JP2003073884A - Method for manufacturing galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively manufacture galvannealed steel sheet having excellent surface appearance, which generates no unevenness caused by surface defects in an original sheet. SOLUTION: A method for manufacturing the galvannealed steel sheet comprises steps of performing electrolysis divided into five times or more, by using an acid plating bath including 20-2,000 ppm of Ni<2+> ions, when electrolyzing the steel sheet as a cathode in the acid plating bath to form a galvannealed layer, and steps of immersing the steel sheet between the above each electrolysis, in the above acid plating bath for 0.2 second or longer. The galvannealed layer contains an Sn content of 10 ppm or less, and a Pb+Cd+In content of 40 ppm or less.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing an electrogalvanized steel sheet, which is used in a wide range of applications such as home electric appliances, automobiles, and building materials, and more specifically, an electrogalvanized steel sheet having an excellent surface appearance. Manufacturing method.

[0002]

BACKGROUND OF THE INVENTION Electrogalvanized steel sheets are used in a wide range of home appliances, automobiles, building materials, etc.
Demand for various chromate-treated electrogalvanized steel sheets used unpainted mainly for home appliances is increasing. Since these various chromate-treated electrogalvanized steel sheets are used without coating, it is required that the steel sheets have excellent surface appearance without surface defects such as uneven appearance.

Regarding the uneven appearance of the plated steel sheet, there are two types, one caused by a defect in the plating apparatus and the other caused by a surface defect of the original plating plate. The former unevenness in appearance caused by the malfunction of the plating apparatus can be improved by correcting the malfunction of the plating apparatus.

As the latter technique for preventing the occurrence of unevenness in appearance due to surface defects of the plated original plate, Japanese Patent Application Laid-Open No. 8-1204
In JP 83, using a plating solution having a lower Zn ion concentration than usual at the beginning of plating, after performing a small amount of plating at a current density higher than the normal current density, the normal Zn ion concentration, at the normal current density The method of plating is disclosed in Japanese Patent Laid-Open No. 8-
Japanese Patent No. 120484 discloses a method of performing three-layer plating by changing the current density and dissolving the second layer by reverse electrolysis.

Further, as a method for preventing the occurrence of appearance unevenness by adding various additives to the plating solution, Japanese Patent Laid-Open No.
In the 256192 publication, a method of adding 100 to 5000 ppm of a fluoro complex ion is disclosed.
The method of adding ~ 10ppm, in JP-A-8-158090, 1 to 5 polyoxyalkylene or its alkyl ether.
The method of adding 00 ppm is Sn, I in JP-A-8-188899.
Each method of adding n, Bi, and Sb is disclosed.

[0006] Further, JP-A-7-76793 and JP-A-8-490.
No. 91, JP-A-8-134688, JP-A-8-188898, Ni, Co, Cr, Ti, Mn, Fe and various other metals are deposited on the original plate by electroplating or vapor phase plating. A method of causing is disclosed.

[0007]

SUMMARY OF THE INVENTION
Since the 8-120483 publication requires two types of plating solutions,
Requires two independent tanks and plating equipment, which increases equipment costs and operating costs. Japanese Patent Laid-Open No. 8-120484 requires a reverse electrolysis facility, which increases the facility cost. Further, since the plating layer is dissolved by reverse electrolysis, it is necessary to deposit more plating than the required deposition amount, which increases the operating cost.

The techniques disclosed in JP-A-9-256192, JP-A-9-195082, JP-A-8-158090, and JP-A-8-188899 do not have sufficient effect of preventing appearance unevenness. .

In the techniques disclosed in JP-A-7-76793, JP-A-8-49091, JP-A-8-134688, and JP-A-8-188898, the appearance unevenness caused by the surface defect of the original plate is considered. Cannot be completely prevented.

As described above, various attempts have heretofore been attempted, but there is still no method for inexpensively producing an electrogalvanized steel sheet capable of completely preventing surface defects such as uneven appearance due to surface defects of the original plating plate. It has not been found and it has not been industrially possible to completely remove the surface defects of the original plate.

An object of the present invention is to provide a method capable of inexpensively producing an electrogalvanized steel sheet having an excellent surface appearance without causing unevenness due to surface defects of an original plate.

[0012]

Means for Solving the Problems The inventors of the present application have studied the cause of the development, aiming at the development of a method for radically eliminating the uneven appearance caused by the surface defects of the original plating plate after electroplating. As a result, the cause of appearance unevenness is caused by trace amounts of oxides such as Si, Al, Ti, Fe, and Ni on the surface of the original plate.
It was found that C was partially concentrated.
It was found that the zinc crystals deposited in these concentrated areas have larger irregularities than the zinc crystals deposited in the unconcentrated areas, which reduces the gloss in appearance and is observed as unevenness. did. In the part where oxides, C, etc. are concentrated, the electrical resistance, hydrogen overvoltage, precipitation overvoltage of Zn, etc. are different from the non-concentrated part, so the crystal nucleus generation behavior at the initial stage of electrolysis is different from that in the non-concentrated part Since the behavior is affected by the distribution and density of crystal nuclei, it is estimated that the final crystal form is different. As described above, at the production stage of the steel sheet, if the oxides, C, etc. can be completely prevented from being partially concentrated, the occurrence of unevenness due to these can be prevented, but the concentration amount is extremely small. , It is impossible to eliminate these completely.

Therefore, the inventors of the present invention suppress the continuous growth of crystals even if the generation behavior of crystal nuclei in the initial stage of electrolysis is non-uniform, and the plating crystals are made uniform and fine to make the appearance of plating uniform. It was thought that the occurrence of unevenness could be prevented, and an effective method for achieving this was examined, and the present invention was completed.

Means of the present invention for solving the above problems are as follows. (1) When electrolyzing a steel sheet as a cathode in an acid plating bath to form an electrogalvanized layer, Ni
Using an acid plating bath containing 20 to 2000 ppm of ²⁺ ions, while performing electrolysis in five or more times, during each of the electrolysis, a steel plate is immersed in the acid plating bath for 0.2 seconds or more. A method for producing an electrogalvanized steel sheet, comprising: (first invention). (2) Sn content of electrogalvanized layer to be formed is 10ppm or less, Pb + Cd + In content is 4
The method for producing an electrogalvanized steel sheet according to (1) above, which is 0 ppm or less (second invention).

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Next, details of the present invention will be described. In the first invention, in forming an electrogalvanized layer by electrolyzing a steel sheet as a cathode in an acid plating bath, using an acid plating bath containing Ni ²⁺ ions of 20 to 2000 ppm, the electrolysis is divided into 5 or more times. It is specified that the method further includes immersing the steel sheet in the plating bath for 0.2 seconds or more during each of the electrolysis. The mechanism by which the appearance unevenness after plating can be effectively prevented by the first invention is estimated as follows.

[0016] An acidic plating bath containing 20 to 2000 ppm of Ni ²⁺ ions is used to electrolyze a steel sheet as a cathode to form a zinc plating layer, and then the steel sheet is subjected to acidic plating containing 20 to 2000 ppm of Ni ²⁺ ions. If there is a step of dipping in a bath, in this step, the galvanized layer formed by electrolysis is
Ni precipitates by substitution. In the subsequent electrolysis, the Ni deposited by substitution and deposition becomes a nucleus to promote the generation of secondary nuclei of zinc crystals and inhibit the continuous growth of zinc plated crystals, thereby making the plated crystals uniform and fine. At the beginning of electrolysis,
Even if non-uniformity of crystal nuclei occurs due to surface defects of the original plate, the dipping step and subsequent electrolysis make the crystals uniform and finer overall, resulting in uneven appearance due to surface defects of the original plate. It can be effectively suppressed. Ni ²⁺
If the ion concentration is less than 20 ppm, this suppressing effect becomes insufficient, and if it exceeds 2000 ppm, the corrosion resistance deteriorates.

In the present invention, when the electrogalvanized layer is formed, the electrolysis is divided into 5 or more times, and during each electrolysis, the steel sheet is dipped in a plating bath for 0.2 seconds or more, thereby galvanizing. Substitution of Ni occurs on the surface of the layer, and the subsequent electrolysis inhibits the continuous growth of the zinc-plated crystal, thereby making the plated crystal uniform and fine, and suppressing the uneven appearance due to the surface defects of the steel sheet.

The effect of suppressing the appearance unevenness caused by the surface defects of the original plate by making the crystals uniform and finer as a whole depends on the immersion time and the number of times. If the immersion time is less than 0.2 seconds, the effect of suppressing the appearance unevenness is insufficient, so the immersion time is 0.2
Must be more than a second. The upper limit is not specified in particular, but when the immersion time becomes long, the amount of dissolution of the plating layer cannot be ignored, and the amount of electrolysis required to obtain the required zinc deposition amount increases, so the electrolysis time is preferably 2 seconds or less. . Also, the number of electrolysis is 4 times or less (the number of immersion is 3 times or less)
In that case, the effect of suppressing the appearance unevenness becomes insufficient, so it is necessary to carry out the electrolysis in 5 or more times. The upper limit of the number of electrolysis is not specified, but in order to prevent an increase in equipment cost, up to 50 times is practical.

According to the method of the present invention, since only one type of plating bath is used and no special equipment such as a reverse electrolysis equipment is required, it is possible to prevent an increase in operation and equipment costs.

The second invention specifies that the electrogalvanized layer to be formed has a Sn content of 10 ppm or less and a Pb + Cd + In content of 40 ppm or less. According to the second aspect of the present invention, it is possible to more effectively suppress unevenness in appearance due to surface defects of the original plate. This is specified for the following reasons.

The eutectoid Sn in the plating layer has a function of significantly changing the morphology of the electrogalvanized crystal and increasing the crystal size even if the amount thereof is extremely small, so the amount is set to 10 ppm or less. By doing so, the effect of making the plating crystals uniform and fine can be maximized, and uneven appearance due to surface defects of the original plate can be more effectively suppressed. Pb, C
Since d and In also have similar functions, although not as much as Sn, by setting the sum of their contents to 40 ppm or less,
The effect of making the galvanized crystals uniform and finer is further enhanced, and unevenness in appearance can be suppressed more effectively.

The amounts of Sn, Pb, Cd, and In that are codeposited in the plating layer depend on the concentration of each element ion in the acidic plating bath and the plating conditions (bath temperature, current density, relative flow velocity, etc.). ,
By adjusting the concentration of each element ion in the acidic plating bath and the plating conditions (bath temperature, current density, relative flow rate, etc.), the amount of Sn, Pb, Cd, In codeposited in the plating layer is within the above range. Can be

In the present invention, as the acidic plating bath,
A sulfuric acid bath, a hydrochloric acid bath, or a mixed bath of both can be applied. Stable presence of Ni ²⁺ ions in the plating bath,
In order to promote substitutional precipitation of Ni, the pH of the plating bath should be 4.5.
The following is desirable. The plating bath may contain, as an additive, a conductivity auxiliary agent such as sodium sulfate, potassium sulfate and ammonium sulfate, and a brightening agent such as aluminum sulfate. Although the bath conditions are not particularly limited, for example, the bath temperature may be 30 to 70 ° C. and the relative flow rate may be 0 to 4.0 m / s. The electrolytic current density is also not particularly limited, for example, 10 ~ 150A
You can use / dm ² .

In the present invention, there is a step of immersing in an acidic plating bath for 0.2 seconds or more between electrolysis. Here, the immersion time is the time during which the steel sheet is immersed in the plating bath without facing the electrode. The dipping process may be continuous, and a non-dipping process may be performed during and / or before and after the dipping process.

It should be noted that the appearance unevenness after plating is also a problem for steel sheets that have been subjected to chromate treatment (coating type, reaction type, electrolytic type) after plating, or resin coating treatment thereon, but these steel sheets Also, the effect of the present invention can be obtained.

[0026]

[Example] As a base plate, a wire is obtained by performing normal zinc plating.
Cold-rolled steel plate (original plate 1) that causes irregularity
Cold-rolled steel sheet (original plate) that causes spot-like unevenness when lead-plated
2) of 2) was used. Alkaline degreasing and acid as pretreatment
After performing the washing process, use the flow cell to remove the acidic plating bath.
Relative flow velocity: While circulating at 1.5 m / s, the original plate became the cathode
And current density: 10-100A / dm²Electrolyze with zinc
20g / m²Forming a galvanized layer of
It was At that time, during the electrolysis, the electrolysis current is applied once or multiple times.
Cut for a predetermined time and leave the steel plate immersed in the acid plating bath.
did. For some steel plates, do not cut the electrolytic current.
It was. As the acidic plating bath, Zn ²⁺Containing 1.5 mol / l
Uses sulfuric acid plating bath (pH 2.0, temperature 50 ° C)
However, all the special grade reagents were used in the plating bath, and the invention example 7
For ~ 12, a certain amount of Ni, Sn, Pb, Cd, In sulfate
Was added as. The Sn, Pb, Cd, In content in the film is
Change the amount of each element added to the plating bath and the electroplating current density.
Control was performed. Manufacturing conditions, plating layer
Table 1 shows each content of Sn, Pb, Cd, and In.

With respect to the electrogalvanized steel sheet produced under the above conditions, the appearance of uneven appearance due to defects in the original sheet was visually confirmed. The evaluation of the appearance of uneven appearance is ◎:
No unevenness in appearance, ○: unevenness in appearance (very slight), Δ: occurrence of unevenness in appearance (mild), ×: occurrence of unevenness in appearance (severe). The survey results are shown in Table 1.

[0028]

【table 1】

In the comparative examples outside the scope of the present invention, the appearance after plating was evaluated as Δ to ×, and the appearance was uneven after plating. On the other hand, the examples of the present invention are evaluated to be ∘ to ∘ in appearance, and are excellent in the effect of preventing appearance unevenness after plating.
In the examples of the present invention, those within the second invention range were evaluated as ⊚ for both the original plate 1 and the original plate 2, and the effect of preventing uneven appearance was more excellent.

[0030]

According to the present invention, it is possible to inexpensively obtain an electrogalvanized steel sheet which has no unevenness due to surface defects of the original plate and has an excellent surface appearance.

Also, an excellent appearance can be obtained for a steel sheet obtained by subjecting the electrogalvanized steel sheet of the present invention to a chromate treatment (coating type, reaction type, electrolytic type) or a resin coating treatment thereon. You can

Continued front page    (72) Inventor Junichi Inagaki             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Masaaki Yamashita             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F-term (reference) 4K023 AA15 BA06 CB26 DA06 DA07                 4K024 AA05 AB01 BA03 BB02 BB15                       BB18 BC01 CA02 CA06 CB17

Claims (2)

[Claims] 1. When forming a galvanized layer by electrolyzing a steel sheet as a cathode in an acid plating bath, Ni ²⁺ ions are ^added .
Using an acidic plating bath containing 20 to 2000 ppm, while performing the electrolysis in five or more times, during each electrolysis, having a step of immersing the steel sheet in the acid plating bath for 0.2 seconds or more, A method for producing a characteristic galvanized steel sheet. 2. The Sn content of the electrogalvanized layer to be formed is 1
2. The method for producing an electrogalvanized steel sheet according to claim 1, wherein the Pb + Cd + In content is 40 ppm or less at 0 ppm or less.