Plated steel sheet

Abstract

According to exemplary embodiments of the present invention, there can be provided a plated steel sheet having excellent workability while having ultra-high corrosion resistance at a level such that the plated steel sheet can be suitably used even in harsh corrosive environments such as marine environments.

Description

galvanized steel sheet

[0001] The present invention relates to plated steel sheets.

[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; by protecting the steel from the oxidizing atmosphere, these corrosion products can improve the steel's corrosion resistance. Due to these advantageous characteristics, the application range of galvanized steel has recently expanded 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 steel is increasingly exposed to complex corrosive conditions, there is a growing need to develop 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 generation facilities are being constructed not only on land but also on coastlines and at sea. Consequently, the corrosion resistance of steel used in power generation facilities is becoming even more critical.

[0004] [Prior Art Literature]

[0005] [Patent Literature]

[0006] (Patent Document 1) Japanese Published Patent Application No. 2000-219950

[0007] The problem that the technical concept of the present invention aims to solve is to provide a plated steel sheet having a level of corrosion resistance suitable for use in various corrosive environments.

[0008] More specifically, the present invention aims to provide a plated steel sheet having excellent corrosion resistance not only in a neutral atmosphere but also in acidic and even alkaline atmospheres. The objectives of the present invention are not limited to those described above. Those skilled in the art to which the present invention pertains will have no difficulty understanding additional objectives of the present invention from the overall details of the specification.

[0009] According to exemplary embodiments for solving the problem of the present invention, a plated steel sheet is provided. The plated steel sheet comprises a base steel sheet and a Zn-Al-Mg-based plating layer disposed on the surface of the base steel sheet, wherein the Zn-Al-Mg-based plating layer comprises an Al phase and a MgZn2 phase, and in a cross-section in the thickness direction, the cross-sectional area of ​​the Al phase is 20 μm, of the total area occupied by the Al phase. 2 The ratio of the first Al phase is 17.0–70.0%.

[0010] The Zn-Al-Mg plating layer may comprise, in the cross-section in the thickness direction, the Al phase: 30.0~80.0%, the MgZn2 phase: 15.0~70.0%, and the remainder phase: 20.0% or less (including 0%).

[0011] The above Zn-Al-Mg-based plating layer has a cross-sectional area of ​​20 μm out of the total area occupied by the Al phase in the cross-section in the thickness direction. 2 The ratio (A1 / A2) of the ratio (A1) of the second Al phase less than the area fraction (A2) of the MgZn2 phase may be in the range of 0.40 to 4.00.

[0012] The above-mentioned plated steel sheet may further include an interfacial alloy layer interposed between the Zn-Al-Mg-based plating layer and the base steel sheet.

[0013] The above interfacial alloy layer may include one or more interfacial alloy phases among FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al.

[0014] The thickness of the above interface alloy layer may be 0.001 to 4.500 μm.

[0015] The above Zn-Al-Mg plating layer may contain, in weight percent, aluminum (Al): 20.0~50.0%, magnesium (Mg): 5.0~22.0%, the remainder being zinc (Zn) and unavoidable impurities.

[0016] The above Zn-Al-Mg-based plating layer may further include one or more elements selected from the following groups i) to iii).

[0017] i) Total content of elements derived from the base steel sheet: 1.000% or less

[0018] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1.000% or less

[0019] iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less

[0020] The elements belonging to i) above are one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B, the elements belonging to ii) above are one or more of Y, Zr, La, Ce, Ca, Sr, and Be, and the elements belonging to iii) above may be one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

[0021] The ratio of Mg content ([Mg]) to Al content ([Al]) of the Zn-Al-Mg plating layer ([Mg] / [Al]) may be 0.20 or more and less than 0.70.

[0022] The thickness of the above Zn-Al-Mg plating layer may be 5 to 70 μm.

[0023] The above first Al phase may be a primary Al phase.

[0024] The above second Al phase may be precipitated inside the above MgZn2 phase.

[0025] According to exemplary embodiments of the present invention, a plated steel sheet having a level of corrosion resistance suitable for use in various corrosive environments can be provided.

[0026] More specifically, the present invention can provide a plated steel sheet having excellent corrosion resistance not only in a neutral atmosphere but also in acidic and even alkaline atmospheres.

[0027] 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.

[0028] Figure 1 is an image of a cross-section of a plated steel sheet according to Invention Example 3, taken using an SEM (magnification: approximately 1500x).

[0029] Figure 2 is an image of a cross-section of a plated steel sheet according to Invention Example 5, taken using an SEM (magnification: approximately 1500x).

[0030] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Prior to this, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings. Instead, based on the principle that the inventor can appropriately define the concepts of terms to best describe his invention, they should be interpreted in a meaning and concept consistent with the technical spirit of the present invention.

[0031] In the following descriptions with reference to the drawings, identical or corresponding components are assigned the same reference numerals, and redundant descriptions thereof will be omitted.

[0032] In the following embodiments, the terms first, second, etc. are used not in a limiting sense, but for the purpose of distinguishing one component from another component.

[0033] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0034] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0035] In the present invention, when indicating the concentration (content) of each element, it means weight % unless specifically otherwise determined.

[0036] In the drawings, the size of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are depicted arbitrarily for convenience of explanation, so the present invention is not necessarily limited to what is illustrated.

[0037] Where an embodiment can be implemented differently, a specific process sequence may be performed differently from the order described. For example, two processes described consecutively may be performed substantially simultaneously or proceed in the reverse order of the description.

[0038] In addition, in describing the present invention, if it is determined that a detailed description of related known components or functions may obscure the essence of the invention, such detailed description is omitted.

[0039] The present invention will be described in detail below through each embodiment. It should be noted that each embodiment described in this specification is not limited to a single embodiment but may also be combined with other embodiments. 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.

[0040] The present invention will be described in detail below through examples. However, it should be noted that the following examples 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.

[0041] [Plated Steel Sheet]

[0042] According to exemplary embodiments, the plated steel sheet may comprise a base steel sheet and a Zn-Al-Mg-based plating layer.

[0043] Any steel suitable for manufacturing plated steel sheets may be used as the base steel sheet. As a non-limiting example, the base steel sheet may be carbon steel containing a certain amount of carbon (C), and various steels such as stainless steel and aluminum sheets may be applied. As one example, if the base steel sheet 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 the base steel sheet for alloy plated steel sheets. Therefore, the alloy composition of the carbon steel is 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 with high or ultra-high strength, no restrictions are placed on the alloy composition of the carbon steel.

[0044] 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 unanesthetized steel sheet that has not undergone the annealing process.

[0045] A Zn-Al-Mg-based plating layer can be disposed on the surface of a base steel sheet. A Zn-Al-Mg-based plating layer can be disposed on at least one surface of the base steel sheet. A Zn-Al-Mg-based plating layer can be disposed on both opposing surfaces of the base steel sheet.

[0046] According to exemplary embodiments, the thickness of the Zn-Al-Mg plating layer may be 5 to 70 μm. In this case, the thickness of the Zn-Al-Mg plating layer refers to the average thickness of one side. If the thickness of the Zn-Al-Mg plating layer is less than 5 μm, it may be difficult to obtain the intended ultra-high corrosion resistance, but the present invention is not necessarily limited thereto. On the other hand, if the thickness of the Zn-Al-Mg plating layer exceeds 70 μm, the adhesion of the plating to the substrate steel sheet is reduced, and problems such as peeling of the plating layer during processing may occur, but the present invention is not necessarily limited thereto.

[0047] As the corrosive environments in which conventional galvanized steel sheets have been used become increasingly harsh and complex, there is a growing demand for galvanized steel sheets capable of exhibiting corrosion resistance in various corrosive environments. Accordingly, the inventors of the present invention have conducted in-depth research to provide galvanized steel sheets suitable for such conditions. As a result, by optimizing the metallic structure of the plating layer, it was confirmed that it is possible to provide a galvanized steel sheet capable of securing corrosion resistance not only in a general neutral atmosphere but also in acidic and even alkaline atmospheres, leading to the completion of the present invention.

[0048] A Zn-Al-Mg plating layer can be formed using a molten plating bath containing elements such as aluminum (Al) and magnesium (Mg) in addition to zinc (Zn). As a result, the plated steel sheet can be provided with improved corrosion resistance in addition to the sacrificial protection provided by Zn by forming a unique microstructure.

[0049] According to exemplary embodiments, the Zn-Al-Mg plating layer may include an Al phase and a MgZn2 phase. In the present invention, the Al phase includes all forms of Al phases, such as primary Al phases and binary eutectic structures. That is, the Al phase may exist alone or may be provided in a form precipitated within another alloy phase. The MgZn2 phase is a phase of an intermetallic compound of Mg and Zn, and may exist alone or be provided in a form such as a eutectic structure with another phase.

[0050] According to exemplary embodiments, in a cross-section in the thickness direction of a Zn-Al-Mg-based plating layer, the Al phase has a cross-sectional area of ​​20 μm 2 A first Al phase with an ideal cross-sectional area of ​​20㎛ 2 It includes a second Al phase less than [amount]. The first Al phase may be a primary Al phase that precipitates first during the solidification process of a Zn-Al-Mg plating layer, but the present invention is not necessarily limited thereto. The second Al phase may be a phase precipitated within the MgZn2 phase, but the present invention is not necessarily limited thereto.

[0051] As a result of conducting in-depth research, the inventors of the present invention discovered that the corrosion resistance in weakly acidic environments can be further improved by controlling the proportion of the coarse first Al phase within the Al phase. In other words, they found that corrosion resistance in acidic environments, as well as neutral environments, can be further improved by appropriately controlling the fraction of the coarse Al phase within the Al phase, rather than simply controlling the area fraction of the Al phase.

[0052] According to exemplary embodiments, in a cross-section in the thickness direction of a Zn-Al-Mg-based plating layer, the cross-sectional area occupied by the Al phase is 20 μm out of the total area. 2The proportion of the first Al phase may be 17.0 to 70.0%. In this way, by sufficiently securing the first Al phase, which is a coarse Al phase, corrosion resistance can be further improved in a weakly acidic corrosive atmosphere (e.g., acid rain in industrial areas). If the proportion of the first Al phase is less than 17.0%, the weakly acidic corrosive medium can easily penetrate through the plating layer to the substrate steel plate, so the effect of improving corrosion resistance may not be sufficient. Therefore, among the Al phases, the proportion of the first Al phase may be 17.0% or more, and in terms of improving corrosion resistance in an acidic atmosphere, a more desirable proportion of the first Al phase may be 20.0% or more. More specifically, the proportion of the first Al phase may be 25.0% or more. If the proportion of the first Al phase exceeds 70.0%, when in contact with an alkaline corrosive medium (e.g., concrete), the coarse first Al phase corrodes rapidly, and the corrosive medium can rapidly reach the substrate steel plate through it. To prevent this, the proportion of the first Al phase may be 70.0% or less. More specifically, the proportion of the first Al phase may be 60.0% or less.

[0053] In the cross-section in the thickness direction of the Zn-Al-Mg plating layer, among the Al phases, the remainder has a cross-sectional area of ​​20㎛ 2 It may be a second Al phase less than

[0054] As such, the Al phase exhibits excellent corrosion resistance in acidic and neutral atmospheres. However, its corrosion resistance may be inferior in alkaline atmospheres. In contrast, the MgZn2 phase can exhibit excellent corrosion resistance in regions where the pH of the corrosive medium is neutral or alkaline, but it can easily corrode in acidic environments. According to exemplary embodiments, the ratio (A1 / A2) of the area fraction (A1) of the second Al phase and the area fraction (A2) of the MgZn2 phase in a cross-section in the thickness direction may satisfy the range of 0.40 to 4.00. According to another example, the ratio (A1 / A2) of the area fraction (A1) of the second Al phase and the area fraction (A2) of the MgZn2 phase in a cross-section in the thickness direction may be 0.8 or higher. The second Al phase is a relatively fine Al phase that may be distributed throughout the Zn-Al-Mg plating layer, but in particular, it may be provided in a form precipitated within the MgZn2 phase. At this time, if the ratio of the second Al phase is excessively high, the second Al phase is excessively formed within the MgZn2 phase, which may cause microcracks at the interface between the second Al phase and the MgZn2 phase during the processing of the plated steel sheet. Since these microcracks act as penetration pathways for corrosive components, there is a concern that the corrosion resistance of the processed part of the plated steel sheet may be reduced. Meanwhile, as another example, the present invention may set the A1 / A2 ratio to 1.8 or less to reduce the absolute fraction of the MgZn2 phase and prevent Al from leaching out from within the MgZn2 phase, thereby improving corrosion resistance in an alkaline environment. On the other hand, if the ratio of the second Al phase is excessively low, the coarse first Al phase is excessively formed, which may cause microcracks during the processing of the plated steel sheet. In this case as well, there is a concern that the corrosion resistance of the processed part of the plated steel sheet may be reduced. However, according to exemplary embodiments, by optimizing the ratio of the second Al phase to the MgZn2 phase, the reduction in corrosion resistance of the processed part caused by the excessive second Al phase can be minimized.

[0055] According to exemplary embodiments, the Zn-Al-Mg plating layer may comprise, in area %, Al phase: 30.0 to 80.0%, MgZn2 phase: 15.0 to 70.0%, and remainder phase: 20.0% or less (including 0%) in a cross-section in the thickness direction. In this way, the Zn-Al-Mg plating layer can provide improved corrosion resistance in various corrosive environments by having a unique cross-sectional structure.

[0056] Al phase: 30.0~80.0%

[0057] Since the Al phase exhibits excellent corrosion resistance in the region where the pH of the corrosive medium is neutral to weakly acidic, the area fraction of the Al phase can be provided at 30.0% or more. More specifically, the area fraction of the Al phase can be 35.0% or more. However, the Al phase corrodes relatively quickly in an alkaline atmosphere. Therefore, there is a concern that if the area fraction of the Al phase is excessively high, it may be difficult to exhibit corrosion resistance in an alkaline environment. To prevent this, the area fraction of the Al phase can be 80.0% or less. More specifically, the area fraction of the Al phase can be 70.0% or less.

[0058] MgZn2 phase: 15.0~70.0%

[0059] Since the MgZn2 phase can exhibit excellent corrosion resistance in the region where the pH of the corrosive medium is neutral or alkaline, the area fraction of the MgZn2 phase may be 15.0% or more. More specifically, the area fraction of the MgZn2 phase may be 30.0% or more. As such, according to the exemplary embodiments, by including both the MgZn2 phase and the Al phase, the corrosion resistance of each metal structure can be mutually complemented. Consequently, the plated steel sheet according to the exemplary embodiments can possess excellent corrosion resistance in various environments. Meanwhile, the MgZn2 phase can corrode relatively easily in acidic environments. Therefore, there is a concern that if the area fraction of the MgZn2 phase is excessively high, sufficient corrosion resistance may not be secured in acidic environments. To prevent this, the area fraction of the MgZn2 phase may be 70.0% or less. More specifically, the area fraction of the MgZn2 phase may be 60.0% or less.

[0060] Remaining balance: 20.0% or less (including 0%)

[0061] The Zn-Al-Mg plating layer may further include alloy phases resulting from the combination of Zn, Al, Mg, and other metal components within the Zn-Al-Mg plating layer as residual phases, in addition to the Al phase and the MgZn2 phase. The residual phase may be a Zn phase or a Zn-Al phase, but the present invention is not necessarily limited thereto. Depending on the composition of the Zn-Al-Mg plating layer, the residual phase may further include any one of the Mg-Si phase, the Al-Zn-Ca phase, or the Al-Si-Ca-Zn phase. That is, the residual phase may be one or more of the Zn phase, the Zn-Al phase, the Mg-Si phase, the Al-Zn-Ca phase, and the Al-Si-Ca-Zn phase, but the present invention is not necessarily limited thereto. The Mg-Si phase may typically be the Mg2Si phase, and the Al-Zn-Ca phase may typically be Al 2.94 CaZn 1.06 It can be an award.

[0062] The residual phase not only fails to significantly contribute to the improvement of corrosion resistance, but the Zn phase, Zn-Al phase, etc., can actually reduce corrosion resistance in various corrosive environments. The Mg2Si phase is a brittle phase that is mainly distributed in the surface layer of the Zn-Al-Mg plating layer and can act as a site for crack initiation. Therefore, it is desirable to suppress the residual phase as much as possible, and its fraction may be 0%. As another example, the above residual phase may be included in an amount of 2.0% or less (including 0%).

[0063] The metallographic phases described above in the present invention can be identified using X-ray diffraction patterns, scanning electron microscopes (SEM), or transmission electron microscopes (TEM). As one example, the composition of the Al phase may be pure Al, but may include less than 30% Zn and less than 10% Mg in terms of atomic ratio (At%). Even if components within the above content range are included, the metallurgical crystal characteristics of the Al phase are not lost, so it is detected as the Al phase in the X-ray diffraction pattern and conforms to the quality characteristics of the Al phase of the present invention. As another example, less than 5% Al in terms of atomic ratio may be included within the MgZn2 phase, but because it possesses the unique metallurgical crystal characteristics of the MgZn2 phase, it is detected as the MgZn2 phase in the X-ray diffraction pattern and conforms to the quality characteristics of the MgZn2 phase of the present invention.

[0064] According to exemplary embodiments, the plated steel sheet may further include an interfacial alloy layer interposed between the Zn-Al-Mg-based plating layer and the substrate steel sheet. The interfacial alloy layer may be formed as Fe leached from the substrate steel sheet reacts with Al in the plating bath during the plating layer formation process. This can further increase the adhesion between the substrate steel sheet and the Zn-Al-Mg-based plating layer.

[0065] According to exemplary embodiments, the thickness of the interfacial alloy layer may be 0.001 to 4.500 μm. If the thickness of the interfacial alloy layer is less than 0.001 μm, the adhesion between the substrate steel sheet and the Zn-Al-Mg plating layer is not sufficiently high, which may cause problems such as the plating layer detaching during processing of the plated steel sheet. Therefore, the thickness of the interfacial alloy layer may be 0.001 μm or more. More specifically, it may be 0.005 μm or more. If the thickness of the interfacial alloy layer exceeds 4.500 μm, the adhesion between the substrate steel sheet and the Zn-Al-Mg plating layer may be excellent, but due to the high hardness of the interfacial alloy layer, the likelihood of powdering occurring during processing may increase. Therefore, the thickness of the interfacial alloy layer may be 4.500 μm or less. More specifically, it may be 3.000 μm or less or 2.500 μm or less.

[0066] There are no specific limitations on the alloy phases constituting the interfacial alloy layer. As a non-limiting example, the interfacial alloy layer may include one or more interfacial alloy phases among FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al. Additionally, depending on the alloy composition of the Zn-Al-Mg plating layer, one or more of Zn, Mg, and metal components within the Zn-Al-Mg plating layer may be additionally contained within the interfacial alloy phase, and even in such cases, there is no difficulty in ensuring adhesion between the substrate steel sheet and the alloy plating layer.

[0067] According to exemplary embodiments, the Zn-Al-Mg plating layer may comprise, in weight percent, aluminum (Al): 20.0–50.0%, magnesium (Mg): 5.0–22.0%, the remainder being zinc (Zn) and unavoidable impurities. In another example, the Zn-Al-Mg plating layer may comprise, in weight percent, aluminum (Al): 20.0–50.0%, magnesium (Mg): 6.0–20.0%, the remainder being zinc (Zn) and unavoidable impurities.

[0068] Aluminum (Al): 20.0~50.0%

[0069] Aluminum (Al) is an element that contributes to improving corrosion resistance and possesses particularly strong properties against acids. Additionally, Al can contribute to suppressing the formation of MgO-based dross by inhibiting the oxidation of Mg within the plating bath. Therefore, the Al content may be 20.0% or higher. More specifically, the Al content may be 22.0%. However, if the Al content is excessively high, there is a concern that corrosion in alkaline environments may increase, and it may raise the melting point of the plating bath. In this case, the temperature of the plating bath must be maintained high during the plating process, which may intensify the erosion of structures within the plating bath. Furthermore, the aforementioned interfacial alloy layer may be formed excessively thickly, leading to powdering where the plating layer detaches during processing. Therefore, the Al content may be 50.0% or lower. More specifically, the Al content may be 40.0% or lower.

[0070] Magnesium (Mg): 5.0~22.0%

[0071] Magnesium (Mg) is an element that contributes to improving corrosion resistance and possesses strong properties, particularly in neutral and alkaline environments. Consequently, it can improve the corrosion resistance of galvanized steel sheets in neutral seawater environments, such as marine environments, and exhibits excellent corrosion resistance even in alkaline corrosive environments like concrete. If the Mg content is less than 5.0%, it may be difficult to secure the level of ultra-high corrosion resistance required in marine environments. Therefore, the Mg content may be 5.0% or higher. More specifically, the Mg content may be 7.0%. However, if the Mg content exceeds 22.0%, it becomes difficult to use in structures in industrial areas subject to acid rain because Mg is susceptible to acid corrosion. Furthermore, as Mg is an element with a high affinity for oxygen, it can oxidize to MgO on the surface of the plating bath in contact with air, forming dross and impairing plating performance. Therefore, the Mg content may be 22.0% or lower. More specifically, the Mg content may be 15.0%.

[0072] Ratio of Mg content ([Mg]) to Al content ([Al]) ([Mg] / [Al]): 0.20 or more and less than 0.70

[0073] Since Mg and Al in the Zn-Al-Mg plating layer possess mutually complementary corrosion resistance characteristics, uniform corrosion resistance can be imparted in all corrosive environments, including acidic, neutral, and alkaline conditions, by controlling the ratio of Mg content ([Mg]) to Al content ([Al]) ([Mg] / [Al]; hereinafter Mg / Al). Even if the Mg and Al content satisfy the ranges proposed in this invention, if Mg / Al is excessively low, there is a concern that corrosion resistance may be reduced in alkaline corrosive environments. Conversely, if Mg / Al is excessively high, there is a concern that corrosion resistance may be inferior in acidic corrosive environments. Furthermore, if Mg / Al is excessively high, the MgZn2 fraction in the Zn-Al-Mg plating layer increases excessively, leading to an increase in the hardness of the Zn-Al-Mg plating layer, which causes cracks to occur during processing and consequently reduces processing corrosion resistance. Therefore, Mg / Al may be between 0.20 and less than 0.70. More specifically, Mg / Al can be between 0.25 and 0.50.

[0074] The Zn-Al-Mg plating layer contains 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, along with the Al and Mg, and the Zn content can be adjusted to match the content of the aforementioned Al and Mg. The Zn-Al-Mg plating layer may contain other unavoidable impurities in addition to the aforementioned alloy composition. Since such impurities can 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.

[0075] Optionally, the Zn-Al-Mg-based plating layer may further include one or more elements selected from the following groups i) to iii). However, since the elements of each of the following groups are not essential for achieving the objectives of the present invention, the lower limit of their content is not restricted. Accordingly, the lower limit of the content of each element may be 0%, even if not specifically mentioned below. The elements of each of the following groups may be intentionally added to the plating bath or derived from the base steel sheet, but the present invention is not necessarily limited thereto.

[0076] i) Total content of elements derived from the base steel sheet: 1.000% or less

[0077] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1.000% or less

[0078] iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less

[0079] The elements of i) above may be elements derived from the base steel sheet. The elements of i) above may contribute to the formation of corrosion resistance or contribute to grain stabilization. However, if the total content of these elements exceeds 1.000%, the surface properties of the plated steel sheet may deteriorate. As a specific example, the elements belonging to group i) above may be one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B.

[0080] The elements of ii) above may be elements added to suppress the formation of Mg oxide in the plating bath. Additionally, the elements of ii) above may contribute to the formation of corrosion resistance of the plated steel sheet. However, if the total content of these elements exceeds 1.000%, the brittleness of the plating layer may increase. As a specific example, the elements belonging to group ii) above may be one or more of Y, Zr, La, Ce, Ca, Sr, and Be.

[0081] The elements of iii) above may be elements added to control the surface quality of the plated steel sheet. Additionally, the elements of iii) above may affect the size of spangles, which are solidification bodies of the Zn-Al-Mg plating layer. However, if the total content of these elements exceeds 1.000%, there is a risk of discoloration of the Zn-Al-Mg plating layer in a humid environment. 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.

[0082] 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.

[0083] [Method for manufacturing galvanized steel sheets]

[0084] According to exemplary embodiments, a method for manufacturing a plated steel sheet may include the steps of: temper rolling; plating; controlling the amount of plating; and cooling.

[0085] However, it should be noted that the following method is included in exemplary embodiments for manufacturing the plated steel sheet described above, and that the plated steel sheet according to the exemplary embodiments must not necessarily be manufactured by the present 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.

[0086] Temper rolling stage

[0087] By performing a skin pass mill (SPM, Skin Pass mill, hereinafter referred to as SPM) on the substrate steel sheet before plating, the surface roughness of the substrate steel sheet can be controlled. As a result, the first Al phase can be controlled in the cross-section of the Zn-Al-Mg-based plating layer.

[0088] Refer to the detailed description regarding the base steel plate. That is, the type and method of providing the base steel plate can be appropriately selected within the scope that does not deviate from the technical concept of the present invention.

[0089] As one example, when the base steel sheet is a hot-rolled steel sheet, SPM treatment can be performed to control the surface roughness immediately after the pickling and rinsing processes to remove oxide scale present on the surface of the hot-rolled steel sheet. As another example, when the base steel sheet is a cold-rolled steel sheet, SPM treatment can be performed during the final stage of cold rolling, specifically by performing temper rolling. That is, for hot-rolled steel sheets, the surface roughness of the SPM rolls and the rolling force are controlled in a separate SPM process after pickling and rinsing, whereas for cold-rolled steel sheets, the surface roughness of the base steel sheet is controlled by adjusting the surface roughness and rolling force of the final roll of cold rolling.

[0090] According to exemplary embodiments, the SPM treatment can be performed to satisfy the following relationship 1, where R (μm) is the surface roughness of the SPM roll, F (ton) is the rolling force of the roll, and BR (μm) is the surface roughness of the base steel sheet after SPM.

[0091] [Relationship 1]

[0092] 84.00 ≤ 48×R + 0.1×F + 50×BR ≤ 102.00

[0093] If the process variables of the SPM treatment do not satisfy the above Equation 1, it may be difficult to control the ratio of the first Al phase and the second Al phase. Since the above Equation 1 is an empirically obtained value, a specific unit may not be set, and it is sufficient if it satisfies the units of each of the process variables of the SPM treatment described above.

[0094] As a non-limiting example, the surface roughness (Rmax, μm) of the temper-rolled base steel sheet can be performed to be 1.00 to 3.00 μm. If the surface roughness of the base steel sheet is less than 1.00 μm, the anchoring effect between the plating layer and the base material is negligible, which may result in poor plating adhesion. If the surface roughness of the base steel sheet exceeds 3.00 μm, the surface may be rough and uneven, making it difficult to control the cross-sectional structure.

[0095] When performing SPM processing, if the surface roughness of the roll is less than 0.20㎛, problems may arise in the manufacturing and management of the roll. If it exceeds 0.50㎛, it is difficult to sufficiently lower the surface roughness of the substrate steel sheet, and there is a concern that the cross-sectional structure may not be sufficiently controlled during the solidification process after plating. Therefore, according to exemplary embodiments, the surface roughness of the SPM roll may be 0.20 to 0.50㎛.

[0096] In addition, if the roll rolling force is less than 150 tons, the effect of controlling the surface roughness of the base steel sheet is low, and the effect of homogenizing the surface roughness after pickling hot-rolled steel sheets or after cold-rolling cold-rolled steel sheets may be reduced. If the roll rolling force exceeds 300 tons, there is a risk of inducing C-curvature, etc. Therefore, according to exemplary embodiments, the rolling force of the SPM roll may be 150 to 300 tons.

[0097] plating step

[0098] A temper-rolled steel sheet can be plated by immersing it in a Zn-Al-Mg-based plating bath.

[0099] The Zn-Al-Mg plating bath may contain, in weight percent, aluminum (Al): 20.0–50.0%, magnesium (Mg): 5.0–22.0%, and the remainder being zinc (Zn) and unavoidable impurities. Additionally, according to exemplary embodiments, the content ratio of Mg to Al in the plating bath (Mg / Al) may be 0.20 or higher and less than 0.70. For the content of each component of the Zn-Al-Mg plating bath, one may refer to the description of the Zn-Al-Mg plating layer described above. That is, one or more elements selected from the groups i) to iii) described above may be further included.

[0100] Meanwhile, the temperature of the plating bath is determined by the composition within the plating bath. 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, components within the substrate steel sheet may leach into the plating bath during the plating process, resulting in a large amount of dross. Furthermore, as the interfacial alloy layer formed between the substrate steel sheet and the plating layer becomes too thick, plating layer powdering may easily occur 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 may become. Therefore, according to exemplary embodiments, the temperature of the Zn-Al-Mg plating bath may be above the melting point (°C) of the plating bath components and below +70°C relative to the melting point of the plating bath components. More specifically, the temperature of the Zn-Al-Mg plating bath may be 450 to 650°C. As another example, the temperature of the Zn-Al-Mg plating bath can be 450 to 550°C.

[0101] Optionally, according to exemplary embodiments, the SPM-treated steel sheet may undergo heat treatment prior to plating. The process of heating the steel sheet may be carried out by passing it through a heat treatment furnace (e.g., a non-oxidizing furnace or a reducing furnace). The temperature at which the steel sheet is heated may be set to be equal to the temperature of the Zn-Al-Mg-based plating bath or 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. As another example, the steel sheet may be heated to about 600°C or higher and then cooled to an appropriate temperature before immersion in the plating bath. As yet another example, if the steel sheet is an un-tendered cold-rolled steel sheet, the steel sheet may be degreased, washed, and dried, then loaded into an annealing furnace controlled by a reducing atmosphere to perform annealing heat treatment, and then cooled to an appropriate temperature. The annealing heat treatment conditions at this time are not specifically limited and may be conditions for annealing conventional cold-rolled steel sheets, and may be an annealing furnace maintained in a reducing atmosphere by controlling the dew point temperature.

[0102] Step to control the plating amount

[0103] The plating amount can be controlled by performing a gas wiping treatment after the plating process. The gas wiping treatment can be performed immediately after the alloy-plated substrate steel sheet exits the plating bath, and can be carried out using commonly used air or nitrogen gas (N2 gas). However, if gas wiping is performed using air on a plating layer containing regions of unsolidified state with high Mg content, Mg and Al within the unsolidified regions may oxidize, causing the surface of the plating layer to become rough, increasing surface roughness, and forming a thick oxide layer on the surface of the plating layer. In this case, the corrosion resistance of the plated steel sheet may be reduced. Therefore, according to exemplary embodiments, the gas wiping treatment can be performed using nitrogen gas. By using nitrogen gas, the formation of a thick oxide layer on the surface of the Zn-Al-Mg plating layer can be suppressed even if it inevitably comes into contact with air with a high oxygen concentration. When performing gas wiping treatment using the above nitrogen gas, nitrogen gas at room temperature may be used, or if the surface quality of the plating layer is to be controlled more strictly, the nitrogen gas may be heated and used.

[0104] Meanwhile, when a large amount of Mg is contained in the Zn-Al-Mg plating bath, even if a wiping treatment using nitrogen gas is performed after the plating treatment, a thick oxide layer may form on the surface of the plated steel sheet if it comes into contact with air with a high oxygen concentration before solidification. Accordingly, according to exemplary embodiments, the gas wiping treatment using nitrogen gas can be performed in a sealing box.

[0105] According to exemplary embodiments, the sealing box may be positioned from the surface of the Zn-Al-Mg plating bath to the point where the gas wiping means for the gas wiping treatment ends or to the point where solidification begins. At this time, one end of the sealing box may be in contact with the surface of the plating bath. That is, when the steel sheet plated in the Zn-Al-Mg plating bath is withdrawn from the plating bath, it can be loaded into the sealing box without coming into contact with air. From this, contact with air with a high oxygen concentration can be prevented. Furthermore, by performing the gas wiping treatment inside the sealing box, contact between the plated steel sheet and air with a high oxygen concentration before and after the gas wiping treatment can be fundamentally prevented. More specifically, when performing the gas wiping treatment using nitrogen gas inside the sealing box, the inside of the sealing box becomes a static pressure state, that is, a state where the pressure inside is higher than the pressure outside the sealing box, thereby suppressing the inflow of external air.

[0106] Meanwhile, by using a sealing box as described above, air can be blocked from the surface of the Zn-Al-Mg plating bath until the point where gas wiping ends. However, since the steel plate must pass through the sealing box after gas wiping, a gap must exist at the top of the sealing box for the steel plate to pass through. Consequently, it is not possible to completely block the inflow of air into the sealing box from the outside. In this case, the oxygen concentration inside the sealing box may increase, and consequently, there is a concern that it may be difficult to obtain the effect of the sealing box. Accordingly, according to exemplary embodiments, the interior of the sealing box can be controlled to a non-oxidizing atmosphere. More specifically, the oxygen concentration inside the sealing box may be 3.00% or less. At this time, the control of the oxygen concentration can be achieved by supplying a non-oxidizing gas (e.g., N2 gas, etc.) into the sealing box through a nozzle provided in the sealing box. If the oxygen concentration inside the sealing box exceeds 3.0%, a thick oxide layer is formed on the surface of the steel plate, increasing the surface roughness, and the corrosion resistance may also be reduced due to the oxide. More specifically, the oxygen concentration inside the sealing box may be 2.00% or less, or 1.50% or less.

[0107] The adhesion amount controlled by gas wiping treatment is not specifically limited; as an example, it is sufficient if the thickness of the Zn-Al-Mg plating layer on the final Zn-Al-Mg plated steel sheet satisfies 5 to 70 μm. As an example, the said adhesion amount is 25 to 350 g / m² on a single side. 2 It could be.

[0108] Cooling stage

[0109] Cooling can be performed in four stages as follows, based on the solidification start temperature (A, °C) and solidification end temperature (B, °C) of the Zn-Al-Mg plating layer. Hereinafter, the cooling rate mentioned in each stage 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 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 one 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, a differential heat peak occurs according to the latent heat of solidification, and at this time, the first peak corresponds to the solidification start temperature and the last peak corresponds to the solidification end temperature.

[0110] Stage 1: Cooling rate of 6.0℃ / s or higher until A(℃)

[0111] Phase 2: From A(°C) to before B(°C), cooling rate of 5.0–14.0°C / s

[0112] Stage 3: From B(°C) to before water cooling, cooling rate of 12.0°C / s or higher

[0113] Phase 4: Initiate water cooling at 250℃ or below

[0114] According to exemplary embodiments, the first stage of cooling may be performed immediately after the gas wiping process. It may be performed from the temperature immediately after the end of the gas wiping process to the solidification start temperature (A, °C) of the Zn-Al-Mg plating layer. In this section, it is necessary to perform cooling at a rapid rate so that the Zn-Al-Mg plating layer, which is still in a liquid state, does not flow down, and considering this, it may be performed at a cooling rate of at least 6.0 °C / s. If the cooling rate of the first stage is less than 6.0 °C / s, the Zn-Al-Mg plating layer in a liquid state may flow down, and flow pattern defects may occur on the surface of the final Zn-Al-Mg plated steel sheet. The upper limit of the cooling rate of the first stage is not specifically limited, but it may be performed at a maximum of 30.0 °C / s considering the specifications of the cooling equipment, etc.

[0115] According to exemplary embodiments, the temperature range in which the second stage of cooling is performed is from the solidification start temperature (A) of the Zn-Al-Mg plating layer to the solidification end temperature (B), and in this range, an alloy phase of the Zn-Al-Mg plating layer components is formed. If the cooling rate is too fast in the temperature range in which the alloy phase is formed, it may be difficult to satisfy the alloy phase fraction proposed in the present invention. That is, if the second stage cooling rate exceeds 14.0°C / s, the area ratio of the MgZn2 phase decreases, and instead, the fraction of the Zn phase, which is one of the remaining phases, may increase. In the present invention, the Zn phase is limited to a maximum of 5% or less because it does not have excellent corrosion resistance in a weak acidic atmosphere and a seawater atmosphere (neutral), but if the second stage cooling rate exceeds 14.0°C / s, the fraction of the Zn phase may exceed 5%. If the cooling rate of the second stage is less than 5.0℃ / s, it is not difficult to form the alloy phase fraction proposed in the present invention, but productivity may be disadvantageous, such as cooling being performed for a long time. Therefore, the cooling rate of the second stage can be limited to 5~14℃ / s.

[0116] After the cooling of the second stage, that is, after the solidification of the molten Zn-Al-Mg plating layer is complete, it may be advantageous to have a faster cooling rate. Generally, immediately after the plating layer solidifies, it is in a solid state but has low hardness, so it is prone to indentations, etc., forming on the surface of the steel plate—that is, on the surface of the solidified plating layer—due to the rolls passing through it during the process of transporting the steel plate for subsequent processes. Therefore, it may be advantageous to increase the hardness of the plating layer by cooling it relatively quickly after the solidification of the plating layer is complete. Accordingly, according to exemplary embodiments, the cooling of the third stage can be performed at a cooling rate of 12.0°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 stage is not specifically limited, it may be performed at a maximum of 70.0°C / s considering the specifications of the cooling equipment, etc.

[0117] According to exemplary embodiments, the cooling of the third stage may be performed from the solidification end temperature (B) of the Zn-Al-Mg 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 250°C or lower, and the cooling of the third stage may be performed up to that starting temperature.

[0118] According to exemplary embodiments, a fourth stage of cooling, corresponding to water cooling, may be performed after the third stage of cooling. The water cooling may be performed by passing the steel plate, which has completed the third stage of cooling, through a water cooling tank containing water (e.g., tap water), or by spraying the water or purified water onto the steel plate. Through the water cooling process, the temperature of the steel plate 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 plated steel plate. The cooling rate of the fourth stage of water cooling is not specifically limited, but as a non-limiting example, it may be performed at a cooling rate of 30 to 100°C / s.

[0119] If the temperature of the steel plate is too high at the time when the fourth stage of cooling begins, defects such as warping of the steel plate may occur. Taking this into consideration, the temperature of the steel plate at the time when the fourth stage of cooling begins may be 250℃ or lower.

[0120] According to exemplary embodiments, when the plating bath temperature is T (°C), the cooling rate in the first stage is C1 (°C / s), the cooling rate in the second stage is C2 (°C / s), and the cooling rate in the third stage is C3 (°C / s), the following relationship 2 can be further satisfied.

[0121] [Relationship 2]

[0122] 0.80 ≤ 0.04×T + 0.90*C1 - C2 + 0.80×C3 -28 ≤ 7.00

[0123] In this way, by optimizing the cooling rate of the subsequent cooling process according to the plating bath temperature, the second Al phase and the MgZn2 phase can be controlled more precisely. That is, if the plating bath temperature condition and the cooling rate do not satisfy the above Equation 2, the area fraction of the MgZn2 phase, the ratio of the first Al phase among the Al phases, and the ratio of the second Al phase can each be controlled, but controlling the ratio of the second Al phase to the MgZn2 phase may become difficult. As another example, the value of the above Equation 2 may be 1.0 or greater. Since the above Equation 2 is an empirically obtained value, a specific unit may not be set, and it is sufficient if it satisfies the units of each process variable.

[0124] [Example]

[0125] Test Example 1: Verification of corrosion resistance according to the first Al phase

[0126] First, a base steel sheet was prepared. As the base steel sheet, a hot-rolled steel sheet composed of, in weight percent, C: about 0.011%, Si: about 0.018%, Mn: about 0.45%, P: about 0.010%, S: about 0.005%, Al: about 0.022%, Nb: about 0.01%, Cr: about 0.005%, Ti: about 0.02%, B: about 0.007%, 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.

[0127] These base steel sheets were subjected to temper rolling, plating, gas wiping, and cooling (stages 1 through 4 cooling) under the conditions shown in Table 2. During the plating process, the composition of the plating bath was designed to produce a plating layer composition as shown in Table 1 below, and the prepared base steel sheets were then plated using each respective plating bath. In performing the alloy plating treatment on each base steel sheet in this manner, the temperature of the base steel sheet immersed in the plating bath was standardized to approximately +10°C relative to the bath temperature, and the adhesion amount during the wiping process after the alloy plating was completed was approximately 150 g / m² per side.2 It was matched identically.

[0128] And, in the cooling process after the above wiping treatment, the fourth stage cooling was initiated by water cooling at the point where the steel plate temperature became 250℃ or lower after the third stage cooling.

[0129] Plating Bath Number Plating Layer Composition (Wet%, remainder Zn and unavoidable impurities) Al Mg Mg / Al Ratio SiCaSn Sr CeMn Other Components 1 34.5 6.8 0.2 00.4 20.11 -0.04 --- 2 30.4 10.2 0.3 40.04 0.2 700.02 30.003 -0.010 - 3 51 20.3 0.4 00.1 90.010 ----- 416.3 5.10.3 10.2 90.2 00 -0.015 -- Fe 0.012

[0130] Relationship 1 for Temper Rolling Conditions, Steel Sheet Surface Roughness after Temper Rolling (Rmax, μm), Plating Bath Conditions, Wiping, Sealing Box, Cooling Conditions, Relationship 2 for Classification, Roll Roughness (Ra, μm), Reduction Force (ton), Composition (Table 1), Temperature (°C), Gas Type, Oil / Oxygen Concentration (Vol%), 1st Stage Speed ​​(°C / s), 2nd Stage Speed ​​(°C / s), 3rd Stage Speed ​​(°C / s) 0.30 230 1.25 9 9.9 15 29 N2 Oil 0.25 9.6 13.3 15.5 0.9 Invention Example 10.2 22 20 5 1.3 39 9.9 15 12 N2 Oil 0.1 19.2 11.8 13.2 0.9 Invention Example 20.30 230 1.25 9 9.9 25 29 N2 Oil 0.25 9.6 7.3 15.5 6.9 Invention Example 30.382571.22104.943578N2U0.298.16.914.87.35Comparative Example 10.332051.33102.844465N2U1.958.511.016.60.53Comparative Example 20.372441.20102.164465N2U0.7510.77.918.47.05Comparative Example 3

[0131] Subsequently, each plated steel sheet was cut to a predetermined size, the plating layer was dissolved, and the composition of the plating layer was confirmed by analyzing each component using ICP (Ion Coupled Plasma Spectrometry). In addition, to confirm the cross-sectional structure of the plating layer of the plated steel sheet and to obtain specimens of a size suitable for measuring XRD diffraction patterns, each plated steel sheet was cut in the thickness direction to prepare specimens, and XRD analysis was performed on each specimen to identify which phases (alloy phases) were present. Furthermore, the surface of the same specimen was photographed using FE-SEM (Field Emission Scanning Electron Microscope), and compositional analysis was performed using EDS (Energy Dispersive X-ray Spectroscopy) to confirm how the phases identified in the XRD were distributed in the FE-SEM image. Finally, to measure the fractions of the Al phase and MgZn2 phase among the observed phases, the surface of the same specimen was photographed using FE-SEM at 1500x magnification, and the area fraction (%) of each phase was measured using an image analyzer (Clemex). At this time, in order to provide representativeness to each phase (alloy phase), 20 random points were measured within a square area of ​​20 mm by 20 mm on each observation specimen, and the average value was taken and applied as the representative value. It was confirmed that the plating layer composition of each inventive example and comparative example was substantially the same as the composition of the plating bath. All of the above results are shown in Table 3 below.

[0132] Classification Al Phase 1 Al Phase (of the total Al phase) 2 Al Phase (of the total Al phase) Invention Example 1 53.2 46.8 Invention Example 2 17.3 82.7 Invention Example 3 53.7 46.3 Comparative Example 1 77.6 22.4 Comparative Example 29.8 90.2 Comparative Example 39.5 90.5

[0133] Meanwhile, to evaluate the corrosion resistance of each galvanized steel sheet, specimens measuring 80mm x 150mm were fabricated, yielding two specimens: one for evaluating the corrosion resistance of the flat section and another for evaluating the corrosion resistance of the processed section. Among these, the specimen for evaluating the corrosion resistance of the processed section was bent at a 90° angle so that the radius of curvature of the processed section was 0.5mm. Subsequently, a Salt Spray Test (SST, 5% NaCl) was performed on each specimen, and corrosion resistance was evaluated by measuring the time until red rust appeared. The evaluation criteria for the corrosion resistance of the flat section and the processed section are as follows. In the evaluation of the corrosion resistance of the flat section in an acidic atmosphere, sulfuric acid was added to a neutral 5% NaCl solution to adjust the pH to 4.0 in order to induce an acidic environment, and the test was conducted. The evaluation criteria for the corrosion resistance were determined as follows, and the results are shown in Table 4 below.

[0134] <Criteria for Evaluating Plate Corrosion Resistance in Acidic Atmospheres>

[0135] ◎: Time required for red-blue discoloration to occur in 5% of the area is 480 hours or more

[0136] ○: Time required for red-blue discoloration to occur in 5% of the area is 360 hours or more but less than 480 hours

[0137] △: Time required for red-blue discoloration to occur in 5% of the area is 200 hours or more but less than 360 hours

[0138] X: Time required for red-blue to occur in 5% of the area is less than 200 hours

[0139] <Evaluation Criteria for Corrosion Resistance of Flat Plates in Neutral Atmospheres>

[0140] ◎: Time required for red-blue discoloration to occur in 5% of the area is 3,000 hours or more

[0141] ○: Time required for 5% of the area to develop red-blue is 2,400 hours or more ~ ​​less than 3,000 hours

[0142] △: Time required for 5% of the area to develop red-blue is 1,500 hours or more to less than 2,400 hours

[0143] X: Time required for red-blue to occur in 5% of the area is less than 1,500 hours

[0144] <Criteria for Evaluating Corrosion Resistance of Processed Parts in a Neutral Atmosphere>

[0145] ◎: Time required for red-blue discoloration to occur in 5% of the area is 2,000 hours or more

[0146] ○: Time required for 5% of the area to develop red-blue is 1,500 hours or more to less than 2,000 hours

[0147] △: Time required for 5% of the area to develop red-blue is 1,000 hours or more to less than 1,500 hours

[0148] X: Time required for red-blue to occur in 5% of the area is less than 1,000 hours

[0149] Classification Plate Corrosion Resistance Processed Part Corrosion Resistance Acidic (pH 4.0) Neutral (pH 6.7) Neutral (pH 6.7) Invention Example 1 ◎◎◎ Invention Example 2 ◎◎◎ Invention Example 3 ◎◎◎ Comparative Example 1 ◎×× Comparative Example 2 ×△× Comparative Example 3 ×△×

[0150] Referring to Tables 1 to 4, it can be seen that if the fraction of the first Al phase satisfies the range proposed in the present invention, it exhibits excellent corrosion resistance not only in a general neutral atmosphere but also in a harsh acidic atmosphere. However, it was confirmed that if the fraction of the first Al phase does not satisfy the range proposed in the present invention, it exhibits inferior corrosion resistance in one or more of an acidic or neutral atmosphere.

[0151] Test Example 2: Confirmation of physical properties according to the ratio of the area fractions of the second Al phase and the MgZn2 phase

[0152] Additional experiments were conducted to verify the physical properties according to the ratio of the area fractions of the second Al phase and the MgZn2 phase. To manufacture the plated steel sheets for this test, some of the plating bath compositions shown in Table 1 above were used, and a new plating bath composition was prepared as shown in Table 5 below. The plating bath compositions used in each example of invention are the same as the numbers shown in Table 6. In addition, the manufacturing conditions are shown in Table 6.

[0153] Conditions not specifically mentioned in this test example were controlled by the conditions described in Test Example 1 above, so a detailed explanation is omitted.

[0154] Plating Bath Number Plating Layer Composition (Wet%, remainder Zn and unavoidable impurities) Al Mg Mg / Al R-SiCaSnSrCeMn Other Components 5 28.3 8.9 0.3 10.1 8 0.00 0 10.03 ---- 6 28.11 3.8 0.4 9 0.2 20.1 20 -0.00 10.00 2 -Ti 0.00 17 4 3.7 14.8 0.3 4 0.4 10.0 60 -0.00 20.00 3 0.02 0Be 0.00 11

[0155] Temper Rolling Conditions Relationship 1 for Steel Sheet Surface Roughness after Temper Rolling (Rmax, μm) Plating Bath Conditions Relationship 2 for Wiping Sealing Box Cooling Conditions Classification Roll Roughness (Ra, μm) Reduction Force (ton) Composition (Table 1 and Table 5) Temperature (°C) Gas Type Oil / Oxygen Concentration (Vol%) 1st Stage Speed ​​(°C / s) 2nd Stage Speed ​​(°C / s) 3rd Stage Speed ​​(°C / s) 0.30 220 0.96 84.45 55 6 N2 Oil 0.1 19.4 10.2 14.8 4.3 4 Invention Example 4 0.30 210 1.2 29 6.4 25 29 N2 Oil 0.48 10.1 11.9 13.7 1.3 1 Invention Example 50.302651.1598.46509N2U0.537.512.820.42.63 Invention Example 60.222870.9988.767553N2U0.849.210.915.13.58 Invention Example 7

[0156] The composition and microstructure distribution of the plating layer were measured using the method described above and are shown in Table 7 below.

[0157] Classification Cross-section Structure (Area %) A1 / A2 Al Phase MgZn2 Phase Remainder 1st Al Phase (of total Al phase) 2nd Al Phase (of total Al phase) Total Al Phase Invention Example 1 53.246.878.115.16.82.42 Invention Example 2 17.382.779.316.54.23.97 Invention Example 3 53.746.352.347.50.20.51 Invention Example 4 18.581.553.246.50.30.93 Invention Example 5 53.446.651.648.10.30.50 Invention Example 6 43.156.947.452.20.40.52 Invention Example 7 45.854.250.949.100.56

[0158] FIG. 1 is an image of a cross-section of a plated steel sheet according to Invention Example 3, taken using SEM (magnification: approximately 1500x). FIG. 2 is an image of a cross-section of a plated steel sheet according to Invention Example 5, taken using SEM (magnification: approximately 1500x). Referring to Table 7 and FIG. 1 and FIG. 2, in the case of Invention Example 3 and Invention Example 5, which satisfy the conditions proposed in the present invention, it can be seen that the first Al phase (10) is properly formed, and the MgZn2 phase (30) and the second Al phase (20) are also formed within the range proposed in the present invention.

[0159] To evaluate the corrosion resistance of each plated steel sheet, specimens were prepared with dimensions of 80mm x 150mm, and two specimens were obtained to evaluate the corrosion resistance of the flat section and the corrosion resistance of the processed section. The criteria for evaluating the corrosion resistance of the flat section and the processed section are the same as those described above. In this case, for the evaluation of the corrosion resistance of the flat section in an alkaline atmosphere, ammonia water was added to a neutral 5% NaCl solution to adjust the pH to 11.0 in order to provide an alkaline environment, and then the test was conducted.

[0160] <Criteria for Evaluating Plate Corrosion Resistance in Alkaline Atmospheres>

[0161] ◎: Time required for red-blue discoloration to occur in 5% of the area is 1,500 hours or more

[0162] ○: Time required for red-blue to occur in 5% of the area is 1,000 hours or more ~ ​​less than 1,500 hours

[0163] △: Time required for red-blue discoloration to occur in 5% of the area is 600 hours or more to less than 1,000 hours

[0164] X: Time required for red-blue to occur in 5% of the area is less than 600 hours

[0165] Classification Plate Corrosion Resistance Processed Part Corrosion Resistance Acidic (pH 4.0) Neutral (pH 6.7) Alkaline (pH 11.0) Neutral (pH 6.7) Invention Example 1 ◎◎X◎ Invention Example 2 ◎◎X◎ Invention Example 3 ◎◎◎◎ Invention Example 4 ○○◎△ Invention Example 5 ◎◎◎◎ Invention Example 6 ◎◎◎◎ Invention Example 7 ◎◎◎△

[0166] Referring to Tables 7 and 8, it can be seen that in the case of Invention Examples 3 to 7, where the ratio of the second Al phase to the MgZn2 phase (A1 / A2) is 0.40 or higher and 1.8 or lower, the corrosion resistance in an alkaline environment is excellent. That is, if the ratio of the second Al phase and the MgZn2 phase is further controlled in addition to controlling the ratio of the first Al phase, excellent corrosion resistance performance can be achieved even in additional corrosive environments.

[0167] Test Example 3: Confirmation of physical properties according to the interfacial alloy layer

[0168] Additional experiments were conducted to verify the physical properties according to the thickness of the interfacial alloy layer formed between the substrate steel sheet and the Zn-Al-Mg plating layer. To manufacture the plated steel sheet for this test, some of the plating bath compositions shown in Tables 1 and 5 above were used, and a new plating bath composition was prepared as shown in Table 9 below. The plating bath compositions used in each inventive example are the same as the numbers shown in Table 6. In addition, the manufacturing conditions are shown in Table 10.

[0169] Conditions not specifically mentioned in this test example were controlled by the conditions described in Test Example 1 above, so a detailed explanation is omitted.

[0170] Plating Bath Number Plating Layer Composition (Wet%, remainder Zn and unavoidable impurities) Al Mg Mg / Al R SiCaSnSrCeMn Other Components 8 2 2.1 9.9 0.4 5 0.2 1 0.00 5----Sb 0.01 0 9 2 9.5 8.1 0.2 8 0.15 0.15 1-----

[0171] Temper Rolling Conditions Steel Sheet Surface Roughness After Temper Rolling (Rmax, μm) Relationship 1 Plating Bath Conditions Wiping Sealing Box Cooling Conditions Relationship 2 Classification Roll Roughness (Ra, μm) Reduction Force (ton) Composition (Table 1, Table 5, and Table 9) Temperature (°C) Gas Type Oil / Oxygen Concentration (Vol%) 1st Stage Speed ​​(°C / s) 2nd Stage Speed ​​(°C / s) 3rd Stage Speed ​​(°C / s) 0.2 228 70.9 98 8.7 6 75 53 N2 Oil 0.8 49.2 10.9 15.1 3.5 8 Invention Example 7 0.2 92 200 98 4.9 22 52 9 N2 Oil 0.8 11.2 10.2 17.1 6.7 2 Invention Example 80.292200.9985.422529N2U0.4510.48.715.05.82 Invention Example 90.292201.0085.926509N2U0.398.410.614.71.08 Invention Example 100.352081.28101.68489N2U1.0710.010.416.23.12 Invention Example 110.272441.19101.669527N2U0.546.28.915.43.7 Invention Example 12

[0172] The composition and microstructure distribution of the plating layer were measured using the method described above and are shown in Table 11 below. Meanwhile, for the interfacial alloy layer, Scanning Electron Microscopy (SEM) images were captured by selecting 10 arbitrary cross-sections of the plating layer. Subsequently, elemental mapping data of Fe and Al was obtained from the captured data using Electron Probe Micro Analyzer (EPMA), and the interfacial alloy layer was identified by comparing the SEM images with the elemental mapping data. The thickness of the interfacial alloy layer was measured using an image analyzer (Clemex Image Analyzer) and averaged.

[0173] Classification Interface Alloy Layer Thickness (㎛) Cross-sectional Microstructure (Area %) A1 / A2 Al Phase MgZn2 Phase Remainder 1st Al Phase (of total Al phase) 2nd Al Phase (of total Al phase) Total Al Phase Invention Example 11.5 53.2 46.8 78.1 5.16.8 2.42 Invention Example 22.3 17.3 82.7 79.3 16.5 4.2 3.97 Invention Example 44.3 18.5 81.5 53.2 46.5 0.3 0.51 Invention Example 73.5 45.8 54.2 50.9 49.1 00.56 Invention Example 81.0 554.2 45.8 51.9 47.8 0.3 0.50 Invention Example 91.0 453.9 46.1 52.3 47.5 0.2 0.51 Invention Example 100.8943.456.647.552.10.40.52 Invention Example 110.4525.174.942.855.61.60.58 Invention Example 120.9160.639.454.744.21.10.49

[0174] The corrosion resistance of each galvanized steel sheet was evaluated using the same criteria as described above and is shown in Table 12 below. In addition, the degree of powdering in the Zn-Al-Mg plating layer of each galvanized steel sheet was measured, and the results are shown in Table 12 below. The criteria for evaluating powdering were determined as follows.

[0175] <Degree of Powdering>

[0176] ◎: Powdering width 0.5mm or less

[0177] ○: Powdering width exceeding 0.5mm and up to 1.0mm

[0178] △: Powdering width exceeding 1mm and up to 3mm

[0179] Х: Powdering width exceeding 3mm

[0180] Classification Powdering Plate Corrosion Resistance Processing Part Corrosion Resistance Acidic (pH 4.0) Neutral (pH 6.7) Alkaline (pH 11.0) Neutral (pH 6.7) Invention Example 1 ◎◎◎X◎ Invention Example 2 O◎◎X◎ Invention Example 4 X○○◎△ Invention Example 7 X◎◎◎△ Invention Example 8 ◎◎◎◎◎ Invention Example 9 ◎◎◎◎◎ Invention Example 10 ◎◎◎◎◎ Invention Example 11 ◎○○◎○ Invention Example 12 ◎◎◎◎◎

[0181] Referring to Tables 11 and 12, it can be seen that in the case of Invention Examples 4 and 7, the thickness of the interfacial alloy layer is excessively formed, resulting in inferior powdering characteristics. However, in the case of Invention Examples 1, 2, and 8 to 12, the interfacial alloy layer is appropriately formed to be 3.00 μm or less, resulting in excellent powdering characteristics. Although the invention has been described with reference to the above examples, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as described in the following claims.

Claims

Base steel plate; and It includes a Zn-Al-Mg-based plating layer disposed on the surface of the above-mentioned base steel plate, and The above Zn-Al-Mg-based plating layer comprises an Al phase and a MgZn2 phase, and In the cross-section in the thickness direction, among the total area occupied by the Al phase, the cross-sectional area is 20㎛ 2 A plated steel sheet in which the ratio of the first Al phase is 17.0~70.0%. In paragraph 1, A plated steel sheet comprising, in a cross-section in the thickness direction, the Zn-Al-Mg plating layer comprising: Al phase: 30.0~80.0%, MgZn2 phase: 15.0~70.0%, and remainder phase: 20.0% or less (including 0%). In Article 1, The above Zn-Al-Mg-based plating layer has a cross-sectional area of ​​20 μm out of the total area occupied by the Al phase in the cross-section in the thickness direction. 2 A plated steel sheet in which the ratio (A1 / A2) of the ratio (A1) of the second Al phase (less than) and the area fraction (A2) of the MgZn2 phase is in the range of 0.40 to 4.

50. In Paragraph 3, A plated steel sheet in which the ratio (A1 / A2) of the ratio (A1) of the second Al phase and the area fraction (A2) of the MgZn2 phase is 1.80 or less. In paragraph 1, The above-mentioned plated steel sheet is, A plated steel sheet further comprising an interfacial alloy layer interposed between the above Zn-Al-Mg-based plating layer and the above base steel sheet. In paragraph 4, The above-mentioned interfacial alloy layer comprises one or more alloy phases selected from FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al, forming a plated steel sheet. In paragraph 5, A plated steel sheet having a thickness of 0.001 to 4.500 μm of the interface alloy layer. In Paragraph 7, A plated steel sheet having a thickness of the above-mentioned interface alloy layer of 3,000 μm or less. In paragraph 1, The above Zn-Al-Mg-based plating layer comprises, in weight percent, aluminum (Al): 20.0~50.0%, magnesium (Mg): 5.0~22.0%, the remainder being zinc (Zn) and unavoidable impurities, in a plated steel sheet. In Paragraph 9, The above Zn-Al-Mg-based plating layer is a 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 galvanized steel sheets: 1,000% or less In Paragraph 10, The elements belonging to i) above are one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B, and The elements belonging to ii) above are one or more of Y, Zr, La, Ce, Ca, Sr, and Be, and A plated steel sheet in which the element belonging to iii) above is one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In. In Paragraph 9, A plated steel sheet in which the ratio ([Mg] / [Al]) of the Mg content ([Mg]) and Al content ([Al]) of the Zn-Al-Mg-based plating layer is 0.20 or more and less than 0.

70. In paragraph 1, A plated steel sheet having a Zn-Al-Mg-based plating layer thickness of 5 to 70 μm. In paragraph 1, The above first Al phase is a plated steel sheet with a primary Al phase. In paragraph 3, The above second Al phase is a plated steel sheet in which the above MgZn2 phase is precipitated inside.

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