JPH0326539A - Anti-corrossive painted steel plate excellent in weldability - Google Patents

Abstract

PURPOSE:To enhance weldability by providing a lower film layer with a specific Cr adhesion amount and an upper film layer with a specific thickness to which an epoxy resin having a specific amount of colloidal silica added thereto is applied and baked. CONSTITUTION:A coating type chromate treatment solution partially reduced so as to set a ratio of Cr<3+>/total Cr to 0.4-0.6 and further containing a reducing agent added in an amount 1-4 equivalent times the content of non-reduced Cr<6+> and not substantially containing colloidal substance is applied to a zinc or zinc type alloy plated steel plate and baked to form a lower film layer with a Cr adhesion amount of 20-100mg/m<2>. Further, a resin solution containing an epoxy resin, to which colloidal silica is added in an amount of 10-25% by wt. of the sum amount of the total resin solid in the resin solution and colloidal silica, as a base resin is applied to the lower film layer and baked to form an upper film layer having a thickness of 0.3-1.6mum. The anti-corrosive painted steel plate wherein the zinc type plate layer, the lower chromate film layer and the upper colloidal silica-containing epoxy resin film layer are successively laminated to the steel plate substrate by this method is excellent in weldability and not inferior to corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rust-proof coated steel sheet with excellent weldability, which is particularly suitable for automobiles.

(Prior Art) Rust-prevention coated steel sheets that can be electrically resistance welded are increasingly being used in Japan, for example, to prevent automobile damage caused by salt sprayed as a snow-melting agent on roads in snow-covered areas. .

Conventional weldable anti-corrosion coated steel plates include steel plates coated with zinc-rich brimer such as Zinchrome Metal (trade name: Diamond Jamrock Co., Ltd., USA). Zinchrome metal is produced by forming a base chromate layer (Dacromet) containing zinc powder and chromic acid as the main ingredients, and a zinc-rich primer layer (Zinchromet) consisting of an epoxy resin containing a large amount of zinc on a steel plate. It is a steel plate with anti-corrosion coating and exhibits significantly superior corrosion resistance compared to cold-rolled steel plate. In general, anti-rust coated steel sheets for automobiles have, in addition to corrosion resistance,
Formability and weldability are required, but the zinc chrome metals mentioned above do not particularly have sufficient formability and corrosion resistance. This is because the zinc clinch/brimer used for these steel sheets contains a large amount of zinc powder, approximately 50% by volume and approximately 85-90% by weight, in order to ensure weldability. This is because the coating film is brittle and easily peels off during processing. This reduces corrosion resistance, and the paint film that falls off adheres to the press mold and causes scratches, which increases the frequency of mold maintenance and deteriorates workability. In addition, zinc-rich brimer has high moisture permeability into the coating film, which also tends to reduce corrosion resistance. Reducing the amount of zinc dust is effective in reducing paint peeling and deterioration of corrosion resistance, but it increases the electrical resistance of the paint film.
Resistance welding becomes disadvantageous or impossible. In addition, conventional zinc chrome metals require baking at a high temperature of 250 to 280°C to bake the zinc rich primer, which has the disadvantage of impairing the bake hardenability of the steel plate. Bake hardenability refers to the property that the yield stress of a steel sheet increases when the user performs processing such as pressing and bakes with electrodeposition coating or the like.

(Intelligence M to be Solved by the Invention) As a result of the inventors' study to eliminate the above-mentioned drawbacks of the zinc clinch and brimer, the present inventors found that using a plated steel plate as a base material,
After coating and baking an aqueous suspension containing chromic acid and colloidal silica as a base layer to form a chromate film, a polyester film containing no pigment or a chromate-based pigment is applied. We previously proposed that by forming the upper clear coating film by applying and baking a hydroxybolyether resin solution, it is possible to obtain a rust-proof coated steel sheet that can be resistance welded and has excellent corrosion resistance and formability. 63-69917). This steel plate can be baked at relatively low temperatures without impairing bake hardenability.
The obtained coated steel sheet stably maintains bake hardenability.Furthermore, as an improvement to the above-mentioned anti-rust coated steel sheet, the present inventors applied an upper layer of at least one of various inorganic fillers and crosslinking agents. By blending it into the resin liquid for film formation, the upper layer/coating film can be made from general-purpose epoxy resin or modified epoxy resin, and excellent corrosion resistance can be obtained without containing chromate pigments. also proposed (Unexamined Japanese Patent Publication No.
-80522).

However, recently, steel sheets for automobiles have come to be required to have a higher level of welding workability, and it may be difficult to meet these demands with conventional weldability levels. In particular, in series spot welding, if the welding electrode tip hits the resin-coated surface in combination to investigate the continuous dot performance, the results may be unsatisfactory.

One way to improve weldability is to include conductive powder, such as the zinc-rich primer mentioned above, but this poses problems in corrosion resistance and press workability. Another method proposed by the present inventors is to ensure good electrical conductivity by providing appropriate protrusions on the surface of the base plated steel sheet according to the thickness of the upper layer clear film. 63-297886). However, this method also has the disadvantage of requiring extra steps such as rolling with dull rolls. (Means for solving the problem) The chromate treatment methods used for the lower layer coating of anti-corrosion coated steel sheets are broadly divided into three types: ■ Spray-on chromate treatment, ■ Reactive chromate treatment, and ■ Electrolytic chromate treatment. Of these, reactive chromate treatment and electrolytic chromate treatment have inferior corrosion resistance compared to coating-type chromate treatment, and the treatment conditions are difficult to control. Therefore, coating-type chromate treatment is generally used. Most coated chromate treatment solutions contain colloidal substances such as colloidal silica. This is aimed at improving corrosion resistance, electrodeposition coating properties, and chromium elution resistance. However, as a result of investigating the effects of colloidal substances in the chromate film and the upper resin film on the weldability of anti-corrosion coated steel sheets, the inventors found that the amount of colloidal substances in the chromate film greatly affected the weldability. I found out that it depends.

In other words, the greater the amount of colloidal material present in the underlying chromate film, the lower the weldability, and when no colloidal material is present in the underlying chromate film, the weldability, especially direct weldability, decreases.
The continuous welding performance of spot welding and series/spot [9 contacts has been greatly improved.

The amount of colloidal substances in the upper layer clear film does not have as great an effect on weldability as the amount of colloidal substances in the chromate film, so by containing colloidal silica only in the upper layer, good weldability can be achieved. It turned out that it was secured. Although the details of the reason for this are unknown, it is presumed that even with the same colloidal silica force, the behavior during welding differs between the chromate film and the resin film. In other words, if colloidal silica exists in the chromate film, since colloidal silica is a substance with considerably higher electrical resistance than chromate, heat generation between the electrode and the steel plate will increase, causing severe damage to the electrode and making continuous dots difficult. is thought to deteriorate. On the other hand, since most of the colloidal silica present in the resin film is blown away to the surroundings as the resin thermally decomposes during welding, it is considered that it does not have much of an effect on weldability.

The basic idea of the present invention is to improve weldability by eliminating the presence of colloidal substances in the chromate treatment liquid, and to remove the colloidal substances from the chromate film, unlike conventional conventional application-type chromate treatment liquids. Corrosion resistance due to
The reduction in electrodeposition coating properties and chromium elution resistance is compensated for by two-stage reduction of chromate and the presence of an appropriate amount of colloidal silica in the upper resin film. By making each layer of the 2N coating in this combination relatively thin, it is possible to ensure excellent weldability without mixing metal powder such as zinc dust, and it has excellent weldability suitable for automobiles. It was found that a rust-proof coated steel plate could be obtained.

The gist of the present invention is to partially reduce a Cr3+/total CrO ratio of 0.4 to 0.6 on a zinc or zinc-based alloy coated steel sheet, and further reduce the Cr3+/total CrO ratio to 1 to 4% with respect to unreduced Cr.
A lower layer film with a Cr adhesion amount of 20 to 100 cm/bo formed by applying and baking a coating-type chromate treatment solution containing substantially no colloidal substances to which an equivalent amount of reducing agent has been added and a resin solution. A film formed by applying and baking a resin liquid based on an epoxy resin containing 10 to 25% by weight of colloidal silica based on the total amount of total resin solid content and colloidal silica.
．． This is a rust-proof coated steel sheet with excellent weldability, characterized by having an upper layer coating of 6-. In a preferred embodiment of the present invention, the reducing agent added to the chromate treatment solution is a polyhydric alcohol and/or a polycarboxylic acid and/or an oxycarboxylic acid, and the chromate treatment solution has a molar ratio of unreduced Cr'+. It may further contain a silane cambling agent of 0.01 times or more. In addition, the resin liquid further contains a crosslinking agent and/or resin in an amount 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 0.1 to 0.2. It may contain resin other than epoxy resin in an amount of 50% by weight or less based on the total solid content. Epoxy resins are modified epoxy resins or one or more types of divalent phenols obtained from mononuclear and dinuclear dihydric phenols.
It may also be a polyhydroxy polyether resin obtained by polycondensation of a hydric phenol and epihalohydrin.

When the base steel plate used is a steel plate having bake hardenability, it is preferable that both the lower layer and the upper layer are baked at a temperature of 200'C or less.

(Function) In the present invention, the reason why the composition of the treatment liquid for each film type is limited as described above and the effect of each effect will be explained in detail.

Chromate Chromate film is formed by reduction of chromic acid and evaporation of water, but in order to efficiently reduce and form the film at low temperature,
A two-stage reduction method described below was used.

That is, the first step is to partially reduce the chromic acid (Cr") in the chromic acid aqueous solution in advance. This reduces the amount of chromic acid reduced during heat drying (baking) and effectively forms a film. The first stage partial reduction rate is Cr3+/total C ``[=Cr3+/(Cr''
+Cr”)] ratio is 0.4 to 0.6.0.4
If it is less than 0.6, the reduction efficiency will be poor, and if it is more than 0.6, Cr”
may become excessive and the stability of the processing solution may be impaired.

The reducing agent used in the first 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. A silane coupling agent may be added to this chromate treatment solution. The silane coupling agent hydrolyzes to produce polysiloxane, thereby strengthening the chromate film and improving its adhesion with the upper layer film. The alcohol liberated by hydrolysis acts as a reducing agent for chromic acid. Examples of silane coupling agents that can be used include vinyltriethoxysilane, vinyllutris(β-methoxyethoxy)silane, T-methacryloxypropyltrimethoxysilane, T-glycidoxypropyltrimethoxysilane, γ-amino Propyltriethoxysilane, N
-β- (ξnoethyl)-T aminobrobyltrimethoxysilane, β-(3.4epoxycyclohexyl)ethyltrimethoxysilane, and the like. When adding Silanka 7 bling agent, the amount added should be at least 0.01 times the molar ratio of unreduced chromic acid; if it is less than 0.01 times, its effect will be small. However, adding too much boron is not only economically disadvantageous, but also may reduce electrodeposition coating properties and corrosion resistance, so the amount added should be no more than 3 times the molar ratio of unreduced chromic acid. That is preferable. The chromate treatment solution may contain phosphoric acid in an amount equal to or less than the same mole based on the total chromic acid in order to improve corrosion resistance.

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.
When the amount exceeds 4 equivalents, not only the reducing action of the reducing agent becomes saturated, but also the reducing agent remains in the chromate film, impairing 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 adivic acid. As oxycarboxylic acid,
These include citric acid and lactic acid. The most desirable time to add this reducing agent is immediately before painting, but at least within a few days before painting.

The reason for this is that the treatment solution to which the second-stage reducing agent has been added tends to gel if left to stand. As mentioned above, the chromate treatment liquid used in the present invention is
It does not substantially contain colloidal substances such as colloidal silica. It is also preferable not to contain chromate-based or other corrosion-resistant pigments or conductive fine particles such as iron phosphide. That is, it is preferable to use a chromate treatment liquid that is substantially in the form of an aqueous solution. In the antirust coated steel sheet of the present invention, the amount of the chromate treatment solution applied is 20 to 100 mg/rd, preferably 40 to 70 .mu./rI'r in terms of chromium amount. Adhesion amount is 20■/
If it is less than 100 cm/bo, the corrosion resistance will be insufficient, and if it exceeds 100 cm/bo, the chip will be severely damaged during welding and weldability will deteriorate.

Coating can be carried out by conventional means such as a roll coater or a bar coater. The baking of chromate treatment liquid is 60~
200°C1 Preferably carried out at 100 to 150°C. If the baking temperature exceeds 200°C, the bake hardenability of the steel sheet will be inhibited. On top of the lower chromate film formed as described above,
In the rust-preventing coated steel sheet of the present invention, a resin solution is applied using an epoxy resin as a base resin and adding colloidal silica in an amount of 10 to 25% by weight based on the total amount of the total resin solid content and colloidal silica in the resin solution. Due to baking and baking, the film thickness is 0.
．． An upper layer film of 3 to 1.6 pea is formed. As the epoxy resin, any glycidyl ether type epoxy resin such as bisphenol A type, novolank type, bisphenol F type 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, and in this specification, these are also included in epoxy resins.

These resins preferably have a molecular weight of 11,000 or more so that the coating film dries to an extent that cooking does not occur even if no curing reaction occurs due to low-temperature baking. Polyhydroxy polyether resin is one of the epoxy resins that can be suitably used in the present invention.
This resin is obtained by polycondensing mononuclear or dinuclear dihydric phenol or a mixed dihydric phenol of mononuclear and dinuclear type with approximately equimolar amount of shrimp halohydrin in the presence of an alkaline catalyst. It is a polymer. Examples of mononuclear dihydric phenols are resorcinol, hydroquinone and catechol, and examples of dinuclear phenol are bisphenol A, which 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 polyhydroxy polyether 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 polyhyde-oxybolyether resins are manufactured from the same raw materials as epoxy resins, they are also included in the epoxy resins in the present invention. However, unlike epoxy resins, this resin does not have a terminal epoxy group and has a number average molecular weight of about 5,000 or more, preferably about 8,000 to so.
, ooo, it is a thermoplastic resin that is considerably larger than normal epoxy resin. A polyhydroxy polyether resin made from bisphenol A and shrimp chlorohydrin is commercially available as phenoxy resin PKH}I from Union Carbide Company of the United States. As is clear from the above structural formula, the polyhydroxy polyether resin contains a large number of ○H groups and 101 groups in the chain. Ordinary epoxy resins also have many OH groups and 101 groups in the chain. Since the OH group forms a hydrogen bond with the substrate, it contributes to an increase in adhesion, and since the 101 group easily causes intramolecular rotation, it contributes to an increase in the flexibility of the resin.

When dihydric phenol is a mononuclear type such as resorcinol rather than a dinuclear type such as bisphenol A, because the molecular weight of dihydric phenol is smaller, the amount of these functional groups per weight is smaller. The number increases. Therefore, as the content of a 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 phenol preferably contains at least a portion of a mononuclear type such as resorcinol, for example, about 50 mol% or more. Similarly, even when using other epoxy resins, it is generally more advantageous to use one with a large number of ○H groups and 101 groups in the resin. One or more types of epoxy resins including the above-mentioned polyhydroxy polyether resins are used, and this is
A resin liquid for coating can be prepared by dissolving it in an appropriate organic solvent selected in consideration of required characteristics such as drying speed and smoothness. Suitable solvents for epoxy resins include cellosolves, ketones, esters, hydrocarbons, halogenated hydrocarbons, and mixed solvents thereof. In the case of polyhydroxy polyether resin, ketones, cellosolves, mixed solvents thereof, etc. can be used.

In the present invention, colloidal silica is added to the resin liquid in an amount of 10 to 25% by weight, preferably 15 to 20% by weight based on the total solid content of the resin. This silica is added to improve corrosion resistance and electrodeposition coating properties. If the amount added is less than 10% by weight, the effect of improving these properties will not be sufficient, and if it exceeds 25% by weight, weldability will deteriorate.

Examples of commercially available colloidal silica products include Aerosil manufactured by Nippon Aerosil, Ludox manufactured by DuPont, Snowtex manufactured by Nissan Chemical, Cataloid and Oscar manufactured by Catalyst Kai, and those manufactured by Asahi Denka, which are used in the present invention. The thing is,
An alcohol type product with an organophilic surface is best. 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 phenolic 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 flexibility of the coating will be lost and the coating may crack during processing, leading to a decrease in corrosion resistance. In addition to the above-mentioned properties, the resin liquid has 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 further 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. Additionally, 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 is
If the amount is too large, the corrosion resistance will deteriorate, so the amount should be about 50% by weight or less based on the total resin solid content in the resin liquid. Additionally, an appropriate amount of coloring pigments or fillers may be added to the upper layer film. It can be carried out by conventional means such as a resin liquid coating machine, a roll coater, a bar coater, etc., and the film thickness is 0.3 to 1.6 after drying.
-, preferably 0.6 to 1.3/JI. If the film thickness is less than 0.3μ, corrosion resistance will be poor, and if it exceeds 1.6μ, weldability will decrease. 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 should be 200 degrees C or less.

In the anti-rust coated steel sheet of the present invention, a zinc or zinc alloy coated steel sheet is used as the base material in order to ensure excellent corrosion resistance. This zinc-based plating may be electroplating, hot-dip galvanizing, or alloyed hot-dip galvanizing. The type of zinc alloy plating is not particularly limited, and Zn-Ni+
Various zinc alloy platings including Zn AQ can be used. Additionally, multi-layer plated steel sheets with two or more zinc-based plating layers can also be used. In order not to reduce workability, it is preferable that the zinc-based plating has a relatively thin basis weight.

The anti-M coated steel sheet of the present invention has a cross-sectional structure in which a zinc-based plating layer, a lower chromate film layer, and an upper colloidal silica-containing epoxy resin film layer are sequentially laminated on a steel sheet substrate. For automobiles, such coating is usually applied only to one side of the steel plate, but depending on the application, the anti-corrosion coated steel plate of the present invention may be coated on both sides. Next, the present invention will be explained by examples. In the examples, percentages and parts are by weight unless otherwise specified.

(Example) (a) One side basis weight 20 g / on a cold rolled steel plate with a base material thickness of 0.8 m
An alloy zinc electroplated steel sheet plated with 12% Ni-Zn at rrT was used as the base material. This plated steel plate is
Before use, it was degreased using Fine Cleaner 4336, an alkaline degreasing liquid manufactured by Nippon Vercalizing. (b
) Ethylene glycol was added as a reducing agent to a chromic acid aqueous solution containing 120 g/' of chromate solution Crys.
Heated at °C for 6 hours. After the reaction is completed, an aqueous chromic acid solution is added to adjust the Cr3+/total Cr ratio, and water is further added to bring the total chromic acid concentration to 40 g/j in terms of Crys!
(=0.4 mol/l). 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, γ-glycidoxypropyltrimethoxycin was added to some of the chromate treatment liquids as a silane coupling agent.

For comparison, we also prepared a chromate treatment solution in which the same colloidal silica used in the resin solution below was added and dispersed. (C) In a flask equipped with a resin liquid cooler, diglycidyl ether of bisphenol A (Ebicoat 828 manufactured by Yuka Ciel Co., Ltd.)
230 parts, 55 parts of resorcinol, 20 parts of methyl ethyl ketone
0 parts, 4 parts by volume of a 5N NaOH aqueous solution were charged, and the mixture was reacted at reflux temperature for 18 hours. The obtained resinous substance is poured into water in a stirrer, stirred, and reprecipitated in water.
A water-insoluble resin was recovered. This was dried under reduced pressure to extract dihydric phenols from resorcinol and bisphenol A (molar ratio l).
/1) Powdered polyhydroxy polyether resin (
Hereinafter referred to as resin A) was obtained.

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 resin solid content of 20% by weight.

In addition, as another polyhydroxy polyether resin, commercially available phenoxy resin PKHH manufactured by Union Carbide Co., Ltd.
(Molecular i130.o00) is also used for the upper layer film type,
The same resin as above was used in liquid form (Resin B). Furthermore, as a general-purpose epoxy resin, Ebicoat 10 manufactured by Yuka Ciel Co., Ltd.
09 (molecular weight 3750) in a mixed solvent of xylene/methyl ethyl ketone (6/4 by weight) was also used for resin C>. Colloidal silica (made by Catalyst Kawei,
Oscar 1432 average particle size 10-20 mμ), crosslinking agent (blocked isocyanate with a dissociation temperature of 80″C for resins A and B, phenolic resin for resin C)
The plasticizer (butyral resin) was added by adding a predetermined amount to the resin solution and stirring and dispersing it. (
b) Preparation of coated steel plate Chromate solution (b) was applied to one side of the plated surface of the degreased steel plate in (a) above using a bar coater in various amounts of Cr, and heated for 30 minutes to a temporary temperature of 140°C. A chromate film was formed by heating for a second. Next, after cooling to room temperature, resin 1 (C) was applied to the chromate film in various thicknesses using a bar coater, and baked by heating for 60 seconds to a board temperature of 140°C to form a resin coating. Formed. The following tests were conducted on the obtained anti-rust coated steel sheets to evaluate weldability, corrosion resistance, electrodeposition coating properties, and chromium elution properties.

(e) Test method ■Weldability test: An AC single spot welder was used on a test piece made by stacking two painted steel plates so that the painted and unpainted surfaces were in contact with each other, and the tip diameter was 6. Spot welding was carried out using an electrode at rtrm under the conditions of a welding current of 10, OOOA, energization 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 evaluation was made based on the number of unstable indentations caused by local current concentration.

FIG. 1 shows a schematic diagram of a good impression and an unstable impression.

B. Electrode diameter after continuous dots 7 The diameter of continuous dots 1ii after 1000 dots was measured using pressure sensitive paper and evaluated according to the following criteria.

○: t pole diameter <7,0 m △l pole diameter -7.0~8.0 ffll ×: electrode diameter > 8.0 m ■ Corrosion resistance test: The test pieces used for the corrosion resistance test are a flat plate and a cylinder with a diameter of 50 mm. It has been subjected to drawing processing. The die shoulder of the cylindrical drawing was cleaned with triclean and polished with No. 120 Emery Gion each time so that the surface roughness was always constant. These test pieces were immersed in degreasing liquid FC-L4410 manufactured by Nippon Barcalizing 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 salt water spraying at 35°C for 4 hours, hot air drying at 60°C for 2 hours, and 50″
C. A composite corrosion test was conducted in which one cycle consisted of 2 hours in a humid atmosphere with a relative humidity of 95% or higher, and the corrosion resistance was evaluated by the area ratio (%) of red rust generated in 200 cycles (test time 1600 hours). ■ Electrodeposition test: After degreasing the test piece in the same way as the corrosion resistance test, apply Nippon Paint's electrocoating paint U-600.
Electrodeposition was applied to a chemically treated cold-rolled steel plate for 20 seconds, and baked at 165°C for 25 minutes. Paint appearance was evaluated using the following criteria. ○: Good appearance, Δ: Severely rough skin, ×: Craters or electrodeposition not possible.

■Chromium elution test: When the test piece was immersed in Nippon Parkerizing's degreasing liquid PC-L4410 at 43°C for 2 minutes and 30 seconds, and then in the same company's zinc phosphate treatment liquid PR-L3080 at 43°C. The amount of chromium eluted after each 2-minute immersion was calculated from the amount of chromium deposited on the coated steel sheet before and after immersion, as measured by X-ray fluorescence analysis. The details of the lower layer coating and upper layer coating are shown in Table 1 below.
The test results are summarized below.

(Effects of the invention) As is clear from the results in Table I, when colloidal silica was added to the lower chromate film as in the past, weldability deteriorated significantly even with a small amount of addition (tests NαI6 to 1).
8). On the other hand, the anti-M coated steel sheet of the present invention, which has colloidal silicide in the upper resin film, has excellent weldability, and also has comparable results in corrosion resistance, electrodepositability, and amount of chromium elution. However, in a comparative example in which a colloidal silica force of more than 25% was present in the upper layer film, weldability was adversely affected (Test N Sumi 24). In addition, among other comparative examples, when no reducing agent was added to the partially reduced chromate treatment solution, when an excessive amount of reducing agent was added, when the amount of colloidal silica added to the resin solution was small, or when either the upper or lower Corrosion resistance was not sufficient when the amount of coating of either layer was small. When the coating amount of either the upper or lower layer was too large, not only the weldability but also the electrodeposition coating properties deteriorated.

As described above, the anti-corrosion 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, electrodeposition coating properties, It also shows sufficiently satisfactory performance in terms of the amount of chromium eluted. Therefore, it is possible to supply rust-proof coated steel sheets with excellent performance, especially suitable for automobiles, at a relatively low cost.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are schematic diagrams showing good indentations and unstable indentations, respectively.

Claims (4)

[Claims] (1) Cr^ on zinc or zinc-based alloy plated steel sheet
A colloidal product that is partially reduced so that the ratio of 3^+/total Cr is 0.4 to 0.6, and a reducing agent that is 1 to 4 times the amount of unreduced Cr^6^+ is added. The amount of Cr deposited is 20 to 100 mg/m formed by applying and baking a coating-type chromate treatment liquid that does not substantially contain substances.
^2 By applying and baking a resin solution based on an epoxy resin containing 10 to 25% by weight of colloidal silica based on the total amount of the total resin solid content and colloidal silica in the resin solution. Formed film thickness 0.3~
A rust-proof coated steel sheet with excellent weldability, characterized by having an upper layer film of 1.6 μm. (2) The chromate treatment solution has a molar ratio of unreduced C^
The anticorrosive coated steel sheet according to claim 1, further comprising a silane coupling agent of 0.01 times or more of 6^+. (3) The resin liquid further contains a crosslinking agent in an amount such that the molar ratio of the functional group of the crosslinking agent to the total amount of epoxy groups and hydroxyl groups in the epoxy resin is 0.1 to 2.0. Item 1 or 2. Rust-preventing coated steel plate. (4) The anticorrosive coated steel sheet according to any one of claims 1 to 3, wherein the resin liquid further contains a resin other than epoxy resin in an amount of 50% by weight or less based on the total solid content of the resin.