WO2019188461A1

WO2019188461A1 - Coated galvanized steel sheet

Info

Publication number: WO2019188461A1
Application number: PCT/JP2019/011038
Authority: WO
Inventors: 山本　哲也; 大輝酒井; 礼士白岩
Original assignee: 株式会社神戸製鋼所
Priority date: 2018-03-29
Filing date: 2019-03-18
Publication date: 2019-10-03

Abstract

One aspect of the present invention relates to a coated galvanized steel sheet which has a resin coating film that contains silica and magnesium hydroxide on the surface of a galvanized steel sheet, and which is configured such that: the total content of the silica and the magnesium hydroxide in the resin coating film is 50-75% by mass; the content of a resin component in the resin coating film is 25-50% by mass; the mass ratio of the magnesium hydroxide to the silica is 0.2-1.5; and the thickness of the resin coating film is 0.3-3.0 μm.

Description

Painted galvanized steel sheet

The present invention relates to a coated galvanized steel sheet having a coating containing an inorganic compound in a resin (hereinafter sometimes referred to as “inorganic coating”) on the surface of the galvanized steel sheet.

The surface-treated steel sheet is plated and painted on the surface of the base material and exhibits excellent corrosion resistance. However, red rust is gradually generated in the atmospheric environment on the end surface of the surface-treated steel plate from which the steel as the base material is exposed. Red rust generated on the end surface of the surface-treated steel sheet not only deteriorates the appearance, but if it occurs in the vicinity of a circuit board such as an electrical product, it may fall off the steel sheet and short circuit, impairing the safety of the product. May cause.

Various studies have been made so far to prevent the occurrence of red rust on the end face of the coated steel sheet. For example, a general pre-coated metal plate has a coating film with a thickness of several tens of μm. The end face corrosion resistance is improved by optimizing the components. On the other hand, a coated steel sheet having a special chemical conversion coating with a coating thickness smaller than that of a pre-coated metal sheet has few rust-preventive components that can be eluted and cannot sufficiently cover the cut end face, so that good end face corrosion resistance can be obtained. Have difficulty. Therefore, in applications where high end face corrosion resistance is required, parts that have been coated or plated after press forming cold-rolled steel sheets are often used.

For galvanizing, it is known that magnesium-based compounds exhibit an antirust effect. In recent years, techniques for highly corrosion resistant coatings containing nano-sized magnesium particles have been developed.

As such a technique, for example, Patent Document 1 discloses a coating made of a composition containing nano magnesium hydroxide particles having an average particle diameter of less than 200 nm.

Further, as a technique for maintaining the corrosion resistance of the film by repairing and passivating the film defect by self-repairing action, Patent Document 2 discloses a rust preventive for metal containing a composite colloid composed of magnesium hydroxide and fine silica. Disclosed is a film formed using an agent.

On the other hand, as a technique using a magnesium-containing film as a film of a chromium-free organic coated steel sheet, Patent Document 3 includes oxide particles, phosphoric acid and / or a phosphoric acid compound, and a magnesium compound on the surface of a zinc-based plated steel sheet. An organic coated steel sheet having a composite oxide film and having an organic film containing a reaction product of an organic resin and an active hydrogen-containing compound and an antirust additive component on the composite oxide film is disclosed.

The coating disclosed in Patent Document 1 has a thickness of 2.5 to 75 μm and is not assumed to be press-molded. Further, the coating disclosed in Patent Document 1 does not exhibit a sufficient antirust effect after press molding when the thickness is several μm or less.

The coating disclosed in Patent Document 2 requires the use of a metal anticorrosive containing a composite colloid composed of magnesium hydroxide and fine silica during film formation. It is stable and prone to problems in the coating process for gelation. Moreover, although the film disclosed in Patent Document 2 is presumed to be effective in corrosion resistance in that it is eluted at the end face, it contains a water-soluble component, so that the water resistance is insufficient, such as condensation or water wetting during transportation. There is a high risk of discoloration due to.

In the organic coated steel sheet disclosed in Patent Document 3, since the magnesium compound is added in the form of water-soluble ions or molecules when forming the composite oxide film, the treatment liquid stability decreases when the addition amount of the magnesium compound is increased. . For this reason, there is a limit in improving the corrosion inhibiting effect of the composite oxide film by increasing the magnesium component. In addition, the organic coated steel sheet disclosed in Patent Document 3 has a problem in that productivity is low and manufacturing cost is high because it is necessary to form an organic film after forming a composite oxide film.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a coated galvanized steel sheet having excellent end face corrosion resistance even when the thickness of the coating is several μm or less.

JP 2016-104574 A JP 2002-322569 A JP 2002-053979 A

One aspect of the present invention is a coated galvanized steel sheet having a resin film containing silica and magnesium hydroxide on the surface of the galvanized steel sheet, wherein the total content of silica and magnesium hydroxide in the resin film is 50 to 50%. 75 mass%, the resin component content of the resin film is 25 to 50 mass%, the mass ratio of the magnesium hydroxide to the silica is 0.2 to 1.5, and the thickness of the resin film is It is a coated galvanized steel sheet having a thickness of 0.3 to 3.0 μm.

The present inventors examined from various angles in order to achieve the above object. As a result, by appropriately adjusting the total content of silica and magnesium hydroxide in the resin film, the mass ratio of magnesium hydroxide to silica, and the thickness of the resin film, the protective effect of the cut end surface of the galvanized steel sheet is enhanced. As a result, the inventors have found that the above object can be achieved brilliantly and completed the present invention.

The coated galvanized steel sheet according to an embodiment of the present invention has a resin film containing silica and magnesium hydroxide on the surface of the galvanized steel sheet. The total content of silica and magnesium hydroxide in the resin film is 50 to 75% by mass. The resin component content of the resin film is 25 to 50% by mass. The mass ratio of the magnesium hydroxide to the silica is 0.2 to 1.5. The resin film has a thickness of 0.3 to 3.0 μm.

According to the above configuration, a coated galvanized steel sheet exhibiting excellent corrosion resistance can be provided even when the thickness of the film is several μm or less.

Hereinafter, the present embodiment will be described more specifically, but the present invention is not limited thereto.

[Total content of silica and magnesium hydroxide: 50 to 75% by mass]
In the present embodiment, the total content of silica and magnesium hydroxide in the resin film is 50 to 75% by mass. It is presumed that silica and magnesium hydroxide in the resin film are eluted from the resin film in a corrosive environment and exhibit an effect of protecting the cut end face. When the total content of silica and magnesium hydroxide in the resin film is less than 50% by mass, the amount of elution is not sufficient, so that the protective action of the end face portion is not ensured. Preferably it is 55 mass% or more, More preferably, it is 60 mass% or more. On the other hand, if the total content of silica and magnesium hydroxide in the resin film exceeds 75% by mass, the resin component serving as the binder is insufficient and the film has many defective portions, so that the corrosion resistance as a flat plate is not ensured. Preferably it is 73 mass% or less, More preferably, it is 70 mass% or less.

In the following description, the inorganic compound contained in the resin film of this embodiment is silica and magnesium hydroxide, and the inorganic film means a resin film containing silica and magnesium hydroxide.

The silica used in the present embodiment is preferably colloidal silica excellent in compatibility with an aqueous resin described later. If the average particle size of the silica is too large, or reduced denseness of the film, there is a possibility that or generating a film defects, it is preferred that the average particle size D ₅₀ is 500nm or less, 450 nm or less It is more preferable that Note that the average particle diameter D ₅₀ of the silica, the integrated value of the silica (integrated value) means the average particle diameter when the 50 mass%.

The magnesium hydroxide used in the present embodiment is not particularly limited as long as it is stable as an aqueous dispersion, and the magnesium hydroxide powder and dispersion method are not particularly limited. The average particle diameter D ₅₀ of the magnesium hydroxide in a state of being dispersed in _water, i.e., the average particle diameter D ₅₀ of magnesium hydroxide in magnesium hydroxide aqueous dispersion is preferably less than that of the resin film thickness, for example, 0. It is preferably 7 μm or less. Thereby, corrosion of the end surface due to insufficient elution amount due to the particulate magnesium hydroxide falling off from the resin film can be suppressed. On the other hand, the lower limit of the average particle diameter D ₅₀ in a state where magnesium hydroxide is dispersed in water is not particularly limited, but if the average particle diameter D ₅₀ becomes too small, the stability of the dispersion (for example, dispersion) is reduced. Since there exists a possibility that it may fall, it is preferable that it is 0.1 micrometer or more. More preferably, it is 0.14 μm or more. Note that the average particle diameter D ₅₀ of the magnesium hydroxide, means the average particle diameter when the cumulative value of magnesium hydroxide (integrated value) of 50 wt%.

When preparing a magnesium hydroxide aqueous dispersion, using a polymer dispersant (for example, a water-soluble acrylic resin, a water-soluble styrene acrylic resin, a nonionic surfactant) that has a small adverse effect on corrosion resistance when a resin film is formed. Also good.

[Content of resin component in resin film: 25 to 50% by mass]
In the present embodiment, the content of the resin component in the resin film is 25 to 50% by mass. As described above, when the resin component in the resin film is insufficient, the film has many defective portions and the corrosion resistance is deteriorated. From such a viewpoint, the content of the resin component in the resin film is 25% by mass or more. Preferably it is 30 mass% or more. However, if the content of the resin component in the resin film is too large, in addition to the deterioration of corrosion resistance due to the decrease in the density of the resin film, the resin film may become soft and the generation of film residue during press molding may increase. From such a viewpoint, the content of the resin component in the resin film is 50% by mass or less. Preferably it is 40 mass% or less.

[Mass ratio of magnesium hydroxide to silica: 0.2 to 1.5]
In this embodiment, the mass ratio of magnesium hydroxide to silica is 0.2 to 1.5. Magnesium hydroxide and silica are both known as rust preventives for zinc plating. The present inventors have found that excellent corrosion resistance can be obtained by blending magnesium hydroxide and silica in a resin film at a specific mass ratio. Excellent corrosion resistance is exhibited when the mass ratio of magnesium hydroxide to silica [Mg (OH) ₂ / SiO ₂ ] is in the range of 0.2 to 1.5. This mass ratio is preferably 0.3 or more, and preferably 1.0 or less.

The mechanism by which the corrosion resistance is improved by adjusting the mass ratio to an appropriate range is unknown, but is probably as follows. That is, it is considered that magnesium ions eluted from magnesium hydroxide stabilized a corrosion product having a high protective action against galvanization produced by silica, and improved the barrier effect by the stabilized corrosion product. The protective action against galvanization means a barrier property that blocks corrosion factors such as water and oxygen. The component eluted from the resin film in a corrosive environment is also considered to be affected by the mass ratio of magnesium hydroxide to silica [Mg (OH) ₂ / SiO ₂ ], and the mass ratio is 0.2 to 1. By making it within the range of 5, it is presumed that the component eluted from the resin film has a composition ratio excellent in end face protection.

In addition, the use of particulate magnesium hydroxide makes it possible to increase the magnesium component addition ratio in the resin film without impairing the stability of the treatment liquid, and as a result, corrosion generation with a high protective action against galvanization. It is presumed that the object will be supplied to the cut end face.

[Resin film thickness: 0.3 to 3.0 μm]
The resin film thickness affects the corrosion resistance of both the flat plate portion and the end surface portion. In the present embodiment, the resin film thickness is 0.3 μm or more. In the flat plate portion, when the resin film thickness is 0.2 μm, a level of corrosion resistance with no practical problem can be obtained. However, in the end face portion, the corrosion resistance is insufficient when the resin film thickness is less than 0.3 μm. On the other hand, the upper limit of the resin film thickness is not particularly required because it depends on the area (plate thickness) of the cut surface. However, since a large-scale drying facility is required when the resin film thickness exceeds 3.0 μm, the resin film thickness is preferably 3.0 μm or less from the viewpoint of manufacturing cost.

[Resin type]
The type of resin used in the present embodiment is not particularly limited, and any of a water-based resin and a non-aqueous resin can be used. When using an aqueous dispersion of magnesium hydroxide or colloidal silica, it is preferable to use an aqueous resin. Such an aqueous resin is not particularly limited, but is preferably mixed with an aqueous dispersion of magnesium hydroxide and colloidal silica. The aqueous resin in the present embodiment refers to a resin that is an aqueous dispersion or a water-soluble resin.

Such water-based resins are preferably polyolefin-based resins, polyurethane-based resins, and polyester-based resins, and among these, polyolefin-based resins and polyurethane-based resins are more preferable. Hereinafter, the polyolefin resin and the polyurethane resin will be specifically described.

[Polyolefin resin]
As the polyolefin resin, an ethylene-unsaturated carboxylic acid copolymer is preferable. As the ethylene-unsaturated carboxylic acid copolymer, for example, those described in JP-A-2005-246953 and JP-A-2006-43913 can be used.

Examples of the unsaturated carboxylic acid include (meth) acrylic acid, crotonic acid, isocrotonic acid, maleic acid, fumaric acid, itaconic acid, and the like. An ethylene-unsaturated carboxylic acid copolymer can be obtained by polymerization using a combination method.

The copolymerization ratio of unsaturated carboxylic acid to ethylene is preferably 10% by mass or more, more preferably 15% by mass or more when unsaturated monomer is 100% by mass. 40% by mass or less, and more preferably 25% by mass or less. When the amount of unsaturated carboxylic acid is less than 10% by mass, the coating strength (emulsion composition), which will be described later, is not stable because the number of carboxyl groups that are the starting point of intermolecular association by ion clusters is small. May be inferior. On the other hand, if the unsaturated carboxylic acid exceeds 40% by mass, the corrosion resistance and water resistance of the resin film may be inferior.

Since the ethylene-unsaturated carboxylic acid copolymer has a carboxyl group, the coating liquid can be emulsified (water dispersion) by neutralization with an organic base or metal ion.

As the organic base, an amine having a boiling point of 100 ° C. or less under atmospheric pressure is preferable from the viewpoint of not significantly reducing the corrosion resistance of the resin film. Specific examples include tertiary amines such as triethylamine; secondary amines such as diethylamine; primary amines such as propylamine, and the like, and one or more of these can be used in combination. Of these, tertiary amines are preferred, and triethylamine is most preferred. Moreover, it is preferable to use a monovalent metal ion together with the amine from the viewpoint of improving solvent resistance and film hardness.

From the viewpoint of ensuring corrosion resistance, the amine is preferably 0.2 mol or more with respect to 1 mol of the carboxyl group in the ethylene-unsaturated carboxylic acid copolymer, and is preferably 0.8 mol or less. preferable. And it is more preferable that it is 0.3 mol or more, and it is more preferable that it is 0.6 mol or less on the other hand.

The amount of monovalent metal ions is preferably 0.02 mol or more with respect to 1 mol of carboxyl groups in the ethylene-unsaturated carboxylic acid copolymer from the viewpoint of ensuring the emulsion stability of the coating liquid. More preferably, it is 0.03 mol or more. On the other hand, from the viewpoint of ensuring corrosion resistance, the amount is preferably 0.4 mol or less, more preferably 0.3 mol or less, based on 1 mol of the carboxyl group in the ethylene-unsaturated carboxylic acid copolymer. The metal compound for imparting monovalent metal ions is preferably NaOH, KOH, LiOH or the like, and NaOH is most preferable because of its best performance.

The ethylene-unsaturated carboxylic acid copolymer is stirred at high speed in a container capable of reacting at a high temperature (about 150 ° C.) and a high pressure (about 5 atm), for example, in the presence of a carboxylic acid polymer described later, if necessary. For 1 to 6 hours to emulsify (emulsify). In emulsification, an appropriate amount of a compound having a surfactant function such as tall oil fatty acid may be added. Further, a hydrophilic organic solvent such as a lower alcohol having about 1 to 5 carbon atoms may be partially added to water.

The mass average molecular weight (Mw) of the ethylene-unsaturated carboxylic acid copolymer is preferably 1,000 or more and 100,000 or less in terms of polystyrene. A more preferable lower limit is 3,000 or more, and further preferably 5,000 or more. On the other hand, a more preferable upper limit value is 70,000 or less, more preferably 30,000 or less. This Mw can be measured by gel permeation chromatography (GPC) using polystyrene as a standard.

A carboxylic acid polymer can also be used as the resin component. As the carboxylic acid polymer, any polymer having an unsaturated carboxylic acid as a constituent unit exemplified as one that can be used for the synthesis of the ethylene-unsaturated carboxylic acid copolymer can be used. Among these, acrylic acid and maleic acid are preferable, and maleic acid is more preferable. The carboxylic acid polymer may contain a constituent unit derived from a monomer other than the unsaturated carboxylic acid, but the constituent unit amount derived from the other monomer is 10% by mass or less in the polymer. More preferably, it is 5 mass% or less, and the carboxylic acid polymer comprised only from unsaturated carboxylic acid is further more preferable. Preferred carboxylic acid polymers include polyacrylic acid, polymethacrylic acid, acrylic acid-maleic acid copolymer, polymaleic acid and the like. Of these, polymaleic acid is more preferable from the viewpoints of resin film adhesion and corrosion resistance. Although the exact mechanism by which the corrosion resistance is improved by using polymaleic acid is unknown, the adhesion between the resin film and the galvanized steel sheet is improved due to the large amount of carboxyl groups. It is done. However, the present invention is not limited to this estimation.

The mass average molecular weight (Mw) of the carboxylic acid polymer used in the present embodiment is preferably 500 or more and 30,000 or less in terms of polystyrene. A more preferable lower limit value is 800 or more, more preferably 900 or more, and most preferably 1,000 or more. A more preferable upper limit value is 10,000 or less, more preferably 3,000 or less, and most preferably 2,000 or less. This Mw can be measured by GPC using polystyrene as a standard.

The content ratio of the ethylene-unsaturated carboxylic acid copolymer and the carboxylic acid polymer is 1,000: 1 to 10: 1, preferably 200: 1 to 20: 1 in terms of mass ratio. If the content ratio of the carboxylic acid polymer is too low, the effect of combining the olefin-acid copolymer and the carboxylic acid polymer may not be sufficiently exhibited. On the contrary, if the content ratio of the carboxylic acid polymer is excessive, the olefin-acid copolymer and the carboxylic acid polymer are phase-separated in the first layer forming coating solution, and a uniform resin film is formed. There is a risk of disappearing.

[Polyurethane resin]
As the polyurethane resin, a carboxyl group-containing polyurethane resin is preferable. As the carboxyl group-containing polyurethane resin, for example, those described in JP-A-2006-43913 can be used.

The carboxyl group-containing polyurethane resin is preferably obtained by chain extension reaction of a urethane prepolymer with a chain extender. The urethane prepolymer is obtained, for example, by reacting a polyisocyanate component and a polyol component.

It is possible to use at least one polyisocyanate selected from the group consisting of tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI) and dicyclohexylmethane diisocyanate (hydrogenated MDI) as the polyisocyanate component. From the viewpoint of obtaining a resin film having excellent stability. In addition to the above polyisocyanates, other polyisocyanates can be used as long as the corrosion resistance and reaction control stability are not lowered. However, the content of the polyisocyanate is preferably 70% by mass or more of the total polyisocyanate component from the viewpoint of ensuring the corrosion resistance of the resin film and the stability of reaction control. Examples of the polyisocyanate other than the above polyisocyanate component include tetramethylene diisocyanate, hexamethylene diisocyanate, dodecane methylene diisocyanate, isophorone diisocyanate, xylene diisocyanate, and phenylene diisocyanate. One or more of these are used. May be.

From the viewpoint of obtaining a resin film having excellent corrosion resistance and slidability, it is preferable to use three types of polyols, 1,4-cyclohexanedimethanol, polyether polyol, and polyol having a carboxyl group, as the polyol component. . As the polyol component, it is more preferable to use three kinds of diols of 1,4-cyclohexanedimethanol, polyether diol, and diol having a carboxyl group. The use of 1,4-cyclohexanedimethanol as the polyol component can enhance the rust prevention effect of the obtained polyurethane resin.

The polyether polyol is not particularly limited as long as it has at least two hydroxyl groups in the molecular chain and the main skeleton is composed of alkylene oxide units. Specific examples include polyoxyethylene glycol, polyoxypropylene glycol, polyoxytetramethylene glycol and the like, and it is preferable to use polyoxypropylene glycol or polytetramethylene ether glycol. The number of functional groups of the polyether polyol is not particularly limited as long as it is at least 2 or more, and for example, it may be trifunctional or tetrafunctional or polyfunctional. The average molecular weight of the polyether polyol is preferably about 400 to 4000 from the viewpoint of obtaining a resin film having an appropriate hardness. The average molecular weight can be determined by measuring the OH value (hydroxyl value).

In the above polyol component, the mass ratio of 1,4-cyclohexanedimethanol: polyether polyol = 1: 1 to 1:19 is preferable from the viewpoint of further enhancing the rust prevention effect of the resin film. The polyol having a carboxyl group is not particularly limited as long as it has at least one carboxyl group and at least two hydroxyl groups. Specific examples include dimethylolpropionic acid, dimethylolbutanoic acid, dihydroxypropionic acid, dihydroxysuccinic acid and the like.

In the above polyol component, in addition to the above three kinds of polyols, other polyols can be used as long as the corrosion resistance is not lowered. However, the content of the three kinds of polyols is preferably 70% by mass or more of the total polyol components from the viewpoint of ensuring the corrosion resistance of the resin film. The polyol other than the above three types of polyol is not particularly limited as long as it has a plurality of hydroxyl groups. For example, a low molecular weight polyol, a high molecular weight polyol, etc. are mentioned. The low molecular weight polyol is a polyol having an average molecular weight of about 500 or less. Specific examples include diols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol; glycerin, trimethylolpropane, hexanetriol, etc. Of the triol. The high molecular weight polyol is a polyol having an average molecular weight exceeding about 500. Specific examples include condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-ε-caprolactone (PCL); polyhexamethylene Polycarbonate polyols such as carbonate; and acrylic polyols.

The chain extender is not particularly limited, and examples thereof include polyamines, low molecular weight polyols, and alkanolamines. As polyamines, aliphatic polyamines such as ethylenediamine, propylenediamine and hexamethylenediamine; aromatic polyamines such as tolylenediamine, xylylenediamine and diaminodiphenylmethane; alicyclic polyamines such as diaminocyclohexylmethane, piperazine and isophoronediamine; hydrazine, And hydrazines such as succinic acid dihydrazide, adipic acid dihydrazide, and phthalic acid dihydrazide. Among these, it is preferable to use ethylenediamine and / or hydrazine as a chain extender component. Examples of the alkanolamine include diethanolamine and monoethanolamine.

The carboxyl group-containing polyurethane resin can be emulsified (emulsified) by a known method, for example, the following method. That is, a method in which a carboxyl group of a carboxyl group-containing urethane prepolymer is neutralized with a base and emulsified and dispersed in an aqueous medium to cause chain extension reaction; a carboxyl group-containing polyurethane resin is emulsified with high shear force in the presence of an emulsifier This is a method of dispersing and chain extending reaction.

The acid value of the carboxyl group-containing polyurethane resin is preferably 10 mgKOH / g or more from the viewpoint of ensuring the stability of the coating liquid, and on the other hand, 60 mgKOH / g or less from the viewpoint of ensuring the corrosion resistance of the resin film. Is preferred. The acid value is measured according to JIS-K0070 (1992).

[Additives in coating liquid]
In the present embodiment, the resin film is applied to the surface of the galvanized steel sheet using a known coating method, that is, a roll coater method, a bar coater method, a spray method, or a curtain flow coater method, and dried by heating. Thus, it can be formed. The coating liquid contains a predetermined amount of silica, magnesium hydroxide and the above resin. The resin solid content in the coating liquid is preferably about 15 to 25% by mass. And a coating liquid may contain various additives in the range which does not inhibit the effect of this invention for the purpose of improving membrane | film | coat performance. Examples of additives include silane coupling agents, elution inhibitors, rust inhibitors, waxes, crosslinking agents, diluents, antiskinning agents, surfactants, emulsifiers, dispersants, leveling agents, antifoaming agents, and penetrants. , Film forming aids, dyes, pigments, thickeners, lubricants and the like.

For example, when a silane coupling agent is used as an additive, the resin film becomes dense and the corrosion resistance is improved. Moreover, the adhesiveness of a galvanized steel plate and a resin film is also improved, and corrosion resistance is improved. And there exists an effect which improves the bond strength of a resin component and colloidal silica, and the toughness of a film | membrane improves. Among them, glycidoxy-based silane coupling agents are highly reactive and have a large effect of improving corrosion resistance. Glycidyl group-containing silane coupling agents include γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxymethyldimethoxysilane, β- (3,4-epoxycyclohexyl) ethyl Examples include trimethoxysilane.

The amount of the silane coupling agent is preferably 0.1 parts by mass or more, more preferably 3 parts by mass or more, with respect to 100 parts by mass in total of the inorganic compound and the resin component in the inorganic film. More preferably, it is 5 parts by mass or more. If the amount is less than 0.1 parts by mass, the adhesion between the galvanized steel sheet and the resin film and the bonding force between the resin component and colloidal silica may be insufficient, and the toughness and corrosion resistance of the film may be insufficient. is there. On the other hand, the amount of the silane coupling agent is preferably 10 parts by mass or less, more preferably 9 parts by mass or less, with respect to 100 parts by mass in total of the inorganic compound and the resin component in the inorganic film. More preferably, it is 7 parts by mass or less. Even if it exceeds 10 parts by mass, not only the effect of improving the adhesion between the metal plate and the resin film is saturated, but also the functional group in the resin may decrease and the paintability may deteriorate. Moreover, it is because silane coupling agents raise | generate a hydrolysis-condensation reaction, the stability of a coating liquid falls and there exists a possibility of causing gelation and precipitation of colloidal silica.

For example, when metavanadate which is an elution inhibitor is used as an additive, dissolution and elution of a galvanized steel sheet are suppressed by elution of metavanadate, and corrosion resistance is improved. Metavanadate is particularly effective in improving the bare corrosion resistance of the galvannealed steel sheet. Examples of the metavanadate include sodium metavanadate (NaVO ₃ ), ammonium metavanadate (NH ₄ VO ₃ ), and potassium metavanadate (KVO ₃ ). These 1 type (s) or 2 or more types can be used.

The amount of metavanadate is preferably 0.5 parts by mass or more and more preferably 1.0 part by mass or more with respect to 100 parts by mass in total of the inorganic compound and the resin component in the inorganic coating. Preferably, it is 1.5 parts by mass or more. This is because if the amount is less than 0.5 parts by mass, the effect of improving the bare corrosion resistance becomes insufficient. On the other hand, the amount of metavanadate is preferably 5.5 parts by mass or less, and 5.0 parts by mass or less with respect to 100 parts by mass in total of the inorganic compound and the resin component in the inorganic coating. Is more preferably 4.5 parts by mass or less. This is because when the amount exceeds 5.5 parts by mass, not only the bare corrosion resistance tends to be slightly lowered but also the film adhesion tends to be remarkably lowered. In addition, the suitable amount of this metavanadate is a V element conversion amount.

[Types of galvanized steel sheets]
The type of galvanized steel sheet used in the present embodiment is not particularly limited, and any of electrogalvanized steel sheet, hot dip galvanized steel sheet, and alloyed hot dip galvanized steel sheet (hereinafter, these may be referred to as “original plates”). Can also be adopted. Moreover, there is no limitation in particular also about the kind of zinc plating layer, An alloying element may be included in a plating layer. The galvanized layer is coated on one side or both sides of the base steel plate, and the resin film is accordingly coated on one side or both sides of the galvanized steel plate.

As described above, the coated galvanized steel sheet according to one aspect of the present invention is a coated galvanized steel sheet having a resin film containing silica and magnesium hydroxide on the surface of the galvanized steel sheet, and the silica in the resin film and The total content of magnesium hydroxide is 50 to 75% by mass, the content of the resin component of the resin film is 25 to 50% by mass, and the mass ratio of the magnesium hydroxide to the silica is 0.2 to 1.%. 5 and the thickness of the resin film is 0.3 to 3.0 μm.

According to this configuration, a coated galvanized steel sheet exhibiting excellent corrosion resistance can be realized even when the thickness of the resin film is several μm or less.

Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the present invention is not limited by the following examples, and can be implemented with modifications within a range that can be adapted to the gist described above and below, all of which are included in the technical scope of the present invention. .

(Preparation of magnesium hydroxide dispersion)
Magnesium hydroxide particles (manufactured by Kyowa Chemical Industry Co., Ltd., trade name: Kisuma 5Q-S) are used as a dispersant and dispersed using a polymer dispersant to obtain an aqueous dispersion (resin solid content: about 30 wt%, average particle size _D 50: 0.69 .mu.m) was formulated.

The average particle diameter D ₅₀ of the magnesium hydroxide in the dispersion was diluted with 0.2 wt% aqueous solution of sodium hexametaphosphate, a laser diffraction scattering particle size distribution measuring device (Microtrac Bell Co., Ltd., trade name: Micro Measurement was performed using a track MT3300EXII).

(resin)
Polyethylene resin manufactured by Toho Chemical Co., Ltd. or urethane resin manufactured by Toho Chemical Industry was used as the resin for forming the resin film.

The above polyethylene resin manufactured by Toho Chemical Co., Ltd. and its aqueous dispersion were prepared by the following method.

In an autoclave having an emulsification facility equipped with a stirrer, a thermometer and a temperature controller, an ethylene-acrylic acid copolymer (manufactured by Dow Chemical Co., Ltd., trade name: Primacol 5990I, structural unit derived from acrylic acid: 20% by mass, mass) Average molecular weight (Mw): 20,000, melt index: 1300, acid value: 150, 200.0 parts by mass, polymaleic acid aqueous solution (manufactured by NOF Corporation, trade name: Non-Paul PMA-50W, Mw: about 1100 (polystyrene conversion) ), 50% by mass) 8.0 parts by mass, 35.5 parts by mass of triethylamine (0.63 equivalent to the carboxyl group of the ethylene-acrylic acid copolymer), 6.9 parts by mass of 48% NaOH aqueous solution (ethylene -0.15 equivalent to the carboxyl group of acrylic acid copolymer), tall oil fatty acid (manufactured by Harima Chemicals Co., Ltd.) : HARTALL FA3) 3.5 parts by weight, and sealed with ion exchange water 792.6 parts by mass, from 3 hours high speed stirring at 0.99 ° C. and 5 atm, and cooled to 30 ° C..

Next, 10.4 parts by mass of γ-glycidoxypropyltrimethoxysilane (Momentive Performance Materials, trade name: TSL8350), polycarbodiimide (Nisshinbo Co., Ltd., trade name: Carbodilite SV-02, Mw) : 2,700, solid content 40% by mass) 31.2 parts by mass and ion-exchanged water 72.8 parts by mass were added and stirred for 10 minutes to emulsify the ethylene-acrylic acid copolymer and mix with each component. A polyethylene resin aqueous dispersion was obtained (resin solid content 20.3% by mass, measured according to JIS K6833 (2014)).

The above urethane resin manufactured by Toho Chemical Co., Ltd. and its aqueous dispersion were prepared by the following method.

In a 0.8 L internal volume synthesizer equipped with a stirrer, thermometer and temperature controller, polytetramethylene ether glycol (Hodogaya Chemical Co., Ltd., average molecular weight 1,000) 60 g, 1,4-cyclohexanedimethanol 14 g Then, 20 g of dimethylolpropionic acid was charged, and 30.0 g of N-methylpyrrolidone was further added. Then, 104 g of tolylene diisocyanate was charged, the temperature was raised from 80 to 85 ° C., and reacted for 5 hours. The obtained prepolymer had an NCO content of 8.9%. Further, 16 g of triethylamine was added for neutralization, a mixed aqueous solution of 16 g of ethylenediamine and 480 g of water was added, emulsified at 50 ° C. for 4 hours, and chain extension reaction was performed to obtain an aqueous urethane resin dispersion (nonvolatile resin component 29 0.1%, acid value 41.4).

(Preparation of coating liquid)
The magnesium hydroxide aqueous dispersion, the polyethylene resin aqueous dispersion or the urethane resin aqueous dispersion, and colloidal silica (manufactured by Nissan Chemical Industries, Ltd., trade name: Snowtex-XS) are mixed to obtain a resin solid content. About 5% by mass of coating liquid was prepared.

(Type of original plate)
Electro-galvanized steel sheet (EG): Thickness 0.8mm, Zinc weight per unit area: Front surface 18g / m ² , Back surface 18g / m ²

(Pretreatment of galvanized steel sheet)
Degreasing: Alkaline degreasing (Nippon Parker Rising Co., Ltd., trade name: Fine Cleaner)
Drying: Hot air drying was performed to evaporate water.

(Painting method)
Method: Bar coater Resin film thickness: The number of bars was selected so that a predetermined film thickness was obtained. About the sample which evaluates the corrosion resistance of an end surface part, the resin film of the same thickness was formed in both surfaces of the front surface and the back surface.

(Drying method)
Time: 1 minute Conditions: Maximum reached temperature of the coated plate 80 ° C (confirmed with thermo label)

Within the above range, various coated galvanized steel sheets (test Nos. 1 to 13) were prepared under various conditions as shown in Table 1 below, and the flat and end surface portions of the obtained coated galvanized steel sheets were prepared. The corrosion resistance was evaluated by the following method.

(Corrosion resistance of flat plate part)
The obtained galvanized steel sheet (sample) was subjected to a salt spray test based on JIS Z2371 (2015) for 96 hours, and the white rust generation rate (100 × area where white rust occurred / The total area of the resin-coated metal plate was calculated. And based on the following reference | standard, (circle) was set as the pass and x was evaluated as failure.

<Evaluation criteria>
○: White rust occurrence rate of 15 area% or less ×: White rust occurrence rate of over 15 area%

Further, a salt spray test based on JIS Z2371 (2015) was performed on the obtained coated galvanized steel sheet (sample) for 480 hours, and the occurrence of red rust on the sample surface was visually observed. And based on the following reference | standard, (circle) was set as the pass and x was evaluated as failure.

<Evaluation criteria>
○: Generation of red rust is not observed ×: Generation of red rust is recognized

(Corrosion resistance of the end face)
In order to align the shape of the burrs on the cut surface, a rectangular test piece having a width of 50 × 120 mm was cut out from a sample for evaluating the corrosion resistance of the end face using the same shearing equipment. The number of cut out test pieces is the test number. In any of 1 to 13, there are 12 sheets. Each test piece was subjected to a salt spray test for 96 hours in accordance with JIS Z2371 (2015) without sealing the end face. The test pieces after the salt spray test are each packed with copy paper and left in an air-conditioned room (temperature: about 23 ° C., humidity: about 30%) for 3 weeks to test the corrosion of the end face. Proceeded. After completion of the room temperature standing test, test no. Photographs of the corrosion status of the end face part with 12 test pieces stacked on each of 1 to 13, and the occurrence rate of red rust generated on each of the lower burr end face and the upper burr end face (100 × area where red rust occurred) / Area of the corresponding end face of the resin-coated metal plate) was calculated, and the average value thereof was determined as the red rust occurrence rate of the end face portion. And based on the following reference | standard, (double-circle) and (circle) were set as the pass, and (triangle | delta) was evaluated as disqualified.

<Evaluation criteria>
◎: Red rust occurrence rate at the end face portion is 15 area% or less ○: Red rust occurrence rate at the end face portion is more than 15 area% and 30 area% or less

The results are shown in Table 1 below together with the conditions (type of resin, composition ratio of resin film, [Mg (OH) ₂ / SiO ₂ ], resin film thickness) when each coated galvanized steel sheet was produced.

As is apparent from the results, the example (No. 8) in which the resin content is 60% by mass has too much resin content, and thus the corrosion resistance of the end face portion is deteriorated. In the case where the resin content is less than 25% by mass (Nos. 9 and 13), since the resin content is too small, film defects are increased, and the corrosion resistance of the end face portion and the white rust corrosion resistance of the flat plate portion are deteriorated. It was. In the examples where the mass ratio [Mg (OH) ₂ / SiO ₂ ] is outside the range of 0.2 to 1.5 (test Nos. 10 and 12), the corrosion resistance of the end face portion was deteriorated. And the example (test No. 11) whose resin film thickness is 0.2 micrometer deteriorated the corrosion resistance of the end surface part, and the white rust corrosion resistance of the flat plate part.

On the other hand, the coated galvanized steel sheet of the present invention (test Nos. 1 to 7) in which the total content, mass ratio [Mg (OH) ₂ / SiO ₂ ], and resin film thickness of silica and magnesium hydroxide were appropriately adjusted Exhibited excellent corrosion resistance in both the flat plate portion and the end face portion.

This application is based on Japanese Patent Application No. 2018-64767 filed on Mar. 29, 2018 and Japanese Patent Application No. 2019-29020 filed on Feb. 21, 2019. The contents thereof are included in the present application.

In order to express the present invention, the present invention has been described appropriately and sufficiently through the embodiments with reference to specific examples and the like. However, those skilled in the art can easily change and / or improve the above-described embodiments. It should be recognized that this is possible. Therefore, unless the modifications or improvements implemented by those skilled in the art are at a level that departs from the scope of the claims recited in the claims, the modifications or improvements are not limited to the scope of the claims. To be construed as inclusive.

The present invention has wide industrial applicability in the technical fields related to steel plates, galvanized steel plates and methods for producing them.

Claims

A coated galvanized steel sheet having a resin film containing silica and magnesium hydroxide on the surface of the galvanized steel sheet,
The total content of silica and magnesium hydroxide in the resin film is 50 to 75% by mass, and the content of the resin component of the resin film is 25 to 50% by mass,
A mass ratio of the magnesium hydroxide to the silica is 0.2 to 1.5;
A coated galvanized steel sheet, wherein the resin film has a thickness of 0.3 to 3.0 μm.