JP3183138B2 - Method for producing Al-Zn-based alloy-coated steel sheet excellent in blackening resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a non-painted aluminum containing 25 to 75% by weight and Si containing Al
Al-Zn-based alloy-coated steel sheet containing 0.5% by weight or more, typically a generally-produced 55% by weight of Al and a plated steel sheet containing about 1.6% by weight of Si (hereinafter referred to as 55% Al -Zn-based alloy-plated steel sheet), it is possible to eliminate the blackening of the steel sheet surface which is likely to occur when stored in a coil state, or when stored in a sheet state, or in a state of being stacked by roll forming or the like. The present invention relates to a method for producing an Al—Zn-based alloy-plated steel sheet having excellent blackening resistance.

[0002]

2. Description of the Related Art A 55% Al-Zn alloy plated steel sheet is
Because the plating appearance is beautiful and excellent in corrosion resistance,
It is formed by roll forming in an unpainted state and used for building roofs, outer walls, and the like. These are required to have long-term corrosion resistance, and since they are used for portions where the plating surface is directly visible, it is required that the original appearance surface be maintained for a long time.

[0005] However, the surface of a 55% Al—Zn alloy-plated steel sheet turns black when stored outdoors for several days in a sheet state or in a state of being stacked by roll forming or the like before construction, and the appearance quality is reduced. May be significantly reduced. Therefore, it is required to have a surface quality that does not change black even when stored outdoors for several days in a stacked state.

[0004] 55% Al-Z used in these applications
As a method for producing an n-type alloy-plated steel sheet, the following method has conventionally been proposed.

[0005] Japanese Patent Publication No. 4-2672 discloses that the surface of a 55% Al-Zn alloy-plated steel sheet is treated with a treatment liquid in which chromic acid and a water-soluble or water-dispersible resin are mixed to provide corrosion resistance and roll. A method for preventing adhesion of plating to a roll which occurs during forming is disclosed. This technology avoids direct contact between the roll and the plating surface by forming a certain amount of resin film on the plating surface,
In addition to preventing plating adhesion to the roll, corrosion resistance is obtained by forming a mixed film with chromic acid.

As a method for preventing black change, Japanese Patent Publication No. 1-53353 discloses a method in which chromic acid directly reacts with plating by treating it with a processing solution in which chromic acid and an acceptor are mixed at a certain ratio or more. A method for preventing the occurrence of blackening and improving blackening resistance is disclosed. In addition, 55% A
As a method for preventing blackening performed on galvanized steel sheets other than the l-Zn alloy-plated steel sheet, that is, a 5% Al-Zn alloy-coated steel sheet, JP-A-59-177381 and JP-A-63-65088 disclose a method for preventing blackening. Prior to performing the chromate treatment after plating described in
The method of processing at o is shown.

[0007]

The blackening resistance of the surface of a 55% Al-based alloy-plated steel sheet can be reduced by using a processing solution prepared by mixing chromic acid and a water-soluble or water-dispersible resin in an appropriate ratio in the prior art. By treating the surface of the Al—Zn-based alloy-plated steel sheet, an improvement effect is recognized in a normal high humidity environment.

[0008] However, the 55% Al-Zn alloy-plated steel sheet is stored in the form of sheets or roll-formed products in a stacked state outdoors during the construction of a roof, an outer wall or the like. In the case where the steel sheet is used, there is a problem that the surface of the steel sheet becomes wet due to dew condensation or the like easily in a natural environment, and the black color is changed even after being left for 1 to 2 days. In addition, Ni, Co
Performing the pre-treatment for example may improve the discoloration resistance, but is disadvantageous in cost because an expensive metal is used.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a manufacturing method capable of manufacturing an Al-Zn steel sheet having excellent blackening resistance.

[0010]

Means for Solving the Problems The inventors of the present invention have found that, for example, when a 55% Al-Zn alloy-plated steel sheet is wet in a sheet state or in a state in which formed materials by roll forming or the like are stacked and the steel sheet surface is wet due to dew condensation. It has been found that the required blackening resistance can be significantly improved by performing a chromate treatment mainly containing Cr ⁶⁺ , further applying a treatment liquid mainly containing an emulsion resin, and drying.

The present invention has been made based on such findings, and the surface of an Al—Zn-based alloy plating film containing 25 to 75% by weight of Al formed on a steel sheet is coated on the surface of the steel sheet with a total chromium content of 6%. The ratio of the valent chromium content is 0.
After applying and drying a chromate treatment liquid of 55 or more, and further applying and drying a treatment liquid containing an emulsion resin, the total chromate adhesion amount is 5 to 40 mg / m ^{2 in} terms of metal chromium, and the emulsion resin adhesion An object of the present invention is to provide a method for producing an Al—Zn-based alloy-plated steel sheet having excellent resistance to blackening, characterized by applying a multilayer coating having an amount of 0.8 to 4.0 g / m ² .

It is another object of the present invention to provide a method for producing an Al—Zn alloy-plated steel sheet having excellent blackening resistance, characterized in that the treatment liquid containing the emulsion resin further contains trivalent chromium.

Further, in any one of the above-mentioned production methods, at least one of the chromate treatment liquid and the treatment liquid containing the emulsion resin contains phosphoric acid, and the phosphoric acid is converted to PO ₄ in the entire film after film formation. And a weight ratio of phosphoric acid to chromium in the case of converting chromic acid to metallic chromium, which is 0.1 to 4.0, and which is excellent in blackening resistance. It is intended to provide a manufacturing method.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically.

First, the results of the study by the present inventors on black discoloration occurring in a 55% Al—Zn alloy plated steel sheet will be described.

In the case of a 55% Al—Zn alloy plated steel sheet, moisture enters between the steel sheets in a state where the steel sheets are stacked,
In the state where a thin water film is formed, blackening easily occurs, in which case,
Blackening occurs not at the edge portion of the steel plate but at the inner portion, that is, the portion where the oxygen supply is likely to be insufficient.

In the blackened portion, a part of Al or Zn in the plating is dissolved to form a hydroxide,
It became clear that the formation of the hydroxide was responsible for the blackening of the steel sheet surface. From this result, 55% Al-Zn
In order to improve the blackening resistance of the base alloy-plated steel sheet, it is led that it is necessary to suppress the plating dissolution reaction in an environment where water is present due to insufficient oxygen supply.

[0018] Therefore, 5 generated in such a special environment.
As a result of studying a method for suppressing a dissolution reaction of a 5% Al-Zn-based alloy-plated steel sheet, it was newly found that blackening resistance was significantly improved when chromate treatment was performed under the following conditions. The following conditions are conditions that are known to promote blackening in a conventional Zn-based plated steel sheet or a 5% Al—Zn-based alloy-plated steel sheet.
It is considered that the effect is exhibited only in the Al-Zn-based alloy-plated steel sheet having a high Al content represented by the l-Zn-based alloy-plated steel sheet. Therefore, in the present invention, the Al content of the Al-Zn-based alloy plating is defined. I have.

First, Cr ⁶⁺ and C
It is possible to include r ³⁺ , but Cr ⁶⁺
When the chromate treatment liquid has a molar ratio of less than 0.55, the effect of improving blackening resistance is insufficient. Although the cause has not been elucidated, it is considered that in an environment where a layer formed by the reaction of Cr ⁶⁺ on the plating surface causes blackening, the layer becomes a passivation layer and blackening resistance is improved. Therefore, according to the present invention, the total Cr in the chromate treatment liquid is
^{The 6+} molar ratio was defined as 0.55 or more.

The chromate treatment conditions are as follows: a treatment solution usually used for Zn plating is sprayed on the surface of a steel sheet while being heated to about 40 to 60 ° C., squeezed by a roll or the like, and then the sheet temperature is 60 to 100 ° C. Can be employed.

However, when the 55% Al-Zn alloy plated steel sheet is subjected to a process such as roll forming and then exposed to an environment in which black color changes, the above-described chromate treatment does not exert its effect sufficiently. This is considered to be because the passivation film having an effect on blackening resistance is destroyed by contact with the roll during processing. Therefore, in the present invention, for the purpose of protecting the passivation film layer during processing, a resin film layer is formed after the chromate treatment.

In order to protect the passivation film layer, it is necessary to coat the surface of the plated steel sheet more uniformly with an organic resin at a certain ratio or more. The surface of the Al—Zn-based alloy-plated steel sheet according to the present invention is composed of an α phase mainly composed of Al and a Zn-rich β phase, and the α phase and the β phase are formed with irregularities. When a resin is applied to the plating surface with a roll coater or the like to form a film, the organic film is filled from the concave portion as the amount of adhesion increases.

The inventors of the present invention have studied the coating ratio of the plating surface with the resin and the roll forming property. As a result, when the resin is applied in an amount of 0.8 g / m ² or more, about 70% or more of the surface is coated. No adhesion at the time of forming, the surface is almost completely covered by applying 1.2 g / m ² or more, and the surface is hardly scratched even when it comes into contact with the roll at the time of roll forming. It was found that the resistance to blackening after processing was improved as the degree of damage was reduced. Also,
It was found that as the degree of surface damage was reduced, the blackening resistance after processing was improved.

However, if the amount of adhesion is excessively large, not only is it economically disadvantageous, but also if the steel sheet is stored in the form of a coil or the like, adhesion occurs between the steel sheets, which is considered to be caused by fusion with the organic resin. It has been found that there are problems with the blocking properties such as the occurrence of uniform unevenness on the steel sheet surface and the peeling of the resin film. Thus, the present inventors have studied the relationship between the type or amount of the coating resin or the amount of the coating resin and the blocking property, and as a result, the following has become clear.

That is, the sheet temperature during storage easily reaches 50 ° C. or higher, and it is effective means to use a resin having a Tg higher than the sheet temperature during storage in order to obtain blocking properties that can withstand the temperature. it is conceivable that. However, increasing the Tg increases the film-forming temperature of the resin, which in turn necessitates a higher film-drying temperature, resulting in an enormous amount of drying and cooling equipment, resulting in higher costs. When a water-soluble resin is used as the resin for coating the surface of the Al—Zn-based alloy-plated steel sheet, these problems occur even with the resin of 0.8 g / m ² , but it is difficult to achieve compatibility with roll forming properties. When an emulsion resin is used, Tg is 50 ° C.
Even when a resin capable of forming a film at a drying temperature of 100 ° C. or less is used, the adhesion amount is 4.0 g / m ² or less, preferably 2.0 g
It has been found that no problem occurs in the blocking property by controlling the g / m ² or less. This is considered to be because the contact area between the resins when the steel sheets are superposed greatly affects each other, and by setting a certain amount of the resin, air enters between the steel sheets and this suppresses a decrease in blocking resistance.

Therefore, in the present invention, an emulsion resin is used as the resin for coating the surface of the chromate-treated Al—Zn-based alloy-plated steel sheet, and the amount of adhesion is 0.8 to 4.
It was specified in the range of 0 g / m ² .

In the present invention, the type of the emulsion resin is not particularly limited, but the film forming temperature, liquid stability, weather resistance, adhesiveness to a steel sheet such as a heat insulating material, and appearance quality, as well as the film forming temperature. It is preferable to select in consideration of the above. In the case of an emulsion resin, a surfactant and an emulsifier are added in order to improve the water dispersibility.
This may cause a decrease in the adhesiveness, paintability, adhesiveness, etc. of the resin film. Therefore, in selecting an emulsion resin, it is necessary to consider a surfactant and an emulsifier. In particular, in the case of manufacturing on a line, immediately after application and drying, the film comes into contact with a roll etc., and if the adhesion of the film after drying is low, the film adheres to the roll etc., which greatly impedes operability. Will come. By applying a resin after chromate treatment and attaching an appropriate amount of chromate, the adhesion of the resin film is improved,
Depending on the emulsion resin, sufficient adhesion may not be obtained.

The present inventors have further studied to further improve the adhesiveness of the resin immediately after drying. As a result, by adding Cr ³⁺ to the resin film, the plating and the resin can be obtained immediately after drying even at a low drying temperature. It has been found that good adhesion to the film can be obtained. This is because Cr ³⁺ is adsorbed on the water-dispersed resin in a hydroxyl group state, and a condensation reaction easily occurs between the adsorbed Cr ³⁺ , and the hydroxyl group enhances the adhesion to the plating surface to form a resin film. It is estimated that the adhesion to plating is improved. Therefore, in a preferred embodiment of the present invention, Cr ³⁺ is added to the solution containing the emulsion resin. As a method of mixing Cr ³⁺ into the resin film, C ³⁺
A method in which a part of r ⁶⁺ is reduced and Cr ³⁺ is mixed with Cr ^{6+ in a} resin solution, or Cr ³⁺
There is a method of mixing only the resin liquid with the resin liquid. The amount of Cr ³⁺ to be added may be the required amount according to the emulsion resin selected.

It is also possible to mix Cr ^{6+ in the} emulsion resin. In this case, even if the formation of the passivation film is partially insufficient, the Cr ⁶⁺ contained in the resin may be contained. Thus, it can be expected that a passivation film is formed after the resin film is formed.However, when the steel sheet is wetted due to excessive mixing, the amount of Cr ⁶⁺ easily dissolved and eluted from the film increases, Environmentally unfavorable. In the present invention, basically, the blackening resistance and the corrosion resistance can be increased without depending on Cr ⁶⁺ in the film, so that an environmentally advantageous film can be formed.

Further, in order to sufficiently exert the effects of the present invention, it is necessary that the total amount of attached chromate (the amount of attached Cr) is in the range of 5 to 40 mg / m ^{2 in} terms of chromium metal. If the amount of Cr attached is less than 5 mg / m ² , the effect of chromate treatment is insufficient, and if it exceeds 40 mg / m ² , coloring becomes conspicuous, and the solubility of the attached Cr tends to increase. Therefore, it is unsuitable when a roof, an outer wall, or the like is mainly used.

In the treatment for forming the resin layer, for example, after coating with a roll coater, the resin layer is dried at a temperature of 60 ° C. or higher, which is considered necessary for evaporating moisture, and cooled with an air cooling device or the like as necessary. And the like. The drying temperature is desirably lower from the viewpoint of manufacturing cost. However, when Cr ⁶⁺ is mixed in the resin layer, high-temperature drying is preferable from the viewpoint of forming a film in which Cr is hardly dissolved.

In this case, if a chromate treatment is performed before the application of the resin, a film excellent in blackening resistance is formed, but if the chromate treatment is not performed, the film is inferior in blackening resistance. Therefore, it is essential to perform a chromate treatment before applying the resin in order to form a film having both excellent resistance to blackening and resistance to dissolution of Cr.

Depending on the resin used, the previously formed chromate-treated film may partially dissolve during the application of the resin, thereby lowering the stability of the resin liquid. By maintaining the pH in a stable range by adding, for example, ammonia or the like, the stability can be ensured.

In the present invention, it is preferable that at least one of the chromate treatment liquid and the treatment liquid containing the emulsion resin contains phosphoric acid. It is known that the addition of phosphoric acid reduces blackening resistance in so-called zinc-based plated steel sheets and 5% Al-Zn-based alloy plated steel sheets. Conversely, it has been found that in the alloy-plated steel sheet, the blackening resistance is further improved. Further, since the coloring of Cr can be suppressed by adding phosphoric acid, it is also desirable from the viewpoint of the color tone of the appearance.

However, the addition amount of phosphoric acid for blackening resistance has an appropriate range for the following reasons.
That is, phosphoric acid is converted to P in the entire film after the film is formed.
When the weight ratio of phosphoric acid to chromium in terms of O ₄ and chromic acid converted to metallic chromium is less than 0.1, the effect of adding phosphoric acid is not sufficiently exerted, and more excellent blackening resistance Cannot be obtained. On the other hand, when the weight ratio exceeds 4.0, the film becomes bluish, which is not preferable in appearance. When phosphoric acid is added for these reasons, the weight ratio between phosphoric acid and chromium is specified in the range of 0.1 or more and 4.0 or less.

The effect of adding phosphoric acid to the treatment solution on blackening resistance is due to the addition of either the chromate treatment solution mainly composed of Cr ⁶⁺ or the treatment solution mainly composed of emulsion resin, or both. Was confirmed in each case.

Further, like phosphoric acid, it is possible to add a compound which is considered to promote the reaction between Cr ⁶⁺ and plating, for example, a sulfuric acid compound or a fluorine compound. There is no particular limitation on the type or amount of, but excessive addition may reduce not only blackening resistance but also corrosion resistance. Therefore, it is necessary to use an additive amount that does not cause such a situation.

In the method of the present invention, Al is 25 to 7
As described above, a chromate-treated layer is applied to an Al—Zn-based alloy-plated steel sheet having a plating layer containing 5% by weight, more preferably 50 to 60% by weight, or a plating layer further containing 0.5% or more of Al. And forming an emulsion resin layer. If the plating layer has the above composition, by forming these two layers thereon, the blackening resistance can be improved. When the plating appearance is particularly important, it is preferable to perform the above-described treatment on an Al—Zn-based alloy-plated steel sheet whose shape has been corrected by a tension leveler without performing skin pass.

[0039]

Examples: Al: 53%, Si: 1.3%, Fe: 1.
Chromate treatment was performed on the Al-Zn-based alloy-plated steel sheet on which a plating film having a composition of 4% and the balance of Zn was formed under the conditions shown in Table 1, and a resin film was formed under the conditions shown in Table 2. .

For comparison, other hot-dip galvanized steel sheets GI (Al: 0.3%, balance Zn) and GF
(Al: 4.5%, balance Zn) was used.

[0041]

[Table 1]

[0042]

[Table 2]

The adhesion amounts of these films were determined, and the following methods were used to evaluate blackening resistance, workability, corrosion resistance, and adhesion immediately after drying the resin film. The amount of the film attached was calculated from the amount of Cr measured using fluorescent X-rays for the chromate-treated film, and calculated from the intensity of C measured using EPMA for the resin film.

(1) Blackening Resistance A 150 mm × 150 mm steel sheet on which a film was formed was placed in a stack state and left in a wet tester (HCT) for 6 days. Denaturation was evaluated visually. For some samples, 2
A test of standing for a week was performed and evaluated in the same manner.

The evaluation criteria at this time are as follows.

A: No change in appearance before and after the test.
(Less than 0%) Δ: There is a planar appearance change part in the test (area 10% to 5)
X: Less than 50% of black discolored portion or planar appearance change portion evident from the test. (2) Workability A 30 mm wide steel sheet on which a film was formed was formed with a bead diameter of 5R, a deformation height of 1 mm, After performing a bead pull-out test under the condition of a pressing load of 100 kgf, the state of the processed material (scratch resistance)
Was evaluated, and then a blackening resistance test was performed.

The evaluation criteria for scratch resistance are as follows.
The blackening resistance after processing was evaluated according to the same criteria as above.

Evaluation criteria score Scratch resistance 5 No damage 4 Less than 3 minor scratches in pull-out direction 3 4-10 minor scratches in pull-out direction 2 4-10 clear scratches in pull-out direction 1 Coating Of the steel plate, galling of the steel plate, and adhesion to the bead are observed.

(3) Corrosion resistance A salt water spray test (JIS-Z-2371) was performed on the cut end face of the sample tape-sealed,
The area of rust occurrence after a lapse of time was examined, and the corrosion resistance was evaluated based on the following criteria.

The evaluation criteria at this time are as follows.

[0051] (4) Adhesion of the film immediately after drying the resin film An adhesive tape was attached to the surface of the sample immediately after leaving the drying oven, peeled off, and evaluated by the degree of peeling of the film. A similar test was performed two hours later and evaluated similarly. The evaluation criteria are shown below.

◎: No peeling ： １: Less than 10% peeled (no peeling after 2 hours in peeling test) △: Less than 10 to 25% peeling (no peeling in 2 hours after peeling test) ×: 25 % Or peeling in a peeling test after 2 hours. Film formation conditions, film adhesion amount, and evaluation results of each property are shown in Tables 3 to 6.

[0053]

[Table 3]

[0054]

[Table 4]

[0055]

[Table 5]

[0056]

[Table 6]

As shown in these tables, No. 1 was treated under the conditions within the range of the present invention. 2-5,7,9,13,1
4,18,22 to 26,28 to 51 were all excellent in blackening resistance, corrosion resistance, workability, blocking resistance, and chromium resistance. Further, in particular, No. 3 in which phosphoric acid was added to the film without the scope of the present invention. 7, 9, 10, 14, 23
~ 26,28 ~ 30,33 ~ 51 showed even better performance than the films without or without the addition of phosphoric acid. Also, in this, No. 14, 15, 31 to 51
Was also excellent in the adhesion of the resin film.

On the other hand, under the conditions deviating from the present invention, one of the above characteristics was inferior. No. Sample No. ⁶ was inferior to blackening resistance due to the low Cr ⁶⁺ ratio of the chromate treatment solution. In addition, No. Samples Nos. 8 and 11 were inferior in blackening resistance, scratch resistance, or blocking resistance after processing because the resin adhesion amount was out of the range of the present invention. No.
Samples Nos. 1, 12, and 15 were inferior in terms of blackening resistance, corrosion resistance, and Cr solubility because the amount of deposited Cr was out of the range of the present invention. In addition, No. Nos. 16 and 17 were cases where the chromate treatment was omitted, and were inferior in blackening resistance. In addition, No. No. 19 was a case where a water-soluble resin was used as the resin. Even when the amount of adhesion was small, the film had a sticky feeling as compared with the emulsion resin, and was inferior in blocking resistance. In addition, No. Samples Nos. 20 and 21 had plating compositions out of the range of the present invention and were poor in blackening resistance. Furthermore, No. In No. 27, the addition range of phosphoric acid exceeded the range shown in the present invention, and the appearance was a bluish film.

[0059]

As described above, according to the present invention,
An Al-Zn steel sheet excellent in blackening resistance, workability, and corrosion resistance can be manufactured at low cost.

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masahiko Sakurai 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yoshiharu Sugimoto 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan (72) Inventor Masaru Sagiyama 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-61-110777 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) B05D 7/14 B32B 15/08 C23C 22/30

Claims (3)

(57) [Claims] 1. The molar ratio of hexavalent chromium content to total chromium content is 0.1% on the surface of an Al—Zn-based alloy plating film containing 25 to 75% by weight of Al formed on a steel plate.
After applying and drying a chromate treatment liquid of 55 or more, and further applying and drying a treatment liquid containing an emulsion resin, the total chromate adhesion amount is 5 to 40 mg / m ^{2 in} terms of metal chromium, and the emulsion resin adhesion manufacturing method of Al-Zn alloy coated steel sheet having excellent blackening resistance, characterized in that the amount is to perform multi-layer film of 0.8~4.0g / m ^2. 2. The method according to claim 1, wherein the treatment liquid containing the emulsion resin further contains trivalent chromium. 3. At least one of the chromate treatment liquid and the treatment liquid containing the emulsion resin contains phosphoric acid, and converts phosphoric acid into PO ₄ and converts chromic acid into metallic chromium in the entire film after film formation. The weight ratio of phosphoric acid to chromium in terms of conversion is 0.1 or more and 4.0 or less, the Al-Zn-based alloy-coated steel sheet having excellent blackening resistance according to claim 1 or 2. Production method.