JP2797036B2 - Organic composite coated steel sheet having excellent external rust resistance and excellent image clarity and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic composite which is excellent in outer rust resistance and sharpness, and is excellent in powdering resistance, weldability and paint adhesion for use in automobile bodies and home electric appliances. The present invention relates to a coated steel sheet and a method for manufacturing the same.

[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 salts for preventing road freezing sprayed in winter has become a major social problem. As one of the measures for preventing rust on automobile bodies, the use of surface-treated steel sheets having excellent corrosion resistance in place of conventional cold-rolled steel sheets is currently increasing.

[0003] Such a surface-treated steel sheet is disclosed in
Examples thereof include organic composite coated steel sheets as disclosed in JP-A-4-8033 and JP-A-2-15177.
These steel sheets are based on a zinc-based plated steel sheet, have a chromate film on the first layer, and a second layer on the first layer and one or more basic nitrogen atoms at the end of the epoxy resin. It has an organic resin film in which silica and a sparingly soluble chromate salt are added at a specific ratio to an organic resin composed of a base resin to which the above primary hydroxyl groups are added and a polyisocyanate compound and a block isocyanate compound. It is an organic composite coated steel sheet having excellent corrosion resistance, weldability, powdering resistance, and paint adhesion.

[0004]

On the other hand, with the recent trend toward higher-grade and individualized automobiles, importance has been placed on finished quality after painting, particularly sharpness. However, the organic composite coated steel sheet as described above has excellent characteristics regarding the perforation resistance required on the inner surface side of the outer plate and the inner plate of the automobile body, but the outer plate and the outer surface of the automobile body. When applied to the outer surface side of the outer plate for the purpose of improving the rust resistance of the outer surface on the outer side, there is a problem that the sharpness is slightly inferior to that of a conventionally used galvanized steel sheet. Further, it is difficult to say that these organic composite coated steel sheets necessarily have sufficient characteristics with respect to the external rust resistance.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and when applied to an outer plate of an automobile body, excellent outer surface rust resistance and sharpness on the outer plate and the outer surface side. It is an object of the present invention to provide an organic composite coated steel sheet having:

[0006]

In order to achieve the above object, the organic composite coated steel sheet of the present invention has the following configuration.

(1) A zinc-plated steel sheet has on its surface a chromate layer having a deposition amount of 10 to 200 mg / m ^{2 in} terms of metal chromium as a first layer, and an epoxy layer as a second layer above the chromate layer.
Modified epoxy obtained by reacting polyfunctional amine with silicone resin
Resin (A), polyol and polyisocyanate
The urethane prepolymer (B) obtained by the reaction
The rust-preventive additive was added to the urethane-modified epoxy resin reacted at the ratio (weight ratio) of / B = 95/5 to 50/50, and the urethane-modified epoxy resin / rust-preventive additive = 90/10 to 40 /
Applying the coating composition added at a ratio of 60 (weight ratio)
An organic composite coated steel sheet having excellent outer rust resistance and sharpness having an organic film having a thickness of 0.2 to 2 μm formed by the above method.

(2) The organic composite coated steel sheet according to the above (1), wherein the rust preventive additive constituting the organic film is silica, which is excellent in external rust resistance and sharpness.

(3) The organic composite coated steel sheet according to the above (1), wherein the rust preventive additive constituting the organic film is a hardly soluble chromate, which has excellent external rust resistance and excellent sharpness. steel sheet.

(4) In the organic composite coated steel sheet according to the above (1), the rust preventive additive constituting the organic film is silica having the following ratio (weight ratio) and hardly soluble chromate, and the external rust resistance is low. Organic composite coated steel sheet with excellent properties and sharpness. Silica / poorly soluble chromate = 90/10 to 10/90 The production method of the present invention for obtaining such an organic composite coated steel sheet has the following configuration.

(5) Chrome on the surface of the zinc-based plated steel sheet
Chromate treatment to apply the coating solution and dry it without washing with water, and then add the polyfunctional amine to the epoxy resin.
Modified epoxy resin (A)
Urethane obtained by reacting
A / B = 95/5 to 50/5 with the prepolymer (B)
The rust-preventive additive was added to the urethane-modified epoxy resin reacted at a ratio of 0 (weight ratio of non-volatile content) to 90/10 to 40/60 (weight ratio of non-volatile content). The method for producing an organic composite coated steel sheet having excellent external rust resistance and sharpness described in the above (1), wherein the coating composition added in a ratio is applied and baked.

(6) The method for producing an organic composite coated steel sheet according to (5), wherein the rust preventive additive constituting the coating composition is silica, which is excellent in external rust resistance and sharpness.

(7) The method according to (5), wherein the rust-preventive additive constituting the coating composition is a hardly soluble chromate and has excellent external rust resistance and excellent sharpness. Manufacturing method.

(8) In the production method according to the above (5), the rust preventive additive constituting the coating composition is silica having the following ratio (weight ratio) and a hardly soluble chromate, and the external rust resistance is low. And a method for producing an organic composite coated steel sheet having excellent clarity. Silica / poorly soluble chromate = 90/10 to 10/90

[0015]

The details of the present invention and the reasons for limiting the same will be described below. The zinc-based coated steel sheet serving as a base includes a zinc-coated steel sheet, a Zn-Ni alloy-coated steel sheet, a Zn-Fe alloy-coated steel sheet, a Zn-Mn alloy-coated steel sheet, a Zn-Al alloy-coated steel sheet, and a Zn-Cr alloy-coated steel sheet. , Zn-Co-
Examples thereof include Cr alloy plated steel sheets, and zinc-based composite plated steel sheets obtained by dispersing and plating metal oxides, hardly soluble chromates, polymers and the like. Further, a multi-layer plated steel sheet obtained by plating two or more layers of the same kind or different kinds of the above platings may be used. As a plating method, any method that can be used among an electrolysis method, a melting method, and a gas phase method can be adopted, but the electrolysis method is advantageous from the viewpoint of selectivity of the underlying cold-rolled steel sheet.

The chromate layer formed on the surface of the galvanized steel sheet suppresses the corrosion of the galvanized steel sheet by a self-repairing action of chromate ions of hexavalent chromium. The amount of the deposited chromate layer is 1 in terms of chromium metal.
If it is less than 0 mg / m ² , sufficient corrosion resistance cannot be expected, while if it exceeds 200 mg / m ² , the weldability will deteriorate. For this reason, the adhesion amount of the chromate layer is 10 to ²⁰⁰ mg / m ^{2 in} terms of chromium metal. Further, in order to satisfy even higher corrosion resistance and weldability, the content is preferably in the range of ²⁰ to 100 mg / m ^{2 in} terms of chromium metal.

As a chromate treatment for forming the chromate layer, any of a reaction type, an electrolytic type and a coating type can be applied. From the viewpoint of corrosion resistance,
A coating type containing a large amount of hexavalent chromium chromate ions in the layer is preferable.

The coating type chromate treatment is mainly composed of a partially reduced aqueous solution of chromic acid, and is a water-soluble or water-dispersible organic resin such as an acrylic resin or a polyester resin. Silica, alumina, titania, zirconia, etc. Oxide colloids and / or powder Acids such as molybdic acid, tungstic acid, and vanadic acid and / or salts thereof Phosphoric acids such as phosphoric acid and polyphosphoric acid Fluoride such as zirconium fluoride, silicofluoride, titanium fluoride, zinc ion, etc. Metal ion of conductive powder of iron phosphide, antimony-type tin oxide, etc. From the above-mentioned components, apply a treatment liquid to which at least one is added as required to a zinc-based plated steel sheet, Dry without washing. The coating type chromate treatment is usually
Although the treatment liquid is applied by a roll coater method, it is also possible to adjust the application amount by an air knife method or a roll drawing method after applying by a dipping method or a spray method.

As described above, on the chromate layer formed on the surface of the galvanized steel sheet, the organic film formed as the second layer is formed of the hexavalent chromate ion in the chromate layer. Excessive elution into a corrosive environment is suppressed, the anticorrosion effect is maintained, and the corrosion resistance is further improved by silica or chromate added to the organic film. If the thickness of the organic film is less than 0.2 μm, sufficient corrosion resistance cannot be expected, while if it exceeds 2 μm, weldability and sharpness deteriorate. Therefore, the thickness of the organic film is 0.2 to 2 μm.
And Further, in order to satisfy higher corrosion resistance, weldability and sharpness, the range of 0.3 to 1.5 μm is preferable.

This organic film is a polyfunctional epoxy resin
To the modified epoxy resin (A) obtained by reacting
And reacting the polyol with the polyisocyanate.
Urethane prepolymer (B) A / B = 95 / 5~
A urethane-modified epoxy resin reacted at a ratio of 50/50 (weight ratio of non-volatile components) was added with a rust preventive additive at a predetermined ratio.
The added coating composition was applied and dried.

The epoxy resin (A) is an epoxy resin obtained by glycidyl etherification of bisphenol A, bisphenol F, novolak, or the like. Glycidyl etherification is obtained by adding propylene oxide or ethylene oxide to bisphenol A. Epoxy resin or the like can be used. Further, aliphatic epoxy resins, alicyclic epoxy resins, and polyether-based epoxy resins can also be used, and two or more of these epoxy resins can be used in combination. Here, the epoxy equivalent of the epoxy resin is 40
0 or more is preferable from the viewpoint of corrosion resistance.

These epoxy resins are modified with polyfunctional amines.
An amine-modified epoxy resin is obtained by reacting a glycidyl group of an epoxy resin with a polyfunctional amine.
Can be Examples of the polyfunctional amine include ethanolamine, propanolamine, isopropanolamine, primary alkanolamines such as butanolamine, propylamine, butylamine, octylamine, primary alkylamines such as decylamine, ethylenediamine, diethylenetriamine, tetraethylenepentamine, Xylene diamine, aminoethylpiperazine, norbornanediaminomethyl, and the like having two or more active hydrogens in one molecule, and two or more of these amines can be used in combination. Alkanolamine is particularly preferred as the polyfunctional amine from the viewpoint of corrosion resistance and paint adhesion.

Examples of the polyol include polyester polyols obtained from caprolactone or glycol and dicarboxylic acid, and polyethylene glycols having a molecular weight of 400 or more, polypropylene glycols, polytetramethylene glycols and the like. Polymer polyols can be exemplified, and two or more of these polyols can be used in combination. Here, as the polyol, use of polyetherpolyol imparts an appropriate hydrophilicity to the organic film,
The compatibility with the cationic electrodeposition paint is improved, and a smooth electrodeposition surface is obtained.

Examples of the polyisocyanate include aliphatic isocyanates such as hexamethylene diisocyanate and trimethylhexamethylene diisocyanate, xylylene diisocyanate, and 2,4-tolylene diisocyanate. ,
Aromatic isocyanates such as 2,6-tolylene diisocyanate, alicyclic isocyanates such as isophorone diisocyanate and norbornane diisocyanate methyl, and mixtures or polynuclears of two or more thereof. Can be illustrated.

The urethane prepolymer is, for example, an isocyanate of polyisocyanate with respect to a hydroxyl group of polyol.
The two groups are mixed and reacted at a ratio of 1.5 to 2.5 equivalents. The reaction temperature is preferably from 30 to 100C. Further, the reaction temperature when the reaction between the epoxy resin and the urethane prepolymer is performed is preferably 30 to 100 ° C. The reaction mainly comprises an addition reaction between a hydroxyl group in the epoxy resin and an isocyanate group of the urethane prepolymer.

The urethane-modified epoxy resin of the present invention comprises the aforementioned epoxy resin (A) and urethane prepolymer (B)
And a reaction ratio of A / B = 95/5 to 50/50 in terms of the weight ratio of nonvolatile components. If the proportion of the epoxy resin (A) exceeds 95/5, there is a problem that the sharpness after the middle / overcoating is deteriorated.
If the proportion of the epoxy resin (A) is less than 50/50, the corrosion resistance is deteriorated. A / B = 90/10 to 60/40 in order to obtain higher sharpness and corrosion resistance.
It is preferable to be within the range.

According to the present invention, the corrosion resistance can be improved by blending silica and / or a sparingly soluble chromate into the urethane-modified epoxy resin. 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 a small amount in corrosive environments to form silicate ions. It is presumed that the anticorrosion effect is exhibited by functioning as a mold corrosion inhibitor. here,
When the weight ratio of the urethane-modified epoxy resin / silica to the non-volatile components exceeds 90/10, the anticorrosion effect of the silica is not sufficiently exerted and the corrosion resistance is poor. On the other hand, when the ratio is less than 40/60, the effect of the urethane-modified epoxy resin as a binder becomes insufficient, and the adhesiveness of the paint deteriorates. Therefore, the weight ratio of the urethane-modified epoxy resin / silica non-volatile content is preferably 90/10 to 40/60.

The silica used in the present invention includes dry silica (for example, AEROSIL 130, manufactured by Nippon Aerosil Co., Ltd.).
AEROSIL 200, AEROSIL 300, AEROSIL 380, AEROSIL R97
2, AEROSIL R811, AEROSIL R805, etc.), and organosilica sols (eg, MA-ST, IPA-ST, Nissan Chemical Industries, Ltd.)
NBA-ST, IBA-ST, EG-ST, XBA-ST, ETC-ST, DMAC-ST
Etc.), sedimentation wet silica (for example, T manufactured by Tokuyama Soda Co., Ltd.)
-32 (S), K-41, F-80, etc.), gel-process wet silica (for example,
Syloid 244, Syloid 150, Syloid 72, Syloid 65, SHIELDEX, etc., manufactured by Fuji Devison Chemical Co., Ltd.) can be used. It is also possible to use a mixture of two or more of the above silicas.

When hydrophobic silica whose surface has been hydrophobized by substituting a silanol group on the silica surface with a methyl group or the like is added to the urethane-modified epoxy resin, the compatibility between the organic coating and the cationic electrodeposition coating material is reduced. It deteriorates, and a smooth electrodeposition surface cannot be obtained, so that the sharpness after middle / overcoating is poor. Therefore, in order to obtain excellent sharpness,
Silica whose surface is not hydrophobized is preferred.

Further, the sparingly soluble chromate added to the organic film is dissolved in a very small amount in a corrosive environment to form
It is considered that the chromium ions release chromic ions and suppress the corrosion of the galvanized steel sheet by the same mechanism as that of the chromate layer. Here, the amount of the hardly soluble chromate added to the urethane-modified epoxy resin is such that the weight ratio of the non-volatile content of the urethane-modified epoxy resin / slightly soluble chromate exceeds 90/10. The anti-corrosion effect of this is not sufficiently exhibited, and the corrosion resistance is poor. On the other hand, when the ratio is less than 40/60, the effect of the urethane-modified epoxy resin as a binder becomes insufficient, and the adhesiveness of the paint deteriorates. Therefore, it is preferable that the weight ratio of the urethane-modified epoxy resin / the hardly soluble chromate to the nonvolatile content is 90/10 to 40/60.

The sparingly soluble chromate used in the present invention includes barium chromate (BaCrO ₄ ), strontium chromate (SrCrO ₄ ), calcium chromate (CaCrO ₄ ), and zinc chromate (ZnCrO ₄ .4Zn (OH ) ₂ ), potassium zinc chromate (K ₂ O ・ 4ZnO
_{· 4CrO 3 · 3H 2 O)} , it can be used fine powder such as lead chromate (PbCrO _4). It is also possible to use a mixture of two or more of the hardly soluble chromates described above. 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-healing effect by chromate ions over a long period of time. Further, from the viewpoint of minimizing the elution of water-soluble chromium from the organic film in the pretreatment step of coating the automobile, barium chromate having low solubility in water is preferable.

In the present invention, the most excellent corrosion resistance can be realized by mixing silica and a sparingly soluble chromate salt in a specific ratio with the urethane-modified epoxy resin, and synergistic effect of both anticorrosion effects. That is, in terms of the weight ratio of silica and the hardly soluble chromate to the nonvolatile matter, urethane-modified epoxy resin / (silica + slightly soluble chromate) = 90/10 to 40/60 silica / slightly soluble chromate = 90/10 When it is blended at a ratio of 10 to 90/90, the most excellent corrosion resistance can be obtained. Here, urethane-modified epoxy resin /
If the ratio of (silica + poorly soluble chromate) exceeds 90/10, the anticorrosion effect of silica and the poorly soluble chromate salt is not sufficiently exerted, resulting in poor corrosion resistance. On the other hand, when the ratio is less than 40/60, the effect of the urethane-modified epoxy resin as a binder becomes insufficient, and the adhesiveness of the paint deteriorates. Also,
Even if the ratio of silica / poorly soluble chromate exceeds 90/10, 1
If it is less than 0/90, the synergistic effect becomes insufficient, and the corrosion resistance is slightly inferior.

In the present invention, the above silica and / or
Alternatively, the hardly soluble chromate salt is a main additive component in the urethane-modified epoxy resin, but in addition, a silane coupling agent, a coloring pigment (for example, a condensed polycyclic organic pigment, a phthalocyanine organic pigment, and the like), Colored dyes (eg, azo dyes, azo metal complex dyes, etc.), lubricants (eg, polyethylene wax, Teflon, graphite, molybdenum disulfide, etc.), rust preventive pigments (eg, aluminum dihydrogen tripolyphosphate, phosphorus) Aluminum molybdate,
One or more of zinc oxide and the like, conductive pigments (eg, iron phosphide, antimony-type tin oxide and the like), surfactants and the like can be further added.

As a method of applying the above-mentioned coating composition to a galvanized steel sheet, the coating composition is usually applied by a roll coater method. However, after being applied by a dipping method or a spray method, an air knife is used. It is also possible to adjust the amount of coating by a method or a roll drawing method. Further, as a heat treatment method after applying the coating composition, a hot blast furnace, a high-frequency induction heating furnace, an infrared furnace, or the like can be used. The heat treatment is performed at the ultimate plate temperature of 50 to 300 ° C, preferably 60 to 250 ° C. Further, when the present invention is applied to a BH steel sheet, a heat treatment at 150 ° C. or lower is preferable.

The organic composite coated steel sheet of the present invention usually has a zinc-based plating film + a chromate layer + an organic film on both surfaces, but is applied to only one surface if necessary, and the other surface is coated on the steel surface. It is also possible to use a system-based plating surface or a zinc-based plating film + chromate film surface.

[0036]

EXAMPLE As a surface-treated steel sheet for an automobile body, a zinc-based plated steel sheet was alkali-degreased, washed with water and dried, and then subjected to chrome plating.
Then, the coating composition was applied by a roll coater and baked. With respect to the obtained organic composite coated steel sheet, each test was performed on outer surface rust resistance, sharpness, paint adhesion, and weldability. The processing conditions of this embodiment 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 platings and used as a treated base sheet.
(See Table 1)

(2) Chromate treatment Coating-type chromate treatment A chromate treatment solution having the following composition was roll-coated.
And dried without washing with water. Chrome
The adhesion amount of the coating layer was adjusted by changing the peripheral speed ratio between 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
A zinc-plated steel sheet was subjected to cathodic electrolysis at a current density of 10 A / dm ² by using a treatment liquid at a temperature of 10 ° C., followed by washing with water and drying. The adhesion amount of the chromate layer was adjusted by controlling the amount of electricity supplied in the cathodic electrolysis treatment.

Reaction type chromate treatment Chromic anhydride 30 g / l, phosphoric acid 10 g / l, NaF
Using a treatment solution of 0.5 g / l, K ₂ TiF ₆ 4 g / l, and a bath temperature of 60 ° C., the zinc-coated steel sheet was spray-treated, 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 urethane-modified epoxy resin shown in the table was prepared by the following method.

Epoxy resin (A) 500 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 1500, 380 parts of xylene, and 3 parts of cyclohexanone were placed in a reactor equipped with a condenser, a stirrer, and a thermometer.
80 parts were added and heated and dissolved under stirring. Further, 6.7 parts of isopropanolamine was added and reacted at 100 ° C. for 5 hours to obtain an epoxy resin A1 having a resin content of 40%.
Adeka Resin EP4000 (made by Asahi Denka Co., Ltd.) using the same device
325 parts and bisphenol A (181 parts) are dissolved in cyclohexyl acetate (69 parts).
To make the epoxy equivalent 7800, and xylene 345
Parts and 345 parts of cyclohexanone, and the resin content is 40%.
The epoxy resin A2 was obtained.

Urethane prepolymer (B) In a reactor equipped with a condenser, a stirrer and a thermometer, 500 parts of polyethylene glycol having a molecular weight of 2,000 and 250 parts of xylene were heated at 60 ° C. with stirring. 2,6-
87 parts of tolylene diisocyanate were added and reacted. Thus, a urethane polymer B1 having an NCO% of 2.5% and a resin content of 60% was obtained. 500 parts of polypropylene glycol having a molecular weight of 4000 under the same apparatus and reaction conditions,
360 parts of xylene and hexamethylene diisocyanate 4
From 2 parts, a urethane prepolymer-B2 having an NCO% of 2.1% and a resin content of 60% was obtained.

Urethane Modified Epoxy Resin An epoxy resin and a urethane prepolymer in the proportions shown in Table 2
Into a reactor equipped with a condenser, stirrer and thermometer, and stirred with dibutyltin dilaurate as a catalyst.
The reaction was carried out at ℃ to obtain a urethane-modified epoxy resin.

Table 3 shows the silica added to the organic film.
Table 4 shows the poorly soluble chromate. Tables 5 to 8 show the configurations of the organic composite coated steel sheets prepared as described above, and the evaluation results of their corrosion resistance (outer rust resistance), sharpness, paint adhesion, and weldability. In addition, the evaluation method of each characteristic is as follows.

(A) Corrosion resistance (outer surface rust resistance) Electrodeposition coating with U-600 manufactured by Nippon Paint Co., Ltd.
5μ), and then KPX-3 manufactured by Kansai Paint Co., Ltd.
6 with a middle coat (30μ), and Kansai Paint Co., Ltd.
A top coat was applied (35 μm) using Lugabake B-531. Cross-cut these test pieces with a cutter knife, and then perform salt spray test, 10 minutes → drying, 155 minutes →
Wet test 75 minutes → dry 160 minutes → wet test 80
The corrosion resistance (external rust resistance) was evaluated based on the swollen width of the corrosion from the cross-cut portion, by performing 300 cycles of a composite corrosion test in which one cycle was defined as 1 cycle. The evaluation criteria are as follows. ◎: less than 2 mm ：: 2 mm or more and less than 4 mm Δ: 4 mm or more, less than 6 mm ×: 6 mm or more

(B) Visibility The test material was subjected to the same electrodeposition coating, intermediate coating, and top coating as in (a) above, and a scalability tester (ICM-IC) manufactured by Suga Test Instruments Co., Ltd.
2DP) and the image clarity C when a 0.5 mm slit was used was evaluated. The evaluation criteria are as follows. ◎: 80 or more ○: less than 80, 75 or more Δ: less than 75, 70 or more ×: less than 70

(C) Coating Adhesion The test material was subjected to the same electrodeposition coating, intermediate coating, and top coating as in (a) above, and these test pieces were immersed in ion-exchanged water at 40 ° C. for 240 hours. Then, remove the test piece,
After standing at room temperature for 24 hours, the coating film was cut into 100 grids at intervals of 2 mm, the adhesive tape was adhered and peeled, and the peeling rate of the coating film was evaluated. The evaluation criteria are as follows. ◎: no peeling ：: less than 3% △: 3% or more and less than 10% ×: 10% or more

(D) Weldability A test for continuous spotting was carried out with a CF electrode, a pressure of 200 kgf, a conduction time of 10 cycles / 50 Hz, and a welding current of 10 kA, and the number of continuous spots was evaluated. The evaluation criteria are as follows. ◎: 5000 points or more ○: 4000 points or more and less than 5000 points △: 3000 points or more and less than 4000 points ×: less than 3000 points

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

[Table 4]

[0054]

[Table 5]

[0055]

[Table 6]

[0056]

[Table 7]

[0057]

[Table 8]

* 1 Emission: Example of the present invention Ratio: Comparative example * 2 See Table 1 * 3 Chromate adhesion in terms of chromium metal * 4 See Table 2 * 5 See Table 3 * 6 See Table 4 * 7 Non-volatile content Weight ratio * 8 Non-volatile content weight ratio

[0059]

As described above, according to the present invention, the use of a specific urethane-modified epoxy resin for the organic coating of the organic composite coated steel sheet provides excellent external rust resistance and sharpness. Further, since it has excellent paint adhesion and weldability, it is extremely useful as a surface-treated steel sheet for automobiles and home appliances.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI // C08G 18/58 C08G 18/58 (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Toyofumi Watanabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yoshio Kikuta 1900 Togo, Togo, Mobara-shi, Chiba Mitsui Toatsu Chemicals Inc. (72) Invention Applicant: Kimio Kobori 1900 Togo, Mobara-shi, Chiba Pref.Mitsui Toatsu Chemicals Co., Ltd. (72) Inventor Hideaki Ogata 1900 Togo, Togo, Mobara-shi, Chiba Pref. Mitsui Toatsu Chemicals Co., Ltd. 134238 (JP, A) JP-A-2-258335 (JP, A) JP-A-3-150396 (JP, A)

Claims (8)

(57) [Claims] 1. A zinc-plated steel sheet having a chromate layer having a coating amount of 10 to 200 mg / m ^{2 in} terms of chromium metal as a first layer on the surface of a zinc-based plated steel sheet, and an epoxy resin as a second layer above the chromate layer.
Modified epoxy tree obtained by reacting polyfunctional amine with fat
Of fatty acid (A), polyol and polyisocyanate
A / B with the urethane prepolymer (B) obtained by
= 95/5 to 50/50 (weight ratio), urethane-modified epoxy resin reacted with urethane-modified epoxy resin / rust-preventive additive = 90/10 to 40/60
Of the coating composition added in the proportion (weight ratio)
An organic composite coated steel sheet having an organic film having a thickness of 0.2 to 2 μm and having excellent external rust resistance and sharpness. 2. The organic composite coated steel sheet according to claim 1, wherein the rust preventive additive constituting the organic film is silica. 3. The organic composite coated steel sheet according to claim 1, wherein the rust preventive additive constituting the organic film is a hardly soluble chromate. 4. The rust-preventive additive constituting the organic film is silica and a sparingly soluble chromate having the following ratio (weight ratio), wherein the rust-preventive additive is excellent in external rust resistance and sharpness. Organic composite coated steel sheet. Silica / poorly soluble chromate = 90/10 to 10/90 5. A chromate treatment in which a chromate solution is applied to the surface of a galvanized steel sheet and dried without washing with water, and then a polyfunctional amine is reacted with the epoxy resin.
Modified epoxy resin (A) obtained by
Urethane prepolymer obtained by reacting polyisocyanate
The rust-preventive additive is added to the urethane-modified epoxy resin obtained by reacting the polymer (B) with A / B at a ratio of 95/5 to 50/50 (weight ratio of nonvolatile components). 2. The rust resistance and freshness of an outer surface according to claim 1, wherein a coating composition added at a ratio of 90/10 to 40/60 (weight ratio of nonvolatile components) is applied and baked. Manufacturing method of organic composite coated steel sheet with excellent cinematography. 6. The method according to claim 5, wherein the rust-preventive additive constituting the coating composition is silica. 7. The method for producing an organic composite coated steel sheet according to claim 5, wherein the rust preventive additive constituting the coating composition is a sparingly soluble chromate. 8. The rust-preventive additive constituting the coating composition is silica and a sparingly soluble chromate having the following proportions (weight ratio), and is excellent in outer rust resistance and sharpness. Manufacturing method of organic composite coated steel sheet. Silica / poorly soluble chromate = 90/10 to 10/90