JP2794675B2 - Organic resin-coated steel sheet for automobile and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to an organic material for automobiles having the corrosion resistance, chromium elution resistance, continuous spot weldability, electrodeposition coating property and processing lubricity required for automobile parts. The present invention relates to a resin-coated steel sheet and a method for producing the same.

(Prior Art) In recent years, the use of galvanized steel sheets or zinc alloy-plated steel sheets instead of conventional cold-rolled steel sheets in automobile bodies has been actively performed to improve the durability of the vehicle bodies. Typical of these galvanized steel sheets are galvanized steel sheets, hot-dip galvanized steel sheets and electrogalvanized steel sheets, and zinc-galvanized steel sheets are Zn-Ni alloys, Zn-Fe alloys, Zn
-Ni-Co alloy, Zn-Co alloy, Zn-Mn alloy or Zn-Fe-
Electric alloy plated steel sheets such as Co alloys and galvannealed steel sheets. For electro-galvanized steel sheet,
In some cases, the upper layer is plated with the above alloy, and the lower layer is plated with another corrosion-resistant metal or alloy.

These galvanized steel sheets and galvanized steel sheets exhibit considerable corrosion resistance if pre-treated and electrodeposited, but depending on the material, the electrodeposition coating film does not stick around like the hemmed part or bag structure part It has a processed part, and in the case of such a member, the corrosion resistance of the plating layer alone is not sufficient. Therefore, a chromate film mainly composed of hexavalent chromium is formed on the entire surface of a zinc or zinc alloy-plated steel sheet for such a member, and an organic resin film is further formed on the entire surface of the chromate film. The production of this organic resin-coated steel sheet is generally performed by subjecting the steel sheet to a chromate treatment, followed by drying, then applying an organic resin with a roll coater, and baking and drying. Since the corrosion resistance can be adjusted by changing the amount of chromium and the thickness of the organic resin film, the corrosion resistance can be adjusted to the level required by the member.

By the way, since the production of members using this organic resin-coated steel sheet is performed in the steps of punching and molding as in the case of cold-rolled steel sheet, processing lubrication to prevent flaws from occurring during processing Need sex. In addition, spot welding must be performed with other members when assembling the vehicle body. Furthermore, organic resin-coated steel sheet members do not require pre-coating treatment before electrodeposition coating, but they are required for cold-rolled steel sheet members assembled together with the car body. When applying, the organic resin coated steel plate member is also degreased, washed with water, surface adjusted,
Since a pretreatment liquid such as a phosphate treatment is sprayed, it is necessary that chromium does not elute from the member of the organic resin-coated steel sheet. This is because when chromium is eluted and mixed into the pretreatment liquid, the oxidizing hexavalent chromium deteriorates the pretreatment liquid in a short period of time, and waste liquid treatment is expensive. In addition, since the electrodeposition coating is applied after the pretreatment, the electrodeposition coating property is also required. As described above, an organic resin-coated steel sheet used for an automobile member needs to have harmonized work lubricity, spot weldability, chromium elution resistance, and electrodeposition coating property.

(Problems to be Solved by the Invention) However, in the case of an organic resin-coated steel sheet, if the resin film is thickened to improve processing lubrication and chromium elution resistance, the electrical conductivity decreases, and spot weldability and electrodeposition are reduced. Since the paintability is reduced, the thickness of the organic resin film must be controlled in a narrow range of 0.8 to 1.0 μm in terms of the dry film thickness in order to harmonize all the properties. However, it is difficult to control the film thickness in such a narrow range by the roll coater coating, and there is a portion that is out of the control range.

The present invention provides an organic resin-coated steel sheet that can satisfy all of the above-mentioned characteristics without performing such strict management of the organic resin film, and a method for producing the same.

(Means for Solving the Problems) The present invention provides an organic resin-coated steel sheet for automobiles by depositing 40 to 150 mg / chromium on a surface of a zinc or zinc alloy plated steel sheet.
m ^{2, and} the discontinuity molar ratio of trivalent chromium and total chromium to form a chromate film is trivalent chromium / total chromium = 0.70 to 0.95, the particle diameter thereon are dispersed following granular resin film 80μm An organic resin film was formed, and a steel sheet having a coverage of 65 to 95% by the organic resin film was obtained. Further, the production of this steel sheet is performed by setting the molar ratio of trivalent chromium to total chromium on the surface of a zinc or zinc alloy-plated steel sheet to trivalent chromium / total chromium = 0.40-0.
A chromate treatment solution of 55 to 40 to 150 mg /
and m ² is applied, without washing, after heating dried 110 to 200 ° C. at a sheet temperature, using an electrostatic atomizing device, the particle size in the dry state was adjusted to 80μm or less water-soluble Or water-dispersible organic resin emulsion particles with a resin coverage of 65 to 95
% And then dried.

(Function) The steel sheet of the present invention is characterized in that the organic resin film formed on the chromate film is converted into a granular discontinuous film, and the exposed portion of the chromate film where the organic resin film is not present secures electrical conductivity, and the organic resin film is formed. It has made management easier. However, when the chromate film is exposed in this way, chromium elutes from the exposed portion, and the amount of hexavalent chromium eluted is larger than that of the conventional organic resin film-coated steel sheet. Therefore, in the present invention, the proportion of trivalent chromium in the chromate film is increased so that the proportion of hexavalent chromium in the eluted chromium is reduced.

However, when the proportion of trivalent chromium in the chromate film is increased, the corrosion resistance is reduced, and when a fine chromate film exposed portion is provided on the steel sheet, the proportion of hexavalent chromium in the eluted chromium is small, If there are many exposed parts, elution of hexavalent chromium cannot be ignored.

Thus, in the present invention, the chromate film and the organic resin film are harmonized with chromium elution resistance, corrosion resistance and electrical conductivity as described below.

That is, it is not problematic to mix the phosphating solution from the organic resin-coated steel plate member when performing the pre-painting treatment on the vehicle body.
The elution amount of valent chromium is 9 mg / m ² or less. The elution amount of hexavalent chromium decreases as the ratio of trivalent chromium / total chromium increases, but the corrosion resistance decreases. Further, the elution amount of hexavalent chromium decreases as the coverage of the organic resin film increases, but the conductivity decreases because the organic resin film is an insulator. The present inventors have conducted experiments on the amount of hexavalent chromium eluted based on the composition of the chromate film and the coverage of the organic resin film, and found that the molar ratio of trivalent chromium / total chromium was 0.70 to 0.95, 65 coating coverage
%, The elution amount of hexavalent chromium can be reduced to 9 m / m ² or less.

However, when the chromate film contains a large amount of trivalent chromium, the corrosion resistance of the exposed portion of the chromate film decreases. Therefore, as a result of conducting an experiment on the amount of chromium to compensate for the decrease in corrosion resistance when the ratio of trivalent chromium / total chromium is within the above range, the amount of chromium was found to be 40 mg / m ² or more.

By the way, since both the chromate film and the organic resin film are insulators, their thickness affects the electrical conductivity. Further, when the particle size of the organic resin film is larger than the electrode thickness, spot welding cannot be continuously performed at an arbitrary position. Therefore, when we experimented on these,
Chromium coating weight of the chromate film 150 mg / m ² or less, the coverage of the organic resin film 95% or less, the particle size of the organic resin film 80μ
m or less.

 The production of the steel sheet of the present invention can be obtained as follows.

First, the formation of a chromate film is performed by attaching a chromate treatment liquid with a molar ratio of trivalent chromium to total chromium of trivalent chromium / total chromium of 0.40 to 0.55 on the surface of a zinc or zinc alloy plated steel sheet. It can be formed by applying 40 to 150 mg / m ^{2 in} quantity and heating and drying at 110 to 200 ° C. at the plate temperature without washing with water. The composition of the chromate treatment liquid and the chromate film are different, but when the chromate treatment liquid is dried, the ratio of trivalent chromium / total chromium increases, and
This is because the higher the drying temperature, the larger the number.

In the present invention, the ratio of trivalent chromium / total chromium in the chromate treatment liquid is set to 0.40 to 0.55, and the drying temperature is set to 110 to 200 ° C.
The reason is as follows.

That is, when the chromate treatment liquid is dried at a temperature higher than 200 ° C., the resulting chromate film cracks,
Decreases corrosion resistance. Therefore, it is necessary to dry the chromate treatment liquid at a temperature of 200 ° C. or less. To dry the chromate treatment liquid at a drying temperature of 200 ° C. to make the ratio of trivalent chromium / total chromium of the chromate film 0.95, It is necessary to make the ratio of trivalent chromium / total chromium of the chromate treatment solution 0.40 or more. On the other hand, in order to use a chromate treatment solution having a ratio of trivalent chromium / total chromium of 0.40 and to make the ratio of trivalent chromium / total chromium of the chromate film 0.70, the drying temperature should be 110 ° C.
It is necessary to be higher than ° C. The chromate treatment solution is
If the ratio of trivalent chromium / total chromium is larger than 0.55,
Since gelation occurs and application becomes difficult, the ratio of trivalent chromium / total chromium must be 0.55 or less.

Chromate treatment liquid is silica sol for the purpose of improving corrosion resistance, chromium elution resistance, hardness of chromate film, etc.
An inorganic sol such as an alumina sol or a zirconia sol may be added. Further, phosphoric acid or boric acid may be added to improve the wettability of the chromate treatment liquid to the steel sheet.

Next, an organic resin film is formed by atomizing and applying a water-soluble or water-dispersible organic resin emulsion by an electrostatic atomizer. The emulsion can be atomized and applied by the spray method, but the emulsion particles are large and the particle size in the dry state cannot be reduced to 80 μm or less. On the other hand, in the case of the electrostatic atomizer, the particle size of the emulsion particles can be arbitrarily controlled by the number of revolutions of the atomizer, and can be uniformly applied. The emulsion particles are adjusted to be 80 μm or less in a dry state, and the coverage is 65-95%.
Apply so that The drying of the emulsion particles may utilize the residual heat obtained by drying the chromate film, or may be heated by hot air.

The type of the organic resin used is not particularly limited, but an epoxy resin, a urethane resin, a polyester resin, an acrylic resin and the like are preferable. If you want to impart lubricity to the resin film, use fluororesin, silicone resin, silicone oil,
A lubricant such as wax, fatty acid-based lubricating oil, graphite, molybdenum disulfide and the like may be added.

The galvanized steel sheet or zinc alloy-coated steel sheet used in the present invention can be used as long as it is conventionally used, but the coating weight is 5 g / m ² or more, preferably 10 to 6
Use the one with 0 g / m ² .

(Example) Example 1 A chromate treatment solution having a composition shown in Table 1 was prepared by mixing silica sol and phosphoric acid with an aqueous solution of chromic anhydride, and was used as an electrolytic Zn-Ni alloy-plated steel sheet (sheet thickness 0.7 mm, The coating weight was 30 g / m ^{2 on} one side) and dried without washing with water. Thereafter, an emulsion of a resin having the composition shown in Table 1 was atomized by an electrostatic atomizer, applied onto the chromate film so as to form a discontinuous film, and dried.

The application by the electrostatic atomizer was performed under constant conditions of a voltage of 100 KV, a spray amount of 100 cc / min, and a cup rotation speed of 20,000 rpm.

On the other hand, as a comparative material, one in which an organic resin film was formed on the entire surface of a chromate film by a roll coater method was manufactured (Comparative Examples Nos. 6 and 7).

Table 2 shows the chromium elution resistance, spot weldability, electrodeposition coating property, lubricity and corrosion resistance of the coated steel sheet. In addition, these characteristic tests were implemented by the following method.

(1) Chromium elution resistance test The amount of chromium adhering to the surface of the coated steel sheet is measured in advance by X-ray fluorescence analysis, and then the steel sheet is degreased, washed, surface-adjusted, phosphated, and washed. Process
The amount of chromium after the treatment was measured in the same manner, and how much the amount of chromium was before the treatment was evaluated based on the following criteria.

○ elution amount 5 mg / m less than ² △ elution 5 mg / m ² or more, × elution amount 9 mg / m ² or more and less than 9mg / m ^{2 (2)} spot weldability test electrodes CF type (4.5mmφ), pressure 200 Kgf, energization Continuous welding was performed without exchanging the electrodes under the conditions of 10 cycles of time and a welding current of 9.8 KA, and the number of hits before breaking the shear was investigated and evaluated according to the following criteria.

○ Exceeding 5000 spots △ 4000 spots or more and 5000 spots or less × 4000 spots or less (3) Electrodeposition coating test After performing cationic electrodeposition coating so that the film thickness becomes 20 μm by the boost method, baking and drying at 165 ° C for 20 minutes Then, the presence or absence of peeling of the coating film by a grid test was evaluated according to the following criteria.

○ No peeling was observed. × Even slight peeling was observed. (4) Lubricity test The dynamic friction coefficient was measured and the limit drawing ratio (LDR) was measured by a cylindrical drawing test.

(5) Corrosion resistance test The occurrence of red rust after the salt spray test was performed for 1500 hours was evaluated according to the following criteria.

 ○ Red rust occurrence rate from flat part is less than 10% × Red rust occurrence rate from flat part is 10% or more Table 2 shows these results.

Example 2 The same chromate treatment solution as in Example 1 was applied to the same steel sheet as the electric Zn-Ni alloy-plated steel sheet used in Example 1 and dried, and trivalent chromium / total chromium = 0.85 (mol) Ratio), a chromate film having a chromium deposition amount of 70 mg / m ² was formed, and then a discontinuous film (particle size: 50 μm) of a mixture of 93% acrylic resin and 7% fluororesin was formed on the chromate film as in Example 1. Formed in the same manner, resin-coated steel sheets having different coverages were manufactured.

FIG. 1 shows the chromium elution amount and the spot weldability due to the change in the organic resin film.

(Effects of the Invention) As described above, the organic resin-coated steel sheet of the present invention harmonizes the corrosion resistance, chromium elution resistance, spot weldability, electrodeposition coating property, and processing lubricity required for manufacturing automobile parts. . In addition, since the production of the organic resin film may be performed by a method of controlling the particle size and the coverage of the applied particles using an electrostatic atomizer, the film management is easier than the conventional roll coater method.

[Brief description of the drawings]

FIG. 1 is a graph showing how the chromium elution resistance and the spot weldability of the organic resin coated steel sheet for automobiles according to the present invention change depending on the coverage of the organic resin film.

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Yamamoto, 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. Steel Research Laboratory Surface Treatment Research Department (72) Inventor Makoto Kawamoto 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Address Nisshin Steel Co., Ltd. Steel Research Laboratory Surface Treatment Research Department (56) References JP-A-60-103185 (JP, A) JP-A-62-276539 (JP, A) JP-A-63-291665 (JP, A) JP-A-63-114635 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 22/00-22/86 B32B 15/08 B05D 7/14 C23C 28/00

Claims (2)

(57) [Claims] 1. A chromate film having a chromium deposition amount of 40 to 150 mg / m ² on a surface of a zinc or zinc alloy plated steel sheet and a molar ratio of trivalent chromium to total chromium of trivalent chromium / total chromium = 0.70 to 0.95. Formed on the substrate, and formed thereon a discontinuous organic resin film in which a granular resin film having a particle size of 80 μm or less is dispersed, and the coverage of the organic resin film is 65 to 95%. Resin coated steel sheet. 2. The surface of a zinc or zinc alloy plated steel sheet
The molar ratio of trivalent chromium to total chromium is trivalent chromium / total chromium =
0.40-0.55 chromate treatment solution with chromium deposition amount of 40
~ 150mg / m2, 110 ~ ² at plate temperature without washing with water
After heating and drying at 00 ° C., using an electrostatic atomizer, the water-soluble or water-dispersible organic resin emulsion particles adjusted to have a particle size in a dry state of 80 μm or less have a resin coverage of 65 to 50 μm. A method for producing an organic resin-coated steel sheet for automobiles, wherein the steel sheet is coated so as to be 95% and dried.