JPS61127891A - Manufacture of galvanized steel sheet - Google Patents

Abstract

PURPOSE:To inhibit the growth of coarse grains and to form a smooth and white galvanized surface by galvanizing a steel sheet at a high current density in an acidic galvanizing bath contg. nonionic polyacrylamide. CONSTITUTION:An acidic galvanizing bath is prepd. by adding 0.7-2.0mol/l zinc ions and 1-100ppm nonionic polyacrylamide having 10<6>-10<8> average mol.wt. or by further adding 0.1-2.0mol/l Ni or Fe ions. The bath is adjusted to 0.5-2.0pH and 40-70 deg.C. A steel sheet is galvanized in the bath at 100-450A/dm<2> current density and >=30m/min relative flow rate. A galvanized steel sheet obtd. by this method has moderate and uniform surface roughness, and when it is coated with paint, a fine appearance is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing electrogalvanized steel sheets.

(Prior Art) Galvanized steel sheets have a wide range of uses, such as automobiles, home appliances, and building materials, due to their edible properties. The manufacturing process for galvanized steel sheets involves electroplating using a cold-rolled steel sheet on an annealing bed as a base plate, and gas reduction and annealing using a hot or cold rolled copper strip as a base plate in a Sendzimer type continuous annealing furnace. It is well known that there are two typical methods of hot-dip plating, in which the metal is then immersed in hot-dip zinc. In electroplating, the electricity cost increases in proportion to the amount of plating deposited (hereinafter referred to as basis weight). In addition, in hot-dip plating, there is a limit to controlling the amount of adhesion by gas wiping;
~50? /m', low basis weight is currently produced by electroplating, and high basis weight is produced by hot dipping.

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, from the point of view of corrosion resistance, hot-dip galvanized steel sheets with easy to thick coatings are advantageous, but they have the disadvantage that high-quality products cannot be manufactured.

On the other hand, in electroplating, it is possible to use a steel plate having a high quality material in advance, so if a thick coating is applied, a product with both corrosion resistance and material quality can be obtained. From this point of view, there is a high need for thick electrogalvanized steel sheets.

When thickening galvanized 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 rapidly removing gas generated within the cell.

For example, (1) jet cell (a method in which the plating solution flows in the opposite direction to the flow of the steel plate), (2) central blow cell (a method in which a slit in the width direction of the plate is installed in the center of the anode to force the plating solution (4) Radial cell and carousel cell (a steel plate is wound around a conductor roll, and the plating solution is rolled around a conductor roll, and the plating solution is flowed from a direction 90 degrees to the direction of movement of the steel plate). (Place the electrodes with appropriate gaps, and then flow the plating solution.)

However, if high current density plating is carried out based only on equipment, problems such as deterioration of surface appearance, especially when the basis weight is increased, and, in practical terms, deterioration of pressability and paint finish arise. 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 completely suppressing the coarse growth of plating crystals and making the crystals finer. A conventional method for this purpose is to add a brightener to the plating bath. Brightening additives for acidic zinc plating baths include aromatic aldehydes, coumaron, S-〇 compounds, amines, imino compounds, imidazolinones, ketone derivatives, thiourea, cresol, dextrin, naphthol derivatives, pyridine, gluconic acid, etc. Low molecular weight compounds or carbonamide substituted products of water bathable long chain polyalkylene compounds (polycarbamoylalkylene compounds) (Japanese Patent Publication No. 39-24027)
, a quaternary amine polymer (Japanese Patent Publication No. 55-41306) and other polymeric compounds having molecules of 11,000 to 10,000 are disclosed. Furthermore, Japanese Patent Publication No. 5-12754 proposes a bright acidic zinc plating bath in which a polyether surfactant, an aromatic carbanyl compound, and a sulfonated casdol 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, conventional brighteners are ineffective in high-current applications, and it is expected that the permissible tortoise flow density will further decrease as the coating becomes thicker. Furthermore, the electrodeposition stress increases due to the gloss, and cracks in the plated layer occur even with slight processing. Especially Zn-Ni
, Zn-Fe, and other alloy platings have a large electrodeposition stress, and it is thought that glossing further promotes this.

As a method for producing electro-galvanized steel sheets with excellent gloss without adding brighteners, the Japanese Patent Publication No. 5B-2
6438 is disclosed. This method uses a total chloride bath and controls the current density and threading speed within a specific range to obtain a V, pneumatic galvanized steel sheet with excellent gloss. However, the upper limit of current density is 250 A/drr? and
As far as we can see from the examples, zinc adhesion is also 40t/rr? It has a glossing effect on the lower basis weight side.

The purpose of the present invention is to add a polymer having a different effect from that of conventional brighteners to a plating bath.
0A/drr? The present invention provides a method for producing an electrogalvanized steel sheet that suppresses the growth of coarse crystal grains during high current density electrogalvanizing and has a smooth, white plated surface without becoming glossy.

(Means for solving the problem) The present inventors have discovered that when increasing the current density of electrogalvanizing,
After conducting basic research on plating quality, we came up with the following basic idea.

] In electrolytic lead plating, the 002 plane grows preferentially. This growth is suppressed, and the zinc plating that has random growth with 102 planes, 103 planes, 112 planes, etc. mixed in will reduce powdering and star grains during pressing.

2. In order to improve the coating finish, especially the sharpness and dot quality after coating, it is necessary to make the plated surface have fine irregularities and give it a white appearance.

By suppressing the growth of 3.002CIIT, the generation of dendrites in the plating layer at the time of thick coating can be suppressed.

However, the preferential growth of the 002 plane is promoted in proportion to the current density, and as a result, the crystal grains become coarse when the weight is thick.

Therefore, by improving the plating bath, 002
The present invention has succeeded in suppressing surface growth, causing random crystallization, and producing an electrogalvanized steel sheet that is smooth and has excellent whiteness.

In the present invention, a polymer resin f capable of dispersing the plating current by thermal molecular motion near the plating surface is added to the plating bath, and this action suppresses the preferential growth of the 002 plane of the zinc plating. At the same time, it was developed to whiten the surface by blocking the growth of coarse crystal grains due to microscopic concentration of plating current and leaving fine crystal irregularities.

As a result of studying the cylindrical molecule street resin for embodying the above plating mechanism, we found that it is a linear superpolymer with an average molecular weight of 106 or more and 108 or less that can form flocs at the interface (diffusion layer) of the plating surface, and that it is suitable for the plating bath. It has been found that a nonionic resin that does not cause strong adhesion to the plating surface and anode surface due to voltage is effective. Furthermore, in order to avoid causing a voltage increase, it is essential that the above effect is achieved by adding ultrafine clouding, and that it is oxidized and decomposed at the anode to become CO□, N2, and N20 and does not remain in the bath. discovered nonionic polyacrylamide.

The characteristic of this polymer is that it is nonionic, so unlike conventional cationic brighteners, which have the effect of adsorbing to the active sites of the plating crystals to make them finer, they do not adsorb to the plating crystals. The purpose is to form flocs near the plating interface to disperse the current and suppress the growth of active sites. For this reason,
It does not achieve the level of gloss that can be obtained with conventional brighteners, but instead provides a smooth matted layer with microscopically minute irregularities remaining.

These fine irregularities are an important quality of galvanized steel sheets. Processability and coating adhesion can be ensured.

The present invention was achieved through detailed study of plating conditions using a matting bath to which nonionic polyacrylamide was added. That is, the present invention.

1) Contains 0.7 to 2.0 mol/l of zinc ions and 1 to 100 ppm of nonionic polyacrylamide with an average molecular weight of 106 to 108, pH 0.5 to 2.0, bath temperature 4.
Current density 1 using acidic electrogalvanizing bath at 0-70℃
00-450A/dffI2. Relative flow velocity 30 m/mi
1. A method for producing an electrogalvanized sheet, which comprises electroplating a steel sheet at a coating temperature of n or more.

2) Zinc ion 0.7-2.0 mol/L and nickel ion or iron ion as the second metal ion 01-;2. Omol/L, and contains 1 to 1100 pp of nonionic polyacrylamide with an average molecular weight of 106 to 108, and has a pH of 0.5 to 2.0. Bath temperature 4
Current density 1 using acidic electrogalvanizing bath at 0-70℃
A method for producing an electrogalvanized steel sheet, comprising electroplating a steel sheet at a rate of 00 to 450 A/di and a relative flow rate 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 composition of the base, tightening bath, and plating conditions are as follows.

Zn S 047H20400f/Z (Zn 2”
1.4 m 01 /Z ) Na2 SO4100?
/l pH 1,2 Bath Temperature: 55°C Relative flow rate: 90 m/min Figure 1 shows the results obtained by electroplating at a current density of 250 A/d using a nonionic polyacrylamide 20 ppm addition bath and a separate beamless bath. Fabric weight 100 f//r
? This is a photograph taken using an optical microscope of a cross section of a galvanized steel sheet. When a separate bath without beams 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.

FIG. 2 shows a current density of 2 s o A/dn? when nonionic polyacrylamide with an average molecular weight of 108 is added. in,
The relationship between the additive concentration and the whiteness and glossiness of the plated surface when performing zinc plating with a basis weight of "@l OO?β" is shown.The whiteness is based on JIS Z8722, and the glossiness is based on JISZ.
According to 8741 Gs 60. Below O,lppm, dendrites occur and the plated surface turns black, resulting in low levels of whiteness and gloss. At 1 to 1 OOppm, dendrites are suppressed, and a plated surface with fine crystals, smooth and highly white, and having a roughness suitable for automobiles and painting can be obtained. If it exceeds lOOppm, the smoothing effect will be reduced, resulting in a plated surface with low whiteness.

That is, under the conditions of the present invention, nonionic polyacrylamide has an optimal roughness range of 1 to 1 O
It has Oppm.

In addition, nonionic polyacrylamide from 1 to 1100p
The plated steel sheet with high whiteness obtained under the conditions of the present invention with P added has fine irregularities remaining on the plated surface, which relieves stress and ensures 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 high whiteness, and has excellent paintability and workability, it is necessary to add 1 to 100 ppm.
It can be seen that it is best to add a very small amount of .

Figure 3 shows a current density of 25o A/dm2 and a coating amount of 100
? /lr? The figure shows the relationship between the molecular weight of added nonionic polyacrylamide and the whiteness of the plated surface when electrolytic galvanizing was performed. The amount added is 20 ppm in each case.

Improvement in whiteness is seen from a molecular weight of about 105, and from 106 to
Stable whiteness was exhibited in the range of 108, but a rapid decrease in whiteness was observed at around 109.

Therefore, the appropriate molecular weight is 106-108.

Figure 4 shows the deposition amount of 100 y/lr by changing the current density.
This figure shows the relationship between current density and whiteness of the plated surface when electrolytic galvanizing is performed. The nonionic polyacrylamide added has an average molecular weight of 108 to 108, and the amount added is 20 ppm. For current densities of 100 to 450 A, /dd, those with a molecular weight of 106 to 108 exhibit stable and high whiteness, and those with a molecular weight of 105 to 200
A/an? The extent is the limit.

As mentioned above, in the electrogalvanizing bath,
08 nonionic polyacrylamide 1~l OOp
100-450A/d- by adding extremely small amount of pm.
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 .

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 was found that it has the effect of smoothing the plating layer and that the alloy composition can be manufactured stably.

Next, the reason for limiting the patent claim @song in the present invention will be explained.

The zinc ion concentration in the plating bath is 0.7 to 2. omo
It is 1/l. If it is less than 0.7 mol/l, the current efficiency will decrease, and if it exceeds 600 t/l, there is a possibility that precipitation of zinc compounds may occur depending on the temperature fluctuation. A more preferable range is 08 to 1.6 mol/l.

The pH of the plating bath is 0.5-20. At pH 0.5, there is a significant decrease in current efficiency and effects on plating equipment such as corrosion of tanks, cells, electrodes, etc. At pH over 2.0, the current density is 25 OA, /drr? If it is particularly high, plating burns occur and good plating quality cannot be ensured. A more preferred range is 10 to 1,8.

The molecular weight and addition amount of nonionic polyacrylamide are 108 to 108 and 1 to 100 ppm, respectively.
Out. If the molecular weight is less than 1 O', the effect of addition will be low, and if it exceeds 108, the effect will be significantly reduced. If the amount added is less than 1 ppm, the addition effect will be low, and if it exceeds 100 ppm, the smoothing effect will be reduced. The bath temperature is 40-70°C. If the temperature is less than 40°C, the solubility of the metal salt is low and the desired concentration of zinc ions cannot be achieved, and if it exceeds 70°C, it is difficult to control the bath concentration. In addition, a large amount of heat source such as steam to maintain the temperature is required. A more preferable range is 50 to 65°C.

The current density is 100-450 A/d-. 100
20 y/rr less than A/dtri'? When applying thicker coatings, it is necessary to reduce the line speed, which reduces productivity and tends to cause uneven plating.

450A/dtt? If it exceeds this, even a nonionic polyacrylamide-added bath will not provide a smooth plated layer. If you are aiming for an eye weight of 601β or higher, choose 1 from the top of the physiological range.
50 A/d- to 450 A/d is preferred.

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. 30
At a relative flow rate of less than m/min, the plating appearance 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 alloy plating will be described below.

The contact range of nickel ions and iron ions is 0.1 to 2.
．． It is 0 mol/A. If it is less than 0.1 mol/, sufficient nickel or iron content cannot be obtained;
When it exceeds Omo1/l, the content becomes too high and the performance as alloy plating deteriorates. The amount of nonionic polyacrylamide added is 1 to 1 OO as in the zinc bath.
It is ppm. The addition of nonionic polyacrylamide has the advantage that the shape of the alloy plating 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, the current density is 200 p, /drr? The use of soluble anodes is possible below. However, at current densities of 200 A, /d 771' or higher, it is necessary to use an insoluble anode. In this case, is chlorine gas generated from the anode? The accompanying gas must be treated.

On the other hand, in the case of a sulfuric acid bath, the only gas generated at the anode under the presence of an insoluble anode is oxygen gas, so there is no need for special treatment equipment, and it is convenient for high current electrolysis. As mentioned above, the nonionic polyacrylamide used in the present invention has a larger molecular weight than the chemical compound used for conventional plating and machining, and its combustion rate due to the oxygen gas generated from the anode is extremely slow when energized. This invention is characterized by a long shelf life relative to the amount of water used, and is effective in sulfuric acid baths.

The present invention also includes the presence of an appropriate amount of a conductive aid such as N, □S04° (NH4)2804, KCJ-, N, CJ-, etc., which are usually added when forming a plating bath.

(Example) (Example 1) Table 1 shows the bath composition, plating conditions, and whiteness of the obtained galvanized steel sheet when electrogalvanizing was performed using a sulfate bath.

In Examples m1 to m16 according to the present invention, smooth plated surfaces with high whiteness were obtained.

Comparative Examples Otori 17 to 26 are cases in which nonionic polyacrylamide is not added at all (1 @ 17 to 20) and cases in which it deviates from the scope of the claims of the present invention (Otori 21 to 26), both of which have low whiteness. Only ÷xyz plating surface was obtained.

(Example 2) ZnCL2136? /l (Zn”l mol/l
), KCj 100t7t weird bath with molecular weight 10?
Using a tightening bath to which 50 ppm of nonionic polyacrylamide was added and the pH was adjusted to 2.0 with HCA, the bath temperature was 55°C, the relative flow rate was 90 m/min, and the current density was 2.
50.350.450 A/drr1' and fabric weight 100
? /rr? electrogalvanized.

The resulting flecks were smooth and had a beautiful appearance with a whiteness of 90 or higher at any current density.

From a plating bath that does not contain polymers, the whiteness is 10 to 20.
Only a very uneven plating with a low and slightly blackish tinge was obtained.

(Example 3) ZnSO, 7H20200? /l (Zn” 0.7
mol/l), N15O.

・6H20263f/l, (Ni” 1.0 m
ol/l), NcL2S○.

Molecular weight 10 in Zn-Ni alloy plating bath of s o y/l
Using a tightening bath to which 100 ppm of nonionic polyacrylamide No. 8 was added and the pH was adjusted to 1.5 with H2SO4, the bath temperature was 60°C, the relative flow rate was 60 m/min, and the current tightness was 150°200, 250, 300 A/ Zn-Ni alloy plating with a basis weight of 30 mm was prepared using drri'. Smooth and beautiful plating was obtained at any current density.

The flecked surface obtained from the polymer-free bath was highly uneven and had a rough appearance.

(Example 4) ZnSO, 7H20200/'L (Zn" 0.
7 mol/L), Fe50.・7H20278
? /l (Fe” 1.0 mol/l) (NH4)
30 ppm of nonionic polyacrylamide with a molecular weight of 108 was added to a Zn-Fe alloy plating bath of 2So450 y/l, C, H8070, 5y/l, and the pH was adjusted to H2S.
Using a tightening bath with O4 to 1.0, the bath temperature is 60°C.
, relative flow velocity 60m/min, current capacity 150.200.
250, 300fi/clrr? So, the basis weight is 30 yA
r? Zn-Fe alloy plating was created. Smooth plating was obtained at any current density. A bath containing no polymer produced only a rough plating layer with severe unevenness.

(Effects of the Invention) According to the bath of the present invention, it is possible to obtain an electrogalvanized steel sheet that exhibits whiteness even at an unprecedentedly high current density of 100 to 450 A/dm'. The present invention is applicable not only to thick coatings but also to low coatings (Zn) commonly used for painting.
30 f/ or less). 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 the drawing]

Figure 1 is a graph showing the relationship between the amount of nonionic polyacrylamide added in a hot bath and the whiteness and gloss of the plated surface.
Graph showing the relationship between molecular weight and whiteness of the plated surface, No. 4
The figure is a graph showing the relationship between current density and whiteness of the plated surface. Figure 2 Article 5 Caro f (ppm) Average molecule 1 Figure 4 Electric current (A/dmz) Procedural amendment order) % formula % 1. Indication of the case 1980 Patent application No. 249643 2. Title of the invention Manufacture method of electrogalvanized steel sheet 3, relationship with the amended case Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (6
65) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4, Agent Address: 1-12-1 Nishi-Shinbashi, Minato-ku, Tokyo, 8th Floor, Daiichi Mori Building March 6, 1985 Date of dispatch of procedural amendment order: March 26, 1985 6. Subject of amendment 4. Brief explanation of drawings and drawings [Fig. 1 (a), (b)) 7. Contents of amendment Attachment (1) Details On page 22, line 13 of the book, after ``photograph,'' it is corrected to read ``illustrative sketch of.'' (2) Correct FIGS. 1(a) and (b) as shown in the attached sheet. Procedural amendment April 2, 1985

Claims (1)

[Claims] 1) A bath containing 0.7 to 2.0 mol/l of zinc ions and 1 to 100 ppm of nonionic polyacrylamide with an average molecular weight of 10^6 to 10^8, pH 0.5 to 2.0, Warm 40
Current density 10 using an acidic electrogalvanizing bath at ~70°C
A method for producing an electrogalvanized steel sheet, which comprises electroplating a steel sheet at a flow rate of 0 to 450 A/dm^2 and a relative flow rate of 30 m/min or more. 2) Zinc ion 0.7-2.0 mol/l and nickel ion or iron ion 0.1-2 mol/l as the second metal ion
．． Using an acidic electrogalvanizing bath containing 1 to 100 ppm of nonionic polyacrylamide with 0 mol/l and an average molecular weight of 10^6 to 10^8, pH 0.5 to 2.0, and bath temperature of 40 to 70 °C, Current density 100-450A/dm
^2. A method for producing an electrogalvanized steel sheet, which comprises electroplating the steel sheet at a relative flow velocity of 30 m/min or more.