Surface-treated member

Abstract

[Problem] To provide a surface-treated member exhibiting excellent corrosion resistance even in soil having a depth of several centimeters from the soil surface. [Solution] A surface-treated member of the present invention is composed of a surface-treated steel material including a steel material as a base material and a zinc-based plating layer. At least a part of the zinc-based plating layer has a steel base exposed portion. When cross-sections obtained by cutting the steel base exposed portion in the thickness direction of the steel material according to the shape of the steel base exposed portion when the zinc-based plating layer is viewed in a plan view from above are observed by a microscope, there is at least one cross section where the steel base exposure rate is within the range of 1.0-40.0%. On a cross section where the steel base exposure rate is within the range of 1.0-40.0%, the shortest distance from an arbitrary point on the surface of the exposed steel material to the zinc-based plating layer is greater than 0 μm and 50.0 μm or less. The total area of the regions that provide cross-sections each having a steel base exposure rate of greater than 40.0% is 5.0% or less of the overall area of the steel base exposed portion.

Description

Surface treatment materials

[0001] The present invention relates to a surface-treated member.

[0002] Zinc-plated steel materials, in which various zinc-based plating layers are provided as surface treatment layers on the surface of a steel material serving as a base material, are widely used materials from the viewpoint of improving the corrosion resistance of structural members in the fields of construction, automobiles, etc. The zinc-plated steel materials as materials are processed into various shapes and then joined together as necessary using fastening members such as screws, bolts, and caulking, or by joining processes using various welding methods, etc., to form surface-treated members having desired shapes.

[0003] The surface-treated members described above are likely to be exposed to various corrosive environments depending on the applications of the surface-treated members, and therefore, various studies have been conducted on the corrosion resistance of the surface-treated steel materials that are the raw materials for the surface-treated members.

[0004] For example, Patent Document 1 below proposes a surface-treated steel provided with a coating containing zinc or a zinc-based alloy and a predetermined amount of at least one of Mg and In, with the aim of achieving excellent corrosion resistance in various corrosive environments such as alkaline corrosive environments, seawater corrosive environments, tap water corrosive environments, high-temperature wet corrosive environments, soil corrosive environments, atmospheric corrosive environments, etc. Patent Document 1 examines corrosion resistance in various corrosive environments by utilizing the excellent sacrificial anticorrosion effect of zinc.

[0005] Japanese Patent Application Publication No. 11-158657

[0006] When a surface-treated member is installed in a desired location, it is assumed that at least a portion of the member is often buried in soil. For a portion of the surface-treated member exposed to the atmosphere, it is easy to visually inspect the corrosion state of the exposed portion. However, for a portion buried in soil, unlike in the atmosphere, it is difficult to visually inspect the corrosion state, and maintenance of the corroded portion is also difficult. Therefore, a surface-treated member used in soil is required to have high corrosion resistance.

[0007] The present inventors have studied the corrosion state of zinc-based plated steel materials in soil. As a result, as will be described later, they have found that the corrosion behavior of zinc-based plated steel materials in soil at a depth of 1 m or more from the soil surface is significantly different from the corrosion behavior of zinc-based plated steel materials in soil at a depth of only a few centimeters from the soil surface, and further that the soil corrosive environment focused on in Patent Document 1 mainly corresponds to the corrosion behavior in soil at a depth of 1 m or more from the soil surface. Furthermore, they have found that in such soil at a depth of only a few centimeters from the soil surface, even if a surface-treated member is manufactured using the surface-treated steel disclosed in Patent Document 1, there is room for improvement in the corrosion resistance of the surface-treated member.

[0008] Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a surface-treated member using a zinc-based plated steel material that can exhibit excellent corrosion resistance even in soil at a depth of about several centimeters from the soil surface.

[0009] To solve the above problems, the present inventors conducted extensive research and came up with the idea of ​​changing the state of a zinc-based plating layer provided as a surface treatment layer depending on the corrosive environment to which the member will be exposed. Based on this idea, the present inventors conducted further research and came up with the idea of ​​a surface-treated member as described below. The gist of the present invention, which was completed based on this idea, is as follows:

[0010] (1) A surface-treated member made of a surface-treated steel material having a base steel material and a zinc-based plating layer located on the surface of the steel material, wherein at least a part of the zinc-based plating layer has an exposed steel base portion, which is a portion where the surface of the steel material is exposed, and when a cross section obtained by cutting the exposed steel base portion in the thickness direction of the steel material according to the shape of the exposed steel base portion when the zinc-based plating layer is viewed in plan from above is observed with a microscope, a steel base exposure ratio, which is the proportion of the portion where the surface of the steel material is exposed in the cross section, is 1.0 or more. (2) A surface-treated member according to (1), wherein there is at least one cross section in which the steel base exposure rate is in the range of 1.0 to 40.0%, and in the cross sections in which the steel base exposure rate is in the range of 1.0 to 40.0%, the shortest distance from any point on the exposed surface of the steel material to the zinc-based plating layer is more than 0 μm and not more than 50.0 μm, and for each of the regions providing the cross sections in which the steel base exposure rate is more than 40.0%, the total area of ​​the regions when viewed in plan from above is 5.0% or less of the total area of ​​the exposed steel base. (3) The exposed steel substrate portions are identified by conducting a color reaction test on the entire surface of the zinc-based plating layer, in which a color reaction occurs at the site where the steel substrate is exposed. When each of the identified exposed steel substrate portions is viewed in plan from above the zinc-based plating layer, the exposed steel substrate portions are each sized to have a short side of 5 mm or more and an area of ​​1 cm or less. ２The rectangular region is virtually partitioned into units, and for the partitions where the area ratio of the exposed steel substrate portion in the rectangular region is 3% or more, the exposed steel substrate rate is measured by observing the cross-section of each partition, and the total area of the partitions from which cross-sections within the range of 1.0 to 40.0% of the exposed steel substrate rate are obtained occupies a ratio of 5.0% or more of the area of the portion buried in the soil, the surface treatment member according to (2). (4) The zinc-based plating layer is a plating layer having a chemical composition containing, in mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and the balance consisting of Zn and impurities, the surface treatment member according to any one of (1) to (3). (5) The zinc-based plating layer is a plating layer containing, in mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and Zn: 60.00% or more, the surface treatment member according to (4). (6) The zinc-based plating layer contains, in mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and further contains one or more selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, and element group F, and the balance consists of Zn and impurities, the surface treatment member according to any one of (1) to (3).[Element group A]: One or two elements selected from the group consisting of Si: 2.50% or less and Fe: 5.00% or less. [Element group B]: One or two or more elements selected from the group consisting of Sb: less than 0.50%, Pb: less than 0.50%, and Sr: less than 0.50%. [Element group C]: One or two or more elements selected from the group consisting of Cu: less than 0.25%, Ti: less than 0.25%, Cr: less than 0.25%, Nb: less than 0.25%, Ni: less than 0.25%, Mn: less than 0.25%, Co: less than 0.25%, and V: less than 0.25%. [Element group D]: One or two or more elements selected from the group consisting of Sn: 20.00% or less, Bi: less than 5.00%, and In: less than 2.00%. [Element group E]: One or more elements selected from the group consisting of Ca: 3.00% or less, La: less than 0.50%, Ce: less than 0.50%, and Y: 0.50% or less [Element group F]: B: less than 0.50% (7) The surface-treated member according to (6), which contains element group A. (8) The surface-treated member according to (6), which contains element group B. (9) The surface-treated member according to (6), which contains element group C. (10) The surface-treated member according to (6), which contains element group D. (11) The surface-treated member according to (6), which contains element group E. (12) The surface-treated member according to (6), which contains element group F. (13) The surface-treated member according to (6), wherein the zinc-based plating layer is a plating layer containing at least, by mass%, Al: 4.0% or more and less than 25.0%, and Mg: 0.3% or more and less than 12.5%. (14) The upper limit of the measured value of the obtained steel substrate exposure rate is expressed as A [%], and the coating weight of the zinc-based plating layer per side is expressed as B [g / m]. ２ A surface-treated member according to any one of (1) to (3), which satisfies both the following formulas (1) and (2) when expressed as A≦0.8×10 －１ ×B+28.0 (1) A≦40.0 (2) (15) The coating weight of the zinc-based plating layer is 50.0 to 150.0 g / m per side. ２ (16) The surface-treated member according to any one of (1) to (3), wherein the coating weight of the zinc-based plating layer is 50.0 to 150.0 g / m per side. ２(17) The surface-treated member according to any one of (1) to (3), wherein there is at least one cross section in which the steel base exposure rate is within a range of 5.0 to 40.0%.

[0011] As described above, according to the present invention, it is possible to exhibit excellent corrosion resistance even in soil several centimeters deep from the soil surface.

[0012] FIG. 1 is a schematic diagram for explaining a structure installed on the soil surface. FIG. 2 is a schematic diagram for explaining the structure of a surface treated member according to an embodiment of the present invention. FIG. 3 is a schematic diagram for explaining the structure of a surface treated member according to the same embodiment. FIG. 4 is a schematic diagram for explaining the structure of a surface treated member according to the same embodiment. FIG. 5 is an explanatory diagram for explaining a steel base exposure rate in the surface treated member according to the same embodiment. FIG. 6 is an explanatory diagram for explaining a steel base exposure rate in the surface treated member according to the same embodiment. FIG. 7 is an explanatory diagram for explaining a modified example of the steel base exposure rate in the surface treated member according to the same embodiment.

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0014] (Corrosion behavior in soil several centimeters deep from the soil surface) Before describing the surface treated member according to an embodiment of the present invention, the corrosion behavior discovered by the present inventors in soil several centimeters deep from the soil surface will be described with reference to Fig. 1. Fig. 1 is a schematic diagram for explaining a structure installed on the soil surface. In the following, "soil several centimeters deep from the soil surface" may be referred to as "surface layer of soil" for convenience.

[0015] As shown schematically in Figure 1, various structures are installed on the soil surface, including, for example, stands on which photovoltaic panels are mounted, road signs, guardrails, traffic lights, etc. In this case, in order to install the structures more stably on the soil surface, as shown in Figure 1, a portion of the structure (for example, the portion surrounded by a dashed line in Figure 1, such as near the lower end of the support) is often buried in the soil.

[0016] Here, various structures are often made of various steel materials, for example, from the viewpoint of production costs, etc. Here, when a part of a structure made of various steel materials is buried in the soil, there is a concern that the steel materials may corrode.

[0017] Soil has two effects on steel corrosion: moisture retention, a corrosion factor, and oxygen diffusion inhibition, another corrosion factor. Moisture retention increases the corrosion rate, while oxygen diffusion inhibition decreases the corrosion rate. In soil, a moist environment (i.e., moisture) is maintained for a longer period of time after rainfall than in the atmosphere. Meanwhile, soil inhibits the penetration and diffusion of oxygen from the atmosphere, slowing the corrosion rate. The actual corrosion rate in soil is determined by the interaction of the two trade-off effects described above. Generally, the inhibition of oxygen diffusion is greater than the influence of moisture retention, so the corrosion rate in soil is slower than in the atmosphere.

[0018] On the other hand, the situation is very different in soil several centimeters deep (surface soil portion). Even in the surface soil portion, moisture is less likely to evaporate from the soil, so the steel remains in contact with moisture for a longer period of time than in the atmosphere. On the other hand, in the surface soil portion, the distance from the atmosphere is short, and oxygen is supplied from the atmosphere quickly, so the effect of soil on inhibiting oxygen diffusion is small. Therefore, combined with the effect of moisture retention, it is thought that the corrosion inhibition effect of soil is almost impossible in the surface soil portion. Thus, the surface soil portion that the inventors focused on can be said to be an extremely severe corrosion environment compared to soil at a depth of 1 m or more from the soil surface.

[0019] The present inventors investigated the corrosion behavior of zinc-based plated steel materials in the surface layer of soil in order to study the corrosion behavior of steel materials in the surface layer of soil, and as a result, found the following: The zinc-based plating layer of zinc-based plated steel materials exhibits high corrosion resistance in the atmosphere by covering the surface of the plating layer with zinc-based corrosion products. However, it was found that the corrosion prevention function of such zinc-based corrosion products is low in the surface layer of soil.

[0020] The present inventors observed zinc-based corrosion products formed on a zinc-based plating layer in the soil surface layer using a scanning electron microscope (SEM) or the like. As a result, it was found that the zinc-based corrosion products formed in the soil surface layer were in a more porous state than the zinc-based corrosion products formed in the atmosphere. This was thought to be because corrosion progresses more rapidly in the soil surface layer than in the atmosphere.

[0021] From the above findings, the present inventors have come to the conclusion that, in order to improve the corrosion resistance of zinc-based plated steel material in the surface layer of soil, it is important to change the corrosion products formed in the zinc-based plated layer in the surface layer of soil into products formed in a dense state.

[0022] In order to realize the generation of dense corrosion products in the surface layer of soil, the inventors conducted further studies and came up with the idea of ​​intentionally providing a portion of the zinc-based coating layer where the surface of the steel material is exposed (hereinafter referred to as "exposed steel substrate portion") at a specific abundance ratio, which is different from the conventional technical idea. It was found that this makes it possible to generate dense corrosion products on the surface of the zinc-based coating layer even in the surface layer of soil.

[0023] In the conventional technical concept, a zinc-based plating layer is provided in a dense state to minimize exposure of the steel substrate, and the steel substrate is protected by utilizing the sacrificial corrosion protection effect of zinc. However, the present inventors have discovered that by intentionally providing portions in the zinc-based plating layer that expose the steel substrate, it is possible to generate corrosion products in a dense state different from that in the atmosphere, starting from such exposed portions. Such corrosion products protect the steel substrate and the zinc-based plating layer in the vicinity thereof.

[0024] When the exposed steel substrate is located in the soil, both the zinc-based plating layer and the steel substrate are exposed to corrosion factors in the soil (i.e., moisture and oxygen). This causes Zn ions to elute from the zinc-based plating layer and Fe ions to elute from the steel substrate. In the atmosphere, the eluted Zn and Fe ions are washed away by rain, etc., and are unlikely to adhere to the exposed portions. On the other hand, because there is no water flow in the soil, the eluted Zn and Fe ions are likely to adhere to the exposed steel substrate, and it is presumed that dense Fe-Zn composite oxides are produced as corrosion products in the exposed steel substrate.

[0025] Furthermore, as a result of investigations by the present inventors, it has been found that when the zinc-based plating layer is an alloy plating layer containing Mg and Al in addition to Zn, Mg ions and Al ions are eluted in soil in addition to Zn ions. It has also been found that such elution produces Fe-Zn-Al-Mg composite oxides, which further improves corrosion resistance.

[0026] Based on the findings described above, the present inventors have conducted further studies and have come up with a surface-treated member according to an embodiment of the present invention, as described in detail below.

[0027] (Regarding the Surface-Treated Member) The surface-treated member according to an embodiment of the present invention will be described in detail below with reference to Figures 2 to 4. Figures 2 to 4 are schematic diagrams for explaining the structure of the surface-treated member according to this embodiment. For convenience, the explanation will be given using the coordinate axes shown in Figures 2 to 4.

[0028] As described in detail below, the surface-treated member according to this embodiment is constructed using a surface-treated steel material having a zinc-based plating layer as its raw material. Here, the specific structure of the surface-treated member according to this embodiment is not particularly limited. The specific structure of the surface-treated member according to this embodiment may be, for example, a box-like structure, or a plate-like structure using plate-shaped steel material, such as the roof or wall of a building. Furthermore, the specific structure of the surface-treated member according to this embodiment may be, for example, a formed body or a joined body using various shaped steel, such as a solar panel mounting frame, a formed body or a joined body using various H-shaped steel or square columns, such as the framework of a structure, or a formed body or a joined body using various steel pipes, such as various supports, signs, traffic lights, guardrails, etc.

[0029] Fig. 2 is a schematic diagram showing a part of the surface-treated member according to this embodiment as viewed from above (in the Z-axis direction in the drawing). As shown in Fig. 2, the surface-treated member 1 according to this embodiment is made of a zinc-based plated steel sheet having a zinc-based plating layer 20 formed on the surface of a steel sheet serving as a base material, as will be described later. At least a part of the surface (corresponding to the XY plane in Fig. 2, the surface of the zinc-based plating layer 20) has an exposed steel base portion 201, which is a portion where the surface of the steel is exposed.

[0030] 2, the entire exposed steel substrate portion 201 is shown schematically as a substantially rectangular shape. However, in the surface-treated member 1 according to this embodiment, the specific shape of the exposed steel substrate portion 201 is not limited to the shape shown in FIG.

[0031] Fig. 3 shows a schematic cross section of the surface-treated member 1 shown in Fig. 2 cut in the Z-axis direction along the cutting line A-A. The cross section shown in Fig. 3 corresponds to a section of the surface-treated member 1 according to this embodiment, in which a portion where the exposed steel base portion 201 does not exist, is cut in the thickness direction of the zinc-based plated steel material that is the raw material for the surface-treated member 1.

[0032] As shown schematically in FIG. 3 , the portion of the surface-treated member 1 according to this embodiment other than the exposed steel base portion 201 is made of a surface-treated steel material (i.e., a zinc-based plated steel material) having a steel material 10 as a base material and a zinc-based plating layer 20 located on the surface of the steel material 10.

[0033] First, the steel material 10 and the zinc-based plating layer 20 will be described in detail below.

[0034] <Regarding the Steel Material 10> The steel material 10 used as the base material of the surface-treated member 1 according to this embodiment is not particularly limited, and various steel materials can be used depending on the mechanical strength (e.g., tensile strength) required of the surface-treated member 1. Examples of such steel materials 10 include various types of Al-killed steel, ultra-low carbon steel containing Ti, Nb, etc., high-strength steel in which ultra-low carbon steel further contains strengthening elements such as P, Si, and Mn, and various steel materials containing various other elements (Cr, N, Cu, B, Ni, Mg, Ca, V, Co, Zn, As, Y, Zr, Mo, Sn, Sb, Ta, W, Pb, Bi, REM, etc.).

[0035] Furthermore, a pre-plated layer may be provided on the steel material 10 before zinc-based plating as described below is performed. Metals that can be used for the pre-plated layer include Ni, Sn, and alloys combining these elements. Using pre-plated steel material that has been pre-plated with these platings in advance can eliminate bare spots (areas where the plated metal is repelled by an oxide film or the like). This is presumably because, when the pre-plated steel material is immersed in a hot-dip plating bath, the metal elements in the hot-dip plating bath react with the pre-plated layer to form Ni(Sn)-Al-Fe-Zn. The coating weight of the pre-plated layer per side is 0.2 to 2.0 g / m２ It is preferable that the coating weight of the pre-plating layer is within the range of 0.2 g / m. ２ By setting the coating weight of the pre-plating layer at 2.0 g / m or more, it is possible to reliably achieve the effect of suppressing unplated areas by the pre-plating. ２ By setting the thickness as follows, it is possible to achieve the above-described effect of suppressing unplated areas while preventing the pre-plated layer from suppressing the elution of Fe, which makes it difficult to generate Fe-Zn-based composite oxides.

[0036] Furthermore, the thickness of the steel material 10 is not particularly limited, and may be set appropriately depending on the mechanical strength required of the surface-treated member 1, etc.

[0037] <Regarding the zinc-based plating layer 20> The zinc-based plating layer 20 is formed over the entire surfaces of both surfaces of the above-described steel material 10. The zinc-based plating layer 20 is not particularly limited as long as it contains at least zinc (Zn), and various known zinc-based platings can be applied.

[0038] Examples of such zinc-based plating include zinc plating, such as hot-dip galvanizing and galvannealed hot-dip galvanizing, zinc-nickel plating, zinc-iron plating, zinc-chromium plating, zinc-aluminum plating, zinc-titanium plating, zinc-magnesium plating, zinc-manganese plating, zinc-aluminum-magnesium plating, and zinc-aluminum-magnesium-silicon plating. Furthermore, zinc-based plating may contain small amounts of different metal elements or impurities, such as cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, and arsenic, or may have inorganic substances, such as silica, alumina, and titania, dispersed therein. The plating method is not particularly limited, and various known plating methods, such as electroplating, hot-dip plating, vapor deposition plating, dispersion plating, and vacuum plating, may be used.

[0039] Among the various zinc-based platings described above, the zinc-based plating layer 20 according to this embodiment is preferably a zinc-based plating layer having, for example, the chemical composition described in detail below. By using a zinc-based plating layer 20 having the chemical composition described in detail below, it is possible to further improve the corrosion resistance and other properties of the surface-treated member 1 according to this embodiment. First, the more preferable chemical composition of the zinc-based plating layer 20 described above will be described in detail below.

[0040] <Regarding the Chemical Composition of the Zinc-Based Plating Layer 20> According to one aspect, the chemical composition of the zinc-based plating layer 20 according to this embodiment contains, in mass %, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and the balance being Zn and impurities.

[0041] In addition, in the chemical composition of a certain aspect of the zinc-based plating layer 20 according to the present embodiment, the zinc-based plating layer 20 is more preferably a plating layer containing, by mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and Zn: 60.00% or more.

[0042] According to another aspect, the chemical composition of the zinc-based plating layer 20 according to this embodiment contains, in mass %, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and further contains one or more elements selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, and element group F, with the balance being Zn and impurities.

[0043] [Element group A]: One or two elements selected from the group consisting of Si: 2.50% or less and Fe: 5.00% or less. [Element group B]: One or two or more elements selected from the group consisting of Sb: less than 0.50%, Pb: less than 0.50%, and Sr: less than 0.50%. [Element group C]: One or two or more elements selected from the group consisting of Cu: less than 0.25%, Ti: less than 0.25%, Cr: less than 0.25%, Nb: less than 0.25%, Ni: less than 0.25%, Mn: less than 0.25%, Co: less than 0.25%, and V: less than 0.25%. [Element group D]: One or two or more elements selected from the group consisting of Sn: 20.00% or less, Bi: less than 5.00%, and In: less than 2.00%. [Element group E]: one or more elements selected from the group consisting of Ca: 3.00% or less, La: less than 0.50%, Ce: less than 0.50%, and Y: 0.50% or less. [Element group F]: B: less than 0.50%.

[0044] [Al: 0.10% by mass or more and less than 40.00% by mass] In a preferred embodiment of the zinc-based plating layer 20 according to this embodiment, Al is an element necessary for constituting the main metal structure (Zn-Al-Mg metal structure). Therefore, Al is preferably contained at a predetermined content or more to ensure corrosion resistance as a plated steel material. If the Al content in the zinc-based plating layer 20 is less than 0.10% by mass, the above-described corrosion resistance may not be ensured. Therefore, in the zinc-based plating layer 20 according to this embodiment, the Al content is preferably 0.10% by mass or more. The Al content is preferably 3.00% by mass or more, more preferably 4.00% by mass or more, and even more preferably 6.00% by mass or more. By ensuring the Al content within the above range, corrosion resistance as a plated steel material can be ensured.

[0045] On the other hand, if the Al content in the zinc-based plating layer 20 is 40.00 mass% or more, the Al phase that functions as a cathode when placed in a corrosive environment will increase excessively, facilitating corrosion of the steel material 10, and there is a possibility that the corrosion resistance of the plated steel material will not be ensured. Therefore, in the zinc-based plating layer 20 according to this embodiment, the Al content is preferably less than 40.00 mass%. The Al content is preferably 25.00 mass% or less, more preferably less than 25.00 mass%, and even more preferably 20.00 mass% or less.

[0046] [Mg: 0.10% by mass or more and less than 15.00% by mass] In a preferred embodiment of the zinc-based plating layer 20 according to this embodiment, Mg is an element necessary for constituting the main metal structure (Zn—Al—Mg-based metal structure). Therefore, Mg is preferably contained in a predetermined amount or more to ensure corrosion resistance as a plated steel material. If the Mg content in the zinc-based plating layer 20 is less than 0.10% by mass, the above-described corrosion resistance may not be ensured. Therefore, in the zinc-based plating layer 20 according to this embodiment, the Mg content is preferably 0.10% by mass or more. The Mg content is preferably 0.30% by mass or more, and more preferably 3.00% by mass or more. By ensuring the Mg content within the above range, it is possible to ensure corrosion resistance as a plated steel material.

[0047] On the other hand, if the Mg content in the zinc-based plating layer 20 is 15.00 mass% or more, anodic dissolution of the zinc-based plating layer is likely to proceed when placed in a corrosive environment, which may result in the corrosion resistance of the plated steel material not being ensured. Therefore, in the zinc-based plating layer 20 according to this embodiment, the Mg content is preferably less than 15.00 mass%. The Mg content is preferably less than 12.50 mass%, and more preferably 12.00 mass% or less. By ensuring that the Mg content falls within the above range, it is possible to ensure the corrosion resistance of the plated steel material.

[0048] In the zinc-based plating layer 20 according to this embodiment, the balance of the Al and Mg is Zn and impurities. In a preferred embodiment of the zinc-based plating layer 20 according to this embodiment, Zn is an element necessary for constituting the main metal structure (Zn-Al-Mg metal structure) and is an important element for improving the corrosion resistance of the plated steel material. Furthermore, when the zinc-based plating layer 20 contains the Al and Mg within the above ranges and further contains Zn, it is possible to ensure the corrosion resistance required of the plated steel material.

[0049] Next, in a preferred embodiment of the zinc-based plating layer 20 according to another aspect of this embodiment, element groups A to F that may be contained in the chemical composition of the zinc-based plating layer 20 will be described in detail.

[0050] In addition, when the zinc-based plating layer 20 according to another aspect of the present embodiment contains at least one of the elements belonging to the following element groups A to F, it is preferable that the at least one of the elements belonging to the following element groups A to F be contained within the following content ranges and in a total content of 60.00 mass% or less.

[0051] By setting the total content of elements belonging to element groups A to F to 60.00% by mass or less, it becomes possible to enjoy the effects exerted by the addition of each element, as described in detail below, without impairing each other. The total content of elements belonging to element groups A to F is preferably 50.00% by mass or less, and more preferably 40.00% by mass or less.

[0052] Element Group A In another aspect of the zinc-based plating layer 20 according to this embodiment, element group A that may be contained in the zinc-based plating layer 20 will be described. At least one element in element group A shown below is an element that may be contained in the zinc-based plating layer 20 in place of a portion of the remaining Zn. [Element Group A]: One or two elements selected from the group consisting of Si: 2.50% or less, and Fe: 5.00% or less

[0053] [Si: 0% by mass or more and 2.50% by mass or less] In another embodiment of the zinc-based plating layer 20 according to the present embodiment, a case where Si is not contained is also conceivable, and therefore the lower limit of the Si content is 0% by mass. On the other hand, Si is an element that can suppress the excessive growth of an Fe-Al-based metallic structure formed at the interface between the zinc-based plating layer 20 and the steel material 10, and further improve the adhesion between the zinc-based plating layer 20 and the steel material 10. When Si is contained in the zinc-based plating layer 20, in order to suppress the excessive growth of an Fe-Al-based metallic structure, the Si content is preferably 0.05% by mass or more, and more preferably 0.20% by mass or more. On the other hand, if the Si content exceeds 2.50% by mass, there is a possibility that high-melting-point intermetallic compounds are formed with Mg in excess, inhibiting the formation of Al-Mg oxides, which have the effect of suppressing Zn evaporation during welding.

[0054] Furthermore, if the Si content in the plating bath for producing the zinc-based plating layer 20 is too high, the viscosity of the plating bath increases more than necessary, which may result in a decrease in operability during the production of plated steel materials (hereinafter referred to as "plating operability"). Therefore, the Si content in the plating bath is adjusted from the viewpoint of plating operability. The Si content in the zinc-based plating layer 20 is preferably 1.50 mass% or less, and more preferably 1.00 mass% or less.

[0055] [Fe: 0% by mass or more and 5.00% by mass or less] Elements constituting the steel material 10, which is the base material, may be mixed into the zinc-based plating layer 20. In particular, in the hot-dip galvanizing method, elements constituting the steel material 10 are likely to be mixed into the zinc-based plating layer 20 due to interdiffusion of elements caused by a solid-liquid reaction between the steel material 10 and the zinc-based plating layer 20. Due to the incorporation of such elements, the zinc-based plating layer 20 often contains a predetermined amount of Fe, and the content thereof is often 0.01% by mass or more. If the interdiffusion is promoted, the adhesion between the steel material 10 and the zinc-based plating layer 20 is improved. From the viewpoint of improving the adhesion between the steel material 10 and the zinc-based plating layer 20, the Fe content in the zinc-based plating layer 20 is preferably 0.20% by mass or more.

[0056] Furthermore, Fe may be intentionally added to the plating bath used to produce the zinc-based plating layer 20, provided that the effects of the present invention are not impaired. However, if the Fe content in the plating bath is increased, high-melting-point intermetallic compounds of Fe and Al are formed in the plating bath. In this case, such high-melting-point intermetallic compounds tend to adhere to the zinc-based plating layer 20 as dross, significantly degrading the appearance quality, which is undesirable. From this perspective, the Fe content in the plating bath is adjusted. The Fe content in the zinc-based plating layer 20 is preferably 5.00% by mass or less. The Fe content in the plating layer 20 is more preferably 3.00% by mass or less, and even more preferably 2.00% by mass or less, 1.00% by mass or less, or 0.50% by mass or less.

[0057] ◇Element Group B In another aspect of the zinc-based plating layer 20 according to this embodiment, element group B that may be contained in the zinc-based plating layer 20 will be described. At least one element in element group B shown below is an element that may be contained in the zinc-based plating layer 20 in place of a portion of the remaining Zn. [Element Group B]: One or more elements selected from the group consisting of Sb: less than 0.50%, Pb: less than 0.50%, and Sr: less than 0.50%

[0058] [Sb: 0% by mass or more and 0.50% by mass or less] [Pb: 0% by mass or more and 0.50% by mass or less] [Sr: 0% by mass or more and 0.50% by mass or less] In another embodiment of the zinc-based plating layer 20 according to this embodiment, it is possible that Sb, Pb, and Sr are not contained. Therefore, the lower limit of the content of these elements is 0% by mass. On the other hand, when at least one of Sb, Pb, and Sr is contained in the zinc-based plating layer 20, spangles are formed on the surface of the zinc-based plating layer 20, thereby improving metallic luster. Therefore, from the viewpoint of further improving the design of the plated steel material, it is preferable that at least one of Sb, Pb, and Sr is contained in the zinc-based plating layer 20. Such an effect of improving the design is achieved when the content of at least one of Sb, Pb, and Sr is 0.05% by mass or more. Therefore, when at least one of Sb, Pb, and Sr is contained in the zinc-based plating layer 20, the content of each of these elements is preferably set independently to 0.05 mass% or more.

[0059] On the other hand, when forming a zinc-based coating layer 20 in which any one of the Sb, Pb, and Sr contents exceeds 0.50 mass%, the amount of dross generated in the coating bath used to form the zinc-based coating layer 20 increases, and there is a possibility that a plated steel material with good coating properties cannot be produced. Therefore, the contents of Sb, Pb, and Sr in the zinc-based coating layer 20 are each preferably independently 0.50 mass% or less. The contents of Sb, Pb, and Sr are each preferably independently 0.20 mass% or less.

[0060] Element Group C In another aspect of the zinc-based plating layer 20 according to this embodiment, element group C that may be contained in the zinc-based plating layer 20 will be described. At least one element in element group C shown below is an element that may be contained in the zinc-based plating layer 20 in place of a portion of the remaining Zn. [Element Group C]: One or more elements selected from the group consisting of Cu: less than 0.25%, Ti: less than 0.25%, Cr: less than 0.25%, Nb: less than 0.25%, Ni: less than 0.25%, Mn: less than 0.25%, Co: less than 0.25%, and V: less than 0.25%

[0061] [Cu: 0% by mass or more and less than 0.25% by mass] [Ti: 0% by mass or more and less than 0.25% by mass] [Cr: 0% by mass or more and less than 0.25% by mass] [Nb: 0% by mass or more and less than 0.25% by mass] [Ni: 0% by mass or more and less than 0.25% by mass] [Mn: 0% by mass or more and less than 0.25% by mass] [Co: 0% by mass or more and less than 0.25% by mass] [V: 0% by mass or more and less than 0.25% by mass] In another aspect of the zinc-based plating layer 20 according to this embodiment, it is possible that Cu, Ti, Cr, Nb, Ni, Mn, Co, and V are not contained, and therefore the lower limit of the content of these elements is 0% by mass. On the other hand, when at least one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is contained in the zinc-based plating layer 20, these elements are incorporated into the Fe-Al-based metal structure produced by welding when such plated steel is welded, thereby further improving the corrosion resistance of the welded joint. This effect of improving the corrosion resistance of the welded joint is realized when the content of at least one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the zinc-based plating layer 20 is 0.05 mass% or more. Therefore, when at least one of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is contained in the zinc-based plating layer 20, the content of each of these elements is preferably independently 0.05 mass% or more.

[0062] On the other hand, when forming a zinc-based plating layer 20 in which any of the elements Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is 0.25% by mass or more, these elements may form various intermetallic compounds in the plating bath used to form the zinc-based plating layer 20, resulting in an increase in the viscosity of the plating bath and making it impossible to produce a plated steel material with good plating properties. Therefore, the contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the zinc-based plating layer 20 are preferably each independently less than 0.25% by mass. The contents of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V are each independently preferably 0.20% by mass or less.

[0063] Element Group D In another aspect of the zinc-based plating layer 20 according to this embodiment, element group D that may be contained in the zinc-based plating layer 20 will be described. At least one element in element group D shown below is an element that may be contained in the zinc-based plating layer 20 in place of a portion of the remaining Zn. [Element Group D]: One or more elements selected from the group consisting of Sn: 20.00% or less, Bi: less than 5.00%, and In: less than 2.00%

[0064] [Sn: 0% by mass or more and 20.00% by mass or less] [Bi: 0% by mass or more and less than 5.00% by mass] [In: 0% by mass or more and less than 2.00% by mass] In another embodiment of the zinc-based plating layer 20 according to the present embodiment, Sn, Bi, and In may not be contained. Therefore, the lower limit of the content of these elements is 0% by mass. On the other hand, Sn, Bi, and In form intermetallic compounds with Mg in the zinc-based plating layer 20, thereby improving the weldability of the zinc-based plating layer 20. Furthermore, since these intermetallic compounds all have high melting points, when plated steel materials are welded, they do not evaporate after welding and remain as intermetallic compounds. The presence of these elements can improve corrosion resistance and corrosion prevention, and can also improve the corrosion resistance of welds during welding. The effect of improving corrosion resistance is achieved when the content of at least one of Sn, Bi, and In in the zinc-based plating layer 20 is 0.05% by mass or more. Therefore, when at least one of Sn, Bi, and In is contained in the zinc-based plating layer 20, the content of each of these elements is preferably set independently to 0.05 mass% or more.

[0065] On the other hand, excessive Sn addition increases the amount of intermetallic compounds formed, which may reduce the corrosion resistance of the zinc-based plating layer 20 after welding. Furthermore, excessive Bi and In addition may make the zinc-based plating layer 20 brittle and prone to peeling, and may also reduce the corrosion resistance of the zinc-based plating layer 20 after welding. These phenomena become more pronounced when the Sn content exceeds 20.00 mass%, when the Bi content is 5.00 mass% or more, or when the In content is 2.00 mass% or more. Therefore, the Sn content is preferably 20.00 mass% or less, the Bi content is preferably less than 5.00 mass%, and the In content is preferably less than 2.00 mass%. The Sn content is more preferably 10.00 mass% or less, the Bi content is more preferably 3.00 mass% or less, and the In content is more preferably 1.00 mass% or less.

[0066] Element Group E In another aspect of the zinc-based plating layer 20 according to this embodiment, element group E that may be contained in the zinc-based plating layer 20 will be described. At least one element in element group E shown below is an element that may be contained in the zinc-based plating layer 20 in place of a portion of the remaining Zn. [Element Group E]: One or more elements selected from the group consisting of Ca: 3.00% or less, La: less than 0.50%, Ce: less than 0.50%, and Y: 0.50% or less

[0067] [Ca: 0% by mass or more and 3.00% by mass or less] In another aspect of the zinc-based plating layer 20 according to the present embodiment, it is possible that no Ca is contained, and therefore the lower limit of the Ca content is 0% by mass. On the other hand, when Ca is contained in the plating bath for producing the zinc-based plating layer 20, dross generated with an increase in the Mg concentration during plating operations can be reduced, and plating operability can be improved.

[0068] Furthermore, when Ca is contained in the zinc-based plating layer 20, it forms an intermetallic compound with Al and Zn. Furthermore, when Si is contained together with Ca in the zinc-based plating layer 20, Ca forms an intermetallic compound with Si. These intermetallic compounds have a high melting point and a stable structure, making it possible to suppress liquid metal embrittlement (LME) when plated steel is welded. When Ca is contained in the zinc-based plating layer 20, the effect of improving plating operability and the effect of suppressing LME during welding are achieved by setting the Ca content to 0.01% by mass or more. The Ca content in the zinc-based plating layer 20 is more preferably 0.05% by mass or more.

[0069] On the other hand, if the Ca content in the zinc-based plating layer 20 exceeds 3.00 mass%, the corrosion resistance of the plated steel material may be reduced. From this viewpoint, the Ca content in the zinc-based plating layer 20 is preferably 3.00 mass% or less. The Ca content in the zinc-based plating layer 20 is preferably 2.00 mass% or less, and more preferably 1.00 mass% or less.

[0070] [La: 0% by mass or more and less than 0.50% by mass] [Ce: 0% by mass or more and less than 0.50% by mass] [Y: 0% by mass or more and less than 0.50% by mass] In another aspect of the zinc-based plating layer 20 according to this embodiment, it is possible that La, Ce, and Y are not contained, so the lower limits of the contents of these elements are 0% by mass. On the other hand, La, Ce, and Y are elements that exhibit effects almost equivalent to those of Ca. This is because the atomic radius of each element is close to the atomic radius of Ca, and when these elements are contained in the zinc-based plating layer 20, they substitute for Ca positions.

[0071] The effect of improving plating operability and the effect of suppressing LME during welding are realized by making the contents of these elements independently 0.01% by mass or more. Therefore, when at least one of La, Ce, and Y is contained, the contents of these elements independently are preferably 0.01% by mass or more. The contents of La, Ce, and Y in the zinc-based plating layer 20 are each independently more preferably 0.05% by mass or more.

[0072] On the other hand, if the La, Ce, and Y contents in the plating bath for producing the zinc-based plating layer 20 are too high, the viscosity of the plating bath may increase more than necessary, potentially reducing plating operability. Therefore, the La, Ce, and Y contents in the plating bath are adjusted from the perspective of plating operability. The La, Ce, and Y contents are preferably each independently less than 0.50 mass%, less than 0.50 mass%, and 0.50 mass% or less. The La, Ce, and Y contents are each independently preferably 0.10 mass% or less.

[0073] Element Group F In another aspect of the zinc-based plating layer 20 according to this embodiment, element group F that may be contained in the zinc-based plating layer 20 will be described. At least one element in element group F shown below is an element that may be contained in the zinc-based plating layer 20 in place of a portion of the remaining Zn. [Element Group F]: B: Less than 0.50%

[0074] [B: 0% by mass or more and less than 0.50% by mass] In another embodiment of the zinc-based plating layer 20 according to the present embodiment, a case where B is not contained is also conceivable, and therefore the lower limit of the B content is 0% by mass. On the other hand, when B is contained in the zinc-based plating layer 20, it has the effect of further suppressing LME. This is presumably because, when B is contained in the zinc-based plating layer 20, B combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compounds. Furthermore, the presence of B in the zinc-based plating layer 20 is presumably effective in further suppressing LME of the steel material 10 through grain boundary strengthening. Furthermore, because the various intermetallic compounds formed with B have extremely high melting points, it is presumed that they also act to suppress Zn evaporation during welding. These improving effects are achieved by including 0.05% by mass or more of B. Therefore, when B is included, the B content is preferably 0.05% by mass or more.

[0075] On the other hand, if an excessive amount of B is added to the plating bath in order to incorporate B into the zinc-based plating layer 20, a rapid rise in the plating melting point may occur, resulting in a deterioration in plating operability and the possibility of not being able to produce a plated steel material with excellent plating properties. Such a deterioration in plating operability becomes significant when the B content is 0.50 mass% or more, so the B content is preferably less than 0.50 mass%. The B content is more preferably 0.10 mass% or less.

[0076] [Method for Measuring Chemical Composition] The chemical composition of the zinc-based plating layer 20 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). ICP-AES is used to analyze chemical compositions down to 0.1 mass%, and ICP-MS is used to analyze trace amounts of chemical compositions less than 0.1 mass%. The plated steel material is immersed in a 10% HCl aqueous solution containing an inhibitor for about 1 minute to peel off the zinc-based plating layer, and a solution in which the zinc-based plating layer is dissolved is prepared. The resulting solution can be analyzed by ICP-AES or ICP-MS to obtain the overall average chemical composition of the zinc-based plating layer.

[0077] <Regarding the coating weight of the zinc-based plating layer 20> The coating weight of the zinc-based plating layer 20 as described above is, for example, 10.0 g / m per one side of the steel material. ２ It is preferable that the content is 50.0 g / m or more. ２ The coating weight of the zinc-based plating layer 20 is, for example, 200.0 g / m per one side of the steel material. ２ Preferably, the total weight is 150.0 g / m or less. ２ When the coating weight of the zinc-based plating layer 20 falls within the above range, the surface-treated member 1 according to this embodiment can exhibit sufficient corrosion resistance.

[0078] The deposition weight of the zinc-based plating layer 20 is determined by cutting a sample measuring 30 mm x 30 mm from the surface-treated member 1 and measuring its mass in advance. A tape seal is attached to one side of the sample to prevent the zinc-based plating layer on that side from dissolving in the next process. The sample is then immersed in a 10% HCl aqueous solution containing an inhibitor to peel off the zinc-based plating layer by pickling, and the mass of the sample after pickling is measured. The deposition weight of the zinc-based plating layer 20 per side can be determined from the change in mass of the sample before and after pickling.

[0079] <<Exposed Steel Base Portion 201 in Zinc-Based Coated Layer 20>> Next, the exposed steel base portion 201 present in at least a part of the zinc-based coated layer 20 according to this embodiment will be described in detail.

[0080] Fig. 4 is a schematic cross-sectional view of the surface-treated member 1 shown in Fig. 2 cut in the Z-axis direction along the cutting line B-B. The cross-sectional view shown in Fig. 4 corresponds to a cross-section of the surface-treated member 1 according to this embodiment, taken in the thickness direction of the zinc-based plated steel material that is the raw material for the surface-treated member 1, at a portion where the exposed steel base portion 201 exists.

[0081] As shown schematically in Fig. 4, the portion of the surface-treated member 1 according to this embodiment where the exposed steel base portion 201 exists is made of a surface-treated steel material (i.e., a zinc-based plated steel material) having a steel material 10 as a base material and a zinc-based plating layer 20 located on the surface of the steel material 10. Furthermore, at least a portion of the zinc-based plating layer 20 has a portion where the surface of the steel material 10 (i.e., the steel base) is exposed.

[0082] As briefly mentioned above, the surface-treated member 1 according to this embodiment intentionally has the exposed steel substrate portion 201 present at a specific abundance ratio in a portion of the zinc-based plating layer 20. As a result, when the portion of the surface-treated member 1 according to this embodiment where the exposed steel substrate portion 201 is provided is buried in soil, dense corrosion products, as explained above, can be produced on the surface of the zinc-based plating layer 20, even in the surface layer of the soil. This allows the surface-treated member 1 according to this embodiment to exhibit excellent corrosion resistance even in soil at a depth of about several centimeters from the soil surface.

[0083] Method for identifying the location of the exposed steel substrate portion 201 Below, we will first explain a method for identifying the location of the exposed steel substrate portion 201 in the zinc-based plating layer 20. The exposed steel substrate portion 201 in the zinc-based plating layer 20 according to this embodiment is a portion where the zinc-based plating layer 20 is missing and the steel substrate is exposed. Therefore, the location can be identified by performing various color tests that cause a color reaction in the portion where the steel substrate is exposed over the entire surface of the zinc-based plating layer 20.

[0084] Such a color test is not particularly limited, and various color tests can be applied. However, it is preferable to use, for example, the ferroxyl test specified in JIS H8626 (1995) or the pinhole test disclosed in a non-patent document (Shozo Matsuda, Tadashi Tanaka, Keiichi Tanigawa, "New Pinhole Test Method for Galvanized Steel Sheets," Metal Surface Technology, Vol. 23 (1972), pp. 142-146). In the above two types of color tests, a reagent for a color reaction is applied to the surface of a test specimen to cause a color reaction, and then the surface of the test specimen is covered with a test paper, and the area where the color reaction occurred is transferred to the test paper. Therefore, by using these color tests, the area where the exposed steel substrate portion 201 exists can be identified regardless of the shape of the surface-treated member 1.

[0085] Here, the various reagents used in the coloration test, the test papers used for transferring the colored areas, and the procedures for the coloration test may be in accordance with those specified in the literature disclosing the respective test methods.

[0086] Regarding the state of the exposed steel substrate portion 201: Next, the state of the exposed steel substrate portion 201 in the zinc-based plating layer 20 according to this embodiment will be described. In the surface-treated member 1 according to this embodiment, the state of the exposed steel substrate portion 201 in the zinc-based plating layer 20 is identified by observing a cross section of the zinc-based plating layer 20. More specifically, the state is identified by observing a cross section obtained by cutting the exposed steel substrate portion 201 in the thickness direction of the steel material 10 according to the shape of the exposed steel substrate portion 201 when the zinc-based plating layer 20 is viewed in plan from above (e.g., from the Z-axis direction in FIG. 2 ). The microscope used in this case is not particularly limited as long as it can achieve a magnification that allows cross-sectional observation. The microscope used for observation may be an optical microscope or an electron microscope such as a SEM.

[0087] A more specific procedure will be described below with reference to Figures 5 to 7. Figures 5 to 7 are explanatory diagrams for explaining the steel base exposure rate in the surface-treated member according to this embodiment.

[0088] When checking the state of the exposed steel substrate portions 201, as shown in the upper left of Fig. 5, first, the coloration test as described above is performed on the entire surface of the zinc-based plating layer 20 to identify the locations of the exposed steel substrate portions 201. Then, as shown in the lower left of Fig. 5, each of the identified exposed steel substrate portions 201 is viewed in plan from above the zinc-based plating layer 20 (the Z-axis direction in Fig. 2), and each exposed steel substrate portion 201 is virtually partitioned. When partitioning, a virtual section with a short side b of 5 mm and an area (area obtained by multiplying the long side a by the short side b) is created. ２ The partitioning is performed using rectangular regions as units.

[0089] Next, for each of the virtually partitioned regions (rectangular regions), the area ratio of the exposed steel substrate 201 to the region is calculated. ２ It is determined whether the area ratio of the exposed steel substrate portion 201 to the total area (area) of the steel substrate 201 is 3% or more for all regions.

[0090] Next, all of the regions (rectangular regions) where the area ratio of the exposed steel base portion 201 was 3% or more were designated as target regions for cross-sectional observation, as shown in the lower right of Figure 5. As schematically shown in the lower right of Figure 5, the surface-treated member 1 corresponding to the region of interest was cut at intervals of △ = 50 μm. All of the obtained cross sections were observed under a microscope, and the steel base exposure rate, which will be described later, was calculated. The cutting direction for obtaining the cross sections shown in the lower right of Figure 5 will be explained again below with reference to Figure 7.

[0091] By the above-described microscopic observation, it is possible to obtain an observation field for each cross section, such as that shown schematically in Fig. 6. Here, for the above-described microscopic observation, an optical microscope (for example, ECLIPSE LV150 manufactured by Nikon Corporation) is used, and a magnification of about 100 times may be selected.

[0092] Within the observation field of view as shown in Figure 6, the exposed steel base portion 201 located at the leftmost position in the field of view and the exposed steel base portion 201 located at the rightmost position in the field of view are identified. Then, as shown in Figure 6, a position 100 µm from the left edge of the leftmost exposed steel base portion 201 in the direction toward the left edge of the field of view is set as the starting point, and a position 100 µm from the right edge of the rightmost exposed steel base portion 201 in the direction toward the right edge of the field of view is set as the end point, and the length between these starting and end points is measured and designated as the reference length L0.

[0093] Furthermore, the length of exposed steel substrate is measured for each exposed steel substrate portion 201 present within the observation field as shown in Fig. 6. The total length of exposed steel substrate is calculated and taken as ΣLi (i is an integer parameter indicating the number of exposed steel substrate portions 201 present within the observation field). In the example shown in Fig. 6, there are three exposed steel substrate portions 201 within the observation field, and ΣLi = L1 + L2 + L3.

[0094] After measuring L0 and ΣLi in this way, the value (unit: %) obtained by (ΣLi / L0)×100 is defined as the steel substrate exposure rate of the cross section of interest.

[0095] The exposed steel substrate portion 201 of the surface-treated member 1 according to this embodiment has at least one cross section in which the exposed steel substrate ratio is in the range of 1.0 to 40.0%. The exposed steel substrate portion 201 being in this state allows for the generation of corrosion products in a dense state different from that in the atmosphere, starting from this exposed portion. These corrosion products protect the steel substrate and the nearby zinc-based plating layer, ensuring the corrosion resistance of the surface-treated member 1 even when the exposed steel substrate portion 201 of the surface-treated member 1 according to this embodiment is buried in soil.

[0096] The more cross sections in which the steel substrate exposure rate is in the range of 1.0 to 40.0%, the better, and there is no particular upper limit. It is more preferable that the steel substrate exposure rate is in the range of 1.0 to 40.0% in all of the obtained cross sections.

[0097] Furthermore, from the viewpoint of the corrosion resistance as described above, it is more preferable that the surface-treated member 1 according to this embodiment has at least one cross section in which the steel base exposure ratio is in the range of 5.0 to 40.0%. The more cross sections in which the steel base exposure ratio is in the range of 5.0 to 40.0%, the better, and there is no particular upper limit. It is also more preferable that the steel base exposure ratio is in the range of 5.0 to 40.0% in all of the obtained cross sections.

[0098] Furthermore, in the surface-treated member 1 according to this embodiment, in a cross section where the steel substrate exposure rate calculated as described above is within the range of 1.0 to 40.0%, the shortest distance from any point on the surface of the exposed steel material 10 to the zinc-based plating layer 20 is greater than 0 μm and not greater than 50.0 μm. For example, in the example shown in FIG. 6 , when focusing on point A on the surface of the exposed steel material 10, the shortest distance to the zinc-based plating layer 20 corresponds to the length Ls in the figure. If this shortest distance exceeds 50.0 μm, it means that the width of the exposed steel substrate portion 201 is too large. In this case, when the exposed steel substrate portion 201 is buried in soil, Zn will not be able to dissolve from the zinc-based plating layer 20 to the exposed steel substrate portion 201 in time, and the corrosion resistance of the surface-treated member 1 cannot be ensured. The length of such a shortest distance is preferably greater than 0 μm and not greater than 40.0 μm, and more preferably greater than 0 μm and not greater than 30.0 μm.

[0099] The various lengths in the observation field as described above can be measured using a length measurement function or the like implemented in the microscope used.

[0100] In the surface-treated member 1 according to this embodiment, the total area of ​​the sections from which cross sections having a steel base exposure rate in the range of 1.0 to 40.0% is obtained is preferably 5.0% or more relative to the area of ​​the portion to be buried in soil. When the total area of ​​the sections is 5.0% or more relative to the area of ​​the intended burial site, the surface-treated member 1 according to this embodiment exhibits superior corrosion resistance. The total area of ​​the sections from which cross sections having a steel base exposure rate in the range of 1.0 to 40.0% is more preferably 10.0% or more relative to the area of ​​the portion to be buried in soil. The upper limit of this ratio is not particularly specified and may be 100%.

[0101] Furthermore, when the cross section of the exposed steel substrate portion 201 is observed as described above, the total area of ​​the regions that give a cross section where the steel substrate exposure rate exceeds 40.0% (the total area when the corresponding regions are viewed in plan from above) is 5% or less of the entire area of ​​the exposed steel substrate portion 201 under consideration. If this total area ratio exceeds 5%, the zinc-based plating layer 20 will peel off excessively, resulting in an excessively large region where the steel substrate is exposed, and the corrosion resistance of the surface-treated member 1 will be reduced. The smaller this total area ratio, the better, and 0% is most preferable.

[0102] Regarding the direction of the cutting line when calculating the steel base exposure ratio, the direction of the cutting line when performing the above-mentioned cross-sectional observation will now be described with reference to Fig. 7. In the surface-treated member 1 according to this embodiment, when determining the steel base exposure ratio, the surface-treated member 1 is cut in the thickness direction of the steel material 10 to obtain the cross-section as described above. In this case, the direction of the cutting line is determined depending on the shape and distribution of the exposed steel base portion 201 when the zinc-based plating layer 20 is viewed in plan from above.

[0103] 7 shows a classification of examples of the shape and distribution of the exposed steel substrate portions 201 when viewed macroscopically in a plan view from above (in the Z-axis direction) the zinc-based plating layer 20. In the surface-treated member 1 according to this embodiment, the shape and distribution of the exposed steel substrate portions 201 are roughly classified into those with directional characteristics, as shown in the group in the upper part of Fig. 7, and those with an isotropic shape or those with no directional shape or distribution, as shown in the group in the lower part of Fig. 7.

[0104] In the group shown in the upper part of Fig. 7, there is a directional orientation (the vertical direction on the paper in the case of the upper part of Fig. 7), such as in the group where the exposed steel substrate portions 201 are approximately elliptical in shape and are distributed so that the long axis directions of the ellipses are approximately parallel (the leftmost drawing in the upper part of Fig. 7), and in the group where the exposed steel substrate portions 201 are approximately rectangular in shape and are distributed so that the long side directions of the ellipses are approximately parallel (the center and rightmost drawings in the upper part of Fig. 7). In such cases, in this embodiment, the direction perpendicular to the directional orientation is taken as the direction of the cutting line.

[0105] Furthermore, the group shown in the lower part of Fig. 7 includes groups in which the exposed steel base portions 201 have an isotropic shape or no directionality in the shape or distribution, such as a group in which circular exposed steel base portions 201 are distributed (the leftmost diagram in the lower part of Fig. 7), a group in which square exposed steel base portions 201 are randomly distributed (the center diagram in the lower part of Fig. 7), and a group in which the zinc-based plating layer 20 is distributed in an island-like manner (the rightmost diagram in the lower part of Fig. 7). In such cases, in this embodiment, the direction of the cutting line is not particularly specified, and any direction may be used as the direction of the cutting line.

[0106] Relationship between the steel substrate exposure rate and coating weight The present inventors actually manufactured surface-treated members while varying the coating weight per side of the zinc-based coating layer 20 and the steel substrate exposure rate, and then buried the manufactured surface-treated members in soil to verify their corrosion resistance. Then, the horizontal axis shows the coating weight per side (unit: g / m ２ The authors investigated how the results of the corrosion resistance evaluation were distributed on a coordinate plane in which the vertical axis represents the upper limit (unit: %) of the measured value of the steel substrate exposure rate. As a result, it was revealed that the surface-treated member 1 according to this embodiment exhibits superior corrosion resistance when both the conditions shown in the following formula (101) and formula (102) are satisfied.

[0107] A ≦ 0.8 × 10 －１ ×B+28.0...Formula (101) A≦40.0...Formula (102)

[0108] In the above formulas (101) and (102), parameter A is the upper limit (unit: %) of the measured value of the obtained steel substrate exposure rate, and parameter B is the coating weight (unit: g / m) of the zinc-based coating layer 20 per side. ２ ) The details of why the establishment of the above formulas (101) and (102) results in better corrosion resistance are not yet clear. However, the inventors presume that this is because the relationship between the area of ​​the exposed steel substrate and the amount of Zn eluted from the zinc-based coating layer 20 commensurate with this area becomes appropriate.

[0109] In addition, the coating weight of the zinc-based plating layer 20 per side is 50.0 to 150.0 g / m ２ When the above formula (101) and formula (102) are both satisfied, the surface-treated member 1 according to this embodiment exhibits even more excellent corrosion resistance.

[0110] The surface treated member 1 according to this embodiment has been described in detail above with reference to FIGS.

[0111] The surface-treated member 1 according to the present embodiment as described above can exhibit excellent corrosion resistance even in soil several centimeters deep from the soil surface by changing the state of the zinc-based plating layer provided as the surface treatment layer depending on the corrosive environment to which it is exposed. The soil in which the surface-treated member 1 according to the present embodiment can be buried is not particularly limited. However, it is presumed that the properties of the surface-treated member 1 according to the present embodiment will be more advantageous in soil that satisfies the following conditions, for example:

[0112] For example, soil suitable for burial includes soil with a pH of about 3 to 12, which allows Fe elution to proceed easily, soil with a Cl ion concentration of 5 mg or more per kg of soil, soil in salt-damaged areas, and soil where snow-melting agents are often sprayed.

[0113] (Modification of the method for calculating the steel base exposure ratio) In the above embodiment, a method for calculating the steel base exposure ratio has been described in which the cross section of the zinc-based plating layer 20 is observed under a microscope. This method is preferable because, by focusing on the cross section, it is possible to specify the steel base exposure ratio regardless of the external shape of the surface-treated member 1 (for example, whether the shape has many flat portions or many curved surfaces).

[0114] Hereinafter, a method for calculating the steel substrate exposure ratio by analyzing the surface of the zinc-based coating layer 20 using an EDX (Energy Dispersive X-ray Spectroscopy) analyzer attached to an SEM (e.g., a JEOL JSM-7000F) and mapping the analysis results, rather than the previously described method of calculating the steel substrate exposure ratio from microscopic observation of the cross section, will be briefly described with reference to FIG. 8 . The calculation method described below is particularly useful when a certain area of ​​the zinc-based coating layer can be easily viewed from above in plan, such as when the outer shape of the surface-treated member 1 is composed of gently curved flat portions or a flat surface. In this case, the method described below may be adopted instead of the previously described method of calculating the steel substrate exposure ratio from the cross section.

[0115] 8 is an explanatory diagram for explaining a modified example of the exposed steel base ratio in the surface-treated member according to the present embodiment. Here, the procedure from conducting a color change test on the surface of the zinc-based plating layer 20 to virtually dividing the exposed steel base portion 201 into rectangular sections is the same as that shown in FIG. 5 , and therefore a detailed description thereof will be omitted below.

[0116] In this modified example, among the virtually partitioned regions, a region (area 1 cm ２ ) in which the area ratio of the exposed steel substrate portion 201 to the total area is 3% or more is to be the subject of EDX analysis.

[0117] In this modification, the entire region to be analyzed is observed from the surface using SEM-EDX, and the distribution of the elements Zn, Al, and Mg in the region to be investigated is analyzed. The SEM-EDX observation conditions may be, for example, an acceleration voltage of 15 kV, a probe current of 0.005 nA, 10 sweeps, and a magnification of 500. During such observation, mapping analysis is performed on at least five elements: O, Zn, Al, Mg, and Fe, and mapping analysis may also be performed on all elements.

[0118] Based on the obtained measurement results, the distribution of the elements Zn, Al, and Mg is two-dimensionally mapped, and within the obtained distribution, regions where the Zn content is 1.0 mass% or less, the Al content is 1.0 mass% or less, and the Mg content is 0.5 mass% or less are treated as exposed steel substrate areas. In this manner, the specific shape and distribution of exposed steel substrate areas within each section can be identified, as schematically shown in the diagram at the lower right of Figure 8. The total area of ​​the exposed steel substrate areas obtained in this manner (in the example of Figure 8, the total area of ​​five exposed steel substrate areas) is calculated.

[0119] Meanwhile, the outermost peripheries of the identified exposed steel base portions are connected to obtain a shape connecting the outermost peripheries, as shown in the diagram at the bottom right of Figure 8. Positions 100 µm further outward from this outermost periphery are connected to identify this shape. The area of ​​this shape connecting the positions 100 µm outward from the outermost periphery is used as the reference area for calculating the steel base exposure rate.

[0120] Then, the value (unit: %) expressed as (total area of ​​exposed steel base portions / reference area)×100 can be treated as the steel base exposure rate.

[0121] In this modified example, the outermost circumference is connected to form a 1 cm ２ If the entire area of ​​the unit area is included in the shape, the area of ​​the unit area (1 cm ２ ) is the reference area. ２ For the above sections, the steel substrate exposure rate shall be calculated.

[0122] The above is a brief explanation of the modified method for calculating the steel substrate exposure rate.

[0123] (Regarding the Manufacturing Method of the Surface-Treated Member) Hereinafter, an example of a manufacturing method of the surface-treated member 1 according to this embodiment will be described.

[0124] <Method for manufacturing surface-treated steel material as a base material> The surface-treated steel material as a base material for the surface-treated member 1 according to this embodiment is manufactured by using the above-described steel material 10 as a base material and forming a zinc-based plating layer on the surface of the steel material 10.

[0125] Here, the zinc-based plating layer can be formed by hot-dip plating, thermal spraying, cold spraying, sputtering, vapor deposition, electroplating, etc. However, hot-dip plating is most preferable in terms of cost.

[0126] Thereafter, a specific processing treatment as described below is performed on the obtained plated steel material (steel material 10 having a zinc-based plating layer) at a desired position (more specifically, a portion where an exposed steel base portion is to be formed) to form an exposed steel base portion in the zinc-based plating layer. This allows the production of a surface-treated steel material that is the raw material for the surface-treated member 1 according to this embodiment.

[0127]

[0033] In the following, an example of a manufacturing method for obtaining a surface-treated steel material according to the present embodiment using a hot-dip galvanizing method will be described in detail. In the manufacturing process of such a surface-treated steel material, first, a steel sheet as an example of a steel material 10 used as a base material is rolled by the Sendzimir method to a desired thickness, and then wound into a coil and placed in a hot-dip galvanizing line.

[0128] In the hot dip plating line, the steel sheet is continuously fed from the coil and passed through the line. At this time, the steel sheet is annealed by an annealing facility installed on the line in an environment where oxidation is unlikely to occur, for example, an oxygen concentration of 20 ppm or less, and N ２ -5% H ２ After heating and reducing at 800°C in a gas atmosphere, the temperature was increased by approximately 20°C to the subsequent plating bath temperature. ２ The plate is cooled with gas and then immersed in a plating bath.

[0129] Here, a molten plating alloy having the above-described chemical components is prepared in the plating bath, and the temperature of the plating bath is set to be equal to or higher than the melting point of the plating alloy (for example, about 460 to 660°C).

[0130] When preparing a plating alloy material, it is preferable to use pure metals (purity of 99% or higher) as the alloying materials. First, predetermined amounts of alloying metals are mixed to obtain the above-described plating layer composition, and the alloy is completely melted in a high-frequency induction furnace, arc furnace, or the like under vacuum or inert gas purging conditions to obtain an alloy. Next, the alloy mixed with the predetermined components (the above-described plating layer composition) is melted in the atmosphere, and the resulting molten material is used as a plating bath.

[0131] In addition, there is no particular restriction on using pure metals for the production of the plating alloys described above, and existing Zn alloys, Mg alloys, and Al alloys may be melted and used. In this case, there is no problem as long as an alloy of a predetermined composition with few impurities is used.

[0132] After immersing the steel sheet in the plating bath, it is pulled up at a predetermined speed. At this time, the steel sheet is pulled up at a predetermined speed so that the zinc-based plating layer to be formed has a desired thickness. ２ The plating coating weight is controlled by wiping gas. Here, with the exception of the bath temperature, general plating operating conditions may be applied, and no special equipment or conditions are required.

[0133] Furthermore, the molten plating alloy located on the steel sheet may be subjected to various heat treatments as required.

[0134] Thereafter, the zinc-based plating layer is subjected to a processing treatment such as scratching or bending at a desired position on the manufactured surface-treated steel material, thereby forming an exposed steel base portion 201 at the processed portion.

[0135] For example, when forming the exposed steel substrate portion 201 by a scratching process, a blade having a length of 50 μm or more and a portion with a tip width of 30 μm or less and sufficiently hard compared to the coating structure is used, and the blade is pressed perpendicularly against the steel sheet. A constant pressure is then applied along the direction in which the blade penetrates the steel sheet. Scratches can be generated on the surface of the zinc-based coating layer by moving the blade linearly under this condition. The steel substrate exposure rate can be changed by varying the number of scratches per unit area. Furthermore, even if other manufacturing conditions are not changed, by using a blade with a wide tip width as described above to generate scratches, it is possible to control the shortest distance from any point in the exposed steel substrate portion to the zinc-based coating layer as desired by utilizing the blade deflection caused by pressure and the blade vibration caused by moving the blade.

[0136] Here, one possible method for applying pressure to the blade is to place a weight of 3 kg or 5 kg on the blade. This pressure is sufficient to scratch the zinc-based plating layer, but is a pressure that barely scratches the steel material. The speed at which the blade is moved is not particularly limited. If the blade and the steel material slip, one method is to set the blade movement speed to 50 cm / sec or less, from the perspective of preventing the blade from being unable to scratch the steel material.

[0137] Furthermore, for example, when forming the exposed steel base portion 201 by bending, a hydraulic bending machine consisting of two parts, a die and a press, may be used. Here, the die is composed of, for example, a convex die having a tip in the shape of a right-angled isosceles triangle, and a concave die having a depression of the same shape as the tip of the convex die, and the tip of the convex die is rounded with a radius R (unit: mm) rather than an acute angle.

[0138] The desired plated steel sheet is placed between the convex and concave dies as described above, and the two dies are pressed together with a press to form the zinc-based plating layer. At this time, by controlling the thickness t (unit: mm) of the plated steel sheet, the radius R (unit: mm) of the tip of the convex die, and the pressing speed V (unit: cm / s) of the press, it is possible to form an exposed steel base portion at a desired position on the zinc-based plating layer.

[0139] In this case, the ratio t / R, which is the ratio of the plate thickness t to the radius R of the tip of the convex die, is controlled to satisfy the relationship 0.6≦t / R≦4.0. By setting these processing conditions, a large elongation is applied to the zinc-based plating layer, causing plating cracks, and the steel substrate exposure rate can be kept within the desired range. In addition, the pressing speed V of the press from the time the convex die contacts the zinc-based plating layer to the completion of processing is set to 1 cm / s or less. In this way, by reducing the processing speed to the minimum, it is possible to suppress the occurrence of large localized cracks and control the size of the exposed steel substrate portion 201.

[0140] An example of the method for manufacturing a surface-treated steel material according to this embodiment has been specifically described above.

[0141] <Method for manufacturing surface-treated member> The surface-treated steel material obtained as described above is used as a raw material to manufacture the surface-treated member according to this embodiment. Here, in order to obtain components for manufacturing the surface-treated member from the surface-treated steel material, various shaping processes can be used, such as various forming processes, joining processes using fastening members such as bolts and caulking, and welding processes. By appropriately combining these processes, a surface-treated member having a desired shape can be manufactured from the surface-treated steel material.

[0142] An example of the method for manufacturing the surface treated member according to this embodiment has been briefly described above.

[0143] In the above description, a case has been described in which a strain-imparting treatment is performed on a surface-treated steel material to form the exposed steel base portion 201, but the strain-imparting treatment may not be performed at the stage of manufacturing the surface-treated steel material, but may be performed as part of the processing performed when manufacturing a surface-treated member from the surface-treated steel material. Furthermore, the processing performed when manufacturing a surface-treated member from the surface-treated steel material may itself be used as a strain-imparting treatment for forming the exposed steel base portion 201.

[0144] The surface-treated member according to the present invention will be specifically described below with reference to examples and comparative examples. Note that the examples shown below are merely examples of the surface-treated member according to the present invention, and the surface-treated member according to the present invention is not limited to the examples below.

[0145] In the test examples shown below, the base sheets for plating (all manufactured by Nippon Steel Corporation) shown in Table 1 below were used, and the base sheets for plating were cut into pieces measuring 100 mm x 200 mm. Then, plating was performed using a batch-type hot-dip plating test apparatus manufactured by our company, and a plurality of plated steel materials having the composition of the plating layer shown in Table 2 below were produced for each level.

[0146]

[0147]

[0148] Each of the obtained plated steel materials was subjected to the above-described scratching or bending process at approximately the center of the surface of the plating layer to form an exposed steel base portion, thereby obtaining a surface-treated member.

[0149] The scratching process was performed by using a blade that was significantly harder than the plating layer to form parallel, linear scratches. The maximum width of the 50 μm range at the tip of the scratching blade and the spacing between parallel scratches were adjusted to obtain the desired steel base exposure rate. The weight used to pressurize the blade during the scratching process was 3 kg, and the blade movement speed was 50 cm / sec. This scratching process resulted in the formation of exposed steel base areas with a directional distribution in the approximate center of the surface of each surface-treated member, as shown in the upper right diagram in the upper row of Figure 7.

[0150] The bending was performed using a hydraulic bending machine. The thickness t (unit: mm) of the plated steel sheet, the radius R (unit: mm) of the tip of the convex die, and the pressing speed V (unit: cm / s) of the press were controlled to obtain a desired steel base exposure rate. This bending process resulted in the formation of an exposed steel base portion with a directional distribution at approximately the center of the surface of each surface-treated member, as shown in the center diagram in the upper part of Figure 7 .

[0151] For the surface-treated members obtained by the above two types of processing, the steel substrate exposure rate, the shortest distance to the zinc-based coating layer, the total area of ​​cross sections with a steel substrate exposure rate in the range of 1.0 to 40.0%, and the total area of ​​cross sections with a steel substrate exposure rate of more than 40.0% were measured using the cross-sectional observation method described above. The color test was a ferroxyl test specified in JIS H8626 (1995). An ECLIPSE LV150 optical microscope manufactured by Nikon Corporation was used at a magnification of 100x.

[0152] The results for the cases where scratching was performed are summarized in Tables 3-1 to 3-3 below, and the results for the cases where bending was performed are summarized in Tables 4-1 to 4-3 below. In Tables 3-1 to 4-3 below, the column "Coating Weight" indicates the coating weight per side, and the column "Steel Base Exposure Rate" indicates the upper limit of the obtained steel base exposure rate. In addition, in Tables 3-1 to 4-3 below, the column "Longest of the Shortest Distances" indicates the value obtained by measuring the shortest distance from the center of the exposed steel base portion to the zinc-based coating layer at the position where the width of the exposed steel base portion is widest.

[0153] The corrosion resistance of each of the surface-treated members obtained was evaluated by the following two test methods.

[0154] (Test method 1: Evaluation of corrosion resistance in general soil) The surface-treated members obtained as described above, with the exposed steel substrate adjusted, were buried in soil to evaluate their corrosion resistance. Silica sand with an average particle size of 300 μm was used as the soil. The cover thickness (distance from the surface of the zinc-based plating layer to the soil surface) was 20 mm. After burial, to simulate general soil, ion-exchanged water containing 0.03 mass% NaCl was added dropwise to the soil, and the moisture content was adjusted to 100%. Then, the following drying and wetting steps were repeated 20 times. Drying step: Stored at 30°C for 7 days Wetting step: Wetted with ion-exchanged water, and the moisture content was adjusted to 100%.

[0155] After the above test, the surface-treated members were pickled with hydrochloric acid to remove the plating layer and corrosion products. The corrosion state of the steel material was then investigated using a shape measuring instrument (Keyence, VR-5000). The evaluation criteria were as follows. Grades A, B, and C were rated as passing. The results are summarized in Table 2 below. Grade A: No pits were formed on the steel material. B: Pits with a depth of less than 10 μm were formed on the steel material. C: Pits with a depth of 10 μm or more and less than 50 μm were formed on the steel material. D: Pits with a depth of 50 μm or more and less than 100 μm were formed on the steel material. E: Pits with a depth of 100 μm or more were formed on the steel material.

[0156] (Test method 2: Evaluation of corrosion resistance in acidic soil) The surface-treated members obtained as described above, with the exposed steel substrate adjusted, were buried in soil to evaluate their corrosion resistance. Silica sand with an average particle size of 100 μm was used as the soil. The cover thickness was 50 mm. After burial, in order to simulate acidic soil, an aqueous HCl solution prepared by adding HCl to ion-exchanged water to adjust the pH to 4 was dripped onto the soil, and the moisture content was adjusted to 100%. Thereafter, the following drying and wetting steps were repeated 20 times. Drying step: Stored at 30°C for 7 days Wetting step: Wetted with ion-exchanged water to adjust the moisture content to 100%

[0157] After the above test, the surface-treated members were pickled with hydrochloric acid to remove the plating layer and corrosion products. The corrosion state of the steel material was then investigated using a shape measuring instrument (Keyence, VR-5000). The evaluation criteria were as follows. Grades A, B, and C were rated as passing. The results are summarized in Table 2 below. Grade A: No pits were formed on the steel material. B: Pits with a depth of less than 10 μm were formed on the steel material. C: Pits with a depth of 10 μm or more and less than 50 μm were formed on the steel material. D: Pits with a depth of 50 μm or more and less than 100 μm were formed on the steel material. E: Pits with a depth of 100 μm or more were formed on the steel material.

[0158]

[0159]

[0160]

[0161]

[0162]

[0163]

[0164] As is clear from Tables 3-1 to 4-3 above, the surface-treated members corresponding to the examples of the present invention exhibited excellent corrosion resistance even in soil, whereas the surface-treated members corresponding to the comparative examples of the present invention were unable to obtain sufficient corrosion resistance.

[0165] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0166] The embodiments disclosed herein are illustrative in all respects and are not limiting. The above-described embodiments may be omitted, substituted, or modified in various ways without departing from the scope of the appended claims, the technical scope of the present invention as described below, and the spirit thereof. For example, the components of the above-described embodiments may be arbitrarily combined within the scope that does not impair the effects of the components. Furthermore, such an arbitrary combination naturally provides the functions and effects of each of the components involved in the combination, and also provides other functions and effects that are apparent to those skilled in the art from the description of this specification.

[0167] Furthermore, the effects described in this specification are merely descriptive or exemplary and are not limiting. In other words, the technology according to the present invention may achieve other effects that will be apparent to those skilled in the art from the description of this specification, in addition to or in place of the above-described effects.

[0168] The following configurations also fall within the technical scope of the present invention: (1) A surface-treated member made of a surface-treated steel material having a base steel material and a zinc-based plating layer located on the surface of the steel material, wherein at least a portion of the zinc-based plating layer has an exposed steel base portion, which is a portion where the surface of the steel material is exposed, and when a cross section obtained by cutting the exposed steel base portion in the thickness direction of the steel material according to the shape of the exposed steel base portion when the zinc-based plating layer is viewed in plan from above is observed under a microscope, there is at least one cross section in which the steel base exposure rate, which is the proportion of the portion where the surface of the steel material is exposed in the cross section, is in the range of 1.0 to 40.0%, and in the cross section in which the steel base exposure rate is in the range of 1.0 to 40.0%, the shortest distance from any point on the exposed surface of the steel material to the zinc-based plating layer is more than 0 μm and not more than 50.0 μm, (2) A surface-treated member according to (1), wherein the total area of ​​each of the regions that provide a cross section in which the steel base exposure rate exceeds 40.0% when viewed in plan from above is 5.0% or less of the total area of ​​the exposed steel base. (3) A surface-treated member according to (1), which is used by burying at least a part of the exposed steel base in soil. (4) The exposed steel base is identified by carrying out a color test over the entire surface of the zinc-based plating layer, in which a color reaction occurs in the part where the steel base is exposed, and when each of the identified exposed steel base is viewed in plan from above the zinc-based plating layer, each of the exposed steel base has a short side dimension of 5 mm or more and an area of ​​1 cm or less. ２and (4) the zinc-based plating layer has a chemical composition of 0.10% or more and less than 40.00% by mass of Al, and 0.10% or more and less than 15.00% by mass of Mg, with the remainder being Zn and impurities. (5) The surface-treated member according to (4), wherein the zinc-based plating layer is a plating layer containing, by mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and Zn: 60.00% or more. (6) The surface-treated member according to any one of (1) to (3), wherein the zinc-based plating layer is a plating layer containing, by mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and further containing one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, and element group F below, with the balance being Zn and impurities.[Element group A]: One or two elements selected from the group consisting of Si: 2.50% or less and Fe: 5.00% or less. [Element group B]: One or two or more elements selected from the group consisting of Sb: less than 0.50%, Pb: less than 0.50%, and Sr: less than 0.50%. [Element group C]: One or two or more elements selected from the group consisting of Cu: less than 0.25%, Ti: less than 0.25%, Cr: less than 0.25%, Nb: less than 0.25%, Ni: less than 0.25%, Mn: less than 0.25%, Co: less than 0.25%, and V: less than 0.25%. [Element group D]: One or two or more elements selected from the group consisting of Sn: 20.00% or less, Bi: less than 5.00%, and In: less than 2.00%. [Element group E]: One or more elements selected from the group consisting of Ca: 3.00% or less, La: less than 0.50%, Ce: less than 0.50%, and Y: 0.50% or less [Element group F]: B: less than 0.50% (7) The surface-treated member according to (6), which contains element group A. (8) The surface-treated member according to (6), which contains element group B. (9) The surface-treated member according to (6), which contains element group C. (10) The surface-treated member according to (6), which contains element group D. (11) The surface-treated member according to (6), which contains element group E. (12) The surface-treated member according to (6), which contains element group F. (13) The surface-treated member according to any one of (6) to (12), wherein the zinc-based plating layer is a plating layer containing, in mass %, at least Al: 4.0% or more and less than 25.0% Mg: 0.3% or more and less than 12.5%. (14) The upper limit of the measured value of the obtained steel base exposure rate is denoted as A [%], and the coating weight of the zinc-based plating layer per side is denoted as B [g / m]. ２ A surface-treated member according to any one of (1) to (13), which satisfies both the following formulas (1) and (2) when expressed as A≦0.8×10 －１ ×B+28.0 (1) A≦40.0 (2) (15) The coating weight of the zinc-based plating layer is 50.0 to 150.0 g / m per side. ２(16) The surface-treated member according to any one of (1) to (15), wherein there is at least one cross section in which the steel base exposure rate is in the range of 5.0 to 40.0%.

[0169] REFERENCE SIGNS LIST 1 surface-treated member 10 steel material 20 zinc-based plating layer 201 exposed steel base

Claims

1. A surface-treated member made of a surface-treated steel material having a base steel material and a zinc-based plating layer located on the surface of the steel material, wherein at least a portion of the zinc-based plating layer has an exposed steel base portion which is a portion where the surface of the steel material is exposed, and when a cross section obtained by cutting the exposed steel base portion in the thickness direction of the steel material according to the shape of the exposed steel base portion when the zinc-based plating layer is viewed in plan from above is observed under a microscope, there is at least one cross section in which the exposed steel base rate, which is the proportion of the portion where the surface of the steel material is exposed in the cross section, is within a range of 1.0 to 40.0%, and in the cross section in which the exposed steel base rate is within a range of 1.0 to 40.0%, the shortest distance from any point on the exposed surface of the steel material to the zinc-based plating layer is more than 0 μm and not more than 50.0 μm, A surface-treated member, wherein for each of the regions giving a cross section in which the steel base exposure rate exceeds 40.0%, the sum of the areas of the regions when viewed in a plan view from above is 5.0% or less of the total area of ​​the steel base exposed portion.

2. The surface-treated member according to claim 1, which is used by burying at least a part of the exposed steel base in soil.

3. The exposed steel base is identified by conducting a color test that causes a color reaction at the site where the steel base is exposed over the entire surface of the zinc-based plating layer, and when each of the identified exposed steel base parts is viewed in plan from above the zinc-based plating layer, the size of the short side of each exposed steel base is 5 mm or more and the area is 1 cm or less. ２ 3. The surface treatment member according to claim 2, wherein a rectangular region having an area ratio of the exposed steel base to the rectangular region is virtually divided into units, and the steel base exposure rate is measured by observing the cross section of each of the sections in which the area ratio of the exposed steel base to the rectangular region is 3% or more, and a ratio of the total area of ​​the sections having cross sections in which the exposed steel base rate is within a range of 1.0 to 40.0% to an area of ​​the portion buried in soil is 5.0% or more.

4. The surface-treated member according to any one of claims 1 to 3, wherein the zinc-based plating layer is a plating layer having a chemical composition containing, by mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, with the balance being Zn and impurities.

5. The surface-treated member according to claim 4, wherein the zinc-based plating layer is a plating layer containing, by mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and Zn: 60.00% or more.

6. The surface treated member according to any one of claims 1 to 3, wherein the zinc-based plating layer contains, by mass%, Al: 0.10% or more and less than 40.00%, Mg: 0.10% or more and less than 15.00%, and further contains one or more elements selected from the group consisting of the following element group A, element group B, element group C, element group D, element group E, and element group F, with the remainder being a plating layer having a chemical composition consisting of Zn and impurities. [Element group A]: One or more selected from the group consisting of Si: 2.50% or less and Fe: 5.00% or less [Element group B]: One or more selected from the group consisting of Sb: less than 0.50%, Pb: less than 0.50%, and Sr: less than 0.50% [Element group C]: One or more selected from the group consisting of Cu: less than 0.25%, Ti: less than 0.25%, Cr: less than 0.25%, Nb: less than 0.25%, Ni: less than 0.25%, Mn: less than 0.25%, Co: less than 0.25%, and V: less than 0.25% [Element group D]: One or more selected from the group consisting of Sn: 20.00% or less, Bi: less than 5.00%, and In: less than 2.00% [Element group E]: one or more selected from the group consisting of Ca: 3.00% or less, La: less than 0.50%, Ce: less than 0.50%, and Y: 0.50% or less [Element group F]: B: less than 0.50% 7. The surface treated member according to claim 6, containing said element group A.

8. The surface treated member according to claim 6, which contains said element group B.

9. The surface treated member according to claim 6, containing said element group C.

10. The surface-treated member according to claim 6, containing said element group D.

11. The surface-treated member according to claim 6, containing said element group E.

12. The surface-treated member according to claim 6, containing the element group F.

13. The surface-treated member according to claim 6, wherein the zinc-based plating layer is a plating layer containing, by mass%, at least: Al: 4.0% or more and less than 25.0% Mg: 0.3% or more and less than 12.5%.

14. The upper limit of the measured value of the steel base exposure rate obtained is expressed as A [%], and the coating weight of the zinc-based coating layer per side is expressed as B [g / m ２ 4. The surface-treated member according to claim 1, wherein, when expressed as A≦0.8×10, the following formula (1) and formula (2) are both satisfied. －１ ×B+28.0...Formula (1) A≦40.0...Formula (2) 15. The coating weight of the zinc-based plating layer is 50.0 to 150.0 g / m per side. ２ The surface-treated member according to any one of claims 1 to 3, 16. The coating weight of the zinc-based plating layer is 50.0 to 150.0 g / m per side. ２ The surface-treated member according to claim 14 , 17. A surface treated member according to any one of claims 1 to 3, in which there is at least one cross section in which the steel base exposure rate is within the range of 5.0 to 40.0%.

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