JPH06228773A - Zn-cr composite plated steel sheet excellent in corrosion resistance after working and its production - Google Patents

Abstract

PURPOSE:To provide a Zn-Cr composite plated steel sheet having superior corrosion resistance after working by making the surface layer part of a plating film, on a steel sheet having a specific Zn-Cr composite plating film, enriched in Cr. CONSTITUTION:In a Zn-Cr composite plated steel sheet, the content of Cr in a plating film is regulated to 0.1-5wt.% and the surface layer part of the plating film on the steel sheet is enriched in Cr, and further, prior to Cr enrichment, 0.1-15wt.%, in total, of at least one or more elements among Fe, Ni, and Co are incorporated into the plating film. The thickness of the surface layer of the plating film is regulated to 0.15-5% of the whole film thickness of the plating film after Cr enrichment. As to the method for Cr enrichment, the plated steel sheet is subjected to immersion etching treatment in an acid or acidic plating bath. By this method, the corrosion resistance of the undercoat plated steel sheet can be improved to a greater extent by means of chromate film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Zn-Cr composite plated steel sheet and a method for producing the same. More specifically, Zn-Cr composite plated steel sheet excellent in corrosion resistance and workability suitable as an anticorrosive steel sheet for automobiles.
The present invention relates to a Cr composite plated steel sheet and a method for manufacturing the same.

[0002]

2. Description of the Related Art At present, various Zn-based plated steel sheets are used as rustproof steel sheets for automobiles. Recently, in the automobile industry, there has been a call for weight reduction of automobiles for the purpose of improving fuel efficiency, and along with this demand, "thinning of steel plate → reduction of corrosion allowance" is required. Therefore, it is necessary to compensate for the reduction in corrosion allowance of the steel sheet, and development of an anticorrosive steel sheet for automobiles having further excellent corrosion resistance is desired. One of such steel plate candidates
One of them is a Zn-plated steel sheet containing Cr. Specifically, for example, a Zn-Cr plated steel sheet described in JP-A-64-55398, JP-A-63-24.
No. 3295, a two-layer plated steel sheet in which Fe or another alloy plated layer is provided on a Zn—Cr based plated layer, and further, Zn described in JP-A-56-20190. Examples include multi-component plated steel sheets such as -Ni-Cr-Co.

[0003]

Problems to be Solved by the Invention Z in which Cr is compounded
The effect of improving the corrosion resistance of the n-based plated steel sheet is remarkable, and when compared with, for example, a salt spray test, depending on the Cr content, it is about several times to several tens of times that of a normal galvanized steel sheet that does not contain Cr. It is said to exhibit corrosion resistance. Generally, such improvement in corrosion resistance is due to the formation of C along with corrosion.
It is believed to be provided by a stable oxide of r. However, on the other hand, because of its stability, when a local defect occurs, the sacrificial anticorrosion ability of Zn is poor,
There is a problem that corrosion of the steel sheet occurs at a relatively early stage.

Defects in the plated film are likely to occur when the plated steel sheet is processed, but since the plated film having a high Cr content is particularly brittle, the damage caused by the processing of the steel sheet is large, and the cracks in the plated film occur. The lack of a coating is likely to be the starting point of steel plate corrosion. Therefore, such a steel sheet has a problem that even if the corrosion resistance of the unprocessed portion is good, the corrosion progresses from the processed portion, and its value as a rustproof steel sheet is significantly impaired. In particular, since the rustproof steel sheet for automobiles is used after being subjected to press working, its corrosion resistance must be considered on the premise of working. Therefore, an object of the present invention is to prevent deterioration of corrosion resistance of a Zn-Cr composite plated steel sheet, which is caused by processing such a steel sheet,
It is to make use of the excellent latent corrosion resistance originally possessed by the n-Cr composite plated steel sheet.

[0005]

The above objects can be achieved by the present invention described below. That is, the present invention is characterized in that the plating film surface layer portion of the steel sheet having a Zn-Cr composite plating film containing 0.1 to 5% by weight of Cr is enriched with Cr, and Zn having excellent post-working corrosion resistance is characterized. -Cr composite plated steel sheet and its manufacturing method.

[0006]

In the following, Zn-C excellent in corrosion resistance after processing according to the present invention
The details of the r composite plated steel sheet will be described. Z of the present invention
In n-Cr composite plated steel sheet, Cr in the plating film
The Cr content before enrichment is 0.1 to 5% by weight.
That is, the effect of improving the corrosion resistance of the steel sheet by including Cr in the Zn-based plating film is that the Cr content is 0.1% by weight.
It will not occur unless it is above. On the other hand, the workability of the plating film tends to decrease when the Cr content in the plating film exceeds 5% by weight, causing cracks in the plating film or chipping of the film. The corrosion resistance of the steel sheet is extremely deteriorated. When the Cr content exceeds 5% by weight, such deterioration in the workability of the plating film is caused by
It is considered that a Zn—Cr alloy phase, which is more brittle than Zn, is formed on the steel sheet. Also, due to alloying with Cr, Z
It is considered that the reduction of the good sacrificial anticorrosion ability that n originally has also contributes to the reduction of the corrosion resistance of the steel sheet in the worked portion.

In a preferred embodiment of the present invention, in addition to Zn and Cr, one or more of iron group metals of Fe, Ni and Co can be further contained as a substance constituting the plating film. By including these iron group metals in the plating film, it is possible to further improve the corrosion resistance and the coatability of the steel sheet. Further, in the aspect in which the iron group metal is contained in the plating film, the iron group metal contained is
0.1% in total in the plating film before Cr enrichment described later.
It is preferable to contain -15% by weight. If it is less than 0.1% by weight, improvement in corrosion resistance and paintability of the steel sheet due to addition of the iron group metal is not recognized, and if it exceeds 15% by weight, the workability of the plating film is deteriorated.

The Zn-Cr composite plated steel sheet of the present invention is characterized in that the plating surface layer portion is richer in Cr than the inner layer portion. By doing so, the amount of Cr in the inner layer portion of the plating film remains 0.1 to 5% by weight, which is the case when the plating film contains more than 5% by weight of Cr. There is no problem of deterioration of workability of the plating film such as generation of cracks in the film and lack of film. On the other hand, in the present invention, since the surface layer portion of the plating film is Cr-riched and a layer having a higher Cr content than the inner layer portion is formed, the inner layer portion is formed by such a Cr-rich layer. Zn is prevented from prematurely eluting from the steel, and the long-term corrosion resistance of the plated steel sheet is improved. Zn of the present invention
In the -Cr composite plated steel sheet, the thickness of the plating film is not particularly specified, but is preferably 2 μm to 10 μm. This is because if it is less than 2 μm, the corrosion resistance of the steel sheet is insufficient, and if it exceeds 10 μm, the workability of the plating film deteriorates.
Further, the surface layer portion of the plating film in the present invention means 0.15 to 5% of the total thickness of the plating film after the enrichment of Cr.
Say the degree. Further, depending on the Cr content contained in the plating film before Cr enrichment, if the Cr-enriched surface layer portion is less than 0.15% of the total film thickness, the intended purpose cannot be achieved and the Cr-rich surface layer cannot be achieved. The totalized film thickness is 5
%, The workability of the plating film will be reduced.

In another preferred embodiment of the present invention, a chromate film layer is provided on the Cr-rich plating film as described above, and an organic resin film layer is further provided thereon. The Zn-Cr composite plated steel sheet of the present invention in this aspect has more excellent corrosion resistance. That is, the chromate film layer formed on the plating film after the Cr enrichment is
Barrier due to the passivation effect of Cr ⁶⁺ contained in the chromate film layer by the chromate ion and the barrier layer covering the plated surface with the chromium hydrated oxide film of Cr ³⁺ , which is a reduction product of chromate ion. Depending on the effect, the corrosion resistance of the base plated steel sheet can be further improved. The amount of this chromate film deposited is 10 to 15 in terms of metallic chromium.
It is preferably 0 mg / m ² . If it is less than 10 mg / m ² , desired corrosion resistance cannot be imparted to the plated steel sheet, while if it exceeds 150 mg / m ² , the weldability of the steel sheet deteriorates. Further, in the present invention, the chromate film may contain an organic resin such as an acrylic resin, an oxide colloid such as silica and alumina, an acid or salt such as molybdic acid, and other anticorrosion enhancing components.

Further, by forming an organic resin film on the chromate film as described above, it is possible to suppress the excessive elution of Cr ⁶⁺ chromate ions in the chromate film into the corrosive environment. It becomes possible to further maintain the anticorrosion effect of the steel sheet. At this time, the film thickness of the organic resin film is preferably 0.3 to 3 μm. If it is less than 0.3 μm, the effect of improving the corrosion resistance by the organic resin film is insufficient,
If it exceeds 3 μm, the weldability of the steel sheet deteriorates. The organic resin used at this time is not particularly limited, but it is preferable to use an epoxy resin or the like as the base, and further, in order to enhance its corrosion resistance, 10 to 60% by weight of a rust preventive additive is added to the organic resin film. It is desirable to include it. Silica and chromate are effective as rust preventive additives in this case.

Next, the manufacturing method of the present invention which can easily manufacture the above Zn-Cr composite plated steel sheet of the present invention will be described in detail. In the method for producing a Zn-Cr composite plated steel sheet of the present invention, first, a Zn-Cr composite plated film containing 0.1 to 5 wt% of Cr is formed on at least one surface of the steel plate, and then a surface layer of the plated film. Part is Cr
Characterized by enrichment. A preferable method for enriching the surface layer portion of the plating film with Cr is to immerse the steel sheet provided with the Zn-Cr composite plating film in an acid or acid plating solution and perform etching treatment on the surface of the plating film to form the plating film. This is a method of eluting Zn in the surface layer portion. That is, when the steel sheet after plating is immersed in an acid or acid plating solution, active Zn is selectively eluted from the surface layer of the plating film and Cr having low solubility remains, so that the Cr-rich layer becomes a plating layer. It is possible to make only the surface layer portion of the plating film formed on the surface of Cr rich in Cr. Further, the Cr content in the plating film surface layer portion that has been Cr-riched by the above etching treatment can be easily controlled by changing the time of immersion in an acid or acid plating solution,
It is preferably 8 to 70%. For example, if the Zn-Cr composite plated steel sheet is immersed in an acid or acidic plating solution for about 0.5 to 6 seconds, only the surface layer portion of the plating film is enriched with Cr.

In the present invention, as a method for forming a Zn-Cr composite plated steel sheet on a steel sheet, any conventionally known method such as electroplating and vapor phase plating can be used. In particular, the electroplating method is used. It is advantageous to manufacture. Further, as a method for etching the surface layer portion of the Zn—Cr composite plating film provided on the steel plate, any method can be preferably used in the present invention as long as the intended purpose can be achieved. For example, when the plated steel sheet of the present invention is manufactured by an electroplating method,
After a plating layer is formed by using a plurality of plating cells, it is passed through the acidic plating solution of the last plating cell (a plurality of plating cells may be used) without electricity to perform the immersion etching treatment. As a result, it is possible to obtain the Zn-Cr composite plated steel sheet of the present invention in which the surface layer portion of the plating film is easily enriched with Cr.
Further, as another aspect of the etching treatment, a tank is separately installed after the plating cell and filled with an acidic aqueous solution (for example, an aqueous solution of sulfuric acid, hydrochloric acid and oxalic acid) or an acidic plating solution. There is also a method of dipping the steel plate after the Zn-Cr composite plating.

As a method for further subjecting the Zn-Cr composite plated steel sheet of the present invention in which the surface layer of the plating film is Cr-rich by the etching treatment after plating by the method as described above, to a chromate treatment, a reaction is performed. Any conventionally known method such as mold, electrolytic type, and coating type can be applied. For example, a desired chromate film layer can be formed by coating with a roll coater. Further, as a method for providing the organic resin film layer on the chromate film layer, similarly to the above, any conventionally known method such as roll coater coating can be used. For example, an epoxy resin or the like with silica and barium chromate added as an anticorrosive additive,
A desired organic resin film layer is provided by applying with a roll coater so as to have a desired thickness.

The plated steel sheet according to the present invention in which the chromate film layer and the organic resin film layer are further formed on the surface of the plating film obtained by the above-mentioned method, the surface of which is subjected to the etching treatment and made to be rich in Cr, It is excellent in corrosion resistance as compared with the case where the conventional plated steel sheet not subjected to the etching treatment is subjected to the chromate treatment and the formation of the organic resin film. This is because the corrosion resistance of the base plating layer is
One of the reasons for this is that the addition of Cr improves, but the plated surface layer that has been subjected to etching is enriched with Cr and at the same time fine irregularities are formed on the surface (also because the surface area increases). ), And good compatibility with the chromate treatment and the resin film layer is presumed to be another reason.

[0015]

EXAMPLES Next, the present invention will be described in more detail with reference to preferred examples and comparative examples. Examples 1 to 15 First, a cold-rolled steel plate having a plate thickness of 0.7 mm was electroplated using an acid plating bath containing zinc sulfate and chromium sulfate as main components. The composition of the plating film to be formed was changed depending on the Cr concentration of the plating solution and the current density. or,
Iron sulphate, nickel sulphate or cobalt sulphate or mixtures thereof were added to the plating baths of some examples. still,
A pH buffering agent, an electric conductivity auxiliary agent, a gloss auxiliary agent, and the like, which are usually used in industrial plating, were appropriately added to the plating bath.
As a result, a Zn-Cr composite plated steel sheet having the plating film composition and plating film thickness shown in Table 1 below was obtained. Next, the plated steel sheet obtained as described above is shown in Table 1.
In the acid or acidic plating solution shown in Fig. 1, the surface layer of the plating film was immersed in C for the time shown in Table 1 for etching treatment.
It became rich. With respect to the Zn-Cr composite plated steel sheets of Examples 1 to 15 of the present invention obtained as described above, the plating film thickness and the Cr content in the surface layer portion of the plating film were measured and shown in Table 1, respectively. It was confirmed that the surface layer portion of the plating film was enriched with Cr.

Example 16 to Example 25 A roll coater is applied to the surface of the plated coating of the Zn-Cr composite plated steel sheet of the present invention obtained in each of Example 2, Example 4, Example 7 and Example 8 described above. Chromate treatment by. Next, an epoxy resin containing silica and barium chromate as a rust preventive additive was further applied thereon with a roll coater to further apply a chromate film layer and an organic resin film layer onto the plating film. The Zn-Cr composite plated steel sheets of the present invention in the modes obtained as described above were set as Examples 16 to 25. Table 3 shows the amount of chromate adhesion converted to metal chromium, the thickness of the organic resin film, the corrosion resistance of the steel sheet, and the continuous spot weldability for each of these steel sheets.

Comparative Examples 1 to 15 A cold-rolled steel sheet having a plate thickness of 0.7 mm similar to that of the example was electroplated using an acid plating bath of zinc sulfate to obtain a Zn-plated steel sheet of Comparative Example 1. It was Further, the same cold-rolled steel sheet was electroplated, and in the same manner as in Example 1, the Zn-Cr composites for comparison of Comparative Examples 2 to 15 having the respective plating film compositions and plating film thicknesses shown in Table 2 were prepared. A plated steel sheet was obtained.
In addition, among the obtained Zn-Cr composite plated steel sheets, Comparative Example 4, Comparative Example 6, Comparative Examples 7 to 8, Comparative Example 10 and Comparative Example 1
For No. 3, the etching treatment was performed under the conditions shown in Table 2 in the same manner as in the example, and the appropriate value of the plating film composition before Cr enrichment was examined. As a result, when the Cr content of the plating film before Cr enrichment is not within the range of 0.1 to 5% by weight in the present invention, Table 2
As shown in, the corrosion resistance of the steel sheet was inferior.

Comparative Examples 16 to 17 On the plated steel sheets produced in Comparative Examples 1 and 2, a chromate film and an organic resin film layer were formed in the same manner as in the examples. Table 4 shows the corrosion resistance and the continuous spot weldability in this case.

The various physical properties shown in Tables 1 to 4 are as follows.
It was measured by the following method. (1) Composition of plating film The bulk (main inner layer) composition was measured by ICP (high frequency inductively coupled plasma emission spectroscopy), and the surface layer composition was measured by EPMA (X-ray microanalyzer).

(2) Corrosion resistance Using the Zn-Cr composite plated steel sheet of the present invention and the comparative plated steel sheet as a test sample 1, the draw bead processing shown in FIG. 1 was performed. The processed steel sheet was subsequently subjected to a corrosion resistance test to evaluate the post-working corrosion resistance of each. Draw bead processing conditions include a pressing pressure of 500 Kgf
And the drawing speed was 200 mm / min. The corrosion resistance test conditions include wet, dry, and salt water immersion for 1 day.
A combined cycle test was repeated. At this time, with respect to the plated steel sheet not subjected to chromate treatment or the like, after carrying out the above corrosion resistance test for 60 cycles, the depth of perforation formed on the surface of the steel sheet was measured and evaluated. For the plated steel sheet subjected to the chromate treatment and the organic resin coating, the corrosion resistance test was performed for 90 cycles, and the depth of perforation formed on the surface of the steel sheet was measured and evaluated.

(3) Continuous Spot Weldability A continuous spot weldability test was conducted on the plated steel sheet that had been subjected to the chromate treatment and the organic resin coating. The welding conditions at this time were that the electrode had a dome shape with a tip diameter of 6 mm and the applied pressure was 25 mm.
0 kg, current 11 KA and energization time 12 cycles. Under the above conditions, the results of testing the continuous spot weldability were evaluated according to the following criteria. ○: Weldable at 2,000 points or more △: Weldable at 500 points or more to less than 2,000 points ×: Weldable at less than 500 points

[0022]

[Table 1] Table 1 Plating composition and corrosion resistance evaluation results of Examples

[0023]

[Table 2] Table 2 Comparative composition of plating composition and evaluation result of corrosion resistance

[0024]

[Table 3] Table 3 Evaluation results of plating composition, corrosion resistance, and continuous spot weldability obtained by further subjecting the film of the example to chromate treatment

[0025]

[Table 4] Table 4 Evaluation results of corrosion resistance and continuous spot weldability of plating compositions that have been subjected to chromate treatment, etc.

[0026]

[Effect] As described above, the Zn-Cr composite plated steel sheet of the present invention has good workability because the inner layer, which is the main (bulk) plating film, has a low Cr content. On the other hand, since the surface layer portion of the plating film is Cr-rich, it is possible to suppress excessive elution of Zn even if the Cr-rich layer is in a corrosive environment, and the good sacrificial anticorrosive ability originally possessed by Zn is maintained. As a result, the Zn-Cr composite plated steel sheet has excellent corrosion resistance after working. Further, Z of the present invention
By forming a chromate film and an organic resin film layer on the plating film of the n-Cr composite plated steel plate, the chromate film further improves the corrosion resistance of the base plated steel plate,
Moreover, the organic resin film layer on the chromate film suppresses excessive elution of hexavalent chromate ions in the chromate film into the corrosive environment, and further achieves a sustained anticorrosion effect. Furthermore, according to the method for producing a Zn—Cr composite plated steel sheet of the present invention, a Zn— having excellent characteristics as described above is formed by a simple method such as immersing the steel sheet in an acid or acid plating solution and performing an etching treatment. A Cr composite plated steel sheet is obtained.

[Brief description of drawings]

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

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Claims (8)

[Claims] 1. Zn-containing 0.1 to 5% by weight of Cr
A Zn-Cr composite plated steel sheet having excellent corrosion resistance after working, characterized in that the surface layer portion of the plating film of the steel sheet having a Cr composite plated coating is enriched with Cr. 2. The plating film before being enriched with Cr further comprises:
The Zn-Cr composite plated steel sheet having excellent post-working corrosion resistance according to claim 1, wherein at least one or more iron group metals of Fe, Ni, and Co are contained in a total amount of 0.1 to 15% by weight. 3. The Zn-Cr composite plated steel sheet excellent in corrosion resistance after working according to claim 1, wherein the surface layer of the plating film is 0.15 to 5% of the total film thickness of the plating film after being enriched with Cr. 4. The Zn-Cr composite plated steel sheet having excellent corrosion resistance after working according to claim 1, wherein the Cr content in the surface layer portion of the plating film is 8 to 70%. 5. On the surface of the Zn—Cr composite plating film,
A chromate film layer of 10 to 150 mg / m ^{2 in} terms of metal chromium is provided, and a thickness of 0.
An organic resin film layer having a thickness of 3 to 3 μm is provided.
~ 4 Zn-Cr composite plated steel sheet excellent in corrosion resistance after processing according to 4 above. 6. A Zn—Cr composite plating film containing 0.1 to 5 wt% of Cr is formed on at least one surface of a steel sheet, and then the surface layer portion of the plating film is enriched with Cr. Zn-Cr with excellent corrosion resistance after processing
Manufacturing method of composite plated steel sheet. 7. The Zn-Cr having excellent corrosion resistance after processing according to claim 6, wherein the method of enriching the surface layer portion of the plating film with Cr is a method of subjecting the plated steel sheet to immersion etching treatment in an acid or acid plating solution. Manufacturing method of composite plated steel sheet. 8. Further, on the surface of the Zn-Cr composite plating film enriched with Cr, 10 to 15 in terms of metallic chromium is calculated.
The Zn-excellent corrosion resistance after processing according to claim 6 or 7, wherein a chromate film layer of 0 mg / m ² is provided, and then an organic resin film layer having a thickness of 0.3 to 3 µm is provided on the chromate film. Method for producing Cr composite plated steel sheet.