KR101613354B1 - Coated steel plate and mehtod for manufacturing the same - Google Patents

Abstract

A coated steel sheet is disclosed. The coated steel sheet according to the present invention is a coated steel sheet having a plated layer formed on its surface, wherein the plated layer is composed of more than 2.3 wt% to less than 4 wt% of Al, less than 2 wt% of Mg and the balance of Zn and other inevitable impurities, Of Al / Mg ≥ 1.5.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coated steel sheet,

The present invention relates to a coated steel sheet and a method of manufacturing the same, and more particularly, to a Zn-Mg-Al-based coated steel sheet having improved surface quality and corrosion resistance of a coated steel sheet and excellent workability and a method of manufacturing the same.

The hot-dip galvanized steel sheet according to the prior art is excellent in self-sacrifice and is widely applied to building materials and household electrical appliances. In case of hot-dip galvanized steel sheet, Zn is lost as a sacrificial anode at the part exposed to corrosive environment, resulting in loss of zinc in the plating layer.

This sacrificial anodic action of zinc plays an excellent role in suppressing the generation of rust of iron in the corrosive environment, but the anode efficiency is somewhat lower. In order to solve these problems, it has been recently proposed to add Zn to Zn in Japan and Europe to produce very dense and stable oxide called Simonkolleite (Zn ₅ (OH) ₈ Cl ₂ ) Improved products are being produced.

However, the coated steel sheet to which Mg is added to Zn has some problems as follows.

As Mg is added to Zn, a brittle intermetallic compound formed between Mg and Zn causes excessive cracking on the surface during processing or peeling of the plating layer, resulting in deterioration of corrosion resistance as well as workability. Organizational control is very important.

In addition, when Mg is added to Zn, the Mg component of the molten metal reacts with oxygen in the air to form an oxide film on the surface of the molten metal. Also, in the process for controlling the thickness of the plating, the surface quality can be obtained only by controlling the deposition amount of the plating using nitrogen gas, so that the processing cost is inevitably increased due to the use of the nitrogen gas.

In addition, Mg added to the molten metal reacts with oxygen to form a large amount of oxides, which causes a large loss of subsidiary materials. In particular, since the Mg oxide film forms a porous oxide, the oxygen reacts with the molten metal continuously in contact with oxygen, and the loss of the subsidiary material due to Mg oxidation is large.

In order to minimize the loss of adhered material by Mg oxidation, elements that are oxidized before the rate at which Mg is oxidized or elements forming dense oxides should be added. Among these elements, there are fine added elements such as Sr, Ca, Li, and Be, but these fine elements generate high temperature images or have high oxygen affinity and become additional defect sources.

In case of Zn-Mg-Al plating system, corrosion resistance is generally improved by Zn and Mg acting as sacrificial and corrosion products generated in the corrosive environment. However, the corrosion product thus formed is weak to thermal shock, so that cracks are easily generated during drying, so that oxygen penetration is facilitated and further corrosion proceeds.

To minimize this phenomenon, the Al resin phase must be fully developed. In addition, Al reacts with oxygen to form oxides that are denser than Mg, so that Mg is prevented from being oxidized if a certain amount is added to the Zn-Mg molten metal. In addition, Mg has a low standard reduction potential and reacts first with moisture to basify the surface. Surface basicity differs depending on the degree of surface oxidation. Surface basicity due to surface oxidation is also a major factor in post-treatment development It is the object of control because it acts.

In order to solve the above-mentioned problems, the present invention aims to provide a coated steel sheet which is effective for improving corrosion resistance and which has excellent plating surface properties, and a method for producing the same.

The coated steel sheet according to a preferred embodiment of the present invention is a coated steel sheet having a plating layer formed on its surface, wherein the plating layer comprises less than 4 wt% of Al, less than 2 wt% of Mg and the balance of Zn and other inevitable impurities And the ratio of Al and Mg is Al / Mg? 1.5.

The oxygen concentration on the surface of the plating layer may be 1 wt% or less.

0.001 wt% to 0.2 wt% of B, 0.001 wt% to 0.2 wt% of B, 0.001 wt% to 0.5 wt% of Si, 0.001 wt% or more and 0.5 wt% or less of Ti, 0.001 wt% or less of Ti, wt% or more and 0.2 wt% or less.

A method of manufacturing a coated steel sheet according to another preferred embodiment of the present invention is characterized in that Al is contained in an amount of more than 2.3 wt%, less than 4 wt%, Mg: less than 2 wt%, and the balance of Zn and other inevitable impurities, Heating a plating composition having a Mg > = 1.5 to 430 DEG C to 500 DEG C to prepare a hot-dip coating bath; dipping the steel sheet in the hot-dip coating bath for 1 to 3 seconds to form a plating layer on the surface of the steel sheet; And cooling the steel sheet having the plated layer formed thereon to a room temperature at a cooling rate of 5 ° C / sec to 30 ° C / sec.

In the step of forming the plating layer, the plating adhesion amount may be 60 g / m ² to 300 g / m ² .

The method of manufacturing the coated steel sheet may further include air wiping using a mixed gas of air and nitrogen to control the deposition amount of the plating layer on the surface of the steel sheet.

The air in the mixed gas may be 80% or less.

Wherein the molten copper bath contains 0.001 to 0.2 wt% of Ti, 0.001 to 0.2 wt% of B, 0.0005 to 0.2 wt% of B, 0.001 to 0.2 wt% of Be, 0.001 to 1 wt% of Si, Or more and 0.5 wt% or less, and TiB: 0.001 wt% or more and 0.2 wt% or less.

According to the coated steel sheet according to the present invention, when the composition of the plating bath is increased by 1.5 times or more as compared to the Mg addition by the amount of Mg, wiping mark defects produced by mixing air and nitrogen gas in the deposition amount control process Can be minimized.

Generally, in order to minimize the oxidation of Mg for controlling the coating amount during the production of the high corrosion resistant coated steel sheet, it is necessary to perform wiping with nitrogen to obtain excellent surface quality.

According to the present invention, excellent surface quality can be obtained without addition of 100% nitrogen wiping by adding Al, which is an additive element that produces denser oxides than Mg, in a molten state at a ratio higher than Mg.

In addition, if the mixing ratio of air and nitrogen is set to a certain amount and the plating adherence amount control process is performed, the surface oxygen concentration can be controlled and the post-treatment performance disadvantage due to the excessive pH increase upon moisture contact with the surface can be prevented. Therefore, it is possible to minimize the cost increase factor by using additional nitrogen.

Therefore, it can be widely commercialized in the production field of high corrosion resistant coated steel sheet by minimizing the loss of subordinate material and quality dislocation caused by Mg oxidation.

FIG. 1 is a photograph of a plating surface after nitrogen wiping with respect to a Zn-3.2% Al-1.5% Mg plating bath composition.
FIG. 2 is a photograph of a plating surface after air wiping for a Zn-1% Al-1.5% Mg plating bath composition.
3 is a photograph of a plating surface after air wiping for a Zn-3.5% Al-1.5% Mg plating bath composition.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims. Like reference numerals refer to like elements throughout the specification.

According to a preferred embodiment of the present invention, the hot dip coating bath containing Al in an amount of more than 1 wt% but less than 4 wt%, Mg in an amount of less than 2 wt%, and the balance of Zn and other inevitable impurities is cooled, In order to minimize the wiping mark due to the oxidation of the plating surface during the air wiping process, a wiping mark generated in the deposition amount control process by adding Al 1.5 times by weight to the Mg addition ratio in weight% Minimize defects

However, as described above, when Al and Mg are added at a certain ratio, the surface appearance can be secured. However, when the oxygen concentration on the surface of the plating increases to 1% or more due to air wiping, The surface pH is rapidly increased due to the generated MgO.

When the pH of the surface rises to 9 or more, the post-treatment performance is rapidly degraded. To prevent this phenomenon, the mixing ratio of air and nitrogen is important in the gas wiping process for controlling the thickness of the plating. In order to control the surface oxygen concentration to 1% or less, it is possible to control the oxygen concentration on the surface to 1% or less when air is mixed with nitrogen at 80% or less.

0.001 wt% to 0.2 wt% of B, 0.0005 wt% to 0.2 wt% of B, 0.001 wt% to 0.2 wt% of Be, 0.001 wt% to 0.5 wt% of Si, 0.001 wt% to 0.2 wt% of TiB % May further be contained as a constituent component of the plating layer. The added elements further serve to refine the texture, so that the roughness of the surface solidified structure is lowered and the smoothness is improved.

FIG. 1 is a photograph of a plating surface after wiping with air and nitrogen mixed at a ratio of 8: 2 with a composition of Zn-3.5% Al-1.5% Mg plating bath having an Al / Mg ratio of 2.3 and no wiping mark was formed on the plating surface .

FIG. 2 shows a wiping mark on the plating surface when the plating bath composition is Zn-1% Al-1.5% Mg and the Al / Mg ratio is 0.66, And the appearance is reduced.

FIG. 3 shows the surface of the plating after N ₂ wiping with respect to the composition of the Zn-3.5% Al-1.5% Mg plating bath. The plating surface has a surface similar to that of FIG. 1 without wiping mark by Mg oxidation. Demonstrating the effect of the invention on the addition rate.

On the other hand, the alloy-plated steel sheet according to an embodiment of the present invention is formed by immersing a steel sheet in a plating bath in which the above-described plating composition is heated and melted to form a plating layer on its surface.

At this time, Al appears in the form of a primer on the plated layer during solidification, and forms a dendritic phase, thereby making the structure dense. It is preferable that Al is added in an amount of 1 wt% or more based on the total weight of the plating bath in order to produce a preliminary Al phase and a three-way process phase of a complex structure acting as passivation of the plating layer. When 4 wt% The coagulation shrinkage structure of the surface is generated during the solidification of the plating layer, thereby deteriorating the appearance.

According to one embodiment of the present invention, Mg is an important element for improving corrosion resistance. In order to produce a three-way process phase contributing to corrosion resistance, Mg must be added in an amount of 0.1 wt% or more, and a polygonal MgZn ₂ In order not to be produced, less than 2% should be added. In the above-mentioned Al and Mg, when the alloy is designed by exceeding 1.5 times of Al relative to Mg addition in weight% by weight, Al in the molten state in the wiping process forms a dense oxide film, Can be minimized.

In general, alloys are designed on the basis of weight percentage, and when the density difference is not large or a small amount is added between alloying design elements, the alloy system of the present invention has a liquid density of 6.57 g / cm ³ of Zn is the major component and the additive element is a light-weight metal having a liquid density 2.37g / cm ³ And 1.58 g / cm < ³ > Mg. If it is designed as weight%, the amount added to the molten metal becomes different.

Considering the density, in order for the ratio of Al and Mg to be added to be 1: 1 by volume, the Al content should be about 1.5 times as high as the Mg content. When Al is added more than 1.5 times, Mg dispersed in the molten metal reacts with oxygen to form a porous oxide film, and Al contacts with oxygen to form a dense oxide film, thereby minimizing additional oxygen contact with Mg.

On the other hand, when the Al addition ratio is less than 1.5 times the Mg addition amount on the basis of the weight percentage, since the ratio of the Al content to the Mg addition amount is small, the oxidation of Mg is difficult due to the addition of oxygen. On the other hand, one embodiment of the present invention provides a method of manufacturing an alloy-coated steel sheet by coating the above-described plating composition on a steel sheet.

A method of coating a plating composition on a steel sheet comprises the steps of heating a plating composition having the above composition to prepare a hot-dip coating bath at 420 to 500 ° C, and dipping a steel sheet in a plating bath to coat the surface of the steel sheet with a plating composition, And cooling the steel sheet having the plated layer formed thereon to a room temperature at a cooling rate of 5 DEG C / sec to 30 DEG C / sec.

Here, the steel sheet may be a cold rolled steel sheet or a hot rolled steel sheet or a steel sheet annealed after cold rolling. The steel sheet is first adjusted to the temperature of the plating bath before being dipped in the plating bath, and then immersed in the plating bath.

After the steel sheet is immersed in the plating bath, the steel sheet is pulled up and air and nitrogen (N2) are mixed and wiped to adjust the plating adhesion amount. If necessary, it may pass through a galvanic annealing furnace. At this time, the plating adhesion amount is adjusted to 60 g / m 2 to 300 g / m 2.

If the temperature of the plating bath molten metal is less than 420 캜, the fluidity of the plating bath deteriorates and the appearance of the plating film becomes poor and the plating adhesion is lowered. On the other hand, when the temperature exceeds 500 캜, the appearance of the coated steel sheet becomes poor due to an excessive oxidation coating, the coating adhesion is deteriorated, and insufficient cooling occurs during the post-plating coagulation process to cause defects such as a flow mark on the plating layer.

When working at a cooling rate of less than 5 ° C / s, surface irregularities are increased in the plating composition near the process composition, and Mg ₂ Zn ₁₁ phase in the form of black spots is formed in a large amount, resulting in a sharp decrease in appearance, And the workability is lowered.

On the other hand, if the cooling rate is higher than 30 DEG C / s, wrinkles are generated on the surface of the plating layer due to the pressure of the cooling air, and not only the appearance is deteriorated, but also cracking occurs in the plating layer due to over-coating. If the coating amount is less than 60 g / m 2, the corrosion resistance is insufficient. If the coating amount exceeds 300 g / m 2, the coating layer becomes excessively thick due to the excessive coating amount, and the adhesion of the coating layer itself decreases and the surface gloss decreases to deteriorate the appearance.

Immersion is carried out for 1 second to 3 seconds. At this time, when the substrate is immersed for less than 1 second, the adhesion of the plating is lowered, and if it exceeds 3 seconds, the alloy layer becomes thicker and the appearance may be deteriorated.

Hereinafter, embodiments of the present invention will be described in detail in comparison with Comparative Examples. However, the following examples are illustrative of the present invention, but the present invention is not limited by the following examples.

Example  One

A cold-rolled steel sheet having a thickness of 0.7 mm was immersed in an alkali solution at 50 占 폚 for 30 minutes and then washed with water to remove foreign matter and oil from the surface. The specimen was annealed and then plated. At this time, the annealing was performed in a reducing atmosphere composed of 10% to 30% of hydrogen and 70% to 90% of nitrogen, and the annealing temperature in the annealing was 700 ° C to 750 ° C.

Coating After annealing the annealed specimen at the plating bath temperature, it was immersed in the plating bath for 2 seconds and then pulled up. Air and nitrogen gas were mixed to adjust the coating amount of wiping to 60g / ㎡ ~ 300g / ㎡. And cooled to room temperature at a cooling rate of ~ 20 ° C / sec to solidify. At this time, the plating bath temperature is 420 ° C to 460 ° C.

The alloy-plated steel sheet was produced under the above conditions, and the surface appearance, paintability, and corrosion resistance of the alloy-plated steel sheet were measured as follows.

<Appearance>

The surface quality of the plating was evaluated as the oxidation wiping mark area produced by air and nitrogen mixed wiping.

◎: Oxidation Wiping mark: less than 5%

○: Oxidation Wiping mark: 5% or more to less than 20%

?: Oxidation Wiping mark: 20 or more to less than 50%

×: Oxidation Wiping mark: Greater than 50%

<Workability>

In the process of controlling the deposition amount of the plating, in the process of wiping with air and nitrogen, the molten metal is oxidized while being cut off from the strip, resulting in a top dross shape.

The incidence of top dross was evaluated with respect to the amount of top dross generated relative to the raw materials used in actual plating.

◎: Top Dross incidence rate 10% or less

○: Top Dross occurrence rate exceeding 10% ~ 20%

△: Top Dross occurrence rate exceeding 10% ~ 30%

×: Top Dross incidence rate exceeded 30%

<Plane corrosion resistance>

According to KS D 9502 (ASTM B-117), the occurrence of rust was evaluated by a salt spray test at 5% NaCl and 3,000 hours at 35 ° C.

At this time, the entire cross section of the specimen was covered on all four sides, and after 3,000 hours, the occurrence of red rust on the surface portion of the specimen was visually observed.

⊚: Red rust occurrence rate of 5% or less

○: Red rust occurrence rate exceeding 5% ~ 10%

?: Red rust occurrence rate exceeding 10% to 30%

×: Red rust occurrence rate exceeding 30%

<Plating surface pH>

After dropping a drop of distilled water on the surface of the plating, the pH of the pH test paper was changed to 5 minutes.

<Post-treatment plate corrosion resistance>

The coating composition (polyurethane system) was applied using a bar coater, and the PMT was dried at 70 to 130 ° C to form a coating layer. The deposition amount of the coating composition was 1,000 mg / m &lt; 2 &gt;.

Then, in accordance with the method described in ASTM B117, the dry film performance of the coating composition is evaluated by evaluating the rate of occurrence of white rust over time of the plate by the following method.

◎: satisfactory corrosion resistance over 120 hours,

○: within 5% for 120 hours,

X: Greater than 5% white rust in 120 hours

division Furtherance Addition ratio  Air% / N2% Appearance Operability Corrosion resistance SST Plating Surface pH After treatment'
Corrosion resistance Zn Al (%) Mg (%) Al / Mg wiping mark Top Dross Occurrence (%) Comparative Example 0 honey. 0.2 0 100/0 ◎ ◎ X 7 ◎ Comparative Example 1-1 honey. 2.3 1.7 1.35 70/30 △ △ ◎ 8 ○ Comparative Example 1-2 honey. 2.5 1.9 1.32 70/30 △ △ ◎ 8 ○ Comparative Example 1-3 honey. 2.8 2.3 1.22 70/30 X X ◎ 8 ○ Comparative Example 1-4 honey. 3.5 2.5 1.40 70/30 X X ◎ 8 ○ Comparative Example 1-5 honey. 2.3 1.2 1.92 100/0 ○ ○ ◎ 9 X Comparative Example 1-6 honey. 3.5 1.8 1.94 100/0 ○ ○ ◎ 9 X Honor
1-1 honey. 2.3 1.2 1.92 70/30 ○ ○ ◎ 8 ◎ Honor
1-2 honey. 2.5 1.6 1.56 70/30 ○ ○ ◎ 8 ◎ Honor
1-3


honey.
2.8
1.4
2.00
70/30
○
○
◎
8
◎
Honor
1-7 honey. 2.5 1.5 1.67 70/30 ◎ ○ ◎ 8 ◎ Examples 1-8 honey. 3.5 1.8 1.94 70/30 ◎ ○ ◎ 8 ◎

As can be seen from Table 1, Inventive Examples 1-1 to 1-3 and 1-7 to 1-8 according to one embodiment of the present invention minimize the occurrence of wiping marks in air and nitrogen mixed wiping conditions, , And the surface pH was controlled to obtain excellent post-treatment corrosion resistance. On the other hand, in Comparative Examples 1-1 to 1-4, the appearance and operability compared to the inventive examples were obtained. Comparative Examples 1-5 to 1-6 in which air was used as 100% showed the result that the pH of the plating surface rose to increase the post-treatment corrosion resistance, whereas Inventive Examples 1-1 and 1-8 were excellent in mixing with air and nitrogen The post-treatment corrosion resistance was obtained.

delete

The inventive examples 1-1 to 1-3, 1-7 and 1-8 have a composition in which the ratio of Al and Mg is 1.5 or more. When air and N ₂ are mixed in an amount of 80% by volume or less when wiping, In the case of Comparative Examples 1-5 and 1-6 in which the Al and Mg addition ratio was 1.5 but the wiping air was used in an amount of 80 vol% or more, Post treatment corrosion resistance.

Example  2

A cold-rolled steel sheet having a thickness of 0.7 mm was immersed in an alkali solution at 50 占 폚 for 30 minutes and then washed with water to remove foreign matter and oil from the surface to prepare a specimen.

The specimens were annealed and plated. The annealing was performed in a reducing atmosphere consisting of 10% to 30% of hydrogen and 70% to 90% of nitrogen, and the annealing temperature was 700 ° C to 750 ° C for annealing.

After annealing, the specimens were cooled to the plating bath temperature, immersed in the plating bath for 2 seconds, then pulled up, and the coating amount of plating was changed from 60 g / ㎡ to 300 g / ㎡ by wiping with air and nitrogen (1: 1) And cooled to room temperature at a cooling rate of 5 to 30 DEG C / s to solidify. At this time, the plating bath temperature was set at 430 ° C to 500 ° C for each composition.

The alloy-plated steel sheet was produced under the above conditions, and the surface roughness of each alloy-plated steel sheet was shown in Table 2 below.

<Smoothness>

The Ra value of the surface of the plating was evaluated using a surface roughness tester.

?: Ra value is 0.5 占 퐉 or less

?: Ra value exceeding 0.5 占 퐉 to below 0.8 占 퐉

DELTA: Ra value is more than 0.8 mu m to 1.2 mu m or less

X: Ra value is 1.2 占 퐉 or more

division Composition for plating (wt%) Smoothness Zn Al Mg Additive element Surface roughness Comparative Example 2-1 Honey. 2.3 0.7 X Comparative Example 2-2 Honey. 2.5 1.6 △ Comparative Example 2-3 Honey. 2.8 1.4 △ Comparative Example 2-4 Honey. 3.5 0.7 X Inventory 2-1 Honey. 2.3 0.7 0.01% Ti ○ Example 2-2 Honey. 2.5 1.6 0.01% B ◎ Examples 2-3 Honey. 2.8 1.4 0.01% Be ◎ Honorable 2-4 Honey. 2.8 1.4 0.01% TiB ◎ Honorable 2-8 Honey. 3.2 0.7 0.05% Si ○

As can be seen from the above Table 2, in the case of Examples 2-1 to 2-4 in which the fine elements were added, as compared with Examples 2-1 to 2-4 in which the fine elements were not added, .

Example  3

A cold-rolled steel sheet having a thickness, a width and a length of 0.7 mm was immersed in an alkali solution at 50 ° C for 30 minutes and then washed with water to remove foreign substances and oil from the surface.

The specimens were annealed and plated. The annealing was performed in a reducing atmosphere composed of 10% to 30% of hydrogen and 70% to 90% of nitrogen, and the annealing temperature was 700 ° C to 750 ° C for annealing.

Plating was performed by cooling the specimens subjected to annealing by the plating bath temperature, immersing them in the plating bath for 2 seconds, pulling them up, adjusting the plating amount from 60 g / m 2 to 300 g / m ₂ by N ₂ wiping, To 35 [deg.] C / s to coagulate.

The temperature of the plating bath was varied within the range of 450 ° C to 550 ° C with different operating temperatures for each composition. The alloy coated steel sheet was manufactured under the above conditions, and the appearance and workability according to the composition cooling rate and plating bath temperature of the produced coated steel sheet were evaluated, and the results are shown in Table 3 below.

<Appearance>

An element which inhibits the appearance of the surface was defined as a black spots and a flow pattern on the surface of the plating, and the best appearance was rated as 5 points and the appearance for the most heat was defined as 1 point.

There is no element that obstructs appearance on the plating surface: 5 points

Black spots on plating surface: - 2 points

When unevenness is formed on the plating surface: - 1 point

Flow pattern on plating surface: -1 point

Crease pattern on plating surface: -2 points

<Processability>

The 1T-thick specimen was bent at 180 ° (bending test), and the cross-section was observed with a microscope to measure the cracking rate per unit length. At this time, the crack was limited to the entire plating layer.

◎: Crack occurrence rate 10% or less

○: Crack generation rate exceeding 10% to 20% or less

?: Crack occurrence rate exceeding 20% to 30% or less

X: Cracking rate exceeding 30%

division Cooling rate
(° C / sec) element Property evaluation Zn Al Mg Appearance Processability Comparative Example 3-1 2 Honey. 2.5 1.6 One × Comparative Example 3-2 2 Honey. 2.8 1.4 One × Comparative Example 3-3 35 Honey. 3.5 0.7 3 △ Example 3-1 10 Honey. 2.5 1.6 5 ○ Example 3-2 15 Honey. 2.8 1.4 5 ○ Honorable 3-3 20 Honey. 3.5 0.7 5 ◎

As a result of examining the effect of cooling rate on the appearance and workability, the results are shown in Table 3.

In the case of Examples 3-1 to 3-3 in which the cooling rate is 10 to 15 ° C / sec as compared with Comparative Examples 3-1 to 3-3 set at 2 ° C / sec and 35 ° C / sec, respectively, Showed excellent results.

This is because if the cooling rate is less than 5 占 폚 / sec (Comparative Example 3-1), unevenness is generated on the surface, and when the cooling rate exceeds 30 占 폚 / sec (Comparative Example 3-3) Wrinkles are generated on the surface to deteriorate the appearance.

Comparative Example 3-1, which was set at a cooling rate of 2 DEG C / sec in the same composition, and Comparative Example 3-3, which was set at a cooling rate of 35 DEG C / sec, 3/3 / ° C / sec, the workability tends to be lowered. When the cooling rate is low, the three-way process phase is excessively grown in the plating layer and the workability is weakened. When the cooling rate is excessively high, And a large amount of cracks are generated in the employment state.

Claims (8)

delete delete delete The plating composition having Al: more than 2.3 wt%, less than 4 wt%, Mg: less than 2 wt%, and the balance of Zn and other inevitable impurities, and the ratio of Al and Mg being Al / Mg? 1.5 is heated to 430 to 500 캜 , A step of producing a hot-dip coating bath;
Immersing the steel sheet in the hot-dip galvanizing bath for 1 to 3 seconds to form a plating layer on the surface of the steel sheet; And
Cooling the steel sheet having the plated layer formed thereon to a room temperature at a cooling rate of 5 ° C / sec to 30 ° C / sec,
Further comprising the step of air wiping using a mixed gas of air and nitrogen in an amount of more than 0 to 80% by volume in order to control the deposition amount of the plating layer on the surface of the steel sheet. 5. The method of claim 4,
In the step of forming the plating layer,
And the plating adhesion amount is 60 g / m ² to 300 g / m ² . delete delete 5. The method of claim 4,
Wherein the molten bath comprises at least one selected from the group consisting of Ti: at least 0.001 wt% and at most 0.2 wt%, B: at least 0.005 wt% and not more than 0.2 wt%, B: at least 0.001 wt% : 0.001 wt% or more and 0.2 wt% or less.

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