JP2007161765A - Clear coated steel sheet and aqueous coating liquid to be used therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a clear coated steel sheet having low steel sheet temperature in baking by the coating in a continuous plating line, resistant to blocking even under a coating condition having high coil winding temperature and having excellent corrosion resistance and processability. <P>SOLUTION: A clear coating film having an application rate of 0.5-8 g/m<SP>2</SP>is formed on the surface of a zinc-plated steel sheet or an aluminum-plated steel sheet by the coating and baking of an aqueous coating liquid containing (A) an acrylic emulsion resin having a Tg of ≥0°C, (B) silica and (C) a zirconium compound and essentially free from hexavalent chromium. The B/A mass ratio is ≥0.08 and the C/A mass ratio is ≥0.05 and ≤0.25. The coating liquid can further contain (D) a wax as a solid lubricant and (E) a vanadium compound at a D/(A+B+C) ratio of ≥0.03 and ≤0.22 and an E/(A+B+C) ratio of ≥0.005 and ≤0.10. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a clear coat steel plate made of a plated steel plate, which is excellent in blocking resistance, workability and corrosion resistance, and a clear coat aqueous coating solution used for the production of this steel plate.

  In recent years, the demand for surface-treated steel sheets as materials for construction and home appliances has increased, and the needs have also diversified. Regarding building materials, demand for zinc-aluminum alloy-plated steel sheets is increasing in place of conventional hot-dip galvanized steel sheets, particularly in areas where salt damage is severe and industrial areas.

  Zinc-aluminum alloy-plated steel sheets that are currently in practical use are roughly classified into those having an Al content of about 5% and those having a content of about 55 wt%. Among them, the Zn-55 wt% Al alloy-plated steel sheet exhibits a high corrosion resistance of 3 to 6 times that of a hot-dip galvanized steel sheet due to the high corrosion resistance of Al and the sacrificial corrosion resistance of Zn. While this Zn-55 wt% Al alloy plated steel sheet is widely used as a base for coated steel sheets, the plating surface is silvery white and beautiful, so from the viewpoint of design, a thin transparent resin film (with antirust property on the surface) Clear coat products having a clear film are also widely used.

  By the way, it has been usual that the clear coating conventionally contains hexavalent chromium as a rust preventive component. However, from the viewpoint of reducing the environmental load in recent years, there has been a demand for the development of clear coat products that do not contain hexavalent chromium, mainly in the home appliance industry.

  As such a technique, JP 2000-248380 A (Patent Document 1) discloses a coating film containing an aqueous resin and at least one of a thiocarbonyl group-containing compound, a vanadate and a phosphate, or A non-chromium treated zinc-based plated steel sheet is proposed in which a coating film containing an aqueous resin, a thiocarbanyl group-containing compound and fine particles is formed, and then this coating film is dried and baked at an ultimate sheet temperature of 50 to 250 ° C Yes.

JP-A-2003-251743 (Patent Document 2) provides good corrosion resistance without using harmful hexavalent chromium, and provides corrosion resistance, appearance, blocking resistance, alkali resistance, film adhesion, water resistance, and lubrication. Resin-coated surface-treated steel sheets having excellent properties and post-coating properties have been proposed. This consists of a silica-modified acrylic emulsion resin or a mixture of this and an acrylic emulsion resin, an emulsion resin having an acid value of 20 or less and a glass transition temperature of 25 to 55 ° C., a solid lubricant of 1 to 30%, and a rust inhibitor. Resin coating composition containing 0.1 to 10% of 2,5-dimercaptothiadiazole, optionally with 0.1 to 10% phosphate and / or 0.1 to 10% vanadate A film is formed from an object.
JP 2000-248380 A JP 2003-251743 A

  In such a clearcoat product, one of the required properties is blocking resistance. For example, in a steel strip wound up in a coil shape or a stacked steel plate, the thin clear coating adheres to each other, and the coating is damaged when the steel strip is rewound or the steel plates are taken out one by one. It is a phenomenon to receive. When blocking occurs, for example, when a customer opens the coil, a loud noise is generated and the performance as a surface-treated steel sheet becomes insufficient.

  One of the reasons why blocking occurs is that the film is not sufficiently formed. The clear coating is formed by applying a clear coat coating solution to a plated steel sheet and drying / baking in an oven. The clear coating is sometimes formed in a separate dedicated coating line after the steel plate is plated, but from the viewpoint of productivity, it is often performed in a post-treatment facility in the plating line. In the latter case, the oven length for drying / baking is often limited due to the space of the production line. Therefore, the final reached plate temperature (PMT) of drying / baking is at most about 70 to 120 ° C., and depending on the components of the coating solution, baking is completed without a clear film sufficiently formed. In many cases, a water-cooling device for lowering the temperature of the steel sheet is not provided after leaving the oven. For this reason, when the coil is wound at a relatively high temperature of about 40 to 70 ° C., blocking is more likely to occur.

  In conventional chromate products, blocking is not necessarily a serious problem because chromate exhibits a film forming acceleration effect in addition to rust prevention. However, it has been found that a clear coating that does not contain chromate, which is undergoing conversion due to chromium-free conversion, is likely to be blocked due to delay in film formation.

  It is an object of the present invention to provide a clear coat product in which a clear film not containing chromate is formed on a plated steel plate such as a 55% Al—Zn alloy plated steel plate, which is less prone to blocking. .

In one aspect, the present invention provides an aqueous coating solution for clear coat that contains (A) an acrylic emulsion resin, (B) silica, and (C) a zirconium compound, and is substantially free of hexavalent chromium. The glass transition temperature of the acrylic emulsion resin (A) is 0 ° C. or higher, and the content of each component (A) to (C) (the amount of the zirconium compound (C) is an amount in terms of ZrO ₂ ) is An aqueous coating liquid characterized by satisfying a mass ratio of (B) / (A) of 0.08 or more and a mass ratio of (C) / (A) of 0.05 or more and 0.25 or less.

From another aspect, the present invention includes (A) an acrylic emulsion resin, (B) silica, and (C) a zirconium compound on the surface of a zinc-based plated steel sheet or an aluminum-based plated steel sheet. A clear-coated steel sheet provided with a clear film formed from an aqueous coating solution substantially free of valent chromium, the glass transition temperature of the acrylic emulsion resin (A) being 0 ° C. or higher, and (A) to (C) (B) / (A) has a mass ratio of 0.08 or more, and (C) / (A) has a mass ratio of 0.0. A clear-coated steel sheet satisfying 05 or more and 0.25 or less and having a coating amount of 0.5 to 8 g / m ² .

The aqueous coating liquid of the present invention may further contain one or both of (D) a solid lubricant and (E) a vanadium compound, and the content of these components in that case (the amount of the vanadium compound (E)) Is the amount in terms of V ₂ O ₅ ),
(D) / [(A) + (B) + (C)] has a mass ratio of 0.03 or more, 0.22 or less,
The mass ratio represented by (E) / [(A) + (B) + (C)] is 0.005 or more, 0.10 or less,
To the amount. Needless to say, (A), (B), (C), (D), and (E) in the formula mean the content (mass%) of each component in the coating solution, and the zirconium compound (C) and the vanadium compound (E) are amounts converted to ZrO ₂ and V ₂ O ₅ , respectively.

Part or all of the acrylic emulsion resin (A) is preferably a silica-modified acrylic emulsion resin.
In the present invention, the galvanized steel sheet means a galvanized steel sheet and a zinc alloy plated steel sheet. Similarly, the aluminum-based plated steel sheet includes an aluminum-plated steel sheet and an aluminum alloy-plated steel sheet.

  In accordance with the present invention, a coating facility commonly found in continuous plating lines with limited oven length and no water cooling means after baking (thus, the dry / baking PMT is as low as around 100 ° C. and the coil winding temperature Even when a clear film is formed at a high temperature of 40 to 70 ° C., it becomes possible to produce a clear coated steel sheet having excellent blocking resistance, and the formed clear film has excellent workability and corrosion resistance. It is excellent, and after processing a clear coat steel plate, excellent processed part corrosion resistance can be secured. Therefore, according to the present invention, it becomes possible to efficiently produce a clear-coated steel sheet having excellent rust prevention ability and good appearance in a continuous plating line of zinc-based or aluminum-based plated steel sheet.

Hereinafter, the present invention will be described in more detail with respect to preferred embodiments. In the following description, “%” means “% by mass” unless otherwise specified.
The aqueous coating liquid of the present invention contains (A) an acrylic emulsion resin, (B) silica, and (C) a zirconium compound as essential components, and further includes (D) a solid lubricant and (E) vanadium as optional components. One or both of the compounds may be contained.

  The acrylic emulsion resin (A) is a base component of the clear film formed according to the present invention. The acrylic emulsion resin is excellent in weather resistance, which is an essential characteristic for building materials, and is also excellent in alkali resistance and acid resistance. In addition, acrylic resins are generally excellent in transparency, and can form a clear film that does not impair the design of the plated steel sheet of the substrate.

  From the viewpoint of improving the blocking property, it is preferable that part or all of the acrylic emulsion resin is a silica-modified acrylic emulsion resin. The silica-modified acrylic emulsion resin is obtained by adding colloidal silica during the emulsion polymerization of the acrylic-modified emulsion resin. The amount of the silica-modified acrylic emulsion resin is preferably at least 3% of the acrylic emulsion resin. This amount is more preferably 4% or more and 12% or less.

  The glass transition temperature (Tg) of the acrylic emulsion resin is 0 ° C. or higher, preferably 10 ° C. or higher, from the viewpoint of improving blocking resistance. If the Tg of the acrylic emulsion resin is too high, the formed film becomes stiff and may affect the work adhesion, etc., so the Tg is preferably 60 ° C. or lower.

  As described above, the acrylic emulsion resin of the present invention may be a mixture of two or more. The Tg of the acrylic emulsion resin in the case of a mixture indicates Tg after mixing. When two or more types of emulsion resins each having a known Tg are mixed, for convenience, the average value (weighted average value) of Tg of the mixed resin system considering the blending ratio is (A) the Tg of the acrylic emulsion resin. Even so, it is not so different from the measured value.

Some or all of the emulsion particles may have a core-shell structure in which the surface layer portion has a higher glass transition temperature than the particle center portion in each emulsion particle.
Silica (B) has the effect of greatly improving the blocking resistance of the clear-coated steel sheet. This silica has a colloidal particle size in order to ensure the transparency of the film. Either colloidal silica (wet silica) or gas phase silica (dry silica) can be used. Silica also has the effect of increasing the rust resistance of the coating.

  As described above, when a silica-modified type resin is used as part or all of the acrylic emulsion resin (A), the blocking resistance is improved. However, since it is difficult to greatly improve the blocking resistance only with the silica-modified acrylic emulsion resin, even when the silica-modified acrylic emulsion resin is used, the silica (B) is separately contained in the coating solution.

  Examples of the zirconium compound (C) include zirconium carbonate and ammonium zirconium carbonate as forms suitable for addition to the coating solution. The blocking resistance is improved by adding a zirconium compound. This is presumably because the adhesion between the clear film and the plating layer therebelow is improved, and the base resin of the film is hardened by the zirconium compound and becomes difficult to adhere. Due to such a mechanism, it is considered that the zirconium compound contributes to the improvement of the corrosion resistance.

Regarding the content of silica (B) and zirconium compound (C) in the coating solution, the respective mass ratios (based on the solid content) with respect to the acrylic emulsion resin (A) are (B) / (A) mass ratio is 0. 08 or more, and (C) / (A) mass ratio should be 0.05 or more. In the present invention, the content of the zirconium compound (C) is represented by a value converted to ZrO ₂ . The (B) / (A) mass ratio is preferably 0.10 or more, and the (C) / (A) mass ratio is preferably 0.07 or more.

  On the other hand, when there is too much content of a zirconium compound (C), it will become easy to gelatinize a coating liquid at the time of the storage. Therefore, the (C) / (A) mass ratio is set to 0.25 or less. The upper limit of the preferred (C) / (A) mass ratio is 0.15. Although not confirmed, since there is a similar concern in (B) silica, the (B) / (A) mass ratio is preferably 0.25 or less.

  The coating liquid of the present invention preferably further contains a solid lubricant (D). Examples of solid lubricants include waxes, fluororesin particles, metal soaps, and the like, but representative ones are waxes such as polyolefin waxes. It is preferable that the content and particle size of the solid lubricant are appropriately selected according to the use, particularly the molding conditions employed. For example:

  As is done in the field of home appliances, when applying clear-coated steel sheets for press forming without applying lubricant, the coating conditions are harsh. As the wax content in the liquid, the mass ratio of the solid lubricant (D) to the total solid mass of (A) to (C), that is, (D) / [(A) + (B) + (C )] Is preferably used in an amount such that the mass ratio of 0.03 to 0.22 is wax, and the mass ratio is more preferably 0.08 to 0.22. In this case, it is preferable to use a wax having a larger particle size of about 1/4 or more than the thickness of the clear film because mold squeezing due to sliding with a press die is reduced.

  On the other hand, roll forming is mainly performed in the building material field. In this case, as a solid lubricant (D), a relatively small amount in which the mass ratio of (D) / [(A) + (B) + (C)] is in the range of 0.03 to 0.08. By including wax in the coating solution, the formed clear film has moderate lubricity, for example, it prevents the clear film and the plating layer below it from rubbing against the roll to show a black streak appearance it can. The particle size of the wax in this case may be smaller than the above.

  If too much wax is used, the stability of the coating solution decreases, and there is a risk of adverse effects when weather resistance is required. Therefore, it is preferable that the amount of wax (solid lubricant in a broader sense) (D) is a small addition amount within a range in which sufficient slidability is obtained in the resin system or film thickness used.

The coating solution of the present invention preferably further contains a vanadium compound (E). An example of a vanadium compound suitable for addition to the coating solution is ammonium vanadate. By containing the vanadium compound, the corrosion resistance of the clear coat steel sheet is improved. In order to obtain this effect, the content of the vanadium compound (E) is the mass ratio with respect to the total amount (A) to (C) based on the solid content, that is, (E) / [(A) + (B) + It is preferable that the mass ratio of (C)] be 0.005 or more. In the present invention, the content of the vanadium compound (E) is an amount converted to V ₂ O ₅ .

  Since the water solubility of the vanadium compound in the aqueous coating solution of the present invention is smaller than that of chromic acid, the coating solution stability is lowered if the content is too large. Therefore, the content of the vanadium compound (E) is set such that the mass ratio of (E) / [(A) + (B) + (C)] is 0.10 or less, preferably 0.06 or less. What is necessary is just to determine content of the vanadium compound in a coating liquid according to the corrosion resistance requested | required and the adhesiveness of a process part.

  In order to further improve the corrosion resistance of the clear coat steel sheet of the present invention, a known rust preventive additive may be further contained in the aqueous coating solution in addition to the vanadium compound (E). Examples of such rust preventive additives include phosphoric acid (including polyphosphates and salts thereof), nitrogen-containing heterocyclic compounds having a mercapto group (for example, mercaptobenzothiazole, 2,5-di-). Mercaptothiadiazole) and the like.

The coating solution of the present invention contains substantially no chromium. “Substantially no chromium” means that the content as chromic anhydride (CrO ₃ ) is less than 0.1% with respect to the solid content of the coating solution. More desirably, it is 0%.

  The aqueous coating solution of the present invention may contain a surfactant. If the clear film has minute defects called pinholes, the corrosion resistance is deteriorated, but the surfactant may be effective in preventing the generation of pinholes. As the surfactant, an anionic surfactant or a nonionic surfactant is preferable. The amount of the surfactant is preferably in the range of 0.1 to 5 parts by mass with respect to 100 parts by mass of the acrylic emulsion resin (A).

  The aqueous coating liquid of the present invention may contain other additives that can be used for acrylic emulsion resin paints in addition to the above components. For example, coloring pigments, dyes, and the like can be added in some cases to such an extent that the appearance of the plating surface is not impaired. Moreover, you may contain the film forming adjuvant for accelerating | stimulating formation of a film | membrane.

  As described above, in the resin coating of the steel plate in the post-processing step in the continuous plating line, the drying / baking temperature is often limited to a relatively low temperature of 70 to 120 ° C. in the final reached plate temperature (PMT). In that case, it is considered that containing a film-forming aid is more effective in expressing the performance of the clear-coated steel sheet of the present invention. As film-forming aids, hydrophilic ethers such as butyl cellosolve and butyl carbitol, dibasic acid esters such as dimethyl adipate, dimethyl glutarate, and dimethyl succinate can be used. The compounding quantity has the preferable range of 1-50 mass parts with respect to 100 mass parts of acrylic emulsion resin (A).

  The coating solution can be prepared by adding the remaining components to the acrylic emulsion resin (A) in an emulsion state together with solvent water as necessary and mixing. The solvent is usually only water, but a small amount of a water-miscible organic solvent can be used in combination. The total solid content concentration of the components in the coating solution is suitably about 10 to 35% by mass.

(substrate)
The substrate of the clear-coated steel sheet of the present invention is a zinc-based plated steel sheet or an aluminum-plated steel sheet having excellent corrosion resistance, but the type and plating basis weight are not particularly limited. The clear coat coating is applied for the purpose of improving the corrosion resistance and workability (eg, mold squeezing property) while making use of the design of the plating appearance. In that sense, a plated steel sheet with excellent appearance design is preferable, for example, a 55% Al—Zn alloy plated steel sheet or an Al plated steel sheet is used. In order to take advantage of the spangle pattern, a hot dip galvanized steel sheet or an alloyed hot dip zinc is used. You may apply to a plated steel plate.

(Application method)
Application of the aqueous coating solution of the present invention to the plated steel sheet can be performed by ordinary means (in the case of a continuous coating line, coating with a roll coater or spraying the coating solution with a ringer roll after spraying). it can. Thus, after apply | coating a coating liquid so that a clear coating film of predetermined adhesion amount may be formed after drying, what is necessary is just to dry (baking) in oven.

  In the case of a continuous plating line, the oven length is limited, and as described above, baking is performed at a relatively low temperature of 70 to 120 ° C. at the final ultimate plate temperature (PMT), and then is not cooled with water. The coil is often wound at a relatively high temperature of 40 to 70 ° C. In the present invention, a clear film exhibiting sufficient blocking resistance and workability can be formed even under such baking and winding conditions. In general, a clear coated steel sheet has a clear coating on both surfaces of the plating, but it is also possible to form a clear coating only on one side.

(Amount of film attached)
The adhesion amount (per side) of the clear film may be adjusted according to the required performance (mainly corrosion resistance). In normal use 0.5 g / m ² or more, preferably with 1 g / m ² or more is more when corrosion resistance is required. The upper limit is also determined mainly by the relationship between required performance and cost. However, if the adhesion amount is too thick, the film tends to be peeled off during processing, and it tends to be disadvantageous in terms of blocking resistance. Therefore, in consideration of particularly strong processing, the adhesion amount is 8 g / m ² or less, preferably 5 g / m ² or less.

(Preparation of coating solution)
Using the following (A) acrylic emulsion resin, (B) silica, (C) zirconium compound, and (D) solid lubricant (wax), various aqueous coating solutions for testing (total solids concentration is 25 % By mass). The ratio of each component in the coating solution was as shown in Table 2:
(A) Acrylic emulsion resin

(B) Silica proxy S1: colloidal silica (Snowtex N manufactured by Nissan Chemical Industries),
Syringe S2: colloidal silica (Snowtex 30 manufactured by Nissan Chemical Co., Ltd.),
Alcohol S3: Gas phase silica (130 manufactured by Nippon Aerosil Co.).

(C) Zirconium compound Alkaline AZ: Zirconium ammonium carbonate.
(D) Wax symbol WA: polyethylene wax (softening point 130 ° C., average particle size 1 μm),
WB: polypropylene wax (softening point 142 ° C., average particle size 1.5 μm).

Supplementally, the acrylic emulsion resin of (A) in Table 2 is a mixture of one non-modified acrylic emulsion resin of A to G in Table 1 and silica modified acrylic emulsion resin of H, The amount of (A) is indicated by its total solid content. The amount of the zirconium compound (C) is shown in terms of ZrO ₂ . Zirconium ammonium carbonate used as the zirconium compound of (C) was added to the coating solution as an aqueous solution dissolved in water.

  Further, in the coating solution, Pluronic L64, which is an ethylene oxide / propylene oxide surfactant, is used as a surfactant, and the solid content mass is 4 with respect to the total solid content of the acrylic emulsion resin (A). The content was 2%. The coating solution was prepared by stirring with a magnetic stirrer.

(Creation of clear-coated steel sheet)
A 0.6 mm thick 55% Al—Zn plated steel sheet (plating adhesion amount: 75 g / m ² per side) was cut into a size of 150 mm × 150 mm, spray degreased with an alkaline degreasing solution (Nippon Parkerizing FCL4480), and then The substrate was washed with water and dried.

Each test coating solution was applied to one side of the substrate by a doctor blade method so that the coating amount was 2.5 g / m ² and baked in a hot air oven at a furnace temperature of 200 ° C. for 10 seconds. The maximum temperature reached by the steel sheet at this time was about 100 ° C.

(Performance evaluation)
The clear-coated steel sheet thus prepared was evaluated for the blocking resistance immediately after baking, and for other performances, after being stood to cool to room temperature, the performance was evaluated by the following method. The results are also shown in Table 2. For any of the performances, a performance of ◯ or higher is acceptable.

Blocking resistance:
A load of ² kgf / mm ² was applied to the evaluation surfaces (clear coat surfaces) of the two clear coat steel plates superimposed on each other, and the temperature of the mold to which pressure was applied was set at 60 ° C. and held for 24 hours. Thereafter, the fusion state between the steel plates was evaluated according to the following criteria. ◎ is desirable, but if it is ○, there is no practical problem.

◎… not fused,
○: Slightly fused, but no separation mark is observed on the evaluation surface after separation.
Δ: Fusing and peeling marks are observed on the evaluation surface after separation,
X: The steel sheets cannot be easily separated from each other by fusing.

Processed part adhesion:
After performing a bending test (in accordance with JIS G33112.2.2) so that the evaluation surface is externally bent, the bending part is subjected to a peeling test with a polyester tape, and the minimum bending thickness at which peeling does not occur (plate thickness (T) Multiple of).

A: 1T or less, B: 2T to 3T, Δ: 4T to 5T, x: 6T or more.
Corrosion resistance:
Evaluation was made based on the white rust area ratio of the evaluation surface after 240 hours of the salt spray test.

A ... 1% or less, B ... More than 1%, 5% or less, B ... More than 5%, 20% or less, X ... More than 20%.
Sliding property:
The pin-on-disk test method described in Japanese Patent Application Laid-Open No. 2003-136151 (a test material is placed on a rotating disk, and the test material is rotated together with the disk while pressing a metal pin-shaped test tool against the test material. In accordance with the sliding friction test method in which the pin is slid in one direction), the friction coefficient of the clear coat steel plate coated with rust preventive oil is measured under the following conditions, and the number of turns until the friction coefficient exceeds 0.2: It was evaluated with.

Test conditions Pressing load: 3kgf
Test tool tip shape: Ball Test tool tip shape Curvature: 2.5 mmR
Test tool tip material: SKD steel Sliding speed: 6300 mm / min (100 rpm)
Friction coefficient μ: Maximum value of average value calculated from six measured values of friction coefficient measured every 0.1 second ◎… 500 laps or more, ○ ... 300 to 499 laps, Δ ... 100 to 299 laps, × ... 99 laps Less than.

(Stability of coating solution)
200 ml of the prepared coating solution was put in a sealed container and kept at 40 ° C., and the state of solidification (gelation) was observed every day (initially every several hours) and evaluated in the period until solidification.

  A: More than 7 days, B: 3-7 days, A: 1-2 days, X: Several hours.

  Contains acrylic resin emulsion (A), silica (B), and zirconium compound (C), and (B) / (A) mass ratio is 0.08 or more, and (C) / (A) mass ratio is 0. The clear film formed from the coating solution whose remarks column is an example in the range of 0.05 to 0.25 had good blocking resistance, and good work adhesion and corrosion resistance.

  More specifically, regarding the acrylic emulsion resin (A), the higher the Tg, the better the blocking resistance (for example, the comparison between No. 18 and No. 29, 31), and the one having a core-shell structure. However, there was a tendency that blocking resistance, work adhesion, and corrosion resistance were improved (for example, comparison between No. 18 and No. 24, 26, and 27).

  Comparative examples in which the coating solution does not contain silica (B) or zirconium compound (C) (No. 1 to 10, 11, 45 to 47) or comparative examples (No. 12, 16, and 17) that contain a trace amount. Then, the blocking resistance was unsatisfactory. In the comparative examples (Nos. 9 to 10) in which the zirconium content was too high, the performance of the clear coat steel plate obtained from the coating solution immediately after the preparation was good, but the coating solution gelled in a very short time.

  Even if the wax (D) was not contained (No. 44) or its content was small (No. 43), very good results were obtained except for the sliding property. Therefore, in applications that do not require slidability, a solid lubricant such as wax can be omitted. As the content of the wax (D) increases, the slidability increases (No. 24, 37 to 44). In particular, the mass ratio of (D) / [(A) + (B) + (C)] is reduced to 0. When it was 10 or more, the stability of the coating solution was slightly deteriorated, but high slidability was obtained.

  An aqueous coating solution having the composition shown in Table 3 was prepared in the same manner as in Example 1. In this example, in addition to the (C) zirconium compound, the following (E) vanadium compound was also used in some cases. The meanings of the symbols in Table 3 are the same as those in the first embodiment.

(E) Vanadium compound:
Alley AV: ammonium vanadate.
Each prepared aqueous coating solution was applied to one side of the substrate and baked in the same manner as in Example 1 to prepare a clear coat steel plate for testing. However, in this example, the adhesion amount of the clear film was varied. Table 3 summarizes the composition of the coating solution, the coating amount, and the results of the performance test.

The more (E) vanadium compound in the coating solution, the better the corrosion resistance. On the other hand, with respect to the film adhesion amount, when it was less than 0.5 g / m ² (No. 56), the corrosion resistance was deteriorated, and when it exceeded 8 g / m ² (No. 62), the work adhesion was deteriorated.

Claims (8)

An aqueous coating solution for clear coating, comprising (A) an acrylic emulsion resin, (B) silica, and (C) a zirconium compound, substantially free of hexavalent chromium,
The glass transition temperature of the acrylic emulsion resin (A) is 0 ° C. or higher, and the content of each component of (A) to (C) (the amount of the zirconium compound (C) is an amount in terms of ZrO ₂ ) is ( B) / (A) mass ratio is 0.08 or more, and (C) / (A) mass ratio is 0.05 or more and 0.25 or less.   Further, (D) a solid lubricant is contained, and the mass ratio represented by (D) / [(A) + (B) + (C)] is 0.03 or more and 0.22 or less. The aqueous coating solution according to claim 1, which is an amount. Further, it contains (E) a vanadium compound, and its content (amount converted to V ₂ O ₅ ) is such that the mass ratio represented by (E) / [(A) + (B) + (C)] is 0. The aqueous coating solution according to claim 1 or 2, wherein the amount is 005 or more and 0.10 or less.   The aqueous coating solution according to any one of claims 1 to 3, wherein a part or all of the acrylic emulsion resin (A) is a silica-modified acrylic emulsion resin. From an aqueous coating solution containing (A) an acrylic emulsion resin, (B) silica, and (C) a zirconium compound and substantially free of hexavalent chromium on the surface of a zinc-based plated steel sheet or an aluminum-based plated steel sheet A clear-coated steel plate provided with a formed clear film,
The glass transition temperature of the acrylic emulsion resin (A) is 0 ° C. or higher, and the contents of the components (A) to (C) (the amount of the zirconium compound (C) is the amount in terms of ZrO ₂ ) is (B ) / (A) mass ratio satisfying 0.08 or more, (C) / (A) mass ratio satisfying 0.05 or more and 0.25 or less, and the coating amount of the clear film is 0.5 to 8 g / m. A clear-coated steel sheet characterized by being ² .   The coating solution further contains (D) a solid lubricant in an amount such that a mass ratio represented by (D) / [(A) + (B) + (C)] is 0.03 or more and 0.22 or less. The clear coat steel plate according to claim 5. The coating solution further contains (E) a vanadium compound, and a mass ratio represented by (E) / [(A) + (B) + (C)] is 0.005 or more and 0.10 or less (vanadium compound (E) The clear-coated steel sheet according to claim 5 or 6, wherein the clear coating steel sheet is contained in an amount that is equivalent to V ₂ O ₅ .   The clear coat steel plate according to any one of claims 5 to 7, wherein a part or all of the acrylic emulsion resin (A) is a silica-modified acrylic emulsion resin.