Plated steel material

Abstract

The present invention employs a plated steel material which comprises a plating layer disposed on a surface of a base steel material, wherein: the plating layer contains, in terms of mass%, 5.0-40.0% of Al, 0.5-15.0% of Mg, and 5.0-40.0% of Fe, with the balance being made up of Zn and impurities; if a cross-section of the plated steel material having a specific length in a direction that is parallel to the surface of the plated steel material is used as an observation area in a cross-section of the plated steel material perpendicular to the surface of the plated steel material, the length L of the boundary line between the plating layer and the base steel material satisfies expression (1); the plating layer includes a first region that has an Fe concentration of less than 5.0 mass% and a second region that has an Fe concentration of 5.0 mass% or more; and the first region contains an Al-containing phase in an area ratio of 5% or more.　(1): (L - L0) / L0 × 100 ≥ 2.0 (%) Herein, L0 is the straight line distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end.

Description

Plated steel

[0001] The present invention relates to a plated steel material.

[0002] For example, steel structures are used in the civil engineering and infrastructure fields. Steel structures are exposed to severely corrosive environments, such as coastal areas and areas where de-icing salt is used. Therefore, stainless steel materials are used to prevent corrosion and maintain steel structures over the long term.

[0003] On the other hand, stainless steel materials use high-cost alloy elements such as Cr and Ni. Therefore, the installation of steel structures using stainless steel materials is costly. Therefore, pre-plated products (e.g., Zn-Al-Mg plated steel materials) have come to be used as a substitute for stainless steel materials.

[0004] However, in the case of large steel structures, pipe-shaped steel structures, etc., the plating layer may disappear due to welding, and the plating layer may also disappear partially due to corrosion from cut end surfaces, etc. Furthermore, it is difficult to manufacture steel parts such as bolts and washers from sheet material in the first place. For these reasons, large steel structures, pipe-shaped steel structures, and steel parts such as bolts and washers are generally manufactured by subjecting steel materials that have been processed into a predetermined shape to a post-plating treatment (so-called hot-dip plating treatment).

[0005] Hot-dip Zn plating is widely used as a post-plating treatment, but recently, Zn-Al-Mg plating has also come into use to improve corrosion resistance.

[0006] Patent Document 1 listed below describes a plated steel material having: a steel material; a surface plating layer disposed on the surface of the steel material and composed of a Zn—Al—Mg alloy layer having an Fe concentration of less than 3 mass %; an intermediate plating layer disposed between the steel material and the surface plating layer and composed of a Zn—Al—Mg alloy layer having an Fe concentration of 3 mass % or more and less than 30 mass %, and a plating layer including: an interfacial alloy layer disposed between the steel material and the intermediate plating layer; wherein the total thickness of the surface plating layer and the intermediate plating layer is 8 μm or more and less than 300 μm; and the plating layer has an average chemical composition, in mass %, of more than 65.00% Zn, more than 6.5% but less than 22.5% Al, more than 3.0% but less than 12% Mg, and impurities; and the Mg concentration, in mass %, of the intermediate plating layer is more than 3.0%.

[0007] Japanese Patent Application Laid-Open No. 2021-4403

[0008] Recently, the use of plated steel, which has been hot-dip plated, has been considered not only in the civil engineering and infrastructure fields but also as a material for automotive parts. When plated steel is used as a material for automotive parts, in addition to red rust resistance, improvements in the adhesion of the plating layer, corrosion resistance to the base steel, and sacrificial corrosion resistance are required.

[0009] The plated steel material described in Patent Document 1 has excellent red rust resistance, but there is no mention whatsoever of the adhesion of the plating layer, the corrosion resistance of the base steel, or the sacrificial corrosion resistance.

[0010] Therefore, an object of the present invention is to provide a plated steel material that has excellent red rust resistance, as well as excellent adhesion of the plating layer, corrosion resistance against the base steel, and sacrificial corrosion resistance.

[0011] In order to solve the above problems, the present invention employs the following configuration: [1] A plated steel product comprising a base steel and a plating layer disposed on the surface of the base steel, wherein the plating layer has a chemical composition, in mass %, of Al: 5.0 to 40.0%, Mg: 0.5 to 15.0%, Fe: 5.0 to 40.0%, Si: 0 to 2.0%, and Ca: 0 to 2.0%, and further contains one or two elements selected from the group consisting of Group A and Group B below, with the balance being Zn and impurities, wherein, in a cross section perpendicular to the surface of the plated steel product, when a cross section of the plated steel product of a predetermined length in a direction parallel to the surface is taken as an observation area, the length L of the boundary line between the plating layer and the base steel product satisfies the following formula (1): The plated steel material has a first region disposed on the surface side of the plated steel material and having an Fe concentration of less than 5.0 mass%, and a second region disposed between the first region and the base steel and having an Fe concentration of 5.0 mass% or more, the first region having a thickness of 5 to 100 μm, the second region having a thickness of 5 to 200 μm, and the first region containing an Al-containing phase containing Zn and 20 to 99 mass% Al at an area ratio of 5% or more. [Group A] Ni: 0 to 1.0%. [Group B] Sb: 0-0.5%, Pb: 0-0.5%, Cu: 0-1.0%, Sn: 0-2.0%, Ti: 0-1.0%, Cr: 0-1.0%, Nb: 0-1.0%, Zr: 0-1.0%, Mn: 0-1.0%, Mo: 0-1.0%, Ag: 0-1.0%, Li: 0-1.0%, La: 0-0.5%, Ce: 0-0.5%, B: 0-0.5%, Y: 0-0.5%, P: 0-0.5%, Sr: 0-0.5%, Co: 0-0.5%, Bi: 0-0.5%, In: 0-0.5%, V: 0-0.5%, W: 0-0.5%. One or more of these is 0-5% in total. (LL ０ ) / L ０ × 100 ≧ 2.0 (%) ... (1) where L in formula (1) ０ is the linear distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end. [2] The plated steel material according to [1], which satisfies the following formula (2): (LL ０ ) / L ０× 100 ≧ 4.0 (%) ... formula (2) where L in formula (2) ０ is the linear distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end. [3] The plated steel material according to [1], which satisfies the following formula (3): (LL ０ ) / L ０ × 100 ≧ 6.0 (%) ... formula (3) where L in formula (3) ０ where λ is the linear distance between one end and the other end of the boundary line in the observation region, and L is the length of the boundary line between the one end and the other end. [4] The plated steel material according to any one of [1] to [3], wherein the thickness of the first region is 15 μm to 100 μm. [5] The plated steel material according to any one of [1] to [3], wherein the area ratio of the Al-containing phase in the first region is 15% or more. [6] The plated steel material according to any one of [1] to [3], wherein the first region contains an Mg—Zn phase containing Zn and 20 to 60 mass% Mg, and wherein the area ratio of the Mg—Zn phase in the first region is 15% or more. [7] The plated steel material according to any one of [1] to [3], wherein the first region contains a binary eutectic structure of an Al phase and an Mg—Zn phase, and wherein the area ratio of the binary eutectic structure in the first region is 5% or more. [8] The plated steel material according to any one of [1] to [3], wherein the thickness of the second region is 15 μm to 200 μm. [9] The plated steel material according to any one of [1] to [3], wherein the second region contains an Mg—Zn phase containing Zn and 20 to 60 mass% of Mg, and the area ratio of the Mg—Zn phase in the second region is 15% or more.

[10] The plated steel material according to any one of [1] to [3], wherein the content of Mg relative to the total amount of Zn and Mg (Mg / (Zn+Mg)(%)) in the chemical composition of the plated layer is 5.0% or more.

[11] The plated steel material according to any one of [1] to [3], wherein the content of Mg relative to the total amount of Zn and Mg (Mg / (Zn+Mg)(%)) in the chemical composition of the plated layer is 6.5% or more.

[12] The plating layer contains Sn at a concentration of 0.02 to 2.0% by mass, and the plating layer contains Mg ２The plated steel material according to any one of [1] to [3], containing an Sn phase.

[0012] According to the present invention, it is possible to provide a plated steel material that has excellent red rust resistance, as well as excellent adhesion of the plating layer, corrosion resistance of the base steel, and sacrificial corrosion resistance.

[0013] Fig. 4 is a schematic cross-sectional view showing a plated steel material according to an embodiment of the present invention. Fig. 5 is a schematic cross-sectional view showing a plated steel material according to an embodiment of the present invention, showing an observation area. Fig. 6 is a schematic cross-sectional view showing a plated steel material according to an embodiment of the present invention, showing a method for determining the ranges of the first area and the second area. Fig. 7 is a schematic cross-sectional view showing a plated steel material according to an embodiment of the present invention, showing a method for determining the ranges of the first area and the second area. Fig. 8 is a schematic cross-sectional view showing a plated steel material according to an embodiment of the present invention, showing an enlarged view of area M in Fig. 4.

[0014] The present inventors have conducted investigations in order to provide a plated steel material that is excellent in red rust resistance, as well as in adhesion of the plating layer, corrosion resistance of the base steel, and sacrificial corrosion resistance.

[0015] The plated steel material described in Patent Document 1 may have an interfacial alloy layer with a thickness of 1 μm to 5 μm. In the plated steel material described in Patent Document 1, the thickness of the interfacial alloy layer is limited to 5 μm or less because a thick interfacial alloy layer significantly reduces the adhesion and workability of the plating layer. However, the present inventors have discovered that a layer containing a large amount of Fe, such as an interfacial alloy layer, can improve the corrosion protection of the base steel by its barrier effect. Therefore, the present inventors have successfully improved the corrosion protection of the base steel without reducing the adhesion of the plating layer, and further increased red rust resistance and sacrificial corrosion protection, by forming a thick Fe-rich layer between the Mg-Al-Zn alloy layer and the base steel and controlling the morphology of the interface between the Fe-rich layer and the base steel.

[0016] More specifically, they found that plated steel with excellent red rust resistance, plating layer adhesion, steel substrate corrosion resistance, and sacrificial corrosion resistance can be obtained by imparting strain to the surface of the base steel before forming a plating layer on the base steel by hot-dip plating and controlling the cooling conditions after pulling up from the plating bath to control the morphology of the interface between the base steel and the plating layer. They also found that precipitating an Al-containing phase in the surface layer of the plating layer and suppressing the Fe concentration to a low level can further improve red rust resistance and steel substrate corrosion resistance.

[0017] Hereinafter, a plated steel material according to an embodiment of the present invention will be described.

[0018] The plated steel material of this embodiment is a plated steel material comprising a base steel material and a plating layer disposed on the surface of the base steel material, wherein the plating layer has a chemical composition, in mass %, of Al: 5.0 to 40.0%, Mg: 0.5 to 15.0%, Fe: 5.0 to 40.0%, Si: 0 to 2.0%, and Ca: 0 to 2.0%, and further contains one or two elements selected from the group consisting of Group A and Group B below, with the balance being Zn and impurities, and when a cross section of the plated steel material of a predetermined length in a direction parallel to the surface is observed in a cross section perpendicular to the surface of the plated steel material, The plated steel material has a boundary line between the plated layer and the base steel material, the length L of which satisfies the following formula (1), the plated layer includes a first region disposed on the surface side of the plated steel material and having an Fe concentration of less than 5.0 mass%, and a second region disposed between the first region and the base steel material and having an Fe concentration of 5.0 mass% or more, the first region having a thickness of 5 to 100 μm, the second region having a thickness of 5 to 200 μm, and the first region containing an Al-containing phase containing Zn and 20 to 99 mass% Al at an area ratio of 5% or more. [Group A] Ni: 0 to 1.0%. [Group B] Sb: 0-0.5%, Pb: 0-0.5%, Cu: 0-1.0%, Sn: 0-2.0%, Ti: 0-1.0%, Cr: 0-1.0%, Nb: 0-1.0%, Zr: 0-1.0%, Mn: 0-1.0%, Mo: 0-1.0%, Ag: 0-1.0%, Li: 0-1.0%, La: 0-0.5%, Ce: 0-0.5%, B: 0-0.5%, Y: 0-0.5%, P: 0-0.5%, Sr: 0-0.5%, Co: 0-0.5%, Bi: 0-0.5%, In: 0-0.5%, V: 0-0.5%, W: 0-0.5%. One or more of these is 0-5% in total. (LL ０ ) / L ０ × 100 ≧ 2.0 (%) ... (1) where L in formula (1) ０ is the linear distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end.

[0019] Furthermore, it is preferable that the plated steel material of this embodiment satisfies the following formula (2): (LL ０ ) / L ０ ×100≧4.0(%) ...Equation (2) Furthermore, it is preferable that the plated steel material of this embodiment satisfies the following equation (3): (LL０ ) / L ０ × 100 ≧ 6.0 (%) ... formula (3) where L in formulas (2) and (3) ０ is the linear distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end.

[0020] The thickness of the first region is preferably 15 μm to 100 μm. The area ratio of the Al-containing phase contained in the first region is preferably 15% or more. The first region preferably contains an Mg—Zn phase containing Zn and 20 to 60 mass% Mg, and the area ratio of the Mg—Zn phase in the first region is preferably 15% or more. The first region preferably contains a binary eutectic structure of an Al phase and an Mg—Zn phase, and the area ratio of the binary eutectic structure in the first region is preferably 5% or more.

[0021] The thickness of the second region is preferably 15 μm to 200 μm. The second region preferably contains an Mg—Zn phase containing Zn and 20 to 60 mass % of Mg, and the area ratio of the Mg—Zn phase in the second region is preferably 15% or more.

[0022] In addition, the content of Mg relative to the total amount of Zn and Mg in the chemical composition of the plating layer (Mg / (Zn+Mg)(%)) is preferably 5.0% or more. In addition, the content of Mg relative to the total amount of Zn and Mg in the chemical composition of the plating layer (Mg / (Zn+Mg)(%)) is preferably 6.5% or more.

[0023] The plating layer contains Sn at a concentration of 0.02 to 2.0% by mass, and the plating layer contains Mg ２ It is preferable that the alloy contains an Sn phase.

[0024] In the following description, the "%" used to indicate the content of each element in the chemical composition means "mass %." The content of an element in the chemical composition may be expressed as the element concentration (e.g., Zn concentration, Mg concentration, etc.). "Adhesion of the plating layer" refers to the property of the plating layer being resistant to peeling. "Red rust resistance" refers to the property of suppressing the occurrence of red rust in the plating layer when it corrodes. "Corrosion protection of the base steel" refers to the property of the plating layer itself being resistant to corrosion. "Sacrificial corrosion protection" refers to the property of suppressing corrosion of the base steel at exposed parts of the base steel (e.g., the cut end surface of the plated steel, parts of the plating layer that crack during processing, parts where the base steel is exposed due to peeling of the plating layer, etc.).

[0025] As shown in FIG. 1 , the plated steel material 1 according to this embodiment includes a base steel material 11. The shape of the base steel material 11 is not particularly limited. Examples of the base steel material 11 include steel plates, angle bars with L-shaped cross sections, and expanded metal. The base steel material 11 may also be formed into a base steel material such as a steel pipe, civil engineering and construction materials (fences, corrugated pipes, drainage ditch covers, sand-flying prevention plates, bolts, wire mesh, guardrails, water-stop walls, etc.), home appliance components (such as the housing of an outdoor unit of an air conditioner), or automobile parts (such as suspension components). The base steel material 11 may also be a formed product obtained by forming a steel plate into a predetermined shape. The base steel material 11 may also be formed into the shape of, for example, an automobile part by welding together two or more formed products obtained by forming a steel plate into a predetermined shape. The forming process may be any of various plastic processing techniques, such as pressing, roll forming, and bending.

[0026] There are no particular limitations on the material of the base steel material 11. The base steel material 11 can be various steel materials such as general steel, Al-killed steel, extra-low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing strengthening elements such as Ni and Cr). The base steel material 11 may also be a hot-rolled steel sheet, hot-rolled steel strip, cold-rolled steel sheet, or cold-rolled steel strip described in JIS G 3302:2010. There are also no particular limitations on the method for manufacturing the steel sheet (hot rolling method, pickling method, cold rolling method, etc.) and the specific manufacturing conditions thereof.

[0027] Furthermore, the base steel material 11 serving as the plating base sheet may be a pre-plated steel material in which a pre-plating is formed on the surface of the base steel material 11. An example of the pre-plated steel material is a Ni pre-plated steel material in which a Ni plating is formed on the surface of the base steel material 11. The pre-plated steel material is obtained, for example, by electrolytic treatment or displacement plating. The electrolytic treatment is performed by immersing the base steel material 11 in a sulfate bath or chloride bath containing metal ions of various pre-plating components. The displacement plating is performed by immersing the base steel material in an aqueous solution containing metal ions of various pre-plating components and having the pH adjusted with sulfuric acid, to cause displacement deposition of the metal.

[0028] The plated steel material 1 according to this embodiment has a plating layer 12 disposed on the surface of a base steel material 11. As described below, the plating layer 12 includes a first region 12A having an Fe concentration of less than 5.0 mass % and a second region 12B having an Fe concentration of 5.0 mass % or more. The plating layer 12 including the first region 12A and the second region 12B has a chemical composition that includes Zn and other alloying elements and may further include impurities. Alternatively, the plating layer 12 may contain Zn and other alloying elements, with the remainder being impurities.

[0029] The chemical composition of the plating layer will be described in detail below. Note that elements whose lower limit of concentration is 0% are not essential for solving the problems of the plated steel material according to this embodiment, but are optional elements that are allowed to be contained in the plating layer for the purpose of improving characteristics, etc.

[0030] <Al: 5.0 to 40.0%> Al contributes to improving red rust resistance, base steel corrosion resistance, and sacrificial corrosion resistance. Therefore, the Al concentration is set to 5.0% or more. The Al concentration may be set to 10.0% or more, 15.0% or more, or 20.0% or more. On the other hand, if the Al concentration is excessive, the Mg concentration and Zn concentration may relatively decrease, which may particularly deteriorate sacrificial corrosion resistance. Furthermore, the appearance of the plating layer may be significantly deteriorated. Therefore, the Al concentration is set to 40.0% or less. The Al concentration may also be set to 35.0% or less, or 30.0% or less.

[0031] <Mg: 0.5 to 15.0%> Mg is an essential element for ensuring the corrosion resistance and sacrificial corrosion resistance of the base steel. ２ Mg is also necessary to crystallize the phase. Therefore, the Mg concentration is set to 0.5% or more. The Mg concentration may be set to 2.0% or more, 3.0% or more, or 4.0% or more. On the other hand, if the Mg concentration is excessive, workability, particularly powdering resistance, may deteriorate, and furthermore, the corrosion resistance of the base steel may deteriorate. Furthermore, the appearance of the coating layer may be significantly deteriorated. Therefore, the Mg concentration is set to 15.0% or less. The Mg concentration may also be set to 10.0% or less, or 8.0% or less.

[0032] <Mg / (Zn+Mg): 5.0% or More, or 6.5% or More> The Mg content (Mg / (Zn+Mg) (%)) relative to the total amount of Zn and Mg in the chemical composition of the plating layer may be 5.0% or more, or 6.5% or more. This can further enhance sacrificial corrosion protection. Mg and Zn in (Mg / (Zn+Mg)) represent the Mg concentration and Zn concentration, respectively, in the plating layer.

[0033] <Fe: 5.0% to 40.0%> In this embodiment, the first and second regions of the coating layer contain a certain amount of Fe. Most of the Fe is incorporated into the coating layer by diffusion from the base steel, which is the original steel sheet used for coating. It has been confirmed that Fe in the coating layer does not adversely affect performance up to a concentration of 40.0% or less. Because most of the Fe is often present as an Al-Fe alloy phase in the second region, the Fe concentration tends to increase as the thickness of the second region increases. If the Fe concentration is less than 5.0%, the corrosion protection of the base steel may deteriorate in the later corrosion stage of the coating layer. Therefore, the Fe concentration is set to a range of 5.0 to 40.0%. The Fe concentration may be 10.0% or more, 15.0% or more, or 20.0% or more. The Fe concentration may also be 30.0% or less or 28.0% or less.

[0034] <Si: 0% to 2.0%> Si is an optional element and may be 0%. However, by including Si, the amount of Mg in the plating layer can be reduced. ２The formation of Si phases, Al-Ca-Si-Zn phases, Mg-Al-Si-Zn phases, etc. can improve the corrosion resistance of the coating layer. Therefore, the Si concentration may be greater than 0%, 0.1% or more, or 0.2% or more. On the other hand, excessive Si concentration may deteriorate the corrosion resistance of the base steel and the sacrificial corrosion resistance. Therefore, the Si concentration is set to 2.0% or less. The Si concentration may also be set to 0.8% or less, or 0.6% or less.

[0035] <Ca: 0% to 2.0%> Ca is an optional element and may be 0%. However, the inclusion of Ca forms phases such as Al-Ca-Zn, Al-Ca-Si-Zn, and Ca-Zn in the coating layer, improving the corrosion resistance of the coating layer. Ca is also an element that can adjust the optimal amount of Mg elution to impart corrosion protection to the steel substrate. Therefore, the Ca concentration may be 0.05% or more, or 0.1% or more. On the other hand, excessive Ca concentration may deteriorate the steel substrate's corrosion protection and workability. Therefore, the Ca concentration is set to 2.0% or less. The Ca concentration may also be set to 1.0% or less.

[0036] Furthermore, the plating layer of this embodiment may contain one or two types selected from the group consisting of Group A and Group B below.

[0037] [Group A] Ni: 0 to 1.0% [Group B] Sb: 0-0.5%, Pb: 0-0.5%, Cu: 0-1.0%, Sn: 0-2.0%, Ti: 0-1.0%, Cr: 0-1.0%, Nb: 0-1.0%, Zr: 0-1.0%, Mn: 0-1.0%, Mo: 0-1.0%, Ag: 0-1.0%, Li: 0-1.0%, La: 0-0.5%, Ce: 0-0.5%, B: 0-0.5%, Y: 0-0.5%, P: 0-0.5%, Sr: 0-0.5%, Co: 0-0.5%, Bi: 0-0.5%, In: 0-0.5%, V: 0-0.5%, W: 0-0.5%. One or more of these is 0-5% in total.

[0038] <Ni: 0 to 1.0%> The Ni concentration in Group A may be 0%. Ni contributes to improving sacrificial corrosion protection. Therefore, the Ni concentration may be 0.001% or more. On the other hand, if the Ni concentration is excessive, the corrosion protection of the base steel may deteriorate. Therefore, the Ni concentration is set to 1.0% or less. The Ni concentration may also be 0.8% or less, 0.6% or less, 0.5% or less, 0.1% or less, or 0.01% or less.

[0039] The plating layer according to this embodiment may contain 0 to 5% in total of one or more of the elements in Group B. If the total content of the elements in Group B exceeds 5%, the corrosion protection or sacrificial corrosion protection of the base steel may be reduced. The elements in Group B are described below.

[0040] <Sb, Pb: 0 to 0.5% each> The concentrations of Sb and Pb may be 0%. Sb and Pb contribute to improving sacrificial corrosion protection. Therefore, the concentrations of Sb and Pb may be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, excessive concentrations of Sb and Pb may deteriorate the corrosion protection of the base steel. Therefore, the concentrations of Sb and Pb are set to 0.5% or less. The concentrations of Sb and Pb may also be 0.3% or less, 0.1% or less, or 0.05% or less.

[0041] <Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li: 0-1.0% each> The concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li may each be 0%. These elements contribute to improving sacrificial corrosion protection. Therefore, the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li may each be 0.005% or more, or 0.01% or more. On the other hand, excessive concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li may degrade the corrosion protection of the base steel. Therefore, the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are each set to 1.0% or less. The concentration of each of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li may be 0.5% or less, 0.1% or less, or 0.05% or less.

[0042] <Sn: 0 to 2.0%> The Sn concentration may be 0%. Sn is an element that forms an intermetallic compound with Mg and improves the sacrificial corrosion protection of the plating layer. Therefore, the Sn concentration may be 0.01% or more, 0.02% or more, or 0.05% or more. However, if the Sn concentration is excessive, the corrosion protection of the base steel may deteriorate. Therefore, the Sn concentration is set to 2.0% or less. The Sn concentration may also be 1.5% or less, 1.0% or less, 0.5% or less, or 0.2% or less. Mg ２ In order to contain the Sn phase, the Sn content is preferably 0.02 to 2.0%.

[0043] <La, Ce, B, Y, P, and Sr: 0 to 0.5% Each> The concentrations of La, Ce, B, Y, P, and Sr may each be 0%. La, Ce, B, Y, P, and Sr contribute to improving sacrificial corrosion protection. Therefore, the concentrations of La, Ce, B, Y, P, and Sr may each be 0.005% or more, or 0.01% or more. On the other hand, excessive concentrations of La, Ce, B, Y, P, and Sr may deteriorate the corrosion protection of the base steel. Therefore, the concentrations of La, Ce, B, Y, P, and Sr each are set to 0.5% or less. The concentrations of La, Ce, B, Y, P, and Sr each may be set to 0.2% or less, 0.1% or less, 0.05% or less, or 0.02% or less.

[0044] <Co, Bi, In, V, W: 0 to 0.5% each> The concentrations of Co, Bi, In, V, and W may each be 0%. Co, Bi, In, V, and W contribute to improving sacrificial corrosion protection. Therefore, the concentrations of Co, Bi, In, V, and W may each be 0.001% or more, 0.002% or more, or 0.004% or more. On the other hand, excessive concentrations of Co, Bi, In, V, and W may degrade the corrosion protection of the base steel. Therefore, the concentrations of Co, Bi, In, V, and W are each set to 0.5% or less. The concentrations of Co, Bi, In, V, and W may each be 0.1% or less, 0.05% or less, 0.02% or less, or 0.01% or less.

[0045] <Balance: Zn and Impurities> The balance of the components of the plating layer according to this embodiment is Zn and impurities. Zn is an element that provides the plating layer with base steel corrosion protection and sacrificial corrosion protection. The Zn concentration does not need to be particularly limited, but may be 15.0% or more, 30.0% or more, or 50.0% or more. Impurities refer to components contained in raw materials or components mixed in during the manufacturing process. For example, trace amounts of components other than Fe may be mixed into the plating layer as impurities due to mutual atomic diffusion between the base steel and the plating bath.

[0046] The chemical composition of the plating layer is measured by the following method. First, an acid containing an inhibitor that suppresses corrosion of the base steel is used, and the plating layer is stripped and dissolved by immersion for 20 minutes at room temperature to obtain an acid solution. Next, the obtained acid solution is quantitatively analyzed by ICP atomic emission spectroscopy. This allows the chemical composition of the plating layer to be obtained. For example, a 10% hydrochloric acid solution containing 0.06 mass% of an inhibitor (Ibit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) can be used as the acid containing the inhibitor. Note that the chemical composition measured by the above-mentioned method is the chemical composition of the entire plating layer.

[0047] Next, the plating layer of this embodiment will be described in more detail. In the plated steel material of this embodiment, the interface between the plating layer and the base steel material is an uneven surface. The uneven surface at the interface between the plating layer and the base steel material significantly improves the adhesion of the plating layer.

[0048] Whether the interface between the plating layer and the base steel is an uneven surface to the extent that the adhesion of the plating layer is improved can be confirmed by checking whether the length L of the boundary line 13 between the plating layer 12 and the base steel 11 satisfies the following formula (1) when a cross section of the plated steel having a predetermined length, for example, 100 μm, in a direction parallel to the surface 1a of the plated steel is taken as the observation region in a cross section perpendicular to the surface of the plated steel, as shown in FIG. 2. Note that in FIG. 2, a cross section of the plated steel having a length of more than 100 μm in a direction parallel to the surface 1a of the plated steel is observed, but the length of the cross section is not limited to 100 μm and can be any length (predetermined length). Specifically, in a cross section perpendicular to the surface 1a of the plated steel, a cross section of the plated steel having a length of 100 μm in a direction parallel to the surface 1a is taken as the observation region. The linear distance L between one end 13a and the other end 13b of the boundary line 13 in the observation region is ０ and the length L of the boundary line 13 between one end 13a and the other end 13b is calculated as ((L - L ０ ) / L ０ As shown in equation (1), the value of (LL ０ ) / L ０ When ×100 is 2.0% or more, the adhesion of the plating layer is improved. The plated steel material of this embodiment may satisfy the following formula (2) or (3) instead of the following formula (1). (LL ０ ) / L ０ There is no particular need for an upper limit of ×100, but if it is too large, the surface smoothness of the plating layer will decrease, so it is preferably 40.0% or less, or alternatively 10.0% or less, or alternatively 9.0% or less.

[0049] (LL ０ ) / L ０ ×100≧2.0(%) …(1) (LL ０ ) / L ０ ×100≧4.0(%) …(2) (LL ０ ) / L ０ × 100 ≧ 6.0 (%) ... (3)

[0050] The observation area is a cross section of the plated steel material of a predetermined length in a direction parallel to the surface 1a of the plated steel material when observed at a magnification of 1000 times or more using an SEM (JEOL "JSM-7000F", acceleration voltage: 15 kV). When the predetermined length is 100 μm, the resolution of the observation area is 2560 pixels or more horizontally and 1920 pixels or more vertically.

[0051] Furthermore, as shown in FIG. 2, when the surface 1a of the plated steel material is not flat in a microscopic region (a region of about 100 μm) of the observation area, it is possible to obtain a flat surface in a wider region of the plated steel material (for example, a region of several mm ２ The direction parallel to the surface in the above-mentioned region (for example, the direction in which the surface of a flat plate extends when a flat plate is placed on it) can be taken as the direction in which the surface 1a extends. Based on this direction, the "cross section perpendicular to the surface 1a" and the "direction parallel to the surface 1a" are specified.

[0052] Furthermore, as shown in FIG. 2, one end 13a and the other end 13b of the boundary line 13 are intersections of the boundary line 13 and a straight line that defines the observation area. ０ is the length of the line connecting one end 13a and the other end 13b. L is the length of the boundary line 13 from one end 13a to the other end 13b. The length L of the boundary line 13 can be measured, for example, using ImageJ, a public domain image processing software. The image obtained by observing the image with a magnification of 1000 times or more using an SEM is used as image data with the above-mentioned resolution or higher. The length of the boundary line 13 is measured using the measurement function of ImageJ for this image data.

[0053] Next, the structure of the plating layer will be described. The proportions of the phases and structures contained in the plating layer of this embodiment affect the red rust resistance, base metal corrosion resistance, and sacrificial corrosion resistance of the plated steel material. Even plating layers with the same component composition can have different properties due to the phases or structures contained in the metal structure depending on the manufacturing method. The metal structure of the plating layer can be easily confirmed by mirror-finishing a cross section perpendicular to the surface of the plated steel material and analyzing the cross section with an electron probe microanalyzer equipped with scanning electron microscope functions (SEM-EPMA, EPMA measuring device: JEOL JXA-8230, acceleration voltage: 15 kV, current: 0.05 μA, irradiation time: 50 ms). The thickness of the plating layer of this embodiment is approximately 10 to 300 μm. With the SEM, the magnification is set to 200 to 5000 times, and the field of view is 36,000 μm. ２ In this embodiment, since the SEM field of view for the plating layer may be a local field of view, 25 points of view are selected from the above cross section to obtain average information on the plating layer. That is, a total of 25 × 36000 μm ２ The metallographic structure in the field of view is observed to determine the area ratio of the phase or structure that constitutes the metallographic structure of the plating layer. If necessary, measurements may be taken on multiple cross sections. It is preferable that the cross sections be near the center of the flat surface of the object to be measured (sample).

[0054] To confirm each phase, elemental analysis by EPMA is performed to confirm the composition of the phase by point analysis, and phases having equivalent components are read from element mapping to identify each phase. In point analysis, the elements Al, Zn, Mg, Fe, and Si are analyzed at 250 pixel x 250 pixel lattice points in the EPMA analysis results. The electron beam diameter is 1 μm or less. Phases having equivalent components can be distinguished by identifying phases with approximately the same composition by element mapping. That is, to confirm each phase, a phase showing a lamellar structure [Al / MgZn ２For regions other than the binary eutectic structure, the phase composition is confirmed by point analysis in SEM-EPMA analysis, and phases with approximately the same components are identified by element mapping or the like. The area of ​​each phase is measured using image analysis software "Image J (Ver. 1.54f)." The percentage of the total area of ​​each phase relative to the area of ​​the entire observation field is taken as the area ratio (%) of each phase.

[0055] The area ratio (area rate) of each phase in the observation field corresponds to the volume rate of the phase in the first region or the second region.

[0056] As already described, the plating layer of this embodiment includes a first region having an Fe concentration of less than 5.0 mass % and a second region having an Fe concentration of 5.0 mass % or more. The phases and structures contained in either or both of the first and second regions will be described below.

[0057] Al-containing phase The Al-containing phase of this embodiment is a phase containing Zn and 20 to 99 mass% Al. The Zn content in the Al-containing phase may be 1 to 80 mass%. The Al-containing phase may also contain 5 mass% or less of other elements in total. Here, the other elements include Mg, Si, etc. The Al-containing phase may be in a form in which Zn is solid-dissolved in Al, or may be an aggregate of a fine Zn phase with a grain size of less than 1 μm and a fine Al phase with a grain size of less than 1 μm. The Al-containing phase can be clearly distinguished from other phases and structures by identifying the phase containing Zn and 20 to 99 mass% Al by elemental mapping using EPMA. In this embodiment, the term [Al / MgZn ２ The Al contained in the binary eutectic structure is not included in the Al-containing phase.

[0058] The Al-containing phase is contained mainly in the first region. A plating layer containing a large amount of the Al-containing phase in the first region has excellent red rust resistance.

[0059] Mg—Zn Phase The Mg—Zn phase is a phase containing Zn and 20 to 60 mass% of Mg. The Zn content in the Mg—Zn phase may be 40 to 80 mass%. The Mg—Zn phase may also contain other elements in a total amount of 5 mass% or less. The other elements here include Mg, Si, etc. Specific examples of the Mg—Zn phase include MgZn２ Phase and Mg ２ Zn １１ The Mg-Zn phase is a phase containing Zn and 20 to 60 mass % of Mg, and can be identified by elemental mapping using EPMA to identify an Al-containing phase, [Al / MgZn ２ It can be clearly distinguished from other binary eutectic structures.

[0060] The Mg—Zn phase is contained in the first region. The Mg—Zn phase may also be contained in the second region. The inclusion of the Mg—Zn phase in the plating layer improves red rust resistance and corrosion resistance of the base steel, and further improves sacrificial corrosion resistance.

[0061] [Al / MgZn ２ Binary eutectic structure] [Al / MgZn ２ The binary eutectic structure of the Al phase and MgZn ２ It is a eutectic structure consisting of Al phase and MgZn phase. ２ It shows a lamellar structure consisting of Al / MgZn phase ２ In the backscattered electron image of the SEM, the binary eutectic structure of Al / MgZn is clearly distinguishable from the Al-containing phase, Mg-Zn phase, and Fe-Al phase contained as the main phase of the plating layer. ２ The structure other than the binary eutectic structure of Al / MgZn shows a lamellar structure. ２ This can be done for the remaining parts excluding the binary eutectic structure.

[0062] [Al / MgZn ２ The binary eutectic structure of [Al / MgZn] may be contained in the first region. ２ By allowing a certain amount of the binary eutectic structure to exist, the sacrificial corrosion protection can be improved.

[0063] Fe—Al alloy phase The Fe—Al alloy phase is a phase containing Al and 20 to 60 mass% of Fe. The Al content in the Fe—Al alloy phase may be 40 to 80 mass%. The Fe—Al alloy phase may also contain other elements in a total amount of 5 mass% or less. Here, the other elements include Si, etc. The Fe—Al alloy phase is a phase containing Al ５ The Fe-Al alloy phase is mainly composed of Al.５ In addition to Fe, AlFe, Al ３ Fe, Al ５ Fe ２ In addition, when the coating layer contains Si, the Fe-Al alloy phase may become an Fe-Al-Si compound phase. The identified Fe-Al-Si compound phase is the AlFeSi phase, and its isomers include α-, β-, q1-, and q2-AlFeSi phases.

[0064] The Fe—Al alloy phase is mainly contained in the second region. The Fe—Al alloy phase has a certain degree of corrosion resistance against Fe, and because the Fe—Al alloy phase is an intermetallic compound phase, it has high insulating properties and corrosion resistance.

[0065] The plating layer may contain other intermetallic compounds as the balance. Examples of the other intermetallic compounds include Mg ２ Si phase, Mg ２ Examples of the phase include an Sn phase, a Zn phase, an Al—Ca—Zn phase, an Al—Ca—Si—Zn phase, a Ca—Zn phase, and an Mg—Al—Si—Zn phase.

[0066] Mg ２ Si phase When Si is contained in the plating layer, Si is ２ It may precipitate as a Si phase. ２ The Si phase has excellent corrosion resistance. ２ The presence of Si can be measured by SEM / EPMA, which will be described later. A phase that satisfies Mg: 50 to 70 at % and Si: 30 to 50 at % is considered to be Mg. ２ It can be determined to be a Si phase.

[0067] Mg ２ Sn phase Mg in the plating layer ２ The inclusion of the Sn phase further improves the red rust resistance and base steel corrosion resistance of the plated steel material. ２ Since the Sn phase is present in a small amount, its presence can be confirmed by X-ray diffraction measurement. ２ In order to contain the Sn phase, it is preferable that the Sn content in the chemical composition of the plating layer is 0.02 to 2.0%. ２The presence of the Sn phase is determined by X-ray diffraction measurement using Cu-Kα radiation at an X-ray output of 50 kV and 300 mA, with the measurement range of 2θ = 10 to 30° and the scan step of 0.02°. When a diffraction peak is detected at 23.4±0.3°, the presence of the Sn phase is determined. ２ It is determined that a Sn phase is present.

[0068] Zn Phase The Zn phase contains more than 80% by mass of Zn, may contain less than 20% by mass of Al, and may contain other elements such as Si and Mg in total of 5% by mass or less. The Zn phase may be contained in the first region. The inclusion of the Zn phase improves sacrificial corrosion protection. The Zn phase appears white in a backscattered electron image of an SEM, and can therefore be clearly distinguished from other phases and structures.

[0069] Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, Mg-Al-Si-Zn phase When Ca or Si is contained in the plating bath, these intermetallic compound phases may precipitate in the plating layer. These intermetallic compound phases are the same as the above-mentioned Mg ２ The presence of these intermetallic compound phases can be confirmed by X-ray diffraction measurement in addition to SEM and EPMA measurements. ２ The same as in the case of the Sn phase may be used.

[0070] As already described, the plating layer of this embodiment includes a first region having an Fe concentration of less than 5.0 mass % and a second region having an Fe concentration of 5.0 mass % or more. The boundary between the first region and the second region is determined as follows.

[0071] First, a cross section perpendicular to the surface of the plated steel material is exposed. The exposed cross section is mirror-finished. The Fe concentration is measured by point analysis using an electron probe microanalyzer (EPMA). The analysis results used to identify each phase of the plating layer described above can be used as the analysis results using EPMA.

[0072] First, as shown in FIG. 3 , in the first-stage measurement, multiple linear analysis lines are set in the cross section of the plating layer 12, parallel to the direction perpendicular to the thickness direction of the plating layer 12. Ten analysis lines are set at equal intervals in the thickness direction of the plating layer. Note that when the thickness of the plating layer exceeds 100 μm, the analysis lines are spaced 10 μm apart. Ten measurement points are set on the analysis lines at 10 μm intervals in the direction perpendicular to the thickness direction. If the length of the analysis line is less than 90 μm and it is not possible to set 10 measurement points at 10 μm intervals, the intervals may be narrowed or the number of measurement points may be reduced as appropriate. In FIG. 3 , the dashed lines represent the analysis lines, and the black dots on the analysis lines represent the measurement points. Note that the analysis lines are set to maximize the number of analysis lines in the cross section of the plating layer 12. If the thickness of the plating layer 12 is 10 μm or less, the first-stage measurement is omitted and the second-stage measurement described below is performed.

[0073] At each measurement point, the Fe concentration (mass%) in the plating layer is measured by point analysis. The output of the electron beam during point analysis is 15 kV, 4 × 10 －７ The spot diameter of the electron beam tip is 0.2 μm. For each analysis line, the average value of the Fe concentration at 10 measurement points is calculated, and this average value is used as the Fe concentration for that analysis line. Analysis line A, which has an Fe concentration less than 5.0% and is closest to 5.0%, and analysis line B, which has an Fe concentration greater than 5.0% and is closest to 5.0%, are then identified.

[0074] Next, as a second-stage measurement, the Fe concentration is analyzed in the region between analysis line A and analysis line B on the cross section of the plating layer 12, as shown in Figures 4 and 5. Figure 4 is a diagram showing analysis lines A and B among the multiple analysis lines. Figure 5 is an enlarged view of region M between analysis line A and analysis line B in Figure 4.

[0075] In the second-stage measurement, as shown in Figures 4 and 5, multiple analysis lines are set at 1 µm intervals in the thickness direction of the plating layer in the region between analysis line A and analysis line B. Ten measurement points are set at 10 µm intervals on the analysis lines. As an example, Figures 4 and 5 show only some of the analysis lines set between analysis line A and analysis line B. Then, at each measurement point, the Fe concentration (mass %) is measured by point analysis. The output of the electron beam and the spot diameter of the tip of the electron beam during point analysis are the same as in the first-stage measurement.

[0076] For each analysis line, the average value of the Fe concentration at 10 measurement points is calculated, and this average value is designated as the Fe concentration for that analysis line. The analysis line with the Fe concentration closest to 5.0% is then identified as the "analysis line F with an Fe concentration of 5.0%" (e.g., analysis line F in FIG. 5). Using the "analysis line F with an Fe concentration of 5.0% by mass" as the boundary, the region closer to the surface 1a of the plated steel material (the analysis line A side) than the analysis line F is designated as the first region 12A, and the region closer to the base steel material 11 than the analysis line F (the analysis line B side) is designated as the second region 12B.

[0077] 4 and 5 , the first region 12A is the region between the surface 1a of the plated steel material and the analysis line F of the 5.0 mass% Fe concentration, and the second region 12B is the region between the analysis line F of the 5.0 mass% Fe concentration and the interface 13 between the plated layer 12 and the base steel material 11. The first region 12A and the second region 12B are formed in layers throughout the plated layer 12. The first region 12A is located closer to the surface 1a of the plated steel material 1 than the second region 12B in the plated layer 12. The second region 12B is located closer to the base steel material 11 than the first region 12A in the plated layer 12, adjacent to the base steel material 11.

[0078] <First Region> The first region is disposed on the surface side of the plated steel material and is a region containing less than 5.0 mass% Fe. The upper surface of the first region constitutes the surface of the plated steel material. The presence of the first region improves the red rust resistance, base steel corrosion resistance, and sacrificial corrosion resistance of the plated steel material, and particularly improves red rust resistance in the early stages of corrosion.

[0079] The first region contains an Al-containing phase at an area ratio of 5% or more. The area ratio of the Al-containing phase may be 10% or more, 15% or more, or 20% or more. It is preferably 15% or more. The more the Al-containing phase, the more Al phase is contained in the first region. The Al phase contained in the plating layer has a passivated surface. The presence of a large amount of such Al phase slows the corrosion rate of the first region in the early stages of corrosion, allowing the first region to remain for a long time. Furthermore, since the Fe concentration in the first region is low, at less than 5.0 mass%, almost no red rust occurs during corrosion. The presence of such a first region on the surface side of the plating layer significantly improves the red rust resistance of the plated steel material. The upper limit of the area ratio of the Al-containing phase in the first region may be, for example, 60% or less.

[0080] The first region may contain an Mg—Zn phase in an area ratio of 15% or more. The Mg—Zn phase has superior sacrificial corrosion protection properties compared to the Al-containing phase. Therefore, from the viewpoint of sacrificial corrosion protection, it is preferable that the first region contains as much Mg—Zn phase as possible. This improves the sacrificial corrosion protection properties of the plated steel material. There is no particular need to limit the upper limit of the Mg—Zn phase in the first region, but for example, 90% or less is preferable.

[0081] The first region is made of [Al / MgZn ２ The area ratio of the binary eutectic structure of Al / MgZn may be 5% or more. ２ The presence of the [Al / MgZn binary eutectic structure] in the first region further enhances the sacrificial corrosion protection of the plated steel material. ２ The upper limit of the binary eutectic structure is not particularly limited, but is preferably 80% or less, and may be 20% or less, for example.

[0082] The first region contains the remainder Mg ２ Si phase, Mg ２ The first region may contain a Sn phase, a Zn phase, an Al—Ca—Zn phase, an Al—Ca—Si—Zn phase, a Ca—Zn phase, an Mg—Al—Si—Zn phase, etc. Furthermore, the first region may contain an Fe—Al alloy phase at an area ratio of 3% or less.

[0083] The thickness of the first region is 5 to 100 μm. If the thickness of the first region is less than 5 μm, red rust resistance decreases. On the other hand, since it may be difficult to make the thickness of the first region exceed 100 μm in manufacturing, the upper limit is set to 100 μm or less. The thickness of the first region may be 70 μm or less, 50 μm or less, or 40 μm or less. The thickness of the first region is defined as the average thickness. The average thickness is defined as the arithmetic mean value of the distance in the thickness direction of the plating layer from the surface of the plating layer to the analysis line F, which indicates an Fe concentration of 5.0%, measured at 10 points spaced 10 μm or more apart in the image obtained by the SEM observation described above.

[0084] <Second Region> The second region is disposed between the first region and the base steel and contains 5.0 mass% or more of Fe. Because the second region contains 5.0 mass% or more of Fe, it contains a large amount of Fe-Al phase. Of the phases or structures contained in the second region, the Fe-Al alloy phase occupies the largest area ratio. Furthermore, the second region may contain an Mg-Zn phase in an amount substantially equal to that of the Fe-Al alloy layer. The presence of the second region containing 5.0 mass% or more of Fe in the coating layer improves the base steel corrosion protection of the coating layer. The reason for the improved base steel corrosion protection is presumed to be that the second region contains 5.0 mass% or more of Fe, resulting in a large amount of Fe-Al alloy phase, which functions as a barrier layer for the base steel.

[0085] Furthermore, as described above, the second region may contain an Mg—Zn phase in addition to the Fe—Al phase. The area ratio of the Mg—Zn phase in the second region may be 15% or more. This further improves the sacrificial protection. The Mg—Zn phase has higher sacrificial protection than the Fe—Al phase. Therefore, from the viewpoint of sacrificial protection, it is preferable that the second region contains as much Mg—Zn phase as possible. MgZn in the second region ２ The upper limit of the phase does not need to be particularly limited, but it is preferably, for example, 50% or less.

[0086] The second region contains, as the balance, an Al-containing phase, Mg ２ Si phase, Mg ２It may contain a Sn phase, a Zn phase, an Al—Ca—Zn phase, an Al—Ca—Si—Zn phase, a Ca—Zn phase, an Mg—Al—Si—Zn phase, etc. The area ratio of these remaining phases is preferably 10% or less in total.

[0087] The thickness of the second region is set to 5 to 200 μm. By setting the thickness of the second region to 5 μm or more, the corrosion protection of the base steel is significantly improved. It is more preferably set to 20 μm or more. However, since it may be difficult to make the thickness of the second region exceed 200 μm in manufacturing, the upper limit is set to 200 μm or less. The thickness of the second region may be set to 70 μm or less, 50 μm or less, or 40 μm or less. Furthermore, the thickness of the second region may be set to 10 μm or more, 20 μm or more, or 30 μm or more. The thickness of the second region is set to the average thickness. The average thickness is determined by measuring the distance in the thickness direction of the plating layer from the analysis line F, where the Fe concentration is 5.0%, to the boundary line between the plating layer and the base steel at 10 points spaced 10 μm or more apart in the image obtained by the SEM observation described above, and taking the arithmetic mean value of the measured values.

[0088] The thickness of the plating layer is the total thickness of the first and second regions, i.e., the thickness of the plating layer is preferably 10 μm to 300 μm.

[0089] In the plated steel material of this embodiment, the base steel material and the second region are preferably in direct contact with each other, i.e., it is preferable that no interfacial alloy layer separate from the second region exists between the base steel material and the second region.

[0090] Next, a method for manufacturing a plated steel material according to this embodiment will be described. The method for manufacturing a plated steel material according to this embodiment includes a blasting step in which the surface of a base steel material is shot blasted, a flux application step in which flux is applied to the base steel material after the blasting step, a plating step in which the base steel material after the flux application step is immersed in a plating bath and then withdrawn, and a cooling step in which the plating layer deposited by the plating step is cooled. The plating method in the plating step is a so-called hot-dip plating method.

[0091] In the blasting process, shot blasting is performed on the surface of the base steel material. This imparts strain to the surface of the base steel material. The conditions for the shot blasting are described below. While various shapes and materials are possible for the shot material, steel shot material conforming to JIS Z 0311:2004 is preferred, with spherical shot being particularly preferred. The center particle size of the shot material should be in the range of 40 to 450 μm. The hardness should be Hv 390 to 510. Specifically, for example, steel shot (TSH-30) manufactured by WINOA IKK JAPAN Co., Ltd. can be used.

[0092] In the blasting process, shots are collided with the surface of the base steel material by centrifugal force or air pressure in accordance with the General Rules for Blasting Treatment Methods for Surface Conditioning (JIS Z 0310:2016). The shot amount is 5 to 400 kg / m. ２ The range of the shot amount is 10 kg / m ２ or more, 15 kg / m ２ The shot amount is 100 kg / m or more. ２ It may be less than 50 kg / m ２ The following is also acceptable.

[0093] The blasting process imparts strain to the surface of the base steel. When hot-dip plating is performed on the strained base steel, the reaction between the base steel and Al in the plating bath becomes more active when the base steel is immersed in the plating bath, resulting in the formation of a relatively large amount of Fe-Al alloy phase. In addition, the impartation of strain creates a distribution in the reaction rate between the base steel and Al, which makes the interface between the plating layer and the base steel more likely to become uneven, resulting in a plated steel that satisfies the relationship of formula (1) above. This improves the adhesion of the plating layer.

[0094] Next, in the flux application process, the base steel material is immersed in a flux solution at 80° C. for 30 seconds, and then pulled out and dried in an air atmosphere at 150° C. The flux is, for example, ZnCl ２ Based on NaCl, KCl, NaF, SnCl ２ , SnCl ４、 BiCl ３A solution containing various salts, surfactants, etc., dissolved therein, and acidified with hydrochloric acid as necessary, is used. By applying flux to the base steel before plating, oxides on the surface of the base steel are removed and the plating reaction is stabilized. Examples of fluxes include 0 to 100 g / L of NaCl, 0 to 100 g / L of KCl, and SnCl. ２ is 0 to 20 g / L, ZnCl ２ An example of the flux is a flux dissolved in water to a concentration of 100 to 300 g / L.

[0095] Next, in the plating process, the base steel material after application of the flux is immersed in a plating bath and then removed. The composition of the plating bath may be substantially the same as the chemical composition of the plating layer. The composition of the plating bath may also be appropriately adjusted so that it falls within the range of the chemical composition of the plating layer of this embodiment. Specifically, it is preferable to increase the amount of Mg, Al, and other alloying elements other than Zn by approximately 1.01 to 1.20 times the target value of the chemical composition of the plating layer so that it falls within the range of the chemical composition of the plating layer of this embodiment.

[0096] The immersion time in the coating bath is preferably in the range of 5 to 300 seconds, for example. This promotes the growth of the second region. If the immersion time is less than 5 seconds, the second region of the coating layer is not sufficiently formed, and the interface between the coating layer and the base steel is less likely to become uneven, resulting in a decrease in the corrosion protection of the base steel and a decrease in the adhesion of the coating layer. On the other hand, if the immersion time exceeds 300 seconds, the second region grows excessively, making the coating layer itself more susceptible to cracking.

[0097] The temperature of the plating bath is preferably in the range of 400°C to 680°C, and may be 460°C to 670°C. If the bath temperature is high, the growth of the Fe-Al alloy phase will be accelerated, resulting in the formation of an excessive amount of the Fe-Al alloy phase, which may result in the formation of another interfacial alloy layer between the second region and the base steel material, and will also result in severe wear on the plating equipment. If the bath temperature is low, plating defects such as non-plating and adhesion of foreign matter will be more likely to occur.

[0098] After removal from the plating bath, N ２ The coating weight is adjusted by the gas spray wiping and the speed at which the base steel is withdrawn from the plating bath.

[0099] Next, in the cooling step, the coating layer is cooled (controlled cooling). In the cooling step, the temperature of the coating layer is cooled from the bath temperature to the controlled cooling stop temperature at an average cooling rate of 5.0°C / sec or more, more preferably 10.0°C / sec or more. There is no particular upper limit to the average cooling rate, but it can be, for example, 15.0°C / sec. The controlled cooling stop temperature is preferably 330°C or less. Cooling is performed, for example, by spraying a cooling gas. When cooling by spraying a cooling gas, a plurality of spray nozzles for the cooling gas may be arranged along the conveying path of the base steel material, and the cooling gas may be sprayed from the nozzles. The type of cooling gas may be air, nitrogen (N ２ ), argon, etc., and preferably nitrogen (N ２ ) gas.

[0100] By cooling, the alloying reaction between Fe and Al is suppressed in the surface layer of the coating layer, and a first region having an Fe concentration of less than 5.0 mass% is formed. Furthermore, by setting the average cooling rate to 5.0°C / sec or more, precipitation of the Al-containing phase occurs preferentially, and the area ratio of the Al-containing phase in the first region is increased. Furthermore, by cooling from the bath temperature to the controlled cooling stop temperature at an average cooling rate of 5.0°C / sec or more, precipitation of the Mg-Zn phase proceeds following the Al-containing phase. Furthermore, the Al phase and MgZn ２ The eutectic reaction of the phases progresses, and the Al phase and MgZn ２ Binary eutectic structure with phase ([Al / MgZn ２ The formation of a binary eutectic structure (combined eutectic structure) is promoted.

[0101] In the cooling step, the controlled cooling stop temperature is preferably set to not more than 330° C. If cooling is stopped at a temperature higher than 330° C. at an average cooling rate of 5.0° C. / sec or more, alloying of the coating layer and the base steel material may progress, and the content of the Al-containing phase in the first region may fall outside the range of the present invention.

[0102] On the other hand, on the base steel side of the coating layer, a reaction between Fe diffused from the base steel and Al in the coating bath progresses, forming a second region with an Fe concentration of 5.0% or more. The second region contains a large amount of Fe—Al alloy phase, and may also contain an Mg—Zn phase.

[0103] In this manner, the plated steel material of this embodiment can be manufactured.

[0104] Furthermore, the plated steel material of this embodiment may be formed by a plating film formation method such as vapor deposition plating, thermal spraying, or cold spraying, and the same effects as when formed by hot-dip plating can be obtained.

[0105] The effects of the present invention will be specifically described below with reference to examples. Base steel materials (SS400 as defined in JIS G 3101) having the following shapes A, B and C were used.

[0106] A: Steel plate with a length of 200 mm, width of 100 mm, and thickness of 3.2 mm. B: An angle bar (length 200 mm, short side length 50 mm, thickness 3.2 mm) with an L-shaped cross section, obtained by bending the steel plate A described above so that the bending angle is 90° at the center of the plate width. C: Expanded metal (XS-62 (thickness 3.2 mm) specified in JIS G 3351:1987). In the case of expanded metal, the "surface of the plated steel material" refers to the surface that corresponds to the plane perpendicular to the plate thickness direction in the overall shape of the plate having a plurality of through holes.

[0107] In the case of the above-mentioned materials B and C, it was difficult to observe and measure the cross-sectional structure and evaluate the plating adhesion. Therefore, in the test examples in which the base steel was material B or material C, plated steel was separately produced by changing the base steel to material A, and observation, measurement, and evaluation were carried out on the plated steel using material A as the base steel.

[0108] A blasting process was carried out on the above-mentioned base steel material. In the blasting process, steel shot (TSH-30) manufactured by WINOA IKK JAPAN Co., Ltd. was used as the shot material, and the shot material was collided with the surface of the base steel material by air pressure in accordance with the General Rules for Blasting Treatment Methods for Surface Preparation (JIS Z 0310:2016). The amount of shot material projected was as shown in Tables 2A and 2B. The projection conditions were a discharge pressure of 0.5 MPa, a distance from the nozzle to the sample (base steel material) of 800 mm, and a projection angle of 90°.

[0109] Next, the base steel material after the blasting process was subjected to a flux application process. In this process, a flux solution (ZnCl２ / NaCl / SnCl ２ After immersion in a solution of 220 g / 20 g / 10 g / L for 30 seconds, the sample was removed and thoroughly dried in a drying oven at 150°C.

[0110] Next, the steel material after the flux application step was immersed in a plating bath and then pulled out. The plating bath temperature and immersion time were as shown in Tables 2A and 2B.

[0111] Next, in the cooling step, compressed air was blown as a cooling gas onto the base steel material pulled out of the coating bath, and the base steel material was cooled while controlling the cooling rate from the coating bath temperature to the controlled cooling stop temperature. The cooling rate and controlled cooling stop temperature were as shown in Tables 2A and 2B. The base steel material was allowed to cool in the temperature range below the controlled cooling stop temperature. In this way, a coated steel material was produced.

[0112] The composition of the plating layer was measured by quantitatively analyzing the elements dissolved in a hydrochloric acid solution by the above-described method using ICP atomic emission spectrometry for a sample cut into a size of 30 mm x 30 mm.

[0113] Whether or not the boundary between the plating layer and the base steel material is an uneven surface that satisfies formula (1) was confirmed as follows.

[0114] First, a small sample piece measuring 20 mm x 15 mm x 3.2 mm was taken from the plated steel material, embedded in resin, and then polished to a mirror finish to expose a cross section perpendicular to the surface of the plated steel material. This test piece was observed with a field emission scanning electron microscope (SEM) (JEOL "JSM-7000F", acceleration voltage: 15 kV) to obtain image data. The observation area was a 100 μm long cross section of the plated steel material parallel to the surface of the plated steel material when observed with the SEM at a magnification of 1000 times or more. The image resolution was 2560 pixels horizontally or more and 1920 pixels vertically or more.

[0115] The image data is subjected to the L ０ The coating layer and the base steel were distinguished based on the Fe concentration of 90% as determined by the EPMA analysis described above, and the portion with an Fe concentration of 90% or more was determined as the base steel. ０It was evaluated whether the relationship between and L satisfied the formula (1). The calculation results (values) of the left side of the formula (1) are shown in Tables 3A and 3B.

[0116] The boundary between the first and second regions of the plating layer was determined as follows: A small sample piece measuring 20 mm × 15 mm × 3.2 mm was taken from the plated steel material, embedded in resin, and then polished to a mirror finish to expose a cross section perpendicular to the surface of the plated steel material. The Fe concentration was measured by point analysis while observing this test piece with an electron probe microanalyzer equipped with scanning electron microscope functions (SEM-EPMA, EPMA measurement device: JEOL JXA-8230, acceleration voltage: 15 kV, current: 0.05 μA, irradiation time: 50 ms).

[0117] First, in the first stage of measurement, as shown in Figure 3, linear analysis lines parallel to the direction perpendicular to the thickness direction of the plating layer were set on the cross section of the exposed plating layer. Ten analysis lines were set at equal intervals along the thickness direction of the plating layer. Ten measurement points were set on the analysis lines at 10 µm intervals in the direction perpendicular to the thickness direction of the plating layer. In Figure 3, the dashed dotted line is the analysis line, and the black dots on the analysis lines are the measurement points.

[0118] The Fe concentration (mass%) in the plating layer was measured by point analysis at the measurement points of each analysis line. The measurement conditions for the point analysis were as described above. For each analysis line, the average value of the Fe concentrations at 10 measurement points was calculated, and this average value was used as the Fe concentration for that analysis line. Analysis line A, which had an Fe concentration less than 5.0% and was closest to 5.0%, and analysis line B, which had an Fe concentration greater than 5.0% and was closest to 5.0%, were identified.

[0119] Next, in the second stage of measurement, the analysis line closest to an Fe concentration of 5.0% was identified by the method described above. The analysis line with an Fe concentration of 5.0% by mass was used as a boundary line, and the region closer to the surface of the plated steel material than the analysis line was designated as the first region, and the region closer to the base steel material than the analysis line was designated as the second region.

[0120] Next, the area ratios of the phases and structures in the first and second regions were determined as follows. The samples used to determine the first and second regions were used as they were, and a cross section in the thickness direction of the plating layer perpendicular to the surface of the base steel was observed. The observation magnification with the SEM was 200 to 5000 times, and a 36000 μm ２ The cross section of the plating layer in the region was confirmed. Since the SEM field of view for the plating layer may observe a local field of view, 25 fields of view were selected from any cross section to obtain average information on the plating layer. That is, a total of 36,000 × 25 μm ２ The metal structure in each field of view was observed to determine the area ratio of the phase or structure that constitutes the metal structure of the plating layer.

[0121] First, the Al phase and MgZn ２ A eutectic structure consisting of phases and exhibiting a lamellar structure is called [Al / MgZn ２ The binary eutectic structure was [Al / MgZn ２ In the region other than the binary eutectic structure, a phase containing Zn and 20 to 99 mass% Al was defined as an Al-containing phase. A phase containing Zn and 20 to 60 mass% Mg was defined as an Mg—Zn phase. A phase containing Al and 20 to 60 mass% Fe was defined as an Fe—Al alloy phase. The area fractions of these phases and structures were then calculated. In the second region, the phases other than the Mg—Zn phase were Fe—Al phases, except for the remaining phase of 10% or less.

[0122] Furthermore, by the above-mentioned method, Mg ２ The presence or absence of the Sn phase was confirmed. When a diffraction peak was detected at 23.4±0.3°, it was determined that the Mg ２ It was determined that a Sn phase was present. The results are shown in Tables 3A and 3B.

[0123] The coating adhesion was evaluated as follows. The coated steel material was subjected to a chipping test. Using a gravelometer, 400 g of crushed stone No. 6 (JIS A500, particle size 5-13 mm) was struck against the test piece at room temperature from a distance of 50 cm with an air pressure of 400 kPa. The projection angle was 90° relative to the surface of the base steel material. The test piece was then subjected to ultrasonic cleaning (room temperature, pure water, 10 minutes), and the weight per unit area (g / m) of the test piece before ultrasonic cleaning was measured. ２) and the weight per unit area of ​​the test piece after ultrasonic cleaning (g / m ２ The plating adhesion was evaluated from the difference (weight loss) between the measured value and the test piece. The evaluation criteria were as follows, with AAA, AA, and A being considered acceptable.

[0124] AAA: Weight loss is 10 g / m ２ Less than AA: Weight loss is 10 g / m ２ 60g / m or more ２ Less than A: Weight loss is 60 g / m ２ 120g / m or more ２ Less than B: Weight loss is 120 g / m ２ End

[0125] Red rust resistance was evaluated as follows. Plated steel materials were subjected to an accelerated corrosion test specified in JASO-CCT-M609. The number of cycles until red rust appeared was then measured. The evaluation criteria were as follows, with AAA, AA, and A being considered pass.

[0126] AAA: Red rust occurs over 1200 cycles. AA: Red rust occurs over 900 cycles and less than 1200 cycles. A: Red rust occurs over 540 cycles and less than 900 cycles. B: Red rust occurs less than 540 cycles.

[0127] The corrosion protection of the base steel was evaluated as follows. Plated steel was cut into samples measuring 150 mm in length and 70 mm in width, and subjected to an accelerated corrosion test specified in JASO-CCT-M609. The corrosion depth (μm) of the sample after 1,560 cycles was measured and evaluated. For the measurement, a 30 mm cross section was taken from a position halfway along the length of the sample (75 mm) and at the center in the width direction, and observed. For the observation, the sample was embedded in resin and mirror-polished, and the field of view was photographed using an optical microscope at 40x magnification. The maximum corrosion depth (μm) was measured at 30 mm in the width direction of the cross section. The evaluation criteria were as follows, with AAA, AA, and A being considered pass.

[0128] AAA: Less than 50 μm AA: 50 μm or more and less than 200 μm A: 200 μm or more and less than 800 μm B: 800 μm or more

[0129] The sacrificial corrosion protection was evaluated as follows. The plated steel material was cut using a fine cutter in a direction perpendicular to the surface of the plated steel material to expose the cut end surface. That is, the cross section of the plating layer and the cross section of the base steel material were exposed at the cut end surface. The cut end surface was subjected to a neutral salt spray test as specified in JIS Z2371:2015, and the time (h) until red rust appeared at the cut end surface was measured. The evaluation criteria were as follows, with AAA, AA, and A being considered pass.

[0130] AAA: 2400 hours or more AA: 1500 hours or more but less than 2400 hours A: 720 hours or more but less than 1500 hours B: Less than 720 hours

[0131] As shown in Tables 1A to 4B, the chemical composition of the plating layer, the thickness of the first region, the area ratio of the Al-containing phase in the first region, and the thickness of the second region were within the ranges of the present invention in Examples 1 to 32. As a result, the plating layer had excellent adhesion, red rust resistance, corrosion resistance against the base steel, and sacrificial corrosion resistance.

[0132] In addition, the first region of Examples 1 to 32 contained the remainder Mg ２ Si phase, Mg ２ The first region of some examples contained one or more of the following phases: Sn phase, Zn phase, Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, and Mg-Al-Si-Zn phase. Furthermore, the first region of some examples contained an Fe-Al alloy phase at an area ratio of 3% or less. Furthermore, the second region of examples 1 to 32 contained an Fe-Al alloy phase as the balance. Furthermore, the balance of the second region contained Mg ２ Si phase, Mg ２ There were cases where one or more of a Sn phase, a Zn phase, an Al—Ca—Zn phase, an Al—Ca—Si—Zn phase, a Ca—Zn phase, and an Mg—Al—Si—Zn phase were contained.

[0133] On the other hand, as shown in Tables 1A to 4B, in Comparative Examples 33 to 43, any one of the chemical composition of the coating layer, the thickness of the first region, the area ratio of the Al-containing phase in the first region, and the thickness of the second region was outside the range of the present invention, resulting in inferior adhesion of the coating layer, at least one of red rust resistance, corrosion resistance of the base steel, and sacrificial corrosion resistance.

[0134] In Comparative Example 33, the Al content of the plating layer was insufficient. As a result, the area ratio of the Al-containing phase in the first region was insufficient, resulting in poor red rust resistance and corrosion protection of the base steel. In Comparative Example 34, the Al content of the plating layer was excessive. As a result, the flux reaction was poor and the appearance of the plating layer was significantly deteriorated.

[0135] In Comparative Example 35, the Mg content of the plating layer was insufficient and the Fe content was excessive, resulting in poor red rust resistance and sacrificial corrosion protection. In Comparative Example 36, the Mg content of the plating layer was excessive, resulting in poor flux reaction and a significant deterioration in appearance.

[0136] In Comparative Example 37, the amount of Si in the plating layer was excessive. Furthermore, the amount of Fe was insufficient. This resulted in an insufficient thickness of the second region, resulting in poor corrosion protection of the base steel. In Comparative Example 38, the amount of Ca in the plating layer was excessive. Additionally, the amount of Fe was insufficient. Furthermore, the flux reaction was poor, resulting in a significant deterioration in appearance.

[0137] In Comparative Example 39, the amount of shots projected in the blasting step was insufficient, which did not satisfy formula (1), resulting in poor adhesion of the plating layer.

[0138] In Comparative Example 40, the cooling rate in the cooling step was insufficient, which resulted in the area ratio of the Al-containing phase in the first region being outside the range of the present invention, resulting in poor red rust resistance.

[0139] In Comparative Example 41, the immersion time in the plating bath was insufficient, which resulted in an insufficient thickness of the second region and inferior corrosion resistance of the base steel.

[0140] Shot blasting was not performed in Comparative Example 42. As a result, sufficient strain could not be introduced into the base steel material before plating, and as a result, formula (1) was not satisfied, resulting in poor adhesion of the plating layer.

[0141] In Comparative Example 43, cooling at an average cooling rate of 5.0°C / s or more was completed at 360°C, and slow cooling was performed at a cooling rate of less than 5.0°C / s from 360°C. As a result, the area ratio of the Al-containing phase in the first region fell outside the range of the present invention, and the red rust resistance was inferior.

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] The plated steel material of the present invention has excellent red rust resistance, as well as excellent adhesion of the plating layer, corrosion resistance of the base steel, and sacrificial corrosion resistance. This has the potential for application in the civil engineering and infrastructure fields, as well as in automotive parts.

[0151] 1...plated steel material, 1a...surface, 11...base steel material, 12...plating layer, 12A...first region, 12B...second region, 13...boundary line (interface).

Claims

1. A plated steel product comprising a base steel material and a plating layer disposed on the surface of the base steel material, wherein the plating layer has a chemical composition, in mass %, of Al: 5.0 to 40.0%, Mg: 0.5 to 15.0%, Fe: 5.0 to 40.0%, Si: 0 to 2.0%, and Ca: 0 to 2.0%, and further contains one or two elements selected from the group consisting of Group A and Group B below, with the balance being Zn and impurities, and wherein, in a cross section perpendicular to the surface of the plated steel product, when a cross section of the plated steel product of a predetermined length in a direction parallel to the surface is taken as an observation area, the length L of the boundary line between the plating layer and the base steel product satisfies the following formula (1): The plated steel material has a first region disposed on the surface side of the plated steel material and having an Fe concentration of less than 5.0 mass%, and a second region disposed between the first region and the base steel and having an Fe concentration of 5.0 mass% or more, the first region having a thickness of 5 to 100 μm, the second region having a thickness of 5 to 200 μm, and the first region containing an Al-containing phase containing Zn and 20 to 99 mass% Al at an area ratio of 5% or more. [Group A] Ni: 0 to 1.0%. [Group B] Sb: 0-0.5%, Pb: 0-0.5%, Cu: 0-1.0%, Sn: 0-2.0%, Ti: 0-1.0%, Cr: 0-1.0%, Nb: 0-1.0%, Zr: 0-1.0%, Mn: 0-1.0%, Mo: 0-1.0%, Ag: 0-1.0%, Li: 0-1.0%, La: 0-0.5%, Ce: 0-0.5%, B: 0-0.5%, Y: 0-0.5%, P: 0-0.5%, Sr: 0-0.5%, Co: 0-0.5%, Bi: 0-0.5%, In: 0-0.5%, V: 0-0.5%, W: 0-0.5%. One or more of these is 0-5% in total. (LL ０ ) / L ０ × 100 ≧ 2.0 (%) ... (1) where L in formula (1) ０ is the linear distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end.

2. The plated steel material according to claim 1, which satisfies the following formula (2): (LL ０ ) / L ０ × 100 ≧ 4.0 (%) ... formula (2) where L in formula (2) ０ is the linear distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end.

3. The plated steel material according to claim 1, which satisfies the following formula (3): (LL ０ ) / L ０ × 100 ≧ 6.0 (%) ... formula (3) where L in formula (3) ０ is the linear distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between the one end and the other end.

4. A plated steel material according to any one of claims 1 to 3, wherein the thickness of the first region is 15 μm to 100 μm.

5. The plated steel material according to any one of claims 1 to 3, wherein the area ratio of the Al-containing phase in the first region is 15% or more.

6. The plated steel material according to any one of claims 1 to 3, wherein the first region contains an Mg-Zn phase containing Zn and 20 to 60 mass% Mg, and the area ratio of the Mg-Zn phase in the first region is 15% or more.

7. The plated steel material according to any one of claims 1 to 3, wherein the first region contains a binary eutectic structure of an Al phase and an Mg-Zn phase, and the area ratio of the binary eutectic structure in the first region is 5% or more.

8. The plated steel material according to any one of claims 1 to 3, wherein the thickness of the second region is 15 μm to 200 μm.

9. The plated steel material according to any one of claims 1 to 3, wherein the second region contains an Mg-Zn phase containing Zn and 20 to 60 mass% Mg, and the area ratio of the Mg-Zn phase in the second region is 15% or more.

10. The plated steel material according to any one of claims 1 to 3, wherein the content of Mg relative to the total amount of Zn and Mg (Mg / (Zn+Mg) (%)) in the chemical composition of the plated layer is 5.0% or more.

11. The plated steel material according to any one of claims 1 to 3, wherein the content of Mg relative to the total amount of Zn and Mg (Mg / (Zn+Mg) (%)) in the chemical composition of the plated layer is 6.5% or more.

12. The plating layer contains Sn at a concentration of 0.02 to 2.0% by mass, and the plating layer contains Mg ２ The plated steel material according to any one of claims 1 to 3, which contains an Sn phase.

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