JPS58189389A - Production of galvanized steel plate with high efficiency - Google Patents

Abstract

PURPOSE:To produce a galvanized steel plate having good quality with high efficiency in electric galvanization wherein an electrolyte is ejected between electrode surfaces and the surface of a strip, by using a low pH plating bath contg. H2SO4, and ZnSO4.7H2O in high concn. CONSTITUTION:An electrolytic device for the above-described electric galvanization is provided with an electrolytic cell 2 into which a strip 1 is passed while the strip is held in a space without immersion into an electrolyte 3. Electrode pads 4 which are disposed in proximity to the surface of the strip so as to face the same and are formed with nozzle holes which inject the electrolyte to the surface of the strip and act static pressure on the surface of the strip with said fluid are provided in the prescribed positions in the cell 2. A plating bath consisting essentially of 300-500g/l ZnSO4 7H2O and 20-60g/l H2SO4 is injected onto the surface of the strip 1, and the electrolysis is effected at >=0.5m/sec relative velocity of the strip 1 and the liquid 3 and 100-250A/dm<2> current density, whereby the galvanized steel plate is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing electrogalvanized steel sheets with high efficiency and good quality.

Due to its excellent properties, electrogalvanized steel sheets are used in a wide variety of applications as anti-rust materials for home appliances, steel furniture, showcases, vending machines, etc. Particularly in recent years, social demands for four types of automobiles, namely improved durability, weight reduction, safety measures, and environmental measures, have rapidly come into focus, and surface-treated steel sheets and high-strength steel sheets are being used as materials to meet these demands. demand is increasing. Currently, commercially available rust-proof steel sheets used for such purposes are mainly galvanized steel sheets.

There are two types of galvanized steel sheets: steel sheets manufactured by hot-dip galvanizing and steel sheets manufactured by electroplating. The former (hot-dip galvanized steel sheet, hereinafter abbreviated as HGI) is suitable for obtaining products with thick plating, but has a manufacturing drawback that makes it difficult to obtain products with thin plating. On the other hand, the latter (hereinafter abbreviated as EGI) can produce products with a thin plating thickness, but products with thick plating may increase electricity costs,
Due to low-speed production, etc., it is a product with high shortness and cine efficiency.

On the other hand, in terms of crystallinity, BGI has advantages such as having less influence on the mechanical properties of the steel material than HG I, and it is easier to obtain single-sided plating. It is an advantageous material for answering questions. However, as mentioned above, electroplating has the basic problems of high power costs and difficulty in increasing the thickness of the plating, and these points are the key points in the development of electroplating. The above problem can only be solved by developing a high-efficiency electrolytic galvanizing method that allows electroplating to be carried out at a high current density and in close contact between the electrodes. The following is a highly efficient electrogalvanizing method.
Citation and explanation of conventionally used methods O The conventionally used electrolytic cell of continuous galvanized steel plate is generally a vertical immersion type cell. In a vertical immersion cell, since the distance between the electrodes is large, the current density is inherently limited due to the relationship between current and voltage, and when the current density is increased, the voltage increases due to passivation of the soluble anode or gas accumulation. In addition, in terms of quality, problems such as discoloration of plating, nonuniformity, coarsening of crystals, pinholes, and poor adhesion occur, so even if the electrolyte is allowed to flow, the current density (hereinafter abbreviated as DK) will be D.
x - = 20 to 50 A/dm2 is the limit. Therefore, the inventors of the present invention solved the above problem by performing close proximity high current density electrolysis and at the same time finding a new galvanizing bath compatible with it. The present invention uses, for example, a cell capable of high current density operation (disclosed in Japanese Patent Application Laid-Open No. 127789/1989), which has already been developed by the present applicant, and uses znso4.7H,,0 as a plating bath.
300-500 g/l, and H2So430-60
It is characterized by using a bath consisting of 11/l.

An example of an electrolytic device for carrying out the present invention will be explained with reference to FIG. The electrolytic device includes an electrolytic cell 2 in which the strip 1 is passed through the electrolytic solution 3 while being held in a space without being immersed in the electrolytic solution 3;
an electrode disposed at a predetermined position in the electrolytic cell 2, facing and close to the surface of the strip 1, and having a nozzle hole formed therein for injecting electrolyte onto the strip surface and applying static pressure to the strip surface by the fluid; In this way, the strip in the electrolytic tank is held in space without being immersed in the electrolyte, and it also functions as a static pressure pad placed at a predetermined position in the tank facing the strip surface. The feature is that electrolytic treatment is carried out by spraying electrolytic plating solution onto the electrode pads and the strip surface.

Therefore, this electrolytic bath is completely different from the conventional immersion type; it is not filled with electrolyte, and the electrode for electrolysis has a hollow box structure, and the electrode is a hollow box structure that extends from the nozzle hole toward the strip. Since the electrolyte is spouted out, various disadvantages seen with the immersion type are eliminated, and the lower roll can also be used as a conductor roll. In addition, it prevents strip vibration and shape defects. This provides the advantage of being able to perform stable sheet threading through correction.

Hereinafter, an example of a plating apparatus that can be used in the present invention has been shown, but the present invention is not limited to this example. Any device that can perform plating at a relative speed of 0.5 m/sec or more between the strip and the electrolyte while holding the plating solution between the plates to be plated may be used.

Another example that meets such conditions is
-An electrolytic plating device that injects an electrolytic solution between the strip and the electrode countercurrently to the traveling direction of the strip, as disclosed in Japanese Patent Application No. 8020; Examples include devices that spray liquid. Further, in the present invention, the closer the distance between the electrodes, the more preferable the distance between the electrodes is, for example, a water-containing electrolyzer equipped with a hydrostatic fluid pad disclosed in Japanese Patent Application No. 57-18836, which is currently pending by the present applicant. The reason for this is, for example, when the current density is 100 A/dm' or 200 A/dm.
”, if the distance between the electrodes is large, the voltage between the electrodes will be 20.
If the voltage exceeds ~30V, the power cost and rectifier capacity will increase, making it impractical. The present invention reduces the voltage between electrodes to at least 20
The target is less than V, preferably less than IOV. Therefore, the closer the distance between poles is (less than 15 mm for light and heavy)
The voltage between the electrodes is reduced, resulting in significant cost savings.

In this way, the smaller the distance between the electrodes, the more advantageous it is in terms of cost, but if the distance between the electrodes is made too small, there is a risk that the strip and the electrode will come into contact with each other. It can be said that there is a limit.

In addition, in the present invention, as described above, plating is performed with the distance between the electrodes as close as possible, but in order to reduce the vibration and positional fluctuation of the strip during plating, or the catenary, to reduce the distance between the electrodes, JP-A-56-12778 described above
9 and Japanese Patent Application No. 57-18836, it is most preferable to perform plating while supporting the strip with a hydrostatic fluid using a plating apparatus having a static pressure fluid pad as shown in Japanese Patent Application No. 57-18836. Since it is automatically centered using pressure, the distance between the electrodes can be greatly shortened, and moreover, plating can be performed stably.

When performing plating while maintaining the liquid between the electrode surface and the surface to be plated by spouting the electrolytic solution between the electrode surface and the surface to be plated using the electrolytic device as explained above, one gas is used. Since there is no accumulation and the distance between the poles can be close, the current carrying resistance is small, making it possible to suppress the rise in voltage. The present inventors have discovered an electrolytic bath capable of high efficiency as shown in FIG. However, it was found that there were still issues to be solved when electrolytic galvanizing was actually carried out.
When electrolytic galvanizing is performed at high current density in close proximity using a highly efficient electrolytic bath, 1) plating burn and plating unevenness occur;
There are problems such as 2) a decrease in current efficiency, and 3) a large increase in voltage due to liquid resistance, and it was found that unevenness is likely to occur on the plated surface, especially depending on the flow velocity distribution. Therefore, with the aim of manufacturing highly efficient and high quality galvanized steel sheets, the present inventors developed a new galvanizing bath suitable for close-in high current density electrolysis, which caused some flow velocity unevenness on the electrode surface. It was also discovered that homogeneous and high quality electrolytic galvanizing can be obtained.

The present invention is characterized by the use of a low-temperature bath with high concentrations of H2SO4 and ZnSO4"7H20. As a result of using this bath and the electrolytic bath described above, the above-mentioned poor plating appearance is completely eliminated. υ, especially the influence of the nozzle that blows out at a high flow rate is completely eliminated.Also, the current efficiency is 93cm, which is comparable to the case of low current density, and because of the high conductivity, the electrolytic voltage is reduced, making it possible to save power. The bath of the present invention breaks the conventional wisdom that current efficiency decreases when sulfuric acid is added, making it possible to obtain high-quality products through high-efficiency plating due to high current density.Figure 2 is based on the present invention ZIISO4・7H20500CEi/l
), H2SO4 = 40C9/l), using a plating bath of -105 and a relative flow rate of 1. When plating with Orry's and conventional plating bath ZnSO4・7H203009/lH,5o
45 g/l This shows the relationship between current efficiency (η) and current density (DK) when similarly plated using pH = 2.0. In the conventional method (curve 2), the current density is high. Although the current efficiency decreases as the current efficiency increases, the method of the present invention (curve 1)
If DK = 100 (A, / dm2) or more 25OA
-/dm2, the current efficiency is about 93% and stable. However, at a DK of 300 A/dm, current efficiency decreases and plating unevenness begins to occur even in the bath of the present invention, so the preferred application DK of the present invention is 100 to 25
0A,/dm 'Optimal DK is 150-200 A/
It is dm2.

Figure 3 shows the effects of the relative velocity (Vrn/s) between the electrolyte and the strip and the current density (DK A / dm) on 6 platings (○ indicates good, △ indicates poor, × indicates extremely poor). The relationship is shown in Figure 3.The shaded area shows the range where the plating appearance is good.
SO4” 7l-(20500C1//l), H!8
0440 (g/l), pH=0.6, conductivity K=
] 80 (mtT/2m) at a temperature of 60°C.
As shown in Figure 3, which is the result of plating in Plating with good appearance can be obtained over a wide range of flow rates and line speeds.
Even if a flow rate of 0.5 m/s or more was applied, a good appearance could not be obtained at such a high current density.Also, even when using the plating bath of the present invention, V = O(rrV's
＼That is, in immersion static plating, DK = 100 (A/d
m') or more will cause discoloration of the plating. From the above, in order to ensure a good plating appearance, the relative velocity V between the electrolyte and the strip must be 0.5 (m/s) or more.In order to ensure the relative velocity V, the flow velocity must be increased. ○ As mentioned above, high current density improves plating appearance, current efficiency,
In order to satisfy low voltage operation, conditions including the upper plating bath, the flow rate between the electrodes, and the electrolytic method are required.The plating bath used in the present invention will be described in further detail below.

FIG. 4 shows the relationship between the amount of H2SO4, conductivity (curve K), and current efficiency (curve η). H2SO.

As the amount increases, the conductivity increases linearly, contributing to a decrease in electrolytic voltage. However, when H2SO4 exceeds 60 g/l-F, the current efficiency decreases rapidly, making it impractical. Figure 5 shows the relationship between the amount of ZnSO4.7H20, conductivity and current efficiency.
-Decreases as H20 increases. Especially 500 E//l
In super, the decline is steeper. On the other hand, the current efficiency is ZnSO4.7
90% or more can be secured with H203009/1 or higher. In addition, increasing the concentration of ZnSO4 has a high risk of precipitating crystals in the electrolyte.
The concentration of is unfavorable for operation. Therefore, in the present invention, ZnSO4*7H20300~500 F//l is required. In Figure 6, H2SO and ZnSO4*7
In Figure 0, which shows the relationship between H20 and plating appearance, ○ indicates good condition, and MU indicates poor condition (staining). do. DK 20 OA/dm
”, the range with good appearance is ZnSO4' from Figure 6.
7H20 is 300 fj/1 or more and H2SO4 is 20 g/1 or more.

As described above in detail about the plating bath of the present invention, 1DK1
In order to ensure good conductivity, current efficiency, and plating appearance at 00 to 250 A/dm', the plating bath concentration should be ZnSO4@7H20300 to 500 fj/l.
It is necessary to secure a minimum of 20 to 60 i/l of H2804. 0 Preferably ZnSO4@7H2040
0±50, li'/l H2SO440±109
/l range is good. As other additives, conventionally used Na25O(N)(4)tso+ and the like may be added.

Example 1 Teznso4-7H2050o (
g/l), (NH4)28045 o(yA), Na
2SO430 (g/l), H2SO44° (9/l
) (7) Using a composition bath at a temperature of 55 to 6 o'cDK = 20
As a result of plating at °A/dm2, electrode spacing of 5 mm, and V=0.5 m/s, the equilibrium voltage was 13 V, and a good plating appearance was obtained.The current efficiency at that time was 93%. For comparison, conventional bath ZnSO4@7H20250 (1/l),
Na2SO43o(9/l), )(,,so, 10
(g/l) under the same conditions as above, the interelectrode voltage was as high as 20 V, the plating appearance was poor, and the current efficiency was 88 cm.

As explained above, the present invention is an epoch-making method for producing highly efficient galvanized steel sheets that solves all of the problems of appearance defects, current efficiency decreases, and voltage increases that occur during galvanizing in close proximity high current density electrolysis.

[Brief explanation of the drawing]

FIG. 1 shows an example of an electrolytic cell used in the present invention. Figure 2 shows the relationship between current density and current efficiency under the conditions of the present invention.
1) and comparison condition (2) are shown. FIG. 3 shows the relationship between relative flow velocity (V) and current density (DK) on plating appearance. The ○ mark indicates a good plating appearance. Figure 4 shows the relationship between sulfuric acid concentration, electrical conductivity (K), and current efficiency (η), and Figure 5 shows the relationship between 7. n S 04・7H2
FIG. 3 is a diagram showing the relationship at 0 concentration. Figure 6 shows ZnSO4 at a current density of 200 A/dm”.
- It is a diagram showing the relationship between 7H20 and H2SO4 concentrations and plating appearance. The areas indicated by circles and diagonal lines in the figure indicate good ranges.葆 /fI Fu2ro Dai 3 Coarse V (Knight) Chi 4 Mouth ρ /(74040g6 /h 5o4(1/l) Fan 5 ztv 4 Zn3()p・. Aot

Claims (1)

[Claims] In the method of electrolytic galvanizing while maintaining the electrolyte between the electrode and the strip by jetting the electrolyte between the electrode surface and the strip surface, znso, @ as the plating bath is used.
7H20300~500 Lj/l S H2SO4
An electrogalvanizing bath containing 20 to 609/l as the main component was used, and the relative velocity between the strip and the electrolyte was set at 0.5 m/sec.
A method for producing a highly efficient galvanized steel sheet characterized by electrolyzing at a current density of 100 to 250 A/dm'