JPH06220648A - Organic composite steel plate excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce the org. composite steel plate having a chromate film as the first layer on the surface of a zinc plated steel plate and the urethane modified epoxy resin containing alpha-Al2O3/SiO2 as a rust preventive pigment on the first layer. CONSTITUTION:The coating composition described subsequently is formed as the second layer in 0.2-3.0mum thickness as a solid film on the steel plate having a 5-150mg/m<2> chromate film as the first layer on the surface of the zinc plated steel plate. (A) The urethane modified epoxy resin content is >=30wt.% of the solid content of the coating. (B) An equivalent ratio of the acrylic compd. containing an oxazoline ring per the epoxy resin is (1/0.8) to (1/1.2). (C) the alpha-Al2O3/SiO2 (<=8mum average grain size) as the rust preventive pigment is 10-40wt.% of the solid content of the coating. In this way, the steel plate remarkably improved in the corrosion resistance is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic composite steel sheet coated with a thin film, and more specifically to an organic composite steel sheet having excellent corrosion resistance as a steel sheet for automobiles.

[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, an organic composite steel sheet whose surface is coated with a thin organic resin to improve corrosion resistance has been developed and put into practical use.

For example, there is Japanese Examined Patent Publication No. 64-11830. The present invention provides a refractory chromate film having a water-soluble content of 5% or less as the first layer of a steel plate plated with zinc, aluminum or zinc-based composite alloy.
˜150 mg / m ² and further coated with a coating composition consisting of the following proportions as a second layer on the upper layer thereof in a thin film of 0.3 to 5 μm for press workability, weldability, The present invention relates to an organic composite steel sheet having excellent electrodeposition coating properties and corrosion resistance. (A) a bisphenol type epoxy resin having a number average molecular weight of 300 to 100,000 in a coating solid content of 30% by weight or more,
(B) At least one curing agent selected from the group consisting of polyisocyanate compounds and blocked polyisocyanate compounds is used in a weight ratio to the epoxy resin solid content of 1 /
10 to 20/10, (C) average particle diameter 0.1 to 100 mμ
5 to 50% by weight of fumed silica in the coating solid content,
(D) A coating composition containing a ketone organic solvent in an amount of 40% by weight or more based on the weight of the coating and having a coating solid content of 10 to 50% by weight. However, it is difficult to balance the corrosion resistance with cationic electrodeposition, press formability, and spot weldability, and if these performances are prioritized, the corrosion resistance will decrease. Conversely, if priority is given to corrosion resistance, other performances will deteriorate. .

[0004]

DISCLOSURE OF THE INVENTION The present invention is intended to improve the corrosion resistance of an organic composite steel sheet without degrading the performances such as cationic electrodeposition, press formability and spot weldability.

[0005]

In order to solve the above problems, the present invention relates to a steel sheet having a chromate film of 5 to 150 mg / m ² as a first layer on the surface of a zinc-based plating, and a coating composition shown below as a second layer. 0.2 to 3.0 as a solid film
The invention has led to the invention of an organic composite steel sheet which is characterized by being formed in a thickness of μm. (A) 30% by weight or more of urethane-modified epoxy resin in the coating solid content (B) Equivalent ratio of the oxazoline ring-containing acrylic compound to the epoxy resin is 1 / 0.8 to 1 / 1.2 (C) Anticorrosion pigment Α-Al ₂ O ₃ / SiO ₂ (average particle size 8 mμ or less) is 10 to 40% by weight in the solid content of the coating as shown in FIG. Zn, Zn-12% Ni, Zn-12% Ni as a Zn-based plating film 2 on the steel plate 1
-1% Co, Zn-15% Fe, Zn-13% Cr, Z
n-9% Cr-2% Ni or the like 5~100g / m ^2, 0.2~ urethane-modified epoxy resin chromate film 3 thereon 5 to 150 mg / m ^2, as a further organic film 4
Adhere to 3.0 μm.

[0006]

ACTION AND EXAMPLES The action and range of action of the constituents of the present invention will be described below from experimental results. (1) Chromate film The chromate film used in the present invention is located between the base plating layer and the organic film and improves the adhesion between the plating film and the organic film. As a result, it is important for ensuring the corrosion resistance of the organic composite steel sheet. Plays a role. 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 will be insufficient with the upper organic film, or the effect of improving the corrosion resistance will not 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. Occurs. From the above, deposition amount of the chromate film must be in the range of ^{5mg / m 2 ~150mg / m 2} , more preferably in the range of the total amount of chromium, 1
It is 5 to 100 mg / m ² .

(2) 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.

[0008]

[Chemical 1]

FIG. 2 shows the effect 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 modification amount of this base epoxy resin with urethane was defined as 60 to 90% from FIG. FIG. 3 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 content of the base resin must be 30% by weight or more based on the solid content of the coating, and if it is 30% by weight or less, the binder action of the base resin on the rust preventive pigment is lowered, making it difficult to form a coating material, and at the same time coating The film is fragile and 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 single time and forms a dense organic film, it suppresses elution of Cr ⁶⁺ from the underlying chromate film as shown in FIG. Fig. 5 shows the curing time of the base resin as the curing agent species and the elution of Cr ⁶⁺ after film formation (film forming property).
It shows the effect on. 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 amount of Cr ⁶⁺ eluted. 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. 6, the compounding amount of the oxazoline ring-containing acrylic compound is an equivalent ratio of 1 to the urethane-modified epoxy resin of the base resin.
/0.8 to 1 / 1.2 is preferable. FIG. 6 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 / 0.6 to 1 /
1.2 shows good corrosion resistance. Next, in the present invention, α-Al ₂ O ₃ / SiO ₂ having an average particle size of 8 mμ is used as a rust preventive pigment in order to impart high corrosion resistance without elution of harmful substances in the degreasing / chemical conversion treatment bath. 10 minutes
-40% by weight is added.

FIG. 7 shows the effect of the difference in the structure and amount of the rust preventive pigment on the corrosion resistance of the organic composite steel sheet.
The horizontal axis represents the amount of Al ₂ O ₃ / SiO ₂ added, and the vertical axis represents the area of red rust after 300 cycles of the cycle corrosion test. In a system in which α-Al ₂ O ₃ / 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,
If it exceeds 4%, the coating film after film formation becomes brittle or the spot weldability deteriorates, which is not preferable. Next, Al ₂
O _3, α-type from was changed to γ-type γ-Al ₂ O ₃ / SiO
_This is a system in which ₂ (colloidal silica) is added as a rust preventive pigment, but even if it is added up to 25%, red rust generation is 50% or more, and the corrosion resistance is insufficient.

Further, SiO₂From colloidal silica
Α-Al converted to fumed silica ₂O₃/ SiO
₂(Fumed silica) added as a rust preventive pigment
However, even if it is added up to 25%, the occurrence of red rust is 75% or more.
And the corrosion resistance is insufficient. Fig. 8 shows α-Al₂O₃And S
iO₂Of the effect of the mixed amount of Cu on the corrosion resistance of the organic composite steel sheet
It is shown. Α-Al on the horizontal axis₂O₃And SiO₂of
Molar mixture ratio, vertical axis shows cycle corrosion test 300 cycles
The red rust generation area is taken later. α-Al₂O₃And Si
O₂Shows the best corrosion resistance with a molar ratio of 3: 2
It Figure 9 shows α-Al₂O₃/ SiO₂Has an average particle size of organic
It shows the effect on the corrosion resistance of the composite steel sheet. side
Α-Al on the shaft₂O₃/ SiO₂The average particle size of
The red rust occurrence area after 300 cycles of the crekle corrosion test
ing. α-Al₂O₃/ SiO₂Has an average particle size of 8 mμ
Shows the best corrosion resistance.

From the viewpoint of press formability of the rustproof steel sheet, polyethylene colloid may be used in the coating composition of the present invention in an amount of 0.1 to 10% by weight based on the solid content of the coating. Figure 10
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,
On the contrary, the amount of the polyethylene colloid is preferably in the range of 0.1 to 10% by weight because it leads to deterioration of other properties such as corrosion resistance.

[0015]

EFFECT OF THE INVENTION The organic composite steel sheet of the present invention as described above has corrosion resistance, which has been a problem in terms of quality in the conventional organic composite steel sheet, without deterioration in performance such as press formability, continuous spot weldability, and electrodeposition. It is a significantly improved epoch-making organic composite steel sheet that fully meets the needs of the market.

[Brief description of drawings]

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

FIG. 2 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. 3 is a diagram showing the influence of the amount of urethane modification of the base epoxy resin on the corrosion resistance of the organic composite steel sheet,

FIG. 4 is a view showing a structure of an oxazoline ring-containing acrylic compound which is a curing agent,

FIG. 5: Curing agent type is curing time of base resin, C after film formation
The figure showing the influence of r ^{6+ on} the dissolution property,

FIG. 6 is a diagram showing a relationship between a base resin / hardening agent equivalent ratio and a red rust occurrence area,

FIG. 7 is a diagram showing the influence of the difference in the structure of the anticorrosion pigment and the addition amount on the corrosion resistance of the organic composite steel sheet,

FIG. 8 is a diagram showing the influence of the mixed amount of α-Al ₂ O ₃ and SiO ₂ on the corrosion resistance of the organic composite steel sheet,

FIG. 9 is a diagram showing the influence of the average particle size of α-Al ₂ O ₃ / SiO ₂ on the corrosion resistance of an organic composite steel sheet,

FIG. 10 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 front page (72) Inventor Yoshio Shindo 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corp. Corporate Technology Development Division

Claims (2)

[Claims] 1. A steel sheet having a chromate film of 5 to 150 mg / m ² as a first layer on the surface of zinc-based plating,
The coating composition shown below as a layer forms a solid film of 0.2
An organic composite steel sheet having excellent corrosion resistance, which is characterized by being formed in a thickness of up to 3.0 μm. (A) 30% by weight or more of urethane-modified epoxy resin in the coating solid content (B) Equivalent ratio of the oxazoline ring-containing acrylic compound to the epoxy resin is 1 / 0.8 to 1 / 1.2 (C) Anticorrosion pigment As α-Al ₂ O ₃ / SiO ₂ (average particle size of 8 mμ or less) is 10 to 40 wt% in the coating solid content. 2. The organic composite steel sheet according to claim 1, wherein the coating composition contains 0.1 to 10% by weight of polyethylene colloid with respect to the coating solid content.