JPH0474872A - Organic composite coated steel sheet having excellent corrosion resistance - Google Patents

Abstract

PURPOSE:To develop the org. composite coated steel sheet having excellent corrosion resistance by forming a lower plating layer of a Zn-Co alloy on the surface of the steel sheet and an upper plating layer consisting of a Zn-Co alloy or Zn-Ni alloy of the different compsn. thereon and further, forming a lower layer chromate film and an upper layer org. resin film thereon. CONSTITUTION:The surface of the base material steel sheet, such as cold rolled aluminum killed steel sheet, is degreased, surface activated and cleaned; thereafter, the Zn-Co alloy contg., by weight %, 0.01 to 2% Co is electroplated as the lower plating layer on at least one surface thereof. The Zn-Co alloy contg. 2 to 20% Co or Zn-Ni alloy contg. 9 to 20% Ni is electroplated as the upper plating layer in the amt. of 0.05 to 10 g/m<2> thereon. The chromate film having 20 to 150 mg/m<2> Cr deposition is then formed thereon and the CR<6+> thereof is so reduced as to attain 0.4 to 0.6 ratio of Cr<3+>/total Cr. One kind of the epoxy resin among an epoxy resin, modified epoxy resin and polyhydroxy polyether resin is applied and baked thereon so as to have 0.3 to 5 mum film thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an organic composite coated steel sheet with excellent corrosion resistance for use in automobiles.

(Prior art) Painted mesh plates that can be electrically resistance welded are gradually being used in Japan, for example, to prevent rust on cars caused by salt that is sprayed as a snow-melting agent on roads in snow-covered areas. be.

Conventional weldable painted steel sheets include Synchrometal (
There is a steel plate coated with zinc clinch primer, which is represented by the product name: Diamond Nyamronok Co., Ltd. (USA).

Synchrometal is a zinc-rich coating consisting of a base chromate layer (Dacrometal) whose main components are zinc powder and chromic acid on a steel plate, and an epoxy resin containing a large amount of zinc.
A primer layer (zinchromet) is formed,
Exhibits significantly superior corrosion resistance compared to cold-rolled steel sheets.

In general, anti-corrosion coated plates for automobiles and the like are required to have moldability, weldability and corrosion resistance, but the above-mentioned synchrometal does not particularly have sufficient moldability and corrosion resistance. This is because the zinc-rich primer used for these steel sheets contains a large amount of zinc powder, around 50% by volume and 85-90% by weight, to ensure weldability. This is because the paint film tends to peel off during processing.Moreover, because the paint film peels off easily, corrosion resistance decreases, and the peeled and fallen paint film adheres to the press mold, causing scratches. This increases the frequency of mold maintenance and deteriorates workability.Also, zinc-rich primers have high moisture permeability into the paint film, which also tends to reduce corrosion resistance.Such paint film peeling occurs. In order to reduce the decrease in corrosion resistance, it is effective to reduce the amount of zinc dust, but this increases the electrical resistance of the coating film, making resistance welding disadvantageous or impossible.

In addition, conventional Synchrometal requires high temperatures of 150 to 180°C to bake the chromate film and 250 to 280°C to bake the zinc clinch primer.
This had the disadvantage that the bake hardenability of the steel plate was impaired. Here, "bake hardenability" refers to a property in which failure stress of a steel sheet increases when the user performs processing such as pressing and bakes with electrodeposition coating or the like.

The present inventors studied to eliminate the above-mentioned drawbacks of zinc-rich primers, and as a result, using plated steel sheets as a base material,
On top of that, an aqueous suspension containing chromic acid and colloidal silica is applied and baked as a base layer to form a chromate film, and then a polyhydroxy film containing no pigment or containing a chromic acid pigment is applied. We previously proposed that by forming an upper clear film by coating and baking a polyether resin solution, a coated steel sheet that can be resistance welded and has excellent corrosion resistance and formability (Japanese Patent Laid-Open No. 62-2
No. 68635).

Since this steel plate can be baked at a relatively low temperature without impairing its bake hardenability, the obtained coated copper plate stably maintains its bake hardenability.

Furthermore, the present inventors have found that by blending at least one of various inorganic fillers and crosslinking agents into the resin liquid for forming the upper layer coating, the upper layer coating can be formed from a general-purpose epoxy resin or a modified epoxy resin. In addition, they proposed an organic composite N steel sheet that can provide excellent corrosion resistance without containing chromate pigments (Japanese Patent Laid-Open No. 1-80522).

(Problems to be Solved by the Invention) Regarding such conventional organic composite coated steel sheets, Znlii
The base material is alloy electroplated toning plate, which is coated with a chromate film and an epoxy resin film, because it has excellent corrosion resistance, electrodeposition coating properties, workability, weldability, etc.
Practical use has already begun. It is often used for the inner panels of automobile doors, doors, and doors. When organic composite coated steel sheets are used in these areas, it is unlikely that deep scratches will penetrate through the electrodeposited coating and reach the steel sheet base.

However, recently, the use of organic composite coated steel sheets for the exterior panels of automobiles has been considered, and when such organic composite coated steel sheets are used for exterior panels, the corrosion resistance of uneven parts of the coating film is important. 6 However, the required performance is ′・11
Organic composite coated copper plates using alloy electroplated steel sheets as a base material do not have sufficient corrosion resistance on uneven parts of the coating film, so it is desired to develop a new organic composite coated copper plate that improves this.

Therefore, an object of the present invention is to provide a material that exhibits not only weldability and paint film adhesion but also sufficient corrosion resistance even when the paint film is damaged, and that is particularly suitable for use on the exterior panels of automobiles. An object of the present invention is to provide a composite coated Y1 steel plate.

(Means for Solving the Problems) In order to achieve the above-mentioned objectives, the present inventors investigated the performance of organic composite coated steel sheets using various plated steel sheets, and found that the Co content was 0.01. It has been found that an organic composite coated copper plate whose base material is a Zn-Co alloy plated steel plate having a weight percent or more and less than a double cover percent has excellent corrosion resistance of painted uneven parts. This is because the Zn-Co alloy plated copper plate has a higher Z
This is thought to be because it has better sacrificial corrosion protection than the n-Ni alloy electroplated steel sheet.

However, simply converting the base material into a Zn Ni alloy electroplated steel sheet with a Co content of 0.01% by weight or more and less than 2% by weight
It has been found that the following problems occur with an organic composite coated steel sheet that is simply replaced with an n-Co alloy plated steel sheet.

■ Decreased corrosion resistance of the flat plate part.

■ Decreased corrosion resistance of processed parts.

■ Decreased adhesion between the plating layer and the upper epoxy resin layer via the chromate film.

On the other hand, the present inventors applied Zn-C with a Co content of 2 to 20% by weight on a Zn-Co alloy plated steel sheet.

It has been found that all of the above-mentioned problems can be solved by using a multi-layer plated steel sheet as a base material, which is coated with an upper layer of a Zn-Ni alloy having a Ni content of 9 to 20% by weight.

In other words, regarding (1) a decrease in the corrosion resistance of the flat plate part and (2) a decrease in the adhesion between the plating layer and the epoxy resin, a Zn-Co alloy plating film with a Co content of less than 2% by weight has excessive reactivity with the chromate treatment solution. Since it is presumed that a certain cause is the cause, it is an effective means to control the reactivity by the above-mentioned upper layer plating.

■ Regarding the decrease in corrosion resistance of processed parts, Co content N2wt%
It is presumed that the cause is that the Zn-Co alloy plated steel sheet is soft and easily causes sliding seizure during machining. By providing this, it is possible to improve the corrosion resistance of the processed part.

Here, the present invention provides Co on at least one side of the base steel plate.
A lower plating layer of ZnCo-based alloy with a content of 0.01% by weight or more and less than 2% by weight, and a ZnCo-based alloy with a Co content of 2 to 20% by weight.
n-Co alloy or Z with Ni content of 9 to 20% by weight
Made of n-Ni alloy, deposit amount 0.05-10 g/
m2 upper plating layer, and on top of this upper plating layer, a lower chromate film with a Cr adhesion amount of 20 to 150 mg an organic composite coated steel sheet having an upper layer resin film with a thickness of 0.3 to 5-5-100 mm formed by coating and baking a resin liquid containing an epoxy resin, the lower layer chromate film having a Cr"/ An organic composite coating with excellent corrosion resistance, which is a film formed by applying and drying a coated chromate treatment solution that is partially unified so that the total CrO ratio is 0.4 to 0.6, and a reducing agent is added. The gist is steel plates.

Many modifications can be made to the implementation of the present invention, and as preferred embodiments, for example, the following modifications are possible.

(2) As a reducing agent added to the chromate treatment solution, a polyhydric alcohol and/or a polycarboxylic acid and/or an oxycarboxylic acid is used in an amount of 1 to 4 times the amount of unreduced Cr.

(2) Add colloidal silica in an amount of 0.1 to 4 times the weight of total chromic acid to the chromate treatment solution.

(2) Add iron phosphide to the chromate treatment solution in a molar ratio of 0.1 to 20 times the weight of the total chromic acid.

■The molar ratio of unreduced Cr to the chromate treatment solution is 0.
．． 01 times or more of the silane coupling agent is added.

■ Add 1% to the resin solid content to the resin liquid for forming the upper resin film.
Add 0-40% by weight of colloidal silica.

(2) A crosslinking agent is added to the resin liquid in an amount such that the molar ratio of functional groups to the total amount of epoxy groups and hydroxyl groups is 0.1 to 2.0.

■As the epoxy resin, we use polyhydroxypolyether resin obtained by MU combination of one or more dihydric phenols selected from mononuclear and trinuclear dihydric phenols and shrimp herohydrin. do.

(2) The resin liquid further contains a resin other than epoxy resin in an amount of 50% by weight or less based on the resin solid content.

(2) Use a base material copper plate that has baking hardenability, and keep the drying temperature of the lower chromate film and the baking temperature of the upper resin film below 200°C.

[Phase] At least one of the lower layer and the upper plating layer is compounded with a metal oxide in an amount of 10% by weight or less in terms of metal.

The organic composite coated steel sheet of the present invention has a cross-sectional structure in which lower and upper multilayer Zn-based plating layers, a lower chromate film layer, and an upper epoxy resin film layer are sequentially laminated on at least one side of a base copper plate.

In coated steel sheets for automobiles, such composite coatings are usually applied to only one side of the steel sheet. Of course, depending on the use, the organic composite coated copper plate of the present invention may have the above organic composite coating on both sides.

(Function) Next, in the present invention, the composition of the upper layer and lower layer plating,
In addition, the reasons for limiting the types of treatment liquids for forming each film as described above and the effects of each component will be explained in detail for each layer.

In the organic composite coated steel sheet of the present invention, the lower layer plating adjacent to the steel sheet has a Co content of 0.01% by weight or more and 2% by weight.
It is a Zn-Co based alloy with less than As a property required for the lower layer plating, it is important to be able to maintain sacrificial corrosion protection against the base material over a long period of time in a corrosive environment, and the above alloys are excellent in this respect. The effect of Co content is small, and the elution rate of the sacrificial anticorrosive film is fast in pure Zn plating that does not contain Co.

Regarding other alloy systems, for example, the Zn-Ni system has slightly inferior sacrificial anticorrosion ability, and the Zn-Fe system has a fast elution rate.
It has the disadvantage that corrosion is accelerated by the corrosion products of e.

The deposited amount of the lower layer plating may be sufficient as long as it exhibits sacrificial anticorrosion ability over a long period of time, and although it varies depending on the environment in which the steel sheet of the present invention is used, it is usually required to be 5 g/m2 or more. In addition, from the viewpoint of workability, weldability, and manufacturing cost, 150 g/
It is better to be less than m2. From the viewpoint of performance and stable supply, it is preferably 10 to 100 g/m2.

-L medical school, 100 tons The upper layer plating provided on the lower Zn-Co alloy plating layer is a Zn-Co alloy with a Co content of 2 to 20% by weight or a vertical layer plating layer with a Ni content of 9 to 2%. The main performance required for this is to suppress the elution rate of the lower layer plating and to control the reactivity to the chromate treatment solution described below, and the plating layer with the above alloy composition is optimal for this purpose. If the Go content is less than 2% by weight or the Ni content is less than 9% by weight, this required performance cannot be fully satisfied.

Further, if the amount of Ni or Co exceeds 20%, the cost will be high.

Conventionally, there have been many cases in which Fe or Fe-based alloy plating has been provided as the upper layer plating, but these platings are good in terms of controlling the reactivity to the chromate treatment solution, but they are difficult to suppress the elution rate of the lower layer plating. It has the disadvantage of being inferior in terms of

The coating weight of the upper layer plating is 0.05 to 10 g/m2.

If the amount of adhesion is less than 0.05 g/m2, the effect will be insufficient, and if it exceeds 10 g/m, it is not desirable from an economical point of view.

From the standpoint of stably manufacturing and supplying steel plates with the above performance,
], g/*” or more is preferable.

A metal oxide may be eutectoid in at least one of the lower and upper plating layers. Corrosion resistance is thereby further improved. As an oxide, AQ. o. , Sin□, Ti
O2, ZrO2, pbo,, Pb2O3, Snow,
5nO1SbzOs, sb. o. , Fe2O3, Fe5
0. etc.

These oxides preferably have an average primary particle size of 2 or less. Those having an average primary particle size of more than 2p may aggregate and become coarse in the plating bath, resulting in decreased corrosion resistance and workability. More preferably, one having a value of 0.5 or less is used. Here, the average primary particle size means the t'·7 diameter of the oxide itself, without considering agglomeration in the plating bath.

The metal oxide is 10% by weight in terms of metal in the plating film.
It is desirable to eutectoid in the following amounts. If it exceeds 10% by weight, processability will decrease.

The Zn-Co alloy used for the lower layer plating, and the Zn-Co alloy or Zn-Ni alloy used for the upper layer plating, are all metal components in the film (excluding metal oxides when metal oxides are eutectoid). ) has the maximum content of Zn.
The second component having the next highest content may be Co or Ni. After the third component, Ni (in the case of Zn-Co system), Co (in the case of Zn-Ni system), AQ,
Si.

Nb. , Mn. Mg=Mo, Ta, Cu. , S
n=Sb, Ti, Cr, Cd.

Pb, TI, In, ν, W, P. There is no functional problem even if one or more of S, B, N, etc. is contained in a smaller amount than the second component, and some performance may be improved by adding the third component or less. be.

Further, it is also possible to make one or both of the lower layer and the upper layer alloy plating layers two or more alloy plating layers within the range of alloy composition defined in the present invention, and such an embodiment is also within the scope of the present invention. include.

For example, the lower plating layer is a Zn-0,7% Co layer and a Zn-1,4% Co layer from the bottom, and the upper plating layer is a Zn-3% Co layer and a Zn-11% layer from the bottom. 2 with N1 layer
You can do layers.

The formation methods of the upper and lower alloy plating layers include electroplating, hot-dip plating, thermal spraying, F'ly process, etc.
Either method may be used.

Chromate ゛Generally, chromate film formation methods include coating type, reaction type,
Although there are electrolytic types, etc., the present invention utilizes a coating type chromate film, which has the best corrosion resistance.

The coating type chromate film is formed by reduction of chromic acid and evaporation of water, but in order to reduce and form the film efficiently at low temperatures, the film is formed using the two-stage reduction method described below.

That is, the -th stage is chromic acid (C) in the chromic acid aqueous solution.
r") is partially reduced in advance. This reduces the amount of chromic acid reduced during heating, drying and baking, making it possible to form a film efficiently.

- The partial reduction rate of stage is 0.4 in Cr-3/total CrO ratio.
It is within the range of ~0.6. If this ratio is less than 0.4, the reduction efficiency will be poor, and if it exceeds 0.6, Cr'' will be excessive, which may impair the stability of the treatment liquid.

The reducing agent used in the - stage reduction is not limited, and polyhydric alcohols, polyhydric carboxylic acids, etc. are used.

Alternatively, a commercially available partially reduced chromate treatment solution may be used.

Colloidal silica may be added to this chromate treatment solution. The amount of colloidal silica added is 0.0% of the total chromic acid.
It is 1 to 4 times (weight). The action of colloidal silica increases the wettability of the chromic acid aqueous solution and helps the film-forming action of the chromate film, but if it is less than 0.1 times the effect of promoting the film-forming action is poor, and if it exceeds 5 times the action, the film becomes weak and cannot be processed. Later corrosion resistance decreases.

Iron phosphide may be added to this chromate treatment solution.

When iron phosphide coexists in the chromate treatment solution, the iron phosphide reacts with M-dissociated hexavalent chromium in the chromate film and reduces its amount, reducing the amount of components that are easily soluble in water, which leads to corrosion. The adhesion of the film in the environment is maintained. Further, since iron phosphide has electrical conductivity, the electrodeposition coating properties and weldability of the obtained organic composite coated steel sheet are also improved. The amount of iron phosphide added is 0.1 to 20 times (by weight) the total amount of chromic acid.
It is necessary that If the amount added is less than 0.1 times, the effect will not be sufficient, and if it exceeds 20 times, the conductive effect of iron phosphide will be too large, and the role of the film as a corrosion barrier will be impaired. . The preferable addition amount is 0.1-1
It is 0 times.

A silane coupling agent may be added to this chromate treatment solution. The silane coupling agent is hydrolyzed,
By producing polysiloxane, the chromate film is strengthened and its adhesion with the upper layer film is improved. The alcohol liberated by hydrolysis acts as a reducing agent for chromic acid. Examples of silane coupling agents that can be used include vinyltriethoxysilane, vinyl-tris(β-methyneethoxy)silane, T-methacryloxypropyltrimethoxysilane, T-glycidoxypropyltrimethoxysilane, γ -aminopropyltriethoxysilane, N
-β-(aminoethyl)-T aminopropyltrimethoxysilane, β-(3,4 epoxine chlorohexyl)ethyltrimethoxysilane, and the like. The amount of these silane coupling agents added is 0.01 times or more in molar ratio to unreduced chromic acid. If it is less than 0.01 times, the effect is small. However, if it is added in too large a quantity, it is not only economically disadvantageous, but also may deteriorate electrodeposition coating properties and corrosion resistance.

Further, in order to improve corrosion resistance, this chromate treatment solution may contain phosphoric acid in an amount equal to or less than the same mole based on the total chromic acid.

A reducing agent is added to the treatment solution that has been subjected to the first reduction in an amount that has a reducing ability of 1 to 4 times the amount of unreduced Cr. As a result, the corrosion resistance and chromium elution resistance of the chromate film become insufficient.
If the amount exceeds 4 equivalents, the reducing action of the reducing agent will not only be saturated (but also the reducing agent will remain in the chromate film and impair the corrosion resistance of the film.

As the reducing agent, it is preferable to use one or more of polyhydric alcohols, polycarboxylic acids, and oxycarboxylic acids. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of polyhydric carboxylic acids include succinic acid, glutaric acid, and adipic acid. As oxycarboxylic acid,
Citric acid, lactic acid, etc.

The most desirable time to add this reducing agent is immediately before painting, but it should be added in a small amount (within a few days before painting).

The reason for this is that the treatment liquid to which the second-stage reducing agent has been added tends to gel if left to stand.

In the organic composite coated steel sheet of the present invention, the amount of the chromate treatment solution deposited is 20 to 150 mg/ml as the amount of chromium,
Preferably 40 to 80 mg/n (with an adhesion amount of 2
If it is less than 0ing/rr, the corrosion resistance will be insufficient,
If it exceeds 150 mg/ml, the chip will be severely damaged during welding and weldability will deteriorate.

The chromate treatment liquid can be applied by a conventional means such as a roll coater or a bar coater.

Drying and baking of chromate treatment liquid is done at 60-200°C1
It is preferably carried out at 100 to 150"C. The drying temperature is 20"C.
If the temperature exceeds 0°C, the bake hardenability of the steel plate will be inhibited.

Ueto Epoxy: By applying a resin liquid containing an epoxy resin to the lower chromate film formed as described above and baking it, an upper resin film with a thickness of 0.3 to 5 μm is formed.

As the epoxy resin, any glycidyl ether type epoxo resin such as bisphenol A type, novolak type, bisphenol F type, etc. can be used.

In addition, epoxy ester resins obtained by reacting the epoxy groups and hydroxyl groups in epoxy resins with carboxyl groups in drying oil fatty acids, urethane-modified epoxy resins obtained by reacting epoxy resins with isocyanates, and epoxy resins obtained by reacting epoxy resins with acrylic acid, methacrylic acid, etc. Modified epoxy resins such as epoxy acrylate resins modified with acids or the like can also be used.

These epoxy resins desirably have a molecular weight of 100 or more so that the coating film dries to an extent that no hardening occurs even if no curing reaction occurs due to low temperature baking.

In the present invention, polyhydroxy polyether resin is one of the epoxy resins that can be suitably used. This resin is obtained by polycondensing mononuclear or trinuclear dihydric phenol or mixed mononuclear and trinuclear dihydric phenol with an approximately equimolar amount of epihalohydrin in the presence of an alkali catalyst. It is a polymer that can be used. Examples of mononuclear dihydric phenols are resorcinol, hydroquinone and catechol, and examples of trinuclear phenols are bisphenol A.
These may be used alone or in combination of two or more. A typical example of shrimp halohydrin is shrimp chlorohydrin.

When the dihydric phenol is a mixture of resorcinol and bisphenol A at a molar ratio of 1/1, the polyhydroxypolyether resin is composed of repeating units represented by the following structural formula.

Further, when the dihydric phenol is resorcinol alone, the repeating unit of the resin is shown by the following structural formula.

Since polyhydroxypolyether resin is manufactured from the same raw materials as epoxy resin, it is also included in the epoxy resin in the present invention. However, unlike epoxy resins, this resin does not have terminal epoxy groups, and is a thermoplastic resin with a number average molecular weight of about 8,000 to 20,000, which is considerably larger than ordinary epoxy resins. A polyhydroxypolyether resin prepared from bisphenol A and epichlorohydrin is commercially available as phenoxo resin PK) 18 from Union Carbide Company, USA.

As is clear from the above structural formula, the polyhydroquine polyether resin contains many OH groups and -0- groups in the chain. Ordinary epoxy resins also have many OH groups and 10-groups in the chain. Since OHM forms hydrogen bonds with the substrate, it contributes to increasing adhesion, and since the -o- group easily causes intramolecular rotation, it contributes to increasing the flexibility of the resin.

When dihydric phenol is a mononuclear type such as resorcinol rather than a trinuclear type such as bisphenol A, the molecular weight of dihydric phenol is smaller, so the amount of these functional groups per weight is smaller. The number increases. Therefore, as the content of mononuclear dihydric phenol such as resorcinol increases, the content of the functional groups in the chain increases, and the adhesion and flexibility of the resulting coating film tend to increase. In this sense, when using polyhydroxy polyether resin, 2
The hydric phenol preferably contains at least a portion of a mononuclear type such as resorcin, for example, about 50 mol% or more, and may be 100% resornon. Similarly, even if other epoxy resins are used, it is generally advantageous to use one with a large number of OH groups and O-groups in the resin.

One or more of the above-mentioned epoxy resins including modified epoxy resins and polyhydroxypolyether resins are used, and these are selected in consideration of required properties such as solubility, drying speed, and smoothness. A resin liquid for coating can be prepared by dissolving it in an organic solvent. As suitable containers, for example, cellosolves, ketones, esters, hydrocarbons, halogenated hydrocarbons, or mixed solvents thereof can be used for dissolving epoxy or modified epoxy resins. To dissolve the polyhydroquine polyether resin, ketones, cellosolves, mixed solvents thereof, etc. can be used.

In the present invention, colloidal silica may be added to the resin liquid in an amount of 10 to 40% by weight based on the total solid content of the resin. This silica is added for the purpose of improving corrosion resistance and electrodeposition coating properties. When the amount added is less than 10% by weight,
These performance improvement effects are not sufficient, and if the content exceeds 40% by weight, weldability will deteriorate.

Examples of commercial products of colloidal sled force include Aerosil manufactured by Nippon Aerosil, Ludox manufactured by DuPont, Snowtex manufactured by Nissan Chemical, Cataloid and Oscar manufactured by Catalyst Kasei, and those manufactured by Asahi Denka. An organosol type colloid having an organophilic colloid surface is preferably used in the present invention.

A crosslinking agent may be added to the resin liquid for the purpose of further improving corrosion resistance. As the crosslinking agent, those known as curing agents for epoxy resin paints can be used, and specific examples include phenol resins, amino resins, polyamides, amines, blocked isocyanates, and acid anhydrides.

The amount of the crosslinking agent added is such that the molar ratio of the functional groups in the crosslinking agent to the total amount of epoxy groups and hydroxyl groups in the epoxy resin is within the range of 0.1 to 2.0. If this molar ratio is less than 0.1, the effect will be small, and if it exceeds 2.0, the coating will lose its flexibility, and the coating will crack during processing.
Corrosion resistance may deteriorate.

In addition to the above ingredients, the resin liquid contains various properties of the film (e.g.
In order to further improve processability, flexibility, lubricity, electrodeposition coating properties, etc., various additives such as resins other than epoxy resins and lubricity imparting agents may be added. . For example, for the purpose of imparting flexibility to the film, a flexible resin such as butyral resin can be added as a type of plasticizer.

Furthermore, a water-soluble resin can be added for the purpose of improving electrodeposition coating properties. Examples of water-soluble resins include polyvinyl alcohol, polyacrylic acid, polyacrylamide, and the like. The amount of resin other than epoxy resin added is preferably in the range of about 1 to 50% by weight of the total resin solid content in the resin liquid, since corrosion resistance and weldability will deteriorate if the amount is too large. A conductive pigment is added to make it moist.

This improves the conductivity of the resin film, improves electrodeposition coating properties,
Improves weldability. The addition of the conductive pigment is particularly effective when the upper resin film is thick. Examples of conductive pigments include zinc powder, iron phosphide, and the like.

Furthermore, a l[8-based chromium-based antirust pigment may be added to this resin liquid. This can be expected to further improve corrosion resistance. In particular, corrosion resistance at uneven portions is improved. Examples of poorly soluble chromium-based rust preventive pigments include 5rCr.
Examples include Oa, BaCrO4, PbCr0n, and the like.

Further, an appropriate amount of a colored pigment or filler can be added to the resin liquid for forming the upper layer film.

Application of the resin liquid for forming the upper layer film can also be carried out by a conventional means such as a roll coater or a bar coater, and after drying, the film thickness is 0.3 to 5 μm, preferably 0.6 to 1.6 μm.
Apply so that a film of

If the film thickness is less than 0.3p, corrosion resistance will be poor, and if it exceeds 5p, weldability will be reduced.

After applying the resin liquid, the coating film is baked at a temperature of 80 to 300°C to form an upper layer film. Within this temperature range, the higher the baking temperature is, the more the crosslinking reaction progresses and a stronger film can be formed.
If heated above C, the bake hardenability of the steel plate will be lost, so
The upper limit of baking temperature is 200°C.

Next, the present invention will be explained by examples. In the examples, percentages and parts are by weight unless otherwise specified. Oxides are shown in metal equivalent amounts.

(Example) (a) Base steel plate A cold-rolled M-killed steel plate with a thickness of 0.8 m was used as the base steel plate, and after rough degreasing with a solvent as a pre-plating treatment, the surface was subjected to electrolytic degreasing, water washing, and hydrochloric acid immersion. Activation and water washing steps were performed.

(b) Lower layer plating/upper layer plating The steel plate treated as described above is used as the base material, and one side of the base material is coated with ZnCo alloy plating as the lower layer plating and Zn-Co or Zn-Ni as the upper layer plating.
A multi-layer plated steel sheet was obtained by plating the alloy. The plating process was performed continuously in the order of lower layer plating, water washing, upper layer plating, and drying.

As a comparison material, one of the upper and lower plating layers was electroplated with Zn-Fe alloy plating, Fe-Zn alloy plating, pure Zn plating, or pure Fe plating, and two layers were formed in the same process as above. Plated multi-layer plated steel sheet,
And a Zn-Co alloy single-layer plated steel plate was prepared.

The plating conditions were as follows.

Sayu Spring Bath Group Precepts Zn-Co alloy plating bath Zn5O<4HzO: 200-400 g/lC
o50n4HzO: 50-400 gelNazS
Oa: 60-100 g/1■Zn-Ni
Alloy plating bath Zn5Oa-7HzO: 200-400 g/l
N15Oa・6)+20: 50-400g/(2
NazSOa: 60-100 g/1■Z
n-Fe, Fe-Zn, pure Fe, pure Zn plating bath Zn
5O<4HzO: O~400 g/j! FeSO
44) 1zo: 0-500 g/INazS
O4: 60-100 g/1 Lili conditions Plating bath pH+ 1.0-2.5 Plating bath temperature: 40-60°C Plating solution flow rate: 0.5-3 m/sec current density
: 40-120 A/dm" In the case of a plating bath containing a tertiary element, the metal's sulfate, carbonate, chloride, molybdate, pyrophosphate, hypophosphite or organic A metal salt or a solution prepared by dissolving the metal in advance in an acid was added to the plating bath in an amount that would give the desired composition.

When forming oxide composite plating, a predetermined metal oxide was added to the bath in the form of a sol. The amount added is 0 as an oxide.
．． It was within the range of 01 to 100 g/μm.

The content of oxides eutectoided in the plating film is I after the film melts.
The amount of metal ions was measured by CP spectroscopy, atomic absorption spectrometry, portammetry, or the like.

The oxide sol used was manufactured by Nissan Chemical ■.

(C) Lower layer chromate film chromate layer ・'i11■ In a chromic acid aqueous solution containing Cry3120g/1,
Ethylene glycol was added as a reducing agent, and the mixture was heated at 80°C for 6 hours. After the reaction, an aqueous chromic acid solution was added to adjust the CS'/total Cr ratio to a predetermined value, and water was further added to adjust the total chromic acid concentration to Cr (40 g/1 in terms of h).
The amount was adjusted to 0.4 mol/j2). Glycerin, which is a polyhydric alcohol, was added as a reducing agent to the obtained partially reduced chromic acid aqueous solution before use. In addition, colloidal silica and/or iron phosphide and/or γ-glycidoxypropyltrimethoxine lan as a silane coupling agent were added to some of the chromate treatment solutions.

A multi-layer plated steel sheet obtained by applying upper layer plating and lower layer plating with chromate (BL) was degreased using an alkaline degreasing liquid (Fine Cleaner 4336) manufactured by Nippon Parkerizing, and then prepared on the plated surface. A coating type chromate treatment liquid was applied using a bar coater so that a predetermined amount of Cr was deposited, and the coating was dried by heating at a plate temperature of 140° C. for 30 seconds to form a chromate film.

(d) Digrindyl ether of bisphenol A (Epicoat 8 manufactured by Yuka Ciel Evokin Co., Ltd.
28) 230 parts of Resol, 2755 parts of Resol, 200 parts of methyl ethyl ketone, and 4 parts by volume of 5N NaOH aqueous solution were charged, and reacted at reflux temperature for 18 hours.

The obtained resinous substance was put into water in a stirrer, stirred, and reprecipitated in water to recover a water-insoluble resin. This was dried under reduced pressure to obtain a powdered polyhydroxypolyether resin in which the dihydric phenols were resorcinol and bisphenol A (molar ratio 1/l). The obtained powdered resin was dissolved in a 1/1 (volume ratio) mixed solvent of cellosolve acetate/cyclohexanone to obtain a resin liquid having a solid content of 20% by weight.

This resin liquid is hereinafter referred to as resin liquid A.

Resin liquid B was prepared using commercially available phenoxy resin PKHH (molecular weight 30,000) manufactured by Union Carbide Co., Ltd. as another polyhydroxypolyether resin, and using the same solvent and solid content as the resin liquid as above.

Furthermore, Epicoat 1009 (molecular weight 3750) manufactured by Yuka Shell Epoxy Co., Ltd., which is a boat-friendly epoxy resin,
xylene/methyl ethyl ketone mixed solvent (weight ratio: 6/
4) to prepare a resin liquid C having a resin solid content of 20% by weight.

Colloidal silica (manufactured by Catalyst Kasei Co., Ltd., Oscar 1432 average particle size 10-20 m), crosslinking agent (blocked isocyanate with a dissociation temperature of 80 ° C for resin solutions A and B,
For resin liquid C, the addition of a phenol resin), a plasticizer (butyral resin), a pigment for lead 1, and a rust preventive pigment was carried out by adding predetermined amounts to the resin liquid and stirring and dispersing them.

After cooling the steel plate that has been chromate-treated as described in MlJJl (C) to room temperature, the resin liquid is applied onto the formed chromate film using a bar coater to a predetermined dry thickness, and the plate is coated. The coating was baked and cured by heating at 140°C for 60 seconds to form a resin film.

The obtained organic composite coated steel sheet was subjected to the following tests to evaluate weldability, corrosion resistance, adhesion after painting, and chromium elution.

(e) Test method - Corrosion resistance test: The test pieces used for the corrosion resistance test are flat plates, cylindrical drawings with a diameter of 50 cm, and cross cuts made in the flat plate part with a cutter knife to reach the steel plate base. .

The die shoulder of the cylindrical drawing was cleaned with triclean and polished with No. 120 emery paper each time so that its surface roughness remained constant. These test pieces were immersed in degreasing liquid FC-L4410 manufactured by Nihon Parkerizing Co., Ltd. at 43°C for 2 minutes and 30 seconds, washed with water, and then baked at 165°C for 25 minutes. Each specimen was then subjected to 4 hours of salt spray at 35°C, 2 hours of warm air drying at 60"C, and 2 hours of humidified atmosphere at 50°C2 with a relative humidity of 95% or higher for 1 hour.
A composite corrosion test was conducted for the flat plate and cylindrical aperture walls for 200 cycles (test time 1600
Corrosion resistance was evaluated based on the area ratio (%) of red rust generated over time. Regarding the cross-cut portion, the maximum red rust width after 25 cycles was measured.

A schematic diagram of the method for measuring the maximum red rust width is shown in Figure 1.

■Warm water secondary adhesion test: After applying 20p thick epoxy-based cationic electrodeposition coating to the prepared organic composite coated steel sheet specimen, an intermediate coating is applied using amino alkyd-based paint.
(40m) and top coat (40/1j11) were applied. This is the same paint used for the exterior of automobiles.

The obtained painted test piece was placed in deionized water at 40°C for 240°C.
After soaking for an hour, make cuts so that there are 100 squares of two squares, and peel them off with cellophane tape.
The remaining rate of the paint film was investigated.

The evaluation was made as follows: 5 or more areas where the coating film of the squares peeled off by 30% or more were evaluated as x, 4 to 1 areas were evaluated as Δ, and 0 areas were evaluated as O.

■Chromic acid elution test: When the test piece was immersed in degreasing liquid FC-L4410 manufactured by Nippon Vercalizing at 43°C for 2 minutes and 30 seconds, and in addition, zinc phosphate chemical conversion treatment liquid P manufactured by the same company
The amount of chromium eluted in each case of immersion in B-L3080 at 43° C. for 2 minutes was calculated from the amount of chromium deposited on the coated steel sheet before and after immersion, which was measured by X-ray fluorescence analysis.

■Weldability test 2 Two specimens of the prepared organic composite coated steel plates were stacked so that the coated and uncoated surfaces were in contact, and an AC singles bond welder was used to weld the two specimens using an electrode with a tip diameter of 6.0 mm. Spot welding was performed under the conditions of a welding current of 10.0 OOA, a current application time of 12 cycles, and a pressing force of 200 kgf. Weldability was evaluated using the following two methods.

A. Stability of conductivity: After 1000 dots, 100 samples were randomly sampled, and the evaluation was based on the number of unstable indentations caused by local current concentration.

FIGS. 2(a) and (b) show a good indentation, respectively.
A schematic diagram of an unstable indentation with localized power concentration is shown.

B. Electrode diameter after continuous dots: The diameter of the electrode after 1000 dots was measured using pressure sensitive paper and evaluated according to the following criteria.

○: Electrode diameter <7.0 am △: Electrode diameter -7.0 to 8.0 (Foundation) ×: Electrode diameter >8.0 ffl11 The details of the upper and lower plating layers and the lower chromate film are shown in Table 1 below. , and the details of the upper epoxy resin film and the test results are shown in Table 2.

In addition, the value immediately before the element symbol in the column for the type of plating film indicates the content of that element in weight%, and for oxide composite plating, the content in terms of metal is indicated in weight%. .

(Left below) As is clear from the results in Table 2, the organic composite coated steel sheet of the present invention not only has excellent adhesion and corrosion resistance of the upper layer resin film, but also has excellent secondary adhesion to hot water (paint coating). It also has excellent post-adhesion) and weldability, and the amount of chromium eluted is small.

(Effects of the Invention) As described above, the organic composite coated steel sheet of the present invention can ensure excellent weldability despite the absence of metal powder that adversely affects workability in the coating, and also has excellent corrosion resistance and It also shows fully satisfactory performance in terms of paint film adhesion and amount of chromium elution. Furthermore, it also has excellent adhesion after coating when an automotive coating is applied on top of the resin film. Therefore, it is possible to supply an organic composite coated net plate with excellent performance, particularly suitable for automobiles, at a relatively low cost.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a method for measuring the maximum red rust width from a cross-cut portion, and FIGS. 2(a) and (b) are schematic diagrams showing a good indentation and an unstable indentation, respectively. (cL) Good f, no indentation determined rt impression

Claims (1)

[Claims] (1) Co content of 0.0 on at least one side of the base steel plate
A lower plating layer of a Zn-Co alloy with a content of 1% by weight or more and less than 2% by weight, and a Zn-Co alloy with a Co content of 2 to 20% by weight or a Zn-Ni alloy with a Ni content of 9 to 20% by weight. It has an upper plating layer with a coating amount of 0.05 to 10 g/m^2, and on top of this upper plating layer, a lower chromate film with a Cr coating amount of 20 to 150 mg/m^2, and an epoxy An organic composite coated steel sheet having an upper layer resin film with a film thickness of 0.3 to 5 μm formed by coating and baking a resin liquid containing an epoxy resin selected from resins, modified epoxy resins, and polyhydroxy polyether resins. Therefore, the lower chromate film is Cr^3
A coating with excellent corrosion resistance that is formed by applying and drying a coated chromate treatment solution that is partially reduced so that the ratio of ^+/total Cr is 0.4 to 0.6 and further contains a reducing agent. Organic composite coated steel sheet.