JPH0874089A - Electrogalvanized steel sheet excellent in whiteness and production of the same - Google Patents

Abstract

PURPOSE: To produce an electrogalvanized steel sheet excellent in whiteness. CONSTITUTION: This production comprises subjecting a steel sheet to plating treatment in a sulfuric acid-acidic Zn plating bath which contains 1 to 20g/l of one or two kinds of glycine and aspartic acid or 1 to 30g/l of a carboxylic acid having at least two carboxyl groups or its salt. Thus, the rate of occurrence of grooves having >=1.5μm depth in the plated surface becomes zero and the whiteness of the plated surface is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to home appliances, automobiles,
The present invention relates to an electrogalvanized steel sheet having excellent whiteness, which is used in a wide range of applications such as building materials, and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, the demand for various chromate-treated electrogalvanizing used without coating for home electric appliances has been increasing, and has become an important application field. In this application, it is required to have an excellent surface appearance because it is used without painting. In addition, after the phosphate treatment, it is usually painted, but if it is a light-colored coating or the coating thickness is thin, if the whiteness of the phosphoric acid original plate is low, the sharpness after coating will be poor and the whiteness will also be high. Is required. A good appearance after painting is that there is no surface defect such as unevenness, but a high whiteness is also required for a good appearance. An appearance with no such unevenness and high whiteness is required after phosphate treatment and various chromate treatments, but of course, the appearance after these chemical conversion treatments depends on the appearance after plating, and zinc before chemical conversion treatment It is necessary that the whiteness of the plated steel sheet is high.

As a proposal for improving the appearance of galvanized steel sheets, there is a method of improving glossiness by adding polyacrylamide or polyvinyl alcohol to a plating bath, as disclosed in Japanese Patent Publication No. 46-38888. As disclosed in Japanese Patent Laid-Open No. 61-244769, a method of performing an oxidation treatment after plating pretreatment to obtain a glare-free appearance,
As disclosed in JP-B-1-36559, a method is disclosed in which nonionic polyacrylamide is added to an acidic zinc plating bath and plating is performed at a high current density to perform smooth and white plating. Among these
Only the 9th publication mentions whiteness, but its whiteness improving effect is limited to high current density of 100 to 450A / dm ² , and the effect is recognized in the plating less than 100A / dm ² which is usually performed. There is a problem in practicability.

[0004]

As described above, although the whiteness of galvanization is an important characteristic, there are few reports on how to improve it. It is considered that the reason is that the factor on the galvanizing side that controls the whiteness is not clear.

As a result of further studies on the factors governing the whiteness of zinc plating, the present inventors have found that the whiteness is governed by the zinc plating crystal morphology. Here, whiteness is a measure of the whiteness, the use of L value in the Lab of Hunter commonly used as a color management method, the whiteness W (Lab) = 100 - [ (100 -L) 2 + a 2 + b 2 ] ^1/2 ... (1) Here, the L value is a lightness index determined by JIS Z 8722, a is a redness index, and b value is a yellowness index.

In the case of an achromatic color such as zinc plating, a
Since the value and the b value are small and can be ignored, the whiteness almost coincides with the lightness index L value. In the measurement of the L value under the conditions C and D of JIS Z 8722, the specularly reflected light is emitted outside the measurement system, so the L value is represented by the intensity of the diffusely reflected light on the surface. Therefore, it can be considered that the diffuse reflection light is a light component obtained by subtracting the specular reflection light and the surface absorption light from the incident light, and in order to increase the diffuse reflection light, that is, the L value (about the same as the whiteness), It is effective to reduce the reflected light and the surface absorbed light. Here, the intensity of the specularly reflected light is the glossiness, and a large reduction in the glossiness is not preferable because it causes a deterioration in the appearance of the plating surface. Therefore, in order to increase the whiteness, it is effective to reduce the light absorption on the plating surface. Here, although there are irregularities due to crystals on the plating surface, it is considered that most of the light absorption occurs at the portions that correspond to the grooves (recesses) of the crystal, so the conclusion is that a plating surface with excellent whiteness is selected. To obtain it, it is important to make the groove of the crystal shallow so as to absorb light as little as possible. Therefore, by electron beam three-dimensional roughness measurement,
The depth of the groove portion of the crystal was measured and its distribution state was evaluated.
FIG. 1 shows the principle of measuring the groove depth of the crystal. As shown in Fig. 1, the surface roughness is measured quantitatively by electron beam three-dimensional roughness measurement, and then the data is processed to cut the surface parallel to the plate from the plating surface at regular intervals. Find the number of. The number obtained by subtracting 1 from the number of cut surfaces is the space where no crystal exists, that is, the number of grooves. As shown in the left diagram of FIG. 1, when the distance from the surface of the cut surface is the horizontal axis and the number of groove portions is the vertical axis, a distribution diagram in the depth direction of the crystal groove portions from the surface is obtained.

2 to 5 show the frequency of existence in the depth direction of the crystal grooves of galvanized steel sheets having different whiteness (L value). 2 and 3 show the existence densities in the depth direction of the crystal grooves of the plated steel sheet having high whiteness of 86.9 and 87.2, respectively. In FIG. 2, there are no grooves deeper than 1 μm, and in FIG. There were no grooves deeper than 1.5 μm. Figure 4,
FIG. 5 shows the frequency of existence in the depth direction of the crystal groove portion of the plated steel sheet having a low whiteness of 76.1 and 80.5, respectively, but in both FIG. 4 and FIG. 5, there is a groove having a depth exceeding 2 μm. . From these results, it can be seen that the whiteness of the plating and the depth of the groove have a correlation. In particular, according to the comparison between FIGS. 2 and 3, the whiteness does not decrease even if there is a groove having a depth of less than 1.5 μm. Further, according to the comparison with FIGS.
It can be seen that the whiteness is greatly reduced when the groove having a depth of m or more is present. Here, in order to further clarify the influence of the depth of the groove portion on the whiteness, the relationship between the whiteness and the abundance ratio of grooves of 1.5 μm or more in a large number of plated steel sheets having different whitenesses was investigated, 6 shows. As is clear from FIG. 6, when the existence ratio of the grooves of 1.5 μm or more is 0, the whiteness shows a value of 86 or more, whereas when the grooves of 1.5 μm or more are present, the whiteness decreases. However, the whiteness also decreased as the abundance ratio increased.

Further, as a result of further studies on the method of obtaining zinc plating having a large whiteness based on the above findings, the present inventors have found that the depth of the crystal groove portion changes depending on the plating bath component, and We have found a plating method that does not exceed 1.5 μm. The present invention has been made based on the above findings, and an object thereof is to provide an electrogalvanized steel sheet having a whiteness value of 86 or more, and a method for producing the same.

[0009]

That is, the electrogalvanized steel sheet of the present invention is an electrogalvanized steel sheet excellent in whiteness, in which any groove on the surface at any position is less than 1.5 μm. The manufacturing method of the electrogalvanized steel sheet is selected from the group of 1 to 20 g / l glycine, 1 to 20 g / l aspartic acid, and 1 to 30 g / l carboxylic acid having two or more carboxylic acid groups or salts thereof. It is a method for producing an electrogalvanized steel sheet having excellent whiteness, which is characterized in that the steel sheet is plated in a sulfated Zn plating bath containing one or two kinds.

[0010]

According to the method of the present invention, glycine and aspartic acid, which are one of the amino acids, and / or a carboxylic acid salt containing two or more carboxylic acid groups in one molecule are added to a sulfuric acid acidic Zn plating bath. The whiteness is greatly improved. The whiteness of the zinc plating obtained from the plating bath containing these additives is improved by 2 to 4 points as compared with the case of no addition. The improvement of whiteness is due to the observation of the plated crystal that the depth of the crystal groove portion became shallow as described above. The mechanism by which these additives change the plating crystal morphology is not clear, but since any amino acid or carboxylic acid forms stable chelates with zinc ions, these chelates near the surface during the process of zinc electrodeposition. It is conceivable that the formation of crystal nuclei suppresses the formation of crystal nuclei to hinder the refinement of crystal grains, and the precipitation overvoltage is increased to make the precipitation uniform and the depth of the crystal grain boundary portion shallow. These complexes and chelates are more stable than hydrated ions, which is the existing form of zinc ions in ordinary sulfuric acid baths, and zinc complexes or chelates are stably present near the electrodeposition surface, and It is estimated that discharge and deposition of only cations occur.

Here, the reason why the carboxylic acid salt having only one carboxylic acid group is not effective in improving the appearance is that, as shown in Table 1, the stability constant of the complex ion with zinc is small, so that it is near the electrode surface. It is considered that this is because complex ions are unlikely to exist stably. On the other hand, the stability constants of carboxylic acids having two or more carboxylic acid groups are generally large, and it is considered that they form stable complex ions or chelates with zinc.

[0012]

[Table 1]

The zinc plating bath used here is a sulfuric acid bath. Chloride bath has advantages such as low plating voltage and easy high current density electrolysis, but it has a serious problem that insoluble anode cannot be used and anode replacement cost is high, so it is gradually used as a galvanizing bath for steel sheets. It's gone.

The plating current density is 40 to 150 A / dm ^2.
It is good to do. This is because the whiteness improving effect is influenced by the current density, and the whiteness improving effect is lost at a current density of less than 40 A / dm ^{2 and more} than 150 A / dm ² . Since the effect of improving whiteness is remarkable at 50 to 120 A / dm ² , it is more desirable to plate in this current density range.

The applicable plating bath pH depends on the anode system. When using an insoluble anode, a plating bath pH of 0.8 to 2.5 is desirable. If the pH is less than 0.8, the stability of the zinc-amino acid chelate is low, so that the whiteness improving effect is insufficient, and the plating efficiency is low, which is not suitable. If the pH exceeds 2.5, the rate of chemical dissolution of metallic zinc, zinc oxide, etc., which is a zinc ion replenishment reaction, is greatly reduced, making ion replenishment difficult. When using a self-dissolving anode, pH 3.0-5.0 is desirable. If the pH is less than 3.0, the chemical dissolution reaction rate of the zinc anode is large and the zinc ion concentration in the plating bath increases, which is not desirable. Above pH 5.0, zinc hydroxide precipitates, which is not suitable.

The amount of amino acid added is 1 to 20 g /
The amount of carboxylic acid added is limited to 1 to 30 g / l. This is because if the addition amount is less than 1 g / l, the whiteness improving effect is insufficient, and if the addition amount exceeds the upper limit, the whiteness improving effect is saturated or lowered, and the plating efficiency decreases. Two types of amino acids and carboxylic acids according to the present invention may be added within the range where the object of the present invention is achieved.

[0017]

【Example】

Example 1 A cold-rolled steel sheet was degreased and pickled by a usual method, and then plated at a coating amount of 20 g / m ^{2 under} the plating bath composition and plating conditions shown in Table 2. The whiteness of the obtained plating was evaluated by the lightness index L value measured by the method (condition d, Hunter method) specified in JIS Z 8722, and shown in FIG. 7. As described above, the whiteness W (Lab) is expressed as follows, and the whiteness W (Lab) = 100 − [(100−L) ² + a ² +
b ² ] ^1/2 L: lightness index L value In the case of galvanizing, since the a value and the b value are small, they can be almost ignored, and the whiteness can be approximated by W (Lab) = L. Therefore, the subsequent whiteness evaluation is based on the lightness index L.
The value was evaluated.

[0018]

[Table 2]

FIG. 7 also shows the existence ratios of the groove portions having a depth of 1.5 μm or more, which are obtained from the measurement results of the three-dimensional surface roughness. As shown in FIG. 7, when glycine was not added to the sulfuric acid plating solution, the existence ratio of grooves having a depth of 1.5 μm or more was 0.03, and the ratio decreased with an increase in the amount of glycine added. It becomes 0 at the addition amount of 20 g / l. Further increase to 30 g / l again increased the ratio to 0.01. Change in whiteness is 1.5 μm deep
Contrary to the above change in the abundance ratio of the groove portion, it increased once with the increase in the amount of glycine added and then decreased with the addition amount of 30 g / l. As is clear from FIG. 7, when the existence ratio of the groove portions having a depth of 1.5 μm or more is 0, the whiteness is 8 in each case.
A high value of 6 or more was shown.

Example 2 As in Example 1, plating was performed under the plating bath composition and plating conditions shown in Table 3 so that the deposition amount was 20 g / m ² . The L value of the obtained plating film is shown in FIG. FIG.
As shown in, the existence ratio of the groove portions having a depth of 1.5 μm or more when aspartic acid is not added to the sulfuric acid plating solution is 0.05, and the ratio decreases with an increase in the amount of aspartic acid added. It becomes 0 at the addition amount of 20 g / l. Further increase to 30 g / l again increased the ratio to 0.01. Change in whiteness is 1.5 μm deep
Contrary to the above change in the abundance ratio of the groove portion, it increased once as the amount of aspartic acid added increased and then decreased with the amount of 30 g / l added. As is clear from FIG. 8, depth 1.
When the existence ratio of the groove portions having a size of 5 μm or more was 0, the whiteness values were all high values of 86 or more.

[0021]

[Table 3]

Example 3 As in Example 1, plating was carried out under the conditions of plating bath composition and plating conditions shown in Table 4 so that the deposition amount would be 20 g / m ² . FIG. 9 shows the existence ratio and L value of the groove portions having a depth of 1.5 μm or more in the obtained plating film. In this example, in order to clarify the influence of the plating current density, the current density was changed to evaluate the existence ratio of the groove portions having a depth of 1.5 μm or more and the whiteness of the plating. As can be seen from FIG. 9, the abundance ratio of the groove portions having a depth of 1.5 μm or more decreases as the current density increases,
The whiteness increased once, but it decreased when the current density was further increased. A high whiteness of 86 or more has a current density of 40 to 150 A / in which the existence ratio of the grooves having a depth of 1.5 μm or more becomes zero.
Got with dm ² . Further, a whiteness of 87 or more is obtained at a current density of 60 to 120 A / dm ² .

[0023]

[Table 4]

Example 4 As in Example 1, plating was carried out under the plating bath composition and plating conditions as shown in Table 5 so that the deposition amount would be 20 g / m ² . FIG. 10 shows the existence ratio and L value of the groove portions having a depth of 1.5 μm or more in the obtained plating film. In this example, in order to clarify the influence of the plating current density as in Example 3, the current density was changed and the existence ratio of the groove portions having a depth of 1.5 μm or more and the whiteness of the plating were evaluated. As can be seen from FIG. 10, the whiteness once increased with the increase of the current density, but it decreased on the contrary when the current density was further increased. A high whiteness of 86 or more is obtained at a current density of 40 to 150 A / dm ² at which the existence ratio of the groove portion having a depth of 1.5 μm or more becomes zero. Further, a whiteness of 87 or more is obtained at a current density of 60 to 120 A / dm ² . The results were the same as in Example 3.

[0025]

[Table 5]

Comparative Example 1 Similar to Examples 1 to 4, the plating bath composition as shown in Table 6
Plating was performed so that the amount of adhesion was 20 g / m ^{2 under} the plating conditions. Although the additives here belong to the same amino acid category as in the examples, the whiteness of the obtained plated film was not improved as can be seen from the table. Correspondingly, there was a groove having a depth of 1.5 μm or more. Therefore, the whiteness improving effect of glycine and aspartic acid of the present invention is not shared by all amino acids and is an effect peculiar to glycine and aspartic acid. In addition, when two kinds of glycine and aspartic acid are added, the same effect as in the case of one kind is exhibited.

[0027]

[Table 6]

Example 5 As in Example 1, plating was performed under the plating bath composition and plating conditions shown in Table 7 so that the deposition amount was 20 g / m ² . The L value of the obtained plating film is shown in FIG. In addition, the depth of 1.5 μm obtained from the measurement result of the three-dimensional surface roughness
The existence ratio of the above groove portions is also shown in FIG. FIG.
As shown in FIG. 1, when the succinic acid is not added to the sulfuric acid plating solution, the existence ratio of the groove portions having a depth of 1.5 μm or more is 0.1.
The ratio is 04, and the ratio decreases with an increase in the amount of succinic acid added, and becomes 0 when the amount added is 1 to 30 g / l. 5 more
Increasing it to 0 g / l again increased the ratio to 0.05. Contrary to the change in the abundance ratio of the grooves having a depth of 1.5 μm or more, the change in whiteness increased once with the increase in the amount of succinic acid added, and then decreased with the addition amount of 50 g / l. As is clear from FIG. 11, when the existence ratio of the groove portions having a depth of 1.5 μm or more was 0, the whiteness values were all high values of 86 or more.

[0029]

[Table 7]

Example 6 As in Example 1, plating was performed under the plating bath composition and plating conditions shown in Table 8 so that the deposition amount would be 20 g / m ² . The L value of the obtained plating film is shown in FIG. In addition, the depth of 1.5 μm obtained from the measurement result of the three-dimensional surface roughness
The presence ratio of the above groove portions is also shown in FIG. FIG.
As shown in FIG. 2, when the citric acid was not added to the sulfuric acid plating solution, the existence ratio of the groove portions having a depth of 1.5 μm or more was 0.
The ratio is 04, and the ratio decreases with an increase in the amount of citric acid added, and becomes 0 when the amount added is 1 to 50 g / l. Contrary to the change in the abundance ratio of the grooves having a depth of 1.5 μm or more, the change in whiteness was once high with the increase of the citric acid addition amount. As is clear from FIG. 12, when the existence ratio of the groove portions having a depth of 1.5 μm or more was 0, the whiteness values were all high values of 86 or more. 30 g / l for citric acid
Although the whiteness did not decrease even with the above addition amount, the electrolytic efficiency was 96 when the addition amount was increased from 30 g / l to 50 g / l.
% To 82%, which is unsuitable in terms of production cost.

Example 7 As in Example 1, plating was performed under the plating bath composition and plating conditions shown in Table 9 so that the deposition amount was 20 g / m ² . The L value of the obtained plating film is shown in FIG. In addition, the depth of 1.5 μm obtained from the measurement result of the three-dimensional surface roughness
The existence ratio of the above groove portions is also shown in FIG. FIG.
As shown in 3, the existence ratio of the groove portion having a depth of 1.5 μm or more in the case where iminodiacetic acid was not added to the sulfuric acid plating solution was 0.07, and the ratio decreased with an increase in the amount of iminodiacetic acid added. It becomes 0 at the added amount of ˜30 g / l.
When the ratio is further increased to 50 g / l, the ratio becomes 0.
It increased to 01. Contrary to the change in the abundance ratio of the groove portions having a depth of 1.5 μm or more, the change in the whiteness was once increased with the increase in the iminodiacetic acid addition amount, and then decreased at the addition amount of 50 g / l. As is clear from FIG. 13, depth 1.
When the existence ratio of the groove portions having a size of 5 μm or more was 0, the whiteness values were all high values of 86 or more. Addition amount is 30g / l
From 50% to 50 g / l, the electrolysis efficiency increases from 96% to 75
%, Which is unsuitable in terms of production cost.

[0032]

[Table 8]

Example 8 As in Example 7, plating was performed under the plating bath composition and plating conditions shown in Table 9 so that the deposition amount was 20 g / m ² . Table 9 also shows the existence ratio of the groove portions having a depth of 1.5 μm or more and the L value of the obtained plating film. As shown in Examples A to M of Table 9, 1 to 30 g of carboxylic acid having two or more carboxylic acid groups in the sulfuric acid plating solution is used.
When 1 is added, the existence ratio of the groove portion having a depth of 1.5 μm or more becomes 0.
Corresponding to this, the whiteness is correspondingly improved, and both are 86
The above L values are shown. On the other hand, as shown in Comparative Examples A to F, when no carboxylic acid is contained, and Comparative Examples G to
As shown in L, the addition of monocarboxylic having one carboxylic acid group had a groove with a depth of 1.5 μm or more, and the whiteness was not improved.

[0034]

[Table 9]

[0035]

EFFECTS OF THE INVENTION By adding glycine, aspartic acid, or a carboxylic acid having two or more carboxylic acid groups or a salt thereof to a sulfuric acid plating solution, a depth of 1.5 μm can be obtained.
The existence ratio of the groove portions of m or more becomes 0, and the whiteness of the plated surface is improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the principle of measuring the number of grooves by three-dimensional roughness measurement, (a) is an explanatory view showing the number of cutting surfaces that the cutting surface crosses, and (b) is the depth of the crystal grooves and the number of cutting surfaces. FIG. 4 is a diagram showing a relationship with the number of groove portions.

FIG. 2 is a diagram showing a roughness distribution (L value: 86.9) of a high whiteness material.

FIG. 3 is another example of the roughness distribution of the high-whiteness material (L value: 87.
The figure which shows 2).

FIG. 4 is a diagram showing a roughness distribution (L value: 76.1) of a low whiteness material.

FIG. 5 is another example of roughness distribution of low whiteness material (L value: 80.
The figure which shows 5).

FIG. 6 is a diagram showing the relationship between whiteness and the number of grooves of 1.5 μm or more.

FIG. 7 is a diagram showing the influence of glycine on the abundance ratio of grooves having a depth of 1.5 μm or more and the plating whiteness.

FIG. 8 is a diagram showing the influence of aspartic acid on the abundance ratio of grooves having a depth of 1.5 μm or more and the plating whiteness.

FIG. 9 is a diagram showing the effect of current density on the abundance ratio of groove portions having a depth of 1.5 μm or more and plating whiteness in a glycine addition bath.

FIG. 10: Depth of 1.5 μm in aspartic acid addition bath
The figure which shows the influence of the current density which affects the existence ratio of the said groove part, and plating whiteness.

FIG. 11 is a diagram showing the effect of succinic acid on the abundance ratio of groove portions having a depth of 1.5 μm or more and the plating whiteness.

FIG. 12 is a diagram showing the effect of citric acid on the abundance ratio of groove portions having a depth of 1.5 μm or more and the plating whiteness.

FIG. 13 is a diagram showing the effect of iminodiacetic acid on the abundance ratio of grooves having a depth of 1.5 μm or more and the plating whiteness.

Claims (2)

[Claims] 1. An electrogalvanized steel sheet having excellent whiteness, in which any groove on the surface at any position is less than 1.5 μm. 2. Glycine of 1 to 20 g / l, 1 to 20 g
/ L aspartic acid, 1 to 30 g / l, the steel sheet is plated in a sulfated Zn plating bath containing one or two selected from the group of carboxylic acids having two or more carboxylic acid groups or salts thereof. A method for producing an electrogalvanized steel sheet having excellent whiteness, which is characterized by the following.