Highly corrosion-resistant plated steel and manufacturing method therefor

Abstract

The present disclosure provides a highly corrosion-resistant plated steel having excellent corrosion resistance and a method of manufacturing the same. According to an embodiment of the present invention, the method of manufacturing the highly corrosion-resistant plated steel includes immersing a base steel in a hot-dip alloy-plating bath including 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities; taking the immersed base steel out of the hot-dip alloy-plating bath to form a hot-dip alloy-plated layer on the base steel; and cooling the base steel having the hot-dip alloy-plated layer thereon, wherein the content ratio of aluminum:magnesium in the hot-dip alloy-plating bath is 2:1 to 6:1.

Description

[0001] The present application is a continuation of PCT Application No. PCT / KR2023 / 020901, filed Dec. 18, 2023, which claims the benefit of KR Patent Application no. 10-2022-0189710 filed Dec. 29, 2022, which applications are incorporated herein by reference in their entirety.FIELD

[0002] The present disclosure relates to a steel, and more particularly to a highly corrosion-resistant plated steel having excellent corrosion resistance and a method of manufacturing the same.BACKGROUND

[0003] Hot-dip zinc-plated steel sheets have excellent self-sacrifice properties and thus are widely used in building materials and home appliances. When hot-dip zinc-plated steel sheets are exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode in the exposed iron portion of a base material, and zinc loss occurs from a plated 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 anode efficiency is reduced. To solve the problem, highly corrosion-resistant plated products are being produced by adding magnesium (Mg) to zinc (Zn) to generate dense corroded products in a corrosive environment, thereby improving anode efficiency and exhibiting excellent corrosion resistance. However, although corrosion resistance increases as magnesium is added to zinc, there is a problem in that corrosion resistance decreases due to a decrease in the processability of a plating layer and an increase in magnesium oxide.SUMMARY

[0004] Therefore, the present disclosure has been made in view of the above problems, and it is one object of the present disclosure to provide a highly corrosion-resistant plated steel having excellent corrosion resistance and a method of manufacturing the same. It will be understood that the technical problems are only provided as examples, and the technical idea of the present disclosure is not limited thereto.

[0005] In accordance with an aspect of the present disclosure, the above and other objects can be accomplished by the provision of a highly corrosion-resistant plated steel having excellent corrosion resistance and a method of manufacturing the same.

[0006] According to an embodiment of the present disclosure, the method of manufacturing a highly corrosion-resistant plated steel includes: immersing a base steel in a hot-dip alloy-plating bath including 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities; taking the immersed base steel out of the hot-dip alloy-plating bath to form a hot-dip alloy-plated layer on the base steel; and cooling the base steel having the hot-dip alloy-plated layer thereon, wherein a content ratio of aluminum:magnesium in the hot-dip alloy-plating bath is 2:1 to 6:1.

[0007] According to an embodiment of the present disclosure, the hot-dip alloy-plating bath may be maintained in a temperature range of 420° C. to 500° C.

[0008] According to an embodiment of the present invention, the hot-dip alloy-plating bath may be maintained at a temperature increased by 20° C. to 50° C. from the melting point of a molten hot-dip alloy.

[0009] According to an embodiment of the present disclosure, the cooling may be performed at a cooling rate of 7° C. / sec to 30° C. / sec.

[0010] According to an embodiment of the present disclosure, the cooling may be performed at a cooling rate of 10° C. / sec to 15° C. / sec.

[0011] According to an embodiment of the present invention, the hot-dip alloy-plated layer may include a plated base layer and a Fe—Al alloy layer, and the total thickness of the hot-dip alloy-plated layer may be twice or more the thickness of the Fe—Al alloy layer.

[0012] According to an embodiment of the present disclosure, the plated base layer may include a primary Al phase, an Al / Zn eutectoid phase, or both.

[0013] According to an embodiment of the present disclosure, the area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both in the hot-dip alloy-plated layer may be in a range of 20% to 60%.

[0014] According to an embodiment of the present disclosure, the highly corrosion-resistant plated steel includes a base steel; and a hot-dip alloy-plated layer formed on the base steel and configured to include 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities.

[0015] According to an embodiment of the present disclosure, the hot-dip alloy-plated layer may include a plated base layer and a Fe—Al alloy layer, the plated base layer may include a primary Al phase, an Al / Zn eutectoid phase, or both, and the area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both in the hot-dip alloy-plated layer may be in a range of 20% to 60%.

[0016] According to an embodiment of the present disclosure, the hot-dip alloy-plated layer includes a plated base layer and a Fe—Al alloy layer, and the total thickness of the hot-dip alloy-plated layer may be twice or more the thickness of the Fe—Al alloy layer.

[0017] According to an embodiment of the present disclosure, a content ratio of aluminum:magnesium in the hot-dip alloy-plated layer may be in a range of 2:1 to 6:1.

[0018] In an aspect, a highly corrosion-resistant plated steel is provide, comprising: 1) a base steel; and 2) a plated layer formed on the base steel comprising 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities, wherein the hot-dip alloy-plated layer comprises a plated base layer and a Fe—Al alloy layer, the plated base layer comprises a primary Al phase, an Al / Zn eutectoid phase, or both, and an area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both in the hot-dip alloy-plated layer is in a range of 20% to 60%.

[0019] Provided is a highly corrosion-resistant plated steel that is formed using a hot-dip alloy-plating bath containing 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities and has a content ratio of aluminum:magnesium in a range of 2:1 to 6:1, thereby generating a primary Al phase or an Al / Zn eutectoid phase, and providing excellent corrosion resistance by suppressing the formation of a Fe—Al alloy layer. For the primary Al phase or the Al / Zn eutectoid phase to be sufficiently formed, the total thickness of a plated layer should be at least twice that of the alloy layer. The fraction of the primary Al phase or the Al / Zn eutectoid phase is preferably 20% to 60% based on the cross-section of the hot-dip alloy-plated layer, which can provide excellent corrosion resistance.

[0020] The effects of the present disclosure are described as examples, and the scope of the present disclosure is not limited by these effects.DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a process flowchart illustrating a method of manufacturing a highly corrosion-resistant plated steel according to an embodiment of the present disclosure.

[0022] FIGS. 2A to 2C are scanning electron microscope photographs illustrating the microstructures of the highly corrosion-resistant plated steel according to an embodiment of the present disclosure.MODE OF THE INVENTION

[0023] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Embodiments of the present disclosure are provided to more completely explain the technical idea of the present disclosure to those skilled in the art, and the following embodiments may be modified in many different forms, but the scope of the technical idea of the present disclosure is not limited to the following embodiments. Rather, the embodiments are provided to make the disclosure thorough and complete and to fully convey the technical idea of the disclosure to those skilled in the art. Like reference numerals in the specification denote like elements. Further, various elements and regions in the drawings are schematically drawn. Therefore, the technical idea of the disclosure is not limited by the relative size or spacing drawn in the accompanying drawings. Hereinafter, the term “hot-dip alloy-plated” is used as a term encompassing “hot-dip zinc-plated.”

[0024] When magnesium is added to zinc, the corrosion resistance of a hot-dip alloy-plated steel increases, but the addition of magnesium may reduce the workability of a plated layer and increase magnesium oxide during manufacturing, so the amount of magnesium added may be limited. To minimize these problems, aluminum is added. When aluminum is added, the oxidation of magnesium in a plating bath may be prevented, thereby increasing production stability. In addition, aluminum forms a primary Al phase or an Al / Zn eutectoid phase in a plated layer, thereby exhibiting excellent corrosion resistance in various environments. Zinc and magnesium, the main components of the plated layer, have excellent sacrificial corrosion resistance and exhibit corrosion resistance through corrosion reduction in a corrosive environment. On the other hand, in the case of the primary Al phase or Al / Zn eutectoid phase of the plated layer, a strong passivation layer is formed on its own without sacrificing, thereby maintaining structurally high corrosion resistance in a corrosive environment.

[0025] The present disclosure provides a plated steel and a method for manufacturing the same, the plated steel being configured to have increased corrosion resistance by controlling the content range and content ratio of aluminum and magnesium and controlling the ratio of a plated layer and an alloy layer to stably form a primary Al phase or an Al / Zn eutectoid phase.

[0026] FIG. 1 illustrates the process flowchart of a method of manufacturing a highly corrosion-resistant plated steel according to an embodiment of the present disclosure.

[0027] Referring to FIG. 1, the method of manufacturing a highly corrosion-resistant plated steel includes a step (S110) of immersing a base steel in a hot-dip alloy-plating bath containing 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), and the remainder being zinc (Zn) and other inevitable impurities; a step (S120) of taking the immersed base steel out of the hot-dip alloy-plating bath to form a hot-dip alloy-plated layer on the base steel; and a step (S130) of cooling the base steel having the hot-dip alloy-plated layer thereon, wherein a content ratio of aluminum:magnesium in the hot-dip alloy-plating bath is 2:1 to 6:1. Accordingly, a plated layer having excellent corrosion resistance may be formed.

[0028] The hot-dip alloy-plating bath suitably may be maintained in a temperature range of for example 420° C. to 500° C.

[0029] The hot-dip alloy-plating bath may be maintained at a temperature range increased by for example 20° C. to 50° C. from the melting point of the molten hot-dip alloy.

[0030] The cooling step suitably may be performed at a cooling rate of for example 7° C. / sec to 30° C. / sec.

[0031] The cooling step suitably may be performed at a cooling rate of 10° C. / sec to 15° C. / sec.

[0032] The hot-dip alloy-plated layer may include a plated base layer and a Fe—Al alloy layer. In aspects, the total thickness of the hot-dip alloy-plated layer may be more than twice the thickness of the Fe—Al alloy layer.

[0033] In aspects, the plated base layer may include a primary Al phase (Zn-containing Al single-phase structure), an Al / Zn eutectoid phase, or both.

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

[0035] A highly corrosion-resistant plated steel manufactured by the method of manufacturing a highly corrosion-resistant plated steel suitably may include a base steel; and a hot-dip alloy-plated layer formed on the base steel, the hot-dip alloy-plated layer including 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), and the remainder being zinc (Zn) and other inevitable impurities. The hot-dip alloy-plated layer suitably may include a plated base layer and a Fe—Al alloy layer, and the total thickness of the hot-dip alloy-plated layer may be more than twice the thickness of the Fe—Al alloy layer.

[0036] The plated base layer suitably may include a primary Al phase, an Al / Zn eutectoid phase, or both, and the area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both in the hot-dip alloy-plated layer may be 20% to 60%.

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

[0038] In aspects, a content ratio of aluminum:magnesium in the hot-dip alloy-plated layer may be 2:1 to 6:1.

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

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

[0041] In addition, when aluminum is added over 18% to the hot-dip alloy-plating bath, the melting temperature increases regardless of the amount of magnesium added, so that the diffusion speed of an alloy layer rapidly increases at the formation stage of a plated layer. Accordingly, the primary Al or the Al / Zn eutectoid phase is not formed.

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

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

[0044] To prevent the reduction of the primary Al phase or the Al / Zn eutectoid phase, a content ratio of aluminum:magnesium is controlled in a range of 2:1 to 6:1. In this content ratio, the primary Al phase or the Al / Zn eutectoid phase may be sufficiently formed. The magnesium acts to prevent the diffusion of elements for the formation of a Fe—Al alloy layer, thereby suppressing the thickness growth of the Fe—Al alloy layer and consequently facilitating the formation of the primary Al phase or the Al / Zn eutectoid phase.

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

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

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

[0048] In addition, to facilitate the formation of the primary Al phase or the Al / Zn eutectoid phase, it is necessary to control the thickness of the alloy layer within the entire plated layer. The total thickness of the hot-dip alloy-plated layer should be at least twice the thickness of the Fe—Al alloy layer, and may range from, for embodiment, 2 to 20 times. When the total thickness of the hot-dip alloy-plated layer is less than twice the thickness of the Fe—Al alloy layer, the fraction of the primary Al phase or the Al / Zn eutectoid phase in the hot-dip alloy-plated layer decreases rapidly, so that the corrosion resistance and the workability of a plated layer decrease simultaneously.

[0049] In the cross-section (e.g., longitudinal section) of the hot-dip alloy-plated layer, the area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both may be in a range of 20% to 60%. The remaining fraction may include a binary eutectic phase composed of two elements among zinc, aluminum, and magnesium, such as MgZn2 phase, Mg2Zn11 phase, or an Al / Zn / Mg ternary eutectic phase composed of three elements, and the area fraction may range from 40% to 80%. This area fraction may exclude the Fe—Al alloy phase. The area fraction refers to an area ratio derived from the microstructure image of the steel using an image analyzer.

[0050] When the area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both is less than 20%, corrosion resistance may not be improved, and when the area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both exceed 60%, MgZn2 phase with brittle characteristics may be excessively formed around the primary Al phase, causing cracks on a plated layer.Experimental Example

[0051] Hereinafter, a preferred experimental example is presented to help understand the present invention. However, the experimental example below is only to help understand the present invention, and the present disclosure is not limited by the experimental example below. Since the contents not described here can be sufficiently technically inferred by those skilled in this technical field, the explanation thereof will be omitted.

[0052] A 1.2 mm cold-rolled steel sheet was prepared as a base steel. The base steel included: carbon (C): 0.15% by weight, silicon (Si): 0.01% by weight, manganese (Mn): 0.6% by weight, phosphorus (P): 0.05% by weight, sulfur(S): 0.05% by weight, the remainder being iron (Fe), and impurities that were inevitably included in a steelmaking process However, the compositions and contents of the base steel are exemplary and the technical idea of the present disclosure is not limited thereto. The base steel was immersed in an alkaline solution at 50° C. for about 30 minutes, then washed with water to remove foreign substances and oil on its surface. The base steel was annealed at 680° C. to 850° C. in a reducing atmosphere consisting of nitrogen gas containing 7% hydrogen gas.

[0053] The annealed base steel was cooled to a temperature not more than +20° C. different from a plating bath and then immersed in a hot-dip alloy-plating bath at 420° C. to 500° C. for 1 to 5 seconds. Next, the total thickness of the hot-dip alloy-plated layer was adjusted to approximately 20 μm by nitrogen wiping. Next, the plated base steel was cooled to produce plated steel.

[0054] The corrosion resistance of the plated steel was evaluated by the time of occurrence of red rust through a salt spray test using a 5% NaCl solution at 35° C. Table 1 shows the compositions of hot-dip alloy-plating baths used to manufacture plated steels of embodiments and comparative examples of the present invention.TABLE 1CoolingrateClassificationAlMgZnAl:Mg(° C. / sec)Comparative0.20Bal.—7Example 1Comparative3.03.0Bal.1:17Example 2Comparative5.02.0Bal.2.5:1  7Example 3Comparative20.05.0Bal.4:17Example 4Comparative18.01.5Bal.9:17Example 5Comparative18.03.0Bal.6:13Example 6Embodiment6.03.0Bal.2:171Embodiment10.03.0Bal.3.3:1  72Embodiment12.04.0Bal.3:173Embodiment15.05.0Bal.3:174Embodiment18.03.0Bal.6:175

[0055] Referring to Table 1, the hot-dip alloy-plating baths of the embodiments contained 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities, wherein a content ratio of aluminum:magnesium satisfied a range of 2:1 to 6:1.

[0056] Comparative Example 1 has a lower aluminum content than the lower limit suggested by the present disclosure and differs in that it does not contain magnesium. Comparative Example 2 has a difference in that the aluminum content is lower than the lower limit suggested by the present invention, and does not satisfy the content ratio of aluminum:magnesium. Comparative Example 3 has differences in that the aluminum content and the magnesium content are lower than the lower limits suggested by the present invention. Comparative Example 4 has a difference in that the aluminum content is higher than the upper limit suggested by the present disclosure and the magnesium content is lower than the lower limit suggested by the present invention. Comparative Example 5 has differences in that the magnesium content is lower than the lower limit suggested by the present disclosure and does not satisfy the content ratio of aluminum:magnesium . Comparative Example 6 has a difference in that the cooling rate is lower than the lower limit suggested by the present invention.

[0057] Here, the overall composition of the hot-dip alloy-plated layer of the plated steel was found to be identical or nearly similar to the composition of the hot-dip alloy-plating bath. Therefore, the compositions of the hot-dip alloy-plating baths in Table 1 can be considered as the composition of the entire plated layer of the plated steel. Table 2 shows the characteristics and corrosion resistance of the hot-dip alloy-plated layers of the plated steels of the embodiments and comparative examples of the present invention.TABLE 2Fraction ofThicknessprimary Alratio ofphase orhot-dipAl / ZnThicknessalloy-Evaluationeutectoidof alloyplatedof red restphaselayerlayer:alloyoccurrenceClassification(%)(μm)layertimeComparative00.2—XExample 1Comparative0120:1ΔExample 2Comparative5120:1ΔExample 3Comparative0121.6:1 ΔExample 4Comparative8111.8:1 ΔExample 5Comparative12111.8:1 ΔExample 6Embodiment22120:1◯1Embodiment261.5 1:1◯2Embodiment32210:1⊚3Embodiment304 5:1⊚4Embodiment3272.8:1 ◯5

[0058] In Table 2, the evaluation of the red rust occurrence time is as follows: ⊚: 2400 hours or more, O: 1800 hours or more and less than 2400 hours, Δ: 800 hours or more and less than 1800 hours, X: less than 600 hours.

[0059] Referring to Table 2, in the embodiments, the area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both was found to be 20% or more, which satisfies the range of 20% to 60% suggested by the present invention. The embodiments showed that the thickness of the Fe—Al alloy layer was less than 7 μm, and the total thickness of the hot-dip alloy-plated layer was twice or more that of the thickness of the Fe—Al alloy layer. The embodiments show excellent corrosion resistance as the red rust occurrence time is 1800 hours or more.

[0060] Comparative Example 1 has a low aluminum content, so a primary Al phase or an Al / Zn eutectoid phase was not formed, and the thickness of the Fe—Al alloy layer was small, but the corrosion resistance was evaluated as poor because the red rust occurrence time was less than 600 hours.

[0061] In Comparative Example 2, a primary Al phase or an Al / Zn eutectoid phase was hardly formed due to the low aluminum content, and the thickness and thickness ratio of the Fe—Al alloy layer were small due to the low aluminum content, but the corrosion resistance was evaluated as poor because the red rust occurrence time was 800 hours or more and less than 1800 hours.

[0062] In Comparative Example 3, a primary Al phase or an Al / Zn eutectoid phase was formed in a small fraction due to the low aluminum content, and the thickness and thickness ratio of the Fe—Al alloy layer were small due to the low aluminum content, but the corrosion resistance was evaluated as poor because the red rust occurrence time was 800 hours or more and less than 1800 hours.

[0063] In Comparative Example 4, a primary Al phase or an Al / Zn eutectoid phase was not formed due to the high aluminum content and low magnesium content. Instead, a thick Fe—Al alloy layer was formed, and the corrosion resistance was evaluated as poor because the red rust occurrence time was 800 hours or more and less than 1800 hours.

[0064] In Comparative Example 5, a primary Al phase or an Al / Zn eutectoid phase was not formed in sufficient quantity due to the low magnesium content. Instead, a thick Fe—Al alloy layer was formed, and the corrosion resistance was evaluated as poor because the red rust occurrence time was 800 hours or more and less than 1800 hours.

[0065] In Comparative Example 6, aluminum and magnesium contents were appropriate, but a primary Al phase or an Al / Zn eutectoid phase was not sufficiently formed due to the low cooling rate. Instead, a thick Fe—Al alloy layer was formed, and the corrosion resistance was evaluated as poor because the red rust occurrence time was 800 hours or more and less than 1800 hours.

[0066] Therefore, for the primary Al phase or the Al / Zn eutectoid phase to be formed in the hot-dip alloy-plated layer, the total thickness of the hot-dip alloy-plated 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-plated layer should be at least twice as thick as the thickness of the Fe—Al alloy layer. Desirably, the fraction of the primary Al phase or Al / Zn eutectoid phase present in the hot-dip alloy-plated layer is present in an area fraction of 20% to 60% based on the cross-section of the hot-dip alloy-plated layer.

[0067] FIGS. 2A to 2C are scanning electron microscope photographs illustrating the microstructure of the highly corrosion-resistant plated steel according to an embodiment of the present disclosure.

[0068] FIG. 2A illustrates Comparative Example 4 where the aluminum content is higher than the upper limit suggested by the present disclosure and the magnesium content is lower than the lower limit suggested by the present invention, so that a Fe—Al alloy layer is formed thickly to a thickness of about 12 μm on a base steel.

[0069] FIG. 2B illustrates Embodiment 2 where a Fe—Al alloy layer was formed thinly to a thickness of about 1.5 μm on a base steel, and a primary Al phase was formed at a fraction of about 26%.

[0070] FIG. 2C illustrates Embodiment 4 where a Fe—Al alloy layer is formed thinly to a thickness of about 4 μm on a base steel, and a primary Al phase and an Al / Zn eutectoid phase are formed at a total fraction of about 30%.

[0071] Therefore, the embodiments show that the corrosion resistance is improved as the formation of a Fe—Al alloy layer is suppressed to the maximum extent and the primary Al phase and the Al / Zn eutectoid phase are sufficiently formed.

[0072] It will be obvious to those skilled in the art, to which the technical idea of the disclosure pertains, that the technical idea of the disclosure described above is not limited to the above-described embodiments and the accompanying drawings and various substitutions, modifications, and changes are possible within the scope of the technical idea of the invention.

[0073] 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 a Fe—Al alloy layer to provide excellent corrosion resistance can be provided.

Claims

1. A method of manufacturing a highly corrosion-resistant plated steel, the method comprising:immersing a base steel in a hot-dip alloy-plating bath comprising 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities;taking the immersed base steel out of the hot-dip alloy-plating bath to form a hot-dip alloy-plated layer on the base steel; andcooling the base steel having the hot-dip alloy-plated layer thereon,wherein a content ratio of aluminum:magnesium in the hot-dip alloy-plating bath is 2:1 to 6:1.

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

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

4. The method according to claim 1, wherein the cooling is performed at a cooling rate of 7° C. / sec to 30° C. / sec.

5. The method according to claim 1, wherein the cooling is performed at a cooling rate of 10° C. / sec to 15° C. / sec.

6. The method according to claim 1, wherein the hot-dip alloy-plated layer comprises a plated base layer and a Fe—Al alloy layer, anda total thickness of the hot-dip alloy-plated layer is twice or more a thickness of the Fe—Al alloy layer.

7. The method according to claim 6, wherein the plated base layer comprises a primary Al phase, an Al / Zn eutectoid phase, or both.

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

9. A highly corrosion-resistant plated steel, comprising:a base steel; anda plated layer formed on the base steel comprising 6% to 18% by weight of aluminum (Al), 3% to 6% by weight of magnesium (Mg), the remainder being zinc (Zn) and other inevitable impurities,wherein the hot-dip alloy-plated layer comprises a plated base layer and a Fe—Al alloy layer,the plated base layer comprises a primary Al phase, an Al / Zn eutectoid phase, or both, andan area fraction of the primary Al phase, the Al / Zn eutectoid phase, or both in the hot-dip alloy-plated layer is in a range of 20% to 60%.

10. The plated steel according to claim 9, wherein the plated layer comprises a plated base layer and a Fe—Al alloy layer, and a total thickness of the hot-dip alloy-plated layer is twice or more a thickness of the Fe—Al alloy layer.

11. The plated steel according to claim 9, wherein a content ratio of aluminum:magnesium in the plated layer is in a range of 2:1 to 6:1.

12. The plated steel according to claim 9 wherein the plated layer is obtained from a hot-dip alloy-plating bath.

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