JPH0625887A - Anti-corrosion steel plate with excellent corrosion resistance and formability - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a rust-preventing steel sheet that has sufficient corrosion resistance and excellent formability. [Structure] An alloy composed of Zn and Cr formed by electrodeposition, having a cubic crystal system and a lattice constant a = 3.00.
A rust-preventing steel plate excellent in corrosion resistance and formability, which is obtained by applying a Zn-Cr alloy plating substantially composed of only a phase having a structure of ~ 3.06Å.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rust-preventing steel plate having excellent properties not only in corrosion resistance but also in formability among various properties required for a rust-preventing steel plate used for automobiles and the like.

[0002]

2. Description of the Related Art Currently, rust-preventing steel sheets for automobiles are put into practical use: electric Zn-plated steel sheets, electric Zn-Ni alloy-plated steel sheets, electric Zn-Fe alloy-plated steel sheets, alloyed hot-dip Zn-plated steel sheets and others. There are various types, but all are Zn-based plated steel sheets. This is Z for steel
It utilizes the sacrificial anticorrosion effect of n. The simplest and clearest way to improve the corrosion resistance is to increase the coating weight (hereinafter referred to as the weight per unit area), but the increase of the weight per unit is accompanied by deterioration of workability, weldability and other quality. So Zn
Attempts have been made to obtain the same corrosion resistance with a smaller basis weight than pure Zn by alloying with other elements. As an effect of alloying, it is expected that the corrosion potential is brought closer to that of steel to suppress the corrosion rate of the plating layer itself and to stabilize the corrosion product.
However, in the conventional Zn-based alloy plated steel sheet, the effect of improving corrosion resistance by alloying is still insufficient.

Therefore, in recent years, attempts have been made to add Cr as an alloying element to the Zn-based plating layer. For example,
Japanese Unexamined Patent Publication No. 1-191797 and Japanese Unexamined Patent Publication No. 3-120393 are proposed. Although the corrosion resistance is certainly improved in the case of Zn-Cr alloy plating compared with the conventional Zn-based alloy plating, even if the coating weight is small, the contribution of the coating weight to the corrosion resistance is still large.

As an example, FIG. 1 shows the result of a test assuming corrosion when used on the outer surface of an automobile. Since the outer surface of the vehicle body is usually painted, corrosion starts from the damaged portion of the coating film caused by stone splashes. The most reliable corrosion test for automobile bodies is the actual vehicle test.However, due to the long evaluation period and cost problems, the coating test pieces were generally scratched. After that, a method of exposing to the atmosphere or a method of using a complex corrosion tester that artificially creates a corrosion-accelerated environment by appropriately combining a salt spray with a drying and wetting cycle is performed. The result of Fig. 1 shows the blister width of the coating film after the zinc phosphate chemical conversion treatment and the coating of 3 coats were performed to scratch the base steel with a cutter knife, and further the complex corrosion test of the cycle shown in Fig. 2 was performed for 2 months. The result is shown.

Pure Zn shown in the figure is a Zn-plated steel sheet prepared by the electric Zn plating method according to a conventional method (hereinafter referred to as EG). GA is a commercially available galvannealed steel sheet. Zn-13 wt% Ni has a Ni content of 13 wt
% Commercial Zn-Ni alloy plated steel sheet (hereinafter Zn
-Ni). Zn-13 wt% Cr is a Zn-Cr alloy plated steel sheet with a Cr content of 13 wt% (hereinafter Z
referred to as n-Cr). Hereinafter, the unit symbol (g / m ² ) may be omitted when indicating the basis weight. For example, when the basis weight is 30 g / m ² , it is indicated as 30 basis weight. As can be seen from FIG. 1, all the alloy-plated steel sheets have improved corrosion resistance as compared with EG having the same basis weight, but the effect is greatest in the Zn—Cr alloy-plated steel sheet.

[0006]

However, since the effect of the basis weight is great, the Zn-Cr alloy plated steel sheet with 10 basis weight is superior to the EG with 20 basis weight, but the EG and Zn-Ni alloy coated steel sheet with 30 basis weight are superior. Inferior. Also,
In order to have corrosion resistance equivalent to that of the alloyed hot-dip galvanized steel sheet which is used most in Japan at present and has a basis weight of 60 g / m ² , 30 g / m ² is required even for Zn-Cr alloy-plated steel sheet. is necessary. As described above, in any of the plating types, the corrosion resistance is improved by the increase of the weight per unit area, but the change is particularly remarkable when the weight per unit area is from 10 to 30 g / m ² . However, particularly in the case of a Zn-Cr alloy-plated steel sheet, the formability rapidly deteriorates with an increase in the weight per unit area, so that there is a problem that the formability is poor and the practicality is low even though the corrosion resistance is excellent.

Therefore, an object of the present invention is to provide a rust preventive steel sheet which is excellent in formability in addition to corrosion resistance.

[0008]

[Means for Solving the Problems] In order to solve the problems described above, the inventors of the present invention have made extensive studies, and as a result,
An alloy composed of Zn and Cr formed by an electrodeposition method, having a cubic crystal system and a lattice constant a = 3.00 to 3.0.
It has been clarified that a Zn-Cr alloy-plated steel sheet having remarkably excellent formability can be obtained by performing Zn-Cr alloy plating substantially composed of only a phase having a structure of 6Å. As a result, the basis weight can be increased without impairing the formability, and as a result, a rust-preventing steel sheet satisfying both corrosion resistance and formability can be obtained.

The present invention will be described in more detail below.
Conventionally, as an intermetallic compound that is stable in a thermal equilibrium state of a binary alloy composed of Zn and Cr, the crystal system is hexagonal and the lattice constant is a = 12.89.
A phase (θ phase) having a structure such that Å and c = 30.5Å has been reported. For example, M. Hansen Constitution of bi
nary alloys. p.571 See the equilibrium diagram in McGRAW-HILL. Although the composition of the θ phase is not always clear, it is said to be in the range of Cr / Cr + Zn = 3.8 to 7 wt%. Other intermetallic compounds have not been reported. That is, 2 of Zn and Cr in the thermal equilibrium state
It is considered that there are only three phases of Zn, which are the η phase, the θ phase, and the Cr phase of Zn in the base alloy.

By the way, generally, in the case of an alloy obtained by the electrodeposition method, it does not always become a thermodynamically stable phase, and a non-equilibrium phase may be generated. Further, various phases appear depending on manufacturing conditions such as plating bath composition and electrolysis conditions.
Therefore, even if the alloy composition is the same, the phase structure may differ. The present inventors believe that there is a correlation between press formability and phase structure. Therefore, it was thought that a plating layer having excellent formability could be obtained by utilizing the characteristics of the electrodeposition method to identify the phase structure.

Regarding the binary alloy of Zn and Cr, there are no reports of such non-equilibrium phase intermetallic compounds, and of course, there is no JCPDS card data. Therefore, the phase structure of the Zn-Cr alloy obtained by the electrodeposition method was investigated in detail. As a method, depending on various manufacturing conditions, Cr / (Cr + Z
n) = 0 to 30% by weight of an alloy having a composition in the range of
The change in the crystal lattice spacing was examined by the X-ray diffraction method. Hereinafter, the amount represented by Cr / (Cr + Zn) (wt%) is referred to as Cr content. In the case of Cr content = 0 wt%, that is, in the case of pure Zn, the crystal system is hexagonal (Hexagona).
l), lattice constant a = 2.665Å, c = 4.947Å η phase.

However, as the Cr content is gradually increased, that is, when Cr is solid-dissolved in the η phase, the crystal system may be maintained and the crystal may be expanded in the a-axis direction and conversely contracted in the c-axis direction. It was found from the change of the lattice spacing based on the line diffraction data. In the vicinity of Cr content = 5 wt%,
It was revealed that only the phase in which the lattice constant was continuously changed by the solid solution of Cr in the phase was present. The present inventors define this phase as ηx.

When the Cr content is further increased, ηx
X-ray diffraction peaks, which are thought to be due to a phase distinctly different from the above, appear. However, the Cr content in which these peaks appear differs depending on the manufacturing conditions. By comparing the results of repeated calculation assuming the crystal system and the lattice constant with the lattice spacing obtained from X-ray diffraction, ηx
In addition to the above, a new crystal system hexagonal (Hexagonal), lattice constant a = 2.72 to 2.78Å, c = 4.43 to 4.
A phase having a structure such as 60Å (defined as a δx phase), a cubic crystal system, and a lattice constant a = 3.00
It was revealed that there is a phase (defined as Γx) having a structure that is 3.06Å. The above results are shown in FIG. The Cr content in which the ηx phase, the δx phase, and the Γx phase appear is different depending on the manufacturing conditions, and therefore cannot be generally stated, but the results obtained under some manufacturing conditions are shown in FIG. 4 as an example. As described above, the phase structure of the electrodeposited Zn-Cr alloy is considered to be composed of only three phases.

Next, Zn--Cr produced under various conditions
When the relationship between the formability of the alloy-plated steel sheet and the basis weight is investigated, it is surprisingly found that the formability of the Zn-Cr alloy-plated steel sheet substantially composed of only the Γx phase is Zn containing the ηx phase or the δx phase. It became clear that it was significantly superior to the Cr alloy plated steel sheet.

That is, an alloy formed of Zn and Cr by the electrodeposition method, and having only a phase having a structure such that the crystal system is cubic and the lattice constant is a = 3.00 to 3.06Å It has been revealed that a Zn-Cr alloy-plated steel sheet having remarkably excellent formability can be obtained by performing the Zn-Cr alloy plating that is substantially constituted.

As described above, Cr for obtaining the Zn--Cr alloy plating substantially composed of the Γx phase only.
Although the range of the content rate cannot be uniquely defined because it varies depending on the manufacturing method, it is preferably 5 to 30 wt%. This is because if the content is less than 5 wt%, the Γx phase does not appear.
This is because if it exceeds 0 wt%, the adhesion of the plating layer itself deteriorates and the effect of the present invention is impaired. Further, the basis weight is preferably 10 to 40 g / m ² . 10 g / m ²
Is less than 40g / m because corrosion resistance is insufficient.
^If it exceeds ² , the formability will deteriorate. Desirably 20-
30 g / m ² has good corrosion resistance and moldability.

Although the manufacturing conditions for obtaining the Zn--Cr alloy plating of the present invention are not necessarily limited, for example, in the case of electrodeposition from a sulfuric acid bath, zinc sulfate and chromium sulfate are used as the main ingredients, and the conduction aid is used. Sodium sulfate, boric acid as a pH buffer, various organic acids, and various surfactants can be added.

In addition, the bath pH, bath temperature, liquid flow rate, electrolysis current density and the like can be appropriately selected. Since all of these factors affect the phase structure, when the combination of these factors is appropriate, a phase structure mainly composed of the Γx phase can be obtained.

In electroplating on an actual industrial scale, there may be a case in which phases other than the Γx phase are inevitably mixed even under the optimum plating conditions, but the same effect as the plating consisting of the pure Γx phase is exhibited. If it is within the range, it does not prevent mixing of some different phases, and it is defined in the present invention including such a range that it is substantially composed of the Γx phase.

[0020]

EXAMPLES The effects of the present invention will be described below with reference to examples. (Example) Table 1 shows manufacturing conditions, weight per unit area, Cr content and phase constitution of the invention examples and comparative examples. In each case, a SPCD having a plate thickness of 0.7 mm was used as the original plate, degreasing and pickling was performed in accordance with a conventional method, and then plating was performed to prepare a sample. In the present invention, all are substantially composed of the Γx phase, while the comparative examples clearly include the ηx phase and the δx phase.
However, those containing up to about 1% of the ηx phase and / or the δx phase were considered to be substantially composed of only the Γx phase. The moldability was evaluated using the samples shown in Table 1. The moldability was evaluated by applying a wrinkle pressing force of 1 ton and a punching speed of 120 mm / min, followed by drawing with a 35 mmφ punch, and determining the limiting drawing ratio (LDR).
In addition to this, a commercially available GA60 (Basis weight 60 g /
m ² GA), Zn-Ni 30 (Zn-Ni alloy plated steel sheet having a basis weight of 30 g / m ² ), EG 30 (a basis weight of 30)
The LDR of g / m ² EG) was also determined.

The results of the above measurements are shown in FIG. It can be seen from FIG. 5 that the moldability of the comparative example rapidly deteriorates as the basis weight increases. As already mentioned, in the case of Zn-Cr alloy plated steel sheet, G which is currently used in the largest amount in Japan
To secure the same corrosion resistance as A60, the basis weight is 30
About g / m ² is necessary. However, in the comparative example, it can be seen that the moldability becomes worse than GA60 when the basis weight is 30 g / m ² or more. On the other hand, when the phase structure of the plating layer is controlled so as to be substantially composed of only the Γx phase, the deterioration of the formability is small even when the basis weight is 30 g / m ² . Considering that the press-formability of the current rust-preventive steel sheet is the best in Zn-Ni alloy-plated steel sheet, EG is slightly inferior, and GA with heavy weight is considered to have further difficulty in formability. It is considered that the Zn-Cr alloy-plated steel sheet has sufficiently excellent formability when placed in the area of the basis weight having excellent corrosion resistance.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

As described above, the present invention provides a rust-preventing steel plate which has sufficient corrosion resistance and is excellent in formability.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a maximum swelling width and a basis weight in appearance rust of various surface-treated steel sheets.

FIG. 2 is a flow chart of a complex corrosion cycle test.

FIG. 3 Phase structure of electrodeposited Zn—Cr alloy (1) ηx,
It is a figure explaining (2) deltax and (3) Γx.

FIG. 4 is a diagram showing changes (1) to (3) in the phase structure depending on the composition of the electrodeposited Zn—Cr binary alloy under the manufacturing conditions 1 to 3 and a phase structure (4) in a thermal equilibrium state.

FIG. 5: Formability of Zn-Cr alloy plated steel sheet (LD
It is a figure which shows the influence of the phase structure which exerts on the relationship between R) and the basis weight.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kei Akira Onuma 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division (72) Inventor Kazuo Mochizuki Chuo-ku, Chiba, Chiba Kawasaki-cho 1 Technical Research Headquarters Kawasaki Steel Co., Ltd. (72) Inventor Morito Extension No. 1 Kawasaki-cho Chuo-ku, Chiba City Chiba Prefecture Kawasaki Steel Co., Ltd. Technical Research Headquarters

Claims (1)

[Claims] 1. An alloy comprising Zn and Cr formed by an electrodeposition method, which has a cubic crystal system and a lattice constant a = 3.
An anticorrosive steel sheet for automobiles, which is excellent in corrosion resistance and formability, and which is plated with a Zn-Cr alloy substantially composed of only a phase having a structure of 00 to 3.06Å.