JP3334522B2 - Al-containing hot-dip galvanized steel sheet excellent in spangle uniformity and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing Al from 20 to 95%.
wt% galvanized steel sheet and its manufacturing method, more specifically, Al having excellent spangle uniformity of 20 to
The present invention relates to a hot-dip galvanized steel sheet containing 95 wt% and a method for producing the same.

[0002]

2. Description of the Related Art Al: 55 wt%, Si: 1.5 wt%
Al-containing hot-dip galvanized steel sheet (hereinafter, Al-containing hot-dip galvanized steel sheet) typified by an Al-containing hot-dip galvanized steel sheet (hereinafter, referred to as a 55% Al-containing hot-dip galvanized steel sheet) substantially including Zn before and after Hot-dip galvanized steel sheets) exhibit excellent corrosion resistance, and their use has been expanding in recent years.

[0003] A hot dip galvanized steel sheet containing Al is hot rolled and then pickled and descaled, or a hot rolled steel sheet obtained by further cold rolling is used as a continuous hot-dip galvanized steel sheet as shown in FIG. The equipment is charged and manufactured as follows.

[0004] The steel sheet 1 first enters an annealing furnace 2 maintained in a reducing atmosphere, and is heated to a predetermined temperature to remove rolling oil and the like adhering to the steel sheet surface at the same time as annealing, and to reduce and remove an oxide film. Then, the lower end passes through the snout 3 immersed in the plating bath 5 and is immersed in the plating bath 5 containing molten zinc containing a predetermined amount of Al. The steel sheet 1 that has been subjected to a predetermined Al-containing hot-dip galvanizing treatment in the plating bath 5 is pulled up above the plating bath 5 via the sink roll 6 and then is moved from the wiping nozzle 7 disposed on the plating bath to the surface of the steel sheet 1. A pressurized gas is sprayed to adjust the amount of plating adhesion, and cooled by the cooling device 8 to obtain an Al-containing hot-dip galvanized steel sheet on which a predetermined plating film has been formed.

[0005] In continuous hot-dip plating equipment, operating conditions such as heat treatment conditions in an annealing furnace, atmosphere conditions in the annealing furnace, plating bath composition, and cooling rate after plating are required to ensure required plating quality and material. It is managed with high accuracy within a predetermined management range.

[0006] A pattern called a spangle corresponding to the solidification structure of the plated molten metal appears on the surface of the plating film of the Al-containing hot-dip galvanized steel sheet manufactured as described above.

A spangle of an Al-containing hot-dip galvanized steel sheet represented by a 55% Al-containing hot-dip galvanized steel sheet has a beautiful glossy appearance. This beautiful glossy appearance has value as a product with design.

[0008]

When the spangle size of an Al-containing hot-dip galvanized steel sheet changes, the glossy surface appearance of the steel sheet is significantly impaired.
Therefore, in order to obtain a beautiful surface appearance, it is necessary that the fluctuation of the spangle size be small.

However, in general, the spangle size of the Al-containing hot-dip galvanized steel sheet depends on the base steel sheet, the plating bath composition,
Since it changes depending on many factors such as the cooling rate after plating, the size of the spangle greatly fluctuates.

[0010] Even if a base steel sheet of the same dimensions manufactured under a certain condition is charged and the operating conditions in the hot-dip galvanizing equipment described above are accurately controlled, if the charging coils are different,
Alternatively, even in the same coil, if the position in the longitudinal direction or the width direction in the coil is different, the size of the spangle after plating may vary significantly. For this reason, even if the steel plates have the same dimensions, a large change in the size of the spangle is inevitable, resulting in a steel plate having a poor surface appearance, which significantly reduces the commercial value.

In general, it is considered that the size of spangles is determined mainly by the state of nucleation in the solidification process of molten metal adhered to the surface of a steel sheet and the growth of dendrites. The factors that affect the phenotype have not been elucidated so far.

Spangles are also found in general hot-dip galvanized steel sheets containing about 0.3 wt% or less of Al (hereinafter referred to as hot-dip galvanized steel sheets). In the case of hot-dip galvanized steel sheet, the spangle itself has Al
It does not have the commercial value of a hot-dip galvanized steel sheet, but rather, for example, a method for reducing the amount of Pb contained as an impurity in a plating bath described in JP-A-54-132436 or JP-B-51-12579. As shown in the method of spraying water mist before the described plating film solidifies, spangles are eliminated to enhance the design of the surface appearance, and the purpose is to eliminate spangles No attempt has been made to stably obtain a spangle of a size as large as possible. Therefore, the technique disclosed for the hot-dip galvanized steel sheet cannot provide useful suggestions for improving the variation in spangle size of the Al-containing hot-dip galvanized steel sheet.

[0013] Therefore, a technique capable of reducing the variation in spangle size as seen in an Al-containing hot-dip galvanized steel sheet has not been developed at present.

The present invention has been made in view of such circumstances, and by appropriately controlling the coiling temperature of a steel sheet during hot rolling according to the composition of the steel, the structure of the steel sheet after plating is improved. By controlling, the variation of the spangle size between the coils and within the coil is reduced, and the spangle uniformity is excellent.
It is an object of the present invention to provide an l-containing hot-dip galvanized steel sheet and a method for producing the same.

[0015]

Means for Solving the Problems The present inventors have conducted intensive studies on the relationship between the solidification phenomenon of the coating film of the Al-containing hot-dip galvanized steel sheet and the spangle or the production factor that affects the size of the spangle.

As a result, the Al content of the plating film was 20
At less than wt%, spangles are formed corresponding to the dendrite structure in which Zn forms a solid solution of Al and solidified, and when the Al content is 20 wt% or more, spangles form a dendrite structure in which Al forms a solid solution of Zn and solidifies. It was found to generate correspondingly.

The spangle produced when the Al content is 20 wt% or more has a beautiful glossy appearance.
The spangle produced when the Al content was less than 20% by weight did not have such a beautiful glossy appearance even if the spangle was uniform. In other words, different metal solidification structures are formed when the Al content is 20 wt%,
It has been found that even with the same spangle, the properties of the surface appearance after plating are significantly different depending on the solidification structure.

It has also been found that when the Al content is 20 wt% or more, spangles tend to be non-uniform and the surface appearance is more likely to be impaired than when the Al content is less than 20 wt%. .

Further, since the size of the spangle after plating may differ significantly from one coil to another, fluctuations in the steel plate structure and surface condition of the base steel sheet to be hot-dip affect the size of the spangle. It was possible. Therefore, when examining the relationship between the steel sheet structure and surface state and the fluctuation state of the spangle size, there is a close relationship between the steel sheet structure at the time of plating and the spangle size of the Al-containing hot-dip galvanized steel sheet. Content of Al and N
By controlling the hot-rolling winding temperature to an appropriate range according to the amount,
It is possible to control the structure of the steel sheet during plating,
It has been found that the variation in the spangle size of a hot-dip galvanized steel sheet between and within coils can be significantly reduced.

The present invention has been made based on this finding, and the features of the invention are as follows .

According to the first invention, the surface of the steel sheet rolled within a range in which the hot-rolling winding temperature T (° C.) and the Al and N in the steel sheet satisfy the following formula ( 6 ) is Al-containing containing 20 to 95 wt% of Al. Al-containing hot-dip galvanized steel sheet having a hot-dip galvanized film and manufactured such that the average spangle diameter d (mm) of the surface of the plated film and the cooling rate C (° C./sec) after the plating satisfy the following formula ( 7 ). An Al-containing hot-dip galvanized steel sheet having excellent spangle uniformity, wherein the spangle uniformity index is in the range of 1.0 to 1.5.

According to a second aspect of the present invention, a hot-rolled steel sheet is wound within a range in which a winding temperature T (° C.) and Al and N in the steel sheet satisfy the following formula ( 6 ), and after pickling or further cold rolling. Thereafter, the method is a method for producing an Al-containing hot-dip galvanized steel sheet having excellent spangle uniformity, wherein the hot-dip galvanizing steel sheet is charged into a continuous hot-dip galvanizing facility to perform Al-containing hot-dip galvanizing containing 20 to 95 wt% of Al.

According to a third aspect of the present invention, a hot-rolled steel sheet is wound within a range in which a winding temperature T (° C.) and Al and N in the steel sheet satisfy the following formula ( 6 ), and after pickling or further cold rolling. After that, it is charged into a continuous hot-dip galvanizing equipment to perform hot-dip galvanizing containing Al containing 20 to 95 wt% of Al, and the cooling rate C (° C./sec) after plating and the average spangle diameter d of the plating film surface.
A method for producing an Al-containing hot-dip galvanized steel sheet having excellent spangle uniformity, characterized in that an Al-containing hot-dip galvanized steel sheet is manufactured so that (mm) satisfies the following formula ( 7 ).

However, T ≦ (1.36-13.5 × A-150 × N) × 10 ³ ... ( 6 ) 0.9-0.13 × (C-9) ^1/2 ≦ d ≦ 1.15-0.13 × (C-9) ^1/2 ( 7 ) A: Al content in steel sheet (wt%) N: N content in steel sheet (wt%) C: Cooling rate (° C./sec) from the time when the steel sheet leaves the plating bath until the solidification of the plating film is completed. The average spangle diameter d and the spangle uniformity index are arbitrarily 10 from a steel sheet sample of 10 cm × 10 cm.
When a location was selected and the number of spangles crossing the length of 1 cm was counted, the average spangle diameter d (mm) = 10
It is defined as 0 / "total of 10 counts" and spangle uniformity index = "maximum count value" / "minimum count value".

The spangle uniformity index is an index indicating local spangle variation in the same coil. If this value is in the range of 1.0 to 1.5, it looks visually uniform. However, if this value exceeds 1.5, the variation of spangle becomes apparent.

(Operation) The reasons for limiting the present invention will be described below.

[0027] The Al content in the plating film must be 20 wt% or more and 95 wt% or less. Al content is 20
When the content is not less than wt%, the spangle has a beautiful luster, but when the Al content is less than 20 wt%, the spangle having the beautiful luster aimed at by the present invention cannot be obtained. If the Al content exceeds 95% by weight, the spangle becomes close to the spangle of the Al plating, and the spangle does not clearly appear on the surface of the steel sheet, so that the beautiful glossy surface intended by the present invention cannot be obtained.

The steel winding temperature after hot rolling, in order to reduce the variation magnitude of the spangle through the control of steel sheet structure of the plating, in response to said Al content and N content in the steel plate (6 ) It is necessary to satisfy the range of the expression.

An investigation was made on the relationship between the spangle size of the Al-containing hot-dip galvanized steel sheet and the beauty of the surface appearance of the steel sheet. As a result, the variation width of the average spangle diameter d defined above was within 0.25 mm. If the spangle uniformity index is in the range of 1.0 to 1.5, the uniformity of the surface appearance required from a practical viewpoint is not impaired, and the variation width of the average spangle diameter d is large. When the value exceeds 0.25 mm or the spangle uniformity index exceeds 1.5, the spangle size greatly fluctuates, and the surface appearance is impaired.

Therefore, in order to improve spangle uniformity, it is necessary to consider both the variation in the average spangle diameter and the spangle uniformity index.

Further, there is a close relationship between the size of the spangle of the Al-containing hot-dip galvanized steel sheet and the structure of the steel sheet after plating, particularly, the crystal grain size and the surface oxide. Becomes large and the spangle becomes small when the crystal structure is small.
It was found that the size of the spangle of the l-containing hot-dip galvanized steel sheet was greatly affected by the change in the crystal grain size of the steel sheet and the change in the surface oxide structure.

Further, paying attention to the fact that the steel sheet structure such as the crystal grain size and the surface oxide structure changes depending on the component composition of the base steel sheet, the manufacturing conditions, and the manufacturing conditions in the pre-plating process, the component composition of the base steel sheet is considered. And the relationship between manufacturing conditions and spangle size.

As a result, the amount of Al, the amount of N and the hot-rolling temperature contained in the steel sheet have a great influence on the fluctuation of the spangle size, and the winding temperature falls within a range satisfying the above-mentioned formula ( 6 ). In this case, the variation of the spangle size of the plating film manufactured under the same cooling condition can be within 0.25 mm and the spangle uniformity index can be 1.0 to 1.5, which is a practical viewpoint. From the results, it was found that the required beautiful surface appearance can be secured.

By controlling the hot-rolling coiling temperature in a range satisfying the above equation ( 6 ) in accordance with the amounts of Al and N contained in the steel sheet, fluctuations in spangle size are reduced, and spangle uniformity is reduced. The reason for becoming better is estimated as follows.

The grain boundaries of the steel sheet are formed in the plating bath /
It is a place where the reaction between steel sheets is active and solidification nuclei of spangles are easily formed. If the coagulation nucleus is large, the spangle becomes small, and if the coagulation nucleus is small, the spangle becomes large. If the crystal grains are small, the grain boundary density increases, so that the solidification nuclei also increase and the spangle becomes small. If the crystal grains are large, the grain boundary density decreases, so that the solidification nuclei decrease and the spangle becomes large.

[0036] N in the steel sheet is converted to Al in the hot rolling step.
To form AlN. When the amount of N contained in the steel sheet changes, the amount of AlN formed changes. In addition, even if the hot-rolling temperature changes, the amount of AlN formed changes. As the amount of N in the steel sheet increases and the hot-rolling winding temperature increases, the amount of AlN formed increases. In the annealing process in the continuous hot-dip plating equipment, AlN has an effect of hindering the grain growth of the steel sheet, so that the size of the crystal grains changes according to the amount of AlN formed.

If the winding temperature is too high with respect to the amount of N in the steel sheet, the amount of AlN formed is large, which hinders grain growth of the steel sheet and reduces spangles.

It is considered that the oxide on the surface of the steel sheet acts as a solidification nucleus of spangles. If the surface oxide is large, the solidification nuclei increase and the spangle becomes small, and if the surface oxide is small, the solidification nuclei decrease and the spangle becomes large.
Also, non-uniformly formed surface oxides increase the spangle uniformity index.

Al in the steel sheet is selectively oxidized during hot rolling and is likely to form an uneven oxide such as Al ₂ O _{3 on the} surface of the steel sheet. When the amount of Al contained in the steel sheet changes, the amount of the oxide and the degree of its non-uniformity change, and as the amount of Al increases, the amount of the oxide and the degree of its non-uniformity increase. In addition, even if the hot-rolling temperature changes, the amount of oxide on the steel sheet surface and the degree of non-uniformity change, and as the hot-rolling temperature increases, the amount of oxide and the degree of non-uniformity increase. Even if the amount of Al contained in the steel sheet changes, the amount of the oxide and the degree of non-uniformity change, and the amount of the oxide and the degree of non-uniformity increase with an increase in the amount of Al.

If the winding temperature is too high with respect to the amount of Al in the steel sheet, a large amount of oxides such as Al ₂ O ₃ are formed and the non-uniformity increases, and the spangles are fine and have a large spangle uniformity index. become.

The coiling temperature of the steel sheet after hot rolling was adjusted to A
By controlling the amount in the range of the formula ( 6 ) according to the amount of l and the amount of N, the variation in the crystal grain size of the base steel sheet is reduced, and Al in the base steel sheet is selectively oxidized so that the steel sheet is selectively oxidized. It is considered that as a result of unevenly forming oxides such as Al ₂ O ₃ on the surface, the variation in spangle size is small and the spangle uniformity index is small.

The Al content and the N content in the steel sheet are more preferably in the following ranges. Al: 0.005 to 0.0
30 wt%. It is preferable to add 0.005 wt% or more for deoxidation during melting. In addition, the addition amount of Al is set to 0.0
When the content is 30 wt% or less, the surface properties of the steel sheet are further stabilized, and the uniformity of spangles becomes more excellent.

N: 0.0035 wt% or less. When the amount of N is 0.
When the amount is less than 0035 wt%, the amount of AlN formed is reduced and the variation of the crystal grains of the steel sheet is reduced, so that the spangle uniformity becomes more excellent.

In the present invention, components other than Al and N do not essentially affect the effects of the present invention. However, when the amount of Si increases, plating defects easily occur. Si: 0.03 wt% or less is preferable. In addition, in order to soften the steel sheet material, C: 0.
05 wt% or less, Mn: 0.30 wt% or less, P: 0.
It is preferable that the content is 03 wt% or less and S: 0.03 wt% or less.

The size of the average spangle diameter is affected by the cooling rate after plating, and is small when the cooling rate is high and large when the cooling rate is low. Therefore, in advance, the relationship between the cooling rate and the average spangle diameter is determined,
By determining an appropriate range of the spangle diameter in accordance with the cooling rate in accordance with the obtained relationship, the fluctuation of the spangle size can be reduced. Cooling rate after plating C
(° C.) and the spangle diameter d (mm) are manufactured so as to satisfy the above equation ( 7 ), the fluctuation width of the spangle size of the plating film manufactured under the same cooling condition is 0.25 mm.
Within and the spangle uniformity index is 1.0 ~
The range is 1.5, and the uniformity of spangles necessary for ensuring a beautiful surface appearance necessary for practical use can be ensured.

FIG. 1 shows a region satisfying the above expression ( 7 ). In FIG. 1, a region sandwiched between two solid lines indicates a region that satisfies the expression ( 7 ). The midpoint of the region satisfying the above equation ( 7 ) is represented by the following equation ( 8 ). d = 1.025-0.13 x (C-9) ^1/2 ... ( 8 )

Therefore, A of a predetermined average spangle diameter
When manufacturing a l-containing hot-dip galvanized steel sheet, the above ( 8 )
By cooling at a cooling rate determined from the formula, an Al-containing hot-dip galvanized steel sheet excellent in spangle uniformity with a predetermined spangle diameter can be obtained.

[0048]

BEST MODE FOR CARRYING OUT THE INVENTION The steel sheet of the present invention is prepared by casting a steel melted by a conventional method under ordinary conditions and hot rolling, and then setting the steel sheet winding temperature according to the Al content and the N content in the steel sheet. Winding is performed so as to satisfy the above formula ( 6 ), then, after pickling, or after further cold rolling, is charged into a continuous hot-dip galvanizing apparatus to perform hot-dip galvanizing containing 20 to 95 wt% of Al.

The average spangle diameter d (m) of the plating film surface
m) and the cooling rate C (° C./sec) after plating satisfying the above formula ( 7 ), the variation of the spangle diameter of the plating film at the same cooling rate is within 0.25 mm and the spangle uniformity. Index is 1.0 to 1.5
And the uniformity of the spangle diameter necessary for ensuring a practically necessary uniform surface appearance can be ensured in practical use.

[0050]

EXAMPLE Steel slabs having the composition shown in Table 1 and the balance of Al and N containing Fe and unavoidable impurities and having different contents were rolled at different temperatures after hot rolling by a conventional method, pickled,
A cold-rolled steel sheet having a thickness of 0.27 to 1.6 mm obtained by cold rolling was charged into a continuous hot-dip galvanizing apparatus to produce an Al-containing hot-dip galvanized steel sheet.

In the hot-dip galvanizing equipment, 55
Al-containing hot-dip galvanizing is performed by a plating bath composed of wt% Al-1.5 wt% Si-balance substantially consisting of Zn, and a plating film having an adhesion amount of 150 to 200 g / m ² is formed on both surfaces, and cooled after plating. The speed was varied.

The average spangle diameter on the surface of the plating film of the Al-containing hot-dip galvanized steel sheet obtained above was examined. Table 1
In addition, the results of investigation of the hot rolling temperature, the cooling rate after plating, and the average spangle diameter are also described. In Table 1, (7) a range with a lower limit of, column upper limit, respectively (7) of 0.9-
013 × (C-9) ^1/2 , 1.15-013 × (C−
9) ^Shows the calculated value of ^1/2 .

[0053]

[Table 1]

FIG. 1 shows the relationship between the cooling rate and the average spangle diameter of the Al-containing hot-dip galvanized steel sheet obtained above, and FIG. 2 shows the relationship between the steel sheet winding temperature and the spangle uniformity index in this embodiment. .

In FIG. 1, a region sandwiched between two solid lines indicates a region satisfying the above equation ( 7 ). Also, in FIG.
The variation range of the average spangle diameter in the conventional Al-containing hot-dip galvanized steel sheet is shown for reference by two broken lines.

In the steel sheet of the present invention satisfying the present invention, the variation of the average spangle diameter at the same cooling rate is 0.25 mm or less, and the average spangle diameter and the cooling rate after plating satisfy the formula ( 7 ). It is within the range, and the spangle uniformity index is 1.5 or less, and the spangle uniformity is excellent.

On the other hand, in the steel sheet of the comparative example, the variation of the average spangle diameter at the same cooling rate exceeded 0.25 mm or the spangle uniformity index was more than 1.5. The speed is out of the range of the above formula ( 7 ), and the spangle uniformity is poor.

The steel sheet of the present invention obtained as described above has excellent surface appearance because the surface appearance does not differ due to the fluctuation of the spangle diameter.

In this embodiment, the steel sheet after cold rolling is used as the base steel sheet. However, similar results can be obtained with a hot-rolled steel sheet that has been descaled after hot rolling.

[0060]

As described above, according to the present invention, an Al-containing hot-dip galvanized steel sheet having excellent spangle uniformity and excellent glossy surface appearance can be obtained.

The steel plate of the present invention has a design property in spangle itself and is excellent in surface appearance, so that it is suitable for use in unpainted applications where this beauty can be utilized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a cooling rate and a spangle diameter.

FIG. 2 is a diagram illustrating a relationship between a steel sheet winding temperature and a spangle uniformity index.

FIG. 3 is a diagram showing a main part of a hot-dip galvanizing facility.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Annealing furnace 3 Snout 4 Plating tank 5 Plating bath 6 Sink roll 7 Wiping nozzle 8 Cooling device

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-9-206831 (JP, A) JP-A-8-100217 (JP, A) JP-A-7-268486 (JP, A) JP-A 9-208 176816 (JP, A) JP-A-9-118970 (JP, A) JP-A-62-23976 (JP, A) (58) Fields studied (Int. Cl. ⁷ , DB name) C23C 2/00-2 / 40

Claims (3)

(57) [Claims] 1. A surface of a steel sheet that has been rolled up so that the hot-rolling winding temperature T (° C.) and Al and N in the steel sheet satisfy the formula ( 1 ),
having an Al-containing hot-dip galvanized film containing 20 to 95 wt% of 1 and an average spangle diameter d (mm) of the surface of the plated film
And an Al-containing hot-dip galvanized steel sheet manufactured so that the cooling rate C (° C./sec) after plating satisfies the formula ( 2 ), wherein the spangle uniformity index is in the range of 1.0 to 1.5. An Al-containing hot-dip galvanized steel sheet having excellent spangle uniformity. T ≦ (1.36-13.5 × A-150 × N) × 10 ³ ( 1 ) where A: Sol. Al content (wt%) N: N content (wt%) contained in the steel sheet 0.9−0.13 × (C-9) ^1/2 ≦ d ≦ 1.15-0.13 × (C-9) ^1/2 ) ( 2 ) where C is the cooling rate (° C / sec) from the time the steel sheet leaves the plating bath until the solidification of the plating film is completed. The average spangle diameter d and the spangle uniformity index are 10 cm. Arbitrarily 10 from a × 10 cm steel plate sample
When a location was selected and the number of spangles crossing the length of 1 cm was counted, the average spangle diameter d (mm) = 10
It is defined as 0 / "total of 10 counts" and spangle uniformity index = "maximum count value" / "minimum count value". 2. A hot-rolled steel sheet is taken up at a winding temperature T (° C.).
And the steel sheet is wound up in a range in which Al and N satisfy the formula ( 3 ).
After the pickling or further cold rolling, the Al-containing hot-dip galvanizing is carried out in a continuous hot-dip galvanizing facility, and the Al-containing hot-dip galvanizing containing 20 to 95 wt% of Al is performed. Manufacturing method of plated steel sheet. T ≦ (1.36-13.5 × A-150 × N) × 10 ³ ( 3 ) where A: amount of Al contained in the steel sheet (wt%) N: amount of N contained in the steel sheet (Wt%) 3. A hot-rolled steel sheet is taken up at a winding temperature T (° C.).
And the steel sheet is wound up in a range where Al and N in the steel sheet satisfy the formula ( 4 ).
After pickling or further cold rolling, it is charged into a continuous hot-dip galvanizing equipment to perform Al-containing hot-dip galvanizing containing 20 to 95 wt% of Al. Average spangle diameter d (mm) of ( 5 )
A having excellent spangle uniformity, characterized by producing an Al-containing hot-dip galvanized steel sheet so as to satisfy the formula.
Production method of l-containing hot-dip galvanized steel sheet. T ≦ (1.36-13.5 × A−150 × N) × 10 ³ ( 4 ) where A: Al amount (wt%) contained in the steel sheet N: N amount contained in the steel sheet (Wt%) 0.9−0.13 × (C-9) ^1/2 ≦ d ≦ 1.15-0.13 × (C-9) ^1/2 ( 5 ) where C: steel sheet Cooling rate (° C./sec) from the time the steel sheet leaves the plating bath until the solidification of the plating film is completed. The average spangle diameter d (mm) = 100 / “total of the counts at 10 locations”
Is defined.