JP4862484B2 - Method for producing electrogalvanized steel sheet - Google Patents

Description

  The present invention relates to a method for producing an electrogalvanized steel sheet having an appearance with high whiteness.

  Electrogalvanized steel sheets are used in a wide range of applications such as home appliances, automobiles, and building materials. In recent years, the demand for various types of chemically treated electrogalvanized steel sheets for home appliances used without coating has increased, and this has become an important application field. In this application, since it is used without coating, it is required to have an excellent surface appearance. Excellent surface appearance conditions require high whiteness in addition to no surface defects such as unevenness. Since the appearance after various chemical conversion treatments is greatly influenced by the appearance of galvanization before chemical conversion treatment, a production technique capable of obtaining an electrogalvanized steel sheet having high whiteness has been demanded.

  As a method for improving the whiteness of the plated steel sheet, there is a method of adding inorganic ions to the plating bath (for example, Patent Document 1).

  Another method is to add an organic substance to the plating bath. Patent Document 2 discloses a method in which one or two selected from the group of glycine, aspartic acid, a carboxylic acid having two or more carboxylic acid groups, or a salt thereof is added to the plating bath. Patent Document 3 discloses at least one selected from the group consisting of a metal salt of naphthenic acid, an aniline derivative, a lower alkanoyl compound, and an organic peroxide as an electrogalvanizing bath additive.

Patent Document 4 discloses a method of cathodic electrolysis in an acidic bath containing a soluble sulfur-containing organic compound, followed by zinc phosphate treatment.
Japanese Patent Laid-Open No. 9-195082 JP-A-8-74089 Japanese Patent Laid-Open No. 10-287992 Japanese Unexamined Patent Publication No. 7-331457

However, the above patent document has the following problems.
In patent document 1, since it is the method of adding an inorganic substance to a plating bath, an inorganic substance co-deposits in a plating film, and the corrosion resistance of the plating film deteriorates due to the eutectoid or the appearance rapidly changes to become glossy. .

  In Patent Documents 2 and 3, there is a problem that the glossiness increases with an increase in whiteness and a manufacturing cost increases due to a decrease in plating current efficiency.

  In patent document 4, it is the technique for raising the glossiness of metal plating, forming a precise | minute and smooth zinc phosphate film, and raising the L value after a zinc phosphate process. That is, the reduction in whiteness due to the zinc phosphate treatment is as small as less than 8. However, with regard to the appearance after plating, the glossiness increases, but there is no effect of increasing the whiteness.

  As described above, any of the conventional methods is insufficient in increasing the whiteness of the plating. In addition, even if the whiteness can be increased, other characteristics such as a decrease in corrosion resistance and glossing are greatly changed, or the current efficiency is greatly decreased.

  In view of the above circumstances, the present invention has an object to provide a method for producing an electrogalvanized steel sheet having high whiteness without greatly changing glossiness and without greatly reducing current efficiency. To do.

  In order to develop a method for producing an electrogalvanized steel sheet having high whiteness by the present inventors, as a result of repeated research, a total of one or more organic compounds having a mercapto group (-SH) is 0.5 to In the electrogalvanizing bath containing 30 mg / l, the electrolysis is divided into 5 or more times, and during each electrolysis, the steel plate is immersed in the acidic plating bath for 0.2 seconds or more, thereby increasing the whiteness. It has been found that an electrogalvanized steel sheet having a degree can be produced. Conventionally, the method of adding an organic compound to the plating bath generally has unavoidable adverse effects such as a glossy appearance, a decrease in current efficiency, or a shortened anode life, but in this method, whitening occurs at a very low addition concentration. Since an effect is obtained, it is possible to manufacture an electrogalvanized steel sheet having high whiteness without incurring the above-described adverse effects.

  The present invention has been made on the basis of such findings, and the means thereof is as follows.

  When forming an electrogalvanized layer by electrolysis using a steel plate as a cathode in an acidic plating bath, acidic plating containing one or more organic compounds having a mercapto group (-SH) in a total amount of 0.5 to 30 mg / l An electrogalvanized steel sheet characterized by having a step of immersing the steel sheet in the bath by dividing it into 5 or more times and immersing the steel sheet in the acid plating bath for 0.2 seconds or more between each electrolysis. Production method.

  An organic compound having a mercapto group (—SH) is adsorbed on the surface of the plating (Zn) during the plating to inhibit the crystal growth of Zn, raises the electrodeposition overvoltage, and promotes the generation of secondary nuclei. Presumed (mercapto group is considered to contribute to adsorption). Therefore, it is considered that the unevenness of the Zn plating crystal becomes finer, and as a result, the whiteness increases.

  In the present invention, the electrolysis is performed by dividing it into 5 or more times, and a step of immersing the steel sheet in the acidic plating bath for 0.2 seconds or more between each electrolysis. Thus, sufficient whiteness can be obtained for the first time. This is considered to be because an organic compound having a mercapto group (-SH) is effectively adsorbed on the surface of the steel sheet in the dipping step.

  According to the present invention, an electrogalvanized steel sheet having high whiteness can be obtained without greatly changing the glossiness and without greatly reducing the current efficiency. The same effect can be obtained with steel sheets that have been subjected to various chemical treatments (coating type, reaction type, electrolytic type) that are chromate-free or chromate-free after electrogalvanizing treatment, and further resin coating treatment on the surface. Thus, a chemical conversion treated galvanized steel sheet excellent in the above can be obtained.

  In the production of electrogalvanized steel sheets, as pretreatment, degreasing treatment and water washing for cleaning the steel plate surface and pickling treatment and water washing for activating the steel plate surface are generally performed. Subsequently, electrogalvanization is performed.

Various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used for the pickling treatment, but sulfuric acid, hydrochloric acid, or a mixture thereof is desirable. The concentration of the acid is not particularly specified, but is preferably about 1 to 20% by mass in consideration of the ability to remove the oxide film, prevention of rough skin by peracid washing, and the like. The pickling treatment liquid may contain an antifoaming agent, an accelerator, an inhibitor, and the like. Typical examples of inhibitors include water-soluble organic compounds with N-type polar groups that have a high adsorption capacity on the steel sheet (Fe) surface. Primary amines (R ₁ NH ₂ ), secondary amines (R ₁ R ₂ NH ), Tertiary amine (R ₁ R ₂ R ₃ N) or quaternary ammonium salt (R ₁ R ₂ R ₃ R ₄ N ⁺ · X ⁻ ). Here, R ₁ , R ₂ , R ₃ and R ₄ represent the same or different hydrocarbon groups, and X ⁻ represents an anion such as Cl ⁻ .

  In the electrogalvanizing step in the present invention, electrolysis is divided into 5 or more times in an electrogalvanizing bath containing 0.5 to 30 mg / l of one or more organic compounds having a mercapto group (-SH). And performing a step of immersing the steel plate in the acidic plating bath for 0.2 seconds or more between each electrolysis.

  The organic compound having a mercapto group (—SH) used in the present invention has a particularly high ability to adsorb to zinc, and it is considered that the effect of refining the unevenness of the plated crystal is very large. Examples of the organic compound having a mercapto group (—SH) include thioglycolic acid, mercaptopropionic acid, mercaptosuccinic acid, sodium 3-mercapto-1-propanesulfonate, 2-mercaptoethanol, propyl mercaptan, and ethanedithiol.

  When the concentration of the organic compound having a mercapto group (—SH) in the electrogalvanizing bath is less than 0.5 mg / l, the whiteness increase effect is insufficient, and when the concentration is 0.5 mg / l or more, the whitening effect of plating is obtained. can get. When the concentration exceeds 30 mg / l, adverse effects such as glossy appearance after plating, increased chemical costs, and shortened life of insoluble anodes become significant. Therefore, the density | concentration of the organic compound which has a mercapto group (-SH) added to an electrogalvanization bath shall be 0.5-30 mg / l. When the concentration exceeds 8 mg / l, the Zn plating current efficiency decreases, and the amount of electricity necessary to obtain a constant amount of Zn deposition increases, resulting in an increase in manufacturing cost. Therefore, 0.5 mg / l to 8 mg / l is a more preferable concentration range. In the density range, the gloss does not change greatly.

  In the present invention, when forming the electrogalvanized layer, the electrolysis is performed by dividing it into 5 or more times, and the steel plate is immersed for 0.2 seconds or more between each electrolysis. In the step of immersing the steel sheet in the plating bath, it is considered that the organic compound having a mercapto group (—SH) is effectively adsorbed on the surface of the galvanizing.

  If this time is less than 0.2 seconds, the whitening effect is insufficient. The upper limit is not particularly specified, but as the time increases, the amount of dissolution of the plating layer increases, and the amount of electrolysis for obtaining the necessary Zn deposition amount increases. Moreover, when electrolysis is made 4 times or less (the number of immersions is 3 times or less), the whitening effect becomes insufficient. The upper limit of the number of times is not stipulated, but up to 50 times is realistic to prevent an increase in equipment costs.

The concentration of the organic compound having a mercapto group (—SH) in the plating bath is limited to the above range, and the electrolysis is further divided into five or more times. There are no particular limitations on the electrogalvanizing bath conditions and electrolysis conditions except that it has a step of immersing in it for 0.2 seconds or longer. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be applied as the plating bath. In addition to Zn ions, the bath may contain additives or impurities as conductivity aids such as sodium sulfate and potassium sulfate, metal ions such as Fe, Ni, Pb, Sn, and Co. The bath conditions are not particularly limited, but for example, the bath temperature may be 30 to 70 ° C., the pH may be 0.5 to 4.5, and the relative flow rate may be 0 to 4.0 m / sec. The electrolytic current density is not particularly limited, but may be, for example, 10 to 150 A / dm ² . Not limited particularly also the adhesion amount of electro-galvanized, it is usually 5 to 40 g / m ² approximately.

The effect of the present invention can also be obtained for steel sheets that have been subjected to various chemical conversion treatment films (coating type, reaction type, electrolytic type) that are chromate-free or chromate-free and further subjected to resin coating treatment on the chromate or chromate-free coating process. The appearance of the chemical conversion treated steel sheet after these treatments is greatly affected by the appearance after electrogalvanizing (before chemical conversion treatment) when used without coating (however, zinc phosphate treatment of 1 g / m ² or more is applied) (Except because the appearance changes greatly). Therefore, the chemical conversion treatment steel plate which has high whiteness can be manufactured by using the electrogalvanized steel plate manufactured by the method of the present invention.

  A cold rolled steel sheet having a thickness of 0.7 mm was used as a plating original sheet. This was degreased with alkali, washed with water, and then pickled under the conditions shown in Tables 1 and 2. The temperature of the pickling solution is 25 ° C. Although some additives were used in the pickling solution, pickling was performed without adding most of the additives.

After subsequent water washing, electrogalvanization was performed under the following conditions. In addition, although various water-soluble organic compounds were added to the plating bath, plating was performed without adding some of them. The types and concentrations of the added additives are shown in Tables 1 and 2.
・ Plating bath: Acidic sulfuric acid bath containing 1.5 mol / l Zn ²⁺ ions, pH 2.0, temperature 50 ° C
・ Relative flow velocity: 1.5m / sec
・ Current density: 20 ~ 120A / dm ²
・ Electricity: 600C / dm ²
During electrogalvanization, the electrolysis current was cut several times during electrolysis, and the steel sheet was immersed in an acid plating bath. In addition, about some steel plates, the electrolysis current was not cut.

About the electrogalvanized steel sheet produced on the above conditions, the whiteness was evaluated by the brightness (L value). The brightness (L value) was measured using a color difference meter (SQ2000) manufactured by Nippon Denshoku Industries Co., Ltd. under conditions (nd) excluding regular reflection (conforms to JIS Z 8722).
The brightness measurement results were evaluated as follows.
◎: 84 ≦ L value ○: 82 ≦ L value <84
Δ: 80 ≦ L value <82
×: 78 ≦ L value <80
Moreover, about the said electrogalvanized steel plate, the zinc plating layer was melt | dissolved with dilute hydrochloric acid, and the zinc adhesion amount (g / m < ² >) was calculated | required by quantifying Zn density | concentration by an atomic absorption method, and the current efficiency was computed.

  The brightness evaluation results and the current efficiency are shown in Tables 1 and 2.

Invention Examples 1 to 23: L value improvement effect is high (L value of 62 or more, evaluation ○ to ◎).
Comparative Examples 1 and 3: The concentration of the organic compound having a mercapto group (-SH) added to the electrogalvanizing bath is less than 0.5 mg / l, and the L value increasing effect is insufficient.
Comparative Example 2: The concentration of the organic compound having a mercapto group (-SH) added to the electrogalvanizing bath is more than 30 mg / l, and there is an L value improving effect, but the glossiness of the appearance is remarkable, It is not desirable from the viewpoint of chemical cost and anode life.
Comparative Examples 4 to 9: The organic compound added to the electrogalvanizing bath does not have a mercapto group (—SH). L value increase effect is not recognized.
Comparative Examples 10 and 11: In this example, the electrolysis is not divided into a plurality of times and is continuously energized, and the effect of increasing the L value is insufficient.
Comparative Examples 12 and 13: Although the electrolysis is divided, the number of times is less than 5. L value increase effect is insufficient.
Comparative Examples 14 and 15: Although the electrolysis is divided, the immersion time between each electrolysis is less than 0.2 seconds. L value increase effect is insufficient.
Comparative Examples 16 and 17: In this example, no water-soluble organic compound is added to the electrogalvanizing bath, and there is no L value increasing effect.

  The electrogalvanized steel sheet of the present invention has high whiteness and excellent surface appearance. Therefore, it is particularly suitable for use in non-painting applications such as home appliances.

Claims (1)

When forming an electrogalvanized layer by electrolysis using a steel plate as a cathode in an acidic plating bath, acidic plating containing one or more organic compounds having a mercapto group (-SH) in a total amount of 0.5 to 30 mg / l An electrogalvanized steel sheet characterized by having a step of immersing the steel sheet in the bath by dividing it into 5 or more times and immersing the steel sheet in the acid plating bath for 0.2 seconds or more between each electrolysis. Production method.