JP2978073B2 - Manufacturing method of electro-galvanized steel sheet excellent in uniform surface appearance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvanized steel sheet, and more particularly to a method for producing a galvanized steel sheet having a uniform surface appearance for home appliances or automobile outer panels. .

[0002]

2. Description of the Related Art The quality requirements for steel products are increasing year by year, and there is a high need for improving corrosion resistance especially for automobiles and home appliances, and accordingly, demand for surface-treated steel sheets is increasing. As surface-treated steel sheets, various types of post-treated steel sheets have been developed and commercialized for automotive use, such as Zn-based plating and organic composite coated steel sheets with an organic coating applied to them, and for household appliances, not only because of corrosion resistance but also because of the need to omit processes. I have. The post-treatment for home electric appliances is mainly a chromate treatment or an organic composite treatment for further providing an organic film, but an electric Zn plating is often used as a 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 are advantages. 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.

Under these circumstances, in the case of home electric appliances, these post-processed steel sheets are often assembled and used as they are, and accordingly, the demand for appearance quality becomes stricter.
The minute appearance stains that have been overlooked so far have become non-negligible. Japanese Patent Application Laid-Open No. 4-74887 discloses a method for producing an electro-galvanized steel sheet having an excellent appearance.
Japanese Patent Application Laid-Open No. 4-74888 discloses a method in which a lower plating is formed in an acidic electro-Zn plating bath containing a conductivity aid and then an upper plating is formed in another Zn plating bath containing no conduction aid. However, it has not yet been able to improve minute appearance stains.

[0004]

The problems in appearance are classified into those generated in an electro-Zn plating process including post-processing and those caused by an original plate. The solution is easy if the cause is specified, but especially in the case of the original sheet, the manufacturing process is extremely complicated, and it is often difficult to specify,
Even if it can be identified, it is difficult to reach a fundamental solution at present. Also, even if it is apparently due to the original plate,
There are many problems that cannot be confirmed at the stage of the original plate and can be confirmed only by performing the electro-Zn plating. For this reason, in the electro-Zn plating process, a method is required that can obtain a stable and good surface appearance without being affected by the state of the original plate, without impairing the original quality characteristics such as corrosion resistance.

[0005]

Means for Solving the Problems The present inventors have developed an electric Zn generated due to minute scratches and dirt which cannot be confirmed on the original plate.
The poor appearance of the plated steel sheet was found to be due to the unevenness of the initially deposited Zn crystal. Therefore, as a result of examining the uniformity of the initial electrodeposited crystal of Zn, the initial Zn plating process was carried out by large current plating (large generation of plating nuclei), medium current plating (growth of plating nuclei and new nucleation), plating It has been found that it is effective to divide into three steps of dissolution (removal of abnormally grown crystals). The present invention has been made based on this finding, and the gist is as follows. Electric Z
In producing an n-plated steel sheet, a plating base sheet subjected to a pretreatment such as degreasing and pickling is applied with a first layer of 100 A / d.
Z adhesion amount 5 to 100 mg / m ² in m ² or more current density
n plating, and then 10 to 100 A /
After applying Zn plating with an adhesion amount of 0.5 to 1 g / m ² at a current density of dm ² , 0.1 to 0.5 g of the second layer was applied with a reverse current.
/ M ² after dissolution, and as a third layer, 10 to 100 A
The present invention provides a method for producing an electrical Zn-plated steel sheet having excellent uniform surface appearance, wherein Zn plating is performed at a current density of / dm ^{2 and} the total plating layer is 5 to 100 g / m ² .

[0006]

According to the present invention, when the electro-Zn plating is performed, the first layer is formed at a high current density of 100 A / dm ² or more.
n is precipitated in an amount of 5 to 100 mg / m ² . The effect of this is that by performing plating under a high overvoltage, the generation of initial plating nuclei is increased, and Zn plating nuclei are generated in flaws and stains in the same manner as in normal parts. Therefore, if the current density is less than 100 A / dm ² , the precipitation overvoltage is insufficient, and the number of Zn plating nuclei generated between the flaws, the stained portion and the normal portion is greatly different, and a uniform appearance cannot be obtained. FIG. 1 shows the Z of the first layer during the electro-Zn plating in the present invention.
FIG. 3 is a diagram showing the influence of the amount of n plating on the appearance contamination rate. In this figure, after the first-layer plating, the second layer 50A / dm ² at 0.8 g / m ^2, followed 3A / dm ²
0.3 g / m ² of the second layer was dissolved by the reverse current of
Total coating weight at 0A / dm ² is created a sample to be 40 g / m ^2. And the current density is 100 A /
dm ² . Thus, the Zn plating amount was 5 mg / m ²
It can be seen that if the amount is less than the initial plating amount, the effect of improvement is not recognized because the amount is too small. Also, when it exceeds 100 mg / m ² , it can be seen that the appearance deteriorates because the plating is burnt.

Next, as the second layer, 10 to 100 A / dm
^At a medium current density of ² , 0.5 to 1 g / m ^{2 of} Zn is deposited. The effect of this is that a new Zn plating nucleus is generated while further growing the Zn plating of the first layer, and the Zn plating crystal is densified. FIG.
The effect of the current density and the amount of Zn plating on the appearance contamination rate is shown in FIG. In this figure, the first layer is 100 A / d
50 mg / m ² m ^2, and the second layer in the second layer opposite the current of the plating after 3A / dm ² 0.3g / m ² was dissolved, the third layer 50
Total coating weight in A / dm ² is created a sample to be 40 g / m ^2. Therefore, the current density is 10 A / dm
^If it is less than ² , generation of new Zn nuclei is unlikely to occur.
The crystal grows large, resulting in poor appearance. Further, at a current density exceeding 100 A / dm ² , 0.5
Precipitation of ｇ1 g / dm ² causes burnt plating, so that the appearance also deteriorates. Zn plating amount is 0.5g /
If it is less than m ² , the amount that can be eluted during the subsequent dissolution of the plating layer will be too small to provide a sufficient effect. On the other hand, if it exceeds 1 g / m ² , abnormally large crystals that cannot be completely dissolved at the time of dissolving the plating layer later are generated, so that the improvement effect is also small.

Next, the second Zn plating layer is dissolved by 0.1 to 0.5 g / dm ² by a reverse current. This effect is achieved by forcibly dissolving the second Zn plating layer.
The purpose is to preferentially dissolve the abnormally grown portion to make the surface uniform. If the current value is too large, the melting of the sound part increases, and as a result, there is a high possibility that the surface becomes non-uniform. Therefore, the current is preferably 5 A / dm ² or less. FIG. 3 shows the effect of the amount of Zn dissolved by the reverse current on the appearance contamination rate. In this figure, the first layer is 50 mg / 100 A / dm ^2.
m ² , the second layer is 0.8 g / m ² at 50 A / dm ² ,
After reverse dissolution of the layer, a sample was prepared so that the third layer was 50 A / dm ² and the total amount of plating was 40 g / m ² . Therefore, if the Zn plating dissolution amount is less than 0.1 g / m ² , the dissolution amount is too small and the improvement effect is small, and 0.5 g / m ^2.
If the amount exceeds, the amount of dissolution is too large, and the dissolution of healthy parts also increases, and as a result, the unevenness of the surface does not disappear,
It can be seen that the appearance is reduced.

Finally, 10 to 100 A / dm as a third layer
² of current density, subjected to Zn plating, made to be 5 to 100 g / m ² as a plating layer total. This effect is to ensure the corrosion resistance required for a Zn-plated product. FIG. 4 shows the effect of the total amount of Zn plating on the red rust generation area after the cyclic corrosion test. In this figure, the first layer is 50 mg / m ² at 100 A / dm ^2.
2, 0.8 g / m ² in the second layer 50A / dm ^2, after 0.3 g / m ² dissolved second layer by reverse current, the third layer 50A /
The plating is performed at dm ² . The red rust generation area is a value after 100 cycles of repeated cyclic corrosion tests of spraying salt water → wetting → drying. Thus, it can be seen that if the total amount of Zn plating is less than 5 g / m ² , sufficient corrosion resistance is not obtained.

FIG. 5 shows the effect of the total amount of Zn plating on the evaluation of powdering resistance as an index of workability. In this figure, the first layer is 50 at 100 A / dm ² .
mg / m ² , the second layer is 0.8 g / m ² at 50 A / dm ^2
2 After dissolving the second layer by 0.3 g / m ² with a reverse current, the third layer is plated at 50 A / dm ² . The higher the score, the better. Therefore, the total amount of Zn plating is 100
It can be seen that if it exceeds g / m ² , the workability will decrease. In addition, it is necessary to reduce the line speed, which is disadvantageous in terms of operation. If the current density is less than 10 A / dm ² ,
In addition to the coarsening of the Zn-plated crystal, which leads to a reduction in quality, the deposition time is long, which is extremely disadvantageous in operation. On the other hand, if it exceeds 100 A / dm ² , burning of the plating will occur, and the quality will also deteriorate.

[0011]

EXAMPLE A hot-rolled material of low carbon steel having a thickness of 4 mm is pickled so that the scale remains thin, and then cold-rolled to a thickness of 0.8 mm, which is annealed to form a plated original sheet. did. For such a plate, a pH containing 1 mol / l of Zn ions
1. Using a sulfuric acid bath at a bath temperature of 60 ° C., electro Zn plating was performed at a relative flow rate of 1 m / sec. As a result, as shown in FIGS. 1 to 5, Zn plating was applied as a first layer with a current density of 100 A / dm ² or more and an adhesion amount of 5 to 100 mg / m ² ,
Next, as the second layer, Zn plating with a current density of 10 to 100 A / dm ² and an adhesion amount of 0.5 to 1 g / m ² is performed.
After dissolving the second layer with a reverse current of 0.1 to 0.5 g / m ² , the third layer is further subjected to Zn plating of 5 to 100 g / m ² at a current density of 10 to 100 A / dm ^2. Was found to be excellent in uniform appearance, and there was no decrease in other quality performance.

[0012]

As described above, the present invention provides a method for efficiently improving fine appearance stains caused by an original plate after electro-Zn plating. In particular, it is suitable as a method for producing an electroplated zinc-coated steel sheet of a surface-treated steel sheet for home electric appliances which is used as it is after being assembled, and in response to increasingly strict demands for appearance quality of customers, It is truly effective.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of Zn plating on a first layer and the appearance contamination rate in the present invention.

FIG. 2 is a diagram illustrating a relationship between a Zn plating amount, a current density, and an appearance contamination generation rate of a second layer.

FIG. 3 is a diagram showing the relationship between the amount of dissolution of a Zn plating layer due to a reverse current and the appearance contamination rate.

FIG. 4 is a diagram showing a relationship between a total amount of Zn plating and a red rust generation area (corrosion resistance).

FIG. 5 is a diagram showing the relationship between the total amount of Zn plating and powdering resistance (workability).

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Katsutoshi Enyama 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside of Nippon Steel Corporation Kimitsu Works (56) References JP-A-4-202786 (JP, A) 63-1690 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C25D 1/00-7/12

Claims (1)

(57) [Claims] When producing a Zn-plated steel sheet, a coating amount of 5 to 1 as a first layer at a current density of 100 A / dm ² or more is applied to a plating base sheet which has been subjected to pretreatment such as degreasing and pickling.
Then, Zn plating of 00 mg / m ² is performed, and then, as a second layer, an adhesion amount of 0.5 to 100 A / dm ² is applied.
After plated with Zn of 1 g / m ^2, after the second layer was 0.1 to 0.5 g / m ² dissolved at a reverse current, further Zn at a current density of 10 to 100 A / dm ² as the third layer A method for producing an electric Zn-plated steel sheet having excellent uniform surface appearance, wherein plating is performed to make the total plating layer 5 to 100 g / m ² .