JP3304808B2 - Chromate treated galvanized or zinc-based alloy-coated steel sheet with excellent blackening resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromate-treated galvanized or zinc-based alloy-plated steel sheet having excellent resistance to blackening and used in applications where the appearance of a household appliance is important.

[0002]

2. Description of the Related Art Galvanized steel sheets subjected to chromate treatment, particularly electrogalvanized steel sheets subjected to reactive chromate treatment, are widely used for chassis and motor covers of household electric appliances, taking advantage of their excellent appearance and corrosion resistance.

[0003] A so-called black discoloration phenomenon in which the surface of a chromate-treated galvanized steel sheet turns black during storage or transportation may occur. The occurrence of the black discoloration phenomenon significantly lowers the commercial value of a steel sheet for household electric appliances in which appearance is particularly important.

The following techniques for improving the blackening resistance of galvanized steel sheets are known. JP-A-60-152680 discloses that the content of Pb ²⁺ which is an impurity in an electrogalvanizing bath is set at 0.
After galvanizing to 2 ppm or less, JP-A-2-8374 discloses that the Pb ²⁺ content of impurities in the electrogalvanizing bath is
And 0.5ppm or less, a Ni ²⁺ containing 100 ~300ppm, Ni ²⁺ / Pb
^It describes a method of improving blackening resistance by performing galvanizing after galvanizing in a galvanizing bath with ²⁺ exceeding 500.

Japanese Patent Application Laid-Open No. 5-279889 discloses the above-mentioned Japanese Patent Application Laid-Open No. 2-79889.
In the method described in No. 8734, when electrogalvanizing is performed in a plating bath to which Ni ²⁺ is added in a plating bath, blackening resistance after chromate treatment is improved, but appearance after phosphate treatment is improved. In consideration of the problem of significant damage, by forming the upper layer of electrogalvanized plating containing 50 to 1000 ppm of cobalt using a plating bath containing 50 to 1000 ppm of cobalt, blackening resistance and phosphorus A method for obtaining an electrogalvanized steel sheet having excellent acid treatment properties is described.

[0006] The blackening phenomenon is considered to be a phenomenon in which zinc on a chromate base is oxidized. To prevent oxidation of zinc, a water-soluble metal salt is sprayed immediately after zinc plating to prevent the blackening phenomenon. A method for performing this is disclosed in, for example, JP-A-7-268592.

[0007]

However, the above technique has the following problems.

In the method described in JP-A-60-152680, it is not easy to always control the Pb ²⁺ content of the plating bath to 1 ppm or less. Particularly in electrogalvanizing using a zinc self-soluble electrode, it is difficult to control the Pb ²⁺ content of the plating bath to 1 ppm or less because it is difficult to eliminate lead contained as an impurity in the zinc electrode. It is. Therefore, the effect of improving blackening resistance cannot be stably obtained.

The method described in Japanese Patent Application Laid-Open No. 2-8374 has the same problems as the method described in Japanese Patent Application Laid-Open No. 60-152680, and also has a problem in phosphatability. Galvanized steel sheets are not always subjected to chromate treatment, but are often used after being subjected to phosphate treatment after galvanization. Considering this point, the method described in this publication using a plating bath having a problem in phosphatability is not an effective measure against blackening.

In the method described in Japanese Patent Application Laid-Open No. 5-279889,
Additional plating equipment for forming an electroplating layer containing cobalt is required, which is disadvantageous in cost.

The method described in Japanese Patent Application Laid-Open No. 7-268592 is disadvantageous in terms of cost because a special treatment facility for spraying a water-soluble metal salt immediately after galvanization is required.

By the way, in the case of a chromate-treated galvanized steel sheet, a reactive type chromate-treated galvanized steel sheet having a different chromium deposition amount is manufactured depending on the purpose and application. It has been empirically known that when producing reactive chromate-treated galvanized steel sheets having different amounts of chromium, the color tone of the steel sheet surface is significantly different due to a slight difference in the amount of chromium adhesion, and color unevenness is likely to occur. For this reason, it is very difficult to control the color tone of the reactive-type chromate-treated galvanized steel sheet, and it is a fact that color unevenness is likely to occur due to a slight difference in the local chromium adhesion amount on the same steel sheet surface.

Color unevenness due to such a slight difference in the local chromium adhesion amount may be regarded as a problem. In order to solve such color unevenness, Japanese Patent Application Laid-Open No. 5-302178 discloses a method for forming a fine protrusion having a height of 50 to 500 nm on the surface of an electrogalvanized steel sheet.
1 × 10 ⁹ 〜1 × 10 ¹² pcs / m ² , with chromium adhesion of 40〜15
A chromate-treated electrogalvanized steel sheet having a color tone stability of 0 mg / m ² and having excellent chroma stability is described.

Since the steel sheet has a convex portion on the surface of the steel sheet,
Although the present invention, which defines the fluctuation state of the chromium deposition amount of the chromate film, is apparently similar in configuration, the present invention is essentially independent of the number of convex portions on the surface. Also,
The publication does not take into account blackening resistance at all. Further, since a large convex portion as large as 500 nm is formed on the surface of the steel plate, the tip of the convex portion is worn intensively during coil winding or processing, and the chromate film or the plating layer is damaged. Therefore, there is a problem that appearance and corrosion resistance are impaired.

The present invention has been made in view of the above problems, and does not require strict regulation of the Pb ²⁺ content of impurities in the plating bath or addition of Ni ²⁺ ions or the like to the plating bath. ,
It is another object of the present invention to provide a chromate-treated galvanized and zinc-based alloy-coated steel sheet having excellent blackening resistance without changing manufacturing equipment such as adding a special processing equipment for spraying a water-soluble metal salt. .

[0016]

In order to solve the above-mentioned problems, the present inventors have developed a blackening of a chromate-treated galvanized steel sheet which has been subjected to electrochromic plating in a normal galvanizing bath and then subjected to reactive chromate treatment. The phenomena were studied diligently. As a result, it was found that the blackening phenomenon was caused by a decrease in brightness due to a light interference phenomenon caused by a chromate film on the zinc plating layer and zinc oxide.

Further, from the above knowledge and understanding of the light interference phenomenon, when the thickness of the chromate film is in an appropriate distribution state,
I think that the blackening phenomenon can be suppressed,
The state of the film thickness distribution of the chromate film was variously changed, and the relationship between the film thickness distribution state and the blackening phenomenon was examined in detail.
As a result, it has been newly found that the blackening resistance of a chromate-treated galvanized or zinc-based alloy-plated steel sheet can be improved when the variation in the amount of chromium adhering to the chromate film is specified in an appropriate range.

Hereinafter, these points will be described in more detail. As described above, the chromate-treated galvanized steel sheet has a problem that the color tone of the surface is apt to fluctuate and a blackening phenomenon is apt to occur. Then, the present inventors investigated in detail the factors that affect the appearance, such as the color tone and black discoloration, of the chromate-treated galvanized steel sheet in order to investigate in detail what causes the color tone fluctuation and blackening phenomenon.

The chromate-treated galvanized steel sheet may be yellowish depending on the thickness of the chromate film and manufacturing conditions. This yellow color was thought to be caused by dd absorption of hexavalent chromium complex in the chromate film and light interference by the chromate film. Generally, hexavalent chromium complex
dd Absorption is 100n in full width around 340nm ultraviolet light
It absorbs light of about m and has a molar absorption coefficient of about 1500 (mol·l ⁻¹ · cm ⁻¹ ) for light having a wavelength of 340 nm. Because of the nature of hexavalent chromium, the color of chromium is often referred to as yellow.

The present inventors quantitatively determined whether or not the yellow color of the chromate-treated galvanized steel sheet was the color of hexavalent chromium. As an example, a reactive type chromate-treated galvanized steel sheet having a yellowish chromium deposition amount of 40 mg / m ² (calculated as chromium, the same applies hereinafter) will be described. According to the reflection spectrum of the spectrophotometer, the steel sheet absorbed about 20% of light having a wavelength of 350 to 400 nm.

The amount of all chromium elements in the chromate film of this steel sheet was measured by X-ray fluorescence analysis, and the proportion of hexavalent chromium in the total amount of chromium was measured by X-ray photoelectron spectroscopy. The abundance of hexavalent chromium atoms in the chromate film was approximately 2-3 mg / m ² in terms of adhesion amount.

Assuming that all of the hexavalent chromium forms a complex that causes dd absorption, the absorptance is calculated by the above molar absorption coefficient, and only about 2% of light having a wavelength of 340 nm is absorbed. However, it is not enough to explain the yellowish color tone of the chromate-treated galvanized steel sheet.

Next, the effect of light interference on the color tone was investigated. A cross-sectional thin film of the steel sheet was prepared by an ultramicrotome, and the thickness of the chromate film observed by TEM was
35 nm. The refractive index of the film measured by an ellipsometer was 2.50, and the film thickness was almost the same as the value measured by TEM. In this case, according to the light interference calculation, the chromate film absorbs about 20% of light having a wavelength of 350 to 400 nm,
It can be easily explained that the appearance of the chromate-treated galvanized steel sheet is yellowish.

Here, a reaction type chromate-treated galvanized steel sheet having a yellowish tint (chromium adhesion amount: 40 mg / m ² ) has been described. However, when the present inventors investigated, the color tone of the chromate-treated galvanized steel sheet was as follows. It has been found that light interference is generally the main cause, not limited to the above-described examples of color tones.

Next, the blackening phenomenon of the chromate-treated galvanized steel sheet, that is, the cause of the decrease in lightness of the steel sheet was investigated.

It is conventionally known that the blackening phenomenon is caused by the growth of zinc oxide at the interface between the chromate film and the zinc plating layer. However, zinc oxide is generally white and basically does not strongly absorb visible light.
Therefore, it is difficult to explain the decrease in brightness, that is, the blackening phenomenon, due to the growth of zinc oxide.

Therefore, the present inventors investigated the cause of the decrease in brightness due to the growth of zinc oxide. As a result, it was clarified that the brightness decreased due to the change in the interference phenomenon caused by the growth of zinc oxide. Hereinafter, this point will be described more specifically.

The chromate-treated galvanized steel sheet having a chromate film having a thickness of 35 nm and a refractive index of 2.50 absorbs light having a wavelength of 350 nm to 400 nm by interference of light. In the light having a wavelength of 350 nm to 400 nm, the phases of the light reflected on the chromate film surface and the light reflected on the galvanized surface are inverted and cancel each other, and a so-called dark line is generated on the reflection spectrum. It has been found that when zinc oxide grows at the interface between the chromate film and the galvanized layer, the dark line moves to the visible light side region, and as a result, the brightness decreases.

Then, a blackening acceleration test was performed on various reactive chromate-treated electrogalvanized steel sheets to grow zinc oxide, and the occurrence of blackening phenomenon, lightness, and reflection spectra before and after the blackening acceleration test were analyzed. Investigation with a photometer revealed the following: (1) For a steel sheet with a marked blackening phenomenon, before the blackening acceleration test,
For a steel sheet that absorbs light with a wavelength of 350 nm to 400 nm and has little blackening phenomenon, the wavelength is 350 nm to 400 nm before the blackening acceleration test.
That the light absorption is small. (2) If the absorption of light having a wavelength of 350 nm to 400 nm is large before the test for promoting blackening, the decrease in lightness in the visible light range becomes large after the test for promoting blackening. Inferior denaturation and a wavelength of 350 nm
If the absorption of light at 400 nm is small, the decrease in brightness in the visible light region after the accelerated blackening test is small, that is, the blackening phenomenon is unlikely to occur and the blackening resistance is excellent. (3) The degree of absorption of light having a wavelength of 350 nm to 400 nm before the accelerated blackening test is affected by the variation in the thickness of the chromate film.

The relationship between the thickness distribution of the chromate film and the change in brightness before and after the blackening acceleration test was examined in detail. As a result, it has been found that when the average value A of the variation in the chromium deposition amount is defined to be 3.0% or more and 8.0% or less of the average chromium deposition amount Cr _mean , the required blackening resistance aimed at by the present invention can be secured.

The present invention is based on such knowledge, and a feature of the present invention is a chromate-treated galvanized or zinc-based alloy-plated steel sheet, which is formed on any straight line on the steel sheet. is excellent chromated galvanized or zinc alloy coated steel sheet blackening resistance, wherein the average value a of the variation in the chromium coating weight is less than 8.0% 3.0% or more of the average chromium coating weight Cr _{me an,} the .

However,

[0033]

(Equation 2)

Here, Cr (r) is a local chromium adhesion amount at the point r. Further, the integration range of the above equation is the analysis area.

Hereinafter, the chromate-treated galvanized steel sheet of the present invention will be described in detail. The variation in the thickness of the chromate film of the chromate-treated galvanized steel sheet can be expressed by the variation in the amount of chromium attached to the film. It is necessary to set the average value A of the variation of the chromium deposition amount to be 3.0% or more and 8.0% or less of the average chromium deposition amount Cr _mean .

When the average value A of the variation in the chromium deposition amount is 3.0 % or more of the average chromium deposition amount Creme, absorption of light having a wavelength of 350 nm to 400 nm in the reflection spectrum of the spectrophotometer is small, and during storage and transportation. Even when the zinc oxide film grows, or when the zinc oxide is grown for 3 hours in an environment where blackening is likely to occur, for example, in a high humidity environment of 80 ° C. and 95% humidity, the spectrophotometer There is almost no change in the reflection spectrum obtained by the meter, and the decrease in brightness in the visible light range is small, the occurrence of blackening phenomenon is suppressed, and the blackening resistance is excellent.

When the variation A of the chromium deposition amount is less than 3.0% of the average chromium deposition amount Cr _mean , the absorption of light having a wavelength of 350 to 400 nm in the reflection spectrum of the spectrophotometer is large,
When the zinc oxide film grows, the decrease in lightness in the visible light region becomes large, and the blackening resistance targeted by the present invention cannot be obtained.

If the variation A of the chromium deposition amount exceeds 8.0% of the average chromium deposition amount Cr _mean , the effect of suppressing the blackening phenomenon is not impaired, but the chromate film becomes non-uniform and the chromate is locally localized. Since the film becomes very thin, the corrosion resistance decreases. For this reason, the variation A of the chromium deposit is limited to 8.0% or less of the average chromium deposit Cr _mean .

In the present invention, the strict control of the Pb ²⁺ content in the zinc plating bath and the addition of a new component such as Ni ²⁺ ions are not required, and the water-soluble metal salt is sprayed immediately after the plating. No special processing equipment is required. Therefore, a chromate-treated galvanized or zinc-based alloy-plated steel sheet having excellent blackening resistance can be manufactured at low cost. In addition, when the chromate treatment is not performed, the phosphatability is not impaired.

Although the above description is for the case of zinc plating, similar effects can be obtained in the case of zinc-based alloy plating.

[0041]

BEST MODE FOR CARRYING OUT THE INVENTION The galvanized or zinc-based alloy-plated steel sheet used in the present invention is a steel sheet which has been subjected to zinc plating or a zinc-based alloy plating to form a zinc-plated layer or a zinc-based alloy-plated layer. I just need. The method for forming the plating layer is not particularly limited, and examples thereof include a hot-dip plating method and an electroplating method. Regarding the chromate treatment method, any method can be used as long as the average value A of the variation of the chromium adhesion amount after the chromate treatment can be set to 3.0% or more and 8.0% or less of the average chromium adhesion amount Cr _mean. Good.

[0042]

Next, the present invention will be described in more detail with reference to examples.

An electrogalvanized steel sheet having a thickness of 0.8 mm and a zinc adhesion amount of 40 g / m ² is degreased and washed with a weak alkali degreasing agent, washed with water, blow-dried, then sprayed with a reactive type chromate treatment solution, washed with water and dried. A reaction type chromate film in which the thickness distribution of the chromate film was changed was formed on the surface of the plated steel sheet (Examples 1 and 5 of the present invention and Comparative Examples 6, 7, and 8).

The chromate treatment conditions were selected so as to change the quality of the chromate film, particularly the thickness distribution of the chromate film. Specifically, the total amount of Cr ions (the sum of Cr ³⁺ and Cr ⁶⁺ ) of the chromate treatment solution is 5000 ppm, and the total amount of zinc ions is
Chromium ion reduction rate (Cr ³⁺ / Total Cr
(Ion) and free acidity were adjusted to the values shown in Table 1 to control the film thickness distribution by adjusting the amounts of fluorine ions, nitrate ions, and sulfate ions. Fluorine ion amount up to 2000ppm
, The maximum amount of nitrate ion is 3000ppm, and the maximum amount of hydrochloric acid ion is 3
000 ppm and sulfate ion were adjusted up to 1000 ppm.

Further, as shown in FIG. 4, some of the test materials were made of a galvanized steel sheet 1 before being subjected to a chromate treatment.
Then, a 75 μm sieve 2 of JIS Z8801 was attached thereto, immersed in the above-mentioned chromate treatment liquid 4 and subjected to the first-stage chromate treatment (immersion time: 1 second), and then the sieve was peeled off. The treatment liquid was sprayed, washed with water and dried, and subjected to a second-stage chromate treatment to control the thickness distribution of the chromate film (Examples 2 to 4 of the present invention).

In order to evaluate the blackening resistance of the galvanized steel sheet subjected to the chromate treatment, a blackening acceleration test was performed. The accelerated blackening test was performed in a high-temperature, high-humidity atmosphere of 80 ° C x 95% humidity.
The test was performed for 3 hours, and the lightness (L value) before and after the test was measured. The L value was measured by a method specified in JIS Z8729 using a handy color tester manufactured by Suga Test Instruments Co., Ltd.

The blackening resistance was evaluated by the change in L value before and after the blackening acceleration test (L value after test-L value before test) ΔL. ΔL in this test is a negative value. It can be said that the smaller the absolute value of ΔL, the better the blackening resistance, and the larger the absolute value, the poorer the blackening resistance.

When the absolute value of ΔL was 4.0 or more, the appearance was clearly deteriorated visually, and the blackening resistance was judged to be unacceptable.

Further, in order to evaluate the thickness distribution of the chromate film, a line analysis of chromium was performed using EPMA (JXA8600MX) manufactured by JEOL Ltd. The measurement conditions of the line analysis are as follows: acceleration voltage 15k
V, current 1 × 10 ^-6 A, acquisition time 20 seconds, sample feed 2
The analysis length was 5 μm and the analysis length was 2.0 mm. PET for EPMA spectral crystals
Was used.

For the chromium line analysis, the chromium K line
Of the chromium, which is about 0.15
A line analysis at the background position was also performed. this
As a result, in all samples,
EPMA intensity is almost constant for the analysis area.
Was. In the analysis, the chromium spot at any point in the analysis interval
Subtract background EPMA intensity from peak EPMA intensity
This was defined as the net intensity ICr (r) of chromium. This chrome
The average value of the net intensity ICr (r) of _meanWhen
The average value of the variation of ICr (r) is defined as A 'as follows:
Justified.

[0051]

(Equation 3)

However, the integration section is an analysis area. In this case, the integration interval is a straight line with a length of 2.0 mm,

[0053]

(Equation 4)

Is as follows.

When the surface analysis is performed and the integral interval is calculated in two dimensions, the average intensity ICr of the EPMA is the same as in the present embodiment.
_{The mean} and the average value A 'of the variation are derived. Such surface analysis is also included in the analysis on any straight line defined by the present invention, and there is no essential difference between the two.

The local chromium adhesion amount Cr (r) and the average chromium adhesion amount of the steel sheet measured by the above method and the like are in a proportional relationship with the respective ICr (r) and ICr _mean . Therefore, by calculating the proportional coefficient, the IC _mean and Cr _mean ,
A can be obtained from A '.

Further, the average value A of the variation of the chromium adhesion amount
The ratio A / Cr _mean of relative to the average chromium coating weight Cr _mean, A '/ I
Since it is equal to Cr _mean , the index R value of the following formula was introduced. R value = A / Cr _mean = A '/ ICr _mean (1) The line analysis is performed on any 10 straight lines on each steel plate under the above conditions, and the R value is calculated by the above procedure. The average value was used as a representative value of the R value of each steel sheet.

Table 1 shows the chromate treatment conditions, the amount of deposited chromium, the R value obtained from the above equation (1), and the change ΔL of the L value before and after the blackening acceleration test.

[0059]

[Table 1]

FIG. 1 shows the relationship between the R value and ΔL in Table 1. As is clear from Table 1 and FIG. 1, the R value falls within the range of the present invention.
In the present invention examples of 0% or more and 8.0% or less, the absolute value of ΔL is smaller than 4.0 and the blackening resistance is excellent, and the R value is out of the range of the present invention. It turns out that it is inferior to blackening.

FIG. 2 shows the local chromium K line of Example 3 of the present invention.
The EPMA intensity ICr (r) is shown in FIG.
4 shows a measurement example of EPMA intensity ICr (r) of a line. 2 and 3, the average intensity ICr (r) of chromium was 50% for both Example 3 and Comparative Example 6.
It turns out that it is almost the same at about 10 counts. this is,
This is because the amount of chromium adhering in the inventive example 3 and the comparative example 6 is almost equal. The distribution of the EPMA intensity ICr (r) of the chromium K-ray was 2
In Comparative Example 6, the variation is small. In Example 3 of the present invention, the variation in the amount of deposited chromium was within the range of the present invention, whereas in Comparative Example 6, the variation was out of the range of the present invention.

Although the present embodiment is directed to the case of a galvanized steel sheet, similar results can be obtained in the case of a zinc-based alloy-coated steel sheet.

The main point of the present invention is to suppress blackening by producing a chromate-treated galvanized or zinc-based alloy-plated steel sheet having a chromate film thickness distribution as in Example 3 of the present invention.

[0064]

According to the present invention, it is possible to obtain a chromate-treated galvanized or zinc-based alloy-plated steel sheet in which the occurrence of a blackening phenomenon, which tends to occur during stocking or transportation, is reduced. This chromate-treated galvanized or zinc-based alloy-plated steel sheet is particularly suitable for use in household electric appliances, which emphasizes appearance.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an R value and a ΔL value.

FIG. 2 is a view showing an EMPA intensity profile of a chromium K line of Example 3 of the present invention.

FIG. 3 is a diagram showing an EMPA intensity profile of a chromium K line of Comparative Example 6.

FIG. 4 is a diagram showing a procedure for performing a chromate treatment using a sieve.

[Explanation of symbols]

 Reference Signs List 1 electrogalvanized steel sheet 2 sieve 3 chromate treatment tank 4 chromate treatment liquid

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yoshiharu Sugimoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-7-26385 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) C23C 22/00-22/86 C23C 28/00 C25D 5/26

Claims (1)

(57) [Claims] 1. A chromate-treated galvanized or zinc-based alloy-coated steel sheet, wherein the average value A of the variation of the chromium adhesion amount on an arbitrary straight line on the steel sheet is at least 3.0% of the average chromium adhesion amount Cr _mean. A chromate-treated galvanized or zinc-based alloy-coated steel sheet having an excellent blackening resistance of 8.0% or less. Where: Here, Cr (r) is a local chromium deposition amount at the point r.