JP2001064782A - Surface treating method imparting high weather resistance to steel and high weather resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treating method imparting high weather resistance to steel by combining hot dip galvanizing treatment building up secure position as already general surface treatment and surface treatment by a sol-gel method and synergistically exhibiting the advantages of both treatment and to provide high weather resistant steel treated thereby. SOLUTION: After the formation of a hot dip galvanized layer on the surface of steel, on the surface of the hot dip galvanized layer, a ceramics coating layer formed of metallic oxide having a single compsn. or metallic oxide added with secondary components or multicomponent system metallic oxide is formed. In particular, the ceramic coating layer is desirably formed of the oxide ceramic of Si, Al or Ti.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment method for imparting high weather resistance to steel and a high weather resistance steel, and more particularly, to forming a composite coating of a hot-dip galvanized layer and a ceramic coating layer on the surface of the steel. The present invention relates to a surface treatment method and a highly weather-resistant steel material obtained by the surface treatment method.

[0002]

2. Description of the Related Art In recent years, many outdoor building structures, such as power transmission towers and parabolic antennas, which have been increasingly expanded, and many outdoor building structures have been increasing. Exposure to acid rain containing gas and nitrogen oxides is expected to increase the degree of corrosion. Also, in Japan, where there are few plains, new expressways constructed to increase traffic volume and transport capacity must be installed in mountainous and coastal areas,
As a result, the snowmelt (sodium chloride, calcium chloride, etc.) sprayed on the road surface as a measure against freezing in winter in the mountainous areas, and the seaside water causes severe corrosion of road facilities in the coastal areas.
Conventional hot-dip galvanizing is in a state where it cannot sufficiently cope. In addition, in order to prevent the whitening of hot-dip galvanizing, the surface is generally subjected to chromate treatment. However, since chromate causes pollution, chromate treatment has recently been shunned. Was.

[0003] Conventionally, as a method of imparting corrosion resistance to power transmission towers, steel materials related to road facilities, and further to steel materials for building structures,
Hot-dip galvanizing is well known. This hot-dip galvanizing is a simple method and is an excellent treatment for improving the corrosion resistance of steel materials. However, a coating having higher corrosion resistance than hot-dip galvanizing has been required. Therefore,
A treatment method for forming a zinc-aluminum alloy plating film having better corrosion resistance than a zinc plating film on a steel material surface,
In other words, a so-called two-bath method in which a zinc-aluminum alloy plating is performed after galvanizing a steel material in advance is proposed,
Some have been put to practical use.

[0004] Further, as one of the methods for improving the high-temperature oxidation property and the corrosion resistance of an aqueous solution of a metal material, it is known to form a ceramic coating layer by a sol-gel method. This sol-gel method was used in Germany in the 1960's to apply an anti-reflection coating to lenses and glass, and is a technology that has been put into practical use. In the 1970's, research into the use of bulk glass or ceramics for synthesis was actively conducted, but was not practically used due to cost and productivity problems. After a long downturn,
In the 1990's, it was reviewed as an application to thin films, and has become a technology that attracts attention again.

[0005]

However, most of the research on corrosion prevention by the conventional sol-gel method relates to the further improvement of corrosion resistance of stainless steel, and other metal materials are directly coated with ceramics on the surface of the metal material. Was applied. As described above, the sol-gel method has been put to practical use only in a special field, and is generally unfamiliar in the field of surface treatment technology, and is hardly a general-purpose surface treatment.

Therefore, the present invention seeks to solve the problem by combining a hot-dip galvanizing treatment, which has already established a solid position as a general-purpose surface treatment, and a surface treatment by a sol-gel method, and synergistically combining the advantages of both treatments. It is an object of the present invention to provide a surface treatment method for imparting high weather resistance to a steel material by exerting the same, and to provide a high weather resistance steel material treated thereby.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the first invention is to form a hot-dip galvanized layer on the surface of a steel material, and then form a single composition by a sol-gel method on the hot-dip galvanized layer. Provided is a surface treatment method for imparting high weather resistance to a steel material having a ceramic coating layer formed of a metal oxide to which a metal oxide or a second component is added or a multicomponent composite metal oxide.

Here, it is preferable that the ceramic coating layer is formed of an oxide ceramic of Si, Al or Ti by a sol-gel method.

Further, the ceramic coating layer is formed by using an alkoxide of Si, Al or Ti as a starting material, forming an alcohol solution, reacting with water, and hydrolyzing the solution. And
It is particularly preferable to form an oxide ceramic film of Si, Al or Ti on the surface of a hot-dip galvanized layer of a steel material by a sol-gel method of drying it.

Further, aluminum isopropoxide has a
-Add propanol and then add hydrochloric acid to bring the pH to 0
3, and an alumina-based sol solution having an aluminum isopropoxide concentration of 0.2 to 0.5 mol / L is preferably used. Here, the sol solution is sprayed on the steel material surface one or more times by a spray method to form a film. Here, in the present invention, “L” indicates liter.

And drying the gel film formed on the surface of the steel material at a temperature from room temperature to 200 ° C. or less.
Particularly preferably, the gel film formed on the steel material surface,
It is naturally dried at room temperature.

During the hydrolysis, a transition metal salt is added as a coloring agent to form an ionic state, and the transition metal is hydrolyzed in the ionic state, and contained in the ceramic coating layer in an amount of 0.01 to 0.5% by weight. It is also preferable to make it.

The second invention uses a surface treatment method for imparting high weather resistance to a steel material by the sol-gel method described above,
An object of the present invention is to provide a highly weather-resistant steel material in which a composite coating of a galvanized layer and a ceramic coating layer is formed on the surface of the steel material.

[0014]

Next, the details of the present invention will be described in more detail based on embodiments.

(Hot-dip galvanizing treatment of steel material) In order to apply hot-dip galvanizing to the surface of a steel material, a conventionally known method can be adopted. In other words, after the steel material is pickled and washed with water,
The flux is treated, immersed in a molten zinc bath set at a predetermined temperature for a predetermined time, pulled up at a predetermined speed, air-cooled or water-cooled, and a hot-dip galvanized film having a predetermined thickness is formed on the surface of the steel material.

Further, the hot-dip galvanized film may have been subjected to a coloring treatment. For this coloring treatment, a patent invention of the present applicant (Japanese Patent No. 2920148) can be used. In other words, the steel material on which the galvanized film is formed is immersed in a chemical conversion solution containing manganese as a main component, subjected to a chemical conversion treatment in which the film surface is colored black, and then washed with water.
Next, this is a method for coloring a galvanized film which is subjected to a post-treatment for imparting corrosion resistance by immersion in a post-treatment solution containing chromium as a main component. Specifically, potassium permanganate (KMn
O ₄ ): 10 to 80 g / l, trisodium phosphate (Na ₃ PO ₄ .12H ₂ O): 20 to 200 g / l, and sodium hydroxide (NaOH): 10 to 100
g / liter of a chemical conversion treatment solution and a treatment solution temperature of 3
A chemical conversion treatment in which a galvanized steel material is immersed under a chemical conversion treatment condition of 0 to 70 ° C. and a treatment time of 2 to 15 minutes;
O ₃ ): 1 to 20 g / liter, pH 1.0 to
Using the treatment liquid prepared at 4.0, the treatment liquid temperature was 30 to 70.
Post-treatment I in which a galvanized steel material is immersed under a treatment condition of 1 ° C. and a treatment time of 1 to 10 minutes.

(Preparation of Sol) A sol is prepared by adding a metal alkoxide, a metal acetylacetonate, or a metal halide to an alcohol which is a common solvent of the alkoxide, the metal acetylacetonate or the metal, and a small amount of water required for hydrolysis and a catalyst. This is done by mixing an acid or a base. Here, in the present invention, an alkoxide of Si, Al, or Ti is used as a metal alkoxide, and the metal alkoxide is used as a starting material. After forming an alcohol solution, the alkoxide is reacted with water and hydrolyzed to form a sol. . When water is directly added to a highly reactive alkoxide, local hydrolysis occurs to cause precipitation, and in this case, a precipitation inhibitor having an effect of suppressing precipitation due to hydrolysis is added to the sol. It is also preferred.

When a metal alkoxide having a single composition is used as a starting material, a ceramic coating layer composed of a metal oxide having a single composition can be obtained, but by adding a second component or element to the starting material, And a ceramic coating layer comprising a metal oxide to which the second component is added or a multicomponent composite metal oxide.

It is known that the microstructure of the sol affects the gel structure and thus the properties of the final ceramic coating layer. The preparation of this sol is important because the microstructure of the sol can be controlled to some extent by the preparation of the sol. Depending on its preparation, the sol becomes a particulate microstructure in which cyclic primary particles are aggregated, or a linear chain microstructure with chains or slight cross-links, and a finer structure of linear chains than in particles. Becomes Then, when the linear chain sol with some crosslinking is gelled, the structure changes to a highly crosslinked structure, and this gel becomes a ceramic coating layer having high strength without heat treatment at a high temperature.

The sol formation process is a competitive reaction between the hydrolysis reaction and the polycondensation reaction using the pH value of the sol as a parameter, and the ratio of the reaction rates of both reactions is an important factor. In the strongly acidic region where the pH value of the sol is low, the hydrolysis reaction rate is high and the polycondensation reaction rate is low, so that the raw material alkoxide is completely hydrolyzed. In the weakly acidic or weakly alkaline region where the pH value of the sol is neutral, the polycondensation reaction rate is conversely high and the hydrolysis reaction rate is low, so that it is difficult to form a sol. For this reason, the hydrolysis is first terminated in the strongly acidic region, and then the preparation is performed by adding an alkaline additive. In addition, in the strongly alkaline region where the pH value of the sol is high, the hydrolysis reaction rate is also large, so that when it is produced by hydrolysis, it immediately undergoes a polycondensation reaction, and large particles are generated. Is not available.

Therefore, it is simplest to prepare a sol in a strongly acidic region, and the sol is stable, which is industrially advantageous. Here, when the sol solution is strongly acidic,
Initially, it was expected that the strong acidity of the sol would have an adverse effect on the durability of the composite coating of the hot-dip galvanized layer and the ceramic coating layer that was subsequently formed, but subsequent experiments revealed no problems. . Conversely, the composite film formed using a weakly acidic or weakly alkaline sol solution, which was considered favorable in the initial preliminary experiments, was insufficient in corrosion resistance as a result of a long time salt spray test. I understood. Therefore, in the present invention, the pH of the sol solution was adjusted to a strongly acidic value of 0 to 3.

Since the reaction rate of the metal alkoxide hydrolysis reaction varies greatly depending on the type of metal, an acid or alkali catalyst is added to control the reaction rate. Representative catalysts include hydrochloric acid, nitric acid, sulfuric acid and organic acids for acids, and ammonia and amines for alkalis. Which catalyst to use depends on the microstructure of the sol.

Also, the polycondensation reaction of the sol greatly varies in reaction rate depending on the type of metal, and is not a problem with Si having a low reaction rate, but cannot be controlled with many elements such as Ti because the reaction rate is too fast. Therefore, a reaction stabilizer having the effect of coordinating to a metal alkoxide or metalloxane chain to reduce the reaction rate to a controllable rate is added to the sol. Typical reaction stabilizers include, for acids, inorganic acids such as hydrochloric acid and organic acids, and for alcohols, cellosolve and amino alcohol.
Examples include acetylacetone, alkylamine, and formamide.

(Sol Coating) As a method of coating a sol on the surface of a steel material, there are a brush coating method, a spray method, and a dipping method. The immersion method is the simplest, but in the case of large steel materials, the immersion tank is inevitably large and a large amount of sol solution is required. The brush coating method and the spray method are preferred because they require only a small amount. In particular, the spray method is preferable because the sol solution can be uniformly applied to the surface of the steel material. When a sol is coated on the surface of a steel material, it is desirable that the thickness of the coating be substantially uniform in any coating method. In the case of the immersion method, the steel material is immersed in the sol tank for a certain time, and then pulled up at a certain speed. In that case, it is known that the thickness of the sol film increases with an increase in the viscosity and pulling rate of the sol, and decreases with an increase in the surface tension and density of the sol. Therefore, in order to make the thickness of the sol film a predetermined thickness, it is necessary to control the viscosity and the pulling speed of the sol. Adjustment of the viscosity of the sol is also important in the brush coating method and the spray method. Regardless of which coating method is used, it is important to obtain a predetermined film thickness by one dipping or coating, but it is important in terms of processing efficiency. If is not obtained, immersion or coating is repeated a minimum number of times.

(Drying / Heat Treatment) When a sol is coated on the surface of a steel material, a wet gel film is formed. When this wet gel film is dried and then subjected to a heat treatment for densifying the dried gel film, a ceramic coating layer having excellent corrosion resistance is formed. Here, in the heat treatment, in addition to the removal of water remaining in the dried gel film, the removal of residual organic substances is performed. When the wet gel film is supercritically dried, it becomes an airgel film with almost no volume shrinkage.

The dry gel film and the airgel film generally form a ceramic coating layer by heat treatment at about 500 ° C. to 1000 ° C. In the present invention, a hot-dip galvanized layer is formed as a base on the surface of a steel material. Therefore, a heat treatment at a temperature as low as possible is preferable in order to prevent deterioration of the steel material due to heating. Therefore, in this embodiment, the temperature of the drying / heating treatment is from room temperature (room temperature) to 200 ° C. at most. Here, in order to remove residual organic substances as carbon dioxide and water vapor by oxidation, when heated in an oxygen atmosphere, the film becomes more dense than in an argon atmosphere or in the air. However, it is industrially very advantageous if a durable ceramic coating layer can be formed only by drying at room temperature. The present invention has found a condition capable of forming a weather-resistant ceramic coating layer that can withstand practical use only by drying at room temperature.
The following heat treatment is performed.

As another method of densifying the gel film at a temperature near normal temperature instead of the heat treatment, there is a method of irradiating the wet gel film with ultraviolet rays. The wet gel film has a wavelength of 185.
A low-pressure mercury lamp (wavelength: 254 nm and 18 nm) is used as a source of ultraviolet light.
5 nm) can be used. That is, when the gel and the wet gel film absorb ultraviolet light having a wavelength of 185 nm, the alkyl group -O
While breaking the bond and forming a metal-O bond,
Densification of the film is performed by oxidation of residual organic matter by ozone and atomic oxygen generated in the atmosphere. Therefore, when the weather resistance suitable for the special use cannot be obtained only by drying at room temperature, it is possible to further improve the weather resistance by executing an ultraviolet irradiation treatment or combining with the above-mentioned heat treatment.

(Control of the thickness of the ceramic coating layer) In order to control the thickness of the ceramic coating layer formed on the surface of the steel material, the steel material is immersed in a sol tank, pulled up, and dried and heated. It may be repeated a desired number of times, and the film thickness increases in proportion to the number of coatings. Incidentally, in order to increase the film thickness in one coating step, the viscosity of the sol and the pulling rate may be increased as described above, but when the film thickness exceeds the critical film thickness, the organic solvent is volatilized from the film. Cracks occur in the film due to tensile stress acting parallel to the surface that occurs, so the film thickness to be coated at one time must be set optimally. To do so, the optimum conditions may be searched for using the viscosity of the sol and the pulling rate as parameters. Similarly, when the sol solution is applied by a spray method, the film thickness can be controlled by the number of times of application. However, industrially, it is advantageous that the number of times of immersion or application is small in both the immersion method and the spray method.

(Characteristics of Ceramic Coating Layer) The properties of the ceramic coating layer, especially the strength and weather resistance, are determined by the pH value, molar concentration, number of times of immersion or application of the sol solution, the fine structure of the sol, and the subsequent drying / heating treatment. It largely depends on.

(Coloring of Ceramic Coating Layer) The ceramic coating layer obtained by the sol-gel method of the present invention has such transparency that the underlying hot-dip galvanized layer can be seen through if the thickness of the film is small. Even if the hot-dip galvanized layer is colored, the surface of the steel material has a colored appearance. However, the colored hot-dip galvanized layer may be discolored by treatment by the sol-gel method, and may become extremely dark.

Therefore, the hot-dip galvanized layer is not colored,
It is also possible to color the ceramic coating layer. That is, at the time of the hydrolysis, a transition metal salt is added as a coloring agent to form an ionic state, the transition metal is hydrolyzed in the ionic state, and the transition metal is colored in the ceramic coating layer. Here, the transition metal salt used for the coloring agent is preferably a nitrate or chloride of Co, Mn or Ni. Further, the transition metal is preferably in the range of 0.01 to 0.5% by weight based on the metal oxide forming the ceramic coating layer after drying and heat treatment. Too much
If it exceeds 0.5% by weight, it becomes dark and dull, and furthermore, it becomes undesirably colloidal or crystalline during heat treatment. The transition metal can be kept in the ionic state by adjusting the pH of the solution to 4 to 6. This coloring treatment for incorporating a transition metal in the ceramic coating layer is an ideal surface treatment because there is no risk of pollution since Cr is not used.

[0032]

EXAMPLES In order to demonstrate the effect of the present invention, 70 mm ×
A 30 mm × 2.3 mm steel plate was subjected to hot-dip galvanizing, and a test material was prepared by subjecting hot-dip galvanizing to a coloring treatment (using a part of the processing of Japanese Patent No. 2920148 by the present applicant). did. That is, the coloring treatment here is performed using potassium permanganate (KMnO ₄ ): 10 to 80.
and g / l, trisodium phosphate (Na ₃ PO ₄ · 1
2H ₂ O): and 20 to 200 g / l, sodium hydroxide (NaOH): using a 10 to 100 g / liter consisting chemical conversion treatment liquid, the treatment liquid temperature 30 to 70 ° C., chemical treatment and processing time of 2-15 minutes The process up to the chemical conversion treatment in which the galvanized steel material is immersed is performed. Then, by changing the treatment conditions on the surface of the test material subjected to the coloring treatment, Si oxide,
A preliminary experiment for forming a ceramic coating layer made of Al oxide and Ti oxide was performed. That is, for each metal component, the concentration of the sol was set to approximately an integral multiple of the unit concentration, the number of times of immersion was set to 1 to 3, and the ceramic coating layer was formed under processing conditions with different firing conditions.

Here, the sol was prepared as follows.
In any case, the pH of the sol solution is in the range of 5-6.

In the case of Si, after mixing ethanol with Si (OC ₂ H ₅ ) ₄ , water, hydrochloric acid and ethanol were mixed and stirred for 1 hour to prepare a sol-like alkoxide solution.

In the case of Al, Al (OCH (CH ₃ ) ₂ ) ₃
Hydrochloric acid was mixed while refluxing to prepare a sol-like alkoxide solution.

In the case of Ti, Ti (OC ₃ H ₇ ) ₃ has 2-
Propanol was mixed and stirred for 1 hour to prepare a sol-like alkoxide solution.

Then, the test material is immersed in these sols,
After drying at room temperature for 30 minutes, a heat treatment was performed at 250 ° C. for 30 minutes. Incidentally, regarding Si, 2 hours at 500 ° C.
A heat treatment at 50 ° C. for 30 minutes was also separately performed. The results are shown in Table 1 below.

[0038]

【table 1】

In this table, cracks and film amount were observed by a scanning electron microscope (SEM), and corrosion resistance was evaluated by a salt spray test (SS).
T). The salt spray test is based on JIS Z 2
371 was evaluated based on the time when white rust was generated in an area ratio of 20% in a portion where the ceramic coating layer was present.

As a result, it was proved that a ceramic coating layer was generally formed on the hot-dip galvanized layer, and it was also confirmed that Al was excellent in corrosion resistance. Based on these results, a preliminary test was further performed on Al. The sol used here was prepared in the same manner as described above. However, in order to form an Al oxide film having a fine and fine structure, the sol concentration was reduced, the number of times of immersion was fixed to three times, and 30 minutes at room temperature. After drying for minutes, a heat treatment was performed at 250 ° C. for 30 minutes. The results are shown in Table 2 below.

[0041]

[Table 2]

As a result, it was confirmed that a ceramic coating layer having a very high corrosion resistance was formed, with a 20% white rust generation time of 96 hours in Sample 2 and 108 hours in Sample 3. This is presumably because the ceramic coating layer is a film having a fine microstructure with few pinholes.

Further, Al was tested by changing the conditions of the heat treatment after drying at room temperature for 30 minutes. The concentration of the sol used here was fixed at 0.1 mol / L and the number of times of immersion was three times, and the conditions of the heat treatment were room temperature, 50 ° C., 100 ° C., 250
C. for 30 minutes and 1 hour, respectively. The results are shown in Table 3 below.

[0044]

[Table 3]

As a result, it was confirmed that the 20% white rust generation time tends to be longer when the heating temperature is lower and the heating time is longer.

Therefore, as a result of repeating experiments to find conditions advantageous for industrial implementation of Al, the optimum range of the pH value, molar concentration, number of times of immersion or application, and drying / heating conditions of the sol solution was specified. did. Here, the preparation of the Al sol solution was performed as follows.

First, 2-propanol was added to 107.1343 g of aluminum isopropoxide (purity: 99.999%, manufactured by Kojundo Chemical Co., Ltd.), and the mixture was heated to reflux. About 45% hydrochloric acid
The mixture was further heated under reflux, and 2-propanol was added to the resulting solution to make the total volume 1000 mL.
A mol / L solution (pH = 0) was prepared. This solution is
-Diluted with propanol, 0.5, 0.4, respectively
0.3 and 0.2 mol / L sol solutions (pH = 0) were prepared.

Using the sol solution prepared as described above, adhere to the surface of the non-colored hot-dip galvanized test material and the surface of the colored hot-dip galvanized test material by dipping and spraying one to three times or by the number of coatings. It was dried at room temperature or subjected to heat treatment after drying at room temperature, and the corrosion resistance was evaluated by a salt spray test. Table 4 shows the results.

[0049]

[Table 4]

Next, the results of the salt spray test of each sample will be briefly described. First, samples 101 to 109
For the change in corrosion resistance due to heating time, the
In the case of 1 mol / L, generation of white rust of 20% or more was confirmed within 72 hours in any treatment time. Concentration 0.3mol
/ L, the 20% white rust generation time greatly depends on the heating time, and the heating time is about 270 hours for 10 minutes and 20 minutes.
In the case of 30 minutes, it was 500 hours or more. Concentration 0.4mo
In the case of l / L, the 20% white rust generation time was about 400 hours with a heating time of 10 minutes, and 500 hours or more with heating for 20 minutes and 30 minutes. When the concentration of the sol solution was as low as 0.1 mol / L, the generation time of 20% white rust was within 72 hours irrespective of the heating time. Excellent corrosion resistance. From the above, if the concentration is 0.3 mol / L or more and the heating time is 10 minutes or more, 2
The 0% white rust generation time was 250 hours or more, indicating that sufficient corrosion resistance could be obtained.

For samples 110 to 113,
It was found that sufficient corrosion resistance was obtained only by drying at room temperature (natural drying) without heat treatment. In this case as well, it was found that the higher the concentration of the sol solution, the better the corrosion resistance. It is noteworthy that the same results were obtained for the corrosion resistance of the heat-treated sample,
In particular, when the concentration is 0.5 or 0.4 mol / L, since the white rust generation area is about 15% even in 408 hours, it is expected that the sample has corrosion resistance comparable to that of the sample subjected to the heat treatment.

For samples 114 to 117,
Even if the heat treatment is not performed, and even if the number of times of immersion is reduced to 1-2 times, at least the concentration is 0.4 or 0.3 mol / L, in each case, the 20% white rust generation time becomes 168 hours or more. , It was found to have sufficient corrosion resistance.
In the case of performing the heat treatment, as the number of times of immersion increases, the more excellent the corrosion resistance is, the more the results are obtained.
Within 20 hours, it was confirmed that 20% white rust was generated within 72 hours. However, it is noteworthy that sufficient corrosion resistance can be obtained even with only one immersion if heat treatment is omitted. . If the number of times of immersion is small, the reason why the corrosion resistance is improved by omitting the heat treatment is that the fine cracks formed in the film by the heat treatment are minimized by omitting the heat treatment. The results of surface microscopy confirm this fact.

Samples 118 to 121 are samples in which the surface of the hot-dip galvanized coating is uncolored, that is, untreated, and a film is formed on the surface by omitting the heat treatment. The time until the occurrence of 20% white rust was in the range of 72 to 128 hours regardless of the concentration of the sol solution.
Compared with the result of the test material subjected to the coloring treatment, it was found that the corrosion resistance was inferior, but it is important that sufficient corrosion resistance is obtained in ordinary applications.

Samples 122 to 125 show the results of the test materials for the non-colored hot-dip galvanizing. It can be seen that the corrosion resistance is excellent as the concentration of the sol solution is high and the number of times of immersion is large. The concentration of the sol solution is 0.4 mol /
When immersed twice in L, the 20% white rust generation time is 15%.
It was 0 hours or more. On the other hand, when the concentration is 0.3 mol / L, 1
In the case of repeated immersion, the same time was 24 hours. But,
In other cases, corrosion resistance of 72 hours or more is obtained, so by adjusting the concentration and the number of times of immersion, it is possible to provide sufficient corrosion resistance that can be used in ordinary applications with the minimum process and minimum cost. It is.

Finally, Samples 126 to 129 are the results of applying the sol solution to the test material by the spray method. In this case, although the concentration of the sol solution is as high as 0.5 mol / L, the 20% white rust generation time is 7 times regardless of the presence or absence of the coloring treatment and the heating treatment by the three coatings.
It was found that at least 2 hours, practically sufficient corrosion resistance was obtained. In particular, it was found that irrespective of the presence or absence of the coloring treatment on the hot-dip galvanized surface, the case where the heat treatment was omitted was superior in corrosion resistance. This means that sufficient corrosion resistance can be obtained only by natural drying at room temperature by employing the spray method, and this is a particularly important result in industrial practice.

[0056]

As described above, the surface treatment method for imparting high weather resistance to the steel material of the present invention comprises forming a hot-dip galvanized layer on the surface of the steel material, and coating the hot-dip galvanized layer with a ceramic coating by a sol-gel method. Since the layer is formed, the galvanized layer has a low but reliable corrosion resistance,
It can be expected to have a synergistic effect with the protective effect of the hot-dip galvanized layer due to the excellent corrosion resistance of the ceramic coating layer, and the ceramic coating layer can be heat-treated at a relatively low temperature of 200 ° C or less, or only dried naturally at room temperature. Since it can be formed, it can also be formed on a hot-dip galvanized layer having a low melting point, and can be applied to a large and complicated-shaped steel material. Further, since this surface treatment method does not contain Cr in the ceramic coating layer, it is an ideal surface treatment that does not cause any pollution caused by hexavalent chromium.

The highly weather-resistant steel material obtained by the surface treatment method of the present invention has a low hot-dip galvanized layer but low reliability because a composite film of a hot-dip galvanized layer and a ceramic coating layer is formed on the surface. High corrosion resistance and
The ceramic coating layer has high weather resistance due to a synergistic effect with the protective action of the hot-dip galvanized layer due to the excellent corrosion resistance.

Claims (9)

[Claims] After a hot-dip galvanized layer is formed on the surface of a steel material, a metal oxide of a single composition, a metal oxide to which a second component is added, or a multi-component metal oxide is formed on the hot-dip galvanized layer by a sol-gel method. A surface treatment method for imparting high weather resistance to a steel material, comprising forming a ceramic coating layer formed of a composite metal oxide. 2. The surface treatment method for imparting high weather resistance to a steel material according to claim 1, wherein the ceramic coating layer is formed of an oxide ceramic of Si, Al or Ti by a sol-gel method. 3. The ceramic coating layer according to claim 1, wherein
The alkoxide of i, Al or Ti is used as a starting material, and after being made into a solution of alcohol, it is hydrolyzed by reacting with water. The surface treatment method for imparting high weather resistance to a steel material according to claim 1 or 2, wherein an oxide ceramic film of Si, Al or Ti is formed on the surface of the galvanized layer of the steel material. 4. Addition of 2-propanol to aluminum isopropoxide and further addition of hydrochloric acid to adjust the pH to 0 to 3 and concentration of aluminum isopropoxide to 0.2
The surface treatment method for imparting high weather resistance to a steel material according to claim 3, wherein an alumina-based sol solution having a concentration of 0.5 mol / L is used. 5. The method according to claim 3, wherein the sol solution is sprayed on the surface of the steel material one or more times by spraying to form a film. 6. The gel film formed on the surface of the steel material,
4. Drying at a temperature from room temperature to 200 ° C. or less.
A surface treatment method for imparting high weather resistance to a steel material according to any one of the above-mentioned items. 7. The method according to claim 7, wherein the gel film formed on the surface of the steel material is
The surface treatment method for imparting high weather resistance to a steel material according to any one of claims 3 to 5, which is naturally dried at room temperature. 8. During the hydrolysis, a transition metal salt is added as a coloring agent to form an ionic state, the transition metal is hydrolyzed in an ionic state, and contained in the ceramic coating layer in an amount of 0.01 to 0.5% by weight. A surface treatment method for imparting high weather resistance to the steel material according to claim 3. 9. A method for forming a composite coating of a hot-dip galvanized layer and a ceramic coating layer on the surface of a steel material by using the surface treatment method for imparting high weather resistance to the steel material according to claim 1. Highly weather-resistant steel material.