JPH0474888A - Method for manufacturing electrogalvanized steel sheets with excellent appearance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing an electrogalvanized steel sheet with excellent appearance, especially brightness.

[Conventional technology]

Electrogalvanized steel sheets are produced by continuously moving a strip-shaped steel sheet through an acidic electrogalvanizing bath in an electrogalvanizing tank, and then connecting the anode plate placed in the electrogalvanizing tank with the moving steel sheet. In the meantime, it is produced by electrogalvanizing the steel sheet through an electrogalvanizing bath.

The electrogalvanizing baths mentioned above mainly include sulfuric acid plating baths containing sulfuric acid as the main component, mixed plating baths containing zinc sulfate and zinc chloride as the main components, and chloride plating baths containing zinc chloride as the main component. It is used.

Incidentally, in recent years, in order to improve productivity, electrolytic galvanizing using high current density has become popular. When a self-fusing zinc anode is used as the anode plate in such electrolytic galvanizing using a high current density, the anode plate is subject to severe wear and tear, resulting in the problem that it must be replaced frequently. For this purpose, insoluble anodes are used as anode plates, which rarely require replacement.

When performing electrogalvanizing with such an insoluble anode, if a mixed plating bath containing zinc sulfate and zinc chloride as the main components or a chloride plating bath containing zinc chloride as the main component is used as the electrogalvanizing bath, Insoluble anodes generate large amounts of harmful chlorine gas. Therefore, when electroplating is performed using an insoluble anode, a sulfuric acid plating bath containing zinc sulfate as a main component is used as the electrogalvanizing bath.

The sulfuric acid dip bath has a lower electrical type conductivity than the mixed plating bath and the chloride plating bath described above. For this reason, when a sulfuric acid plating bath is used, a conductivity auxiliary agent has traditionally been added to the plating bath. Such conductivity auxiliary agents increase the conductivity of the plating bath and contribute to lowering the plating voltage. Therefore, with the addition of conductivity aids,
Power saving can be achieved.

[Problem to be solved by the invention]

When a steel sheet is electrogalvanized using a sulfuric acid plating bath to which a conductivity additive is added, particularly at a high current density of 60 A/dm' or more, the appearance of the plated steel sheet becomes slightly darkish.

The reason for this is as follows.

That is, when a conductivity auxiliary agent is added to a sulfuric acid plating bath, the viscosity coefficient of the plating bath increases, and it participates in electrophoresis at the cathode surface, thereby affecting the limiting current density and lowering the limiting current. Such a decrease in limiting current affects the orientation of the plating crystals. That is, the smooth (0002) planes that tend to precipitate at low overvoltages are reduced, and the pyramidal (11?0) and (10-1) planes are strongly oriented. As a result, the unevenness of the plating crystal becomes severe.

The appearance of the plating layer is affected by the plating crystal, and if the plating crystal has irregularities, it will absorb light more easily, reducing the brightness of the plating layer.

Therefore, when a steel plate is electrogalvanized at high current density using a sulfuric acid plating bath to which a conductivity additive has been added, severe unevenness occurs in the plating crystals, and the brightness of the plating layer decreases to L,
The brightness index value of the a and b color system decreases to about 70 to 80.

Electrogalvanized steel sheets whose surfaces are coated, such as steel sheets for automobiles and home appliances, or electrogalvanized steel sheets where the appearance is not a problem, such as those used in areas that are not visible even without painting. In the case of a plated steel sheet, there is almost no problem even if the brightness of the plated layer is low as described above.

However, demand has increased in recent years for electrogalvanizing, which is used on visible parts of home appliances and other appliances, and which has been given excellent fingerprint and corrosion resistance through chromate treatment, without painting the surface. In the case of steel sheets, the above-mentioned reduction in the brightness of the plating layer causes a major product defect.

As a method for improving the appearance of an electrogalvanized steel sheet, for example, Japanese Patent Application Laid-Open No. 64-52099 discloses a method comprising subjecting an electrogalvanized steel sheet to temper rolling. However, in the case of this method, a problem arises in that the material quality of the electrogalvanized steel sheet deteriorates due to temper rolling.

Further, Japanese Patent Application Laid-Open No. 14890/1989 discloses a method in which an organic additive is added to an electrogalvanizing bath and a steel sheet is electrolytically galvanized using such a plating bath. However, in this method,
Continuous control of the concentration of the organic additive is difficult, and the problem arises that it adversely affects the life of the insoluble electrode.

Furthermore, none of the above-mentioned prior art is aimed at improving the brightness of the plating layer.

Therefore, an object of the present invention is to provide a method for producing an electrogalvanized steel sheet with an excellent appearance in which the brightness of the plating layer does not decrease even when the surface of the steel sheet is electrogalvanized at high current density. There is a particular thing.

[Means to solve the problem]

The present inventors have conducted extensive research in order to solve the above-mentioned problems. As a result, an electrogalvanized layer was formed as a lower layer on the surface of the steel sheet in an acidic zinc electroplating bath containing zinc sulfate as a main component and a conductivity additive, and then zinc sulfate and zinc chloride were added to the surface of the steel sheet. In another mixed acidic zinc electroplating method, which is made mainly of It has been discovered that excellent electrogalvanized steel sheets can be manufactured.

The present invention was made based on the above-mentioned findings, and is based on an acidic electrolytic galvanizing bath containing zinc sulfate as a main component and at least one of sodium sulfate, ammonium sulfate, and potassium sulfate as a conductivity auxiliary agent. In,
An electrogalvanized layer is formed as a lower layer on the surface of the steel sheet by using a high current density of 60 A/dm' or more, and then
In a separate mixed acidic electrogalvanizing bath containing zinc sulfate and zinc chloride as main components, a high current density of 50 A/dm' or more is applied to the electrogalvanized layer as the lower layer.
It is characterized by forming an electrogalvanized layer as an upper layer having a plating amount of 1 g/m2 or more.

The plating bath for the electrogalvanized layer as the lower layer contains zinc sulfate (ZnSo, 7H, O), sodium sulfate (NazSOt), potassium sulfate (K2SO,), and ammonium sulfate as conductivity aids. ((N
A plating bath containing at least one of H, 1, SO,) is used. Such conductivity auxiliary agents increase the conductivity of the plating bath and contribute to lowering the plating voltage. The content of sodium sulfate is preferably 20 to 100 g#', the content of ammonium sulfate is 10 to 60 g/l, and the content of potassium sulfate is preferably 10 to 70 g/l.

A mixed acidic electrogalvanizing bath containing zinc sulfate and zinc chloride as main components is used as the plating bath for the electrogalvanized upper layer. The crystals of the plating layer formed by such a mixed acidic electrogalvanizing bath are smooth, and in particular, compared to the crystals formed by a sulfuric acid plating bath, Cf- ions contained in the plating bath are adsorbed to the cathode. hand,
Reduce overvoltage. As a result, it has a high brightness CL value with few smooth (0002) planes that tend to precipitate at low overvoltages.

5 or more) is obtained.

Furthermore, the plated surface formed by the mixed acidic electrogalvanizing bath described above has short wavelength (10 to 100 μm) undulations that affect the appearance, especially the gloss and gloss. Therefore, the brightness of this plated surface is higher than that of a plated surface formed by a sulfuric acid plating bath.

The plating current density when forming the electrogalvanized layer as the upper layer needs to be 50 A/dm2 or more. If the plating current density is less than 5 OA/d1, the crystals will not become dense due to low overvoltage, and the brightness cannot be increased.

The plating amount of the electrogalvanized layer as the upper layer is I g7
It needs to be more than m2. The amount of plating is 1 g/
If it is less than lr+2, the lower layer cannot be sufficiently coated and therefore the brightness cannot be increased. As mentioned above, the plating bath for forming the electrogalvanized layer as the upper layer is zinc sulfate and zinc chloride. This is a mixed acidic electrogalvanizing bath containing as the main components. Therefore, a self-soluble zinc anode must be used. For this reason, it is necessary to replace the anode, so it is preferable to form the upper layer in the final plating bath.

Next, the present invention will be explained based on examples.

〔Example〕

A horizontal continuous electroplating device consisting of a JO-based horizontal plating bath is used to coat the surface of a steel plate as described below.
Electrogalvanized specimens of the present invention Na 1 to 16
was prepared.

Nα1-9 plating bath Plating bath Zinc sulfate (ZnSO, 7H20)...450g/
l Sodium sulfate (NazSOt)・+ −・−−70g
/l anode Insoluble anode 2 pieces per 1 plating tank Length: 171 pieces
/l zinc chloride (ZnC12)...100g/
L, a, b of the plating layer formed in this way
.

The brightness index of the color system was measured using a multi-light source spectrophotometer (MSC-!S-28 manufactured by Suga Test Instruments Co., Ltd.). Table 1 shows the plating conditions and brightness index of each specimen. show.

[Comparative example]

Using the same plating apparatus as in the example, electrogalvanizing was performed on the surface of the steel sheet as described below to prepare comparative specimens NQ 1 to 16.

Nα1~1O plating bath Plating bath Zinc sulfate (ZnSO+・71(20) + 6 + 8
+ 450 g/l sodium sulfate (Na2so<)-+
+ H+ + 70g/12 anodes Insoluble anode 2 pieces per plating tank Length 1m/] L, a, b of the plating layer thus formed
The brightness index of the color system was measured using a multi-light source spectrophotometer (MSC-Is-2B manufactured by Suga Test Instruments Co., Ltd.). Table 2 shows the plating conditions and brightness index of each specimen. .

As is clear from Table 2, the brightness of the plating layer of the comparison specimen was low, ranging from 70 to 80, and its appearance was darkish. On the other hand, the brightness of the plating layer of the specimen of the present invention is
A high and beautiful plating surface was obtained at a value of 85 or higher.

〔Effect of the invention〕

As described above, according to the present invention, an electrogalvanized steel sheet with a dull appearance in which the brightness of the plating layer does not decrease even when electrogalvanizing is applied on the surface of the steel sheet at high current density. This brings about excellent industrial effects.

Claims (2)

[Claims] (1) In an acidic electrogalvanizing bath containing zinc sulfate as the main component and at least one of sodium sulfate, ammonium sulfate, and potassium sulfate as a conductivity auxiliary agent, with a high current density of 60 A/dm^2 or more, An electrogalvanized layer is formed as a lower layer on the surface of the steel sheet, and then the electrogalvanized layer is heated to a high temperature of 50 A/dm^2 or more in another mixed acidic electrogalvanized bath containing zinc sulfate and zinc chloride as main components. Electrolytic zinc with an excellent appearance, characterized by forming an electrogalvanized layer as an upper layer with a plating amount of 1 g/m^2 or more on the electrogalvanized layer as a lower layer by using a current density. A method for producing plated steel sheets. (2) The content of the sodium sulfate as the conductivity auxiliary agent is 20 to 100 g/l, the ammonium sulfate content is 10 to 60 g/l, and the potassium content is , 10-70 g/l.