a non-chromium compound in the indicated amount to this base paint, a container containing 20 g of glass beads with respect to 100 g of the paint mass is stirred for 40 minutes with a hybrid mixer to uniformly disperse the pigment. Was made.

The coating film having a thickness of 10 μm thus obtained was peeled from the above plate, and the electrical conductivity after immersion in ion-exchanged water was measured as follows. The test results are also shown in Table 21.
(1) Ion exchange water immersion test (electric conductivity of immersion water)
(1-1) With respect to each sample of the coating film, 100 pieces of test pieces each having a 0.5 cm × 4.5 cm strip size (the sum of end face lengths is 10 cm) are cut out with a cutter knife (test in each sample) The total end face length of the piece is 10 m).
(1-2) These 100 test pieces are immersed together in 200 ml of 50 ° C. ion exchange water in a beaker placed in an ultrasonic vibration device.
(1-3) While maintaining a temperature of 50 ° C., a 40 kHz ultrasonic vibration is applied to the beaker for 30 minutes (Usage device: US CLEANER manufactured by ASONE).
(1-4) After the application of ultrasonic vibration is completed, the test piece is immediately removed, and the resulting aqueous solution (immersion water) is used to conduct electricity with an electric conductivity meter (D-54SE manufactured by Horiba, Ltd.). Measure the degree.

  As can be seen from the results shown in Table 21, in the system containing highly soluble alkali metal phosphate or sodium chloride, which is a chloride, the electrical conductivity is very high, similar to the test results on the coated steel sheet. On the other hand, in the system containing other rust preventive pigments, the electric conductivity was low as in the case of the coated steel sheet. Therefore, it was found that it is possible to evaluate whether or not the coating film is a coating material suitable for manufacturing a coated steel plate having excellent resistance to red rust on end faces even in a coating film immersion test.

Claims (14)

A chromium-free painted steel sheet having two or more coating films on both sides of a coated base material made of a plated steel sheet having a Zn-containing plated layer, and the coating film of the outermost layer on at least one surface of the coated base material is A content of at least one non-chromium compound satisfying the requirements of A) and (B), the outermost layer coating film having a thickness of 5 to 30 μm, and the innermost layer coating film content of the non-chromium compound Is 100 mass%, 100 samples of this coated steel plate cut into a rectangle of 0.5 cm × 4.5 cm are placed in 200 ml of 50 ° C. ion exchange water (electric conductivity: 4 μS / cm or less) with a frequency of 40 kHz. A chromium-free coated steel sheet characterized by having an electric conductivity of immersion water of 30 μS / cm or more when immersed for 30 minutes under application of ultrasonic vibration.
(A) 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,
(B) No thermal decomposition occurs up to 200 ° C. A chromium-free coated steel sheet having two or more coating films on each side of a coated base material comprising a plated steel sheet having a Zn-containing plated layer, wherein at least one layer other than the innermost layer is formed on at least one side of the coated base material. The coating film contains at least one non-chromium compound that satisfies the following requirements (A) and (B), the coating thickness of the outermost layer is 5 to 30 μm, and the non-chromium of the innermost coating layer The content of the compound is 1% by mass or less, and 100 samples cut into a rectangle of 0.5 cm × 4.5 cm after covering the front and back surfaces of the coated steel sheet with a polyester film were subjected to ion exchange water at 50 ° C. ( (Electric conductivity: 4 μS / cm or less) Chromium-free coated steel characterized in that the electric conductivity of immersion water is 10 μS / cm or more when immersed in 200 ml for 30 minutes under application of ultrasonic vibration at a frequency of 40 kHz. .
(A) 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,
(B) No thermal decomposition occurs up to 200 ° C.   The chromium-free painted steel sheet according to claim 1 or 2, wherein the P concentration of the immersion water is 0.5 µg / ml or more. A chromium-free coated steel sheet having two or more coating films on both surfaces of a coated base material made of a plated steel sheet having a Zn-containing plating layer, and the coating film of the outermost layer on at least one surface has the following (A) and ( at least one non-chromium compound meets the requirements of B) in an amount of 0.5 to 30 wt%, the film thickness of the coating film of the outermost layer is Ri der less 5 to 30 [mu] m, the innermost layer of the coating chrome-free coated steel sheet that content is characterized der Rukoto most 1 mass% of the non-chromium compound.
(A) 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,
(B) No thermal decomposition occurs up to 200 ° C. A chromium-free coated steel sheet having two or more coating films on both surfaces of a coated base material comprising a plated steel sheet having a Zn-containing plating layer, and at least one layer of the coating film other than the innermost layer is at least on one side, It contains at least one non-chromium compound satisfying the requirements of (A) and (B) in an amount of 5 to 30% by mass, and the film thickness of at least one layer other than the innermost layer is 5 μm or more and 50 μm or less. , and the and chromium-free coated steel sheet thickness of the outermost layer of the coating film is Ri 5~30μm der, the content of the chromium-free compound of the innermost layer of the coating, characterized in der Rukoto than 1 mass% .
(A) 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,
(B) No thermal decomposition occurs up to 200 ° C.   The non-chromium compound is one or two or more selected from the group consisting of alkali metal phosphates and chlorides and alkaline earth metal hypophosphites. Chrome-free painted steel sheet as described.   The alkali metal phosphate is sodium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate, potassium dihydrogen phosphate The chromium-free painted steel sheet according to claim 6, which is at least one selected alkali metal phosphate.   The chromium-free painted steel plate according to any one of claims 1 to 7, wherein the one surface is a back surface as a coated steel plate.   The coated steel sheet according to any one of claims 1 to 8, wherein the coated steel sheet has a painted base treatment film that does not contain chromium between the coated substrate and the two or more coating films.   The coated steel sheet according to any one of claims 1 to 9, wherein the innermost coating film of the two or more coating films does not contain a non-chromium compound that satisfies the requirements (A) and (B).   The coated steel sheet according to any one of claims 1 to 10, wherein a thickness of the coated substrate is 0.5 mm or more and 2.0 mm or less. The coated steel sheet according to any one of claims 1 to 11, wherein a zinc-based plating adhesion amount of the coating substrate is 100 g / m ² or less in terms of one-side average adhesion amount.   The coated steel sheet according to claim 4, wherein at least one coating film other than the innermost layer contains 20 to 50 mass% of a coloring pigment in addition to the non-chromium compound.   The housing | casing obtained by the shaping | molding process of the chromium free coating steel plate in any one of Claims 1-13.

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