JP2727861B2 - Electrogalvanized steel sheet with excellent blackening resistance and good appearance after chemical conversion treatment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to prevent the blackening phenomenon which occurs in a galvanized steel sheet subjected to a chromate treatment, and to provide a white surface tone after a chemical conversion treatment such as a phosphate treatment and a blackening resistance. Electro zinc which is excellent and has good appearance after chemical conversion treatment
It is related to a steel plate .

[0002]

2. Description of the Related Art When an electrogalvanized steel sheet is used without painting, a chromate film is generally formed on the electrogalvanized layer by a chromate treatment. The formation of such a chromate film significantly improves the corrosion resistance of the electrogalvanized steel sheet. However, during storage in a hot and humid environment, the chromate-treated electrogalvanized steel sheet may undergo a so-called blackening phenomenon in which part or all of its surface turns gray-black or brownish-brown, resulting in a commercial value. It decreases significantly. Although the cause of the blackening phenomenon of the electrogalvanized steel sheet is not necessarily clear, it is thought to be due to corrosion of the electrogalvanized layer surface, and in particular, lead (Pb) contained in the electrogalvanized layer. ) Is said to promote the occurrence of blackening.

The following method is known as a method for preventing blackening of an electrogalvanized steel sheet. JP-A No. 60-152680 By subjecting a steel sheet to electrogalvanization with an electrogalvanizing bath having a Pb ²⁺ content of 0.2 ppm or less as an impurity, the blackening resistance of the electrogalvanized steel sheet is improved ( Hereinafter, it is referred to as prior art 1).

Japanese Patent Application Laid-Open No. 60-77988 An electric power containing 5 to 500 times the content of Pb ²⁺ as an impurity and containing not more than 1/25 of the amount of zinc ions and not more than 10 g / l of nickel ions. The steel sheet is electrogalvanized by a galvanizing bath and then subjected to chromate treatment to improve the blackening resistance of the electrogalvanized steel sheet (hereinafter referred to as Prior Art 2).

[0005] Japanese Patent Publication No. 2-8374 contains 100 to 300 ppm of Ni ²⁺ , the content of Pb ²⁺ as an impurity is 0.5 ppm or less, and N
The steel sheet is subjected to electrogalvanizing with an electrogalvanizing bath having i ²⁺ / Pb ²⁺ of more than 500, and then subjected to chromate treatment to improve the blackening resistance of the electrogalvanized steel sheet (hereinafter referred to as Prior Art 3). ).

On the other hand, the chromate-treated electrogalvanized steel sheet is often used as a material for products that can be seen from the outside, such as household electrical products, so that its appearance is regarded as extremely important, and the white surface tone is high. Is required.

The appearance of an electrogalvanized steel sheet subjected to a chromate treatment is greatly affected by the appearance of an as-plated steel sheet. That is, the surface tone of the electrogalvanized layer of the as-plated steel sheet is whiter, and the better the surface gloss is, the more beautiful the appearance after the chromate treatment is. The appearance of the electrogalvanized layer is easily affected by impurities in the plating bath. In particular, nickel ions added to the plating bath to improve blackening resistance deteriorate the appearance of the electrogalvanized layer even if the addition amount is very small.

[0008] In general, when an electrogalvanized steel sheet is used by painting, it is generally applied after a chemical conversion treatment such as a phosphate treatment in order to improve the adhesion of the coating film. The film thickness of the coating film is often reduced to reduce the manufacturing cost. In the case where the film thickness is small, the whiter the surface color after the chemical conversion treatment, the more beautiful the coating appearance.

The following method is known as a method for improving the surface color of a galvanized steel sheet subjected to a phosphate treatment. JP-A-63-4099 The presence of 300 ppm of Ni ²⁺ in a pure galvanizing bath reduces the whiteness of a pure galvanized steel sheet after phosphate treatment to 50 or less. Of Ni ²⁺ (hereinafter referred to as Prior Art 4)
).

[0010] The surface tone of the electrogalvanized steel sheet after phosphate treatment is kept white by controlling Ni ²⁺ in the zinc plating bath to 10 ppm or less (hereinafter referred to as Prior Art 5). .

[0011]

Prior art 1 has the following problems. That is, the lead content in the plating layer greatly changes depending on the plating conditions such as the composition of the plating bath, the pH value, the temperature, the flow state, and the plating current density.
Therefore, even if the content of Pb ²⁺ in the plating bath is reduced to 0.2 ppm or less, black discoloration may occur. Therefore, according to Prior Art 1, black discoloration cannot be completely prevented.

Further, the content of Pb ²⁺ in the plating bath is adjusted to 0.1.
In order to make it 2 ppm or less, a large amount of strontium carbonate must be added to the plating bath. Therefore, equipment such as a strontium carbonate addition device and a replacement reaction tank, and a device for separating the removed product are required, and a large amount of relatively expensive strontium carbonate is consumed. Therefore, the manufacturing cost of the galvanized steel sheet increases.

Prior arts 2 and 3 have the following problems. That is, the addition of nickel ions into the plating bath improves the blackening resistance of the galvanized steel sheet,
On the other hand, the nickel present in the plating layer deteriorates the appearance of the electrogalvanized steel sheet after the chemical conversion treatment, particularly the phosphate treatment, and significantly impairs the commercial value of the product.

Prior arts 4 and 5 have the following problems. That is, although the appearance of the electrogalvanized steel sheet after the phosphate treatment can be prevented from being deteriorated, its blackening resistance cannot be improved.

Accordingly, an object of the present invention is to solve the above-mentioned problems, to produce no black discoloration under any conditions, and not to deteriorate the appearance after chemical conversion treatment, and to be excellent in black discoloration resistance. Electrogalvanized steel sheet with good appearance after chemical conversion treatment
Is to provide .

[0016]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors first examined the effect of the lead content in the electrogalvanized layer on blackening. As a result, the blackening that occurs in the electrogalvanized steel sheet depends on the lead content in the electrogalvanized layer. Therefore, if the lead content in the electrogalvanized layer is regulated to 10 ppm or less, it can be manufactured under any conditions. However, it was found that no blackening occurred.

[0017] However, the lead content of the electro-galvanized layer, in order to regulate the 10ppm or less, it is necessary to manage the Pb ²⁺ content in the plating bath at a low concentration, Pb ²⁺ content in the plating bath Although the concentration of lead changes depending on the composition of the plating bath and the plating conditions, the lead content in the electrogalvanized layer is always restricted to 10 ppm or less in the continuous electroplating line even if the concentration is controlled to a low concentration. It is difficult to do.

Therefore, an electrogalvanized steel sheet which does not cause black discoloration under any conditions, has a good appearance after chemical conversion treatment, has excellent blackening resistance, and has a good appearance after chemical conversion treatment. We worked diligently to develop the manufacturing method. As a result, the electric zinc
And then a predetermined amount of Co ²⁺ is added.
Electrogalvanized using an electrogalvanized bath
Over the electrogalvanized layer as a predetermined amount as the upper layer
If an electrogalvanized layer containing cobalt is formed ,
No blackening occurs under any conditions, and
It has been found that an electrogalvanized steel sheet having a good appearance after the chemical conversion treatment can be obtained.

[0019] This invention was made based on the above findings, electro-galvanized steel sheet of the present invention, the steel sheet
As an underlayer formed on at least one surface of
Electrogalvanized layer and electrogalvanized as said lower layer
Of less than 50 to less than 1000 ppm formed on the layer
0.5g / m ² or more plating amount containing cobalt
And an electrogalvanized layer as an upper layer .

[0020]

In the present invention, at least one of the steel sheets
Electrogalvanized layer as a lower layer formed on the surface
An electrogalvanizing bath containing a predetermined amount of Co ²⁺
Electrogalvanized, containing a certain amount of cobalt
It is necessary to form an electrogalvanized layer as an upper layer . Thus, using an electrogalvanizing bath containing a predetermined amount of Co ²⁺ , an electrogalvanizing bath as a lower layer is used.
By electric galvanizing Tsu Ki layer is formed steel, cobalt is contained in the upper layer plating layer, further, the results of lead content of the upper <br/> plating layer is reduced, the occurrence of blackening and is prevented from both excellent corrosion resistance is exhibited. Although the reason why blackening is prevented by the cobalt in the plating layer is not clear, it is considered that the cobalt in the plating layer suppresses the oxidation of zinc on the plating surface.

FIG. 1 shows the Co ²⁺ content in the plating bath when Co ²⁺ was added to the electrogalvanizing bath containing 0.7 ppm of lead while changing the amount thereof, and by the plating bath, It is a graph which shows the relationship with the lead content in the electrogalvanized layer formed on the surface of the steel plate. As is apparent from FIG. 1, as the Co ²⁺ content in the plating bath increases, the lead content in the electrogalvanized layer decreases, and blackening is prevented. And, even if the electrogalvanized layer contains 10 ppm or more of lead, 5%
If 0 ppm or more of cobalt is contained, the occurrence of blackening is suppressed by cobalt.

As described above, N is contained in the electrogalvanizing bath.
Even when i ²⁺ is contained, precipitation of lead in the plating layer can be suppressed, but nickel is more easily precipitated in the plating layer than cobalt. Therefore, even if the content of Ni ^{2+ in} the plating bath is small, the appearance of the plating layer and the surface tone after the chemical conversion treatment become dark, and the appearance deteriorates.

On the other hand, cobalt is less likely to precipitate in the plating layer than nickel. Therefore, even if cobalt is added to the plating bath in an amount of less than 5000 ppm, the appearance of the plating layer and the surface tone after chemical conversion treatment such as phosphate treatment are not darkened, and the appearance is not deteriorated. For example, amount of cobalt deposited on the plating layer in the case of electro-galvanized using electro-galvanized bath containing Co ^2+, use electrolytic zinc plating bath containing a Ni ²⁺ of Co ²⁺ in the same amount It is about 1/10 to 1/2 of the amount of nickel precipitated in the plating layer when electrogalvanizing.

FIG. 2 shows the appearance of an electrogalvanized steel sheet on which an electrogalvanized layer containing cobalt or nickel, which had been subjected to a phosphate treatment, was evaluated using a lightness index L value in a Lab color system. 6 is a graph showing a relationship between the content of cobalt or nickel in a plating layer and a lightness index L value. As is clear from FIG.
Even when the same amount of nickel or cobalt is contained, cobalt does not adversely affect the appearance after phosphating.

In the electrogalvanized steel sheet of the present invention,
It is necessary that the cobalt content in the electrogalvanized layer as the upper layer formed on the lower layer be limited to the range of 50 to less than 1000 ppm. If the cobalt content in the upper plating layer is less than 50 ppm, the effect of preventing blackening cannot be obtained. On the other hand, when the cobalt content in the upper plating layer is 1
When the content is 000 ppm or more, the appearance of the electrogalvanized steel sheet after the chromate treatment or the chemical conversion treatment as it is is easily deteriorated.

Further, in the electrogalvanized steel sheet of the present invention, the amount of plating of the upper electrogalvanized layer is 0.5 g /
m ² or more. If the plating amount of the upper plating layer is less than 0.5 g / m ² , the lower electrogalvanizing layer cannot sufficiently cover the lower electrogalvanizing layer, and the effect of preventing blackening will be insufficient.

The plating amount of the lower electrogalvanized layer is not particularly limited, but is preferably in the range of 5 to 40 g / m ² . The plating amount of the lower electrogalvanized layer is 5 g
If it is less than / m ² , the corrosion resistance may be deteriorated. On the other hand, when the plating amount of the lower electrogalvanized layer exceeds 40 g / m ² , the press formability may be deteriorated.

In the present invention , in order to keep the cobalt content in the upper plating layer within the range of 50 to less than 1000 ppm, it is necessary to use an electrogalvanizing bath containing at least one of zinc sulfate and zinc chloride as a main component. , 50-5000
it is necessary to contain ^{Co 2+} in an amount ranging from less than ppm. Cobalt exhibits an abnormal deposition type electrodeposition morphology, and easily deposits in the plating layer at a high current density and a low plating flow rate. Considering the production conditions of the continuous electroplating line, the cobalt content in the upper plating layer is set to 50 to 1
To the range of less than 000ppm are the ^{Co 2+} content of electrolytic zinc plating bath, 50 to 5000 ppm Not
It is necessary in the range of full. If the Co ²⁺ content is less than 50 ppm or 5000 ppm or more , cobalt in the above range cannot be contained in the upper plating layer.

[0029]

Next, the present invention will be described based on embodiments. Cold-rolled steel sheet is degreased and pickled in the usual manner, then electrogalvanized to form a lower layer on the steel sheet surface.
The electro-galvanized layer is formed of Te, then the then the lower layer
On the electrogalvanized layer, electrogalvanizing is performed under the following conditions to form an electrogalvanized layer containing cobalt as an upper layer, and then the following (A), (B) and (C) Chromate treatment is performed under any of the conditions described above to form a chromate film on the electrogalvanized layer as the upper layer, or phosphate treatment is performed under the conditions described below, and the electrogalvanized layer as the upper layer is formed. A phosphate coating was formed on the substrate. Thus, specimens of the electrogalvanized steel sheet of the present invention shown in Table 1 (hereinafter, referred to as the present invention specimens)
o. 1 to 9 was prepared.

Electrogalvanizing conditions: Composition of plating bath: zinc sulfate: 400 g / l Sodium sulfate: 50 g / l pH value of plating bath: 1.5 Temperature of plating bath: 40-60 ° C. Flow rate of plating bath: 1 5m / s plating current density: 50-200A / dm²  Plating amount: 20 g / m²

Chromate treatment conditions: (A) Silica-containing coating type chromate treatment Chromium adhesion amount: 40 to 60 mg / m²  SiO₂/ Cr: 3-4 (B) After the reactive chromate treatment, a thin film organic resin coating treatment Chromium adhesion amount: 20-25 mg / m²  Resin film thickness: 1.5-2 μm (C) Electrolytic chromate treatment Chromium adhesion amount: 20-25 mg / m²  Nickel adhesion amount: 25 to 45 mg / m²  Silica adhesion amount: 30 to 50 mg / m²

Phosphate treatment conditions: (A) Spray-type zinc phosphate treatment Phosphate coating adhesion amount: 1.5 to 2.5 g / m ²

For comparison, a test piece of an electrogalvanized steel sheet having an electrogalvanized layer outside the scope of the present invention (hereinafter referred to as a comparative test piece) No. 1
~ 15 were prepared.

[0034]

[Table 1]

With respect to each of the specimen of the present invention and the comparative specimen in which a chromate film was formed on the electrogalvanized layer, the resistance to blackening was examined by a performance test described below. The appearance after phosphate treatment was examined by the performance test described below for each of the present invention specimen and the comparative specimen on which the phosphate film was formed, and the examination results are shown in Table 2 . .

Blacking resistance test: A plurality of each of the test sample of the present invention and the comparative test sample were stacked, and the stacked test samples were subjected to a temperature of 50 ° C.
A RH 95% wet test was performed for 60 days. like this,
JIS Z 87 of each specimen before and after the wet test
The change amount (ΔL) of the lightness index L value of the Lab color system defined in No. 30 was determined, and the change amount (ΔL) obtained and visually observed were evaluated as follows. ：: ΔL ≧ -2 (no black discoloration) ×: ΔL <-2 (discolored)

Appearance test after phosphate treatment: Each of the test sample of the present invention and the test sample for comparison, which were untreated and oiled,
After degreasing, spray-type phosphate treatment was performed, and using a multi-light source spectrophotometer MSC-1S-2B manufactured by Suga Test Instruments, L
The appearance was evaluated as follows by the lightness index L value of the ab color system. ：: L value ≧ 60 ×: L value <60

[0038]

[Table 2]

As is clear from Tables 1 and 2 , the comparative sample No. having a single electrogalvanized layer and containing no cobalt in the scope of the present invention. In Nos. 1 to 4 , blacking resistance was poor.

[0040] The comparative sample No. 1 containing nickel instead of cobalt in the electrogalvanized layer. 5 to 7
In Examples, the appearance after blackening resistance or phosphate treatment, or both, were poor.

Even when the electrogalvanized layer is a multilayer and the upper plating layer contains cobalt, the amount of plating of the upper plating layer is less than the range of the present invention and is comparatively small. In Nos. 8 to 11 , the blackening resistance was poor depending on the chromate treatment method. Electrogalvanized layer is a multilayer, even if cobalt is contained in the upper plating layer,
The comparative sample No. having a small cobalt content outside the range of the present invention. In Comparative Sample Nos. 12 and 13 , the blackening resistance was inferior, and the cobalt content was large outside the range of the present invention. In Nos . 14 and 15 , the appearance after the phosphate treatment was inferior.

On the other hand, the specimen No. 1-9
In any case, the blackening resistance and the appearance after the phosphate treatment were excellent irrespective of the chromate treatment conditions.

[0043]

As described above, according to the present invention, blackening does not occur even when manufactured under chromate treatment conditions or any other conditions, and the appearance after chemical conversion treatment is not deteriorated. Excellent appearance and good appearance after chemical conversion treatment
An industrially useful effect is obtained in which an excellent electrogalvanized steel sheet can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a cobalt content in an electrogalvanizing bath and a cobalt content in an electrogalvanizing layer.

FIG. 2 is a graph showing a relationship between a content of cobalt or nickel in an electrogalvanized layer and a lightness index L value.

Claims (1)

(57) [Claims] 1. The method according to claim 1, wherein the at least one surface is formed on a steel sheet.
Electrogalvanized layer as a lower layer, and
50-100 formed on all electrogalvanized layers
0.5 g / m, containing less than 0 ppm cobalt
An electrogalvanized layer as an upper layer with a plating amount of ² or more
Excellent blackening resistance, chemical conversion treatment
Electrogalvanized steel sheet with good appearance .