JP2621751B2 - Surface treated steel sheet for automobiles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automobile having excellent cationic electrodeposition coating properties, remarkably improved sharpness after coating, and corrosion resistance, low temperature chipping resistance, spot weldability, press formability, etc. The present invention also relates to a surface-treated steel sheet for automobiles that also has favorable other properties required for automotive steel sheets.

[0002]

2. Description of the Related Art In recent years, the requirements for rust prevention performance of automobile bodies have become increasingly severe over the years, and various surface-treated steel sheets have been developed to meet this demand. Recently, as a part of measures to enhance rust prevention performance especially on the exterior surface of automobiles, zinc-plated or zinc-alloy-plated (eg, Zn-Ni or Zn-Fe alloy-plated) steel sheets have been used as base materials, which have been subjected to chromate treatment and clear coating. The application of an organic composite surface-treated steel sheet coated with a thin organic resin (eg, an epoxy resin paint containing a crosslinking agent) has been rapidly progressing.

However, in the conventional organic composite surface-treated steel sheet, the cationic organic electrodeposition coating property is not sufficient even in a thin film because the conductivity of the upper organic resin film is poor. Since it is expected that the requirements for the appearance of automobiles will become more and more advanced in the future as well as the rust prevention performance, it has been urgently required to improve the electrodeposition coating properties which affect the appearance.

As a measure for improving the electrodeposition coating property of an organic composite surface-treated steel sheet, Japanese Patent Publication No. 3-32638 proposes that an upper organic resin film is formed from an epoxy resin containing a hydrophilic polyamide resin. Have been. This surface-treated steel sheet showed excellent cationic electrodeposition coating properties compared to conventional products, and had satisfactory performance when considered for automotive inner plates, but was used for outer plates. Then, there were the following problems, and it had to be said that it was unsuitable for use in automobile exteriors.

That is, when the organic composite surface-treated steel sheet is subjected to cationic electrodeposition coating, the zinc-coated steel sheet used as the base material of the surface-treated steel sheet is subjected to cationic electrodeposition coating under the same conditions while being naked. The electrodeposition coating film thickness is 1 to 8 μm
It will be about thick. For this reason, when used as an automotive outer panel, in areas where the organic resin film or chromate film has been scraped off, such as in the care for press flaws, the electrodeposition coating film thickness is thinner than its surroundings, and the appearance of the coating is significantly reduced. I do.

Apart from this, it has been pointed out that the conventional organic composite surface-treated steel sheet does not have sufficient low-temperature chipping resistance. The corrosion resistance of the organic composite surface-treated steel sheet is mainly ensured by the lowermost layer of zinc or zinc alloy plating film, but the zinc-based plating film retains the large residual stress generated during plating, so low-temperature chipping ( In cold regions such as Europe and North America, there is a tendency for the plating layer to easily peel off from the interface with the steel sheet due to the impact of stone splashes. For surface-treated steel sheets for automobile exteriors, it is important to prevent low-temperature chipping, which significantly degrades the corrosion resistance of automobiles and leads to deterioration of life and appearance.

As a countermeasure against this problem, Japanese Patent Application Laid-Open No. 46-78832
Japanese Patent Application Laid-Open Publication No. H11-163873 proposes to form a soft (glass transition point of 55 ° C. or lower) resin film effective as a buffer material on a zinc-based plating surface. However, the soft resin has a lower corrosion resistance than a crosslinked epoxy resin or the like, so that the improvement of the corrosion resistance, which is the main purpose of the surface-treated steel sheet, is insufficient. Further, similarly to the above, the problem of poor appearance due to thickening of the electrodeposition coating was inevitable.

An important factor influencing the appearance of an automobile is the sharpness after painting. Conventionally, as a means for improving the sharpness after coating, a technique of rolling a steel sheet with a roll that has been subjected to laser processing to form a dull pattern is known (for example, JP-A-62-168602). Such) . This technology is
The steel sheet is dulled in order to provide oil retaining properties necessary for lubrication at the time of press forming while securing the flatness of the steel sheet to some extent. Although the so-called bright steel plate without dull is considered to be ideal for sharpness after painting, the bright steel plate has no oil retention and cannot be press-formed. A dull with irregularities had to be formed.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide an organic composite surface-treated steel sheet for an automobile, which satisfies all the required performances on the inner side and the outer side of an automobile exterior material.

A specific object of the present invention is to provide excellent corrosion resistance,
Of course, it has press formability and spot weldability,
An object of the present invention is to provide an organic composite surface-treated steel sheet for automobiles, which has significantly improved cationic electrodeposition coating property, low-temperature chipping resistance, and sharpness after coating.

[0011]

Means for Solving the Problems The inventors of the present invention made use of a steel plate of Ra ≦ 0.3 μm (a so-called bright steel plate), which is an ideal base material for improving the sharpness, and provided a lubricity on the surface. As a result of repeated studies to obtain an organic composite surface-treated steel sheet that can be press-formed by forming an excellent organic resin film and that satisfies the above-described various performances, the present invention has been achieved.

In the present invention, at least one side of the steel sheet has a surface roughness of 0.3 μm or less in Ra, and a zinc or zinc alloy plating layer is formed on this surface in order from the lower layer, and a metal Cr of 20 to 200 mg / m ² chromate film, 0.1-20% by weight melamine cyanuric acid adduct and 1
An organic resin film having a thickness of 0.1 to 2.0 μm comprising a crosslinked epoxy resin containing 2020% by weight of a polyolefin wax, and further having a chromate film for converting silica to SiO ₂ /
Cr content in an amount of 0.05 to 4.0 by weight, and / or
Alternatively, the gist of the present invention is a surface-treated steel sheet for an automobile, wherein the organic resin film contains silica in an amount of 5 to 40% by weight of SiO ₂ .

The main features of the organic composite surface-treated steel sheet of the present invention are as follows (a) to (e). (a) A bright steel plate will be used as the base steel plate for the ultimate improvement in post-painting sharpness. (b) In addition to the underlying zinc-based plating layer and the intermediate chromate film, the outermost resin film is a dense and hard crosslinked epoxy resin film to improve corrosion resistance. (c) Melamine cyanuric acid adduct (hereinafter abbreviated as MCA) having excellent lubricity and a polyolefin wax are added to the resin film to impart high slidability to the film. As a result, press forming can be easily performed even when the bright steel sheet is the base material. (d) Due to the effect of the MCA blended in the resin film, the electrodeposition coatability is significantly improved without reducing the corrosion resistance and spot weldability. (e) Regarding the problem of low-temperature chipping of the zinc-based plating layer, silica is blended in one or both of the chromate film and the resin film to improve the low-temperature chipping resistance.

As a comprehensive result, the inner and outer surfaces of a steel sheet for automobiles such as cationic electrodeposition coating properties and sharpness after coating are remarkably improved and corrosion resistance, low temperature chipping resistance, spot weldability, press formability, etc. Thus, an organic composite surface-treated steel sheet satisfying all of the various performances required for the above can be obtained.

[0015]

The reason why the above-described structure is employed in the organic composite surface-treated steel sheet of the present invention will be described below together with its operation.

(1) Base steel sheet The surface roughness, which is an ideal material for securing sharpness after coating, is Ra.
Use a steel sheet (bright steel sheet) of ≤0.3 μm. Conventionally, since there is no oil retention and press molding is not possible,
Although a bright steel sheet was considered to be unsuitable as a base steel sheet, in the present invention, a bright steel sheet can be applied by imparting a high degree of lubricity to an upper organic resin film. If the surface roughness of the steel sheet exceeds 0.3 μm in Ra, the effect of improving the sharpness after painting becomes unclear.

Since the sharpness is the performance required on the outer side, it is sufficient that only one side has the above-mentioned surface roughness. However, a bright steel sheet having Ra ≦ 0.3 μm on both sides is required due to the manufacturing process. You may. The multi-layer surface treatment coating composed of plating + chromate film + organic resin film, which will be described below, basically only needs to be formed on one surface on the outer surface side, and the pressability also improves the slidability on the die surface side. As a result, the effect is sufficiently exhibited only on one side of the outer surface,
Since the perforation resistance and the like required on the inner surface side are also improved, it can be applied to both surfaces.

(2) Underplating The underplating, which is the lowermost layer of the organic composite surface treatment, is a zinc or zinc alloy plating (hereinafter referred to as zinc-based plating) film having excellent sacrificial corrosion resistance in order to ensure corrosion resistance. That is, as a base steel sheet of the surface-treated steel sheet of the present invention, a zinc-based plated steel sheet subjected to zinc-based plating is used. The basis weight of the zinc-based plating is selected according to the required level of corrosion resistance, but is preferably 10 g / m ² or more per side for the purpose of improving the external rust resistance. If the basis weight is too large, the press formability will decrease, so the upper limit is 60 g /
preferably with m ² approximately.

The type of zinc alloy plating is not particularly limited.
Zn-Ni alloy plating, Zn-Fe alloy plating, Zn-Al plating,
Alternatively, any of known zinc-based alloy platings further added with Co, Ti, etc. may be applied. The zinc-based plating method is not particularly limited. Electroplating is preferred because the basis weight is relatively small, but hot-dip galvanizing or galvannealing may be used. Further, the zinc-based plating film may be a multilayer plating film of two or more layers.

(3) Chromate film A chromate film is provided on the zinc-based plating layer. The chromate film has an effect of further improving the corrosion resistance of a surface-treated steel sheet having a zinc-based plated steel sheet as a base material. In addition, when silica is contained in the chromate film, an effect of improving low-temperature chipping resistance is also exerted. The method for forming the chromate film may be any of a coating type, a reaction type and an electrolytic type, but a coating type chromate film having particularly excellent corrosion resistance is preferred.

In the chromate film, besides silica which can be arbitrarily added, other additives conventionally added to the chromate film can be blended. Examples of such additives include silane coupling agents, rust preventive pigments, phosphoric acid, hydrofluoric acid, and the like.

The amount of the chromate film adhered is ²⁰ to 200 mg / m ^{2 in} terms of metal Cr. Is less than 20 mg / m ^2, can not be ensured the level of corrosion resistance and low temperature chipping resistance required for automotive exterior, the adhesion amount is more than 200 mg / m ^2, the press formability and weldability deteriorate In addition, the amount of Cr eluted during the degreasing / chemical conversion treatment before coating increases. The preferred coating amount of the chromate film is 20 to 150 mg / m ² .

(4) Organic resin film Organic resin film on chromate film (clear film)
Is a barrier layer that prevents Cr from being eluted from the lower chromate film during degreasing and chemical conversion treatment of the surface-treated steel sheet, and also functions as a barrier layer that prevents invasion of corrosive substances from the outside. And a lubricating film that imparts slidability during press molding. In the present invention, a crosslinked resin film mainly composed of a dense epoxy resin having excellent corrosion resistance is used.

As the epoxy resin, any glycidyl ether type epoxy resin such as bisphenol A type, bisphenol F type, novolak type, brominated epoxy and the like, and the epoxy group and hydroxyl group in these epoxy resins can be used in drying oil fatty acids. Various modified epoxy resins such as an epoxy ester resin reacted with a carboxyl group and a urethane-modified epoxy resin obtained by reacting with an isocyanate can also be used. Further, the present invention also relates to a resin generally referred to as a polyhydroxypolyether resin obtained by polycondensing a dihydric phenol (eg, resorcinol and / or bisphenols) with an approximately equimolar amount of epihalohydrin in the presence of an alkali catalyst. Of epoxy resins.

This epoxy resin contains a small amount of
Other resins (eg, acrylic resin, polyester resin, alkyd resin, polyvinyl butyral resin, various water-soluble resins) may be blended.

In order to improve the corrosion resistance by densifying the resin film, the epoxy resin is cross-linked by a cross-linking agent during the heating and drying of the coating film to form a cross-linked resin film. As a crosslinking agent, a phenol resin, an amino resin, a polyamide, an aminopolyamide, an amine, a blocked isocyanate, an acid anhydride, etc., which are known as a curing agent for an epoxy resin, can be used. The amount of the crosslinking agent is selected so that the flexibility of the crosslinked epoxy resin film is not excessively lost.

In the present invention, the crosslinked epoxy resin film contains MCA and a polyolefin wax. To improve low-temperature chipping resistance, silica may be further contained. Other optional components can also be included in the epoxy resin film. For example,
Silane coupling agents, inorganic fillers other than silica (e.g.,
Various silicate minerals, alumina, calcium carbonate, etc.),
One or more metal chromate-based rust preventive pigments (eg, barium chromate or strontium) can be added within a range that does not adversely affect the performance of the organic resin film.

The organic resin film may be colored with an organic or inorganic coloring pigment or dye for the purpose of giving the front and back distinctiveness. For example, when applying the organic composite surface treatment of the present invention to only one side of a steel sheet (eg,
If only the outer side of the car exterior is treated)
By coloring the organic resin film, the distinction between the front and the back can be made clear at a glance, and the work by the user becomes easy.

The thickness of the organic resin film is 0.1 to 2.0 μm. If the film thickness is less than 0.1 μm, the above-mentioned effects of the organic resin film cannot be sufficiently obtained. If the film thickness exceeds 2.0 μm, the insulating property of this film is increased, the spot weldability is reduced, and there is a possibility that the chip may be damaged at the time of welding or current may not be supplied.

The organic resin film is formed by diluting an epoxy resin with an appropriate organic solvent, if necessary, to prepare a resin solution, and adding a cross-linking agent, MCA, polyolefin wax to the resin solution, if necessary. Additional components (e.g., silica sol,
(A silane coupling agent, etc.) in a prescribed amount with respect to the total weight of the non-volatile components (all components other than the solvent), dissolve or disperse them, and apply them on the chromate film by appropriate means. Performed by When this coating film is then baked to dry and crosslink the epoxy resin, the desired crosslinked organic resin film is obtained. The baking temperature is generally from 80 to 290 ° C, preferably from about 120 to 200 ° C. However, an appropriate baking temperature is determined by a crosslinking reaction temperature of a combination of a crosslinking agent and a base resin to be added. When the base steel sheet is a bake hardening (BH) steel sheet, the baking temperature is preferably set to 150 ° C. or lower so as not to impair the bake hardenability.

(5) MCA (melamine cyanuric acid adduct) MCA is contained in the organic resin film. MCA is a fine white powder crystal having an average primary particle size of about 1 to 2 μm, and is considered to have the chemical structure shown in FIG. It has been known that this compound has lubricating properties and is useful as a solid lubricant, but it has not been known that it is effective for improving electrodeposition coating properties.

When MCA is contained in the organic resin film,
The resulting organic composite surface-treated steel sheet, while maintaining good corrosion resistance and spot weldability, has significantly improved cationic electrodeposition paintability compared to the same type of conventional surface-treated steel sheet, and was electrodeposited on bare plated steel sheet It has been found that electrodeposition coating can be performed with almost the same thickness as in the case. The reason is considered as follows.

MCA has a very strong hydrophilicity, facilitates the penetration of the electrodeposition paint into the organic resin film, and can exert the effect of reducing the electric resistance of the organic resin film during the electrodeposition coating. Moreover, since MCA is present in the form of fine particles dispersed in the organic resin film, the electrodeposition paint is applied to the resin film at the time of electrodeposition coating compared to a hydrophilic resin soluble in the organic resin paint such as hydrophilic polyamide. The effect of infiltrating into the electrodeposition is remarkably large, and more energizing paths can be secured, so that the electrodeposition coating property is remarkably improved.

As a result, even if there is a part where the organic resin film or the chromate film is cut off, such as a part for repairing a press flaw, the thickness is uniform to such an extent that a difference from this part is not recognized. Thus, an electrodeposited coating film is formed, and it is possible to obtain a sufficiently satisfactory paint appearance for use on the exterior of an automobile.

Moreover, the lubricity inherent in the MCA can significantly improve the slidability (press formability) during press forming of the organic resin film. In the present invention, MC
By adding a polyolefin wax to the organic resin film in addition to A, an organic composite surface-treated steel sheet that can be easily press-formed can be obtained regardless of whether the base material is a bright steel sheet that cannot be press-formed as it is. It is done.

The content of MCA in the organic resin film is in the range of 0.1 to 20% by weight based on the weight of the dried film. If the compounding amount of MCA is less than 0.1% by weight, the desired effect of improving the electrodeposition coating property and the sliding property cannot be obtained, and if it exceeds 20% by weight, the dispersion stability of MCA in the resin solution used for coating is not obtained. This makes it difficult to distribute MCA uniformly in the film, and is economically disadvantageous. The preferred amount of MCA is 0.5 to 15% by weight.

(6) Polyolefin Wax Since the base steel sheet is a bright steel sheet, the organic resin film has M
The slidability required for press forming cannot be ensured in the obtained organic composite surface-treated steel sheet only by adding CA. Therefore, in order to further improve the slidability during press molding, in addition to MCA, a polyolefin wax (eg,
Polyethylene wax) is also incorporated into the organic resin film.

The amount of the polyolefin wax in the organic resin film is in the range of 1 to 20% by weight, preferably 1 to 15% by weight, based on the weight of the dry film. 1% by weight
When the amount is less than the above, it is difficult to press-form the surface-treated steel sheet even when used in combination with MCA, and when the amount exceeds 20% by weight, the adhesion of the electrodeposition coating film decreases.

(7) Silica As described above, in order to improve the low-temperature chipping resistance of the surface-treated steel sheet of the present invention having a zinc-based plated steel sheet as a base material, one of a chromate film and an organic resin film thereon is provided. Both contain silica. The mechanism by which the low-temperature chipping resistance of the organic composite surface-treated steel sheet obtained by containing silica is remarkably improved is not necessarily clear,
It is considered that the presence or absence of silica particles causes some dispersion and absorption of impact energy.

Silica in Chromate Film The silica to be mixed in the chromate film may be either silica sol (aqueous colloidal silica) or fumed silica (gas phase colloidal silica). The silica used preferably has an average primary particle size of 5 to 50 μm.

When silica is compounded in the chromate film,
The silica content in the coating is 0.05 to 4.0 in terms of SiO ₂ / Cr weight ratio,
Preferably, the amount is 0.1 to 3.0. If this ratio is less than 0.05, the effect of improving the low-temperature chipping resistance is not sufficient,
On the other hand, if it exceeds 4.0, the insulating property of the chromate film becomes large, and the spot weldability is reduced (tip damage at welding and current supply failure are likely to occur).

Silica in Organic Resin Film When silica is blended in the organic resin film, both silica sol and fumed silica can be used, but it is particularly preferable to use silica sol having excellent weldability and corrosion resistance. Preferred silica sols are those in which the average primary particle size of the silica particles is in the range of 2-20 μm. When silica is blended in the organic resin film, the silica is contained so that the content of silica (SiO ₂ ) in the dried film is 5 to 40% by weight, preferably 5 to 30% by weight. If the amount is less than 5% by weight, the desired effect of improving the low-temperature chipping resistance cannot be obtained, while if it exceeds 40% by weight, the spot weldability is reduced as described above.

(8) Silane Coupling Agent As described above, one or both of the chromate film and the organic resin film may contain a silane coupling agent as an optional additive. When a silane coupling agent is added, the adhesion between the silica particles in the coating and the coating component is increased. As a result, the adhesion between the plating / chromate / organic resin coatings and the adhesion with the electrodeposition coating are improved. Even better results can be obtained in improving low-temperature chipping resistance. Therefore, when silica is contained in the chromate film and / or the organic resin film, a silane coupling agent may be added together.

The amount of the silane coupling agent is preferably at least 1 × 10 ⁻⁵ times the weight of the silica contained in the chromate film or the organic resin film. The type of the silane coupling agent may be appropriately selected from existing ones.

[0045]

Next, the present invention will be illustrated by way of examples. In Examples,% is% by weight unless otherwise specified.

Example 1 Two types of cold-rolled steel sheets (sheet thickness 0.8 mm) with Ra = 0.14 μm and Ra = 0.8 μm on both sides, and Zn-13% Ni on both sides.
Alloy electroplating (sulfate bath) was applied. The basis weight of this plating film was 30 g / m ² per one side.

A coating type chromate solution containing gas-phase silica (average primary particle size: 15 μm) in a weight ratio of SiO ₂ / Cr = 1 on the plating film on both surfaces was chromated at 60 mg / m ^{2 in} terms of metal Cr. The coating was applied with a bar coater so that a film was formed, and heated for 60 seconds at a plate temperature of 150 ° C. to form a chromate film.

A coating solution containing a urethane-modified epoxy resin as a main component and a blocked isocyanate as a cross-linking agent is applied on the chromate film with a bar coater while changing the film thickness. After heating for 2 seconds, the coating film was dried to crosslink the resin, and an organic composite surface-treated steel sheet was obtained. In addition to the resin and the crosslinking agent, the coating solution used contained 4% of MCA or 20% of the total weight of the nonvolatile components.
% Of a hydrophilic polyamide resin and 10% of a polyethylene wax, and were sufficiently stirred and dispersed before use.

The cationic electrodeposition coatability of each of the obtained organic composite surface-treated steel sheets was tested as follows. Cut out a 150 mm × 70 mm test piece from each test steel plate, and halve one side
Only (75 mm × 70 mm) was polished with No. 600 abrasive paper to remove the resin film and the chromate film, exposing the plated surface. Then, after applying zinc phosphate treatment to the entire surface as a coating base, using a commercially available cationic electrodeposition paint under the condition that an electrodeposition coating film having a film thickness of 18 μm is formed on the polished portion. The electrodeposition coating was performed, and the thickness of the electrodeposition coating film in the non-polishing portion when the electrodeposition coating film having a thickness of 18 μm was formed in the polishing portion was measured. The smaller the difference in the thickness of the electrodeposition coating film between the polished portion and the non-polished portion, the better the electrodeposition coating property is evaluated.

FIG. 2 shows the test results as a relationship between the thickness of the organic resin film and the thickness of the electrodeposited film on the non-polished portion. As can be seen from this figure, when a surface-treated steel sheet containing a hydrophilic polyamide resin contained in an organic resin film, which is proposed in Japanese Patent Publication No. Hei 3-32638, is electrodeposited, a polished portion and a non-polished portion are obtained. The electrodeposited film thickness difference was larger, and the electrodeposited film thickness difference became larger as the film thickness of the organic resin film was larger. Therefore, the electrodeposited film becomes thinner at a care portion such as a press flaw corresponding to the polished portion, resulting in uneven appearance after the electrodeposition coating.

On the other hand, when MCA is contained in the organic resin film according to the present invention, regardless of the film thickness of the organic resin film, the non-polished portion can always be electrodeposited with the same thickness as the polished portion. It was excellent. This means that appearance unevenness does not occur after electrodeposition coating even if there is a care for press flaws.

The difference in Ra of the base steel sheet does not affect the electrodeposition coating property, and the Ra of the base steel sheet used is 0.14 μm and 0.1 mm.
Approximately the same results were obtained with any of the 8 μm.
FIG. 2 shows the case where Ra is 0.14 μm.

Further, a test steel sheet having an organic resin film containing MCA having a thickness of 0.5 μm was subjected to an intermediate coating (35 μm thick) using a commercially available automotive paint after the electrodeposition coating.
m) and a top coat (film thickness 35 μm) were applied, and the paint clarity of the unpolished portion was measured by a portable clarity gloss meter (PGD IV type). As a result, the PGD value when the steel sheet having Ra = 0.14 μm was used as the base material was 0.9, while the Ra = 0.8
The PGD value when using a μm steel sheet as the base material was 0.7.
Apparently, when a steel sheet having a small Ra was used as the base material, the sharpness after coating was significantly improved.

Example 2 An organic composite surface-treated steel sheet was produced in the same manner as in Example 1 except that a cold rolled steel sheet having Ra = 0.14 μm was used as a base material. However, the organic resin film contains MCA in various ratios, and the film thickness is
It was kept constant at 1.0 μm. The electrodeposition coating properties of the obtained organic composite surface-treated steel sheet were tested in the same manner as in Example 1, and the results were compared with the MCA content in the organic resin film (the amount of MCA added to the total amount of nonvolatile components in the coating solution). FIG. 3 shows the relationship between the electrodeposited film thicknesses in the above.

From the results shown in FIG. 3, when MCA is contained in the organic resin film in an amount of 0.1% or more, it becomes possible to perform electrodeposition coating with the same film thickness as that of the polished portion. It turned out that it became possible to demonstrate.

Example 3 An organic composite surface-treated steel sheet was produced in the same manner as in Example 1 using a cold-rolled steel sheet (sheet thickness 0.7 mm) having Ra = 0.14 μm on both surfaces as a base material. The base steel plate used was TS = 300 N / mm ² , YP = 130
It had tensile properties of N / mm ² and El = 47%. Zn-13% Ni
The basis weight of the alloy plating film was set to 20 g / m ² per side, and a chromate film containing silica at a weight ratio of SiO ₂ / Cr = 1 was deposited on the alloy plating film at an amount of 50 mg / m ^{2 in} terms of metal Cr. It was formed so that it might become. The uppermost organic resin film layer was formed by changing the content of MCA and polyethylene wax (the amount added to the total amount of nonvolatile components in the coating solution).

The press formability of the obtained organic composite surface-treated steel sheet was evaluated by the presence or absence of cracks in a cylindrical drawing test with a drawing ratio of 2.0. In addition, in order to evaluate the secondary adhesion of the electrodeposition coating film, a test piece (unpolished) of each surface-treated steel sheet was treated with zinc phosphate, and then treated with a base material of Zn-13% Ni alloy-plated steel sheet. Cationic electrodeposition coating was performed under the condition that an electrodeposition coating film having a film thickness of 20 μm was formed. After immersing the electrodeposition coated test piece in warm water of 50 ° C. for 240 hours, a rubbish test (1 mm interval × 100
Mass, cellophane tape peeling), and the secondary adhesion was evaluated based on the residual ratio of the coating film.

The test results are shown in Table 1 below together with the contents of MCA and wax in the organic resin film. As can be seen from the test results, according to the present invention, the organic resin film
When both CA 0.1% or more and polyethylene wax 1% or more were contained, good press-forming without cracks was possible. However, if at least one of these additional components is missing, cracks occur during press-forming. Occurred. That is, when a bright steel plate is used as the base material, it is understood that the organic resin film needs to contain both MCA and wax. However, when the wax content exceeded the upper limit of 20%, the press-formability was ensured, but the adhesion of the electrodeposition coating film was reduced.

[0059]

[Table 1]

Example 4 For a cold-rolled steel sheet (sheet thickness 0.8 mm) with Ra = 0.18 μm on both sides, Zn-13% Ni alloy plating or Zn-9% Fe alloy plating with the basis weight shown in Table 2 on both sides Was applied by an electroplating method using a sulfate bath. Then, in the same manner as in Example 1,
A coating type chromate film and an organic resin film containing an epoxy resin as a main component were formed on the plating film to produce an organic composite surface-treated steel sheet. The adhesion amount of the chromate film (in terms of Cr) and the thickness of the resin film were changed as shown in Table 2.

To the used chromate solution, one or both of colloidal silica (average primary particle size of 10 μm) and a silane coupling agent (γ-crysidoxypropyltrimethoxysilane) were added as the case may be. Addition amount of silica (SiO
₂ / Cr weight ratio) and the amount of silane coupling agent added (vs. SiO
₂ are shown in Table 2.

The coating liquid used for forming the organic resin film is as follows:
A coating material containing a bisphenol A-based epoxy resin and a blocked isocyanate as a cross-linking agent, in which 5% of MCA and 3% of polyethylene wax are dispersed with respect to the total amount of non-volatile components. One or both of the same silane coupling agent as described above were blended. The amount of SiO ₂ added to this coating solution
(% Based on nonvolatile content) and the amount of silane coupling agent added
(Weight ratio to SiO ₂ ) is as shown in Table 2.

The low-temperature chipping resistance, Cr elution amount, external rust resistance, and spot weldability of the obtained organic composite surface-treated steel sheet were examined by the following test methods. The test results are also shown in Table 2.

(Low-temperature chipping resistance)
After coating with 2 coats and 2 bake coats of 40 μm and an automotive top coat, the mixture was cooled to −20 ° C. and subjected to impact of pebbles with a gravure tester, and this portion was peeled off with cellophane tape. The peel diameter at the plating / steel plate interface after peeling was measured.

(Cr elution amount) A test piece was treated with an alkaline degreasing solution at 40 ° C.
(Nihon Parkerizing Co., Ltd. FC4410) was immersed for 2 minutes to perform degreasing, and the amount of Cr adhering to the test piece before and after that was measured by X-ray fluorescence to calculate the Cr elution amount.

(Outer surface rust resistance) A test piece was coated with 2 coats and 2 bake coats of 30 µm of electrodeposition coating and 40 µm of automotive top coat, and then a cross-cut flaw having a depth reaching the base steel plate was made with a cutter knife. 3 outdoor exposures, including twice weekly salting
After a month, the occurrence of red rust from the cut portion was visually determined.

(Spot Weldability) Two single test pieces of the same organic composite surface-treated steel sheet were overlapped, and an AC single spot welder (50 mm) having a Cu-Cr alloy electrode tip having a tip diameter of 6 mm was used.
Hz), welding current 10 kA, squeeze time 20 cycles, energization time 12 cycles, holding 5 cycles, pressure 200
A spot welding continuous spot test was conducted under the condition of Kg. The spot weldability was evaluated based on the number of points at which welding peeling occurred due to shear tension.

[0068]

[Table 2]

As can be seen from Table 2, the organic composite surface-treated steel sheet according to the present invention was excellent in all the performances tested. On the other hand, in the comparative examples, the amount of the attached chromate film or the thickness of the organic resin film is out of the range of the present invention, both the chromate film and the organic resin film do not contain silica, or the silica An example in which the content is out of the range of the present invention is shown. In these comparative examples, at least one of low-temperature chipping resistance, Cr elution amount, outer surface rust resistance, and spot weldability was inferior, and an organic composite surface-treated steel sheet satisfying all the performances could not be obtained. In addition, the electrodeposition coating properties of the organic composite surface-treated steel sheets produced in this example were all good.

[0070]

As described in detail above, the organic composite surface-treated steel sheet of the present invention is ideal for improving the sharpness, but it is difficult to press-form, so that Ra which has not been used before has been used. ≤0.3 μm bright steel sheet as a base material, and the uppermost organic resin film contains both MCA and polyolefin wax, so that the film slides to the extent that good press forming is possible even with a bright steel sheet as the base material. It has succeeded in improving the character. As a result, it has become possible to achieve excellent post-painting clarity superior to the conventional level, which is derived from a bright steel plate.

Further, due to the action of the MCA present in the organic resin film made of the crosslinked epoxy resin and the silica contained in at least one of the resin film and the lower chromate film, excellent corrosion resistance and spot weldability are maintained. In addition, an organic composite surface-treated steel sheet for automobiles is obtained, in which the cationic electrodeposition coating property and the low-temperature chipping resistance are remarkably improved, and all the performances required for both the inside and outside of the automobile exterior steel sheet can be satisfied. Therefore, when an automobile body is manufactured from the surface-treated steel sheet of the present invention, a remarkable improvement in the appearance and durability of the automobile can be expected, and an extremely useful effect in industry is brought.

[Brief description of the drawings]

FIG. 1 shows the chemical structure of MCA (melamine cyanuric acid adduct).

FIG. 2 is a graph showing the electrodeposition coatability of an organic composite surface-treated steel sheet having an organic resin film containing a hydrophilic polyamide or MCA as a relationship between the thickness of the organic resin film and the electrodeposition film thickness of a non-polished portion. .

FIG. 3 shows the electrodeposition coatability of an organic composite surface-treated steel sheet having an organic resin film containing MCA, and the MC in the organic resin film.
4 is a graph showing the relationship between the A content and the electrodeposition film thickness of a non-polished portion.

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Claims (1)

(57) [Claims] At least one side of a steel plate has a Ra of 0.3 or more.
μm or less surface roughness, zinc or zinc alloy plating layer on this surface in order from the bottom, chromate film of 20 to 200 mg / m ^{2 in} terms of metal Cr, 0.1 to 20% by weight of melamine cyanuric acid adduct And 1
An organic resin film having a thickness of 0.1 to 2.0 μm comprising a crosslinked epoxy resin containing 2020% by weight of a polyolefin wax, and further having a chromate film for converting silica to SiO ₂ /
Cr content in an amount of 0.05 to 4.0 by weight, and / or
Or an organic resin film is characterized in that it contains in the amount of SiO ₂ of 5 to 40 wt% of silica, surface treated steel sheet for automobiles.