JP2000345387A - Electrogalvanized steel sheet having excellent appearance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an electrogalvanized steel sheet free from surface defects such as unevenness and high in whiteness by forming an electrogalvanizing layer on a tin layer of a specified precipitating amt. formed on a steel sheet by pickling. SOLUTION: The precipitating amt. of tin formed by pickling is 0.5 to 10 mg/m2. At first, a steel sheet is passed through a pickling soln., and pretreatment to plating is executed. The picking soln. has been added with tin sulfate, e.g. by 0.1 to 20 g/l, and tin is precipitated onto the surface of a plating original sheet. As the pickling soln., a sulfuric acid soln. is desirable. Next, the pretreated steel sheet is electroplated with an acidic plating bath contg. H2SO4. The plating current density is not particularly limited, and generally, by 30 to 150 A/dm2 current density at which it is industrially produced for electrogalvanizing a steel sheet or by the higher current density, e.g. 200 A/dm2, the effect of dissolving unevenness and improving whiteness can be obtd. The applicable pH in the plating both is different in accordance with the anode system, but, in the case an insoluble anode is used, the pH in the plating bath is desirably controlled to 0.8 to 2.5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to home appliances, automobiles,
The present invention relates to a galvanized steel sheet used in a wide range of applications such as building materials.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for various chromate-treated electrogalvanized steel sheets which are used as unpainted electric home galvanized steel sheets, and have become an important field of application. In this application, it is required to have an excellent surface appearance because it is used without painting. As a condition of excellent surface appearance, firstly, there is no surface defect such as unevenness, but high whiteness is also an important condition. In addition, after the phosphate treatment, the paint is usually applied, but when the color of the light-colored paint or the film thickness is small, the whiteness of the phosphoric acid base plate is low and the sharpness after the coating is poor, so the whiteness is also high. Is required. Naturally, the appearance after these various chemical conversion treatments depends on the appearance after plating, and it is necessary that the galvanized steel sheet before chemical conversion treatment has no unevenness and high whiteness.

The unevenness of a plated steel sheet includes unevenness caused by a failure of a plating apparatus and unevenness caused by a surface defect of an original plate. The former unevenness can be improved by correcting the defect of the plating apparatus. However, the latter unevenness should be solved by improving the surface defects of the original sheet, but it is industrially difficult to completely eliminate the surface defects of the original sheet, and practical improvement measures have been reported. Not.

As a proposal for improving the appearance of a galvanized steel sheet, a method of improving glossiness by adding polyacrylamide or polyvinyl alcohol to a plating bath, as disclosed in Japanese Patent Publication No. 46-38888, has been proposed. A method of performing an oxidation treatment after plating pretreatment to obtain an appearance without glare as disclosed in Japanese Patent Application Laid-Open No. 61-244469, an acidic zinc plating bath as disclosed in Japanese Patent Publication No. 01-36559. A non-ionic polyacrylamide is added thereto, and plating is performed at a high current density, whereby a smooth white plating is disclosed.
Among these, Japanese Patent Publication No. 01-36559 only mentions whiteness, but the whiteness improvement effect is limited to a high current density of 100 to 450 A / dm ² , and 100 A / dm, which is usually performed. ^No effect is observed with plating less than ² , and there is a problem in practicality. As described above, an electrogalvanized steel sheet that has no surface defects such as unevenness and high whiteness has not yet been proposed.

[0005] A technique for adding tin to the pickling solution is disclosed in Japanese Patent Publication No. 57-14758. However, as shown in the specification, in the present invention, in electrogalvanizing using an insoluble lead electrode, trace amounts of lead eluted from the lead anode are eutectoid to the plating, and the corrosion resistance and heat resistance when heated are reduced. There is no description about plating appearance for the purpose of preventing plating. Further, in this method, in a pickling solution, electrolysis is performed using a steel sheet as a cathode, and the amount of deposited tin is 10 to 40 mg /
It is as large as m ² .

[0006]

DISCLOSURE OF THE INVENTION The present inventors have studied the unevenness caused by the surface defects of the original plate, and the cause of the unevenness is a very small amount of silica, alumina, titania, etc. concentrated on the surface of the original plate. I found that. It was found that the zinc crystals precipitated in the portions where these oxides were concentrated were finer than the zinc crystals deposited in the portions where the oxides were not concentrated, which caused a difference in appearance and unevenness was observed. It is not clear why zinc oxide becomes finer in the areas where these oxides are concentrated, but it is expected that the electrical resistance will increase in these areas due to the formation of oxides. It is considered that nucleation and crystal growth are affected. As described above, if the segregation of these oxides can be completely prevented, the occurrence of unevenness due to these can be prevented.However, the amount of segregation is extremely small, and it is impossible to completely prevent this. is there. Therefore, the present inventors have studied a manufacturing method that does not cause plating unevenness even if these oxides are segregated in a trace amount on the base plate.As a result, the plating unevenness is reduced by depositing a trace amount of tin prior to plating. Was found to no longer occur. The mechanism by which unevenness does not occur is that tin is extremely noble compared to zinc and is also a noble metal for iron, so it is easy to precipitate, and it is easy to deposit on surfaces with and without oxide. It is considered that a uniform tin-deposited layer is formed by depositing on the surface, and as a result, the surface when zinc is deposited becomes uniform and no unevenness occurs. Further, the inventors of the present invention have conducted studies for applying the method for improving unevenness by precipitation of a trace amount of tin at low cost industrially, and as a result, tin is added to a pickling solution performed as a pretreatment for zinc plating. As a result, the present inventors have invented an improved method that can be manufactured as it is with the current galvanizing equipment and hardly increases the manufacturing cost.

The present invention has been made based on the above findings, and has as its object to provide an electrogalvanized steel sheet which has no surface defects such as unevenness and a high whiteness.

[0008]

The electrogalvanized steel sheet of the present invention that achieves this object has a precipitation amount of 0.5 mg on the steel sheet.
It has an excellent appearance in which an electrogalvanized layer is provided on a tin layer deposited by pickling at a concentration of not less than 10 mg / m ^{2 and not} more than 10 mg / m ² .

The method for producing an electroplated steel sheet according to the present invention
0.5mg / m precipitation amount on the plate by pickling ^TwoMore than 10mg / m
^TwoDepositing and forming a tin layer less than
Excellent appearance with the step of forming a galvanized layer
Can be manufactured.

[0010]

According to the present invention, first, a steel plate is passed through a pickling solution to perform a plating pretreatment. The pickling solution contains tin sulfate, for example, 0.1-2.
0 g / l, and tin was added to the surface of
mg / m ² or more 10 mg / m ² than to precipitate. By adding tin, the surface of the original plate at the time of zinc crystal precipitation is made uniform by tin precipitation, zinc crystals are also uniform, the depth of crystal grain boundaries becomes shallow, light absorption is reduced, and the intensity of diffuse reflected light is increased. For this reason, even if segregation of silica, alumina, titania, or the like occurs, no unevenness occurs, and the whiteness is improved.

[0011] reason for the tin to be 0.5 mg / m ² or more 10 mg / m precipitation less than ² to be plated surface, 0.5 mg /
If it is less than m ² , the effect of preventing the occurrence of unevenness is insufficient and the effect of improving whiteness is also insufficient, while if it is 10 mg / m ² or more, a non-uniform appearance which is considered to be caused by tin precipitation is caused. . In order to obtain such an amount of deposited tin, it is necessary to control the tin concentration in the pickling solution. However, since the amount of deposited tin is affected by the acid concentration, the processing time, the processing temperature, etc. The concentration of tin in the pickling solution may be determined in consideration of the conditions. For example, under general pickling conditions (100 g / l of sulfuric acid, 5
0.5 mg / m ² or more at 10 ° C for 10 seconds
The amount of tin sulfate added to the pickling solution to obtain a tin deposition amount of less than g / m ² was 0.1 to 20 g / l. Here, the amount of tin added to the pickling solution is based on tin sulfate. However, the effect of tin is due to tin ions (II valence). Considering only addition of tin, a compound that becomes tin ions (II valence) in the pickling solution,
The same effect can be obtained with other salts, for example, divalent tin salts such as tin chloride and tin diphosphate.

The pickling solution is preferably a sulfuric acid solution. The sulfuric acid concentration is not particularly limited, and generally used 15 to 100 g /
The effect of adding tin was confirmed at a concentration of 1. Further, iron dissolved from the steel sheet gradually increases in the pickling solution, but the effect of adding tin was confirmed up to an iron concentration of 20 g / l.

[0013] Pickling can be performed by immersion instead of electrolytic treatment. In the case of electrolytic treatment, in the usual electrolytic pickling performed by continuous zinc plating of a strip, an energization method called grid energization is often applied, but in this method, the electrodes alternately become an anode and a cathode, so that a cathode is used. Tin precipitates on the electrode which has become. When the amount of deposited tin increases, the tin peels off from the electrode and is caught between the strip and the roll to form a press wound, thereby inducing spark in the current-carrying roll. Therefore, in grid energization, it is necessary to change the polarity before tin deposited on the cathode is peeled off and dissolve tin deposited in anode energization, and it is necessary to switch the polarity in a short time.

Next, the pretreated steel sheet is electroplated in a sulfuric acid acid plating bath.

The chloride bath has advantages such as low plating voltage and easy high current density electrolysis. However, it has a serious problem that an insoluble anode cannot be used and the anode replacement cost is high. Not appropriate.

The plating current density is not particularly limited, and is generally 30 to 15 which is industrially produced as electrogalvanized steel sheet.
Not only the current density of 0 A / dm ² but also a higher current density, for example, 200 A / dm ^2, can achieve the effect of eliminating unevenness and improving whiteness.

The applicable plating bath pH depends on the anode system. When an insoluble anode is used, the plating bath pH is desirably 0.8 to 2.5. If the pH is less than 0.8, the plating efficiency is low, which is not desirable. When the pH exceeds 2.5, the chemical dissolution rate of zinc metal, zinc oxide, etc., which is a zinc ion replenishment reaction, is greatly reduced, so that ion replenishment becomes difficult. If a self-soluble anode is used, pH 3.
0 to 5.0 is desirable. If the pH is less than 3.0, the chemical dissolution reaction rate of the zinc anode is large, and the zinc ion concentration in the plating bath is undesirably increased. If the pH is more than 5.0, zinc hydroxide precipitates, which is not suitable.

The applicable plating temperature is not particularly limited.
40 to 6 generally applied as electrogalvanizing of steel sheet
At 0 ° C., the effect of the present invention was confirmed.

As described above, according to the present invention, zinc plating having improved whiteness of the plating, having no unevenness, and having an ideal appearance having a high whiteness can be obtained.

[0020]

Example 1 A cold rolled steel sheet which had a segregation of silica, alumina, titania, etc. in the original sheet and which caused linear unevenness when subjected to normal zinc plating was degreased by a normal method. Plating was performed under an appropriate pickling condition, plating bath composition and plating condition so as to obtain an adhesion amount of 20 g / m ² . The state of occurrence of unevenness of the obtained plating was visually evaluated, and the whiteness was evaluated by a lightness index L value measured by a method (condition d, Hunter method) specified in JISZ8722, and shown in Table 2.

Comparative Examples A to E in Table 2 are cases in which tin was not added, streak-like unevenness due to oxide segregation of the original plate occurred, and the whiteness was less than 85. In Comparative Examples F and G, the amount of tin added was 0.0
5 g / l, which is small, and the line-like unevenness is slight and the whiteness is slightly improved as compared with Comparative Examples A to E.
The improvement is insufficient. In Comparative Examples H and I, the tin addition amount was as large as 50 g / l, the whiteness was improved to 87 or more, and no streak-like unevenness due to oxide segregation of the original plate was observed, but large unevenness was observed on the entire plating surface. occured. The reason for this is not clear, but is considered to be that if the amount of deposited tin increases, the deposited state becomes uneven.

On the other hand, in Examples A to V, the amount of tin added was 0.1.
1 to 20 g / l, tin deposition amount 0.5 mg / m ² or more 1
Within the range of less than 0 mg / m ^2, there was no unevenness and the whiteness was as high as 87 or more, showing a good appearance.

Example 2 The same original plate as in Example 1 was used, and in the same manner as in Example 1, 20 g under pickling conditions, plating bath compositions and plating conditions as shown in Table 3.
The plating was performed so that the adhesion amount was / m ² . The state of occurrence of unevenness of the obtained plating was visually evaluated, and the whiteness was measured according to JIS.
Table 3 shows the evaluation results based on the lightness index L value measured by the method specified in Z8722 (condition d, Hunter method).

Table 3 shows the effect of tin when the components of the plating solution are different. In any case, no unevenness occurred in the example in which tin was added, as compared to the comparative example in which tin was not added,
In addition, the whiteness is improved by 2 points or more.

[0025]

According to the present invention, the pre-plating of the original sheet is carried out with a pickling solution, and the amount of precipitation on the steel sheet is 0.5 mg / m ² or more.
By performing electroplating in a sulfuric acid plating bath after forming a tin layer of less than mg / m ^{2, a} galvanized steel sheet having excellent whiteness without unevenness due to surface defects can be obtained.

[0026]

[Table 1]

[Table 2]

[Table 3]

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toru Segawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Toyofumi Watanabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun Honko Tube Co., Ltd.

Claims (2)

[Claims] 1. The amount of precipitation formed on a steel sheet is 0.5 mg /
and m ² or more 10 mg / m layer of tin was deposited in less than ² pickling, and a electro-galvanized layer formed on the tin layer, electro-galvanized steel sheet with excellent appearance. 2. A precipitation amount of 0.5 mg / m ² by pickling on a steel plate.
Depositing and forming a tin layer of less than 10 mg / m ² ,
Forming an electrogalvanized layer on the tin layer, the method comprising the steps of: