JP3068976B2 - Organic composite steel sheet with excellent corrosion resistance and cationic electrodeposition properties - Google Patents

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 particularly to an organic composite steel sheet having excellent corrosion resistance and cationic electrodeposition as a steel sheet for automobiles.

[0002]

2. Description of the Related Art In recent years, there has been an increasing market need for zinc-coated steel sheets for automobiles and home appliances for higher quality, higher functionality, and lower cost, and research and development of new products in response thereto. Has been popular recently. However, since galvanized steel sheets rely on corrosion resistance due to sacrificial corrosion protection, if corrosion resistance is further improved, an increase in the amount of plating applied is unavoidable, and as a result, press formability and spot weldability deteriorate. Also,
Zinc and Ni, as corrosion-resistant steel sheets with low coating weight
Coated steel sheets and multi-layer coated steel sheets in which Co, Cr, Fe, Mn and the like are alloyed have been developed. However, it does not have sufficient corrosion resistance to a hem portion or a bag structure portion exposed to more severe corrosion conditions in an automobile body. Under such circumstances, an organic composite steel sheet having improved corrosion resistance by coating the surface with a thin organic resin has been developed and put into practical use.

For example, there is Japanese Patent Publication No. 64-11830. According to the present invention, as a first layer of a steel sheet plated with zinc, aluminum, or a zinc-based composite alloy, a hardly soluble chromate film having a water-soluble content of 5% or less is used.
To 150 mg / m ² was formed, further press formability, characterized in that formed by 0.3~5μm thin coating paint composition comprising the proportion below as a second layer thereon as a solid coating, 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 a polyisocyanate compound and a blocked polyisocyanate compound in a weight ratio of 1 /
10-20 / 10, (C) average particle size 0.1-100 mμ
5 to 50% by weight of the fumed silica in the paint solids,
(D) A coating composition containing a ketone-based organic solvent in an amount of 40% by weight or more based on the weight of the coating material and a solid content of the coating material of 10 to 50% by weight. However, corrosion resistance, cationic electrodeposition and press moldability,
It is difficult to balance the spot weldability, and if these performances are prioritized, the corrosion resistance is reduced. Conversely, if the corrosion resistance is prioritized, other performances are reduced.

[0004]

DISCLOSURE OF THE INVENTION The present invention realizes an improvement in corrosion resistance and cationic electrodeposition of an organic composite steel sheet without deteriorating performance such as press formability and spot weldability.

[0005]

In order to solve the above-mentioned problems, the present invention relates to a steel sheet having a chromate film as a first layer on a zinc-based plating surface having a thickness of 5 to 150 mg / m ² , and a coating composition shown below as a second layer. 0.2 to 3.0 as a solid film
The present invention has led to the invention of an organic composite steel sheet characterized by being formed in μm. (A) The urethane-modified epoxy resin is 30% by weight or more in the solid content of the coating composition. (B) The oxazoline ring-containing acrylic compound is 1 / 0.8 to 1 / 1.2 in equivalent ratio to the epoxy resin. SiO ₂ (colloidal silica, the average particle diameter 8mμ below) paint solid content 10 to 40 wt% (D) a melamine cyanurate paint solids in a 0.5 to 1
0% by weight

FIG. 1 shows the structure of this film. As shown in FIG. 1, a Zn-based plating film 2 is formed on the surface of a steel sheet 1, for example, Zn, Zn-12% Ni, Zn-12% Ni-1% C.
o, Zn-15% Fe, Zn-13% Cr, Zn-9%
Plated 5 to 100 g / m ² of Cr-2% Ni, thereon a chromate film 3 5 to 150 mg / m ² is formed, and further 0.2 to urethane-modified epoxy resin 4 is an organic coating thereon Apply 3.0 μm.

[0007]

Functions and Examples The effects and the range of action of the constituent factors of the present invention will be described below based on 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 an important role. The type of chromate may be any of the known electrolytic chromate and coating type chromate, and the treatment method is not particularly limited. If the amount of the chromate film adhered is less than 5 mg / m ^{2 in} terms of the total amount of chromium, the adhesion to the upper organic film will be insufficient, or the effect of improving the corrosion resistance will not be obtained. On the other hand, when the total chromium content exceeds 150 mg / m ² , the adhesion between the plating film and the organic film is significantly reduced due to cohesive failure of the chromate film due to press working and the like, and adverse effects are also exerted on continuous spotting 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
5 to 100 mg / m ² .

(2) Organic film The base resin used for forming the organic film of the present invention is:
It is a urethane-modified epoxy resin. The molecular structure is shown in Chemical formula 1. It has a structure in which a bisphenol A-type epoxy resin-based polycarbonate having a carboxyl group is polymerized with a urethane bond, and has amine groups at both ends. When the film thickness is 0.2 μm or less, the corrosion resistance is insufficient, and when the film thickness is 3.0 μm or more, the spot weldability decreases.

[0009]

Embedded image

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 indicates the type of resin, and the vertical axis indicates the area where red rust occurs after 300 cycles of the cyclic corrosion test. Those using a urethane-modified epoxy resin as a base resin show the most excellent corrosion resistance. The amount of modification 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 indicates the amount of urethane modification, and the vertical axis indicates the red rust generation area after 300 cycles of the cyclic corrosion test. Good corrosion resistance was shown at 60 to 90%, and the best corrosion resistance was shown at 75%.
The amount of the base resin must be 30% by weight or more based on the solid content of the paint. If the amount is less than 30% by weight, the binder effect of the base resin on the rust-preventive pigment is reduced, making it difficult to form a paint and The film is brittle and the working adhesion becomes insufficient.

Next, the curing agent is an oxazoline ring-containing acrylic compound. FIG. 4 shows the structure. Since this compound reacts with the base resin in a single hour and forms a dense organic film, it suppresses the elution of Cr ⁶⁺ from the base chromate film as shown in FIG. Fig. 5 shows the curing time of the base resin and the dissolution of Cr ⁶⁺ after film formation (film formation).
It shows the effect on The abscissa indicates the type of the curing agent, the ordinate indicates the curing time, and the ordinate indicates the elution amount of Cr ⁶⁺ . The oxazoline ring-containing acrylic compound has excellent curing time and resistance to elution of Cr ⁶⁺ . In addition, since this curing agent reacts with the base resin even at a low temperature of 150 ° C. or lower, there is no problem in application to a BH steel sheet having improved press formability.

The oxazoline ring-containing acrylic compound is preferably compounded in an equivalent ratio of 1 / 0.8 to 1 / 1.2 with respect to the urethane-modified epoxy resin as the base resin as shown in FIG. FIG. 6 shows the effect of the mixing ratio of the base resin and the curing agent on the corrosion resistance of the organically coated steel sheet. The horizontal axis indicates the mixing equivalent ratio of the base resin and the curing agent, and the vertical axis indicates the red rust generation area after 300 cycles of the cyclic corrosion test. Good corrosion resistance is shown at an equivalent ratio of 1 / 0.8 to 1 / 1.2. Next, in the present invention, in order to impart high corrosion resistance without harmful substances being eluted in the degreasing / chemical conversion bath, SiO ₂ having an average particle size of 8 μm or less is used as a rust preventive pigment in the paint solids in an amount of 10 to 40% by weight. % Is added.

FIG. 7 shows the effect of the difference in the structure and addition amount of SiO ₂ used as the rust preventive pigment on the corrosion resistance of the organic composite steel sheet. The horizontal axis indicates the amount of added SiO ₂ , and the vertical axis indicates the red rust generation area after 300 cycles of the cyclic corrosion test. In a system in which SiO ₂ (colloidal silica) was added as a rust preventive pigment, no red rust was generated at an addition amount of 10% or more, indicating good corrosion resistance. However, 40%
Exceeding this is not preferable because the coating film after film formation becomes brittle or the spot weldability decreases. Next, SiO
_{2 This} is a system in which (fumed silica) is added as a rust preventive pigment, but even if it is added up to 25%, the generation of red rust is 80% or more and the corrosion resistance is insufficient.

FIG. 8 shows the effect of the average particle size of SiO ₂ on the corrosion resistance of the organic composite steel sheet. Si on the horizontal axis
The average particle size of O ₂ is plotted on the vertical axis with the area of red rust occurrence after 300 cycles of the cyclic corrosion test. When the average particle size of SiO ₂ is 8 μm, the best corrosion resistance is exhibited. Further, in the present invention, melamine cyanurate is used as a conductivity-imparting agent in order to improve the cationic electrodeposition property with a paint solid content of 0.5.
-10% by weight. Melamine cyanurate has a structure as shown in FIG. 9 and promptly causes three-dimensional charge transfer, so that the resistance value at the initial stage of cation electrodeposition is reduced, and as a result, the appearance after electrodeposition is improved. Think to bring.

FIG. 10 shows the effect of melamine cyanurate on the initial resistance in cationic electrodeposition. The abscissa indicates the amount of melamine cyanurate added, and the ordinate indicates the initial resistance at the time of cationic electrodeposition. It can be seen that the addition of melamine cyanurate lowers the initial resistance value of the cation electrodeposition and improves the electrodeposition property. But 10%
The above addition is not preferable because the coating film becomes brittle. FIG.
No. 1 shows the effect of melamine cyanurate on the appearance of cationic electrodeposition. The horizontal axis indicates the amount of melamine cyanurate added, and the vertical axis indicates the score of the surface appearance after cationic electrodeposition. It is confirmed that the addition of melamine cyanurate in an amount of 0.5% or more eliminates gaspin and yuzu skin and improves the electrodeposition appearance.

From the viewpoint of press formability of the rust-preventive steel sheet, the coating composition of the present invention may contain 0.1 to 10% by weight of polyethylene colloid based on the solid content of the coating. FIG.
Shows the effect of the amount of polyethylene colloid on cylindrical press workability. The abscissa indicates the amount of the added polyethylene colloid, and the ordinate indicates the weight reduction during cylindrical pressing. It can be seen that the addition reduces the weight loss due to powdering and galling during processing and improves the press workability. However, the effect is not improved even if it is added by 10% or more.
On the contrary, the amount of the polyethylene colloid is preferably in the range of 0.1 to 10% by weight because other properties such as corrosion resistance are reduced.

[0017]

As described above, the organic composite steel sheet of the present invention, which has a problem in quality as compared with the conventional organic composite steel sheet, deteriorates in corrosion resistance and cationic electrodeposition properties such as press formability and continuous spot weldability. It is a revolutionary organic composite steel sheet that has been greatly improved without compromising the needs of the market.

[Brief description of the drawings]

FIG. 1 is a view showing a film configuration according to the present invention;

FIG. 2 is a diagram showing the effect of modification of a base epoxy resin on the corrosion resistance of an organic composite steel sheet,

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

FIG. 4 is a view showing the structure of an oxazoline ring-containing acrylic compound as a curing agent;

FIG. 5 is a graph showing the curing time of a base resin and the C after film formation.
Diagram showing the effect of r ⁶⁺ on dissolution,

FIG. 6 is a view showing the effect of the mixing ratio of a base resin and a curing agent on the corrosion resistance of an organic-coated steel sheet;

FIG. 7 is a diagram showing the effect of the difference in structure and addition amount of SiO ₂ used as a rust preventive pigment on the corrosion resistance of an organic composite steel sheet;

FIG. 8 is a diagram showing the effect of the average particle size of SiO ₂ on the corrosion resistance of an organic composite steel sheet,

FIG. 9 shows the structure of melamine cyanurate,

FIG. 10 shows the effect of melamine cyanurate on initial resistance in cationic electrodeposition,

FIG. 11 is a diagram showing the effect of melamine cyanurate on the appearance of cationic electrodeposition,

FIG. 12 is a graph showing the effect of the amount of polyethylene colloid on cylindrical press workability.

[Explanation of symbols]

 Reference Signs List 1 steel sheet 2 Zn-based plating film 3 chromate film 4 organic film

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI C23C 22/24 C23C 22/24 (72) Inventor Yoshio Shindo 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development (56) References JP-A-5-202487 (JP, A) JP-A-6-155656 (JP, A) JP-A-63-35798 (JP, A) JP-A-64-297 (JP, A) JP-A-64-8034 (JP, A) JP-A-4-280980 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 22/00-22/86 B05D 3 / 10 B05D 7/14 B05D 7/24 302 B32B 15/08

Claims (2)

(57) [Claims] 1. 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,
The coating composition shown below was used as a solid
An organic composite steel sheet having excellent corrosion resistance, wherein the organic composite steel sheet is formed to have a thickness of about 3.0 μm. (A) The urethane-modified epoxy resin is 30% by weight or more in the solid content of the coating composition. (B) The oxazoline ring-containing acrylic compound is 1 / 0.8 to 1 / 1.2 in equivalent ratio to the epoxy resin. SiO ₂ (colloidal silica, the average particle diameter 8mμ below) paint solid content 10 to 40 wt% (D) a melamine cyanurate paint solids in a 0.5 to 1
0% by weight 2. The organic composite steel sheet according to claim 1, wherein the coating composition contains 0.1 to 10% by weight of a polyethylene colloid based on the solid content of the coating.