JP2707168B2 - 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.
Modification obtained by reacting polyfunctional amine with phosphoric acid
Polyol as a crosslinking agent for the reactive epoxy resin (A)
Reaction of polyisocyanate and blocking agent
Resulting block urethane (B) A / B = 95 / 5~
The rust-preventive additive was added to the block urethane-modified epoxy resin mixed at a ratio of 50/50 (weight ratio) = 90/10 to 40/6.
Applying the coating composition added at a ratio of 0 (weight ratio)
An organic composite coated steel sheet having a formed 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 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) The surface of the zinc-based plated steel sheet is subjected to a chromate treatment in which a chromate solution is applied and dried without washing with water, and then a polyfunctional amine and epoxy resin are added to the epoxy resin.
To the modified epoxy resin (A) obtained by reacting
And polyols, polyisocyanates and
Urethane obtained by reacting
(B) A / B = 95/5 to 50/50 (weight ratio of non-volatile content) mixed in a block urethane-modified epoxy resin, and a rust-preventive additive was added to the block-urethane-modified epoxy resin / rust-preventive additive = The rust resistance and clear image appearance described in (1) above, wherein the coating composition added at a ratio of 90/10 to 40/60 (weight ratio of nonvolatile components) is applied and baked. For producing organic composite coated steel sheet with excellent heat resistance.

(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
Modified epoxy resin obtained by reacting min and phosphoric acid
(A), polyol, polyisocyanate and
Block urethane obtained by reacting a blocking agent
A coating composition in which a rust-preventive additive is added at a predetermined ratio to a block urethane-modified epoxy resin obtained by mixing (B) at a ratio of A / B = 95/5 to 50/50 (weight ratio of nonvolatile components) .
And dried.

As the epoxy resin, bisphenol A, bisphenol F, novolak and the like can be used.
A tertiary epoxy resin, glycidyl etherified epoxy resin obtained by adding propylene oxide or ethylene oxide to bisphenol A, or the like can be used. Furthermore, aliphatic epoxy resins, alicyclic epoxy resins, and polyether-based epoxy resins can also be used,
Two or more of these epoxy resins can be used in combination. Here, the epoxy equivalent of the epoxy resin is preferably 400 or more from the viewpoint of corrosion resistance.

The modified epoxy resin (A) can be obtained by reacting a glycidyl group of these epoxy resins with a polyfunctional amine and phosphoric acid. Examples of the polyfunctional amine include primary alkanolamines such as ethanolamine, propanolamine, isopropanolamine and butanolamine, primary alkylamines such as propylamine, butylamine, octylamine and decylamine, ethylenediamine, diethylenetriamine, Tetraethylenepentamine, xylenediamine, aminoethylpiperazine, norbornanediaminomethyl, and the like having two or more active hydrogens in one molecule are exemplified.
More than one species can be used in combination. As the polyfunctional amine, alkanolamine is particularly preferred from the viewpoint of corrosion resistance and paint adhesion.

Phosphoric acid has at least one P in one molecule.
It has an -OH group and includes, for example, orthophosphoric acid, pyrophosphoric acid, phosphorous acid, and polyphosphoric acid. Examples of the phosphoric acid ester include the above-mentioned alkyl esters and hydroxyalkyl esters of phosphoric acid. Modification with phosphoric acid improves the adhesion to the steel sheet.

As an example of the synthesis of the modified epoxy resin (A), the active hydrogen of the polyfunctional amine is 0.5 to 0.9 times equivalent to the glycidyl group of the epoxy resin, and the P-OH group of the phosphoric acid is 0.1 to 0.1. The two were mixed to an equivalent of 0.5 times, and 7
The reaction can be carried out at 0 to 150 ° C. for 4 to 10 hours, or both can be reacted stepwise.

The block urethane of the present invention is a compound in which the isocyanate group having a strong activity of the isocyanate compound is protected by a suitable compound to render it inactive. When heated, the blocking agent is dissociated and the isocyanate is easily dissociated. It regenerates the activity of the group. That is, it has a role as a curing agent for the modified epoxy resin.

As the polyol, ethylene glycol
Propylene glycol, 1,6-hexane di-
Divalent alcohols such as diethylene glycol and triethylene glycol, trivalent alcohols such as glycerin and trimethylolpropane, low molecular weight polyols such as pentaerythritol, and further caprolactone or low molecular weight Polyester polyols obtained from polyols and dicarboxylic acids, and high molecular weight polyols such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol having a molecular weight of 400 or more can be exemplified. Two or more kinds of polyols can be used in combination. Here, as the polyol, when a high molecular weight polyol is used, an appropriate hydrophilicity is imparted to the organic film, the affinity with the cationic electrodeposition paint is improved, and a smooth electrodeposition surface is obtained. Excellent in image clarity after overcoating and preferred.

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

Examples of the blocking agent include phenol compounds such as phenol, lactam compounds such as ε-caprolactam, oxime compounds such as methyl ethyl ketoxime,
An imine compound such as ethyleneimine can be used, and a mixture of two or more of these can be used.

The block isocyanate (B) is prepared by mixing and reacting the polyisocyanate so that the isocyanate group of the polyisocyanate is in excess with respect to the hydroxyl group of the polyol. It is obtained by protecting the residual isocyanate group of the prepolymer with a blocking agent. In addition, these reaction temperatures are 30-100 degreeC.

The block urethane-modified epoxy resin of the present invention can be obtained by mixing the above-mentioned modified epoxy resin (A) and block urethane (B). The mixing ratio (weight ratio of nonvolatile components) is in the range of A / B = 95/5 to 50/50. If the ratio of the modified epoxy resin (A) to the block urethane (B) exceeds 95/5, there is a problem that the sharpness after the middle / overcoating is deteriorated. When the ratio of the modified epoxy resin (A) is less than 50/50, the corrosion resistance is deteriorated. Further, in order to obtain higher sharpness and corrosion resistance, it is preferable that A / B is in the range of 90/10 to 60/40.

In the present invention, the corrosion resistance can be improved by blending silica and / or a sparingly soluble chromate with the above-mentioned block 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, the addition amount of silica in the block urethane-modified epoxy resin is as follows:
If the weight ratio of the non-volatile content of silica exceeds 90/10, the anticorrosion effect of silica 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 block urethane-modified epoxy resin as a binder becomes insufficient, and the adhesiveness of the paint is deteriorated. Therefore, the weight ratio of the non-volatile content of the block urethane-modified epoxy resin / silica is 9
It is preferably 0/10 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 is hydrophobized by substituting a silanol group on the silica surface with a methyl group or the like is added to the block urethane-modified epoxy resin,
The compatibility between the organic film and the cationic electrodeposition coating material is poor, and a smooth electrodeposition coating surface cannot be obtained. Therefore, in order to obtain excellent sharpness, silica whose surface is not hydrophobized is more preferable.

In addition, 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 addition amount of the hardly soluble chromate in the block urethane-modified epoxy resin is such that when the weight ratio of the non-volatile content of the block urethane-modified epoxy resin / slightly soluble chromate exceeds 90/10, The anticorrosion effect of the acid salt is not sufficiently exhibited, and the corrosion resistance is poor. On the other hand, 40 /
If it is less than 60, the effect of the block urethane-modified epoxy resin as a binder will be insufficient, and the adhesion of the paint will be degraded. Therefore, the weight ratio of the non-volatile content of the block urethane-modified epoxy resin / slightly soluble chromate is 90 /
It is preferably 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 blending silica and a sparingly soluble chromate salt in a specific ratio to the block urethane-modified epoxy resin, and synergistic effect of both anticorrosion effects. That is, in terms of the weight ratio of silica and the sparingly soluble chromate to the nonvolatile matter, block urethane-modified epoxy resin / (silica + poorly soluble chromate) = 90/10 to 40/60 silica / poorly soluble chromate = 90 / When it is blended at a ratio of 10 to 10/90, the most excellent corrosion resistance can be obtained. Here, if the ratio of the block urethane-modified epoxy resin / (silica + poorly soluble chromate) exceeds 90/10, the anticorrosive effect of silica and the poorly soluble chromate salt is not sufficiently exerted, and the corrosion resistance is poor. On the other hand, 40/60
If the amount is less than the above range, the effect of the block urethane-modified epoxy resin as a binder becomes insufficient, and the adhesiveness of the paint is deteriorated. In addition, silica / poorly soluble chromate is 90/10
If the ratio exceeds 10/90, the synergistic effect becomes insufficient, and the corrosion resistance is slightly inferior.

In the present invention, the above silica and / or
Alternatively, a sparingly soluble chromate salt is a main additive component in the block urethane-modified epoxy resin. In addition, a silane coupling agent, a coloring pigment (for example, a condensed polycyclic organic pigment, a phthalocyanine organic pigment, etc.) , Coloring dyes (eg, azo dyes, azo metal complex salt dyes, etc.), lubricants (eg, polyethylene wax, Teflon, graphite, molybdenum disulfide, etc.), rust preventive pigments (eg, aluminum dihydrogen tripolyphosphate, It is also possible to further blend one or more of aluminum phosphate molybdate, zinc phosphate, etc., conductive pigments (eg, iron phosphide, antimony-type tin oxide, etc.), surfactants and the like.

As a method for applying the above-mentioned coating composition to a galvanized steel sheet, the coating composition is usually applied by a roll coater method. However, after applying 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.

[0040]

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 30 g / l chromic anhydride, 0.2 g / l sulfuric acid, 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 resins used. The modified epoxy resin (A) and block urethane (B) shown in the table were prepared by the following method.

Modified epoxy resin (A) In a reactor equipped with a condenser, a stirrer and a thermometer, 500 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 2500, 385 parts of xylene, and 3 parts of cyclohexanone
85 parts were added and heated and dissolved under stirring. Further, 5 parts of monoethanolamine was added, 7.6 parts of dibutyl phosphoric acid was added, and the mixture was reacted at 100 ° C. for 5 hours to obtain a modified epoxy resin A having a resin content of 40%.

Block urethane (B) 440 parts of polyethylene glycol having a molecular weight of 1,000 and 125 parts of xylene were placed in a reactor equipped with a condenser, a stirrer and a thermometer, and heated at 60 ° C. with stirring. 6-
153 parts of tolylene diisocyanate were added. The NCO% of this intermediate was 4.8%. Further, 106 parts of ε-caprolactam was added to continue the reaction, and the NCO% was reduced to 0%.
Then, 175 parts of butanol was added to obtain block urethane B1 having a resin content of 70%. An intermediate having an NCO% of 1.2 was obtained from 500 parts of polypropylene glycol having a molecular weight of 4000, 300 parts of xylene and 42 parts of hexamethylene diisocyanate under the same apparatus and reaction conditions. Further, 23 parts of methyl ethyl ketoxime was added to continue the reaction, and after confirming that the NCO% was 0, 77 parts of butanol was added to obtain block urethane B2 having a resin content of 60%.

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 (outside rust resistance) Electrodeposition coating (2) on a test material 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) Sharpness 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 CF-type electrode, a pressure: 200 kgf, an energizing time: 10 cycles / 50 Hz, a welding current: 10 kA, were tested for continuous spotting, and evaluated by the number of continuous spots. 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

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

[0056]

[Table 4]

[0057]

[Table 5]

[0058]

[Table 6]

[0059]

[Table 7]

[0060]

[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

[0062]

As described above, according to the present invention, the use of a specific block urethane-modified epoxy resin for the organic film 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.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toyofumi Watanabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yoshio Kikuta 1900 Togo, Mobara City, Chiba Prefecture Mitsui Toatsu Chemicals In-house (72) Inventor Kimio Kobori 1900 Togo, Mobara City, Chiba Prefecture Inside Mitsui Toatsu Chemicals Co., Ltd. (72) Inventor Hideaki Ogata 1900 Togo Togo, Mobara City, Chiba Prefecture Mitsui Toatsu Chemicals Inc. Document JP-A-4-323383 (JP, A) JP-A-60-94466 (JP, A) JP-A-4-197473 (JP, A) JP-A-60-103185 (JP, A) JP-A-63-1988 99938 (JP, A) JP-A-64-297 (JP, A) JP-A-63-35798 (JP, A) JP-A-2-134238 (JP, A) JP-A-2-263633 (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.
Fatty acid reacts with polyfunctional amine and phosphoric acid
A polyol as a cross-linking agent for the epoxy resin (A);
Obtained by reacting a polyisocyanate and a blocking agent.
Block urethane (B) A / B = 95 / 5~50
A rust preventive additive is added to a block urethane-modified epoxy resin mixed at a ratio (weight ratio) of 90/10 to 40/60 = 90/10 to 40/60. Formed by applying a paint composition
An organic composite coated steel sheet having an organic film having a thickness of 0.2 to 2 μm and excellent in 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 the polyfunctional amine and phosphoric acid are added to the epoxy resin.
To the modified epoxy resin (A) obtained by reacting
As crosslinking agents, polyols, polyisocyanates and
Block urethane obtained by reacting a blocking agent
(B) A / B = 95/5 to 50/50 (weight ratio of non-volatile content) mixed in a block urethane-modified epoxy resin, and a rust-preventive additive was added to the block-urethane-modified epoxy resin / rust-preventive additive = The rust resistance and clarity of the outer surface according to claim 1, wherein the coating composition added at a ratio of 90/10 to 40/60 (weight ratio of nonvolatile components) is applied and baked. Method of manufacturing organic composite coated steel sheet excellent in quality. 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