JPH11200002A - Production of hot dip al-zn base alloy coated steel sheet excellent in surface appearance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot dip Al-Zn base alloy coated steel sheet containing 4-5 wt.% Al and having excellent surface appearance. SOLUTION: After annealing the steel sheet at >=750 deg.C annealing temp., the hot dip Al-Zn base alloy coating containing 4-5 wt.% Al is applied. Otherwise, after annealing the steel sheet having <=0.5 μm surface roughness Ra and/or washed with alkali, the hot dip Al-Zn base alloy coating containing 4-5 wt.% Al is applied. Uneven growth of coated alloy layer caused by rolling oil residue is prevented, and traverse wrinkle defect peculiar to the hot dip Al-Zn base alloy coated steel sheet containing 4-5 wt.% Al can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing Al by adding 4 to 5 wt.
The present invention relates to a method for producing an Al-Zn-based hot-dip coated steel sheet having excellent surface appearance, and particularly to a method for manufacturing an Al-Zn-based hot-dip coated steel sheet having 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
A steel alloy-plated (hereinafter, simply referred to as a hot-dip Al-Zn-based alloy) steel sheet is more excellent in corrosion resistance than a hot-dip galvanized steel sheet, and thus its use in building materials is increasing. In these applications, some are used naked, but many are used for painting.

[0003] A hot-dip Al-Zn alloy-plated steel sheet is usually charged into a continuous hot-dip galvanizing equipment as shown in FIG. 2, the rolling oil is removed, the surface oxides are reduced in the non-oxidizing furnace 3 to the radiant tube heating furnace 4, the surface oxides are reduced, and annealing is performed at the same time. Al--Zn alloy plating containing 4.5 wt% is performed, adjusted to a predetermined plating adhesion amount by a gas wiping device 9, and then cooled to manufacture.

[0004] Hot-dip Al-Zn alloy-coated steel sheets are more susceptible to plating defects than hot-dip galvanized steel sheets.
Poor surface appearance. Fused Al-Zn with poor surface appearance
When painted on a base alloy-plated steel sheet, poor surface appearance due to plating property defects after coating becomes more noticeable. In particular, in the case of a high-gloss coated steel sheet, the above-described poor surface appearance becomes more conspicuous.

[0005] 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 coating. Therefore, melting A
For coated steel sheets made from l-Zn alloy plated steel sheets, in order to make the surface appearance defects after coating less noticeable,
Aggregates and the like are blended to form a low-gloss coating film.

[0006] However, with the expansion of applications of hot-dip Al-Zn alloy plated steel sheets, high gloss coated steel sheets are sometimes required from the viewpoint of design. Accordingly, there is a need for a hot-dip Al-Zn-based alloy-plated steel sheet capable of stably obtaining an excellent surface appearance even with a high gloss coated steel sheet.

[0007] Plating defects generated in the hot-dip coated steel sheet include the adhesion of plated metal oxide to the steel sheet, which occurs when controlling the amount of adhesion using a gas wiping nozzle.
Alternatively, fumes of the plating metal evaporated from the plating bath surface in the snout or a compound thereof (such as an oxide) adhere to the steel strip.

[0008] As for the former, a method of sealing the peripheral portion of the gas wiping nozzle from the plating bath surface and performing gas wiping in a non-oxidizing atmosphere to prevent the generation of oxidized dross (Japanese Patent Application Laid-Open No. 8-337863). For the latter, a gas containing fumes and compounds of the plating metal evaporated in the snout is sucked out of the snout, and after removing the metal fumes and the compounds from the gas sucked by the metal fume or compound removal device, A number of defect prevention methods have been proposed, such as a method of returning a cleaned gas into a snout (Japanese Patent Laid-Open No. 6-306555).
With respect to the 1-Zn-based alloy-plated steel sheet as well, it is possible to prevent poor surface appearance due to such defects.

[0009]

However, molten Al-Z
The n-type alloy plated steel sheet has a coating weight on one side of 100
g / m ² or more, a specific defect on the steel sheet surface after plating, that is, a horizontal wrinkle-like uneven pattern parallel to the steel sheet width direction (hereinafter, referred to as a horizontal wrinkle defect) occurs, There is a problem that the surface appearance becomes poor. Heretofore, no method for preventing this type of plating defect has been studied.

[0010] It is an object of the present invention to provide a method for producing a hot-dip Al-Zn-based alloy-plated steel sheet which prevents the lateral wrinkle defect and has excellent surface appearance.

[0011]

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

(1) After a steel sheet is annealed at an annealing temperature of 750 ° C. or more, a molten Al-Zn alloy containing 4 to 5 wt% of Al is plated to obtain a molten steel having an excellent surface appearance.
Manufacturing method of 1-Zn alloy plated steel sheet (first invention).

(2) Surface appearance characterized by subjecting a steel plate having a surface roughness Ra of 0.5 μm or less and / or an alkali-washed steel sheet to plating with a molten Al—Zn alloy containing 4 to 5 wt% of Al. Of producing a hot-dip Al-Zn-based alloy-plated steel sheet having excellent heat resistance (second invention).

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

In the first invention, the reduction of the steel strip is enhanced,
The rolling oil residue is reduced, and the surface appearance can be improved. Second
In the invention, since the residual amount of the rolling oil is small, the influence of the residual rolling oil component after annealing is reduced, and the surface appearance can be improved even at a normal annealing temperature. In the third invention, since the steel sheet with a small amount of the rolling oil residue is annealed while strengthening the reduction, the above-mentioned effect is further improved.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below.

FIG. 1 shows that cold-rolled steel sheets having different surface roughnesses were annealed at different annealing temperatures, and the amount of plating applied was 100 to 1 on one side.
It is a figure which shows the relationship between an annealing temperature and a lateral wrinkle defect about the case where 35g / m < ² > hot-dip Al-Zn type alloy plating is performed. For lateral wrinkle defects, visually observe the steel plate surface after plating,
The degree of defect was evaluated on a five-point scale according to the following criteria. In order to obtain a good surface appearance with a coated steel sheet using a high gloss paint, it is desirable that the rating is 4 or more, more preferably 5.

1: Those with remarkable lateral wrinkle defects 2: Those with clear lateral wrinkle defects 3: Those with light lateral wrinkle defects 4: Those with extremely light horizontal wrinkle defects 5: Lateral wrinkles No defects found

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

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 coated with a molten Al—Zn alloy having a coating weight of 110 to 135 g / m ^{2 on} one side. FIG. 4 is a diagram showing a relationship between a residual amount of rolling oil and a lateral wrinkle defect generated after plating. If the residual amount of rolling oil in the steel sheet before plating is reduced, the lateral wrinkle defect becomes lighter,
It can be seen that when the residual amount of the rolling oil is 250 mg / m ² or less, the score of the lateral wrinkle defect can be made 4 to 5.

FIG. 3 shows the surface roughness Ra of the steel sheet after cold rolling.
FIG. 4 is a diagram showing a relationship between the amount of residual rolling oil. In the case of a cold-rolled steel sheet (normal material, symbol: ○), reducing the surface roughness reduces the amount of rolling oil residue. Therefore, by reducing the surface roughness of the plating base sheet, the amount of rolling oil remaining can be suppressed, and lateral wrinkle defects can be prevented. The surface roughness Ra is set to 0.
When the thickness is 5 μm or less, the rolling oil residue is reduced to 250 mg / m, which gives a surface appearance with a rating of 4 to 5 required for high gloss coating applications.
² or less. Further, the steel sheet (symbol: ●) that has been subjected to alkali cleaning after cold rolling has a large effect of preventing lateral wrinkle defects since the residual amount of rolling oil is further reduced.

Reducing the surface roughness of the plating base sheet;
FIG. 1 shows the effect of improving the surface appearance due to the alkali cleaning. In FIG. 1, the surface roughness Ra is 0.5
The as-rolled steel sheet (symbol: □) having a size 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 more.

A steel sheet of normal roughness (symbol: ●) which has been alkali-cleaned 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 more. Further, a steel plate having a surface roughness Ra of 0.6 μm (symbol:
(3) is 5 even at the normal annealing temperature of 710 to 730 ° C.
Thus, a hot-dip Al-Zn-based alloy-plated steel sheet having an excellent surface appearance can be reliably obtained.

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

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

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

When the surface roughness Ra of the steel sheet is 0.5 μm or less, the amount of the rolling oil is reduced, and the surface of the steel strip after reducing the steel strip in the non-oxidizing atmosphere of the hot-dip galvanizing line has C, O on the steel strip surface.
The growth of the Fe-Al alloy layer that occurs when the steel strip enters the plating bath becomes uniform,
Parallel wrinkle-like uneven patterns can be prevented. If the steel sheet is alkali-cleaned before plating, the amount of rolling oil adhering is reduced, and thus has the same effect as when the surface roughness of the steel sheet is reduced.

When at least the two conditions of annealing at 750 ° C. or more, reducing the surface roughness Ra of the steel sheet to 0.5 μm or less, and washing with alkali are satisfied, the effect of improving the surface appearance is obtained. Excellent, in particular, excellent surface appearance required for high gloss coating applications can be stably obtained.

The surface roughness Ra of the original sheet before plating can be reduced to 0.5 μm or less by adjusting the surface roughness of a work roll used for a final rolling stand in cold rolling. The washing conditions for the alkali washing are not particularly limited, and generally-used washing conditions may be used.

[0030]

EXAMPLE A cold-rolled steel sheet was rolled as it was, or after alkali washing in a washing line, charged into a continuous hot-dip galvanizing equipment shown in FIG.
In the radiation tube type heating furnace 4, the rolling oil was removed, the surface oxides were reduced, and annealing was performed at the same time. Next, the cooling furnace 5 and the snout 6
Through the hot-dip plating tank 7 containing Al: 4.5 wt%
Zn-based alloy plating is performed, adjusted to a predetermined plating adhesion amount by a gas wiping device 9, and then cooled to form a molten Al-
Zn-based alloy-plated steel sheets were manufactured, and the occurrence of lateral wrinkle defects was investigated. Table 1 shows the manufacturing conditions and the inspection results.

[0031]

[Table 1]

The conventional steel sheet has a horizontal wrinkle defect score of 1 to 1.
2. The steel sheet of Inventive Example 1, which satisfies the constitutional requirements of the first invention of the present invention, has a score of 3 or more in the result of the lateral wrinkle, and has an excellent surface appearance as compared with the conventional example.

The steel sheet of Inventive Example 2, which satisfies the constitutional requirements of the second invention of the present invention, has a rating of lateral wrinkle defects of 4 or more, and particularly has a surface roughness Ra of 0.5 μm or less, and is alkali-washed No. 2 The steel sheet No. 9 has a rating of 5 for horizontal wrinkle defects, and further has an excellent surface appearance.

The steel sheet of Inventive Example 3, which satisfies the constitutional requirements of the third invention of the present invention, has a rating of horizontal wrinkle defect of 5, and has excellent surface appearance.

[0035]

According to the present invention, in particular, the amount of adhesion is 100 g.
/ M ² or more can prevent the occurrence of lateral wrinkle defects peculiar to a hot-dip Al-Zn-based alloy-plated steel sheet containing 4 to 5 wt% of Al. The hot-dip Al-Zn-based alloy plated steel sheet produced by the present invention has an excellent surface appearance. The steel sheets of the second and third inventions are more excellent in surface appearance and are particularly suitable for high gloss coating applications.

[Brief description of the drawings]

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

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

FIG. 3 is a diagram showing the relationship between the surface roughness Ra of an original sheet before plating and the remaining 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]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Annealing equipment 3 Non-oxidizing furnace 4 Radiation tube heating furnace 5 Cooling furnace 6 Snout 7 Hot-dip plating tank 8 Sink roll 9 Gas wiping nozzle 10 Plating bath

Claims (3)

[Claims] The present invention is characterized in that a steel sheet is annealed at an annealing temperature of 750 ° C. or higher, and then subjected to hot-dip Al—Zn-based alloy plating containing 4 to 5 wt% of Al.
A method for producing a Zn-based alloy-plated steel sheet. 2. A surface roughness Ra of 0.5 μm or less or /
And after annealing the alkali-washed steel sheet,
A method for producing a hot-dip Al-Zn alloy-plated steel sheet having excellent surface appearance, characterized by applying hot-dip Al-Zn-based alloy plating containing 5 wt%. 3. A surface roughness Ra of 0.5 μm or less or /
And an alkali-washed steel sheet is annealed at an annealing temperature of 750 ° C. or higher, and then subjected to hot-dip Al—Zn-based alloy plating containing 4 to 5 wt% of Al. Production method.