JPH05302178A - Chromate treated electrogalvanized steel sheet excellent in hue stability - Google Patents

Abstract

PURPOSE:To improve hue stability by forming a chromate treated layer having a prescribed fine projecting part on the surface of an electrogalvanized steel sheet to prevent the interference of the chromate layer. CONSTITUTION:The electrogalvanized steel sheet is chromate treated by immersing into a solution containing e.g. 1-20g/l 6-valent Cr ion. And the surface is covered with the chromate treated layer having the projecting part 50-500nm in height and of 1X10<9>-1X10<12> number/m<2> by 40-150mg/m<2> in terms of Cr. The hue is stabilized by preventing the interference between light, which passes through the chromate layer and is reflected by a plating surface, and light, which is reflected by the surface of the chromate layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reactive chromate-treated electrogalvanized steel sheet mainly used for home appliances.

[0002]

2. Description of the Related Art Electrochromic galvanized steel sheets that have undergone a reactive chromate treatment are widely used for home electric appliances such as chassis and motor covers because of their excellent corrosion resistance and lubricity during processing. The electrochromic galvanized steel sheet treated with the reaction type chromate contained hexavalent Cr ions after immersing the steel sheet in a solution containing Zn ions and depositing metal zinc by applying a negative charge. By contacting with a reactive chromate solution, the metal zinc plating layer is dissolved, and at the same time, chromium chromate composed of reduced and generated trivalent Cr ions and hexavalent Cr ions is precipitated in a gel form. It is manufactured by forming a chromate film on the plating. Here, since the corrosion resistance is exerted in proportion to the amount of the chromate film deposited on the plating layer, reactive chromate-treated electrogalvanized steel sheets with various amounts of Cr deposited are manufactured depending on the purpose and application.

However, in the case of producing such reactive-type chromate-treated electrogalvanized steel sheets with different Cr deposits, the problem that the color tone of the steel sheet surface is remarkably different due to a slight difference in the Cr deposits (edited by Japan Metal Surface Technology Association). Metal Surface Technology Handbook "51-11-30, Nikkan Kogyo Shimbun, p.756)
Had essentially. Therefore, the color tone of the reactive chromate-treated electrogalvanized steel sheet is extremely unstable, and at the same time, a slight difference in the amount of Cr deposited on the steel sheet causes a remarkable color unevenness. Especially Cr content is 70mg / m ² to 100
In the range of mg / m ^2, the surface of the steel sheet may have a bluish interference color, and oil stains due to pressing and the like were noticeable, impairing the appearance.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object thereof is to provide a reactive chromate-treated electrogalvanized steel sheet having a stable color tone.

[0005]

According to the present invention, 1 × 10 5 microscopic protrusions having a height of 50 to 500 nm are formed on the surface of an electrogalvanized steel sheet.
A chromate-treated electrogalvanized steel sheet having excellent color stability, which is characterized by coating a chromate-treated layer having ⁹ to 1 × 10 ¹² pieces / m ^{2 in} terms of chromium in an amount of 40 to 150 mg / m ² .

[0006]

In general, the chromate coating layer itself mainly selectively absorbs blue color, so that the object color is yellow, which is its complementary color. Therefore, as the adhesion amount increases, the steel sheet also becomes yellow. However, when the usual chromate treatment is applied, the Cr content is 40 mg.
/ M ² to 150 mg / m ² range, especially 70 mg / m ² to 120
In the range of mg / m ² , the steel sheet surface may exhibit a bluish interference color. This is because the chromate layer on the plating layer exists as a uniform and parallel thin film, so that a so-called equiclination interference phenomenon occurs. That is, the light incident on the surface of the steel sheet is transmitted through the chromate layer, is reflected by the plating surface, and is again transmitted through the chromate layer.
It is reflected as light in which the components reflected on the surface of the chromate layer are superposed, and exhibits a bluish color. Therefore, in order to stabilize the color tone, it is necessary to prevent this interference action.

As a result of earnest research on this interference color prevention measure, the present inventors have found that this problem can be solved by forming a predetermined convex portion on a chromate-treated electroplated steel sheet to cause irregular reflection. That is, any surface of the galvanized steel sheet after chromate treatment has 1 × 10 ⁹ to 1 × 10 ¹² ridges / m ^{2 with a} height of 50 to 500 nm.
By having this, a stable yellowish color tone is obtained. Here, the projections of less than 50 nm are not enough to form an interference thin film and prevent the interference effect of the chromate layer, and the projections of more than 500 nm have insufficient coverage of the chromate layer, leading to deterioration of corrosion resistance. To do.

If the number of convex portions is less than 1 × 10 ⁹ pieces / m ² , it is not enough to form an interference thin film and prevent the interference effect of the chromate layer, which is 1 × 10 ¹² pieces / m 2. ^When it is more than ² , the appearance becomes black, which is rather detrimental to the appearance. The amount of the chromate layer deposited is 40 to 150 mg / m ^{2 in} terms of chromium, which is subjected to chromate treatment. Cr amount 40 mg
If it is less than / m ² , the coating is not sufficient and the corrosion resistance is poor, and if it exceeds 150 mg / m ² , the corrosion resistance is not further improved and it is not economical, and it has a striking golden color and has an appearance. Spoil.

Next, a method for manufacturing the plated steel sheet of the present invention will be described. A thin steel sheet is used as the starting material of the present invention. After heat treatment for obtaining the required material, electrogalvanization is performed.
Electrogalvanizing is generally performed in a plating bath used for industrial production. That is, it is composed of an additive that contains Zn ions and is added as needed for the purpose of improving conductivity and imparting gloss. The counter ion of Zn ion may be a commonly used sulfate ion or chlorine ion, or a bath in which both are mixed may be used.

As the plating tank, either carousel plating, which is one-sided sequential plating, horizontal cell, which is simultaneous plating on both sides, or vertical cell may be used. In this way, the electrogalvanized steel sheet is washed with water and then subjected to reactive chromate treatment. The reactive chromate treatment method may be either immersion treatment or spray treatment, or both may be used in combination.

As the chromate treatment bath, a solution containing 1 to 20 g / l of hexavalent Cr ions is used. Hexavalent Cr ions are the basic components that supply chromium ions by a reduction reaction to the chromate film formation. When the amount is less than 1 g / l, the chromate film is not sufficiently formed, and when it exceeds 20 g / l, it is in the treatment liquid. Precipitation is likely to occur, which is not preferable in operation.

Examples of the anion include chlorine ion, fluorine ion, nitrate ion and sulfate ion, and these can be used alone or in combination. In order to form the above-mentioned convex portions on the surface of the chromate-treated layer, for example, a method of containing 10 g / l or more of Zn ²⁺ ions in the chromate-treated liquid can be used. However, the present invention is not limited to this.

[0013]

EXAMPLES Examples of the present invention will be described below. A cold-rolled steel plate SPCC having a plate thickness of 0.7 mm was used as a material. The electrogalvanization was performed in the following plating bath using a fluidized bed cell with platinum as a counter electrode. Plating bath composition ZnSO ₄ 7H ₂ O 430 g / l K ₂ SO ₄ 30 g / l Na ₂ SO ₄ 30 g / l Plating bath pH 1.5 (adjusted with H ₂ SO ₄ ) Bath temperature 60 ° C

The steps are electrolytic degreasing → 5% sulfuric acid pickling → plating → water washing → drying (dryer), and the current density is 1
It has been so as to be 20 g / m ² as a coating weight in 00A / dm ^2. Immediately after plating, the steel sheet contains Zn ²⁺ ions in an amount of 10 to 30 g /
Chromate treatment was performed using the reaction type chromate treatment liquid containing 1 of the above. The chromate treatment was performed by spraying, followed by washing with water immediately after the treatment and drying with a dryer.

The color tone after the chromate treatment is performed by measuring the b value with a color difference meter (TC-1800 SZ-Σ80 type manufactured by Nippon Denshoku Industries Co., Ltd.), where the b value is less than 0 is blue and 0 or more is yellow. did. The surface roughness after the chromate treatment is measured by using an atomic force microscope (AFM: Nano Scope II manufactured by US Digital Instrumental Corp.),
The number of convex portions at 500 nm was measured.

The corrosion resistance was determined by visually observing the state of white rust generation after the salt spray test. The evaluation criteria are as follows. ◯: White rust occurrence time is 48 hours or more Δ: White rust occurrence time is 24 to 48 hours ×: White rust occurrence time is less than 24 hours Table 1 shows the results.

[0017]

[Table 1]

As is apparent from the table, all the chromate-treated electrogalvanized steel sheets of the present invention exhibited a stable color tone.

[0019]

As described above, according to the present invention, a chromate-treated electrogalvanized steel sheet excellent in color tone stability can be obtained, and its industrial value is extremely high.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Toru Honjo, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. (72) Koji Yamato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division

Claims (1)

[Claims] 1. A height of 50 on the surface of the electrogalvanized steel sheet.
The fine convex portions of ~500nm 1 × 10 ⁹ ~1 × 10 chromate layer having ¹² / m ^2, 40~150mg / m ² in terms of chromium
A chromate-treated electrogalvanized steel sheet having excellent color stability, which is characterized by being coated.