JPH0380874B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rust-preventing steel plate for multilayer coating with two or more coats, in which the undercoat is a cationic electrodeposition coating.

In recent years, there has been an increasing demand for steel sheets with excellent corrosion resistance as steel sheets for automobile bodies, and for this reason, there is a strong tendency to use surface-treated steel sheets with high corrosion resistance in place of conventionally used cold-rolled steel sheets.

The first example of such surface-treated steel sheets is galvanized steel sheets, but with this type of steel sheet, it is necessary to increase the amount of zinc deposited in order to improve corrosion resistance, and along with this, workability and weldability There is a problem of deterioration. In order to improve these problems, zinc alloy plated steel sheets and multi-layer plated steel sheets are being researched and developed to which one or more elements such as Ni, Fe, Mn, Mo, Co, Al, Cr, etc. are added. Compared to the above-mentioned galvanized steel sheet, the steel sheet can have improved corrosion resistance without deteriorating weldability and workability. However, when a steel plate is applied to the bag structure or bent part (hemming part) of an automobile body plate, a high degree of corrosion resistance is required on the surface, and zinc alloy plated steel plates and multilayer plated steel plates as mentioned above are required. Steel plates have a problem in that their corrosion resistance is not quite sufficient. As a steel plate with a high degree of corrosion resistance, special
Research and development has been carried out on anti-corrosion steel sheets coated with zinc-rich coatings, such as the one seen in No. 6882, and the representative one is known as Zinchrome Metal.
However, even with this anti-corrosion coated steel sheet, peeling of the coating may occur in the processed parts such as press forming, and it is still not fully satisfactory as a highly corrosion-resistant and anti-rust coated steel sheet that meets the requirements for automobile body materials. It's hard to call it a thing.

For this reason, composite coated steel sheets made of zinc-plated steel sheets with a two-layer coating of a chromate film and an organic composite silicate film and a method for manufacturing the same have been published in Japanese Patent Laid-Open Nos. 57-108292 and 1983. −
It has been proposed in No. 224174, etc. However, although this type of composite coated steel sheet has superior corrosion resistance and workability compared to conventional surface-treated steel sheets, when it is used for automobile exterior panels, paint adhesion is poor, and paint corrosion resistance is not fully satisfactory. It's hard to say. In particular, for automobile bodies, it is becoming common to apply two or more coats on top of the cationic electrodeposition coating, but the adhesion of such two or more coats is extremely poor. There is a problem.

The present invention was researched and developed in view of these problems, and aims to provide a method for manufacturing a rust-proof steel sheet that can ensure high corrosion resistance, workability, and good adhesion to two or more coats of paint. It is something to do.

Therefore, in the present invention, after chromate treatment is applied to the surface of a steel plate coated with zinc-based plating or aluminum-based plating, the epoxy resin accounts for 35% or more of the total solid content, and the ratio of SiO ₂ /resin is 10/90. Treatment is performed with an organic composite silicate solution adjusted to ~60/40, preferably 20/80 to 50/50, and then heated to 300 °C above 250 °C.
Its basic feature is that it is heat-treated at a temperature below ℃. Another basic feature is that a curing agent is added to the organic composite silicate solution, treated with the solution, and then heat-treated at a temperature of 260°C or higher and 350°C or lower.

In the present invention, after chromate treatment of the surface of a steel plate coated with zinc-based plating or aluminum-based plating,
Epoxy resin is 35wt% or more of the total solid content
Treated with organic composite silicate welding with a SiO ₂ /resin ratio of 10/90 to 60/40 and a hardening agent added if necessary, and then heated to temperatures exceeding 250℃ and 300℃.
Temperature range below, 260℃ if hardener is added
As described above, the structure is such that heat treatment is performed in a temperature range of 350° C. or lower.

In the present invention, a zinc-plated steel plate or an aluminum-plated steel plate is used as the material steel plate. The above-mentioned galvanized steel sheets include zinc-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-nickel alloy-plated steel sheets, zinc-manganese alloy-plated steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-cobalt-chromium alloy-plated steel sheets, and One or more elements such as Ni, Fe, Mn, Mo, Co, Al, Cr, etc. can be added to the plating components of any of these steel sheets, and the same type of plating among the above-mentioned platings can be used. Alternatively, it may be a composite plated steel plate with two or more layers of different types applied.

As a method for plating these zinc-based plated steel sheets, any practicable method among electrolytic method, melting method, vapor phase method, etc. can be adopted. However, the rust-proof steel sheets that are the object of the present invention are mainly used for automobile bodies, and in such applications, it is important not to damage the material of the cold-rolled steel sheets that are plated. Therefore, it can be said that electroplating, which does not generate heat, is advantageous.

In the present invention, the above plated surface is subjected to chromate treatment to form a chromate film. This chromate film has a chromium deposition amount (dry) of 1
It is appropriate to set the content to about 1000 mg/m ² , preferably about 10 to 200 mg/m ² (calculated as metal chromium).
If the amount of chromium deposited exceeds 1000 mg/m ² , workability and weldability deteriorate, and if it is less than 1 mg/m ² , the film may become non-uniform, which is not preferable. Further, it is preferable that hexavalent Cr be present in the chromate film. This is because the action of Cr ⁶⁺ promotes crosslinking of the film in the next step of organic composite silicate treatment, thereby strengthening the film. Cr ⁶⁺ also has a repairing effect, and when a steel plate is damaged, it acts to suppress corrosion from there.

The chromate treatment for such a base film may be performed by any known method such as a reaction type or a coating type.

The coating-type chromate treatment liquid has a partially reduced chromic acid solution as its main component, and if necessary, it may also contain water-dispersible or water-soluble organic resin such as acrylic resin and/or silica with a thickness of several tens to several thousand angstroms. It contains particles (silica sol, fumed silica). In this case, the ratio of Cr ³⁺ /Cr ⁶⁺ is 1/1 to 1/
3. PH is preferably 1.5 to 4.0 (more preferably 2 to 3). The ratio of Cr ³⁺ /Cr ⁶⁺ is adjusted to a predetermined ratio using a general organic reducing agent (eg, sugar, alcohol, etc.) or an inorganic reducing agent. Further, as the coating type chromate treatment, any method such as a roll coater method, a dipping method, a spray method, etc. may be used. In the coating type chromator treatment, a film is obtained by drying without washing with water after the chromate treatment. The reason for drying without washing with water is that Cr ⁶⁺ is removed by normal washing with water, so the ratio of Cr ³⁺ /Cr ⁶⁺ is maintained stably and the organic composite silicate is used in the next process. This is for sealing by treatment with a solution.

On the other hand, in electrolytic chromate treatment, cathodic electrolysis treatment is performed in a bath containing chromic anhydride and one or more of anions such as sulfuric acid, phosphoric acid, fluoride, or halogen oxygen acid, and then washed with water and dried to form a film. to form.

Comparing the chromate films produced by the above two treatment methods, the coated chromate film contains more hexavalent chromium in the film than the electrolytic chromate film, so it has superior corrosion resistance. When heat treated, the film becomes dense and strong, resulting in better corrosion resistance than electrolytic chromate. On the other hand, electrolytic chromate has the advantage that the film has a high degree of completion regardless of the presence or absence of heat treatment, and also has the advantage that the amount of film deposited can be easily controlled. In consideration of corrosion resistance, coated chromate is most preferred. Further, rust-preventing steel sheets for automobiles are often single-sided treated steel sheets, and from this point of view, coating type and electrolytic type are preferable.

After the chromate treatment, in the present invention, a chromate film is obtained by drying without washing with water. Following the above chromate treatment, treatment with an organic composite silicate solution is performed to form an organic composite silicate film on the chromate film. In the present invention, in order to ensure adhesion with two or more coats of paint, including cationic electrodeposition paint, the organic composite silicate solution has a SiO ₂ /resin ratio by weight.
The ratio is 10/90 to 60/40, preferably 20/80 to 50/50, and the epoxy resin is an essential component and is adjusted to contain 35 wt% or more of the total solid content, and the treatment is performed using this.

In the coated steel sheet adopted in the present invention, that is, the steel sheet consisting of a plating original plate + chromate film + organic composite silicate film, the adhesion of the cationic electrodeposition paint and the top coat is the same as that of the top layer of the organic composite silicate film. This is determined by the properties, and in this respect, sufficient adhesion could not be expected with conventional steel plates of this kind as mentioned above.
Regarding this problem, as a result of examining the relationship between the properties of organic composite silicate and paint adhesion, we determined that the ratio of SiO ₂ /resin in the organic composite silicate solution was set within a predetermined range, and the epoxy resin content in the resin was set within a predetermined range. It has been discovered that by setting the ratio at least as high as 100% and setting the heating temperature after solution treatment at a high value, a film having high corrosion resistance and extremely excellent adhesion to multilayer coatings can be obtained.

To explain this in detail, firstly, the SiO ₂ part in the organic composite silicate component is effective for corrosion resistance, and the resin part is effective for paint adhesion, so that both the corrosion resistance and adhesion that are the objectives of the present invention are satisfied. In order to achieve this, it is necessary to regulate the ratio of both. _SiO2
When the weight ratio of resin to resin exceeds 60/40,
Corrosion resistance increases, but paint adhesion becomes insufficient, and conversely, if it is less than 10/90, corrosion resistance deteriorates. Therefore, the ratio of SiO ₂ /resin is
It is adjusted to 10/90 to 60/40, preferably 20/80 to 50/50.

Furthermore, in the present invention, the proportion of the epoxy resin in the organic composite silicate to the total solid content is set to a predetermined amount (35 wt%) or more, and the heating temperature is set to be high, thereby improving paint adhesion. If the epoxy resin in the organic composite silicate is less than 35 wt% of the total solid content, sufficient adhesion for multilayer coating, which is the object of the present invention, cannot be obtained.

This improvement in paint adhesion is due to the following reasons. First, cationic electrodeposition paints contain epoxy resins, so by including a certain amount or more of epoxy resin in the organic composite silicate, a strong bond between the epoxy resin and the cationic electrodeposition paints can be created. Interaction is obtained, and high adhesion is obtained at the interface between the cationic electrodeposition paint and the organic composite silicate. Second, when the epoxy resin content in the organic composite silicate is increased, the decomposition limit temperature of the resin shifts to a higher temperature side, making it possible to perform heat treatment at a higher temperature.This high temperature heat treatment accelerates the crosslinking reaction of the organic composite silicate. is promoted, resulting in improved adhesion. To explain this in detail, cationic electrodeposition coating has the characteristic that the coating interface becomes an alkaline environment, but organic composite silicate films are generally weak in an alkaline environment and soften and swell due to the alkalization of the interface by cationic electrodeposition coating. This is a major cause of impeding paint adhesion. In addition, in the case of multilayer coating with two or more coats, such as applying a topcoat on top of the cationic electrodeposition paint as described above, the thickness of the coating on the steel sheet increases, which increases the internal stress and reduces the adhesion. This is a major cause of deterioration. In response to such adhesion problems, the present invention promotes crosslinking to increase the crosslinking density as described above, thereby reducing the decrease in paint adhesion caused by softening and swelling in an alkaline environment and internal stress. It was designed to suppress the

Further, in the present invention, a curing agent such as melamine can be added to the organic composite silicate solution as described above. By adding such a curing agent, the decomposition limit temperature of the organic composite silicate film by heating shifts to a higher temperature side, thus making it possible to perform heat treatment in a higher temperature range. By performing heating at a higher temperature in this manner, the adhesion is further improved for the reasons mentioned above. As a result, better paint adhesion can be obtained even with more multilayer coatings (for example, 3 coats or more) than in the case where no curing agent is added.

Examples of the curing agent to be added include blocked isocyanate, urea, melamine, phenol, and others such as polyamide, amino resin, amine, organic acid, inorganic acid, alcohol, mercaptan, acid anhydride, and the like. The addition ratio of this curing agent is based on the epoxy resin content in the organic composite silicate.
0.1 to 100 parts, preferably 0.3 to 100 parts
50 copies. If the addition ratio of hardening agent is too large,
Free curing agent that does not react with the epoxy resin impairs adhesion, and therefore it is preferable to add the curing agent at a maximum of 100 parts per 100 parts of the epoxy resin.

The coating weight (dry) of the organic composite silicate as described above is in the range of 0.5g/m ² to 4.0g/m ² , more preferably in the range of 1.0g/m ² to 3.0g/m ² . . If the amount of adhesion is less than 0.5 g/m ² , sufficient corrosion resistance cannot be obtained, and if it is more than 4.0 g/m ² , spot weldability deteriorates, which is not preferable.

The organic composite silicate has water-dispersible silica as an essential component, and a water-soluble or water-dispersible organic polymer resin is mixed therein in the presence of a silane compound to produce a product at a temperature of 10°C or higher and below the boiling point, preferably 50 to 90°C. It can be obtained by reacting at a temperature range of . The water-dispersible silica is so-called silica sol or colloidal silica and has a particle diameter of several tens to several thousand angstroms. The silane compound is used as a reaction accelerator when silica and organic resin are combined. As this silane compound, a commercially available silane coupling agent may be used.
For example, vinyltriethoxysilane, vinyltris(β-methoxyethoxy)silane, γ-glycidoxypropyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, N=β
Examples include trialkoxysilane compounds such as (aminoethyl)-γ-aminopropyltrimethoxysilane and γ-aminopropyltriethoxysilane.

Examples of water-soluble or water-dispersible organic polymer resins include polyvinyl alcohol, hydroxyethyl cellulose, polyester, alkyd, epoxy, and acrylic copolymers, but as described above, epoxy resins are essential in the present invention. Become. Examples of epoxy resins include fatty acid-modified epoxy resins, polybasic acid-modified epoxy resins, acrylic resin-modified epoxy resins, alkyd resin-modified epoxy resins, phenol resin-modified epoxy resins, polybutadiene resin-modified epoxy resins, and amine-modified epoxy resins. In order to make the above organic resin water-soluble or water-dispersible, an amine compound or ammonia may be added.

As mentioned above, the blending ratio of water-dispersible silica and water-soluble or water-dispersible organic resin in the organic composite silicate is from 10/90 to 60/1 in terms of solid weight percentage.
40, preferably 20/80 to 50/50. Further, the addition ratio of the silane compound is 0.5 to 15 wt% based on the total solid weight of silica and organic resin.

The organic composite silicates obtained as described above may be used alone or in combination of two or more. Furthermore, molybdenum, tungsten, or vanadium oxygen acids or salts thereof, or titanium or zirconium alkoxide chelate compounds may be added. Corrosion resistance can be improved by adding one or more of these additives to 14 wt% or less, preferably 0.2 to 8 wt%, based on the total solid content of the silica sol and organic resin.

The organic composite silicate can be applied by any method such as a roll coater method or a spray method, and a film is formed by drying after application.

After the solution treatment as described above, in the present invention, the temperature is 250°C.
300℃ or less, or 260℃ or more if a curing agent is added to the organic composite silicate solution.
Heat treatment is performed at a temperature of 350℃ or less. The reason for limiting the heating temperature in this way is that sufficient corrosion resistance and paint adhesion cannot be obtained at temperatures below the above lower limit, and when the upper limit of 300°C or 350°C is exceeded, the organic composite silicate film will thermally decompose, resulting in weight loss. This is because it will cause a decrease. As mentioned above, by adding a curing agent, the allowable limit temperature for pyrolysis shifts to the higher temperature side, and the maximum temperature is increased.
It is possible to heat up to about 350℃.

Looking at the above heat treatment from the perspective of improving corrosion resistance, first of all, corrosion resistance is improved by densification of the underlying chromate film, that is, a dense chromic chromate film is formed by reduction of Cr ⁶⁺ by heating, dehydration reaction, etc. It will be done. In addition, when silica or organic resin or both are present in the chromate film,
A crosslinking reaction occurs between chromium and these components by heating, and a more dense chromate film is formed. Furthermore, Cr ⁶⁺ on the surface layer of the chromate film reacts with polar groups such as hydroxyl groups and carboxyl groups in the composite organic silicate film to strengthen the bond between the two layers, which also improves corrosion resistance.

Note that after heating the plate to a predetermined temperature, it is preferable to hold the plate for several seconds or minutes. Holding for a long time is not only economically disadvantageous but also undesirable as it may lead to performance deterioration.

A series of coating treatments such as the above-mentioned Metsky-chromate treatment-organic composite silicate solution treatment can be applied to both or one side of the steel plate. Examples of the embodiments of the steel plate manufactured according to the present invention include the following.

(1) One side...metallic chromate film - organic composite silicate film One side...Fe side (2) One side...metallic chromate film - organic composite silicate film One side...metallic surface (3) Both sides...metallic chromate film - organic composite Silicate film surface Next, examples of the present invention will be described.

Inventive materials and comparative materials were produced as steel plates suitable for the inner surface of automobile bodies under the conditions shown in Tables 1-a to 2-b, and adhesion tests and corrosion resistance tests were conducted on them.

The plating components of each steel plate are as follows, and for each steel plate with a chromate film and an organic composite silicate film in the table, the plating steel plate is degreased with alkaline, washed with water, dried, and then coated with a coating type chromate treatment solution. An electrolytic chromate film was formed by coating with a coater or dipping in an electrolytic chromate treatment bath, and after drying, an organic composite silicate treatment liquid was applied as a second layer with a roll coater. After further drying, it was heat-treated and air-cooled.

Ni-Zn alloy electroplating...Ni content 12% Fe-Zn alloy electroplating...Fe content 25% Mn-Zn alloy electroplating...Mn content 60% Zn-Al alloy electroplating...Al content 5% Details of the coating type chromate treatment, electrolytic chromate conditions, and organic composite silicate treatment liquid are as follows.

○zu fin pattern