JPS62268636A - Coated steel plate having excellent corrosion resistance andworkability - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a coated steel plate that has excellent corrosion resistance and workability, and particularly to a coated steel plate that can be welded for use in automobiles.

(Prior Art) Electric resistance weldable O-clad plates have gradually come to be used in Japan, for example, to prevent automobiles from rusting due to salt, which is spread as a snow-melting agent on roads in snow-covered areas. There is.

As a conventional two-bag steel plate that can be welded, Synchrometal (
Synchrometal is a steel plate coated with a zinc-clinch primer (product name: Diamond Shamlofta Co., Ltd., USA). ) and a single layer of zinc-rich primer (zinchromet) made of epoxy resin containing a large amount of zinc, exhibiting significantly superior corrosion resistance compared to cold-rolled steel sheets. A steel plate on which a zinc clinch primer is applied and hardened is known as a Z-coated steel sheet. Synchrometal, especially its underlying chromate layer, is described in Japanese Patent Publication Nos. 47-6882 and 52.
-904 and 52-4286, and JP-A-49-74137, JP-A-49-74138 and JP-A-4
No. 9-74139 proposes that, in addition to zinc and chromic acid, one or more of a primary agent, a chromate, an amphoteric metal oxide or hydroxide, and a hydrophilic colloid should be added to the base chromate treatment solution. has been done.

Generally, moldability, weldability, and corrosion resistance are required for anticorrosive coatings for automobiles, etc., but the above-mentioned Synchro Metal and Z Coat@kari do not particularly have sufficient moldability. This is because the zinc-rich primer used for these steel plates has a 50%
This is because it contains a large amount of zinc powder, about 85 to 9094% by weight, and the film formed is dangerous and tends to peel off during processing. As a result, corrosion resistance deteriorates, and the paint film that falls off adheres to the press mold and causes scratches, which increases the frequency of cleaning the mold and deteriorates workability. In addition, zinc chloride and chloride primers have a high i3 transmissibility of moisture into the coating film, which also leads to a decrease in corrosion resistance (intermediate).To reduce the peeling of the coating film and the decrease in corrosion resistance, reduce the amount of zinc dust. However, the increased electrical resistance of the coating film makes 1° welding disadvantageous or impossible.

In addition, conventional Synchrometal requires baking at high temperatures of 150 to 180°C for chromate and 250 to 280°C for zinc clinching primer, which has the disadvantage that the bake hardenability of the steel sheet is deteriorated. was.

Incidentally, 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.

(Problems to be Solved by the Invention) As a result of studies to solve the above-mentioned drawbacks of zinc-rich primers, the present inventors found that using a plating plate as a base material,
On top of this, an aqueous suspension containing chromic acid and iron phosphide is applied as a base layer and dried to form a chromate film containing iron phosphide. We previously proposed 12 proposals that, when cured, a coated steel sheet with excellent corrosion resistance without blistering or 4t11 peeling could be obtained (Japanese Patent Application No. 58-19 = 10
No. 72). However, as the upper layer paint, conventional resins such as Evokin, polyamide, and acrylic resin = PT are used (for example, in the example, the same Evokin zinc-rich primer as before was used), and the paint film is not easily formed. Since the flexibility and adhesion were still insufficient, there was still room for improvement in corrosion resistance and formability even with this coated steel sheet.

Sarashiko, depending on the case: Requires baking at a higher temperature,
It was difficult to stably obtain bake hardenability, and it was necessary to enable low-temperature baking from the viewpoint of energy saving.

(Means for Solving the Problems) Here, the gist of the present invention is to apply partially reduced chromic acid and 0.1 to 5 times the weight of total chromic acid onto a zinc or zinc-based alloy coated steel sheet. A lower layer film formed by applying an aqueous suspension containing colloidal silica and drying it by heating, and one or more dihydric phenols selected from mononuclear and dinuclear dihydric phenols. A resin liquid containing a polyhydroxy polyether resin obtained by polycondensation of phenol and epihalohydrin is applied, and 80%
Film thickness 0 formed by heating drying at ℃ or higher and lower than 300℃
．． This is a coated steel sheet with excellent corrosion resistance and workability, characterized by having a lower layer coating of 3 to 10 μm.

(Function) Next, the reason why the lower layer film and the upper layer film are limited as described above in the present invention and the action of each component will be explained in detail.

Chromate film is formed by reduction of chromic acid and evaporation of water in engineering, but in order to efficiently reduce and form a film at low temperatures,
The following means were used.

l) Partially reduce the chromic acid in the chromic acid aqueous solution and add colloidal silica at the same time. By performing partial reduction, the amount of chromic acid reduced during heat drying is reduced,
This effectively forms a film.

The partial reduction rate is Cr'°/(Cr 0 Cr'°
) =0.1 to 0.6, preferably 0.3 to 0.6. If it is less than 0.1, the reduction efficiency will be poor, and if it exceeds 0.6, the stability of the liquid will deteriorate due to the large amount of Cr'°.

The amount of colloidal silica added is 0.1 to 5 times the weight of total chromic acid. The action of colloidal silica increases the wettability of the chromic acid aqueous solution and assists in the film-forming action of the chromate film, but if it is less than 0.1 times, the film-forming action is poor, and if it exceeds 5 times, the film will become weak.

2) Effect of silane coupling agent
-methoxyethoxy)silane, γ-methacryloxypropyltrimethoxysilane, T-glycidoxypropyltrimethoxysilane, γ-aminopropyltriethoxynolane, N-β-(aminoethyl)-r-aminopropyltrimethoxysilane Silane coupling agents such as β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane can hydrolyze themselves to form polysiloxane, thereby strengthening chromate coatings containing colloidal silica. , improves adhesion with the upper coating.

The alcohol produced by hydrolysis acts as a reducing agent for chromic acid. The amount added is 0.0 relative to unreduced chromic acid.
If the amount is less than 1 mole, the effect will be small, and if it is added in a large amount, it will be economically disadvantageous.

3) Effects of polyhydric alcohols, polycarboxylic acids, and oxycarboxylic acids In order to increase the efficiency of reducing chromium at low temperatures, polyhydric alcohols, polycarboxylic acids, and oxycarboxylic acids may be used as reducing agents.

Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin.

Examples of polyhydric carboxylic acids include succinic acid, glutaric acid, and adipic acid.

Examples of oxycarboxylic acids include citric acid and lactic acid.

Furthermore, it is also possible to partially replace the above reducing agent with saccharide.

The amount added is preferably 0.01 to 2.0 times the mole of unreduced chromic acid. If it is less than 0.01 times the mole, the reducing effect is small, and if it exceeds 2.0 times the mole, the action of the reducing agent not only becomes saturated, but also remains in the chromate film, impairing the water resistance of the film.

4) Effect of iron phosphide When iron phosphide is present in the chromate treatment solution, the iron phosphide reacts with the free hexavalent chromium in the chromate film and reduces the amount of free hexavalent chromium. Easily soluble components are reduced, thereby preserving the adhesion of the coating in corrosive environments. Further, since iron sulfuride has electrical conductivity, resistance welding of the obtained coated steel sheet becomes easy even without the presence of zinc powder, and welding workability is also good.

Iron phosphide is water-insoluble and in a suspended state in an aqueous chromic acid solution, so in order to react efficiently with free hexavalent chromium, it must be present in an amount of at least 0.1 times the weight of the total chromic acid. be. On the other hand, if a large amount of iron phosphide is added, the iron phosphide will not be firmly adhered to the chromate coating, causing the coating to peel off during processing, resulting in a decrease in workability and corrosion resistance.

Furthermore, since iron phosphide has electrical conductivity as described above, if it is added in excess, current will easily flow between the base material and the coating surface, impairing its role as a corrosion barrier. These phenomena become noticeable when the weight of iron phosphide exceeds 20 times the weight of total chromic acid.Therefore, in the present invention, the amount of iron phosphide is 0.1 to 20 times the weight of total chromic acid. twice the weight, preferably 1 to 10 times the weight.

In addition to the typical PetP, iron phosphide includes FeP, F
Those having compositions such as ezP and FeP2 are also known, and any of these can be used. Further, it is preferable to use iron phosphide having a fine particle size with an average particle size of less than 5 μm.

5) Effect of Metal Chromate Preferably, the aqueous solution containing chromic acid and iron phosphide used to form the underlayer coating further contains metal chromate. The presence of metal chromate in the chromate coating acts as a rust-preventing pigment that enhances the rust-preventing power. That is, since metal chromates passivate zinc and iron in the base plated steel sheet, their dissolution in a corrosive environment is suppressed, contributing to further improvement in the corrosion resistance of the coated steel sheet. Examples of metal chromates include zinc and strontium salts of chromic acid.

Instead of the metal chromates mentioned above, zinc or strontium oxides or hydroxides may be used as precursors thereof. Such oxides or hydroxides are converted to metal chromates by reacting with chromate ions in a chromic acid-containing liquid.

Excessive addition of the metal chromate inhibits the film-forming action of the chromate film, resulting in a decrease in the adhesion between the steel sheet and the film. Therefore, when adding metal chromate, the amount added should be equal to or less than chromic acid, and when added as an oxide or hydroxide, it will react with chromic acid and cause chromic acid to react with chromic acid. Therefore, it is preferable to keep the amount to a small amount of 0.5 times the mole of chromic acid or less.

6) In addition, in order to further improve the adhesion between the chromate film and the zinc-plated steel sheet, the above-mentioned aqueous suspension containing chromic acid may contain phosphoric acid in an amount equal to or less than the molar amount of chromic acid. It's okay.

In the coated steel sheet of the present invention, the amount of the aqueous suspension applied is preferably 30 to 600 mgl rd, more preferably 60 to 300 mgl m in terms of Cr content.

However, this amount of Cr means only that derived from chromic acid, and excludes Cr derived from metal chromates. Cr1i
If l is less than 30 mg/m, corrosion resistance may be insufficient. On the other hand, a thick chromate film with a CfJ exceeding 600 mglm is likely to peel off during post-painting processing. However, for applications that do not undergo deformation after painting (eg, structures), it is possible to apply more than 600 mg/rtr. Coating can be carried out by conventional means such as a bar coater or a roll coke.

The zinc-plated 'F4 plate coated with the above aqueous suspension is then heated and dried to render the coating insoluble. This heating is
Preferably, the temperature is about 60 to 200'C, more preferably 100 to 150'C.

In the present invention, an upper layer film is further applied to the lower layer film obtained as described above in the following manner.

1) In the coated steel sheet of the present invention, a coating film of polyhydroxypolyether resin is formed on the chromate coating, which is the lower coating formed as described above. The polyhydroxypolyether resin used in the present invention is prepared by adding mononuclear or dinuclear dihydric phenol or mixed dihydric phenol of mononuclear type and dinuclear type to epihalohydrin in an approximately equimolar amount in the presence of an alkali catalyst. It is a polymer obtained by polycondensation,
It is also called polyphenoxy resin. Examples of mononuclear dihydric phenols are resorcinol, hydroquinone and catechol, and examples of dinuclear phenols are bisphenol A, which can be used alone or in combination with
More than one species may be used in combination. A typical example of epihalohydrin is epichlorohydrin. A polyhydroxy polyether resin in which the dihydric phenol is a mixture of resorcinol and bisphenol A at a molar ratio of 1/1 is represented by the following structural formula.

Moreover, when the dihydric phenol is resorcinol, it is shown by the following structural formula.

Thus, polyhydroxypolyether resins are manufactured from the same raw materials as epoxy resins, but unlike epoxy resins, they do not have terminal epoxy groups and have a number average molecular weight of about 8.
000 to 20,000, which is a fairly large thermoplastic resin. Polyhydroxypolyether resin manufactured from bisphenol A and epichlorohydrin is commercially available from Union Carbide Company (trade name: PKII).
H).

As is clear from the above structural formula, the polyhydroxy polyether resin contains many OH groups and 10-groups in the chain. Since the 011 group forms a hydrogen bond with the substrate, it contributes to an increase in adhesion. On the other hand, since the -0- group easily causes intramolecular rotation, it contributes to increasing the flexibility of the resin. These functional groups have a lower molecular weight per weight when the dihydric phenol is a mononuclear type such as resorcinol than a dihydric phenol such as bisphenol A. The number of functional groups increases. For example, the molar ratio of resorcin and bisphenol A is 0/1.1.
The numerator when /1.110! The number of functional groups per 100 is shown in Table 1 below.

Strategy table 0/1 0 0.35 0.7
01/1 23 0.44 0.
89110 66 0.60 1
．． 20 Therefore, the higher the content of mononuclear dihydric phenol such as resorcinol, the higher the content of the functional groups in the chain, which tends to increase the adhesion and flexibility of the resulting coating film. . Therefore, in the present invention, the dihydric phenol preferably contains 50% or more of a mononuclear type such as resorcinol, and may be 100% resorcinol.

Further, this resin can be modified with acrylic acid, methacrylic acid, etc., or a small amount of other resin can be added.

The above polyhydroxy polyether resin contains alcohol,
It is insoluble in hydrocarbons, etc., but soluble in cellosolves, ketones, glycol-ethers, mixed solvents of these, etc., so an appropriate solvent should be selected taking into consideration the required characteristics such as drying speed and smoothness. A resin solution for cloth can be prepared by dissolving 1. A chromate such as the aforementioned strontium chromate or zinc chromate, which acts as a rust-preventing pigment, may be added to this resin liquid in an amount of 1 to 50% by weight of the resin, thereby further improving corrosion resistance. can be achieved.

This resin liquid can also be applied by a conventional means such as a bar coater or a roll coater, and after drying, the film thickness is 0.3 to 1.
Apply so that a coating film of 0 μm is obtained. Film thickness is 0.3
If it is less than μm, corrosion resistance and film removability during processing will be poor;
If it exceeds 10 μm, resistance welding becomes difficult or impossible.

Here, in the present invention, the reason for drying the upper coating film as described above is not only to dry the upper coating film quickly, but also to promote the reduction of the chromate coating that is the lower coating to make the coating tough. be. In the present invention, since partially reduced chromic acid is used to form the chromate film, it is possible to use a polyhydroxyether resin that dries at low temperatures as the upper layer film.

Regarding the drying temperature, it is desirable to heat the solvent to a temperature higher than the boiling point of the solvent in order to prevent blocking of the product.
No blocking occurs.

Therefore, when the 'fl19 thickness after drying is 5 to 10 μm, the temperature is above the boiling point of the solvent to 300 °C, and when the thickness is below 5 μm, the temperature is 80 to 300 °C.
Heat to ℃. Naturally, if the baking temperature is high, a uniform film will be formed and the corrosion resistance and workability will be improved, but if the baking temperature is higher than 200° C., the baking hardenability of the steel plate will disappear.

2) In order to increase the flexibility of the coating film, there is a method of adding a plasticizer. As is well known, polyvinyl chloride is a rigid resin, but it is made softer by adding a plasticizer. However, plasticizers tend to bleed out depending on temperature and time. As the plasticizer used in the present invention, acrylic acid and/or methacrylic acid ester is used as a reactive plasticizer. The unsaturated bonds contained in the ester are cleaved and cross-linked over time, resulting in
Fixed. Further, after processing, the plasticizer is fixed immediately by further heating such as painting or baking.

The thus obtained /ilI steel plate has a cross-sectional structure in which a zinc-based plating layer, a lower chromate coating layer, and an upper resin coating layer are sequentially layered on a steel plate substrate.

For automobiles, such double coating is usually applied to only one side of the steel plate, but depending on the application, the 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, % and parts are by weight unless otherwise specified.

Flame coaling +91 base material thickness 0.8 mm cold rolled steel plate with area weight 120 g/m on one side
An alloy zinc electroplated FI temporary electroplated with 12% Ni-Zn was used as the base material. This plating EfJiE
Before use, the sample was degreased with Fine Cleaner 4336 (manufactured by Nippon Parriki Rising Co., Ltd.) to clean the surface.

Further, an alloy electroplated steel plate obtained by plating a bake-hardenable steel plate in the same manner was also used as the base material.

An aqueous solution of ethylene glycol was added as a reducing agent to an aqueous solution of chromic acid containing Cr (h 120g/7!) and heated at 80°C for 16 hours.After the reaction was completed, Cr''/C
Cr (h 40g/1
It was diluted with an aqueous solution of

Further water was added to give a CrodA total of 40g/l (=0.
4mo+/C) partially reduced chromic acid aqueous solution was prepared.

7 volumes of this chrome mature tree; colloidal silica (Aerosil 200 manufactured by Nippon Aerosil Co., Ltd., average particle size 12 μm)
A predetermined amount of was added. Furthermore, as various additives, silane coupling agent vinyl etquin silane (manufactured by Nippon Unicar Co., Ltd.: A 15
1) T-glycidoxypropyltrimethoxysilane (Nippon Unicar Co., Ltd.?!8!: A 187) γ-Aminopropyltriethoxysilane (Nippon Unicar Co., Ltd.: A-1
100) Polyhydric alcohol glycerin (first class reagent) Oxycarboxylic acid citric acid (i class reagent) Iron phosphide (manufactured by Hooker Chemical Company: 1IR5-213
2. Average particle size: 3 μm) Total chromate strontium chromate (special grade type II drug) was added in a predetermined amount, stirred with a high-speed disperser, and used as an aqueous suspension.

(C1 In a flask equipped with a resin liquid cooler, diglycidyl ether of resorcinol (Sumi Epoxy E A-130 manufactured by Sumitomo Chemical Co., Ltd.),
135.8 parts of a reaction product of 1 mole of resorcin and 2 moles of shrimp chlorohydrin), 55 parts of resorcin, 190.8 parts of methyl ethyl ketone, and 4 parts by volume of a 5N Na011 aqueous solution were charged and reacted at reflux temperature for 18 hours. The obtained resinous substance was put into water in a stirrer, stirred, reprecipitated in water, and water-insoluble resin was recovered. This resin was dried under reduced pressure to obtain a polyhydroxy polyether whose powdered dihydric phenol was resorcinol.

The obtained powdered resin was dissolved in a 1/1 (volume ratio) mixed solvent of cellosolve acetate/cyclohexanone to obtain a resin solution with a resin solid content of 20% by weight.

MEK was used as a solvent for low temperature heating of 100° C. or lower.

A commercially available polyhydroxypolyether resin made of trinuclear dihydric phenol (manufactured by Union Carbide: PK
)Ill) was used.

As a reactive plasticizer, pentaerythritol acrylate (manufactured by Toagosei Kagaku Co., Ltd.: Alonisox-305
) was used.

(Preparation of dl coated steel plate) Apply the above [base treatment liquid fb to a clean steel plate of al] with a bar coater at various Cr adhesion amounts, heat for 30 seconds to reach a predetermined plate temperature, and apply iron phosphide containing A chromate film was formed. Next, after cooling to room temperature, the resin liquid (C
No. 1 was coated with various types using a bar 4-1, and baked by heating for 60 seconds to a predetermined board temperature to form a resin film. The following tests were conducted on the obtained painted steel sheets to evaluate corrosion resistance, workability, and weldability.

+e+ test) Corrosion resistance test: The corrosion resistance of the coated steel plate was evaluated by a dry-wet cycle test consisting of one cycle of immersion in 5% saline at 35°C for 1 hour, and then 1 hour of hot air drying at 50°C. Corrosion resistance was evaluated based on the blistering area of the coating film after 480 test cycles (testing time: 960 hours) and the average diameter of blisters.

(ii) Work adhesion test: In order to evaluate the workability of the painted steel sheet, a beaded zero bend ironing test as shown in Figure 1 was conducted (since it is symmetrical, only one side is shown in Figure 1). . A test piece 2 is placed on the die 1 shown in FIG. 1 with the coating film 3 facing the die side, and the test piece is supported by a temporary presser 5 with a spacer 4 in between. Next, the punch 6 is pushed in the direction of the arrow at a constant speed to squeeze the test piece between the die and the punch to zero-bend the steel plate. As shown in FIG. 2, the evaluation is performed based on the area of 11 parts 7 of the coating film squeezed with a die, and the peeled area is determined by the following formula.

2I! Although only one side is shown in FIGS. 1 and 2, the peeling area is calculated using the above formula based on the results on both sides of the beam bending test piece. In addition, in order to keep the surface roughness of the die shoulder constant, cleaning with triclean and polishing with No. 120 emery paper was performed after each measurement.

(iii)? Acceptance test: Using an AC single spot welder, welded with an electrode with a tip diameter of 5.0 mm, welded two layers of paint under the conditions of 13000 A, 10 cycles of current, and a pressure load of 200 kg. 8 spots of the steel plate test piece) were made. The two test pieces were placed one on top of the other so that the painted and unpainted surfaces were in contact. Weldability was evaluated based on the following criteria.

○: If the product does not fail after welding.

Δ: If welding is possible.

× - If there is unwelded or it is impossible to weld.

(1v) After giving 2% elongation to the bake hardenability test piece, 170°C x 30m1n
held. Measuring the mechanical strength of ￥AIyi after heating,
Bake hardenability was evaluated by (yield stress after heating - yield stress before heating).

Table 1 summarizes the coating film composition and manufacturing conditions of the coated steel sheet according to the present invention, together with those of comparative examples.

Table 2 summarizes the test results of corrosion resistance, work adhesion, and weldability of each sample material, and Table 3 shows the results of the bake hardenability test conducted on some of the test materials (O). are shown together with the base material components.

Table 2 (Table 1 continued)

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram showing an example of a beaded U-bending ironing test, and FIG. 2 is a schematic explanatory diagram showing a method for evaluating the ironing peeling area. 1: Dice, 2: Test piece, 3- Paint film, 4 Varnish spacer, 5- Plate holder, 6: Punch, 72 Peeling part

Claims (9)

[Claims] (1) Applying an aqueous suspension containing partially reduced chromic acid and colloidal silica in an amount of 0.1 to 5 times the weight of total chromic acid onto a zinc or zinc-based alloy coated steel sheet and heating and drying. A resin containing a polyhydroxypolyether resin obtained by polycondensation of the lower layer film formed by and epihalohydrin and one or more dihydric phenols selected from mononuclear and dinuclear dihydric phenols. A coated steel sheet with excellent corrosion resistance and workability, characterized by having an upper layer film having a thickness of 0.3 to 10 μm formed by applying a liquid and heating and drying at 80° C. or higher and lower than 300° C. (2) The coated steel sheet according to claim 1, wherein the aqueous suspension contains a silane coupling agent in an amount of 0.01 times or more mole of unreduced chromic acid. (3) The aqueous suspension contains 0.01 to 2% of unreduced chromic acid.
．． The coated steel sheet according to claim 1, which contains 0 times the mole of polyhydric alcohol and/or polycarboxylic acid and/or oxycarboxylic acid. (4) The aqueous suspension further contains 0% of unreduced chromic acid.
．． The coated steel sheet according to claim 2, which contains 01 to 2.0 times the mole of polyhydric alcohol and/or polycarboxylic acid and/or oxycarboxylic acid. (5) The coated steel sheet according to any one of claims 1 to 4, wherein the aqueous suspension contains iron phosphide in an amount of 0.1 to 20 times the weight of total chromic acid. (6) The coated steel sheet according to claim 1, which contains 1 to 20% by weight of acrylic acid/methacrylic ester based on the resin in the resin liquid. (7) For the lower layer coating, 1% of the resin in the resin liquid is
The coated steel sheet according to any one of claims 2 to 5, which uses a resin liquid containing ~20% by weight of acrylic acid/methacrylic ester. (8) The coated steel plate according to any one of claims 1 to 7, wherein the plated steel plate is a bake-hardenable steel plate, the lower layer coating is applied, and the baking temperature is 200° C. or less. . (9) The coated steel sheet according to any one of claims 1 to 9, wherein the aqueous suspension and/or resin liquid further contains a metal chromate.