JPH0796699B2 - Hot-dip galvanized steel sheet with excellent resistance to blackening - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is a hot-dip zinc-based plating excellent in black resistance in which a cobalt oxide film is formed on the surface of a plating layer and a chromate film is formed on the cobalt oxide film. Regarding steel plates.

(Prior Art) Conventionally, hot-dip galvanized steel sheets have been mainly used for roofs and walls, but in recent years, applications have expanded to interiors, equipment, and other areas that require a high degree of appearance, shape, and workability. .
For this reason, in the production of hot-dip galvanized steel sheet, the spangle is minimized by spraying water droplets on the unsolidified coating layer immediately after adjusting the coating amount after plating to quench the spangle, or after solidification of the coating layer. Mechanical treatment, such as buffing, leveler or skin pass, is applied to improve the appearance, shape and workability.

(Problems to be solved by the invention) However, if minimized spangles or mechanically processed spangles are subjected to chromate treatment to improve corrosion resistance and coating adhesion, normal storage is not possible. Even in the state, there was a problem that the surface turned grayish black (hereinafter referred to as black discoloration) in a relatively short period of several months, and the silvery white appearance was lost.

This black discoloration is a film that easily absorbs light (hydroxide mainly composed of Al, Cr, Zn) due to changes in the element existence state and crystal orientation on the surface of the plating layer due to minimization of spangles and mechanical processing. It is presumed that this film appears grayish black, but the blackening was remarkable especially when chromate treatment was applied. For this reason, a non-chromic acid type post-treatment may be used to make the blackening light or inconspicuous, but the non-chromic acid type post-treatment has a drawback that the treatment cost is high.

On the other hand, as a method of preventing black discoloration even if chromate treatment is used for the post-treatment, there is a method of treating with an aqueous solution containing Ni ions or Co ions or both after mechanical treatment and before chromate treatment (JP 59-177381 gazette). However, since this method deposits Ni and Co from the treatment liquid by displacement plating, the adhesion of the deposited metal was weak. For this reason, when winding coils, stacking or handling of cut plates, slips occur between the plates during handling, or if they are squeezed by roll forming, pressing, bending, or other processing, they are easily removed. In addition, black discoloration occurred in the form of dots, stripes, or stripes.

In addition, if the cleaning after the treatment is insufficient, the residual acid or alkali becomes a corrosion factor and rather induces blackening or corrosion,
It causes white rust, and the deposited Ni and Co are Zn.
A local cell was formed between the and, causing microscopic corrosion.

(Means for Solving Problems) Therefore, the present inventors have developed a hot-dip galvanized steel sheet that does not cause blackening even if slip occurs between the plates or is subject to ironing due to processing, and there is no problem of corrosion. As a result of various studies to achieve this, the surface of the hot dip zinc or zinc alloy plated steel sheet is treated with an aqueous solution containing one or both of cobalt nitrate and cobalt chloride, and the cobalt salt adhered is thermally decomposed to form a cobalt oxide film. The inventors have succeeded in developing a hot-dip galvanized steel sheet having the above-mentioned precipitates and formed on it and a hot-dip galvanized steel sheet having a chromate film formed on the cobalt oxide film. Here, in order to deposit and form an oxide film of cobalt on the surface of the steel sheet by thermal decomposition of the cobalt salt, it is necessary to use cobalt nitrate or cobalt chloride or these in an aqueous solution sprayed on the unsolidified plating layer when minimizing the conventional spangle. Both of them are contained, and the coating layer immediately after hot-dip galvanizing is applied to the surface of the hot-dip galvanized steel sheet by the water droplets of the aqueous solution so that the cobalt salt adheres to the steel sheet surface and the cobalt salt is heated to a high temperature. By thermally decomposing in the plating layer, a cobalt oxide film is deposited and produced on the surface of the plating layer.

In the present invention, the black oxide is prevented by forming an oxide film of cobalt because the metal oxide film other than cobalt (oxide film of Ni, Fe, Zn, Ti, W, Nb, etc. when cold) is resistant to This is because it has little effect on blackening suppression, or exhibits the opposite effect.

Also, it is not possible to use cobalt nitrate or cobalt chloride or both to form an oxide film of cobalt and to coat the plating layer with other cobalt salts (eg cobalt acetate, cobalt sulfate, cobalt fluoride, etc.). If it is sprayed on the solidified plating layer, a cobalt oxide film is formed, but the effect of suppressing blackening is poor, it is chemically unstable, and it is easy to elute, and residual unreacted substances and decomposed substances corrode. This is because it becomes a factor and inhibits blackening resistance.

In contrast, cobalt nitrate or cobalt chloride does not have such a problem. For example, when the aqueous solution is a cobalt nitrate solution, NO ₂ is generated by thermal decomposition, and Co ₃ O ₄ , CoO or a spinel-type CoAl ₂ O ₄ single film or two of them are instantly generated.
It becomes a composite film of more than one kind. The coating is deposited on the plating layer surface, since that is integral with the plating layer becomes a barrier of blackening film growth comprising _{Al 2 O 3, Cr 2 O} 3, MgAl 2 O 4 or the like made mainly of ZnO ₂ , Suppress blackening film growth. Further, since this film is integrated with the plating layer as described above, it is not easily removed even if mechanical treatment such as leveler or skin pass is performed after solidification of the plating layer, and it is chemically removed. Since it is stable, it will not dissolve even after chromate treatment and protects the plating layer for a long time even in stock. In this respect, a conventional method for precipitating Co and Ni by displacement plating by treating with an aqueous solution containing Co and Ni ions is disclosed in JP-A-59-17.
This is significantly different from the hot-dip galvanized steel sheet produced by the method of Japanese Patent No. 7381.

It should be noted that there is no problem even if the cobalt oxide film contains a minimized spangle agent such as phosphate or salt which has been conventionally used for uniform minimization of spangle.

Cobalt oxide film amount is 1 to 100m in terms of cobalt metal
It should be g / m ² . When the coating amount is less than 1 mg / m ² , the blackening preventing effect is insufficient, and when it exceeds 100 mg / m ² ,
Because it will be colored.

The cobalt oxide film suppresses blackening even when chromated, so it is possible to form a chromate film on the cobalt oxide film in order to further improve corrosion resistance and coating adhesion. This chromate film is mainly composed of chromic anhydride, and Cr ³⁺ / (Cr ³⁺ + Cr ⁶⁺ ) = 0
It is most desirable to use a chromate treatment solution adjusted to ~ 0.6. The chromate film also has a film modifier (eg
Al ₂ O ₃ , SiO _2, etc.) does not impair the blackening inhibiting effect of the cobalt oxide film.

The present invention will be described below with reference to examples.

(Example) Example 1 A steel sheet was hot-dipped in a plating bath (460 ° C) containing Al 0.17%, Pb 0.30%, Fe 0.03%, the balance Zn and inevitable impurities in a weight percentage, and then gas expansion was performed. The amount of plating adhered by the method
It was adjusted to 0 to 250 g / m ² , and immediately thereafter, the plating layer was rapidly cooled with the aqueous solution and spraying method shown in Table 1 before the plating layer solidified, and at the same time as minimization of spangle, Co, Ni,
An oxide film of Fe was formed to cover the plating layer. After that, a test piece was taken from the hot-dip galvanized steel sheet obtained,
A blackening acceleration test was carried out by immersing in boiling deionized water for 5 hours. Table 1 shows the results of this accelerated test.

The blackening was judged according to the following criteria.

◎ No blackening ○ Very light gray △ △ Black area is less than 30% of total × Black area is more than 30% Example 2 Al 4%, Mg 0.1%, Pb 0.02%, Fe 0.02%, balance by weight%
After hot-dip-plating the steel sheet in a plating bath (470 ° C.) containing Zn and unavoidable impurities, the coating weight was adjusted in the same manner as in Example 1, and immediately after that, the aqueous solution and the spray shown in Table 2 were added before the plating layer solidified. The plating layer was rapidly cooled by the attachment method, and the spangle was minimized and at the same time an oxide film of Co, Ni, and Fe was formed, and the plating layer was coated. Then Example 1
In the same manner, the black discoloration acceleration test was conducted to determine the black discoloration. The results are shown in Table 2. The determination was made on the same basis as in Example 1.

Example 3 Hot-dip galvanizing was carried out in the same manner as in Example 1, and the plating layer was rapidly cooled by the aqueous solution and spraying method shown in Table 3 to minimize the spangle and simultaneously form an oxide film of Co, Ni and Fe. Was generated to cover the plating layer. Then, after the plating layer was solidified, it was immersed in a chromic anhydride aqueous solution of CrO ₃ 20 g / temperature of 40 ° C. for 5 seconds and subjected to chromate treatment to form a chromate film. Next, a test piece was taken from the hot dip galvanized steel sheet on which this chromate film was formed, and the temperature was 50 ± 1.
The test for accelerating blackening was conducted by leaving it in an atmosphere of ℃ and relative humidity of 98% or more for 30 days, and the same criteria as in Example 1 were evaluated. Table 3 shows the results of this accelerated test.

Example 4 Hot-dip galvanizing was performed in the same manner as in Example 2, and the plating layer was rapidly cooled by the aqueous solution and spraying method shown in Table 3 to minimize the spangle and simultaneously form an oxide film of Co, Ni, and Fe. It was generated and covered the plating layer. Then, after the plating layer was solidified, a chromate film was formed and a blackening acceleration test was conducted in the same manner as in Example 3, and the same criteria as in Example 1 were evaluated.
Table 4 shows the results of this accelerated test.

As is clear from Examples 1 to 4, a hot-dip galvanized steel sheet coated with a cobalt oxide film by forming an oxide film of cobalt by thermal decomposition of cobalt nitrate and cobalt chloride and coating the plated layer is resistant to corrosion. Excellent in blackening. Also, the problem of blackening does not occur even if a chromate film is formed.

(Effects) As described above, the hot-dip galvanized steel sheet of the present invention has a cobalt oxide film formed on the surface and is coated with a plating layer. The cobalt oxide film becomes a barrier to blackening growth. Therefore, blackening growth can be suppressed.

Further, since the cobalt oxide film is integrated with the plating layer, it is strong against mechanical processing and can withstand interplate slip that occurs during coil winding and ironing during molding processing.

Furthermore, since the oxide film of the coil is chemically stable, it does not form a local cell with zinc and cause micro corrosion.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Sanae Watanabe 1 at 7 Takatani Shinmachi, Ichikawa City, Chiba Nisshin Steel Co., Ltd. Ichikawa Research Institute (72) Inventor Kazuko Uchida 1 of 7 Takatani Shinmachi, Ichikawa City, Chiba Prefecture Nisshin Steel Co., Ltd. Ichikawa Laboratory (56) Reference JP 59-177381 (JP, A) JP 55-45514 (JP, A) Actual development 56-29680 (JP, U) Actual development Sho 59-193278 (JP, U) Actual development Sho 59-193279 (JP, U) Japanese Patent Sho 53-11260 (JP, B2)

Claims (10)

[Claims] 1. A surface of a hot-dip zinc or zinc alloy-plated steel sheet is treated with an aqueous solution containing one or both of cobalt nitrate and cobalt chloride, and the deposited cobalt salt is thermally decomposed to form a cobalt oxide film. A hot-dip galvanized steel sheet with excellent blackening resistance, which is characterized by being deposited and formed on the surface. 2. A hot-dip galvanized steel sheet excellent in blackening resistance according to claim 1, characterized in that the cobalt oxide film is a CoO film. 3. A hot-dip galvanized steel sheet excellent in blackening resistance according to claim 1, wherein the cobalt oxide film is a Co ₃ O ₄ film. 4. A hot-dip galvanized steel sheet excellent in blackening resistance according to claim 1, wherein the cobalt oxide film is a CoAl ₂ O ₄ film. 5. The hot-dip galvanized steel sheet excellent in blackening resistance according to claim 1, wherein the cobalt oxide film is a composite film of CoO and Co ₃ O ₄ . 6. The cobalt oxide film is a composite film of Co ₃ O ₄ and CoAl ₂ O ₄ , wherein the oxide film is cobalt.
The hot-dip galvanized steel sheet having excellent blackening resistance according to the item. 7. The hot-dip galvanized steel sheet excellent in blackening resistance according to claim 1, wherein the cobalt oxide film is a composite film of CoO and CoAl ₂ O ₄ . 8. A cobalt oxide film comprising CoO, Co ₃ O ₄ and
A hot-dip galvanized steel sheet excellent in blackening resistance according to claim 1, which is a composite coating of CoAl ₂ O ₄ . 9. A cobalt oxide film in terms of cobalt 1.
The hot-dip galvanized steel sheet having excellent blackening resistance according to claim 1, characterized in that the hot-dip galvanized steel sheet is formed to have a thickness of 0 to 100 mg / m ² . 10. A surface of a hot dip zinc or zinc alloy plated steel sheet is treated with an aqueous solution containing one or both of cobalt nitrate and cobalt chloride, and the cobalt salt adhered is thermally decomposed to form a cobalt oxide film. A hot-dip galvanized steel sheet excellent in blackening resistance, which is characterized in that it is deposited and formed on the surface, and a chromate film is formed on the cobalt oxide film.