JPH08104995A - Production of electrogalvanized steel sheet excellent in appearance - Google Patents

Abstract

PURPOSE: To produce an electrogalvanized steel sheet having a good plating appearance at the time of electrogalvanizing a steel sheet with an acidic bath by pickling the steel sheet before plating to control the pickling loss to a specified value. CONSTITUTION: When a steel sheet is electrogalvanized with an acidic bath such as a sulfuric acid bath and a chloride bath, the steel sheet to be plated is pickled before plating to control the pickling loss to >=0.60g/m<2> , and then plating is applied. 2 Besides, the pickling loss is preferably controlled to >=0.90g/m<2> to completely remove the whole heterogeneous layer of the steel sheet. Consequently, an electrogalvanized steel sheet without any defect such as stripes, having good luster and excellent in appearance is obtained.

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 having an excellent appearance by using an acid bath such as a sulfuric acid bath or a chloride bath.

[0002]

2. Description of the Related Art Generally, electrogalvanized steel materials are widely used for automobiles, home appliances, building materials, etc. because of their excellent film uniformity and appearance as compared with hot-dip galvanized steel materials. As a plating bath used for this electrogalvanizing, an acid bath such as a sulfuric acid bath or a chloride bath is mainly used in a high-speed plating line for the purpose of improving productivity.

However, in these acidic plating baths, the crystal orientation of the plating is strongly influenced by the base steel sheet. Therefore, if the base steel sheet has a non-uniform portion or a defective portion, the crystal orientation of the plating is as it is. And the unevenness and defective parts have different crystal orientation of plating from other parts,
The plating often had a non-uniform plating appearance with a pattern. This is a major drawback of the acid bath, and the gloss is not as good as that of the cyan bath.

The appearance of galvanizing is an important item that cannot be ignored by the manufacturer of plated steel sheets for the following reasons. Conventionally, galvanized steel sheets have been used as coating bases, but recently, due to requests from users to omit coating, chromate and organic resin coatings have been applied onto zinc plating to provide a surface with excellent corrosion resistance and fingerprint resistance. It is increasingly used as a treated steel sheet without any treatment. In this case, the appearance of galvanization is directly reflected as the appearance of the product. Therefore, how to perform plating with a beautiful appearance is an important issue.

Therefore, for example, as a method for improving the gloss of zinc plating in a sulfuric acid acid bath, (1) a method of adding an organic additive such as glycerin, glucose, dextrin to the plating bath (Japanese Patent Laid-Open No. 64-14890). (2) A method of adding SrCO ₃ or BaCO ₃ to the plating bath to purify the plating bath (Japanese Patent Publication No. 53-18170) has been proposed. In addition, as a method of improving the plating appearance defect, (3) the ferrite grain size of the steel sheet as the plated material was changed from 10 μm to 35 μm.
Method for defining in the range of μm (Japanese Patent Laid-Open No. 61-166992), (4) Method for defining the surface roughness of a steel plate as a material to be plated (Japanese Patent Publication No. 5-43796), (5) Electric zinc Method of temper rolling a plated steel sheet (Japanese Patent Laid-Open No. 64-52099)
Etc. have been proposed.

Although pickling of the base metal sheet before electrogalvanizing is routinely carried out, removal of surface oxides such as temper color after annealing (at most several hundred Å at most) is the main method. For the purpose, the pickling weight loss of steel sheet is also 0.3g / m ² or less, and it is not used for improving the appearance of electrogalvanizing. On the other hand, pickling of the base steel sheet before electroplating with electrogalvanic alloy plating is performed by preventing plating defects such as pinholes by removing the surface-enriched layer (Japanese Patent Laid-Open No. 61-159590) or by pickling steel sheet. It has been attempted to improve the coating film creep and frozen chipping property of the flaw part by the anchor effect of the surface (JP-A-63-38597), but from the viewpoint of removing the uneven texture of the steel sheet, The base steel sheet before plating is not pickled.

[0007]

[Problems to be Solved by the Invention]
In the method of (1), the organic additive is adsorbed on the cathode interface,
The leveling effect has some effect of improving gloss, but it is not perfect. In addition, the concentration control range of additives is narrow,
Particularly in high-speed, high-current-density plating in which the composition of the plating bath tends to fluctuate during operation, there is a problem that product homogeneity is not guaranteed. In addition, since the additive itself is reduced at the cathode interface and is actually incorporated into the zinc film, the corrosion resistance of the plating,
There is a problem of degrading the chromate reactivity. Further, the method (2) also has a problem that the concentration control range of the added chemicals is difficult and a small amount of Sr and Ba are co-deposited in zinc, resulting in deterioration of the film characteristics.

In addition, the method (3) is extremely difficult for various users to manage the ferrite crystals of the steel plate in a narrow range of 10 μm to 35 μm in addition to the requirements for the steel plate, and the appearance is very difficult. The effect of improvement is small. Similarly, in the method (4), it is difficult to control the surface roughness of the steel sheet, and the effect of improving the appearance is small. The method (5) has a problem that the material of the plated steel sheet is deteriorated by temper rolling.

Furthermore, the pickling of the base metal steel sheet before plating is routinely carried out, but the main purpose is to remove oxides on the surface of the steel sheet (thickness of several hundred Å at most). Weight loss
It is less than 0.3g / m ² and is not used for improving the plating appearance. It is an object of the present invention to provide a method for producing an electrogalvanized steel sheet which is free of defects such as streaks and has a good gloss and excellent plating appearance even when electrogalvanizing the steel sheet using an acidic bath. Is.

[0010]

Means for Solving the Problems The present invention, when electrogalvanizing a steel sheet using an acid bath, a steel sheet as a material to be plated has a pickling loss of 0.60 g / m ² or more by pickling before plating. It is a method for producing an electrogalvanized steel sheet having an excellent appearance, which is characterized in that it is pickled and then plated.

[0011]

[Working] As a result of diligent study on the influence of the plating base metal side factor on the appearance of electrogalvanizing, the present inventors have found that the electrogalvanizing crystals show ferrite crystal grain size of the base steel sheet and tens of mm square. It was found that, regardless of the crystal orientation in the range, it is strongly influenced by the texture of the surface of the base steel sheet in the range of several hundreds of μm square that is visible, and that the plating appearance is determined by the plating crystal. Came to.

That is, according to the present invention, a steel sheet as a material to be plated is subjected to pickling with a pickling reduction amount of 0.60 g / m ² (per surface) or more before pickling to obtain zinc-plated crystals on the surface of the steel sheet. Since the plating is performed after removing the non-uniform layer that affects the orientation, it is possible to manufacture an electrogalvanized steel sheet that has no defects such as streak patterns and has a good gloss and an excellent appearance. As the acidic electrogalvanizing bath, a sulfuric acid bath, a chloride bath, a mixed bath of a sulfuric acid bath and a chloride bath, and the like can be used.

Next, the reasons for limiting the pickling loss of the steel sheet before plating according to the present invention will be explained. The reason why the pickling loss of the base steel sheet before plating is specified to be 0.60 g / m ² or more is that the uneven layer on the surface of the steel sheet that affects the plating crystal can be removed for most of the steel sheets. Desirably, it is 0.90 g / m ² or more, which can completely remove the non-uniform layer of all the steel sheets. The pickling of the steel sheet before plating is performed with sulfuric acid in plating in a sulfuric acid bath and hydrochloric acid in plating in a chloride bath, which is preferable from the viewpoint of controlling the plating bath composition, but is not particularly limited to this, phosphoric acid, Various substances such as nitric acid can also be used. In order to obtain the pickling weight loss of the steel sheet of the present invention, if the acid concentration cannot be increased from the point of the apparatus, the immersion time of the steel sheet is lengthened, and if the immersion time cannot be increased from the point of the apparatus, a method of increasing the acid concentration is used. It is good to collect.

In general, the crystal form of zinc electrodeposited on a polycrystal exhibits a fault (hexagonal) in the case of a sulfuric acid bath (without addition of additives). When the fault crystals with a certain directionality grow large, they are shimmering and the appearance is inferior. On the other hand, if the fault crystals are fine as a whole, the appearance is fine, the appearance is smooth, and the gloss is excellent. On the other hand, when zinc is electrodeposited, the crystal growth orientation is affected by the crystal orientation of the base (base material), and a phenomenon similar to the epitaxial effect often observed in the metal oxidation phenomenon occurs. The crystals of the base material having this epitaxial effect are those of the outermost layer.

When the plating base material is a polycrystal such as a cold-rolled steel plate, zinc crystals grow while competing for each ferrite crystal grain. At this time, if a small portion of the ferrite crystal grains (for example, 10 μm or less) is accumulated on a specific part of the steel plate surface, competition for crystal growth will be strengthened, resulting in a fine crystal morphology and a plating appearance different from other parts. Present. Alternatively, if a portion having large ferrite crystal grains (for example, 40 μm or more) is accumulated in a specific portion, competition for crystal growth is weakened, resulting in a large crystal morphology, and a plating appearance different from other portions is exhibited. However, such accumulation of ferrite crystal grains is rarely found in ordinary steel plates and is not a problem.

However, when the ferrite crystals of a specific portion on the surface of the steel sheet are accumulated in a specific orientation, a plating appearance different from other portions is exhibited. The accumulation of ferrite crystallographic orientations of specific parts on the surface of the steel sheet occurs in cold rolling or temper rolling because the texture of the steel sheet surface is determined by uneven rolling on the rolling rolls. It is unavoidable in the production of sheet steel. The thickness of this abnormal layer on the surface of the steel sheet is at most about 0.12 μm (pickling loss
It can be removed at 0.60 g / m ² or more), and if this layer is removed by pickling before plating, a uniform plating crystal can be obtained.
It is possible to obtain an electrogalvanized steel sheet which has no appearance defect due to the difference in plating crystal depending on the location on the surface of the steel sheet, has fine plating crystal, and has good gloss and excellent appearance.

[0017]

EXAMPLES The method for producing an electrogalvanized steel sheet according to the present invention, which has an excellent appearance, will be described in detail below. Thickness 0.8m
After alkaline degreasing of the cold rolled steel sheet of m, various pickling loss before plating of the plating base material was performed, and electrogalvanization was performed in a sulfuric acid bath and a chloride bath under the following conditions. (Digital gonio gloss meter) was evaluated. (Sulfuric acid bath) Zn ²⁺ : 100 g / l Na ⁺ : 3 g / l pH: 1.8 Temperature: 60 ° C Current density: 100 A / dm ² Unit weight: 20 g / m ² (Chloride bath) Zn ²⁺ : 110 g / l K ⁺ : 180 g / l pH: 4.5 Temperature: 50 ℃ Current density: 80 A / dm ² Unit weight: 20 g / m ² Plating appearance defects are evaluated from 0 to 5 The number of defects such as streak patterns per 100 × 150 mm steel sheet was evaluated as follows.

5: 0 line 4: 1 line 3: 2 to 3 line 2: 4 to 9 line 1:10 to 15 line 0:16 or more The results are shown in FIG. Case 2
Shown in

As is clear from FIGS. 1 and 2, by adjusting the pickling loss of the steel plate before plating of the plating base material to 0.60 g / m ² or more, there is no appearance defect and the electrical appearance is excellent in gloss. Galvanized steel sheet can be manufactured.

[0020]

EFFECTS OF THE INVENTION According to the present invention, an electrogalvanized steel sheet excellent in a glossy plating appearance without using defects such as streaks that impair the plating appearance while using an acidic bath such as a sulfuric acid bath or a chloride bath Can be manufactured. In addition, the present invention has an advantage that the production efficiency is improved because a good plating appearance can be obtained because the abnormal portion is dissolved and removed by pickling before plating even with a steel sheet having a material defect. Also, because it improves the plating appearance without changing the characteristics of the coating itself, there is no problem of deterioration of plating characteristics such as corrosion resistance and chromate treatment, and it can be applied to high-speed (high current density) plating lines. There are advantages.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the pre-plating pickling loss of a material to be plated, the occurrence rate of electrogalvanizing appearance defects, and the glossiness in the case of a sulfuric acid bath.

FIG. 2 is a graph showing the relationship between the pre-plating pickling loss of a material to be plated, the occurrence rate of electrogalvanizing appearance defects, and the glossiness in the case of a chloride bath.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Nobuo Totsuka 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.

Claims (1)

[Claims] 1. When electrogalvanizing a steel sheet using an acid bath, the steel sheet to be plated is pickled by pickling before plating so that the pickling loss is 0.60 g / m ² or more, and then A method for producing an electrogalvanized steel sheet having an excellent appearance characterized by plating.