JPS61104086A - Chromated steel sheet - Google Patents

Abstract

PURPOSE:To provide superior corrosion resistance, superior adhesion to a paint film and high weldability to a plated steel sheet by successively forming a reacted chromate film and a coated chromate film on the steel sheet by the specified extent of sticking each. CONSTITUTION:The 1st Cr<3+>-base chromate film as an under layer is formed on a steel sheet plated with a Zn alloy, an Al alloy or the like by reaction type chromating by 5-50mg/m<2> Cr. The 2nd chromate film contg. Cr<3+>, Cr<6+> and SiO2 as principal components is formed on the 1st chromate film by coating type chromating by 20-150mg/m<2> Cr. By this treatment, the corrosion resistance of the steel sheet, the adhesion to a paint film and the weldability are improved, and blackening is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a chromate-treated steel sheet in which a chromate film is formed on a metal plating film formed on the surface of the steel sheet.

[Prior art and its problems]

Galvanized steel sheets usually have - on the surface of the galvanized film.
Next, a chromate film is formed to prevent rust. Conventionally, the purpose of forming such a romate film was to prevent white rust from forming on the surface of a galvanized steel sheet before it is used by a user.

However, in recent years, electrical appliances and steel furniture, etc.

As formed galvanized steel sheets are often used as they are, the chromate film formed on the surface of galvanized steel sheets is not only used for primary rust prevention as mentioned above, but also for rust prevention after forming and as a base for painting. It has become necessary to have better corrosion resistance and coating adhesion than before.

As a method for improving the corrosion resistance of the chromate film as described above, for example, Japanese Patent Publication No. 42-14050 and Japanese Patent Application Laid-Open No. 54-161549 disclose that chromate treatment solution is made by adding silica sol or a small amount of trivalent chromium to chromic acid. A chromate treatment method (hereinafter referred to as prior art) has been disclosed, which comprises applying an aqueous solution containing ions (Cr3+) to the surface of a galvanized film and drying it without washing with water.

However, the chromate-treated steel sheet manufactured by the above-mentioned prior art had a white rust generation time of 24 to 72 hours in a salt spray test according to JIS Z2371, and its corrosion resistance was insufficient, and further improvement thereof was desired.

The reason why the corrosion resistance improvement effect of the Chronaut treated steel sheet manufactured by the above-mentioned prior art is not sufficient is unknown, but
The following may be considered. That is, the chromate treatment solution used in the above-mentioned prior art is different from reactive chromate treatment solutions9 in that it does not have any reactivity with the galvanized film formed on the surface of the steel sheet, and is simply applied to the surface of the galvanized film. ,
Since it is only dried, it is not compatible with the galvanized film. Therefore, it is not possible to form a uniform and sufficiently adherent chromate film on the surface of the galvanized film, and defects are likely to occur in the chromate film. As a result, corrosion progresses from the defective parts of the chromate film. Corrosion resistance of the entire film is impaired.

4. As a means of solving the problems of the prior art described above and improving the corrosion resistance of the entire plating film, it is possible to increase the amount of chromium deposited on the surface of the galvanized film, that is, to reduce the thickness of the chromate film. 2. If the thickness of the chromate film is increased, the corrosion resistance will be improved accordingly, but on the other hand, the chromate film of the prior art has hygroscopicity because its main component is Cr. The film easily dissolves due to moisture absorption. Further, the chromate film of the prior art is formed by applying a chromate treatment solution onto the surface of the plating film, so the adhesion to the galvanized film is weak, and the chromate film easily peels off from the galvanized film. For example, when a chromate-treated steel plate is cut with a shearing device, the chromate film on the cut portions peels off, significantly reducing corrosion resistance. Such moisture absorption and peeling phenomena of the romate film become more severe as the amount of chromium deposited increases, so even if the amount of chromium deposited is increased as described above, corrosion resistance cannot be improved.

It was also formed according to the prior art described above. The chromate film produced by application-type chromate treatment has insufficient adhesion with the paint film applied on the surface, and the paint film is prone to scratches, so-called poor scratch resistance. It did not fully function as a base for painting, and black discoloration sometimes occurred. Furthermore, depending on the content of silica sol in the chromate film,
There was also the problem that weldability deteriorated due to an increase in electrical resistance.

[Purpose of the invention]

Therefore, an object of the present invention is to provide a romate-treated steel sheet that has good corrosion resistance and coating adhesion, and also has good weldability.

[Summary of the invention]

The present invention provides a chromate-treated steel sheet having a metal plating film on the surface of the steel sheet and a chromate film adhering to the metal plating film, wherein the chromate film comprises:
a first chromate film as a lower layer formed on the metal plating film by reactive chromate treatment in which the amount of Cr mainly composed of Cr is in the range of 5 to 50 my/m'; The amount of Cr deposited on the first chromate film, the main components of which are Cr3+, Cr6+ and 5in2, is within the range of 20 to 15.01□.
It is characterized by a second chromate film as an upper layer formed by coating-type chromate treatment.

[Structure of the invention]

In this invention, the metal plating film formed on the surface of the steel sheet is zinc plating or Zn-Fe.

Zn-Ni, Zn-At, Zn-Mn, Zn-Co
Zinc alloy plating, aluminum plating, etc.
Multi-layer plating using the same or different types of plating is preferred.

Such a plating film may be formed by any known method such as an electrolysis method, a melting method, a vapor phase method, or a thermal spraying method, and is not particularly limited.

Preferred components of the zinc-based alloy plating film are as follows.

Fe amount in Zn-Fe alloy plating: 5 to 90 wt, % (preferably 5 to 35 wt, Ni amount in Zn-Ni alloy plating: 5 to 20 wt, % Zn
- Volume of Mn alloy plating: 230-85wt, %Zn-A
At amount of A alloy plating 2-60wt, %Zn-Co-
Co amount of Cr alloy plating: 0.01 to 15 wt, %
Same <Cr amount: 0.01 to 1 wt, % If the components of the zinc-based alloy plating film are outside the above range, it is not preferable because the desired corrosion resistance cannot be obtained. In addition, also in the case of multilayer plating with two or more layers, the components of the plating film of each layer are preferably within the above range.

In this invention, the chromate film is formed by a first chromate film as a lower layer, which is a reactive chromate treatment formed on a metal plating film, and a coating-type chromate treatment, which is formed on the first chromate film. The reason for having two layers of the second black and mate films as the upper layer is as follows.

In other words, since the first chromate film as a lower layer on the metal plating film is subjected to reactive chromate treatment, it has good wettability (compatibility) and excellent adhesion to the metal plating film. A thin, even layer is formed on the top. Therefore, the coating-type chromate treatment liquid that forms the second chromate film as an upper layer can be evenly and uniformly applied onto the first chromate film, and the coating-type chromate treatment solution can be applied evenly and uniformly to the surface of the metal plating film. This is because a chromate film with excellent paint film adhesion can be constructed with high adhesion.

Furthermore, since the first chromate film requires a water washing process after the reactive chromate treatment, the film components are mostly Cr3+.
It is estimated that This Cr3+ has an anti-corrosion function through passivation, and due to the synergistic effect with the self-rust preventive function of Cr6+, which is a component of the second chromate film,
This is because corrosion resistance can be improved.

In this invention, the first chromate film as a lower layer formed on the metal plating film described above is made of chromic acid or dichromic acid, cynic acid or nitric acid, sulfuric acid or hydrogen fluoride as a reaction accelerator. C consisting of fluoride such as acid
It is a film formed by a reactive chromate treatment with r3''f as the main component, and the amount of Cr attached should be within the range of 5 to 50 degrees.

That is, if the amount of Cr attached is less than 5 myA'', the desired corrosion resistance cannot be obtained, whereas if the amount of Cr attached exceeds No. 509,
The problem arises that the paint film adhesion (Erichsen test value after painting) decreases and the quality of the product deteriorates.

The chromenide treatment solution for forming the first chromate film as the lower layer contains chromic acid or dichromic acid, cynic acid or nitric acid, and a fluoride such as sulfuric acid or hydrofluoric acid as a reaction accelerator. It is necessary to use a reactive chromate treatment solution containing as the main components.

Next, an example of such a reactive chromate treatment liquid will be shown.

CrO3: 5-30 t/L, H3PO4: 0.6? / or more, preferably 10μ
.

Any one of H2SO, HF, F-20, 1-2
02A Note that cobalt, nickel, titanium, tungsten, and vanadium are added to the above treatment solution in order to improve the density and uniform adhesion of the resulting film.

A salt of a metal selected from the group consisting of molybdenum and beryllium or a transparent conductor thereof may be added.

The chromate treatment using such a reactive chromate treatment liquid may be performed by a known method such as a spray method or a dipping method. Note that after the chromate treatment, it is necessary to perform sufficient washing with water.

A second chromate film is formed as an upper layer on the first chromate film.
The chromate film is a film formed by a coating type chromate treatment whose main components are Cr3+, Cr6+, and silica sol, and the amount of Cr deposited is between 20 and 15
Ci q/m".

That is, is the amount of Cr deposited 20 rnV'tr? If the amount is less than 150, the desired corrosion resistance cannot be obtained. On the other hand, if the amount of Cr attached exceeds 150, there is no problem with corrosion resistance in the case of a flat plate (non-formed plate), but the chromate film becomes thick and the manufacturing cost increases. Not only does it become expensive, but the film also tends to peel off during molding, resulting in problems of deterioration of the corrosion resistance and coating adhesion of the molded plate.

The chromate treatment solution used to form the second chromate film as the upper layer is composed of chromic acid (Cr6+), chromic acid reduction product (Cr""), and silica sol (Si).
It is necessary to use a coating type chromate treatment liquid containing 02) as the main component.

The ratio of Cr to Cr in the second chromate film is preferably within the range of 1:0.6 to 0.8 in terms of Cr. If the ratio of Cr3+ to Cr'+ is less than 0.6, there will be too much Cr6+ in the chromate film, and the film will become hygroscopic and easily peel off, and when alkaline degreasing etc. are performed, Cr6+ will be eluted. , the problem arises that the corrosion resistance after degreasing deteriorates. On the other hand, Cr6+
When the ratio of Cr''+ to exceeds 1:'o,sl,
When applying a chromate treatment liquid, the degree of polymerization of reduced chromate in the treatment liquid increases and precipitates form in the treatment liquid, resulting in a problem that makes it difficult to apply the chromate treatment liquid thinly and uniformly. .

7Ricasol (SiO□) in the second chromate film has the effect of improving coating film adhesion. The reason for this is not clear in detail, but it is presumed that the silica sol lowers the surface free energy, which improves the elasticity.

The ratio of SiO20 to the total amount of Cr3+ and Cr6+ (in terms of Cr03) in the second chromate film is 1:
It is preferably within the range of 0.1 to 2. Cr
and Cr H.

If the ratio of SiO2 to the total amount of
If the ratio exceeds 1:2, the electrical resistance of the chromate film increases, resulting in a problem of poor weldability.

Chromate treatment using such a coating type chromate treatment liquid is performed by a roll coater method, a spray method, or a method in which the excess treatment liquid is squeezed out using a squeezing roll after dipping.

Any known means such as the law may be used.

[Embodiments of the invention]

Next, the method of the present invention will be further explained using Examples and comparing with Comparative Examples.

20 tAr on the surface of the steel plate? A first chromate film as a lower layer formed by a reactive chromate treatment and an upper layer formed by a coating type chromate treatment within the scope of this invention shown in the attached table. A second chromate film was formed, and its corrosion resistance (white rust occurrence in a salt spray test), film adhesion, and weldability were measured. The measurement results are shown in the attached table together with comparative examples outside the scope of the present invention. .

Corrosion resistance was determined by salt spray test according to JIS z2137.
The test was carried out for 6 hours and evaluated based on the presence or absence of white rust. Paint film adhesion was determined by applying Amiranku 2000 (manufactured by Kansai Paint) to a thickness of 30 μm on the surface of the chromate film, baking it, adhering it with Cicero tape, and then forcing it with '. It was evaluated by whether the coating film peeled off or not.

Weldability was evaluated by performing a spot welding test under the following conditions and determining whether the welding result was good or bad.

a, Electrode tip Nichrome copper RWMA, C1ass
2 equivalent, shape C b, electrode tip diameter: 5.4 mm C0 electric rod pressure crab 200 Kq d1 energizing time: 12 cycles e, current: 8 KA As is clear from the attached table, one layer of reactive chromate treatment In Comparative Example 1 consisting of a chromate film,
White rust occurs, resulting in poor corrosion resistance and poor paint film adhesion.

Even if there are two chromate films, the amount of Cr deposited in the upper second chromate film is less than the range of the present invention.
Ar? In Comparative Example 2, white rust occurred and the corrosion resistance was poor. In Comparative Examples 3 and 4, which consisted of a single layer of chromate film formed by coating-type chromate treatment, the weldability was poor, and when the amount of Cr deposited and the amount of SiO□ was small, white rust occurred and the corrosion resistance deteriorated. Bad<, When the amount of Cr attached and the amount of SiO□ are large, the coating film adhesion is not good.

In addition, Comparative Example 5 is composed of one layer of chromate film formed by reactive chromate treatment and has a Cr adhesion amount of 100 mμ-, and even if there are two chromate films, the Cr adhesion amount of the first chromate film in the lower layer is 100 rq/rr which is more than the scope of the present invention? In Comparative Examples 6 and 7, the coating film adhesion was not good.

On the other hand, Examples 1 to 9 of the present invention are excellent in corrosion resistance, coating film adhesion, and weldability.

〔Effect of the invention〕 .

As described above, according to the chromate-treated steel sheet of the present invention, the chromate film is formed by forming a first chromate film as a lower layer formed on a metal plating film by reactive chromate treatment, and a first chromate film formed on the metal plating film as a lower layer. By forming a two-layer coating with a second chromate film as an upper layer formed on top of the coated chromate treatment, its corrosion resistance and coating adhesion are greatly improved, and it also has good weldability and is free from blackening. Many excellent effects can be brought about, such as the fact that it never occurs.

Claims (2)

[Claims] (1) In a chromate-treated steel sheet having a metal plating film on the surface of the steel plate and a chromate film adhering to the metal plating film, the chromate film is formed on the metal plating film. , the amount of Cr deposited mainly consisting of Cr^3^+ is 5
A first chromate film as a lower layer and Cr^3^+ and Cr^6^+ formed on the first chromate film by reactive chromate treatment within a range of from 50 mg/m^2 to 50 mg/m^2. and a second chromate film as an upper layer formed by coating-type chromate treatment in which the amount of Cr deposited is within the range of 20 to 150 mg/m^2 and whose main components are and SiO_2. Treated steel plate. (2) Cr in the second chromate film as the upper layer
The ratio of Cr^3^+ to ^6^+ is 1 in terms of Cr.
: within the range of 0.6 to 0.8, and Cr^3^
According to claim (1), the ratio of SiO_2 to the total amount of + and Cr^6^+ (in terms of CrO_3) is within the range of 1:0.1 to 2. chromate-treated steel sheet.