JPH0776792A - Production of electrogalvanized steel sheet excellent in surface appearance - Google Patents

Abstract

PURPOSE:To provide a plated steel sheet having an excellent surface appearance by applying a small amt. of plating to a steel sheet at a specified relative flow rate by using an acidic galvanizing bath contg. water-soluble organic compounds and thereafter applying a prescribed amt. of plating thereto by using a plating bath contg. no water-soluble organic compounds. CONSTITUTION:At first, an acidic galvanizing bath contg. water-soluble organic compounds (such as cationic organic compounds contg. quaternary amine salt and oxy compounds contg. oxygen) is prepd. By using this plating bath, a steel sheet is stuck with Zn by 0.1 to 5g/m<2> coating weight at >=0.5m/sec relative flow velocity. Then, even into the parts of microflaws and stains on the surface of the steel sheet, uniform Zn is precipitated similarly to the case in the normal part. Next, by using an acidic plating bath contg. no water-soluble organic compounds, galvanizing is applied to regulate the total with the initial stuck zinc into >=10g/m<2>. Thus, the galvanizing is uniformly grown as the nucleus of the initial plating to form a uniform appearance.

Description

Detailed Description of the Invention

[0001]

The present invention has an excellent surface appearance,
Particularly, the present invention relates to a method for producing an electrogalvanized steel sheet suitable as a surface-treated steel sheet used for home electric appliances and the like.

[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 automobiles and home appliances, and the demand for surface-treated steel sheets is increasing accordingly. As surface-treated steel sheets, Zn-based alloy plating and organic composite-plated steel sheets with an organic coating applied to it for automobiles are developed, and various post-treated steel sheets are developed and put to practical use for household appliances not only because of corrosion resistance but also because of the need for process omissions. Has been done. As post-treatments for home appliances, chromate treatments and organic composite treatments that further apply an organic film are the main ones, but electrogalvanizing is often used as the base plating. Compared with Zn-based alloy plating developed for automobiles, electrogalvanization is inferior in corrosion resistance with the same amount of adhesion, but it is easy to manufacture and is also advantageous in terms of cost, and has the advantage of compatibility with chromate treatment. There is. For this reason, the base plating is fixed to electrogalvanizing, and post-treatments according to needs are applied to this, thereby responding to diversifying needs such as corrosion resistance and omission of steps in the field of home appliances.

Under such circumstances, in post-processing steel sheets, the post-treated steel sheets are often assembled and used as they are for home electric appliances. As a result, the requirements for appearance quality become stricter, and the minute appearances that have been overlooked so far. Dirt is becoming ignorable. As a method for producing an electrogalvanized steel sheet having an excellent appearance, a lower layer plating is formed with an acidic electrogalvanizing bath containing a conductivity aid in JP-A-4-74887 and JP-A-4-74888. Then, a method of forming the upper layer plating in another galvanizing bath containing no conductivity auxiliary agent is disclosed, but it has not yet been possible to improve fine appearance stains.

[0004]

Problems in appearance are distinguished between those occurring in the electrogalvanizing process including post-treatment and those caused by the original plate. If the cause is identified, the solution is easy, but especially in the case of the original plate, the manufacturing process is extremely complicated, so it is often difficult to identify,
Even if they can be identified, the current situation is that they cannot reach a radical solution. Moreover, even if it is apparently due to the original plate, it cannot be confirmed at the stage of the original plate, and there are many problems of the type that can be confirmed only by performing electrogalvanizing. Therefore, in the electrogalvanizing process, 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 impairing the original quality characteristics such as corrosion resistance. The present invention is intended to solve the above problems, and an object of the present invention is to provide a method for producing an electrogalvanized steel sheet having an excellent surface appearance.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have found that the poor appearance of an electrogalvanized steel sheet caused by minute flaws or stains that cannot be confirmed on the original plate is due to a subtle difference in the initial electrodeposition of Zn. I found him. Therefore, as a result of earnest studies on the homogenization of the initial Zn electrodeposition, it was found that it is effective to perform initial Zn plating in an acidic zinc plating bath containing a water-soluble organic compound. The present invention has been made based on this finding, and the summary thereof is as follows.

[0006] When galvanizing a steel sheet,
First, 0.1 to Zn plating is performed at a relative flow rate of 0.5 m / sec or more using an acidic zinc plating bath containing a water-soluble organic compound.
5 g / m ^{2 is} deposited, and then Zn plating is performed so as to obtain a total of 10 g / m ² or more with the Zn plating first deposited using an acidic zinc plating bath containing no water-soluble organic compound. There is a method for producing an electrogalvanized steel sheet having an excellent surface appearance.

[0007]

The present invention has the following advantages when electrogalvanizing
First, the initial Zn plating is performed using an acidic zinc plating bath containing a water-soluble organic compound. The function and effect are to improve the poor appearance caused after the electrogalvanization due to minute flaws and stains on the surface of the steel sheet. The defective appearance portion has a different color tone by visual observation, and it can be confirmed by microscopic observation that the size and orientation of the plated crystal are not uniform as compared with the normal portion. It is considered that, in the micro-defects, the zinc plating deposition, especially the initial deposition, becomes non-uniform due to its shape or segregation of steel components. On the other hand, when the initial Zn plating is performed using the acidic zinc plating bath containing the water-soluble organic compound, Zn is uniformly deposited even in the micro-defects as in the normal part.

Next, when main Zn plating is performed using an acidic zinc plating bath containing no water-soluble organic compound, the Zn plating can grow uniformly with the initial plating uniformly deposited as a nucleus, and a uniform appearance is obtained. Come to present. Although the entire Zn plating may be performed in an acidic zinc plating bath containing a water-soluble organic compound, the action of the water-soluble organic compound causes the plated crystals to become finer and finally gives a glossy appearance different from the conventional one. An acidic zinc plating bath containing a water-soluble organic compound is applied to the formation of the initial Zn plating in order to make the appearance of the product comparable to the conventional one while improving the appearance stain caused by micro-defects. As the water-soluble organic compound of the present invention, a compound effective for suppressing local current concentration on the steel sheet and increasing nucleation density of Zn plating is used. Specific examples include cationic organic compounds containing quaternary amine salts, oxy compounds containing oxygen, and sugars. The average molecular weight is preferably about 10 ² to 10 ⁷ . By containing such a water-soluble organic compound, the initial Zn plating is uniformly formed, but in order to further enhance this effect, it is necessary to consider the liquid flow rate.

FIG. 1 shows the effect of the initial Zn plating amount on the appearance defect due to micro-defects. Actual microscopic flaws occur indefinitely.Therefore, a steel plate is irradiated with a laser beam in a simulated manner, electrogalvanization is performed using this as a base plate. It was confirmed. By repeating this process, the irradiation conditions were set such that the appearance stains could be confirmed by electrogalvanizing, 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 a steel plate having a micro-defect that has been simulated in this way as a base plate, and initial Zn plating is performed in an acidic zinc plating bath containing a water-soluble organic compound, and then Zn plating is performed in an acidic zinc plating bath containing no water-soluble organic compound. This is a summary of the relationship between the initial Zn plating amount and the occurrence rate of appearance defects when performing.

The initial Zn plating was carried out using a sulfuric acid plating bath having a pH of 2 containing 60 ppm of Zn ions and 20 ppm of polyacrylamide having an average molecular weight of 10 ⁶ as a water-soluble organic compound and a bath temperature of 60 ° C., and a relative flow rate of 1 m / sec. The current density was 10 to 100 A / dm ² , and the main Zn plating was a zinc ion 70 g / l, pH 1, sulfuric acid acidic plating bath with a bath temperature of 55 ° C., a relative flow rate of 1.5 m / sec, a current density of 70 A /
dm ² and the amount of adhesion is 20 in total with the initial Zn plating.
It was made to be g / m ² . Processing 10 sheets under the same conditions,
The rate of appearance defects was calculated. As is clear from FIG. 1, the occurrence rate is 1 even at the mild level 1 in the initial Zn plating 0.
Although it is 00%, the occurrence rate starts to decrease at the initial Zn plating of 0.1 g / m ² and decreases to 10% or less at 0.5 g / m ² or more. Further, at level 2, it is reduced to 10% or less at 1 g / m ² or more.

FIG. 2 shows the influence of the relative flow velocity at the time of initial Zn plating on the appearance defect due to micro-defects. The initial Zn plating and the main Zn plating conditions are the same as in FIG. 1, the initial Zn plating amount is 1 g / m ² , and the main Zn plating amount is 10 g.
/ M ² . The generation rate tends to decrease as the liquid flow velocity increases, and at 0.5 m / sec or higher, the generation rate drops to 50% or less even with severe dirt.

As described above, by performing the initial Zn plating in the acidic zinc plating bath containing the water-soluble organic compound, the appearance stain due to the original plate of the electrogalvanized steel sheet is reduced. The range of the initial Zn plating amount is 0.1 to 5 g
/ M ² is good. If it is less than 0.1 g / m ² , there is no effect on appearance stain as shown in FIG. Even if it exceeds 5 g / m ² , the effect on the appearance stain is maintained, but the final appearance becomes different from the conventional one, which is not preferable. A more preferable range is 0.5 to 3 g / m ² . Early Zn
The relative flow velocity at the time of plating is preferably 0.5 m / sec or more. If it is less than 0.5 m / sec, the effect on the appearance stain is small as shown in FIG.

Here, the relative flow velocity is the difference between the liquid flow velocity and the strip passing velocity in consideration of the flow direction of the liquid and the strip passing direction of the steel sheet. When the liquid is passed in the opposite direction to the traveling direction of the steel sheet, the relative flow velocity is However, even if 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 plate passing speed of the steel sheet should be sufficiently increased to make the relative The effect of the present invention is exhibited if a flow velocity of 0.5 m / sec or more is secured. The composition conditions of the initial Zn plating bath are not particularly limited, but Zn ions 10 to 100
g / l, water-soluble organic compound 0.01 to 10 g / l, pH
A 1.5 to 4 sulfuric acid bath is effective and the current density is 1
0-100 A / dm ² is preferable.

The total amount of the main Zn plating and the initial plating is preferably 10 g / m ² or more. When it is less than 10 g / m ² , the effect of the initial Zn plating becomes small, and the effect of the initial Zn plating on the final appearance becomes large. As the plating bath, a sulfuric acid acidic bath containing Zn ions in an amount of 40 to 100 g / l and having a pH of 0.5 to 2 is preferable. In addition to Zn ions, total Ni ions and Fe ions are 0.5 to 5 g /
If it is included, the effect of the present invention is further enhanced, and the occurrence rate of level 2 in FIG. 1 is further reduced by 10% or more.

[0015]

【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 electrogalvanizing (main Zn plating) of 2 were performed, appearance stains were generated on all 10 sheets, and the degree was more severe as the plate thickness increased. Next, using a sulfuric acid bath of pH 3 containing 30 g / l of Zn ions and 1 g / l of polyethylene glycol having an average molecular weight of 10 ³ as a water-soluble organic compound, a bath temperature of 40 ° C., a current density of 15 A / dm ² , and a relative flow rate of 3 m / sec. After performing initial Zn plating of 0.5 g / m ² and performing 10 main Zn platings under the above conditions, a plate thickness of 0.8 m
At m, there is no appearance stain, and at 1.0 mm, 2 sheets,
At 1.2 mm, four sheets had stains on their appearance, but the extent was lighter than that without initial Zn plating.

Example 2 The same original plate as in Example 1 was prepared by adding Zn ions 70 g / l and dextrin 2 g / l as a water-soluble organic compound to pH 1.
5. Using sulfuric acid bath with bath temperature of 50 ℃, current density 60A / dm
² , 10 g of 17 g / m ² main Zn plating was performed under the conditions of Example 1 after performing initial Zn plating of 3 g / m ² at a relative flow rate of 1 m / sec. Nothing at all, 1 piece at 1.0 mm, 3 pieces at 1.2 mm
Although the appearance stains on the sheet were generated, the extent of the stain was lighter than that without the initial Zn plating. Under the same conditions, initial Zn plating of 5 g / m ² + main Zn plating 15 g / m ² and initial Zn plating of 10 g / m ² + main Zn plating of 10 g / m ² were applied. The number of stains on appearance was reduced to 1 and 2, respectively. However, the surface appearance of the initial Zn plating of 10 g / m ² was more glossy than the others.

EXAMPLE 3 A sulfuric acid bath having a pH of 2 containing 50 g / l of Zn ions and 1 g / l of a polyamine cation polymer having an average molecular weight of 10 ⁴ as a water-soluble organic compound was used on the same original plate as the water density of 40 A / dm. ^{2. After} applying ² g / m ² of main Zn plating at a relative flow rate of 2 m / sec, Zn ions 70 g / l, N
pH containing i ion 0.5 g / l and Fe ion 2 g / l
1.5, using a sulfuric acid bath with a bath temperature of 50 ° C., a relative flow rate of 1 m /
When 10 sheets of electrogalvanizing were applied at a current density of 40 A / dm ² and an adhesion amount of 20 g / m ² , the appearance of stains was completely absent at any plate thickness.

[0018]

As described above, the present invention provides a method for improving fine appearance stains after electrogalvanizing caused by an original plate. In particular, it is suitable as a manufacturing method for electrogalvanizing a surface of a surface-treated steel sheet for home electric appliances which is used after being assembled and used as it is, and is extremely effective in responding to 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 micro-defects and the initial Zn plating amount,

FIG. 2 is a diagram showing the relationship between the appearance defect occurrence rate and the relative flow velocity at the time of initial Zn plating.

Claims (1)

[Claims] 1. When electrogalvanizing a steel sheet, first, Zn plating is performed at a relative flow rate of 0.5 m / sec or more using an acidic zinc plating bath containing a water-soluble organic compound.
1 to 5 g / m ² deposited, then Z deposited first using an acidic galvanizing bath containing no water-soluble organic compounds
Zn so that the total with n plating is 10 g / m ² or more
A method for producing an electrogalvanized steel sheet having an excellent surface appearance, which is characterized by plating.