HOT-DIPPED Zn-Al-Mg PLATED STEEL MATERIAL

Abstract

This hot-dipped Zn-Al-Mg plated steel material comprises a steel material and a plating layer disposed on a surface of the steel material, wherein: the plating layer includes a Zn-Al-Mg-based alloy layer provided above the steel material; the Zn-Al-Mg-based alloy layer has a chemical composition containing, in terms of mass%, 4.5%-30.5% of Al, and 1.05%-10.6% of Mg, with the balance being Zn and impurities; and the Zn-Al-Mg-based alloy layer includes, in terms of area%, 0% to 41.5% of an Al phase, 0%-75.5% of an Al-Zn phase mainly containing Al, 0%-43.4% of a Zn-Al phase mainly containing Zn, 0% to 6.0% of a η+β phase, 0.1%-56.0% of a MgZn2 phase, and 0%-95.2% of a residual phase.

Description

Hot-dip Zn-Al-Mg plated steel

[0001] This disclosure relates to a hot-dip Zn—Al—Mg plated steel material. This application claims priority to Japanese Patent Application No. 2024-074940, filed May 2, 2024, the contents of which are incorporated herein by reference.

[0002] Hot-dip galvanized steel is used in a variety of fields where rust prevention is required, such as civil engineering, building materials, automobiles, and home appliances, as a means of inexpensively preventing rust on steel.

[0003] For example, Patent Documents 1 and 2 disclose Zn-Al-Mg plated steel sheets that have recently attracted attention as highly corrosion-resistant plated steel sheets.

[0004] The performance and specifications required for steel materials vary across different fields. For example, the thickness of plate materials used in the civil engineering and building materials fields generally tends to be thicker than that used in automobiles and home appliances. However, in recent years, high-tensile steel, which can be made thinner and therefore lighter, has begun to be used in the building materials field for the purpose of labor saving, etc. As such, in recent years, there has been a growing demand across fields for plating high-performance steel materials such as high-tensile steel.

[0005] The performance required of the plating layer mainly includes corrosion resistance and sacrificial corrosion protection to inhibit corrosion of the steel material, as well as prevention of scratches during forming (high hardness), processability during severe processing such as drawing (soft hardness), durability in high-temperature environments (high-temperature properties), and design.

[0006] Regarding corrosion resistance, in the case of thick plated steel sheets, plated steel sheets with high sacrificial corrosion resistance are often used to prevent corrosion from the exposed parts of the steel sheet (exposed parts of the base metal) that occur during cutting or heavy processing. On the other hand, in the case of thin plated steel sheets, plated steel sheets that prioritize corrosion resistance of flat parts over sacrificial corrosion resistance are often used.

[0007] Furthermore, to prevent scratches during forming, such as roll forming, and handling, a high-hardness plating layer with excellent slip properties is required, while a low-hardness plating layer that can follow the plastic deformation of the steel sheet is required for adaptability to severe processing, such as thin sheet drawing and extrusion. These performance characteristics are largely determined by the alloy composition of the plating layer.

[0008] Japanese Patent No. 6428974 Japanese Patent No. 6365807

[0009] As described above, in recent years, high levels of performance have been required across a wide range of applications for the plating layer of plated steel materials, regardless of the field of application. In particular, for plated steel materials with a relatively large thickness, high sacrificial corrosion protection is required because the area that will be processed during subsequent processing is large. In addition, for example, when plated steel materials are assumed to be used as outdoor structures, heat resistance (high-temperature corrosion resistance), scratch resistance, or an appearance that does not conspicuously show white rust, etc., may be required. Thus, there is a strong demand for plated steel materials with high-performance plating layers that can exhibit not only sacrificial corrosion protection but also various other performances at high levels.

[0010] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a hot-dip Zn—Al—Mg plated steel material having excellent sacrificial corrosion protection properties.

[0011] In order to solve the above problems, the present disclosure employs the following configuration. [1] A plated steel material according to one embodiment of the present disclosure is a hot-dip Zn-Al-Mg plated steel material having a steel material and a plating layer disposed on a surface of the steel material, wherein the plating layer includes a Zn-Al-Mg alloy layer disposed above the steel material, and the Zn-Al-Mg alloy layer has a chemical composition containing, by mass%, Al: 4.5% to 30.5% and Mg: 1.05% to 10.6%, with the balance being Zn and impurities, and the Zn-Al-Mg alloy layer contains, by area%, Al phase: 0 to 41.5%, Al-Zn phase containing Al as the main component: 0 to 75.5%, Zn-Al phase containing Zn as the main component: 0 to 43.4%, η+β phase: 0 to 6.0%, and MgZn. ２[2] The hot-dip Zn-Al-Mg plated steel material according to the above item [1], wherein the chemical composition of the Zn-Al-Mg based alloy layer is such that when Al is 11 to 13%, Mg is 5.7 to 7.3%, when Al is more than 13% and not more than 15%, Mg is 5.3 to 7.5%, when Al is more than 15% and not more than 20%, Mg is 5.0 to 9.0%, and when Al is more than 20.0% and not more than 30.5%, Mg is 6.8 to 10.5%, and in the Zn-Al-Mg based alloy layer, ２ [3] In the hot-dip Zn-Al-Mg plated steel material according to the above [1] or [2], the chemical composition of the Zn-Al-Mg based alloy layer satisfies the following formulas (1) and (2), and in the Zn-Al-Mg based alloy layer, ２ phase: 50% or more, and a sum of area ratios of the Al—Zn phase and the Zn—Al phase may be 40% or less. Mg≧−0.2222×Al+30.3333 (Al<15%) (1) Mg≧0.32308×Al+11.5754 (15%≦Al) (2) [4] In the hot-dip Zn-Al-Mg plated steel material according to the above item [1], the chemical composition of the Zn-Al-Mg based alloy layer may be: Al: 23 to 26.5%, and Mg: 8.5 to 10.5%.

[0012] According to an embodiment of the present invention, a hot-dip Zn—Al—Mg plated steel material having excellent sacrificial corrosion protection can be provided.

[0013] Hereinafter, a hot-dip Zn—Al—Mg plated steel material (hereinafter simply referred to as hot-dip plated steel material) according to one embodiment of the present invention will be described.

[0014] In this specification, the "%" used to indicate the content of each element in the chemical composition of the plating layer means "mass %" unless otherwise specified. Furthermore, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. Furthermore, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit.

[0015] As used herein, "planar corrosion resistance" refers to the corrosion-resistant properties of the plating layer itself. Zn-based plating layers have a sacrificial corrosion protection effect on steel (base steel). Therefore, the corrosion process of plated steel materials with Zn-based plating layers progresses as follows: first, the plating layer corrodes and turns into white rust before the steel material corrodes; then, the white-rusted plating layer disappears; and then, the steel material corrodes and red rust appears.

[0016] As used herein, "sacrificial corrosion protection" refers to the property of suppressing corrosion of steel material at exposed portions of the steel material (base steel) (for example, at cut end surfaces of plated steel material or at portions where the steel material is exposed due to cracking of the hot-dip coating layer during processing). Details of each property will be described later.

[0017] First, the results of the inventors' investigation into the relationship between sacrificial corrosion resistance, heat resistance, and hardness and the alloy composition constituting the plating layer will be described below.

[0018] The present inventors first investigated the alloy components of the coating layer of hot-dip galvanized steel materials used in various fields such as building materials, automobiles, home appliances, etc. Steel materials used in these fields can be broadly classified by, for example, plate thickness, and can be used according to the purpose, application, and required performance.

[0019] The present inventors have investigated the alloy compositions of various plating layers, and have investigated the melting points of the plating layers, observed their cross sections, evaluated their corrosion resistance, investigated their sacrificial corrosion protection (corrosion protection at cut edges), and evaluated their hardness. As a result, they have obtained the following new findings.

[0020] (i) The properties of a Zn-Al-Mg alloy plating layer depend on the concentrations of Al and Mg contained in Zn. (ii) When the phases constituting a Zn-Al-Mg alloy plating layer are classified into six types based on the concentrations of the constituent elements, the amounts of the six types of phases (area ratios of each phase in cross-sectional observation) govern the properties of the plating layer. (iii) The morphologies (dispersion, aggregation) of the six types of constituent phases of a Zn-Al-Mg alloy plating layer may change depending on manufacturing conditions, but the influence of these morphologies on the properties is minor compared to the area ratios.

[0021] The plating layer investigated this time refers to the Zn-Al-Mg alloy plating layer excluding the interfacial reaction layer (less than 5 μm) with the base iron (steel material). In other words, the Zn-Al-Mg alloy plating layer, which is the main constituent layer of the plating layer of the present invention, has reduced influence of elements diffused from the steel material.

[0022] Hereinafter, more specific embodiments of the hot-dip plated steel material of the present invention, which have been obtained based on the above-mentioned new findings, will be described in detail.

[0023] The hot-dip plated steel material of this embodiment has a steel material and a plating layer disposed on the surface of the steel material.

[0024] (Steel Material) The steel material to be plated will be described. The steel material is, for example, a steel plate. When the steel material is a steel plate, there is no particular limitation on its size. The steel plate may be any steel plate that can be applied to a normal hot-dip galvanizing process. Specifically, this applies to steel plates that can be applied to a process in which the steel plate is immersed in molten metal and solidified, such as a continuous hot-dip galvanizing line (CGL). As the steel material, for example, a steel plate having a thickness of 10 mm or less and a width of 2000 mm or less can be suitably applied, but the size of the steel plate is not limited to this.

[0025] There are no particular limitations on the quality of the steel material, and various types of steel plates such as general steel, Al-killed steel, extra-low carbon steel, high carbon steel, various types of high-tensile steel, some high alloy steels (steels containing elements that strengthen corrosion resistance, such as Ni and Cr), steel for bolts, and steel wire rods for bridge cables can be used. More specifically, for example, cold-rolled steel sheets specified in JIS G 3131 and 3141, those included in general structural rolled steel materials corresponding to so-called SS materials, so-called general steels included in hot-rolled steel sheets specified in JIS G3193, pre-plated steels thinly plated with various metals such as JIS H8641, JIS G 3302, 3303, 3313, 3314, 3315, 3317, and 3321, JIS G 3136 Al-killed steel, extra-low carbon steel, high carbon steel, and various high-tensile steels described in JIS G 3113, 3134, and 3135 are applicable.

[0026] (Plating Layer) Next, the plating layer provided on the steel material will be described. The plating layer according to this embodiment includes a Zn-Al-Mg alloy layer provided on the steel material. For example, a pre-plating layer, a layered intermetallic compound formed after the reaction between the steel material (base steel) and the plating layer, or an oxide layer inevitably contained in the manufacturing process (but which does not affect the adhesion of the plating layer to the steel material) may be provided between the steel material and the Zn-Al-Mg alloy layer. A Zn-Al-Mg alloy layer may be provided on the steel material. The Zn-Al-Mg alloy layer may contain, for example, a Zn phase, an Al phase, an MgZn phase, or the like. ２ The phases are contained.

[0027] When Zn contains alloying elements such as Al and Mg, the flat corrosion resistance is improved. Therefore, in the case of a plating layer containing such a Zn phase, even a thin film (for example, about half the thickness of a normal Zn plating layer) can exhibit corrosion resistance equivalent to that of a normal Zn plating layer. Similarly, even when the plating layer of this embodiment is thin, it ensures flat corrosion resistance equivalent to or greater than that of a conventional Zn plating layer. The Zn-Al-Mg alloy layer is the main part of the plating layer in this embodiment.

[0028] The Zn—Al—Mg alloy layer is made of a Zn—Al—Mg alloy. The term “Zn—Al—Mg alloy” refers to a ternary alloy containing Zn, Al, and Mg.

[0029] The thickness of the entire plating layer is not particularly limited. For example, when the Zn—Al—Mg alloy layer is formed by a continuous hot-dip plating method, the thickness of the plating layer may be, for example, 100 μm or less.

[0030] <Chemical Composition> Next, the average chemical composition of the Zn—Al—Mg alloy layer will be described. The Zn—Al—Mg alloy layer has a chemical composition containing, by mass, 4.5% to 30.5% Al and 1.05% to 10.6% Mg, with the balance being Zn and impurities. The balance may be 40% or more Zn and 0.1% or less 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 95% or more.

[0031] Al: 4.5% or more, 30.5% or less Like Zn, Al is an element that mainly constitutes the coating layer. If the Al content is less than 4.5%, the desired phase described below may not be obtained. Furthermore, if the Al content is low, sufficient corrosion resistance may not be ensured as a highly corrosion-resistant hot-dip plated steel material. Therefore, the Al content is set to 4.5% or more. Preferably, it is set to 6.0% or more. On the other hand, if the Al content exceeds 30.5%, the desired phase described below may not be obtained. Furthermore, if the Al content is excessively high, the sacrificial corrosion protection of the coating layer itself may be reduced, and sufficient sacrificial corrosion protection may not be ensured as a highly corrosion-resistant hot-dip plated steel material in cut-processed portions, bent-processed portions, etc. Therefore, the Al content is set to 30.5% or less. Preferably, it is set to 25.0% or less.

[0032] Mg: more than 1.05% and not more than 10.6% Mg is an element that ensures the corrosion resistance of the coating layer. If the Mg content is 1.05% or less, the desired phase described below may not be obtained. Furthermore, if the Mg content is too low, sufficient corrosion resistance may not be ensured as a highly corrosion-resistant hot-dip coated steel material. Therefore, the Mg content is made more than 1.05%, and preferably 2.0% or more. On the other hand, if the Mg content is too high, the desired phase described below may not be obtained. Furthermore, if the Mg content is too high, sufficient corrosion resistance may not be obtained. Therefore, the Mg content is made 10.6% or less, and preferably 9.0% or less.

[0033] Balance: Zn and impurities In the hot-dip plated steel material of this embodiment, Zn, like Al, is an element that mainly constitutes the coating layer. That is, the balance other than the above-mentioned Al and Mg may be Zn and impurities. In the coating layer according to this embodiment, Zn is the element contained in the largest amount. Note that Zn is an element that provides sacrificial corrosion protection to the coating layer, so the Zn content is preferably 40% or more.

[0034] The plating layer may contain trace amounts of impurities such as Fe due to, for example, mutual atomic diffusion between the steel material (base steel) and the plating bath or due to the raw materials. Furthermore, Zn, Al, and Mg metals with a purity of 3N are often used to produce the plating layer. Therefore, the total concentration of impurities in the plating layer may be approximately 0.1% or less.

[0035] To identify the average chemical composition of the plating layer, the plating layer is stripped and dissolved using an acid containing an inhibitor that suppresses corrosion of the base steel (steel material) to obtain an acid solution. The resulting acid solution is then measured using ICP atomic emission spectroscopy or ICP-MS to obtain the chemical composition. There are no particular restrictions on the type of acid, as long as it is an acid that can dissolve the plating layer. By measuring the area and weight before and after stripping, the plating adhesion amount (g / m ２ ) can also be obtained at the same time.

[0036] <Constituent Phases> The performance and properties of each component of a plating layer are affected by the form and amount of the constituent phases. In other words, desired characteristics can be achieved by optimizing the types and ratios of the constituent phases of a plating layer (particularly a Zn-Al-Mg alloy layer). Specifically, for example, one of the factors affecting the flat corrosion resistance of a plating layer is the corrosion rate of each phase. Because the various phases within a plating layer are mixed, the corrosion rate of the entire mixed phase must be considered when determining the overall corrosion rate of the plating layer. Similarly, when determining the sacrificial corrosion protection of a plating layer, the potential of the entire mixed phase must be considered when determining the overall potential of the plating layer. Furthermore, while each phase has its own inherent hardness, the hardness of the entire mixed phase must be considered. Below, the individual phases constituting a Zn-Al-Mg alloy layer are described.

[0037] The Zn-Al-Mg alloy layer according to this embodiment has, in area %, Al phase: 0 to 41.5%, Al-Zn phase containing Al as the main component: 0 to 75.5%, Zn-Al phase containing Zn as the main component: 0 to 43.4%, η+β phase: 0 to 6.0%, MgZn ２ The remaining phase: 0 to 95.2%. Each phase can be classified according to the ratio of components and concentration ratio. Each phase will be described in detail below.

[0038] [Al Phase] The Al phase is a phase containing Al as the main component. Generally, in plating layers of binary or higher elements, such as Zn-Al-based plating, a certain amount of Zn is often dissolved in Al, so almost complete Al (pure Al) does not exist in the plating layer. However, since the composition range that has the same properties as this pure Al is "Al: more than 99.0% and Zn: 1% or less," a phase having this composition range is considered to be the Al phase. Note that, in the plating composition range described above, Mg often combines with Zn and does not form a solid solution in the Al phase. In other words, the Mg concentration in the Al phase in this embodiment is 1% or less. As will be described in detail later, the Mg concentration is also 1% or less in the α phase, β phase, and η+β phase contained in the Zn-Al-Mg-based alloy layer.

[0039] The properties of the Al phase in this embodiment are similar to those of pure Al. Pure Al itself exhibits a very low potential, but in a general corrosive environment, Al is present on the Al surface. ２ O ３ The passivation effect of the coating is manifested, resulting in a very low corrosion rate. However, the inherent sacrificial corrosion protection against Fe cannot be expected to be very significant. The Al phase is also highly ductile, soft, and has low hardness. On the other hand, the melting point of the Al phase is 660°C, the same as pure Al, and higher than 419°C of Zn.

[0040] As described above, the Al phase is a phase that is not expected to provide significant sacrificial corrosion protection against Fe. Therefore, in this embodiment, the area ratio of the Al phase may be 0%. Furthermore, once exposed portions appear, it becomes difficult to ensure sacrificial corrosion protection, so it is necessary to mix a phase other than the Al phase to ensure a certain degree of sacrificial corrosion protection. Therefore, the area ratio of the Al phase is set to 41.5% or less, preferably 35.0% or less, and more preferably 30.0% or less. On the other hand, by increasing the content of the Al phase in the plating layer, excellent flat surface corrosion resistance can be ensured. To achieve these effects, the area ratio of the Al phase may be set to 0.1% or more, preferably 10.0% or more, and more preferably 20.0% or more. Furthermore, because the Al phase is soft, it can suppress the occurrence of cracks and other defects in processed portions even when subjected to severe processing. Furthermore, even when the plated steel material is formed into a complex shape, the inclusion of a soft Al phase causes the plated steel material to thin, allowing it to follow the processing, and as a result, this followability reduces the exposure of the base steel material.

[0041] [α Phase] In this embodiment, the α phase is defined as a phase having a composition range of "Zn: 1 to 40.5%, Al: more than 59.5% to 99.0% or less, Mg: less than 1%." That is, the α phase is a phase containing Al as the main component, similar to the Al phase, but has a slightly lower Al concentration and a higher Zn concentration than the Al phase. The α phase is also called "Al-Zn phase containing Al as the main component."

[0042] During the solidification process of the coating layer, a large amount of Zn dissolves in Al in the high temperature region, and if the Al content of the coating layer is high, this α phase may crystallize or precipitate and remain. Such α phase exhibits properties similar to Al in the coating layer, and Al ２ O ３ However, this stability is not as strong as that of the Al phase, and in terms of the order of corrosion, the α phase has a slightly lower potential than the Al phase and a slightly higher corrosion rate than the Al phase. From these facts, it can be said that the α phase has a slightly higher ability to improve sacrificial corrosion protection than the Al phase. Furthermore, with regard to hardness, the α phase has more dislocations contained therein, so it has a higher hardness than the Al phase, specifically, a value slightly higher than the intermediate value between Al and Zn.

[0043] Generally, pure Al plating provides almost no sacrificial corrosion protection. However, by mixing the α phase as in this embodiment, a certain level of sacrificial corrosion protection can be obtained, and high flat surface corrosion resistance can also be obtained. The α phase content is preferably 20.0 to 40.0%, more preferably 40.0 to 60.0%, and even more preferably 60.0% or more.

[0044] However, the α phase is ２ The α phase is a phase that cannot be expected to have sacrificial corrosion protection. Therefore, in this embodiment, the area ratio of the α phase may be 0%. Therefore, the area ratio of the α phase is set to 75.5% or less.

[0045] [β Phase] In this embodiment, the β phase is defined as a phase having a composition range of "Zn: 40.5 to 90.0%, Al: 10.0 to 59.5% or less, Mg: less than 1%." In other words, the β phase can be said to be a phase having a lower Al concentration and a higher Zn concentration than the α phase. The β phase is also called "Zn-Al phase containing 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 a stable phase even in high temperature ranges (around 250-300°C), but as the plating is eventually cooled to room temperature, the β phase separates into Al and Zn. In other words, the β phase eventually undergoes component separation within the solid phase, becoming fine precipitates and taking the form of a mixed phase of fine Zn phase and Al phase, so the final β phase retains traces of the β phase in high temperature ranges and exhibits unique performance. 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 in the β phase, Al ２ O ３ is not sufficiently formed. As a result, the potential of the β phase shifts toward the extremely base potential of -1.66 V, which is inherently possessed by Al, making the potential of the β phase lower than that of Zn, resulting in a phase with excellent sacrificial corrosion protection. The β phase has the lowest potential among the constituent phases of the plating layer of this embodiment. Therefore, by setting the Al content in the plating layer within an appropriate range and ensuring a sufficient amount of β phase, it is possible to improve the sacrificial corrosion protection. On the other hand, the β phase, which has a base potential, causes an increase in coupling current between it and surrounding phases, and therefore tends to have inferior flat corrosion resistance. The hardness of the β phase is slightly higher than that of soft Al and Zn, and can be considered to be approximately equivalent to that of the α phase.

[0047] As described above, the β phase contributes to improving sacrificial corrosion protection. However, if planar corrosion resistance is to be improved in addition to sacrificial corrosion protection, it is not preferable to include an excessive amount of β phase. Therefore, the area ratio of the β phase is set to 43.4% or less. Preferably, it is set to 40.0% or less, and more preferably, it is set to 35.0% or less. The area ratio of the β phase may be 0%. However, if it is desired to improve sacrificial corrosion protection, the area ratio of the β phase is preferably set to 1.0% or more, and more preferably, it is set to 5.0% or more.

[0048] [η + β Phase] The η + β phase is a phase primarily composed of Zn. Specifically, it is a mixed phase of pure Zn (η phase) and β phase. Generally, in plating layers of binary or higher systems, such as Zn-Al-based plating, a certain amount of Al is often dissolved in Zn, so complete Zn (pure Zn) is almost absent in the plating layer. However, since the composition range with properties equivalent to this pure Zn can be considered to be "Zn: 90.0% or more, Al: 8.0% or more, less than 10.0%," in this embodiment, a phase having this composition range is considered to be the pure Zn phase (η phase). Strictly defined, the η + β phase is a mixed structure of a structure equivalent to a fine β phase and a fine η phase of the same quality as 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, less than 10.0%."

[0049] The properties of the η + β phase in this embodiment are similar to those of pure Zn. The corrosion rate of the flat surface of the η + β phase is the highest among the constituent phases of the plating layer (i.e., higher than the β phase), making it the phase most susceptible to corrosion. On the other hand, the η + β phase has higher sacrificial corrosion protection than the α phase but lower than the β phase. The hardness of the η + β phase is almost the same as that of pure metal Zn, and is therefore very low, similar to that of the Al phase.

[0050] As described above, the η + β phase contributes to improving sacrificial corrosion protection, but because of its high corrosion rate, it is not preferable to include it in excess. Therefore, the area ratio of the η + β phase is set to 6.0% or less, preferably 5.0% or less, and more preferably 4.0% or less. The area ratio of the η + β phase may be 0%. However, if it is desired to improve sacrificial corrosion protection, the area ratio of the η + β phase may be set to 1.0% or more.

[0051] [MgZn ２ In the Zn-Al-Mg based plating layer, most of the contained Mg is bonded to Zn, and the resulting intermetallic compound is MgZn. ２ This MgZn ２ When coarsened to a certain extent, a single phase consisting of intermetallic compounds, i.e., MgZn ２Specifically, in the case of a plating layer having the chemical composition described above, the Al-Zn phase, which is the origin of the formation of the Al phase, α phase, β phase, and η+β phase, crystallizes first, and as this Al-Zn phase grows, the concentrations of Al and Zn in the surrounding liquid phase decrease, resulting in the formation of MgZn ２ The precipitated MgZn ２ The phase grows to surround the Al phase, α phase, β phase, and η + β phase, and grows into a large phase, MgZn ２ The performance of Al phase, α phase, β phase, η+β phase and MgZn ２ When the phase is sufficiently crystallized, the final liquid phase is the "residual phase" described later, that is, the ternary eutectic structure (Zn-Al-MgZn ２ ) may be formed.

[0052] MgZn ２ The MgZn phase has excellent insulating properties, and its inclusion provides high corrosion resistance on flat surfaces. Its effect is higher than that of the β phase, but lower than that of the α phase. In terms of sacrificial corrosion protection, it has the second lowest potential after the β phase, and provides excellent sacrificial corrosion protection in the plating layer. In addition, MgZn ２ is an extremely hard material, and MgZn ２ When a large amount of this phase is contained, the hardness of the plating layer is significantly improved.

[0053] As mentioned above, MgZn ２ The MgZn phase also contributes to improving sacrificial corrosion protection. ２ The area ratio of the MgZn phase is 0.1% or more, preferably 1.0% or more, and more preferably 5.0% or more. ２ Since the MgZn phase is a very hard phase, it is not preferable to include it in excess. ２ The area ratio of the phase is set to 56.0% or less. However, by setting the concentration appropriately, a plating layer with high hardness and workability can be obtained. The area ratio is preferably 35.0 to 45.0%, and more preferably 45.0 to 56.0%.

[0054] In addition, from the viewpoint of sacrificial corrosion protection, β phase, MgZn ２ It is preferable that any one of the phases is contained in an amount of 20% or more. More preferably, MgZn ２It is preferable that the area ratio of the β phase is larger than the area ratio of the β phase.

[0055] Here, MgZn in this embodiment ２ The phase is an intermetallic compound, MgZn ２ Specifically, MgZn ２ The MgZn phase is defined as a phase having a composition in which the Mg concentration is 15.48±5%, the Al concentration is less than 1%, and the balance is Zn. ２ MgZn phase ２ is the equivalent circular diameter of 5 μm ２ and an axial length a in the longest linear direction within the phase ｍａｘ and the maximum value b of the axial length b in the linear direction perpendicular to it. ｍaｘ Ratio a ｍａｘ / b ｍaｘ ≦3. That is, MgZn ２ The phase is a massive, relatively circular MgZn ２ MgZn in a ternary eutectic structure that exists in a lamellar state ２ is excluded.

[0056] [Others (remaining phase)] As described above, Al phase, α phase, β phase, η+β phase and MgZn ２ After the phase origin material (Al-Zn) crystallizes, the liquid phase finally solidifies, and most of it forms a ternary eutectic structure (Zn-Al-MgZn ２ ) In some cases, Zn and other elements may grow and coarse Zn grains may also be included as part of the remaining phase. The overall composition of the remaining phase is close to Zn-5%Al-2%Mg, but if coarse Zn grains are included as part of the remaining phase, the Zn ratio may become higher.

[0057] Regarding the properties of the remaining phase, which is mainly a ternary eutectic structure, it has superior corrosion resistance to the flat surface compared to the η+β phase and β phase. ２ This is because the MgZn phase, which has relatively high corrosion resistance in the Zn-Al-Mg alloy layer, ２ The sacrificial corrosion resistance is also slightly better than that of the α phase and the η+β phase, but MgZn ２The proportion of the ternary eutectic structure is small compared to the ternary eutectic structure. The balance phase may be 0%, but from the viewpoint of ensuring both flat surface corrosion resistance and sacrificial corrosion protection, the proportion of the ternary eutectic structure may be increased. Specifically, the proportion of the balance phase including the ternary eutectic structure may be 95.2% or less.

[0058] Next, a method for classifying the constituent phases is described. For classification, it is preferable to measure the phase fraction using a plating layer with a thickness of 20 μm or more. This is because measurement error is reduced when the plating thickness is 20 μm or more. First, a 20 mm square test piece is cut from the plated steel material so that it includes the plating layer. This test piece is embedded in resin so that the cross section along the thickness direction of the plating layer becomes the observation surface, and the observation surface is then polished. After polishing, the cross section is observed using EPMA. For example, if an Al-Fe alloy layer or the like is formed at the interface between the steel material and the plating layer, this is excluded, and a region 1 μm thick from the interface toward the plating layer is excluded from the evaluation. Next, an SEM image of the observation surface is obtained, and an EPMA composition map image of Zn, Al, and Mg is obtained from the SEM image. The area ratio of each constituent phase is determined based on the obtained composition map image and the above-mentioned definition of the composition range of each layer. The reason for measuring the area ratio in a cross section along the thickness direction of the coating layer rather than in a cross section parallel to the surface of the coating layer (a horizontal cross section) is that a structure with biased components is formed in a horizontal cross section. That is, in a typical coating layer, solidification occurs from the surface, so the final solidification portion is located near the interface between the base steel and the coating layer, which can result in biased components in the thickness direction of the coating layer. For this reason, in this embodiment, the area ratio is measured in a cross section along the thickness direction of the coating layer.

[0059] To confirm the area ratio, the area was checked with an SEM at a magnification of 500, and the total area of ​​the plating layer was 100,000 μm ２ The measurement is repeated until the plated layer portion is trimmed from the obtained EPMA map image. Since histograms of the concentrations of Zn, Al, and Mg are obtained, the Al phase, α phase, β phase, η+β phase, and MgZn ２ The map area of ​​the component corresponding to the phase is cut out, and the part that remains finally corresponds to the remaining phase.

[0060] Furthermore, by determining the threshold value of the components from the mapping of the Zn concentration distribution, it is possible to distinguish between the ternary eutectic structure, β phase, MgZn ２ The position of the phase can be easily identified. By superimposing this on the SEM image, it is possible to identify the location. Regarding the remaining phase, there is no need to set a particular threshold value, and the Al phase, α phase, β phase, η + β phase, and MgZn ２ If the phase can be accurately extracted, the remainder can be regarded as the remaining phase. The above operation is checked in at least two fields of view for at least five samples of different platings with the same composition, and at least 10 samples for each are checked, and the area ratio of each phase is calculated from the average value.

[0061] As described above, in this embodiment, the β phase and MgZn ２ By providing a Zn-Al-Mg alloy layer mainly composed of Zn-Al-Mg phases, a plating layer with excellent sacrificial corrosion resistance can be obtained. Therefore, it is more suitable for use when the steel plate thickness is relatively thick. From this perspective, a Zn-plated layer is ideal; however, when Zn-plated steel having a thickness exceeding 1.6 mm, such as for outdoor structures, is processed, the corrosion resistance of the processed portion deteriorates significantly. For components that require a long service life, corrosion resistance at the processed portion that is equal to or greater than that of Zn-plated steel is required. Furthermore, while sacrificial corrosion resistance and flat surface corrosion resistance are mutually exclusive properties, it is necessary to optimize the constituent components of the Zn-Al-Mg-plated layer to fall within the above-mentioned composition range so as to achieve sacrificial corrosion protection equal to or greater than that of Zn-plated steel while avoiding inferior flat surface corrosion resistance.

[0062] Furthermore, when a plating layer is formed on a thicker steel plate as a substrate, it is preferable that the following chemical composition and phase ratios are satisfied, which can further improve sacrificial corrosion protection and further improve corrosion resistance and scratch resistance in processed parts even for thicker steel plates.

[0063] That is, in the chemical composition of the Zn-Al-Mg alloy layer, when Al is 11 to 13%, Mg is 5.7 to 7.3%, when Al is more than 13% and 15% or less, Mg is 5.3 to 7.5%, when Al is more than 15% and 20% or less, Mg is 5.0 to 9.0%, when Al is more than 20.0% and 30.5% or less, Mg is 6.8 to 10.5%, and in the Zn-Al-Mg alloy layer, Zn-Al phase (β phase), and MgZn ２ The total area ratio of the phases is preferably 55% or more.

[0064] In order to improve the sacrificial corrosion protection, it is effective to keep the potential low. Specifically, among the constituent phases of the Zn-Al-Mg alloy layer, the β phase, which is the most susceptible to corrosion, and MgZn ２ It is preferable to increase the ratio of the β phase and MgZn phase from the viewpoint of improving sacrificial corrosion protection. ２ When the total amount of the phases is 55% or more, not only is better sacrificial corrosion protection achieved, but also high corrosion resistance in the processed portion is obtained.

[0065] In order to further enhance the sacrificial corrosion resistance, the chemical composition is set to Al: 23 to 26.5%, Mg: 8.5 to 10.5%, and satisfies the following formulas (1) and (2), and MgZn ２It is effective to set the area ratio of the α phase to 50% or more, and the sum of the area ratios of the α phase and the β phase to 40% or less. Mg≧-0.2222×Al+30.3333 (1) (Al<15%) Mg≧0.32308×Al+11.5754 (2) (Al≧15%). However, formula (1) applies when the Al content in the chemical composition of the coating layer is less than 15%, while formula (2) applies when the Al content is 15% or more. In other words, when the Al content is less than 15%, satisfying formula (1) can further enhance sacrificial corrosion protection, while when the Al content is 15% or more, satisfying formula (2) can further enhance sacrificial corrosion protection. The reason why the tendency for the Mg content to be controlled depending on the Al content changes is thought to be because the ratio of the constituent phases of the coating layer is significantly affected by the balance between the Al content and the Mg content. In other words, the inventors have found that the tendency of the Mg content to be controlled changes when the Al content reaches 15%, and as a result, by satisfying the above two formulas selectively depending on the Al content, a suitable phase ratio can be obtained, and higher sacrificial corrosion protection can be achieved.

[0066] The chemical composition of the plating layer of this embodiment is mainly composed of Zn. Therefore, it is difficult to obtain the same level of flat corrosion resistance as a plating layer mainly composed of Al. However, by setting the chemical composition to the range of Al: 24 to 30.5% and Mg: 8.0 to 10.6%, a sufficient Al phase can be formed even in a plating layer mainly composed of Zn. Specifically, when the Al phase is 0.1% or more, very good flat corrosion resistance can be obtained. More preferably, the Al phase is 10% or more, and even more preferably 20% or more. In addition to the Al phase, the constituent phases of this embodiment include a β phase and MgZn ２ When used in combination with the Al phase, a very favorable corrosion prevention mechanism is formed, and it is possible to exhibit significantly higher corrosion resistance than pure Al plating.

[0067] In addition, in order to ensure a certain level of flat surface corrosion resistance while improving sacrificial corrosion protection, it is preferable to increase the proportion of the α phase. The α phase tends to have slightly inferior flat surface corrosion resistance compared to the Al phase. However, in order to obtain excellent flat surface corrosion resistance while ensuring a certain level of sacrificial corrosion protection, it is effective to increase the proportion of the α phase.

[0068] Specifically, when Al is 17-20%, Mg is 7.5-9.0%, and when Al is 20-30.5%, Mg is preferably 1.5-10.6%. This allows the area ratio of the α phase to be increased to 20% or more. More preferably, Al is 26.5% or more, which allows the α phase to be increased to 40% or more, resulting in higher flat corrosion resistance. Even more preferably, the α phase is 60% or more.

[0069] Here, increasing the hardness of the plating layer improves scratch resistance. ２ This can be increased by increasing the ratio of the phase. ２ When the MgZn phase is contained in an amount of 35% or more, the hardness can be increased sufficiently, and therefore the scratch resistance can be improved. ２ The MgZn phase is more preferably 45% or more. ２ If the ratio of the MgZn phase is excessively increased, the plastic deformability of the plating layer itself will be significantly deteriorated, which is not preferable. ２ The Al content is preferably 56% or less. To obtain such a constituent phase, the chemical composition range of the plating layer is preferably 12% or more Al content and 5% or more Mg content, more preferably 7% or more Mg content.

[0070] The corrosion resistance and poor performance of a coating layer at high temperatures tend to depend on the melting point of the coating layer. This is because the lowest melting point portion of the coating layer melts when exposed to high temperatures. In this embodiment, the low-melting-point substance is a ternary eutectic structure. Therefore, the proportion of this ternary eutectic structure is preferably 10% or less, 5% or less, or even 0%. It is also effective to select a chemical composition of the coating layer that is far from the eutectic composition. For example, by satisfying 23.5%≦Al+Mg, more preferably 32.9%≦Al+Mg, the proportion of the ternary eutectic structure can be brought close to 0%.

[0071] <Characteristics> In this embodiment, by optimizing the chemical composition of the plating layer and the content (area ratio) of each constituent phase, it is possible to control the flat surface corrosion resistance, sacrificial corrosion protection (corrosion resistance at cut end surfaces), and hardness (workability) of the plating layer to desired levels. Various characteristics and performances are described below.

[0072] [Flat surface corrosion resistance] Flat surface corrosion resistance indicates the corrosion resistance of the plating layer itself. ２ O ３ MgZn with excellent insulating properties ２ MgZn with intermetallic compounds such as ２ By increasing the proportion of the β phase and decreasing the β+η phase, the corrosion resistance of the flat surface can be improved.

[0073] The corrosion resistance of flat surfaces is evaluated using an accelerated corrosion test. Specifically, a cyclic corrosion tester is used to evaluate the corrosion weight loss of the plating before and after the corrosion test. After the corrosion test, the corrosion products formed on the surface of the plated steel material are removed with a 30% aqueous solution of chromic acid (VI). The corrosion test is preferably conducted under conditions with a low salt concentration to approximate an atmospheric environment. However, if a salt concentration is too low, it may not be possible to corrode the plated steel sheet, so it is preferable to adjust the salt concentration depending on the composition of the plating layer to be measured.

[0074] [Sacrificial corrosion protection] Sacrificial corrosion protection is an essential performance when manufacturing products (processed or molded parts) using continuous hot-dip galvanized steel. Optimizing the sacrificial corrosion protection of plated steel makes it possible to suppress the progression of corrosion and the development of red rust in processed areas and cut edges. In severely processed areas and cut edges, the coating layer is often destroyed, leaving exposed areas of the base steel, making it necessary to prevent the development of red rust through sacrificial corrosion protection. If the sacrificial corrosion protection of the coating layer is weak, red rust will develop in processed areas and cut edges, and this red rust will accelerate the corrosion of the surrounding coating layer. As plate thickness increases to a certain extent, the degree of corrosion acceleration increases, requiring higher sacrificial corrosion protection.

[0075] However, whether or not processed areas can be protected from corrosion depends not only on sacrificial corrosion protection but also on the plastic deformability of the plating layer. If a plating layer with greater plastic deformability is used, the plating layer will follow the plastic deformation of the steel, reducing its thickness, which in turn reduces the exposed area of ​​the steel. In other words, if the plating layer has high plastic deformability, it can exhibit sufficient corrosion resistance for a certain degree of processing, even if its sacrificial corrosion protection is somewhat low. Furthermore, even if the plating layer has poor plastic deformability, if it has a strong sacrificial corrosion protection effect, it can similarly suppress the progression of corrosion in processed areas.

[0076] The corrosion resistance (sacrificial corrosion protection) of the processed part (exposed part) can be confirmed by preparing a test piece obtained by a 1T bending test (bending 180°, leaving a space equivalent to the thickness of the steel material between the bent parts), conducting a severe corrosion test such as an accelerated corrosion test on this, and observing the progress of corrosion at the top part. In other words, the corrosion resistance of the processed part can be confirmed by checking the red rust generation cycle at the top part of the test piece.

[0077] [Hardness] High hardness of the plating layer generally leads to a deterioration in plastic deformability. On the other hand, high-hardness plating layers have some desirable aspects, such as resistance to scratching and improved sliding properties during mold contact due to a reduced dynamic friction coefficient. Low-hardness plating layers also have excellent plastic deformability and can improve the coverage of the plating layer in the processed area. On the other hand, low-hardness plating layers are prone to scratching and may result in reduced sliding properties during mold contact. Generally, thin steel materials are often processed into complex shapes, so soft plating layers are preferred for plating layers formed on thin steel materials. On the other hand, thick steel materials are often processed into simple shapes, so hard plating layers are often preferred for plating layers formed on thick steel materials.

[0078] [Scratch Resistance] The scratch resistance of the plating layer can be evaluated using a friction and wear tester. #400 emery paper is sandwiched between two plated steel materials to be evaluated, and the paper is subjected to 10 reciprocating motions over a predetermined distance, and the scratch resistance is evaluated based on the magnitude of the applied load.

[0079] [High-Temperature Corrosion Resistance] Due to the manufacturing process characteristics of hot-dip plated steel, the plating layer remelts when exposed to high temperatures, limiting the upper temperature limit of the usage environment. In particular, Zn-based plated steel is generally limited to applications in low-temperature environments, such as below 100°C. On the other hand, Al-plated steel may be used up to approximately 300°C, for example, in home appliances. The melting point of the plating layer is a characteristic determined by the alloy components that make it up. In the case of the plating layer of this embodiment, by satisfying the average chemical composition described above, it can be used suitably even in environments exposed to high temperatures. However, when heated in the atmosphere, any plating layer will undergo atmospheric oxidation to some extent, and a certain degree of deterioration in corrosion resistance is usually observed.

[0080] The high-temperature corrosion resistance of a coating layer is evaluated by examining the corrosion resistance of the coated steel material after use in a high-temperature environment. Specifically, the coated steel material is left in an atmospheric heating furnace at a specified temperature for a specified time, and then its corrosion resistance is evaluated using accelerated corrosion tests or other methods.

[0081] (Manufacturing Method) Next, a preferred manufacturing method of the hot-dip plated steel material of this embodiment will be described. The manufacturing method of the hot-dip plated steel material of this embodiment is not particularly limited as long as the plating layer as described above can be obtained. A preferred mode for manufacturing the hot-dip plated steel material of this embodiment will be described below.

[0082] First, a steel material (base steel) for forming a plating layer is prepared. Suitable steel materials are as described above, and examples thereof include general steel, high-tensile steel, and low-carbon steel.

[0083] The performance of a plating layer depends on the composition and constituent phases within the layer, so it is effective to adjust the plating bath and the desired plating layer so that their compositions are roughly similar.

[0084] Next, a pre-plating layer (metal layer) consisting of a multi-layer structure of a Cr pre-plating layer and a Ni pre-plating layer is formed on the surface of the steel material in advance. The plating method may be hot-dip plating, electroplating, displacement plating, or vapor deposition (PVD, etc.). Furthermore, the Cr-Ni-based pre-plating layer may be heated and alloyed. The steel material on which this Cr-Ni-based pre-plating layer is formed is used as the base sheet for hot-dip plating. The reason for providing a Cr-Ni-based pre-plating layer on the surface of the steel material is explained below.

[0085] Generally, when a steel material reacts with a plating bath at 500°C or higher, Fe atoms from the steel material diffuse into the plating bath due to mutual diffusion between the steel material and the plating bath, and components constituting the plating bath diffuse into the steel material. This diffusion reaction is particularly active in plating baths containing 10% or more Al. The interfacial alloy layer and Fe-diffused layer formed by this diffusion reaction have an anchoring effect and contribute to improving the adhesion of the plating layer. However, they may also reduce corrosion resistance and affect the wear of internal components of the plating layer, resulting in reduced uniformity of plating performance.

[0086] As a result of investigating the corrosion resistance and uniformity of such a plating layer, it was found that if a plated steel material is produced according to the following procedure, it is possible to sufficiently suppress Fe diffusion during immersion in a plating bath, and also to obtain sufficient wettability and adhesion of the plating layer.

[0087] First, a Cr plating layer is formed on the surface of the plated original sheet by electroplating, for example, at 1 g / m ２ The Cr plating bath is, for example, CrO ３ (100g / L), H ２ SO ４ (1.5 g / L), bath temperature 50°C, 20 A / dm ２ By applying current for 40 seconds or less, it is possible to deposit a Cr plating layer of about 1 g. The deposition amount is 3 g / m ２ There are no limitations on the method of plating as long as metallic Cr adheres to the steel sheet, and the conditions are the most common Cr plating method known as a Sargent bath, but other baths such as a fluoride bath and a trivalent oxalic acid bath are also possible.

[0088] Then, a Ni plating layer of 1 g / m was applied by electroplating. ２ Formation (maximum 3 g / m ２ It is acceptable to allow the Ni to adhere to a certain extent.) The bath used for the Ni electroplating in this embodiment is a plating bath under acidic conditions, unlike general conditions such as a Watts bath. Specifically, by suppressing the formation of an oxide film on the Cr plating layer and then plating a Ni layer on top of it, it is possible to maintain the Ni plating layer on the top surface of the plated original sheet, and the Cr plating layer below that, which suppresses Fe diffusion. The conditions for Ni electroplating are as follows: ４ ・6H ２ O) 400g / L, Na ２ SO ４ (anhydrous) 100 g / L, pH = 1.4, bath temperature 60 °C, 5 A / dm ２ By applying current for about 10 seconds, it is possible to deposit about 1 g of Ni plating layer. The pH of the plating bath is H ２ SO ４ If you want to increase the amount of adhesion, simply double or triple the current application time.

[0089] Next, the obtained base sheet is used to produce a plated steel material by a conventional plating process (continuous reduction method using the Sendzimir method). The hot-dip plating bath used is a Zn-Al-Mg-based plating bath adjusted to satisfy the average chemical composition of the plating layer described above. The Zn-Al-Mg-based plating bath may also be a plating bath containing predetermined amounts of desired metals.

[0090] After plating, the plate is removed by wiping, and then rapidly cooled from the bath temperature to room temperature at an average cooling rate of 10 to 50° C. / sec.

[0091] By manufacturing under the above conditions, the hot-dip plated steel material of this embodiment can be manufactured.

[0092] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0093] Details of the examples are shown in Table 1. First, a thoroughly pickled cold-rolled steel sheet (0.1% C-0.3% Si-1.0% Mn) with a thickness of 1.6 mm was prepared as the base steel sheet to be plated. This base steel was cut into a size of 200 mm x 200 mm (x thickness), and then hot-dip galvanization was carried out continuously using a batch-type hot-dip galvanizing test apparatus.

[0094] A pre-plated layer (metal layer) consisting of a multi-layer of a Cr pre-plated layer and a Ni pre-plated layer was produced between the original plate and the Zn-Al-Mg alloy layer by the following method.

[0095] First, a Cr plating layer was applied to the surface of the plated base sheet by electroplating at 1 g / m. ２ The Cr plating bath is, for example, CrO ３ (100g / L), H ２ SO ４ (1.5 g / L), bath temperature 50°C, 20 A / dm ２ A current was passed through the wire for 30 seconds to deposit 1 g of a Cr plating layer.

[0096] Then, a Ni plating layer of 1 g / m was applied by electroplating. ２ The conditions for electroplating Ni were Ni sulfate (NiSO ４ ・6H ２ O) 400g / L, Na ２ SO ４ (anhydrous) 100 g / L, pH = 1.4, bath temperature 60 °C, 5 A / dm ２ The plating bath was heated to 100°C for 10 seconds to deposit 1 g of Ni plating layer. ２ SO ４ It was adjusted with.

[0097] The plating bath was prepared by mixing pure metals (purity 3N or higher) of Zn, Al, and Mg to prepare a Zn-Al-Mg hot-dip plating bath. The plating bath temperature was 520°C when Al was less than 20%, and 600°C when Al was 20% or more.

[0098] Each pure metal (3N or more) was added to this plating bath to prepare a plating bath (15 L) having the desired composition.

[0099] The temperature history during production was managed using a sample in which a K-type thermocouple was welded to a single center point of the plated steel sheet. Specifically, first, electricity was passed through the plated steel sheet equipped with the K-type thermocouple to generate Joule heat and heat the plated steel sheet. Then, based on the amount of electricity passed at this time and the thermal history acquired by the K-type thermocouple, the deviation from the target temperature was corrected by adjusting the amount of electricity passed. In this way, the temperature history during production of the Zn-Al-Mg alloy layer was managed.

[0100] The plating base sheet on which the pre-plating layer was formed was ２ -H ２ The plated original sheet was heated at a rate of 10°C / sec in a reducing atmosphere with a dew point of -60°C, and then held at 800°C for 1 minute. ２ The temperature was lowered to the plating bath temperature by gas blowing, and the plate was placed in the plating bath. After immersion in the plating bath for 3 seconds, it was pulled out and ２ Gas wiping was carried out to adjust the plating film thickness. ２ Gas blow cooling was carried out, and the material was cooled from the plating bath temperature to 100°C at an average cooling rate of 10 to 15°C / sec.

[0101] Performance evaluation samples were taken from the resulting plated steel sheets and evaluated for flat surface corrosion resistance, sacrificial corrosion protection, scratch resistance, and high-temperature corrosion resistance.

[0102] [Corrosion Resistance of Flat Surfaces] The corrosion resistance of flat surfaces was evaluated by measuring the corrosion weight loss of the plating before and after the corrosion test using a saltwater combined cyclic corrosion tester. After the corrosion test, the corrosion products formed on the surface of the plated steel material were removed with a 30% aqueous solution of chromic acid (VI).

[0103] Specifically, the combined cycle corrosion test (CCT) was performed in accordance with JASO (M609-91). The salt concentration was 0.5% NaCl. The corrosion weight loss was calculated after 150 cycles. The evaluation criteria were as follows: "A", "B", "C", and "D" were considered pass, and "E" was considered fail.

[0104] Corrosion weight loss: 100g / m ２ If the result is more than: "E" grade Corrosion weight loss is 75 to 100 g / m ２ Less than: "D" rating: corrosion weight loss of 50-75g / m２ Less than: "C" rating: corrosion weight loss 25-50g / m ２ Less than: "B" rating Corrosion weight loss 25g / m ２ Under: "A" rating

[0105] [Sacrificial corrosion protection] The corrosion resistance (sacrificial corrosion protection) of the processed portion of the plating layer was evaluated by a combination of a 1T bending test and a saltwater combined cyclic corrosion test (CCT, JASO M609-91). The salt concentration was 0.5% NaCl.

[0106] First, a 1T bending test piece was prepared. A sample measuring 40 mm x 120 mm x 3.2 mm was taken and bent at the center of the 40 mm width. Specifically, a 1T (the thickness of one steel plate) was sandwiched inside, bent 180° in a jig, and pressed to prepare a 1T bending test piece with a space equivalent to one steel plate inside.

[0107] The 1T bend test specimen was then placed upright in a CCT machine and observed for the occurrence of red rust. The top of the 1T bend was subjected to severe processing, which inevitably resulted in cracks in the plating layer, resulting in exposed areas of the base steel. In this example, photographs were taken of the top of the 1T bend test specimen (top of the 1T bend) at each specified cycle, and the test specimen was evaluated based on the number of cycles at which the area of ​​red rust on the evaluation surface from the vertical direction reached 5%. The "evaluation surface" refers to a 120 mm region of the top of the 1T bend in the longitudinal direction of the test specimen. The evaluation criteria were as follows: "AAA," "AA," "A," "BBB," "BB," "B," "C," and "D" were considered pass, and "E" was considered fail.

[0108] If the number of cycles in which red rust occurs is less than 90, it is judged as "E." If the number of cycles in which red rust occurs is 90 to 120, it is judged as "D." If the number of cycles in which red rust occurs is 120 to 150, it is judged as "C." If the number of cycles in which red rust occurs is 150 to 180, it is judged as "B." If the number of cycles in which red rust occurs is 180 to 210, it is judged as "BB." If the number of cycles in which red rust occurs is 210 to 240, it is judged as "BBB." If the number of cycles in which red rust occurs is 240 to 270, it is judged as "A." If the number of cycles in which red rust occurs is 270 to 300, it is judged as "AA." If the number of cycles in which red rust occurs is more than 300, it is judged as "AAA."

[0109] [Scratch Resistance] The scratch resistance of the plating layer was evaluated using a friction and wear tester. First, #400 SiC emery paper was sandwiched between two plated steel materials to be evaluated, and one of the plated steel materials was moved back and forth 10 times over a predetermined distance of 30 mm. The moving speed was 50 mm / min, and the scratch resistance was evaluated based on the load when the area of ​​scratches occurred reached more than 50% of the area of ​​the contact surface. The load was measured using a load measuring device installed in the friction and wear tester. It can be said that the larger the load at which the scratch area exceeded 50%, the more excellent the scratch resistance of the sample. The evaluation criteria were as follows, with "AA," "A," "B," "C," and "D" being considered pass, and "E" being considered fail.

[0110] The load at which the flaw area becomes more than 50% is less than 200g: Graded "E" The load at which the flaw area becomes more than 50% is 200-300g: Graded "D" The load at which the flaw area becomes more than 50% is 300-400g: Graded "C" The load at which the flaw area becomes more than 50% is 400-500g: Graded "B" The load at which the flaw area becomes more than 50% is 500-600g: Graded "A" The load at which the flaw area becomes more than 50% is 600g or more: Graded "AA"

[0111] [High-temperature corrosion resistance] First, the plated steel material was left in an atmospheric furnace at 150°C for 240 hours. Then, a combined cyclic corrosion test (CCT) was carried out in accordance with JASO (M609-91). The salt concentration was 0.5% NaCl. After 60 cycles, the corrosion weight loss was determined. The evaluation criteria were as follows, with "AA", "A", "B", "C" and "D" being considered pass and "E" being considered fail.

[0112] Corrosion weight loss: 60g / m ２ If the result is more than: "E" grade Corrosion weight loss is 50-60g / m ２ Less than: "D" rating: corrosion weight loss of 40-50g / m ２ Less than: "C" rating: corrosion weight loss 30-40g / m ２ Less than: "B" rating: corrosion weight loss 20-30g / m ２ Less than: "A" rating Corrosion weight loss 20g / m ２ Under: "AA" rating

[0113]

[0114]

[0115]

[0116]

[0117]

[0118] According to the above aspect of the present invention, it is possible to provide a hot-dip Zn—Al—Mg plated steel material having excellent sacrificial corrosion protection properties.

Claims

1. A hot-dip Zn-Al-Mg plated steel material comprising: a steel material; and a plating layer disposed on the surface of the steel material, wherein the plating layer includes a Zn-Al-Mg alloy layer disposed above the steel material, and the Zn-Al-Mg alloy layer has a chemical composition containing, in mass%, Al: 4.5% to 30.5%, and Mg: 1.05% to 10.6%, with the balance being Zn and impurities, and the Zn-Al-Mg alloy layer contains, in area%, Al phase: 0 to 41.5%, Al-Zn phase containing Al as the main component: 0 to 75.5%, Zn-Al phase containing Zn as the main component: 0 to 43.4%, η+β phase: 0 to 6.0%, MgZn ２ phase: 0.1 to 56.0%, and the remainder phase: 0 to 95.2%.

2. In the chemical composition of the Zn-Al-Mg alloy layer, when Al is 11 to 13%, Mg is 5.7 to 7.3%, when Al is more than 13% and not more than 15%, Mg is 5.3 to 7.5%, when Al is more than 15% and not more than 20%, Mg is 5.0 to 9.0%, when Al is more than 20.0% and not more than 30.5%, Mg is 6.8 to 10.5%, and in the Zn-Al-Mg alloy layer, ２ 2. The hot-dip Zn-Al-Mg plated steel material according to claim 1, wherein a total area ratio of the phases is 55% or more.

3. The chemical composition of the Zn—Al—Mg alloy layer satisfies the following formulas (1) and (2), and in the Zn—Al—Mg alloy layer, ２ 2. The hot-dip Zn-Al-Mg plated steel material according to claim 1, characterized in that the total area ratio of the Al-Zn phase and the Zn-Al phase is 50% or more, and the sum of the area ratios of the Al-Zn phase and the Zn-Al phase is 40% or less. Mg≧−0.2222×Al+30.3333 (Al<15%) (1) Mg≧0.32308×Al+11.5754 (15%≦Al) (2) 4. The hot-dip Zn-Al-Mg plated steel material according to claim 1, characterized in that the chemical composition of the Zn-Al-Mg alloy layer is Al: 23 to 26.5% and Mg: 8.5 to 10.5%.

Images