JPH08260190A - Production of electrogalvanized steel sheet excellent in surface appearance - Google Patents

Abstract

PURPOSE: To enable a steel sheet to correspond to a request for the quality in the appearance by previously executing sticking of Co plating to it at a specified relative flow rate. CONSTITUTION: For example, simulatively, a steel sheet applied with microflaws is used as an original sheet, and Co plating and electrogalvanizing are executed. Then, at the time of coating it with Co plating by 5 to 300mg/m<2> coating weight at >=0.5m/sec relative flow rate, the generation of inferiority in the appearance is made small. Namely, in the case of >=0.5m/sec, the generating ratio of stains in the appearance is made small. In the case of the coating weight of <5mg/m<2> , the generating ratio of inferiority in the appearance is made high, and in the case of >=5mg/m<2> , it is made small. Furthermore, even in the case of >300mg/m<2> , the effect on stains in the appearance can be maintained, but its corrosion resistance, particularly, corrosion from the edge parts becomes the cause of fear, and it is not preferable. Thus, microstains in the appearance after electrogalvanizing caused by the original sheet can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric Zn-plated steel sheet which has an excellent surface appearance and is suitable as a surface-treated steel sheet particularly used for home electric appliances and the like.

[0002]

2. Description of the Related Art The quality required for steel products is becoming higher year by year, and there is a great need for improving corrosion resistance, especially for automobiles and home appliances, and the demand for surface-treated steel sheets is increasing accordingly. As surface-treated steel sheets, for automobiles, Zn-based alloy plating and organic composite-plated steel sheets with an organic coating applied to it are developed. Has been converted.
As post-treatments for home appliances, chromate treatments and organic composite treatments that add an organic coating are the main
In many cases, electric Zn plating is adopted as the undercoat. Compared to Zn-based alloy plating developed for automobiles, electro-Zn plating is inferior in corrosion resistance at the same deposition amount, but is easy to manufacture and advantageous in terms of cost, and has an advantage such as good compatibility with chromate treatment. There is. Therefore, the base plating is fixed to the electric Zn plating, and the post-treatment according to the needs is applied to this to meet the diversifying needs such as corrosion resistance and the omission of steps in the field of home appliances.

Under these circumstances, in post-processing household appliances, these post-treated steel sheets are often assembled and used as they are, and accordingly, the demand for appearance quality becomes strict, and the minute appearance that has been overlooked until now. Dirt is becoming ignorable. As a method for producing an electric Zn-plated steel sheet having an excellent appearance, JP-A-4-74887 is known.
Japanese Patent Publication No. 74888 discloses a method of forming a lower layer plating in an acidic electric Zn plating bath containing a conductivity auxiliary agent and then forming an upper layer plating in another Zn plating bath containing no conductivity auxiliary agent. However, it is not enough to cover minute appearance stains.

[0004]

The appearance problems are classified into those occurring in the electric Zn plating process including post-treatment and those caused by the original plate. It is easy to solve if the cause is identified, but it is often difficult to identify because the manufacturing process is extremely complicated, especially if it is due to the original plate, and even if it is possible, it is drastic. The current situation is that it is difficult to resolve. Also, even if obviously due to the original plate,
There are many problems that cannot be confirmed at the original plate stage, but can be confirmed only by performing electric Zn plating. Therefore, there has been a need for a manufacturing method capable of stably obtaining a good surface appearance without being affected by the original plate state without deteriorating the original quality characteristics such as corrosion resistance in the electric Zn plating process.
The present invention is for solving the above problems, and an object of the present invention is to provide a method for producing an electric Zn-plated steel sheet having an excellent surface appearance.

[0005]

The inventors of the present invention have found that the electric Z generated due to minute flaws or stains that cannot be confirmed on the original plate.
It was determined that the poor appearance of the n-plated steel sheet was due to a subtle difference in the initial Zn electrodeposition. Therefore, as a result of earnest studies on making the initial electrodeposition of Zn uniform, it was found that it is effective to apply a small amount of Co plating beforehand. The present invention has been made based on this finding, and the summary thereof is as follows. A method for producing an electric Zn-plated steel sheet having an excellent surface appearance, which comprises depositing Co plating in advance at a relative flow rate of 0.5 m / sec or more in an amount of 5 to 300 mg / m ² in performing the electric Zn plating on the steel sheet.

[0006]

In the present invention, when performing electric Zn plating,
It is characterized in that a small amount of Co plating is performed in advance under specific conditions. The function and effect are to improve the poor appearance caused after the electric Zn plating due to minute scale flaws and stains on the surface of the steel sheet. It is possible to confirm that the defective appearance portion has a different color tone by visual observation and that the size and orientation of the plated crystal are not uniform when compared with the normal portion when microscopic observation is performed. It is considered that in the micro-defect portion, the Zn plating deposition, especially the initial deposition, becomes non-uniform due to the current situation or segregation of steel components. On the other hand, Co plating deposits uniformly on the micro-defects as well as on the normal parts, and
It is presumed that as a result of becoming a nucleus of precipitation and Zn being electrodeposited uniformly, appearance stains are difficult to be observed. This is because the uniform precipitation of Co is higher than that of Zn.
In order to further enhance the effect of Co, it is necessary to consider the liquid flow rate.

FIG. 1 shows Co for the appearance defect due to micro-defects.
The effect of the plating amount is shown. Actual microscopic flaws occur indefinitely.Therefore, a steel plate is simulated to irradiate a laser beam, and electroplating is performed using this as a master plate. It was confirmed. By repeating this process, the irradiation conditions were set such that the appearance stains could be confirmed by the electric Zn plating, which could not be visually confirmed on the original plate. The degree of appearance stain was set to level 1 which is relatively mild and level 2 which is severe stain.

FIG. 1 shows the amount of Co plating and the occurrence rate of appearance defects when Co plating and electric Zn plating are carried out using a steel plate thus simulated as a micro flaw as a base plate. For Co plating, Co ion is 60 g / l, pH
2. Using a sulfuric acid acidic plating bath having a bath temperature of 60 ° C., a relative flow rate of 1 m / sec, and a current density of 10 to 100 A / dm ² ,
n plating is Zn ion 70g / l, pH 1.2, bath temperature 5
Using sulfuric acid acidic plating bath at 5 ° C, relative flow rate 1.5m /
Sec., Current density 70 A / dm ² , adhesion amount 20 g /
It was set to m ² . Ten sheets were processed under the same conditions, and the occurrence rate of appearance defects was calculated. As is clear from FIG. 1, Co plating 0
The occurrence rate is 100% even in the mild level 1, but in the level 1 the Co plating is reduced to 50% or less at 5 mg / m ² and 10% or less at 10 mg / m ² or more. Further, at level 2, it is reduced to 10% or less at 50 mg / m ² or more.

FIG. 2 shows the appearance defect due to micro-defects.
The influence of the flow velocity at the time of Co plating is shown. Co plating, Zn plating
The plating conditions are the same as in Fig. 1, and the Co plating amount is 25 mg.
/ M ² , Zn plating amount is 10 g / m² And Liquid flow rate
With the increase, the incidence tends to decrease, 0.5m /
If it is more than a second, the occurrence rate will be 50% or less, even if it is severe
descend. As described above, a small amount of Co plating is applied first.
The appearance of the Zn-plated steel sheet caused by the
This is reduced. The range of Co plating amount is 5 to 3
00 mg / m² Is good. 5 mg / m² Less than shown in Figure 1
As mentioned above, there is no effect on appearance stains. 300 mg
/ M² Even if it exceeds, the effect on appearance stain is maintained
However, there are concerns about corrosion resistance, especially from the edges.
It is not preferable. More preferable range is 10 to 100
mg / m² Is.

The relative flow velocity for Co plating is preferably 0.5 m / sec or more. If it is less than 0.5 m / sec, the effect on appearance stain is small as shown in FIG. Here, the relative flow velocity is the difference between the liquid flow velocity and the plate passing velocity that take into consideration the liquid flow direction and the steel plate passing direction, and the relative flow velocity increases when the liquid flows in the opposite direction to the traveling direction of the steel plate. Although advantageous, for example, even when the plating solution is circulated at a low flow rate of about several meters / minute in a dipping type plating tank such as a vertical type, the steel sheet passing speed is sufficiently increased to a relative speed of 0. .5m
If / sec or more is secured, the effect of the present invention is exhibited. The composition conditions of the Co plating bath are not particularly limited, but a sulfuric acid acidic bath having Co ions of 3 to 100 g / l and pH of 1.5 to 6 is effective, and the current density is 10 to 100 A.
/ Dm ² is preferred. There is no particular limitation on the amount of Zn plating, but the effect of Co plating is large in the range of 5 to 40 g / m ² . As a plating bath, 40 Zn ions are used.
A sulfuric acid acidic bath containing 0.5 to 100 g / l and having a pH of 0.5 to 2 is preferable.

[0011]

【Example】

Example 1 A hot rolled material of low carbon steel having a plate thickness of 4 mm was pickled so that the scale remained thin, and then cold rolled to obtain a plate thickness of 0.8, 1.
The thickness was set to 0, 1.2 mm, and this was annealed to obtain a plating original plate. A relative flow rate of 1 was applied to such an original plate using a sulfuric acid bath containing Zn ions at 70 g / l at a pH of 1 and a bath temperature of 60 ° C.
m / sec, current density 100 A / dm ² and adhesion amount 20 g / m
^When 10 sheets of electro-Zn plating of ² were performed, appearance stains were generated on all 10 sheets, and the degree of the stain was more severe as the plate thickness was thicker. A sulfuric acid bath having a pH of 5 containing Co ions of 5 g / l and a bath temperature of 40 ° C. was used to obtain a current density of 10 A / dm ² and a relative flow rate of 5 m /
After applying Co plating of 20 mg / m ² per ^second , the electric Zn
When 10 plates were plated, appearance stains were completely absent at a plate thickness of 0.8 mm, 3 stains at 1.0 mm, and 5 stains at 1.2 mm. It was light.

Example 2 The same original plate as in Example 1 was added with p containing 60 g / l of Co ions.
H2.5, bath temperature 50 ℃ sulfuric acid bath, current density 60A
/ Dm² , 100 mg / m at a relative flow rate of 1 m / s ² Co
After plating, electroplating Zn under the conditions of Example 1
When 10 sheets were made, all the appearance stains were observed at a plate thickness of 0.8 mm.
None, 2 sheets at 1.0 mm, 3 sheets at 1.2 mm
Appearance was observed, but the degree is comparable to that without Co plating
And it was light. Co plating 300 under the same conditions,
500 mg / m² When the plate thickness is 1.0, 1.2 mm
The appearance stains were reduced to 1 and 2 sheets, respectively. C to these
r amount 15 mg / m² Chromate treatment, and then
After applying Lamin alkyd paint with a thickness of 20 μm,
Salt spray test with the end exposed (JIS Z237
1) was performed for 500 hours. Bulging width of coating film from the edge
Is Co deposition amount 0.100 mg / m² Then there is no difference,
Co 300 mg / m² Then the swelling is a little big
But Co500mg / m² Then the swelling width compared to these
Was quite big.

[0013]

Industrial Applicability As described above, the present invention provides a method for improving the microscopic appearance stain after the electric Zn plating caused by the original plate. In particular, it is suitable as a method for manufacturing a base Zn plating of a surface-treated steel sheet for home electric appliances which is used as it is after being assembled and processed, and is very effective in meeting the increasingly strict requirements for appearance quality.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the appearance defect occurrence rate due to minute defects and the Co plating amount,

FIG. 2 is a diagram showing a relationship between a defective appearance rate and a relative flow velocity during Co plating.

Claims (1)

[Claims] 1. A galvanized steel sheet having an excellent surface appearance, characterized by depositing Co plating in advance at a relative flow rate of 0.5 m / sec or more in an amount of 5 to 300 mg / m ^{2 when} galvanizing the steel sheet. Production method.