JPH0665760A - High corrosion resistance multi-ply electroplated steel sheet - Google Patents

Abstract

PURPOSE:To provide a multi-ply electroplated steel sheet having excellent impact resistive adhesion and corrosion resistance after coating and contg. chromium and iron group metals. CONSTITUTION:The high corrosion resistance multi-ply electroplated steel sheet comprises two or three layers. The first layer is a Zn plated layer having 0.1 to 3g/m<2> plated weight and contg. Cr and at least one metal selected from Fe, Ni and Co in a total amount of below 5wt.%. The second layer is a Zn based composite electroplated layer containing 5 to 30wt.% Cr, 0.1 to 10wt.% of at least one metal selected from Fe, Ni and Co and 0.001 to 5wt.% organic polymer. Optionally, a Zn or Zn based alloy plated layer is formed as the third layer to further improve the corrosion resistance after coating.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high corrosion resistant multi-layer electroplated steel sheet having excellent impact resistance and corrosion resistance after coating and applicable to various applications such as an anticorrosion steel sheet for automobiles.

[0002]

2. Description of the Related Art As a method of improving the corrosion resistance of galvanized steel sheets, there is a means of increasing the zinc coating amount (adhesion amount), but this has many problems in terms of weldability and workability. Therefore, many alloy platings have been proposed as a method of suppressing the dissolution of zinc itself and extending the life of zinc plating. Above all, Zn-based alloy plating containing an iron group metal such as Fe, Co, or Ni as an alloy component has been recognized for its good corrosion resistance and has been put to practical use. Further, for the purpose of further improving the corrosion resistance, attempts have been made to add Cr, for example, JP-A-61-270398 and JP-A-65-24094.
No. 9 gazette is disclosed, the Cr content is as low as 5% or less, and the corrosion resistance does not exceed that of the conventional Zn-based alloy plating, and a plated steel sheet having higher corrosion resistance is desired.

On the other hand, in these Zn-based alloy platings, the internal stress of the plating layer is higher than that of the Zn plating, and therefore the adhesion to the steel sheet is essentially weaker than that of the Zn plating. When applied to the outer surface of an automobile body, usually 3 coats of coating are applied with a total thickness of 100μ or more, and the shrinkage stress at the time of baking acts on the plating layer, and the plating adhesion becomes even lower than before coating. Become. Further, in cold regions in winter, the temperature drops below the freezing point, and the shrinkage of the coating film progresses due to this effect, so the stress acting on the plating layer becomes even greater, and the plating adhesion becomes even lower. Under such severe conditions, when pebbles or sprayed rock salt collides with a running vehicle (this phenomenon is called chipping), the impact force causes the plating layer to peel off. To solve such a problem, as described in Japanese Patent Laid-Open No. 59-200789, a method of depositing a deposit made of a specific metal on a steel sheet and then performing Zn-based alloy plating, and Zn-Ni alloy plating are described below. JP-A-1-225790 discloses a method of plating a lower layer with a Zn-Ni alloy having a low Ni content. However, both methods
The adhesion of plating under the severe condition of chipping at low temperature after coating is not improved to a sufficient level.

[0004]

The present inventors have added a specific organic polymer as a Cr deposition accelerator to a plating bath so that a high content of Cr and a trace amount of the organic polymer, which have never existed before, are contained. Thus, a prospect for obtaining a Zn-based electroplated steel sheet having epoch-making corrosion resistance was obtained. However, this new Zn
Similar to conventional Zn-based alloy plating, system-based electroplating cannot avoid stripping of the plating under severe conditions, and it is necessary to solve this problem. Further, even if the above-mentioned method disclosed for the purpose of improving the adhesiveness of Zn-based alloy plating is applied, it is insufficient, and it is necessary to find a means suitable for the Zn-based electroplating. In view of such circumstances, the present invention provides a multi-layer electroplated steel sheet having excellent impact resistance after coating and excellent corrosion resistance.

[0005]

The gist of the present invention is as follows. (1) Cr and F as the first plating layer on the steel plate surface
A Zn-based plating layer containing at least one of e, Co, and Ni in a total amount of less than 5% by weight and having a deposition amount of 0.1 to 3 g / m ² is formed, on which Cr is added as a second plating layer. ~ 30 wt%, at least one of Fe, Co, Ni
A high corrosion-resistant multi-layer electroplated steel sheet, characterized in that a Zn-based composite electroplated layer containing 0.1 to 10% by weight of seeds and 0.001 to 5% by weight of C is formed. (2) The high corrosion-resistant multi-layer electroplated steel sheet according to (1), wherein a Zn or Zn-based alloy plating layer having an adhesion amount of 0.1 to 5 g / m ² is formed as a third plating layer on the second plating layer. (3) The high corrosion-resistant multi-layer electroplated steel sheet according to (1) or (2), wherein the organic polymer contained in the second plating layer is a cationic polymer.

[0006]

The multi-layer electroplated steel sheet of the present invention comprises two to three plating layers. That is, the first plating layer contains Cr, and at least one or more of Fe, Co, and Ni in a total amount of less than 5% by weight, and an adhesion amount of 0.1 to 3 g /
m ² Zn plating layer, which significantly improves impact resistance adhesion of the second plating layer. The second plating layer contains Cr of 5 to 30
% Zn, a Zn-based composite electroplating layer containing 0.1 to 10% by weight of at least one of Fe, Co and Ni and 0.001 to 5% by weight of an organic polymer, and exhibits excellent corrosion resistance. To do. The third plating layer is a Zn or Zn-based alloy plating layer having an adhesion amount of 0.1 to 5 g / m ² which is applied as necessary, and improves the corrosion resistance after coating.

First, the first plating layer will be described. Conventional Zn-based alloy plating and Zn-based composite electroplating targeted by the present invention have higher internal stress than simple Zn plating, that is, the plating layer is hard and is less likely to undergo plastic deformation. Therefore, when a shock such as chipping is received, the shock force cannot be absorbed, the shock force reaches the interface with the base iron, and the plating layer is separated from the base iron interface. However, by providing a Zn-based plating layer containing a small amount of alloy components as the first plating layer, the impact resistance is greatly improved. Since the Zn-based plating layer containing less than 5% by weight of alloy is soft, it is plastically deformed to absorb the impact force even when it receives an impact such as chipping, and the adhesion to the base metal and the second plating layer is also strong. , It does not easily cause peeling of plating even under severe conditions. The alloy component contains Cr as an essential component, contains at least one kind of Fe, Co, and Ni, and the total amount thereof is less than 5% by weight. If it is 5% by weight or more, the impact resistance cannot be improved. From the viewpoint of corrosion resistance, Fe, Co, Ni
It is desirable that the total content of at least one of these is less than the Cr content.

The amount of the first plating layer deposited is 0.1 to 3 g / m
² is preferred. If it is less than 0.1 g / m ² , effective impact adhesion cannot be improved. Further, if it exceeds 3 g / m ² , the corrosion resistance of the second plating layer is impaired, which is not preferable. The plating component of the first plating layer is preferably the same as that of the second plating layer. By doing so, in a continuous electroplating line having a large number of electroplating cells, the plating baths for forming the first plating layer and the second plating layer can be the same, and complicated work such as management of the plating baths can be performed. Can be avoided. In this case, composition control of each of the first plating layer and the second plating layer can be performed by controlling the current density and the liquid flow rate.

The second plating layer has a high degree of corrosion resistance and constitutes the main layer of the multi-layer electroplated steel sheet of the present invention. The Cr content in the second plating layer is preferably 5 to 30% by weight. 5% by weight
If the amount is less than the above, the effect of improving the corrosion resistance is recognized, but the tendency that red rust easily occurs remains, and the corrosion resistance is insufficient. When the content is 5% by weight or more, the generation of red rust is extremely suppressed, and red rust does not easily occur even if a salt spray test is conducted for 500 hours or more in a bare state as it is, and conventionally known alloys such as Zn-Ni and Zn-Fe. Exhibits high corrosion resistance superior to plating. It is presumed that the inclusion of Cr may be the reason why high corrosion resistance is exerted because the corrosion product forms a hardly soluble protective film to cover the surface and suppress the progress of corrosion. Even if the Cr content exceeds 30% by weight, it has a high degree of corrosion resistance, but even if it has the eutectoid effect of an organic polymer that promotes Cr precipitation such as the cationic polymer described later, it prevents deterioration of the powdering property during processing. It is difficult to apply in practice.

The second plating layer further contains an iron group metal such as Fe, Co or Ni as a metal component. The function of the iron group metal is to further improve the stability of the corrosion product and improve the corrosion resistance by the interaction with Cr. The content of these is preferably 0.1 to 10% by weight in total of one kind or two or more kinds. If it is less than 0.1% by weight, the above effect is not remarkable, and if it exceeds 10% by weight, the properties of the iron group metal are strengthened and the effect of Cr is reduced, so that the corrosion resistance tends to decrease. Considering the powdering resistance during processing, the total amount of Cr and iron group metal is preferably 30% by weight or less. In addition,
Among the iron group metals, Ni is also effective for corrosion resistance and is most advantageous.

The organic polymer contained in the second plating layer is added to the plating bath as a Cr deposition accelerator and has a function of depositing Cr ³⁺ ions as metallic Cr. By co-depositing this together with Cr in the plating layer, the powdering resistance during processing can be improved. It is presumed that such an eutectoid effect of the organic polymer lies in the fact that Cr hinders the uniform electrodeposition growth of Zn and prevents the plating structure from lacking uniformity and smoothness. That is, through the co-deposited organic polymer,
It is considered that a dense plating layer in which Zn and Cr are uniformly mixed or alloyed is formed. The content of the organic polymer is preferably 0.001 to 5% by weight. If it is less than 0.001% by weight, the effect of improving the powdering resistance is poor, and it is 5% by weight.
Not only is it difficult to obtain a content higher than the above even if the concentration of the organic polymer in the plating bath is increased, but when a large amount of eutectoid is returned, the plating adhesion is lowered. In order to secure the powdering resistance, it is desirable to co-deposit the organic polymer in a Cr content of 1/1000 or more. As the organic polymer, a water-soluble cationic polymer is effective, and among them, a quaternary amine salt having a molecular weight of 10 ³ to
10 ⁶ polymers are effective.

[0012]

[Chemical 1]

Specifically, the following polyamine (PA) or polyamine sulfone (PAS) polymer is Cr
Most effective as a precipitation accelerator. It is considered that the contribution of the amine group to the cathode surface, the suppression of Zn ion precipitation by the alkyl group, and the coordinate bond of Cr ³⁺ ion to the sulfone group are considered to contribute.

[0014]

[Chemical 2]

Besides these, polymers of 1, 2, and 3 amines are also effective as Cr precipitation accelerators although they do not reach the quaternary amine polymers. Other than the cationic polymer, polyoxyalkylene derivatives, especially polyethylene glycol (PEG) shown below are effective.

[0016]

[Chemical 3]

The coating amount of the second plating layer is not particularly limited, but it is 10 g / m ² or more in view of corrosion resistance, and 50 g / m 2 in cost for production in an electroplating line.
² or less is preferable. The third plating layer further improves the corrosion resistance after coating by the action of improving the phosphoric acid treatment property of the second plating layer. Since the second plating layer contains a large amount of Cr, it is difficult to form a dense phosphatized film by the present phosphate treatment. Even in such a state, the corrosion resistance after coating is superior to that of conventionally known Zn-Ni, Zn-Fe, etc. due to the effect of Cr, but Zn plating or Zn-based plating in which a dense phosphatized film is formed as the third plating layer When alloy plating is provided, the corrosion resistance after painting is further enhanced. The Zn alloy plating used here is, for example, 2
An alloy plating containing 0% or less of Ni or 90% or less of Fe and excellent in phosphating property. The adhesion amount of the third plating layer is preferably 0.1 to 5 g / m ² . If it is less than 0.1 g / m ^2, it is difficult to form a dense phosphatized film and it is 5 g.
If it exceeds / m ² , the property of the third plating layer becomes strong and the corrosion resistance after coating is lowered, which is not preferable. From the viewpoint of phosphating property and corrosion resistance after coating, the range of 0.5 to 3 g / m ² is more preferable.

The multi-layer electroplated steel sheet of the present invention is prepared by subjecting the steel sheet to pretreatment such as degreasing and pickling, and then applying Zn as the first plating layer.
-Based plating, Zn-based composite electroplating as the second plating layer,
It can be obtained by applying Zn or a Zn-based alloy plating as the third plating layer if necessary. The first plating layer and the second plating layer are Zn ²⁺ , Cr ³⁺ , further Co ²⁺ , Fe.
²⁺ and Ni ²⁺ are each contained in an amount of 10 to 100 g / l, and a cation polymer typified by PAS or an organic polymer such as PEG as a Cr precipitation accelerator is added in an amount of 0.
PH of 0.5 to 3, containing 01 to 20 g / l, bath temperature of 40 to
A 70 ° C. sulfuric acid acidic bath may be used. Here, a plating bath having a different composition may be used for each of the first plating layer and the second plating layer, or the same plating bath may be used to control the respective plating compositions by changing the current density and the liquid flow rate. May be. A conductivity aid such as Na ⁺ and NH ₄ ⁺ and a buffer such as boric acid may be added to the plating bath. Third
The method for forming the plating layer is not particularly limited, but for the purpose of uniformly coating the surface of the steel sheet with a small adhesion amount, it is best to use a sulfuric acid acid bath at a current density of 10 to 100 A / dm ² . The structure of the present invention may be applied to both sides of the steel sheet, or may be applied to only one side and the other side may be the steel sheet side or another plating layer. It may also be applied to the undercoating of an organic composite plated steel sheet having an organic coating on the upper layer.

[0019]

Example: Cold-rolled steel sheet was degreased with alkali and 5% sulfuric acid solution was added.
After pickling with a liquid, electroplating was performed under the following conditions.
The first plating layer and the second plating layer are Zn²⁺, Cr³⁺And C
o ²⁺, Fe²⁺, Ni²⁺One or more of these, and even organic
Molecules (PA with average molecular weight 100,000, average molecular weight 3500
PAS, PEG with an average molecular weight of 1500), and Na⁺
Using a sulfuric acid acidic bath with a pH of 1-2 including 55 and a bath temperature of 55 ° C
I made it. The third plating layer is Zn²⁺Or even F
e²⁺, Ni²⁺PH of 1.5 including, sulfuric acid acid at bath temperature 60 ℃
Formed using a bath. The plating composition is the plating bath composition and
Controlled by current density and liquid flow rate, and adhered amount controlled by energization amount
did. Table 1 shows the plating composition, impact adhesion and resistance after painting.
The relationship of food habits is shown. The evaluation method is as follows.

(1) Impact resistance Adhesion resistance Test piece was immersed in phosphate treatment, cationic electrodeposition coating 30μ
Then, 40 μm of each of the intermediate coating and the top coating was applied to give a total film thickness of 110 μm. After that, a low temperature chipping test was conducted in which 250 g of JIS No. 7 crushed stone was collided with the test piece at a speed of 150 km / hr at a test piece temperature of -20 ° C, and the plating peeled area after tape peeling was evaluated in 5 levels. Rating 5 (good: plating peeling 0.1% or less) -rating 1 (poor: plating peeling more than 10%)

(2) Corrosion resistance after coating Corrosion resistance of chipping flaw The test piece evaluated for impact resistance was subjected to outdoor exposure while spraying salt water twice a week as it was, and evaluated for the area where red rust occurred after 3 months. Rating 5 (Good: No red rust) -Rating 1 (Poor: Red rust greater than 10%) Corrosion resistance of cross-cut flaws Perform the same coating as in (1), insert a cross-cut flaw that reaches the base iron with a cutter knife, The cycle corrosion test was conducted for 150 cycles, and the coating film bulge width was evaluated. Cycle corrosion test SST 4 hours-dry 2 hours-wet 2
Time Score 5 (Good: Blister width of 2 mm or less) -Score 1 (Poor: Blister width of more than 10 mm)

In Table 1, in Comparative Example 1, the total content of components other than Zn in the first plating layer is too high, and in Comparative Example 2, the adhesion amount of the first plating layer is too small. Inadequate adhesion and corrosion resistance of chipping flaws. In Comparative Example 3, the adhesion amount of the first plating layer was too large, and therefore the corrosion resistance after coating was insufficient. Comparative Example 4 is Cr of the second plating layer.
Since the content rate is too low, the corrosion resistance after coating is poor, and in Comparative Example 5, the Cr content rate of the second plating layer is too high, so the impact resistance adhesion and the corrosion resistance of chipping flaws are poor.
In Comparative Example 6, since the Ni content of the second plating layer is too low,
The corrosion resistance of the cross-cut flaw is insufficient, and in Comparative Example 7, since the Ni content of the second plating layer is too high, the impact resistance and the corrosion resistance after coating are insufficient.

In Comparative Example 8, in forming the second plating layer,
Cr was deposited by adding extremely high current density without adding organic polymer to the plating bath, but Cr was deposited as an oxide rather than a metal, and both impact resistance and corrosion resistance after painting It is bad. In Comparative Example 9, the adhesion amount of the third plating layer was too large, and therefore the corrosion resistance after coating was insufficient. In Comparative Example 10, Ni was applied as the first plating layer, but the impact resistance was good, but the corrosion resistance of chipping flaws was insufficient. Inventive Examples 1 to 6 and 10 to 15 are those in which the first plating layer and the second plating layer are formed using the same plating bath, and Inventive Examples 7 to 9 and 16 to 18 use different plating baths. All of them have good impact resistance and good corrosion resistance after coating. Further, Examples 10 to 18 of the present invention in which the third plating layer was provided were much more excellent in the corrosion resistance of cross-cut flaws.

[0024]

[Table 1]

[0025]

As described above, the layer-coated electroplated steel sheet of the present invention has excellent impact resistance adhesion at low temperature after coating,
It is a high-performance plated steel sheet that is excellent not only in bare corrosion resistance but also in corrosion resistance after painting, and is particularly suitable as an anticorrosion steel sheet for automobiles.

Claims (3)

[Claims] 1. A steel plate surface comprising Cr as a first plating layer,
And a Zn-based plating layer containing less than 5% by weight of at least one of Fe, Co, and Ni in a total amount of 0.1 to 3 g / m ² and formed with a second plating layer containing Cr as a second plating layer. A Zn-based composite electroplating layer containing 5 to 30% by weight, 0.1 to 10% by weight of at least one of Fe, Co, and Ni and 0.001 to 5% by weight of an organic polymer was formed. High corrosion resistance multi-layer electroplated steel sheet characterized by. 2. The highly corrosion-resistant multi-layered electric layer according to claim 1, wherein a Zn or Zn-based alloy plating layer having a deposition amount of 0.1 to 5 g / m ² is formed on the ^second plating layer as a third plating layer. Plated steel sheet. 3. The high corrosion-resistant multi-layer electroplated steel sheet according to claim 1, wherein the organic polymer contained in the second plating layer is a cationic polymer.