JPH051397A - Production of two-layer plating steel sheet having iron-based electroplating layer on galvanized sheet - Google Patents

Abstract

PURPOSE:To provide the method for manufacturing a two layers galvanized sheet to improve electroprecipitation efficiency and to decrease deterioration of a plating solution caused by increase of Fe<3+> content when iron electroplating on an upper layer of a galvanized sheet such as an alloyed galvanized sheet. CONSTITUTION:In the case of iron electroplating on the galvanized sheet, an electroprecipitation efficiency is improved up to 15-25% under a condition where non-addition of conductivity assistant is applied and, 30-60 deg.C of the plating bath temp. >=100A/dm<2> of current density, <=1m/sec of the liquid flow rate are held. High electroprecipitation efficiency is attained with respect to the iron electroplating layer on the galvanized sheet. As a result, in the mass production, improvement of line speed is attained, consumption of electric power is decreased to reduce cost, and since temp. of bath is lowered and flow rate of the liquid is decreased, increase of Fe<3+> content is suppressed to improve operability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a double-layer plated steel sheet having an iron-based electroplated layer on a hot-dip galvanized steel sheet.

[0002]

2. Description of the Related Art In general, steel sheets for automobiles are required to be plated steel sheets having excellent rust resistance, corrosion resistance, workability, chemical conversion treatment property, and paintability. As a conventional surface-treated steel sheet, there is an alloyed hot-dip galvanized steel sheet obtained by heat-treating a galvanized steel sheet. Although this is excellent in rust resistance and corrosion resistance, it is inferior in adhesion to a chemical conversion treatment layer required for automobile steel sheets, paintability, and workability during press molding. Therefore, a method of forming a thin and uniform iron-zinc alloy plating layer having a high iron content by electroplating on a galvannealed steel sheet is known (see Japanese Patent Publication No. 58-15554). The plating conditions include JP-A-58-5249.
2 and JP-A-60-149794.

[0003]

In the prior art, when iron-based plating is applied to the upper layer of a zinc-based plated steel sheet by electroplating such as electrozinc-iron plating and electrozinc-nickel plating, the plated layer is efficiently formed. It is known to be obtained. However, it has been found that when galvanized galvanizing, particularly iron-based galvanizing on a galvannealed steel sheet, the electrodeposition efficiency is significantly lower than that on other steel sheets. Therefore, the iron-based plating is rate-determining on the continuous plating line and the productivity is reduced. Furthermore, since the electrodeposition efficiency is low, a large amount of input current is required, and the power cost of iron-based electroplating is high. Therefore, it is necessary to improve the electrodeposition efficiency in the iron-based plating on the galvannealed steel sheet.

Further, in iron-based plating, Fe²⁺Is due to air
Fe by oxidation and oxidation at the anode ³⁺Therefore, Fe
³⁺The concentration is too high and the plating solution is likely to deteriorate. Conventional
In technology, Fe³⁺Sludge generation due to large increase in
Filter clogging, adverse effect on plating appearance,
Problems such as poor adhesion have occurred. Therefore, F
e²⁺To Fe³⁺Deteriorates the rate of oxidation to
It is necessary to suppress

The present invention improves the electrodeposition efficiency when iron-based electroplating is performed on the upper layer of hot dip galvanized steel sheet such as alloyed hot-dip galvanized steel sheet and reduces the deterioration of plating solution due to the increase of Fe ³⁺ concentration. It is an object of the present invention to provide a method for manufacturing a double-layer plated steel sheet as intended.

[0006]

In a conventional continuous electroplating line for steel sheets, a plating bath to which a conductivity aid such as an alkali metal or an alkaline earth metal is added is used. The conductivity aid increases the electric conductivity of the plating bath, and has the effect of lowering the electric resistance during energization.
Therefore, the voltage is low and the power cost is low. In normal electric Zn plating and electric Zn-Ni plating, Na ₂
Although conductivity aids such as SO ₄ and (NH ₄ ) ₂ SO ₄ are added, there is almost no effect on the electrodeposition efficiency. However, in the case of hot-dip galvanized steel sheet, especially iron-based electroplating on alloyed hot-dip galvanized steel sheet, the addition of the conductivity aid increases the conductivity, but the deposition efficiency is significantly reduced. I found a new one.

In the conventional electroplating, it is known that the higher the bath temperature, the higher the electrodeposition efficiency. It is considered that this is because the higher the bath temperature, the faster the diffusion of the ions, so that the influence of the metal ion diffusion rate control becomes smaller. However, in the case of iron-based plating on a hot dip galvanized steel sheet, it was newly found that when the bath temperature is increased, the electrodeposition efficiency decreases, and conversely, when the bath temperature is low, the electrodeposition efficiency improves. The reason for this is not clear, but the electrodeposition of iron is not controlled by the diffusion rate of metal ions, but the hydroxide of iron is generated and adsorbed due to the increase in the pH of the cathode interface, and iron is electrodeposited from iron hydroxide. This is because it can be considered.

That is, according to the present invention, in order to achieve the above object, when performing the iron-based electroplating on the hot dip galvanized steel sheet, without adding the conductivity auxiliary agent in the iron-based electroplating bath. Plating bath temperature 30-60 ℃, current density 10
An insoluble anode is used under the conditions of 0 A / dm ² or more and a liquid flow rate of 1 m / sec or less to perform electroplating on the hot dip galvanized steel sheet, and an iron electroplated layer is formed on the hot dip galvanized steel sheet. A method for producing a double-layer plated steel sheet having the same is provided.

The iron-based electroplating bath is a sulfate bath, and has an Fe ²⁺ ion concentration of 70 g / l or more and a solubility limit or less, a plating metal ion weight ratio Zn ²⁺ / Fe ^{2+ of} 0.1 or less, The Fe ³⁺ concentration is preferably less than 7 g / l and the pH is 1.8 to 2.2.

The iron-based electroplating bath is a sulfate bath and has an Fe ²⁺ ion concentration of 70 g / l or more and a solubility limit or less, a total phosphorus concentration of 0.5 g / l or less, and an Fe ³⁺ concentration of 7 g / l.
Preferably, the pH is less than 1.8 to 2.2.

The iron-based electroplating bath is a sulfate bath and has a metal ion concentration of 70 g / l or more and a solubility limit or less, a plating metal ion weight ratio Zn ²⁺ / Fe ^{2+ of} 0.1 or less, and total phosphorus. Concentration 0.5g / l or less, Fe ³⁺ concentration 7g / l
Less, preferably pH 1.8 to 2.2.

The amount of iron-based electroplating deposited on the hot dip galvanized steel sheet is preferably 1 to 5 g / m ² .

The present invention will be described in more detail below.

In the present invention, the hot dip galvanized steel sheet is
Typical examples are hot-dip galvanizing, alloying hot-dip galvanizing, 5% aluminium-galvanizing, and 55% aluminium-galvanizing, and in particular, remarkable effects are obtained in hot-dip galvannealed steel sheets, and these are included. Fe-Zn, Fe-P, Fe-Zn-P plating etc. can be mentioned as a kind of the iron type electroplating layer provided on the hot-dip galvanized steel sheet in the present invention. In the case of Fe-Zn plating, the composition of these plating layers is such that the Fe content is 70 wt% or more and 100% or more.
It is preferably less than wt%, and in the case of Fe-P, the P content is preferably in the range of 0.01 to 1 wt%, and when it is less than or more than the above range, the phosphate chemical conversion treatment property and the cationic electrodeposition coating property are deteriorated. In the present invention, the amount of the iron-based electroplating applied on the hot-dip galvanized steel sheet is preferably 1 to 5 g / m ² , and when it is less than the above range, the phosphoric acid conversion treatment property and the cationic electrodeposition coating property deteriorate, If it exceeds the range, the blister resistance is further deteriorated. In the present invention, it is important not to intentionally add a conductivity aid such as an alkali metal salt, which is naturally added in the conventional electroplating, to the iron-based electroplating bath. In the present invention, the plating bath temperature is set to 30 to 60 ° C. It is not preferable because it is difficult to control the plating bath temperature at a temperature of less than 30 ° C. and the plating adhesion and appearance are adversely affected. Further, at 60 ° C. or higher, the electrodeposition efficiency becomes low, which is not suitable. Further, if the bath temperature is high, the air oxidation rate of Fe ²⁺ is also accelerated, and Fe ³⁺ which adversely affects the plating adhesion and operability is rapidly increased. Therefore, from the viewpoint of electrodeposition efficiency and Fe ²⁺ air oxidation rate, it is important to perform plating at 30 to 60 ° C. 40-55 degreeC is more preferable.

In electroplating, it is generally known that the lower the current density, the higher the electrodeposition efficiency. It is considered that this is because when the current density becomes high, the supply of metal ions becomes insufficient, the current density approaches the limit current, and hydrogen is easily generated. However, in the case of hot-dip galvanized steel sheet, especially iron-based plating on alloyed hot-dip galvanized steel sheet, lowering the current density rather lowers the electrodeposition efficiency, but rather increases the current density to significantly improve the electrodeposition efficiency. I found what to do. In the present invention, it has been found that the plating layer can be obtained with high electrodeposition efficiency, especially by increasing the current density to 100 A / dm ² or more. When the current density is less than 100 A / dm ² , unlike other steel sheets, the galvanized galvanized steel sheet, in particular, the iron-based electroplating efficiency on the galvannealed steel sheet has a low electrodeposition efficiency, and its fluctuation is large and unstable. Therefore, the current density needs to be 100 A / dm ² or more. More preferably 1
25A / dm ² or more is preferable. Usually, electroplating in a continuous plating line is performed under a high plating solution flow rate. This has the effect of reducing the influence of the diffusion limit and improving the electrodeposition efficiency by activating the supply of metal ions.
However, in iron-based plating on a hot-dip galvanized steel sheet, it was newly found that the higher the flow rate, the lower the electrodeposition efficiency, and the slower the flow rate, the higher the electrodeposition efficiency. This is considered to be because, as in the case of the bath temperature, the hydroxide of Fe was generated and adsorbed due to the increase of the pH at the cathode interface, so that Fe was electrodeposited. Electrodeposition efficiency is low when the liquid flow velocity is 1 m / sec or more. Further, the air oxidation rate increases in proportion to the liquid flow rate and the Fe ³⁺ concentration rapidly increases, so the liquid flow rate is set to 1 m / sec or less.

[0016] Electrodeposition of metal from an aqueous solution is always a competitive reaction with hydrogen generation. Therefore, if the absolute amount of metal ion is small, the electrodeposition efficiency is considered to be low. As a result of the experiment, it was found that the electrodeposition efficiency was improved by increasing the metal ion concentration in the plating solution. Fe ²⁺ ion 70g / l
If less than 60%, the electrodeposition efficiency is low, so the Fe ²⁺ ion concentration is 70
The solubility is preferably g / l or more and the solubility limit or less, and preferably 80 g / l or more and the solubility limit or less. However, in order to secure quality such as workability of galvanized steel sheet, chemical conversion treatment property and paintability, Zn
It is necessary to keep the content rate and P content rate. As mentioned above, F
Fe content in e-Zn plating is 70 wt% or more 10
It is preferably less than 0 wt% and the P content in Fe-P plating is preferably 0.01 to 1 wt%. Therefore, Fe-
In Zn plating, the metal ion weight ratio is Zn ²⁺ / Fe
²⁺ is 0.1 or less, and in Fe-P plating, the total P concentration is preferably 0.5 g / l or less. For the same reason, in Fe-Zn-P plating, the metal ion weight ratio is
Zn ²⁺ / Fe ²⁺ is 0.1 or less, total P concentration is 0.5 g / l
The following is preferable.

Since Fe ³⁺ is produced by air oxidation or anodic oxidation, some generation cannot be prevented. It has been conventionally known that when the Fe ²⁺ concentration becomes high, sludge is generated in the plating solution, which hinders the operation. However, this time, it was found that the deposition efficiency decreases due to the increase of Fe ³⁺ concentration in addition to the generation of sludge. On the other hand, if the Fe ³⁺ concentration exceeds 7 g / l, the plating adhesion and the plating appearance are adversely affected, and sludge is likely to be generated, which is not preferable in operation. Therefore, it is preferable to control to 7 g / l or less. In ordinary plating, since the electrodeposition efficiency is originally high, the influence of pH on the electrodeposition efficiency is small. However, it has been found that iron-based plating on an alloyed hot-dip galvanized steel sheet has a pH dependency that has not been seen until now. Therefore, it has been found that the electrodeposition efficiency is greatly improved by increasing the pH. When the pH is less than 1.8, the electrodeposition efficiency is about 70%. If the pH exceeds 2.2, sludge is likely to be generated, which is not preferable. Therefore, pH 1.8 to 2.2
It is preferable to keep

By performing iron-based electroplating on a hot-dip galvanized steel sheet based on the above conditions, the electrodeposition efficiency is improved by 15 to 25% as compared with the conventional method. Also,
By lowering the bath temperature and slowing the liquid flow rate, Fe
The amount of increase of ³⁺ decreases compared to the conventional one and the operation becomes stable. Also,
Fe-Zn, Fe-P,
The plating composition such as Fe-Zn-P is stable and the quality is secured.

[0019]

EXAMPLES The present invention will be specifically described below based on examples.

(Examples 1 to 13) Fe-Zn plating was applied to a 100 x 175 mm alloyed hot-dip galvanized steel sheet (plating thickness 60 g / m ² , alloying degree 9%). The plating bath composition was adjusted to that of Comparative Example 1 below. The metal ion concentration was adjusted using ferrous sulfate, ferric sulfate and zinc sulfate. Fe ²⁺ ion weight ratio is Zn ²⁺ / Fe ²⁺
It was set to 0.07. Comparative Examples 1, 3, 4, 7, 8, 12, 1
Sodium sulfate was used for 3, 16 and 17 as a conductivity aid. Table 1 shows the conditions and results of Examples 1 to 13 and Comparative Examples 1 to 18.

Plating bath composition (Comparative Example 1) FeSO ₄ .7H ₂ O 350 g / l ZnSO ₄ .7H ₂ O 23 g / l Fe ₂ (SO ₄ ) ₃ .xH ₂ O 15 g / l Na ₂ SO ₄ 15 g / l

(Examples 14 to 22) 100 × 175 mm
Fe-P plating was applied to the alloyed hot-dip galvanized steel sheet (plating thickness 60 g / m ² , alloying degree 9%). The plating bath composition was adjusted to that of Comparative Example 19 below. The metal ion concentration was adjusted using ferrous sulfate, ferric sulfate, and hypophosphorous acid. The total P concentration was 0.1 to 0.15 g / l. Comparative Examples 19, 21, 22, 25, 26, 29, 3
In 2 and 33, sodium sulfate was used as a conductivity aid. Table 2 shows each condition and results of Examples 14 to 22 and Comparative Examples 19 to 34.

Plating bath composition (Comparative Example 19) FeSO ₄ .7H ₂ O 350 g / l NaH ₂ PO ₂ .H ₂ O 0.4 g / l Fe ₂ (SO ₄ ) ₃ .xH ₂ O 15 g / l Na ₂ SO ₄ 15 g / l

(Examples 23 to 33) 100 × 175 mm
Alloyed hot-dip galvanized steel sheet (plating thickness 60g / m²,alloy
Fe-Zn-P plating was performed on the chemical conversion degree 9%). Plating
The bath composition was that of Comparative Example 35 below. Na
The metal ion concentrations are ferrous sulfate, ferric sulfate, and sulfuric acid.
Adjusted with zinc and hypophosphorous acid. Fe²⁺ion
The weight ratio is Zn²⁺/ Fe ²⁺0.07, and the total P concentration is 0.
It was set to 1 to 0.15 g / l. Comparative Examples 35, 37, 38,
41, 42, 45 and 48 are sodium sulfate as a conductivity aid.
Lithium was used. Examples 23-33 and Comparative Examples 35-
Table 3 shows each condition of 49 and the result.

Plating Bath Composition (Comparative Example 35) FeSO ₄ .7H ₂ O 350 g / l ZnSO ₄ .7H ₂ O 23 g / l NaH ₂ PO ₂ .H ₂ O 0.4 g / l Fe ₂ (SO ₄ ) ₃・xH ₂ O 15 g / l Na ₂ SO ₄ 15 g / l

From Tables 1 to 3, no conductivity auxiliary agent added, low bath temperature,
It can be seen that the electrodeposition efficiency is good at high current density and low flow rate. At that time, the Fe ²⁺ ion concentration is 70 g / l or more, the Fe ³⁺ concentration is 7 g / l or less, and the pH is 1.8.
Was set to 2.2.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

[Table 5]

[0032]

[Table 6]

[0033]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, it is possible to perform plating without addition of a conductivity aid, low bath temperature, high current density and low flow rate. The effect is that the iron-based electroplating layer can be obtained with high electrodeposition efficiency. Therefore, it is possible to improve the line speed in mass production, further reduce the power consumption, and reduce the cost. Further, since the bath temperature is lowered and the liquid flow velocity is slowed down, an increase in Fe ³⁺ concentration is reduced and the operability is improved.

Continued front page    (72) Inventor Akira Yasuda             1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.             Corporate Technology Research Division (72) Inventor Koji Yamato             1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.             Corporate Technology Research Division

Claims (5)

[Claims] 1. When performing iron-based electroplating on a hot dip galvanized steel sheet, a plating bath temperature of 30 to 60 ° C. and a current density of 1 are added to the iron-based electroplating bath without adding a conductivity aid.
An iron-based electroplating layer is formed on the hot-dip galvanized steel sheet, which is characterized by electroplating on the hot-dip galvanized steel sheet using an insoluble anode under the conditions of 00 A / dm ² or more and a liquid flow rate of 1 m / sec or less. A method for manufacturing a double-layer plated steel sheet having the same. 2. The iron-based electroplating bath is a sulfate bath, and has an Fe ²⁺ ion concentration of 70 g / l or more and a solubility limit or less,
Plating metal ion weight ratio Zn ²⁺ / Fe ²⁺ 0.1 or less, F
The method for producing a double-layer plated steel sheet having an iron-based electroplated layer on the hot-dip galvanized steel sheet according to claim 1, wherein the e ³⁺ concentration is less than 7 g / l and the pH is 1.8 to 2.2. 3. The iron-based electroplating bath is a sulfate bath, the Fe ²⁺ ion concentration is 70 g / l or more and the solubility limit or less,
The total phosphorus concentration is 0.5 g / l or less, the Fe ³⁺ concentration is less than 7 g / l, and the pH is 1.8 to 2.2. Two layers having an iron-based electroplated layer on the hot dip galvanized steel sheet according to claim 1. Manufacturing method of plated steel sheet. 4. The iron-based electroplating bath is a sulfate bath, and the Fe ²⁺ ion concentration is 70 g / l or more and the solubility limit or less,
Plating metal ion weight ratio Zn ²⁺ / Fe ²⁺ 0.1 or less, total phosphorus concentration 0.5 g / l or less, Fe ³⁺ concentration 7 g / l or less,
The method for producing a double-layer plated steel sheet having an iron-based electroplated layer on the hot-dip galvanized steel sheet according to claim 1, which has a pH of 1.8 to 2.2. 5. The hot-dip galvanized steel sheet according to claim 1, wherein the amount of iron-based electroplating deposited on the hot-dip galvanized steel sheet is 1 to 5 g / m ^2. A method for manufacturing a double-layer plated steel sheet having an electroplated layer.