JPH0730457B2 - Chromate-treated plated steel sheet and its manufacturing method - Google Patents

Description

Description: [Industrial field of application] The present invention relates to a surface-treated steel sheet obtained by forming a chromate film on a zinc-plated or zinc-based alloy-plated steel sheet.

[Prior Art and Problems] It is known to form a coating film containing Si, P and an organic resin with chromium as a main sentence as a chromate treatment for a conventional galvanized steel sheet.

For example, Japanese Patent Publication No.
-58552, Japanese Patent Publication No. 61-1508, Japanese Patent Laid-Open No. 60-13079
There is a bulletin, etc. Further, as a material to which phosphoric acid is added as P, there is JP-B-53-41621, or as an additive to which both Si and P are added, there is JP-B-60-18751. These are intended to further improve the corrosion resistance of the conventional chromium-only chromate coating and to impart adhesion to the coating.

On the other hand, steel sheets used in home appliances, office equipment, etc. in recent years are used without or with zinc or zinc alloy plated steel sheets. That is, the properties required for a steel sheet are corrosion resistance without paint, appearance (color tone, color unevenness), fingerprint resistance during production and use of the product, and adhesion of the paint when applied.

Chromate coating by chromic acid treatment has long been to simply provide corrosion resistance, but Si, P, metal ions have been added to the treatment liquid as the above various characteristics are required, Furthermore, in order to improve the fingerprint resistance, the method disclosed in
As disclosed in JP-A-61-207579 and JP-A-59-140050, organic resins have come to be added.

However, none of them can completely satisfy all the required performances, and they are merely dealt with by balancing some performances.

In particular, a chromate film obtained by applying a chromate treatment liquid containing P on a zinc or zinc alloy plating film has excellent corrosion resistance, but when a top coat paint is applied and baked, the adhesion of the paint depends on the type of paint. However, there is a problem that it is limited to unpainted use.

[Object of the Invention] In view of the above points, the present invention is directed to corrosion resistance, paint adhesion, and corrosion resistance of a chromate film on a galvanized and zinc-based alloy-plated steel sheet.
An object of the present invention is to provide a chemical conversion treated steel sheet having a chromate film structure with improved fingerprint resistance and color unevenness, and a method for producing the same.

[Means for Solving the Problems] The present invention advantageously solves the above-mentioned problems, and the gist thereof is: 1 a zinc- or zinc-alloy-plated steel sheet, a water-soluble chromium compound and a water-soluble phosphate compound, Furthermore, by treating with a treatment liquid containing or not containing silica, the first layer has a Cr ³⁺ / Cr ⁶⁺ ratio of 0.1.
~ 2.0Cr and Cr / P ratio of 0.1 to 10 in terms of P When silica is included, the Cr / Si ratio is 0.1 to 10 and the amount of adhesion (Cr / + P + Si) ≧ 5 mg / m ²
After forming the chromate film on the plating surface, the Cr ³⁺ / Cr ⁶⁺ ratio is 0.25 to 1.0 and the Cr / Si ratio is
A chromate-treated plated steel sheet having a second-layer chromate coating containing 0.1 to 10 chromium compounds and silica in an amount of 0.5 to 50 mg / m ^{2 in} terms of an amount of adhesion (Cr + Si).

2 A water-soluble chromium compound is applied to the surface of zinc or zinc alloy plated steel sheet, and the Cr concentration of the first layer of the chromate film is Cr ³⁺ / Cr.
⁶⁺ ratio 0.1 to 2.0 1 to 100 g / l in terms of CrO ₃ , water-soluble phosphate compound in the ratio Cr / P of 0.1 to 10, and silica in the ratio Cr / Si of 0.1 to 1
0 After applying an aqueous solution containing or not containing it, after forcibly drying it to a plate temperature ≧ 50 ° C without going through a water washing step, the second layer of the chromate film has a Cr concentration of Cr ³⁺ / Cr ^{6+ of} 0.25 to 1.0. 0.1 to 50 g / l in terms of CrO ₃ and Cr / Si ratio 0.1 to 10
The method for producing a chromate-treated plated steel sheet is characterized in that the treatment liquid containing the above-mentioned silica is applied and forcedly dried to a plate temperature ≧ 50 ° C.

The present invention will be described in detail below.

In the present invention, the inventors conducted a detailed study on how the above performances are affected by the constituent components of the coating. As a result, Cr is effective for corrosion resistance, but it is likely to cause color unevenness and impairs the appearance. It is also disadvantageous in paint adhesion and fingerprint resistance. It was found that Si is particularly effective for the primary adhesion of the paint, but is disadvantageous in the secondary adhesion of the paint and fingerprint resistance, and P is advantageous in the corrosion resistance and fingerprint resistance but is disadvantageous in the primary adhesion of the paint. The primary adhesion of paint means the adhesion between the surface of the steel sheet immediately after applying the paint and the paint.
It is evaluated by making 100 notches (10 × 10) incisions of m and peeling with an adhesive tape. What is paint secondary adhesion?
The paint adhesion is shown after the coated steel sheet is aged under a certain environment, and in the present invention, it is evaluated by the goosen tape method after being kept in a 100% humidity tank for one week.

If we carefully consider the contents of various properties, we think that the corrosion resistance and color unevenness are the properties of the entire chromate film, while the paint adhesion and fingerprint resistance are the properties shown by the outermost layer of the film. To be In other words, if the outermost layer has a structure advantageous in paint adhesion and fingerprint resistance and the inner layer has a structure excellent in corrosion resistance, it should be possible to satisfy all of the paint adhesion, fingerprint resistance, and corrosion resistance. In addition, Cr, which easily causes color unevenness,
By stopping the component only in the inner layer, the appearance problem can be solved by removing it from the outermost layer.

According to the present invention, the outermost layer (second layer) is composed of a chromate film containing Si containing no P and the inner layer (first layer) is composed of Cr and P effective for corrosion resistance. This solves all the problems of the conventional chromate film.

The coating of the first layer of the present invention is a chromate coating containing chromate and phosphate as main components in order to maintain corrosion resistance. The Cr / P ratio is 0.1 to 10, and the Cr / P ratios 1 to 4 give the best corrosion resistance. If the Cr / P ratio is less than 0.1, the fixation of the phosphate by Cr is insufficient, so that it easily dissolves in water and the corrosion resistance deteriorates. In addition, since the amount of etching on the plating surface is large, uneven appearance is likely to occur. If the Cr / P ratio exceeds 10, it is difficult to recognize the effect of phosphoric acid on improving corrosion resistance. From this viewpoint, Cr / P ratio is 1
The best quality can be obtained with ~ 4. Adhesion amount is Cr
The quality of the first layer is determined by + P, and 5 mg / m ² or more is required. If it is less than 5 mg / m ² , the corrosion resistance is insufficient and not practical.
The upper limit of the amount of Cr + P deposited varies depending on the application, but is preferably 200 mg / m ² or less when used without coating. 10 for painting
0 mg / m ² or less is desirable.

The Cr compound in the first layer coating is either Cr ⁶⁺ alone or Cr ³⁺ .
A Cr ⁶⁺ composite coating can be applied. Cr ³⁺ / Cr ⁶⁺ ratio 0.1 to 2.0
By controlling to 1, a highly corrosion resistant poorly water-soluble chromate film is obtained, and an interaction with the chromate of the second layer described later is exhibited. That is, in the case of a chromate coating of Cr ⁶⁺ alone, if the amount of chromate coating adhered is high, it easily dissolves in water,
Both the first and second layers are affected. But Cr ³⁺ and Cr
It can be made less soluble and improved by compounding ⁶⁺ . Cr ³⁺ /
The Cr ⁶⁺ ratio of 0.1 is the reason for the above setting, and if it exceeds 2.0, the corrosion resistance deteriorates due to lack of Cr ⁶⁺ , which is not preferable. More optimal
The Cr ³⁺ / Cr ⁶⁺ ratio can be narrowed down to 0.25 to 1.0.

Silica formed from silica sol on the chromate film composed of Cr and P in the first layer (herein represented as Si)
By further combining as necessary, a more excellent corrosion resistance and poorly soluble chromate coating can be obtained. Si
The compounding amount of is within the range of the present invention when the Cr / Si ratio is 0.1 to 10.
The preferable range of the Cr / Si ratio is 0.2 to 1.0. The reason is that if the Cr / Si ratio is less than 0.1, the silica fixing ratio due to Cr is low, and the hydrophilicity of the silica is adversely affected, so that the secondary adhesion of the coating material is deteriorated, which is not preferable.

If the Cr / Si ratio exceeds 10, the effect of Si is weak. The amount of chromate coating containing Si should be at least 5 mg / m ² . It is preferably 10 to 200 mg / m ² , and the reason is the same as for Cr + P.

In addition, by compounding a water-soluble or water-dispersible organic resin in the coating of the first layer, it is possible to promote insolubilization of the coating and obtain better corrosion resistance.

The second layer (outer layer) is intended to improve the primary and secondary adhesion of the paint. As described above, the first layer has a structure of Cr + P or Cr + P + Si centering on corrosion resistance, and there is no silanol group of Si molecule which is advantageous for paint adhesion, or it is in a state of being adsorbed to another molecule. Therefore, it has excellent corrosion resistance, but is disadvantageous in paint adhesion.

The second layer has a Cr ³⁺ / Cr ⁶⁺ ratio of 0.25 to 1.0 and a Cr / Si ratio of 0.1 to 1
It is possible to obtain a well-balanced and appropriate chromate film by depositing a film of 0 and Cr + Si of 0.5 to 50 mg / m ² by paint baking. ^Below Cr ³⁺ / Cr ⁶⁺ 0.25,
The water resistance of the chromate coating is poor, and if it exceeds 1, chromium hydroxide (Cr (OH) ₃ ) precipitates in the treatment liquid, resulting in an uneven appearance. Cr
The most preferred range of ^{3 +} / ^{Cr6 +} ratio is 0.4 to 0.7. Cr / S
An i ratio of less than 0.1 is disadvantageous because the silanol groups effective for paint adhesion are insufficient. On the other hand, if the Cr / Si ratio exceeds 10, the silanol groups are excessive and the secondary adhesion of the coating deteriorates. The most preferable Cr / Si ratio is 0.2 to 1.0. Further, the amount of Cr + Si deposited is in the range of 0.5 to 50 mg / m ² , preferably ² to 20 mg / m ² . If it is less than 0.5, the effect of improving the coating adhesion of the first layer coating cannot be obtained, and if it exceeds 50 mg / m ² , on the contrary, the cohesive failure of the chromate coating and the secondary coating adhesion of the coating deteriorate.

That is, since the second-layer chromate coating of the present invention has the chromate coating underneath, the reduction reaction by the metal such as zinc does not usually occur. Therefore, the chromate treatment bath must be self-cured by drying and is limited to the bath composition and the coating amount described above.

The production method of the present invention will be described below.

The plated steel sheet targeted by the present invention is zinc produced by a method such as electroplating, hot dipping, vapor deposition plating and thermal spraying,
It is a metal-plated steel plate of zinc-iron group, zinc-aluminum, zinc-tin, zinc-manganese, zinc-chromium, zinc-magnesium and the like.

The chromate treatment liquid for the first layer is applied to the surface of these plated steel plates, and the plates are forcibly dried to a plate temperature ≥50 ° C. The chromate treatment liquid contains water-soluble chromium compounds and phosphoric acid compounds as main components. The water-soluble chromium compound can be easily contained in the first layer forming treatment liquid by dissolving chromic anhydride, chromate salt, and dichromate salt in water at a concentration of 1 to 100 g / l. It is possible to include in the inside. At this time, a reducing agent was added to reduce Cr to trivalent as described above, and the ratio of trivalent Cr to hexavalent Cr in the first layer coating, that is, Cr ³⁺ / Cr
By using an aqueous solution of chromic anhydride partially reduced to a ratio of ⁶⁺ of 0.1 to 2.0, preferably 0.25 to 1.0,
Chromate coatings with a Cr ³⁺ / Cr ⁶⁺ ratio can be obtained. The partially reduced chromic acid is prepared by adding a reducing agent to a chromic acid aqueous solution by a known method. The reducing agent is preferably an organic compound in which decomposition reaction products hardly remain. For example, starch, sugars, and alcohols are added little by little to an aqueous solution in which concentrated chromic anhydride is dissolved, and reduced while being decomposed into water and carbon dioxide. At this time, better results are obtained by previously adding the phosphoric acid compound described below to the aqueous solution of chromic anhydride.

As described above, the concentration of the water-soluble chromium compound is 1-100 g / l in terms of CrO ₃ . If it is less than 1 g / l, it is difficult to obtain a proper amount of adhesion, and the reaction rate with the plating surface is low, so that it is difficult to obtain a high-quality coating. On the other hand, if it exceeds 100 g / l, on the contrary, the reaction rate with the plating surface is strong and the appearance of the chromate and the aging of the bath due to the eluted metal ions are rapid, and precipitation or gelation is likely to occur. CrO ₃ concentrations preferably adequate adhesion amount Cr + P ≧ 5mg / m ² is determined by applying a method for obtaining a ^{Cr + P = 10~100mg / m 2} .

As the water-soluble phosphoric acid compound, phosphoric acid, condensed phosphoric acid, or phosphate can be used. The concentration is added at a ratio of 0.1 to 10 in the above-mentioned coating composition ratio, that is, the Cr / P ratio. The plate temperature is 50 ° C. or higher, preferably 200 ° C. or lower, and the drying temperature is compulsorily heated by hot air, infrared rays, an electric heater or gas heating. If it is less than 50 ° C, it tends to deteriorate when the second layer chromate coating is applied. On the other hand, if the temperature exceeds 200 ° C, the chromate film deteriorates due to heat, and the performance deteriorates due to cracking and reduction of Cr ⁶⁺ . In addition, when the chromate solution for the second layer is applied, cooling cannot be done in time, which leads to unevenness and deterioration due to application at a high temperature, and a plate temperature of 100 ° C. or lower is optimal.

Subsequently, the chromate treatment of the second layer is performed. The treatment method will be described below. The chromate treatment liquid for the second layer is bathed so as to have the above-described chromate film composition for the second layer. That is, the water-soluble chromium compound obtained by partially reducing chromic acid by the above-mentioned method with a Cr ³⁺ / Cr ⁶⁺ ratio of 0.25 to 1.0 is 0.1 to 50 g / l in terms of CrO ₃ and a Cr / Si ratio of 0.1 to 10 It consists of added silica sol. The silica sol is preferably an acidic type silica sol having a particle size of 1 to 100 mμ, but dry silica powder,
A silane coupling agent, an alkoxide compound, and various silicates may be dissolved or dispersed in water before use.

If the concentration of CrO ₃ deviates from the upper or lower limit, the lower chromate film cannot be improved and rather deteriorates. The most preferable method is to control the plate temperature to 30 to 60 ° C. after the first layer chromate treatment, and apply the second layer chromate solution by a roll. The reason is that the gel-like chromate film of the first layer is quickly dried and hardened in a short time contact. The reason for setting the plate temperature is below 30 ° C because of the second layer chromate solution,
The chromate solution of the first layer dissolves, and the function of the thin chromate layer of the second layer deteriorates. On the other hand, if it exceeds 60 ° C, chromate unevenness may occur due to rapid drying, which is not preferable. The drying temperature of the coating of the second layer is the same as the condition of the first layer, forcibly drying at a plate temperature of ≥50 ° C, preferably 200 ° C or less.

Si used for the second layer in the chromate treatment solution for the first layer is Cr / Si
By adding 0.1 to 10, preferably 0.2 to 1.0 in the ratio, the above-mentioned Si-combined chromate film can be obtained.

[Examples] Examples will be described below.

The conditions in the examples were evaluated by the following methods unless otherwise specified. Each amount of adhesion was measured by fluorescent X-ray, and the amount was shown in mg / m ² unit.

The performance evaluation is as follows.

1) Corrosion resistance ... Chromate-treated plated steel plate is cut into 75 × 150 mm and sealed on the back and end faces with a dense wax, and then white rust with an area ratio of 5% occurs in a salt spray test (JIS Z-2371, continuous method). It was observed every day and the time until white rust was generated was scored as follows.

7: 2 20 hours or more 6: 196 hours 5: 168 hours 4:96 to 144 hours 3: 48-72 hours 2:24 hours 1:10 hours or less 2) Paint adhesion The coated plate is a commercially available melamine alkyd resin-based enamel (White) was coated with a bar coater to a dry film thickness of 25 μm and baked under predetermined conditions (90 ° C., 30 minutes). The primary adhesion of the paint was evaluated by making 100 mm (10 x 10) 1 mm cuts with a knife in the paint film of the coated steel sheet and then using the number of peeled cuts when peeled off with an adhesive tape. It is a thing (Gobang eye tape method). In the present invention, the secondary adhesion of the paint is evaluated by the goosen tape method after holding the coated steel plate in a humidity tank having a relative humidity of 100% for one week in the present invention.

The scores are as follows.

5: 0 (no peeling) 4: 3 or less 3:10 or less 2:50 or less 1:50 or more Example 1 First layer shown in Table 1 using an electrogalvanized steel sheet (zinc coverage 20 g / m ² ) as a base plate. After spraying the chromate treatment of No. 1 and homogenizing with an air knife, forcibly drying with hot air to a plate temperature of 80 ° C, and then cooling with air to a plate temperature of 30 to 40 ° C, the second layer chromate treatment liquid shown in Table 1 is applied. A chromate-treated plated steel sheet coated with a natural coater and dried at a plate temperature of 60 ° C. was prepared. Corrosion resistance and paint adhesion were evaluated under standard conditions.

Nos. 1 to 5 are treatment solutions in which phosphoric acid is changed to 5 g / l to 30 g / l in an aqueous solution of chromic anhydride reduced with starch to form a first layer by the air knife method, and then the same. Partially reduced chromic acid and silica sol (average particle size 20-30mμ)
The results are obtained by roll coating the aqueous solution of No. 2 and chromate-treating the second layer. Comparative Examples R-1 and R-2 in which corrosion resistance is improved in proportion to the amount of phosphoric acid added and the second layer chromate is not applied
In this example, the deterioration of the adhesiveness of the primary paint is improved.

Nos. 6 and 7 are examples of low adhesion of the first and second layers, but the corrosion resistance is slightly reduced, but the paint adhesion is excellent in both primary and secondary.

No.8, 9 and 10 are examples in which the amount of chromate adhered to the second layer was changed.No.8, which has a low amount of adhesion, has reduced primary adhesion, and No. 10 on the high adhesion side has secondary adhesion. Is reduced. The best results are shown by No. 9 and No. 2 with the proper second layer adhesion amount.

Nos. 11 and 12 are obtained by changing the reduction rate (Cr ³⁺ / Cr ⁶⁺ ) of the second layer chromate. If the reduction rate is lowered (No. 11), the secondary adhesion tends to decrease.

The non-reducing Comparative Examples Nos. R-3 and R-4 have poor secondary adhesion and the high adhesion of the second layer, No. R-4, is also inferior to the present invention in primary adhesion.

Comparative example No. R-5 is an example in which the silica sol in the second layer chromate film is not included, and both primary and secondary adhesion are insufficient, whereas No. 13, 14, 15 in which the silica sol is gradually increased is improved. is doing. No. 15 has a large amount of silica, and the secondary adhesion tends to be inferior to the others. No.16 and 17 have plate temperature of 100 ℃ and 200 ℃
In Example No. 17 in which the second layer was baked at 200 ° C, the corrosion resistance and the primary adhesion were deteriorated, but those at 100 ° C showed good results.

Example 2 Chromate treatment of the first layer shown in Table 2 was uniformly applied with a reverse coater using an electrogalvanized steel sheet (zinc adhesion amount: 20 g / m ² ) as a base sheet, and forcedly dried with hot air to a sheet temperature of 80 ° C. After that, it was cooled to 30 to 40 ° C. with air, and the chromate treatment liquid of the second layer shown in Table 2 was forcibly dried to a plate temperature of 80 ° C. coated with a natural coater. The evaluation was performed under standard conditions.

No.18 to 21 are examples of chromate film containing silica obtained by applying a treatment liquid containing silica sol (particle size 10 to 20 mμ) to the chromate treatment liquid of the first layer, and No.22 does not contain silica. Here is an example. Corrosion resistance is improved by compounding silica. R-7 is a comparative example in which the second layer (upper layer) chromate is not applied, and the primary adhesion is poor, and R-8 is a comparative example of the upper chromate film of CrO ₃ alone, and the primary and secondary adhesion is poor. Comparative Example R-9 is an example in which the amount of the second layer chromate coating deposited was too large. The primary and secondary adhesion is deteriorated.

Example 3 Hot-dip galvanized steel sheet (containing 0.2% Al, 0.1% Pb, adhesion amount 60 g / m ² : symbol G), hot-dip aluminum alloy-plated steel sheet (4.9% under the conditions shown in Table 2 No. 18 of Example 2) Al, 0.2% Mg
It was contained and deposited 60 g / m ² : symbol GA). Chromate-treated galvanized steel sheets with the same amount and composition ratio as in No. 18 were obtained. The corrosion resistance of the G treated material was 6, and the GA treated material was 7. Both of them had a primary paint adhesion score of 5, and a secondary paint adhesion score of 5 for both G and GA.

Example 4 Galvannealed iron-zinc alloy-plated steel sheet (10% Fe-containing basis weight 40 g /
m ² ). The amount of chromate deposited and the composition ratio were almost the same as those of No. 19. The corrosion resistance was 6, and the paint adhesion was 5 for both primary and secondary.

Example 5 Example 2, 10% Ni-Zn alloy electroplated steel sheet (deposition amount 20 g / m ² : symbol N), 20% Fe-Zn alloy electroplated steel sheet (deposition amount 20 g) under the conditions of No. 18 in Table ² / m ² : treated with symbol F).

Excellent performance was obtained with a corrosion resistance of 7 for both N and F and a paint adhesion of 5 for both primary and secondary.

[Advantages of the Invention] The present invention divides the functions into a first layer intended for corrosion resistance and a second layer intended for paint adhesion and fingerprint resistance, and divides the functions into two layers (upper layer). It improves the adhesion of paint containing Cr. Therefore, by combining the first layer and the second layer, it is possible to provide a chemical conversion treatment plated steel sheet having excellent performance in all of corrosion resistance, paint adhesion and fingerprint resistance.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masato Nakazawa 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa Inside the 2nd Technical Research Laboratory, Nippon Steel Corp. (56) Reference JP-A-61-104086 (JP, Sho A) JP-A-54-31043 (JP, A)

Claims (2)

[Claims] 1. A zinc or zinc alloy-plated steel sheet is treated with a treatment liquid containing a water-soluble chromium compound and a water-soluble phosphoric acid compound, and further containing or not containing silica, to form Cr ³⁺ / as a first layer.
Cr ⁶⁺ ratio 0.1 to 2.0 Cr and Cr / P ratio in terms of P 0.1 to 10 When silica is included, the Cr / Si ratio is 0.1 to 10 and the adhesion amount (Cr + P + Si)
After forming a chromate film of ≧ 5 mg / m ^{2 on} the plating surface, a chromium compound and silica with a Cr ³⁺ / Cr ⁶⁺ ratio of 0.25 to 1.0 and a Cr / Si ratio of 0.1 to 10 without undergoing a water washing step. Chromate film of the second layer containing is 0.5 in terms of adhesion amount (Cr + Si)
Chromate treated plated steel sheet characterized by having ~ 50 mg / m ² . 2. A water-soluble chromium compound is added to the surface of a zinc or zinc alloy plated steel sheet so that the Cr concentration of the first layer of the chromate film is
Cr ³⁺ / Cr ⁶⁺ ratio 0.1 to 2.0 1 to 100 g / l in terms of CrO ₃ , water-soluble phosphate compound in Cr / P ratio of 0.1 to 10, and silica in Cr / Si ratio of 0.1 to 10 Or, after applying an aqueous solution that does not contain it, after forcibly drying to a plate temperature ≥ 50 ° C without going through a water washing step, the water-soluble chromium compound with a Cr concentration Cr ³⁺ / Cr ⁶⁺ ratio of 0.25 to 1.0 in the second layer of the chromate film. 0.1 to 50 g / l and Cr / Si in terms of CrO ₃
Applying a treatment liquid containing silica with a ratio of 0.1 to 10 and applying plate temperature ≥ 5
A method for producing a chromate-treated plated steel sheet, which comprises forcibly drying at 0 ° C.