JPH0625889A - Rust preventive steel sheet excellent in external rust resistance as well as in corrosion resistance - Google Patents

Abstract

PURPOSE:To provide superior external rust resistance as well as corrosion resistance by applying Zn-Cr alloy plating, which has a structure having hexagonal crystal system and specific lattice constant, to a steel sheet by an electrodeposition method. CONSTITUTION:Zn-Cr alloy plating is applied to the surface of a steel sheet by an electrodeposition method. This Zn-Cr alloy is practically constituted only of the phase which has a structure having hexagonal crystal system and lattice constant satisfying [(a)=2.66 to 2.74Angstrom ] and [(c)=4.61 to 4.95Angstrom ]. It is desirable to regulate Cr content to about 1-15wt.% and it is also desirable to regulate coating weight to about (10 to 40)g/m<2> By this method, the rust preventive steel sheet excellent in resistance to external rust of automobile body can be obtained.

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 quality not only in corrosion resistance but also in external rust resistance 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.

Therefore, it has been attempted to alloy Zn with other elements to obtain equivalent corrosion resistance to pure Zn even with a smaller basis weight. As an effect of alloying, it is expected that the corrosion potential will be closer to that of steel to suppress the corrosion rate of the plating layer itself and that the corrosion products will be stabilized. 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, in the Zn-based plating layer, Cr
Attempts have been made to add Al as an alloying element. For example, Japanese Patent Laid-Open Nos. 1-191797 and 3-120393
Etc. have been proposed. Certainly, if it is limited to bare corrosion resistance,
By increasing the Cr content, the Zn-Cr alloy plating exhibits better corrosion resistance than the conventional Zn-based alloy plating.

As an example, FIG. 1 shows the results of examining the number of days until the occurrence of 2% of red rust by conducting a salt spray test according to JIS Z2371. In the case of an automobile body, since it is usually used after being processed, a test piece subjected to a 17% tensile processing was used. 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.
The EG30 shown in the figure is a commercially available electroplated Zn-plated steel sheet having a unit weight of 30. GA60 is a commercially available alloyed hot-dip Zn-plated steel sheet having a basis weight of 60. Zn-Ni30
Indicates a commercially available Zn-Ni alloy-plated steel sheet having a unit weight of 30 and a Ni content of 13 wt%. The amount of Zn-Cr deposited is all 20
It is a unit weight.

From FIG. 1, it can be seen that the bare corrosion resistance of the Zn-Cr alloy-plated steel sheet is improved almost linearly as the Cr content in the alloy increases. Cr /
If (Cr + Zn) = 2 wt% or more, EG with a large basis weight
It can be seen that it has better bare corrosion resistance than 30 and GA60. As described above, the bare corrosion resistance of the Zn-Cr alloy-plated steel sheet is excellent because the surface oxide film of Cr remarkably suppresses the dissolved oxygen reduction reaction in a corrosive environment to reduce the corrosion current density, that is, the corrosion rate becomes slow. It is thought to be for the reason.

[0006]

The above experimental results are mainly tests assuming corrosion in a portion of the inner surface of the vehicle body where the electrodeposition coating is poorly distributed and is left naked. However, the corrosion resistance of various surface-treated steel sheets is highly dependent on the corrosive environment, and the order of the corrosion resistance may be changed depending on the change in the environment. In recent years, the demand for corrosion resistance against rust generated on the outer surface of a vehicle body has become stricter as the quality of automobiles has increased. The outer surface rust mainly progresses under the coating film starting from a damaged portion of the coating film due to running stones and the like, and damages the appearance as red rust and swelling of the coating film.

As described above, in a corrosive environment on the inner surface of the vehicle body, the corrosion resistance of the Zn--Cr alloy-plated steel sheet is linearly improved by increasing the Cr content. However, the rust resistance on the outer surface of the vehicle body does not always improve with the increase of the Cr content, but rather tends to deteriorate with the increase of the Cr content. Therefore, the Zn-Cr alloy-plated steel sheet has a problem that it is superior in bare corrosion resistance to other Zn-based plated steel sheets but inferior in vehicle body outer surface rust resistance.

Therefore, it is an object of the present invention to provide a rust preventive steel sheet which is excellent not only in corrosion resistance but also in external surface rust resistance.

[0009]

[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,
It is an alloy composed of Zn and Cr formed by an electrodeposition method, and has a hexagonal crystal system and a lattice constant a = 2.66 to 2.7.
A Zn-Cr alloy-plated steel sheet excellent in external surface rust resistance is obtained by performing Zn-Cr alloy plating which is substantially composed of only phases having a structure of 4Å, c = 4.61 to 4.95Å. It became clear that it could be obtained.

[0010]

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

By the way, generally, in the case of an alloy obtained by an electrodeposition method, a thermodynamically stable phase is not always obtained, 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 inventors of the present application consider that there is a correlation between the rust resistance on the outer surface of the vehicle body and the phase structure. Therefore, it was thought that a plating layer having excellent external surface rust resistance could be obtained by utilizing the characteristics of the electrodeposition method and specifying 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 wt% of an alloy having a composition in the range of
The change of the crystal lattice spacing was investigated by the line diffraction method. Since then C
The amount represented by r / (Cr + Zn) (wt%) is called Cr content. Cr content = 0 wt%, that is, pure Zn
In the case of, the crystal system is hexagonal and the lattice constant a = 2.665.
Å, c = 4.947 Å η phase.

However, as the Cr content is gradually increased, that is, when Cr is solid-dissolved in the η phase, the crystal system is maintained and the a-axis extends and the c-axis contracts. It was found from the change of the lattice spacing by the diffraction data. Until the Cr content = 5 wt%, the C in the η phase is
It was clarified that only the phase in which the lattice constant continuously changes by the solid solution of r exists. 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, a new hexagonal crystal system in addition to ηx, a lattice constant a = 2 .72-
2.78Å, a phase having a structure such that c = 4.43 to 4.60Å (defined as δx phase), and the crystal system is cubic and the lattice constant is a = 3.00 to 3.06Å It was revealed that there is a phase (defined as Γx) with a different structure. The above results are shown in FIG. The Cr content in which the ηx phase, the δx phase, and the Γx phase appear differs depending on the manufacturing conditions, and therefore cannot be generally stated, but the results obtained under some manufacturing conditions are shown in FIG. 3 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 car body outer surface rust resistance of the alloy-plated steel sheet and the alloy composition was investigated, surprisingly, the outer surface rust resistance of the Zn-Cr alloy-plated steel sheet consisting essentially of ηx phase was δ.
It was revealed that it was remarkably superior to the Zn-Cr alloy plated steel sheet containing the x phase and the Γx phase.

That is, it is an alloy composed of Zn and Cr formed by the electrodeposition method, the crystal system is hexagonal, and the lattice constant is a = 2.66 to 2.74Å, c = 4.61 to 4.95.
It has been clarified that a Zn-Cr alloy-plated steel sheet having excellent rust resistance on the outer surface of the vehicle body can be obtained by performing Zn-Cr alloy plating that is substantially composed of only the phase having a structure such as Å.

As already mentioned, Cr for obtaining a Zn--Cr alloy plating consisting essentially of the ηx phase only.
The range of the content ratio cannot be uniquely defined because it varies depending on the manufacturing method, but it is preferably 1 to 15 wt%.
This is because if it is less than 1 wt%, the corrosion resistance is insufficient.
This is because if the wt% is exceeded, the δx phase or Γx phase will appear and it will be difficult to obtain a plating layer that consists essentially of the ηx phase. As the basis weight of 10 to 40 g / m ² is desirable, is less than 10 g / m ² is because corrosion resistance is insufficient, the 40 g / m ² greater because cost merit is eliminated.

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 as the main ingredients, and a conductivity assistant. Sodium sulfate as an agent, boric acid or various organic acids as a pH buffer, and various surfactants can be added.

In addition, a desired phase constitution is obtained by appropriately selecting the bath pH, bath temperature, liquid flow rate, electrolysis current density and the like. Since all of these conditions affect the phase structure, an alloy plating consisting essentially of ηx is obtained when a combination of these conditions is appropriate.

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

[0021]

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. The examples of the present invention are substantially composed of only the ηx phase, while the comparative examples clearly include the δx phase and the Γx phase. However, those containing up to about 1% of the δx phase and / or the Γx phase were considered to consist essentially of the ηx phase. Using the samples shown in Table 1, the outer surface rust resistance of the vehicle body was evaluated. The exterior rust resistance of the vehicle body is evaluated by 150 mm x 70 m
m test piece was subjected to the same zinc phosphate chemical conversion treatment as that used for ordinary cold-rolled steel sheets for automobiles, followed by cationic electrodeposition coating (film thickness 20 μm), intermediate coating (film thickness 40 μm), top coating (film thickness 40 μm) 3) coating is applied, a scratch that reaches the base material is made with a cutter knife, and after being exposed to the corrosive environment of the cycle shown in FIG. 4 for one month using a complex corrosion tester,
The measurement was performed by measuring the swelling width from the damaged part.

The above measurement results are shown in FIG. From FIG. 5, the rust resistance of the Zn-Cr alloy-plated steel sheet satisfying the conditions of the present invention on the outer surface of the vehicle body is EG30 (EG with a basis weight of 30 g / m ² )
-Ni30 (Zn-Ni alloy plated steel sheet with a basis weight of 30 g / m ² ) is superior to GA60 (G with a basis weight of 60 g / m ² ).
A) It turns out that it is comparable. Comparative Example 1
Although the phase composition is only the η phase, it does not contain Cr, so its corrosion resistance is insufficient. Since Comparative Example 2 and later have a phase structure substantially containing the δx phase and / or the Γx phase, Cr
It indicates that the external rust resistance deteriorates due to the increase in the content rate.

[0023]

[Table 1]

[0024]

As described above, the present invention provides an anticorrosive steel sheet for automobiles having excellent rust resistance on the outer surface of the vehicle body.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between bare corrosion resistance and an alloy composition of a Zn—Cr alloy plated steel sheet.

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

FIG. 3 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. 4 is a flow chart of a complex corrosion cycle test.

FIG. 5 is a diagram showing a relationship between a vehicle body outer surface rust resistance of a Zn—Cr alloy plated steel sheet and an alloy composition.

Front page continuation (72) Inventor Michio Katayama 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. (72) Inventor Kazuo Mochizuki 1 Kawasaki-cho, Chuo-ku, Chiba Address Kawasaki Iron & Steel Co., Ltd. Technical Research Headquarters (72) Inventor Morito Extension No. 1 Kawasaki-cho, Chuo-ku, Chiba City Chiba Prefecture Kawasaki Iron & Steel Co., Ltd. Technical Research Headquarters

Claims (1)

[Claims] 1. An alloy comprising Zn and Cr formed by an electrodeposition method, wherein the crystal system is hexagonal and the lattice constant is a = 2.
66-2.74Å, c = 4.61 to 4.95Å consisting essentially of only a phase having a structure
Anti-corrosion steel plate with excellent corrosion resistance and outer surface rust resistance, which is plated with Cr alloy.