JPS6320316B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a first layer that is effective against perforation, which is a form of automobile body corrosion.
This invention relates to a surface-treated steel sheet that has a two-layer structure of Ni-containing Zn alloy plating and a second layer Fe-containing Zn alloy plating, and has excellent corrosion resistance after painting.

Corrosion of automobile bodies has become a major problem in recent years, and corrosion of automobile bodies can be broadly divided into (1)
Deterioration of appearance and (2) safety and functional deterioration should be taken into consideration, but conventionally, emphasis has been placed on point (1), and for this reason, the difference between car body steel plates and car body paint coatings should be considered. Attention has been focused on the adhesion of the paint and the blistering phenomenon of the paint film due to corrosion after painting. However, in recent years, point (2), such as collision safety, has been receiving a lot of attention, and countermeasures have become necessary in conjunction with thinner steel plates for vehicle bodies to reduce weight. .

Therefore, the form of corrosion that causes safety and functional deterioration is
There are two types of corrosion: corrosion that involves a reduction in the thickness of an automobile steel plate over a flat area, and pitting corrosion, where holes are formed in the thickness direction within a narrow planar area. The corrosion resistance of the latter is evaluated based on the maximum corrosion depth of the steel plate, and therefore, high evaluations in these two areas are required for automotive steel plates from the viewpoint of safety and prevention of functional deterioration.

On the other hand, when automotive steel sheets are used in automobiles, they are increasingly being painted rather than being used as bare steel sheets, and due to advancements in painting technology, car bodies that previously could not be coated with paint are now being used. This is because paint can now be applied to the bag structure, etc. through electrodeposition coating, and this means that steel plates are more likely to be exposed to a corrosive environment in a painted form than if they were used bare. It means getting bigger.

Therefore, considering the above-mentioned forms of corrosion, it is important to improve the corrosion resistance of the steel plate after it has been painted rather than in its bare state, that is, to improve both the corrosion weight loss and the maximum corrosion depth. The corrosion resistance of a bare steel plate and the corrosion resistance of a painted steel plate do not match, and even if the corrosion resistance of a bare steel plate is good, the corrosion resistance of a painted steel plate is not the same. It cannot necessarily be said that there is. The reason for this is that corrosion of steel plates after painting occurs due to pinholes or scratches in the paint film, or even scratches caused by collisions with foreign objects while driving, and the corrosion is caused by the physical presence of the paint film, which causes the corrosion to occur when the paint film is exposed. This is because corrosion proceeds through a chemically or electrically different mechanism and form from that of steel plates. Therefore, it became necessary to study the corrosion resistance of coated steel plates from a completely different perspective from the conventional study of the corrosion resistance of bare steel plates.

Conventional rust-preventing steel sheets for automobiles are made of a single layer that has a sacrificial corrosion-preventing effect on the base steel.
Zn or Zn alloy plating is often used.

However, the present inventors conducted various studies on the well-known Zn-plated steel sheets from the viewpoint of corrosion resistance after painting, and found that hole corrosion after painting = maximum corrosion depth. However, in terms of corrosion accompanied by a decrease in plate thickness = corrosion loss, the loss is too large, so in a single layer of Zn plating or Zn alloy plating layer, the maximum corrosion depth and corrosion loss are It was discovered that the characteristics for both cannot be satisfied.

As a result of repeated research on Zn alloy plating layers consisting of multiple layers, the present inventors found that the first layer was a Zn-Ni alloy plating layer containing Ni, and the second layer above was a Zn-Ni alloy plating layer containing Fe. By combining the plating of the two contained Zn-Fe alloy plating layers and specifying the Ni content in the first layer and the Fe content in the second layer, it is possible to By specifying the quantitative relationship between the adhesion amounts of the two layers, it was discovered that a steel plate with excellent properties for both maximum corrosion depth and corrosion loss could be obtained, and the present invention developed a surface with excellent corrosion resistance after painting. They completed the process of producing treated steel sheets.

That is, the surface-treated steel sheet with excellent corrosion resistance after painting according to the present invention is characterized by electroplating of a Zn--Ni alloy containing 2wt% to 7wt% of Ni as a first layer on one or both sides of the steel sheet. The second layer thereon is a Zn--Fe alloy electroplated layer containing 65 wt% or more of Fe.

In addition, the adhesion amount of the second layer Zn-Fe alloy electroplated layer is 2 g/m2 or ^more , and the Zn-Ni alloy layer of the first layer is
Zn of the second layer relative to the amount of alloy electroplated layer
-The ratio of the amount of the Fe alloy electroplated layer (second layer amount/first layer amount) is set to 0.1 to 1.0.

Surface-treated steel sheet with excellent corrosion resistance after painting according to the present invention (hereinafter sometimes simply referred to as the steel sheet of the present invention. Also, electroplating is sometimes simply referred to as plating).
will be explained in detail.

The steel sheet of the present invention has two types of Zn alloy electroplated layers on one or both sides of the steel sheet, and single-layer Zn or Zn alloy electroplating is difficult to achieve both maximum corrosion depth and corrosion loss. However, it has excellent properties that cannot be satisfied with other materials.

In the steel plate of the present invention, the first layer on the steel plate substrate
The Zn alloy plating layer is an important plating layer that determines the maximum corrosion depth and corrosion loss.
In terms of bare corrosion resistance of plated steel sheets, Zn
-Ni alloy plating is the most suitable plating because it has higher corrosion resistance than other Zn alloy plating.For example,
Judging from the corrosion loss after painting, Zn-Fe alloy plating,
Zn alloy plating such as Zn-Al alloy plating and Zn-Cr alloy plating has a larger corrosion loss than Zn-Ni alloy plating.

Therefore, in order to achieve excellent corrosion resistance after painting in terms of maximum corrosion depth and corrosion loss, Zn-
This is largely due to the Ni content of the Ni alloy plating layer; in other words, when the Ni content exceeds 7wt%, the maximum corrosion depth increases and the puncture resistance deteriorates; If it is less than that, the corrosion weight loss will increase. Therefore, the Ni of the first Zn-Ni alloy plating layer
The content is 2wt% to 7wt%.

Next, the second Zn-Fe alloy plating layer on the first Zn-Ni alloy plating layer in the steel sheet of the present invention maintains the corrosion-preventing effect of the first Zn-Ni alloy plating layer. This is important in maintaining time, and even if the first Zn-Ni alloy plating layer has excellent corrosion resistance after painting, if the corrosion resistance deteriorates over time, corrosion protection may be necessary. This is not desirable in terms of effectiveness, and therefore, a second Zn-Fe alloy plating layer is provided on top of the first Zn-Ni alloy plating layer to prevent economic deterioration. be.

In addition, this Zn-Fe alloy plating has better adhesion to the paint film than other alloy platings, and has a synergistic effect with the first layer of Zn-Ni alloy plating to prevent sacrificial corrosion on steel sheets. The effect can be maintained for a longer time. This sacrificial corrosion prevention effect contributes to improving the corrosion resistance after painting, especially preventing the maximum corrosion depth.If this sacrificial corrosion prevention effect is small, corrosion in the thickness direction of the steel plate will be accelerated, and the maximum corrosion depth will be increased. Depth increases. Furthermore, the second Zn--Fe alloy plating layer has not only a sacrificial corrosion-preventing effect but also corrosion resistance after painting. Therefore, in order to achieve the various effects described above, the Fe of the second Zn-Fe alloy plating layer must be
The content must be at least 67wt%, and
If the content is less than 65 wt%, the corrosion loss will significantly increase. The reason for this is that the Zn content is 35wt%
If the
On the contrary, it is presumed that the synergistic effect with the first layer of Zn--Ni alloy plating layer is lost and its own corrosion resistance is impaired.

In addition, in the steel sheet of the present invention, in order to further improve the corrosion resistance after coating, the amount of the second Zn-Fe alloy plating layer is set to 2 g/m ² or more, and the first layer
Adhesion amount of Zn-Ni alloy plating layer and Zn of second layer
- The ratio of the Fe alloy plating to the deposited amount (second layer deposited amount/first layer deposited amount) is preferably 0.1 to 1.0, and for severe corrosive environments, it is recommended to be in the range of 0.4 to 0.9. .

The adhesion amount of this second Zn-Fe alloy plating layer is
The reason why the coating amount is 2 g/m ² or more is that if the coating amount is less than 2 g/m ² , it will approximate the case of a single Zn-Ni alloy plating layer, so the maximum corrosion depth after painting = hole depth will increase. Therefore, these corrosion prevention effects cannot be expected, and
First layer of Zn-Ni alloy plating layer and second layer of Zn-
The total amount of Fe alloy plating layer deposited is generally determined by the corrosive environment in which the steel plate is used, but if the corrosive environment is severe, it is necessary to deposit it thickly at 40 g/m ² or more. Therefore, in less severe corrosive environments, it is best to use a thin coating weight of about 20 g/m ² in consideration of the workability and weldability of the steel plate.

Next, we will explain the ratio between the amount of Zn--Ni alloy plating in the first layer and the amount of Zn--Fe alloy plating in the second layer.If this ratio is less than 0.1 or exceeds 1.0, In either case, it is difficult to guarantee the effect of preventing the maximum erosion depth.

An example of a surface-treated steel sheet with excellent corrosion resistance after painting according to the present invention will be described.

Example 1 A cold rolled steel plate having a thickness of 0.8 mm was electrolytically degreased and pickled using a conventional method, and then a first layer of Zn--Ni alloy was electroplated under the conditions shown below.

Plating bath composition: Zinc sulfate, nickel sulfate Plating bath temperature: 60℃ Plating bath PH: 1.5 Current density: 30A/dm ² Anode: Pt Based on these plating processing conditions, adhesion can be achieved by adjusting the bath composition. The amount was set constant at 17g/ ^m2 , but
Zn-Ni alloy electroplated steel sheets with different Ni contents were fabricated.

After washing these Zn-Ni alloy electroplated steel sheets with different Ni contents, a second plated plate was applied under the plating conditions shown below.
A layer of Zn-Fe alloy was electroplated.

Plating bath composition: ferrous sulfate, zinc sulfate, ammonium sulfate Plating bath temperature: 60℃ Plating bath PH: 2.0 Current density: 30A/dm ² Plating coverage: 7g/m ² Zn-Fe alloy electroplating Fe content of the layer: 85wt% In this way, the steel sheets subjected to the alloy electroplating of the first and second layers were subjected to immersion zinc phosphate treatment and cationic electrodeposition coating according to the automotive painting process. After that, a cross cut is made in the coating film to reach the substrate.
5% salt water spray (50℃×16Hr) → Drying (50℃×
A composite test consisting of a corrosion cycle of 3 hours) → 5% salt water immersion (50℃ x 16 hours) → natural standing (2 hours) was conducted for 30 days.
I did the cycle.

After removing the paint film and rust from the steel plate after the test, the depth of the corrosion hole was measured using a dial gauge to determine the maximum corrosion depth. The results are shown in FIG. In Figure 1, there are seven types of test materials, and the circle mark indicates the Zn of the first layer.
-Ni content of Ni alloy plating layer is 2wt%, 3.8wt
%%, 6.5wt%% of the present invention steel plate, ● mark indicates
Comparative materials with Ni contents of 0.5wt%, 8.5wt%, 11wt%, and 14wt%, which are outside the range of the Ni content of the steel sheet of the present invention, are shown. In addition, a steel plate having a single layer of Zn-Ni alloy plating layer marked with △ in Fig. 1 was subjected to an automobile painting process and a combined cycle test in the same manner as the steel plate of the present invention, and this steel plate (plating amount:
24g/m ² , the Ni content was changed in the same way as marked with ○ and marked with ●. ) results are also shown. This first
As can be seen from the figure, comparative materials and
The maximum corrosion depth of both the Zn-Ni single layer alloy coated steel sheets is larger than that of the steel sheet of the present invention, proving that the steel sheet of the present invention has excellent corrosion resistance at the maximum corrosion depth. .

In addition, the weight of the steel plate with the Zn-Fe alloy plating layer and the Zn-Ni alloy plating layer with the paint film and rust removed was measured, and the corrosion loss of the steel plate was measured by a combined cycle test. The results are shown in Figure 2. show. As is clear from FIG. 2, when the Ni content of the first Zn-Ni alloy plated layer is less than 2wt%, the corrosion weight loss tends to increase rapidly, and when the Ni content exceeds 2wt%, the corrosion loss occurs in the steel sheet of the present invention. It can be seen that it has excellent corrosion resistance against weight loss.

Example 2 A first Zn--Ni alloy plating layer (deposition amount: 20 g/m ² , Ni content: 6.5 wt%) was applied to a cold-rolled steel plate with a thickness of 0.8 mm using the same method as in Example 1. 2nd layer Zn-
Fe alloy plating layer (coating amount: 4g/m ² , Fe content:
A steel plate of the present invention having a corrosion resistance of 85 wt%) was produced, and the sacrificial corrosion protection effect of the plating layer on the steel plate was investigated.
The results are shown in FIG. Moreover, a schematic diagram of the test apparatus is shown in FIG. In the test apparatus shown in Fig. 6,
A is the steel plate of the present invention (anode), B is a bare cold-rolled steel plate (cathode), these are connected through a 100Ω resistor, and C is a 5% NaCl aqueous solution (PH: 7,
(room temperature), and the current flowing from the cold-rolled steel sheet to the plated steel sheet is measured as a potential difference using a potentiometer D. These two steel plates are sealed from the NaCl aqueous solution except for the shaded area S (cm ² ), and the coupling current is calculated by the following equation.

Coupling current (A/cm ² ) = Potential difference generated by resistance (V) / 100Ω ÷ S (cm ² ) In Figure 3, the decrease in coupling current as the steel plate immerses time elapses, ○ indicates the steel plate of the present invention, and △ indicates the Zn-
Steel plate with single layer of Ni alloy plating (coating amount:
20g/m ² , Ni content: 6.5wt%).

As is clear from FIG. 3, the sacrificial corrosion protection effect of the steel plate of the present invention lasts for about 168 hours, but the comparative material with a single layer of Zn-Ni alloy plating lasts only about 108 hours, and the sacrificial corrosion protection effect of the steel plate of the present invention lasts only about 108 hours. The sacrificial anti-corrosion effect of the coating layer that protects the steel plate has an excellent retention time over a very long period of time.

Example 3 A first layer of Zn-Nii alloy was electroplated on a cold-rolled steel plate with a thickness of 0.8 mm using the same method as in Example 1 (coating amount: 17 g/m ² , Ni content: 6.5 wt).
%), then the second layer provided on the first layer of Zn-Ni alloy plating layer has an Fe content of 23 wt%,
The deposited amount of different Zn-Fe alloy electroplated layers of 45wt%, 69wt%, and 85wt% was kept constant at 7g/ ^m2 . The Fe content was adjusted by adjusting the amount of zinc sulfate in the plating bath.

These plated steel plates were subjected to dipping phosphate treatment and cationic electrodeposition coating in the same manner as in Example 1, and then a combined cycle test was conducted for 30 cycles to investigate the weight loss due to corrosion of the steel plates. This result is the fourth
As shown in the figure. In this figure 4, the circle mark indicates the second layer.
Fe content of Zn-Fe alloy plating layer is 69wt%, 85wt
% of the steel sheet of the present invention, the ● mark indicates that the Fe content is 23wt%,
As can be seen from Fig. 4, the steel sheet of the present invention with an Fe content of 65 wt% or more has excellent properties against corrosion loss.
It is clear that comparative materials with Fe content of less than 65 wt% are inferior.

Example 4 A cold-rolled steel plate with a thickness of 0.8 mm was coated with a Zn-Ni alloy electroplated layer (Ni
A steel plate of the present invention was prepared with a second Zn-Fe alloy electroplated layer (Fe content: 85 wt%). At this time, the total deposited amount of the two layers was kept constant at 24g/ ^m2 , and the first layer Zn-Ni alloy plated layer and the second layer
Steel sheets with different ratios of the amount of Zn-Fe alloy plating layer deposited were used. That is, the ratio of second layer adhesion amount/first layer adhesion amount is 0 (Zn-Ni alloy plated single layer), 0.2,
It was changed to 0.4, 0.7, 1.0, 1.4, and 2.0.

These steel plates were subjected to immersion phosphate treatment and cationic electrodeposition coating in the same manner as in Example 1, and then subjected to a combined cycle test for 30 cycles to measure the maximum corrosion depth. The results are shown in FIG. It is clear from Fig. 5 that the maximum erosion depth is the lowest when the ratio of the adhesion amount of the first layer to the second layer is in the range of 0.1 to 1.0, and this range has an excellent effect on the depth of re-major erosion. I understand that.

As explained above, the surface-treated steel sheet with excellent corrosion resistance after painting according to the present invention has the above structure, so it has excellent corrosion resistance after painting and maximum corrosion depth. It is particularly effective as a rust-proof steel plate for automobiles, and is extremely useful industrially.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the Ni content of the Zn-Ni alloy plating layer and the maximum corrosion depth, and Figure 2 is a diagram showing the relationship between the Ni content and corrosion loss of the Zn-Ni alloy plating layer. , Figure 3 shows the temporal change in coupling current, and Figure 4 shows the change in the coupling current over time.
Figure 5 shows the relationship between Fe content and corrosion loss.
Figure 6 is a diagram showing the relationship between the ratio of Zn--Ni alloy plating layer (amount of adhesion) and maximum corrosion depth, and is a schematic diagram of a coupling current measuring device. A...Steel plate of the present invention, B...Cold rolled steel plate, C...
...NaCl aqueous solution, D...potentiometer, S...
...Exposed part.

Claims (1)

[Claims] 1. Ni is applied as a first layer on one or both sides of a steel plate.
After painting, which is characterized by having a Zn--Ni alloy electroplated layer containing 2 wt% to 7 wt%, and a Zn--Fe alloy electroplated layer containing 65 wt% or more of Fe as a second layer thereon. A surface-treated steel sheet with excellent corrosion resistance. 2. The amount of Zn--Fe alloy electroplated layer of the second layer is 2 g/m2 ^or more, and the amount of Zn--Fe alloy of the second layer is relative to the amount of electroplated Zn--Ni alloy layer of the first layer. Ratio of electroplated layer adhesion amount (second layer adhesion amount/
The surface-treated steel sheet with excellent corrosion resistance after painting as claimed in claim 1, wherein the coating weight of the first layer is 0.1 to 1.0.