JPH0427539A - Surface-treated steel sheet excellent in low temperature chipping resistance and water resistant secondary adhesion - Google Patents

Abstract

PURPOSE:To obtain the surface-treated steel sheet excellent in low temperature chipping resistance and water resistant secondary adhesion by forming a chromate film and the resin layer with the glass transition temperature of 60-120 deg.C on the zinc-plated layer formed 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 chromate film 24 with the coating build up of 5-400mg/m<2> by the value converted into metallic chrome on such zinc-plated layer 12. The surface-treated steel sheet forms the resin layer 22 which has the coating build up of 0.1-2.5g/m<2> and the glass transition temperature of 60-120 deg.C on such zinc- plated layer 12. By such a construction, very excellent low temperature chipping resistance and water resistant secondary adhesion may be obtained. As the resin used for the resin layer 22, epoxy resin, urethane resin, acryl resin, phenol resin, 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 and water-resistant secondary adhesion, which is particularly suitably applied 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 types of surface-treated steel sheets have been developed and put into practical use to improve their rust prevention properties.

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 galvanized steel sheet is a steel sheet with excellent rust resistance under high humidity conditions, and is used in high humidity environments such as the bag structure of automobile doors. Nil
It exhibits high bare rust prevention properties even when used in applications where it is exposed to one condition. Therefore, this surface-treated steel sheet is treated with chromate treatment, organic film formation, etc. on the upper layer of the zinc-based plating, and is particularly suitable for areas such as the inner surface of the car body, which tend to have insufficient coating, such as the bag structure. 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 use zinc-nickel plating, which has poor chemical conversion treatment (phosphate treatment), on the outer surface of the car body, and ordinary steel sheets for automobiles are plated only on one side. A non-scorched 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 is high.

Incidentally, surface-treated steel sheets for exterior use, particularly the steel sheet surfaces that serve as the exterior surfaces of automobiles, are required to have performances that do not need to be considered at all on the inside surfaces.

As mentioned above, such performance includes various things such as image clarity and paintability, but one of the important performances is low-temperature chipping resistance.

Chipping is a phenomenon in which a pebble or the like hits the car body while the car is running, and the impact causes the plating layer to peel off, exposing the steel sheet base, and is particularly likely to occur at low temperatures.

The above-mentioned zinc-based plated steel sheet, especially the zinc-nickel plated crucian carp steel sheet, has poor plating adhesion. Therefore, after coating is applied on the plating layer, the stress of the coating film (electrodeposition coating, intermediate coating, top coating, etc.) is applied directly to the plating layer. Peeling is likely to occur. In other words, although zinc-based alloy coated steel sheets have significantly superior corrosion resistance, especially rust prevention, compared to the conventionally used 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 joint adhesion of zinc-based plated steel sheets and improve their low-temperature chipping resistance.

As a method for improving this low-temperature chipping resistance,
The initial plating disclosed in Japanese Patent No. 3-143286 is performed at a low current density to form an epitaxial layer of zinc-nickel on the surface of the steel sheet, and then the current density is increased to form a Zn-nickel epitaxial layer.
A method of applying Ni plating, a method characterized by preferentially dissolving and removing Zn in a plating layer disclosed in JP-A-1-108400, and a method characterized by preferentially dissolving and removing Zn in a plating layer disclosed in JP-A-1-176091. 0.9<N i/ (Z
n+N i ) < 1.0 (molar ratio), 50 to 10 under the conditions of PH2 or less and current density of 5 to 200 A/d m2
Various methods regarding the structure of the plating itself have been disclosed, such as a method in which Zn-Ni plating is performed at a concentration of 0.00 mg/m'' and then Zn-Ni plating is applied at a predetermined Ni content.

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.

Further, a chromate or phosphate treatment film layer is provided on the plating disclosed in JP-A No. 64-78832,
Furthermore, an organic resin film layer with a thickness of 0.1 to 2.0 μm is provided on top of it, and has a Tg point of 55°C or less and a hardness of pencil hardness at room temperature after baking of H to 2B. A method is disclosed.

This method certainly reduces peeling.
However, when we investigated the peeling area in more detail, we found that although it was true that the peeling between the plating and the base metal was decreasing, at the Tg point of 55°C or lower, which is an essential requirement for this method, the peeling between the resin and the electrical sign coating film was Considerable peeling was observed, and it was found that it was not practical as an outer panel for automobiles. This is presumed to be because the adhesion between the resin and the electrodeposited coating becomes weak when the Tg point is below 55°C.

In addition to ensuring such low-temperature chimping resistance and paint adhesion, steel plates for exterior use are also desired to have excellent corrosion resistance and paintability as described above.

The purpose of the present invention is to solve the problems of the prior art described above, and to provide a galvanized surface-treated steel sheet that has excellent low-temperature chipping resistance, corrosion resistance, and water-resistant secondary adhesion, and is easy to manufacture. It is about providing.

Means for Solving the Problem> In order to achieve the above-mentioned goal, the inventors of the present invention have made extensive studies and have unexpectedly found that phosphoric acid, which is commonly used in zinc-plated surface-treated steel sheets, has been developed. It has been found that salt treatment reduces the low temperature chipping resistance. In zinc-plated surface-treated steel sheets for automobiles, paint adhesion (secondary adhesion)
In order to improve this, a water-insoluble phosphate film is produced by performing a phosphate treatment after the graining layer is formed. However, research by the present inventors revealed for the first time that by having a phosphate film, although the secondary adhesion of the obtained surface-treated steel sheet is improved, the low-temperature chipping resistance is reduced.

In addition, the present inventors have further investigated and found that a predetermined resin layer, that is, a resin layer with a glass transition temperature of 60 to 120°C, is applied on the plating layer of a zinc-based plated surface-treated steel sheet by 0.1 to 2.5°C.
g/m2 (dry weight), which not only prevents the plating from peeling off in low-temperature chipping tests, but also prevents the separation between the resin and the electrodeposited coating! IJ ill was also found to be significantly improved.

Moreover, although such a resin layer is necessary and has sufficient paint adhesion, before forming this resin layer, 50% of the amount obtained in terms of metallic chromium obtained by chromate treatment is
By having a chromate film of ~400 mg/m', better paint adhesion can be obtained,
Excellent corrosion resistance can also be achieved.

Furthermore, by having such a romate film and resin layer, even if phosphate treatment is performed, substantial formation of a phosphate film that reduces low-temperature chipping resistance can be prevented.

That is, the present invention has a zinc-based plating layer on at least one side, and has a coating amount of 5 to 400 in terms of metallic chromium.
It is characterized by having a chromate film of m g/m', and a resin layer having an adhesion amount or dry weight of 01 to 2.5 g/m2 and a glass transition temperature of 60 to 120°C. Provided is a surface-treated steel sheet with excellent low-temperature chipping resistance and water-resistant secondary adhesion.

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 paint, that is, a cationic electrodeposition coating layer 6 is applied thereon. , the adhesion of the intermediate coat layer 18 and the top coat layer 20 (
It has a phosphate film 14 formed by phosphate treatment to improve secondary adhesion.

Conventional surface-treated steel sheets with such a configuration have poor low-temperature chipping resistance, and various improvement methods as mentioned above have been proposed, and none of the methods can provide the desired low-temperature chipping resistance. As mentioned above.

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 in No. 4, the phosphate crystals are large and the surface roughness is large, so that the paint can obtain a physical anchoring effect. Therefore, the phosphate crystals increase the contact area between the electrodeposition coating and the plating, which promotes the transfer of residual stress in the paint, causing distortion in the plating and reducing plating adhesion.

However, if the cationic electrodeposition coating layer 6, intermediate coat layer 18, top coat layer 20, etc. are simply formed without forming 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 treatment 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 pencil hardness H to 2B at room temperature after baking is 0.1 to 2B. Although there was no peeling due to plating on the steel plate with a thickness of 2.0 μm, the resin-'I! The coating peels off during low-temperature chipping, which is thought to be due to a decrease in the adhesion between the Ls coatings, which is also not practical.

In contrast, the surface-treated steel sheet of the present invention with excellent low-temperature chipping resistance (hereinafter referred to as surface-treated steel sheet) substantially has this phosphate film 14, and has excellent low-temperature chipping resistance and water resistance. A predetermined resin layer 22 for improving secondary adhesion, that is, a glass transition temperature of 60 to 120°C
The resin layer of 0, 1~2, 5 g/m2 (
dry weight).

In other words, as a result of further intensive studies, the present inventors have found that by forming the resin layer 22 on the plating layer 12, the transmission of the residual stress of each paint layer to the plating layer can be alleviated. It has been found that by setting the Tg point of the resin layer to 60 to 120°C, it is possible to improve the peeling resistance of the coating film.

Moreover, by having such a resin layer 22 and a predetermined chromate film, even if phosphate treatment is performed, a phosphate film (second number 14) will not be substantially formed. I found it.

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 chromate layer @24 and resin 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 that reduce low-temperature chipping, so there is no need to make any changes to the automotive painting process.

Therefore, the surface-treated steel sheet of the present invention has extremely excellent low-temperature chipping resistance, in combination with the fact that it does not substantially have the phosphate film 14 that reduces low-temperature chipping resistance as described above. .

Moreover, in the surface-treated steel sheet of the present invention, as the lower layer of the resin layer 22, the adhesion amount is 5 to 40.0 mg/metal chromium.
By having the chromate film 24 of m2, it is possible to obtain excellent corrosion resistance, especially water-resistant adhesion, and it is possible to exhibit very excellent performance as a steel plate for automobile exteriors.

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 10. 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.

The method for forming the plating layer 12 is not particularly limited, 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 higher Ni content. Preferably, it is formed.

The surface-treated steel sheet of the present invention has a coating amount of 5 to 400 mg in terms of metallic chromium on such a zinc-based plating layer.
/ m2 chromate skin lI! 24.

By having such a romate film @ 24, a phosphate film is not formed, and in addition to the high low temperature chipping resistance obtained by having the resin layer 22, good paint adhesion, especially water resistant secondary It is possible to obtain a surface-treated steel sheet that has good adhesion and excellent corrosion resistance.

In the surface-treated steel sheet of the present invention, the chromate coating lI@2
If the adhesion amount of 4 is less than 5 mg/m'' in terms of metallic chromium, it will be difficult to form a uniform romate skin 11!24, and excellent water-resistant secondary adhesion cannot be obtained.
In addition, the amount of chromate film 24 deposited is 400 m g
/ m' is unfavorable from an environmental standpoint because chromium elution occurs when the ratio is too large.

There is no particular limitation on the method of forming the chromate skin@24 on the surface-treated steel sheet of the present invention, that is, the method of chromate treatment, and any known chromate treatment method such as reaction type, coating type, electrolytic type, etc. can be applied. . Moreover, various additives such as silica may be added to the chromate treatment liquid as necessary.

The surface-treated steel sheet of the present invention has a resin layer 22 of 0.1 to 2.5 g/m 2 and a glass transition temperature of 60 to 120° C. on top of the plating layer 12 made of such zinc-based plating. With such a configuration, very good low temperature chipping resistance and water resistant secondary adhesion can be obtained. Moreover, with conventional surface-treated steel sheets, a phosphate film is formed, which is necessary to obtain sufficient adhesion (secondary adhesion) of the paint layer formed as the upper layer, and which leads to a decrease in low-temperature chipping resistance. Sufficient water-resistant secondary adhesion can be obtained even without this, and further improvement in low-temperature chipping resistance has been realized.

Moreover, even if phosphate treatment is performed, formation of a phosphate film (second number 14) can be substantially prevented. In the present invention, "substantially no phosphate film is formed" also means a case where the amount of the phosphate film formed by the phosphate treatment is 0.2 g/m2 or less.

The resin applied to the present invention has a glass transition temperature of 60 to 1
The temperature is 20°C. When the glass transition temperature is less than 60°C, the residual stress of each paint layer will not be applied to the plating layer 12,
In addition, since the transmission of impact is stopped by the resin layer, this plating layer 12
The resin layer 2 does not peel off from the base steel plate, but the resin layer 2
However, as mentioned above, peeling occurs between the two and it is not possible to obtain a good surface-treated steel sheet. On the other hand, if the glass transition temperature of the resin layer 22 exceeds 120°C, the viscosity of the resin is high and the solubility in various solvents is also reduced.
It is difficult to apply this uniformly, and a good surface-treated steel sheet cannot be obtained.

In the present invention, it is more preferable that the glass transition temperature of the resin layer 22 is 80 to 120°C, and in this case, the glass transition temperature of the resin layer 22 is preferably 80 to 120°C. better results can be obtained with

Various resins can be used as the resin for the resin layer 22 of the present invention 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.

Incidentally, the above resins may be used in a mixture of 2 fi or more.

Furthermore, even if the resin has a glass transition point of less than 60°C, it is possible to add additives such as various curing agents and modifiers to make the glass transition point higher than 60°C.

In the present invention, the amount of this resin layer 22 is 0 on a dry weight basis.
．． 1 to 2.5 g/m". The amount of the resin layer 22 is 0.
If the amount is less than 1 g/m, it is difficult to uniformly form the resin layer 22 on the chromate skin WA24, and the chromium elution resistance decreases.
If it exceeds 2, the electrodeposition coating 6 that is normally performed after forming the resin layer 22 cannot be performed. 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 resin layer 22.
A conventional method may be used in which the resin is dissolved in a variety of solvents depending on the resin to be applied, applied by spray coating, dip coating, roll coating, etc., and then dried by heating, ultraviolet irradiation, etc. .

Basically, the surface-treated steel sheet of the present invention having such a structure has an intermediate coating layer 18, a top coating layer 20, etc. applied on the resin 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 described above, the surface-treated steel sheet of the present invention has excellent coating film adhesion (secondary adhesion) 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] After pickling and degreasing a cold-rolled steel sheet, Zn-Ni plating was performed, and Z
An n-Ni plated steel plate was created.

In addition, some of the Zn-Ni plated steel sheets created were plated at a low current density in the initial stage, and then the current density was increased to reach an average Ni content of 12% and a coating weight of 20%.
Zn-Ni plating of g/m2 was applied to improve plating adhesion.

Table 1 shows the basis weight of the Zn-Ni plated steel sheets prepared and whether or not measures were taken to improve plating adhesion.

Chromate treatment was performed on the surfaces of several Zn-Ni plated steel sheets prepared as described in 2 under the following conditions to form a chromate film.

Chromate treatment The chromate treatment was performed by applying a chromate treatment liquid using a roll coater and then drying it using an electric furnace.

A resin layer was formed on the surface of several Zn-Ni plated steel sheets prepared in this manner using the following resin. The resin layer was formed by dissolving the resin to be applied in a suitable solvent, applying it with a bar coater, and then drying it in an electric furnace.

Resin A (glass transition point 120°C) Bisphenol A type epoxy resin Resin B (glass transition point 106°C) Phenol resin resin C (glass transition point 85°C) Acrylic resin resin D (glass transition point 63°C) Urethane-epoxy resin resin E (glass transition point 54°C) Urethane resin resin F (glass transition point 22°C) Modified epoxy resin G (glass transition point -12°C) Type of resin in the resin layer formed with polyolefin resin, glass tipping temperature, and resin coating The amounts are shown in Table 1.

Note that samples No. 12 and 6 were subjected to phosphate treatment after resin formation, and samples No. 18.19 and 20 were treated with phosphate after resin formation.
did not form this resin layer and performed phosphoric acid treatment.
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 a coating weight of 0.2 g/m'
Those above were treated as having a coating, and those less than that were treated as having no coating.

Here, the amount of chromium eluted from each sample obtained before and after the alkaline degreasing step before electrodeposition coating was measured.

In the measurement, the amount of chromium deposited on the surface of the steel plate before and after degreasing was measured by fluorescent X-ray analysis, and the difference was regarded as the amount of dissolved chromium a for evaluation.

The evaluation criteria are as follows.

0 1 m g/m ' or less Of - less than 2 mg/m2 △ 2-5 m g/m 2 less than 5 m
g/m′ results are shown in Table 1.

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.

In addition to the sample after electrodeposition coating, we also added automotive intermediate coating paint,
A top coat of paint was applied by spray painting.

Using each sample thus obtained, a diamond shot test and a water resistant secondary adhesion test were conducted.

[Diamond shot test] Diamond: 10mg (±1mg) Test temperature 2
-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 area of the peeled areas of 10 points.

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 taking out each sample, within 10 minutes, 2
100 scratches were made at mm intervals using a cutter knife, and a peel test was performed using the following.

This was done by measuring.

◎, no peeling O1 peeling number 1 or less △: Number of peeled pieces 2 to 10 X: Number of peeled pieces 11 or more The results of each test are shown in Table 1.

As for the cellophane tape evaluation, the number of peeled pieces was determined from the results shown in Table 1 above.According to the surface-treated steel sheet of the present invention, compared to the conventional surface-treated steel sheet that does not have a resin layer,
It can be seen that it has far superior low-temperature chipping resistance. 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, the results shown in Table 1 also show that the conventional surface-treated steel sheet having a phosphate coating has extremely low low-temperature chipping resistance.

Moreover, it can be seen that the surface-treated steel sheet of the present invention has excellent water-resistant secondary adhesion.

Furthermore, it can be seen that by having a predetermined resin layer and chromate film, a phosphate film is not substantially formed even if phosphate treatment is performed.

Although the above-mentioned examples were about Zn-Ni plated surface-treated steel sheets, the effects 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, it is the same whether it is something or not.

<Effects of the Invention> As explained in detail above, the surface-treated steel sheet of the present invention, which has excellent low-temperature chipping resistance and water-resistant secondary adhesion, has much better low-temperature chipping resistance than conventional surface-treated steel sheets. It is something that you have. Furthermore, it is possible to eliminate the need for a phosphate film that degrades low-temperature chipping resistance, which was conventionally necessary to improve secondary adhesion.

In addition, it has excellent water resistant secondary adhesion, and together with the above-mentioned low temperature chipping resistance, it can be well used in applications such as automobile steel plates.

[Brief explanation of drawings]

FIG. 1 is a diagram conceptually showing a surface-treated steel sheet of the present invention that is excellent in temperature chipping resistance and water resistant secondary adhesion. 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... Resin layer, 24... Chromate film■

Claims (1)

[Claims] (1) Has a zinc-based plating layer on at least one side, has a chromate film on it with an adhesion amount of 5 to 400 mg/m^2 in terms of metallic chromium, and has an adhesion amount of 0 on dry weight.
．． 1~2.5gg/m^2, glass transition temperature 60~1
A surface-treated steel sheet with excellent low-temperature chipping resistance and water-resistant secondary adhesion, characterized by having a 20°C resin layer.