JPH0427536A - Surface-treated steel sheet excellent in low temperature chipping resistance, coating peeling resistance and electrocoating property - Google Patents

Abstract

PURPOSE:To obtain the surface-treated steel sheet excellent in low temperature chipping resistance, coating-peeling resistance and the electrocoating property by providing the layer of mixture which contains the resin of glass transition temperature of 30-120 deg.C and silicon dioxide on the zinc-plated layer on at least one surface of the steel sheet. CONSTITUTION:A surface-treated steel sheet has the plated layer 12 composed of zinc-plating on at least one surface of the steel sheet 10. The surface-treated steel sheet has the layer of prescribed mixture i.e., the mixture-layer 22 with the coated amount of 0.1-2.5g/m<2> (dried weight) which contains the resin with glass transition temperature of 30-120 deg.C and silicon dioxide on the upper layer of such zinc-plated layer 12. By such construction, very excellent low temperature chipping resistance is obtained. As the resin used for the layer 22 of mixture, urethane resin, acryl resin, phenol resin and polyolefin resin, etc., are used.

Description

Detailed Description of the Invention <Industrial Application Field> The present invention relates to a surface-treated steel sheet with excellent low-temperature chipping resistance, coating peeling resistance, and electrodeposition coating property, which is particularly suitable for use as a steel sheet for automobiles. .

<Conventional technology> The rust prevention requirements for various steel plates are increasing year by year. In particular, rust prevention requirements for automobile steel sheets, electrical products, etc. are severe, and various surface-treated steel sheets that have been subjected to surface treatments to improve rust prevention properties have been developed and put into practical use.

One of such surface-treated steel sheets is a zinc-based plated steel sheet that is coated with zinc-based alloy plating such as zinc plating or zinc-nickel plating.

This zinc-plated steel sheet has excellent rust prevention properties under high humidity conditions, and also exhibits high bare rust prevention properties when applied to applications exposed to high humidity conditions, such as the bag structure of automobile doors. do. Therefore, this surface-treated steel sheet has chromate treatment, organic film formation, etc. applied to the upper layer of zinc-based plating, and is particularly suitable for areas such as the inner surface of the car body, where areas such as bag structures tend to be insufficiently coated. It is suitably applied for various purposes.

On the other hand, when it comes to steel plate surfaces that make up exterior surfaces, such as the exterior surfaces of automobiles, emphasis has traditionally been placed on appearance, such as sharpness and paintability. Therefore, it is undesirable to apply zinc-nickel plating, which has poor chemical conversion (phosphate treatment) properties, to the outer surface of the car body, and ordinary steel sheets for automobiles are plated only on one side. , A non-plated surface (cold-rolled steel plate) is used for the outer surface of the vehicle body. However, in recent years, high rust prevention properties have been required on the outer surface as well, and with improvements in chemical conversion treatment solutions, surface-treated steel sheets are increasingly being used in place of conventional cold-rolled steel sheets. The percentage of people who do so is increasing.

By the way, surface-treated steel sheets for exterior use, especially the steel sheet surfaces that serve as the exterior surfaces of automobiles, require performance that does not need to be considered on the inside surface.One of the important performances is low-temperature chipping resistance. Chipping is a phenomenon in which pebbles or the like hit the car body while the car is running, and the impact causes the plating layer to peel off and expose the steel sheet base, and it is particularly likely to occur at low temperatures.

The above-mentioned zinc-based plated steel sheets, especially zinc-nickel plated steel sheets, have poor plating adhesion. Therefore, after coating is applied on the plating layer, the coating film (electrodeposition coating, intermediate coating, top coating, etc.)
Since this stress is applied directly to the plating layer, low-temperature chipping of galvanized steel sheets tends to cause peeling between the steel sheet base and the plating layer. In other words, although zinc-based alloy coated steel sheets have significantly superior corrosion resistance, especially rust prevention, compared to conventionally applied cold rolled steel sheets, cold rolling It has the problem that it is inferior to steel plate.

<Problems to be Solved by the Invention> In order to solve these problems, various improvements have been made to improve the plating adhesion of zinc-based plated steel sheets and improve their low-temperature chipping resistance.

As a method for improving this low-temperature chipping resistance,
3-143286 discloses a method in which initial plating is performed at a low current density, a Zn-Ni epitaxial layer is formed on the surface of the steel sheet, and then zinc-nickel plating is performed by increasing the current density, A method characterized by preferentially and selectively dissolving and removing Zn in a plating layer disclosed in JP-A-1-108400, JP-A-1-176091
0, 9 < Ni
/ (Z n + N i ) < 1, 0 (molar ratio)% pH 2 or less, current density 5 to 200 A/drn
The structure of the plating itself, such as the method of applying Zn-Ni plating at 50 to 1000 mg/s*'' under the conditions of Various methods have been disclosed.

According to each of the above-mentioned methods, the low-temperature chipping resistance is certainly improved compared to a normal galvanized steel sheet. However, with either method, its low-temperature chipping resistance does not reach the level of cold-rolled steel sheets, which have excellent properties.
Further improvement in low temperature chipping resistance is desired.

In addition, Japanese Patent Application Laid-Open No. 64-78832 discloses that a chromate or phosphate treated film layer is provided on the plating, and the Tg point is 55°C or less, and the hardness at room temperature after baking is equal to that of a pencil hardness. A steel sheet is disclosed in which the low-temperature chipping resistance is improved by providing a bound resin film layer of H to 2B with a thickness of 0.1 to 2.0 μm.

According to this method, plating peeling is certainly reduced. However, when we investigated the peeled area in more detail, it was found that although the peeling between the plating and the base metal was indeed reduced, if the Tg point of the resin is below 55°C, which is an essential requirement for this method, the resin
Significant peeling between the electrodeposited coatings was observed, and it was found that it was not practical as an outer panel for automobiles. This is presumed to be because the adhesiveness between the resin and the electrodeposited coating becomes weak at temperatures below 55° C. at the 7g point of the resin in this invention.

Further, steel plates for automobile exteriors are processed into a predetermined shape by a user and then painted. In recent years, this coating has often been carried out in a so-called three-coat manner, in which a surface-treated steel sheet is electro-deposited, an intermediate coat is applied, and a top coat is further applied. Therefore, in order to perform a good coating and obtain an excellent product, it is necessary to perform the electrodeposition coating, which is the lowest layer of the coating layer, well. Furthermore, it is desired that a surface-treated steel sheet with excellent coating film peeling resistance be developed.

The purpose of the present invention is to solve the problems of the prior art, and to provide a zinc-plated surface-treated steel sheet having excellent low-temperature chipping resistance, electrodeposition coating properties, and coating peeling resistance. be.

Means for Solving the Problem> In order to achieve the above object, the present inventors have made extensive studies and have surprisingly found that phosphate treatment, which is commonly performed on zinc-based surface-treated steel sheets, It was found that the low temperature chipping resistance was decreased. In zinc-plated surface-treated steel sheets for automobiles, in order to improve paint adhesion (secondary adhesion), phosphate treatment is performed after the plating layer is formed to form a water-insoluble phosphate film. However, research by the present inventors has shown that when phosphate treatment is performed to form a phosphate film, the secondary adhesion of the resulting surface-treated steel sheet improves, but the low-temperature chipping resistance decreases. revealed for the first time.

In addition, the present inventors have further studied and found that a predetermined thin film layer, that is, a mixture layer containing a resin with a glass transition temperature of 30 to 120°C and silicon dioxide, is formed on the plating layer of a zinc-based plated surface-treated steel sheet. 1 to 2.5 g/m" (dry weight), not only the plating peeling in the low-temperature chipping property test but also the peeling between the resin and the electrodeposited coating was greatly improved, and it also showed very good electrodeposition. It has been found that paintability can also be obtained.

Furthermore, it has also been found that sufficient paint adhesion (water resistant secondary adhesion) can be obtained without forming a phosphate film that reduces low temperature chipping resistance.

That is, the present invention has a zinc-based plating layer on at least one side, and a mixture layer containing silicon dioxide and a resin having a glass transition temperature of 30 to 120°C on top of the zinc plating layer, with a dry weight of 0.1 to Provided is a surface-treated steel sheet with excellent low-temperature chipping resistance, coating peeling resistance, and electrodeposition coating property, characterized by having a coating thickness of 2.5 g/m''.

Moreover, it is preferable that the content of the silicon dioxide in the mixture layer is 50% by weight or less.

The present invention will be explained in detail below.

The present invention was achieved based on the above-mentioned knowledge, and this point will be explained in more detail with reference to the drawings.

As shown in FIG. 2, in the conventional surface-treated steel sheet, a plating layer 12 such as zinc-nickel plating is formed on a cold-rolled steel sheet 10, and a coating layer 12, such as a cationic electrodeposition coater 6, is applied thereon. Adhesion of intermediate coat layer 18 and top coat layer 20 (
A phosphate film 14 is formed by phosphate treatment to improve secondary adhesion.

Conventional surface-treated steel sheets with such a configuration have low low-temperature chipping resistance (although various improvement methods have been proposed, the reason why none of these methods can achieve the desired low-temperature chipping resistance is the above-mentioned problem). That's right.

As a result of extensive studies, the inventors of the present invention found that the low-temperature chipping resistance of conventional surface-treated steel sheets is reduced due to the phosphate film 14 formed by phosphate treatment. I found it.

Although the reason is not clear, it is thought to be as follows.

The commonly used phosphate treatment allows the paint to adhere firmly to it. This is the phosphate film 1
It is thought that this is because the phosphate crystals in No. 4 are thick (and the surface roughness is large, so that the paint can obtain a physical anchoring effect. Therefore, the phosphate crystals provide a physical anchoring effect. It is thought that because the contact area between the applied paint and the plating increases, the transmission of residual stress in the paint is promoted, causing distortion in the plating and reducing plating adhesion.

However, if the cationic electrodeposition coating device 6, intermediate coating layer 18, top coating layer 20, etc. are simply formed without providing this phosphate film, sufficient secondary adhesion, especially waterproof secondary adhesion, cannot be obtained. First, it is well known that it is impossible to put it to practical use as a steel plate for automobiles, especially as a steel plate for exterior use.

In addition, a chromate or phosphate treated film layer is provided on the plating, and an organic resin film layer with a Tg point of 55°C or less and a hardness of H to 2B of pencil hardness at room temperature after baking is applied to the plating. Regarding the steel plate provided with ~2.0 μm,
Although the plating does not peel off, the coating peels off during low-temperature chipping, which is thought to be due to a decrease in the adhesion between the resin and the electrodeposited coating, which is still not practical.

In contrast, the surface-treated steel sheet (hereinafter referred to as surface-treated steel sheet) that has excellent low-temperature chipping resistance, paint peeling resistance, and electrodeposition coating property of the present invention has substantially no phosphate coating 14. In addition, the predetermined mixture layer 22 for improving low temperature chipping resistance, that is, the glass transition temperature is 3.
It has a thin film layer of 0.1 to 2.5 g/m'' (dry weight) consisting of a mixture of resin and silicon dioxide at 0 to 120°C.

In other words, as a result of further intensive studies, the present inventors have found that by forming such a mixture layer 22 on the plating layer 12, the transmission of the residual stress of each paint layer to the plating layer is alleviated.
In addition, by forming the mixture layer 22 with a resin containing silicon dioxide with a Tg point of 30 to 120°C, the peeling resistance of the coating film is improved, and by adding silicon dioxide, electrodeposition The present invention was completed by discovering that it is also possible to improve paintability.

Moreover, by having such a mixture layer 22,
It was also found at the same time that even if phosphate treatment was performed, a phosphate film (reference numeral 14 in FIG. 2) was not substantially formed.

On automobile production lines, the painting process, including phosphate chemical treatment, is generally carried out after each part is assembled into the shape of the car body, and is not necessary on the outside of the car body due to its resistance to low-temperature chipping. Since a phosphate film is also required in other parts, the phosphate chemical conversion process itself cannot be eliminated.

In contrast, according to the present invention, by forming a predetermined mixture layer 22 on the plating layer 12 as shown in FIG. It also has the undesirable effect of preventing the substantial formation of phosphate films, which eliminate the need for any changes to the automobile painting process.

Therefore, the surface-treated steel sheet of the present invention has extremely excellent low-temperature chipping resistance and resistance, in addition to substantially not having the phosphate film 14 that reduces low-temperature chipping resistance as described above. Has coating peelability and electrodeposition coating properties.

The present invention will be explained in more detail below.

The surface-treated steel sheet of the present invention has a plating layer 12 made of zinc-based plating on at least one side of the steel sheet 1o. In addition,
The zinc-based plating in the present invention is a zinc-based plating that has the inherent problem of low-temperature chipping, and a typical example thereof is Zn-Ni alloy plating.

There is no particular limitation on the method of forming the plating layer 12, and various methods such as electroplating and hot-dip plating can be applied.

In addition, when the applied plating is Zn-Ni plating, in order to improve the adhesion between the steel plate 10 and the plating layer 12, the lower part of the plating layer has a high Ni content (so that Preferably, it is formed.

The surface-treated steel sheet of the present invention has a predetermined mixture layer 22, that is, a mixture layer 22 having a resin having a glass transition temperature of 30 to 120°C and silicon dioxide on the top layer of the plating layer 12 made of such zinc-based plating. , 0.1-2.5g/
m” (dry weight).
With such a configuration, very good low-temperature chipping resistance can be obtained. In addition, it has good paint adhesion, especially excellent electrostatic coating properties, so it can resist low-temperature chipping, which is necessary to obtain sufficient adhesion (secondary adhesion) of the paint layer with conventional surface-treated steel sheets. Sufficient water-resistant secondary adhesion can be obtained without forming a phosphate film that would lead to a decrease in properties, and combined with the low-temperature chipping resistance provided by the mixture layer 22, extremely excellent low-temperature resistance is achieved. Chipping properties can be obtained.

Further, even if phosphate treatment is performed, substantial formation of a phosphate film (second row number 14) can be prevented. In the present invention, "substantially no phosphate film is formed" means that there is no phosphate film at all, and that the amount of the phosphate film formed by phosphate treatment is 0.2 g/m" It also means that the resin applied to the present invention has a glass transition temperature of 30 to 120°C.
It is. When the glass transition temperature is less than 30°C, the residual stress of each paint layer will not be applied to the plating layer 12, and the plating layer 12 will not peel off from the steel plate base.
The adhesion between the mixture layer 22 and the coating layer formed thereon is low, and peeling occurs between the two, making it impossible to obtain a good surface-treated steel sheet. On the other hand, if the glass transition temperature of the resin exceeds 120°C, the viscosity of the resin is high and the solubility in various solvents is also reduced, making it difficult to uniformly apply the mixture layer 22, which also results in a good coating. Surface-treated steel sheets cannot be obtained. In addition, in the present invention,
The glass transition temperature of the resin applied to the mixture layer 22 is 60
It is more preferable that the glass transition temperature is 80 to 120°C, and even more preferable that the glass transition temperature is 80 to 120°C. , better results can be obtained in terms of various performances such as powdering property.

As the resin to be applied to the mixture layer 22 of the present invention, various resins can be used as long as they have the above-mentioned glass transition point, and examples include epoxy resins, urethane resins, acrylic resins, phenolic resins, and polyolefin resins. Illustrated.

Note that two or more of the above resins may be used in combination.

Furthermore, even if the resin has a glass transition point of less than 30°C, it is possible to add additives such as various curing agents and modifiers to make the glass transition point higher than 30°C.

The mixture layer 22 of the surface-treated steel sheet of the present invention contains silicon dioxide (silica) together with such a resin. By having silica in the mixture layer 22, in addition to the above-mentioned low-temperature chipping resistance, it is possible to obtain excellent electrostatic coating properties.

There are no particular limitations on the silica that can be applied to the present invention.
Any of the various known silicas can be used, and
It may be either colloidal silica or fumed silica, and has a secondary particle size of several to several ±nm.

The content of such silica in the mixture layer 22 is preferably 50% by weight or less.

By setting the silica content in the mixture layer 22 to 50% by weight or less, gelation due to an increase in the viscosity of the paint can be reliably prevented, and better low-temperature chipping resistance can be obtained. Further, the silica content is more preferably 5 to 50% by weight. By setting the silica content to 5% or more, better electrodeposition coating properties can be obtained near the upper limit of the film thickness, which is more preferable.

``In the present invention, the amount of this mixture layer 22 is 0.1 to 2.5 g/m'' in terms of dry weight.

If the amount of the mixture layer 22 is less than 0.1 g/m'', it is difficult to uniformly form the mixture layer 22 on the plating layer 12, and when phosphate treatment is performed, a portion of the phosphate film may be substantially removed. In addition, if the amount of the mixture layer 22 is 2.5 g/m
If it exceeds s, the coating properties of the electrodeposition coating device 6, which is usually carried out after the formation of the mixture layer 22, will deteriorate, making it impossible to obtain a good surface-treated steel sheet. Furthermore, as mentioned above,
When using a mixture of multiple resins, reduce the total amount to 0.
．． 1 to 2.5 g/m''.

There is no particular limitation on the method of forming such a mixture layer 22, and the resin may be dissolved in various solvents depending on the resin to be applied, and then applied by a method such as spray coating, dip coating, or roll coating. Any conventional drying method may be used, such as drying by heating, ultraviolet irradiation, or the like.

Basically, the surface-treated steel sheet of the present invention having such a structure is coated with an intermediate coat layer 18, a top coat layer 20, etc. on the mixture layer 22 using a cationic electrodeposition coating device 6, and various paints, and is suitable for use in automobiles. Suitable for various uses such as steel plates.

As mentioned above, the surface-treated steel sheet of the present invention has good coating film adhesion (secondary adhesion), particularly excellent electrodeposition coating properties, even without a phosphate film. .

<Example> Hereinafter, the present invention will be explained in more detail by giving specific examples of the present invention.

[Example] A cold rolled steel sheet was subjected to Zn-Ni plating to produce a Zn-Ni plated steel sheet.

Of the Zn-Ni plated steel sheets produced, some were plated at a low current density in the initial stage, and then the current density was increased to achieve an average Ni content of 12% and a basis weight of 20 g.
/m'' Zn-Ni plating was applied to improve plating adhesion.

Table 1 shows the basis weight of the produced Zn-Ni plated steel sheets and the presence or absence of plating adhesion measures.

A mixture layer was formed on the surfaces of several Zn-Ni plated steel sheets produced in this way using a mixture of the following resin and silicon dioxide. The mixture layer is formed by dissolving the resin to be applied in a solvent suitable for each resin, adding a predetermined amount of silicon dioxide to the mixture, and then coating it on the plating layer using a bar coater. This was done by drying in an electric oven.

Resin A (glass transition temperature 120°C) Bisphenol A type epoxy resin Resin B (glass transition temperature 106°C) Phenol resin resin C (glass transition temperature 85°C) Acrylic resin resin D (glass transition temperature 63°C) Urethane-epoxy resin resin E (glass transition temperature 54°C) Urethane resin resin F (glass transition temperature 35°C) Modified epoxy resin resin G (glass transition temperature 22°C) Modified epoxy resin resin H (glass transition temperature -12°C) Mixture layer formed of polyolefin resin Table 1 shows the type of resin, the glass transition temperature, the content of silicon dioxide in the mixture, and the amount of coating of this mixture layer (coating amount).

Note that samples No. 2.6 and 18 were subjected to phosphate treatment after forming the mixture layer, and sample No. 22.2
Nos. 3 and 24 were treated with phosphoric acid without forming this mixture layer. The processing conditions are as shown below.

Phosphate treatment: Perform normal spray-type alkaline degreasing, followed by phosphate treatment (Palbond 30 manufactured by Nippon Vercalizing Co., Ltd.).
20 for 120 seconds).

Table 1 shows the presence or absence of phosphate treatment and the presence or absence of phosphate film.
Shown below. In addition, the phosphate film has an adhesion amount of 0.2 g/m2.
The above cases were judged as having a film, and those less than 50.2 g/m'' were judged as not having a film.

Power top U-
600 (manufactured by Nippon Paint ■) was applied at an electrodeposition voltage of 250 V and a bath temperature of 28° C. for 180 seconds, and baked at 170° C. for 20 minutes.

After electrocoating, the electrocoatability of each steel plate was examined.

The electrodeposition coating property was evaluated by measuring the number of craters generated on the coating film.

Less than 01 pieces/　c　m　” 01~6 pieces/　c　　　　　” △　7~10 pieces/　c　m　” X more than 10 pieces / c m” The results are shown in Table 1.

In addition to the sample after electrodeposition coating, we also added automotive intermediate coating paint,
A top coat was applied by spray painting, and a diamond shot test and a water resistant secondary adhesion test were conducted.

[Diamond shot test] Diamond: 10 mg (±1 mg,) Test temperature: -20°C Shot speed: 210 km/h A diamond shot test was conducted under each of the above conditions. The evaluation method was the total peeled area of 10 shots.

Note that the shot speed of 210 km/h is stricter than the normal test conditions in Japan (usually the shot speed is 170 km/h or less).

[Water Resistance Secondary Adhesion Test] Each sample was immersed in 40°C warm water for 240 hours. After collecting each sample, within 10 minutes, 2
100 scratches were made at mm intervals with a cutter knife, and a peel test was performed using cellophane tape. Evaluation was performed by measuring the number of peeled pieces.

0: No peeling ○: Number of peeled pieces is 1 or less △Number of peeled pieces 2 to 10 ×; Number of peeled pieces: 11 or more The results of each test are shown in Table 1.

From the results shown in Table 1 above, it can be seen that the surface-treated steel sheet of the present invention has much better low-temperature chipping resistance than the conventional surface-treated steel sheet that does not have a mixture layer. Furthermore, even if no measures are taken to improve plating adhesion, some steel sheets have far superior low-temperature chipping resistance than cold-rolled steel sheets, which are conventionally thought to have high low-temperature chipping resistance. Furthermore, from the results shown in Table 1, it can be seen that the conventional surface-treated steel sheet having a phosphate film has extremely low low-temperature chipping resistance.

It can also be seen that by having a predetermined mixture layer, a phosphate film is not substantially formed even if phosphate treatment is performed.

In addition, although the above-mentioned example was about a Zn-Ni plated surface-treated steel sheet, such an effect of the present invention can be applied to any of the above-mentioned zinc-based plated steel sheets (zinc-based alloy plating, zinc-based composite plating, etc.). Of course, the same applies to things as well.

<Effects of the Invention> As explained in detail above, the surface-treated steel sheet of the present invention, which is excellent in low-temperature chipping resistance, paint peeling resistance, and electrostatic storage coating property, has a predetermined mixture layer, and in addition, conventionally two It is also possible to virtually eliminate the need for a phosphate film that reduces low-temperature chipping resistance, which was required to improve adhesion, resulting in significantly superior low-temperature resistance compared to conventional surface-treated steel sheets. It has chipping resistance and coating peeling resistance.

Moreover, it has excellent electrostatic coating properties.

[Brief explanation of drawings]

FIG. 1 is a diagram conceptually showing a surface-treated steel sheet of the present invention that is excellent in low-temperature chipping resistance, paint film peeling resistance, and electrostatic storage coating property. FIG. 2 is a conceptual diagram showing a conventional surface-treated steel sheet. Explanation of symbols 10...Cold rolled steel plate, 12-Zn-Ni plating layer, 14...Phosphate film, 16...Electrodeposition coating, 18...Intermediate coating layer, 20... Top coat layer, 22...mixture layer agent

Claims (2)

[Claims] (1) At least one side has a zinc-based plating layer, and on top of that, a mixture layer containing a resin with a glass transition temperature of 30 to 120°C and silicon dioxide is applied in an amount of 0.1 to 2 by dry weight.
．． 5 g/m^2 A surface-treated steel sheet having excellent low-temperature chipping resistance, coating peeling resistance, and electrodeposition coating property. (2) The surface-treated steel sheet with excellent low-temperature chipping resistance, coating peeling resistance, and electrodeposition coating properties according to claim 1, wherein the content of the silicon dioxide in the mixture layer is 50% by weight or less.