JPH06306686A - Galvanized sheet excellent in blacking resistance and production thereof - Google Patents

Abstract

PURPOSE:To provide a galvanized sheet excellent in blacking resistance and a producing method for the galvanized sheet capable of producing only by chromate treating without necessitating excess additives corresponding to the optional change of line speed. CONSTITUTION:The galvanized sheet excellent in blacking resistance has a chromate coating film composed of a metal chromium layer as the under layer and a hydrated oxide layer of Cr<3+> and Cr<6+> as the upper layer on the surface of the galvanized sheet and the total Cr coating weight in the chromate coating film is 10-200mg/m<2> expressed in terms of metal Cr and the element ratio of (metal Cr)/(total Cr) in the coating film is 0.4-0.9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having excellent resistance to blackening and a method for producing the same, and more specifically, in the fields of household electric appliances, construction, construction, etc. Alternatively, it takes a long time from production to use, and therefore a hot-dip galvanized steel sheet and a method for producing such a hot-dip galvanized steel sheet are required that the appearance of the surface does not change for a relatively long time. Regarding

[0002]

2. Description of the Related Art Conventionally, hot-dip galvanized steel sheets have been used in household electric appliances, construction, and construction fields because of the demand for high corrosion resistance of steel sheets, but in recent years, the demand thereof is increasing more and more. In addition, in recent years, not only corrosion resistance,
Further performance improvements such as uniformity of appearance and flatness have been demanded. Therefore, in order to meet such demands, recently, after hot dip galvanizing, spraying water droplets in the form of mist before zinc solidifies, zero spangle treatment for making zinc crystals fine, and skin pass rolling after cooling are performed. Has been done.

On the other hand, conventionally, chromate treatment has been carried out for the purpose of preventing white rust from occurring on a galvanized steel sheet and improving the adhesion of a coating film. However, in this hot-dip Zn-plated steel sheet that has been subjected to this chromate treatment, in particular, in the hot-dip Zn-plated steel sheet that has been subjected to chrome treatment after skin pass rolling, the plated surface is grayish black during storage after manufacturing and after construction. A phenomenon of discoloration (hereinafter referred to as blackening) is often recognized.
Further, this black discoloration is likely to occur in a state of being packaged with a coil or being stacked with a cutting plate, and since it partially occurs in spots, the appearance is inferior.

Regarding the cause and mechanism of such black discoloration,
Although there is no established theory, the chromate film suppresses the permeation of oxygen, and non-stoichiometric zinc oxide is generated, or normal zinc oxide or basic zinc carbonate is needle-shaped to absorb visible light. It is said to be based on things. Various methods for preventing this black discoloration have been conventionally proposed. For example,
After plating and skin pass rolling, cobalt, nickel,
A method in which iron or the like is deposited on the plating surface by dipping or spraying, and then a chromate film is formed (JP-A-59-177381), and a treatment liquid containing cobalt ions after plating and before the metal solidifies. Is sprayed on the surface to form a film of the oxide (Japanese Patent Laid-Open No. Sho 62-62).
No. 50474), a method in which ions of nickel, cobalt, molybdenum, etc. are contained in a chromate bath, and the above metal ions are simultaneously deposited on the plating surface at the same time as the chromate film by cathode electrolysis (Japanese Patent Publication No. 3-36918 and Japanese Patent Publication No. 3-36918). Japanese Patent Publication No. 3-61759, etc.) has been proposed.

As described above, the treatment method of depositing cobalt, nickel, iron or the like on the plating surface before the chromate treatment is effective by coating the starting point of corrosion product generation on the plating surface with such a metal. It is said to occur. However, since the hot dip coating line has an oxidation and reduction furnace before plating, the line speed changes depending on the plate thickness. Therefore, since the time for the treatment varies regardless of the treatment itself, it is difficult to strictly control the amount of the metal deposited, and in some cases, deterioration of corrosion resistance or poor appearance may occur. is there. Furthermore, the above treatment method necessarily requires two steps, a pretreatment step and a chromate treatment step, which leads to an increase in cost. In addition, if the washing with water after the above treatment is insufficient, impurities may be brought into the subsequent process, which may cause various problems.

Next, a treatment method of spraying a treatment liquid containing cobalt ions on the surface after plating and before metal is solidified is as follows.
It is said that a barrier effect is produced by forming an oxide film of cobalt. However, when performing a skin pass rolling, since it is after the above processing,
The oxide film may be destroyed and the effect may be reduced. Further, since it is difficult to uniformly form an oxide film of cobalt, it is feared that harmful gases such as nitrogen oxide and chlorine are generated.

Finally, in the treatment method in which a metal other than chromium is simultaneously deposited by cathodic electrolysis at the same time as the chromate coating, the electrolytic current density is changed in accordance with the change in the line speed to deposit the chromate coating. Since it is necessary to control the amount, it is difficult to keep the amount of the metal other than chromium attached and the ratio with the amount of the chromate film attached constant. Furthermore, when the number of kinds of additives to the chromate bath is increased, the control of bath composition becomes complicated.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems in the conventionally known methods for preventing black discoloration, in which an extra additive is added only by chromate treatment. A hot-dip galvanized steel sheet excellent in blackening resistance that can be produced without any need and capable of responding to an arbitrary change in line speed, and a method for producing a hot-dip galvanized steel sheet having excellent blackening resistance. The purpose is to provide.

[0009]

A hot dip galvanized steel sheet excellent in blackening resistance according to the present invention comprises a metallic Cr layer as a lower layer and Cr ³⁺ and C as an upper layer on the surface of the galvanized steel sheet.
It has a chromate coating consisting of a hydrated oxide layer of r ^{6+, and} the total Cr deposition in terms of metallic Cr in the chromate coating is in the range of 10 to 200 mg / m ² and [metal Cr in the coating is present. ] / [Total Cr] elemental ratio is in the range of 0.4 to 0.9.

According to the present invention, such a hot-dip galvanized steel sheet excellent in blackening resistance is (a) at least one selected from chromic anhydride, chromate and dichromate in terms of CrO ₃ . 5 to 150 g / 1, (b) SO ₄ ²⁻ ,
At least one anion selected from F ⁻ , C 1 ⁻ , NO ₃ ^−, and SiF ₆ ²⁻ in anion weight / CrO ₃ weight ratio of 0.0005 to 0.1, and (c) Cr ³⁺ , Zn ²⁺ , A1
It can be obtained by subjecting a zinc-plated steel sheet to cathodic electrolytic treatment in a bath containing inevitable impurities generated by treatment such as ³⁺ .

Further, according to the present invention, if necessary, after subjecting the zinc-based plated steel sheet to cathodic electrolytic treatment as described above, a treatment liquid mainly containing Cr ⁶⁺ and Cr ³⁺ is applied. , And can be baked to form a film having further excellent corrosion resistance. Since the present inventors develop a hot-dip galvanized steel sheet excellent in blackening resistance, it is possible to easily control the adhered amount by following changes in the line speed, and complicated bath composition and impurity management. As a treatment capable of imparting corrosion resistance and coating adhesion to the hot-dip galvanized steel sheet by a single step, it is of course possible to improve the blackening resistance without increasing the chromate bath. In this, electrolytic chromate treatment for cathodic electrolysis of hot dip galvanized steel sheet was investigated.

As a result, the inventors of the present invention have earnestly studied bath conditions, electrolysis conditions, and coating composition for the above chromate treatment, and as a result, by setting appropriate treatment conditions, the surface of the hot-dip galvanized steel sheet was set. In addition, total C converted to metallic Cr
The chromate film is formed in the range of 10 to ²⁰⁰ mg / m ^{2 in} the amount of r deposition, and [metal C
By forming metallic Cr at the coating and plating interface so that the element ratio of [r] / [total Cr] is in the range of 0.4 to 0.9, blackening resistance is improved, and corrosion resistance and coating are further improved. The present invention has been completed by finding that it is possible to obtain a hot-dip galvanized steel sheet having excellent film adhesion.

As described above, the reason why the blackening resistance of the hot-dip galvanized steel sheet is improved by depositing the metal Cr on the surface of the hot-dip galvanized steel sheet to form the metal Cr layer as the lower layer is still unsolved. Although not always clear, it is considered as follows. That is, non-stoichiometric,
Alternatively, if acicular corrosion products are formed, microscopic anodes and cathodes are formed on the plating surface, but metal C
This is because r preferentially coats this part that should be the anode or cathode, and prevents the reaction in which corrosion products occur.

In the present invention, the total amount of Cr deposited in the chromate coating on the surface of the galvanized steel sheet is 10 to 200.
It is in the range of mg / m ² . When the total Cr adhesion of the chromate coating on the surface of this hot dip galvanized steel sheet is less than 10 mg / m ² , the original corrosion resistance and coating adhesion of the chromate coating become insufficient, while When the total amount of Cr deposited in the chromate film on the surface exceeds 200 mg / m ² , the appearance defect at the time of performing the chromate treatment is likely to occur.

Further, in the present invention, the element ratio of [metal Cr] / [total Cr] in the chromate film is 0.4.
The range is from to 0.9. When the elemental ratio of [metal Cr] / [total Cr] is smaller than 0.4, the blackening resistance of the treated steel sheet obtained is insufficiently improved. On the other hand, when it exceeds 0.9, it is chromated. This is because the original corrosion resistance of the coating becomes insufficient.

In the present invention, if necessary, C
SO ₄ ²⁻ , S in the hydrated oxide consisting of r ⁶⁺ and Cr ³⁺
iF ₆ ²⁻ , F ⁻ , C1 ⁻ , NO ₃ ⁻ , PO ₄ ³⁻ , CH ₃
One or more anions such as COO ⁻ , cations such as CO ²⁺ , Ni ²⁺ and Ti ⁴⁺ , and colloidal compounds such as SiO ₂ colloid may be appropriately contained. Next, in the method of the present invention, chromic anhydride in the chromate treatment bath,
The concentration of chromate and dichromate in terms of CrO ₃ is in the range of 5 to 150 g / 1. When the above concentration in the chromate treatment bath is lower than 5 g / 1, the required amount of metallic chromium does not precipitate. On the other hand, when the above concentration in the chromate treatment bath exceeds 150 g / 1, Cr ⁶⁺ and Cr ³⁺
The hydrated oxide of is easily dissolved in the bath, and the original corrosion resistance of the chromate coating is likely to be insufficient.

Further, in the method of the present invention, the chroma
SO in treatment bath_Four ^2-, F^-, NO ₃ ^-, SiF₆ ^2-,
C1^-The concentration of anions such as₃Weight ratio (ani
On weight / CrO₃) In the range of 0.0005 to 0.1
It This ratio in the chromate bath is better than 0.0005
When it is small, the required amount of metallic chromium does not precipitate. other
On the other hand, when 0.1 is exceeded, when chromate treatment is performed
Poor appearance at points is likely to occur.

Further, in the treatment bath, CO ²⁺ , Ni are appropriately ^added .
^If desired, one or more cations such as ²⁺ and Ti ⁴⁺ and colloidal compounds such as SiO ₂ colloid may be contained. In the method of the present invention, the treatment temperature is not particularly limited, but in consideration of easiness of bath management and economical efficiency, the range of normal temperature to 70 ° C. is usually suitable for practical use. The current density is not particularly limited either, but when it is less than 20 A / dm ² , it takes a long time to deposit the metal chromium, so that it is usually preferably 20 A / dm ² or more. Further, the electrolysis time is not subject to any restriction, but normally, the range of 10 seconds or less is industrially suitable.

According to the present invention, (a) as described above,
By subjecting the zinc-based plated steel sheet to cathodic electrolysis in a bath containing components (b) and (c), the lower layer is a metallic Cr layer and the upper layer is Cr ^{3+ on} the surface of the zinc-based plated steel sheet. Cr ⁶⁺
Of the hydrated oxide layer, the total amount of Cr deposited in the chromate film is 1 in terms of metallic Cr.
It is possible to form a coating having an elemental ratio of [metal Cr] / [total Cr] of 0.4 to 0.9 in the range of 0 to 200 mg / m ² , but If higher corrosion resistance is required, after forming a coating film by the cathodic electrolytic treatment, a treatment liquid mainly containing Cr ⁶⁺ and Cr ³⁺ within the above composition range is applied and baked. Good. Furthermore, in the treatment liquid, if necessary,
SO ₄ ²⁻ , SiF ₆ ²⁻ , F ⁻ , C1 ⁻ , NO ₃ ⁻ , PO
₄ ^3- , anions such as CH ₃ COO ⁻ , CO ²⁺ , Ni ²⁺ ,
One or more kinds of cations such as Ti ⁴⁺ and colloidal compounds such as SiO ₂ colloid may be contained.

[0020]

As described above, according to the method of the present invention,
On the surface of the zinc-based plating, a lower layer is a metallic Cr layer and an upper layer is a chromate coating consisting of a hydrated oxide layer of Cr ³⁺ and Cr ^6+. In the range of 10 to 200 mg / m ² and [metal Cr] / [total Cr] in the chromate film
By controlling the element ratio in the range of 0.4 to 0.9, it has excellent blackening resistance without bath management and process complexity, and also has corrosion resistance and coating adhesion. It is possible to obtain a hot-dip galvanized steel sheet that also has properties.

[0021]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples. As the plated steel sheet to be treated, there were used a hot-dip galvanized steel sheet and a hot-dip zinc-aluminum alloy-plated steel sheet having a predetermined amount of plating and subjected to skin pass rolling of fine spangles. After subjecting these zinc-plated steel sheets to chromate treatment according to the present invention, they are dried at 60 ° C. for 10 minutes,
It was subjected to the blackening resistance, corrosion resistance and coating film adhesion tests described below.

Tables 1 and 2 collectively show various blackening resistances, corrosion resistances, and coating adhesions of the obtained chromate-treated steel sheets, together with the treatment steps, treatment conditions, and Cr deposits in Examples and Comparative Examples. Here, the coating process of the chromate solution,
The quantitative determination of the amount of metallic Cr and the total amount of Cr deposited, the corrosion resistance and the coating film adhesion test were carried out by the following methods.

Coating Treatment Step: The plated steel sheet to be treated or the plated steel sheet to be treated after cathodic electrolysis is immersed in a commercially available coating type chromate treatment liquid containing Cr ⁶⁺ as a main component, and after pulling up, excess treatment is carried out by a Ringer roll. The liquid was squeezed. The amount of adhesion was adjusted by the tightening pressure of the roll. Coating treatment process ^{Dip the} plated steel sheet to be treated or the plated steel sheet to be treated after cathodic electrolytic treatment in a commercially available coating type chromate treatment solution containing Cr ⁶⁺ as the main component, pull it up, and squeeze the excess treatment solution with a ringer roll. It was The amount of adhesion was adjusted by the tightening pressure of the roll.

Metal Cr, Total Cr Adhesion Amount Metal Cr: The treated hot-dip plated steel sheet was immersed in a strong alkali to dissolve chromium hydrate oxide by heating. Then, the Cr remaining on the surface was dissolved with an acid and quantified by atomic absorption spectrometry. Total Cr adhesion amount: Cr extracted in the above-mentioned alkali was quantified by atomic absorption spectrometry, and the above-mentioned metallic Cr amount was added to this to obtain the total Cr adhesion amount.

The blackening resistance-treated hot-dip plated steel sheet was stored for 168 hours under conditions of a temperature of 50 ° C. and a relative humidity of 98%, and the degree of black discoloration on the plated surface was visually observed. ⊚ indicates that no black change was observed at all, ∘ indicates that only a slight change was observed, Δ indicates that some black change was observed, and x indicates that most were observed.

Corrosion resistance The treated hot-dip galvanized steel sheet was subjected to a salt spray test according to the method specified in JIS Z-2371, the time until the white rust occurrence area ratio became 1% was measured, and evaluated by this time. . That is, 1% white rust generation time is ⊚ for 120 hours or more, ∘ for 80 to 120 hours, Δ for 50 to 80 hours, and x for 50 hours or less.

Coating adhesion The alkyd melamine-based paint was applied to the treated hot-dip plated steel sheet with a bar coater and baked to form a coating having a thickness of 20 μm, and a coating adhesion test was conducted. Adhesion of the coating film, 1mm × 1mm squares on the surface of the coating film with a cutter knife, and 6mm extruded with an Erichsen tester, stick an adhesive tape, then peel off the tape, The residual rate of the coating film at that time was evaluated. ⊚ indicates that there is no peeling of the coating film, ◯ indicates that there is almost no peeling, Δ indicates that some peeling is observed, and × indicates that most of the peeling occurs.

[0028]

[Table 1]

[0029]

[Table 2]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomio Kajita 2222, Ikeda, Ikeda, Ikeda, Onoe-machi, Kakogawa-shi, Hyogo Prefecture Kakogawa Research Area, Kobe Steel Co., Ltd.

Claims (3)

[Claims] 1. A lower layer of metal C on the surface of a zinc-based plated steel sheet.
It consists of r layer and the upper layer is Cr³⁺And Cr ⁶⁺The hydrated oxide layer
It has a chromate coating consisting of
The total Cr deposition amount calculated from metallic Cr is 10 to 200 mg / m²
In the range of [metal Cr] / [total
The element ratio of Cr] is in the range of 0.4 to 0.9.
Hot-dip galvanized steel sheet with excellent blackening resistance. 2. (a) 5 to 150 g / 1 of at least one selected from chromic anhydride, chromate and dichromate in terms of CrO ₃ , (b) SO ₄ ²⁻ , F ⁻ , C1 ^-, NO ₃ ^- and SiF ₆ ^2-
Containing at least one anion selected from the following in anion weight / CrO ₃ weight ratio of 0.0005 to 0.1, and (c) Cr ³⁺ , Zn ²⁺ , A 1 ^{3+ and the} like unavoidable impurities produced by the treatment. A method for producing a hot dip galvanized steel sheet having excellent blackening resistance, which comprises subjecting the galvanized steel sheet to a cathodic electrolysis treatment in a bath. 3. (a) 5 to 150 g / 1 in terms of CrO _{3 of} at least one selected from chromic anhydride, chromate and dichromate, (b) SO ₄ ²⁻ , F ⁻ , C1 ^-, NO ₃ ^- and SiF ₆ ^2-
Containing at least one anion selected from the following in anion weight / CrO ₃ weight ratio of 0.0005 to 0.1, and (c) Cr ³⁺ , Zn ²⁺ , A 1 ^{3+ and the} like unavoidable impurities produced by the treatment. Hot-dip galvanized plating excellent in blackening resistance, characterized in that a zinc-based plated steel sheet is subjected to cathodic electrolytic treatment in a bath, and then a treatment liquid mainly containing Cr ⁶⁺ and Cr ³⁺ is applied and baked. Steel plate manufacturing method.