JPH08252525A - Organic composite coated steel sheet excellent in contact rusting resistance and electrodeposition coating property - Google Patents

Abstract

PURPOSE: To improve contact rusting resistance and electrodeposition coating property of an organic composite coated steel sheet. CONSTITUTION: A chromate coating film with 5-200mg/m<2> coating weight is formed on a galvanized steel sheet and, in the upper layer of the chromate coating film, 1-110 pts.wt. (solid portion) hydrophilic polyamide resin (B) 50-1000 in degree of polymerization and 5-80 pts.wt. (solid portion) polyfunctional polyisocyanate (C) having at least three isocyanate groups in a molecule per 100 pts.wt. (solid portion) urethane modified epoxy ester resin (A) having the number average molecular weight of 300-100000 are contained. Further, a resin coating film 0.2-3.0μm in dry film thickness is formed thereon by applying an organic solvent based coating material composition further containing a rust preventive agent (D) in the ratio ((A)+(B)+((C))/(D) of 90/10 to 40/60 by the weight of solid portion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic composite-coated steel sheet suitable for automobile bodies and having excellent rust resistance and electrodeposition coating property.

[0002]

2. Description of the Related Art In recent years, in cold regions such as North America and Northern Europe, corrosion of automobile bodies due to salt used for road freezing prevention in winter has become a serious social problem. Therefore, as one of rust preventive measures for automobile bodies, there is an increasing tendency to use a surface-treated steel sheet having excellent corrosion resistance in place of the conventional cold-rolled steel sheet. As such a surface-treated steel sheet, for example, in JP-A-4-236786, an organic composite excellent in corrosion resistance having a coating type chromate film on the surface of a Zn-plated steel sheet and a resin film having a film thickness of about 1 μm as an upper layer thereon. Coated steel sheets have been proposed. The organic composite-coated steel sheet disclosed in Japanese Patent Laid-Open No. 236786/1992 has a hydrophilic polyamide resin for securing the smoothness of an electrodeposition coating film, and a silica fine powder for ensuring adhesion and corrosion resistance, and hydrophilicity. In order to improve the electrodeposition coating property while ensuring good lubricity by combining polyamide resin and fine silica powder, a high corrosion resistance epoxy resin binder is used as an essential component, and these high corrosion resistance epoxy resin and hydrophilic polyamide are used. It has a resin film made of a solvent-type organic polymer resin containing a resin and fine silica powder.

[0003]

[Problems to be Solved by the Invention] On the other hand, recently, corrosion resistance in an environment where iron rust coexists in a corrosive environment (hereinafter referred to as anti-corrosion resistance) has begun to be a problem (CAMP-ISIJ vo
l.5 (1992), p.1693). That is, it has been pointed out that when the organic composite-coated steel sheet is used in such an environment, iron rust adheres to the surface of the organic coating, and the original excellent corrosion resistance of the organic composite-coated steel sheet is reduced. However, it has been found that the above-mentioned organic composite-coated steel sheet of Japanese Patent Laid-Open No. 4-236786 does not always exhibit sufficient performance with respect to this rust resistance. This is because the resin film of the coated steel sheet is the addition of a known curing agent (resole type phenol resin, melamine resin, general polyisocyanate) even if the curing agent is not added, or even if added. It is considered that this is because the crosslink density during bake hardening is not sufficiently high.

Incidentally, the known document "GALVATECH '92, p. 3
72 ”describes that if the amount of the curing agent in the organic resin is reduced to lower the crosslink density, the rust resistance of the organic composite-coated steel sheet decreases. No specific measures are shown to improve it. In view of such circumstances, the present invention aims to provide an organic composite coated steel sheet excellent in corrosion resistance (perforation resistance), paint adhesion and electrodeposition paintability as well as resistance to rust. And

[0005]

The organic composite-coated steel sheet of the present invention has been studied by the present inventors in order to achieve the above-mentioned object. It was made by discovering that two conditions, that is, the combined use of silica and a sparingly soluble chromate, are extremely effective as an anticorrosive additive, by increasing the crosslink density of the organic coating by polyfunctionalizing the polyisocyanate compound. That is, the organic composite coated steel sheet of the present invention has the following configuration.

(1) A chromate film having an adhesion amount of 5 to 200 mg / m ^{2 in} terms of metallic chromium is formed on the surface of a Zn-based plated steel sheet, and a urethane epoxy resin having a number average molecular weight of 300 to 100,000 is formed on the chromate film. 1 to 1 of the hydrophilic polyamide resin [B] having a degree of polymerization of 50 to 1000 relative to 100 parts by weight (solid content) of the ester resin [A].
10 parts by weight (solid content), 5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [C] having at least 3 isocyanate groups in one molecule, and a rust preventive additive [D] ([A] + [B] + [C]) /
An organic solvent-based coating composition, which is contained in a proportion of 90/10 to 40/60 by weight of the solid content of [D], is applied to form a resin film having a dry film thickness of 0.2 to 3.0 μm. An organic composite coated steel sheet having excellent rust resistance and electrodeposition paintability, characterized by:

(2) In the organic composite coated steel sheet of (1) above, the rust preventive additive [D] constituting the resin coating is silica or / and a sparingly soluble chromate. Organic composite coated steel sheet with excellent properties. (3) In the organic composite coated steel sheet according to (1) above, the rust preventive additive [D] constituting the resin film is silica and a sparingly soluble chromate having the following weight ratios. Organic composite coated steel sheet with excellent properties. Silica / Slightly soluble chromate = 35/5 to 1/39

(4) In the organic composite coated steel sheet of (1) to (3), the polyfunctional polyisocyanate compound [C] is a polyfunctional polyisocyanate compound having at least 4 isocyanate groups in one molecule. , Organic composite coated steel sheet with excellent rust resistance and electrodeposition paintability. (5) In the organic composite-coated steel sheet according to the above (1) to (3), the polyfunctional polyisocyanate compound [C] is a polyfunctional polyisocyanate compound having at least 6 isocyanate groups in one molecule. Organic composite coated steel sheet with excellent rust resistance and electrodeposition paintability. (6) In the organic composite coated steel sheet of (1) to (3) above, the polyfunctional polyisocyanate compound [C] is a polyfunctional hexamethylene diisocyanate having at least 6 isocyanate groups in one molecule, Organic composite coated steel sheet with excellent rust resistance and electrodeposition paintability.

[0009]

[Function] The resin film formed as the second layer on top of the chromate film formed on the surface of the zinc-based plated steel sheet suppresses excessive elution of hexavalent chromate ions in the chromate film into the corrosive environment. It has an effect of maintaining the anticorrosion effect, but in the present invention, in particular, it is formed by reacting a specific base resin with a polyfunctional polyisocyanate compound having at least three isocyanate groups in one molecule as a curing agent. Due to the action of the resin film with high cross-linking density and the rust preventive additive added in a specific ratio in the resin film (especially, the synergistic action when silica and the sparingly soluble chromate are added together), conventional organic Compared with the composite coated steel sheet, the corrosion resistance is significantly improved.

The details and reasons for limitation of the present invention will be described below. Examples of the zinc-based plated steel sheet as the base include a zinc-plated steel sheet, a Zn-Ni alloy-plated steel sheet, a Zn-Fe alloy-plated steel sheet, a Zn-Mn alloy-plated steel sheet, a Zn-Al alloy-plated steel sheet, a Zn-Cr alloy-plated steel sheet, and a Zn-Cr alloy-plated steel sheet. -Co-
Cr alloy-plated steel sheets, Zn-Cr-Ni alloy-plated steel sheets, Zn-Cr-Fe alloy-plated steel sheets, and zinc-based composite-plated steel sheets in which metal oxides, sparingly soluble chromates, polymers and the like are dispersion-plated Can be mentioned. Further, it may be a multi-layer plated steel sheet obtained by plating two or more layers of the same or different types of plating as described above. As the plating method, it is possible to employ any method that can be carried out among the electrolytic method, the melting method, and the vapor phase method,
From the selectivity of the cold-rolled steel sheet as the base, the electrolytic method is advantageous.

The chromate film formed on the surface of the zinc-plated steel sheet suppresses corrosion of the zinc-plated steel sheet by the self-repairing action of hexavalent chromium by chromate ions. If the adhesion amount of this chromate film is less than 10 mg / m ^{2 in} terms of metal chromium, sufficient corrosion resistance cannot be expected, while if it exceeds 200 mg / m ² , weldability deteriorates. For this reason, the amount of the chromate film deposited is set to 10 to 200 mg / m ^{2 in} terms of metallic chromium. Further, in order to satisfy higher corrosion resistance and weldability, it is preferably in the range of ²⁰ to 100 mg / m ^{2 in} terms of metallic chromium. As the chromate treatment for forming the chromate film, any of reaction type, electrolytic type, and coating type methods can be applied. From the viewpoint of corrosion resistance, the coating type in which the chromate film contains a large amount of hexavalent chromium chromate ions is preferable.

In the coating-type chromate treatment, a treatment liquid containing a partially reduced chromic acid aqueous solution as a main component and one or more of the following components, if necessary, is added to a zinc-based plated steel sheet. Apply to and dry without washing with water. Water-soluble or water-dispersible organic resin such as acrylic resin, polyester resin, etc. Colloids of oxides such as silica, alumina, titania, zirconia, zinc oxide and / or powders Acids such as molybdic acid, tungstic acid, vanadic acid and / or Or salts thereof Phosphoric acids such as phosphoric acid and polyphosphoric acid Zirconium fluoride, fluorides such as silicofluoride and titanium fluoride Metal ions such as zinc ion Iron phosphide, conductive fine powder such as antimony-doped tin oxide Hydrogen fluoride Silane coupling agent In the coating type chromate treatment, the treatment liquid is usually applied by a roll coater method, but it is also possible to adjust the coating amount by an air knife method or a roll squeezing method after applying by a dipping method or a spray method. .

In the upper layer of the chromate film, the degree of polymerization is 5 with respect to 100 parts by weight (solid content) of the urethanized epoxy ester resin [A] having a number average molecular weight of 300 to 100,000.
0 to 1000 of hydrophilic polyamide resin [B] is added to 1 to 11
0 to 50 parts by weight (solid content), 5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [C] having at least 3 isocyanate groups in one molecule, and a rust preventive additive [D] A resin film is formed by applying an organic solvent-based coating composition containing the solid content of ([A] + [B] + [C]) / [D] in a weight ratio of 90/10 to 40/60. Form.

The urethanated epoxy ester resin [A] which is a binder resin is an epoxy resin containing 50 wt% or more of phenol in the molecule as a solid content,
There is an epoxy ester resin obtained by reacting this epoxy resin with a dicarboxylic acid in the presence or absence of an amine catalyst, and a urethanized epoxy ester resin obtained by reacting this epoxy ester resin with a partially blocked isocyanate compound, These urethanized epoxy ester resins are used alone or as a mixture. When the number average molecular weight of the urethanized epoxy ester resin [A] is less than 300, the adhesiveness and the corrosion resistance are deteriorated. On the other hand, the number average molecular weight that can be used as a coating by dissolving the resin in an organic solvent is 100,000.

The hydrophilic polyamide resin [B] forms the most important skeleton in the resin film, is a highly hydrophilic and highly hygroscopic organic resin having many acid amide bonds, and has a large degree of polymerization and an extremely high molecular weight. It has great features. As hydrophilic polyamide resin, nylon 6 and nylon 6
6 or copolymers thereof with other nylons; polyether polyol-modified nylon, polyesterol-modified nylon or polybutadiene polyol-modified nylon; aromatic polyamides such as polymetaphenylene isophthalamide and polyparaphenylene terephthalamide. be able to. By including such a hydrophilic polyamide resin in the resin film, the electrodeposition liquid permeates into the coating film during electrodeposition coating, and the electric resistance value of the coating film decreases, resulting in coating electrodeposition and electrodeposition coating film. Appearance (preventing generation of gas pins, scratched skin, etc.) is significantly improved. In addition, addition of the polyamide resin to the resin film is also desirable in order to impart flexibility and high processability to the resin film.

The fact that the hydrophilic polyamide resin has a high degree of polymerization and a high molecular weight is effective in preventing the swelling and dissolution of the resin film during pretreatment for electrodeposition coating (particularly during alkali degreasing) or during electrodeposition coating. is there. If the degree of polymerization of the hydrophilic polyamide resin is less than 50, alkali degreasing prior to electrodeposition coating or dissolution of the resin film during electrodeposition coating will result in inability to obtain a uniform appearance after coating and decrease in acid resistance. The degree of polymerization is 1
If it exceeds 000, the polymer becomes too high in molecular weight and is not suitable for the present invention. The degree of polymerization of the hydrophilic polyamide resin is 50 to 5
A range of 00 is particularly preferable. When the compounding amount (solid content) of the hydrophilic polyamide resin [B] is less than 1 part by weight with respect to 100 parts by weight (solid content) of the urethanized epoxy ester resin [A], uniform electrodeposition property (prevention of generation of gas pins and skin damage) However, if the blending amount exceeds 110 parts by weight, the adhesion of the electrodeposition coating film will deteriorate, which is not preferable.

In the present invention, as a means for ensuring excellent anticorrosion and rust resistance, a curing agent is added as an essential component, and a specific polyfunctional polyisocyanate compound [C] is added as the curing agent to prepare a base resin. It is characterized by causing a urethanization reaction between the hydroxyl group and the isocyanate group in the polyisocyanate.

The polyisocyanate compound used as the curing agent for the resin film of the present invention has three or more isocyanate groups in one molecule (even if these isocyanate groups are blocked, for the purpose of improving rust resistance. It is a polyfunctional isocyanate compound having That is, a monoisocyanate compound having one isocyanate group in one molecule or a diisocyanate compound having two isocyanate groups in one molecule cannot give sufficient rust resistance to the resin film. On the other hand, in the present invention, a polyfunctional isocyanate compound having 3 or more isocyanate groups in one molecule, more preferably 4
It has been found that a polyfunctional isocyanate compound having one or more, and particularly preferably six or more, isocyanate groups can impart much more excellent resistance to rusting than a monoisocyanate compound or a diisocyanate compound.

Among the polyfunctional polyisocyanate compounds having 6 or more isocyanate groups in one molecule, the hexamethylene diisocyanate polyfunctional compound is most effective in resisting rusting. The polyfunctional polyisocyanate compound used in the present invention may be a mixture of homologous compounds having different numbers of isocyanate groups in one molecule. In addition, the above polyfunctional polyisocyanate compound
You may use together more than one kind.

As such a polyfunctional polyisocyanate compound having three or more isocyanate groups in one molecule, a compound having three or more isocyanate groups in one molecule, or at least two or more isocyanates in one molecule There are compounds obtained by reacting a compound having a group with a polyhydric alcohol, or compounds such as a buret type adduct and an isocyanuric ring type adduct thereof. For example, triphenylmethane-4,4 ', 4 "-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4
Polyisocyanate compounds having 3 or more isocyanate groups such as 6-triisocyanatotoluene, 4,4'-dimethyldiphenylmethane-2,2 ', 5,5'-tetraisocyanate; ethylene glycol, propylene glycol, 1,4 An adduct obtained by reacting a polyisocyanate compound in an amount such that an isocyanate group is in excess with respect to a hydroxyl group of a polyol such as butylene glycol, polyalkylene glycol, trimethylolpropane and hexanetriol; hexamethylene diisocyanate, isophorone diisocyanate, tri Biuret type adduct such as diisocyanate, xylylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-methylenebis (cyclohexyl isocyanate) or And the like Soshianuru ring type adducts.

In the adduct obtained by reacting the polyisocyanate compound in an amount such that the isocyanate group becomes an excess amount with respect to the hydroxyl group of the polyol, the polyisocyanate compound is a polyisocyanate compound having three or more isocyanate groups. And aliphatic diisocyanate compounds such as hexamethylene diisocyanate, 1,4-tetramethylene diisocyanate, dimer acid diisocyanate and lysine diisocyanate; isophorone diisocyanate, 4,4'-methylenebis (cyclohexyl isocyanate), methylcyclohexane-
2,4- (or -2,6-) diisocyanate, 1,3
Alicyclic diisocyanate compounds such as-(or 1,4-) di (isocyanatomethyl) cyclohexane; and xylylene diisocyanate, tolylene diisocyanate, m
Aromatic diisocyanate compounds such as-(or p-) phenylene diisocyanate, diphenylmethane diisocyanate, and bis (4-isocyanatophenyl) sulfone can be exemplified.

In order to stably store the film-forming product, it is necessary to protect the curing agent, isocyanate. As this method, a protecting method in which a protecting group (blocking agent) is desorbed during heat curing and an isocyanate group is regenerated can be adopted. Examples of the protective agent (blocking agent) include (1) aliphatic monoalcohols such as methanol, ethanol, propanol, butanol, and octyl alcohol (2) monoethers of ethylene glycol and / or diethylene glycol, such as methyl and ethyl , Propyl (n-, iso), butyl (n-, iso, sec) and other monoethers (3) Phenols, cresols and other aromatic alcohols (4) Acetoximes, methyl ethyl ketone oximes and other oximes By reacting one or more of the isocyanate compounds with the isocyanate compound, an isocyanate compound that is stably protected at room temperature is obtained.

If the compounding amount (solid content) of the polyfunctional polyisocyanate compound [C] is less than 5 parts by weight with respect to 100 parts by weight (solid content) of the urethanized epoxy ester resin [A], the crosslinking density of the formed film is insufficient. Therefore, the effect of improving rust resistance is small. On the other hand, if the amount is more than 80 parts by weight, the unreacted residual isocyanate absorbs water and not only has no effect on rust resistance, but also adversely affects corrosion resistance (perforation resistance) and adhesion. The compounding amount of the polyfunctional polyisocyanate compound is particularly preferably in the range of 10 to 50 parts by weight.

Further, as a cross-linking agent, a part or all of the methylol compound obtained by reacting one or more selected from melamine, urea and benzoguanamine with formaldehyde is reacted with a monohydric alcohol having 1 to 5 carbon atoms. The alkyl etherified amino resin thus obtained may be used in combination with the above isocyanate compound. The resin is sufficiently cross-linked with the above-mentioned cross-linking agent, but it is desirable to use a known curing-accelerating catalyst in order to further increase the low temperature cross-linking property. As the curing accelerating catalyst, for example, N-ethylmorpholine, dibutyltin dilaurate, cobalt naphthenate, stannous chloride, zinc naphthenate, bismuth nitrate or the like can be used. In addition, for the purpose of slightly improving the physical properties such as adhesion, the known acrylic resin for the resin composition,
Resins such as alkyd and polyester can also be added.

As the organic solvent species used in the coating composition, one or a mixture of two or more organic solvents commonly used in the coating industry can be used. For that purpose, an alcohol solvent having a high boiling point is used. Is preferably avoided. Examples thereof include ethylene glycol or diethylene glycol, monoalkyl ethers, and alcohols having a primary hydroxyl group of C ₅ or more. Such a solvent hinders the curing reaction of the film. Recommended solvents include hydrocarbons, ketones (methyl isobutyl ketone,
Acetone, ciflohexanone, isophorone, etc.), S-based solvents, ether-based solvents, and alcohols having a low molecular weight of C ₄ or less, or alcohols having secondary or tertiary hydroxyl groups are also suitable.

In the present invention, a rust preventive additive [C] is added to the resin film. As the rust preventive additive [C], silica and a sparingly soluble chromate are particularly preferable. Either one of these silica and the sparingly soluble chromate may be added alone, but by adding both of them in a specific ratio, excellent anti-corrosion and puncture resistance can be realized. Silica has the effect of promoting the formation of basic zinc chloride, which is effective in suppressing corrosion, among the corrosion products of galvanized steel sheets, and dissolves in trace amounts in the corrosive environment to form silicate ions. It is presumed that the anticorrosion effect is exerted by functioning as a type corrosion inhibitor.

The silica used in the present invention is dry silica (for example, AEROS manufactured by Nippon Aerosil Co., Ltd.).
IL 130, AEROSIL 200, AEROSIL
300, AEROSIL 380, AEROSIL R
972, AEROSIL R811, AEROSIL R
805), organosilica sol (for example, MA-ST, IPA-ST, NBA-S manufactured by Nissan Chemical Industries, Ltd.).
T, IBA-ST, EG-ST, XBA-ST, ETC
-ST, DMAC-ST, etc., precipitated wet silica (eg, T-32 (S) manufactured by Tokuyama Soda Co., Ltd., K-41, F-).
80 etc.), gel wet silica (for example, Syloid 244, Syloid 150, Syloid 72, Syloid 65, SHIELDEX manufactured by Fuji Davisson Chemical Co., Ltd.)
Etc. can be mentioned. Further, two or more kinds of the above silica can be mixed and used.

These silicas include hydrophilic silicas and hydrophobic silicas. Of these, hydrophilic silica can be expected to have an effect of improving resistance to rust, but as described later, hydrophobic silica significantly improves resistance to rust. Hydrophilic silica has a hydroxyl group (

Embedded image ), It is hydrophilic. Since this silanol group is highly reactive, it easily reacts with various organic compounds, and the surface of silica can be organized. Hydrophobic silica is one in which silanol groups on the surface of such hydrophilic silica are partially or almost entirely substituted with a methyl group or an alkyl group to make the silica surface hydrophobic.

There are various methods for producing hydrophobic silica, and the representative ones are reactions of organic solvents such as alcohols, ketones and esters, silanes, silazanes, polysiloxanes and the like. Examples of the method include a reaction pressurization method in an organic solvent and a catalyst heating method. Silica has an excellent anticorrosion effect, but especially, hydrophobic silica has a large effect of improving rust resistance by resisting. The reason for this is that hydrophilic silica is apt to cause permeation of iron ions or iron oxides in iron rust due to its strong hydrophilicity, which is presumed to be the reason why it is less effective in improving rust resistance. Therefore, in the present invention, it is preferable to use hydrophobic silica.

Further, the sparingly soluble chromate added to the organic film is dissolved in a corrosive environment in a trace amount to give 6
It is thought that it releases valent chromate ions and suppresses corrosion of the zinc-based plated steel sheet by a mechanism similar to that of the chromate film. The sparingly soluble chromate used in the present invention includes barium chromate (BaCrO ₄ ), strontium chromate (SrCrO ₄ ), and calcium chromate (CaCr).
O ₄ ), zinc chromate (ZnCrO ₄ .4Zn (O
H) ₂ ), zinc potassium chromate (K ₂ O ・ 4ZnO ・ 4
Fine powders such as CrO ₃ .3H ₂ O) and lead chromate (PbCrO ₄ ) can be used. It is also possible to use a mixture of two or more of the above-mentioned hardly soluble chromates. However, from the viewpoint of corrosion resistance, it is preferable to use barium chromate or strontium chromate, which can be expected to have a self-repairing effect by chromate ions for a long period of time. In addition, barium chromate, which has a low solubility in water, is preferable from the viewpoint of minimizing the elution of water-soluble chromium from the organic film in the pretreatment process for coating on automobiles.

In the present invention, silica and a sparingly soluble chromate are blended in a specific ratio in the resin composition comprising the above-mentioned specific base resin and polyfunctional polyisocyanate compound, whereby synergistic anticorrosive effects of both are obtained. Depending on the effect
The best resistance to rust can be achieved. That is, the weight ratio of silica and sparingly soluble chromate to the nonvolatile content is (urethaned epoxy ester resin [A] + hydrophilic polyamide resin [B] + polyfunctional polyisocyanate compound [C]) / (silica + sparingly soluble) Chromate = 90/10
40/60 Silica / Slightly soluble chromate = 35/5 to 1/39 In the case of being compounded in the ratio, the most excellent corrosion resistance can be obtained. Here, when (urethane-ized epoxy ester resin [A] + hydrophilic polyamide resin [B] + polyfunctional polyisocyanate compound [C]) / (silica + slightly soluble chromate) exceeds 90/10, silica and Corrosion resistance is inferior because the anticorrosion effect of the poorly soluble chromate is not fully exerted. On the other hand, if it is less than 40/60,
The effect of the resin as a binder becomes insufficient, and the paint adhesion and the like deteriorate. Further, if the silica / slightly soluble chromate exceeds 35/5, or if it is less than 1/39, the synergistic effect becomes insufficient, and the rust resistance is slightly deteriorated.

As described above, silica suppresses corrosion due to rust due to the effect of promoting the formation of stable corrosion products, while the poorly soluble chromate causes defects in the organic film formed by rust. It has the effect of repairing due to the effect of hexavalent chromate ions, and it is the first time that excellent corrosion resistance rust can be obtained by using silica and a sparingly soluble chromate in combination, which have different mechanisms for suppressing corrosion due to such corrosion rust. It was possible to achieve the sex.

FIG. 1 shows an organic resin (No. 2 in Table 2) consisting of the above-mentioned specific base resin and an isophorone diisocyanate-based 6-functional polyisocyanate compound, in which the ratio of silica and sparingly soluble chromate is changed. Unpainted corrosion resistance (evaluation after 200 cycles of puncture resistance test) and resistance to rust (wear rust test 7)
The relationship between the evaluation after the cycle) and the weight ratio of silica / hardly soluble chromate is shown. According to this, if the weight ratio of silica / hardly soluble chromate exceeds 35/5, the rust resistance deteriorates, and if it is less than 1/39, the normal unpainted corrosion resistance (perforation resistance) deteriorates. To do. Therefore, the compounding ratio of silica / slightly soluble chromate is preferably 35/5 to 1/39, and is 20 / when particularly the rust resistance is important.
The range of 20 to 1/39 is preferably set to the range of 20/20 to 5/35 in order to obtain the most excellent corrosion resistance (bearing rust resistance and perforation resistance). Further, FIG. 2 shows, for comparison, the relationship between the rust resistance and the weight ratio of silica / poorly soluble chromate when a conventional diisocyanate compound (HMDI) is used as a curing agent. From FIG. 1 and FIG. 2, it can be seen that the present invention can be achieved for the first time by the synergistic effect of the combination of the polyfunctional polyisocyanate compound, the specific ratio of silica and the sparingly soluble chromate.

Further, in FIG. 4, unpainted corrosion resistance (penetration resistance test 2 in the case of using hexamethylene diisocyanate-based 6-functional polyisocyanate compound as a curing agent)
The relationship between the evaluation after 00 cycles) and the weather rust resistance (evaluation after 15 cycles of the weather rust resistance test) and the weight ratio of silica / hardly soluble chromate is shown. According to this,
As in the case of using an isophorone diisocyanate-based hexafunctional polyisocyanate compound, the compounding ratio of silica / hardly soluble chromate is preferably 35/5 to 1/39, and
Especially when giving importance to rust resistance, 20/20 to 1 /
The range of 39 is 20/20 in order to obtain the best corrosion resistance (corrosion resistance, rust resistance and puncture resistance).
It is preferably in the range of 5 to 35. Furthermore, examples described later (for example, comparison of No. 70 and No. 73)
As is clear from the above description, when the hexamethylene diisocyanate-based 6-functional polyisocyanate compound and the isophorone diisocyanate-based 6-functional polyisocyanate compound are compared, when the mixing ratio of silica / hardly soluble chromate is the same, In the case of using a hexamethylene diisocyanate-based hexafunctional polyisocyanate compound, more excellent resistance to rust can be obtained.

In the present invention, the above-mentioned silica and sparingly soluble chromate are the main additive components to the resin composition, but in addition to them, a silane coupling agent and a coloring pigment (for example, a condensed polycyclic ring). Type organic pigments, phthalocyanine type organic pigments, etc.), coloring dyes (eg, azo type dyes, azo type metal complex salt dyes, etc.), rust preventive pigments (eg, aluminum tripolyphosphate dihydrogen phosphate, aluminum phosphomolybdate, zinc phosphate, etc.) ), A conductive pigment (for example, iron phosphide, antimony-doped tin oxide, etc.), a surfactant, and the like, may be further blended. In addition, the workability can be improved by adding at least one of the following as a lubricant. Polyolefin wax (polyethylene wax, polypropylene wax, etc.) Fluorine resin (tetrafluoroethylene resin, tetrafluoroethylene-hexafluoropropylene copolymer resin, fluoroethylene-
Perfluoroalkyl vinyl ether copolymer resin, tetrafluoroethylene-ethylene copolymer resin, trifluoroethylene chloride resin, vinylidene fluoride resin, etc.) Graphite molybdenum disulfide boron nitride May be.

The organic film as described above is formed on the chromate film by 0.2 to 3.0 μm, preferably 0.5 to 2.0 μm.
It is formed with a film thickness of m. When the film thickness is less than 0.2 μm, sufficient rust resistance cannot be obtained, while 3.0 μm
If it exceeds the range, the weldability (particularly continuous spotting property) deteriorates. FIG.
The results of examining the relationship between the film thickness of the organic coating and the spot weldability (continuous spotting property) are shown in FIG. According to this, the film thickness is 3.
It can be seen that if it exceeds 0 μm, the spot weldability deteriorates.

As a method of applying the above coating composition to a zinc-based plated steel sheet, the coating composition is usually applied by a roll coater method, but after applying by a dipping method or a spray method, an air knife method or a roll drawing method is applied. It is also possible to adjust the coating amount by. Further, as the heat treatment method after applying the coating composition, a hot air oven, a high frequency induction heating oven, an infrared oven, or the like can be used. The heat treatment is performed at an ultimate plate temperature of 80 to 250 ° C, preferably 100 to 200 ° C.
It is desirable to carry out within the range. Furthermore, when the present invention is applied to a BH steel sheet, heat treatment at 150 ° C. or lower is desirable. The steel sheet of the present invention has a great feature that it can be obtained by such low temperature baking.

If the baking temperature is less than 80 ° C., the cross-linking of the film does not proceed and sufficient corrosion resistance cannot be obtained.
When it is baked at a high temperature of over 50 ° C, the corrosion resistance deteriorates. This is due to the volatilization of water contained in the chromate film components and the hydroxyl groups (

Embedded image ) The rapid progress of the dehydration condensation reaction between the two promotes the destruction of the chromate film due to the occurrence of cracks in the chromate film, and the reduction of hexavalent chromium to reduce the passivation effect of hexavalent chromium. It is estimated that Although the car body is subjected to cationic electrodeposition coating, the wet electrical resistance of the chromate film + organic film is 200
If it exceeds kΩ / cm ² , there is a problem that the cationic electrodeposition coating is not formed well. Therefore, in the steel sheet of the present invention mainly used for automobile bodies, the wetting resistance of the chromate film + resin composition film is 200 kΩ / cm ² or less. It is preferable to form both films so that

The present invention includes a steel sheet having the coating structure as described above on both sides or one side. Examples of aspects of the steel sheet of the present invention include the following. (1) One side: plating film-chromate film-organic film One side: Fe surface (2) One side: plating film-chromate film-organic film One side: plating film (3) Both sides ... plating film-chromate film-organic film The organic composite coated steel sheet of the invention is not limited to automobiles,
It can also be used for home appliances, building materials, etc.

[0040]

[Examples] As an organic composite coated steel sheet for automobile bodies, a zinc-based plated steel sheet was degreased with alkali, washed with water and dried, subjected to chromate treatment, and then the coating composition was applied by a roll coater and baked. The obtained organic composite coated steel sheet was tested for puncture resistance, rust resistance, paint adhesion, and weldability. The results are shown in Table 5 to Table 15.
Shown in The manufacturing conditions of this example are as follows. (1) Zinc-based plated steel sheet A cold-rolled steel sheet having a thickness of 0.8 mm and a surface roughness (Ra) of 1.0 μm was subjected to various zinc-based plating and used as a processed original plate.
(See Table 1)

(2) Chromate Treatment Coating Chromate Treatment A chromate treatment liquid having the following liquid composition was applied by a roll coater and dried without washing with water. The amount of the chromate layer deposited was adjusted by changing the peripheral speed ratio of the pickup roll of the roll coater and the applicator roll. Chromic anhydride: 20 g / l Phosphate ion: 4 g / l Zirconium fluoride ion: 1 g / l Zinc ion: 1 g / l Hexavalent chromium / Trivalent chromium: 3/3 (weight ratio) Chromic anhydride / zirconium fluoride Ion: 20/1
(Weight ratio)

Electrolytic chromate treatment Chromic anhydride 30 g / l, sulfuric acid 0.2 g / l, bath temperature 40
The zinc-plated steel sheet was subjected to cathodic electrolysis at a current density of 10 A / dm ² using a treatment liquid at 0 ° C., washed with water and dried.
The amount of the chromate layer deposited was adjusted by controlling the amount of electricity applied in the cathodic electrolysis treatment. Reactive chromate treatment Chromic anhydride 30g / l, phosphoric acid 10g / l, NaF
A zinc-based plated steel sheet was spray-treated with a treatment liquid of 0.5 g / l, K ₂ TiF ₆ 4 g / l and a bath temperature of 60 ° C., washed with water and dried. The adhesion amount of the chromate layer was adjusted by changing the treatment time.

(3) Organic Resin Table 2 shows the organic resin used. The base resin (urethane epoxy resin) and the curing agent (polyisocyanate) shown in the same table were prepared by the following method.

[Production of Urethaned Epoxy Ester Resin] Production of Isocyanate Compound Hexamethylene diisocyanate: 168 parts and methyl isobutyl ketone: 168 parts were added to a reactor equipped with a reflux condenser, a stirrer, a thermometer and a nitrogen gas blowing device. After weighing it out, dissolving it uniformly and raising the temperature to 80 ° C., a solution of 87 parts of methyl ethyl ketone oxime and 87 parts of methyl isobutyl ketone was added dropwise over 1 hour, and further 80 ° C.
And reacted for 3 hours to obtain an isocyanate compound X having a nonvolatile content of 50%. The isocyanate equivalent of this isocyanate compound X was 510.

Urethaned Epoxy Ester Resin A1 A reactor equipped with a reflux condenser, a stirrer, a thermometer and a nitrogen gas blowing device was used, and 250 parts of Epicoat 1001 (Epoxy Shell Epoxy Co., Ltd .: epoxy equivalent 500) was xylol. After dissolving in 50 parts and 63 parts of cellosolve acetate, 39.2 parts of azelaic acid and 4.3 parts of diethanolamine were added and reacted at 145 ° C. for 6 hours,
When the acid value of the resin solid content reached 1.1, 110 parts of xylol and 68 parts of methyl ethyl ketone were added and cooled. Next, the isocyanate compound X obtained above
200 parts (solid content) and cellosolve acetate 200
And 100 parts of isopropyl alcohol was added and cooled to obtain urethane-modified epoxy ester resin A1.

Urethane-ized epoxy ester resin A2 475 parts of Epicoat 1004 (Epoxy Shell Epoxy Co., Ltd .: epoxy equivalent 950) was xylol in a reactor equipped with a reflux condenser, a stirrer, a thermometer and a nitrogen gas blowing device. After dissolving in 95 parts and 119 parts of cellosolve acetate, 29.2 parts of adipic acid and 8.3 parts of diethanolamine were added, and the mixture was reacted at 145 ° C. for 6 hours,
When the acid value of the resin solid content reached 1.1, 209 parts of xylol and 13 parts of methyl ethyl ketone were added and cooled. Next, the isocyanate compound X obtained above
150 parts (solid content) and cellosolve acetate 150
And 100 parts of isopropyl alcohol was added and cooled to obtain urethane-modified epoxy ester resin A2.

[Curing agent] 6-functional isocyanate (curing agent C1) 222 parts of isophorone diisocyanate and 34 parts of methyl isobutyl ketone were weighed into a reaction vessel equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel, After being uniformly dissolved, 87 parts of methyl ethyl ketone oxime was added dropwise from the dropping funnel to the stirred isocyanate solution kept at 70 ° C. for 2 hours. Then, 30.4 parts of sorbitol was added, the temperature was raised to 120 ° C., and the reaction was carried out at 120 ° C. After that, IR measurement of this reaction product is performed,

[Equation 1] It was confirmed that there was no absorption due to the isocyanate group, and 50.4 parts of butyl cellosolve was added to obtain a curing agent C1. The active ingredient of this curing agent C1 was 80%.

Tetrafunctional isocyanate (curing agent C2) 222 parts of isophorone diisocyanate and 34 parts of methyl isobutyl ketone were weighed and uniformly dissolved in a reaction vessel equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel. Then, 87 parts of methyl ethyl ketone oxime was added dropwise from the dropping funnel to the stirred isocyanate solution kept at 70 ° C. for 2 hours. Then, 34 parts of pentaerythritol was added, the temperature was raised to 120 ° C, and the reaction was carried out at 120 ° C. After that, IR measurement of this reaction product is performed,

[Equation 2] It was confirmed that there was no absorption due to the isocyanate group of, and 52 parts of butyl cellosolve was added to obtain a curing agent C2. The active ingredient of this curing agent C2 was 80%.

Trifunctional isocyanate (curing agent C3) Duranate TPA-100 (HMD) was added to a reaction vessel equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel.
I isocyanuric ring type; manufactured by Asahi Kasei Co., Ltd. 550 parts and methyl isobutyl ketone 34 parts were weighed out and uniformly dissolved, and then 270 parts of methyl ethyl ketone oxime was kept at 70 ° C. from the dropping funnel to prepare an agitated isocyanate solution. It dripped in 2 hours. After that, IR measurement of this reaction product is performed,

(Equation 3) It was confirmed that there was no absorption by the isocyanate group of, and 47 parts of butyl cellosolve was added to obtain a curing agent C3. The active ingredient of this curing agent C3 was 90%.

Bifunctional Isocyanate (Curing Agent C4) Takenate B-870N (MEK oxime block of IPDI; Takeda Pharmaceutical Co., Ltd.) was used as a curing agent C4. Hexamethylene diisocyanate-based 6-functional isocyanate (hardening agent C5) Hexamethylene diisocyanate-based 6-functional isocyanate compound Duranate MF-B80M (HMD
I-type hexafunctional isocyanate oxime block: manufactured by Asahi Kasei Co., Ltd. was used as a curing agent C5.

The evaluation method of each characteristic is as follows. (A) Corrosion resistance test (perforation resistance) After tape-sealing the edge and the back surface of an unpainted test piece, a cross cut was put on the lower half surface of the test piece, and a corrosion acceleration test of the following complex corrosion test cycle was performed. The evaluation was made based on the degree of progress of corrosion after 200 cycles. 5% NaCl spray, 35 ° C. (4 hours) ↓ Drying, 60 ° C. (2 hours) ↓ 95% RH wetness, 50 ° C. (4 hours) The evaluation criteria are as follows. ◎: No red rust generation ○ +: Red rust area ratio less than 5% ○: Red rust area ratio 5% or more and less than 10% ○-: Red rust area ratio 10% or more and less than 20% △: Red rust area ratio 20% or more, 50% Less than ×: Red rust area ratio 50% or more

(B) Anti-corrosion rust resistance After tape-sealing the edge and the back surface of an unpainted test piece, an accelerated corrosion test was conducted by the following complex corrosion test cycle in the presence of iron rust. 7 for 1-69
The degree of rust generation after the cycle is also shown in Example No. 70-
For 124, the degree of rust generation after 15 cycles was evaluated. In the presence of iron rust (* Note) 5% NaCl immersion · 50 ° C (18 hours) ↓ 95% RH wet · 50 ° C (3 hours) ↓ Drying · 60 ° C (3 hours) (* Note) Iron rust supply method: A cold-rolled steel sheet having an area of 10 cm ² was soaked per liter of salt water. The evaluation criteria are as follows. ◎: No red rust generation ○: Red rust area ratio less than 10% ○-: Red rust area ratio 10% or more, less than 20% △: Red rust area ratio 20% or more, less than 50% ×: Red rust area ratio 50% or more

(C) Paint adhesion After degreasing the test piece with alkali, Nippon Paint Co., Ltd. U-
Electrodeposition coating (thickness: 25 μm) was performed at 600, and then topcoat coating (thickness: 35 μm) was performed with LugaBake B-531 manufactured by Kansai Paint Co., Ltd. These tests were immersed in ion exchange water at 40 ° C. for 240 hours. Then, the test piece was taken out and allowed to stand at room temperature for 24 hours. Then, 100 cross-cuts were cut on the coating film at intervals of 2 mm, the adhesive tape was adhered and peeled off, and the peeling rate of the coating film was evaluated. The evaluation criteria are as follows. ◎: No peeling ○: Peeling rate less than 3% △: Peeling rate 3% or more and less than 10% ×: Peeling rate 10% or more

(D) Weldability A CF type electrode, a pressure of 200 kgf, an energizing time of 10 cycles / 50 Hz, and a welding current of 10 kA were subjected to a continuous spotting property test, and the continuous spotting number was evaluated. The evaluation criteria are as follows. ◎: 2000 points or more ○: 1000 points or more, less than 2000 points ×: less than 1000 points

(E) Electrodeposition coating property The test piece was degreased in the same manner as in the corrosion resistance test, and then electrodeposition coating (film thickness 20 μm) was performed with the electrodeposition coating power top U-600 manufactured by Nippon Paint Co., Ltd. Baking at 165 ° C for 25 minutes. Electrodeposition coatability is confirmed by visual inspection of the coating appearance.
The following criteria evaluated. ◎: Smooth and uniform appearance ○: Very slight surface roughness but good appearance △: Large surface roughness or craters ×: Many craters or non-electrodepositable

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

[Table 4]

[0060]

[Table 5]

[0061]

[Table 6]

[0062]

[Table 7]

[0063]

[Table 8]

[0064]

[Table 9]

[0065]

[Table 10]

[0066]

[Table 11]

[0067]

[Table 12]

[0068]

[Table 13]

[0069]

[Table 14]

[0070]

[Table 15]

* 1 Origin: Inventive example, ratio: Comparative example * 2 No. * 3 Chromate adhesion amount in terms of metallic chromium * 4 No. of organic resins listed in Table 2 * 5 No. of silica listed in Table 3 * 6 No. of poorly soluble chromate described in Table 4 * 7 Nonvolatile content weight ratio * 8 Nonvolatile content weight ratio

As is clear from the above examples, the organic composite-coated steel sheet of the present invention exhibits excellent corrosive rust resistance. Further, particularly when a polyfunctional polyisocyanate having at least 4, preferably 6 or more isocyanate groups in one molecule is used as a curing agent, it exhibits more excellent corrosive rust resistance. Example No. 70 and the example Nos. Comparing No. 73, it can be seen that the use of a hexamethylene diisocyanate-based 6-functional polyisocyanate compound as a curing agent provides better resistance to rust than the use of an isophorone diisocyanate-based 6-functional polyisocyanate compound.

The above-mentioned organic composite coated steel sheet of the present invention is excellent in rust resistance and puncture resistance, and also has coating adhesion,
It is also extremely useful as a surface-treated steel sheet for automobile bodies because it has excellent electrodeposition paintability and weldability.

[Brief description of drawings]

FIG. 1 shows silica and a sparingly soluble chromate added at different ratios to an organic resin composed of a specific base resin defined by the present invention and a polyfunctional polyisocyanate compound (isophorone diisocyanate-based 6-functional polyisocyanate compound). In the case of the above, the weight ratio of silica / hardly soluble chromate and normal unpainted corrosion resistance and rust resistance (proof rust test 7
Graph showing the relationship with the evaluation after the cycle)

FIG. 2 shows the weight ratio of silica / hardly soluble chromate and rust resistance when silica and poorly soluble chromate are added in different ratios to an organic resin using a conventional diisocyanate compound as a curing agent. Graph showing the relationship with

FIG. 3 is a graph showing the relationship between the film thickness of the organic film and the weldability.

FIG. 4 shows silica / silica and sparingly soluble chromate added in different ratios to an organic resin composed of a specific base resin defined by the present invention and a hexamethylene diisocyanate-based hexafunctional polyisocyanate compound. A graph showing the relationship between the weight ratio of the poorly soluble chromate and ordinary unpainted corrosion resistance and rust resistance (evaluation after 15 cycles of rust resistance test).

Claims (6)

[Claims] 1. A chromate film having an adhesion amount of 5 to 200 mg / m ^{2 in} terms of metallic chromium is formed on the surface of a Zn-based plated steel sheet, and a number average molecular weight of 3 is formed on the chromate film.
The degree of polymerization is 50 with respect to 100 parts by weight (solid content) of the urethane-modified epoxy ester resin [A] of 100 to 100,000.
1 to 110 of hydrophilic polyamide resin [B]
5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [C] having at least 3 isocyanate groups in one molecule is contained, and a rust preventive additive [D] ( [A] + [B] + [C]) / [D] solid content weight ratio of 90/10 to 40/60 is applied to the organic solvent-based coating composition, and the dry film thickness is 0. .2
A resin film having a thickness of up to 3.0 μm is formed,
Organic composite coated steel sheet with excellent rust resistance and electrodeposition paintability. 2. A rust preventive additive [D] which constitutes a resin film.
Is silica or / and a sparingly soluble chromate, and the organic composite-coated steel sheet excellent in rust resistance and electrodeposition coating property according to claim 1. 3. A rust preventive additive [D] which constitutes a resin film.
Is a silica and a sparingly soluble chromate having the following weight ratios, and the organic composite-coated steel sheet excellent in rust resistance and electrodeposition coating property according to claim 1. Silica / Slightly soluble chromate = 35/5 to 1/39 4. A polyfunctional polyisocyanate compound [C]
Is a polyfunctional polyisocyanate compound having at least 4 isocyanate groups in one molecule, the organic composite-coated steel sheet having excellent rust resistance and electrodeposition coating property according to claim 1, 2 or 3. 5. A polyfunctional polyisocyanate compound [C]
Is a polyfunctional polyisocyanate compound having at least 6 isocyanate groups in one molecule, the organic composite-coated steel sheet excellent in rust resistance and electrodeposition coating property according to claim 1, 2 or 3. 6. A polyfunctional polyisocyanate compound [C]
Is a polyfunctional product of hexamethylene diisocyanate having at least 6 isocyanate groups in one molecule, wherein the organic composite-coated steel sheet having excellent rust resistance and electrodeposition coatability according to claim 1, 2 or 3. .