JPS6393891A - Manufacturing method of rust-proof steel plate for automobiles - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention not only has excellent rust prevention performance but also has excellent plating adhesion, and is suitable not only for the inner part of an automobile body but also for the outer panel. The present invention relates to a method of manufacturing a Zn-Ni alloy plated steel sheet.

BACKGROUND TECHNOLOGY In recent years, requirements for anti-rust performance of automobile bodies have become increasingly strict, and countermeasures have also become more serious. In particular, various automobile manufacturers have been proposing one after another to raise the rust prevention target in order to prolong the period of suppressing holes and external rust in automobile bodies, and measures to improve the rust prevention performance of automobile rust-preventive steel sheets are an urgent issue. It becomes.

By the way, currently the mainstream of rust-proof steel sheets for automobiles is Zn-Fe alloy plated steel sheet and Zn-Ni alloy plated steel sheet because of their excellent paintability and corrosion resistance.
There is a shift to n-based alloy plated steel sheets. However, these Zn-based alloy-plated steel sheets have been used mainly for interior parts of automobiles due to their particularly excellent resistance to large-scale corrosion. Zn-Fe alloy plating or Zn-N
Consideration has begun to apply i-alloy plating to the exterior of vehicle bodies.
Some attempts have been made to put Zn--Fe alloy plating into practical use.

However, Z is said to be superior in terms of corrosion resistance.
N-Ni alloy plated steel sheets have the following problems with respect to the performance required as outer panels for automobiles, and it is currently desired to improve these problems.

That is, in normal Zn-Ni alloy plating, the N in the plating layer
Good corrosion resistance is obtained in the range of 1:10 to 16% by weight of i, but the alloy in this range is a hard intermetallic compound with a single phase of T, so the steel sheet cannot be used as an outer panel for automobiles. When used as a vehicle, the impact from flying stones, etc. that tends to occur when driving tends to cause peeling of the paint film and even peeling of the plating film (chipping phenomenon), and in addition, red rust is likely to occur in areas where the plating has peeled off due to the chipping phenomenon. Therefore, these problems were fatal for use as an automobile exterior panel, where occurrence of exterior cracking is highly discouraged.

Note that chipping resistance is related to plating adhesion. In order to improve the adhesion of plating, a proposal has been made in the past to apply multilayer plating of Zn-Ni alloys with different Ni contents (Japanese Patent Laid-Open No. 58-20419
Publication No. 6). However, in order to perform multilayer plating of such a Zn-Ni alloy, it is necessary to prepare plating baths with various concentration ratios of z n 2 + and Ni'', and it is necessary to maintain and manage these plating bath concentrations. Considering this, it required a serious investment in equipment, and in addition, there were many operational difficulties that could not be avoided.

Means for Solving the Problems> From the above-mentioned viewpoint, the present inventors have developed a material that not only has excellent rust prevention performance but also has sufficient plating adhesion that leads to improved chipping resistance. In order to stably manufacture surface-treated steel sheets that can be used as both the inner and outer panels of automobile bodies without reducing the conventional production efficiency of plating, we have developed an approach that requires special attention to the advantages of Zn-Ni alloy plating. As a result of repeated research aimed at improving the chipping resistance (plating adhesion), which is a problem in addition to utilizing the corrosion resistance of The main purpose of pickling treatment is to reduce the etching amount of the base steel plate to Ig/rr during this pickling treatment.
If the pickling loss is greater than r than the conventional one (pickling loss is more than 0.5 g/rrl and much less than Ig/nf), preferential etching of grain boundaries becomes noticeable and many fine irregularities are formed on the surface of the steel sheet. As a result, the area of electrodeposition increases in the subsequent plating process, which reduces the impact energy per unit area of the plating interface against external impact, and Zn-Ni alloy plating has some problems with adhesion. The unexpected finding was that even if the coating was applied, the apparent adhesion would improve and the plating would be less likely to peel off.

This invention was made based on the above knowledge, and the amount of etching is 1 to 6 g/n as pre-plating treatment. Zn- with a Ni content of 8 to 16% by weight
This method is characterized by the fact that by applying Ni-based alloy plating, it is possible to easily and reliably manufacture a rust-proof steel sheet for automobiles that has both excellent corrosion resistance and plating adhesion.

Here, as the pickling treatment means, as usual, nitric acid or picric acid, or two of sulfuric acid, hydrochloric acid, nitric acid and picric acid are used.
Electrical M2 using a steel plate as an anode in a mixed acid solution containing more than
0 to 20 QC/dn+'', or sulfuric acid solution (concentration of about 5% or more) or hydrochloric acid solution (concentration of about 5% or more) at room temperature to about 80°C.

Either method of immersion treatment in a nitric acid solution (concentration of about 2% or more) or a picric acid solution (concentration of about 2% or more) for 1 to 2 seconds may be adopted. This pickling treatment not only removes the oxide film and surface deposits of the steel sheet to create an active surface, but also etches the steel sheet at a rate of 1 to 6 g/n (1 to 6 g/n), making the grain boundaries of the steel sheet sufficiently clear. As a result, the surface becomes more irregular and the adhesion of the interface is improved due to an increase in the area of plating electrodeposition.

Further, the Zn--Ni alloy plating performed subsequent to the above pretreatment does not require any special means, and it is sufficient to apply a conventional method known for a long time.

The reason why the "etching amount during pickling" and the "Ni content of Zn--Ni alloy plating" are numerically limited as described above in the method of the present invention is as follows.

(a) Etching amount during pickling Is the etching amount during pickling Ig/rr? If it is below the desired plating adhesion (
On the other hand, if the etching amount is 6 g/n? Exceeding this will cause the surface of the steel plate to become rough, which will have a negative effect on the appearance roughness and the sharpness of the image after painting.
The amount of etching during pickling was determined to be 1 to 6 g/m.

(Ni content of Zn-Ni alloy plating) In the method for producing rust-proof steel sheets for automobiles according to the present invention, following pickling, Zn, which has the best corrosion resistance among Zn-based platings,
Although n-Ni alloy plating is applied, the desired corrosion resistance (large crack resistance, external rust resistance, etc.) cannot be secured even if the Ni content in the plating is less than 8% by weight or more than 16% by weight. Therefore, the Ni content of Zn-Ni alloy plating is 8 to 1
It was set at 6% by weight.

Furthermore, Table 1 shows the chemical treatment using Nippon Parriki Rising Co., Ltd.'s "B T-3020 (trade name)" as a base treatment for Zn-Ni alloy plated cold-rolled steel sheets for automobiles with various Ni contents. Treatment and 20 μ
1 After applying a 3-coat process consisting of a surface-thick cationic electrodeposition coating and a 40 μm intermediate coat and top coat for melamine/alkyd baking, crosscuts were applied, and 5% NaCl was applied twice a week. This table shows the evaluation results of external rust resistance when exposed outdoors for half a year while being sprayed. From the results shown in Table 1, it is clear that the Ni content of Zn-Ni alloy plating is less than 8% by weight. It is clear that if the content exceeds 16% by weight, sufficient corrosion resistance cannot be obtained.

Hereinafter, the present invention will be explained using examples and comparing with comparative examples.

<Example> First, a cold-rolled steel plate with a plate thickness of 0.8 fin was subjected to alkaline electrolytic degreasing in the usual manner, and then pickled under the pickling conditions shown in Table 2.
Next, the plating bath composition: Zn"...30g/β, Ni"...
...60g/l, Sodium sulfate...50g/f.

Sodium acetate...20g/β, plating bath temperature: 50-60°C, plating bath pH: 1.5-2.5, Ni content: 12% by weight Zn-Ni alloy 20
A basis weight of 8/rrr was electroplated in quantity.

In addition, in the column "Presence or absence of grain boundary etching" shown in Table 2, the presence or absence of grain boundary etching indicates that the grain boundaries of the steel sheet are etched and SE
M (Scanning Electron Micro
Grain boundaries can be confirmed by os-cope observation, none.
The degree of pickling is indicated as: - The degree of etching is small and grain boundaries cannot be confirmed by SEM observation; - Unevenness: - Fine irregularities within the crystal grains (microscopic etching) are observed. .

Next, the Zn-Ni alloy plated steel sheet obtained in this way was coated with Nippon Parkerizing Co., Ltd. as a base treatment for painting.
Chemical treatment using “B T-3080 (product name)” from Nippon Paint Co., Ltd. and “Power Top U-50” from Nippon Paint Co., Ltd.
(Product name) 20μ thick cationic electrodeposition coating using ゛ and melamine alkyd paint 35~40cm thick respectively.
After applying a three-coat process consisting of an intermediate coat and a top coat of μ, it was cooled to -20°C, and a chipping test was performed using a gravelometer under the conditions specified in ASTM D-3170-74, and the plating at that time was Chipping resistance was evaluated based on the presence or absence of peeling.

The results are also shown in Table 2.

The chipping resistance is evaluated on a three-level scale, and Table 2 shows: ○... Peelability on par with cold-rolled steel sheets, △... Slightly inferior to cold-rolled steel sheets, ×... Inferior to cold-rolled steel sheets. Level, expressed as .

From the results shown in Table 2, it is clear that the method of the present invention provides a rust-proof steel plate that has excellent chipping resistance and can exhibit sufficiently excellent performance even when used as an outer panel of an automobile body. On the other hand, if the manufacturing conditions deviate from the conditions specified in this invention, the resulting Zn-Ni alloy-plated steel sheet will not exhibit sufficient chi-noping resistance, and therefore will not be suitable for use in automobiles, which is a future target. It is clear that the product is unsatisfactory as a rust-proof steel plate.

Overall Effects> As described above, according to the present invention, it is possible to produce rust-proof steel sheets for automobiles with excellent performance that is sufficiently satisfactory for use as outer panels of car bodies, without the need for expensive equipment. This makes it possible to stably mass-produce the product without the need for additional processes, and it brings extremely useful industrial effects, such as further improving the durability of automobiles.

Claims (1)

[Claims] As a pre-plating treatment, pickling with an etching amount of 1 to 6 g/m^2 was performed, followed by Zn with a Ni content of 8 to 16% by weight.
- A method for producing a rust-proof steel sheet for automobiles with excellent plating adhesion, characterized by applying Ni-based alloy plating.