Zn-al-mg-based alloy-plated steel sheet and manufacturing method therefor

Abstract

The present invention provides: an alloy-plated steel sheet having excellent resistance to low-temperature joint embrittlement while having a level of ultra-high corrosion resistance suitable for use even in harsh corrosive environments such as marine environments, as well as in land environments; and a manufacturing method therefor.

Description

Zn-Al-Mg alloy plated steel sheet and method for manufacturing the same

[0001] The present invention relates to a Zn-Al-Mg alloy plated steel sheet and a method for manufacturing the same.

[0002] Galvanized steel sheets possess a so-called sacrificial protection characteristic, in which zinc (Zn), having a lower oxidation-reduction potential than iron (Fe), corrodes first when exposed to a corrosive environment, thereby inhibiting the corrosion of the steel. Furthermore, as Zn, the main component of the plating layer, oxidizes, it forms dense corrosion products on the surface of the steel; these products protect the steel from oxidizing atmospheres, thereby improving the steel's corrosion resistance. Due to these advantageous characteristics, the application range of zinc-based galvanized steel has recently been expanding to include construction materials, home appliances, and automotive materials.

[0003] However, corrosive environments are gradually deteriorating due to increased air pollution resulting from the diversification and advancement of industries. As we face increasingly complex corrosive conditions, there is a growing need for the development of steel materials that possess superior corrosion resistance in various environments compared to conventional galvanized steel sheets. Furthermore, with the recent strengthening of carbon neutrality policies leading to an increase in solar power generation as an eco-friendly energy source, power facilities are being constructed not only on land but also on coastlines and at sea; consequently, the corrosion resistance of steel materials used in these facilities is becoming even more critical.

[0004] Meanwhile, in addition to galvanized steel sheets, aluminum (Al)-based plated steel sheets such as Al plating and Al-Zn plating (Galvalume) are being presented. However, while the above-mentioned aluminum-based plated steel sheets exhibit excellent corrosion resistance of the plating layer itself in weakly acidic or neutral corrosive environments, there is a problem in that when the base steel sheet is exposed to a corrosive environment at the cut section, the sacrificial corrosion protection ability of the Al-based plating layer against the base steel sheet (Fe) is lost, making it prone to corrosion at the cut section where the base steel sheet is exposed.

[0005] As one of the measures to improve these problems, various studies are being conducted on manufacturing technology for zinc alloy-based plated steel sheets, which can further improve the corrosion resistance of the plated steel sheets produced therefrom by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc (Zn) plating bath.

[0006] As a representative example, Patent Document 1 discloses a hot-dip galvanized steel sheet having good corrosion resistance after coating, wherein the composition of the hot-dip galvanized layer consists of aluminum (Al): 3.0~6.0 wt%, magnesium (Mg): 1.0~7.0 wt%, and the remainder is Zn and unavoidable impurities, and the microstructure of the galvanized layer consists of a Zn primary phase and a Zn-Al-Mg ternary eutectic phase. Additionally, Patent Document 2 discloses a coated galvanized steel material for concrete structures having a galvanized layer composed of Zn, Mg: 2~10 wt%, Al: 4~20 wt%, and Si: 0.01~2 wt%, as well as Mg: 2~10 wt%, Al: 4~20 wt%, and Si: 0.01~2 wt%, as a galvanized steel sheet capable of securing sufficient corrosion resistance in an environment in contact with concrete. This document discloses that by configuring the structure of the plating layer into a structure in which Mg2Si phase and Al phase are mixed within a ternary eutectic structure of Al / Zn / MgZn2, it possesses strong alkalinity and exhibits excellent corrosion resistance even in concrete environments. However, the plated steel sheets according to the above patent documents 1 and 2 have somewhat low Al and Mg content, so there are limitations in achieving the level of corrosion resistance required in marine environments.

[0007] Meanwhile, Patent Document 3 discloses a plated steel sheet in which the composition of the plating layer consists of Zn: 20~60%, Al: 0.3~15.0%, and the remainder being Mg and impurities, and further improves corrosion resistance by including a quasicrystalline phase at the interface between the plating layer and the substrate / plating layer. When the plating layer structure includes a quasicrystalline phase, the corrosion resistance is superior compared to when it includes a crystalline phase; however, as disclosed in Patent Document 3, the hardness of the quasicrystalline phase is significantly higher, so when the crystalline phase and the quasicrystalline phase are mixed, cracks are prone to occur at the interphase boundaries or within the quasicrystalline phase during processing of the plated steel sheet. Consequently, there is a risk that the plating layer may detach or peel off when the plated steel sheet is formed into a structure, which has the disadvantage of not being suitable as a material for structures requiring deep processing.

[0008] Patent Document 4 discloses a plated steel sheet in which the plating layer composition consists of Zn: 11~80 mass%, Mg: 8~45 mass%, and Al: 3~80 mass%, and contains a quasicrystalline phase in a volume fraction of 5% or more within the plating layer. While the plated steel sheet of the present document has high levels of Mg and Al and contains a quasicrystalline phase, thereby greatly improving corrosion resistance, the quasicrystalline phase is susceptible to processing. Therefore, when deep processing is required, the plating layer peels off during the processing process, and there is a problem in that local corrosion resistance is significantly degraded in that area.

[0009] As such, in the case of Zn-Al-Mg plated steel sheets containing Al and Mg in addition to Zn, a higher content of Mg and / or Al in the plating layer is advantageous in terms of improving the corrosion resistance of the steel sheet; however, as the hardness of the plating layer increases, cracks are prone to occur in the plating layer during the processing of products requiring complex machining, and if such cracks are severe, there is a problem in that corrosion resistance actually decreases.

[0010] Furthermore, when the plating layer has a hexagonal closed-packing structure, the slip system is limited and twin deformation does not occur when stretched along the C-axis (height direction of the crystal structure). Consequently, it is not only vulnerable to tension, but also, while brittle, grain boundary, and ductile fractures act together at temperatures above room temperature, only brittle (cleavage) fracture occurs at low temperatures, making it susceptible to failure caused by external impact. In other words, as the orientation ratio of the (00l) planes of the hexagonal structure within the plating layer increases, the ductility of the plating layer decreases, leading to a problem where brittle fracture becomes more likely.

[0011] (Patent Document 1) Japanese Published Patent Application No. 2000-219950

[0012] (Patent Document 2) Japanese Published Patent Application No. 2011-219791

[0013] (Patent Document 3) Japanese Registered Patent Publication No. 6075513

[0014] (Patent Document 4) Japanese Published Patent Application No. 2017-066459

[0015] One aspect of the present invention is to provide an alloy-plated steel sheet having excellent low-temperature bonding brittleness and ultra-high corrosion resistance suitable for use in harsh corrosive environments such as marine environments as well as terrestrial environments, and a method for manufacturing the same.

[0016] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0017] According to one aspect of the present invention, the invention comprises a base steel plate; an Fe-Al alloy layer provided on at least one surface of the base steel plate; and a Zn-Al-Mg alloy plating layer provided on the alloy layer.

[0018] The above Zn-Al-Mg alloy plating layer includes an MgZn2 phase, and the MgZn2 phase satisfies the following relationship 1 when analyzed by X-ray diffraction, thereby providing a Zn-Al-Mg alloy plated steel sheet.

[0019] [Relationship 1]

[0020] I1∑(MgZn2) / I0∑(MgZn2) ≥ 0.87

[0021] (In Equation 1, I0∑(MgZn2) represents the sum of the X-ray diffraction peak intensities of the (100) plane, (002) plane, (101) plane, (102) plane, (103) plane, (110) plane, (112) plane, (201) plane, (004) plane, (203) plane, (213) plane, (220) plane, (313) plane, and (402) plane on MgZn2, and I1∑(MgZn2) represents the sum of the X-ray diffraction peak intensities of the (100) plane, (101) plane, (102) plane, (103) plane, (110) plane, (112) plane, (201) plane, (203) plane, (213) plane, (220) plane, (313) plane, and (402) plane on MgZn2 (means sum)

[0022] As such, a Zn-Al-Mg alloy-plated steel sheet having a Zn-Al-Mg alloy plating layer having an alloy composition and a metallic structure according to one embodiment of the present invention may have excellent corrosion resistance, particularly in a range extending not only to acidic environments but also to neutral and alkaline environments. Furthermore, the Zn-Al-Mg alloy-plated steel sheet may have excellent low-temperature bonding brittleness.

[0023] In one embodiment of the present invention, the Zn-Al-Mg alloy plating layer may comprise, in terms of area fraction, an Al phase: 20.0~80.0%, a MgZn2 phase: 20.0~65.0%, and a remainder phase: 40.0% or less (including 0%).

[0024] In one embodiment of the present invention, the remainder phase may be one or more of the Zn phase, Al-Zn phase, Mg-Si phase, Al-Zn-Ca phase, and Al-Si-Ca-Zn phase.

[0025] In one embodiment of the present invention, the Zn-Al-Mg alloy plating layer may comprise, in weight percent, aluminum (Al): 20~50%, magnesium (Mg): 8.0~15.0%, silicon (Si): 0.02~1.00%, calcium (Ca): 0.005~0.500%, and the remainder being zinc (Zn) and unavoidable impurities.

[0026] In one embodiment of the present invention, the Zn-Al-Mg alloy plating layer may further include one or more elements selected from the following groups i) to iii).

[0027] i) Total content of elements derived from the base steel sheet: 1,000% or less

[0028] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1,000% or less

[0029] iii) Total content of elements added to control the surface quality of alloy-plated steel sheets: 1,000% or less

[0030] In one embodiment of the present invention, the element belonging to i) may be one or more of Cr, Mn, Co, Ti, Ni, Cu, V, Nb, Mo, W, and B; the element belonging to ii) may be one or more of Y, Zr, La, Ce, Sr, and Be; and the element belonging to iii) may be one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

[0031] In one embodiment of the present invention, the thickness of the Zn-Al-Mg alloy plating layer may be 6 to 50 μm, and the thickness of the Fe-Al alloy layer may be 0.01 to 7.00 μm.

[0032] In one embodiment of the present invention, the Fe-Al alloy layer is FeAl, FeAl2, FeAl3, FeAl4, Fe2Al 5, It can be composed of one or more alloy phases among Fe2Al and Fe3Al.

[0033] According to the present invention, an alloy-plated steel sheet with significantly improved corrosion resistance can be provided. In particular, the alloy-plated steel sheet of the present invention has the effect of being advantageously applicable not only to structures in onshore areas but also to environments requiring ultra-high corrosion resistance, such as beaches or offshore areas. Furthermore, the alloy-plated steel sheet of the present invention exhibits excellent low-temperature bonding brittleness, making it suitable for application as a material for structures and components used in environments with extremely low external temperatures, such as bridges, various pipelines, and gas storage tanks.

[0034] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0035] Figure 1 shows a photograph of the cross-sectional structure of the alloy plating layer of a Zn-Al-Mg alloy-plated steel sheet according to one embodiment of the present invention, observed using FE-SEM.

[0036] Figure 2 shows the X-ray measurement results (peak intensity graph) of the culture surface of the MgZn2 phase within the alloy plating layer of a Zn-Al-Mg alloy-plated steel sheet according to one embodiment of the present invention.

[0037] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0038] In addition, embodiments of the present invention are provided to more fully explain the invention to those with average knowledge in the relevant technical field.

[0039] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0040] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or practices of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0041] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0042] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0043] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0044] As the corrosive environments in which conventional galvanized steel sheets have been used have become increasingly harsh and complex, leading to a growing demand for galvanized steel sheets with ultra-high corrosion resistance, the inventors of the present invention have conducted in-depth research to provide galvanized steel sheets suitable for such needs. As a result, by optimizing the alloy composition and metal structure of the plating layer, it was confirmed that it is possible to provide an alloy-plated steel sheet that not only possesses ultra-high corrosion resistance but also exhibits excellent low-temperature bonding brittleness, thereby completing the present invention.

[0045] The present invention will be described in detail below.

[0046] In the following description, an alloy-plated steel sheet according to one aspect of the present invention is described. Unless otherwise specifically stated, the content of each element of the alloy composition constituting the alloy-plated steel sheet is based on weight. Additionally, it should be noted that when describing the microstructure in terms of ratios, the basis is area.

[0047] According to one aspect of the present invention, a Zn-Al-Mg alloy plated steel sheet is provided.

[0048] In one embodiment of the present invention, a Zn-Al-Mg alloy-plated steel sheet may comprise a base steel sheet; an Fe-Al alloy layer provided on at least one surface of the base steel sheet; and a Zn-Al-Mg alloy plating layer provided on the alloy layer.

[0049] In one embodiment of the present invention, the base steel plate may be any steel suitable for manufacturing plated steel plates. As a non-limiting example, the base steel plate may be carbon steel containing a certain amount of carbon (C), and various steels such as stainless steel and aluminum plates may be applied. As one example, if the base steel plate is carbon steel, it may be ultra-low carbon steel, medium-low carbon steel, low carbon steel, general carbon steel, high carbon steel, etc., which are well known in the steel industry, and all of these can exhibit similar effects when used as base steel plates for alloy plated steel plates. Therefore, the alloy components of the carbon steel are not specifically limited. In particular, since there is almost no influence from elements such as manganese (Mn), silicon (Si), titanium (Ti), niobium (Nb), and boron (B), which are actively added to obtain carbon steel having high strength or ultra-high strength, no limitation is placed on the alloy components of the carbon steel.

[0050] In one embodiment of the present invention, the base steel sheet may be a hot-rolled steel sheet manufactured through a series of hot-rolling processes, or a cold-rolled steel sheet manufactured through a series of cold-rolling processes on the hot-rolled steel sheet. Additionally, the cold-rolled steel sheet may be an annealed steel sheet that has undergone an annealing process performed after the cold-rolling, or an un-anesthetized steel sheet that has not undergone the annealing process.

[0051] An alloy-plated steel sheet according to one embodiment of the present invention may include a plating layer on at least one surface of the base steel sheet. In one embodiment of the present invention, the plating layer may be an alloy plating layer, specifically a Zn-Al-Mg-based alloy plating layer.

[0052] As will be explained in detail below, the above Zn-Al-Mg alloy plating layer can be formed using a molten plating bath containing elements such as aluminum (Al) and magnesium (Mg) in addition to zinc (Zn). From this, the alloy-plated steel sheet having the above Zn-Al-Mg alloy plating layer can be provided with corrosion resistance due to Al and Mg in addition to sacrificial protection due to Zn. In particular, according to one embodiment of the present invention, the corrosion resistance of the alloy-plated steel sheet can be further improved by limiting the composition of the above Zn-Al-Mg alloy plating layer as follows.

[0053] Specifically, a Zn-Al-Mg alloy plating layer according to one embodiment of the present invention may be composed of, in weight percent, aluminum (Al): 20~50%, magnesium (Mg): 8.0~15.0%, silicon (Si): 0.02~1.00%, calcium (Ca): 0.005~0.500%, iron (Fe): 0.300% or less, and the remainder being zinc (Zn) and unavoidable impurities. The Zn-Al-Mg alloy plating layer may be formed using a Zn-Al-Mg plating bath, and the content of Al, Mg, Si, Ca, Fe, and Zn in the plating bath may match the content of each element in the Zn-Al-Mg alloy plating layer formed therefrom.

[0054] Al and Mg in the above Zn-Al-Mg alloy plating layer are elements that play a role in improving the corrosion resistance of the alloy-plated steel sheet. Among these, since Al exhibits strong resistance to acid, the higher the content of Al in the alloy plating layer, the greater the improvement in corrosion resistance in acidic environments. However, as the higher the content of Al, the greater the tendency for corrosion in alkaline environments to increase, it is necessary to appropriately limit the content. In addition, Al has the effect of inhibiting the oxidation of Mg in the plating bath, so the higher the content, the more advantageous it is for inhibiting the formation of MgO-based dross. However, if the content of Al is excessive, the melting point of the plating bath rises, requiring the temperature of the plating bath to be maintained high during the plating process. In this case, in addition to the problem of intensified erosion of structures within the plating bath, an Fe-Al alloy layer may be formed excessively thickly at the interface between the base steel sheet and the plating layer, which may cause plating peeling during processing. Taking all of this into consideration, in one embodiment of the present invention, the Al may be included in an amount of 20 to 50%. According to another embodiment of the present invention, the Al may be 22% or more, 25% or more, and according to yet another embodiment, the Al may be 45% or less, 40% or less, 38% or less.

[0055] If the Mg content in the Zn-Al-Mg alloy plating layer is less than 8.0%, it is not possible to secure the level of ultra-high corrosion resistance required in a marine environment. Similar to Al, a higher Mg content is advantageous for improving corrosion resistance; however, if the content exceeds 15.0%, the corrosion resistance effect becomes saturated, and if the content exceeds 20.0%, corrosion resistance in a marine environment may actually decrease. Furthermore, Mg is an element with a high affinity for oxygen, and it oxidizes to MgO on the surface of the plating bath in contact with air, thereby impairing plating performance. Considering this, in one embodiment of the present invention, the Mg may be included in an amount of 8.0 to 15.0%. According to another embodiment of the present invention, the Mg may be 8.5% or more and 9.0% or more, and according to yet another embodiment, the Mg may be 14.5% or less and 14.0% or less.

[0056] As described above, in order to improve the corrosion resistance of an alloy-plated steel sheet according to one embodiment of the present invention, a Zn-Al-Mg alloy plating layer containing a certain amount of Al is provided on a base steel sheet. During the plating process to obtain such a Zn-Al-Mg alloy plating layer, Fe in the base steel sheet reacts with Al in the plating bath to form an Fe-Al alloy layer at the interface between the base steel sheet and the plating layer. If the Fe-Al alloy layer is formed too thickly, there is a risk of impeding the adhesion between the base steel sheet and the plating layer, and the interfacial strength between the base steel sheet and the plating layer may be reduced. Considering this, one embodiment of the present invention includes some Si in the Zn-Al-Mg alloy plating layer. At this time, by including Si at a minimum of 0.02%, an effect of inhibiting the growth of the Fe-Al alloy layer can be obtained. However, as the Si content increases, the melting point of the plating bath rises, requiring the temperature of the plating bath to be maintained high during the plating process, which leads to a problem of intensified erosion of structures within the plating bath. In addition, if the temperature of the plating bath is increased, the reaction between the steel sheet and the plating bath during the plating process may become severe, and a thick Fe-Al alloy layer may be formed. This causes severe powdering of the plating layer during processing of the final alloy-plated steel sheet, and in that case, there is a problem of poor corrosion resistance after processing. Considering this, the above Si may be included in an amount of 1.00% or less.

[0057] In one embodiment of the present invention, Ca may be further included to more effectively suppress the formation of MgO oxides in a plating bath containing Mg. To sufficiently obtain the aforementioned effect, Ca may be included in the plating bath at a concentration of 0.005% or more, and thereby Ca may be included in the Zn-Al-Mg alloy plating layer at a concentration of 0.005% or more. However, if the content is too excessive, the formation of CaO-based oxides in the plating bath increases, and these form on the surface of the plating bath together with MgO oxides and adhere to the substrate steel plate during the plating process, thereby causing dross adhesion defects. Considering this, Ca may be included at a concentration of 0.500% or less.

[0058] Meanwhile, the Zn-Al-Mg alloy plating layer according to one embodiment of the present invention may contain some Fe. During the process of forming the Zn-Al-Mg alloy plating layer, that is, during the process of the substrate steel sheet passing through the plating bath, Fe leached from the substrate steel sheet into the plating bath exists in a solid solution state or as fine spherical particles, and remains in the plating layer as the plating solution is plated onto the surface of the substrate steel sheet. Since a large amount of such Fe may impair corrosion resistance, it is advantageous to include it at a level that does not impair the corrosion resistance of the alloy-plated steel sheet. Accordingly, in one embodiment of the present invention, the Fe may be included in an amount of 0.300% or less, and it is acceptable for it to be 0.000%. It should be noted that the Fe content at this time excludes the Fe content constituting the Fe-Al alloy layer to be described later.

[0059] A Zn-Al-Mg alloy plating layer according to one embodiment of the present invention comprises Zn and unavoidable impurities as residual components in addition to the aforementioned elements. The Zn is an important element for ensuring the corrosion resistance of the alloy-plated steel sheet together with the Al and Mg, and the content of Zn can be adjusted to match the content of the aforementioned Al, Mg, Si, Ca, etc. The Zn-Al-Mg alloy plating layer according to one embodiment of the present invention may include other unavoidable impurities in addition to the aforementioned alloy composition. Since such impurities may be unintentionally incorporated from raw materials or the surrounding environment during the conventional steel manufacturing process, they cannot be completely eliminated. As any skilled technician in the conventional steel manufacturing process would be aware of these impurities, the present invention does not specifically mention all such details.

[0060] However, the Zn-Al-Mg alloy plating layer according to one embodiment of the present invention may further include the following components, and the following elements may be 0.000%.

[0061] i) Total content of elements derived from the base steel sheet: 1,000% or less

[0062] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1,000% or less

[0063] iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less

[0064] The elements of i) above are elements that mainly constitute the base steel sheet, and are elements that exist in the plating bath while leaching from the base steel sheet during the plating process and are incorporated into the alloy plating layer. The elements of i) above may contribute to the formation of corrosion resistance or contribute to grain stabilization. These elements do not have a significant effect on the physical properties intended in the present invention up to a total content of up to 1.000%, but if the content exceeds 1.000%, there is a risk that the plating appearance characteristics may be impaired. As a specific example, the elements belonging to group i) above may be one or more of Cr, Mn, Co, Ti, Ni, Cu, V, Nb, Mo, W, and B.

[0065] The elements of ii) above can inhibit the oxidation of Mg in the plating bath and slightly improve the corrosion resistance of the alloy-plated steel sheet. However, since there is a concern that the brittleness of the alloy plating layer may increase if the total content exceeds 1.000%, they may be included in an amount of 1.000% or less. As a specific example, the elements belonging to group ii) above may be one or more of Y, Z, La, Ce, Si, and Be. Meanwhile, as another embodiment, the elements of ii) above may be included in an amount of 0.200% or less.

[0066] The elements of iii) above are elements that affect the size of spangles, which are the solidification structure of the alloy plating layer, and may be included in a certain amount if spangles of a specific size are to be obtained. However, if the total content exceeds 1.000%, there is a risk that the alloy-plated steel sheet may easily discolor when exposed to a humid environment, so they may be included in an amount of 1.000% or less. As a specific example, the elements belonging to group iii) above may be one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In. Meanwhile, as another embodiment, the elements of iii) above may be included in an amount of 0.500% or less.

[0067] Furthermore, the Zn-Al-Mg-based plating layer may include additional elements other than the alloying elements described above within the scope of the technical concept of the present invention, and the route of addition of these elements (addition to the plating bath, origin from the base steel sheet, etc.) is not particularly limited.

[0068] In one embodiment of the present invention, the Zn-Al-Mg alloy plating layer having the aforementioned alloy composition may have a thickness of 6 to 50 μm on one side. If the thickness of one side of the alloy plating layer is less than 6 μm, the intended ultra-high corrosion resistance cannot be obtained, whereas if it exceeds 50 μm, the solidification of the plating layer may occur unevenly, causing problems such as the occurrence of flow patterns.

[0069] Hereinafter, the metal structure of a Zn-Al-Mg alloy plating layer according to one embodiment of the present invention will be described in detail.

[0070] Specifically, the metallographic structure of the Zn-Al-Mg alloy plating layer may comprise, in terms of area fraction, an Al phase of 20.0–80.0%, a MgZn2 phase of 20.0–65.0%, and a remainder of 40.0% or less (including 0%). The metallographic structure of the Zn-Al-Mg alloy plating layer is represented based on the thickness cross-section of the alloy plating layer, and as one example, it can be represented by measuring the fraction of each phase observed in the thickness cross-section of the alloy plating layer as shown in FIG. 1.

[0071] In one embodiment of the present invention, the Al phase in the metal structure of the Zn-Al-Mg alloy plating layer exhibits excellent corrosion resistance in the pH range of the corrosive medium (neutral and acidic regions), while the MgZn2 phase exhibits excellent corrosion resistance in the alkaline region. Accordingly, by appropriately controlling the fractions of the Al phase and the MgZn2 phase, excellent corrosion resistance can be secured in most environments ranging from acidic to neutral and alkaline.

[0072] In addition, since the above Al phase has a cubic structure, the Zn-Al-Mg alloy plated steel sheet according to one embodiment of the present invention can have excellent processability.

[0073] In one embodiment of the present invention, the Al phase plays a role in preventing corrosion agents from rapidly penetrating into the steel plate in an acidic atmosphere, so it is advantageous for its fraction to be 20.0% or more. However, if the fraction of the Al phase is excessive, excellent corrosion resistance in an alkaline environment cannot be achieved, so it may be included at 80.0% or less.

[0074] Meanwhile, in one embodiment of the present invention, Zn may be dissolved in the Al phase in an amount of 3.0 to 35.0% and Mg in an amount of 0.1 to 20.0%, but even if Zn and Mg are contained in this way, the crystal structure of the Al phase is not changed and can maintain a cubic structure.

[0075] In one embodiment of the present invention, the MgZn2 phase may be included along with the aforementioned Al phase. As previously mentioned, the MgZn2 phase has excellent corrosion resistance in an alkaline atmosphere, and in order to obtain this effect, the MgZn2 phase may be included in an area fraction of 20.0% or more. However, if the fraction of the MgZn2 phase is excessive and exceeds 65.0%, the fraction of the Al phase is relatively reduced, and corrosion resistance may be inferior in a neutral atmosphere, particularly in a saltwater environment or an alkaline atmosphere.

[0076] The above MgZn2 phase has a hexagonal structure, and when the alloy plating layer has such a hexagonal structure, the higher the cultivation ratio of the (OO1) plane within the hexagonal structure, the lower the ductility of the alloy plating layer becomes, making it prone to brittle fracture. Accordingly, in one embodiment of the present invention, the MgZn2 phase can satisfy the following relationship 1 during X-ray diffraction analysis.

[0077] [Relationship 1]

[0078] I1∑(MgZn2) / I0∑(MgZn2) ≥ 0.87

[0079] (In Equation 1, I0∑(MgZn2) represents the sum of the X-ray diffraction peak intensities of the (100) plane, (002) plane, (101) plane, (102) plane, (103) plane, (110) plane, (112) plane, (201) plane, (004) plane, (203) plane, (213) plane, (220) plane, (313) plane, and (402) plane on MgZn2, and I1∑(MgZn2) represents the sum of the X-ray diffraction peak intensities of the (100) plane, (101) plane, (102) plane, (103) plane, (110) plane, (112) plane, (201) plane, (203) plane, (213) plane, (220) plane, (313) plane, and (402) plane on MgZn2 (means sum)

[0080] In one embodiment of the present invention, the method for measuring the culture ratio of the (00l) plane on the MgZn2 phase is not specifically limited. However, as a non-limiting example, peak intensity values ​​measured through X-ray diffraction analysis may be used. As one example, if X-ray diffraction analysis is performed on the surface of a Zn-Al-Mg alloy-plated steel sheet according to one embodiment of the present invention, peak intensity corresponding to each plane can be obtained, and the value can be calculated by substituting the peak intensity value of each plane into the above-mentioned Equation 1. The X-ray diffraction analysis may use Cu-K rays and may also be measured under conditions of X-ray output of 40 kV and 200 mA.

[0081] In one embodiment of the present invention, if the value of the above relationship 1 is less than 0.87, the ratio of the (002) plane and the (004) plane increases, making it difficult to obtain brittleness resistance in a cryogenic environment.

[0082] A Zn-Al-Mg alloy plating layer according to one embodiment of the present invention may further include a remainder phase of 40.0% or less (including 0%) in addition to the Al phase and the MgZn2 phase. The type of the remainder phase is not particularly limited and may be determined according to the constituent elements constituting the Zn-Al-Mg alloy plating layer. As a non-limiting example, the remainder phase may be one or more of the Zn phase, Al-Zn phase, Mg-Si phase, Al-Zn-Ca phase, and Al-Si-Ca-Zn phase. Here, the Mg-Si phase may be typically the Mg2Si phase, and the Al-Zn-Ca phase may be typically Al 2.94 CaZn 1.06 It may be a phase. If the fraction of these residual phases exceeds 40.0%, the fractions of the aforementioned Al phase and MgZn2 phase decrease relatively, making it difficult to obtain the target corrosion resistance.

[0083] A Zn-Al-Mg alloy-plated steel sheet according to one embodiment of the present invention may include an Fe-Al alloy layer between the aforementioned base steel sheet and the Zn-Al-Mg alloy plating layer. That is, the Fe-Al alloy layer is provided on at least one surface of the base steel sheet, and the Zn-Al-Mg alloy plating layer may be provided on this alloy layer.

[0084] As previously mentioned, the Fe-Al alloy layer is formed between the base steel plate and the alloy plating layer during the process of plating the base steel plate by immersing it in an Al-containing plating bath, when Fe leached from the base steel plate reacts with Al in the plating bath. In this way, the presence of the Fe-Al alloy layer enhances the adhesion between the base steel plate and the alloy plating layer.

[0085] In one embodiment of the present invention, the Fe-Al alloy layer may have a thickness of 0.01 to 7.00 μm. If the thickness of the Fe-Al alloy layer is less than 0.01 μm, the adhesion between the base steel sheet and the alloy plating layer is not sufficiently high, which may cause problems such as the plating layer detaching during processing of the alloy-plated steel sheet. On the other hand, if the thickness of the Fe-Al alloy layer exceeds 7.00 μm, the adhesion between the base steel sheet and the alloy plating layer is excellent, but due to the high hardness of the Fe-Al alloy layer, powdering is likely to occur during processing.

[0086] Meanwhile, the alloy phase constituting the above Fe-Al alloy layer is not specifically limited, but as an example, it may be one or more alloy phases among FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al. In addition, one or more of Zn, Mg, Si, Ca, Ni, and Mn may be additionally contained in the above Fe-Al alloy phase in an amount of 5 weight% or less, and even in such cases, there is no difficulty in ensuring adhesion between the substrate steel sheet and the alloy plating layer.

[0087] Hereinafter, a method for manufacturing a Zn-Al-Mg alloy-plated steel sheet according to another aspect of the present invention will be described in detail.

[0088] However, it should be noted that the following method is merely one example for manufacturing a Zn-Al-Mg alloy-plated steel sheet, and that the Zn-Al-Mg alloy-plated steel sheet according to one embodiment of the present invention must not necessarily be manufactured by this manufacturing method, and that any manufacturing method that satisfies the claims of the present invention may be used to implement each embodiment of the present invention without any problem.

[0089] According to one embodiment of the present invention, a Zn-Al-Mg alloy-plated steel sheet can be manufactured by including the steps of: preparing a base steel sheet; alloy plating the base steel sheet; gas wiping treatment after alloy plating; and cooling after controlling the amount of plating.

[0090] In one embodiment of the present invention, the base steel sheet for obtaining the plated steel sheet may be the aforementioned base steel sheet, and it should be noted that there are no specific limitations on its composition and that the foregoing details serve as a substitute. As a non-limiting example, the base steel sheet may be carbon steel, and the carbon steel may be a hot-rolled steel sheet or a cold-rolled steel sheet, and the cold-rolled steel sheet may be an annealed steel sheet or an un-anesthetized steel sheet. Here, the hot-rolled steel sheet may be pickled for the purpose of removing scale, etc., formed on its surface during the hot rolling process. The pickling treatment may be performed under normal conditions, and such conditions are not specifically mentioned. In addition, the cold-rolled steel sheet may be degreased. In this case, the degreasing treatment may be performed by immersing the cold-rolled steel sheet in an alkaline solution, or by spray and / or electrolytic degreasing methods.

[0091] After preparing the aforementioned base steel plate, the base steel plate is immersed in an alloy Zn-Al-Mg plating bath and subjected to alloy plating treatment, thereby obtaining an alloy-plated steel plate having a Zn-Al-Mg alloy plating layer on at least one surface of the base steel plate.

[0092] In one embodiment of the present invention, the Zn-Al-Mg plating bath may be composed, in weight percent, of aluminum (Al): 20.0~50.0%, magnesium (Mg): 8.0~15.0%, silicon (Si): 0.02~1.00%, calcium (Ca): 0.005~0.500%, and the remainder being zinc (Zn) and unavoidable impurities. The content of each component in the plating bath is replaced by the previously mentioned details. Additionally, in the process of immersing the substrate steel plate in the Zn-Al-Mg plating bath to perform plating, Fe leached from the substrate steel plate may be present in the plating bath. At this time, the content of Fe present in the plating bath is not specifically limited, and it is acceptable as long as the Fe is present in the formed Zn-Mg-Al alloy plating layer at a level of 0.300% or less.

[0093] In one embodiment of the present invention, the substrate steel plate can be immersed in the Zn-Al-Mg plating bath to perform alloy plating treatment. When immersing the substrate steel plate in the Zn-Al-Mg plating bath, if the temperature of the substrate steel plate is lower than the temperature of the plating bath, the plating adhesion is reduced, and the target Zn-Al-Mg alloy plating layer cannot be obtained. Accordingly, the substrate steel plate can be heated to a certain temperature before being immersed in the Zn-Al-Mg plating bath.

[0094] In one embodiment of the present invention, the process of heating the base steel sheet may be performed by passing it through a heat treatment furnace (e.g., an oxidation-free furnace or a reduction furnace). The temperature at which the base steel sheet is heated may be the same as the temperature of the Zn-Al-Mg plating bath or may be set to a temperature higher than the temperature of the plating bath, and the temperature is not specifically limited. As one example, it may be set at least 10°C higher than the temperature of the plating bath, and as another example, it may be heated to around 600°C and then cooled to an appropriate temperature before immersion in the plating bath. As yet another example, if the base steel sheet is an un-heated cold-rolled steel sheet, it may be charged into an annealing furnace controlled by a reducing atmosphere after the washing and drying treatment above, and then cooled to an appropriate temperature after performing annealing heat treatment. The annealing heat treatment conditions at this time are not specifically limited and may be conditions for annealing a normal cold-rolled steel sheet, and may be an annealing furnace maintained in a reducing atmosphere by controlling the dew point temperature.

[0095] Meanwhile, in one embodiment of the present invention, a washing step may be further performed prior to alloy plating the base steel plate. At this time, the washing treatment may be performed before the base steel plate passes through a heat treatment furnace and is immersed in a plating bath. Depending on the type of base steel plate, the washing treatment may be performed for the following purposes. As one example, if the base steel plate is a hot-rolled steel plate, it may be performed to remove acid components remaining on the surface immediately after a pickling process to remove oxide scale present on the surface of the hot-rolled steel plate. As another example, if the base steel plate is a cold-rolled steel plate, it may be performed to remove degreasing solution remaining on the surface immediately after a degreasing process performed to remove foreign substances, such as rust-preventive oil, applied to the surface of the cold-rolled steel plate.

[0096] In one embodiment of the present invention, the washing treatment may be performed using water. Typically, water may be tap water, purified water, stored water, groundwater, etc., and trace amounts of impurities may be present in such water, and these impurities may be physically or chemically attached to the surface of the base steel sheet. If subsequent drying and heating processes are performed while impurities remain on the surface of the base steel sheet, the impurities react with the base steel sheet at high temperatures and remain in the form of black spots during the subsequent plating process, causing surface defects. Furthermore, the washing treatment affects the surface condition of the base steel sheet, and as the surface condition changes, the formation behavior of the interfacial Fe-Al alloy phase changes, which may affect the structure of the alloy plating layer on top of the Fe-Al alloy layer.

[0097] Accordingly, the inventors have conducted various studies and discovered that the aforementioned problem can be prevented by controlling the content of sodium (Na), potassium (K), and calcium (Ca) among the impurities present in the water used for washing. Thus, in one embodiment of the present invention, the washing treatment can be performed using water in which the content of Na is 80 ppm or less, the content of K is 60 ppm or less, the content of Ca is 100 ppm or less, and the total content of Na, K, and Ca is 180 ppm or less.

[0098] In one embodiment of the present invention, the washed steel sheet may be subjected to a drying treatment. The drying treatment may be performed to remove water remaining on the steel sheet after the washing treatment. The conditions for the drying treatment are not specifically limited and may be performed under normal drying conditions. Furthermore, any appropriate temperature capable of evaporating the water is acceptable.

[0099] Meanwhile, the temperature of the plating bath is determined by the composition within the plating bath. If the temperature of the plating bath is maintained at a level similar to the melting point, the plating layer may solidify during the wiping process to control the plating amount after plating, making it difficult to control the amount of plating. On the other hand, the higher the temperature of the plating bath is above the melting point, the better the fluidity of the plating bath becomes, allowing for a smooth plating layer to be obtained after plating. However, if the temperature becomes excessively high, there is a problem in that components within the base steel sheet leach into the plating bath during the plating process, resulting in a large amount of dross. Furthermore, as the Fe-Al alloy layer formed between the base steel sheet and the plating layer becomes too thick, it becomes prone to peeling of the plating layer during subsequent processing. Moreover, the higher the temperature of the plating bath, the more severe the erosion of structures such as sink rolls provided within the plating bath becomes. Accordingly, in one embodiment of the present invention, the temperature of the Zn-Al-Mg plating bath may be above the melting point (°C) according to the plating bath components and below +70°C relative to the melting point according to the plating bath components. More specifically, the temperature of the Zn-Al-Mg plating bath may be 450 to 650°C. In another embodiment, the temperature of the Zn-Al-Mg plating bath may be below +50°C relative to the melting point.

[0100] In one embodiment of the present invention, after the alloy plating process is completed, a gas wiping process may be performed to control the plating amount. The gas wiping process may be performed immediately after the alloy-plated substrate steel sheet exits the plating bath, and may be performed using commonly used air or nitrogen gas (N2 gas). However, if gas wiping is performed using air on an alloy plating layer that has an unsolidified region with a high Mg content, the Mg within the unsolidified region oxidizes, causing the surface of the alloy plating layer to become rough, increasing surface roughness, and forming a thick oxide layer on the surface of the alloy plating layer, thereby reducing corrosion resistance. Accordingly, in one embodiment of the present invention, the gas wiping process may be performed using nitrogen gas. When performing the gas wiping process using nitrogen gas, nitrogen gas at room temperature may be used, or if stricter control of the surface quality of the plating layer is desired, the nitrogen gas may be heated before use.

[0101] In one embodiment of the present invention, the amount of adhesion controlled by the gas wiping treatment is not particularly limited, and it is sufficient if the thickness of the Zn-Al-Mg alloy plating layer in the final Zn-Al-Mg alloy plated steel sheet satisfies 6 to 50 μm. As one example, the amount of adhesion is 25 to 300 g / m² on a single side. 2 It could be.

[0102] In one embodiment of the present invention, by controlling the plating amount through the gas wiping treatment and then performing cooling to solidify the plating layer in a liquid state, a Zn-Al-Mg alloy plated steel sheet having an alloy plating layer composed of a desired metal structure can be obtained.

[0103] Specifically, it is advantageous for the Zn-Al-Mg alloy plating layer according to one embodiment of the present invention to have a microstructure that is mainly cubic. However, if the solidification rate of the molten Zn-Al-Mg alloy plating layer is too fast, some or all of the microstructure of the alloy plating layer may be formed in an amorphous or quasicrystalline phase. In this case, while it is advantageous for securing ultra-high corrosion resistance of the Zn-Al-Mg alloy-plated steel sheet, the alloy plating layer is hard and brittle, which may cause problems such as the plating layer detaching during processing. Furthermore, if the solidification rate is very fast, the proportion of the (00l) orientation plane of the hexagonal MgZn2 phase increases, which reduces the ductility of the alloy plating layer in cryogenic environments and leads to brittle fracture.

[0104] Accordingly, in one embodiment of the present invention, the cooling can be performed in the first to fourth steps as follows, based on the solidification start temperature (A, °C) and solidification end temperature (B, °C) of the Zn-Al-Mg alloy plating layer. Hereinafter, the cooling rate mentioned in each step refers to the average cooling rate in the corresponding temperature range. Meanwhile, the method for measuring the solidification start temperature (A) and solidification end temperature (B) of the Zn-Al-Mg alloy plating layer is not specifically limited. However, as a non-limiting example, the solidification start temperature and solidification end temperature of the plating composition can be determined from the differential scanning calorimetry method using a differential scanning calorimeter (DSC). As an example, when a plating bath sample is placed in a differential scanning calorimeter (DSC), heated above the melting point of the plating bath, and then cooled to near room temperature at a cooling rate of 5°C / min, differential heat peaks occur according to the latent heat of solidification, where the first peak corresponds to the solidification start temperature and the last peak corresponds to the solidification end temperature.

[0105] Stage 1: Cooling rate of 6.0℃ / s or higher until A(℃)

[0106] Phase 2: From A(°C) to before B(°C), cooling rate of 6.0–11.0°C / s

[0107] Stage 3: From B(°C) to before water cooling, cooling rate of 9.0°C / s or higher

[0108] Phase 4: Start at 200℃ or below, water cooling

[0109] (A above represents the solidification start temperature (°C) of the plating layer, and B represents the solidification end temperature (°C) of the plating layer.)

[0110] In one embodiment of the present invention, the cooling of the first step is performed immediately after the gas wiping process, and can be performed from the temperature immediately after the end of the gas wiping process until the solidification start temperature (A, °C) of the Zn-Al-Mg alloy plating layer. In this section, it is necessary to perform cooling at a rapid speed so that the Zn-Al-Mg alloy plating layer, which is still in a liquid state, does not flow down, and considering this, it can be performed at a cooling rate of at least 6.0 °C / s. If the cooling rate of the first step is less than 6.0 °C / s, the Zn-Al-Mg alloy plating layer in a liquid state may flow down, and flow pattern defects may occur on the surface of the final Zn-Al-Mg alloy plated steel sheet. Although the upper limit of the cooling rate of the first step is not specifically limited, it can be performed at a maximum of 30.0 °C / s considering the specifications of the cooling equipment, etc.

[0111] In one embodiment of the present invention, the temperature range in which the second stage of cooling is performed is from the solidification start temperature (A) of the Zn-Al-Mg alloy plating layer to the solidification end temperature (B), and an alloy phase is formed in this range. As previously mentioned, if the solidification rate is too fast in the temperature range where the alloy phase is formed, the ratio of the (00l) orientation planes of the hexagonal MgZn2 phase increases, and the ductility of the alloy plating layer decreases in a cryogenic environment, which may lead to brittle fracture; therefore, the second stage of cooling can be performed at a somewhat slow cooling rate. Accordingly, in one embodiment of the present invention, the second stage of cooling may be 6.0 to 11.0°C / s. If the second stage of cooling is less than 6.0°C / s, it is not difficult to form an alloy phase with a cubic structure, but productivity may be disadvantageous, such as cooling being performed for a long time. On the other hand, if the above cooling rate exceeds 11.0℃ / s, the ductility of the alloy plating layer decreases as the ratio of (00l) of the MgZn2 phase in the alloy plating layer increases, and there is a possibility that low-temperature bonding brittleness may occur.

[0112] In one embodiment of the present invention, after the cooling of the second step, that is, after the solidification of the molten Zn-Al-Mg alloy plating layer is completed, it is advantageous for the cooling rate to be faster. Generally, immediately after the plating layer solidifies, it is in a solid state but has low hardness, so it is easy for dents, etc., to form on the surface of the steel plate—that is, on the surface of the solidified plating layer—due to the changing rolls during the process of transporting the steel plate for subsequent processes. Therefore, it is advantageous to increase the hardness of the plating layer by cooling it relatively quickly after the solidification of the plating layer is completed. Accordingly, in one embodiment of the present invention, the cooling of the third step can be performed at a cooling rate of 9.0°C / s or higher. At this time, to more advantageously increase the cooling rate, air mist, etc., may be used. Although the upper limit of the cooling rate of the third step is not specifically limited, it can be performed at a maximum of 70.0°C / s considering the specifications of the cooling equipment, etc.

[0113] In one embodiment of the present invention, the cooling of the third stage may be performed from the solidification end temperature (B) of the Zn-Al-Mg alloy plating layer, which is the end temperature of the second stage, and may be performed up to the point immediately before the water cooling of the fourth stage is performed. Accordingly, the end temperature of the cooling of the third stage is not specifically set. As one example, the cooling of the fourth stage described below may be started at 200°C or lower, and the cooling of the third stage may be performed up to that starting temperature.

[0114] In one embodiment of the present invention, when performing the cooling of the first to third stages as described above, the cooling rate of the third stage may be faster than the cooling rate of the first or second stage, although this is not necessarily limited thereto.

[0115] In one embodiment of the present invention, after the cooling of the third stage, a fourth stage of cooling corresponding to water cooling may be performed. The water cooling may be performed by passing the Zn-Al-Mg alloy-plated steel sheet, which has completed the cooling of the third stage, through a water cooling tank containing water or purified water, or by spraying the water or purified water onto the alloy-plated steel sheet. Through the water cooling process, the temperature of the Zn-Al-Mg alloy-plated steel sheet can be rapidly cooled to near room temperature, thereby achieving the effect of reducing the manufacturing time of the final plating material, i.e., the final Zn-Al-Mg alloy-plated steel sheet. The cooling speed of the fourth stage of the water cooling process is not specifically limited, but as an example, it may be performed at a cooling speed of 30 to 100°C / s.

[0116] Meanwhile, if the temperature of the Zn-Al-Mg alloy-plated steel sheet is too high at the time when the cooling of the fourth stage is initiated, defects such as warping of the alloy-plated steel sheet may occur. Taking this into consideration, the temperature of the alloy-plated steel sheet at the time when the cooling of the fourth stage is initiated may be 200℃ or lower.

[0117] As described above, by undergoing the aforementioned series of cooling processes, the metallographic structure of the Zn-Al-Mg alloy plating layer of the final Zn-Al-Mg alloy-plated steel sheet can be configured as previously mentioned. Specifically, the metallographic structure of the Zn-Al-Mg alloy plating layer mainly has a cubic structure, and more specifically, a MgZn2 phase with a controlled ratio of the cubic Al phase is formed. Thus, the Zn-Al-Mg alloy-plated steel sheet according to one embodiment of the present invention not only exhibits excellent corrosion resistance in various corrosive environments such as acidic, neutral, and alkaline conditions, but also possesses excellent processability and excellent low-temperature bonding brittleness.

[0118] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0119] (Example)

[0120] First, the composition of the plating bath was designed to have a plating layer composition as shown in Table 1 below, and then the prepared steel sheets were alloy-plated using each plating bath. As the steel sheets, a hot-rolled steel sheet composed of, in weight%, C: 0.015%, Si: 0.011%, Mn: 0.23%, P: 0.009%, S: 0.007%, Al: 0.025%, Nb: 0.02%, Cr: 0.05%, Ti: 0.02%, B: 0.0002%, and the remainder being Fe and other unavoidable impurities, and a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet were used.

[0121] In this case, when using hot-rolled steel sheets, pickling treatment using an aqueous hydrochloric acid solution was performed before the alloy plating treatment to remove iron oxides remaining on the surface of the hot-rolled steel sheets, and then washing and drying were performed under the conditions shown in Table 2 below. Afterwards, the sheets were charged into a furnace in a reducing atmosphere with a dew point temperature of -25℃ and heated to 610℃, after which a subsequent alloy plating process was carried out.

[0122] Meanwhile, when using cold-rolled steel sheets, in order to remove rolling oil, iron powder, and other foreign substances remaining on the surface before alloy plating treatment, the cold-rolled steel sheets were immersed in an alkaline electrolyte and subjected to electrolytic degreasing, and then washed and dried under the conditions shown in Table 2 below. Afterward, they were charged into an annealing furnace in a reducing atmosphere with a dew point temperature of -26°C and heat-treated at 810°C, and then a subsequent alloy plating process was carried out.

[0123] In performing the alloy plating process on each of the above-mentioned base steel sheets, the temperature of the base steel sheet immersed in the plating bath was standardized to +10℃ relative to the temperature of the plating bath. When wiping is performed after the alloy plating is completed, the adhesion amount is 160g / m² per side. 2 It was matched identically.

[0124] In addition, the temperature of the steel plate at the time when the fourth stage of cooling (water cooling) begins in the cooling process after the above wiping treatment was controlled to be 200℃ or lower.

[0125] Plating Bath Number Plating Layer Composition (Wet%, remainder Zn and unavoidable impurities) AlMgFeSiCaSnSrCeMn ① 25 17.20.00 20.05 --0.00 1 -- ② 43 13.50.00 30.50 0.00 50.00 1 -- ③ 32 11.80.00 10.03 0.00 8 --0.00 10.01 ④ 36 10.60.00 80.100.100 --0.00 20.00 1 -- ⑤ 40 14.30.00 70 .300.0200.010-0.010-⑥707.10.0050.040.060----⑦229.20.2000.021.300-0.0010.0020.010⑧4312.50.0013.300.300----⑨176.60.0060.080.050---0.020⑩4515.20.090-0.200----

[0126]

[0127] Steel Type Rinsing (ppm) Plating Bath Conditions Wiping Cooling Conditions Classification NaKCa Composition (Table 1) Temperature (°C) Gas Type Stage 1 Stage 2 Stage 3 Speed ​​(°C / s) Speed ​​(°C / s) Speed ​​(°C / s) H-1 13 10 24 ② 50 3 N 28.3 6.3 11.5 Invention Example 1 H-2 13 10 24 ② 50 4 N 27.2 8.8 10.8 Invention Example 2 H-3 15 12 33 ③ 53 8 N 29.1 6.5 12.2 Invention Example 3 H-4 22 4 ④ 52 2 N 27.4 7.2 10.6 Invention Example 4 H-5 64 3 ⑤ 53 6 N 28.8 6.5 11.1 Invention Example 5 H-6 20 611 ⑦ 50 1 N 29.6 8.1 6.7 Comparative Example 1H-7825133⑧539N23.37.512.3Comparative Example 2H-81643⑨495N27.57.415.4Comparative Example 3H-94156⑩544N27.810.113.3Comparative Example 4F-1175611⑧558N28.110.512.5Comparative Example 5F-2101433①503N28.57.610.7Comparative Example 6F-3221218⑦501N27.18.911.5Comparative Example 7F-414829⑥600N29.39.213.4Comparative Example 8F-511841③513N28.614.214.1Comparative Example 9H is It refers to hot-rolled steel sheets, and F refers to cold-rolled steel sheets. The washing conditions indicate the content of impurities (Na, K, Ca) present in the water used during washing.

[0128] Subsequently, in order to confirm the cross-sectional structure of the alloy plating layer of each alloy-plated steel sheet, specimens were prepared by cutting each alloy-plated steel sheet in the thickness direction to a size suitable for measuring XRD diffraction patterns. Then, XRD analysis was performed on the thickness cross-section of each specimen to determine which phases (alloy phases) were present and to measure the ratio (Equation 1) from the peak intensity of the orientation plane of a specific phase. The conditions for the XRD measurement were as follows. Cu-K α rays targeting Cu were used as the X-ray source, and the X-ray output was set to voltage: 40kV, current: 200mA, scan speed: 4degree / min, step width: 0.02degree, and scanned in the range of 5~90°.

[0129] In addition, the thickness cross-section of the same specimen was photographed using a Field Emission Scanning Electron Microscope (FE-SEM), and compositional analysis was performed using Energy Dispersive X-ray Spectroscopy (EDS) to confirm how the phases identified earlier in XRD were distributed in the FE-SEM image. Furthermore, to measure the fractions of the Al phase, MgZn2 phase, etc. among the observed phases, the thickness cross-section of the same specimen was photographed using an FE-SEM at 5000x magnification, and the area fraction (%) of each phase was measured using an image analyzer. At this time, to assign representativeness to each phase (alloy phase), measurements were taken at 20 random locations within the thickness cross-section based on a specimen length of 20 mm (meaning the length in the direction parallel to the rolling direction), and the average value was taken and applied as the representative value.

[0130] In addition, the thickness of the Fe-Al alloy layer observed in the thickness cross-section of the same specimen was measured.

[0131] All of the above results are shown in Table 3 below.

[0132] Classification Fe-Al alloy layer thickness (㎛) Zn-Al-Mg alloy plating layer thickness Cross-sectional microstructure (fraction %) Relationship 1 Al phase MgZn2 phase Remainder phase Invention Example 1 4.6 4 2.3 5 0.4 7.3 0.91 Invention Example 2 4.7 4 2.1 4 0.7 1 7.2 0.93 Invention Example 36.4 3 7.2 4 1.9 2 0.9 0.92 Invention Example 45.7 3 9.5 3 0.6 2 9.9 0.90 Invention Example 5 6.2 4 0.6 4 5.4 1 4.0 0.95 Comparative Example 13.9 3 3.1 4 8.9 1 8.0 0.94 Comparative Example 29.4 1 2.5 4 0.2 4 7.3 0.93 Comparative Example 33.3 1 1.2 2 4.6 6 4.2 0.92 Comparative Example 49.314.744.840.50.92 Comparative Example 59.512.340.747.00.94 Comparative Example 64.510.623.166.30.93 Comparative Example 74.013.448.737.90.95 Comparative Example 810.248.314.237.50.96 Comparative Example 95.237.044.718.30.71 The remainder is one or more of Zn, Al-Zn, Mg-Si, and Al-Si-Ca-Zn.

[0133] In addition, the surface of the alloy plating layer of each alloy-plated steel sheet was observed to evaluate the presence of defects. At this time, the degree of defects caused by dross adhesion, the presence or absence of dents caused by rolls during the steel sheet transport process, the degree of flow pattern occurrence, the degree of black spot defect occurrence, and the degree of powdering of the plating layer were measured, and the results are shown in Table 4 below. Each evaluation criterion was determined as follows. At this time, the degree of powdering was evaluated by performing a 90° bend on the alloy-plated steel sheet specimen, attaching a transparent tape to the inner coil, peeling it off, measuring the width of the plating material adhering to the tape, and determining a criterion based on that width.

[0134] · Degree of dross adhesion defect occurrence: None (Good: ◎), 1 or more per 200㎡ surface area (Poor: ×)

[0135] · Degree of flow pattern occurrence: None (Good: ◎), flow patterns observed when visually inspected from a distance of more than 5m from the plating surface (Poor: ×)

[0136] · Severity of sunspots: None (Good: ◎), 1–100 per 1m² on the surface (Severe: △), More than 100 per 1m² on the surface (Extremely Severe: ×)

[0137] · Degree of powdering: Powdering width 1.0mm or less (Good: ◎), Powdering width exceeding 1.0mm (Poor: ×)

[0138] In addition, an Impact Peel Test was used to evaluate the low-temperature bonding brittleness of each alloy-plated steel sheet. Specifically, two identical specimens of each alloy-plated steel sheet were prepared in a size of 20 mm × 90 mm, processed to ensure there were no burrs on the bonding surface, and then an adhesive was applied between the specimens in a size of 20 mm × 30 mm and butted together. After applying an impact to the bonded specimens at -45°C, the two specimens were forcibly separated. At this time, if delamination occurred within the adhesive, the low-temperature bonding brittleness was evaluated as good and was labeled as 'non-delamination'; if delamination occurred within the alloy plating layer of the specimen or at the interface between the alloy plating layer of the specimen and the base steel sheet, the low-temperature bonding brittleness was evaluated as inferior and was labeled as 'delamination'.

[0139] Meanwhile, to evaluate the corrosion resistance of the flat sections of each alloy-plated steel sheet, specimens were prepared by cutting them to a size of 75 mm × 150 mm. The corrosion resistance was evaluated by measuring the time until red rust appeared after conducting a Salt Spray Test (SST, 5% NaCl) on each specimen. At this time, to provide an acidic environment, sulfuric acid was added to a neutral 5% NaCl solution to adjust the pH to 4.0 before the test was conducted. Additionally, to provide an alkaline environment, ammonia water was added to a neutral 5% NaCl solution to adjust the pH to 11.0 before the test was conducted.

[0140] The evaluation criteria for the above corrosion resistance were determined as follows, and each result is shown in Table 4 below.

[0141] <Criteria for Evaluating Corrosion Resistance in Acidic Atmospheres>

[0142] ◎: Time required for red-blue discoloration to occur in 5% of the area is 360 hours or more

[0143] △: Time required for red-blue discoloration to occur in 5% of the area is 200 hours or more but less than 360 hours

[0144] ×: Time required for red-blue discoloration to occur in 5% of the area is less than 200 hours

[0145] Evaluation Criteria for Corrosion Resistance in Neutral Atmospheres

[0146] ◎: Time required for red-blue discoloration to occur in 5% of the area is 2,400 hours or more

[0147] △: Time required for 5% of the area to develop red-blue is 1,500 hours or more to less than 2,400 hours

[0148] ×: Time required for red-blue to occur in 5% of the area is less than 1,500 hours

[0149] Evaluation Criteria for Corrosion Resistance in Alkaline Atmospheres

[0150] ◎: Time required for red-blue discoloration to occur in 5% of the area is 1,400 hours or more

[0151] △: Time required for red-blue to occur in 5% of the area is 800 hours or more to less than 1,400 hours

[0152] ×: Time required for red-blue to occur in 5% of the area is less than 800 hours

[0153] Classification Dross Adhesion Defect Presence / Absence of Indentation Flow Pattern Defect Black Spot Defect Powdering Low-Temperature Bonding Brittleness Flat Plate Corrosion Resistance Acidic (pH 4.0) Neutral (pH 6.7) Alkaline (pH 11.0) Invention Example 1 ◎ No ◎◎◎ Non-peeling ◎◎◎ Invention Example 2 ◎ No ◎◎◎ Non-peeling ◎◎◎ Invention Example 3 ◎ No ◎◎◎ Non-peeling ◎◎◎ Invention Example 4 ◎ No ◎◎◎ Non-peeling ◎◎◎ Invention Example 5 ◎ No ◎◎◎ Non-peeling ◎◎◎ Comparative Example 1 × Yes ◎◎◎ Non-peeling ××◎ Comparative Example 2 ◎ No ××◎ Non-peeling ◎△ ×× Comparative Example 3 ◎ No ◎◎◎ Non-peeling ××◎ Comparative Example 4 ◎ No ◎△ × Non-peeling ◎◎◎ Comparative Example 5 ◎ No ◎△ × Non-peeling ◎◎◎ Comparative Example 6 × No ◎◎◎ Non-peeling △△◎ Comparative Example 7×No◎◎◎Non-peeling××◎Comparative Example 8◎No◎◎×Non-peeling◎△×Comparative Example 9◎No◎◎◎Peeling◎◎◎

[0154] As shown in Tables 1 to 4 above, Invention Examples 1 to 5, in which the plating bath components were composed to form a Zn-Al-Mg alloy plating layer of the intended composition and the washing, gas wiping, and cooling processes were performed according to the conditions of an embodiment of the present invention, exhibited excellent corrosion resistance and processability. In particular, the above invention examples showed excellent low-temperature bonding brittleness and excellent corrosion resistance in corrosive environments ranging from acidic to neutral and alkaline, as the metal structure of the Zn-Al-Mg alloy plating layer was mostly formed as a cubic structure and the Al phase and MgZn2 phase were formed in appropriate fractions, and the (00l) plane fraction of the MgZn2 phase was formed exactly as intended.

[0155] Meanwhile, in comparative examples where the plating bath components deviated from the range according to one embodiment of the present invention, or where the conditions of the washing, gas wiping, and cooling processes deviated from the conditions according to one embodiment of the present invention, the metallic structure of the alloy plating layer was not formed as intended, or it did not exhibit corrosion resistance in various corrosive environments. In addition, in some comparative examples, the surface characteristics were inferior or the processability was inferior.

[0156] Specifically, Comparative Example 1 is a case where the Ca content in the plating bath is excessive, resulting in the excessive formation of CaO oxide during the plating process, which caused dross adhesion defects and inferior corrosion resistance in acidic and neutral atmospheres. In addition, the cooling rate of the third stage during cooling after plating was too slow, causing the plated steel sheet to pass through the roll while maintained at a high temperature, resulting in dents on its surface.

[0157] In Comparative Example 2, the Si content in the plating bath composition was excessive, which significantly increased the melting point of the plating bath and required maintaining a high plating bath temperature. Consequently, the Fe-Al alloy layer was formed too thickly, and the residual phase was excessively formed, resulting in poor corrosion resistance in neutral and alkaline atmospheres. Furthermore, as the cooling in the first stage proceeded too slowly, it took a long time for the plating layer to solidify, causing severe flow patterns to occur as the plating bath flowed down the surface of the substrate steel plate in the direction of gravity. In addition, black spot defects were induced on the surface of the final alloy-plated steel plate by using water with an excessive Ca content during rinsing.

[0158] Comparative Example 3 is a case where the content of Al and Mg in the plating bath is low, and as the Al phase is not formed in a sufficient fraction and the remainder phase is formed too excessively, the corrosion resistance in acidic and neutral environments is inferior.

[0159] In Comparative Example 4, because Si was not contained in the plating bath, severe leaching of Fe occurred within the plating bath, and an Fe-Al alloy layer was formed too thickly at the interface between the base steel sheet and the plating layer. As a result, severe powdering occurred. In addition, black spot defects were induced on the surface of the final alloy-plated steel sheet by using water with a high Na content during rinsing.

[0160] In Comparative Example 5, the plating bath temperature had to be maintained at a high level because the melting point of the plating bath increased significantly due to the excessive Si content in the plating bath composition, and consequently, the Fe-Al alloy layer was formed too thickly. As a result, the brittleness of the plating layer increased significantly, and the powdering performance was inferior. Furthermore, black spot defects were induced on the surface of the final alloy-plated steel sheet by using water with an excessive K content during rinsing.

[0161] Comparative Example 6 is a case in which Ca was not added to the plating bath, and as the effect of inhibiting MgO oxide during the plating process was insufficient, excessive MgO oxide was formed in the bath, causing dross adhesion defects. In addition, the Mg content in the plating bath was excessive, resulting in a relatively small fraction of Al phase and an excessive amount of residual phase, which led to poor corrosion resistance in acidic and neutral atmospheres.

[0162] Comparative Example 7 is a case where the Ca content in the plating bath was excessive, and excessive CaO oxide was formed during the plating process, causing dross adhesion defects. In addition, as the Al phase was formed in too small a fraction, corrosion resistance in acidic and neutral atmospheres was inferior.

[0163] Comparative Example 8 is a case where the Al content in the plating bath is excessive and the Mg content is low, and the MgZn2 phase is not sufficiently formed, resulting in inferior corrosion resistance in neutral and alkaline atmospheres. In addition, as the Al content in the plating bath increased, the temperature of the plating bath had to be maintained at a high level, and as a result, the base steel sheet and Al reacted excessively in the plating bath, causing the Fe-Al alloy layer to be formed too thickly at the interface between the base steel sheet and the plating layer, and the powdering performance was inferior.

[0164] Comparative Example 9 did not satisfy Equation 1 because the proportion of the (00l) plane of the hexagonal MgZn2 phase increased as the cooling of the second stage proceeded rapidly, and the bonding brittleness in a cryogenic environment was inferior.

[0165] FIG. 1 shows a photograph of the cross-sectional structure of the alloy plating layer of a Zn-Al-Mg alloy-plated steel sheet (Invention Example 4) according to one embodiment of the present invention, observed using FE-SEM. As shown in FIG. 1, an Fe-Al alloy layer is formed with an appropriate thickness between the base steel sheet and the plating layer, and it can be confirmed that Al phase and MgZn2 phase are formed within the plating layer.

[0166] FIG. 2 shows the X-ray measurement results (peak intensity graph) of the culture surface of the MgZn2 phase within the alloy plating layer of a Zn-Al-Mg alloy plated steel sheet (Invention Example 1) according to one embodiment of the present invention. As shown in FIG. 2, it can be confirmed that the intensity of the (002) and (004) planes, which correspond to the (001) plane among the culture surfaces of the MgZn2 phase, is lower than that of the other planes.

Claims

1. Base steel plate; An Fe-Al alloy layer provided on at least one surface of the above-mentioned base steel plate; and It includes a Zn-Al-Mg alloy plating layer provided on the above alloy layer, A Zn-Al-Mg alloy plated steel sheet, wherein the Zn-Al-Mg alloy plating layer comprises an MgZn2 phase, and the MgZn2 phase satisfies the following Equation 1 upon X-ray diffraction analysis. [Relationship 1] I1∑(MgZn2) / I0∑(MgZn2) ≥ 0.87 (In Equation 1, I0∑(MgZn2) represents the sum of the X-ray diffraction peak intensities of the (100) plane, (002) plane, (101) plane, (102) plane, (103) plane, (110) plane, (112) plane, (201) plane, (004) plane, (203) plane, (213) plane, (220) plane, (313) plane, and (402) plane on MgZn2, and I1∑(MgZn2) represents the sum of the X-ray diffraction peak intensities of the (100) plane, (101) plane, (102) plane, (103) plane, (110) plane, (112) plane, (201) plane, (203) plane, (213) plane, (220) plane, (313) plane, and (402) plane on MgZn2 (means sum) 2. In Paragraph 1, A Zn-Al-Mg alloy plated steel sheet, wherein the Zn-Al-Mg alloy plating layer comprises, in terms of area fraction, Al phase: 20.0~80.0%, MgZn2 phase: 20.0~65.0%, and remainder phase: 40.0% or less (including 0%).

3. In Paragraph 2, The above-mentioned residual phase is one or more of the Zn phase, Al-Zn phase, Mg-Si phase, Al-Zn-Ca phase, and Al-Si-Ca-Zn phase, forming a Zn-Al-Mg alloy plated steel sheet.

4. In Paragraph 1, The Zn-Al-Mg alloy plating layer comprises, in weight percent, aluminum (Al): 20~50%, magnesium (Mg): 8.0~15.0%, silicon (Si): 0.02~1.00%, calcium (Ca): 0.005~0.500%, and the remainder being zinc (Zn) and unavoidable impurities, a Zn-Al-Mg alloy plated steel sheet.

5. In Paragraph 4, The Zn-Al-Mg alloy plating layer above is a Zn-Al-Mg alloy plated steel sheet further comprising one or more elements selected from the following groups i) to iii). i) Total content of elements derived from the base steel sheet: 1,000% or less ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1,000% or less iii) Total content of elements added to control the surface quality of alloy-plated steel sheets: 1,000% or less 6. In Paragraph 5, The elements belonging to i) above are one or more of Cr, Mn, Co, Ti, Ni, Cu, V, Nb, Mo, W, and B, The elements belonging to ii) above are one or more of Y, Zr, La, Ce, Sr, and Be, and A Zn-Al-Mg alloy plated steel sheet in which the element belonging to iii) above is one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

7. In Paragraph 1, A Zn-Al-Mg alloy plated steel sheet having a Zn-Al-Mg alloy plating layer thickness of 6 to 50 μm.

8. In Paragraph 1, Zn-Al-Mg alloy-plated steel sheet having a thickness of 0.01 to 7.00 μm of the Fe-Al alloy layer.

9. In Paragraph 1, The above Fe-Al alloy layer is FeAl, FeAl2, FeAl3, FeAl4, Fe2Al 5, Zn-Al-Mg alloy-plated steel sheet composed of one or more alloy phases among Fe2Al and Fe3Al.

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