JPH05295560A - Organic composite galvanized steel sheet for external ornament - Google Patents

Abstract

PURPOSE:To provide an org. composite galvanized steel sheet capable of obtaining the uniform and beautiful finished appearance of electrodeposition coating even if there is a damaged film and having excellent rust preventing performance. CONSTITUTION:This org. composite steel sheet is provided with a galvenized film as a first layer on the steel sheet, a chromate film as a second layer and an org. resin film as a third layer. In this case, the resistance (R-R1) obtained by subtracting the resistance R1 at the time of cationically electrodeposition- coating the zinc phosphate plating film from the resistance R when a maximum current is applied to cationically electrodeposition-coating the steel sheet or the resistance (R-R2) obtained by subtracting the resistance R2 when the zinc phosphated steel sheet is cationically coated from the R, is controlled to be -650 to +1450mOMEGA.cm<2>, expressed in terms of the surface area of the material to be coated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic composite zinc-based plated steel sheet for exterior outer surfaces having good electrodeposition coatability.

[0002]

2. Description of the Related Art In recent years, organic composite zinc-based plated steel sheets exhibiting excellent corrosion resistance have been developed and applied to automobile inner hole corrosion resistant inner plate materials, etc., and have achieved great results in improving their life. ..

This "organic composite zinc-based plated steel sheet" means
It is a multi-layered rust-proof steel plate having a zinc-based plating (Zn or Zn alloy plating) film as the first layer, a chromium film as the second layer, and an organic resin-based film as the third layer on the steel plate surface. Recently, with the aim of further improving the external surface rust resistance (for example, the rust resistance of impact parts of bounced pebbles), there has been a movement to apply organic composite zinc-based plated steel sheets to the exterior surface of automobiles. (Japanese Patent Laid-Open No. 64-78832)
(See the bulletin, etc.)

However, the organic composite zinc-based plated steel sheet is
Originally it was developed as an inner plate material for automobiles,
Although it has excellent cleating resistance during cation electrodeposition coating, it has a "press indentation flaw inspection part by grinding with a grindstone after press forming" or "grinder polishing of indentation flaw" which is unique when used as an exterior material. There was a problem in the electrodeposition coating finish of the damaged part of the organic composite film on the surface such as "care part". That is, it is pointed out that if the damaged portion of the organic composite film as described above exists, it appears as an uneven defect of the coating film during electrodeposition coating, and remains inside or after the top coating, which significantly deteriorates the appearance quality as an automobile. It had been done.

In view of the above, the object of the present invention is to obtain a uniform and beautiful appearance of a finished electrodeposition coating even if there are damaged portions of the film, and to have excellent rust preventive performance as it is. It was to realize an organic composite zinc-based plated steel sheet.

[0006]

The inventors of the present invention have conducted research from various viewpoints in order to achieve the above object, and as a result,
`` The electrodeposition coating on the organic composite zinc-based plated steel sheet after press forming is usually carried out after applying zinc phosphate treatment after pressing, and also on the zinc-based plating surface such as the film damage part by press or the press scratch maintenance part. It is performed after the zinc phosphate coating is formed on the exposed portion of the steel sheet surface or the steel sheet surface. In this case, the electrical resistance at the time of maximum current application during the cationic electrodeposition coating of the organic composite zinc-based plated steel sheet is " For the same electrical resistance of "base steel plate surface treated with zinc phosphate" and "first layer zinc plated film surface treated with zinc phosphate", a value in a specific range converted to the surface area of the electrodeposited coating By controlling the composition and film thickness of the third layer, the organic resin film, so that the damaged parts of the film such as surface polishing and maintenance can be electrodeposited without any problem in appearance. " Gained new knowledge with It could be.

The present invention has been completed after further studies based on the above findings and the like. "A zinc-based plating film as the first layer, a chromium film as the second layer, and a third layer on the steel sheet" As an organic composite zinc-based plated steel sheet with an organic resin-based coating, the "electrical resistance (R) at maximum current application during cationic electrodeposition coating" The value obtained by subtracting the electric resistance (R ₁ ) when cationic electrodeposition coating is applied to the zinc oxide treated product (R-R ₁ ), or from the “R” to “phosphoric acid in the base steel sheet” electrical resistance when performing cationic electrodeposition coating against those subjected to zinc treatment (R ₂₎ -650~ in both is the electrodeposition coating material the surface area in terms of "a value obtained by subtracting (R-R ₂₎ + 1450
By adopting a coating composition that is adjusted within the range of mΩ · cm ² , it is possible to obtain a uniform and beautiful finished appearance by electro-deposition coating that is sufficiently satisfactory for exterior use even if there are damaged parts. It has a great feature in "point".

[0008]

That is, according to the present invention, "In the conventional organic composite zinc-based plated steel sheet, if there is a damaged portion of the coating, the difference in electrical resistance between the damaged portion and the non-damaged portion during electrodeposition coating is large,
Therefore, the current concentrates on the part where the electric resistance is small, and the electrodeposition film thickness of the part becomes relatively thick, and the appearance after electrodeposition becomes poor. " By adopting the above, the electrodeposition coatability of the organic composite zinc-based plated steel sheet is improved.

Here, the reason why the "electrical resistance of the organic composite zinc-based plated steel sheet" as the calculation standard of the electric resistance difference is limited to "the one at the time when the maximum current for cationic electrodeposition coating is applied" is limited to that of the electrodeposition coating. This is because the organic film swells due to the influence of the hydrophilic group and the electric resistance is different from that in the dry state. In addition, as a reference for calculating the electric resistance difference, especially for the "first layer zinc-based plating film" or "base material steel plate" which causes unevenness of the electrodeposition coating property by being exposed at the damaged portion of the film, especially phosphorus. What specified the electrical resistance value of what was treated with zinc oxide,
As mentioned above, the zinc phosphate coating is usually formed on the damaged portion and is used for electrodeposition coating.Therefore, the surface of the zinc-based plating coating or the base steel plate exposed by the damage is electrodeposition coated. This is because it is sometimes covered with a zinc phosphate film.

The surface area conversion resistance value [R] of the object to be electrodeposited at the time of maximum energization during the cationic electrodeposition coating is V (V) for electrodeposition voltage, I (A) for maximum current, and When the surface area of the coated object is S (cm ² ), the formula It is the value calculated in.

The electric resistance value can be measured by measuring the voltage and current value during energization during normal electrodeposition coating. For example, for an object with a surface area of 420 cm ² , the distance between the object and the electrode is set to 10 cm and the voltage is 32
Fig. 1 shows an example of the measurement results of voltage and current when the dokan is energized at 0 V. The values of "I" and "V" in the equation (1) are [I] and [V] in Fig. 1. It is sufficient to catch and calculate R. It should be noted that the value of V to be measured naturally includes, for example, a voltage drop caused by the electrodeposition liquid, but the present invention defines that the base (base material) steel sheet and the zinc-based plated steel sheet are phosphoric acid. Since there is a difference in electrical resistance from the zinc-treated product, there is no problem in actual measurement under the same electrodeposition conditions.

As described above, in the present invention, the coating composition of the organic composite zinc-based plated steel sheet is adjusted so as to reduce the electric resistance difference between the damaged portion and the non-damaged portion during electrodeposition coating, and the cationic electrodeposition coating is performed. The current flowing at times is made uniform to improve the appearance of electrodeposition coating and improve the suitability as an exterior material. However, a value obtained by subtracting the "electrical resistance R _{1 of} the material obtained by subjecting the zinc-based plating of the first layer to zinc phosphate treatment" from the "electrical resistance R of the article to be electrodeposited (organic composite zinc-based plated steel sheet)" (R -R _1), or "the R" minus "electric resistance R ₂ of the material to the base material steel plate treated with zinc phosphate" from the value (R-
One of R ₂ ) is -650 m in terms of surface area of electrodeposited coating
If it is less than Ω · cm ² , the electrical resistance of the undamaged part becomes larger than that of the damaged part of the film, and the electrodeposition film thickness of the undamaged part becomes thicker than the damaged part of the film, resulting in uneven coating after electrodeposition coating. Since it stands out, it is not suitable as an exterior material. On the other hand, if the electric resistance difference exceeds +1450 mΩ · cm ² , the electrical conductivity of the undamaged area will be reduced, and local discharge (pinhole-like microscopic film defects) during electrodeposition coating will cause concentrated discharge. The tendency is to increase, and the occurrence of cratering increases, making it unsuitable as an exterior material.

By the way, in the present invention, the coating composition of the organic composite zinc-based plated steel sheet is defined by the specific electrical resistance difference. As means, A) a method of controlling the film thickness (usually around 1 μm) of the organic coating as the third layer, and B) addition of a hydrophilic resin contained in the organic coating as the third layer or a hydrophilic group such as silica. The method of controlling the addition amount of the substance, and the like can be mentioned.

As the chromate treatment carried out when the organic composite zinc-based plated steel sheet is manufactured, general reaction type chromate treatment, coating type chromate treatment or electrolytic chromate treatment, or Each chromate treatment or the like performed by adding silica to the treatment liquid used for these can be applied. As the organic coating that is the third layer of the organic composite zinc-based plated steel sheet, an epoxy-based or acrylic-based resin, or a resin to which silica or a hydrophilic resin is added can be applied.

Next, the present invention will be described more specifically by way of examples.

[Example] First, a Zn-13wt% Ni alloy-plated steel sheet (weight per unit area:
30 g / m ² ) as the base, and “Cr / SiO ₂ = 1” on top of it.
_{/ 1, Cr / PO 4 =} 1/1 chromate having a composition of "- coating type as Cr deposition amount using the preparative treatment liquid becomes 60 mg / m ² chromate - was subjected to preparative process.

Subsequently, the film thickness is changed by using a) urethane modified epoxy clear (adding 20% of SiO _{2 and} adding 10% of hydrophilic polyamide resin) as an organic resin film on the chromate film. The electric resistance was changed by changing the electric resistance. (B) Acrylic resin-based clear (with 15% of SiO ₂ added) was used, and the electric resistance was controlled by changing the addition amount of the hydrophilic polyamide resin to be added. The coating was performed to produce an "organic composite zinc-based plated steel sheet".

Next, a sample having the dimensions shown in FIG. 2 was taken from each of the obtained organic composite zinc-based plated steel sheets, and the electrodeposition coatability was investigated. When investigating the electrocoating property, a # 120 sand paper was used for the above sample to form a "film damage part" as shown in FIG. 2, and then the electrocoating was performed in the process shown in FIG. The electrodeposition appearance was evaluated by.

The results of these investigations are summarized in FIG.
In addition, "electrical resistance difference" in FIG. 4 means "a base material steel sheet treated with zinc phosphate" or "a Zn-Ni alloy plated steel sheet treated with zinc phosphate" and "organic coating formed" (That is, “organic composite zinc-based plated steel sheet”) ”, the one having the larger absolute value of the difference is adopted.
The results of "evaluation of appearance after electrodeposition coating" in Fig. 4 are indicated by "◎: excellent, ○: good, Δ: inferior, x: extremely inferior". From the results shown in FIG. 4, it can be confirmed that each of the organic composite zinc-based plated steel sheets according to the present invention has a cationic electrodeposition coating property which is sufficiently satisfactory as an exterior surface material.

[0019]

[Summary of Effects] As described above, according to the present invention, it is possible to realize an organic composite zinc-based alloy-plated steel sheet exhibiting extremely excellent electrodeposition coating properties. Industrially useful effects such as the application of composite zinc-based alloy plated steel sheets become possible.

[Brief description of drawings]

FIG. 1 is an explanatory diagram regarding how to capture an electric resistance value (R) and a voltage value (V) during electrodeposition coating.

FIG. 2 is an explanatory diagram of a sample for electrodeposition coating property investigation in Examples.

FIG. 3 is an explanatory diagram regarding a manufacturing process of an organic composite zinc-based alloy plated steel sheet in an example.

FIG. 4 is a graph summarizing the results of investigations on electrodeposition paintability in Examples.

Claims (1)

[Claims] 1. An organic composite zinc-based plated steel sheet comprising a zinc-based plated coating as a first layer, a chrome coating as a second layer, and an organic resin-based coating as a third layer on a steel sheet,
From "Electrical resistance (R) at maximum current when applying cationic electro-deposition coating to this", "Cation electro-deposition coating is applied to the first layer of zinc-based plating film treated with zinc phosphate. The value obtained by subtracting the electrical resistance (R ₁ ) at the time (R-R ₁ ), or from "the above R" when "the base metal sheet is treated with zinc phosphate is subjected to cationic electrodeposition coating. Of the electrical resistance (R ₂ ) ”(R−R ₂ ) is adjusted within the range of −650 to +1450 mΩ · cm ^{2 in} terms of the surface area of the electrodeposition coated object. Organic composite zinc-based plated steel sheet for exterior.