JPS6247488A - Phosphate treatment method for electrogalvanized steel sheets - Google Patents

Abstract

PURPOSE:To obtain a phosphated steel sheet having high whiteness stably by continuously phosphating a galvanized steel sheet with a replenisher soln. having a specified weight ratio of Zn/Ni. CONSTITUTION:When a galvanized steel sheet is continuously phosphated so as to improve the corrosion resistance of the steel sheet, the adhesion to paint and the corrosion resistance after painting, a replenisher soln. for phosphating having 10-20wt. ratio of Zn/Ni is used. The concns. of Zn and Ni in a phosphating bath used are kept constant, and even when the area of the steel sheet to be phosphated is increased, a phosphated steel sheet having a high whiteness such as >=about 60 L value, high water resistance and adhesion after painting is stably obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a post-treatment technique for electrogalvanized steel sheets, and more particularly to a phosphoric acid jjQ treatment method that yields steel sheets with high whiteness.

Conventional technology and its problems Post-treatment of electrogalvanized steel sheets is carried out for the purpose of improving the corrosion resistance and adhesion of galvanized steel sheets, as well as the corrosion resistance after painting. Representative and necessary. Of these, phosphate treatment is a chemical conversion treatment applied when electrogalvanized steel sheets are used as a base for painting.
A phosphate treatment film (1 to 3 (1/rd)) is formed on the galvanized layer using a treatment solution containing phosphate as a main component.

Figure 3 is a block diagram showing the phosphate treatment process. After the treated steel sheet (7) leaves the galvanizing tank (1), it undergoes alkaline cleaning called surface conditioning in the pre-treatment tank (2). Thereafter, it is sprayed or dipped in a phosphate treatment tank (3), rinsed with water in a subsequent washing tank (4), sealed with a thin chromic acid in a sealing tank (5), and dried in a dryer (6). ) to create a phosphate treatment film. Conventionally, the phosphate treatment solution in the phosphate treatment tank is mainly composed of zinc phosphate, and a small amount of nickel is added to it to improve coating performance. In addition, small amounts of nitrate ions and trace amounts of fluorine ions are added for the purpose of promoting film formation.

Electrogalvanized steel sheets generally become black due to phosphate treatment, and the whiteness of the surface decreases. However, recently, users are demanding steel sheets with high whiteness, and product steel sheets are required to have a whiteness of 60 or more (L value on a color difference meter). In order to meet this demand, a method of obtaining a steel plate with high whiteness by using a treatment liquid with a high concentration of nitric acid has been proposed (Japanese Patent Application Laid-Open No. 107084/1984).

In this method, a treated steel sheet with a whiteness of 60 or more can be obtained by setting the nitric acid concentration in the treatment liquid to 2.5 q/I or more. However, when electrogalvanized steel sheets are continuously treated with phosphates, as the treated area increases, the whiteness of the phosphates decreases even if a replenisher is added, and the steel sheets with high whiteness continuously The disadvantage is that it is not possible to obtain

Purpose of the Invention The present invention was made in order to eliminate the above-mentioned drawbacks of the conventional method, and it is a method of phosphoric acid for electrogalvanized steel sheets that can obtain treated steel sheets with high whiteness even in continuous processing by improving the composition of the replenishing solution. The purpose of this paper is to propose a salt treatment method.

Structure of the Invention The method for phosphating an electrogalvanized steel sheet according to the present invention includes phosphate treatment replenishment to achieve a Zn/Ni weight ratio of 10 to 20 when continuously phosphating an electrogalvanized steel sheet. By using the solution, even if the phosphate treatment area increases, the Zn/Ni ratio in the treatment solution can be kept constant and a treated steel sheet with high whiteness can be obtained.

The method of this invention will be explained in detail below.

The inventors investigated the cause of the decrease in whiteness when phosphate treatment is performed continuously using a normal treatment solution, and found that each component in the phosphate treatment bath (Pop-,
F-, Zn2 Ni″: xo;

NH5, etc.), it was determined that the Zn'' and N1'' concentrations were influential. In other words, it has been found that the whiteness of the product decreases with a decrease in 7-rr'' and an increase in N1''.

However, with conventional replenishment liquids, when continuous processing is performed over several hundred thousand times, the values of 7n" and Ni" in the processing bath cannot be maintained within a constant range and fluctuate.

Therefore, in order to prevent a decrease in whiteness in continuous processing, it is necessary to prevent fluctuations between ln and N+1 degrees.

This invention was made based on the above knowledge, and is a method for preventing fluctuations in Zn and N+iU degree in continuous processing. It was discovered that by using a replenishment solution with a volume ratio of 10 to 20, it was possible to maintain the values of 7-n゛゛ and N1''+ in the treatment solution within a certain range even if the phosphate treatment area increased. It's been a while since I've been in the middle of a long time.

Here, the reason why the Zn/Ni weight ratio of the replenishing liquid was limited to 10 to 20 is because if the Zn/l'Ji weight ratio is less than 10, the whiteness will decrease, and if the same weight ratio exceeds 20, the whiteness will deteriorate after painting. This is because the water resistance adhesion decreases, and although the degree of whiteness is high, the product price (directness is lower).

Example 10 (
] / A double-sided electrogalvanized steel sheet at the bottom was treated with phosphate, and the zn/n in the replenishment solution was changed as shown in Table 2. Zn
/l'4i was investigated.

FIG. 1 shows the relationship between the weight ratio in the replenisher and color tone change, FIG. 2 shows the change in Zn/Ni in the treatment bath, and the water resistant adhesion after painting is also shown in Table 2.

In addition, for water-resistant adhesion after painting, apply melamine paint (product name: Shinto Glycine 100) at 25μ, then apply 120'C
After baking for 25 minutes, soak in warm water at 80°C for 3 hours,
#r! Baseline tests were conducted at intervals and evaluated based on the coating film survival rate at that time.

In this example of the invention, as is clear from FIG. 2, the Zn/N ratio in the treatment bath could be kept almost constant.
As is clear from FIG. 1, treated steel plates with a whiteness of 60 or more were obtained, and as is clear from Table 2, all of the obtained treated steel plates had excellent water resistant adhesion after painting.

Below, the margins are as follows: Table 1 Table 2 ○: 90% or more: 60-89% /Ni weight ratio 10-2
Zn and Ni in the treatment bath can be
The concentration can be maintained within a certain range, so even if the phosphate treatment area increases, the whiteness is high (L value 60 or more).
It is possible to obtain treated steel plates stably, and has excellent water-resistant adhesion after painting, and has the effect of mass-producing products of 1 q. which fully satisfies user requirements.

[Brief explanation of drawings]

FIG. 1 shows Zn/Ni in the replenishment solution in an embodiment of the present invention.
Figure 2 is a diagram showing the relationship between the ratio and color tone change. Figure 2 is a diagram showing the transition of the Zn/Ni ratio in the treatment bath in the same example. Figure 3 is a block diagram showing the phosphate treatment process for electrogalvanized steel sheets. It is. 1... Galvanizing tank, 2... Surface treatment tank, 3... Phosphate treatment tank, 4... Washing tank, 5... Sealing tank, 6... Dryer, 7... ...Steel plate. Figure 1 Processing area (xtom) Figure 2 Processing area (X 10'm2) Figure 3

Claims (1)

[Claims] A method for phosphate treatment of electrogalvanized steel sheets, characterized in that a phosphate treatment replenishment solution having a Zn/Ni weight ratio of 10 to 20 is used when electrogalvanized steel sheets are continuously phosphate treated.