JPS59182987A - Double plated steel sheet - Google Patents

Abstract

PURPOSE:To obtain a steel sheet having sufficient suitability to chemical conversion treatment with respect to the secondary adhesion of a paint film and showing superior red rust resistance after coating by depositing a specified amount of tin on the surface of a steel sheet having a corrosion preventive layer and an Fe or Fe-Zn alloy surface layer formed by plating in order. CONSTITUTION:Tin is deposited on the surface of the surface layer of a double plated steel sheet by 1-100mg/m<2>. The steel sheet has an Fe or Fe-Zn alloy layer formed by electroplating as the surface layer on at least one side and a corrosion preventive layer formed by plating just under the surface layer. Said deposition is carried out by flash-coating tin by electroless plating, electroplating or other method. Thus, the futile consumption of tin due to direct addition to an Fe or Fe-Zn plating bath can be avoided, and the desired steel sheet can be obtd. economically.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a temporary multi-layer plated net that has particularly excellent chemical properties and exhibits extremely high coating and water-resistant adhesion after painting.

Recently, the application of corrosion-resistant plated steel sheets to automobile bodies used under severe corrosive conditions has been studied and promoted.

Currently, the plated steel plate for this heel is Zn-based! Or Zn-
1'Ji series, Zn-Cr series, Zn-Fe series, Zn-C
Steel sheets having a Zn combination plating film such as d series, Zn-5n series, ZZn-1 VI series, Zn-1 meta series, Zn-1, and I series have become common.

Now, in the case of automobile body applications, a coating is usually applied before actual use, but in recent years, this coating has been combined with use on roads where anti-freezing agents are sprayed, especially in cold regions, and has been coated with cationic paint. Electrodeposition coating has become the norm, but both Zn-based and Zn alloy-based plated steel sheets tend to have poor secondary adhesion to the cationic electrodeposition coating. The quality of this adhesion is determined by the damage caused by damage to the paint film due to chipping, etc. during single-layer application.
This relates to the occurrence and progression of red rust, also known as "exterior rust," and is one of the most important properties for painting applications.

Recently, multilayer plating has been proposed as a measure to improve the above-mentioned inferiority. In other words, a FEF or Fe-Zn alloy plating layer (about 60 wt% Fe) (hereinafter referred to as sFe-rich surface layer, S: ) is applied on the anti-corrosion plating layer (inner layer) as described above. Proposed. However, this has the following problems. The above Fe1J
The red rust resistance tends to deteriorate in the etch surface layer, and in this relationship, it is difficult as a practical matter to improve the secondary adhesion of the paint film while maintaining the red rust resistance. That is, 1) Adhesion with the paint film largely depends on the state of the chemical conversion film as the base for painting. In other words, in the chemical conversion coating, 7 osulfuric acid I-(Zn2Fe(PO4)2
The more 4I (20, denoted P), the better the steel sheet shows secondary coating adhesion;
As Zn3(PO4)2.48 (indicated by αH) increases, the secondary adhesion of the coating film tends to deteriorate. In general, the phosphoophierite (content) in the chemical conversion coating, that is, P/(P+H
) ratio (hereinafter referred to as P value) is understood as a property of chemical conversion treatment of the steel sheet.

The Fe-rich surface layer dissolves and supplies Fe in the chemical conversion treatment, contributes to the formation of dense phosphoopherite, and results in the formation of a chemical conversion film with a high P value.

However, in reality, this surface layer is discontinuous, and in order to stably obtain a sufficient P value (approximately 80% or higher) from the viewpoint of secondary adhesion of the paint film, the amount of adhesion must exceed at least 52%. Preferably, 10 shoulders are required.

This can be explained as follows. Fe or td Fe-
Zn-based alloy plating is formed by electroplating, but in general, in areas where the amount of electroplated film is small due to its deposition mechanism, the coverage of the plating tends to decrease as the amount of adhesion decreases. However, on the other hand, when the coverage of the surface layer decreases, the effect of improving the P value of the chemical conversion coating decreases. The portions left uncovered by the surface layer cause preferential elution of the inner layer, which has a more base potential than the Fe1J surface layer, during the chemical conversion treatment, preventing effective Fe supply from the surface layer, and forming a dense phosphor layer. This is thought to be because it results in inhibiting the formation of phoopherite.

In other words, if the amount of adhesion is too small, the coverage of the surface layer will decrease and the effect of improving the P value will tend to be lost.Therefore, in order to obtain a high P value, a minimum of 52
It can be considered that the amount of adhesion is required to exceed the eyebrows.

2) However, on the other hand, this surface layer has a high Fe content,
If the amount of adhesion is large, it is likely to become a source of red rust if the paint film becomes scratched. In other words, from the point of view of red rust resistance, F
The amount of the e-rich surface layer attached should be as small as possible, specifically 1 to 6 fAr? It is preferable to limit it to a certain extent.

In this way, the means of providing a Fe1J surface layer is considered to be contradictory to chemical conversion treatment and maintaining red rust resistance as described above, and it must be said that it is extremely difficult to achieve both in practice. .

The object of the present invention is to provide a multilayer plating structure capable of obtaining a plated steel sheet that has sufficient chemical conversion treatment properties in terms of secondary coating adhesion and also exhibits excellent red rust resistance after coating. It is.

The present inventors have developed a multi-layer plated steel sheet having the above-mentioned Fe-rich surface layer, reducing the amount of surface layer adhesion to a level that poses no problem from the viewpoint of red rust resistance, while maintaining its original excellent chemical conversion treatment property. A previous study found that it is effective to incorporate a small amount of Sn into the Fe'J surface layer as a means to make the most of it. The mechanism of improvement in chemical conversion treatment properties due to the inclusion of Sn in the surface layer has not yet been fully investigated and cannot be said with certainty, but it is thought to be as follows.

■ Sn forms a microbattery with Fe in the surface layer and acts as a cathode in chemical conversion treatment, not only promoting the dissolution of the Fe but also increasing the nucleation and formation of chemical crystals. Through this action, it promotes the formation of phosphoophelite chemical crystals.

(2) Also, by shifting the potential of the surface layer itself toward the shore side, it acts to promote elution of Fe from the surface layer during chemical conversion treatment.

In this way, the agile addition of Sn to the Fe-rich surface layer...
This is effective in ensuring excellent chemical conversion treatment properties with a small amount of Fe-rich surface layer deposited. However, this addition of Sn to the surface layer is carried out by adding Sn to the plating bath when depositing the Fe-Zn alloy film from the plating bath whose main components are Fe2+ and Zn"+. However, in this type of electroplating, the following cost disadvantage cannot be avoided. Namely, in an acidic aqueous solution, nsn takes the form of a multivalent ion Sn29Sn'+,
However, Sn added to the Fe-Zn based plating bath
exists in the form of Sn, Sn2, whereas in this type of plating bath, that is, containing Fe2+, Fe3
The production of + easily progresses, and the concentration of Fe3+ increases over time. As a result, in the plating bath, Fe and Sn become Fe + Sn − + F
Exhibiting the reaction e +Sn, Sn2+→Sn'+
As the replacement progresses, the amount of Sn'+ in the bath gradually increases on top of the initial amount of Sn'+. Sn'+ originally does not have high solubility in acidic aqueous solutions, and as a result, Sn'+ in the bath increases over time, becoming a colloidal hydrate of 5n (004, etc.) and turning into suspended sediment Ki. In other words, in a Fe-Zn-based plating bath with Sn added, a part of the added Sn does not contribute to Sn electrodeposition and becomes 5n(
This means that Sn is wasted by forming colloidal precipitates containing OH, etc., and the amount of Sn consumed by this accounts for a non-negligible proportion (approximately 30 to 95 inches) of the total amount of Sn used. This consumption is a major factor contributing to high costs, as is expensive.

Since the addition of the Fe'J surface layer surface layer n involves such economical disadvantages, the present inventors set out to find an economically excellent means to obtain the same effect as the addition of Sn. , further experiments and research were carried out, and as a result, the above Fe
It has been found that the SnD effect can be similarly brought out by attaching a small amount of Sn to the surface of the rich surface layer, that is, by flash coating it.

If Sn is to be deposited, it is sufficient to flash-coat Sn on the surface layer in a completely separate process after the Fe-rich surface layer is deposited.
It is possible to eliminate the useless consumption of Sn due to the direct addition of Sn to the Zn-based plating bath, and the amount of S mold used in the implementation can be reduced accordingly.

The present invention has been made based on the above findings, and the gist thereof is to have an sFe-based or Fe-Zn-based alloy electroplated layer as a surface layer on at least one side, and a layer immediately below the electroplated layer ( Hereinafter, the term "inner layer" refers to this, but in a multi-layer plated steel sheet having two or more plating layers, including an anti-corrosion plating layer, Sn is added to the surface of the surface layer.
1~100'Q/rr? (hereinafter, this layer is referred to as a Sn flash layer).

In the present invention, the adhesion amount of the Sn flash layer is 1 to 1.
This is because of the following reasons: If this amount of adhesion is less than l"f/rn', especially Fe
If the amount of the surface layer made of Fe-Zn alloy or Fe-Zn alloy is small (approximately 1 to 62 mm), the effect of improving SnO chemical conversion treatment properties is insufficient, and it is difficult to form a good chemical conversion coating with a high P value in chemical conversion treatment. I can't expect it to generate.On the other hand, this is 100
"If f/W is exceeded, as is clear from the description of the examples below, the chemical crystals will have uneven spots or what is commonly called "
Uneven adhesion, called sagging, may occur. This S
Sn has a random, discrete, micro-island distribution on the surface layer, but the function of this Sn is considered to be the same as when it is contained in the surface layer, as mentioned earlier. It will be done. However, regarding this attached Sn, is it metallic Sn0, or is it attached to the active site in the form of SnH or SnP' oxide/hydroxide?
The details of its state of existence are not clear.

Methods for flash coating Sn include known methods such as electroplating, electroless plating, vacuum evaporation, vapor phase plating, plasma spraying, and hot-dip plating. , deplating is preferred. Sn flash with electroless plating and electroplating
An example of the plating conditions for coating is as follows.

(Electroless plating) Immersion treatment in a hydrochloric acidic 5N (J210-100?/J solution).

(Electroplating) The same alkaline bath, halogen bath, and ferrostane bath used for tin plated steel sheets are used. For example, phenol sulfonic acid 15 eye1 phenol sulfonic acid 10 f/71. sn2+ 30/71 (ferrostane bath), current density 0.05-5 A/dm", 3
Electrodeposit at 0-3!5'C.

Next, the surface layer must be plated with Fe-based or Fe-Znn alloy metal. This is because the surface layer is intended to improve chemical conversion treatment properties and, ultimately, the secondary adhesion of the coating film, and Fe-based 5Fe-Zn alloy plating is effective in the sense of t. Among them, the Fe-Zn type has a much faster chemical conversion reaction rate in chemical conversion treatment than the Fe type, and is more suitable for practical use from the viewpoint of productivity, and its adoption is recommended. The composition of the Fe-Zn alloy plating used as the surface layer is Fe
The content is preferably 50wt1 or more. That is, Fe3
If it is less than 0 wt%, the effect of improving the chemical conversion treatment properties of the surface layer tends to be insufficient, and there is a possibility that precipitation of phosphoopherite, which is effective for secondary adhesion of the coating film, may be insufficient in the chemical conversion treatment. Furthermore, when a corrosion test is conducted with cross cuts (eaves) made in the coating film, the corrosion resistance from the eaves tends to deteriorate.

However, if this exceeds 95 wt%, the chemical conversion reaction rate tends to be slow, similar to the above-mentioned Fe system, so 95 wt% or less is practical. Further, within this Fe content range, it is easy to obtain good plating with less unevenness caused by stirring the plating solution by electroplating, and manufacturing control is also easy.

In the case of the plated steel sheet of the present invention, the surface layer has a minimum coating amount of
If/rn" is also sufficiently effective. In other words, 1 ff/m" is sufficient to obtain a chemical conversion film with a high P value in chemical conversion treatment. This is due to the 5ty7) action on the surface layer mentioned above. However, considering the adhesion variation in actual line production, 1~62~ is preferable. Therefore, in the plated steel sheet of the present invention, the surface layer adhesion amount is 6 g/r+? It is possible to simultaneously ensure red rust resistance and excellent corrosion treatment properties by reducing the amount to a level below.

Regarding the inner layer immediately below the surface layer, there are no particular limitations on its composition or amount of adhesion. This layer ensures the original anti-corrosion performance, but in that sense, any anti-corrosion plating, such as sZn plating, sZn alloy plating, etc., can be used. However, in order to make the surface layer effectively contribute to chemical conversion treatment, it is best to suppress elution from the inner layer as much as possible, and from this point of view, it is preferable to use Zn alloy plating, which has a higher potential. Yes.

That is, N13-20wt%, γ4■-based Zn+
Ti-based alloy electroplating, Zn 60-90wt% Zn
-Fe based alloy electroplating, A715-60wt% Zn-Al based alloy hot dip plating, etc. are suitable. Note that in the inner layer of such a Zn alloy, a small amount of Cr%Fe%Co%NisCu%A
71% There is no difference in effectiveness even if one or more types of IVlg and hemorrhoids are contained. The amount of this inner layer adhered may be determined as appropriate depending on the application, but for automobile bodies, it is approximately 20%. ~4 obscenity level is appropriate.

The multi-layer plated steel sheet of the present invention has the above-mentioned Sn flash coat, a surface layer, and an inner layer, but as long as it has this plating structure, for example, it has a steel sheet base further inside the above-mentioned inner layer. There is no problem in having a Cu plating layer to improve adhesion, a Ni plating layer to prevent micro-cracks in the plating layer, Cr plating or other anti-common plating to further improve corrosion resistance. .

Note that the multilayer plating structure according to the present invention does not necessarily have to be applied to both sides of the steel plate, but it is possible to adopt this structure only for one side and leave the other side bare, or It goes without saying that the plated surface may be used in a different configuration.

Next, examples of the present invention will be described.

Steel plates having various plating structures shown in Table 1 were subjected to the following tests. The Sn flash layer of (4) to 1) is
Obtained by the electroplating method described above. Regarding the surface layer, inner layer, and layers below, the * mark in the table is a hot-dip plating film, and all others are electroplating films. The amount of adhesion of the surface layer was determined to a level that did not substantially affect red rust resistance (6
f/m2 or less).

<Chemical conversion treatment test> Phosphate-based chemical conversion treatment [immersion, Ti manufactured by Nippon Barker Co., Ltd.]
The (100) plane strength of phosphoopherite and the (020) plane strength of hovite were detected by diffraction of the chemical crystals in the obtained chemical conversion film. The crystal structure was investigated using the method of calculating the ratio (P value).

Secondary coating adhesion test> Cationic electrodeposition coating (20 μ) was applied to the steel plate after the above chemical conversion treatment.
- Intermediate coat (30 μ) - Top coat (30 μ), apply the same coating as the practical exterior painting of automobiles, soak in ion-exchanged water at 50°C for 10 days, and then add 10 x 10 base loss scribes in 2% increments. The peeling of the paint film was investigated using Scotch tape to check the degree of adhesion of the paint film.

The above test results are shown in the right column of Table 1.

475- In Table 1, Zn-Ni alloy with sNi 8-20wt% or Zn-Fe-based alloy electroplated steel sheets (1) and (2) with Zn60-90wt% have a low P value, and as a result, the coating film is peeling off. It is also large, and the secondary adhesion of the coating film is poor. (3
) is an example of a two-layer plated steel sheet with a Fe-Zn alloy plating layer as the surface layer, but this also does not provide a sufficient P value due to the coverage of the surface layer plating, and therefore the secondary adhesion of the coating film is poor. In contrast, the present invention's tZIJ (4) to ( 10'/''i, all showed extremely high P values, and the P values during coating film peeling were much smaller than those of the above-mentioned conventional example, and both chemical conversion treatment properties and secondary coating adhesion were extremely good.

As a comparative example, Q7) (E) Since the amount of USN flash layer is less than 1η/m, satisfactory results cannot be obtained in terms of P value and rotation during paint film peeling. (18
), (II, L2'fH times) Since the amount of adhesion exceeded 100 W/m, poor chemical conversion (unevenness, smearing, etc.) occurred during the chemical conversion treatment, and the results were also poor in terms of 71 paint film peeling.

As is clear from the above explanation, in the multilayer plated steel sheet of the present invention, Fe'! It is possible to obtain excellent chemical conversion treatment properties and secondary adhesion of the coating film even if the amount of the Fe-Zn alloy plating surface layer is quite small, so it has excellent chemical conversion treatment properties and post-painting needle corrosion resistance at the same time. It is possible to
The plated steel sheet of the present invention is economically advantageous, and therefore has extremely high utility value especially for use in automobile bodies.

Claims (1)

[Claims] (1) Fe-based or F-based surface layer on at least one side
In a multi-layer plated steel sheet having an e-Zn alloy electroplated layer and two or more plating layers having an anti-corrosion plating layer immediately below it, the surface of the surface plating film has Snl~10
A tough steel plate characterized by having a layer of 0η/rn'.