JPH0136559B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method of manufacturing an electrogalvanized steel sheet. (Prior Art) Galvanized steel sheets have a wide range of uses, such as automobiles, home appliances, and building materials, due to their good corrosion resistance. The manufacturing process for galvanized steel sheets includes electroplating using an annealed cold-rolled steel sheet as the base plate, and gas reduction and annealing using a hot or cold rolled steel strip as the base plate in a Sendzimer type continuous annealing furnace. It is well known that there are two typical hot-dip galvanizing methods, which involve immersing the metal in molten zinc. In electroplating, the amount of plating deposit (undercoat weight)
Electricity costs increase in proportion to In addition, in hot melt plating, there are limits to controlling the deposit amount using gas wiping, and at present, low basis weights are produced using electroplating, and ^high basis weights are produced using hot melt plating. be. In recent years, there has been a strong demand for highly corrosion-resistant steel sheets and high-strength steel sheets, especially as automobile body materials. It is generally known that the corrosion resistance of galvanized steel sheets improves in proportion to the basis weight. Therefore, in terms of corrosion resistance, hot-dip galvanized steel sheets are advantageous because they can be easily thickened, but they have the disadvantage that they cannot produce high-grade products.
On the other hand, in electroplating, a steel plate having a high quality material can be used in advance, so if thick coating is performed, a product with both corrosion resistance and material quality can be obtained.
From this point of view, there is a high need for thick galvanized steel sheets. When thickening electrogalvanized steel sheets, productivity deteriorates due to a decrease in line speed. To solve this problem, high-speed plating with high current density is required. In order to increase the current density of electroplating, various types of cells have been released for the purpose of reducing the plating voltage and quickly removing gas generated within the cell. For example, (1) a jet cell (a method in which the plating liquid flows in the opposite direction to the flow of the steel plate), (2) a central blow cell (a method in which a slit is provided in the center of the anode in the width direction of the plate, and the plating liquid is forced into it). (3) Side-blown cell (a method in which the plating liquid is flowed from a direction 90° to the direction of movement of the steel plate) (4) Radial cell and carousel cell (a steel plate is wound around a conductor roll, and the plating liquid is applied to one side in a circular pattern) (Place the electrodes while maintaining a suitable gap, and then flow the plating solution.) However, if high current density plating is carried out with only these equipment measures in place, there will be problems such as blackening of the surface appearance, especially when the basis weight is increased, and, in practical terms, a decrease in pressability and paint finish. The cause of this is
This is because the plating crystal becomes coarse and dendrite-like due to local concentration of current at the crystal growth point. Therefore, there is a need for a method of suppressing the coarse growth of plated crystals and making the crystals finer. A conventional method for this is the addition of brighteners to the plating bath. Brightening additives for acidic galvanizing baths include aromatic aldehydes, coumarins, S
Low molecular weight compounds such as -O compounds, amines, imino compounds, imidazolinones, ketone derivatives, thioureas, cresols, dextrins, naphthol derivatives, pyridine, gluconic acid, or carbonamide substituted products of water-soluble long-chain polyalkylene compounds ( Polycarbamoylalkylene compound) (Special Publication 1977)
-24027), quaternary amine polymer (Special Publication 55-
41306) and other polymer compounds having a molecular weight of 1000 to 10000 are disclosed. Furthermore, Japanese Patent Publication No. 59-12754 proposes a bright acidic zinc plating bath to which a polyether surfactant, an aromatic carbanyl compound, and sulfonated castor oil are added in combination. The action of these brighteners is thought to be that they adsorb to atomic metals precipitated on the base metal, increase cathodic polarization, prevent crystallization, and provide a smooth plated surface, thereby making the surface brighter. There is.
Therefore, many examples of known brighteners include compounds having cationic groups that are easily adsorbed onto the cathode. (Problems to be Solved by the Invention) However, none of the conventional brighteners is effective at high current densities, and it is expected that the allowable current density will further decrease with thickening. Furthermore, the electrodeposition stress increases due to the gloss, and cracks in the plated layer occur even with slight processing. In particular, electrodeposition stress is large in alloy plating such as Zn-Ni and Zn-Fe, and glossing is thought to further promote this. Japanese Patent Publication No. 58-26438 discloses a method for producing electrogalvanized steel sheets with excellent gloss without adding brighteners. This method uses a total chloride bath and controls the current density and threading speed within a specific range to obtain electrogalvanized steel sheets with excellent gloss. However, the upper limit of current density is
250 A/dm ² , and as far as the examples show, the amount of zinc deposited is 40 g/m ² or less, which has a glossing effect on the low basis weight side. The purpose of the present invention is to add a polymer to the plating bath that has a different effect from that of conventional brighteners, thereby improving galvanizing properties during electrogalvanizing at high current densities of 100 to 450 A/ ^dm2 . The present invention provides a method for producing an electrogalvanized steel sheet that suppresses the growth of coarse grains and has a smooth, white plated surface without becoming glossy. (Means for Solving the Problems) The present inventors conducted basic research on plating quality when increasing current density in electrolytic galvanizing, and discovered the following basic idea. 1 In electrogalvanizing, the 002 plane grows preferentially. This growth is suppressed and 102, 103, 112, etc. are mixed in, and the randomly grown zinc plating reduces powdering and star grain during pressing. 2. In order to improve the paint finish, especially the sharpness and color tone after painting, it is necessary to create fine irregularities on the plating surface to make the appearance whiter. By suppressing the growth of the 3002 plane, the generation of dendrites in the plating layer can be suppressed when the coating is thick. However, the preferential growth of the 002 plane is promoted in proportion to the current density, and as a result, the crystal grains become coarser when the weight is thicker. Therefore, by improving the plating bath, the growth of the 002 plane at high current density was suppressed, and random crystallization was achieved.
The present invention has succeeded in producing an electrogalvanized steel sheet that is smooth and has excellent whiteness. In the present invention, a polymer resin capable of dispersing the plating current by thermal molecular motion near the plating surface is added to the plating bath, and this function suppresses the preferential growth of the 002 plane of the galvanized plating. At the same time, by preventing the growth of coarse crystal grains due to microscopic concentration of plating current and leaving fine crystal irregularities,
It was developed for the purpose of whitening the surface. As a result of examining polymer resins to embody the above-mentioned plating mechanism, we found that a linear superpolymer with an average molecular weight of 10 ⁶ or more and 10 ⁸ or less is capable of forming flocs at the interface (diffusion layer) of the plating surface. It was discovered that a nonionic resin that does not cause strong adhesion to the plating surface and anode surface due to the plating bath voltage is effective. Furthermore, in order to avoid causing a voltage increase, it is essential that the above effects be achieved with a very small amount of addition, and that CO ₂ , N ₂ , and H ₂ O are oxidized and decomposed at the anode and do not remain in the bath. We found nonionic polyacrylamide as a polymer that satisfies this requirement. The feature of this polymer is that it is nonionic,
Unlike conventional cationic brighteners, which make the crystals finer by adsorbing to the active sites of the plating crystals, they do not adsorb to the plating crystals, but instead form flocs near the plating interfaces to conduct current. The point is that it disperses and suppresses the growth of active sites. For this reason, a smooth plated layer with microscopically minute irregularities remaining is obtained, without achieving the level of gloss that can be obtained with conventional brighteners. These fine irregularities ensure workability and coating adhesion, which are important qualities of galvanized steel sheets. The present invention was achieved through detailed study of plating conditions using a plating bath to which nonionic polyacrylamide was added. That is, the present invention provides: (1) Zinc ions of 0.7 to 2.0 mol/and an average molecular weight of 10 ⁶
~ ¹⁰⁸ nonionic polyacrylamide 1~
Contains 100 ppm, uses an acidic electrogalvanizing bath with a pH of 0.5 to 2.0 and a bath temperature of 40 to 70°C, with a current density of 100 to 100.
A method for producing an electrogalvanized steel sheet, comprising electroplating the steel sheet at a flow rate of 450 A/dm ² and a relative flow velocity of 30 m/min or more. (2) 0.7 to 2.0 mol/zinc ion and 0.1 nickel ion or iron ion as the second metal ion.
~2.0 mol/containing 1 to 100 ppm of nonionic polyacrylamide with an average molecular weight of 10 ⁶ to 10 ⁸ , current density using an acidic electrogalvanizing bath with pH 0.5 to 2.0 and bath temperature of 40 to 70°C. 100~450A/ ^dm2 ,
A method for producing an electrogalvanized steel sheet, comprising electroplating the steel sheet at a relative flow velocity of 30 m/min or more. It is. (Function) The present invention will be specifically explained below by showing some of the experimental results that led to the present invention. The base, plating bath composition, and plating conditions are as follows. ZnSO ₄ 7H ₂ O 400g/(Zn ²⁺ 1.4mol/) Na ₂ SO ₄ 100g/ PH 1.2 Bath temperature 55℃ Relative flow rate 90m/min Figure 1 shows nonionic polyacrylamide
Current density using 20ppm additive bath and non-additive bath
This is a photograph taken with an optical microscope of a cross section of a galvanized steel sheet having a basis weight of 100 g/m ² obtained by electroplating at 250 A/dm ² . When an additive-free bath is used, dendrites grow in columnar shapes and the plating layer is severely cracked. The matte layer obtained from the bath containing 20 ppm of nonionic polyacrylamide was free from cracks and a smooth plated layer was obtained. Figure 2 shows the addition of nonionic polyacrylamide with an average molecular weight of 10 ⁷ and a current density of 250 A/dm ² .
The relationship between the additive concentration and the whiteness and gloss of the plated surface is shown when galvanizing with a basis weight of 100 g/m ² is performed. In addition, whiteness is based on JIS Z8722, glossiness is based on JIS Z8722.
By Z8741Gs60. Below 0.1 ppm, dendrites occur and the plated surface turns black, resulting in low whiteness and gloss. At 1 to 100 ppm, dendrites are suppressed, and a plated surface with fine crystals, smooth, high whiteness, and roughness suitable for use in automobiles and painting can be obtained. If it exceeds 100 ppm, the smoothing effect will decrease, resulting in a plated surface with low whiteness. That is, under the conditions of the present invention, nonionic polyacrylamide is in the optimum concentration range 1 that provides appropriate roughness.
It has ~100ppm. Also, nonionic polyacrylamide from 1 to
The plated steel sheet with high whiteness obtained under the conditions of the present invention with 100 ppm added has fine irregularities remaining on the plated surface, which contributes to stress relaxation and ensuring paint adhesion, and improves press workability and post-painting. The appearance and coating adhesion were good. Therefore, in order to obtain an electrogalvanized steel sheet that is smooth, has a high degree of whiteness, and has excellent paintability and workability, steps 1 to 1 are required.
It can be seen that adding a very small amount of 100 ppm is best. Figure 3 shows a current density of 250 A/dm ² and a coating amount of 100.
This figure shows the relationship between the molecular weight of added nonionic polyacrylamide and the whiteness of the plated surface when electrolytic galvanizing was performed at a rate of g/m ² . Any amount added
It is 20ppm. An improvement in whiteness was observed from a molecular weight of about 10 ⁵ , and a stable whiteness was exhibited in the range of 10 ⁶ to 10 ⁸ , but a rapid decrease in whiteness was observed at a molecular weight of about 10 ⁹ . Therefore, the appropriate molecular weight is 10 ⁶ to 10 ⁸ . Figure 4 shows the deposition amount of 100g/m ² by changing the current density.
This figure shows the relationship between current density and whiteness of the plated surface during electrogalvanizing. The added nonionic polyacrylamide has an average molecular weight of 10 ⁵ to ^{10 8}
The amount added is 20 ppm in both cases. 100~450A/
For a current density of dm ² , those with a molecular weight of 10 ⁶ to 10 ⁸ exhibit stable and high whiteness, and for a molecular weight of 10 ⁵ , the limit is about 200 A/dm ² . As mentioned above, in the electrogalvanizing bath, molecular weight
1 to 10 ⁶ to 10 ⁸ nonionic polyacrylamide
By adding a very small amount of 100ppm, it can generate 100~450A
It can be seen that an excellent electrogalvanized steel sheet with smoothness and high whiteness can be obtained even at a high current density of ^dm2 . In addition, this additive is effective not only when electrogalvanizing is performed at high current density, but also when Zn-Ni alloy plating or Zn-Fe alloy plating is performed at high current density. It has a smoothing effect on the sticking layer,
It was found that the alloy composition can be manufactured stably. Next, the reason for limiting the scope of claims in the present invention will be explained. The zinc ion concentration in the plating bath is 0.7~
It is 2.0 mol/. If it is less than 0.7 mol/, the current efficiency will decrease, and if it exceeds 2.0 mol/, there is a risk that zinc compounds will be deposited due to temperature fluctuations. A more preferable range is 0.8 to 1.6 mol/. The pH of the plating bath is 0.5 to 2.0. If the pH is less than 0.5, there will be a significant decrease in current efficiency and effects on plating equipment such as corrosion of tanks, cells, electrodes, etc. When the current density is particularly high, such as 250 A/dm ² or more, when the pH exceeds 2.0, plating and burning occur, and good plating quality cannot be ensured. A more preferable range is 1.0 to 1.8. The molecular weight and amount of nonionic polyacrylamide added are 10 ⁶ to 10 ⁸ and 1 to 100 ppm, respectively.
When the molecular weight is less than ¹⁰⁶ , the effect of addition is low;
Moreover, even if it exceeds 10 ⁸ , the effect is significantly reduced.
If the amount added is less than 1 ppm, the effect of the addition is low;
If it exceeds 100 ppm, the smoothing effect will decrease. The bath temperature is 40-70°C. Below 40°C, the solubility of the metal salt is low and a predetermined zinc ion concentration cannot be achieved, and above 70°C, it is difficult to control the bath concentration. Furthermore, a large amount of heat source such as steam is required to maintain the temperature. A more preferable range is 50 to 65°C. The current density is 100-450 A/ ^dm2 . 100A/
When carrying out thick coatings of 20 g/m ² or more at less than dm ² , it is necessary to lower the line speed, which reduces productivity and tends to cause uneven plating. If it exceeds 450 A/dm ² , a smooth plated layer cannot be obtained even with a bath containing nonionic polyacrylamide. When aiming for a basis weight of 60 g/m ² or more, 150 A/dm ² to 450 A/dm ² is preferable from the viewpoint of productivity. The relative flow velocity will be explained in detail below. In high current density electrolysis, it is necessary to ensure a relative flow rate in order to remove gas generated at the anode and ensure plating quality. Also in the bath of the present invention, the higher the relative flow rate, the smoother the plating can be obtained. The lower limit relative flow velocity in the present invention is 30 m/min. At a relative flow velocity of less than 30 m/min, the appearance of the plating becomes black and there is a high probability that dendrites will precipitate in the current density range of the present invention.
The upper limit is only limited by hardware. A case where the present invention is applied to Zn--Ni alloy plating and Zn--Fe galvanizing will be described below. The concentration range of nickel ions and iron ions is
It is 0.1 to 2.0 mol/. At less than 0.1 mol/
Not having enough nickel or iron content,
If it exceeds 2.0mol/, the content will be too high,
Performance as alloy plating deteriorates. The amount of nonionic polyacrylamide added is 1 to 100 ppm, similar to the zinc bath. The addition of nonionic polyacrylamide has the advantage that the shape of the galvanized plate becomes smooth and that the alloy composition can be stably manufactured. As described above, the present invention is a manufacturing method for high-speed plating with a high current density, and the plating bath is an acidic plating bath such as a halogen bath or a sulfuric acid bath. In the case of a halogen bath, it is possible to use a soluble anode at a current density of 200 A/dm ² or less. However, the current density
Above 200 A/dm ² it is necessary to use an insoluble anode. In this case, chlorine gas is generated from the anode, and the gas must be treated. On the other hand, in the case of a sulfuric acid bath, the only gas generated at the anode under an insoluble anode is oxygen gas, so there is no need for special treatment equipment, which is advantageous in high current density electrolysis. As mentioned above, the nonionic polyacrylamide used in the present invention has a larger molecular weight than compounds used as conventional plating additives, and its burning rate due to oxygen gas generated from the anode is extremely slow, resulting in a short effective life for the amount of current applied. This invention has the characteristic of having a long sulfuric acid bath, and exhibits its characteristics in sulfuric acid baths. Usually added in forming plating baths
The present invention also includes cases where a suitable amount of a conductive aid such as Na ₂ SO ₄ , (NH ₄ ) ₂ SO ₄ , KCl, NaCl, etc. is present. (Examples) Example 1 Table 1 shows the bath composition, plating conditions, and whiteness of the obtained galvanized steel sheet when electrolytic galvanizing was performed using a sulfate bath. In all of Examples Nos. 1 to 16 according to the present invention, smooth plated surfaces with high whiteness were obtained. Comparative Examples Nos. 17 to 26 are cases in which nonionic polyacrylamide is not added at all (Nos. 17 to 20) and cases in which it deviates from the scope of the claims of the present invention (Nos. 21 to 26).
In either case, only a rough plated surface with low whiteness was obtained.

[Table] Example 2 ZnCl ₂ 136g/(Zn ²⁺ 1mol/), KCl100
50 ppm of nonionic polyacrylamide with a molecular weight of ¹⁰⁷ was added to a chloride bath of
2.0 using a tightening bath, bath temperature 55℃, relative flow rate 90m/min, current density 250, 350, 450A/dm ²
An electrogalvanized plate with a basis weight of 100 g/m ² was prepared. The resulting glitter was smooth and had a beautiful appearance with a whiteness of 90 or higher at any current density. The whiteness of plating baths that do not contain polymers is
All I could get was a low, slightly blackish, highly uneven plating of 10 to 20. Example 3 ZnSO ₄ 7H ₂ O 200g/(Zn ²⁺ 0.7mol/),
NiSO ₄ 6H ₂ O263g/, (Ni ²⁺ 1.0mol/),
Na ₂ SO ₄ 80g/molecular weight in Zn-Ni alloy plating bath
Using a tightening bath to which 100ppm of nonionic polyacrylamide of ¹⁰⁶ _was added and the pH was adjusted to 1.5 with _H2SO4 , the bath temperature was 60℃, the relative flow rate was 60m/min, and the current density was
150, 200, 250, 300A/ ^dm2 , area weight 30g/ ^m2
Zn-Ni alloy plating was created. Smooth and beautiful plating was obtained at any current density. The glazed surface obtained from a polymer-free bath is
It had severe unevenness and a rough appearance. Example 4 ZnSO ₄ 7H ₂ O 200g/(Zn ²⁺ 0.7mol/),
_FeSO4・_7H2O278g /(Fe2 ⁺ 1.0mol/)
(NH ₄ ) ₂ SO ₄ 50g/, C ₆ H ₈ O ₇ 0.5g/Zn−
30 ppm of nonionic polyacrylamide with a molecular weight of ¹⁰⁸ was added to an Fe alloy plating bath, and the pH was adjusted to 1.0 with H ₂ SO ₄ using a plating bath. Bath temperature was 60°C, relative flow rate was 60 m/min, and current density was 150. , 200, 250,
A Zn-Fe alloy plating with a basis weight of 30 g/m ² was prepared at 300 A/dm ² . Smooth plating was obtained at any current density. Baths that did not contain polymers gave only a highly uneven and coarsely plated layer. (Effects of the Invention) According to the bath of the present invention, an electrogalvanized steel sheet that is smooth and has excellent whiteness even at an unprecedentedly high current density of 100 to 450 A/dm ² can be obtained. The present invention is effective not only for smoothing thick coatings but also for low coating weights (30 g/Zn or less) that are generally used for coating. In recent years, there has been a demand for improved image clarity after painting, and the present invention makes it possible to obtain an excellent painted appearance with a surface having a moderately uniform roughness.
Therefore, the present invention can provide a high-quality electrogalvanized steel sheet with high productivity, and its industrial utility value is extremely high.

[Brief explanation of drawings]

Figure 1 shows micrographs of the cross-sectional appearance of electrogalvanized steel sheets obtained from (a) a bath with nonionic polyacrylamide added and (b) a bath without it. Figure 2 shows non-ionic polyacrylamide in the bath of the present invention. A graph showing the relationship between the amount of ionic polyacrylamide added and the whiteness and gloss of the plated surface, Figure 3 is a graph showing the relationship between the molecular weight and the whiteness of the plated surface, and Figure 4 is It is a graph showing the relationship between current density and whiteness of a plated surface.

Claims (1)

[Claims] 1. Zinc ion 0.7 to 2.0 mol/and average molecular weight 10 ⁶
~ ¹⁰⁸ nonionic polyacrylamide 1~
Contains 100ppm, uses an acidic electrogalvanizing bath with a pH of 0.5 to 2.0 and a bath temperature of 40 to 70°C, with a current density of 100 to 450A/
dm ² and a relative flow velocity of 30 m/min or more. 2 0.7 to 2.0 mol/zinc ion and 0.1 to 2.0 mol/nickel ion or iron ion as the second metal ion.
2.0 mol/contains 1 to 100 ppm of nonionic polyacrylamide with an average molecular weight of 10 ⁶ to 10 ⁸ , pH 0.5
~2.0, using an acidic electrogalvanizing bath with a bath temperature of 40 to 70°C, current density of 100 to 450 A/ ^dm2 , relative flow rate of 30
A method for producing an electrogalvanized steel sheet, comprising electroplating the steel sheet at a speed of at least m/min.