JPH0225593A - Production of taint-free steel sheet having uniform chromium oxidative film - Google Patents

Abstract

PURPOSE:To produce a taint-free steel sheet having a uniform chromium oxidative film by performing formation of the chromium oxidative film layer while separately performing both low current treatment in an initial stage and main electrolytic treatment in the case of forming a chromium plating layer and the chromium oxidative film layer on the surface of the steel sheet. CONSTITUTION:A metallic chromium plating layer having about 50-200mg/m<2> is formed by performing cathodic treatment on a thin steel sheet in an aq. soln. incorporating chromium and performing electroplating wherein metallic chromium is a main body. Then in the case of forming a chromium oxidative film layer by performing electrolytic chromic acid treatment in the aq. soln. incorporating chromium, firstly the aq. soln. wherein proper quantities of F<-> and SiF, etc., are added to a low-concn. chromic acid bath contg. about 10-100g/l chromium oxide is used and low current treatment in an initial stage is performed at about 30-50 deg.C bath temp. at about 0.1-2.0A/dm<2> low current density and thereby the chromium oxidative film having about 5-30mg/m<2> is formed. Then the chromium oxidative film having the prescribed film thickness is formed by performing main electrolytic treatment by a known method. Thereby thickness unevenness of the chromium oxidative film is extremely reduced and the taint-free steel sheet which has the uniform chromium oxidative film and is excellent in the secondary adhesive properties is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for forming a chromium oxide film on the surface of a steel sheet, and more particularly to a method for manufacturing a tin-free steel sheet having a uniform chromium oxide film.

<Prior art> Electrolytic chromate-treated steel sheets are called tin-free steel chromium type (hereinafter referred to as TFS), and have been recognized for their properties as a material for cans that can replace tinplate, and their usage has been increasing in recent years. .

Recently, TFS has been used not only for cold packs such as carbonated drinks and beer, where the contents are filled at low temperatures, but also for fruit juice and coffee.
It has come to be used for hot bags, in which the contents are sterilized at high temperatures before being filled, and for retort bags, in which the contents are sterilized at high temperatures after the bag is filled.

Since TFS has metallic chromium and a chromium oxide film on its surface, it does not have sufficient welding performance, and when making cans, the can body is coated with epoxyphenol resin and then joined with polyamide adhesive. Therefore, especially in TFS cans for hot bags and retort bags, there may be a problem that the can body is torn.

One of the causes of this can body tearing is thought to be that due to the uneven chromium oxide film on the TFS surface, the adhesion at the interface between the paint film and TFS partially deteriorates and peels off. .

Various methods have been considered for making this chromium oxide film uniform.

In the invention disclosed in JP-A No. 55-28313 regarding current balance for solving the macroscopic non-uniformity of the chromium oxide film due to fluctuations in line speed, the microscopic non-uniformity of the chromium oxide film is was insufficient.

<Problems to be Solved by the Invention> TFS has a metallic chromium layer on a steel substrate and a chromium oxide film layer on top of the metal chromium layer. As described above, the thickness of this chromium oxide film is non-uniform and tends to form so-called thickness unevenness. This thickness unevenness is considered to be one of the causes of peeling of the TFS adhesive interface during retort processing.

Therefore, the present invention aims to improve the microuniformity of the chromium oxide film on the TFS surface, and provides a method for reducing thickness unevenness.

In order to reduce the unevenness in the thickness of the chromium oxide film and obtain a uniform chromium oxide film, we used the conventional TFS manufacturing method to produce an electrolytic chromium oxide film. It has been found that it is effective to carry out acid treatment separately into low current treatment and main electrolysis, leading to the present invention.

That is, when manufacturing a tin-free steel sheet having a chromium plating layer and a chromium oxide film layer on the surface of the steel sheet, the present invention provides cathodic treatment for forming the chromium oxide film layer by an initial low current treatment and a main electrolytic treatment. A method for producing a tin-free steel sheet having a uniform chromium oxide film is provided.

At this time, the current density of the initial low current treatment is 0.1 to 2.
It is preferable to carry out in the range of 0 A/d m''.

Hereinafter, the present invention will be explained in detail.

In the present invention, a thin steel sheet is subjected to cathodic treatment in an aqueous solution containing chromium, electroplated mainly with metallic chromium, and then subjected to electrolytic chromic acid treatment in an aqueous solution containing chromium. A uniform chromium oxide film can be created by performing current treatment and then main electrolytic treatment.

In the present invention, the steel plate is not particularly limited, and ordinary butted steel plates, tin-free steel plates, etc. are used.

In the present invention, the method for forming the metallic chromium layer on the steel sheet is arbitrary, and a known electroplating method may be used, such as a method of cathodic electrolysis in an aqueous solution containing chromic anhydride as the main component and to which a quantity of 504'-F- is added. Just use it.

In addition, the metal chromium layer is applied to the thin steel plate at a concentration of 50 to 200 mg/m.
Just do the second wipe. The metal chromium layer is 50 mg
/ m ”, corrosion resistance is poor; less than 200 m g
/m', no improvement in corrosion resistance can be expected. So, the normal TFS is 50-200 mg/m2
It has chromium metal.

In the electrolytic chromic acid treatment, a low current treatment is first performed, and then a main electrolytic treatment is performed.

The electrolytic chromic acid treatment method uses a known bath composition, for example, a low-concentration chromic acid bath with chromium oxide of about 10 to 100 g/fL, an aqueous solution containing an appropriate amount of F-SiFg'-, etc., and a bath temperature of 30 g/fL. Electrolytic treatment may be performed at ~50°C.

The chromium oxide film on the chromium layer has a thickness of 5 to 30 mg.
/ m ”. The chromium oxide film should be 5
If it is less than 30 mg/m2, the desired paint adhesion cannot be obtained, and if it exceeds 30 mg/m2, the appearance will be poor.
I don't like it very much.

When securing this chromium oxide film, a current is applied according to the line speed. In the present invention, a low current density of 0.1 to 20 A/dm'' is suitable for initial energization.

This low current is essential for uniformity of the oxide film, but
0. IA/dm', less than 20 A/dm
``At super speeds, the effect is poor.

Next, a main electrolytic treatment is performed. The main electrolytic treatment may be performed by a known method, and the main electrolytic treatment may be performed according to the difference between the film thickness formed by the low current treatment of the chromium oxide film and the desired amount of the chromium oxide film. Ordinary electrolysis can be performed.

In order to maintain further retort treatment resistance, it is desirable to perform reverse electrolytic treatment (anodic treatment) in the solution after the electroplating mainly consisting of metallic chromium.

<Example> Hereinafter, the present invention will be specifically explained based on Examples.

(Examples 1 to 4 and Comparative Examples!, 2) Cold rolled steel plate (T4-C
A) was pretreated with 5% NaOH solution at 70°C.
8 A/d m2 cathodic treatment, 3% after water washing
After immersion in H2SO4 and washing with water, the steel plate was subjected to the following treatment at a line speed equivalent to 250 mpm.

(1) Chromium plating bath composition CrOs 180 g/
J! ) i, 504 0. 75g/lNa2S
iF6 8g/11bath
Temperature: 50°C Electrolytic treatment current density: 80 A/dm' (2) Using the reverse electrolytic treated steel plate as the anode, current density: 3.5 A/dm'' (3) Electrolytic chromic acid treatment bath composition: CrOs 60 g/11
NazSiFa 0, 15 g/1 bath
Temperature: 40°C Electrolytic treatment Initial current density: 0. IA/dm'~20A/dm' Main Ti, flow density approximately 25 A/dm2
Here, in the electrolytic chromic acid treatment, one bath was immersion, the second bath was low current treatment, and the main electrolytic treatment was performed in three baths and four baths.

In the TFS that underwent the steps (1) to (3) above, the unevenness of the chromium oxide film thickness was observed using an optical microscope with various initial current densities, and the scores are shown in Table 1.

In addition, the evaluation of secondary adhesion by the actual can test for each is also appended.

's1 As is clear from Table 1, initial low current treatment example 1
As for samples 4 to 4, almost no unevenness in the thickness of the chromium oxide film was observed, and a TFS having a uniform chromium oxide film was obtained. Further, secondary adhesion was also improved.

<Effects of the Invention> According to the present invention, a TFS with a uniform chromium oxide film and excellent secondary adhesion, with significantly reduced thickness unevenness of the chromium oxide film.
can be manufactured.

same

Claims (1)

[Scope of Claims] (1) When manufacturing a tin-free steel sheet having a chromium plating layer and a chromium oxide film layer on the surface of the steel sheet, cathodic treatment for forming the chromium oxide film layer is performed by initial low current treatment and A method for producing a tin-free steel sheet having a uniform chromium oxide film, characterized in that main electrolysis is carried out separately. (2) The method for manufacturing a tin-free steel sheet having a uniform chromium oxide film according to claim 1, wherein the initial low current treatment is performed at a current density in the range of 0.1 to 20 A/dm^2.