JPH06228789A - Cr system composite plating steel plate excellent in corrosion resistance after work - Google Patents

Abstract

PURPOSE:To obtain the Cr system composite plating steel plate excellent in corrosion resistance after work by decreasing the Cr content of the plating film of the first layer and controlling a film thickness in a specified value in the Cr system composite plating. CONSTITUTION:The plating layer on the steel plate is a two layer constitution, the first layer is the Zn-Cr composite plating layer which has 2-10mum film thickness and contains 0.1-5wt.% Cr. The second layer is the Zr-Cr composite plating layer which has 0.01-2mum film thickness and contains 5-60wt.% Cr. By compositing the Cr in a suitable ratio in the first layer like this, the sacrificial corrosion preventive period of Zn is turned into durable state. By incorporating at least one kind of ferrous group metal among Fe, Ni and Co beside Zn and Cr in the plating film of the first layer, the corrosion resistance and the weldability of the first layer is improved. Because the second plating film layer rich in Cr content is readily stripped off when the layer is too thick, <=2mum thickness is preferable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cr-based composite plated steel sheet, and more particularly to a Cr-based composite plated steel sheet which is suitable as an anticorrosion steel sheet for automobiles and has excellent corrosion resistance and workability.

[0002]

2. Description of the Related Art Conventionally, as a steel sheet for automobiles, a surface-treated steel sheet that withstands red rust and perforation for a predetermined time even under a severe corrosive environment such as rock salt or calcium chloride as an antifreezing agent used in winter cold regions. Is becoming desired. Further, from the viewpoint of spot weldability, workability, etc., it is desired to develop a rust-preventive steel sheet having a smaller amount of coating and having higher corrosion resistance than conventional materials. Conventionally, Zn-plated steel sheets have been used as surface-treated steel sheets for automobiles, but the corrosion rate of Zn is relatively high, and if a long-term rust prevention is expected, a larger coating amount is required. Is.

Therefore, in order to suppress the activity of the Zn plating layer, Zn-Ni or Zn-Fe alloy plating, which is an alloy of Ni or Fe and Zn, has been put into practical use, and is now used for most of the vehicle body. And the usage rate is increasing. Especially in alloy electroplating, Zn-Ni alloy plating is mainly used because the corrosion resistance of the steel sheet when the organic composite film is formed is very good. Further, in recent years, as a coating capable of remarkably improving the corrosion resistance by plating with a thin coating, for example, as described in Japanese Patent Publication No. 2-51996, a zinc-chromium eutectoid plating layer and a Zn-
A technique of forming a multi-layer plating layer or the like made of a plating layer of Ti or the like is disclosed.

[0004]

[Problems to be Solved by the Invention] As in the prior art described above, the effect of improving the corrosion resistance of the Zn-based plated steel sheet by the compounding of Cr is remarkable, and when compared in the salt spray test or the like, it depends on the Cr content. Indicates a corrosion resistance that is several times to several tens of times that of a normal galvanized steel sheet. Generally, such improvement in corrosion resistance is considered to be brought about by a stable oxide of Cr generated by corrosion, but on the other hand, when a local defect is caused due to the stability, the sacrifice by Zn is sacrificed. There is a problem that the corrosion resistance is poor and the steel sheet is locally corroded relatively early.

Defects in the Cr-based composite plating film are likely to occur when the plated steel sheet is processed. In particular, since a coating having a high Cr content is brittle, damage caused by working is large, and cracks in the coating, lack of coating, and the like are the starting points of steel sheet corrosion. Therefore, in such a steel plate, even if the unprocessed part has good corrosion resistance, corrosion progresses from the processed part, and the value as a rust-proof steel plate is significantly impaired. Further, since the rustproof steel sheet for automobiles is used after being subjected to press working, its corrosion resistance should be considered on the premise of working. Therefore, the object of the present invention is to prevent the deterioration of the corrosion resistance caused by such processing, and
It is intended to provide a Cr-based composite plated steel sheet which is excellent in corrosion resistance after working by utilizing the excellent potential of the original corrosion resistance of the system-based composite plating.

[0006]

The above object can be achieved by the present invention described below. That is, according to the present invention, as a first layer, a plating film having a thickness of 2 to 10 μm and Cr of 0.1 is formed on a steel plate.
A Zn-Cr composite plating layer containing 5 to 5 wt% and a Zn-Cr composite plating layer having a plating film thickness of 0.01 to 2 μm and a Cr content of 5 to 60 wt% as the second layer. Is a Cr-based composite plated steel sheet having excellent corrosion resistance after working.

[0007]

The plating layer on the steel sheet has a two-layer structure, and the first layer is a Zn-Cr composite plating layer having a film thickness of 2 to 10 μm and containing Cr in an amount of 0.1 to 5% by weight, and its second layer. Layers,
Plating film thickness is 0.01-2 μm, Cr content is 5-6
The Zn-Cr composite plating layer of 0% by weight prevents a decrease in corrosion resistance caused by processing,
Further, it is possible to provide a Cr-based composite plated steel sheet having excellent post-working corrosion resistance, which originally takes advantage of the excellent potential for corrosion resistance of the Cr-based composite plating layer. Furthermore, in a preferred embodiment of the present invention, the first layer is a Zn-Cr-iron group metal composite plating layer containing 0.1 to 25% by weight of one or more of Fe, Ni and Co in total. , The first layer of Zn
As a result, the high corrosion resistance of the steel sheet after processing can be obtained.

In the conventional Cr-based composite plated steel sheet, C
The plating film in which r is compounded with Zn forms a relatively stable corrosion product at the early stage of corrosion. While this corrosion product is present on the plating surface layer, the corrosion reaction is inactive, and this period is very long compared to Zn plating, etc.
Excellent corrosion resistance is realized. However, conventional Cr
When the Zn-Cr coating composited with Zn was observed after being subjected to processing such as press molding, fine cracks reaching the steel sheet interface were found in the plating layer. When such a coating is subjected to a corrosion test, corrosion products are not formed sufficiently to suppress the corrosion reaction on the steel sheet, so that the excellent corrosion resistance originally possessed by the Cr-based composite plating layer cannot be exhibited.

On the other hand, in the present invention, when the plating film has a two-layer structure and Cr is contained in the first layer in an appropriate ratio, corrosion of the steel sheet is prevented by the good sacrificial anticorrosion ability of Zn. It In this case, the plating film of the first layer is Z
If only n, corrosion of the steel sheet is not sufficiently prevented.
This is because the period during which Zn plating can bear the sacrificial corrosion is very short. Therefore, in the present invention, the sacrificial anticorrosion period can be sustained by compounding Cr in the first plating layer in an appropriate ratio. In the present invention, it is not clear why the sacrificial corrosion protection period of Zn is sustained by compounding Cr in the first plating layer in an appropriate ratio.
It is considered that this is due to the effect that the cathode protective reaction is suppressed due to the formation of an insulating protective film due to. If the Cr content of the first plating layer of the steel sheet of the present invention is not less than 0.1% by weight, the effect of sustaining the sacrificial anticorrosion period as described above cannot be obtained. Further, when the Cr content of the first plating layer exceeds 5% by weight, a very stable corrosion product is formed. On the contrary, the sacrificial anticorrosion ability of Zn is not sufficiently exerted, The amount of cracks in the plating film during processing is extremely increased, and the plating film that should bear the anticorrosion can remain insufficiently on the steel sheet. As a result, an improvement in the corrosion resistance of the steel sheet after working can no longer be expected. It is considered that the above-mentioned increase in crack formation that occurs when the Cr content exceeds 5% by weight is due to the formation of a Zn—Cr alloy phase that is more brittle than Zn.

Therefore, in the steel sheet of the present invention, it is necessary that the Cr content of the first layer plating film is 0.1 to 5% by weight. The thickness of the plating film of the first layer is appropriately 2 μm to 10 μm. If the thickness of the first plating layer is less than 2 μm, the corrosion resistance is insufficient, while if the thickness exceeds 10 μm, the workability of the plating layer of the steel sheet deteriorates. In a preferred embodiment of the present invention, the plating film of the first layer is made of F and F in addition to Zn and Cr.
One or more of the iron group metals e, Ni and Co can be included. By including these alloy components, the obtained steel sheet is further improved in corrosion resistance and weldability. The reason why the corrosion resistance of the obtained plated steel sheet is improved by adding the iron group metal to the first layer is F
Although different depending on the type of e, Ni, or Co, these iron group metals provide the cathode active sites in the first layer plating film and accelerate the oxidation reaction of the plating layer, and as a result, control the sacrificial corrosion resistance of Zn. It is considered that this is due to the action of forming the protective film quickly and uniformly, or the action of blocking the active sites of the cathode on the steel sheet by the hydroxide of Co or the like as with Cr.

When these iron group metals are contained, the content range is suitably 0.1 to 25% by weight. Furthermore, the reason why the weldability of the steel sheet is improved by the addition of these iron group metals is that the melting point is increased by the alloying of the iron group metal, the contact resistance is improved by the addition of the conductive component, or the alloying reaction with the electrode tip is suppressed. The effects such as When adding these metals of the iron group, if the metal component is less than 0.1% by weight,
No improvement in corrosion resistance and weldability of the obtained plated steel sheet is observed. On the other hand, if it exceeds 25% by weight, cracks are likely to occur in the plating film and the workability of the plating layer is extremely lowered, resulting in deterioration of the post-working corrosion resistance of the steel sheet. In the present invention, as described above, a specific ratio of Cr is contained in the film of the first layer.
It is possible to improve the corrosion resistance of the steel sheet due to the effect of suppressing the activity of Zn by compounding
By providing the r-rich second plating film layer, it is possible to further increase the corrosion resistance of the steel sheet.

However, when the steel sheet is processed,
If the Cr-rich coating that is the second layer is thick, the second layer is easily peeled off, and the effect of the second layer cannot be obtained. Therefore, it is necessary to make the film thickness of the second layer as thin as possible. For this reason, the thickness of the second layer is preferably 0.01 to 2 μm. The plating thickness of the second layer is 0.01 μm
If it is smaller, the effect as a film cannot be obtained, and if it exceeds 2 μm, not only the workability of the second layer becomes poor, but also the sacrificial anticorrosion ability of the first layer is sufficiently exerted due to the influence of the second layer. Disappear. The Cr content of the second layer is 5 to 60% by weight.
Is effective. When the Cr content of the second layer is less than 5% by weight, the contribution to the corrosion resistance is insufficient, and when it exceeds 60% by weight, the workability of the second layer becomes poor and the second layer peels off during processing. Not only the effect on the corrosion resistance becomes smaller, but it also causes surface defects such as scratches.

The plating films of the first and second layers in the steel sheet of the present invention are formed as follows. The first-layer plated layer is obtained by using an acidic plating solution containing a predetermined amount of Cr ³⁺ ions and Zn ²⁺ ions and forming it on at least one surface of the steel sheet by an ordinary electroplating method.
Further, in order to form a film containing Fe, Co or Ni or a plurality of these in a predetermined amount, Fe ²⁺ , Co ²⁺ or Ni may be further added.
²⁺ or a plurality of these are contained in the plating solution. At this time, a cation such as K ⁺ , Na ⁺ or NH ₄ ⁺ may be added to the plating solution used in order to impart electric conductivity. As the anion, SO ₄ ²⁻ , Cl ^−, or BF
₄ ^- or the like can be used either a. Furthermore, other C
It is also possible to add a precipitation accelerator.

In order to form the second layer on the first layer, a plating solution having a higher Cr concentration than that of the first layer may be used, or the Zn-Cr plated layer may contain Cr. The rate depends on the current density during electrolysis, and if electrolysis is performed at a higher current density and a lower relative flow rate between the plating solution strips, the Cr content tends to be higher, so that the production facility is simplified. It is also possible to form the second layer by changing the current density and the relative flow velocity.

[0015]

EXAMPLES Next, the present invention will be described more specifically with reference to Examples and Comparative Examples. Example 1 A cold-rolled steel sheet having a plate thickness of 0.7 mm was electroplated under the conditions shown in Table 1 below to form Zn-Cr electroplated layers of a first layer and a second layer on the surface of the steel sheet, The plated steel sheets of the present invention and the comparative example were obtained. The composition of the plating film was adjusted by changing Zn ²⁺ ions and Cr ³⁺ ions in the plating solution.

[0016]

[Table 1]

The plated steel sheet having the first and second plating layers thus obtained was subjected to a draw bead test assuming processing. In the draw bead test, as shown in the schematic cross-sectional view shown in FIG. 1, a male die 2 having a horizontal convex portion of a predetermined length (40 mm) and a female die 3 facing the male die 2 having a width of 30 mm were used. While pressing the plated steel plate sample piece 1 at a pressure of 500 Kgf, press it upwards to
00 mm / min. Pull out the sample piece by 100 mm at the speed of. Then, the sample piece was removed, the oil on the surface was removed with a solvent, and the end and the back surface were sealed to perform a corrosion test.

Corrosion test is wet (50 ° C., RH 80%)
→ Dry (50 ℃, RH30%) → Leave at room temperature (20 ℃, R
H60%) → salt water immersion (0.5% NaCl, 1 minute) is a corrosion acceleration test in which one cycle is performed per day. Corrosion resistance evaluation is that after 60 cycles, the plating film is peeled off from the steel sheet.
The depth of the holes formed in the cold rolled steel sheet was measured. Tables 2-4
The above measurement results are shown in the table. From Tables 2 to 4, it can be seen that the products of the present invention have better corrosion resistance than the comparative products. In addition, the workability of the sample was determined by measuring the film weight before and after pulling out the sample piece in the draw bead test shown above, and calculating the film peeling amount per unit area from these values and the area where the bead passed, and compared. did. From Tables 2 to 4, it can be seen that the workability of the product equivalent to the present invention is significantly superior to that of the comparative product.

[0019]

Table 2 Examples of the present invention

[0020]

Table 3 (Continued from Table 3) Examples of the present invention

[0021]

[Table 5] Comparative example

[0022]

[Effects] According to the present invention as described above, it is possible to prevent the deterioration of the corrosion resistance of the plated steel sheet caused by the processing and to obtain the excellent corrosion resistance after processing by utilizing the excellent potential corrosion resistance originally possessed by the Cr-based composite plating film. A Cr-based composite plated steel sheet can be provided.

[0023]

[Brief description of drawings]

FIG. 1 is a schematic sectional view illustrating a draw bead test.

Claims (2)

[Claims] 1. A Zn having a plating film thickness of 2 to 10 μm and a Cr content of 0.1 to 5 wt% as a first layer on a steel plate.
-Z as a Cr composite plating layer and a second layer having a plating film thickness of 0.01 to 2 μm and a Cr content of 5 to 60% by weight.
A Cr-based composite plated steel sheet having excellent corrosion resistance after working, which is provided with an n-Cr composite plated layer. 2. The first layer further comprises Fe and Ni in addition to Cr.
The Cr-based composite plated steel sheet having excellent post-work corrosion resistance according to claim 1, which is a Zn-Cr-iron group metal composite plating layer containing 0.1 to 25% by weight in total of one or more of Co and Co.