Highly corrosion-resistant plated steel and method for manufacturing same

Abstract

Provided are a highly corrosion-resistant plated steel having excellent corrosion resistance and a method for manufacturing the same. According to an embodiment of the present invention, a method for manufacturing a highly corrosion resistant plated steel comprises the steps of: immersing a base material steel into a hot dip alloy bath comprising, by weight%, aluminum (Al): 6% to 18%, magnesium (Mg): 3% to 6%, and a remainder of zinc (Zn) and other unavoidable impurities; the immersed base material steel is taken out of the hot-dip alloy plating bath, and a hot-dip alloy plating layer is formed on the base material steel; and cooling the base steel having the formed hot dip alloy layer formed thereon, wherein the content ratio of aluminum to magnesium in the hot dip alloy bath ranges from 2: 1 to 6: 1.

Description

Technical Field

[0001] The present invention relates to a steel, and more particularly to a highly corrosion-resistant plated steel having excellent corrosion resistance and a method for manufacturing the same. Background Art

[0002] Hot-dip galvanized steel sheets have excellent self-sacrificing properties and are thus widely used in building materials and household appliances. When hot-dip galvanized steel sheets are exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode in the exposed iron portion of the base material, and zinc loss occurs in the plating layer. The sacrificial anode action of zinc plays an excellent role in suppressing the rusting of iron in the base material in a corrosive environment, but the anodic efficiency decreases. To solve this problem, a highly corrosion-resistant plated product is produced by adding magnesium (Mg) to zinc (Zn) to produce dense corrosion products in a corrosive environment, thereby improving the anodic efficiency and exhibiting excellent corrosion resistance. However, although the corrosion resistance is improved by adding magnesium to zinc, there is a problem that the corrosion resistance decreases due to a decrease in the processability of the plating layer and an increase in magnesium oxide. Summary of the Invention

[0003] Technical Problem

[0004] Therefore, the present invention has been completed in view of the above problems, and an object of the present invention is to provide a highly corrosion-resistant plated steel having excellent corrosion resistance and a method for manufacturing the same. It will be understood that the technical problem is provided only as an example, and the technical idea of the present application is not limited thereto.

[0005] Technical Solution

[0006] According to an aspect of the present invention, the above object and other objects can be achieved by providing a highly corrosion-resistant plated steel having excellent corrosion resistance and a method for manufacturing the same.

[0007] According to an embodiment of the present invention, a method for manufacturing a highly corrosion-resistant plated steel includes: immersing a base steel into a hot-dip alloy bath containing 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), the balance of zinc (Zn), and other inevitable impurities; taking out the immersed base steel from the hot-dip alloy bath to form a hot-dip alloy layer on the base steel; and cooling the base steel having the hot-dip alloy layer thereon, wherein the content ratio of aluminum to magnesium in the hot-dip alloy bath is 2:1 to 6:1.

[0008] According to an embodiment of the present invention, the hot-dip alloy bath may be maintained in a temperature range of 420°C to 500°C.

[0009] According to an embodiment of the present invention, the hot-dip alloy bath may be maintained at a temperature that is 20°C to 50°C higher than the melting point of the molten hot-dip alloy.

[0010] According to an embodiment of the present invention, the cooling can be carried out at a cooling rate of 7 °C / s to 30 °C / s.

[0011] According to an embodiment of the present invention, the cooling can be carried out at a cooling rate of 10 °C / s to 15 °C / s.

[0012] According to an embodiment of the present invention, the hot-dip alloy layer may include a plating base layer and an Fe-Al alloy layer, and the total thickness of the hot-dip alloy layer may be twice or more than twice the thickness of the Fe-Al alloy layer.

[0013] According to an embodiment of the present invention, the plating base layer may contain primary Al phase, Al / Zn eutectic phase or both.

[0014] According to an embodiment of the present invention, the area fraction of the primary Al phase, Al / Zn eutectic phase or both in the hot-dip alloy layer may be in the range of 20% to 60%.

[0015] According to an embodiment of the present invention, the highly corrosion-resistant plated steel includes a base steel and a hot-dip alloy layer formed on the base steel and configured to contain 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), and the balance being zinc (Zn) and other inevitable impurities.

[0016] According to an embodiment of the present invention, the hot-dip alloy layer may include a plating base layer and an Fe-Al alloy layer, the plating base layer may contain primary Al phase, Al / Zn eutectic phase or both, and the area fraction of the primary Al phase, Al / Zn eutectic phase or both in the hot-dip alloy layer may be in the range of 20% to 60%.

[0017] According to an embodiment of the present invention, the hot-dip alloy layer includes a plating base layer and an Fe-Al alloy layer, and the total thickness of the hot-dip alloy layer may be twice or more than twice the thickness of the Fe-Al alloy layer.

[0018] According to an embodiment of the present invention, the content ratio of aluminum to magnesium in the hot-dip alloy layer may be in the range of 2:1 to 6:1.

[0019] Advantageous effects

[0020] According to the technical idea of the present invention, a highly corrosion-resistant plated steel is provided. The highly corrosion-resistant plated steel is formed using a hot-dip alloy bath, the hot-dip alloy bath containing 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), the balance of zinc (Zn) and other inevitable impurities, and the content ratio of aluminum:magnesium being in the range of 2:1 to 6:1, thereby producing a primary Al phase or an Al / Zn eutectoid phase, and providing excellent corrosion resistance by suppressing the formation of an Fe-Al alloy layer. In order to sufficiently form a primary Al phase or an Al / Zn eutectoid phase, the total thickness of the plating layer should be at least twice the thickness of the alloy layer. Based on the cross-section of the hot-dip alloy layer, the fraction of the primary Al phase or the Al / Zn eutectoid phase is preferably 20% to 60%, which can provide excellent corrosion resistance. The effects of the present invention are described by way of example, and the scope of the present invention is not limited by these effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A process flow diagram showing a method for manufacturing a highly corrosion-resistant plated steel according to an embodiment of the present invention.

[0022] Figures 2a to 2c A scanning electron microscope photograph showing the microstructure of a highly corrosion-resistant plated steel according to an embodiment of the present invention. DETAILED DESCRIPTION

[0023] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The embodiments of the present application are provided to more completely explain the technical idea of the present application to those skilled in the art, and the following embodiments can be modified in many different forms, but the scope of the technical idea of the present application is not limited to the following embodiments. On the contrary, the embodiments are provided to make the present application thorough and complete, and to fully convey the technical idea of the present application to those skilled in the art. In the specification, the same reference numerals denote the same elements. In addition, various elements and regions in the drawings are schematically drawn. Therefore, the technical idea of the present invention is not limited by the relative sizes or spacings drawn in the accompanying drawings. Hereinafter, the term "hot-dip alloy" is used as a term including "hot-dip galvanized".

[0024] When magnesium is added to zinc, the corrosion resistance of hot-dip galvanized steel is improved. However, the addition of magnesium may reduce the workability of the coating and increase the amount of magnesium oxide during the manufacturing process. Therefore, the amount of magnesium added may be limited. To minimize these problems, aluminum is added. When aluminum is added, oxidation of magnesium in the plating bath can be prevented, thereby improving production stability. In addition, aluminum forms primary Al phase or Al / Zn eutectic phase in the coating, thus exhibiting excellent corrosion resistance in various environments. Zinc and magnesium (the main components of the coating) have excellent sacrificial corrosion resistance and exhibit corrosion resistance by reducing corrosion in a corrosive environment. On the other hand, in the case of the primary Al phase or Al / Zn eutectic phase of the coating, a strong passive layer is independently formed without sacrifice, thereby maintaining high structural corrosion resistance in a corrosive environment.

[0025] The present invention provides a coated steel and a method for manufacturing the same. By controlling the content range and content ratio of aluminum and magnesium and controlling the ratio of the coating layer to the alloy layer, a primary Al phase or Al / Zn eutectic phase is stably formed, so that the coated steel is configured to have improved corrosion resistance.

[0026] Figure 1 A process flow chart showing a method for manufacturing a highly corrosion-resistant coated steel according to an embodiment of the present invention is shown.

[0027] Reference Figure 1 , the method for manufacturing a highly corrosion-resistant coated steel includes the step (S110) of immersing a base steel into a hot-dip alloy bath, the hot-dip alloy bath containing 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), and the balance of zinc (Zn) and other inevitable impurities; the step (S120) of taking out the immersed base steel from the hot-dip alloy bath to form a hot-dip alloy layer on the base steel; and the step (S130) of cooling the base steel having the hot-dip alloy layer thereon, wherein the content ratio of aluminum to magnesium in the hot-dip alloy bath is 2:1 to 6:1. Therefore, a coating layer having excellent corrosion resistance can be formed.

[0028] The hot-dip alloy bath can be maintained within a temperature range of 420 °C to 500 °C.

[0029] The hot-dip alloy bath can be maintained within a temperature range that is 20 °C to 50 °C higher than the melting point of the molten hot-dip alloy.

[0030] The cooling step can be performed at a cooling rate of 7 °C / s to 30 °C / s.

[0031] The cooling step can be performed at a cooling rate of 10 °C / s to 15 °C / s.

[0032] The hot-dip alloy layer may include a plating base layer and an Fe-Al alloy layer. The total thickness of the hot-dip alloy layer may be more than twice the thickness of the Fe-Al alloy layer.

[0033] The plating base layer may contain primary Al phase (Al single-phase structure containing Zn), Al / Zn eutectic phase, or both.

[0034] In the hot-dip alloy layer, the area fraction of the primary Al phase, Al / Zn eutectic phase, or both may be 20% to 60%.

[0035] The highly corrosion-resistant plated steel obtained by a method for manufacturing highly corrosion-resistant plated steel may include a base steel and a hot-dip alloy layer formed on the base steel. The hot-dip alloy layer contains 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), and the balance of zinc (Zn) and other inevitable impurities. The hot-dip alloy layer may include a plating base layer and an Fe-Al alloy layer, and the total thickness of the hot-dip alloy layer may be more than twice the thickness of the Fe-Al alloy layer.

[0036] The plating base layer may contain primary Al phase, Al / Zn eutectic phase, or both, and the area fraction of the primary Al phase, Al / Zn eutectic phase, or both in the hot-dip alloy layer may be 20% to 60%.

[0037] Aluminum is put into the hot-dip alloy bath to prevent the oxidation of magnesium during the plating process. When the amount of magnesium added to the hot-dip alloy bath is 3% or more, the amount of aluminum added should be at least twice the amount of magnesium added to prevent the oxidation of magnesium in the hot-dip alloy bath and reduce oxide scum.

[0038] The content ratio of aluminum to magnesium in the hot-dip alloy layer may be 2:1 to 6:1.

[0039] When aluminum is added to the hot-dip alloy bath in an amount of 6% or more, during the curing process after plating, a primary Al phase or an Al / Zn eutectic phase may be formed in the hot-dip alloy layer formed on the base steel. The primary Al phase or Al / Zn eutectic phase is a phase that forms a well-structured passivation layer in a corrosive environment, thereby improving the corrosion resistance of the coating and providing crack resistance to the coating. In the composition range of the present invention, a processed phase mainly having a layered structure with high hardness is formed, so the crack resistance is improved by the primary Al having excellent elongation.

[0040] However, in order to form primary Al phase or Al / Zn eutectoid phase, formation of Fe-Al alloy layer between the base steel and the hot-dip alloy layer should be prevented or at least minimized. When more than 18% aluminum is added to the hot-dip alloy bath, the Fe-Al alloy layer grows excessively, so that the aluminum content required for forming primary Al phase or Al / Zn eutectoid phase becomes insufficient, resulting in a rapid decrease in the amount of primary Al phase or Al / Zn eutectoid phase formed in the hot-dip alloy layer.

[0041] In addition, when more than 18% aluminum is added to the hot-dip alloy bath, regardless of the amount of magnesium added, the melting temperature increases, so that the diffusion rate of the alloy layer during the coating formation stage increases rapidly. Therefore, primary Al phase or Al / Zn eutectoid phase will not be formed.

[0042] On the other hand, when aluminum is added to the hot-dip alloy bath in an amount less than 6%, due to insufficient aluminum content, primary Al phase or Al / Zn eutectoid phase may not be formed sufficiently.

[0043] When the amount of magnesium is less than 3% by weight, its contribution to corrosion resistance may be small, and when the amount of magnesium exceeds 5% by weight, deterioration of the steel may occur due to magnesium oxide scum.

[0044] In order to prevent reduction of primary Al phase or Al / Zn eutectoid phase, the content ratio of aluminum to magnesium is controlled within the range of 2:1 to 6:1. At this content ratio, primary Al phase or Al / Zn eutectoid phase can be formed sufficiently. Magnesium is used to prevent diffusion of elements that form Fe-Al alloy layer, thereby suppressing the thickness growth of Fe-Al alloy layer and thus promoting the formation of primary Al phase or Al / Zn eutectoid phase.

[0045] The hot-dip alloy layer may include a plating base layer and an Fe-Al alloy layer. The plating base layer refers to the layer formed on the base steel by the components of the hot-dip alloy bath and has a composition similar to the composition range of the hot-dip alloy bath. The Fe-Al alloy layer is formed by alloying iron contained in the base steel with aluminum contained in the hot-dip alloy bath and mainly refers to the layer formed at the interface between the base steel and the hot-dip alloy layer. In order to form the highly corrosion-resistant plated steel of the present invention, it is desirable to prevent or suppress the formation of Fe-Al alloy layer as much as possible.

[0046] The formation of such Fe-Al alloy layer may be affected by the temperature of the hot-dip alloy bath. The hot-dip alloy bath can be maintained within the temperature range of 420 °C to 500 °C. The hot-dip alloy bath can be maintained at a temperature 20 °C to 50 °C higher than the melting point of the molten hot-dip alloy.

[0047] Since the formation of the Fe-Al alloy layer also occurs during the cooling step after the formation of the hot-dip alloy layer, a cooling rate that can minimize the formation of the Fe-Al alloy layer is required. Therefore, the cooling step can be carried out at a cooling rate of 7 °C / s to 30 °C / s. Preferably, due to a cooling rate of 10 °C / s to 15 °C / s, a plated steel having excellent corrosion resistance and plating processability can be manufactured. When the cooling rate is less than 7 °C / s, the Fe-Al alloy layer continues to grow during solidification, making it difficult to form the primary Al phase or the Al / Zn eutectic phase. When the cooling rate exceeds 30 °C / s, the appearance of the plated surface may solidify unevenly, so the surface quality of the steel may deteriorate. A lower cooling rate promotes the formation of the primary Al phase, while a higher cooling rate promotes the formation of the Al / Zn eutectic phase.

[0048] In addition, in order to promote the formation of the primary Al phase or the Al / Zn eutectic phase, it is necessary to control the thickness of the alloy layer throughout the coating. The total thickness of the hot-dip alloy layer should be at least twice the thickness of the Fe-Al alloy layer, and the range can be, for example, 2 times to 20 times. When the total thickness of the hot-dip alloy layer is less than twice the thickness of the Fe-Al alloy layer, the fraction of the primary Al phase or the Al / Zn eutectic phase in the hot-dip alloy layer rapidly decreases, resulting in a simultaneous decrease in the corrosion resistance and processability of the coating.

[0049] In the cross-section (e.g., longitudinal section) of the hot-dip alloy layer, the area fraction of the primary Al phase, the Al / Zn eutectic phase, or both can be in the range of 20% to 60%. The remaining fraction can include a binary eutectic phase (e.g., MgZn2 phase, Mg2Zn 11 phase) composed of two elements among zinc, aluminum, and magnesium or an Al / Zn / Mg ternary eutectic phase composed of three elements, and the range of the area fraction can be 40% to 80%. This area fraction may not include the Fe-Al alloy phase. The area fraction refers to the area ratio obtained from the microstructure image of the steel using an image analyzer. When the area fraction of the primary Al phase, the Al / Zn eutectic phase, or both is less than 20%, the corrosion resistance may not be improved, and when the area fraction of the primary Al phase, the Al / Zn eutectic phase, or both exceeds 60%, the MgZn2 phase with brittle characteristics may be excessively formed around the primary Al phase, resulting in cracks on the coating.

[0050] Experimental Example

[0051] The following preferred experimental examples are shown to help understand the present invention. However, the following experimental examples are only for helping to understand the present invention, and the present invention is not limited to the following experimental examples. Since those skilled in the art can technically infer sufficiently what is not described herein, the description thereof will be omitted.

[0052] Prepare a cold-rolled steel sheet with a thickness of 1.2 mm as the base steel. The base steel contains: carbon (C): 0.15 wt%, silicon (Si): 0.01 wt%, manganese (Mn): 0.6 wt%, phosphorus (P): 0.05 wt%, sulfur (S): 0.05 wt%, the balance being iron (Fe) and impurities inevitably contained in the steelmaking process. However, the composition and content of the base steel are exemplary, and the technical idea of the present invention is not limited thereto.

[0053] Immerse the base steel in an alkaline solution at 50 °C for about 30 minutes, and then wash it with water to remove foreign substances and oil on its surface. Anneal the base steel at 680 °C to 850 °C in a reducing atmosphere composed of nitrogen containing 7% hydrogen.

[0054] Cool the annealed base steel to a temperature not exceeding ±20 °C different from the plating bath, and then immerse it in a hot-dip alloy bath at 420 °C to 500 °C for 1 second to 5 seconds. Then, adjust the total thickness of the hot-dip alloy layer to about 20 μm by nitrogen wiping. Then, cool the plated base steel to produce the plated steel.

[0055] Evaluate the corrosion resistance of the plated steel by the time of the appearance of red rust through a salt spray test using a 5% NaCl solution at 35 °C.

[0056] Table 1 shows the composition of the hot-dip alloy bath for manufacturing the plated steel of the examples and comparative examples of the present invention.

[0057] [Table 1]

[0058]

[0059] Referring to Table 1, the hot-dip alloy bath of the example contains 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), the balance being zinc (Zn) and other inevitable impurities, wherein the content ratio of aluminum:magnesium satisfies the range of 2:1 to 6:1.

[0060] In Comparative Example 1, the aluminum content is lower than the lower limit recommended by the present invention, and the difference is that it does not contain magnesium. Comparative Example 2 is different in that the aluminum content is lower than the lower limit recommended by the present invention and does not satisfy the content ratio of aluminum:magnesium. Comparative Example 3 is different in that the aluminum content and the magnesium content are lower than the lower limit recommended by the present invention. Comparative Example 4 is different in that the aluminum content is higher than the upper limit recommended by the present invention and the magnesium content is lower than the lower limit recommended by the present invention. Comparative Example 5 is different in that the magnesium content is lower than the lower limit recommended by the present invention and does not satisfy the content ratio of aluminum:magnesium. Comparative Example 6 is different in that the cooling rate is less than the lower limit recommended by the present invention.

[0061] Here, it was found that the overall composition of the hot-dip alloy layer of the plated steel is the same as or almost similar to the composition of the hot-dip alloy bath. Therefore, the composition of the hot-dip alloy bath in Table 1 can be considered as the composition of the overall plating layer of the plated steel.

[0062] Table 2 shows the characteristics and corrosion resistance of the hot-dip alloy layer of the plated steel of the examples and comparative examples of the present invention.

[0063] [Table 2]

[0064]

[0065] In Table 2, the evaluation of the red rust appearance time is as follows: ◎: 2400 hours or longer, O: 1800 hours or longer and less than 2400 hours, △: 800 hours or longer and less than 1800 hours, ×: less than 600 hours.

[0066] Referring to Table 2, in the examples, it was found that the area fraction of the primary Al phase, Al / Zn eutectic phase, or both is 20% or more, which satisfies the range of 20% to 60% proposed by the present invention. The examples show that the thickness of the Fe-Al alloy layer is less than 7 μm, and the total thickness of the hot-dip alloy layer is twice or more than twice the thickness of the Fe-Al alloy layer. The examples show excellent corrosion resistance because the red rust appearance time is 1800 hours or longer.

[0067] Comparative Example 1 has a lower aluminum content, so the primary Al phase or Al / Zn eutectic phase is not formed, and the thickness of the Fe-Al alloy layer is small, but the corrosion resistance is evaluated as poor because the red rust appearance time is less than 600 hours.

[0068] In Comparative Example 2, due to the lower aluminum content, it is difficult to form the primary Al phase or Al / Zn eutectic phase, and due to the lower aluminum content, the thickness and thickness ratio of the Fe-Al alloy layer are small, but the corrosion resistance is evaluated as poor because the red rust appearance time is 800 hours or longer and less than 1800 hours.

[0069] In Comparative Example 3, due to the lower aluminum content, the primary Al phase or Al / Zn eutectic phase is formed in a smaller fraction, and due to the lower aluminum content, the thickness and thickness ratio of the Fe-Al alloy layer are small, but the corrosion resistance is evaluated as poor because the red rust appearance time is 800 hours or longer and less than 1800 hours.

[0070] In Comparative Example 4, due to the higher aluminum content and lower magnesium content, the primary Al phase or Al / Zn eutectic phase is not formed. Instead, a thicker Fe-Al alloy layer is formed, and the corrosion resistance is evaluated as poor because the red rust appearance time is 800 hours or longer and less than 1800 hours.

[0071] In Comparative Example 5, due to the low magnesium content, the primary Al phase or the Al / Zn eutectoid phase was not formed in sufficient amounts. Instead, a relatively thick Fe-Al alloy layer was formed, and the corrosion resistance was evaluated as poor because the time to red rust appearance was 800 hours or longer and less than 1800 hours.

[0072] In Comparative Example 6, the aluminum content and the magnesium content were appropriate, but due to the low cooling rate, the primary Al phase or the Al / Zn eutectoid phase was not formed sufficiently. Instead, a relatively thick Fe-Al alloy layer was formed, and the corrosion resistance was evaluated as poor because the time to red rust appearance was 800 hours or longer and less than 1800 hours.

[0073] Therefore, in order to form the primary Al phase or the Al / Zn eutectoid phase in the hot-dip alloy layer, the total thickness of the hot-dip alloy layer: the thickness of the Fe-Al alloy layer should be at least 2:1. That is, the total thickness of the hot-dip alloy layer should be at least twice the thickness of the Fe-Al alloy layer. Ideally, based on the cross-section of the hot-dip alloy layer, the primary Al phase or the Al / Zn eutectoid phase present in the hot-dip alloy layer is present in an area fraction of 20% to 60%.

[0074] Figures 2a to 2c Scanning electron microscope photographs showing the microstructure of the highly corrosion-resistant plated steel according to an embodiment of the present invention.

[0075] Figure 2a Comparative Example 4 is shown, where the aluminum content is higher than the upper limit recommended by the present invention and the magnesium content is lower than the lower limit recommended by the present invention, such that the Fe-Al alloy layer is formed relatively thickly on the base steel to a thickness of about 12 μm.

[0076] Figure 2b Example 2 is shown, where the Fe-Al alloy layer is formed relatively thinly on the base steel to a thickness of about 1.5 μm, and the primary Al phase is formed at a fraction of about 26%.

[0077] Figure 2c Example 4 is shown, where the Fe-Al alloy layer is formed relatively thinly on the base steel to a thickness of about 4 μm, and the primary Al phase and the Al / Zn eutectoid phase are formed at a total fraction of about 30%.

[0078] Therefore, the examples show that the corrosion resistance is improved because the formation of the Fe-Al alloy layer is maximally suppressed and the primary Al phase and the Al / Zn eutectoid phase are formed sufficiently.

[0079] It will be apparent to those skilled in the art to which the technical idea of the present invention pertains that the technical idea of the present invention as described above is not limited to the above-described embodiments and the accompanying drawings, and that various alternatives, modifications, and changes are possible within the scope of the technical idea of the present invention.

[0080] Industrial Applicability

[0081] According to an embodiment of the present invention, a highly corrosion-resistant plated steel capable of forming a primary Al phase or an Al / Zn eutectoid phase and suppressing the formation of an Fe-Al alloy layer to provide excellent corrosion resistance can be provided.

Claims

1. A method for manufacturing highly corrosion-resistant plated steel, the method comprising: immersing a substrate steel into a hot-dip alloy bath, the hot-dip alloy bath comprising 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), the balance of zinc (Zn) and other inevitable impurities; removing the immersed substrate steel from the hot-dip alloy bath to form a hot-dip alloy layer on the substrate steel; and cooling the substrate steel having the hot-dip alloy layer thereon, wherein the content ratio of aluminum to magnesium in the hot-dip alloy bath is 2:1 to 6:

1.

2. The method according to claim 1, wherein The hot-dip alloy bath is maintained within a temperature range of 420 °C to 500 °C.

3. The method according to claim 1, wherein, The hot-dip alloy bath is maintained at a temperature that is 20 °C to 50 °C higher than the melting point of the molten hot-dip alloy.

4. The method according to claim 1, wherein The cooling is carried out at a cooling rate of 7 °C / s to 30 °C / s.

5. The method according to claim 1, wherein The cooling is carried out at a cooling rate of 10 °C / s to 15 °C / s.

6. The method according to claim 1, wherein The hot-dip alloy layer comprises a plating base layer and an Fe-Al alloy layer, and the total thickness of the hot-dip alloy layer is two times or more the thickness of the Fe-Al alloy layer.

7. The method according to claim 6, wherein, The plating base layer comprises primary Al phase, Al / Zn eutectic phase or both.

8. The method according to claim 7, wherein The area fraction of the primary Al phase, Al / Zn eutectic phase or both in the hot-dip alloy layer is in the range of 20% to 60%.

9. A highly corrosion-resistant plated steel, the highly corrosion-resistant plated steel comprising: a substrate steel; and a hot-dip alloy layer formed on the substrate steel and configured to comprise 6 wt% to 18 wt% of aluminum (Al), 3 wt% to 6 wt% of magnesium (Mg), the balance of zinc (Zn) and other inevitable impurities, wherein the hot-dip alloy layer comprises a plating base layer and an Fe-Al alloy layer, the plating base layer comprises primary Al phase, Al / Zn eutectic phase or both, and the area fraction of the primary Al phase, Al / Zn eutectic phase or both in the hot-dip alloy layer is in the range of 20% to 60%.

10. The highly corrosion-resistant plated steel according to claim 9, wherein, The hot-dip alloy layer comprises a plating base layer and an Fe-Al alloy layer, and the total thickness of the hot-dip alloy layer is two times or more the thickness of the Fe-Al alloy layer.

11. The highly corrosion-resistant plated steel according to claim 9, wherein, The content ratio of aluminum to magnesium in the hot-dip alloy layer is in the range of 2:1 to 6:1.

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