JPH06272059A - Production of thin-film organic composite steel sheet excellent in coating material adhesion property - Google Patents

Abstract

PURPOSE:To obtain an org. composite steel sheet excellent in coat material adhesion property by forming a chromate film on the surface of galvanized steel sheet as a first layer and forming a urethane-modified epoxy resin film contg. Sin. dispersed in water with hexametaphosphoric acid as a rust-inhibiting pigment on the chromate film. CONSTITUTION:A chromate film is formed on the surface of a galvanized steel sheet at 5-150mg/m<2> as a first layer, and a solid film of a coating composition mentioned below is formed on the first layer in 0.2-3.0mum thickness as a second layer. Namely, the composition contains >=30wt.% of a urethane-modified epoxy resin based on the solid coating material, an oxazoline ring-contg. acrylic compd. in the 1/0.8 to 1/1.2 equivalent ratio to the epoxy resin and 10-40wt.% of the SiO2 (having 8mum average grain diameter) dispersed in water with hexametaohosphoric acid based on the solid coating material as a rust-inhibiting pigment.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing an organic composite steel sheet coated with a thin film, and more specifically, it has excellent adhesion to paint after cationic electrodeposition coating / intermediate coating / top coating as a steel sheet for automobiles. The present invention provides a method for manufacturing an organic composite steel sheet.

[0002]

2. Description of the Related Art In recent years, market needs for product quality improvement, high functionality, and cost reduction have been increasing for zinc-based plated steel sheets for automobiles and home appliances, and research and development of new products in response to this Has been active recently. However, since the galvanized steel sheet relies on the corrosion resistance due to the sacrificial anticorrosion action, if the corrosion resistance is further improved, an increase in the coating adhesion amount cannot be avoided, resulting in the deterioration of press formability and spot weldability. Also,
As a corrosion-resistant steel plate with low coating weight, zinc and Ni,
Coated steel sheets and multi-layered steel sheets, which are alloyed with Co, Cr, Fe, Mn, etc., have been developed. However, it does not have sufficient corrosion resistance with respect to the hem portion and the bag structure portion which are exposed to more severe corrosion conditions in the automobile body. Under such circumstances, a thin film organic composite steel sheet whose surface is coated with a thin organic resin has been developed and put into practical use.

For example, as disclosed in Japanese Patent Publication No. 3-80874, after the surface of a steel plate plated with zinc or aluminum is chromated, the epoxy resin content is 35% or more in the total solid content and SiO ₂ / Resin ratio is adjusted to 10/90 to 60/40 and treated with an organic composite silicate solution, then 250 ° C. and 300
There is a method for producing a rust-preventive steel sheet for multi-layer coating, wherein the undercoat coating is cationic electrodeposition coating, which is characterized by performing heat treatment at a temperature of ℃ or less.

However, since an attempt is made to improve the adhesiveness of the paint only with the resin, the SiO dispersed in water by the conventional method is used.
₂ acts as a negative factor, the adhesion of the paint after applying the intermediate coating material and the top coating material on the cationic electrodeposition coating film is insufficient, and further, the secondary adhesion is very poor, and it is for automobiles. There is a problem that it cannot be used as an outer plate.

[0005]

DISCLOSURE OF THE INVENTION The present invention has realized improvement in coating adhesion when an organic composite steel sheet is used as an outer plate without degrading the quality performance required for a steel plate for automobiles.

[0006]

In order to solve the above problems, the present invention relates to a zinc-based plated steel sheet on which a chromate film of 5 to 150 mg / m ^{2 is} formed as the first layer, and hexametaphosphoric acid as the second layer. A water-soluble urethane-modified epoxy resin containing 10 to 40% by weight of SiO ₂ water-dispersed as a coating material in a solid film of 0.2 to 3.0 μm
The present invention has led to the invention of a method for producing an organic composite steel sheet having excellent paint adhesion, which is characterized by being formed.

FIG. 1 and FIG. 2 show this film structure. As shown in the figure, a Zn-based plating film 2 is formed on the surface of the steel plate 1, for example, Zn, Zn-12% Ni, Zn-12% Ni-1%.
Co, Zn-15% Fe, Zn-13% Cr, Zn-9
% Of Cr-2% Ni plated 5 to 100 g / m ^2, the chromate film 3 5 to 150 mg / m ² is formed on a urethane-modified epoxy resin film 4 having a solid coating is more organic coating thereon To form 0.2 to 3.0 μm.

[0008]

ACTION AND EXAMPLES The action and range of action of the constituents of the present invention will be described below from experimental results. (1) Manufacturing method The thin film organic composite steel sheet according to the present invention has a chromate film of 5 to 150 mg / m ² as a first layer on the surface of a zinc-based plated steel sheet.
On the produced steel sheet, SiO ₂ water-dispersed with hexametaphosphoric acid was used as the second layer in the paint as solid content
It is produced by coating a water-soluble urethane-modified epoxy resin containing 40% by weight to form a solid film having a thickness of 0.2 to 3.0 μm.

(2) Chromate film The chromate film used in the present invention is located between the undercoating layer and the organic film, improves the adhesion between the plating film and the organic film, and as a result ensures the corrosion resistance of the organic composite steel sheet. Plays an important role on. The type of chromate may be any of known electrochromate and coating type chromate, and the treatment method thereof is not particularly limited. If the total amount of chromium is less than 5 mg / m ² , the adhesion of the chromate film is not preferable because the adhesion with the upper organic film is insufficient or the effect of improving the corrosion resistance cannot be obtained. On the other hand, when the total chromium amount exceeds 150 mg / m ² , the adhesion between the plating film and the organic film is significantly reduced due to the cohesive failure of the chromate film due to press working, etc., and it also has an adverse effect on continuous spot welding during continuous spot welding. It is not preferable because it occurs. From the above, the amount of chromate film deposited is 5 mg / m ^{2 to} 150 mg
Should be in the range of / m ^2, more preferably in the range of the total chromium content is 15~100mg / m ^2.

(3) Organic Film The base resin used for forming the organic coating film of the present invention is
It is a urethane-modified epoxy resin. The chemical structure is shown in Chemical formula 1. A bisphenol A type epoxy resin-based polycarbonate having a carboxyl group is polymerized by urethane bonds, and has a structure having amine groups at both ends. If the film thickness is 0.2 μm or less, the corrosion resistance is insufficient, and if it is 3.0 μm or more, the spot weldability deteriorates. Therefore, the thickness must be in the range of 0.2 to 3.0 μm.

[0011]

[Chemical 1]

[0012]

[Chemical 2]

FIG. 3 shows the influence of the modification of the base epoxy resin on the corrosion resistance of the organic composite steel sheet. The horizontal axis shows the type of resin, and the vertical axis shows the red rust generation area after 300 cycles of the cycle corrosion test. The one with urethane-modified epoxy resin as the base resin shows the best corrosion resistance.

The amount of modification of the base epoxy resin with urethane was defined as 60 to 90% from FIG. FIG. 4 shows the effect of the urethane modification amount of the base epoxy resin on the corrosion resistance of the organic composite steel sheet. The horizontal axis shows the amount of urethane modification, and the vertical axis shows the area of red rust after 300 cycles of the cycle corrosion test. 60 to 90% showed good corrosion resistance, and 75% showed the best corrosion resistance. The blending amount of this base resin is 30% by weight in the solid content of the paint.
If the amount is 30% by weight or less, the binder action of the base resin with respect to the anticorrosive pigment is lowered, making it difficult to form a coating material, and at the same time, the coating film is brittle and the processing adhesion is insufficient.

Next, the curing agent is an oxazoline ring-containing acrylic compound. The structure is shown in FIG. Since this compound reacts with the base resin in a short time and forms a dense organic film, the elution of Cr ⁶⁺ from the underlying chromate film is suppressed as shown in FIG. FIG. 6 shows the influence of the curing agent species on the curing time of the base resin and the elution property (film forming property) of Cr ⁶⁺ after film formation. The axis of abscissa indicates the type of the curing agent, the axis of ordinate indicates the curing time, and the axis of ordinate indicates the elution amount of Cr ⁶⁺ . The acrylic compound containing an oxazoline ring is excellent in curing time and resistance to Cr ⁶⁺ elution.

Further, since this curing agent reacts with the base resin even at a low temperature of 150 ° C. or lower, there is no problem in applying it to a BH steel sheet having improved press formability. As shown in FIG. 7, the compounding amount of the oxazoline ring-containing acrylic compound is preferably 1 / 0.8 to 1 / 1.2 with respect to the urethane modified epoxy resin of the base resin. FIG. 8 shows the effect of the mixing ratio of the base resin and the curing agent on the corrosion resistance of the organic coated steel sheet. The horizontal axis represents the mixing equivalent ratio of the base resin and the curing agent, and the vertical axis represents the red rust generation area after 300 cycles of the cycle corrosion test. Equivalent ratio 1 / 0.8 to 1 /
1.2 shows good corrosion resistance.

Next, in the present invention, SiO ₂ having an average particle size of 8 mμ dispersed with hexametaphosphoric acid is used as a rust preventive pigment in order to impart high corrosion resistance without leaching harmful substances in the degreasing / chemical conversion treatment bath. It is added in an amount of 10 to 40% by weight in the coating solid content. FIG. 8 shows the method of dispersing SiO ₂ and ED
It shows the relationship of paint adhesion after coating (25 μm) / intermediate coating (40 μm) / top coating (35 μm) and dipping in an aqueous solution at 40 ° C. for 10 days. The horizontal axis represents the dispersion method, and the vertical axis represents the 2 mm cut edge pattern test (10
The number of peeled coating films after 0) is taken. Those using SiO ₂ dispersed with hexametaphosphoric acid did not peel off the coating film and showed good results. This is because hexametaphosphoric acid neutralizes the alkaline component generated at the time of cation electrodeposition, which is the cause of the decrease in paint adhesion.

FIG. 9 shows the effect of the difference in the structure and addition amount of SiO ₂ used as a rust preventive pigment on the corrosion resistance of the organic composite steel sheet. The horizontal axis shows the amount of SiO ₂ added, and the vertical axis shows the area of red rust after 300 cycles of the cycle corrosion test. In the system in which SiO ₂ (colloidal silica) was added as a rust preventive pigment, red rust generation was 0 at an addition amount of 10% or more, and good corrosion resistance is exhibited. However, 40%
If it exceeds, the coating film after film formation becomes brittle or the spot weldability deteriorates, which is not preferable. However, in the system in which the rust preventive pigment is changed to SiO ₂ (fumed silica),
Even if added up to 25%, the generation of red rust is 80% or more, and the corrosion resistance is insufficient.

FIG. 10 shows the effect of the average particle size of SiO ₂ on the corrosion resistance of the organic composite steel sheet. S on the horizontal axis
The average particle size of iO ₂ is plotted on the vertical axis, and the area where red rust occurs after 300 cycles of the cycle corrosion test. The average particle size of SiO ₂ is 8 mμ, which shows the best corrosion resistance. From the viewpoint of press formability of anticorrosion steel sheet, polyethylene colloid is contained in the coating composition of the present invention in an amount of 0.1% based on the coating solid content.
It may be used in an amount of 10 to 10% by weight.

FIG. 11 shows the effect of the amount of polyethylene colloid on the cylindrical press workability. The horizontal axis shows the amount of polyethylene colloid added, and the vertical axis shows the weight reduction during cylindrical pressing. It can be seen that the addition reduces powdering during processing and reduces weight loss due to galling, and improves press workability. However, addition of 10% or more does not improve the effect, but rather deteriorates other properties such as corrosion resistance. Therefore, the amount of polyethylene colloid is 0.1 to 1
A range of 0% by weight is preferred.

[0021]

EFFECTS OF THE INVENTION The present invention, which has been made as described above, significantly improves the adhesion of the paint after cationic electrodeposition / intermediate coating / top coating, which is problematic when an organic surface is used as an outer plate, without deteriorating other performances. It is an improved epoch-making method of manufacturing an organic composite steel sheet, which sufficiently meets the needs of the market.

[Brief description of drawings]

FIG. 1 is a diagram showing a single-sided film structure according to the present invention,

FIG. 2 is a diagram showing a double-sided coating structure according to the present invention,

FIG. 3 is a diagram showing the influence of the modification of the base epoxy resin on the corrosion resistance of the organic composite steel sheet,

FIG. 4 is a diagram showing the effect of the amount of urethane modification of the base epoxy resin on the corrosion resistance of the organic composite steel sheet,

FIG. 5 is a view showing a structure of an oxazoline ring-containing acrylic compound,

[FIG. 6] Curing agent type is the curing time of the base resin, C after film formation
A figure showing the influence of r ^{6+ on} the elution property,

FIG. 7 is a diagram showing a relationship between an equivalent ratio and a red rust occurrence area with respect to a urethane-modified epoxy resin of a base resin,

FIG. 8 is a diagram showing the effect of the mixing ratio of the base resin and the curing agent on the corrosion resistance of the organic coated steel sheet,

FIG. 9 is a diagram showing the influence of the difference in the structure and the amount of SiO ₂ used as a rust preventive pigment on the corrosion resistance of the organic composite steel sheet,

FIG. 10 is a diagram showing the influence of the average particle diameter of SiO ₂ on the corrosion resistance of an organic composite steel sheet,

FIG. 11 is a view showing the influence of the amount of polyethylene colloid on the cylindrical press workability.

[Explanation of symbols]

 1 Steel plate 2 Zn-based plating film 3 Chromate film 4 Organic film

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location C25D 13/20 C (72) Inventor Fumio Yamazaki 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Kimitsu Works

Claims (2)

[Claims] Claim: What is claimed is: 1. A zinc-plated steel sheet having a chromate film as a first layer formed on the surface thereof in an amount of 5 to 150 mg / m < ² >.
SiO 2 water-dispersed with hexametaphosphoric acid as the second layer
Water-soluble urethane-modified epoxy resin containing ₂ to 10% by weight as a coating solid content of 0.2 to
A method for producing an organic composite steel sheet having excellent paint adhesion, which is characterized in that it is formed to have a thickness of 3.0 μm. 2. The method for producing an organic composite steel sheet according to claim 1, wherein the coating composition contains polyethylene colloid in an amount of 0.1 to 10% by weight based on the coating solid content.