JPH05302193A - Zn-cr plated steel sheet excellent in powdering property and corrosion resistance after working and its production - Google Patents

Abstract

(57) [Summary] [Purpose] Excellent powdering and corrosion resistance after processing
Zn-Cr-based plated steel sheets for automobiles, home appliances, building materials, etc.
And the manufacturing method thereof. [Structure] Having a Zn-Cr alloy plating layer on the steel plate surface,
The plating layer is a 3% NaCl aqueous solution (pH = 4 to 8, 25
Cathode and anode content measured in atmosphere (° C)
Corrosion potential and corrosion current density obtained by extrapolation from polar curves
The value of degree is corrosion potential = -900 to -1100 mV
vs S.C.E. and corrosion current density ≦ 1μA / cm² In the range of
And the current density (μA / cm) on the X-axis (horizontal axis)² )
The logarithm of the logarithm and the potential (mV vs. S.C.E.) on the Y-axis (vertical axis)
When taken, the shape of the anodic polarization curve is X = −3 to +3.
Is a linear monotonic increase with no inflection point in the range
Zn-Cr based plating with excellent ringability and corrosion resistance after processing
Plain steel plate. This steel plate is Zn ²⁺And Cr³⁺As the main component
The malic acid and / or malate as Cr³⁺Against
Use a bath with a molar ratio of 0.1 or more and a pH of 0.5 to 3.
It is obtained by electrolyzing a steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention mainly relates to rust-preventing steel plates for automobiles, home appliances, building materials, etc., which have been required to have stronger rust-preventing power in recent years, and particularly have excellent powdering properties and corrosion resistance after processing. The present invention relates to a rusted steel plate and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, a Zn-plated steel sheet has been generally used as a rustproof steel sheet. This utilizes the sacrificial anticorrosion effect of the Zn plating layer, which is lower in potential than the base steel sheet in the atmosphere. By the way, the sacrificial anticorrosive effect of the Zn plating layer lasts as long as the Zn plating layer is sufficiently present on the surface of the steel sheet, but can no longer be expected after the Zn plating layer is eluted. However, in the case of pure Zn plating, there is a problem that the corrosion rate of the plating layer itself is high, and therefore the period for which a sufficient sacrificial anticorrosive effect can be maintained is short.

For this reason, when pure Zn plating is used, it is common practice to secure corrosion resistance by increasing the basis weight. However, in the case of using a thick basis weight, there is a problem in economic efficiency, and in the case of using it as an automobile outer plate, powdering is likely to occur such that the plating layer peels off during press forming, and there is a problem in workability.

Therefore, Zn-Ni alloy-plated steel sheets, which have better corrosion resistance even with a lighter weight than pure Zn plating, have been partially adopted. However, since the corrosion potential of Zn-Ni alloy plating is closer to that of a steel plate than that of pure Zn plating, the corrosion resistance of the plating layer itself is good, but it cannot be said that the sacrificial anticorrosion ability is sufficient. If exposed, red rust is likely to occur.

Therefore, Zn-Cr type plated steel sheets have been developed for the purpose of further improving the corrosion resistance.
For example, in Japanese Examined Patent Publication No. 62-6758, the plating layer contains 0.1
˜30 wt% Cr and 0.01% alumina
A plated steel sheet containing ~ 3 wt% is disclosed. Further, JP-A-60-50195 discloses a plated steel sheet containing 10% or more of Cr in the plating layer. In addition to this, as a Zn-Cr-based plated steel sheet, JP-A-64-5533.
No. 97 etc. are disclosed.

[0006]

Certainly, these Zn-
The Cr-based plated steel sheet has sufficient corrosion resistance as long as it is not processed. However, in any of these plated steel sheets, the corrosion resistance is improved by the increase of the Cr content in the plated layer, but the powdering property is deteriorated. In addition, since cracks are likely to occur during processing, the exposed area of the base steel increases, and the sacrificial anticorrosion effect of the plating layer does not fully reach, and corrosion resistance deteriorates.

Accordingly, an object of the present invention is to provide automobiles, home appliances, which are excellent in powdering property and corrosion resistance after processing,
An object is to provide a rustproof steel sheet for building materials and the like and a method for producing the same.

[0008]

That is, the present invention has a Zn—Cr alloy plating layer on the surface of a steel sheet, and the plating layer is a 3% NaCl aqueous solution (pH = 4 to 8, 25 ° C., under air). The values of the corrosion potential and the corrosion current density obtained by extrapolation from the cathode and anode polarization curves measured in are the corrosion potential = -900 to -1100 mV vs SC, respectively.
E. and corrosion current density ≤ 1 μA / cm ² , the X axis (horizontal axis) is the logarithm of the absolute value of the current density (μA / cm ² ), and the Y axis (vertical axis) is the potential (mV vs. When SCE) is taken, the shape of the anodic polarization curve is a linear monotonic increase with no inflection point in the range of X = −3 to +3, and it has excellent powdering property and corrosion resistance after processing. Zn-C
An r-based plated steel sheet is provided. The present invention also provides
In manufacturing the Zn-Cr-based plated steel sheet, Zn
²⁺ and Cr ³⁺ as main components, containing malic acid and / or malate in a molar ratio of at least 0.1 with ^respect to Cr ³⁺ ,
Z having excellent powdering property and corrosion resistance after working, characterized by electrolyzing a steel sheet using a bath having a pH of 0.5 to 3
The present invention provides a method for manufacturing an n-Cr-based plated steel sheet.

The present invention will be described in more detail below. The Zn-Cr-based plated steel sheet of the present invention preferably has a Zn-Cr-based plating adhesion amount of 10 to 40 g / m ² . The reason is that if it is less than 10 g / m ² , the corrosion resistance is insufficient, and if it exceeds 40 g / m ² , the economical superiority to the pure Zn-plated steel sheet having a high basis weight is lost and the powdering property is deteriorated. 20 to 30 g / m ² is optimum.
Further, the Cr content in the plating layer is appropriately 1 to 30 wt%. The reason is that if it is less than 1 wt%, the corrosion resistance is insufficient, and if it exceeds 30 wt%, the powdering property deteriorates. Desirably, 10 to 20 wt% is optimum.

The Zn-Cr system plating of the present invention is Zn-C.
r binary alloy plating may also be used, Ni,
Ti, Fe, Sn, Pb, Cu, C, Mg, P, Mn,
Multi-component alloy plating containing a third element such as Co, Mo, Al or Si may be used. Alternatively, Al ₂ O ₃ , Si is contained in the matrix of the Zn-Cr binary or multi-component alloy plating layer.
Dispersion plating containing various dispersed particles such as O ₂ , TiO ₂ , BaCrO ₄ , and ZrO ₂ may also be used. Essentially, as can be seen from FIG. 1, 3% NaCl aqueous solution (pH = 4-8, 2,
Corrosion potential and corrosion current density values obtained by extrapolation from cathode and anodic polarization curves measured at 5 ° C. in air) are corrosion potential = −900 to −1000 mV, respectively.
vs SCE and corrosion current density ≤ 1 μA / dm ² , the X axis (horizontal axis) is the logarithm of the absolute value of the current density (μA / dm ² ), and the Y axis (vertical axis) is the potential (mV vs. When SCE) is taken, the shape of the anodic polarization curve shows a property such that it has a linear monotonic increase with no inflection point in the range of X = −3 to +3.

Next, the effect of the present invention will be described. Figure 1
As an example, Zn-13 wt% Cr alloy-plated steel sheet, pure Zn-plated steel sheet and Zn-13 wt% according to the present invention
The polarization curves of Ni alloy plated steel sheets are shown for comparison. Further, FIG. 2 shows Zn-13 wt% Cr manufactured by the conventional technique.
Alloy-plated steel sheet and Zn-13 wt% according to the present invention
The polarization curves of Cr alloy plated steel sheets are shown for comparison.

The following can be seen from FIG. The corrosion potential of the Zn-13 wt% Cr alloy-plated steel sheet according to the present invention is slightly higher than that of the pure Zn-plated steel sheet,
It is baser than Zn-13 wt% Ni alloy plated steel sheet.
Therefore, the Zn-Cr alloy-plated steel sheet according to the present invention is Zn-
The sacrificial anticorrosion effect on the base steel sheet is greater than that on the Ni alloy plated steel sheet, and the occurrence of red rust can be suppressed. The corrosion current density of the Zn-13 wt% Cr alloy-plated steel sheet according to the present invention is significantly smaller than that of the pure Zn-plated steel sheet. That is, the corrosion rate of the plating layer is slow, and the sacrificial anticorrosion effect can be maintained for a long period of time. Zn-13 wt% of the present invention
Since the anode polarization curve of the Cr alloy plated steel sheet shows a linear rise, it is considered that the corrosion reaction is dominated by the cathode, and the cathode current is significantly suppressed as compared with the pure Zn plated steel sheet, so the corrosion current is It is considered that the density becomes smaller.

As shown in FIG. 2, Zn-- manufactured by the conventional technique.
It can be seen that the 13 wt% Cr alloy-plated steel sheet has a corrosion potential and a corrosion current density value within the scope of the present invention, but the shape of the anode polarization curve is different from that of the present invention. The anodic polarization curve of the present invention rises linearly, whereas the anodic polarization curve of the conventional method once rises in a downward convex shape and then has an inflection point and becomes an upward convex shape.

According to the knowledge of the inventors of the present application, in any Zn-Cr alloy-plated steel sheet showing such an inflection point on the anode polarization curve, the powdering property is rapidly increased due to the increase of the Cr content in the plating layer. In addition, the deterioration of corrosion resistance due to processing is remarkable. The deterioration of corrosion resistance due to processing is considered to be due to the increase of the exposed area of the base steel due to the occurrence of cracks. Although the reason why the rising of the anodic polarization curve is not linear is not clear yet, the inventors of the present application presume that it may be due to nonuniformity of the plating layer or co-deposition of impurities in the plating layer. ..

FIG. 3 shows Zn-13 wt% C according to the present invention.
Depth direction analysis result of r alloy plated steel sheet by GDS,
FIG. 4 shows Zn-13 wt% Cr manufactured by the conventional technique.
An example of the depth direction analysis result by GDS of an alloy plated steel sheet is shown. 3 and 4 are both 20g / plated coating weight
This is the result of measurement under the same discharge condition for the m ² sample. By comparing FIG. 3 and FIG. 4, Zn-
The plating layer of the 13 wt% Cr alloy-plated steel sheet shows a uniform distribution of Zn and Cr in the depth direction, whereas the Zn-13 wt% Cr alloy-plated steel sheet manufactured by the conventional technique has a bulk coating layer. Z near the surface compared to
It can be seen that the n concentration seems to be high, S eutectoid is observed in the plating layer, and the sputtering rate is slow. By the way, S was not detected in the Zn-Cr alloy plated steel sheet of the present invention. Of course GD
Although it should not be determined only by the data of S, Zn of the conventional method
One of the reasons why the powdering property of the -Cr-plated steel sheet significantly deteriorates when the Cr content in the plating layer increases is the poor adhesion of the plating layer due to such eutectoid impurities.

Next, the manufacturing conditions of the Zn-Cr type plated steel sheet of the present invention will be described. In order to produce a Zn-Cr-based plated steel sheet that exhibits a polarization curve that satisfies the requirements of the present invention, malic acid and / or malate is added to a plating bath containing Zn ²⁺ and Cr ³⁺ as main components. It is necessary. The amount of addition varies depending on the concentration and pH of the plating bath, the electrolysis conditions, etc., and therefore cannot be generally stated, but at least a molar ratio of at least 0.1 to Cr ³⁺ in the bath is necessary. A generally known bath such as a sulfuric acid bath or a chloride bath can be used as the supporting electrolyte, but it is necessary that the supporting electrolyte is an acidic bath having a pH range of 0.5 to 3. Particularly in the case of a sulfuric acid bath having a relatively lower conductivity than that of a chloride bath, it is effective to add various sulfates as a conduction aid, but it is not always necessary.

In addition, it is possible to add various kinds of surfactants which are generally known to have the effect of refining the crystal grains of Zn plating. The most important thing is to add malic acid and / or malic acid salt to the plating bath. If these chemicals are not added, Zn-Cr type plated steel sheet showing a polarization curve satisfying the requirements of the present invention. Cannot be manufactured. The plating current density can be appropriately selected in the range of 40 to 200 A / dm ² , but 60 to 120 A / dm ² is desirable in view of productivity. The bath temperature is preferably 30 to 70 ° C., and the liquid is preferably stirred at a relative flow rate of about 0.5 to 3 mps with respect to the steel plate.

[0018]

EXAMPLES The effects of the present invention will be described more specifically with reference to the following examples.

(Example) Table 1 shows the plating conditions of the plating baths used in the examples of the present invention and comparative examples, the alloy plating composition obtained, and the shape of the anode polarization curve of the obtained sample.

Comparative Example 1 is Zn ²⁺ , Cr ³⁺ , a supporting electrolyte,
In this example, the plating was performed from a bath containing only the conductive additive and the conduction aid, but the Cr deposition efficiency was low and the Cr content in the plating layer was 1
Only a sample of less than 0 wt% could be prepared. The shapes of the anodic polarization curves measured using the sample of Comparative Example 1 all had the same inflection point as shown in FIG.

Comparative Example 2 is a case where a surfactant was added to the bath of Comparative Example 1. The precipitation efficiency of Cr was improved as compared with Comparative Example 1, but the shape of the anode polarization curve of the sample of Comparative Example 2 had an inflection point as in Comparative Example 1.

Inventive examples are cases where malic acid and / or sodium malate were added in a molar ratio of 0.1 or more with respect to Cr ³⁺ . It showed a linear rise as shown in.

FIG. 5 shows the results of measuring the amount of weight reduction as the amount of powdering by peeling the processed portion with tape after performing the hat drawing with a diameter of 55 mm. In the comparative example, when the Cr content in the plating layer increases, the powdering amount rapidly increases, and when the Cr content is ≧ 10 wt%, Zn-1
It can be seen that the powdering property is worse than that of the 3% Ni alloy plated steel sheet. On the other hand, in the invention example of the present application, even if the Cr content is increased, the powdering property is less deteriorated and is 25 wt%.
It can be seen that the Cr content is equal to or higher than that of the Zn-13 wt% Ni alloy plated steel sheet.

FIG. 6 shows the results of a complex corrosion test of the following cycle performed on the sample subjected to the above-mentioned hat drawing. In the present invention example, it can be seen that the corrosion resistance is improved due to the increase in the Cr content in the plating layer, and that the Cr content of 7 wt% or more shows superior corrosion resistance to the Zn-13 wt% Ni alloy plated steel sheet. On the other hand, in the comparative example, C
Even if the r content is increased, the corrosion resistance is not significantly improved. The reason for this is that in the case of the comparative example, the powdering property deteriorates due to an increase in the Cr content, so that even with the same Cr content, the cracks in the plating layer due to processing are larger and the exposed area of the base steel is larger than in the present invention example. It is thought to be because of this.

(Complex corrosion test cycle) Salt spray (4
Time) → drying (60 ° C., 60% RH, 2 hours) → wetting (50 ° C., 98% RH, 2 hours) was set as one cycle, and this was repeated.

[0026]

[Table 1]

[0027]

As described above, the Zn of the present invention is
Since the Cr-plated steel sheet has a good corrosion resistance of the plating layer itself and has a strong sacrificial anticorrosion effect on the base steel sheet and also has an excellent powdering property, good corrosion resistance can be maintained even after working.

[Brief description of drawings]

FIG. 1 is a diagram showing cathode and anode polarization curves of a Zn-13 wt% Cr plated steel sheet (Example of the present invention), a Zn-13 wt% Ni plated steel sheet, and a pure Zn plated steel sheet.

FIG. 2 Zn-13 wt% according to the present invention and prior art
It is a figure which shows the cathode and anode polarization curve of Cr alloy plating steel plate.

FIG. 3 is a view showing a depth direction profile by GDS of a Zn-13 wt% Cr alloy-plated steel sheet according to the present invention.

FIG. 4 is a view showing a depth-direction profile by GDS of a Zn-13 wt% Cr alloy-plated steel sheet according to a conventional technique.

FIG. 5: Powdering amount and Zn by hat type drawing
It is a figure which shows the relationship with -Cr alloy plating composition.

FIG. 6 is a diagram showing the relationship between bare corrosion resistance of a Zn—Cr alloy-plated steel sheet after hat-type drawing and the Cr content in the plating layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuhiro Hasegawa 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division (72) Inventor Kazuo Mochizuki Kawasaki, Chuo-ku, Chiba-shi, Chiba Town No. 1 Kawasaki Iron & Steel Co., Ltd. Technical Research Division (72) Nobuyuki Morito No. 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki Steel Co., Ltd. Technical Research Division

Claims (2)

[Claims] 1. A Zn-Cr alloy plating layer is provided on the surface of a steel sheet, and the plating layer is a 3% NaCl aqueous solution (pH = 4 to 8,
The values of the corrosion potential and the corrosion current density obtained by extrapolation from the cathode and anode polarization curves measured at 25 ° C. in the atmosphere) are corrosion potential = −900 to −1100, respectively.
Within the range of mV vs SCE and corrosion current density ≤ 1 μA / cm ² , and the current density (μA / cm ² ) on the X-axis (horizontal axis)
The logarithm of the absolute value of is the potential (mV vs SC) on the Y-axis (vertical axis).
E.), the shape of the anodic polarization curve is X = -3
A Zn-Cr-based plated steel sheet excellent in powdering property and corrosion resistance after working, which has a linear monotonic increase with no inflection point in the range of +3. 2. A process for producing the Zn—Cr-based plated steel sheet according to claim 1, wherein Zn ²⁺ and Cr ³⁺ are the main components, and malic acid and / or malate is a mole relative to Cr ^3+. A method for producing a Zn-Cr-based plated steel sheet excellent in powdering property and corrosion resistance after working, characterized in that the steel sheet is electrolyzed using a bath having a ratio of 0.1 or more and a pH of 0.5 to 3.