JPH06256971A - Electrogalvanized steel sheet excellent in image clarity of coating and having high corrosion resistance - Google Patents

Abstract

PURPOSE:To provide an electrogalvanized steel sheet which is an electroplated steel sheet having a Cr-containing Zr type plating layer on a steel sheet of specific roughness and has superior corrosion resistance and can be suitably used for a substrate for image clarity coating. CONSTITUTION:This highly corrosion resistant electrogalvanized steel sheet excellent in image clarity of coating is prepared by forming a Zn type plating layer containing 5-30wt.% Cr on a steel sheet of <=0.35mum average surface roughness Ra. A chromate film or a phosphate film may be formed, as pretreatment for coating, on the plating layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrogalvanized steel sheet which has excellent paint clarity and corrosion resistance and is suitable for home electric appliances and automobile materials.

[0002]

2. Description of the Related Art The quality requirements for steel products are becoming higher year by year, and there is a strong need for improved corrosion resistance, especially for automotive applications.
Along with this, the demand for surface-treated steel sheets is increasing. In such a movement, the appearance after painting has begun to be emphasized as a factor that directly appeals to consumers the quality of home appliances and automobiles.
A surface-treated steel sheet having excellent paint clarity is desired. Heretofore, improvement in clearness of paint has been an issue on the paint side, and studies have been conducted to improve paint methods and paints. As a result, although the paint clarity has been improved steadily, recently, the influence of the steel sheet side, which is the object to be coated, has also been examined, and it is becoming clear that the roughness of the steel sheet surface is also an important factor.

Usually, the control of the roughness of the surface of the steel sheet is carried out by transferring the dullness imparted to the rolling roll, but in the case of the surface-treated steel sheet, a surface-treated layer such as zinc plating is further formed. Therefore, the roughness of the surface treatment layer becomes dominant. In particular, if the plating amount is increased to improve the corrosion resistance, this tendency is intensified, and even if a steel plate with a low roughness is used for the coating sharpness, the desired coating sharpness is obtained after the coated steel sheet is formed. Is often not obtained. With respect to such a problem, in the alloyed hot-dip plated steel sheet, Japanese Patent Laid-Open No. 2-2
As disclosed in Japanese Patent No. 74860, a method of adjusting the surface roughness by a skin pass after plating is disclosed.

[0004]

However, in the method of adjusting the surface roughness after plating, it is difficult to avoid damage to the plating layer such as cracks. In particular, when the amount of plating is relatively small like electroplating, such damage is a factor that significantly reduces the corrosion resistance, and is therefore extremely difficult to apply. The present invention has been made in view of such circumstances, and an object of the present invention is to provide an electroplated steel sheet having excellent coating image clarity and corrosion resistance.

[0005]

[Means for Solving the Problems] That is, the present invention uses a steel sheet whose surface roughness has been adjusted in advance as a base, and has a dense crystal which is advantageous in terms of image clarity as compared with conventional galvanizing or the like. And an electroplated steel sheet having a Cr-containing Zn-based plating layer having a high degree of corrosion resistance, the gist of which is as follows. (1) C on a steel plate having a surface roughness Ra of 0.35 μm or less
A high corrosion-resistant galvanized steel sheet having excellent coating image clarity, characterized in that a Zn-based plating layer containing 5 to 30% by weight of r was formed. (2) Further, there is a galvanized steel sheet having a high corrosion resistance and excellent in the image clarity of coating, which is characterized in that a chromate film or a phosphate film is formed thereon as a pretreatment for coating.

[0006]

The present invention will be described with reference to the drawings. FIG. 1 shows the relationship between the centerline average roughness Ra of the steel sheet surface and the coating clarity. After pre-treatment for plating such as degreasing and pickling, the steel plate whose roughness is adjusted is electroplated to form Zn plating, Ni1
Zn-Ni alloy plating containing 2% and Zn-Cr alloy plating containing 10% of Cr were applied at 30 g / m ² respectively. After applying a chromate treatment and a top coat paint for refrigerators of 30 μm to these electroplated steel sheets, the image clarity was measured. An image clarity measuring instrument NSIC type manufactured by Suga Test Instruments Co., Ltd. was used for measuring the image clarity. In any of the electroplated steel sheets, the relationship between the coating image clarity and the Ra of the steel sheet surface is recognized, but especially Zn-at Ra of 0.35 μm or less
The image clarity of the Cr alloy plating is extremely excellent with an image clarity of 80 or more.

FIG. 2 shows the relationship between the Cr content of Zn-based electroplating, the coating clarity, and the corrosion resistance. R of steel plate surface
a is 0.3 μm, the plating amount is 30 g / m ² , and the coating image clarity is measured as in FIG. The corrosion resistance was evaluated by putting a cross-cut on a coated plate and observing a coating blister and red rust after 1000 hours of a salt spray test. Both the coating sharpness and the corrosion resistance are improved by the content of Cr, but when it is 5% by weight or more, it reaches a level exceeding that of Zn-Ni alloy plating. Cr
Even if the content exceeds 30% by weight, the coating image clarity and the corrosion resistance are still at a high level, but the workability is deteriorated, which is not preferable.

The Zn-based electroplating layer containing a large amount of Cr of the present invention cannot be obtained from a plating bath consisting only of inorganic substances such as metal ions, and a specific organic polymer such as polyamine sulfone or polyethylene glycol is added to the plating bath. It is obtained by doing. This is because these organic substances act as a Cr depositing agent. The plating film thus obtained is Zn plating or Zn-Ni, Z obtained by electroplating.
Compared with conventional alloy plating such as n-Fe, the crystals are extremely dense and have a glossy appearance. Usually, the surface roughness of the plating film obtained by electroplating tends to be higher than that of the base steel sheet, but in the case of the plating film, such a tendency is extremely small due to the dense crystals. It is thought that this is the reason why the clear image clarity is obtained.

In terms of corrosion resistance, Cr does not passivate in the presence of Zn and exhibits a sacrificial anticorrosive action together with Zn, and the corrosion product forms a protective film of a poor solubility to cover the surface and promote corrosion. It is presumed that the suppression of the corrosion resistance may be the reason why it exerts an epoch-making high corrosion resistance. The plating amount is
Since the corrosion resistance is good, a relatively small amount of 10 to 40 g / m ² is sufficient, and this point is also advantageous for coating image clarity.

In the Zn-based electroplating layer, in addition to Cr, an iron group metal such as Ni, Fe, Co, or Mn, C
A metal component such as u or corrosion resistant fine particles such as SiO ₂ or BaCrO ₄ may be contained as the third component. The inclusion of the metal component does not essentially affect the coating image clarity, but a large amount of the component induces a decrease in processability, so that the content of 0.
It is preferable that the content is 1 to 10% by weight and the total content with Cr is 30% by weight or less. Among the metal components, particularly the iron group metal has a high melting point, so it acts favorably on spot weldability,
Ni, in particular, makes a large contribution to the improvement of corrosion resistance.
Greatly effective as an ingredient. Regarding the corrosion-resistant particles, if the content is high, the roughness of the plating surface may be increased rather, so 5% by weight or less is preferable. Even if the Cr depositing agent added to the plating bath is co-deposited in the plating layer in any form, it does not hinder the present invention.

In the present invention, the user only needs to paint.
A steel plate that has been pre-treated before painting
Also include. Chromate treatment or pretreatment for painting
Phosphate treatment is preferred. As a method,
Cr for the heat treatment⁶⁺, Cr ³⁺Si as the main component
O₂And TiO₂Inorganic colloids such as phosphoric acid and molybde
Acids such as acids, their salts, fluorides, water-soluble or
Marjoon type organic resin, anion such as sulfuric acid and halogen
Applicable to those containing metal ions such as Co, Zn, etc.
Yes, any method of electrolysis, etching reaction, coating baking
Is also applicable. Chromate coating amount is total Cr
5-100mg / m in quantity²Is preferred. Phosphating
Contains zinc phosphate as the main component, and also contains Ni, Mn, etc.
Also applicable are chemical reaction methods such as spraying and dipping
Either electrolytic method is possible. As the phosphate film amount,
0.1-3g / m²Is preferred.

[0012]

Example A low carbon steel having various surface roughnesses was used as an original plate, and after degreasing and pickling, electroplating was performed using a sulfuric acid acidic plating bath. As the Cr depositing agent, polyamine sulfone or polyethylene glycol was used. After the chromate treatment or the phosphate treatment on the plated steel sheet thus obtained, the coating for household appliances is 30 μm, or the total coating for automobiles is 90 μm (cationic electrodeposition 20 μm).
m, middle coating 35 μm, top coating 35 μm), and the coating clarity and corrosion resistance were evaluated.

For the measurement of the paint clarity, an image sharpness measuring instrument NSIC type manufactured by Suga Test Instruments Co., Ltd. was used, and a rating of 4 levels was given. (Map definition of 80 or more: 4, 80 to 70: 3, 7
0 to 60: 2, less than 60: 1) Regarding corrosion resistance, those subjected to coating for household appliances have a salt spray test 1 after cross-cutting.
Those coated with 3 coats for automobiles for 000 hours were exposed to salt water spraying (twice weekly for salt water) for 1 year after cross-cutting, and four grades were evaluated based on swelling of coating film and red rust. (Good: Score 4-1: Poor)

Table 1 shows these results. Invention Examples 1 to 1
6 and Comparative Examples 1 and 2 were applied for home electric appliances, and Inventive Examples 7 to 15 and Comparative Example 3 were applied for automobile 3 coat coating.
Comparative Example 1 is an example in which the Cr content in the Zn-based electroplating layer is low, and the coating image clarity and corrosion resistance are insufficient. Comparative Example 2 is an example in which the surface roughness of the steel sheet is large, and the coating image clarity is insufficient. Comparative Example 3 is a simple electroplating with Zn, the coating image clarity is still a step away, and the corrosion resistance is poor. The examples of the present invention include the case of containing a component other than Cr such as Ni,
Good paint clarity and corrosion resistance.

[0015]

[Table 1]

[0016]

EFFECTS OF THE INVENTION According to the present invention, an electroplated steel sheet having both excellent paint clarity and corrosion resistance can be obtained, and exterior plates for household appliances and automobiles that require not only corrosion resistance but also high-grade paint appearance. It can be used in a wide range of applications, mainly for outer panels.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a center line average roughness Ra of a steel plate surface and coating sharpness (image clarity).

FIG. 2 is a diagram showing a relationship between a Cr content in a Zn-based electroplated layer, coating clarity (image clarity), and corrosion resistance.

Claims (2)

[Claims] 1. A high corrosion resistance excellent in coating sharpness, characterized in that a Zn-based plating layer containing 5 to 30% by weight of Cr is formed on a steel sheet having a surface roughness Ra of 0.35 μm or less. Electrogalvanized steel sheet. 2. A galvanized steel sheet having high corrosion resistance and excellent corrosion resistance according to claim 1, wherein a chromate film or a phosphate film is formed on the Zn-based plating layer.