JP3437663B2 - Manufacturing method of electro-galvanized steel sheet with excellent surface gloss - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an electro-galvanized steel sheet having excellent surface gloss and particularly suitable as a surface-treated steel sheet used for home electric appliances and the like. Things. [0002] The required quality of steel products is becoming higher year by year, and there is a high need for improvement of corrosion resistance especially for automobiles and home electric appliances. Accordingly, demand for surface-treated steel sheets is increasing. As surface-treated steel sheets, Zn-based alloy plating and organic composite coated steel sheets with an organic coating applied to them have been developed for automobiles, and various post-treated steel sheets have been developed and developed for household appliances for not only corrosion resistance but also the need for omission of processes. Has been
As post-processing for home appliances, mainly chromate treatment and organic composite treatment to add an organic coating are mainly used.
In many cases, electric Zn plating is adopted as the base plating. Compared to Zn-based alloy plating developed for automobiles, electro-Zn plating is inferior in corrosion resistance with the same coating weight, but is easy to manufacture and advantageous in cost, and has good compatibility with chromate treatment. There is. For this reason, the underlying plating is fixed to electrical Zn plating, and post-processing according to the needs is applied thereto, thereby responding to diversified needs such as corrosion resistance and omission of steps in the field of home appliances. [0003] Under these circumstances, in the case of home electric appliances, these post-treated steel sheets are often assembled and used as they are, and accordingly, the demand for appearance quality is becoming stricter. At the same time, the appearance quality requirements for the plated steel sheet, which is the raw material, have been increasing, but it has been difficult to produce a highly glossy Zn-plated steel sheet with a normal acidic electric Zn plating bath. As a method for producing an electro-galvanized steel sheet having excellent gloss, Japanese Patent Application Laid-Open No.
Japanese Patent No. 247790 discloses a method of forming a plating layer using an acidic electro-Zn plating bath containing an organic additive, but there remains a problem of plating bath management when using an additive. [0004] A high-gloss electric Zn-plated steel sheet can be realized by using a special plating bath. However, it is necessary to take account of ease of bath management, operation stability and safety. Sometimes there are few that are industrially feasible. For this reason,
Instead of a special plating bath in the electro-Zn plating process, a manufacturing method that can stably obtain a high surface gloss was required. The present invention is to solve the above problems,
An object of the present invention is to provide a method for producing an electro-galvanized steel sheet having excellent surface gloss. SUMMARY OF THE INVENTION The present inventors have developed an electric Zn
It was found that the surface gloss of the plated steel sheet was due to a slight difference in the initial deposition of Zn. Therefore, as a result of intensive studies on the uniformization of the initial deposition of Zn, it was found that it is effective to apply Zn plating by increasing the current density in a plating bath having a low Zn ion concentration in the plating bath. The present invention
This was achieved based on this finding, and the gist is as follows. Before applying electric Zn plating to a steel sheet, as a pretreatment, Zn ions 0.4
Current density of 16 with a plating solution containing 0.8 mol / l
0 ~200A / dm ² in 0.5~5g the Zn-plated / m ²
Electric Zn excellent in surface gloss characterized by being adhered
Manufacturing method of plated steel sheet. The present invention is characterized in that a small amount of Zn plating is performed in advance at a high current density in a plating bath having a low Zn ion concentration before performing the electro-Zn plating. Hereinafter, this plating layer is referred to as initial Zn plating. The low Zn ion concentration and the high current density have the effect of increasing the overvoltage during electrodeposition, and the crystal nuclei formed at the initial stage of plating are finely and uniformly formed on the steel sheet. This functions as a nucleus during the subsequent Zn plating at a high ion concentration and a low current density (hereinafter, this plating layer is referred to as main Zn plating), so that current concentration does not occur, and as a result, coarsening of Zn crystal grains is prevented. It is thought that. For example, when an electro-galvanized steel sheet having a current density of 50 A / dm ² and a current density of 20 g / m ² is manufactured in a plating bath having a Zn ion concentration of 1 mol / l, the Zn crystal grains become coarse and the plating appearance becomes dark. (60 °), JIS Z874
1] is around 30, but the Zn ion concentration is 0.4 to 0.1.
The current density is 160 to 200 in an 8 mol / l plating bath.
After depositing 0.5 to 5 g / m ² of Zn plating at A / dm ² , then, in a plating bath containing 0.8 to ² mol / l, normal Zn plating is performed at a current density of 20 to 150 A / dm ^2. When applied, coarsening of Zn crystal grains is prevented, and gloss 45
An electrical Zn-plated steel sheet exhibiting the above excellent surface appearance is obtained. [0007] On the other hand, the initial Zn plating is a plating step under a condition where low Zn ion concentration, high current density and so-called plating burn easily occur, so that the range of plating conditions is limited. When plating burns occur, the glossiness is rather reduced, so that the condition setting at the time of initial Zn plating formation needs to be studied in detail. As a result of the examination, the plating conditions for the initial Zn plating were such that the Zn ion concentration in the plating bath was 0.4 to 0.1.
8 mol / l, current density of 160 to 200 A / dm ² , and adhesion amount of 0.5 to 5 g / m ² are good. Under these conditions, the glossiness does not increase at a high ion concentration, a low current density, and a low adhesion amount, and plating burns occur at a low ion concentration, a high current density, and a high adhesion amount, and conversely, the glossiness decreases. The plating conditions for the main Zn plating are not particularly limited.
Preferably, the ion concentration is 0.8 to ² mol / l, the pH is 0.8 to 6, and the current density is 20 to 150 A / dm ² , and the adhesion amount is preferably 10 to 40 g / m ^{2 in} total with the initial Zn plating. [0008] A cold-rolled steel sheet having a thickness of 0.8 mm is alkali-degreased, pickled with a 5% sulfuric acid aqueous solution, subjected to initial Zn plating under the plating conditions shown in Tables 1 to 3, and then to Zn ions. 1 mo
current density of 100 A / dm ^{2 in} a plating bath containing 1 / l
, An initial Zn plating and a total of 20 g / m ² of electric Zn plating were performed. The plating bath was controlled at a pH of 0.8 to 1.2 and a bath temperature of 40 to 60 ° C. for both the initial plating and the main plating. Tables 1 to 3 also show the results of the gloss evaluation after the main Zn plating. The gloss was evaluated as follows: Gloss [Gs (60 °), based on JIS Z8741] Exceeding 50: 〜 Exceeding 45 to 50: 超 Exceeding 35 to 45: △ 35 or less: x did. [Table 1] [Table 2] [Table 3] [Table 4] As shown in Tables 1 to 3, the Zn ion concentration in the plating bath is 0.4 to 0.8 mol / l and the current density is 160 to
It can be seen that an electro-galvanized steel sheet having excellent gloss can be obtained in the range of 200 A / dm ² and the adhesion amount of 0.5 to 5 g / m ² . Table 4 shows a comparative example. Comparative examples 2, 3, and 5
Has a low Zn concentration, a high current density, an insufficient amount of adhesion, and is not a pretreatment for obtaining bright plating, so that sufficient gloss cannot be obtained. In Comparative Examples 1, 4, and 6, plating burns occurred due to excessively low Zn ion concentration and excessively high current density, and plating time was extended to increase the amount of adhesion, resulting in extremely low glossiness. Things. As described above, the present invention provides a method for producing an electro-galvanized steel sheet having excellent surface gloss. In particular, it is suitable as a method for producing a base Zn plating of a surface-treated steel sheet for home appliances which is used as it is after being assembled, and is particularly effective in responding to increasingly strict requirements for appearance quality.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C25D 5/36 C25D 5/26 C25D 5/10

Claims (1)

(57) [Claims] [Claim 1] When performing electro-Zn plating on a steel sheet, a Zn ion of 0.4 is initially set as a pre-treatment.
Current density of 16 with a plating solution containing 0.8 mol / l
0 ~200A / dm ² in 0.5~5g the Zn-plated / m ²
Electric Zn excellent in surface gloss characterized by being adhered
Manufacturing method of plated steel sheet.