JP2812228B2 - Electrogalvanized steel sheet and its manufacturing method - Google Patents

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 for efficiently producing the steel sheet.

[0002]

2. Description of the Related Art Galvanized steel sheets have excellent corrosion resistance because the galvanized film exerts a sacrificial anticorrosion action on the base steel sheet, and is therefore used as an outer plate material for automobiles and household electric appliances or as a building material. Widely used. Commonly used as a method for producing such a galvanized steel sheet is a galvanizing method of forming a galvanized film on a steel sheet surface by immersing the steel sheet in a hot-dip galvanizing bath, and including zinc ions. This 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.

[0003] Among them, the hot-dip galvanizing method has an advantage that a thicker coating layer can be easily formed and has excellent corrosion resistance. It is difficult to obtain a large amount of plating and dross or the like may adhere to the plating surface, and there are some difficulties in uniformity and surface appearance. On the other hand, in the electrogalvanizing method, the uniformity and surface appearance are better than the hot-dip galvanizing method, but the power cost increases in proportion to the amount of plating, so that the thicker the material, the higher the cost. There is an economic problem.

[0004] However, most of the conventional zinc plating is used as a base material for painting, and the appearance defect of the galvanized layer itself is concealed by the overcoating. It is considered that the hot-dip galvanizing method is still the mainstream and it is for these reasons.

[0005] However, in recent years, in response to a request from the consumer side to omit the coating process, a special chemical treatment in which the galvanized steel sheet is subjected to a chemical conversion treatment such as a chromate treatment or a transparent resin coating to improve corrosion resistance, fingerprint resistance, lubricity, etc. There is an increasing trend to use bare galvanized steel sheets. Then, the appearance of the galvanized steel sheet at the time of manufacturing the galvanized steel sheet has a great influence on the appearance of the final product as it is, so that the requirements for the surface quality of the galvanized steel sheet itself become very strict.

[0006] In such barely used applications, hot-dip galvanized steel sheets that do not provide a very good surface appearance are unsuitable, and beautiful and highly uniform electrogalvanized steel sheets are used. . However, as the field of application for bare applications expands, the demands of the customers on the surface quality of galvanized steel sheets become more and more severe, and recently, even conventional electro-galvanized steel sheets cannot satisfy the demands of the users. That is the fact.

[0007] 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, unevenness is observed in the appearance after plating between thick and thin portions of the film. Even if impurities such as Ni, Si, and C segregate or adhere to the surface of the steel sheet, plating unevenness occurs. In addition to such uneven plating, even when the crystal grain size on the surface of the steel sheet is large, there arises a problem that the plated crystals grow large and the appearance looks shiny (glitter).

Therefore, as means for improving the surface appearance of the electrogalvanized layer, 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, pre-treatment conditions (electrolytic degreasing conditions, pickling conditions, etc.) and post-treatment conditions of zinc plating have been reported.

For example, Japanese Patent Application Laid-Open No. 61-166992 discloses that the crystal of a zinc plating layer is strongly affected by the orientation and size of crystal grains on the surface of a steel material, and zinc plating crystals grow epitaxially along the crystal grains. Utilizing this phenomenon, a method of making the zinc-plated crystal fine and smooth by controlling the diameter of ferrite crystal grains on the surface of the steel material within a range of 10 to 35 μm is disclosed. In this way, in the method of adjusting the grain size of the steel material surface, there is no sharp and sharp change in the glossiness with respect to the grain size of the material to be plated, but the surface grain size of the steel material is in a range suitable for all applications. It is difficult to control, and when a material to be plated having a partially different crystal grain size is used, there arises a problem that the galvanized layer eventually has a different color tone and 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 crystal grain size on the steel material surface is set to be 7.0 to 7.0. 10 (ie, crystal grain size: 35 to 10 μm)
m), a method of improving the plating appearance by setting the elongation percentage to 1.0 to 2.0 and adjusting the pH of the plating bath to a range of 1.0 to 2.5. Further, JP-A-57-11
No. 0692 is intended to obtain an electrogalvanized steel sheet having an excellent surface appearance by removing stains on a plated surface and a non-plated surface and preventing discoloration. This is a method of wetting the steel sheet surface with the above-mentioned 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 provide 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 ions) or Sn ions to precipitate these metal ions on the steel sheet first, and the deposited metal elements become growth nuclei for zinc plating crystals to disperse the initial precipitation of zinc. Discloses a method for making zinc-plated crystals 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-mentioned circumstances, and an object of the present invention is to provide an electrogalvanized steel sheet having an excellent appearance without uneven plating and glittering luster. Another object of the present invention is to provide a method capable of efficiently manufacturing the steel sheet.

[0013]

Means for Solving the Problems The constitution of the electrogalvanized steel sheet according to the present invention, which has achieved the above-mentioned objects, is based on a steel sheet having an average crystal grain size of 38 μm or less on the surface.
The gist of the invention is that it is a zinc-plated one containing 0.0008 to 0.05% by weight in total of at least one selected from the group consisting of n, In, Bi and Sb.

[0014] Further, the structure of the method of manufacturing an electrogalvanized steel sheet according to the present invention, which can achieve the above object, has an average crystal grain size of 38 μm when forming a galvanized layer on the steel sheet surface by the electroplating method. The gist is to perform electrogalvanizing in a galvanizing 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 sheet. .

[0015]

The present invention has been made by paying attention to the fact that the occurrence of uneven plating and glittering in an electrogalvanized steel sheet largely depends on the size and orientation of galvanized crystals.

The crystal structure of zinc is a hexagonal close-packed crystal. In general, the crystal form of zinc plating is a hexagonal (0001) basal plane hexagonal plate-like crystal laminated with a slight inclination with respect to the material to be plated. doing.

There are various types of unevenness in the plating appearance of the electrogalvanized steel sheet, and they are roughly classified into grain-like unevenness, glittering gloss, and reversal unevenness. Among these, the grain-like unevenness refers to unevenness that looks like a streak like a grain of wood, and when the crystal structure of the grain portion is observed in detail, hexagonal plate-like crystals of zinc are coarsened. I understand. In addition, the glitter luster is one in which the appearance appears to be glittering, and its crystal structure is such that hexagonal plate-like crystals of zinc grow large in a certain direction for each crystal grain of the steel sheet. Further, inversion inversion means that the color tone of black and white appears to be inverted depending on the angle of observation. This is because the orientation of the crystal plane of zinc differs depending on the position of the steel sheet, so that the white and black are different depending on the degree of light reflection. Is considered to be inverted.

Since the appearance unevenness of the plating is greatly affected by the size and orientation of the zinc-plated crystal, if the zinc-plated crystal can be made finer and the crystal orientation can be made uniform. It is considered that the problems such as the plating unevenness can be solved. As a method for reducing the size of the plating crystal, it is conceivable to raise the deposition potential during the plating step, but there is a limit in actual operation, which is inappropriate.

Therefore, as a result of diligent studies on a method of miniaturizing the crystal of zinc plating and making the orientation of the crystal uniform, as a result, the crystal grain size of the steel sheet was reduced, and a metal which was more noble in potential than iron. It has been found that the steel sheet may be pickled in a sulfuric acid pickling bath containing ions (Ni, Co, Cu and / or Pb) and has already been reported. That is, since the deposition potential of the above 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 precipitation of zinc. Can be made fine and smooth,
In order to exert such an effect more effectively, the crystal grain size of the steel sheet was further defined. In the present invention, other than the above-mentioned elements, metal elements which are more noble than iron have been studied from the same viewpoint. Hereinafter, the configuration of the present invention will be described in detail with reference to the history of experiments that have reached the present invention.

The following experiments were carried out 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 .2 m / sec Plating weight: 20 g / m ²

First, the influence of the Sn content in the plating layer on the appearance of the galvanized coating was examined by changing the Sn content in the range of 0 to 0.01% by weight. The plating appearance was evaluated as follows. In the following evaluation items, the unevenness of the plating surface includes both grain-like unevenness and reversal unevenness. ：: There is no unevenness or glitter on the plating surface. ○: Slight unevenness or glitter on the plating surface. △: A little unevenness or glitter on the plating surface.

FIG. 1 shows the results thus obtained.
As is clear from FIG. 1, the Sn concentration in the plating layer was set to 0.0
It was found that when the content was 008% by weight or more, the appearance of the plating layer became good. Preferably it is 0.001% by weight or more.

Next, for In, Bi and Sb,
The effects of these elements on the galvanized appearance were examined in the same manner as for Sn. The results obtained in this way 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 improved. 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 metal element in this way is considered as follows.

That is, the metal element (S) used in the present invention
Since n, In, Bi and Sb) are all noble elements than iron and have a higher deposition potential than zinc, these metal elements can serve as growth nuclei for zinc-plated crystals to make the zinc-plated crystals finer. Of course you can. However,
As in the present invention, in the case where the aim is to make the crystal orientation uniform in addition to the miniaturization of the galvanized crystal, while precipitating the fine zinc-plated crystal using the above-mentioned metal element as a nucleus, the plating and the epitaxy of the steel sheet are reduced. It is necessary to grow the zinc plating crystal at random. If the lower limit of the amount of the metal element used in the present invention is specified in this manner, the above-described function as a nucleus can be obtained, and these metal ions are co-deposited, albeit slightly, in the plating layer, so that the steel sheet and the steel sheet can be used. It is considered that the epitaxy with the zinc plating crystal is reduced, the growth direction becomes random, and coarse generation of zinc crystal can be suppressed.

However, even if it is added in excess of 0.05% by weight, such an effect is saturated and at the same time, since each of the metal elements is a metal nobleer than iron which is a base steel sheet,
Corrosion rate of the galvanized layer is remarkably increased, and corrosion resistance is deteriorated, such as early generation of red rust caused by zinc. 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 from 08 to 0.05% by weight. Among these, Sn is most recommended from the viewpoint of easy and stable precipitation in the plating layer.

In the present invention, it is necessary to further specify the average crystal grain size on the steel sheet surface. That is, the crystal form of the galvanized coating greatly 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 falls within the range specified in the present invention. Even if it does, the galvanized crystal is coarsened and the plating appearance is not improved.

From such a viewpoint, in order to determine the optimum average crystal grain size on the surface of the steel sheet in the present invention, an electrogalvanized steel sheet containing 0.002% by weight of Sn in a galvanized layer under the above-mentioned plating conditions. And the average crystal grain size of the steel sheet was changed from 5 to 50 μm to evaluate the plating appearance 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 an SEM observation image of the plating surface, and calculating the particle size (assuming the crystal grains are circular) from the area per one crystal grain. Calculated.

FIG. 5 shows the results obtained in this manner.
As is clear from FIG. 5, the average crystal grain size of the steel sheet is 38 μm.
Below, the galvanized appearance becomes good. Preferably it is 35 μm or less.

The steel sheet of the present invention defines the average crystal grain size on the steel sheet surface, the type and the content of the metal element in the plating layer, and other requirements are not limited at all. No, but it is briefly explained below.

The amount of galvanized coating is not limited at all, and the range from a low coating amount to a high coating amount may be determined in consideration of the degree of corrosion resistance required according to the application. / M ² , the original corrosion resistance of the galvanized coating cannot be sufficiently exhibited.
Even if the amount of adhesion is excessively increased beyond g / m ² , no further improvement in corrosion resistance can be obtained, but only an increase in power cost at the time of manufacture is wasteful.

Further, improvement of corrosion resistance, improvement of surface scratch resistance,
For the purpose of improving various performances such as improvement of fingerprint resistance and processability, one or more chemical conversion treatment layers may be provided on the surface of the galvanized layer of the above-described electrogalvanized steel sheet of the present invention. 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 them, the chromate film layer includes a reaction type chromate film or a coating type chromate film. Particularly, in the case of a coating type chromate film, phosphoric acid, colloidal silica, and an organic resin are added to further improve corrosion resistance. Is also possible. Further, as the organic film layer, for example, polyethylene resin,
An epoxy resin, an acrylic resin, a urethane resin, a polyester resin, a fluororesin, a mixture thereof, a copolymer, a modified resin, or the like can be appropriately selected and used. For the purpose of further improving corrosion resistance, silica gel, colloidal silica, or the like is added to the organic film,
It is also possible to add a small amount of various wax components for the purpose of improving the processability after the application of the coating, or to add a silane coupling agent for the purpose of improving the adhesion of the coating. Examples of the inorganic coating layer include silicate and the like.

The amount of the chromate film layer to be deposited is not particularly limited, but is preferably 5 mg / m ² from the viewpoint of corrosion resistance.
It is preferable to make the above. However, 300mg /
If it exceeds m ² , the effect of improving the corrosion resistance is saturated, and the production cost is increased, which is wasteful. Regarding the adhesion amount of the organic film layer and the inorganic film layer, 0.5 g / m ² or more and 20 g
/ M ² or less.

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 performing electroplating on the steel sheet in a galvanizing bath containing a specified amount of the above element. . Here, the total amount of metal elements added to the galvanizing bath is 0.08 to 25 ppm.
It is necessary to contain it. The preferred lower limit is 0.1
0 ppm, more preferably 0.50 ppm, even more preferably 1.00 ppm, and the preferred upper limit is 10 ppm.
ppm, more preferably 5 ppm, even more preferably 3 ppm.

The addition form of the metal element used in the present invention is not limited at all, and it may be added in a metal state, or may be added in the form of a 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 in from a plating facility such as an electrode.

The steel sheet used in the present invention includes:
Various types of steel such as ordinary steel, Al-killed steel, and high-strength steel plate are applicable, and as described above, the crystal grain size of the surface is 35 μm.
There is no limitation other than the following.

The plating bath used for electrogalvanizing is as follows:
From the viewpoint of providing 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 alkali spray degreasing, alkali electrolytic degreasing, pickling treatment or the like before electrogalvanizing.

[0038]

EXAMPLES Hereinafter, the structure and operation and effect of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and the present invention is applicable to the above and following points. It is, of course, possible to implement the present invention with appropriate modifications within the scope of the invention, and all of them are 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 a metal element (Sn, In, Bi, Sb) to be added to an electrogalvanizing bath was 0.0. 02-30
The surface was cleaned by electrolytic degreasing and pickling in an electroplating line while changing the average crystal grain size on the surface of the steel sheet while changing it in the range of ppm, followed by electrogalvanizing. These processing conditions are as follows.

(Electrolytic degreasing conditions) Current density: 50 A / dm ² Bath temperature: 60 ° C. Energization method: Indirect energization Energization time: 2 sec Bath composition: Na orthosilicate (30 g / liter) (Pickling treatment conditions) Bath composition: H ₂ SO ₄ (10 g / l) Bath temperature: 50 ° C. Immersion time: 2 sec (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 Sn, At least one of In, Bi, and Sb: 0.02 to 30 ppm (electrogalvanizing treatment conditions) Current density: 100 A / dm ² Temperature: 60 ° C. Flow rate: 1.2 m / sec Plating weight: 20 g / m ²

With respect to the surface appearance of the electrogalvanized steel sheet thus obtained, three items, that is, grain-like unevenness, glittering luster, and reversal unevenness were evaluated according to the following criteria. Wood-like unevenness The degree of unevenness that looks like a streak like a so-called wood grain was evaluated. ：: No grain-like unevenness at all. ○: Slight grain-like unevenness. Δ: Slight grain-like unevenness. ×: Significant grain-like unevenness. Glitter gloss. The degree to which the surface looks glittering was evaluated. ：: no glitter at all ○: slight glitter attrition △: slight glitter at ×: remarkable glitter atversion unevenness The degree of unevenness in which white and black appear to be inverted depending on the observation direction was evaluated. ：: no inversion unevenness at all ○: slight inversion unevenness Δ: slight inversion unevenness ×: remarkable inversion unevenness

Further, the electrogalvanized steel sheet was left bare and J
It was subjected to a salt spray test according to IS Z 2371, and the corrosion resistance was evaluated by measuring the time until 1% 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. 1-34 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-like unevenness, glittering luster, and reversal unevenness. In addition, the corrosion resistance was good.

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

In addition, No. 37 to 41 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 is smaller than the range of the present invention. , A little inversion inversion occurred. Among them, No. 1 containing no metal element at all. In No. 41, all of these irregularities occurred remarkably.

Further, No. 42 to 45 are comparative examples in which the average crystal grain size of the steel sheet surface 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 grain-like unevenness, glittering luster, A small amount of reversal unevenness occurred, and the corrosion resistance was reduced.

[0049]

Since the electrogalvanized steel sheet of the present invention is constituted as described above, it has a very excellent surface appearance free from grain-like unevenness, glittering luster and reversal unevenness without impairing corrosion resistance and the like. Is obtained. Further, by using the production method of the present invention, it is possible to efficiently produce an electrogalvanized steel sheet having excellent appearance, and it is possible to obtain an advantage that it can be applied to a high current density plating line.

[Brief description of the drawings]

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

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

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

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

FIG. 5 is a graph showing the influence of the average crystal grain size of the steel sheet on the galvanized appearance.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-124295 (JP, A) JP-A-3-281794 (JP, A) JP-A-4-285194 (JP, A) JP-A-3-284 115594 (JP, A) JP-A-7-233498 (JP, A) JP-A-7-331483 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C25D 3/00-5 / 52

Claims (2)

(57) [Claims] 1. A steel sheet having a surface with an average crystal grain size of 38 μm or less, wherein at least one selected from the group consisting of Sn, In, Bi and Sb is 0.0008 in total.
An electrogalvanized steel sheet characterized by being subjected to galvanization containing up to 0.05% by weight. 2. When forming a galvanized layer on the surface of a steel sheet by an electroplating method, the average crystal grain size on the surface is 38.
μm or less, Sn, In, Bi and S
at least one member selected from the group consisting of b
A method for producing an electrogalvanized steel sheet, wherein electrogalvanizing is performed in a galvanizing bath containing about 25 ppm.