JPH07331483A - Production of electrogalvanized steel sheet - Google Patents

Abstract

PURPOSE:To produce an electrogalvanized steel sheet excellent in surface appearance by subjecting a steel sheet of fine surface crystals to immersing treatment into a sulfuric acid pickling bath contg. specified amounts of Ni, Co, Cu, Pb or the like to apply an electrogalvanizing treatment thereto. CONSTITUTION:As the material to be plated, a steel sheet in which the average grain size in the surface is regulated to <=25mum is used. This steel sheet is immersed into a sulfuric acid pickling bath with about 10 to 300g/>= H2SO4 concn. contg. one or more kinds among Ni, Co, Cu and Pb by >=100ppm or into a galvanizing bath. Thus, the surface of the steel sheet is melted by 10mg to 1g/m<2>. After that, this steel sheet is subjected to electrogalvanizing treatment. Thus, the electrogalvanized steel sheet free from defects such as irregularity in plating, flaw in gloss and excellent in surface appearance can be obtd. without causing deterioration in plating characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an electrogalvanized steel sheet, and more particularly to a method for producing an electrogalvanized steel sheet having an excellent appearance without surface unevenness.

[0002]

2. Description of the Related Art Conventionally, electrogalvanized steel sheets have been widely used in automobiles, home appliances, building materials, etc. because of their excellent coating layer uniformity and appearance compared to hot-dip galvanized steel sheets. There is.

Recently, because of the necessity of omitting the step of coating a galvanized steel sheet with a paint, a chromate treatment is performed on the galvanized layer, and a clear resin coating is further applied to the galvanized steel sheet for corrosion resistance and fingerprint resistance. In many cases, the surface-treated steel sheet having excellent heat resistance is used as it is, that is, in a bare state.

In this case, the appearance of the galvanized layer is reflected as it is even after the post-treatment.
It is an important technical subject to perform a beautiful galvanization with a uniform appearance on the surface.

However, the appearance of the zinc plating layer is easily influenced by the surface of the plating base material. For example, if the thickness of the oxide film layer on the surface of the base material is not constant, the thick part and the thin part of the oxide film layer are not constant. In the case, unevenness is observed in the appearance after galvanizing.

In addition, impurities such as Ni, Si and C segregate or adhere to the surface of the base material,
As described above, unevenness occurs on the galvanized layer surface.

Further, in addition to the occurrence of such unevenness on the surface of the zinc plating layer, if the crystal grains on the surface of the base material are large, the plating crystals also grow large, and the surface of the plating layer looks so shiny. The problem of poor gloss occurs.

Therefore, in order to improve the appearance of the galvanized layer, zinc plating such as pickling after hot rolling, electrolytic cleaning after cold rolling, base material manufacturing conditions such as annealing, electrolytic degreasing and pickling conditions. Pre-treatment conditions, plating conditions, post-treatment conditions, etc.
Japanese Patent Publication No. 03-031795, JP-A-57-110
Although it is proposed in Japanese Patent No. 692, it has not been possible to obtain a stable and good appearance.

[0008]

DISCLOSURE OF THE INVENTION In order to solve various problems in the conventional method for producing a galvanized steel sheet described above, the present invention has been earnestly studied by the present inventors, and as a result of repeated studies, We have found that defects such as galvanized layer unevenness and glittering in the manufactured electrogalvanized steel sheet, so-called defective gloss, depend on the size and orientation of the galvanized crystal, and the surface appearance is based on these findings. Has developed a method for manufacturing an excellent electrogalvanized steel sheet.

[0009]

In the method for producing an electrogalvanized steel sheet according to the present invention, the average grain size on the surface is 25 μm.
The following steel sheets are used as the plated materials, Ni, Co, C
The surface of the steel sheet was immersed in a sulfuric acid pickling bath containing 100 ppm or more of one or more of u and Pb, and the surface of the steel sheet was 10 mg / m ²
~ 1 g / m ^{2 After the} melting, electrogalvanizing treatment is carried out, the method for producing an electrogalvanized steel sheet is defined as the first invention, and a steel sheet having a surface average crystal grain size of 25 µm or less is used as a plated material Used, 1 of Ni, Co, Cu, Pb
The steel plate surface by immersing it in a galvanizing bath containing 100 ppm or more of two or more kinds, and 10 mg / m ^{2 to 1} g / m ^{2 of the} steel plate surface.
A second invention is a method for producing an electrogalvanized steel sheet, which is characterized by performing electrogalvanizing treatment after melting.
It consists of two inventions.

The method for producing an electrogalvanized steel sheet according to the present invention will be described in detail below. That is, the appearance unevenness of the galvanized layer of the galvanized steel sheet produced by the method for producing an electrogalvanized steel sheet according to the present invention, defects such as poor gloss are dependent on the size of the galvanized crystal and the orientation. It was

That is, the zinc crystal structure of the zinc-plated layer is a hexagonal close-packed crystal, and in general, the zinc-plated crystal form is a hexagonal (0001) basal plane hexagonal plate-like crystal with respect to the base material. The layers are stacked with a slight inclination.

There are various types of unevenness in the appearance of the galvanized layer, one of which is so-called unevenness that looks like a grain of wood, and is a hexagonal plate of zinc in the grain part. The crystals are coarse.

Further, the zinc plating layer has a glittering appearance, that is, a so-called poor gloss is observed. When the zinc plating crystal is confirmed, a hexagonal plate crystal has a certain direction and has grown greatly. I understand.

In addition, depending on the angle at which the galvanized layer is observed, white and black tones may appear to be reversed, resulting in unevenness of the type.
It is considered that this is because the orientation of the crystal plane of zinc differs depending on the position of the sample, and the white and black tones appear to be reversed depending on the degree of light reflection.

As described above, the appearance of the zinc-plated layer is affected by the size and orientation of the zinc-plated crystal, so if the zinc-plated crystal is miniaturized and the crystal orientation is made uniform. It is considered that defects such as uneven appearance and poor gloss of the galvanized layer can be eliminated. Then, as a method of refining the zinc plating crystal, it is possible to increase the deposition overvoltage of the plating, but there is a limit in itself in actual operation.

Therefore, the zinc plating crystal is made finer,
Various studies were conducted on a method of making the crystal orientation uniform, and it was found that the crystal grain size of the steel sheet was made small and the metal ions (Ni, Co, Cu, Pb ions) which were more noble than iron were contained. It has been found that the steel plate may be pickled with a sulfuric acid pickling bath.

FIG. 1 shows the effect of the Ni ²⁺ concentration in the pickling bath on the appearance of the galvanized layer. It can be seen that the appearance of this galvanized layer is good when the Ni ²⁺ concentration in the pickling bath is 100 ppm or more. Note that Co and C other than Ni
Even when u and Pb ions were contained in the pickling bath, they showed the same tendency as Ni ions, and Ni, Co and
It can be seen that even when two or more Cu and Pb ions are contained, if the total concentration is 100 ppm or more, the zinc plating layer has a good appearance. In FIG. 1, the sulfuric acid pickling bath is H ₂ SO ₄ ; 70 g / l, the pickling dissolution amount of the steel sheet is 3
It is 0 mg / m ² .

FIG. 2 shows the relationship between the appearance of the galvanized layer and the amount of dissolution of pickling steel sheet. It can be seen that the appearance of this galvanized layer is good when the pickling weight loss of the steel sheet is within the range of 10 mg / m ^{2 to 1} g / m ² . If the pickling dissolution loss is less than 10 mg / m ² , the zinc plating layer appearance will be deteriorated, and if it exceeds 1 g / m ^{2, it} will be per-pickled and the zinc plating layer appearance will be adversely affected. In FIG. 2, the sulfuric acid pickling bath is H ₂
SO ₄ ; 70 g / l, contained ion is Ni ²⁺ .

The reason why the appearance of the galvanized layer is improved by pickling the steel sheet containing Ni, Co, Cu, and Pb in the pickling bath is unknown, but it can be inferred as follows. .

That is, the zinc-plated crystal grows in accordance with the crystal orientation of the base material. If there is an oxide film or segregation of impurities on the outermost surface of the base material, the crystal growth of zinc plating is affected, and the orientation becomes non-uniform.

Further, if the crystal grain size of the base material is large, zinc plating also grows greatly. However, when a steel sheet is pickled with a sulfuric acid pickling bath containing a metal ion that is more noble than iron, the oxide film on the surface of the base metal is removed and at the same time the metals that are nobler than iron in the pickling bath are removed. It is considered that since the ions are substitutionally deposited on the surface of the steel sheet, the zinc plating crystal becomes fine and the orientation becomes uniform.

However, if the crystal grain size of the base material becomes too large, even if the steel sheet is pickled with a sulfuric acid pickling bath containing a metal ion that is more precious than iron, the zinc plating crystals will only coarsen. Yes, it cannot be expected to improve the surface appearance of the galvanized layer.

FIG. 3 shows the influence of the crystal grain size of the steel sheet on the appearance of the galvanized layer. It can be seen from FIG. 3 that the appearance of the galvanized layer is good when the average grain size of the steel sheet is less than 25 μm. The average crystal grain size of the steel sheet was calculated by counting the number of crystal grains in a certain area from an SEM observation image and calculating the grain size from the area per grain (assuming the crystal grain is a circle). The sulfuric acid pickling bath in this FIG. 3 was H ₂ SO ₄ ; 70 g / l, and the pickling dissolution amount of the steel sheet was 30 mg / m ² .

The pickling bath has a sulfuric acid concentration of 10 to 300 g / l.
Ni, Co, C using a common sulfuric acid pickling bath
One or more u and Pb ions are contained in an amount of 100 ppm or more (see FIG. 1).

Ni, Co, Cu, and Pb may be added to the pickling bath in a metallic state such as granular (chip), lump, and powder, and nickel sulfate, cobalt sulfate, copper sulfate, and lead sulfate may be added. Sulfate, nickel chloride, cobalt chloride,
May be added in the form of copper chloride, chloride of lead chloride, nickel phosphate, cobalt phosphate, copper phosphate, phosphate of lead phosphate, nickel oxide, cobalt oxide, copper oxide, lead oxide, etc. There is no limit. There is no problem even if it is mixed from plating equipment such as electrodes.

In the method for producing an electrogalvanized steel sheet according to the present invention, the pickling is usually performed in-line with the electroplating line, but there is no problem even if it is performed off-line, for example, in an annealing line for steel sheets, and the like. Also, when it is carried out in-line in the electroplating line, it may be carried out in a normal pickling tank or may be carried out in the plating tank.

When the steel sheet is pickled in a plating tank, one or more Ni, Co, Cu and Pb ions are added to the zinc plating bath so as to contain 100 ppm or more (see FIG. 2). The pickling is performed using the first one or two or more of the plurality of plating treatment tanks, and then zinc plating is performed in the remaining plating tanks.

The electrogalvanizing is carried out by usual electroplating. As the base steel sheet as a base material, various steel sheets such as ordinary steel, aluminum killed steel, and high-tensile steel sheet can be used, and the average crystal grain size on the surface of these steel sheets is 25 μm or less (see FIG. 3). ).

[0028]

[Examples] Examples of the method for producing an electrogalvanized steel sheet according to the present invention will be described.

A cold rolled steel sheet having a thickness of 0.7 mm and a width of 1419 mm was electrolytically degreased 2 and washed with water 3 in the electroplating line shown in FIG. 4 and then pickled 4 (from iron) under the conditions shown in Table 1. Noble ions were added), followed by washing with water 5, followed by electrogalvanizing.

Electrogalvanizing conditions Plating bath composition ZnSO ₄ .7H ₂ O; 350 g / l Na ₂ SO ₄ ; 70 g / l H ₂ SO ₄ ; 30 g / l

Electrolysis conditions Current density; 100 A / dm ₂ temperature; 60 ° C. Flow rate: 1.2 m / min Adhesion amount; 20 g / m ²

With respect to the electrogalvanized steel sheet thus manufactured, the items described below were evaluated. Table 1 shows this result.

(1) Surface Appearance Wood-like unevenness The degree of unevenness that looks like so-called wood-like streaks was evaluated. ◎: No wood grain unevenness ○: Slight wood grain unevenness △: Slight wood grain unevenness ×: Significant wood grain unevenness

Defect in gloss: The degree of so-called defect in gloss, in which the galvanized surface was shining, was evaluated. ◎: No gloss defect at all ○: Slight gloss defect occurred slightly △: Gloss defect slightly generated × ： Gloss defect significantly occurred

Reversal unevenness The degree of unevenness in which white and black appear to be reversed depending on the direction of observation was evaluated. ◎: No reversal unevenness at all ○: Slight reversal unevenness occurred △: Little reversal unevenness ×: Significant reversal unevenness

[0036]

[Table 1]

From Table 1, the electrogalvanized steel sheet produced by the method for producing an electrogalvanized steel sheet according to the present invention satisfies the prescribed conditions described in the claims, so that grain-like unevenness, poor gloss and reversal There is no unevenness,
The surface appearance of the electrogalvanized layer is extremely good, and on the contrary, the comparative example does not satisfy the above-specified conditions, so that wood grain-like unevenness, poor gloss and reversal unevenness are severely generated.
The surface appearance of electrogalvanized steel sheet is poor.

[0038]

[Example 2] A cold rolled steel sheet having a thickness of 0.7 mm and a width of 1219 mm was electrolytically degreased 2, washed with water 3 and then pickled 4 (sulfuric acid bath) in the electroplating line shown in FIG. After washing with water 5, the steel plate 1 was dipped at 10 mg / in a first galvanizing tank 6 or two tanks 7 or more by non-current immersion.
After dissolution of m ^{2 to 1} g / m ² , electrogalvanization was subsequently performed in the remaining galvanizing tank 8.

[0039] Zinc galvanizing conditions galvanizing bath composition _{ZnSO 4 · 7H 2 O; 350g} / l Na 2 SO 4; 70g / l H 2 SO 4; 30g / l additive elements; shown in Table 2

Electrolysis conditions Current density: 100 A / dm ² Temperature: 55 ° C. Flow rate: 1.2 m / min Adhesion amount: 20 g / m ²

The produced galvanized steel sheet was evaluated for grain-like unevenness, poor gloss and uneven reversal in the same manner as in Example 1. Table 2 shows the results. From this Table 2, since the electrogalvanized steel sheet manufactured by the method for manufacturing an electrogalvanized steel sheet according to the present invention satisfies the specified conditions described in the claims, wood grain-like unevenness, poor gloss and reversal unevenness In contrast, the electrogalvanized layer had a good surface appearance, and the comparative example, on the contrary, did not satisfy the above-specified conditions.Therefore, wood grain-like unevenness, poor gloss and reversal unevenness were severely generated. The galvanized steel sheet surface appearance is poor.

[0042]

[Table 2]

[0043]

[Example 3] A cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1419 mm was pickled under the conditions shown in Table 3 on the outlet side of the continuous annealing line for the steel sheet, and then electrozinc-coated at the electroplating line. Plated. The electrogalvanizing conditions are the same as in Example 1. With respect to the produced electrogalvanized steel sheet, the surface appearance of grain-like unevenness, poor gloss and uneven reversal was evaluated by the same method as in Example 1. The results are shown in Table 3.

From Table 3, the electrogalvanized steel sheet produced by the method for producing an electrogalvanized steel sheet according to the present invention satisfies the prescribed conditions of the claims, so that grain-like unevenness, poor gloss and reversal There is no unevenness,
The surface appearance of the electrogalvanized layer was extremely good, and on the contrary, the comparative example did not satisfy the specified conditions of the claims, so that wood grain-like unevenness, poor gloss and uneven reversal often occurred. The surface appearance of the plated steel sheet is poor.

[0045]

[Table 3]

[0046]

As described above, since the method for producing an electrogalvanized steel sheet according to the present invention has the above-described structure, the surface crystal grain size of the base steel sheet is reduced and before the electrogalvanization treatment is performed. By performing the pickling treatment, it is possible to eliminate uneven plating of the electrogalvanized layer due to the base steel sheet and defective gloss, and further, without changing the characteristics of the coating,
The appearance of the electrogalvanized layer can be improved, and corrosion resistance, chromate treatment and other plating characteristics do not deteriorate,
Furthermore, it can be applied to a high current density plating line.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the Ni ²⁺ concentration in a pickling bath and the appearance of a plating layer.

FIG. 2 is a diagram showing the relationship between the amount of dissolution of pickling steel sheet and the appearance of a plating layer.

FIG. 3 is a diagram showing the relationship between the average crystal grain size of a steel sheet and the appearance of a plated layer.

FIG. 4 is a schematic view showing an electrogalvanizing process of Example 1.

5 is a schematic view showing an electrogalvanizing process of Example 2. FIG.

[Explanation of symbols]

 1 ... steel plate 2 ... electrolytic degreasing 3 ... water washing 4 ... pickling 5 ... water washing 6 ... electrogalvanizing bath 7,8 ...

Claims (2)

[Claims] 1. A steel sheet having an average crystal grain size of 25 μm or less on the surface is used as a material to be plated and immersed in a sulfuric acid pickling bath containing 100 ppm or more of one or more of Ni, Co, Cu and Pb. The steel plate surface at 10 mg / m ^{2 to 1} g / m ²
A method for producing an electrogalvanized steel sheet, which comprises performing electrogalvanizing treatment after melting. 2. A steel plate having an average crystal grain size of 25 μm or less on the surface is used as a material to be plated and immersed in a zinc plating bath containing 100 ppm or more of one or more of Ni, Co, Cu and Pb. 10 mm / m ^{2 to 1} g /
A method for producing an electrogalvanized steel sheet, which comprises performing electrogalvanizing treatment after melting m ² .