JP3371787B2 - Method for producing hot-dip Al-Zn-based alloy-coated steel sheet having excellent surface appearance - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to Al of 4-5 wt.
%, It relates to a method for producing an Al-Zn hot-dip galvanized steel sheet having an excellent surface appearance, and particularly to a method for producing an Al-Zn hot-dip steel sheet having an excellent surface appearance suitable for high gloss coating applications.

[0002]

2. Description of the Related Art Molten Al-Zn containing 4 to 5 wt% of Al
Steel alloy-plated steel sheets (hereinafter, simply referred to as hot-dip Al-Zn alloy-plated steel sheets) have higher corrosion resistance than hot-dip galvanized steel sheets, and therefore their usage in building materials and the like is increasing. In these applications, some are used naked, but many are used painted.

As the hot-dip Al-Zn alloy plated steel sheet, a cold-rolled steel sheet or a descaled hot-rolled steel sheet is usually charged into a continuous hot-dip galvanizing equipment as shown in FIG. 2, the rolling oil is removed in the non-oxidizing furnace 3 to the radiant tube type heating furnace 4, the surface oxide is reduced, and at the same time, after annealing, the cooling furnace 5 and the snout 6 are passed through the hot dip bath 7 for Al: It is manufactured by performing Al-Zn alloy plating containing 4.5 wt%, adjusting the coating amount to a predetermined amount by the gas wiping device 9, and then cooling.

Compared with hot-dip galvanized steel sheets, hot-dip Al-Zn alloy plated steel sheets are more likely to cause plating defects,
The surface appearance is inferior. Molten Al-Zn with poor surface appearance
When coated on the base alloy-plated steel sheet, the surface appearance defect due to the plating property defect becomes more noticeable after coating. In particular, with a high-gloss coated steel sheet, the above-mentioned poor surface appearance becomes more noticeable.

For these reasons, even if a high-gloss paint is applied to a hot dip Al-Zn alloy plated steel sheet, an excellent surface appearance cannot be stably obtained after the application. Therefore, melt A
Regarding the coated steel sheet using the 1-Zn alloy plated steel sheet as a material, in order to make the surface appearance defect after coating inconspicuous,
A low-gloss coating film is prepared by mixing aggregates.

[0006] However, as the applications of the hot-dip Al-Zn alloy plated steel sheet are expanded, a high-gloss coated steel sheet may be required from the viewpoint of designability. Therefore, there is a demand for a hot-dip Al-Zn alloy-plated steel sheet that can stably obtain an excellent surface appearance even with a high-gloss coated steel sheet.

Plating defects that occur on the hot-dip galvanized steel sheet include the adhesion of oxide dross of the plating metal to the steel sheet, which occurs when the deposition amount is controlled by the gas wiping nozzle.
Alternatively, this is mainly due to the deposition of fumes and their compounds (such as oxides) of the plating metal evaporated from the plating bath surface in the snout onto the steel strip.

In the former method, the periphery of the gas wiping nozzle is sealed from the plating bath surface and gas wiping is performed in a non-oxidizing atmosphere to prevent the generation of oxidized dross (Japanese Patent Laid-Open No. 8-337863). , For the latter, the gas containing the fumes and compounds of the plating metal evaporated in the snout is sucked out of the snout, and after removing the metal fume and its compound from the gas sucked by the device for removing the metal fumes and compounds, A number of methods for preventing the occurrence of defects have been proposed, such as a method of returning the cleaned gas in the snout (Japanese Patent Laid-Open No. 6-306555), and the melt A
With respect to the 1-Zn alloy plated steel sheet, it has become possible to prevent the surface appearance defect due to this kind of defect.

[0009]

However, molten Al-Z
The n-based alloy plated steel sheet has a coating amount of 100 on one side.
When it is g / m ² or more, a peculiar defect on the surface of the steel plate after plating, that is, a horizontal wrinkle-like uneven pattern parallel to the width direction of the steel plate (hereinafter referred to as horizontal wrinkle defect) occurs, There is a problem that the surface appearance becomes poor. Heretofore, a method of preventing this type of plating defect has not been studied.

It is an object of the present invention to provide a method for producing a hot-dip Al-Zn alloy plated steel sheet which is excellent in surface appearance by preventing the lateral wrinkle defect.

[0011]

The present inventor has made various studies on the cause of the defects. As a result, the defects are
This is due to uneven growth of the Fe-Al alloy layer. If the growth of the alloy layer is uneven, a horizontal wrinkle-like uneven pattern parallel to the steel sheet width direction appears during gas wiping. It was clarified that the uneven growth is affected by the residue of the rolling oil component during plating. Therefore, various studies have been made on methods for reducing these effects. The present invention was born from this study, and means of the present invention for solving the above problems are as follows.

[0012]

( 1 ) Surface appearance characterized in that a steel sheet having a surface roughness Ra of 0.5 μm or less or / and an alkali washed is annealed, and then hot-dip Al—Zn alloy plating containing 4 to 5 wt% of Al is applied. A method for producing a hot-dip Al-Zn alloy-plated steel sheet excellent in heat resistance ( first invention).

( 2 ) Molten Al-Z containing Al in an amount of 4 to 5 wt% after annealing a steel sheet having a surface roughness Ra of 0.5 μm or less and / or alkali washed at an annealing temperature of 750 ° C. or more.
A method for producing a hot-dip Al-Zn alloy-plated steel sheet having an excellent surface appearance, characterized by performing n-based alloy plating ( second invention).

[0015] In the first invention, since a small rolling oil remaining amount, the influence of residue of rolling oil component after annealing is reduced, even an ordinary annealing temperature, it can improve the surface appearance. Second
In the invention, since the steel sheet having a small amount of residual rolling oil is annealed by strengthening the reduction, the above effect is further excellent.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below.

In FIG. 1, cold-rolled steel sheets having different surface roughness are annealed at different annealing temperatures, and the amount of coating adhered is 100 to 1 on one side.
It is a figure which shows the relationship between an annealing temperature and a lateral wrinkle defect, when 35 g / m < ² > of molten Al-Zn type alloy plating is given. Horizontal wrinkle defects are visually observed on the surface of the steel plate after plating,
The degree of defects was evaluated on a scale of 5 according to the following criteria. In order to obtain a good surface appearance on a coated steel sheet using a high-gloss paint, it is desirable that the score is 4 or more, more preferably 5.

[0018] 1: Remarkable lateral wrinkle defect 2: Clear lateral wrinkle defect is recognized 3: Light lateral wrinkle defect is recognized 4: Ultra-light lateral wrinkle defect is recognized 5: No horizontal wrinkle defect

A cold-rolled steel sheet having a surface roughness Ra of 1.0 to 1.3 μm was charged in a hot dip galvanizing facility, and 700
〜730 ° C Annealed conventional material (symbol: ○)
Two horizontal wrinkle defects are recognized. When the annealing temperature is increased, the lateral wrinkle defects become lighter, and by setting the annealing temperature to 750 ° C. or higher, the score of the lateral wrinkle defects becomes 3 or more, and the surface appearance can be improved. However, a rating of 4 required for high gloss coating applications
The above surface appearance cannot be stably obtained.

FIG. 2 shows a case where cold-rolled steel sheets having different surface roughnesses are annealed at an annealing temperature of 700 to 730 ° C. and then subjected to hot dip Al-Zn alloy plating with a coating weight of 110 to 135 g / m ^{2 on} one side. FIG. 4 is a diagram showing the relationship between the residual amount of rolling oil and a horizontal wrinkle defect generated after plating. If the amount of rolling oil remaining on the steel sheet before plating is reduced, lateral wrinkle defects are reduced,
It can be seen that when the residual amount of rolling oil is 250 mg / m ² or less, the lateral wrinkle defect score can be set to 4 to 5.

FIG. 3 shows the surface roughness Ra of the steel sheet after cold rolling.
It is a figure which shows the relationship between rolling oil residual amount. In the case of a cold-rolled steel sheet (normal material, symbol: ◯), reducing the surface roughness reduces the residual amount of rolling oil. Therefore, by reducing the surface roughness of the original plating plate, it is possible to suppress the residual amount of rolling oil and prevent lateral wrinkle defects. The surface roughness Ra is 0.
When the thickness is 5 μm or less, the residual amount of rolling oil is 250 mg / m, which gives a surface appearance with a rating of 4 to 5 required for high gloss coating applications.
Can be ² or less. Further, after cold rolling, the steel sheet subjected to alkali cleaning (symbol: ●) has a further reduced rolling oil residual amount, and therefore has a great effect of preventing lateral wrinkle defects.

The surface roughness of the plating original plate is reduced,
The effect of improving the surface appearance by the alkali cleaning is shown in FIG. In FIG. 1, the surface roughness Ra is 0.5.
The as-rolled steel sheet (symbol: □) having a thickness of μm or less has a rating of 4 to 5 at a normal annealing temperature of 710 to 730 ° C. and a rating of 5 at an annealing temperature of 750 ° C. or higher.

A steel sheet of normal roughness (symbol: ●) that has been subjected to alkali cleaning before annealing has a rating of 4 to 5 at a normal annealing temperature of 710 to 730 ° C. and a rating of 5 at an annealing temperature of 750 ° C. or higher. Further, a steel plate having a surface roughness Ra of 0.6 μm (symbol:
■) has a rating of 5 even at a normal annealing temperature of 710 to 730 ° C.
Therefore, it is possible to reliably obtain a hot-dip Al-Zn alloy plated steel sheet having an excellent surface appearance.

The reason why the lateral wrinkle defect can be reduced by the above conditions is considered as follows.

When the surface roughness of the original plate before plating becomes large, the amount of rolling oil components such as ester adhered to the surface of the steel plate during cold rolling increases, and the steel strip is reduced in the non-oxidizing atmosphere of the hot dip galvanizing line. However, many elements such as C and O remain on the surface of the steel strip. This steel strip penetrates into the plating bath and melts Z
When reacting with n-Al, the growth of the Fe-Al alloy layer differs depending on the density of residual elements such as C and O, and appears as a horizontal wrinkle-like uneven pattern parallel to the width direction of the steel strip during wiping.

By setting the annealing temperature to 750 ° C. or higher, the amount of elements such as C and O remaining after the reduction is reduced.
Horizontal wrinkle defects can be reduced.

When the surface roughness Ra of the steel sheet becomes 0.5 μm or less, the amount of rolling oil attached decreases, and C, O is formed on the surface of the steel strip after reducing the steel strip in the non-oxidizing atmosphere of the hot dip galvanizing line.
The remaining of elements such as, and the like, the growth of the Fe-Al alloy layer that occurs when the steel strip enters the plating bath becomes uniform,
It is possible to prevent parallel wrinkle-like uneven patterns. If the steel sheet is alkali-cleaned before plating, the amount of rolling oil adhered is reduced, so that the same effect as when the surface roughness of the steel sheet is reduced is obtained.

When at least two conditions of annealing at 750 ° C. or higher, surface roughness Ra of the steel sheet of 0.5 μm or less, and alkali cleaning are satisfied, the effect of improving the surface appearance is obtained. It is possible to stably obtain an excellent surface appearance which is particularly excellent for high gloss coating applications.

The surface roughness Ra of the original plate before plating can be set to 0.5 μm or less by adjusting the surface roughness of the work roll used in the final rolling stand in cold rolling. The cleaning conditions for the alkaline cleaning are not particularly limited, and the cleaning conditions generally used may be used.

[0030]

Example A cold-rolled steel sheet is as-rolled or after alkali cleaning in a cleaning line, charged into a continuous hot-dip galvanizing equipment shown in FIG.
In the radiant tube type heating furnace 4, the rolling oil was removed, the surface oxides were reduced, and simultaneously annealed. Next, cooling furnace 5, snout 6
Al-containing 4.5 wt% Al in the hot dip bath 7
Zn-based alloy plating is performed, the amount of deposited plating is adjusted to a predetermined amount by the gas wiping device 9, and then cooled, and molten Al-
A Zn-based alloy-plated steel sheet was manufactured and the occurrence of lateral wrinkle defects was investigated. Table 1 shows the manufacturing conditions and the survey results.

[0031]

[Table 1]

The steel sheet of the conventional example has a lateral wrinkle defect rating of 1 to
It is 2 .

The steel sheet of Inventive Example 1 satisfying the constitutional requirements of the first invention of the present invention has a score of lateral wrinkle defect of 4 or more, and in particular, has a surface roughness Ra of 0.5 μm or less and is alkali-cleaned No. The steel plate of No. 9 has a lateral wrinkle defect rating of 5, and the surface appearance is further excellent.

The steel sheet of Inventive Example 2 satisfying the constituent requirements of the second invention of the present invention has a lateral wrinkle defect score of 5, and has an excellent surface appearance.

[0035]

EFFECTS OF THE INVENTION According to the present invention, in particular, the adhesion amount is 100 g.
It is possible to prevent the occurrence of horizontal wrinkle defects peculiar to the molten Al-Zn alloy plated steel sheet containing 4 to 5 wt% of Al of 4 / m ² or more. Molten Al-Zn alloy coated steel sheet produced by the present invention, since superior in Table Mengaikan, is particularly suitable for high gloss coating applications.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an annealing temperature and a degree of a horizontal wrinkle defect.

FIG. 2 is a diagram showing a relationship between a rolling oil residual amount of a steel sheet before plating and a degree of a lateral wrinkle defect after plating.

FIG. 3 is a diagram showing a relationship between a surface roughness Ra of an original plate before plating and a residual amount of rolling oil.

FIG. 4 is a diagram showing a main part of a hot dip galvanizing facility used in an example of the present invention.

[Explanation of symbols]

1 steel plate 2 Annealing equipment 3 non-oxidizing furnace 4 Radiant tube type heating furnace 5 cooling furnace 6 snouts 7 Hot dip bath 8 sink rolls 9 Gas wiping nozzle 10 plating bath

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Claims (2)

(57) [Claims] 1. A surface roughness Ra of 0.5 μm or less or /
After annealing the steel sheet that has been alkali washed, Al
A method for producing a hot-dip Al-Zn alloy-plated steel sheet having an excellent surface appearance, which comprises performing hot-dip Al-Zn-based alloy plating containing 5 wt%. 2. The surface roughness Ra is 0.5 μm or less or /
And a hot-dip Al-Zn alloy plated steel sheet having an excellent surface appearance, characterized by performing hot-dip Al-Zn alloy plating containing Al in an amount of 4 to 5 wt% after annealing the steel sheet subjected to alkali cleaning at an annealing temperature of 750 ° C or higher. Production method.