JPH0427538A - Surface-treated steel sheet excellent in low temperature chipping resistance and film peeling resistance - Google Patents

Abstract

PURPOSE:To obtain the surface-treated steel sheet excellent in low temperature chipping resistance and film peeling resistance by providing the resin layer which has the coated amount of 0.1-2.5g/m<2> by dried weight and the glass transition temperature of 60-120 deg.C on the zinc-coated 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 a steel sheet 10. 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 is formed on the upper layer of said zinc-plated layer 12. By such construction, very excellent low temperature chipping resistance may be obtained. As the resin used for the resin layer 22 of this invention, any various resins with said glass transition temperature may be used, and they are epoxy resin, urethane resin, acryl resin, phenol resin, polyolefin resin etc. Cation electro-coating 16, the intercoating layer 18 by the coating by various kinds and top coat 20 etc., are applied onto said resin layer 22. Said surface-treated steel sheet is preferably used for various uses of the steel sheet of an automobile.

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 coating peeling resistance, 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 that have been subjected to surface treatments to improve rust prevention 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 is subjected to chromate treatment, organic film formation, etc. on the upper layer of zinc-based plating, especially on the inner surface of the car body, where areas such as bag structures are prone to be insufficiently coated. It is suitably applied to the following applications.

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 automotive steel sheets are plated on only 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 instead of conventional cold-rolled steel sheets. 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. One of its important properties 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 plating layer peels off due to the impact, exposing the steel sheet base, and 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 stress of the coating film (first coat, intermediate coat, top coat, 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 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,
The initial plating disclosed in Japanese Patent No. 3-143286 is performed at a low current density to form a zinc-nickel epitaxial layer on the surface of the steel sheet, and then the current density is increased to form a zinc-nickel epitaxial layer.
A method of applying nickel plating, a method characterized by preferentially and selectively dissolving and removing Zn in a plating layer disclosed in JP-A-1-108400, JP-A-1-17609
0.9<Ni/(
50 to 1000 m under the conditions of Zn+Ni) < 1.0 (molar ratio), pH 2 or less, and current density 5 to 200 A/dm.
Various methods relating to the structure of the plating itself have been disclosed, such as a method in which Zn-Ni plating is performed at a rate of 1.5 g/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.

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 method is disclosed in which an organic resin film layer of H to 2B is provided 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, when the Tg point is below 55°C, which is an essential requirement for this method, the gap between the resin and the electrodeposited coating was reduced. It was found that the peeling was considerably thickened and it was not practical as an outer panel for an automobile. This is the Tg point of 55℃
It is presumed that this is because the following resins weaken the adhesion between the resin and the electrodeposited coating.

An object of the present invention is to solve the problems of the prior art described above, and to provide a zinc-plated surface-treated steel sheet that has excellent low-temperature chipping resistance and is easy to manufacture.

Means for Solving the Problem> In order to achieve the above object, the inventors of the present invention have conducted extensive research and have surprisingly found that the l/- It has been found that the phosphate film formed by salt treatment reduces the low temperature chipping resistance. In order to improve paint adhesion (secondary adhesion) in zinc-plated surface-treated steel sheets for automobiles, 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 further investigated and found that 01 to 2.5 g of a predetermined resin layer, that is, a resin layer with a glass transition temperature of 60 to 120°C, was applied on the plating layer of a galvanized surface-treated steel sheet.
/m2 (dry weight), it has been found that not only the plating peeling but also the peeling between the resin and the electrodeposited coating film is significantly improved in the low-temperature chimbling property test. Moreover, by having such a resin layer,
At the same time, we have also discovered that sufficient paint 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 has a coating amount of 0.1 to 2.5 g on dry weight.
The present invention provides a surface-treated steel sheet having excellent low-temperature chipping resistance and coating peeling resistance, characterized by having a resin layer having a glass transition temperature of 60 to 120°C.

The present invention will be explained in detail below.

The present invention has been achieved by obtaining 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 coating layer 6, is applied thereon. Adhesion of intermediate coat layer 18 and top coat layer 20 (
It has a phosphate film 14 formed by phosphate treatment to improve secondary sealability.

Conventional surface-treated steel sheets with such a configuration have low low-temperature chipping resistance, and various improvement methods as mentioned above have been proposed, but none of these methods can provide the desired low-temperature chipping resistance. is as described 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, it is thought that the phosphate crystals increase the contact area between the electrodeposition coating and the plating, thereby accelerating the transmission 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, can be obtained. It is well known that it is impossible to put this waste into practical use as a steel plate for automobiles, especially as a steel plate for exterior use.

In addition, a chromate or phosphate treatment film is provided on the plate, 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 a thickness of 0.1 to 20 μm. Regarding the steel plate to be installed, although there was no plating peeling, peeling of the coating occurred during low-temperature chipping, which is thought to be due to a decrease in the adhesion between the resin and the electrodeposited coating.
It's still 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 Predetermined resin 122 for improving paint film removability, that is, glass transition temperature is 60 to 120'e
It has a resin layer of 0.1 to 2.5 g/m2 (dry weight).

In other words, as a result of further intensive studies, the present inventors found that the plating layer 1
By forming the resin layer 22 on the coating layer 2, the transfer of the residual stress of each paint layer to the plating layer is alleviated, and by setting the Tg point of the resin layer to 60 to 120°C, the coating film peeling resistance is improved. The present invention was completed based on the discovery that it is also possible to improve the

Moreover, it has also been found that by having such a resin layer 22, even if phosphate treatment is performed, a phosphate film (second row number 14) is 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 low-temperature chipping resistance. 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 resin layer 22 on the plating layer 12 as shown in FIG. 1, even if phosphate treatment is performed thereon, the low temperature chipping resistance is reduced. 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, in addition to being substantially free of the phosphate film 14 that reduces low-temperature chipping resistance as described above. .

Moreover, by having this resin layer 22, necessary and sufficient coating film peeling resistance (secondary adhesion) can be obtained even without the phosphate film 14.

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 refers to zinc-based plating that has inherent problems in 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 commonly used plating is Zn-Ni plating, in order to improve the adhesion between the steel plate 1o and the plating layer 12, the lower part of the plating layer is formed to have a high Ni content. Preferably.

The surface-treated steel sheet of the present invention has a resin layer 22 of 0.1 to 2.5 g/m' 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 can be obtained. Moreover, in conventional surface-treated steel sheets, it is necessary to obtain sufficient adhesion (secondary adhesion) of the paint layer formed as the upper layer.
It is possible to eliminate the need for the formation of a phosphate film that leads to a decrease in low-temperature chipping resistance, and it is also possible to obtain necessary and sufficient secondary adhesion even without forming a phosphate film.

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" also means that the amount of the phosphate film formed by 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 is not applied to the plating layer 12,
In addition, since the transmission of impact is stopped at the resin layer 22, peeling of the plating layer 12 from the steel plate base will not occur.
As described above, the adhesion between the resin 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 layer 22 exceeds 120°C, the viscosity of the resin will be high and the solubility in various solvents will also decrease, making it difficult to apply it uniformly and, again, to achieve a good result. It is not possible to obtain surface-treated steel sheets.

In the present invention, it is preferable that the glass transition temperature of the resin layer 22 is 80 to 120°C. Better results can be obtained in various performances.

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 niboxy resin, urethane resin, acrylic resin, phenol resin, and polyolefin resin. Illustrated.

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

As will be detailed later, in this case as well, the total amount of resin is 0.1
2.5 g/m'.

Further, even if the resin has a glass 718 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 o
, i~2.5g/m2. The amount of resin layer 22 is 0.
If it is less than 1 g/m2, it is difficult to uniformly form the resin layer 22 on the plating layer 12, and if the phosphate treatment is performed through an existing painting line that includes a phosphate treatment process, the phosphate treatment A salt film is partially formed, making it impossible to obtain good low-temperature chipping resistance. Furthermore, if the amount of the resin layer 22 exceeds 2.5 g/m2, the coating properties of the electrodeposition coating device 6, which is usually performed after the formation of the resin layer 22, will deteriorate, making it difficult to obtain a good surface-treated steel sheet. Can not.

There is no particular limitation on the method of forming such a resin layer 22.
Using various solvents depending on the resin to be applied, the resin is dissolved in it and applied by a method such as tin-no coating, dip coating, roll coating, etc., and then dried by heating, ultraviolet irradiation, etc. by the usual method. The wrinkles are good.

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 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 good 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 produced.

In addition, some of the Zn-Ni plated steel sheets produced were coated with Zn with an average Ni content of 12% and a basis weight of 120 g/m' by performing the initial plating at a low current density and then increasing the current density. Measures for plating adhesion have been taken by applying low N1 plating.

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

A resin layer was formed on the surface of several Zn-Ni plated steel sheets produced in this way using the following resin. The 'H11 fat layer was formed by dissolving a common resin in a suitable solvent, coating 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 resin G (glass transition point -12°C) Type of resin, glass transition temperature, and resin coating of the resin layer formed with polyolefin resin The amounts (coating film amounts) are shown in Table 1.

In addition, samples No. 2.6 and 13 were subjected to phosphate treatment under the following conditions, and sample No. 17.18
and No. 19, the phosphoric acid treatment was performed without forming a resin 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.
Those above were treated as having a coating, and those less than that were treated as having no coating.

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 electrodeposition coating, each sample was further spray-painted with an automotive intermediate paint and a top coat.

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

[Diamond shot test co-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.

Please 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 with a cutter knife, and a peel test was performed using cellophane tape. Evaluation was performed by measuring the number of peeled pieces.

◎: No peeling O: 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, 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, 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 is also seen that by having a predetermined resin layer, a phosphate film that deteriorates low-temperature chipping resistance is not substantially formed even if phosphate treatment is performed.

Although the above-mentioned embodiments were related to Zn-Ni plated surface-treated steel sheets, the effects of the present invention are applicable to any of the above-mentioned zinc-based plated steel sheets (zinc-based alloy plating, zinc-based composite plating, etc.). Of course, even if it belongs to the Sono-sama, it belongs to Sono-sama.

<Effects of the Invention> As explained in detail above, the surface-treated steel sheet of the present invention with excellent low-temperature chipping resistance has significantly superior low-temperature chipping and nobbing resistance and paint peeling resistance compared to conventional surface-treated steel sheets. It is something that has a nature. Moreover, it is no longer necessary to use a phosphate film that reduces low-temperature chipping resistance, which was conventionally necessary to improve secondary adhesion. It is possible to prevent the formation of a phosphate film, and also to obtain the necessary and sufficient secondary adhesion.

[Brief explanation of the drawing]

FIG. 1 is a diagram conceptually showing a surface-treated steel sheet having excellent low-temperature chipping resistance and coating peeling resistance according to the present invention. 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

Claims (1)

[Claims] (1) Has a zinc-based plating layer on at least one side, and has an adhesion amount of 0.1 to 2.5 g/dry weight on it.
A surface-treated steel sheet having excellent low-temperature chipping resistance and coating peeling resistance, comprising a resin layer having a glass transition temperature of 60 to 120°C.