JP5609344B2 - Method for producing electrogalvanized steel sheet - Google Patents

Description

  The present invention relates to a method for manufacturing an electrogalvanized steel sheet with reduced manufacturing costs.

Electrogalvanized steel sheets are widely used in automobiles, home appliances, building materials, and the like because they are excellent in film uniformity and appearance.
Electrogalvanized steel sheets are currently manufactured in an electrogalvanizing line (EGL) facility, and reducing the electrolysis voltage is effective to reduce manufacturing costs. Lowering the electrolysis voltage reduces the power cost (= electrolysis voltage x quantity of electricity), leading to overall cost reduction.
In order to reduce the electrolysis voltage, it is effective to increase the electrical conductivity and the kinematic viscosity of the plating bath. When the electrical conductivity increases, the voltage consumed by the resistance of the plating bath decreases, so the overall electrolytic voltage decreases. Further, when the kinematic viscosity is lowered, it becomes possible to efficiently remove bubbles generated at the anode, and the electrolysis voltage is lowered.

  Thus, in order to keep the electrolysis voltage low, it is effective to use a plating bath with high electrical conductivity and low kinematic viscosity, and the following methods have been proposed as conventional techniques for realizing this. ing.

In Patent Document 1, sodium sulfate + ammonium sulfate: 40 g / L or less, anhydrous sulfuric acid: 30 to 60 g / L, Zn: 75 to 130 g / L, Ni: 50 to 250 ppm, Fe: 200 to 3000 ppm, Sn: 0.01 to 5 ppm Preferably, using a plating bath having an electrical conductivity measured at 50 ° C. of 140 mS or more and a kinematic viscosity coefficient of 0.013 × 10 ⁻² cm ² / s or less, and a current density of 70 to 600 A / dm ² And the manufacturing method of the galvanized steel plate which performs the plating process of the adhesion amount 2-120g / m < ² > is disclosed.

Patent Document 2 contains 400 to 600 g / L in terms of ZnSO ₄ · 7H ₂ O, 200 to 300 A / dm ² in a plating bath having a bath temperature of 60 to 80 ° C., a pH of 0 to 1, and a flow rate of 2 m / s or more. It is described that the steel sheet is electrogalvanized with the current density to form an electrogalvanized layer in an amount of 10 to 100 g / m ² per side of the steel sheet on at least one surface of the steel sheet.

Patent Document 3 includes nonionic polyacrylamide having a zinc ion of 0.7 to 2.0 mol / L, a second metal ion of 0.1 to 2.0 mol / L of nickel ion or iron ion, and an average molecular weight of 10 ⁶ to 10 ^8. Electricity for electroplating steel plates with a current density of 100-450 A / dm ² and a relative flow rate of 30 m / min or more using an acidic electrogalvanizing bath containing 1-100 ppm, pH 0.5-2.0, bath temperature 40-70 ° C A method for producing a galvanized steel sheet is disclosed. Here, the amount of sulfuric acid required to adjust the pH to 0.5 to 2.0 is about 2 to 35 g / L.

Japanese Patent Laid-Open No. 11-200087 Japanese Patent Laid-Open No. 6-2193 JP 61-127891

However, in Patent Document 1, Zn: 75 to 130 g / L is 330 to 572 g / L in terms of ZnSO ₄ · 7H ₂ O, and since the amount of zinc is large, low kinematic viscosity cannot be achieved. . Moreover, since there is little sulfuric acid addition amount, high electrical conductivity cannot be achieved. Furthermore, since Ni, Fe, and Sn ions are contained in the bath, Ni, Fe, and Sn are co-deposited in the plating film, and the corrosion resistance is deteriorated.
In Patent Document 2, since the amount of zinc is large, a low kinematic viscosity cannot be achieved.
In Patent Document 3, sulfuric acid is estimated to be about 35 g / L or less, and since the amount of sulfuric acid added is small, high electrical conductivity cannot be achieved. Moreover, since the organic substance is contained in the bath, the organic substance is co-deposited in the plating film, and the hardness of the plating film is increased.

  As described above, in the prior art, it was impossible to produce an electrogalvanized steel sheet using an electrogalvanizing bath that has both high electrical conductivity and low kinematic viscosity.

  In view of such circumstances, the present invention aims to provide a method for producing an electrogalvanized steel sheet in which the power cost is reduced and the production cost is reduced by using a plating bath having both high electrical conductivity and low kinematic viscosity. And

The inventors of the present invention have made extensive studies to solve the above problems. As a result, by controlling the Zn concentration, sulfuric acid concentration, and temperature of the plating bath within the appropriate ranges, both high electrical conductivity and low kinematic viscosity can be achieved. It has been found that electrogalvanized steel sheets can be produced with reduced costs and manufacturing costs.
Conventionally, in order to increase the electrical conductivity of an electrogalvanizing bath, it is common to increase the ZnSO ₄ concentration or to add an electrical conductivity auxiliary agent such as NaSO ₄ . On the other hand, in order to lower the kinematic viscosity, it has been considered effective to reduce the H ₂ SO ₄ concentration and the ZnSO ₄ concentration.
Therefore, the present inventor has intensively studied for the development of a bath having both high electrical conductivity and low kinematic viscosity. As a result, first, in order to increase the electrical conductivity of the plating bath, it is possible to significantly increase the H ₂ SO ₄ concentration compared to the conventional case, compared to the conventional ZnSO ₄ concentration increase or the addition of electrical conductivity assistant. I found it effective. Furthermore, it has been found that, in the H ₂ SO ₄ high concentration bath (60 g / L or more), the electrical conductivity can be further improved and the kinematic viscosity can be reduced at the same time by lowering the ZnSO ₄ concentration. . This made it possible to achieve both high electrical conductivity and low kinematic viscosity.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In a method for producing an electrogalvanized steel sheet, in which an electrogalvanized steel sheet is produced by performing electrogalvanizing treatment using the steel sheet as a cathode,
Contains Zn to ZnSO ₄ · 7H ₂ O 100 to 280 g / L, sulfuric acid 60 to 140 g / L in terms of H ₂ SO ₄ , electrical conductivity 20 S / m or more, kinematic viscosity 1.0 mm ² / s or less Using a plating bath whose bath temperature is 50 ° C or higher,
A method for producing an electrogalvanized steel sheet, comprising forming an electrogalvanized layer of 5 to 30 g / m ² on one surface of the steel sheet.

  According to the present invention, a plating bath having both high electrical conductivity and low kinematic viscosity can be used for performing electrogalvanizing treatment. As a result, a reduction in electrolytic voltage is realized, and an effect of reducing power costs can be obtained. As mentioned above, it becomes possible to provide the manufacturing method of the electrogalvanized steel plate with which manufacturing cost was reduced.

  The plated steel sheet which is the subject of the present invention is an electrogalvanized steel sheet obtained by electrogalvanizing using an acidic bath. From the balance of performance (corrosion resistance, workability, whiteness, etc.) and operation, the preferable range of the zinc content in the plating film is 98 mass% or more.

And, in the present invention, in producing the electrogalvanized steel sheet, Zn is contained in 100 to 280 g / L in terms of ZnSO ₄ · 7H ₂ O, sulfuric acid in 60 to 140 g / L in terms of H ₂ SO ₄ , Using a plating bath with a temperature of 20 S / m or more, a kinematic viscosity of 1.0 mm ² / s or less, and a bath temperature of 50 ° C. or more, the steel plate is electrogalvanized as a cathode, and the surface of the steel plate is 5-30 g / side. An electrogalvanized layer of m ² shall be formed. Moreover, it is preferable to carry out the electrogalvanizing process at a current density of 200 to 500 A / dm ² . These are important requirements of the present invention.

Details of the present invention will be described below.
In the present invention, it is possible to realize a plating bath having both high electrical conductivity and low kinematic viscosity by controlling the Zn concentration, sulfuric acid concentration, and bath temperature within predetermined ranges.

In the plating bath, Zn is contained in an amount of 100 to 280 g / L in terms of ZnSO ₄ .7H ₂ O, and sulfuric acid is contained in an amount of 60 to 140 g / L in terms of H ₂ SO ₄ .
If Zn is less than 100 g / L in terms of ZnSO ₄ · 7H ₂ O, zinc ions at the steel sheet interface are deficient during electrolysis, resulting in plating burn. When plating burn occurs, the lightness (L value) decreases and the current efficiency decreases, so that the power cost necessary for plating with the target adhesion amount increases. If it exceeds 280 g / L, the electrical conductivity will decrease and the kinematic viscosity will increase. From the above, Zn is in the range of 100 to 280 g / L in terms of ZnSO ₄ .7H ₂ O.
When the sulfuric acid concentration is less than 60 g / L in terms of H ₂ SO ₄ , high electrical conductivity cannot be achieved. On the other hand, if it is present in a larger amount than 140 g / L, the amount of hydrogen generated during electrolysis increases and plating burn occurs. From the above, the sulfuric acid concentration is in the range of 60 to 140 g / L in terms of H ₂ SO ₄ . Preferably, it is 60-120 g / L.

The temperature of the plating bath is 50 ° C or higher.
Below 50 ° C, the electrical conductivity of the bath decreases and the kinematic viscosity increases. Although the upper limit is not particularly defined, since the evaporation amount of the plating bath increases as the temperature rises, a temperature up to about 90 ° C. is realistic and preferable.

Electrical conductivity is achieved by satisfying the conditions of these plating baths: Zn is 100 to 280 g / L in terms of ZnSO ₄ · 7H ₂ O, sulfuric acid is 60 to 140 g / L in terms of H ₂ SO ₄ , and the bath temperature is 50 ° C or higher. A plating bath having a degree of 20 S / m or more and a kinematic viscosity of 1.0 mm ² / s or less is obtained. Thereby, the fall of electrolysis voltage is implement | achieved and it becomes possible to reduce the power cost in manufacturing an electrogalvanized steel plate.

In the present invention, electrogalvanizing treatment is performed using a steel plate as a cathode, using the above-described plating bath.
Current density is preferably 200~500A / dm ^2. In the present invention, a remarkable effect is obtained particularly when electrogalvanizing is performed at a high current density of 200 A / dm ² or more. On the other hand, if the current density is 500 A / dm ² or less, it is preferable because zinc ions cannot be diffused in time and plating burn does not occur.
In addition, when performing an electrogalvanization process, it does not specifically limit except the plating bath and electric current density mentioned above, It is preferable to carry out on the following conditions.
The type of the electrode (anode) is not particularly limited, but it is preferable to use an iridium oxide electrode in view of dissolution of impurities in the plating bath.
The plating bath flow rate is preferably 1.0 m / min or more from the viewpoint of thinning the diffusion layer at the electrolytic interface.
In addition, some elements such as impurities that inevitably enter the electrogalvanizing bath may be added, but they are applicable as long as the effects of the present invention are not impaired.

As a result, an electrogalvanized steel sheet having an electrogalvanized layer of 5 to 30 g / m ² per side is produced on the steel sheet surface. The amount of plating is 5-30 g / m ² per side. If it is less than 5 g / m ² , good corrosion resistance cannot be obtained, and if it is more than 30 g / m ² , the cost increases.

Furthermore, the electrogalvanized steel sheet of the present invention can be a surface-treated steel sheet by having a chemical conversion film and / or a film containing an organic resin on the surface.
The chemical conversion treatment film can be formed, for example, by a chromate treatment or a chromium-free chemical conversion treatment in which a heat drying treatment is performed at a steel plate temperature of 80 to 300 ° C. without applying a chromate treatment solution or a chromium-free chemical treatment solution and washing with water. These chemical conversion treatment films may be a single layer or multiple layers, and in the case of multiple layers, a plurality of chemical conversion treatments may be performed sequentially.

  Moreover, the electrogalvanized steel sheet of this invention can form the single layer or multiple layer coating film containing an organic resin on the surface of a plating layer or a chemical conversion treatment film according to a use. Examples of the coating film include a polyester resin coating film, an epoxy resin coating film, an acrylic resin coating film, a urethane resin coating film, and a fluorine resin coating film. Further, for example, an epoxy-modified polyester resin coating film in which a part of the resin is modified with another resin can be applied. Further, a curing agent, a curing catalyst, a pigment, an additive and the like can be added to the resin as necessary.

Although the coating method for forming the coating film is not particularly defined, examples of the coating method include roll coater coating, curtain flow coating, and spray coating. After coating a paint containing an organic resin, the coating film can be formed by heating and drying by means of hot air drying, infrared heating, induction heating or the like.
However, the manufacturing method of the said surface treatment steel plate is an example, and is not limited to this.

Next, the present invention will be described in more detail with reference to examples.
A cold rolled steel sheet having a thickness of 0.7 mm manufactured by a conventional method is subjected to a degreasing treatment and a pickling treatment, followed by an electrogalvanizing treatment with the plating bath composition shown in Table 1 and the conditions shown below. A plated steel sheet was produced. In addition, the zinc plating adhesion amount per one side was calculated | required by melt | dissolving zinc plating with dilute sulfuric acid, measuring the zinc concentration in a solution with an ICP (Inductively Coupled Plasma) mass spectrometer, and converting into the adhesion amount.
Electrolytic condition current density: bath temperature shown in Table 1, galvanized coating amount: electrode shown in Table 1: iridium oxide flow rate: 2.0 m / sec
For the electrogalvanized steel sheet obtained as described above, the power cost, L value, and current efficiency were determined and evaluated as shown below.
Electricity cost When using an ordinary galvanized bath (electric conductivity 10 S / m, kinematic viscosity 1.0 mm ² / s), otherwise producing an electrogalvanized steel sheet with the same zinc coverage under the same conditions as above It was evaluated how much electricity cost was required compared with the electricity cost of
○: 80% or less ×: 80% to 100%
Lightness (L value)
Using a spectral color difference meter (SD5000, manufactured by Nippon Denshoku Industries Co., Ltd.), brightness (L value) was measured according to JIS Z 8722: 2009 by SCE (regular reflection light removal) and evaluated as follows. did.
◎: 80 ≦ L value ○: 78 ≦ L value <80
×: L value <78

From Table 1, it can be seen that the power cost is kept low in the example of the present invention, and the electrogalvanized steel sheet can be manufactured at a reduced manufacturing cost.
On the other hand, in the comparative example, when manufacturing an electrogalvanized steel sheet having a high L value, the power cost is high and the manufacturing cost cannot be reduced. Or, plating burn occurs, the L value decreases, and the power cost cannot be reduced.

Claims (1)

In the method for producing an electrogalvanized steel sheet, which produces an electrogalvanized steel sheet by subjecting the steel sheet to an electrogalvanizing treatment as a cathode,
Contains Zn to ZnSO ₄ · 7H ₂ O 100 to 280 g / L, sulfuric acid 60 to 140 g / L in terms of H ₂ SO ₄ , electrical conductivity 20 S / m or more, kinematic viscosity 1.0 mm ² / s or less Using a plating bath whose bath temperature is 50 ° C or higher,
A method for producing an electrogalvanized steel sheet, wherein an electrogalvanized layer having a current density of 200 to 500 A / dm ² and 5 to 30 g / m ² per side is formed on the steel sheet surface.