Melt-coated Zn-Al-Mg steel

Abstract

The molten Zn-Al-Mg coated steel has a steel material and a coating layer disposed on the surface of the steel material; the coating layer includes a Zn-Al-Mg alloy layer disposed on the steel material; and the Zn-Al-Mg alloy layer has the following chemical composition: by mass % including: Al: greater than 30.5% and less than 54.0% and Mg: more than 1.09% and less than 10.0%; and the remainder being Zn and impurities; and the Zn-Al-Mg alloy layer comprises, by area %: Al phase: 0~53.0%; Al-Zn phase with Al as the main component: 4.0~75.0%; Zn-Al phase with Zn as the main component: 0~18%; η+β phase: 0~9.2%; MgZn2 phase: 0.1~50.3%; and the remaining phase: 0~36.2%.

Description

[Technical Field]

[0001] This disclosure relates to a molten Zn-Al-Mg coated steel. This application claims priority based on Japanese Patent Application No. 2024-074933, filed on May 2, 2024, the contents of which are incorporated herein by reference. [Previous Technology]

[0002] In various fields such as civil engineering, building materials, automobiles, and home appliances, where rust prevention is required, molten steel coating is used as a low-cost means of rust prevention for steel.

[0003] For example, Patent Documents 1 and 2 disclose a Zn-Al-Mg coated steel sheet that has attracted attention in recent years as a highly corrosion-resistant coated steel.

[0004] The required performance and specifications of steel vary widely across different fields. For example, generally speaking, the sheet metal used in the civil engineering and construction industry tends to be thicker than that used in the automotive and home appliance industries. However, in recent years, high-tensile steel, which can be produced by reducing sheet thickness, is increasingly being used in the construction industry for purposes such as saving labor. As mentioned above, in recent years, this demand has crossed over into other fields, creating a need for galvanizing high-tensile steel, a type of high-performance steel.

[0005] Here, regarding the performance requirements of the coating, the main requirements include corrosion resistance to inhibit steel corrosion and sacrificial corrosion resistance. In addition, the requirements include preventing scratches during forming (high hardness), processing compliance during harsh processing such as drawing (softness and hardness), durability in high-temperature environments (high-temperature characteristics) and design.

[0006] Regarding the corrosion resistance of the processed parts, when the steel sheet is thick, in order to prevent corrosion from the exposed part (exposed base iron part) of the steel sheet during cutting or heavy processing, steel sheets with high sacrificial corrosion resistance are mostly used. On the other hand, when the steel sheet is thin, steel sheets with higher emphasis on the corrosion resistance of the flat part are mostly used.

[0007] Furthermore, to prevent scratches during forming or processing such as roll forming, a high-hardness coating with excellent slip properties is required. On the other hand, considering the applicability to demanding processes such as drawing and extrusion of thin sheets, a low-hardness coating that can follow the plastic deformation of the steel sheet is required. These properties are significantly affected by the alloy composition of the coating. (Previous Art Documents, Patent Documents)

[0008] Patent Document 1: Japanese Patent No. 6428974; Patent Document 2: Japanese Patent No. 6365807 [Summary of the Invention]

[0009] The problem to be solved by the invention is as described above. In recent years, high standards have been demanded for various properties of the coating on coated steel, regardless of the application field. In particular, there is an increasing demand for coatings with excellent planar corrosion resistance and low hardness that can adapt to the plastic deformation of steel even under harsh processing. For example, for thin steel materials such as roofing materials and lightweight roofs (lightweight steel frame components, LGS), high planar corrosion resistance is required. In other cases, such as for thin sheet metal used in kitchens for the purpose of lightweighting, in addition to high planar corrosion resistance, characteristics that can withstand complex processing (processability) or inconspicuous appearance such as white rust on the processed parts are sometimes required. As mentioned above, there is an urgent need for a coated steel with a high-performance coating that not only exhibits planar corrosion resistance but also performs a variety of high-level properties.

[0010] This disclosure is made in view of the above circumstances, and the object is to provide a molten Zn-Al-Mg coated steel with excellent corrosion resistance of the planar part and the processed part.

[0011] Means for solving the problem In order to solve the above-mentioned problem, this disclosure adopts the following structure. [1] This invention discloses a type of plated steel, which is a molten Zn-Al-Mg plated steel, having a steel material and a plating layer disposed on the surface of the steel material; the plating layer includes a Zn-Al-Mg alloy layer disposed on the steel material; and the Zn-Al-Mg alloy layer has the following chemical composition: comprising, by mass %: Al: greater than 30.5% and less than 54.0%; and Mg: more than 1.09% and less than 10.0%; and the remainder is Zn and impurities; and the Zn-Al-Mg alloy layer comprises, by area %: Al phase: 0~53.0%; Al-Zn phase with Al as the main component: 4.0~75.0% (however, when the Al phase is not present, the Al-Zn phase is 48.5% or more); Zn-Al phase with Zn as the main component: 0~18%; η+β phase: 0~9.2%; MgZn2 phase: 0.1~50.3%; and the remaining phase: 0~36.2%. [2] The molten Zn-Al-Mg coated steel as described in [1] above can also be: In the aforementioned chemical composition of the Zn-Al-Mg alloy layer: Al: 30.65~30.95%, Mg: 8.55~9.20%; In the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is 5.0% or more and the area ratio of the aforementioned Al-Zn phase is 21.0% or more; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is 20.0% or more; and when the proportion of the aforementioned Al-Zn phase is less than 60.0%, the aforementioned Al phase is at least 7.0% or more. [3] The molten Zn-Al-Mg coated steel as described in [1] above can also be: In the aforementioned chemical composition of the Zn-Al-Mg alloy layer: Al: 33.0~37.5%, Mg: 7.5% or more and less than 9.1%; In the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is 5.0% or more and the area ratio of the aforementioned Al-Zn phase is 21.0% or more; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is 20.0% or more; and when the proportion of the aforementioned Al-Zn phase is less than 60.0%, the aforementioned Al phase is at least 7.0% or more.[4] The molten Zn-Al-Mg coated steel as described in [1] above can also be: In the aforementioned chemical composition of the Zn-Al-Mg alloy layer: Al: greater than 37.5% and less than 41.5%, Mg: greater than 7.0% and less than 8.2%; In the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is greater than 5.0% and the area ratio of the aforementioned Al-Zn phase is greater than 21.0%; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is greater than 20.0%; When the proportion of the aforementioned Al-Zn phase is less than 60.0%, at least the aforementioned Al phase is greater than 7.0%. [5] The molten Zn-Al-Mg coated steel as described in [1] above can also be: In the aforementioned chemical composition of the aforementioned Zn-Al-Mg alloy layer: Al: greater than 41.5% and less than 45.25%, Mg: greater than 4.0% and less than 8.0%; In the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is greater than 5.0% and the area ratio of the aforementioned Al-Zn phase is greater than 21.0%; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is greater than 20.0%; and when the proportion of the aforementioned Al-Zn phase is less than 60.0%, the aforementioned Al phase is at least greater than 7.0%. [6] The molten Zn-Al-Mg coated steel as described in [1] above can also be: In the aforementioned chemical composition of the Zn-Al-Mg alloy layer: Al: greater than 45.25% and less than 54.0%, Mg: greater than 1.2% and less than 5.3%; In the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is 5.0% or more and the area ratio of the aforementioned Al-Zn phase is 21.0% or more; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is 20.0% or more; When the proportion of the aforementioned Al-Zn phase is less than 60.0%, the aforementioned Al phase is at least 7.0% or more. [7] The molten Zn-Al-Mg coated steel as described in [1] above can also be: In the aforementioned chemical composition of the Zn-Al-Mg alloy layer: Al: 30.0~44.0%, Mg: 7.0~10.0%; and the area fraction of the aforementioned MgZn2 phase in the aforementioned Zn-Al-Mg alloy layer is 35.0~50.3%.

[0012] Effects of the invention According to the embodiments of the present invention, a molten Zn-Al-Mg coated steel with excellent corrosion resistance of the planar part and the processed part can be provided.

Implementation Method

[0013] Hereinafter, a description will be given regarding a molten Zn-Al-Mg coated steel (hereinafter also referred to as molten coated steel) according to an embodiment of the present invention.

[0014] Furthermore, unless otherwise stated, in this specification, the "%" designation for the content of each element in the chemical composition of the plating layer means "mass %". Also, the numerical range indicated by "~" means the range including the values ​​before and after "~" as lower and upper limits. Furthermore, the numerical ranges accompanied by "greater than" or "less than" before and after "~" indicate the range excluding those values ​​as lower or upper limits.

[0015] The term "planar corrosion resistance" in this specification refers to the property that the coating itself is not easily corroded. Zn-based coatings provide sacrificial corrosion protection to the steel (base steel). Therefore, regarding the corrosion process of coated steel with Zn-based coatings, firstly, the coating will corrode and turn white rust before the steel corrodes. Only after the white rusted coating disappears will the steel corrode and produce red rust.

[0016] In this specification, "corrosion resistance of the processed area" refers to the property of inhibiting corrosion of the steel at exposed parts of the steel (base steel) (e.g., where the steel is exposed due to the cut end of the plated steel or the cracking of the molten plating layer during processing). Furthermore, "machinability" refers to the property of excellent ductility of the plating layer, which allows it to follow the processing of the steel even under harsh processing conditions. If a certain degree of "sacrificial corrosion protection" is achieved by minimizing the cracking of the plating layer and reducing the exposed area, then corrosion of the steel from the base material can be sufficiently inhibited. In other words, while the corrosion protection methods of the raw materials differ, the excellent machinability allows the white rust used for sacrificial corrosion protection to be minimized, maintaining an appearance of integrity from the initial stage of corrosion. This results in long-term corrosion protection of the processed area; in other words, excellent corrosion resistance of the processed area. Further details regarding each characteristic will be provided later.

[0017] First, the inventors have studied the relationship between the corrosion resistance of the plane, the hardness and the corrosion resistance of the processed part, and the alloy composition constituting the plating layer, and the results of their study are described below.

[0018] The inventors of this case first investigated the alloy composition of the coating of molten coated steel used in various fields such as building materials, automobiles, and home appliances. In addition, the steel used in these fields can be roughly distinguished by plate thickness, and can be differentiated according to purpose or use and required performance.

[0019] After studying the alloy composition of various coatings, the inventors investigated the melting point of the coatings, observed the cross-sections, evaluated the corrosion resistance, investigated the sacrificial corrosion resistance (corrosion resistance at the cut end face), and evaluated the hardness. As a result, the following new insights were obtained.

[0020] (i) The properties of Zn-Al-Mg alloy coatings depend on the concentrations of Al and Mg contained in Zn. (ii) When the phases constituting Zn-Al-Mg alloy coatings are classified into six phases according to the concentrations of the constituent elements, the amount of the six phases (the area ratio of each phase when viewed in cross-section) will affect the properties of the coating. (iii) The morphology (dispersion, aggregation) of the six constituent phases of Zn-Al-Mg alloy coatings may sometimes change depending on the manufacturing conditions, but the influence of these morphologies on the properties is less significant than that on the area ratio.

[0021] Furthermore, the coating in this investigation refers to a portion of the Zn-Al-Mg alloy coating that does not include an interfacial reaction layer (less than 5µm) with the base iron (steel). That is, the Zn-Al-Mg alloy coating, which is the main constituent layer of the coating of the present invention, has reduced the influence of diffusing elements from the steel.

[0022] Hereinafter, a more specific embodiment of the hot-dip galvanized steel of the present invention, based on the above-mentioned new knowledge and insights, will be described in detail.

[0023] The molten-coated steel of this embodiment has a steel material and a coating layer disposed on the surface of the steel material.

[0024] (Steel) The steel to be plated will be explained. For example, steel sheet. When the steel is a sheet, its dimensions are not particularly limited. The steel sheet can be used in a general hot-dip galvanizing process. Specifically, steel sheets suitable for use in continuous hot-dip galvanizing (CGL) lines, where the steel is immersed in molten metal and solidified, are acceptable. For example, steel sheets with a thickness of 10 mm or less and a width of 2000 mm or less are suitable, but the dimensions of the steel sheet are not limited by this.

[0025] There are no special restrictions on the material of the steel. The steel that can be used includes, for example: general steel, aluminum deoxidized steel, very low carbon steel, high carbon steel, various high tensile steels, some high alloy steels (including steels containing corrosion-resistant strengthening elements such as Ni and Cr), bolt steel, steel wire for bridge cables, and various steel plates. More specifically, the following can be applied: cold-rolled steel sheets as specified in JIS G 3131 and 3141; general structural rolled steels included in the so-called SS material; general steels included in the hot-rolled steel sheets shown in JIS G 3193; pre-plated steels such as JIS H8641, JIS G 3302, 3303, 3313, 3314, 3315, 3317, and 3321, which are thinly coated with various metals; aluminum deoxidized steel in JIS G 3136; very low carbon steel; high carbon steel; and various high-tensile steels described in JIS G 3113, 3134, and 3135.

[0026] (Coating Layer) Next, the coating layer provided on the steel will be described. The coating layer of this embodiment includes a Zn-Al-Mg alloy layer provided on the steel. For example, a pre-plating layer, a layered intermetallic compound formed after the steel as the base steel reacts with the coating layer, or an oxide layer that is unavoidable in manufacturing (but does not affect the adhesion of the coating layer to the steel) may also be provided between the steel and the Zn-Al-Mg alloy layer. It is also acceptable to provide a Zn-Al-Mg alloy layer on the steel. The Zn-Al-Mg alloy layer contains, for example, a Zn phase, an Al phase, and a MgZn2 phase.

[0027] When Zn contains alloying elements such as Al and Mg, its planar corrosion resistance is improved. Therefore, if the coating layer contains the Zn phase, even if it becomes a thin film (for example, about half the thickness of a typical Zn coating layer), it can still exhibit the same corrosion resistance as a typical Zn coating layer. Similarly, the coating layer of this embodiment, even when formed into a thin film, can still ensure planar corrosion resistance equal to or better than that of conventional Zn coating layers. In this embodiment, the Zn-Al-Mg alloy layer forms the main body of the coating layer.

[0028] The Zn-Al-Mg alloy system is composed of Zn-Al-Mg alloys. The so-called "Zn-Al-Mg alloy" refers to a ternary alloy containing Zn, Al and Mg.

[0029] There is no particular limitation on the overall thickness of the coating. For example, when a Zn-Al-Mg alloy layer is formed by a continuous melt deposition method, the thickness of the coating may be, for example, less than 100µm.

[0030] <Chemical Composition> Next, the average chemical composition of the Zn-Al-Mg alloy layer will be explained. The Zn-Al-Mg alloy layer has the following chemical composition: by mass%, it contains more than 30.5% and less than 54.0% and Mg: more than 1.09% and less than 10.0%; and the remainder is Zn and impurities. The remainder may be more than 40% Zn and less than 0.1% impurities. That is, in the Zn-Al-Mg alloy layer, Zn, Al and Mg are the main elements, and the total content of Zn, Al and Mg in the Zn-Al-Mg alloy layer may be more than 95%.

[0031] Al: Al content is greater than 30.5% and less than 54.0%, and like Zn, it is a major element constituting the coating. When the Al content is less than 30.5%, the desired phase described later cannot be obtained. Furthermore, if the Al content is low, sufficient corrosion resistance for highly corrosion-resistant molten-coated steel cannot be ensured. Therefore, the Al content is set to be greater than 30.5%, preferably 31.0% or more. On the other hand, if the Al content is greater than 54.0%, the desired phase described later cannot be obtained. Furthermore, if the Al content is too high, the sacrificial corrosion resistance of the coating itself will decrease, and in cutting / bending sections, sufficient sacrificial corrosion resistance for highly corrosion-resistant molten-coated steel cannot be ensured. Therefore, the Al content is set to be less than 54.0%, preferably 50.0% or less.

[0032] Mg: 1.09% or more and 10.0% or less. Mg is an element used to ensure the corrosion resistance of the coating. When the Mg content is less than 1.09%, the desired phase described later cannot be obtained. Furthermore, if the Mg content is too low, sufficient corrosion resistance for hot-dip galvanized steel with high corrosion resistance cannot be ensured. Therefore, the Mg content is set at 1.09% or more. Preferably, it is 2.0% or more. On the other hand, when the Mg content is too high, the desired phase described later cannot be obtained. Furthermore, if the Mg content is too high, sufficient corrosion resistance cannot be obtained. Therefore, the Mg content is set at 10.0% or less. Preferably, it is 9.0% or less.

[0033] Remaining portion: Zn and impurities. In the molten steel of this embodiment, Zn, like Al, is a major element constituting the coating layer. That is, the remaining portion other than Al and Mg can also be Zn and impurities. In the coating layer of this embodiment, the element contained in the largest quantity is Zn. In addition, Zn is an element that sacrifices corrosion resistance to the coating layer, so the Zn content should preferably be 40% or more.

[0034] Furthermore, in the plating layer, trace amounts of Fe and other components may sometimes be introduced as impurities due to atomic diffusion between the steel (base iron) and the plating bath, or from the raw materials. Also, for the manufacture of the plating layer, Zn, Al, and Mg are mostly metals with a purity of 3N. Therefore, the concentration of impurities in the plating layer can be set to approximately 0.1% or less in total.

[0035] In determining the average chemical composition of the plating layer, an acid solution containing an inhibitor that suppresses corrosion of the base iron (steel) is used to obtain the acid solution after the plating layer has been peeled off and dissolved. The chemical composition is then determined by ICP-MS or ICP-luminescence spectrophotometry. There are no particular restrictions on the type of acid; it can be any acid capable of dissolving the plating layer. If the area and weight before and after peeling are measured in advance, the plating adhesion amount (g / m²) can also be obtained simultaneously.

[0036] <Constituent Phases> The various properties and characteristics of a coating are influenced by the morphology and quantity of its constituent phases. That is, by optimizing the types or ratios of the phases constituting the coating (especially Zn-Al-Mg alloy layers), desired characteristics can be obtained. Specifically, one factor affecting the corrosion resistance of the planar portion of the coating is the corrosion rate of each phase. Since the coating is a mixture of phases, the overall corrosion rate of the mixed phases is considered from the perspective of the overall corrosion rate of the coating. Similarly, regarding the sacrificial corrosion resistance of the coating, the overall potential of the mixed phases is considered from the perspective of the overall potential of the coating. Furthermore, regarding the hardness of the coating, although each phase has inherent hardness, the hardness must still be considered as a whole for the mixed phases. The following will explain the phases constituting the Zn-Al-Mg alloy layers.

[0037] The Zn-Al-Mg alloy layer of this embodiment comprises, by area %: Al phase: 0~53.0%; Al-Zn phase with Al as the main component: 4.0~75.0%; (However, when the Al phase is not present, the Al-Zn phase is 48.5% or more); Zn-Al phase with Zn as the main component: 0~18%; η+β phase: 0~9.2%; MgZn2 phase: 0.1~50.3%; and the remaining phase: 0~36.2%. Each phase can be classified according to its composition ratio and concentration ratio. The following is a detailed description of each phase.

[0038] [Al Phase] The Al phase is a phase with Al as the main component. Generally, in binary or higher-component plating layers such as Zn-Al plating, a certain amount of Zn is usually dissolved in Al, so there is almost no completely pure Al in the plating layer. However, the composition range with the same properties as pure Al is "Al: greater than 99.0%, and Zn: less than 1%", so phases with this composition range are considered Al phases. In addition, when the plating composition is within the range described above, Mg is usually bonded to Zn and therefore does not dissolve in the Al phase. That is, the Mg concentration of the Al phase in this embodiment is less than 1%. In addition, the Mg concentration in the α phase, β phase, and η+β phase contained in the Zn-Al-Mg alloy layer is also less than 1%, which will be described in detail later.

[0039] The properties of the Al phase in this embodiment are the same as those of pure Al. Pure Al itself exhibits a very low potential, but under normal corrosive conditions, the passivation effect brought about by the Al2O3 film will manifest on the Al surface, resulting in a very low corrosion rate. On the other hand, the original sacrificial corrosion protection against Fe cannot be expected as well. Furthermore, the Al phase is ductile and soft with low hardness. On the other hand, the melting point of the Al phase is the same as that of pure Al, 660℃, which is higher than that of Zn, 419℃.

[0040] The Al phase possesses the properties described above. Therefore, by increasing the Al phase content in the coating, excellent corrosion resistance of planar areas can be ensured. Furthermore, because the Al phase is soft, even under harsh processing, cracks in the processed areas can be suppressed. Even when forming complex shapes, the coated steel can be thinned to follow the processing, thereby reducing the exposure of the base steel. This is because increasing the Al phase relatively reduces the presence of MgZn2 or ternary eutectic structures (others) that would worsen processability. To achieve these effects, the Al phase area ratio is set to 0.1% or more. On the other hand, once exposed areas appear, it becomes difficult to ensure sacrificial corrosion resistance. Therefore, it is necessary to mix in phases other than the Al phase to ensure a certain degree of sacrificial corrosion resistance. In other words, since the degree of processability varies with plate thickness, ensuring a certain amount of phases other than the Al phase while suppressing the Al phase is sometimes preferable. Therefore, the area fraction of the Al phase is set to be below 53.0%.

[0041] [α phase] In this embodiment, the α phase is defined as a phase having a composition range of "Zn: 1~40.5%, Al: greater than 59.5% and less than 99.0%, Mg: less than 1%". That is, the α phase, like the Al phase described above, is a phase with Al as the main component, and it can be described as a phase with a slightly lower Al concentration and a higher Zn concentration than the Al phase described above. The α phase is also called "Al-Zn phase with Al as the main component".

[0042] During the solidification process of the coating, a large amount of Zn melts into Al in the high-temperature region. Therefore, when the Al content of the coating is high, the α phase may crystallize or precipitate and remain. As mentioned above, the α phase in the coating exhibits properties close to Al, and Al2O3 is also stable. However, its stability is not as strong as that of the Al phase. In terms of corrosion sequence, the α phase has a slightly lower potential than the Al phase, and its corrosion rate is also slightly higher than that of the Al phase. From these circumstances, it can be said that the α phase has a slightly higher performance in improving sacrificial corrosion resistance than the Al phase, but on the other hand, its corrosion resistance in planar areas is slightly worse than that of the Al phase. Furthermore, regarding hardness, the α phase has a higher hardness than the Al phase due to the increased internal differential arrangement, specifically exhibiting a value slightly larger than the midpoint between Al and Zn. If the coating contains the Al phase, the corrosion resistance in planar areas will be significantly improved, but even without the Al phase, the α phase can still sufficiently improve the corrosion resistance in planar areas. At this point, it is advisable to have a content of 48.5% or higher.

[0043] Generally speaking, it is almost impossible to obtain sacrificial corrosion resistance when using pure Al plating. However, in this embodiment, by mixing the Al phase with the α phase, a certain degree of sacrificial corrosion resistance can be obtained, and high corrosion resistance of the planar portion can also be achieved. That is, by increasing the amount of Al phase and α phase in the plating layer, an optimal plating layer can still be obtained even in the case of thinner steel.

[0044] To achieve the aforementioned effect, the area fraction of the α phase is 4.0% or more. When particular emphasis is placed on the corrosion resistance of planar portions, the area fraction of the α phase has an optimal range, preferably 20.0% or more, and more preferably 40.0% or more. On the other hand, once exposed portions appear, it becomes difficult to ensure sacrificial corrosion resistance; therefore, it is necessary to mix the Al phase with phases other than the α phase to ensure a certain degree of sacrificial corrosion resistance. In this embodiment, the area fraction of the α phase is set to 75.0% or less. Preferably 60.0% or less, and more preferably 55.0% or less.

[0045] [β phase] In this embodiment, the β phase is defined as a phase with a composition range of "Zn: 40.5~90.0%, Al: 10.0% and below 59.5%, Mg: less than 1%". That is, the β phase can be described as a phase with a lower Al concentration and a higher Zn concentration than the α phase. The β phase is also called "Zn-Al phase with Zn as the main component". The formation process of the β phase is the same as that of the α phase.

[0046] The β phase itself is stable in the high-temperature region (around 250~300℃), but because the plating is eventually cooled to room temperature, the β phase separates into Al and Zn. That is, the β phase eventually undergoes component separation in the solid phase, becoming fine precipitates and a mixed phase of fine Zn and Al phases. Therefore, the final β phase will remain as a trace of the β phase morphology in the high-temperature region, exhibiting unique properties. The β phase exhibits properties closer to Zn than Al. As mentioned above, the β phase has a lower Al concentration and a higher Zn concentration than the α phase, so Al2O3 will not be fully formed in the β phase. Therefore, the potential of the β phase will shift towards the extremely low potential of Al, -1.66V, so the potential of the β phase will be lower than that of Zn, resulting in a phase with sacrificial corrosion resistance. In addition, the β phase has the lowest potential among the constituent phases of the plating layer in this embodiment. Therefore, by setting the Al content in the coating to an appropriate range and ensuring sufficient β phase content, sacrificial corrosion resistance can also be improved. On the other hand, the low potential of the β phase can cause an increase in coupling current with the surrounding phases, thus tending to result in poor corrosion resistance in planar areas. In addition, the hardness of the β phase is slightly higher than that of the soft Al and Zn phases, and can be considered almost equal to that of the α phase.

[0047] As described above, the β phase is a phase that helps improve sacrificial corrosion resistance, but from the viewpoint of improving planar corrosion resistance, it is advisable not to contain too much of it. Therefore, the area fraction of the β phase is set to 18.0% or less. It is preferable to be 15.0% or less, and more preferably 10.0% or less. The area fraction of the β phase can also be 0%. In addition, when it is desired to improve sacrificial corrosion resistance in addition to planar corrosion resistance, the area fraction of the β phase can also be set to 1.0% or more.

[0048] [η+β phase] The η+β phase is a phase with Zn as the main component. Specifically, it is a mixed phase of pure Zn phase (η phase) and β phase. Generally, in binary or higher-order plating layers such as Zn-Al plating, a certain amount of Al is usually dissolved in Zn, so there is almost no completely pure Zn in the plating layer. However, the composition range with the same level of properties as pure Zn is considered to be "Zn: 90.0% or more, Al: 8.0% or more and less than 10.0%", so in this embodiment, the phase with this composition range is considered to be pure Zn phase (η+β phase). If the η+β phase is strictly defined, it is a mixed structure of micro-β phase and micro-η phase homogeneous with pure Zn, and the average composition of the η+β phase as a whole falls within the range of "Zn: 90.0% or more, Al: 8.0% or more and less than 10.0%".

[0049] The properties of the η+β phase in this embodiment are the same as those of pure Zn. The corrosion rate of the planar portion of the η+β phase is the highest among the constituent phases of the coating (i.e., greater than that of the β phase), making it an easily corroded phase. On the other hand, regarding sacrificial corrosion resistance, the η+β phase is higher than that of the α phase but lower than that of the β phase. The hardness of the η+β phase is almost the same as that of pure metallic Zn, and therefore very low, similar to that of the Al phase.

[0050] As described above, the η+β phase is a phase that helps improve sacrificial corrosion resistance, but because of its high corrosion rate, it should not be present in excessive amounts. Therefore, the area fraction of the η+β phase is set to 9.2% or less. It is preferably 5.0% or less, and more preferably 4.0% or less. The area fraction of the η+β phase can be 0%. In addition, when it is desired to improve sacrificial corrosion resistance in addition to planar corrosion resistance, the area fraction of the η+β phase can also be set to 1.0% or more.

[0051] [MgZn2 phase] In Zn-Al-Mg system plating layers, almost all the Mg contained therein will bond with Zn, resulting in an intermetallic compound called MgZn2. If this MgZn2 grows to a certain size, it will become a single phase composed of intermetallic compounds, namely the MgZn2 phase. Specifically, in plating layers with the chemical composition described above, although the Al-Zn phase, from which the Al, α, β, and η+β phases originate, will crystallize first, the concentration of Al and Zn in the surrounding liquid phase will decrease as this Al-Zn phase grows, thus creating a situation where MgZn2 is easily formed. At this time, the precipitated MgZn2 phase will grow by surrounding the Al, α, β, and η+β phases, and grow into a large phase, exhibiting the properties of MgZn2. Furthermore, if the Al phase, α phase, β phase, η+β phase and MgZn2 phase are fully crystallized, the final liquid phase may sometimes form the "residual phase" described later, that is, mainly forming a ternary eutectic structure (Zn-Al-MgZn2) as the residual phase.

[0052] The MgZn2 phase exhibits excellent insulation properties, and its high planar corrosion resistance is achieved through its presence. This effect is higher than that of the β phase and lower than that of the α phase. Regarding sacrificial corrosion resistance, its potential is lower than that of the β phase, resulting in excellent sacrificial corrosion resistance in the coating. From this perspective, the area fraction of the MgZn2 phase should be 0.1% or more, preferably 20.0% or more. Furthermore, MgZn2 is an extremely hard substance; therefore, a large amount of MgZn2 phase can significantly increase the hardness of the coating, resulting in excellent resistance to damage. From this perspective, the area fraction of the MgZn2 phase should preferably be 30.0% or more, and more preferably 35.0% or more.

[0053] As described above, the MgZn2 phase also helps to improve sacrificial corrosion resistance, but because it is a very hard phase, it should not be contained in excess. Furthermore, by controlling the area ratio of the MgZn2 phase in the coating and ensuring that other structures (e.g., ternary eutectic structures) contain a majority of MgZn2, a coating that achieves a balance between workability and sacrificial corrosion resistance can be obtained. From this perspective, the MgZn2 phase content should be set to 50.3% or less, and preferably 45.0% or less.

[0054] Here, the MgZn2 phase system in this embodiment is a phase composed of the intermetallic compound MgZn2. Specifically, the MgZn2 phase system is defined as a phase having the following composition: Mg concentration: 15.48±5%, Al concentration: less than 1%, and the remainder being Zn. However, the MgZn2 constituting the MgZn2 phase of this embodiment has an area of ​​5µm2 or more in terms of the equivalent diameter of a circle, and the ratio amax / bmax of the longest straight axis length amax in the longest straight direction and the axis length b in the straight direction orthogonal to it satisfies the maximum value bma. That is, the MgZn2 phase system mentioned here is composed of blocky MgZn2 with a ratio close to that of a circle, and excludes MgZn2 in the ternary eutectic structure existing in a lamellar state.

[0055] In this embodiment, by setting a Zn-Al-Mg alloy layer mainly composed of Al phase and α phase, a coating layer with excellent planar corrosion resistance and processability can be obtained. However, by increasing the ratio of MgZn2 phase to β phase, a coating layer that can take into account corrosion resistance can also be obtained, which can also be used for thicker steel plates.

[0056] [Other (Residual Phases)] As mentioned above, after the Al phase, α phase, β phase, η+β phase, and MgZn2 phase (Al-Zn) crystallize, the liquid phase eventually solidifies, and it mostly becomes a ternary eutectic structure (Zn-Al-MgZn2). In addition, depending on the situation, Zn may grow and even coarse Zn grains may be included as residual phases. Although the overall composition of the residual phase is close to Zn-5%Al-2%Mg, when coarse Zn grains are included as residual phases, the Zn ratio may be higher.

[0057] Regarding the properties of the residual phase, which is mainly composed of a ternary eutectic structure, it tends to have better corrosion resistance in the planar portion than the η+β phase or the β phase, and slightly worse than the MgZn2 phase. This is because the Zn-Al-Mg alloy layer also contains the MgZn2 or Al phase, which has higher corrosion resistance. Regarding sacrificial corrosion resistance, although it is slightly better than the α phase or the Zn phase, it is still smaller than the MgZn2 phase. The residual phase can be 0%. In order to optimize the content of each of the above-mentioned constituent phases, it is not advisable to maintain high corrosion resistance in the planar portion with the Al phase and the α phase, and to have an excessive amount of residual phase. Therefore, the residual phase is set to 36.2% or less.

[0058] Next, the classification method for the constituent phases will be explained. Furthermore, when classifying, it is advisable to use a coating layer with a thickness of 20µm or more to determine the phase fraction. This is because the measurement error decreases when the coating thickness is 20µm or more. First, a 20mm square test piece is cut from the coated steel, including the coating layer. The test piece is embedded in resin with the cross-section along the thickness direction of the coating layer as the observation surface, and the observation surface is ground. After grinding, the cross-section is observed using EPMA. For example, if an Al-Fe alloy layer is formed at the interface between the steel and the coating layer, it will be excluded; therefore, a region with a thickness of 1µm from that interface towards the coating layer direction is excluded from the judgment. Next, a SEM image of the observation surface is obtained, and the Zn, Al, and Mg composition distribution image obtained using EPMA in the SEM image is obtained. Based on the obtained composition distribution image and the definition of the composition range of each layer, the area fraction of each constituent phase is calculated. Furthermore, the reason for measuring the area ratio along the thickness direction of the plating layer, rather than in a cross-section parallel to the surface of the plating layer (a horizontal cross-section), is that a horizontal cross-section would result in a compositionally uneven structure. That is, in a typical plating layer, since it solidifies from the surface, the final solidified portion will exist near the interface between the base steel and the plating layer, leading to compositional unevenness along the thickness direction of the plating layer. Therefore, in this embodiment, the area ratio is measured along a cross-section along the thickness direction of the plating layer.

[0059] To confirm the area ratio, SEM was used at 500x magnification, and measurements were repeated until the total field of view of the coating layer reached 100,000 µm². The coating layer was trimmed and cropped from the obtained EPMA distribution image. Since histograms of the concentrations of Zn, Al, and Mg were obtained, the distribution areas of the components corresponding to the Al phase, α phase, β phase, η+β phase, and MgZn2 were selected. The remaining portion corresponds to the remaining phase.

[0060] Furthermore, based on the Zn concentration distribution spectrum, the positions of the ternary eutectic structure, β phase, and MgZn2 phase can be easily identified by defining a threshold for the composition. These positions can be identified by overlaying them onto an SEM image. Regarding the remaining phases, no specific threshold needs to be set; if the Al phase, α phase, β phase, η+β phase, and MgZn2 phase can be correctly selected, the remaining portion can be considered as the remaining phase. In this embodiment, at least five samples taken from different samples with the same composition of coating are subjected to the above operation in two or more fields of view, and the area ratio of each phase is calculated from the average value of the entire field of view. Furthermore, regarding the phases obtained in the general plating process, apart from the aforementioned phases, the rest are ternary eutectic structures (an aggregate of fine η phase, MgZn2 phase, and pure Al phase). The composition ranges of the aforementioned phases have been defined by their respective definitions, so other phases will almost certainly be ternary eutectic structures. Depending on the circumstances, the MgZn2 phase may sometimes contain Mg2Zn11 (an equilibrium phase precipitated from MgZn2) due to the holding time after the plating step (around 50~100℃), but the amount is trace. Also, sometimes the plating surface may form an oxide film or an undefined structure directly beneath it due to solidification segregation, but these are also trace amounts and can be considered as "residual phases" without issue.

[0061] As described above, in this embodiment, by setting a Zn-Al-Mg alloy layer mainly composed of Al and α phases, a coating with excellent planar corrosion resistance and processability can be obtained. Therefore, it is more suitable for applications with thinner steel plates. From this point of view, although an Al coating layer is ideal, since the sacrificial corrosion resistance of pure Al coating is extremely low, it is desirable to ensure a certain level of sacrificial corrosion resistance from the perspective of the overall corrosion resistance of the coating layer. That is, as mentioned above, it is effective to include not only the Al phase but also other phases in appropriate proportions.

[0062] Furthermore, based on the viewpoint of balancing the corrosion resistance of the planar portion and sacrificial corrosion protection, the inventors have newly discovered that the chemical composition of the Zn-Al-Mg alloy layer exhibits an optimal balance in terms of Al and Mg content. Specifically, when the coating is applied to a thinner steel substrate, the following chemical composition and ratio should be satisfied. This allows for the formation of a large amount of Al and α phases, thereby further improving the corrosion resistance of the planar portion. Even with thinner steel plates, the corrosion resistance in the processed portion can be sufficiently suppressed through sacrificial corrosion protection.

[0063] In the chemical composition of the Zn-Al-Mg alloy layer: when Al: 30.65~30.95%, Mg: 8.55% or more and 9.20% or less; when Al: 33.0~37.5%, Mg: 7.5% or more and less than 9.1%; when Al: greater than 37.5% and less than 41.5%, Mg: 7.0% or more and less than 8.2%; when Al: greater than 41.5% and less than 45.25%, Mg: 4.0% or more and less than 8.0%; when Al: greater than 45.25% and less than 54.0%, Mg: greater than 1.2% and less than 5.3%; when the area fraction of the Al phase is 5.0% or more and the area fraction of the α phase (Al-Zn phase) is 21.0% or more, and the total proportion of the Al phase and the α phase (Al-Zn phase) is less than 50.0%, the α phase is 20.0% or more; When the proportion of α phase is less than 60.0%, the proportion of Al phase must be at least 7.0%.

[0064] To further improve the corrosion resistance of the planar portion, the main phase of the coating layer should be composed of a phase that can significantly improve the corrosion resistance of the planar portion. Specifically, it is sufficient to configure the coating so that the total ratio of Al phase, α phase, and MgZn2 phase is 80% or more. Furthermore, by setting the α phase to be 60% or more and the Al phase to be 10% or more as the main phase of the coating layer, even higher corrosion resistance of the planar portion can be obtained.

[0065] Furthermore, by setting the MgZn2 phase to 20% or more, both high sacrificial corrosion resistance and planar corrosion resistance can be obtained.

[0066] Regarding the range of chemical composition of the coating, from the perspective of corrosion resistance of the planar part, it is advisable to set it as: Al content: 48~54%, Mg content: 1.3% or more, and more preferably as: Al content: 48.5~50%, Mg content: 3.5~4.5%.

[0067] Furthermore, when the coating is soft, it can exhibit the plastic deformation capacity suitable for steel, thus allowing it to be covered without exposing the base steel during relatively gentle processing. This superior plastic deformation capacity of the coated steel further prevents corrosion degradation in the processed parts, therefore, high standards are required, especially for coated steels intended for coating after plating or for thin sheet coated steels where appearance is important.

[0068] To soften the plating layer and improve its processability, it is effective to keep the area fraction of the hard MgZn2 phase at 10% or less. Similarly, the ternary eutectic structure (Zn-Al-MgZn2) should also be set at 5% or less. To obtain the aforementioned constituent phases, the chemical composition range of the plating layer should preferably be: Al content: 51%~54%, Mg content: 1.8% or less.

[0069] On the other hand, increasing the hardness of the coating improves scratch resistance. The hardness of the coating can be increased by increasing the proportion of the MgZn2 phase in the coating. In particular, if the MgZn2 phase is present in the range of 35 to 50.0%, the hardness can be sufficiently increased, thus improving scratch resistance. To obtain the aforementioned constituent phase, the chemical composition range of the coating is preferably set as follows: Al content: greater than 30.5% and up to 44.0%, Mg content: 7.0 to 10.0%. More preferably, when the Al content is greater than 30.5% and less than 35.0%, the Mg content is set to 8.0% to 9.7%; or, when the Al content is 35.0% to 37.0%, the Mg content is set to 9.0% to 9.7%.

[0070] <Characteristics> In this embodiment, by optimizing the chemical composition of the plating layer and the content (area ratio) of each constituent phase, the corrosion resistance of the processed part can be controlled to a desired level through the corrosion resistance of the planar portion of the plating layer, sacrificial corrosion resistance (corrosion resistance of exposed portions such as cut ends), or processability (improved inhibition of initial corrosion due to increased plating coverage), the hardness of the plating layer, and the combined effect of these. The various characteristics and properties will be explained below.

[0071] [Corrosion Resistance of Planar Sections] Corrosion resistance of planar sections refers to the characteristic that the coating itself is not easily corroded. Corrosion resistance of planar sections can be improved by increasing the ratio of Al phase or α phase with Al2O3 film, or MgZn2 phase with excellent insulation properties such as MgZn2 intermetallic compound, and reducing β phase or β+η phase.

[0072] The corrosion resistance of the flat surface is evaluated using a corrosion-promoting test. Specifically, a composite cyclic corrosion tester is used to evaluate the corrosion weight loss of the plating before and after the corrosion test. After the corrosion test, corrosion products formed on the surface of the plated steel are removed with a 30% chromic acid (VI) aqueous solution. To make the corrosion test approximate the atmospheric environment, it is advisable to conduct it under conditions of low salt concentration. On the other hand, if too low a salt concentration is used, depending on the situation, the plated steel may not be able to corrode. Therefore, it is advisable to adjust the concentration according to the composition of the plating layer of the test object. In particular, for plating layers with high Al content, the inhibition of corrosion caused by Al2O3 is very obvious.

[0073] [Sacrificial Corrosion Resistance] Sacrificial corrosion resistance is an indispensable property when manufacturing products (processed parts, formed parts) from continuously melt-plated steel as raw material. Optimizing the sacrificial corrosion resistance of the plated steel can suppress corrosion or red rust formation in machined areas and cut-off surfaces. In heavily machined areas or cut-off surfaces subjected to harsh processing, the plating layer is often damaged, exposing the base steel; therefore, sacrificial corrosion resistance is necessary to prevent red rust formation. If the sacrificial corrosion resistance of the plating layer is insufficient, red rust will form from the machined areas or cut-off surfaces, becoming a major cause of corrosion in the surrounding plating layer. If the plate thickness reaches a certain level, the degree of corrosion increases, thus requiring higher sacrificial corrosion resistance.

[0074] However, the effectiveness of corrosion protection for the processed parts depends not only on the sacrifice of corrosion resistance but also on the plastic deformation capacity of the coating. If a coating with greater plastic deformation capacity is used, the coating will follow the plastic deformation capacity, and the thickness of the coating will be reduced, thereby reducing the exposed area of ​​the steel. That is, when the coating has high plastic deformation capacity, even if the sacrifice of corrosion resistance is small, it can still provide sufficient corrosion resistance for a certain degree of processing. Furthermore, even if the coating lacks plastic deformation capacity, if the sacrifice of corrosion resistance is large, the corrosion of the processed parts can be suppressed in the same way.

[0075] The corrosion resistance (sacrificial corrosion resistance) of the machined part (exposed part) can be confirmed by the following method: A test piece obtained through a 1T bending test (180° bending with a space of just the thickness of steel between the bent components) is prepared, and rigorous corrosion tests such as corrosion promotion tests are performed on it to observe the corrosion process on the top part. That is, the corrosion resistance of the machined part can be confirmed by confirming the cycle of red rust formation on the top part of the test piece.

[0076] [Appearance Maintenance of the Processed Part (Initial Rusting of the Processed Part)] Here, when it is a soft plating layer, the plating layer will undergo sufficient plastic deformation on the top part of the test piece, so it is not easy for exposed parts to appear. On the other hand, when it is a hard plating layer, cracks will occur on the top part of the test piece during processing, so the base steel is easily exposed. When evaluating the sacrificial corrosion resistance by the above-mentioned 1T bending test, when the plating layer completely covers the processed part (that is, when it is a softer plating layer), the occurrence of initial white rust is greatly suppressed, so the corrosion resistance (processability) of the processed part is excellent regardless of the size of the sacrificial corrosion resistance. On the other hand, when cracks occur on the top part (that is, when it is a harder plating layer), the exposed base steel will be subject to the sacrificial corrosion resistance of the plating layer, so white rust will inevitably occur, and there is a tendency for the white rust area ratio to increase. Furthermore, when the plate thickness is the same and the processing of the processing part is rigorous, there is a tendency for the white rust area ratio to increase with the exposed area of ​​the base steel.

[0077] The aforementioned characteristics particularly affect the formation of white rust on the plated steel sheet (processed product) immediately after actual use. If the plated steel has excellent appearance and can suppress the formation of initial white rust in the processed area, it can prevent changes in appearance of the processed product immediately after use, such as dull luster caused by cracks in the processed area, and maintain the metallic luster. Furthermore, it can also suppress partial corrosion of cracks in the processed area, which is ideal.

[0078] The appearance immediately after use can be evaluated by confirming the appearance of white rust after 9 cycles in the same corrosion test as the 1T bending test mentioned above for [sacrificial corrosion resistance].

[0079] [Scratch Resistance] The scratch resistance of the coating can be evaluated using a friction and abrasion tester. #400 sandpaper is sandwiched between two pieces of coated steel of the test object, and the object is subjected to a back-and-forth movement of a predetermined distance 10 times. The scratch resistance is then evaluated based on the magnitude of the set load.

[0080] (Manufacturing Method) Next, a suitable manufacturing method for the molten-plated steel of this embodiment will be described. There are no particular limitations on the manufacturing method of the molten-plated steel of this embodiment; any plating layer as described above is acceptable. Hereinafter, an ideal form for manufacturing the molten-plated steel of this embodiment will be described.

[0081] First, prepare the steel (base steel) to form the coating. Suitable steels are as described above, such as general steel, high-tensile steel and low-carbon steel.

[0082] The properties of the plating layer depend on the composition and structure of the layer. Therefore, it is effective to adjust the plating bath to have a composition that is substantially the same as that of the desired plating layer.

[0083] Next, a pre-plating layer (metal layer) consisting of a Cr pre-plating layer and a Ni pre-plating layer is formed on the surface of the steel. The plating method can be melt plating, electroplating, displacement plating, or vapor deposition (PVD, etc.). In addition, the Cr-Ni pre-plating layer can also be alloyed by heating. The steel with the Cr-Ni pre-plating layer formed thereon is used as the base plate for melt plating. The reasons for setting the Cr-Ni pre-plating layer on the surface of the steel will be explained below.

[0084] Generally speaking, when steel reacts with a plating bath at a temperature above 500°C, Fe atoms originating from the steel diffuse into the plating bath through mutual diffusion between the steel and the plating bath, and the components constituting the plating bath also diffuse into the steel. In particular, this diffusion reaction is very vigorous when the plating bath contains more than 10% Al. The interfacial alloy layer and Fe diffusion layer formed by this diffusion reaction have anchoring effects, which help improve the adhesion of the plating layer. However, on the other hand, it can affect the reduction of corrosion resistance or the loss of internal components of the plating layer, resulting in a decrease in the uniformity of plating performance.

[0085] We studied how to ensure the corrosion resistance and uniformity of the coating and found that if the coated steel is manufactured according to the following procedure, the diffusion of Fe can be fully suppressed during immersion in the plating bath, and the wettability and adhesion of the coating can also be fully achieved.

[0086] First, a Cr plating layer of 1 g / m² is formed on the surface of the substrate by electroplating. For the Cr plating bath, for example, CrO3 (100 g / L) or H₂SO₄ (1.5 g / L), the bath temperature is set to 50°C, and an current is applied at 20 A / dm² for approximately 40 seconds, thereby allowing approximately 1 g of Cr plating layer to adhere. An adhesion amount of approximately 3 g / m² is also acceptable. If this plating layer adheres metallic Cr to the steel plate, there are no restrictions on the method of application. This condition is the most common Cr plating method known as the Sargent bath. Other methods include fluoride baths or trivalent oxalic acid baths.

[0087] Then, a Ni plating layer of 1 g / m² is applied using Ni electroplating (attachment up to approximately 3 g / m² is also acceptable). In this embodiment, the Ni electroplating bath differs from general conditions such as Watt's baths; it is an acidic plating bath. Specifically, by suppressing the formation of an oxide film on the Cr plating layer and forming a Ni plating layer thereon, the outermost surface of the plating substrate can maintain the Ni plating layer, and below it, a Cr plating layer that suppresses Fe diffusion can be maintained. The Ni electroplating conditions are set as follows: Ni sulfate (NiSO4・6H2O) 400 g / L, Na2SO4 (anhydrous) 100 g / L, pH=1.4, bath temperature 60°C, and an energizer of 5 A / dm² for approximately 10 seconds, thereby allowing approximately 1 g of Ni plating layer to adhere. The pH of the plating bath can be adjusted using H2SO4.

[0088] Next, using the obtained plating substrate, plated steel is manufactured using a conventional plating process (a continuous reduction method utilizing the Sendzimir process). The molten plating bath used is a Zn-Al-Mg based plating bath that has been adjusted to meet the average chemical composition of the aforementioned plating layer. Furthermore, the Zn-Al-Mg based plating bath can also be prepared as a plating bath mixed with a predetermined amount of a desired metal.

[0089] After plating, the plated plate is lifted by wiping and then rapidly cooled to room temperature from the bath temperature at an average cooling rate of 10 to 50°C / second.

[0090] By manufacturing according to the above conditions, the molten coated steel of this embodiment can be manufactured.

[0091] Embodiments Next, embodiments of the present invention will be described. However, the conditions in the embodiments are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as the purpose of the present invention can be achieved without departing from its spirit.

[0092] The details of the embodiments are listed in Table 1. First, a 1.6 mm thick cold-rolled steel sheet (0.1%C-0.3%Si-1.0%Mn) that has been thoroughly pickled is prepared as the base plate (substrate steel) for plating. After the base steel is cut into 200 mm × 200 mm (× sheet thickness) pieces, continuous melt plating is performed using a batch melt plating test device.

[0093] Furthermore, a pre-plating layer (metal layer) consisting of a Cr pre-plating layer and a Ni pre-plating layer is formed between the plating substrate and the Zn-Al-Mg alloy layer by the following method.

[0094] First, a Cr plating layer of 1 g / m2 is formed on the surface of the substrate by electroplating. For example, the Cr plating bath is set to CrO3 (100 g / L), H2SO4 (1.5 g / L), the bath temperature is set to 50°C, and an electric current of 20 A / dm2 is applied for 30 seconds to deposit a 1 g Cr plating layer.

[0095] Subsequently, a Ni plating layer of 1 g / m² was formed by electroplating. The Ni electroplating conditions were set as follows: Ni sulfate (NiSO₄・6H₂O) 400 g / L, Na₂SO₄ (anhydrous) 100 g / L, pH=1.4, bath temperature 60°C, and an electric current of 5 A / dm² for 10 seconds, thereby allowing 1 g of Ni plating layer to adhere. The pH of the plating bath was adjusted using H₂SO₄.

[0096] The plating bath system is a Zn-Al-Mg molten plating bath prepared by mixing pure metals (purity ≥3N) of Zn, Al and Mg. The plating bath temperature is set to 520℃ when Al is less than 20% and 600℃ when Al is ≥20%.

[0097] Various pure metals (3N or more) are added to the plating bath to produce a plating bath (15L) with the desired composition.

[0098] Furthermore, the temperature process during manufacturing is managed using a sample with a K thermocouple fused to a central point on the plated steel sheet. Specifically, firstly, an electric current is applied to the plated steel sheet equipped with the K thermocouple to generate Joule heat, thereby heating the plated steel sheet. Then, based on the current applied and the thermal process obtained through the K thermocouple, the deviation from the target temperature is corrected by adjusting the current applied. In this way, the temperature process of the Zn-Al-Mg alloy layer during manufacturing is managed.

[0099] The substrate with the above-mentioned pre-plating layer is heated at 10°C / second in a N2-H2 (5%) reducing gas environment at a dew point of -60°C and maintained at 800°C for 1 minute. Then, N2 gas is blown to cool the substrate to the plating bath temperature and it is then immersed in the plating bath. After immersion in the plating bath for 3 seconds, it is lifted out and wiped with N2 gas to adjust the plating film thickness. Then, N2 gas is blown to cool it down at an average cooling rate of 10~15°C / second up to the plating bath temperature of ~100°C.

[0100] Performance evaluation samples were taken from the obtained coated steel sheet and evaluated for corrosion resistance of the flat part, sacrificial corrosion resistance, appearance maintenance of the machined part (initial rusting of the machined part) and resistance to damage.

[0101] [Corrosion Resistance of Flat Sections] The corrosion resistance of flat sections was assessed using a brine composite cyclic corrosion test apparatus, with the weight loss of the plating before and after the corrosion test. After the corrosion test, corrosion products formed on the surface of the plated steel were removed with a 30% chromic acid (VI) aqueous solution.

[0102] Specifically, the composite cyclic corrosion test (CCT) was performed according to JASO (M609-91). The salt concentration was set to 0.5% NaCl. Furthermore, the pH was set to a range of 5.0 ± 0.2 using a low concentration of hydrochloric acid. The corrosion weight loss after 150 cycles was calculated. The evaluation criteria are as follows: "A", "B", "C", and "D" are considered acceptable, and "E" is considered unacceptable.

[0103] Corrosion weight loss of 60 g / m² or more: classified as "E". Corrosion weight loss of 50 to less than 60 g / m²: classified as "D". Corrosion weight loss of 40 to less than 50 g / m²: classified as "C". Corrosion weight loss of 30 to less than 40 g / m²: classified as "B". Corrosion weight loss of less than 30 g / m²: classified as "A".

[0104] [Sacrificial Corrosion Resistance] The method for confirming the corrosion resistance (sacrificial corrosion resistance) of the processed parts of the coating is to combine a 1T bending test and a brine combined cycling corrosion test (CCT, JASO M609-91) for evaluation. The salt concentration is set to 0.5% NaCl. The pH of the brine is not adjusted.

[0105] First, a 1T bending test piece is made. A sample measuring 40mm × 120mm × 1.2mm is taken and bent in the center of a 40mm width. Specifically, a 1T (the thickness of exactly one steel plate) is sandwiched inside, and after bending and pressing with a jig at 180°, a 1T bending test piece with a space of exactly one steel plate inside is made.

[0106] Next, the 1T bending test piece is placed against the CCT device to observe the formation of red rust. The top part of the 1T bending test piece is subjected to intense processing, resulting in cracks in the plating layer and inevitably exposing the base steel. In this embodiment, photographs are taken of the top part of the 1T bending test piece (the top part of the 1T bending test piece) according to a prescribed cycle, and the evaluation is based on the number of cycles in which the area of ​​red rust formation in the evaluation surface, measured from the vertical direction, reaches 5%. The "evaluation surface" refers to a 120mm area along the length of the test piece in the top part of the 1T bending test piece. The evaluation criteria are as follows: "A", "B", and "C" are considered acceptable, and "E" is considered unacceptable.

[0107] When the number of cycles in which red rust is generated is less than 60: "E" judgment. When the number of cycles in which red rust is generated is 60-90: "C" judgment. When the number of cycles in which red rust is generated is 90-120: "B" judgment. When the number of cycles in which red rust is generated is more than 120: "A" judgment.

[0108] [Appearance Maintenance in the Processed Part (Initial Rusting in the Processed Part)] Regarding the appearance of the processed part, it is evaluated by confirming the condition (appearance) of white rust (initial rusting) after 9 cycles in the same corrosion test as the 1T bending test mentioned above for [Sacrificial Corrosion Resistance]. After 9 cycles, the condition in which white rust has formed on the top of the 1T bending test is judged as "unacceptable (×)", and the condition in which no white rust has formed and the metallic luster is maintained is judged as "acceptable (○)".

[0109] [Scratch Resistance] The scratch resistance of the coating was evaluated using a tribological abrasion tester. First, #400 SiC sandpaper was sandwiched between two coated steel samples, and one of the coated steel samples was moved back and forth 10 times at a predetermined distance of 30 mm. The moving speed was set to 50 mm / min, and the scratch resistance was assessed under load when the area of ​​the contact surface exceeded 50%. The load was measured using a load measuring device installed on the tribological abrasion tester. A higher load when the scratch area exceeded 50% of the sample indicates better scratch resistance. The evaluation criteria are as follows: "AA", "A", "B", "C", and "D" are considered acceptable, and "E" is considered unacceptable.

[0110] Load less than 200g with more than 50% of the wound area: Classified as "E". Load of 200-300g with more than 50% of the wound area: Classified as "D". Load of 300-400g with more than 50% of the wound area: Classified as "C". Load of 400-500g with more than 50% of the wound area: Classified as "B". Load of 500-600g with more than 50% of the wound area: Classified as "A". Load of 600g or more with more than 50% of the wound area: Classified as "AA".

[0111] [Table 1]

[0112] [Table 2]

[0113] Industrial Applicability According to the above-described form of the present invention, a molten Zn-Al-Mg coated steel with excellent corrosion resistance of both the planar portion and the processed portion can be provided.

Claims

1. A molten Zn-Al-Mg coated steel, comprising: steel; and a coating layer disposed on the surface of the steel; the coating layer comprising a Zn-Al-Mg alloy layer disposed above the steel; and the Zn-Al-Mg alloy layer having the following chemical composition: comprising, by mass %: Al: 32.1% or more and 54.0% or less; and Mg: 1.09% or more and 10.0% or less; with the remainder being Zn and impurities; and the Zn-Al-Mg alloy layer comprising, by area %: Al phase: 0 to 53.0%; Al-Zn phase with Al as the main component: 20.0 to 75.0%; (however, when the aforementioned Al phase is not present, the aforementioned Al-Zn phase is 48.5% or more); Zn-Al phase with Zn as the main component: 0 to 18.0%; η+β phase: 0 to 9.2%; MgZn2 phase: 0.1~50.3%; and the remaining phase: 0~36.2%.

2. For the molten Zn-Al-Mg coated steel as claimed in claim 1, wherein the aforementioned chemical composition of the Zn-Al-Mg alloy layer is as follows: Al: 33.0~37.5%, Mg: 7.5% or more and less than 9.1%; in the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is 5.0% or more and the area ratio of the aforementioned Al-Zn phase is 21.0% or more; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is 20.0% or more; and when the proportion of the aforementioned Al-Zn phase is less than 60.0%, the aforementioned Al phase is at least 7.0% or more.

3. For the molten Zn-Al-Mg coated steel as claimed in claim 1, wherein the aforementioned chemical composition of the Zn-Al-Mg alloy layer is as follows: Al: greater than 37.5% and less than 41.5%, Mg: greater than 7.0% and less than 8.2%; in the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is greater than 5.0% and the area ratio of the aforementioned Al-Zn phase is greater than 21.0%; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is greater than 20.0%; and when the proportion of the aforementioned Al-Zn phase is less than 60.0%, at least the aforementioned Al phase is greater than 7.0%.

4. For the molten Zn-Al-Mg coated steel as claimed in claim 1, wherein the aforementioned chemical composition of the Zn-Al-Mg alloy layer is as follows: Al: greater than 41.5% and less than 45.25%, Mg: greater than 4.0% and less than 8.0%; in the aforementioned Zn-Al-Mg alloy layer, the area ratio of the aforementioned Al phase is greater than 5.0% and the area ratio of the aforementioned Al-Zn phase is greater than 21.0%; and when the total proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is greater than 20.0%; and when the proportion of the aforementioned Al-Zn phase is less than 60.0%, the aforementioned Al phase is at least greater than 7.0%.

5. For the molten Zn-Al-Mg coated steel as claimed in Item 1, wherein the aforementioned chemical composition of the Zn-Al-Mg alloy layer is as follows: Al: greater than 45.25% and less than 54.0%, Mg: greater than 1.2% and less than 5.3%; in the aforementioned Zn-Al-Mg alloy layer, the area fraction of the aforementioned Al phase is 5.0% or more and the area fraction of the aforementioned Al-Zn phase is 21.0% or more; and when the combined proportion of the aforementioned Al phase and the aforementioned Al-Zn phase is less than 50.0%, the aforementioned Al-Zn phase is 20.0% or more; when the proportion of the aforementioned Al-Zn phase is less than 60.0%, the aforementioned Al phase is at least 7.0% or more.

6. The molten Zn-Al-Mg coated steel as claimed in claim 1, wherein the aforementioned Zn-Al-Mg alloy layer has the following chemical composition: Al: 32.1~44.0%, Mg: 7.0~10.0%; and the area fraction of the aforementioned MgZn2 phase in the aforementioned Zn-Al-Mg alloy layer is 35.0~50.3%.