WO2018186474A1

WO2018186474A1 - Precoated steel sheet having good scratch resistance and moisture absorption resistance in a coating film and excellent end-surface rust corrosion resistance

Info

Publication number: WO2018186474A1
Application number: PCT/JP2018/014622
Authority: WO
Inventors: 古川　博康; 保明河村
Original assignee: 新日鐵住金株式会社
Priority date: 2017-04-05
Filing date: 2018-04-05
Publication date: 2018-10-11
Also published as: SG10201914074YA; SG11201908797VA; KR20190125377A; JP6760489B2; CN110520290A; JPWO2018186474A1

Abstract

A chromate-free-type precoated steel sheet having excellent end-surface rust corrosion resistance and good scratch resistance and moisture absorption resistance in a coating film, the precoated steel sheet characterized by having on both surfaces of a galvanized steel sheet a chromate-free-type chemical conversion treatment layer and a plurality of coating films including at least a top-layer coating film and an underlayer coating film adjacent thereto, the coating films being formed on the chromate-free-type chemical conversion treatment layer, the top-layer coating film on one surface satisfying the conditions of (1) the contact angle of a coating film surface with respect to water being 65-75°, (2) the thickness of the coating film being 0.5-5 µm, (3) the coating film including a first pigment, and the pigment content being 15-70% by mass with respect to the dry mass of the coating film, (4) the total amount of pigment having an average primary particle diameter of 1 µm or greater among the first pigment being 5% by mass or more with respect to the dray mass of the coating film, and (5) the coating film containing 1-10% by mass of a particulate wax having an average particle diameter of 1-8 µm with respect to the dry mass of the coating film, at least a portion of the particulate wax particles being in a state of partially protruding from the coating film surface, and the underlayer coating film containing a second pigment that is one or more species of deliquescent rust preventive pigment.

Description

Pre-coated steel sheet with excellent scratch resistance and moisture absorption resistance and excellent red rust corrosion resistance

The present invention relates to a pre-coated steel sheet that is excellent in red rust corrosion resistance on the end face and has good scratch resistance and moisture absorption resistance of the coating film. More specifically, the present invention is a chromate-free pre-coated steel sheet used in an environment where it gets wet with water, particularly in an outdoor environment exposed to rainwater, for example, in the field of electrical products, architecture, etc. The present invention relates to a chromate-free pre-coated steel sheet having excellent scratch resistance as well as excellent resistance to scratches.

When the pre-coated steel sheet installed outdoors is exposed to high temperature and heavy rain environment such as rainy season, there is a problem that red rust is generated in a short time from a cut end face without a coating film. In order to solve this problem, a pre-coated steel sheet having end face red rust corrosion resistance has been proposed.

As an example, Patent Document 1 describes an end face red rust-suppressing precoated steel sheet in which a rust preventive pigment exhibiting a red rust suppressing effect is added to a coating film on at least one side of a plated steel sheet. When the end surface of the precoated steel sheet is immersed in water, the rust preventive pigment is eluted from the coating film, and the component protects the end surface, thereby suppressing the occurrence of red rust. Sodium tungstate is given as an example of a practical application that is highly effective as a rust preventive pigment. In addition to this, various rust preventive pigments exhibiting a red rust inhibitory effect have been found. New pre-coated steel sheets have been proposed. Similarly, in Patent Document 2 describing a chromium-free coated steel sheet having excellent red rust corrosion resistance, alkali metal phosphates and chlorides and alkaline earth metal hypophosphites have a red rust inhibitory effect. It is described that it can be used as a rust preventive pigment.

In the pre-coated steel sheets described in Patent Documents 1 and 2, since it is premised that all of the rust preventive pigments are eluted from the coating film due to water wetting, these rust preventive pigments are highly water-soluble and highly deliquescent. They are common in that they are substances having The coating film containing such a deliquescent rust preventive pigment easily absorbs moisture in a high humidity environment, and the problem that the surface of the coating film becomes wet has been revealed. For example, when using a pre-coated steel sheet as a housing for equipment, if the surface of the coating film becomes wet, sticking a caution sticker or foaming material for improving airtightness to the painted surface with an adhesive There was a problem that the adhesive could not adhere.

Patent Document 3 describes a pre-coated steel sheet in which a coating layer (hereinafter referred to as a “water-permeable coating”) that easily transmits water is coated on one surface of a plated steel sheet. This is because water easily penetrates into the film and reaches the steel plate surface of the original plate, so that a couple current is positively generated between the ground metal and the plating on the cut end surface of the steel plate, and the sacrificial anticorrosive effect of zinc makes the end surface It is intended to suppress the occurrence of red rust from. As a water-permeable film, a predetermined amount of pigment is contained in a relatively thin film, and at least a part of the film is a pigment having a relatively large particle diameter, so that moisture from the outside can pass through the coating film to form zinc. A coating designed to easily reach the plating layer has been proposed.

In the pre-coated steel sheet described in Patent Document 3, there is no problem that the film tends to absorb moisture, as in the films of Patent Documents 1 and 2, but only the sacrificial anticorrosive effect of zinc is insufficient for the effect of suppressing red rust on the end face, Moreover, generation of white rust from the steel sheet surface due to dissolution of zinc cannot be avoided.

The present inventors have developed a precoated steel sheet described in Patent Document 4 as a precoated steel sheet that has solved the problems in the techniques of Patent Documents 1 to 3. This is because the surface of the coating film containing anti-rust pigment effective for preventing red rust on the end face allows liquid water to pass through in order to achieve the effect of the anti-rust pigment. It is a pre-coated steel sheet that is covered with an uppermost layer coating film (water permeable / water vapor impermeable coating film) that does not allow gas water (water vapor) that causes it to permeate.

A pre-coated steel sheet that is shipped after a coating film is formed in advance and is formed and processed without a coating process is generally required to have good workability (bending workability, press workability, etc.). As a technique to meet such a demand, it is known to add a wax to a coating film. For example, Patent Document 4 discloses a water-permeable / water-vapor-impermeable coating provided on a coating film containing an antirust pigment. It is described that the film contains a component such as wax.
In Patent Document 5, wax is added to a hydrophilized coating film, and the ratio of the projected area of the wax onto the coating film surface is controlled to exhibit excellent moldability by the wax, and tetraalkoxysilane. It has been proposed to exhibit excellent raindrop resistance improvement by the partial hydrolysis condensate. In the coated steel sheet described in Patent Document 5, the end surface corrosion resistance of the steel sheet is not considered.

International Publication No. 2011/071175 JP 2009-045923 A JP 2010-083075 A Japanese Unexamined Patent Publication No. 2016-193593 JP 2013-018263 A

In order to improve the workability of the pre-coated steel sheet described in Patent Document 4 developed by the inventors of the present application, water permeability and water vapor impermeability on a coating film containing a rust preventive pigment are improved. The coating film contained a general low melting point type wax. However, it has been found that the red rust corrosion resistance of the end face of the precoated steel sheet is lost when the content of such low melting point type wax is increased so as to be close to a normal amount so that the effect can be obtained. That is, it has been found that the amount of the wax contained in the water-permeable / water-vapor-impermeable coating film of the pre-coated steel sheet of Patent Document 4 that has been developed needs to be limited.

On the other hand, it was found that when the amount of the wax contained in the water-permeable / water-vapor-impermeable coating film of the precoated steel sheet of Patent Document 4 is limited, sufficient scratch resistance cannot be obtained. Specifically, for example, when the coating on the design side of the pre-coated steel sheet (the side on which the coating does not contain antirust pigments for developing red rust corrosion resistance) is thin, it is stacked for shipment to the user or coiled In the pre-coated steel sheet, phenomena such as peeling of the coating film on the design side from the base steel sheet due to sliding during transportation, and galling of the coating film during forming by the user were observed.

Thus, the pre-coated steel sheet of Patent Document 4 not only has excellent moisture absorption to prevent wetting of the coating film surface containing a rust preventive pigment for the development of end face red rust corrosion resistance, but also the scratch resistance of the coating film, In particular, it has been revealed that there are problems that the front and back coating film must have scratch resistance for sliding during transportation and that the coating film must have excellent scratch resistance during molding.

The present invention aims to solve this problem.

The inventors of the present invention have a water permeability on a coating film (hereinafter referred to as “underlayer coating film”) containing a rust prevention pigment (hereinafter sometimes simply referred to as “rust prevention pigment”) for developing red rust corrosion resistance on the end face.・ The reason why the effect of antirust pigment on the end face is affected by the addition of wax to the water vapor-impermeable top layer coating (hereinafter sometimes referred to simply as “top layer coating”) is as follows. Considered.

If a general wax having a low melting point is contained in the uppermost layer coating film, the wax melts during heating and drying to form the uppermost layer coating film. Since the wax is hydrophobic and has a low surface tension, the melted wax wets and spreads on the surface of the coating film or enters the water permeation path in the coating film to block it. Therefore, when the precoated steel sheet gets wet, water cannot permeate the uppermost coating film, and the elution of the anticorrosive pigment in the lower coating film is hindered, thereby inhibiting the expression of end face red rust corrosion resistance.

Based on this consideration, the inventors continued the study by using a wax having a form that is granular and protrudes from the coating film and is scattered in an island shape after the coating film is formed, rather than a wax type that melts by heating. The present inventors have found that it is possible to improve the slidability in order to ensure the corrosion resistance of the end face and at the same time ensure the scratch resistance. In addition to this, the wax used is granular with an average particle size of 1 to 8 μm, and the content is 1 to 10% by mass with respect to the dry coating film mass, and a part of the wax protrudes from the coating film surface. It was found that is suitable. The present invention has been completed based on these findings.

The pre-coated steel sheet of the present invention thus completed corresponds to a coating film thinner than the water-permeable / water-vapor-impermeable coating film of the pre-coated steel sheet of Patent Document 4 and the above-mentioned granular wax added, and specifically, The summary is as follows.

[1] A chromate-free pre-coated steel sheet having a chromate-free chemical conversion treatment layer on both surfaces of a zinc-based plated steel sheet, and a plurality of coating films including at least the uppermost coating film and the lower coating film adjacent thereto. Because
The condition of the top layer coating on one side is as follows:
(1) The contact angle with water on the surface of the uppermost layer coating film is 65 to 75 °,
(2) The film thickness of the uppermost coating film is 0.5-5 μm,
(3) The uppermost layer coating film contains the first pigment, and the content of the pigment is 15 to 70% by mass with respect to the dry coating film mass;
(4) Among the first pigments, the total amount of those having an average primary particle size of 1 μm or more is 5% by mass or more based on the dry coating mass, and (5) the uppermost coating layer has an average particle size of 1 to 8 μm. The granular wax is contained in a content of 1 to 10% by mass with respect to the dry coating film mass, and at least a part of the granular wax particles is present in a state of partially protruding from the coating film surface.
And the lower layer coating adjacent to the uppermost layer coating contains a second pigment which is one or more deliquescent rust preventive pigments effective for preventing red rust on the end face. A chromate-free pre-coated steel sheet.
[2] The chromate-free precoated steel sheet according to [1], wherein the wax is selected from the group consisting of PTFE wax, polyethylene / PTFE co-dispersed wax, and high-molecular polyethylene wax.
[3] The second pigment is selected from the group consisting of tungstates and silicates, alkali metal phosphates and chlorides, and alkaline earth metal hypophosphites. The chromate-free precoated steel sheet according to [1] or [2].
[4] The chromate-free type according to [3], wherein the tungstate is sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, magnesium tungstate or a mixture thereof. Pre-coated steel sheet.
[5] The chromate-free precoated steel sheet according to [3], wherein the silicate is sodium silicate, potassium silicate, lithium silicate, or a mixture thereof.
[6] The alkali metal phosphate is sodium dihydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate diwater The chromate-free precoated steel sheet according to [3] above, which is a Japanese product or a mixture thereof.
[7] The chromate-free precoated steel sheet as described in [3] above, wherein the alkali metal chloride is sodium chloride.
[8] The chromate-free precoated steel sheet according to [3], wherein the hypophosphite is calcium hypophosphite.

According to the present invention, there is no generation of white rust from the surface of the steel sheet, and it is possible to avoid wetness of the coating film in a high humidity environment while exhibiting the effect of suppressing red rust on the end face. It is possible to use a chromate-free precoated steel sheet having sufficient scratch resistance such as galling and scratch resistance for front and back coating films for sliding during transportation.

It is a figure which shows the wetting evaluation by moisture absorption of the back surface coating film in an Example.

In the pre-coated steel sheet of the present invention aiming at improving the scratch resistance of the coating film, the chromate-free chemical conversion treatment layer formed on the surface of the zinc-based plated steel sheet and the end face red rust formed on the chromate-free chemical conversion treatment layer In contact with the upper surface of the lower layer coating film on the coating layer (hereinafter referred to as “lower layer coating”) containing a rust preventive pigment (hereinafter also referred to as “second pigment”) for the development of corrosion resistance. And a water-permeable / water vapor-impermeable top layer coating film formed as described above.
In the present invention, it is important that the uppermost coating film satisfies the following conditions (1) to (5), particularly (5).
(1) The contact angle with water on the surface of the uppermost layer coating film is 65 to 75 °,
(2) The film thickness of the uppermost coating film is 0.5-5 μm,
(3) The uppermost layer coating film contains the first pigment, and the content of the first pigment is 15 to 70% by mass with respect to the dry coating film mass,
(4) Among the first pigments, the total amount of those having an average primary particle size of 1 μm or more is 5% by mass or more with respect to the dry coating mass, and (5) the uppermost coating layer is granular having an average particle size of 1 to 8 μm. The wax is contained at a content of 1 to 10% by mass with respect to the dry coating film mass, and at least some of the granular wax particles are present in a state of partially protruding from the coating film surface; The lower layer coating film adjacent to the uppermost layer coating film contains a second pigment that is one or more deliquescent rust preventive pigments effective for preventing the occurrence of red rust on the end face.
Therefore, in the precoated steel sheet of the present invention, according to this condition, at least a part of the wax particles in the uppermost layer coating film is dispersed and scattered in an island shape in a state of partially protruding from the coating film surface. As a result, most of the surface of the top layer coating film does not lose water permeability, so that when the pre-coated steel sheet gets wet, the rust inhibitor can elute through the top layer coating film, ensuring sufficient red rust corrosion resistance at the end face. Is done.

On the other hand, when the coating film is rubbed with the object, the wax protruding from the coating film is partially cracked to obtain slidability. Thereby, it is possible to prevent problems such as peeling of the design-side coating film from the base steel plate due to sliding during transportation in the pre-coated steel plates stacked or coiled, and occurrence of coating galling during the forming process by the user.

At least a part of the granular wax used in the uppermost coating film of the precoated steel sheet of the present invention partially protrudes from the coating film and is scattered in an island shape after forming the coating film. In order to satisfy this requirement, the particulate wax has a melting point exceeding the heating and drying temperature for forming the coating film, or even when melted during the heating and drying of the coating film, it protrudes from the coating film into an island shape after film formation. Use one having a high melt viscosity to such an extent that the granular form that can be scattered is not lost. Since the heat drying temperature at the time of coating film formation is generally about 200 ° C., the melting point of the wax is preferably over 200 ° C., more preferably over 250 ° C., and even more preferably over 300 ° C. On the other hand, the wax having a melt viscosity that does not lose the granular form even when melted at the time of heating and drying of the coating film is 2000 mPa · s or more, preferably 2500 mPa · s or more, more preferably at the temperature at which the coating film is heated and dried. Indicates a viscosity of 3000 mPa · s or more.

Examples of waxes that meet these conditions include polytetrafluoroethylene (PTFE) wax, polyethylene / PTFE co-dispersed wax, and high-molecular polyethylene wax. As an example, wax of SST series supplied from Shamrock Technologies (USA) can be mentioned.

The granular wax used in the present invention preferably has an average particle diameter of 1 to 8 μm. It is difficult to obtain a granular wax having an average particle size of less than 1 μm, and it is difficult to obtain a sufficient protruding height from the coating film surface. An average particle diameter exceeding 8 μm is not preferable because the wax protrudes greatly from the surface of the coated film after film formation, and the wax is likely to fall off when the coated film is rubbed. A more preferable average particle diameter of the granular wax is 3 to 5 μm.

The height at which the granular wax protrudes from the surface of the uppermost coating film (distance between the coating film surface and the end of the wax particle that is farthest from the coating film surface) is preferably about 0.1 to 3 μm. If it is less than 0.1 μm, it is difficult to obtain slidability due to cracking of the protruding portion, and if it exceeds 3 μm, the wax is transferred to the mold during the press forming of the steel sheet, and it is necessary to frequently care for the mold. A more preferable protrusion height is 0.5 to 2 μm.

The state of presence of the granular wax in the uppermost layer coating film and the film thickness of the coating film of each layer can be confirmed by embedding a test piece of the precoated steel sheet in a resin, polishing the section, and observing the section of the coating film. Here, the film thickness of the uppermost layer coating film is the average value of the film thicknesses of the portions where no granular wax is present.

The content of the particulate wax in the uppermost layer coating film is preferably 1 to 10% by mass with respect to the dry coating film mass. If the amount is less than 1% by mass, the effect of adding the wax is not sufficiently exhibited. If the amount exceeds 10% by mass, the cohesive force of the coating film is lowered and film formation is difficult. Since the uppermost coating film in the present invention contains a larger amount of the first pigment than usual in order to obtain water permeability and water vapor impermeability, the cohesive force is essentially small and a large amount of wax is contained. Addition further reduces the cohesive strength of the coating film, which is particularly inconvenient. A more preferable content of the particulate wax is about 1 to 5% by mass with respect to the dry coating film mass.

The precoated steel sheet of the present invention will be further described. It is a chromate-free chemical conversion treatment layer on at least both surfaces of a zinc-based plated steel sheet, a lower coating film formed thereon, and an uppermost layer formed adjacent to the lower coating film. A coating film. In these coatings, a chromate-free chemical conversion treatment layer, a lower layer coating, and an uppermost layer coating are formed in this order from the surface side of the galvanized steel sheet, and the uppermost layer coating becomes a lower layer coating. The structure is not limited as long as it is provided adjacent to the upper surface of the lower layer coating film. For example, another coating film may be provided between the chromate-free chemical conversion treatment layer and the lower layer coating film. Specifically, the primer coating layer may be provided on the chromate-free chemical conversion treatment layer, and a plurality of coatings such as one or more upper coating layers may be provided thereon.

In the pre-coated steel sheet of the present invention, the uppermost layer coating on one side satisfies the conditions (1) to (5) above, and the lower layer coating adjacent to the galvanized steel plate side of the uppermost layer coating is effective for end face red rust corrosion resistance. 1 type, or 2 or more types of the 2nd pigment which is a deliquescent rust preventive pigment is contained. In the present invention, it is necessary that the lower layer coating film contains a deliquescent rust preventive pigment. However, as long as this requirement is satisfied, any other coating film positioned below the uppermost layer coating film is the above-mentioned rust prevention coating. It is also possible to contain pigments.

The second pigment, which is a deliquescent rust preventive pigment effective for end face corrosion resistance, is composed of tungstate and silicate, alkali metal phosphate and chloride, and alkaline earth metal hypophosphite. The 1 type (s) or 2 or more types chosen from a group can be mentioned.

As the tungstate, for example, sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, or magnesium tungstate can be used, or a mixture thereof may be used. The tungstate can be mixed in the coating film by about 6 to 50 wt%. When the amount is less than 6 wt%, the end face red rust corrosion resistance is insufficient. When the amount is more than 50 wt%, tungstate is excessively eluted when the coated film is exposed to a wet environment, Peeling may occur. The tungstate content in the coating is more preferably 7 to 40 wt%, most preferably 10 to 30 wt%.

As the silicate, for example, sodium silicate, potassium silicate, or lithium silicate can be used, or a mixture thereof may be used. Silicates can be mixed in the coating film by about 5 to 50 wt%. If it is less than 5 wt%, the end face red rust corrosion resistance is insufficient, and if it exceeds 50 wt%, the silicate is excessively eluted when the film after film formation is exposed to a wet environment, and the film is swollen. Peeling may occur. The silicate content in the coating film is more preferably 7 to 40 wt%, and most preferably 10 to 30 wt%.

The tungstate or silicate used in the present invention is a powder at room temperature, and the paint is applied until the maximum particle size becomes 5 to 20 μm, more preferably about 5 to 10 μm in consideration of the thickness of the mixed coating film. It is preferable to use it in a dispersed manner.

In the present invention, alkali metal phosphates and chlorides, or alkaline earth metal hypophosphites can be used as other rust preventive pigments that exhibit end face red rust corrosion resistance. Examples of the alkali metal phosphate include sodium dihydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, sodium dihydrogen phosphate monohydrate or sodium dihydrogen phosphate dihydrate. Products may be used, or mixtures thereof may be used. As the alkali metal chloride, for example, sodium chloride can be used. As hypophosphite, for example, calcium hypophosphite can be used. The preferred content of these anticorrosive pigments in the coating film is 0.5-30 wt%, more preferably 1-20 wt%.

The anticorrosive pigment used in the present invention can be any combination of the above-mentioned substances.

The constituent components other than the anti-corrosion pigment effective in developing the end face red rust corrosion resistance in the lower layer coating adjacent to the uppermost layer coating may be those used in the coating layer of a normal precoated steel sheet.

The lower coating film preferably has a thickness of 2 to 20 μm. If it is less than 2 μm, the absolute amount of the added anticorrosive pigment is insufficient and sufficient end face red rust corrosion resistance cannot be obtained. If it exceeds 20 μm, the effect of end face red rust corrosion resistance is saturated and uneconomical, and at the same time during processing The adhesiveness of is reduced. The film thickness is more preferably 3 to 15 μm, still more preferably 5 to 10 μm. The film thickness referred to here is the average film thickness of the coating layer, that is, the average of the distance between the interface between the upper layer and the lower layer of the coating layer. The film thickness can be measured by the following method. A pre-coated steel sheet is embedded and cured in an epoxy resin and polished from the cross-sectional direction, and then the cross-sectional observation of the coating film is performed to measure the distance between the upper and lower interfaces. Considering that the distance between the upper and lower interfaces varies depending on the measurement location, it is desirable that the cross-sectional microscope observation range is at least 10 mm. When the particle size of the added rust preventive pigment is large and protrudes from the resin of the coating film, the protruding pigment surface is regarded as an interface with the upper and lower layers of the coating film layer.

As described above, the uppermost layer coating film adjacent to the lower layer coating film containing the second pigment is in addition to the above condition (5) of [1],
(1) The contact angle of water on the coating surface with water is 65 to 75 °,
(2) The film thickness of the coating film is 0.5-5 μm,
(3) The coating film contains the first pigment, and the content of the first pigment is 15 to 70% by mass with respect to the dry coating film mass. (4) Among the first pigments, the average primary particle size is 1 μm. The total amount of the above is 5% by mass or more based on the dry coating film mass,
It is necessary to satisfy the condition. Here, the “first pigment” refers to the entire pigment for the purpose of coloring or rust prevention, but does not include the deliquescent rust preventive pigment existing in the lower layer. Moreover, other components, such as a binder, a pigment, and a bead, are mentioned as a component which comprises dry coating-film mass (mass in the dry state of a coating film). The film thickness referred to here is the average film thickness of the coating film layer, that is, the average of the distance between the surface of the uppermost coating film layer excluding the protruding portion of the granular wax and the interface between the lower coating film. The film thickness can be combined with the above-mentioned measurement of the film thickness of the lower layer coating film. In addition, when the particle size of the anticorrosive pigment of the second pigment added to the lower layer coating film is large and protrudes from the resin of the lower layer coating film into the uppermost layer coating film, the protruding pigment surface is defined as the lower layer coating film layer. It is considered as the interface.

The contact angle of water on the coating surface with water needs to be 65 to 75 °. If it is less than 65 °, the effect as a barrier layer against water vapor is insufficient, and moisture absorption due to the deliquescent action of the rust preventive pigment in the adjacent lower layer coating film cannot be sufficiently suppressed, and if it exceeds 75 °, it is immersed in water. This is because elution of the rust preventive pigment from the lower layer coating is inhibited, and sufficient end face red rust corrosion resistance cannot be obtained.

For the measurement of the contact angle, a test piece cut out in a size of 30 mm × 60 mm by shearing from the prepared coated steel plate is left for 15 hours in wet test conditions (temperature 49 ° C., relative humidity 90-95%), and then a blower is sufficient. After drying, measure the contact angle with the water of the top layer coating film (water permeable / water vapor impermeable coating film) on the surface provided with the coating film containing anti-rust pigment effective for red rust corrosion resistance at the end face. . A general-purpose contact angle measuring instrument (for example, CA-A type contact angle measuring instrument manufactured by Kyowa Interface Science Co., Ltd.) is used to measure the contact angle with water 30 seconds after dropping water droplets from the microsyringe onto the uppermost coating film. Use to measure.

Satisfy the above conditions (1) to (4) to make the surface of the uppermost coating film hydrophilic, and to contain a predetermined amount of the first pigment in the relatively thin uppermost coating film, and at least one of them. The part is made of a pigment having a relatively large particle diameter, so that the uppermost layer coating film becomes a barrier layer against water (water vapor) as a gas in contact with the uppermost layer coating film, and the second layer in the adjacent lower layer coating film While suppressing the moisture absorption due to the deliquescent action of the rust preventive pigment, the liquid water in contact with the uppermost coating film can reach the surface of the lower coating film by the capillary action of this coating film. The effect of preventing red rust on the end face by elution of the pigment is enabled. At the same time, liquid water that reaches the surface of the lower layer coating film containing anti-corrosion pigment does not come into contact with the galvanized layer on the steel sheet surface, thereby avoiding the occurrence of white rust from the steel sheet surface due to zinc dissolution. The

The uppermost coating film that is water-permeable and water-vapor-impermeable is mainly composed of a binder, a first pigment, and the above-described granular wax, and contains other components as necessary. .

The binder is composed of a binder resin as a main component, a curing agent, and other components.

The type of binder resin is practically selected according to the performance required as a coated steel sheet, and various resins that have been used in coated steel sheets such as epoxy resins, polyurethane resins, and polyester resins are used. be able to. However, in the present invention, the contact angle of water on the surface of the uppermost layer coating film needs to be 65 to 75 °, and therefore the contact angle is within this range when the uppermost layer coating film is formed by adding other components. It is required to use a resin that can be Examples of such a resin include a resin containing a partially hydrolyzed condensate of tetraalkoxysilane using an epoxy resin, a polyurethane resin, or a polyester resin as a base resin.

Examples of tetraalkoxysilanes include commercially available products such as methyl silicate 51, ethyl silicate 40, ethyl silicate 48 (manufactured by Colcoat), MKC silicate MS51, MS56 (manufactured by Mitsubishi Chemical Corporation), and the like. Further, water and a catalyst may be added to a monomer such as tetramethoxysilane, tetraethoxysilane, or tetrapropoxysilane to cause hydrolysis and condensation.

Examples of the curing agent combined with the binder resin include melamine resin and polyisocyanate compound. If necessary, a crosslinking catalyst may be blended.

The content of the binder component with respect to the dry coating film mass is preferably 30 to 85% by mass. When the content of the binder component is excessively large, the content of the first pigment is relatively lowered, and the number of pores that can be used for liquid permeation is insufficient, and the corrosion resistance is reduced. Or coloring in a desired color tone becomes difficult. On the other hand, when the content is excessively low, the hardness of the coating film itself is increased and the scratch resistance is improved, but the elongation of the coating film itself is decreased, so that the workability is decreased. A particularly preferable range of the content is 40 to 60% by mass with respect to the dry coating film mass.

The kind of the first pigment is not particularly limited except that the kind of the first pigment is not the same as that of the second pigment. Examples of anti-corrosion pigments include aluminum tripolyphosphate, phosphoric acid and phosphorous acid Zn, Mg, Al, Ti, Zr, and Ce salts, Ca ion exchanged silica, and oil absorption of 100 to 1000 ml / 100 g, specific surface area of 200 ~ 1000m ² / g, like amorphous silica particles having an average particle size of 2 ~ 30 [mu] m is.

Examples of coloring pigments 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 uppermost coating film in the present invention contains a larger amount of the first pigment than usual in order to obtain water permeability and water vapor impermeability. The content of the first pigment is 15 to 70% by mass with respect to the dry coating film mass. If the pigment content is less than 15% by mass, liquid water is difficult to permeate the coating film, so that the elution of the anticorrosive pigment in the lower layer coating film is hindered, and the effect of preventing red rust on the end face is less likely to appear. . On the other hand, when the pigment content exceeds 70% by mass, it becomes difficult to satisfy other characteristics required for the precoated steel sheet, such as workability.

Further, among the first pigments, the total content of pigments having an average primary particle diameter of 1 μm or more with respect to the dry coating film mass needs to be 5% by mass or more. The presence of a pigment having such a relatively large particle size is important for allowing liquid water to permeate. The upper limit of the total content of the pigment having an average primary particle size of 1 μm or more with respect to the dry coating mass is not particularly set, but is preferably 70% by mass or less for the reason of maintaining workability after coating. On the other hand, the total content of pigments having an average primary particle size of 1 μm or more with respect to the dry coating mass is more preferably 5 to 30% by mass, and particularly preferably 5 to 15% by mass.

The average primary particle size of the first pigment is preferably smaller than the thickness of the coating film. When the average primary particle diameter of the pigment is excessively larger than the thickness of the coating film, there is a concern that the corrosion resistance of the painted surface is lowered.

In addition to the binder and the first pigment component, the uppermost layer coating film includes a leveling agent, beads for obtaining irregularities in appearance, conductive powder for improving weldability and electromagnetic wave shielding properties, etc., if necessary. Other components may be contained. However, when the content of these components increases, the original properties of the uppermost layer coating such as the permeability of liquid water and the barrier effect against gaseous water are deteriorated. The total content with respect to the mass is preferably 20% by mass or less, and particularly preferably 10% by mass or less.

The thickness of the uppermost layer coating is 0.5-5 μm. When the thickness is less than 0.5 μm, the barrier effect against gaseous water (water vapor) is reduced. On the other hand, since the uppermost layer coating film of the present invention contains a larger amount of the first pigment than usual in order to obtain water permeability and water vapor impermeability, the cohesive force is essentially small. This is because when the film thickness is increased, the coating film is liable to cohesive failure without being able to withstand the shearing force caused by rubbing on the surface of the coating film. Fragments of the back surface coating film peeled off by rubbing against the front surface coating film also scratch the front surface coating film, resulting in scratches on both the front and back surfaces. Moreover, since water permeability falls when the film thickness of a top layer coating film becomes thick, elution of the deliquescent rust preventive from a lower layer is inhibited, and end face red rust corrosion resistance falls. For the above reasons, the thickness of the uppermost layer coating film is 1 to 5 μm, and more preferably less than 1 to 2 μm. In addition, as mentioned above, it is preferable that the thickness of the coating film is not less than the average primary particle diameter of the first pigment contained.

The coating film used as the uppermost coating film of the present invention is similar to the coating film described in Patent Document 3 as being permeable to water. The role of the water-permeable coating film in Patent Document 3 is that when water as a liquid touches the coating film, water freely passes through the coating film to fill the voids in the coating film, and plating under the coating film Wetting the surface with liquid (water). In the present invention, the lower layer adjacent to the uppermost layer coating film is not a plating layer but a coating layer containing a deliquescent rust preventive pigment, but the role of the coating film to permeate liquid water into the lower layer is described in Patent Literature The same as in 3. In other words, when pre-coated steel sheet products installed outdoors are wet with water due to rain, etc., in order to suppress the occurrence of red rust from the cut end surface, the deliquescence rust prevention effect that exhibits red rust prevention effect by leaching into water It is necessary to ensure that the deliquescent rust preventive pigment is eluted from the lower coating film containing the pigment and to act on the cut end face. For this reason, the top layer coating film has a role of freely diffusing the deliquescent rust preventive pigment that has been impregnated with water into the coating layer and leaching from the lower layer to the water that wets the surface of the precoated steel sheet product. I carry it.

On the other hand, in the present invention, in addition to the above-mentioned role, the uppermost layer coating film has another role. When water (water vapor) as a gas touches the uppermost layer coating film, the uppermost layer coating layer serves as a barrier layer and suppresses moisture absorption of the deliquescent rust preventive pigment contained in the lower layer. is there. As described above, in the present invention, the lower layer adjacent to the uppermost layer coating film is not a plating layer but a coating layer containing a deliquescent rust preventive pigment. When the lower layer is a coating layer, the adhesion between the uppermost layer coating and the lower coating layer is better than when the lower layer is a plating layer. Then, the gap that allows water to permeate in the upper layer has a structure in which one end is sealed with the lower layer coating layer, and water can reach the lower layer coating layer by capillary action. Since the lower part of the space | gap of the uppermost layer coating film in the site | part which contact | connects a film | membrane is sealed, the gas in a space | gap is hard to substitute. For this reason, it is considered that a new mechanism that liquid water permeates but gas does not permeate appears. As a result, by providing the above-mentioned uppermost coating film on the upper layer of the deliquescent rust preventive pigment-containing layer, the surface of the pre-coated steel sheet product that directly touches the environment is not easily wetted even in a high humidity environment. Can be.

As described above, in the present invention, in addition to the conventional function, the water-permeable coating film of Patent Document 3 has a new function that can be said to have a reverse effect. In other words, when liquid water touches the uppermost coating film, it does not interfere with elution of the deliquescent rust-preventing pigment from the lower layer (conventional function), and does not easily elution when gaseous water touches it. (New function).

In the chromate-free precoated steel sheet of the present invention, other than the uppermost coating film satisfying the conditions (1) to (5) of [1] above provided on one side and the lower coating film containing a deliquescent rust preventive pigment adjacent thereto. The coating film (including all coating films on the opposite surface) can be formed from a common material used for the production of chromate-free precoated steel sheets. On the other hand, all the coating films on both sides can be easily formed by ordinary production equipment and production methods, and they need not be described in detail here.

As the steel sheet that is the base material of the precoated steel sheet of the present invention, a zinc-based plated steel sheet is used. As the galvanized steel sheet, for example, any one such as hot dip galvanized steel sheet, electrogalvanized steel sheet, Al-galvanized steel sheet, Al-Mg-Si-galvanized steel sheet can be used. In particular, when an Al—Mg—Si—galvanized steel sheet is used, the improvement in red rust resistance is particularly remarkable and preferable.

On the surface of the galvanized steel sheet, a normal chromate-free chemical conversion treatment layer is provided as a pretreatment layer for applying a paint for coating film. As an example, the chemical conversion treatment layer can use a film containing two or more of silica, a silane coupling agent, tannin or tannic acid, a zirconium compound, and a titanium compound and a resin. The chemical conversion treatment layer can be formed by dip coating, roll coater coating, ringer roll coating, brush coating, spray coating or the like.

Next, the present invention will be described with reference to examples. Needless to say, the following examples are for illustrative purposes, and the present invention is not limited to the following examples.

A pre-coated steel sheet as a product is generally a surface on the outer side of the finished product that meets the design requirements required by consumers who process it to produce the finished product. Supplied to the customer in the form provided on the surface, the coating on the other side is not required to have the design characteristics required for the coating on the other side, and the coating on the other side. Contains components that do not directly affect the design demanded by customers, such as end face red rust corrosion resistant pigments. In general, the surface on which a sticker for precautionary notes or a foam material for improving airtightness is attached with an adhesive is the other surface that is not touched by human eyes in the final product. Therefore, in the following, for the sake of convenience, the surface with the coating film with the performance required by the customer is the “front surface”, and the surface with the end surface red rust corrosion-resistant pigment-containing coating film and the uppermost coating film adjacent thereto is the “back surface”. I will call it.

As original plates, 11% Al-3% Mg-0.2% Si-galvanized steel sheet (0.6 mm thickness, single-sided plating coverage 80 g / m ² ) (hereinafter referred to as SD), and hot-dip galvanized steel sheet (0. 6 mm thick, single-sided plating adhesion 80 g / m ² ) (hereinafter referred to as GI) was used. The surfaces of these original plates were degreased with alkali, and then subjected to chemical conversion treatment (attachment amount: 100 mg / m ^{2 on} one side) with a chromate-free chemical conversion treatment solution (silane treatment agent, tannic acid, silica, and polyester resin mixed treatment solution) ( Hereinafter, referred to as process A). By this chemical conversion treatment, a chromate-free chemical conversion treatment layer was formed on the surface layer of the galvanized steel sheet.

On the front side, on the chromate-free chemical conversion treatment layer of the chemical-treated original plate, a general undercoat layer (a polyester / melamine + isocyanate combined curing type FLC687 resin paint (general pigment contained) manufactured by Nippon Fine Coatings Co., Ltd.) ) And dried with hot air at a dry film thickness of 2 μm and PMT: ultimate plate temperature of 215 ° C. Furthermore, as a general top paint, polymer polyester / melamine curable paint (FLC8500 resin paint (light beige color) manufactured by Nippon Fine Coatings Co., Ltd.) is dried with hot air at a dry film thickness of 10 μm, PMT: ultimate plate temperature of 230 ° C. A top coating was formed.

On the back surface, three types of coatings were applied as lower coatings on the chromate-free chemical conversion treatment layer of the chemical-treated original plate. Two of them contain 18 wt% sodium tungstate or potassium dihydrogen phosphate as a deliquescent end face red rust corrosion resistant pigment (second pigment). The remaining one is a general coating film (same paint as the above-mentioned front side subbing layer) that does not contain a deliquescent end face red rust corrosion resistant pigment but contains only a general pigment. The resin used for these paints is a polyester / melamine + isocyanate combined curable resin (FLC687 resin manufactured by Nippon Fine Coatings). These paints were applied and formed (PMT: hot air drying at a reaching plate temperature of 215 ° C.) so that the lower layer coating film had a predetermined dry film thickness.

The base coating material used for the uppermost coating film formed on the lower coating film (the coating material prior to the addition of wax) is the same as the coating material used for the water-permeable / water vapor-impermeable film described in Patent Document 4 above. Applicable. Of these, the base paint indicated as Type I in Table 1 is NKC1200SC Clear (low-contamination imparted (silanol group surface thickening) baking type polyester resin clear type paint made by Nippon Fine Coatings Co., Ltd.) and titania (Ishihara). A paint prepared by blending 40 wt% of Taipek CR-90 manufactured by Sangyo Co., Ltd., 5 wt% of aluminum tripolyphosphate (K-WHITE # 82 manufactured by Teika), and 5 wt% of porous silica (H-33 manufactured by Dokai Chemical Industries, Ltd.) (The pigment content is 50 wt% with respect to the dry coating film mass). The total amount of pigments having an average primary particle size of 1 μm or more in the paint was calculated to be 10% with respect to the dry coating film weight. Other base paints of type II to type IX satisfy the conditions of the uppermost layer coating film of the present application, but are prepared so that various conditions are variously changed.

A coating material in which various waxes shown in Table 1 were contained in these base coating materials so that the content varied variously was used as the coating material for the uppermost layer coating film. “PTFE”, “PE / PTFE”, and “polymer PE” in the table are respectively SST series polytetrafluoroethylene (PTFE) wax (average particle size 2,4,10 μm, melting point 321 ° C.) of Shamrock Technologies. FLUOROSLIP series polyethylene / PTFE co-dispersed wax (average particle size 4, 6, 13 μm, melting point 110-130 ° C.) from Shamrock Technologies, TEXTURE series high molecular weight polyethylene wax (average particle size 5,30 μm, The melting point of the microcrystalline wax and the carnauba wax was 90 ° C. and 80 ° C., respectively.

The contact angle of the formed uppermost layer coating film was measured with a CA-A type contact angle measuring instrument manufactured by Kyowa Interface Science Co., Ltd. according to the measurement method described above. As a result, no change was observed in the contact angle with the base coating material when any wax was added.

On the other hand, polyester / melamine curable FLC100HQ resin manufactured by Nippon Fine Coatings Co., Ltd. was used as a paint resin for general coating films other than types I to IX. The uppermost layer coating film on the back surface was formed with the film thickness shown in Table 1 as a dry film thickness (PMT: hot air drying at a reaching plate temperature of 230 ° C.).

Table 1 shows an overview of the pre-coated steel sheets produced.

[Evaluation 1] Wetting due to moisture absorption of the back coating film A test piece of 50 × 100 mm cut out from the pre-coated steel sheets of Examples and Comparative Examples prepared was exposed to a test environment adjusted to various temperatures and relative humidity for 30 minutes, The wetness of the coating film due to moisture absorption on the back surface was observed. The results were summarized into three patterns shown in FIGS. 1 (a) to 1 (c) and evaluated as ◎, ○, and ×, respectively. A summary of the test results is shown in Table 2.

[Evaluation 2] Adhesiveness of adhesive material under high humidity The adhesiveness of the adhesive material under high humidity conditions to the back coating film of each test piece was tested. Each test piece is left in an environment of 20 ° C. and relative humidity 95% for 30 minutes, and then a sticking material (adhesive tape) (15 mm width) used in the air conditioner field is attached to the coating film of the test piece, which is applied immediately thereafter. The peel load was measured by pulling one end of the material in the direction perpendicular to the coating film surface with only a spring. The case where the peeling load was 1.96 N or more was evaluated as “◯”, and the case where it was less than 1.96 N was evaluated as “X”. The results are shown in Table 2.

[Evaluation 3] End face red rust corrosion resistance A distilled water immersion test was conducted. It has been found that the resistance to red rust generated in a short time in a non-electrolyte moist environment can be easily evaluated by a distilled water immersion test. About each precoat steel plate of an Example and a comparative example, 20 test pieces of 10x40mm were each immersed in 40 ml of distilled water put into a petri dish with a diameter of 80 mm, and change of red rust generation on the end face was observed.

Normally, it is known that there is no problem in actual use when red rust does not occur in 168 hours. In this case, red rust does not occur at all even if the immersion time in distilled water exceeds 500 hours, and very slight red rust (5 spot rusts with a diameter of 1 mm or less) occurs in 500 hours, but does not occur in 168 hours. ◎ to ○, red rust (six or more spot rusts with a diameter of 1 mm or less) occurred in 500 hours, but no rust in 168 hours, and rust generated from less than 168 hours was evaluated as x. The results are shown in Table 2.

[Evaluation 4] Scratch resistance of front and back coating film When pre-coated steel sheets are stacked or transported in the state of being wound around a coil, scratch resistance to the front and back coating film caused by sliding of the front and back coating film, and user's molding process The anti-galling property of the coating film that sometimes occurs was evaluated by the following method. Two sheets of pre-coated steel sheets of each example and comparative example cut to 50 × 50 mm were prepared, one front side and another back side were overlapped and pressed with a load of 98 N from above and below, After rotating one steel plate 90 °, the load was released and both rubbed painted surfaces were observed.
◎ When both coating surfaces are not scratched on the coating film,
A case where scratches are seen on the coating film on either side, but the original plate is not exposed,
The scratches are seen on the coatings on both sides, but none of the original plates are exposed.
A case where scratches are seen on both sides of the coating and the back side has been exposed to the original plate, but the front side has not been exposed to the original plate, and the scratches have remained in the top layer and have not reached the primer layer. ,
A case where scratches are seen on the coating film on both sides and the back side has reached the exposure of the original plate, but the front side has not reached the exposure of the original plate, but the scratches have reached the primer layer.
When the scratches leading to the exposure of the original plate are seen on both sides of the coating film ×
It was evaluated. A case where the evaluation was Δ or more was regarded as acceptable. The results are shown in Table 2.

The results of the evaluation of the wax detachability are shown in the remarks column of Table 2. When the coating film on the back surface side of each Example and Comparative Example was scraped off with a metal blade, x was marked as the wax particles were observed to fall off. In such a case, when the press molding is actually performed with a mold, it is not practically preferable because the wax is dropped and transferred to the mold and the mold needs to be frequently maintained.

Claims

A chromate-free pre-coated steel sheet having a chromate-free chemical conversion treatment layer on both surfaces of a zinc-based plated steel sheet, and a plurality of coating films including at least the uppermost coating film and the lower coating film adjacent thereto. ,
The condition of the top layer coating on one side is as follows:
(1) The contact angle with water on the surface of the uppermost layer coating film is 65 to 75 °,
(2) The film thickness of the uppermost coating film is 0.5-5 μm,
(3) The uppermost layer coating film contains the first pigment, and the content of the pigment is 15 to 70% by mass with respect to the dry coating film mass;
(4) Among the first pigments, the total amount of those having an average primary particle size of 1 μm or more is 5% by mass or more based on the dry coating mass, and (5) the uppermost coating layer has an average particle size of 1 to 8 μm. The granular wax is contained in a content of 1 to 10% by mass with respect to the dry coating film mass, and at least a part of the granular wax particles is present in a state of partially protruding from the coating film surface.
And the lower layer coating adjacent to the uppermost layer coating contains a second pigment which is one or more deliquescent rust preventive pigments effective for preventing red rust on the end face. A chromate-free pre-coated steel sheet.
2. The chromate-free precoated steel sheet according to claim 1, wherein the wax is selected from the group consisting of PTFE wax, polyethylene / PTFE co-dispersed wax, and polymer polyethylene wax.
The second pigment is selected from the group consisting of tungstates and silicates, alkali metal phosphates and chlorides, and alkaline earth metal hypophosphites. The chromate-free precoated steel sheet according to 1 or 2.
The chromate-free precoated steel sheet according to claim 3, wherein the tungstate is sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, magnesium tungstate or a mixture thereof.
The chromate-free precoated steel sheet according to claim 3, wherein the silicate is sodium silicate, potassium silicate, lithium silicate or a mixture thereof.
The alkali metal phosphate is sodium dihydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate or The chromate-free pre-coated steel sheet according to claim 3, which is a mixture thereof.
The chromate-free precoated steel sheet according to claim 3, wherein the alkali metal chloride is sodium chloride.
The steel sheet according to claim 3, wherein the hypophosphite is calcium hypophosphite.