JPH05171456A - Steel sheet for exterior of automobile excellent in chipping resistance - Google Patents

Abstract

PURPOSE:To provide a steel sheet for the exterior of an automobile not only having corrosion resistance, coating suitability, spot weldability or the like but also excellent in the exterior and provided with low temp. chipping resistance as well. CONSTITUTION:The objective steel sheet for the exterior of an automobile is constituted in the following order; a) a steel sheet coated with a zinc alloy plating layer with >=10g/m<2> coating weight, b) chromate film coated with 20 to 200mg/m<2> coating weight expressed in terms of metallic chromium contg. SiO2 by 0.05 t0 4.0 in the weight ratio of SiO2/Cr and c) an organic resinous film coated with 0.05 to 2.0mu film thickness (if required, contg. 5 to 40wt.% SiO2).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rust preventive steel sheet for automobile exteriors which exhibits excellent chipping resistance at low temperatures.

[0002]

2. Description of the Related Art In recent years, various kinds of surface-treated steel sheets have been developed in order to meet the ever-increasingly strict demands for improving the rust preventive performance of automobile bodies. Zinc-based alloy plating (Zn-Ni alloy plating, Zn-Fe alloy plating, etc.) as one of the rust prevention technologies related to the outer surface
Is rapidly being applied.

However, the zinc-based alloy plating has a large residual stress in the plating film. Therefore, when the zinc-based alloy plating is applied to the outer surface of a steel sheet for automobile exteriors, "low temperature chipping (stone splashes in cold regions such as Europe and North America) There is a problem that the plating layer is liable to be peeled from the interface with the steel sheet due to the impact (1).

However, in order to address the above problems, (a) a method of improving the chipping resistance by strengthening the interface between the plating layer and the steel sheet (Japanese Patent Laid-Open Nos. 62-213397 and 6-62197).
No. 3-93891, JP-A-63-130794, JP-A-2-19491, JP-A-2-70091), (2) A method of forming a soft resin film as a buffer on the plating surface (JP-A-64-78832). No.), etc. have been developed.

However, these techniques have not always been sufficiently effective as a countermeasure against chipping of automobile exterior materials. Moreover, in the case of the "method of relaxing the impact on the plated surface with a resin film" as described in the above item (b), it is necessary to select a soft resin (glass transition temperature of 55 ° C or less) as the resin to be coated. Therefore, a highly crosslinked resin having good corrosion resistance cannot be applied, resulting in a disadvantage in terms of corrosion resistance.

Therefore, the object of the present invention is to obtain sufficient corrosion resistance for automobile exteriors,
It was an object of the present invention to provide a steel sheet for automobile exterior which not only has paintability and spot weldability, but also has excellent low temperature chipping resistance on the outer surface.

[0007]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the present inventors have devised a method capable of significantly improving the low temperature chipping resistance of a zinc-based alloy-plated steel sheet which is excellent in rust prevention and paintability. As a result of earnestly conducting research in earnest, "a predetermined SiO ₂ -containing chromium film is formed on a zinc-based alloy-plated steel sheet, and a protective resin film such as a clear film is further provided on the outer surface of the zinc-alloy-plated steel plate, or the above-mentioned protective resin film is formed. When a multilayer coating steel sheet is made to contain SiO ₂ in a specific ratio, the multilayer coating steel sheet retains excellent corrosion resistance, paintability and spot weldability, and at the same time, a soft resin is particularly used for the protective resin coating. Delivers significantly improved low temperature chipping resistance even without use. "
I was able to obtain the knowledge.

The present invention has been completed on the basis of the above-mentioned findings and the like, and "a) a zinc-based alloy plating layer having a basis weight of 10 g / m ² or more, b) an adhesion amount of metallic chromium. 20-200 mg / m ^{2 in} terms of conversion
Of chromate containing SiO ₂ of 0.05 to 4.0 in SiO ₂ / Cr weight ratio - DOO film, an organic resin film (SiO ₂ 5-40 wt% depending on the needs of c) thickness 0.05 to 2.0Myu By including these in this order in order to form a steel sheet for automobile exteriors, not only satisfactory corrosion resistance, paintability, spot weldability but also excellent low temperature chipping resistance can be exhibited. It has a big feature in "the point that it did."

Here, the "zinc alloy plating" is Zn
-Ni alloy plating or Zn-Fe alloy plating, or Co,
Any known rust-preventive plating containing Ti or the like or dispersion plating may be applied. As the "organic resin" coated on the chromate film, various types such as epoxy type, urethane type and acrylic type can be applied, and the glass transition degree of this resin film is not particularly limited. Absent.

The SiO ₂ contained in the chromate film and the organic resin film can be roughly classified into vapor phase silica, liquid phase colloidal silica, and aqueous agglomerated silica depending on the manufacturing method. It has already been confirmed that an excellent effect of improving low temperature chipping resistance can be obtained.

[0011]

As described above, the steel sheet for automobile exterior according to the present invention, which is provided with the zinc alloy plating layer to ensure the excellent corrosion resistance, has the low temperature chipping resistance remarkably improved by having the chrome coating containing SiO _2. It has achieved the effect, and by forming an organic resin film on the upper layer, it reduced the amount of Cr elution during degreasing and chemical conversion treatment of the steel sheet and ensured excellent adhesion with the electrodeposition coating film. It should be noted that in the chromate film or in this and the organic resin film at a specified ratio.
This is because the inclusion of SiO ₂ gives extremely excellent low temperature chipping resistance (the organic resin film also contains SiO 2).
_{When 2} is added, the effect of improving low temperature chipping resistance is further improved compared to when it is added only to the underlying chromate film).

Si in the chromate film or the organic resin film
While O ₂ is not necessarily clear the mechanism to significantly improve the low temperature chipping resistance of the anticorrosive steel sheets, SiO ₂
It is considered that the presence of particles exerts some effect of dispersing and absorbing impact energy.

A report has been made on a material for an automobile inner panel in which a zinc alloy-plated steel sheet is provided with a chromium coating containing SiO ₂ and an organic resin coating in order to improve the rust preventive power against pitting corrosion. Sometimes, but for anti-rust steel
SiO ₂ containing chromate - reported that the provision of the bets film or SiO ₂ containing organic resin film low temperature chipping resistance of significant improvement in anticorrosive steel plates come true is none in the past, actively finds its significant effect Applied to the material for the exterior of automobiles,
The effect of the present invention that exerted the performance superior to that of the conventional material was unexpected.

Hereinafter, in the present invention, "amount of coating weight",
"Chromate - DOO attachment amount", "chromate - SiO ₂ content in preparative coating", "organic resin film thickness" and "in the organic resin film
The reason for limiting the “SiO ₂ content” as described above will be explained.

A) Coating weight The zinc-based alloy plating layer is necessary for improving the outer surface rust resistance (securing the sacrificial anticorrosion effect of Zn), but if the coating weight is less than 10 g / m ^2, it is a rust-proof steel plate. The outer surface rust resistance of is insufficient. Therefore, the basis weight of the zinc-based alloy plating is set to 10 g / m ² or more.

B) Chromate adhesion amount The chrome coating film containing SiO ₂ is a layer necessary for improving the low temperature chipping resistance, but the adhesion amount is 20 mg / m 2.
^If it is less than ² , the desired effect of improving chipping resistance cannot be obtained, while if the amount of adhesion exceeds 200 mg / m ² , the amount of Cr elution during degreasing / chemical conversion treatment becomes large, which is not preferable. Therefore, the amount of chromate deposited is 20-200 mg / m
I decided to ² .

[0017] C) chromate - SiO ₂ content in preparative coating chromate - SiO ₂ content in preparative coating, it is necessary to adjust in order to ensure the desired low temperature chipping resistance improving effect, SiO ₂ content If the ratio is less than 0.05 in terms of “SiO ₂ / Cr weight ratio”, the effect of improving low temperature chipping resistance is not sufficient, while if the weight ratio exceeds 4.0, spot weldability deteriorates (insulating property of film Becomes large and the damage to the tip during welding becomes large, or it becomes impossible to energize).
Therefore, the weight ratio of SiO ₂ / Cr in the chromate film is 0.05 to
It was decided to adjust to 4.0.

D) Film thickness of organic resin film The organic resin film is Cr during degreasing and chemical conversion treatment of rust-proof steel plate.
It is necessary in order to reduce the amount of elution and to secure the adhesion to the electrodeposition coating film, and when SiO ₂ is contained, the effect of further improving the low temperature chipping resistance is exhibited. However, if the film thickness is less than 0.05μ, the desired effect due to the above action cannot be obtained, while if the film thickness exceeds 2.0μ, the spot weldability is deteriorated. It causes damage to the chip and the inability to energize), and the thickness of the organic resin film was set to 0.05 to 2.0μ.

[0019] E) SiO in the SiO ₂ in the content organic resin in the organic resin film since the inclusion of SiO ₂ low temperature chipping resistance of the anticorrosive steel sheet is further improved, if necessary
_The organic resin coating containing ₂ is applied, but if the content of SiO ₂ in the organic resin film is less than 5% by weight, the desired chipping resistance improving effect cannot be obtained, while 40% by weight is obtained.
If it is contained in excess of 1.0, the spot weldability is deteriorated. Therefore, the content of SiO ₂ in the organic resin film is set to 5 to 40% by weight.

As mentioned above, one of the purposes of forming the organic resin film is to secure the adhesion with the electrodeposition coating film and to elute Cr in the degreasing / chemical conversion treatment of the rustproof steel plate. Is to reduce. Therefore, although there is no regulation on the resin system itself of the organic resin film, it is naturally preferable to select the resin film in consideration of the adhesiveness with the chromate film and the electrodeposition coating film. From this viewpoint, it is preferable to add a silane coupling agent to one or both of the chromate film and the organic resin film. This further improves the adhesion between the chromate film and the organic resin film.

In this case, the addition amount of the silane coupling agent is preferably 1 × 10 ^-5 times or more the weight of SiO ₂ contained in the chromate film or the organic resin film. Thereby, the adhesion between the chromate film and the organic resin film can be improved, so that the low temperature chipping resistance can be further improved.

Examples of silane coupling agents include vinyltrichlorosilane, vinyltris (βmethoxyethoxy) silane, vinyltriethoxysilane, vinyltrimethoxysilane, β- (3,4 epoxycyclohexyl).
Ethyltrimethoxysilane, γ-chrysidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, N-β (aminoethyl) γ-aminopropyltrimethoxysilane, N-β (aminoethyl)
Existing products such as γ-aminopropylmethyldimethoxysilane, γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane and γ-mercaptopropyltrimethoxysilane can be used, but these are used. An appropriate one may be selected depending on the combination with the organic resin.

Next, the present invention will be described with reference to examples.

[Examples] First, the coating weight "Zn-13wt" shown in Table 1 was used.
% Ni-plated steel sheet "and" Zn-9wt% Fe-plated steel sheet "were prepared. Then, these plated steel sheets were subjected to chromate treatment and clear film coating, and the zinc-based alloy plating shown in Table 1 above was applied on the steel sheet. A multi-layer coated steel sheet having a layer, a chromate film and an organic resin film was obtained.

In the above chromate treatment, Cr ³⁺ / (Cr ³⁺ + Cr ⁶⁺ ) = 0.7, PO ₄ ^{3− /Cr=0.5} (however, Cr = Cr ³⁺ + Cr ⁶⁺ ) The prepared chromate solution was used as a base, and a treatment solution prepared by appropriately adding SiO ₂ and a silane coupling agent was used.

Further, the clear film is made of urethane-modified epoxy resin as a base to be crosslinked with an isocyanate, and SiO ₂ and a silane coupling agent are appropriately added so that the glass transition point becomes 83 ° C. after baking. It was formed by the resin paint adjusted to.

Next, for each Zn-Ni alloy plated steel sheet produced in this manner, "low temperature chipping resistance" and "Cr
"Elution amount", "outer surface rust resistance" and "weldability" were investigated, and the results are shown in Table 2.

[0027]

[Table 1]

[0028]

[Table 2]

The following method was adopted for the performance evaluation of the plated steel sheet. [Low-temperature chipping resistance] After applying 2 coats and 2 baking treatments of 30 μ of electrodeposition coating and 40 μ of top coating for automobiles, -2
Cool to 0 ° C and give a pebbles impact with a gravure tester,
Attempts to peel cellophane tape at this location and measure the "peeling diameter at the plating-steel plate interface".

[Cr elution amount] 40 with an alkaline degreasing solution [using FC4410 (trade name) of Nippon Parkerizing Co.]
Immersion degreasing for 2 minutes at ℃
The adhered amount was measured by fluorescent X-ray and the elution amount was calculated.

[Rust resistance on the outer surface] After being subjected to a two-coat / two-bake treatment with an electrodeposition coating film of 30 μ and an automobile top coating composition of 40 μ,
After putting cross-cut flaws with a cutter knife and performing outdoor exposure including spraying salt water twice a week for 3 months, it was visually confirmed whether red rust was generated from the cut portion.

[Welding Property] A Cu-Cr alloy tip having a tip tip diameter of 6 mmφ was used, and squeeze time: 20 cycles, energization: 12 cycles, holding: 5 cycles (50 Hz), welding current: 10 kA, pressure: 200 kg. A spot welding continuous welding point test is performed, and the weldability is defined as the number of welding points where welding peeling occurs due to shear tension.

From the results shown in Table 2, it can be seen that the rust-preventing steel plate satisfying the conditions specified in the present invention is excellent in any of the above-mentioned performances, while the rust-preventing steel plate has the following constitutional conditions. If it does not meet the requirements of the invention, low temperature chipping resistance and Cr resistance
It is clear that the realization of a rust-proof steel sheet that has sufficient dissolution, outer surface rust resistance, and spot weldability is unstable.

[0034]

[Summary of Effects] As described above, according to the present invention, excellent corrosion resistance, paintability, and spot weldability are provided, and high resistance to low temperature chipping caused by stone splashes in cold regions is provided. It becomes possible to realize the rust-preventing steel plate that has been provided, and it is possible to expect a remarkable improvement in durability when applied to the steel plate for exteriors of automobile bodies.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasushi Hosoda No.3 Hikari, Kashima-machi, Kashima-gun, Kashima-gun, Ibaraki Prefecture Sumitomo Metal Industries, Ltd. Kashima Works

Claims (2)

[Claims] 1. A zinc-based alloy plating layer having a basis weight of 10 g / m ² or more on a steel plate, and b) an adhesion amount of 20 to 200 mg / m ^{2 in} terms of metallic chromium.
Characterized by comprising a preparative coating, c) an organic resin film having a thickness of 0.05 to 2.0Myu, the in this order - of chromate containing SiO ₂ of 0.05 to 4.0 in SiO ₂ / Cr weight ratio , Steel sheet for automobile exterior with excellent chipping resistance. 2. An organic resin film of 5 to 40% by weight of SiO ₂
A steel sheet for automobile exterior having excellent chipping resistance according to claim 1, which comprises: