JPH09324251A - Hot dip galvanized steel sheet having excellent appearance after coating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve surface ruggedness characteristic and also to improve the image clarity of coated film by regulating the coating weight and spangle diameter of galvanizing layer of a galvanized steel sheet to be coated, respectively. SOLUTION: The coating weight C(g/m<2> ) of a steel sheet and the spangle diameter D(mm) of a plating layer are regulated so that they satisfy the relation of inequality 50<=C<=-100×D+200, where D in the inequality means the mean value represented by L/n when a straight line of a length of L(mm), drawn in an arbitrary direction on the surface of the plating layer, crosses (n) pieces of spangles. Although it is effective to improve surface smoothness to a greater extent by performing dull skin pass rolling after plating, the area ratio of the dull skin pass reduction region in the average projection drawing of the plating layer is regulated to <=70%, preferably <=50%. Moreover, Pb content and Sb content in the plating layer are regulated to <=0.04% and <=0.05%, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot dip galvanized steel sheet, and more particularly to a hot dip galvanized steel sheet which, when used as a coating base, provides a painted galvanized steel sheet having excellent image clarity after painting.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets are widely used in the fields of automobiles, home appliances, construction and civil engineering for the purpose of rust prevention of steel sheets, but in recent years, the surface appearance has been improved and further corrosion resistance has been improved. From the viewpoint of improving the quality of the product, it is often used by coating the surface of galvanized steel sheet.
For example, when it is applied to an environment such as a roof or a wall material that is exposed to the atmosphere and easily corroded, it is often painted.

The coating may be carried out by the user after processing the hot-dip galvanized steel sheet into a product shape, or may be pre-painted on the surface by a hot-dip galvanized steel sheet manufacturer, the latter being the user's side. Since there is an advantage that the coating process can be omitted, it tends to be particularly frequently used in recent years.

[0004] By the way, the hot-dip galvanized steel sheet has a large blooming flower pattern called "spangle" due to crystal growth during solidification of the coating layer. There is a zero spangle material, but when the former regular spangle material is used as the coating base, not only is there a possibility that the spangle pattern may rise to the surface of the coating film, but it is also difficult to obtain good coating adhesion. Therefore, the latter zero spangle material is often used as the coating base.

Regular spangle materials are produced by adding Pb or Sb having a spangle growth effect (effect of increasing spangle size) to a plating bath, while zero spangle materials are Pb or Sb in the plating bath.
It can be produced by lowering the concentration (for example, in Japanese Patent Publication No. 36-20560), A1 is 0.2% or less, Pb is 0.05% or less, Sb is 0.02% or less, and Sb is 0.02% or less.
n is limited to 0.8% or less) has been confirmed.
It is also possible to produce a zero spangle material on a line for producing a regular spangle material. In this case, Pb or S having spangle growth action in the plating bath is produced.
Since b is added, just before the plating layer is solidified,
A method of preventing spangle growth by spraying a coolant (water, or an aqueous solution of a chemical such as ammonium chloride added to water, or a mixture of zinc powder and air, etc.) on the surface of the plating layer to increase the solidification rate is adopted. ing.

[0006]

By the way, as the preference for high quality of products increases, the corrosion resistance of the coated steel sheet is of course, and further improvement of the appearance after painting, which is responsible for the high quality and high quality of the final product. Is required. As for the appearance after painting, first of all, the gloss and depth of the coating are important.
These characteristics have been remarkably improved by the development and improvement of various paints such as metallic paints and mica paints. It has been confirmed that another important required characteristic is that an image is clearly reflected on the coating film, that is, that the image clarity after coating is good, and for that purpose, the coating film should be thickened. That is, if the coating film is made thicker, the coating material levels the irregularities on the surface of the base steel sheet and smoothes the coating film surface. However, thickening the coating directly leads to higher costs such as an increase in the basic unit of coating and a decrease in productivity due to the extension of the drying time, so the coating thickness remains the same and the surface properties of the base steel sheet are improved. There is a demand for the development of a method for improving the clarity after painting.

When the above-mentioned zero spangle material is coated, a spangle pattern does not appear up to the surface of the coating film unlike the regular spangle material, but the surface of the coating film still has a yuzu-like appearance, No satisfactory post-painting sharpness is obtained. In the case of zero spangle material, convex defects may occur on the surface of the plating layer due to dross in the molten zinc bath.Therefore, when using this as a coating base plate, skin pass rolling using a dull roll is performed after plating. However, even after that, the surface after coating still has a yuzu-like appearance, and good image clarity after coating cannot be obtained. If skin pass rolling is performed using a bright roll, the plating layer surface can be smoothed, and the coating film surface after coating can be smoothed to obtain good post-painting clarity.
On the other hand, when skin pass rolling is performed using a bright roll, scratches, push marks and the like are likely to occur on the surface of the plating layer, and the yield of the product is remarkably reduced, so that practical application is difficult.

The present invention has been made by paying attention to the problems of the prior art as described above. Particularly, it is aimed at a zero spangle material, and by improving the surface property, the surface after coating is a citron skin. An object of the present invention is to provide a hot-dip galvanized steel sheet that gives excellent post-painting image clarity without causing defects such as shape.

[0009]

[MEANS FOR SOLVING THE PROBLEMS] To solve the above problems.
The hot-dip galvanized steel sheet according to the present invention is
In hot dip galvanized steel sheet that is painted after
Adhesion amount: C (g / m ^Two ) And the average spang of the plating layer
Diameter: D (mm) satisfies the relationship of the following formula (1)
That characteristic exists where. 50 ≦ C ≦ −100 × D + 200 (1) However, D in the formula is drawn in an arbitrary direction on the surface of the plating layer.
Straight line of length L (mm) crosses n (pieces) spangle
Means the average of the values represented by L / n.

That is, in the present invention, by adjusting the coating amount and the average spangle diameter in the hot dip galvanized steel sheet so as to satisfy the relationship of the above formula (1),
The surface of the plated steel sheet can be smoothed as much as possible, thereby significantly improving the image clarity after coating.

In the present invention described above, it is also effective to further enhance the surface smoothness by performing dull skin pass rolling after hot dip galvanizing, but in this case, it is possible to suppress deterioration in workability due to dull skin pass rolling. In order to suppress the undulations formed on the surface of the plating layer by rolling, in the plan view of the plating layer, the area ratio of the region rolled down by the dulskin pass rolling is preferably 70% or less, more preferably 50% or less. . Further, in carrying out the present invention, it is desirable to minimize Pb and Sb exhibiting spangle growth action in the hot-dip galvanized layer, the Pb content is 0.04 wt% or less, and the Sb content is 0.05%. It is better to keep it to less than weight%.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have found that the amount of plated coating and the surface property of the plated layer after coating are applied to hot-dip galvanized steel sheets produced under various conditions using a continuous hot-dip galvanizing line (CGL). The effect on the image clarity was examined.

First of all, the present inventors investigated the effect of the amount of plating deposited on the image clarity after coating. In CGL, a tandem cold-rolled steel plate is usually used as the base steel plate.
In tandem cold rolling, a dull roll is generally used for the final stand, but the dull roll surface has unevenness with a large pitch (waviness) in addition to unevenness with a small pitch (roughness), and these are transferred to the steel plate surface. . When this undulation affects the plating layer surface and causes large irregularities on the plating layer surface, a smooth coating film surface cannot be obtained even if coating is performed on the plating layer surface. The after-image clarity cannot be obtained.

Then, hot dip galvanizing was carried out at various plating thicknesses, and the relationship between the plating thickness and the post-painting image clarity was investigated.
As a result, it was first confirmed that in order to obtain good post-painting image clarity, the coating weight must be 50 g / m ² or more. That is, the hot-dip galvanized layer has the function of smoothing the waviness of the base steel sheet, and the coating weight is 50 g /
When it is set to m ² or more, more preferably 70 g / m ² or more, the above-mentioned waviness smoothing effect of the plating layer is effectively exhibited.

However, during the above investigation, even if the coating amount is the same, the image clarity after coating may be inferior or poor, and the image clarity after coating is uniquely determined only by the amount of coating deposition. Since it was confirmed that this was not the case, further study was conducted on the effect of the surface properties of the hot-dip galvanized layer on the image clarity after coating.

As described above, for example, the concentration of Pb and Sb having a spangle growth effect in the plated layer is set to 0.
If the content is suppressed to about 04% by weight or less and about 0.05% by weight or less, the spangle on the plating surface becomes so small that it is difficult to visually identify it. However, when the surface is observed with a microscope, it is observed that small spangles are formed in a flower pattern. Therefore, the effect of the small spangles appearing in the pattern on the image clarity after painting was examined in detail.

At any position on the surface of the hot dip galvanized layer,
When a straight line of length: L (mm) crosses n (pieces) of spangles, L / n (mm) is the average spangle diameter: D (m
m), the average spangle diameter is usually 1 to 1.5
The size is mm. On the other hand, as in the case of producing a zero spangle material in a plating bath containing Pb or Sb, which has a spangle growth effect, if a coolant is sprayed on the surface of the plating layer before it solidifies, it will average depending on the cooling conditions. The spangle diameter can be reduced to a minimum of about 0.2 mm. When manufactured under the same conditions, almost no variation in the average spangle diameter due to the position of the plated surface was observed.

Next, in order to investigate the unevenness of the surface of the plating layer, cross-section curves of hot-dip galvanized steel sheets having various spangle diameters were examined using a stylus roughness meter.
As a result, as shown in, for example, FIG. 1 (a cross-sectional view of the surface layer of the plating layer and the lines appearing in the vertical direction represent the grain boundaries of each spangle), the surface of the plating layer having a small spangle diameter is compared. However, as the spangle diameter increased, relatively large irregularities were observed for each spangle.

Therefore, the measurement length of 5 m at any part
The difference between the highest convex portion and the lowest concave portion in the cross section of m is the maximum unevenness (m
m) and the relationship with the average spangle diameter was investigated, the results shown in FIG. 2 were obtained. As is clear from this figure, there was a correlation between the maximum unevenness and the average spangle diameter, and it was confirmed that the maximum unevenness also increased as the average spangle diameter increased.

Next, among many hot-dip galvanized steel sheets, the relationship between the coating amount and the maximum unevenness (mm) was investigated for those having approximately the same average spangle diameter. The result is shown in Figure 3.
It can be seen that, even if the average spangle diameters are almost the same, the maximum unevenness (mm) increases as the coating amount increases. Therefore, various hot dip galvanized steel sheets with different coating weights and average spangle diameters were coated, and the relationship with the image clarity after coating was investigated. As a result, the coating weight C (g / m ² ) and the average spangle diameter D
It was confirmed that those having a (mm) satisfying the relationship of the following formula stably exhibit excellent image clarity after coating. C ≦ −100 × D + 200

The reason why such a result is obtained is considered as follows. That is, it is considered that if the above expression is satisfied, the unevenness caused by each spangle is smoothed by the paint during coating.If the relationship of the above expression is not satisfied, the pitch of the unevenness or the height difference of the unevenness becomes large, and the plating It is considered that the unevenness is not sufficiently smoothed even if the surface is painted.

As a means for satisfying the relation of the above equation, there is exemplified a method of appropriately controlling the cooling conditions when the plating layer is solidified, according to the amount of the deposited coating. When the surface temperature is 424 ± 3 ° C, the refrigerant is sprayed onto the surface of the plating layer from a position within a distance of about 150 mm at an air pressure of about 2 kg / cm ² or more, and the amount of the refrigerant sprayed per unit area on the surface of the plating layer is in the refrigerant. Water volume 1
Examples include a method in which the solid content is about 0 cc / m ² or more and the solid content is about 100 mg / m ² or more.

Next, after the plating, the elongation rate was changed and dull skin pass rolling was performed to examine the relationship between the surface properties of the plated layer and the clearness after painting. As the surface texture, in the plan view of the plating layer surface, the area ratio of the region pressed by the skin path was defined as the SKP pressed area ratio S (%) and measured. As a result, it was confirmed that if the SKP rolling area ratio S is suppressed to 70% or less, the image clarity after coating can be more surely improved. The reason is considered as follows.

That is, if the SKP rolling area ratio exceeds 70%, the base steel sheet undergoes work hardening to deteriorate the workability, and not only cracks are likely to occur during processing into a product shape, but also a normal dull roll is used. In the skin pass rolling used, S
When the KP rolling area ratio exceeds 70%, the undulations existing on the roll surface are transferred to the plating surface, and even if the undulations of the base steel sheet are smoothed by the plating layer, new undulations are generated on the surface of the plating layer and the coating layer is fresh after coating. This is because the image quality is deteriorated. Therefore, if skin pass rolling is performed after plating, SKP
The reduction area ratio is 70% or less, more preferably 50% or less,
More preferably, it is desirable to suppress it to 40% or less.

As described above, according to the present invention, basically, a zero spangle material in which the spangle is made as small as possible is basically targeted, and the coating adhesion amount: C (g / m ² ) and the average spangle diameter of the plating layer: D (mm ) Is characterized by specifying the relationship. Furthermore, regarding Pb and Sb added to grow spangles in the plating layer, the Pb content is 0.04 wt% or less, more preferably 0.02 wt% or less, and the Sb content is 0.05 wt%. It is desirable to control the content to be below, more preferably 0.02% by weight or less. However, Pb and Sb are elements that have a spangle growth effect,
This is because if the method of adding in bath → cooling after plating is adopted, the spangle diameter becomes large and the spangle pattern is likely to be raised on the plating surface. However, if the zero treatment refrigerant is sprayed when the plating layer is solidified, the spangle diameter can be made sufficiently small. In this case, therefore, some Pb and Sb may be contained.

The presence or absence of unevenness on the surface of the plating layer varies depending on the presence or absence of Pb or Sb. However, if the addition amount of these is reduced, the unevenness will also decrease, and Pb and It is preferable that the amount of Sb is small.

[0027]

EXAMPLES Hereinafter, the constitution and effects of the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples and may be adapted to the gist of the preceding and following. It is of course possible to carry out appropriate modifications within the scope, and all of them are included in the technical scope of the present invention.

Example Using a continuous hot-dip galvanizing line (CGL), the composition is Al: 0.2 wt%, Pb: 0.005 wt%, F
A hot-dip galvanized steel sheet was produced under the following conditions using a bath consisting of e: 0.02 wt% and the balance: Zn. (Plating conditions) Plating bath temperature: 460 ° C. Bath entry plate temperature: 460 ° C. Line speed: 70 to 140 m / min Steel plate thickness: 0.6 to 1.6 mm Plating adhesion amount: 30 to 140 g / m ²

The amount of plating adhered is adjusted by gas wiping, and an aqueous solution of ammonium dihydrogen phosphate is sprayed onto the surface of the plating layer together with air before the plating layer is solidified to increase the cooling rate of the plating layer. It was At this time, the spray conditions such as the spray amount, spray pressure, and spray position were changed. After plating, the elongation rate was changed and dulskin pass rolling was performed, and the SKP rolling area ratio was determined by an image analysis method.

Each hot-dip galvanized steel sheet thus obtained was coated under the following conditions, and the post-painting image clarity was evaluated by the following method. (Coating conditions) Coating type chromate: Cr adhesion amount 50 mg / m ² , drying temperature 120 ° C. Undercoat: polyester paint (white), film thickness 5 μm, drying temperature 210 ° C Top coat: polyester paint (white), film thickness 20 μm,
Drying temperature 230 ℃ (Evaluation method for image clarity after painting) PDG meter (Portable
Distinctness of GlossMete
r) was used and evaluated according to the following criteria. ◯: 0.6 ≦ PDG value Δ: 0.4 ≦ PDG value <0.6 ×: PDG value <0.4 The results are shown in Table 1.

[0031]

[Table 1]

As is clear from Table 1, in Examples satisfying the requirements of the present invention, excellent image clarity after coating was obtained, whereas in Comparative Examples lacking the requirements of the present invention. ,
Satisfactory image clarity after painting is not obtained.

FIG. 4 is a graph showing the effects of the coating weight and the average spangle diameter on the image clarity after painting, among many experimental data including the data shown in Table 1 above. In the region where the coating adhesion amount (C) and the average spangle diameter (D) satisfy the requirement of the above formula (1), excellent post-painting image clarity is obtained in all cases.
It can be seen that the image clarity after coating becomes insufficient if the condition (1) is not satisfied.

[0034]

EFFECTS OF THE INVENTION The present invention is constituted as described above, and by defining the coating adhesion amount, the average spangle diameter, and the SKP rolling area ratio, the post-painting visual appearance is excellent and the post-painting appearance is excellent. It has become possible to provide a hot-dip galvanized steel sheet that gives

[Brief description of drawings]

FIG. 1 is a schematic view illustrating the cross-sectional shape of a steel plate.

FIG. 2 is a graph showing the relationship between the average spangle system and the maximum unevenness of a steel sheet.

FIG. 3 is a graph showing the relationship between the plating deposition amount and the maximum unevenness.

FIG. 4 is a graph summarizing the effects of the coating weight and the average spangle diameter on the image clarity after coating.

Claims (3)

[Claims] 1. In a hot-dip galvanized steel sheet coated after plating, the coating weight: C (g / m ² ) and the average spangle diameter of the plating layer: D (mm) satisfy the following formula: A hot-dip galvanized steel sheet that gives an excellent post-painting appearance characterized by the following. 50 ≦ C ≦ −100 × D + 200 However, D in the formula is L / n when a straight line of length L (mm) drawn in an arbitrary direction on the plating layer surface crosses n (pieces) of spangle. Means the average of the values represented. 2. A dull skin pass rolling after hot dip galvanizing, and in a plan view of the plating layer,
Area ratio of the area rolled down by Dulskin Pass rolling is 7
The hot dip galvanized steel sheet according to claim 1, which is 0% or less. 3. The hot-dip galvanized steel sheet according to claim 1, wherein the Pb content in the hot-dip galvanized layer is 0.04% by weight or less and the Sb content is 0.05% by weight or less.