JPH08188899A - Electrogalvanized steel sheet and its production - Google Patents

Abstract

PURPOSE: To provide an electrogalvanized steel sheet free of nonuniform plating, glittering luster, etc., and having an excellent appearance and to provide the method capable of efficiently producing the steel sheet. CONSTITUTION: A zinc plating contg. 0.0008-0.05wt.% at least one kind selected from a group consisting of Sn, In, Bi and Sb in total is applied on a steel sheet with the average crystal grain diameter of the surface controlled to <=38μm. Meanwhile, a steel sheet with the average crystal grain diameter of the surface controlled to <=38μm is used at the time of forming a zinc plating layer on the steel sheet surface by electroplating, and the steel sheet is electrogalvanized in a zinc plating bath contg. 0.08-25ppm at least one kind selected from a group consisting of Sn, In, Bi and Sb.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Galvanized steel sheets have excellent corrosion resistance due to the sacrificial anticorrosion effect of the galvanized coating on the base steel sheet. Therefore, they are used as outer panels or building materials for automobiles and household electric appliances. Widely used. Commonly used as a method for producing such a galvanized steel sheet includes a hot dip galvanizing method of forming a galvanized film on the steel sheet surface by dipping the steel sheet in a hot dip galvanizing bath, and a zinc ion. It is an electrogalvanizing method in which a steel sheet is immersed in an acidic solution and a galvanized film is deposited on the surface of the steel sheet by an electrochemical reaction.

Of these methods, the hot dip galvanizing method has the advantage that it can easily form a thick coating layer and is excellent in corrosion resistance, but on the other hand, since it is dipped in a molten metal bath, it is uniform. It is difficult to obtain a large amount of plating, and dross or the like may adhere to the surface of the plating, which is somewhat difficult in terms of uniformity and surface appearance. On the other hand, in the electrogalvanizing method, the uniformity and surface appearance are better than those in the hot dip galvanizing method, but the electric power cost increases in proportion to the amount of the applied plating, and therefore the cost increases when the weight is increased. There is an economic problem.

However, the conventional zinc plating is almost always used as a coating base, and defects in the appearance of the zinc plating layer itself are hidden by the overcoating. This is the reason why the hot dip galvanizing method is still predominant without any problem.

However, in recent years, due to a demand from the customer side to omit the coating process, a special chemical treatment for improving corrosion resistance, fingerprint resistance, lubricity, etc. by subjecting galvanized steel sheet to chemical treatment such as chromate treatment or transparent resin coating. The trend of using bare galvanized steel sheets is increasing. If this happens, the appearance of the galvanized steel sheet during manufacturing will directly affect the appearance of the final product, and therefore the requirements for the surface quality of the galvanized steel sheet itself will become very strict.

[0006] In such a bare application, a hot-dip galvanized steel sheet that does not give a very good surface appearance is unsuitable, and a beautiful and highly electrogalvanized steel sheet is used. . However, as the field of application for naked applications has expanded, the demands of consumers for the surface quality of galvanized steel sheets have become more and more stringent, and recently, even with conventional electrogalvanized steel sheets, the demands of consumers cannot be satisfied. Is the reality.

In general, the appearance of a galvanized steel sheet is easily affected by the surface of the steel sheet which is the material to be plated. For example, if the thickness of the oxide film layer on the surface of the steel sheet is not uniform, the appearance after plating is uneven between the thick part and the thin part of the film. Further, even if impurities such as Ni, Si and C segregate or adhere to the surface of the steel sheet, uneven plating occurs. In addition to such uneven plating, even when the crystal grain size on the surface of the steel sheet is large, problems such as large growth of plated crystals and a glittering appearance (glitter luster) occur.

Therefore, as means for improving the surface appearance of the electrogalvanized layer, the production conditions of the material to be plated (pickling conditions after hot rolling, electrolytic cleaning conditions after cold rolling, annealing conditions, etc.), Methods for controlling zinc plating conditions, pretreatment conditions for zinc plating (electrolytic degreasing conditions, pickling conditions, etc.) and post-treatment conditions have been reported.

For example, in Japanese Patent Laid-Open No. 61-166992, the crystals of the zinc plating layer are strongly influenced by the orientation and size of the crystal grains on the surface of the steel material, and the zinc plating crystals grow epitaxially along the crystal grains. It discloses a method of making a zinc-plated crystal fine and smooth by controlling the diameter of ferrite crystal grains on the surface of the steel material in the range of 10 to 35 μm by utilizing such phenomenon. In this way, in the method of adjusting the grain size of the steel material surface, a sharp and sharp change in the glossiness with respect to the crystal grain size of the plated material is not seen, but the surface grain size of the steel material in a range suitable for all applications It is difficult to control, and when a material to be plated having a partially different grain size is used, a problem arises that the galvanized layer eventually has different color tone / gloss.

Japanese Patent Publication No. 3-31795 further defines the plating conditions in addition to the above-mentioned factors on the steel sheet side. Specifically, the ferrite grain size on the surface of the steel material is No. 7.0. 10 (that is, crystal grain size: 35 to 10 μm)
m), the spread rate is 1.0 to 2.0, and the plating bath pH is in the range of 1.0 to 2.5. Furthermore, JP-A-57-11
Japanese Patent No. 0692 is intended to obtain an electrogalvanized steel sheet having an excellent surface appearance by removing stains on the plated surface and the non-plated surface and preventing discoloration. It is a method of wetting the steel sheet surface with the above alkaline aqueous solution or acidic aqueous solution having a pH of 2.5 or less. However, none of these methods has been able to stably obtain a good plating appearance.

In addition, JP-A-61-147894 and JP-A-4-124295 disclose iron group ions (Fe, N) having a lower deposition potential than zinc in an electrogalvanizing bath.
(i and / or Co ion) or Sn ion is added first to precipitate these metal ions on the steel sheet, and the precipitated metal element serves as a growth nucleus of the zinc plating crystal to disperse the initial precipitation of zinc. Discloses a method of making a zinc plated crystal fine and smooth. However, these methods have a problem that a sufficient crystal refining effect is not recognized.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is an electrogalvanized steel sheet having an excellent appearance without uneven plating, glitter luster, etc., Another object of the present invention is to provide a method capable of efficiently producing the steel sheet.

[0013]

The constitution of the electrogalvanized steel sheet according to the present invention, which has been able to achieve the above-mentioned object, is that the average grain size of the surface of the steel sheet is 38 μm or less.
The gist is that it is galvanized containing at least one selected from the group consisting of n, In, Bi and Sb in a total amount of 0.0008 to 0.05% by weight.

Further, in the constitution of the method for manufacturing an electrogalvanized steel sheet according to the present invention, which has been able to achieve the above-mentioned object, the average crystal grain size of the surface is 38 μm when the galvanized layer is formed on the surface of the steel sheet by the electroplating method. It has a gist to perform electrogalvanizing in a zinc plating bath containing 0.08 to 25 ppm of at least one selected from the group consisting of Sn, In, Bi and Sb using the following steel sheets. .

[0015]

The present invention has been made by paying attention to the fact that the occurrence of plating unevenness, glitter luster, etc. in electrogalvanized steel sheets largely depends on the size and orientation of the galvanized crystals.

The crystal structure of zinc is hexagonal close-packed crystal, and the crystal form of zinc plating is generally such that hexagonal (0001) basal plane hexagonal plate-like crystals are laminated with a slight inclination to the material to be plated. are doing.

There are various types of unevenness in the plating appearance of the electrogalvanized steel sheet, which are roughly classified into wood grain-like unevenness, glitter luster, and reversal unevenness. Among them, the grain-like unevenness refers to the unevenness that looks like a streak like a grain of wood, and when the crystal structure of the grain part is observed in detail, the hexagonal plate crystals of zinc may become coarse. I understand. Glitter luster means that the appearance looks shimmering, and its crystal structure is a hexagonal plate-like crystal of zinc grown largely with a certain directionality for each crystal grain of the steel sheet. In addition, inversion unevenness is that the color tones of white and black appear to be inverted depending on the observation angle.This is because the orientation of the crystal plane of zinc differs depending on the position of the steel plate, so white and black depend on the degree of light reflection. It is considered that the color tone of the appears to be reversed.

Since the unevenness of the plating appearance is strongly influenced by the size and the orientation of the zinc-plated crystal as described above, if the zinc-plated crystal can be made fine and the orientation of the crystal can be made uniform. It is considered that the above problems such as uneven plating can be solved. As a method for reducing the size of the plating crystal, increasing the deposition potential during the plating process may be considered, but it is not suitable because it has a limit in practical operation.

Then, as a result of diligent study on a method for making the zinc plating crystal finer and making the crystal orientation uniform, as a result, the crystal grain size of the steel sheet is made smaller and a metal which is more noble than the iron in terms of potential. It has already been reported and found that the steel sheet may be pickled with a sulfuric acid pickling bath containing ions (Ni, Co, Cu and / or Pb). That is, since the deposition potential of the metal element is lower than that of zinc, these metal elements are first deposited on the steel sheet, and this becomes the growth nucleus of the zinc plating crystal to disperse the initial deposition of zinc, thereby forming the zinc plating crystal. Can be made fine and smooth,
In order to exert such an effect more effectively, the grain size of the steel sheet is further defined. The present invention was made by studying a metal element other than the above-mentioned elements, which is more noble than iron, from the same viewpoint, and the constitution of the present invention will be described in detail below with the history of the experiments that reached the present invention.

The following experiments were conducted based on the following electrogalvanizing bath composition and plating conditions. (Plating bath _{composition) ZnSO 4 · 7H 2 O:} 350g / l Na ₂ SO _4: 70g / l H ₂ SO _4: 30g / l (Plating conditions) Current density: 100A / dm ² Bath temperature: 60 ° C. Flow rate: 1 0.2m / sec Plating coverage: 20g / m ²

First, the influence of the Sn content in the plated layer on the appearance of zinc plating was examined by changing the Sn content in the range of 0 to 0.01% by weight. The plating appearance was evaluated as follows. Regarding the following evaluation items, the unevenness of the plating surface includes both wood grain-like unevenness and reversal unevenness. ◎: No unevenness or glitter on the plating surface ○: Slight unevenness or glitter on the plating surface △: Slight unevenness or glitter on the plating surface ×: Very large unevenness or glitter on the plating surface

The results thus obtained are shown in FIG.
As is clear from FIG. 1, the Sn concentration in the plating layer is 0.0
It was found that when the content was 008% by weight or more, the appearance of the plating layer was good. It is preferably 0.001% by weight or more.

Next, for In, Bi and Sb,
The effect of these elements on the galvanized appearance was examined in the same manner as Sn. The results thus obtained are shown in FIGS. As is clear from these figures, I in the plating layer
It was found that when the concentrations of n, Bi and Sb were all 0.0008% by weight or more, the appearance of the plating layer was good. The preferred concentration of In, Bi and Sb is 0.001% by weight or more. The reason why an excellent plating appearance can be obtained by using a specific amount of the above metal element is considered as follows.

That is, the metal element (S
Since n, In, Bi and Sb) are all elements nobler than iron and have a higher deposition potential than zinc, it is possible that these metal elements serve as growth nuclei for zinc plating crystals to miniaturize the zinc plating crystals. Of course it is possible. However,
As in the present invention, when the aim is to make the crystal orientation uniform in addition to the refinement of the zinc plating crystal, the fine zinc plating crystal is deposited with the metal element as a nucleus, and the epitaxy of the plating and the steel sheet is reduced. Therefore, it is necessary to randomly grow zinc-plated crystals. If the lower limit of the amount of the metal element used in the present invention is specified in this way, the action as the nucleus described above can be obtained, and by co-depositing these metal ions in the plating layer to a small extent, It is considered that the epitaxy with the zinc-plated crystal is reduced, the growth direction becomes random, and coarse formation of the zinc crystal can be suppressed.

However, even if added in excess of 0.05% by weight, such an effect is saturated, and since all the metal elements are noble metals than iron, which is the base steel sheet,
Corrosion rate of the galvanized layer is remarkably increased, red corrosion caused by zinc occurs early, and corrosion resistance deteriorates. The upper limit is preferably 0.01% by weight or less, more preferably 0.05% by weight or less.

Each of these metal elements may be contained alone in the plating layer, but it is also possible to use two or more of them in combination, that is, the total amount in the plating layer is 0.00.
It may be 08 to 0.05% by weight. Of these, Sn is most recommended from the viewpoint of being easily and stably deposited in the plating layer.

In the present invention, it is necessary to further specify the average crystal grain size on the surface of the steel sheet. That is, the crystal morphology of galvanization also largely depends on the crystal grain size of the steel sheet, and when the crystal grain size of the steel sheet is large, the content of the metal element in the plating layer is within the specified range of the present invention. Even if it does, the zinc plating crystal becomes coarse, and the plating appearance is not improved.

From such a viewpoint, in order to determine the optimum average grain size of the steel sheet surface in the present invention, the electrogalvanized steel sheet containing 0.002% by weight of Sn in the galvanized layer under the above-mentioned plating conditions. Using, the average crystal grain size of the steel sheet was changed to 5 to 50 μm and the plating appearance was evaluated in the same manner. The average crystal grain size of the steel sheet is obtained by counting the number of crystal grains within a certain area from the SEM observation image of the plating surface and determining the grain size (assuming the crystal grains are circles) from the area per one crystal grain. It was calculated.

The results thus obtained are shown in FIG.
As is clear from FIG. 5, the average crystal grain size of the steel sheet is 38 μm.
The zinc-plated appearance will be good if it becomes the following. It is preferably 35 μm or less.

The steel sheet of the present invention thus defines the average crystal grain size on the surface of the steel sheet and the type and content of the metal element in the plating layer, and the other requirements are not limited in any way. No, but here's a brief explanation.

The amount of zinc plating applied is not limited in any way, and the range from a low amount of adhesion to a high amount of adhesion may be determined in consideration of the degree of corrosion resistance required for the application, but 1 g If it is less than / m ² , the original corrosion resistance of galvanization cannot be fully exhibited, while 100
Even if the adhesion amount is excessively increased in excess of g / m ² , the effect of improving the corrosion resistance cannot be further obtained, and the electric power cost at the time of manufacturing is increased, which is wasteful.

Furthermore, the corrosion resistance is improved, the surface scratch resistance is improved,
One or two or more chemical conversion treatment layers may be provided on the surface of the galvanized layer of the electrogalvanized steel sheet of the present invention described above for the purpose of improving various properties such as fingerprint resistance and workability. It is possible. Examples of such a chemical conversion treatment layer include a chromate film layer, an organic film layer, an inorganic film layer, and a phosphate film layer. Among these, as the chromate film layer, a reaction type chromate film or a coating type chromate film can be mentioned. Especially in the case of a coating type chromate film, phosphoric acid, colloidal silica and an organic resin should be added in order to further improve the corrosion resistance. Is also possible. As the organic film layer, for example, polyethylene resin,
It is also possible to appropriately select and use an epoxy resin, an acrylic resin, a urethane resin, a polyester resin, a fluorine resin, a mixture thereof, a copolymer, a modified resin, or the like. Further, for the purpose of improving the corrosion resistance, silica gel, colloidal silica or the like is added to the organic film,
It is possible to add a small amount of various wax components for the purpose of improving the processability after applying the coating film, or to add a silane coupling agent for the purpose of improving the adhesion of the coating film. Examples of the inorganic coating layer include silicate and the like.

The amount of the above chromate film layer deposited is not particularly limited, but from the viewpoint of corrosion resistance, it is 5 mg / m ^2.
It is preferable to make the above. However, 300 mg /
When it exceeds m ² , the effect of improving the corrosion resistance is saturated and the manufacturing cost is increased, which is wasteful. The amount of the organic coating layer and the inorganic coating layer deposited is 0.5 g / m ² or more and 20 g or more for the same reason as the chromate coating layer.
/ M ² or less is preferable.

The electrogalvanized steel sheet of the present invention is obtained by using a steel sheet having a specified crystal grain size as described above and carrying out electroplating on the steel sheet in a galvanizing bath containing the specified amounts of the above elements. . Here, the total amount of metal elements added to the zinc plating bath is 0.08 to 25 ppm.
It is necessary to include it. The preferred lower limit is 0.1
0 ppm, more preferably 0.50 ppm, even more preferably 1.00 ppm, with a preferred upper limit of 10
ppm, more preferably 5 ppm, even more preferably 3 ppm.

The form of addition of the metal element used in the present invention is not limited at all, and it may be added in the metal state, or may be added in the form of sulfate, chloride, phosphate, carbonate, oxide or the like. It may be added in any form. Alternatively, there is no problem even if it is mixed from plating equipment such as electrodes.

Further, as the steel sheet used in the present invention,
Various materials such as ordinary steel, Al-killed steel, and high-strength steel can be applied. As described above, the surface grain size is 35 μm.
There is no limitation other than the following.

The plating bath used for electrogalvanizing is
From the viewpoint of giving a high current density, it is preferable to use an acidic bath such as a sulfate bath or a chloride bath. Further, for the purpose of cleaning the surface of the steel sheet, it is also effective to perform alkaline spray degreasing, alkaline electrolytic degreasing, pickling treatment, etc. before electrogalvanizing.

[0038]

EXAMPLES Hereinafter, the constitution and working effects of the present invention will be described more specifically with reference to examples, but the present invention is not limited by the following examples, and is applicable to the gist of the preceding and the following. It is needless to say that appropriate modifications can be made within the range to be obtained, and they are all included in the technical scope of the present invention.

Example 1 A cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1419 mm was used as a steel material to be plated, and the amount of metal elements (Sn, In, Bi, Sb) added to the electrogalvanizing bath was 0. 02-30
While changing the range of ppm and changing the average crystal grain size on the surface of the steel sheet, the surface was cleaned by electrolytic degreasing and pickling treatment in the electroplating line, and then electrogalvanization was performed. These processing conditions are as follows.

(Electrolytic degreasing conditions) Current density: 50 A / dm ² Bath temperature: 60 ° C. Energizing method: Indirect energizing Energizing time: 2 sec Bath composition: Na orthosilicate (30 g / liter) (Pickling treatment condition) Bath composition: H ₂ SO ₄ (10g / l) bath temperature: 50 ° C. immersion time: a time of 2 sec (plating bath _{composition) ZnSO 4 · 7H 2 O:} 350g / l Na ₂ SO _4: 70g / l H ₂ SO _4: 30g / l Sn, One or more of In, Bi, Sb: 0.02 to 30 ppm (electrogalvanizing treatment condition) Current density: 100 A / dm ² Temperature: 60 ° C. Flow velocity: 1.2 m / sec Plating adhesion amount: 20 g / m ²

With respect to the surface appearance of the electrogalvanized steel sheet thus obtained, three items of grain-like unevenness, glitter luster and reversal unevenness were evaluated according to the following criteria. Grain-like unevenness The degree of unevenness that looks like so-called grain-like streaks was evaluated. ⊚: No wood grain unevenness ○: Slight wood grain unevenness Δ: Slight wood grain unevenness ×: Significant wood grain unevenness Glitter gloss The degree to which the surface looks glittering was evaluated. ⊚: No glitter gloss ○: Glitter was slightly generated Δ: Glitter was slightly generated ×: Glitter was significantly generated ⊚: No reversal unevenness ○: Slight reversal unevenness Δ: Little reversal unevenness ×: Significant reversal unevenness

Furthermore, J
Corrosion resistance was evaluated by subjecting to a salt spray test according to IS Z 2371 and measuring the time until 1% of red rust was generated. The obtained results are shown in Tables 1 and 2.

[0043]

[Table 1]

[0044]

[Table 2]

The following can be considered from Tables 1 and 2.
No. Nos. 1 to 34 are examples satisfying all the requirements of the present invention, and the surface appearance of the obtained galvanized layer is very excellent without any grain unevenness, glitter luster, or reversal unevenness. At the same time, the corrosion resistance was good.

On the other hand, in No. Nos. 35 and 36 are comparative examples in which the average crystal grain size on the surface of the steel sheet exceeds the range of the present invention, and all of wood grain-like unevenness, glitter luster, and inversion unevenness occurred, and the appearance was very poor.

No. Nos. 37 to 41 are comparative examples in which the average grain size on the surface of the steel sheet satisfies the range of the present invention, but the content of the metal element in the galvanized layer is less than the range of the present invention. , Inversion unevenness was slightly generated. Of these, No. 1 containing no metal element described above. In No. 41, all these irregularities were remarkably generated.

Further, in No. Nos. 42 to 45 are comparative examples in which the average crystal grain size on the surface of the steel sheet satisfies the range of the present invention, but the content of the metal element in the galvanized layer exceeds the range of the present invention, and wood grain unevenness, glitter luster, Some inversion inversion occurred and the corrosion resistance also decreased.

[0049]

EFFECT OF THE INVENTION Since the electrogalvanized steel sheet of the present invention is configured as described above, it has a very excellent surface appearance without causing wood grain-like unevenness, glitter luster, reversal unevenness, etc. without impairing corrosion resistance and the like. A steel sheet having is obtained. Further, by using the manufacturing method of the present invention, it is possible to efficiently manufacture an electrogalvanized steel sheet having an excellent appearance, and it is also possible to obtain the advantage that it can be applied to a high current density plating line.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the amount of Sn in a plated layer on the appearance of zinc plating.

FIG. 2 is a graph showing the influence of the amount of In in the plating layer on the appearance of zinc plating.

FIG. 3 is a graph showing the influence of the amount of Bi in the plating layer on the zinc plating appearance.

FIG. 4 is a graph showing the influence of the amount of Sb in the plated layer on the appearance of zinc plating.

FIG. 5 is a graph showing the effect of the average grain size of the steel sheet on the appearance of galvanizing.

Claims (2)

[Claims] 1. A total amount of at least one selected from the group consisting of Sn, In, Bi and Sb is 0.0008 on a steel sheet having an average crystal grain size of 38 μm or less on the surface.
An electrogalvanized steel sheet, characterized in that the galvanized steel sheet is contained in an amount of up to 0.05% by weight. 2. When forming a galvanized layer on the surface of a steel sheet by electroplating, the average crystal grain size on the surface is 38.
Sn, In, Bi and S
0.08 at least one selected from the group consisting of b
A method for producing an electrogalvanized steel sheet, which comprises performing electrogalvanizing in a galvanizing bath containing ˜25 ppm.