JP2013079426A - Production method for zinc-electroplated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method for a zinc-electroplated steel sheet with a chromate-free chemical conversion film having both corrosion resistance and high conductivity.SOLUTION: A zinc-electroplating process 2 is carried out on the surface of a steel sheet 1 in a sulfate zinc-electroplating bath containing one or more elements selected from sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate in an amount of ≥0.75 mol/L in total and having a pH value of ≥1.2 and ≤2.5, at a current density of ≥220 A/dmand ≤500 A/dmto form a zinc-electroplated steel sheet. Further, a chromate-free chemical conversion film 3 having an average thickness of ≥0.05 μm and ≤1.0 μm is formed as a top layer, thereby a zinc-electroplated chromate-free chemical conversion-treated steel sheet is formed.

Description

  The present invention relates to a method for producing an electrogalvanized steel sheet, and more particularly to an improvement in conductivity of a chromate-free chemical conversion electrogalvanized steel sheet.

  In recent years, with the development of digital home appliances typified by flat-screen televisions, leakage of electromagnetic waves from these electric and electronic products has become a major problem. Electromagnetic waves leaking from electrical / electronic products act as interference waves for other electrical / electronic products and cause malfunctions. For this reason, standards for electromagnetic wave leakage of electrical and electronic products have been established both in Japan and overseas, and CISPR (International Committee for Radio Interference) and VCCI (Voluntary Control Council for Radio Interferences such as Information Processing Equipment) have a frequency of 30 to 1000 MHz. The upper limit value of the electric field strength is specified for the electromagnetic wave leaking at. In addition, it is necessary to take measures to prevent failures such as malfunctions due to external electrical interference.

  As measures against these problems, for example, thin-screen televisions such as liquid crystal displays and PDPs (plasma display panels) use a steel plate having electromagnetic wave shielding properties as a material for a shield box or the like for the purpose of electromagnetically shielding a built-in circuit board. ing. Here, the electromagnetic wave shielding property of the steel sheet depends on the electrical conductivity of the steel sheet, and the higher the electrical conductivity, the better the electromagnetic wave shielding property. Therefore, steel plates used as constituent members of electric / electronic products are required to have excellent conductivity. On the other hand, since electric / electronic products are used in various corrosive environments, it is important that these components have desired corrosion resistance in an actual use environment. In particular, design properties such as fingerprint resistance are also required in shield boxes for flat-screen televisions.

  As described above, components of electrical and electronic products such as flat-screen TVs need to be applied with materials that have high conductivity as well as excellent corrosion resistance and fingerprint resistance (design properties). Has been. Therefore, as a material of members constituting electric / electronic products, chemical conversion treatment is applied to electrogalvanized steel sheets that have electromagnetic wave shielding properties and excellent corrosion resistance, in order to further improve characteristics such as corrosion resistance and fingerprint resistance. The applied electrogalvanized base chemical conversion steel sheet is widely used.

  Conventionally, chromate treatment has been widely used as the chemical conversion treatment described above, but a change to chromate-free chemical conversion treatment is being promoted from the viewpoint of reducing the environmental load. However, in order to develop sufficient corrosion resistance by the chromate-free chemical conversion treatment, it is necessary to increase the film thickness of the treatment film as compared with the chromate treatment. Since the chemical conversion treatment film is usually insulative, the electrogalvanized steel sheet provided with the chromate-free chemical conversion treatment film has a significantly lower conductivity than the electrogalvanized steel sheet provided with the chromate film.

  Here, in many cases, the structural members of electric and electronic products such as a shielded box of a thin television are formed in a desired shape by joining steel plates as raw materials. Therefore, when the electrogalvanized base chemical conversion treated steel sheet having a significantly reduced conductivity as described above is applied to a component of an electrical / electronic product, the contact surfaces between the electrogalvanized base chemical treated steel sheets, that is, the chemical conversion coatings A sufficient electromagnetic shielding characteristic cannot be ensured at the portion where the contact is made, and the electromagnetic wave leaks.

For the above reasons, there is a strong demand for a technique for improving conductivity in the electrogalvanized base chemical conversion treated steel sheet.
In order to increase the electrical conductivity of a steel sheet provided with an insulating film, it is extremely effective to control the coverage of the chemical conversion film, and various techniques have been proposed so far.

For example, Patent Document 1 discloses that silica is added to an upper layer of a metal plate, a chromate-coated metal plate or a chromate-coated plated metal plate having a surface roughness of 1.0 μm or less in the center line average roughness Ra (JIS B 0601). A technique has been proposed in which an organic composite film containing 35% and having an adhesion amount of 0.25 to 1 g / m ² is formed so that the covering area ratio is 70 to 99%. Patent Document 2 discloses that a solid lubricant is applied to an upper layer of a metal plate, a chromate-coated metal plate or a chromate-coated plated metal plate having a surface roughness of 1.0 μm or less in centerline average roughness Ra (JIS B 0601). There has been proposed a technique for forming an organic composite film containing 1 to 30% by weight so that the covering area ratio is 70 to 99%. And according to these technologies, the surface of the original plate (metal plate, chromate-coated metal plate or chromate-coated plated metal plate) before coating the organic composite film is provided with irregularities so that the organic composite film enters the concave portions of the original plate surface. By providing a part of the original plate that is not covered with the organic composite film (that is, the part where the underlying metal or chromate film is exposed from the organic composite film), even a steel plate with an insulating film has excellent conductivity. It is said that sex can be obtained.

  However, in JIS B 0601 (and JIS B 0651 cited in the same JIS), the cut-off value when measuring the center line average roughness Ra is defined as 0.08 mm or more. As is clear from this cut-off value, the techniques proposed in Patent Documents 1 and 2 give relatively long wavelength irregularities to the surface of the metal plate before forming the organic composite film, and As shown in FIG. 2, the plating layer 21 and the chromate film 22 are formed along the contour of the irregularities on the surface of the metal plate 10 as shown in FIG. Some are intended to be exposed from the organic composite film 30. Therefore, although the area ratio of the exposed portion of the metal or chromate film itself is as low as 1 to 30%, there is a problem that the area per exposed portion becomes large and sufficient corrosion resistance cannot be obtained.

  Patent Document 3 discloses a technique for controlling the surface form of a metal material so that the coverage of the surface treatment film is 70% or more and less than 100%, specifically, rolling a metal plate with a roll having a recess. A technique has been proposed in which the surface treatment film is formed to have a coverage of 70% or more and less than 100% by forming a surface treatment film after providing a convex portion having a height of 0.5 to 30 μm on the surface. And according to this technique, it is said that the electrical conductivity which was excellent even if it was the steel plate provided with the insulating film by exposing the convex part of the surface of a metal plate from a surface treatment film.

  However, it is impossible to form irregularities with short wavelengths of the order of microns or less on the surface of the metal plate by rolling the metal plate with a roll having concave portions as described above. Therefore, even with the technique proposed in Patent Document 3, the unevenness imparted to the original plate (metal plate) before the surface treatment film is formed is uneven with a relatively long wavelength, and the techniques proposed in Patent Documents 1 and 2 Similarly, there was a problem that sufficient corrosion resistance could not be obtained. In Patent Document 3, it is said that a desired convex portion can be provided on the surface of the metal plate by adjusting the deposition conditions and depositing the electroplating in a granular manner on the metal plate. Does not describe any specific electroplating conditions.

  As described above, in order to develop electrical conductivity in the electrogalvanized base chemical conversion treated steel plate, which is a steel plate provided with an insulating coating, it is necessary to form a portion where the electrogalvanizing is exposed from the insulating coating. However, each of the above prior arts intentionally controls the surface irregularities of the base steel plate before plating, so that the base steel plate after plating (follows the irregularities of the base steel plate and is formed with a substantially constant film thickness). A thin part or an exposed part of the chemical conversion film was formed on the convex part of the film. Here, the wavelength of the unevenness of the base steel plate is usually on the order of 10 μm or more although it depends on the shape of the skin pass roll. Therefore, in the conventional technology, the surface unevenness of the base steel sheet becomes unevenness of a relatively long wavelength, so the area per one exposed portion becomes large, and although the conductivity is improved, the corrosion resistance, which is an inherent important characteristic, is improved. There was a problem of being lowered.

With respect to such a problem, Patent Document 4 relates to an electrogalvanized steel sheet with a chemical conversion coating, and has a maximum height roughness (Rz) of 0.6 to 1.1 μm (provided that the cut-off value is 0.01 mm) on the steel sheet surface. By forming an electrogalvanized layer and forming a chemical conversion treatment film having an average film thickness of 0.05 to 1.0 μm on the upper layer of the electrogalvanization layer, the area ratio of the exposed portion with respect to the chemical conversion treatment film is determined as the electrogalvanized coating area. The technology which makes 0.3 to 1.0% of is proposed. Moreover, as a specific means for forming an electrogalvanized layer having a maximum height roughness (Rz) of 0.6 to 1.1 μm (however, a cut-off value: 0.01 mm) on the steel sheet surface, the electrolytic bath is an aqueous sulfuric acid solution, Means for setting the electrolytic current density in the range of 100 to 200 A / dm ² have been proposed. The maximum height roughness Rz is the “maximum height roughness” defined in JIS B 0601 (2001). The reason why the cut-off value λc is set to 0.01 mm (10 μm) in the above is to extract and evaluate the micro unevenness of the electrogalvanized crystal from the unevenness having a wavelength of 10 μm or more due to the surface steel plate. .

JP-A-10-330955 JP-A-10-330956 JP 2005-139551 A JP 2011-32528 A

  The technique proposed in Patent Document 4 is a technique for imparting fine irregularities (wavelength: irregularities with a short wavelength on the order of micron to submicron order) due to zinc crystals to the surface of the electrogalvanized layer. As shown, the convex portion of the fine irregularities on the surface of the electrogalvanized layer 2 formed on the steel plate 1 is to be exposed from the chemical conversion coating 3. Even if there is no relatively large zinc exposed portion (FIG. 2) corresponding to the unevenness of the underlying steel plate formed by the skin pass, a zinc exposed portion of a submicron level (FIG. 2) formed by minute unevenness caused by the zinc crystal. Even in 1), it is possible to ensure high conductivity. Moreover, if the zinc exposure part is very fine as shown in FIG. 1, the influence on corrosion resistance can also be suppressed very small. Therefore, according to the technique proposed in Patent Document 4, the conductivity and corrosion resistance of the electrogalvanized steel sheet with a chemical conversion coating can be highly compatible.

However, in the technique proposed in Patent Document 4, the current when forming an electrogalvanized layer having a maximum height roughness (Rz) of 0.6 to 1.1 μm (however, a cutoff value λc: 0.01 mm) on the surface of the steel sheet. The density is at most 200 A / dm ² . Therefore, in order to further improve productivity, a technique for forming an electrogalvanized layer having a maximum current roughness (Rz) of 0.6 to 1.1 μm at a higher current density has been desired.
That is, for electronic and electrical products such as flat-screen TVs that are mass-produced, it is required to stably produce and supply the chemical conversion electrogalvanized steel sheet as the material with high productivity. In the technique proposed in No. 4, although a high-quality electrogalvanized base chemical conversion treated steel sheet having desired characteristics can be obtained, productivity is limited, and a technique for further improving productivity has been demanded.

  The present invention was developed based on the above-mentioned present situation, and the object of the present invention relates to a method for producing an electrogalvanized steel sheet with a chromate-free chemical conversion coating film that has both excellent conductivity and corrosion resistance, and is intended to increase productivity. It is to provide a possible method.

In the technique proposed in Patent Document 4, when the current density when forming the electrogalvanized layer on the steel sheet surface exceeds 200 A / dm ² , the maximum height roughness Rz (cutoff value λc: 0.01 mm) is less than the desired value, and it is said that the electrogalvanized steel sheet after the chemical conversion treatment is not sufficiently conductive.
Therefore, in order to achieve higher productivity, the present inventors, when forming an electrogalvanized layer on the steel sheet surface, have a maximum height roughness even when a high current density exceeding 200 A / dm ² is used. The inventors have intensively studied means for stably forming an electrogalvanized layer having an Rz of 0.6 to 1.1 μm (cutoff value λc: 0.01 mm).

As a result, when forming an electrogalvanized layer on the steel sheet surface, the electrogalvanizing bath contains a predetermined amount of one or more selected from sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate, and has a predetermined pH. Electricity with a desired maximum height roughness Rz: 0.6-1.1 μm (cut-off value λc: 0.01 mm) by using a sulfuric acid electrogalvanizing bath and further setting the current density to 220-500 A / dm ² It has been found that a galvanized layer can be obtained.
Furthermore, when the chromate-free chemical conversion treatment is applied to the electrogalvanized steel sheet thus obtained, the electrogalvanization with high conductivity and corrosion resistance is achieved by controlling the film thickness of the chromate-free treatment film to an appropriate range. It was found that a base chromate-free chemical conversion treated steel sheet can be obtained stably.

The present invention has been made based on these findings, and the gist thereof is as follows.
[1] In the method for producing an electrogalvanized steel sheet by subjecting the surface of the steel sheet to electrogalvanizing treatment to obtain an electrogalvanized steel sheet, the electrogalvanizing treatment is one or two selected from sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate. It is characterized in that the current density is 220 A / dm ² or more and 500 A / dm ² or less in a sulfuric acid electrogalvanizing bath containing 0.75 mol / L or more in total and having a pH of 1.2 to 2.5. A method for producing an electrogalvanized steel sheet.

[2] Electrogalvanizing characterized in that a chromate-free chemical conversion coating film having an average thickness of 0.05 μm or more and 1.0 μm or less is further formed as an upper layer on the electrogalvanized steel sheet produced by the method described in [1]. A method of manufacturing a steel sheet.

  According to the method of the present invention, an electrogalvanized base chromate-free chemically treated steel sheet excellent in both corrosion resistance and conductivity, suitable for use in home appliances, electronic / electric equipment, etc., can be stably produced at an industrial level. It becomes possible to produce.

It is the figure which showed typically the surface part of the electrogalvanization base chromate-free chemical conversion treatment steel plate (cross section) manufactured according to the method of this invention. It is the figure which showed typically the surface part of the steel plate with an insulating film (cross section) manufactured according to the conventional method.

Hereinafter, the present invention will be described in detail.
The present invention controls the precipitation form of zinc crystals formed during the electrogalvanization process, so that the maximum height roughness Rz (JIS B 0601 (2001) “maximum height roughness” ”) Is 0.6 to 1.1 μm (however, cut-off value λc: 0.01 mm), that is, an electrogalvanized layer having fine (short wavelength) irregularities due to zinc crystals as shown in FIG. It is something to try. And this invention is the manufacturing method of the electrogalvanized steel sheet which electrogalvanizes to the steel plate surface, and makes the electrogalvanized steel sheet, The said electrogalvanization process is chosen from sodium sulfate, potassium sulfate, ammonium sulfate, and magnesium sulfate This is a treatment to make the current density 220 A / dm ² or more and 500 A / dm ² or less in a sulfuric acid electrogalvanizing bath containing one or two or more in total of 0.75 mol / L or more and having a pH of 1.2 to 2.5. It is characterized by that.

The steel plate used as the original plate in the present invention is a conventionally known steel plate for plating, and any of a cold-rolled plate and a hot-rolled plate can be suitably used. The plate thickness is preferably 0.2 to 4.0 mm.
In the present invention, a predetermined electrogalvanizing treatment is performed on the steel plate as the original plate, but if necessary, a pretreatment prior to the electrogalvanizing treatment, for example, a degreasing treatment and water washing for cleaning the steel plate surface It is preferable to perform a pickling treatment for activating the steel plate surface and a pretreatment for washing with water.

Subsequently, the steel sheet that has been pretreated as described above is subjected to electrogalvanizing treatment as described above. In the present invention, the plating bath used in electrogalvanizing is sodium sulfate, potassium sulfate, ammonium sulfate, and magnesium sulfate. A sulfuric acid electrogalvanizing bath containing a total of 0.75 mol / L or more of one or more selected from the above and having a pH of 1.2 to 2.5. As a result, when electrolysis is performed at an appropriate current density (220 A / dm ² or more and 500 A / dm ² or less), the surface of the electrogalvanized layer can be provided with irregularities having a wavelength of micron order or less due to zinc crystals. An electrogalvanized layer having an appropriate surface roughness, that is, a maximum height roughness Rz of 0.6 to 1.1 μm when measured at a cutoff value λc of 0.01 mm can be obtained.

When a sulfuric acid bath adjusted to a predetermined zinc concentration is used as a plating bath as in the technique proposed in Patent Document 4, if the current density exceeds 200 A / dm ² , the unevenness of zinc crystals on the surface of the electrogalvanized layer Becomes too fine. Therefore, the maximum height roughness Rz of the electrogalvanized layer measured at a cutoff value λc: 0.01 mm is less than a desired value (less than 0.6 μm). The reason why the unevenness of the zinc crystal becomes finer as the current density is increased is that the nucleation rate of the zinc crystal is increased as the current density is increased as compared with the growth rate of the zinc crystal. Is presumed to be too fine.

Therefore, in the present invention, for the purpose of adjusting the precipitation form of zinc crystals, the sulfuric acid bath adjusted to a predetermined zinc concentration contains one or more selected from sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate. By using such a sulfuric acid electrogalvanizing bath, when the electrogalvanizing process is performed at a current density exceeding 200 A / dm ² , the unevenness caused by the zinc crystals is not excessively refined, and the maximum height roughness An electrogalvanized layer having a thickness of 0.6 to 1.1 μm is obtained. Although the reason why such an effect is obtained is not clear, a sulfuric acid bath adjusted to a predetermined zinc concentration contains one or more selected from sodium sulfate, potassium sulfate, ammonium sulfate, and magnesium sulfate, When plating at a high current density of 220 A / dm ² or more, the overvoltage during electrogalvanization is greatly increased, and the preferential orientation plane of zinc hexagonal dense crystals in galvanization changes from the (0002) plane to the following plane. Presumed to be.

  When the total content of one or more selected from the above-mentioned sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate is less than 0.75 mol / L, the above-mentioned effects cannot be fully exhibited, and the cutoff value λc : The maximum height roughness Rz of the electrogalvanized layer measured at 0.01 mm is less than a desired value (less than 0.6 μm). Therefore, in the present invention, one or two or more kinds selected from sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate are contained in total of 0.75 mol / L or more. Taking into account variations in the concentration of components in the electrogalvanizing bath under actual operation, the content is preferably 1.0 mol / L or more.

  When the total content of one or more selected from the above-mentioned sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate is excessive, these components are not dissolved in the sulfuric acid electrogalvanizing bath and become powdery. It will remain. Therefore, the upper limit of the total content of one or more selected from the above-mentioned sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate is preferably not more than the solubility of these components, 2.0 mol / L or less It is preferable that

  The pH of the sulfuric acid electrogalvanizing bath needs to be in the range of 1.2 to 2.5. If the pH is less than 1.2, the unevenness caused by the zinc crystals formed on the surface of the electrogalvanized layer becomes too small, and the required Rz cannot be obtained. On the other hand, if the pH exceeds 2.5, the current efficiency of zinc electrodeposition is reduced, and the amount of electricity required to obtain a predetermined amount of zinc deposition increases, leading to an increase in manufacturing cost (electricity cost). In consideration of variations in the component concentration and the like in the electrogalvanizing bath under actual operation, the pH is more preferably in the range of 1.6 to 2.2.

Electrolytic current density in the electrolytic zinc plating treatment, it is necessary to be 220 A / dm ² or more 500A / dm ² or less. As described above, when using a sulfuric acid electrogalvanizing bath containing 0.75 mol / L or more in total of one or more selected from sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate, at less than 220 A / dm ² , The unevenness of the zinc crystal on the surface of the electrogalvanized layer becomes too small, and the desired Rz cannot be obtained. On the other hand, when the electrolytic current density exceeds 500 A / dm ² , the current efficiency of zinc electrodeposition is drastically lowered. Therefore, in the present invention, the electrolytic current density in the electrolytic zinc plating treatment, and 220 A / dm ² or more 500A / dm ² or less. Preferably, it is 250 A / dm ² or more and 400 A / dm ² or less.

The zinc concentration in the sulfuric acid electrogalvanizing bath is desirably 1 mol / L or more. If it is less than 1 mol / L, plating burn occurs at a high current density of 220 A / dm ² or more, and the plating appearance tends to be non-uniform. The upper limit of the zinc concentration is practically about 2.5 mol / L in order to prevent the zinc sulfate from precipitating due to a decrease in solubility when the temperature is lowered.

In the present invention, other plating bath conditions are not particularly defined, but it is preferable that the plating bath temperature is 30 to 70 ° C. and the relative flow rate is 0 to 4 m / s. Moreover, electrogalvanizing can also divide electrolysis into multiple times. As zinc adhesion amount per one side of electrogalvanization, 5-30 g / m < ² > is desirable.

As described above, by following the method of the present invention, the electrogalvanizing provided with an electrogalvanizing layer having a maximum height roughness Rz of 0.6 to 1.1 μm (however, a cutoff value λc: 0.01 mm) on the steel sheet surface. Steel sheets can be manufactured.
In addition, the method for producing an electrogalvanized steel sheet with a chromate-free chemical conversion coating of the present invention comprises a chromate-free chemical conversion sheet having an average thickness of 0.05 μm or more and 1.0 μm or less as an upper layer on the electrogalvanized steel sheet produced according to the method of the present invention. A treatment film is formed. As a result, an electrogalvanized steel sheet with a chromate-free chemical conversion treatment film as shown in FIG. 1, that is, a chromate-free chemical conversion treatment in which the projections and depressions of the zinc crystals formed on the surface of the electrogalvanization layer are exposed from the chromate-free chemical conversion treatment film. A coated electrogalvanized steel sheet can be obtained.

  The average film thickness of the chromate-free chemical conversion film in the present invention needs to be 0.05 μm or more and 1 μm or less. If it is less than 0.05 μm, the exposed portion of zinc increases and sufficient corrosion resistance cannot be obtained. On the other hand, when the thickness exceeds 1 μm, the exposed zinc portion decreases and sufficient conductivity cannot be obtained.

  As for the average film thickness of the above chromate-free chemical conversion film, the film thickness is arbitrarily measured at at least 5 points, and the arithmetic average is taken.

  In the present invention, as long as the average film thickness of the chromate-free chemical conversion film is 0.05 μm or more and 1 μm or less, there is no particular limitation on the configuration of the chromate-free chemical conversion treatment film and the chromate-free chemical conversion treatment conditions. The manufacturing method is applicable. That is, the matrix of an inorganic compound or an organic resin may contain an organic pigment or an inorganic pigment depending on the application, and the film may be a single layer or a layer in which a plurality of layers are sequentially laminated. Good.

Alkaline electrolytic degreasing is performed as a pretreatment on an electrogalvanized base plate (cold rolled steel plate with a thickness of 0.7 mm), washed with water, and immersed in a pickling solution at 25 to 30 ° C. to which 50 g / L of sulfuric acid has been added for 5 seconds. Then, it was pickled, then washed with water, and electrogalvanized to give an electrogalvanized steel sheet. The electrogalvanizing bath is based on a zinc sulfate aqueous solution, and one or two kinds selected from sodium sulfate, potassium sulfate, magnesium sulfate and ammonium sulfate are added. It was a bath. Zinc concentration (mol / L) of each plating bath, types and concentrations (mol / L) of additives (sodium sulfate, potassium sulfate, magnesium sulfate, ammonium sulfate), pH, plating bath temperature, current density of electrogalvanizing treatment Table 1 shows (A / dm ² ) and the amount of adhesion (g / m ² ) per one side of the electrogalvanized layer. Moreover, the electrogalvanization was performed in a state where the plating bath was flowed, and the relative flow rate of the electrogalvanizing original plate and the electrogalvanizing bath was 1.5 m / sec.

  For the electrogalvanized steel sheet obtained as described above, the maximum height roughness Rz (cut-off value λc: 0.01 mm) of the electrogalvanized layer was measured using an electron beam three-dimensional roughness analyzer (3D-SEM). Obtained from secondary electron images. Here, the maximum height roughness Rz is “maximum height roughness” defined in JIS B 0601 (2001), and a specific measuring method is as follows.

<Maximum roughness Rz>
Using 3D-SEM (ERAIONX ERA = 8800FE), acceleration voltage: 5kV, measurement area: 120 × 90μm, measurement interval: 0.2μm, cut-off wavelength λc: 0.01mm high-pass filtered height Distribution data was obtained and the maximum height roughness Rz was calculated.
Table 1 shows the maximum height roughness Rz of various electrogalvanized steel sheets.

Further, the various electrogalvanized steel sheets obtained as described above were subjected to chromate-free chemical conversion treatment to obtain electrogalvanized base chromate-free chemical conversion steel sheets. After applying a chromate-free chemical conversion treatment solution containing 0.32 mol / L of primary phosphoric acid in terms of P ₂ O ₅ , 0.50 mol / L of colloidal silica in terms of SiO ₂ and 0.16 mol / L of Mn with a bar coater And dried at 140 ° C. to form a chromate-free treated film (first layer). Mn was supplied as primary phosphate. Next, an organic resin solution containing an epoxy resin was applied on the first layer and baked at 140 ° C. to form a silica-containing organic resin film (second layer).
About the obtained electrogalvanized steel sheet with various chromate-free chemical conversion coatings, the total average film thickness of the first layer and the second layer was measured by the method described above (optionally measured at 5 points). The measurement results are shown in Table 1.

The electrogalvanized steel sheet with chromate-free chemical conversion coating obtained as described above was evaluated for corrosion resistance and conductivity. The evaluation method is as follows.
<Corrosion resistance>
A salt spray test according to JIS Z 2371 (2000) was conducted, and the white rust generation area after 48 hours was visually evaluated. The evaluation criteria are as follows.
◎: No white rust occurrence ○: White rust occurrence area ratio over 0% to 5% or less △: White rust occurrence area ratio over 5% to 20% or less ×: White rust occurrence area ratio over 20%

<Conductivity>
In accordance with JIS K 7194, press the company's four-probe probe ESP probe connected to a low resistance measuring device ("Loresta GP" manufactured by Mitsubishi Chemical Corporation) against the electrogalvanized steel sheet with chromate-free chemical conversion coating, The surface resistance value was measured. In the measurement, the surface resistance was measured while increasing the probe pressing load by 20 g / s, and the pressing load when the surface resistance value was 10 ⁻⁴ Ω or less was defined as the load during energization. This operation was repeated 10 times for each steel plate, the average load during energization was calculated, and evaluated according to the following criteria.
◎: Average energization load is 200 g or less ○: Average energization load is more than 200 g and less than 300 g ×: Evaluation results with average energization load exceeding 300 g or more are shown in Table 1.

As is apparent from Table 1, according to the present invention, an electrogalvanized steel sheet comprising an electrogalvanized layer having a maximum height roughness Rz of 0.6 to 1.1 μm measured at a cutoff value λc: 0.01 mm. In addition, it can be manufactured at a high current density of 220 A / dm ² or more at the time of electrogalvanizing treatment. In addition, by forming a chromate-free chemical conversion treatment film having a predetermined average thickness on the electrogalvanized steel sheet thus obtained, the electrogalvanized steel sheet with a chromate-free chemical conversion treatment film having both good corrosion resistance and conductivity Is obtained.

1… steel plate
2… Electrogalvanized layer
3… Chemical conversion coating
10… Metal plate
21… Plating layer
22… Chromate film
30… Organic composite coating

Claims (2)

In the method for producing an electrogalvanized steel sheet by subjecting the steel sheet surface to an electrogalvanized steel sheet, the electrogalvanized steel sheet comprises one or more selected from sodium sulfate, potassium sulfate, ammonium sulfate and magnesium sulfate. Electricity characterized in that the current density is 220 A / dm ² or more and 500 A / dm ² or less in a sulfuric acid electrogalvanizing bath containing a total of 0.75 mol / L or more and having a pH of 1.2 to 2.5. Manufacturing method of galvanized steel sheet. A method for producing an electrogalvanized steel sheet, comprising forming a chromate-free chemical conversion film having an average thickness of 0.05 μm or more and 1.0 μm or less as an upper layer on the electrogalvanized steel sheet produced by the method according to claim 1. .

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