Plated steel material

Abstract

A plated steel material comprising a metal coating layer disposed on the surface of a steel material is used. In the metal coating layer, a lower alloy layer containing 22.5% or more of Fe, an intermediate alloy layer containing 20.0% or more of Fe and 0.5-10.0% of Ni, and an upper plating layer comprising a Zn-based alloy containing 0-5.0% of Fe are disposed in this order from the steel material side, the thickness of the lower alloy layer being 5 μm or more, the thickness of the intermediate alloy layer being 1.0 μm or more, and the total thickness thereof being 6 μm or more. The lower alloy layer comprises Zn: 5.0-30.0%, Al: 30.0-55.0%, Fe: 22.5-50.0%, Si: 0-10.0%, Ni: 0% to less than 0.5%, Mg: 0-1.0%, and the balance: impurities; and the intermediate alloy layer comprises Zn: 5.0-35.0%, Al: 25.0-60.0%, Fe: 20.0-45.0%, Si: 0-10.0%, Ni: 0.5-10.0%, Mg: 0-1.0%, and the balance: impurities.

Description

Plated steel

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

[0002] Steel structures are used in the civil engineering and infrastructure fields. In particular, in areas with severe corrosive environments such as coastal areas and areas where snow-melting salt is used, stainless steel is used as the material for steel structures to prevent corrosion and maintain them for the long term.

[0003] On the other hand, stainless steel materials contain expensive alloying elements such as Cr and Ni, and therefore, the installation of steel structures using stainless steel materials is costly. For this reason, cheaper plated steel sheets (e.g., Zn-Al-Mg plated steel materials) have recently come to be used as the material for steel structures.

[0004] When manufacturing large or pipe-shaped steel structures using plated steel sheets as raw materials, the steel structures must be manufactured by cutting the plated steel sheets into a predetermined shape and joining the cut pieces by welding or the like. However, the plating layer at the welded portion may be lost during welding, which may result in insufficient corrosion resistance of the completed steel structure. Furthermore, the cut edge of the plated steel sheet may remain on the steel structure, which may cause corrosion to occur from the cut edge. Therefore, in order to ensure the corrosion resistance of steel structures, a post-plating process (hot-dip plating process) is performed after the steel structure is manufactured using steel sheets, steel pipes, structural steel, or the like as raw materials.

[0005] As such a post-plating treatment, hot-dip Zn plating has been widely used up to now, but Zn-Al-Mg plating has also come to be used to improve corrosion resistance.

[0006] On the other hand, in post-plating treatments other than hot-dip Zn plating (such as Zn—Al—Mg-based plating treatments), plating components such as Al and Mg can inhibit the reactivity between the plating bath and the flux, causing poor adhesion of the plating layer. For this reason, a two-stage plating method has been adopted, as seen in Patent Document 1 (JP 2010-70810 A) or Patent Document 2 (JP 2002-47548 A), in which a steel material is subjected to hot-dip Zn plating as a first plating stage, and then the steel material is immersed in a Zn—Al—Mg-based plating bath as a second plating stage.

[0007] However, a disadvantage of the two-stage plating method is that the plating layer formed in the first stage plating process is somehow involved in the plating layer formation reaction during the second stage plating process. As a result, the structure of the alloy layer of the plating layer adjacent to the steel material is not a homogeneous structure made of an Al-Fe alloy, but a mixed phase structure of island-like or mesh-like Al-Fe alloy and Zn-Al-Mg alloy. This reduces the barrier effect exerted by the homogeneous Al-Fe alloy structure, and the corrosion resistance of the entire plating layer may be reduced.

[0008] Furthermore, the two-stage plating method requires a step of immersing the steel material twice in two different plating baths, which can result in higher costs than the one-stage plating method.

[0009] Several methods for applying Zn-Al-Mg-based plating by a one-stage method have also been disclosed. For example, Patent Document 3 (JP 2017-66524 A) reveals that the growth of the alloy layer can be controlled by adding Cr. However, adding Cr to a plating bath is difficult, and concentration control can be difficult.

[0010] JP 2010-70810 A JP 2002-47548 A JP 2017-66524 A

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a plated steel material having better corrosion resistance than conventional plated steel materials.

[0012] In order to solve the above problems, the present invention employs the following configuration. [1] A steel material and a metal coating layer disposed on a surface of the steel material, wherein the metal coating layer is composed of, from the steel material side, a lower alloy layer containing 22.5% or more of Fe, an intermediate alloy layer containing 20.0% or more of Fe and 0.5 to 10.0% of Ni, and an upper plating layer made of a Zn-based alloy containing 0 to 5.0% of Fe, arranged in this order, wherein the thickness of the lower alloy layer is 5 μm or more, the thickness of the intermediate alloy layer is 1.0 μm or more, and the total thickness of the lower alloy layer and the intermediate alloy layer is 6 μm or more, and the chemical composition of the lower alloy layer is, in mass %, Zn: 5.0 to 30.0%, Al: 30.0 to 55.0%, Fe: 22.5 to 50.0%, Si: 0 to 10.0%, Ni: 0 to less than 0.5%, Mg: 0 to 1.0%, the balance being impurities, and the chemical composition of the intermediate alloy layer is, in mass %, Zn: 5.0 to 35.0%, Al: 25.0 to 60.0%, Fe: 20.0 to 45.0%, Si: 0 to 10.0%, Ni: 0.5 to 10.0%, Mg: 0 to 1.0%, a total of one or more elements selected from the group consisting of Ca, Sn, Bi, In, Y, La, Ce, Sr, B, P, Cr, Ti, Co, V, Nb, Cu, Mn, Mo, W, Zr, Ag, Li, Na, K, and Ba: 0 to 2.0%, and the balance being impurities.[2] The chemical composition of the upper plating layer is, in mass %, Al: 6.0% to 25.0%, Mg: more than 3.0% to 12.5%, Ni: 0.001% to less than 0.5%, Si: 0% to 1.5%, Ca: 0% to 3.0%, Sn: 0% to 3.0%, Bi: 0% to 1.0%, In: 0% to 1.0%, Y: 0% to 0.5%, La: 0% to 0.5%, Ce: 0% to 0.5%, Sr: 0% to 0.5%, B: 0% to 1.0%, P: 0% to 0.5%, Cr: 0% to 0.25%, Ti: 0% to 0.25%, Co: 0% to 0.25%, V: 0% to 0.25%, The plated steel material according to [1], containing Nb: 0% to 0.25%, Cu: 0% to 1.0%, Mn: 0% to 0.25%, Mo: 0% to 0.25%, W: 0% to 0.25%, Zr: 0% to 0.25%, Ag: 0% to 1.0%, Li: 0% to 0.5%, Na: 0% to 0.05%, K: 0% to 0.05%, Ba: 0% to 0.25%, Fe: 0% to 5.0%, and the balance: more than 50% of Zn and impurities. [3] The plated steel material according to [2], wherein the upper plating layer satisfies the following formulas (1) and (2): Mg / Al≦0.5 (1) 2×Ca+Sr+Y+La+Ce≧0.05 (2) In the formulas (1) and (2), Mg, Al, Ca, Sr, Y, La, and Ce each represent the content (mass %) of each element in the upper plating layer, and 0 is substituted if the element is not contained.

[0013] The present invention provides a Zn-Al-Mg plated steel material with superior corrosion resistance compared to conventional steel materials, thereby enabling the realization of inexpensive steel structures that can exhibit stable corrosion resistance even in harsh corrosion-requiring environments.

[0014] 1 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing an SEM photograph of a cross section of the metal coating layer. 2 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 3 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 4 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 5 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 6 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 7 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 8 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 9 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 10 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 11 is a diagram illustrating a method for measuring the total thickness of the intermediate alloy layer and the lower alloy layer, showing a schematic diagram of a cross section of the metal coating layer. 12 is a cross-sectional SEM photograph of the metal coating layer of Example No. 12. 13 is an elemental mapping analysis result of Fe contained in the metal coating layer of Example No. 12. Elemental mapping analysis results of Ni contained in the metal coating layer of Example No. 12. Elemental mapping analysis results of Zn contained in the metal coating layer of Example No. 12. Elemental mapping analysis results of Al contained in the metal coating layer of Example No. 12. Elemental mapping analysis results of Mg contained in the metal coating layer of Example No. 12.

[0015] Generally, when a Zn-based plating layer is formed by hot-dip plating, the surface of the steel material is treated with a flux to increase the reactivity between the steel material and the plating bath. In a typical flux treatment, an aqueous solution containing zinc chloride and ammonium chloride is applied to the steel material surface to form a flux coating on the steel material surface. Then, the steel material after the flux treatment is immersed in a hot-dip plating bath to obtain a hot-dip plating layer. However, the use of ammonium chloride (NH ４When a Zn-Al-Mg-based coating layer is formed using a flux containing chlorine (Cl), Mg in the coating bath reacts with Cl in the flux to form halides such as magnesium chloride and aluminum chloride. These halides may firmly adhere to the steel surface as flux residue, resulting in the formation of unplated areas. Furthermore, flux residues such as halogen compounds may remain on the coating layer, reducing the corrosion resistance of the coating layer. Therefore, in order to consistently produce a Zn-Al-Mg-based coating layer with excellent corrosion resistance without producing unplated areas, conventionally, plated steel has been produced using a so-called two-stage coating method, in which a hot-dip galvanized layer is first formed on the surface of the steel, and then a Zn-Al-Mg-based coating layer is formed by hot-dip galvanization.

[0016] The present inventors have investigated the production of Zn-Al-Mg plated steel materials with excellent corrosion resistance by a one-stage plating method in order to reduce the burden involved in producing plated steel materials. As a result, they have found that by adding a small amount of Ni to the hot-dip plating bath, it is possible to suppress the occurrence of bare spots and to significantly improve the corrosion resistance of the plating layer compared to conventional methods.

[0017] Hereinafter, a plated steel material according to an embodiment of the present invention will be described. The plated steel material according to this embodiment comprises a steel material and a metal coating layer disposed on the surface of the steel material, the metal coating layer comprising, from the steel material side, a lower alloy layer containing 22.5% or more Fe, an intermediate alloy layer containing 20.0% or more Fe and 0.5 to 10.0% Ni, and an upper plating layer made of a Zn-based alloy containing 0 to 5.0% Fe, arranged in this order, the lower alloy layer is 5 μm or more thick, the intermediate alloy layer is 1.0 μm or more thick, the total thickness of the lower alloy layer and the intermediate alloy layer is 6 μm or more, and the chemical composition of the lower alloy layer is, in mass %, Zn: 5.0 to 30.0%, Al: 30.0 to 55.0%, and Fe: 22.5 to 50.0%. 0%, Si: 0-10.0%, Ni: 0-less than 0.5%, Mg: 0-1.0%, balance: impurities of 1% or less, and the chemical composition of the intermediate alloy layer is, in mass %, Zn: 5.0-35.0%, Al: 25.0-60.0%, Fe: 20.0-45.0%, Si: 0-10.0%, Ni: 0.5-10.0%, Mg: 0-1.0%, a total of one or more elements selected from the group consisting of Ca, Sn, Bi, In, Y, La, Ce, Sr, B, P, Cr, Ti, Co, V, Nb, Cu, Mn, Mo, W, Zr, Ag, Li, Na, K, and Ba: 0-2.0%, balance: impurities.

[0018] Furthermore, in the plated steel material of this embodiment, the chemical composition of the upper plated layer is, in mass %, Al: 6.0% to 25.0%, Mg: more than 3.0% to 12.5%, Ni: 0.001% to less than 0.5%, Si: 0% to 1.5%, Ca: 0% to 3.0%, Sn: 0% to 3.0%, Bi: 0% to 1.0%, In: 0% to 1.0%, Y: 0% to 0.5%, La: 0% to 0.5%, Ce: 0% to 0.5%, Sr: 0% to 0.5%, B: 0% to 1.0%, P: 0% to 0.5%, and Cr: 0% to 0. 0.25%, Ti: 0% to 0.25%, Co: 0% to 0.25%, V: 0% to 0.25%, Nb: 0% to 0.25%, Cu: 0% to 1.0%, Mn: 0% to 0.25%, Mo: 0% to 0.25%, W: 0% to 0.25%, Zr: 0% to 0.25%, Ag: 0% to 1.0%, Li: 0% to 0.5%, Na: 0% to 0.05%, K: 0% to 0.05%, Ba: 0% to 0.25%, Fe: 0% to 5.0%, and the balance: more than 50% Zn and impurities.

[0019] In addition, in the plated steel material of this embodiment, it is preferable that the upper plated layer satisfies the following formulas (1) and (2).

[0020] Mg / Al≦0.5…(1) 2×Ca+Sr+Y+La+Ce≧0.05…(2)

[0021] In the formulas (1) and (2), Mg, Al, Ca, Sr, Y, La, and Ce each represent the content (mass%) of the upper plating layer, and 0 is substituted when the element is not contained.

[0022] The plated steel material of this embodiment includes a steel material and a metal coating layer formed on the surface of the steel material. The metal coating layer includes a lower alloy layer, an intermediate alloy layer, and an upper plating layer, arranged in this order from the steel material side toward the surface side of the plated steel material. A chemical conversion treatment layer may be formed on the upper plating layer of the metal coating layer.

[0023] In the plated steel material of this embodiment, the surface of the steel material is coated with a lower alloy layer mainly composed of an Al-Fe alloy. An intermediate alloy layer containing Al, Fe, and Ni is formed on the lower alloy layer. An upper plating layer with excellent sacrificial corrosion protection is further formed on the intermediate alloy layer. This improves the corrosion resistance of the plated steel material. More specifically, the intermediate alloy layer containing Al, Fe, and Ni suppresses the generation of red rust. Furthermore, the presence of the intermediate alloy layer makes it difficult for the corrosion reaction to progress locally in the depth direction of the metal coating layer, thereby suppressing early corrosion of the steel material. Furthermore, an upper plating layer mainly composed of Zn is formed on the intermediate alloy layer, and this upper plating layer has excellent sacrificial corrosion protection. Furthermore, since the upper plating layer also has excellent chemical conversion treatability, the durability of the steel material can be further improved by chemical conversion treatment.

[0024] The shape of the steel material constituting the plated steel material of this embodiment is not particularly limited, and various shapes can be used, such as a linear shape such as a steel wire, a plate shape such as a steel plate, a mesh shape, a cylindrical shape such as a steel pipe, and a three-dimensional shape such as a rod. For example, the steel material can be used for a wide range of substrates, from small substrates such as bolts, nuts, and power transmission fittings to large substrates such as balustrades, main posts, bridge guardrails, road signs, road card fences, river fences, rockfall prevention nets, and steel pipes.

[0025] The quality of the steel material is not particularly limited. Various types of steel material can be used, such as general steel, Ni-preplated steel, Al-killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing strengthening elements such as Ni and Cr). Furthermore, the conditions for the steel material, such as the manufacturing method for the steel material and the manufacturing method for the steel sheet (hot rolling, pickling, cold rolling, etc.), are not particularly limited. Furthermore, the steel material may be a steel material on which a metal or alloy film of 1 μm or less is formed, such as Zn, Ni, Sn, Bi, or an alloy thereof.

[0026] Next, the lower alloy layer, intermediate alloy layer, and upper plating layer that constitute the metal coating layer will be described in detail.

[0027] In the following description, the "%" used to indicate the content of each element in the chemical composition means "mass %." Furthermore, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. When the numerical values ​​before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit.

[0028] "Corrosion resistance" refers to the corrosion resistance of the metal coating layer itself. Because Zn-based plating layers have a sacrificial corrosion protection effect on steel materials, the upper plating layer corrodes and turns to white rust before the steel material corrodes. After the white-rusted upper plating layer disappears, corrosion progresses in the order of the intermediate alloy layer and the lower alloy layer, and finally the steel material corrodes and turns to red rust. This is the corrosion process of plated steel materials.

[0029] The upper plating layer is a layer mainly composed of a Zn-based alloy formed on the metal coating layer on the surface side of the plated steel material. The upper plating layer is formed by solidifying the plating bath components on the surface of the steel material, and has almost the same component concentrations as the plating bath, except for some elements.

[0030] The lower alloy layer is formed on the surface of the steel material and is a layer that is continuous with the surface of the steel material and mainly contains an Al—Fe alloy.

[0031] The intermediate alloy layer is formed between the lower alloy layer and the upper plating layer and mainly contains an Al-Fe-Ni alloy.

[0032] In the following description, the lower alloy layer and the intermediate alloy layer may be collectively referred to as the alloy layer.

[0033] The Zn-based alloy constituting the upper plating layer may be a Zn-Al-Mg-based alloy in which alloying elements such as Al and Mg are added to Zn. A plating layer made of a Zn-Al-Mg-based alloy has improved corrosion resistance compared to a normal Zn-plated layer. For example, a plating layer made of a Zn-Al-Mg-based alloy has corrosion resistance equivalent to that of a Zn-plated layer even if it is about half the thickness of the Zn-plated layer. Therefore, it is preferable that the upper plating layer be made of a Zn-Al-Mg-based alloy. A Zn-Al-Mg-based alloy is mainly composed of a ternary eutectic structure of a Zn phase, an Al phase, an Mg2Zn phase, and [Zn / Al / MgZn2].

[0034] When the upper plating layer is made of a Zn-Al-Mg alloy, the preferred ranges of component concentrations are as follows:

[0035] In explaining the component concentration ranges of the upper plating layer, the plating bath used to produce the metal coating layer will be described. The composition of the upper plating layer is essentially the same as that of the plating bath, with the exception of some elements. Elements whose composition differs from that of the plating bath include Al and Ni. The Al and Ni contents in the upper plating layer are lower than those in the plating bath. This is because Al and Ni migrate from the plating bath to the alloy layer (intermediate alloy layer and lower alloy layer) between the time the steel material is pulled out of the plating bath and the time the plating bath attached to the steel material solidifies. In addition to the major elements such as Al, Mg, and Zn, elements such as Si, Cr, Mn, and Mo tend to migrate to the alloy layer, and like Al and Ni, the contents of these elements in the upper plating layer are lower than those in the plating bath.

[0036] Al: 6.0% to 25.0% When the plating layer is made of a Zn-Al-Mg alloy, Al exists as an Al phase or a ternary eutectic structure such as [Zn / Al / MgZn2]. Although Al has little sacrificial corrosion protection, the inclusion of Al in the plating layer improves corrosion resistance. Furthermore, Al is added to the plating bath because, without Al, Mg cannot be stably maintained in the plating bath. The preferred Al content of the upper plating layer is 6.0% or more, which is the amount necessary to ensure a certain level of corrosion resistance. If the content is less than this amount, it becomes difficult to ensure the corrosion resistance of the upper plating layer. Furthermore, by setting the Al concentration of the upper plating layer to 6.0% or more, the alloy layer growth is activated, ensuring a stable alloy layer thickness. On the other hand, an excessive Al content causes excessive growth of the lower alloy layer during the manufacturing process, making it difficult to control the growth and increasing the likelihood of defects such as the alloy layer reaching the plating surface and plating discoloration. Therefore, the Al content of the upper plating layer is set to 25.0% or less.

[0037] Mg: over 3.0% to 12.5% ​​Mg is an element that has a sacrificial anticorrosion effect and enhances the corrosion resistance of the upper plating layer. By containing more than a certain amount of Mg, the upper plating layer ２ The higher the Mg content in the top plating layer, the more the MgZn phase is formed. ２ A larger amount of phase is formed. The Mg content is set to more than 3.0% because this is the concentration necessary for exhibiting corrosion resistance; if Mg is 3.0% or less, sufficient corrosion resistance cannot be obtained. On the other hand, an excessive Mg content makes it difficult to manufacture the upper plating layer, so the upper limit is set to 12.5% ​​or less. A more preferred Mg content is more than 5.0% and 12.5%, or may be more than 5.0% and 10.0%, or may be more than 5.0% and 8.0% or less.

[0038] Ni: 0.001% to less than 0.5% Ni is contained in the metal coating layer to form an Al-Fe-Ni alloy, but Ni tends to concentrate in the intermediate alloy layer when the plating bath solidifies. Therefore, the Ni concentration in the upper plating layer is 0.001 to less than 0.5%. Because the Ni concentration in the upper plating layer is thus low, no Al-Fe-Ni alloy is present in the upper plating layer, or even if it is present, the amount is very small.

[0039] Si: 0% to 1.5% Si is a semimetallic element. In the upper plating layer, Si is present in a single Si phase or in a Mg ２ Si is precipitated. Furthermore, when Ca is contained in the plating layer together with Si, an Al-Ca-Si compound is precipitated. This improves the corrosion resistance of the upper plating layer. There is an upper limit to the Si content, and if the upper limit is exceeded, adhesion of dross and the like increases and corrosion resistance tends to deteriorate overall. Furthermore, if excessive Si is contained in the plating bath, the formation of the alloy layer described below is significantly suppressed, so there is an appropriate range for the Si concentration contained in the upper plating layer. Therefore, the Si content is set to 0% or more and 1.5% or less, preferably more than 0% and 1.0% or less.

[0040] Sn: 0% to 3.0% Bi: 0% to 1.0% In: 0% to 1.0% Sn, Bi, and In improve the sacrificial corrosion protection of the upper plating layer. That is, they have the effect of suppressing the formation of red rust from the steel when localized corrosion progresses. However, there is an upper limit to the content of these elements. If the upper limit is exceeded, adhesion of dross and the like increases, and the cost does not justify the performance improvement. Therefore, Sn is set to 0 to 3.0%, more preferably more than 0% and less than 3.0%. Bi is set to 0% to 1.0%, more preferably more than 0% and less than 1.0%. In is set to 0% to 1.0%, more preferably more than 0% and less than 1.0%.

[0041] Ca: 0% to 3.0% Y: 0% to 0.5% La: 0% to 0.5% Ce: 0% to 0.5% Sr: 0% to 0.5% Ca, Y, La, Ce, and Sr are easily oxidized in the atmosphere. When these elements are present in the coating bath, they form a dense oxide film on the bath surface, preventing the oxidation of Mg. This effect stabilizes the Mg concentration and facilitates the production of an upper coating layer with the target composition. To optimally achieve this effect, the content of these elements is set to greater than 0%, more preferably 0.01% or greater. However, there is an upper limit to the content of each element. Exceeding the upper limit tends to make the preparation of the coating bath more difficult. Furthermore, the deposition of dross and other substances tends to increase, and weldability tends to deteriorate. Therefore, the Ca content is set to 0% to 3.0%. The content of each of Y, La, Ce, and Sr is set to 0% to 0.5%, preferably more than 0% and less than 0.5%, and more preferably 0.01% or more and less than 0.5%.

[0042] B: 0% to 1.0% P: 0% to 0.5% B and P are elements that belong to the semi-metallic group and improve the corrosion resistance of the upper coating layer. These elements also generally affect corrosion resistance, but there is an upper limit to the content of each element, and if the upper limit is exceeded, dross and other substances tend to adhere more, resulting in a deterioration in corrosion resistance. Therefore, the contents of B and P are set to 0% to 1.0% and 0% to 0.5%, respectively.

[0043] Cr: 0% to 0.25% Ti: 0% to 0.25% Co: 0% to 0.25% V: 0% to 0.25% Nb: 0% to 0.25% Cu: 0% to 1.0% Mn: 0% to 0.25% Mo: 0% to 0.25% W: 0% to 0.25% Zr: 0% to 0.25% Ag: 0% to 1.0% Li: 0% to 0.5% Na: 0% to 0.05% K: 0% to 0.05% Cr, Ti, Co, V, Nb, Cu, Mn, Mo, W, Zr, Ag, Li, Na, and K are metallic elements that, when incorporated into the upper plating layer, form substitution solid solutions and new high-melting-point intermetallic compounds, improving corrosion resistance. Each element has an upper limit to its content, and exceeding the upper limit tends to increase the adhesion of dross, etc. Therefore, Na and K are each limited to 0% to 0.05%, preferably more than 0% and less than 0.05%. Cr, Ti, Co, V, Nb, Mn, Mo, W, and Zr are each limited to 0% to 0.25%, preferably more than 0% and less than 0.25%. Li is limited to 0% to 0.5%, preferably more than 0% and less than 0.5%. Cu and Ag are each limited to 0% to 1.0%, preferably more than 0% and less than 1.0%.

[0044] Ba: 0% to 0.25% Ba is an element with properties similar to those of Zn. Therefore, the inclusion of Ba does not generally provide any special effects, but it does have effects such as making it easier to form a spangle pattern on the plating's appearance. However, excessive inclusion of Ba may reduce the corrosion resistance of the upper plating layer. Therefore, the Ba content is set to 0% to 0.25%, and preferably to more than 0% and less than 0.25%.

[0045] Fe: 0% to 5.0% Fe may be unavoidably contained in the upper plating layer. This is because Fe may diffuse from the base steel into the upper plating layer during plating production. It has been confirmed that Fe does not affect the corrosion resistance of the upper plating layer if the Fe content is 5.0% or less. Therefore, the Fe content is 0% to 5.0% or less, and may be more than 0% and less than 5.0%.

[0046] The balance: Zn and impurities. Zn is a metal with a low melting point. When the upper plating layer is made of a Zn-Al-Mg alloy, it can form a Zn phase, an Mg2Zn phase, or a ternary eutectic structure of [Zn / Al / MgZn2], or even an Mg2Zn phase. １１ The Zn content is present in an equal amount. Zn is an element necessary for ensuring the corrosion resistance of the upper plating layer and for obtaining sacrificial corrosion protection for the steel material. Zn is the balance, and it is preferably contained in an amount of 50.00% or more, more preferably more than 50.00%. If the Zn content is less than 50.00% or 50.00% or less, the sacrificial corrosion protection properties of the plating layer are reduced. In other words, when corrosion progresses locally, it becomes difficult to suppress the generation of red rust from the steel material. The Zn content is more preferably 65.00% or more, or 70.00% or more. The upper limit of the Zn content is the amount that is the balance excluding elements other than Zn and impurities.

[0047] Impurities in the upper plating layer refer to components contained in the 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 steel material (base steel) and the plating bath.

[0048] In addition, in the plated steel material of this embodiment, it is preferable that the upper plated layer satisfies the following formulas (1) and (2).

[0049] Mg / Al≦0.5…(1) 2×Ca+Sr+Y+La+Ce≧0.05…(2)

[0050] However, Mg, Al, Ca, Sr, Y, La, and Ce in formulas (1) and (2) are the contents (mass%) of the upper plating layer, and if the element is not contained in the upper plating layer, 0 is substituted.

[0051] Mg has the effect of suppressing the alloy reaction of Al--Fe, and by satisfying formula (1), the thickness of the upper plating layer can be ensured.

[0052] Furthermore, Ca, Cr, Y, La, and Ce are easily oxidized in the atmosphere, and when present in the plating bath, they form a dense oxide film on the bath surface, which has the effect of preventing oxidation of Mg. When the contents of these elements satisfy formula (2), the Mg concentration in the upper plating layer is stabilized, and an upper plating layer with the target composition can be formed.

[0053] The average chemical composition of the upper plating layer can be identified by measuring it using ICP optical emission spectroscopy or ICP-MS. Specifically, the upper plating layer is stripped and dissolved using an acid or alkaline solution containing an inhibitor that suppresses corrosion of the alloy layer (intermediate alloy layer, lower alloy layer). More specifically, when stripping and dissolving using acid, an acid solution containing 0.01% by mass of the commercially available inhibitor IBIT No. 700AS added to a 10% by mass aqueous hydrochloric acid solution can be used. When stripping using an alkaline solution, a 5% by mass aqueous NaOH solution can be used. Next, if the resulting solution is an acid solution, it is measured using optical emission spectroscopy or ICP-MS. If the solution is an alkaline solution, it is subjected to acid treatment and then measured using optical emission spectroscopy or ICP-MS.

[0054] The thickness of the upper plating layer can be determined by subtracting the thickness of the alloy layer (described later) from the thickness of the metal coating layer. The thickness of the metal coating layer is derived by observing the cross-sectional structure. Specifically, a cross section perpendicular to the surface of the plated steel material is exposed. The exposed cross section is mirror-finished. In an SEM backscattered electron image of the cross section of the metal coating layer revealed in the cross section, the distance in the thickness direction between the outermost surface of the metal coating layer (the surface of the plated steel material) and the surface of the steel material is measured at 10 points, and the average value is calculated as the thickness of the metal coating layer. It is desirable that the distance between the measurement points is approximately 100 μm. The thickness of the upper plating layer can then be derived by subtracting the thickness of the alloy layer (described later) from the thickness of the metal coating layer.

[0055] It is desirable that the upper coating layer contains as few quasicrystalline phases as possible. When a steel material is immersed in a coating bath containing Zn, Al, and Mg, and cooled immediately after being pulled out, if the cooling rate becomes high, the structure of the coating layer will contain Mg. ３２ (Zn, Al)４９ In some cases, a quasicrystalline phase having a composition similar to the above may appear. Since the quasicrystalline phase is a brittle structure, if the upper plating layer of this embodiment contains a quasicrystalline phase, cracks may occur in the upper plating layer, adversely affecting corrosion resistance. Therefore, from the viewpoint of further improving corrosion resistance, when the upper plating layer contains a quasicrystalline phase, the area ratio of the quasicrystalline phase is preferably 0.5% or less. The area ratio of the quasicrystalline phase may be 0%.

[0056] The area ratio of the quasicrystalline phase can be determined by observing the cross-sectional structure. First, a cross section perpendicular to the surface of the plated steel material is exposed. The exposed cross section is mirror-finished. In the exposed cross section, the measurement area of ​​the upper plating layer is identified. When the thickness of the upper plating layer is 20 μm, the vertical range of the measurement area of ​​the upper plating layer is from the surface of the plated steel material to a depth of 16 μm. Furthermore, the horizontal range of the measurement area of ​​the upper plating layer is a range of 40 μm parallel to the surface of the plated steel material. The number of measurement areas is five.

[0057] Next, an electron probe microanalyzer (EPMA) is used to perform elemental mapping of Mg in the measurement region of the upper plating layer. As a result, regions with an Mg concentration of 19% by mass or more are identified as pure crystalline phases, and the area percentage of the quasicrystalline phase is determined for each of the five measurement regions. The average area percentage of the quasicrystalline phase in the five measurement regions is then taken as the area percentage of the quasicrystalline phase.

[0058] The EPMA measurement conditions are an electron probe microanalyzer JXA-8230 manufactured by JEOL Ltd., an electron beam output of 15 kV, 50 nA, and an irradiation time of 50 milliseconds.

[0059] The quasicrystalline phase is a phase containing quasicrystals. Quasicrystals are crystal structures first discovered by Daniel Schuchtman in 1982, and have an atomic arrangement of a regular icosahedron (icosahedron). This crystal structure is an aperiodic crystal structure with a unique rotational symmetry, e.g., five-fold symmetry, that cannot be obtained in ordinary metals or alloys, and is known as a crystal structure equivalent to aperiodic structures typified by three-dimensional Penrose patterns. To identify this metallic substance, for example, electron beam observation using a TEM is performed to obtain an electron beam diffraction image of a radial regular decagon resulting from the icosahedron structure from the phase.

[0060] The quasicrystalline phase that may be formed by the composition of the plating bath of this embodiment can be simply described as Mg ３２ (Zn, Al) ４９ The phase exhibits diffraction peaks that can be identified by X-ray diffraction using JCPDS card: PDF #00-019-0029 or #00-039-0951. Diffraction peaks are often observed around 36.3 to 8°. However, in this embodiment, it is acceptable to identify the region where the Mg concentration is 19% by mass or more as the pure crystalline phase.

[0061] Next, the alloy layer (lower alloy layer, intermediate alloy layer) will be described. The thickness and form of the alloy layer vary depending on the type of steel used and the manufacturing conditions of the metal coating layer, but it is formed as a layer mainly composed of an Al-Fe alloy or an alloy containing an Al-Fe alloy with substitution elements or solid solution elements such as Ni and Si. The form of the alloy layer can be broadly divided into two types. Specifically, when observing the cross-sectional structure, the alloy layer can be divided into a homogeneous structure in which the Al-Fe alloy is formed as a homogeneous layer, and a mixed-phase structure in which the Al-Fe alloy is formed in an island or network shape and a Zn-based alloy is present between the alloy, as if the plating bath had solidified almost intact.

[0062] The structure of the alloy layer formed when manufactured by a one-stage plating method such as the present embodiment differs from that formed by a two-stage plating method, and this may allow the manufacturing method of the product to be distinguished. This is due to the behavior of the alloy layer formation. In the one-stage plating method, the alloy layer formed by the reaction between the steel material and the plating bath containing Al is formed as a homogeneous layer containing an Al-Fe alloy. On the other hand, when a plated material on which an Fe-Zn alloy layer has already been formed is immersed in a plating bath containing Al, as in the two-stage plating method, the Fe-Zn alloy layer changes to an Al-Fe alloy layer. The change from an Fe-Zn alloy to an Al-Fe alloy is accompanied by volumetric shrinkage, resulting in the formation of an island-like or network-like Al-Fe alloy.

[0063] Differences in the morphology of Al-Fe alloys appear as differences in the average composition of the alloy layer. That is, when the alloy layer composition is measured by elemental mapping analysis using EPMA on a cross-sectional structure, in the case of a homogeneous structure, the composition shows an analytical value close to the Al-Fe alloy composition analyzed by point analysis. On the other hand, when a multiphase structure is subjected to area analysis, since it contains a Zn-based alloy, the result is a higher Zn concentration and a lower Fe concentration compared to a homogeneous structure.

[0064] The lower alloy layer will be described below. The lower alloy layer is formed on the surface of the steel material. Therefore, the region on the surface of the steel material that mainly contains an Al—Fe alloy can be the lower alloy layer.

[0065] Although the thickness and form of the lower alloy layer containing an Al-Fe alloy vary depending on the type of steel used and the manufacturing conditions for the metal coating layer, when manufactured by a one-stage plating method such as that of this embodiment, the lower alloy layer is formed as a homogeneous layer containing an Al-Fe alloy.

[0066] The lower alloy layer is formed on the surface of the steel material and has an Al structure. ５The lower alloy layer may contain an Al-Fe alloy phase as the main phase. The lower alloy layer may have a structure consisting of an Al-Fe alloy that accounts for the largest area or volume percentage. This Al-Fe alloy is formed by mutual atomic diffusion between the base steel (steel material) and the coating bath. However, when a hot-dip coating method is used as the manufacturing method, an Al-Fe alloy is likely to be formed when the steel material is immersed in a coating bath containing Al, but if the coating bath contains Al at a certain concentration or above, the Al-Fe alloy may be easily formed. ５ The Fe2 phase is formed most frequently. However, atomic diffusion takes time, and the Fe concentration is high in some areas near the base steel. Therefore, the Al-Fe alloy contained in the lower alloy layer is partially composed of AlFe phase, Al ３ Fe phase, Al ５ In some cases, the plating bath contains a certain concentration of Zn, and therefore, in the Al-Fe alloy that constitutes the lower alloy layer, Zn is dissolved or a portion of the Al is substituted with Zn. Therefore, the lower alloy layer also contains Zn.

[0067] Furthermore, when Si is contained in the metal coating layer, Si is easily incorporated into the Al-Fe alloy constituting the lower alloy layer, and may form an Al-Fe-Si intermetallic compound phase in the lower alloy layer. Specific intermetallic compound phases include the AlFeSi phase, and isomers include α-, β-, q1-, and q2-AlFeSi phases. Therefore, these AlFeSi phases may be detected in the Al-Fe alloy.

[0068] The lower alloy layer has a chemical composition including Zn: 5.0 to 30.0%, Al: 30.0 to 55.0%, Fe: 22.5 to 50.0%, Si: 0 to 10.0%, Ni: 0 to less than 0.5%, Mg: 0 to 1.0%, and the balance: impurities.

[0069] The Zn in the lower alloy layer is contained in the Al-Fe alloy. While Zn-free Al-Fe alloys form red rust when corroded, Zn-containing Al-Fe alloys do not form red rust, but turn to yellow rust or black rust instead. To prevent the formation of red rust, the lower alloy layer must contain 5.0% or more Zn. Meanwhile, the upper limit of the Zn concentration contained in an Al-Fe alloy is approximately 30.0%, and if the Al-Fe alloy has a homogeneous structure, it is difficult to contain more than this amount. Therefore, the Zn content in the lower alloy layer should be 30.0% or less. Therefore, the Zn contained in the lower alloy layer should be in the range of 5.0 to 30.0%.

[0070] Al in the lower alloy layer is a main element constituting the Al-Fe alloy. The upper and lower limits of the Al concentration are determined by the phase structure of the Al-Fe alloy to be formed, but in this embodiment, it is preferable that the lower alloy layer contains Al in a proportion of 30.0 to 55.0%.

[0071] Fe in the lower alloy layer is a main element constituting the Al-Fe alloy. The upper and lower limits of the Fe concentration are determined by the phase structure of the Al-Fe alloy to be formed, but in this embodiment, the lower alloy layer preferably contains Fe in a proportion of 22.5 to 50.0%.

[0072] The inclusion of Si in the plating bath has the effect of suppressing the alloying reaction between Al and Fe. Therefore, Si may be added to the plating bath to control the alloying reaction between Al and Fe. However, if the Si concentration in the plating bath exceeds approximately 1.5%, the lower alloy layer will hardly grow at bath temperatures below 540°C, the barrier properties of the Al-Fe alloy will not be ensured, and corrosion resistance will be reduced. Furthermore, Si tends to concentrate in Al-Fe alloys or Al-Fe-Ni alloys, and the Si concentration in the lower alloy layer may be higher than that in the upper plating layer. Even so, as long as the Si concentration in the plating bath is 1.5% or less, the Si content in the lower alloy layer will not exceed 10.0%. Therefore, the upper limit of Si in the lower alloy layer is set to 10.0% or less. Note that if the plating bath does not contain Si, the Si content in the lower alloy layer will be 0%. Therefore, the Si content in the lower alloy layer is set to 0-10.0%.

[0073] When plated steel materials are produced by a two-stage plating method using a plating bath containing Si, a mixed-phase structure may be obtained in which an Al-Fe-based alloy and a Zn-based alloy that has solidified almost as is from the plating bath are solidified. However, in such a mixed-phase structure, corrosion progresses through the Zn-based alloy that has solidified almost as is from the plating bath, so that the corrosion resistance of the Al-Fe-based alloy is not utilized and the corrosion resistance of the metal coating layer itself is reduced.

[0074] Ni is contained in the metal coating layer to form an Al-Fe-Ni alloy, but Ni tends to concentrate in the intermediate alloy layer when the plating bath solidifies. Therefore, the Ni concentration in the lower alloy layer is 0 to less than 0.5%. Because the Ni concentration in the lower alloy layer is thus low, no Al-Fe-Ni alloy is present in the lower alloy layer, or even if it is present, the amount is very small.

[0075] The lower alloy layer may contain Mg, but since Mg has a low solid solubility in Al-Fe alloys, if the Al-Fe alloy does not form a mixed phase structure with the solidified structure derived from the plating bath, the Mg content is 1.0% or less. Therefore, the Mg content is set to the range of 0 to 1.0%.

[0076] The remainder of the lower alloy layer is impurities. As with the upper plating layer, the impurities in the lower alloy layer refer to components contained in the raw materials or components mixed in during the manufacturing process. For example, trace amounts of components other than Fe may be mixed into the lower alloy layer as impurities due to mutual atomic diffusion between the steel material (base steel) and the plating bath.

[0077] The lower alloy layer may also contain one or more elements selected from the group consisting of Ca, Sn, Bi, In, Y, La, Ce, Sr, B, P, Cr, Ti, Co, V, Nb, Cu, Mn, Mo, W, Zr, Ag, Li, Na, K, and Ba in a total amount of 0 to 2.0%. These elements are components of the upper plating layer, but small amounts of them may be mixed into the lower alloy layer.

[0078] The average chemical composition of the lower alloy layer can be identified by analyzing a cross section of the lower alloy layer with an electron probe microanalyzer (EPMA) and quantitatively analyzing the elements contained in the region of the lower alloy layer, as will be described in detail later.

[0079] The lower alloy layer needs to have a certain thickness from the viewpoint of corrosion resistance, and in order to ensure sufficient corrosion resistance, the thickness is preferably 5 μm or more, may be 30 μm or more, and more preferably 50 μm or more. The method for measuring the thickness will be described later.

[0080] Next, the intermediate alloy layer will be described. The intermediate alloy layer is disposed between the lower alloy layer and the upper plating layer. That is, the intermediate alloy layer is formed closer to the upper plating layer than the lower alloy layer. The intermediate alloy layer contains an Al-Fe-Ni alloy. The Al-Fe-Ni alloy is an Al-Fe alloy containing Ni. The Al-Fe-Ni alloy is formed by producing a metal coating layer using a plating bath containing Ni.

[0081] When plated steel materials are produced by a two-stage plating method using a plating bath containing Ni, a mixed-phase structure may be obtained in which an Al-Fe-Ni-based alloy and a Zn-based alloy that is solidified almost directly from the plating bath are solidified. However, in such a mixed-phase structure, corrosion progresses through the Zn-based alloy, and the corrosion resistance of the Al-Fe-Ni-based alloy is not utilized, which may result in a decrease in the corrosion resistance of the metal coating layer itself, which is undesirable.

[0082] The intermediate alloy layer may be entirely composed of an Al-Fe-Ni alloy structure, or may contain a structure other than an Al-Fe-Ni alloy structure, in which case it is preferable that the Al-Fe-Ni alloy structure occupies the largest area or volume ratio.

[0083] Although the intermediate alloy layer is similar to the lower alloy layer in that it contains Al and Fe, it can be distinguished from the lower alloy layer by the presence or absence of 0.5% or more Ni. Specifically, this can be distinguished by mapping the regions containing Fe, Al, and Ni using EPMA. Because of its Ni content, the Al-Fe-Ni alloy has higher corrosion resistance than the Al-Fe alloy that constitutes the lower alloy layer. The intermediate alloy layer containing such an Al-Fe-Ni alloy is distributed approximately parallel to the surface of the steel material and exists so as to cover the lower alloy layer, thereby achieving a high barrier effect. Furthermore, because Al-Fe-Ni alloys have poor wettability with flux residue, when an Al-Fe-Ni alloy is formed, the flux residue quickly detaches from the surface of the steel material and floats to the surface of the plating bath. In other words, flux detachment is improved. This suppresses problems such as non-plating and reduced corrosion resistance due to remaining flux residue.

[0084] Al-Fe-Ni alloys are Al containing Ni. ３ Fe phase or Al ５ Fe ２ phase, but Al-Fe-Ni alloys are partially composed of AlFe phase, Al ３ Fe phase, Al ５ In some cases, the alloy contains Fe phases and the like. Furthermore, since the plating bath contains a certain concentration of Zn, the Al-Fe-Ni alloy also contains Zn. Zn may be present in the form of substitution with Al, solid solution, or both. Furthermore, when Si is contained in the metal coating layer, an Al-Fe-Ni-Si intermetallic compound phase may be formed. The intermetallic compound phase identified in this case is the AlFeSi phase, and isomers such as α-, β-, q1-, and q2-AlFeSi phases exist. Therefore, these AlFeSi phases and the like may be detected in Al-Fe-Ni alloys.

[0085] The composition of the Al-Fe-Ni alloy contained in the intermediate alloy layer is preferably Zn: 5.0 to 30.0%, Al: 30 to 60%, Fe: 30 to 50%, Si: 0 to 10%, Ni: 0.5 to 10%, Mg: 0 to 1.0%, and the balance: impurities.

[0086] The intermediate alloy layer has a chemical composition including Zn: 5.0 to 35.0%, Al: 25.0 to 60.0%, Fe: 20.0 to 45.0%, Si: 0 to 10.0%, Ni: 0.5 to 10.0%, Mg: 0 to 1.0%, and the balance: impurities.

[0087] The Zn in the intermediate alloy layer is mainly contained in the Al-Fe-Ni alloy. While Zn-free Al-Fe-Ni alloys form red rust when corroded, Zn-containing Al-Fe-Ni alloys do not form red rust, but turn to yellow rust or black rust instead. To prevent the formation of red rust, the intermediate alloy layer must contain 5.0% or more Zn.

[0088] On the other hand, since the upper limit of the Zn concentration contained in an Al-Fe-Ni alloy is about 30.0%, when the Al-Fe-Ni alloy has a homogeneous structure, the plating bath solidification phase occupying the intermediate alloy layer is not so large, and therefore, the Zn content in the intermediate alloy layer is set to 35.0% or less.

[0089] The Al content in the intermediate alloy layer is a major element constituting the Al-Fe-Ni alloy, and the upper and lower limits of the Al content are determined by the phase structure of the Al-Fe alloy to be formed. In this embodiment, the Al content in the intermediate alloy layer is preferably 25.0 to 60.0%.

[0090] The Fe in the intermediate alloy layer is a main element constituting the Al-Fe-Ni alloy, and the upper and lower limits of the Fe concentration are determined by the phase structure of the Al-Fe-Ni alloy to be formed. In this embodiment, the Fe content in the intermediate alloy layer is preferably 20.0 to 45.0%.

[0091] Like the Si in the Al-Fe-Ni alloy of the lower alloy layer, the Si in the intermediate alloy layer tends to concentrate in the Al-Fe-Ni alloy. Therefore, the Si concentration in the intermediate alloy layer may be higher than that in the upper plating layer. Even so, if the upper limit of the Si concentration in the plating bath is 1.5%, the Si concentration in the intermediate alloy layer will not exceed 10.0%, so the upper limit of Si is set to 10.0%. Note that if the plating bath does not contain Si, the Si content in the intermediate alloy layer will be 0%. Therefore, the Si content in the intermediate alloy layer is set to 0-10.0%.

[0092] Because Ni is concentrated in the intermediate alloy layer when the metal coating layer solidifies, the Ni concentration in the intermediate alloy layer is higher than the Ni concentration in the lower alloy layer and the Ni concentration in the upper plating layer. If the upper plating layer contains 0.5% or more Ni, the corrosion resistance of the upper plating layer deteriorates, so the Ni content added to the plating bath must be less than 3.0%. In this case, it is difficult to achieve a Ni concentration in the intermediate alloy layer of more than 10%, so the Ni concentration in the intermediate alloy layer is 10.0% or less. Furthermore, since a Ni content of 0.5% or more in the intermediate alloy layer allows sufficient formation of an Al-Fe-Ni alloy, the Ni content is set to 0.5% or more.

[0093] The intermediate alloy layer may contain Mg, but the Mg content is limited to 1.0% or less because the solid solubility of Mg in Al-Fe-Ni alloys is low. Therefore, the Mg content is limited to the range of 0 to 1.0%.

[0094] The intermediate alloy layer may contain one or more elements selected from the group consisting of Ca, Sn, Bi, In, Y, La, Ce, Sr, B, P, Cr, Ti, Co, V, Nb, Cu, Mn, Mo, W, Zr, Ag, Li, Na, K, and Ba in a total amount of 0 to 2.0%. These elements are elements that constitute the upper plating layer, but small amounts of them may be mixed into the intermediate alloy layer.

[0095] The remainder of the intermediate alloy layer is impurities. As with the upper plating layer and the lower alloy layer, the impurities in the intermediate alloy layer refer to components contained in the raw materials or components mixed in during the manufacturing process. For example, trace amounts of components other than Fe may be mixed into the intermediate alloy layer as impurities due to mutual atomic diffusion between the steel material (base steel) and the plating bath.

[0096] The intermediate alloy layer preferably has a thickness of 1.0 μm or more. However, since it is difficult to make the thickness of the intermediate alloy layer exceed 40 μm, the thickness of the intermediate alloy layer is set to 1.0 μm or more and 40 μm or less.

[0097] The total thickness of the metal coating layer is preferably 25 to 200 μm. Generally, the thickness of a metal coating layer such as a plated film correlates with corrosion resistance, with a thicker layer providing better corrosion resistance and greater resistance to scratches. On the other hand, the thickness of the plated film is limited by the lifting of the plating bath, and applying a thicker plated film requires a large amount of plating metal, which increases costs. Therefore, in this embodiment, the thickness of the metal coating layer is preferably 200 μm or less. Furthermore, if the thickness of the metal coating layer is less than 25 μm, corrosion resistance decreases, so the thickness of the metal coating layer should be 25 μm or more.

[0098] Various measurement methods will be described below.

[0099] The thickness of the alloy layer (total thickness of the lower alloy layer and the intermediate alloy layer) is determined by determining the boundary between the upper plating layer and the alloy layer from an SEM backscattered electron image of the cross section of the metal coating layer. When determining the boundary between the upper plating layer and the alloy layer, attention is paid to the Al-Fe-Ni alloy and Al-Fe alloy contained in the alloy layer.

[0100] First, a cross section perpendicular to the surface of the plated steel material is exposed. The exposed cross section is mirror-finished. FIG. 1A shows an SEM image of the cross section perpendicular to the surface of the plated steel material. The magnification of the SEM image is 1000 times. Note that FIG. 1A is a backscattered electron image of the SEM. FIG. 1B shows a schematic diagram of FIG. 1A. In FIG. 1B, reference numeral 1 denotes a steel material, reference numeral 2 denotes a metal coating layer, reference numeral 2a denotes the surface of the plated steel material (the surface of the metal coating layer), reference numeral 2b denotes an upper plating layer, reference numeral 2c denotes an alloy layer (an intermediate alloy layer and a lower alloy layer), and reference numeral 3 denotes a cross-sectional contour of the interface between the steel material 1 and the metal coating layer 2. Furthermore, the curved line indicated by reference numeral 4 is the boundary between the dark gray region and the white or light gray region in the SEM photograph, and is a line presumed to be the boundary line between the upper plating layer 2b and the alloy layer 2c.

[0101] 1B, a cross-sectional contour line 3 of the interface between the steel material 1 and the metal coating layer 2 is identified. In this embodiment, the interface between the steel material 1 and the metal coating layer 2 is an uneven surface, and therefore the cross-sectional contour line 3 is a continuous irregular curve.

[0102] Next, as shown in FIG. 1C , the center line CL of the cross-sectional contour 3 of the interface is identified. Specifically, the cross-sectional contour 3 of the interface is defined as a roughness curve, which is a line roughness curve expressed with a cutoff value λc = 0.8 mm. Then, a straight line CL with a length of 40 μm that is approximately parallel to the roughness curve (cross-sectional contour 3) is superimposed on the roughness curve (cross-sectional contour 3). By superimposing the roughness curve (cross-sectional contour 3) and the straight line CL, the cross-sectional contour 3 and the straight line CL have many intersections, resulting in multiple regions surrounded by the cross-sectional contour 3 and the straight line CL, and these multiple regions are located above (the side opposite the steel material side) and below (the steel material side) the straight line CL. The straight line CL is then positioned so that the total area of ​​the region located above the straight line CL is equal to the total area of ​​the region located below the straight line CL. The positioned straight line CL is defined as the center line of the roughness curve of the interface.

[0103] Next, as shown in Fig. 1D, a plurality of first imaginary lines K1, each having a length of 40 µm, are set parallel to the center line CL of the interface roughness curve. The first imaginary lines K1 are set at intervals of 10 µm in the thickness direction of the metal coating layer.

[0104] Next, the overlap length between the Al-Fe-Ni alloy or Al-Fe alloy (hereinafter collectively referred to as the measurement target alloy) and the first imaginary line K1 is measured for each first imaginary line K1. Furthermore, for each first imaginary line K1, the ratio (%) of the overlap length with the measurement target alloy to the total length (40 μm) of the first imaginary line K1 is calculated.

[0105] The length ratio (%) of the first virtual line K1 overlapping the target alloy relative to the total length of the first virtual line K1 is derived by obtaining the brightness distribution on the first virtual line K1 in the SEM backscattered electron image. Phases consisting of Al-Fe-Ni alloys or Al-Fe alloys appear darker than Zn phases and steel and brighter than Al phases and Mg phases, making them easily distinguishable from these phases. The ratio of brightness corresponding to the target alloy on the first virtual line K1 is calculated using image processing software, and this is used as the length ratio of the first virtual line K1 overlapping the target alloy. For example, public domain image processing software such as ImageJ can be used as the image software. Images observed with an SEM at a magnification of 1000x or greater are used as image data, and the measurement function of ImageJ is used to measure the length ratio (%) of the first virtual line K1 overlapping the target alloy relative to the total length of the first virtual line K1. The image processing software is not limited to ImageJ; any software with equivalent functionality to ImageJ may be used.

[0106] The alloys to be measured refer to the Al-Fe-Ni alloy contained in the intermediate alloy layer and the Al-Fe alloy contained in the lower intermediate layer. As described in the description of the intermediate alloy layer and the lower alloy layer, these alloys may contain Zn, Si, Ni, etc.

[0107] Next, as shown in Figure 1E, among the multiple first virtual lines K1, a virtual line K50L that overlaps with the alloy to be measured by less than 50% and a virtual line K50U that overlaps with the alloy to be measured by 50% or more are identified. If there are multiple virtual lines K50L that overlap with the alloy to be measured by less than 50%, the virtual line closest to the steel material is identified. These virtual lines K50L and K50U are used as reference lines.

[0108] 1F, a plurality of second imaginary lines K2, each 40 μm long and parallel to the reference lines K50L and K50U, are set at intervals of 1 μm between the reference lines K50L and K50U. Hereinafter, the reference lines K50L and K50U are included in the second imaginary line K2.

[0109] Next, the overlap length between the alloy to be measured and the second imaginary line K2 is measured for each second imaginary line K2. Furthermore, for each second imaginary line K2, the ratio (%) of the overlap length between the alloy to be measured and the total length of the second imaginary line K2 (40 μm) is calculated. The method for measuring the ratio (%) of the overlap length between the alloy to be measured and the total length of the second imaginary line K2 is the same as for the first imaginary line K1.

[0110] 1G, a second virtual line K50 is identified from among the plurality of second virtual lines K2, the second virtual line K50 having an overlapping ratio with the alloy to be measured of less than 50%, and then a virtual line K50A located closest to the steel material side of the identified second virtual lines K50 is identified.Furthermore, a virtual line K50B located closest to the virtual line K50A on the steel material side of the virtual line K50A is identified.

[0111] As shown in FIG. 1H, the distance t between the imaginary line 50B and the straight line CL is defined as the thickness of the alloy layer.

[0112] The above operation is carried out for five SEM images, and the average value of the thickness of the alloy layer obtained in each SEM image is taken as the total thickness of the alloy layer, that is, the intermediate alloy layer and the lower alloy layer.

[0113] The thickness of the upper plating layer is determined by subtracting the thickness of the alloy layer from the thickness of the metal coating layer.

[0114] The thickness of the intermediate alloy layer is measured from an EPMA image of the metal coating layer in a cross section perpendicular to the surface of the plated steel material. Specifically, using the SEM image used to measure the thickness of the alloy layer, elemental mapping of Ni is performed with an electron probe microanalyzer (EPMA), and regions where Ni is 0.5% or more are extracted by image processing. The measurement conditions for EPMA are the same as those for determining the area ratio of the quasicrystalline phase.

[0115] Next, in the same manner as in the measurement of the thickness of the alloy layer, an imaginary line 40 μm long is drawn parallel to the center line CL of the roughness curve of the steel surface, and the imaginary line that overlaps with the region (Al—Fe—Ni-based alloy) where Ni is 0.5% or more by 50% or more is identified, and from among these, the imaginary line closest to the center line CL of the roughness curve is taken as the boundary line of the intermediate alloy layer. The thickness of the intermediate alloy layer is determined by measuring the distances between the boundary line of the intermediate alloy layer in the SEM images of five fields of view and the imaginary line 50B identified in Figures 1B to 1G and averaging the distances.

[0116] The composition of the intermediate alloy layer is determined by drawing perpendicular lines from both ends of the boundary line of the intermediate alloy layer in the surface thickness direction in five fields of view, and analyzing the elements in the area surrounded by the imaginary line 50B using EPMA, and deriving the average of the results to determine the composition of the intermediate alloy layer. The measurement conditions for EPMA are the same as those for determining the area ratio of the quasicrystalline phase.

[0117] The thickness of the lower alloy layer is calculated by dividing the thickness of the intermediate alloy layer by the thickness of the alloy layer. The composition of the lower alloy layer is determined by drawing a perpendicular line from the center line CL of the roughness curve of the steel surface to the boundary line of the intermediate alloy layer, and the elements in the lower alloy layer are analyzed by EPMA. The average of the results is used to determine the composition of the lower alloy layer. The measurement conditions for EPMA are the same as those for determining the area ratio of the quasicrystalline phase.

[0118] A method for confirming the morphology of the intermediate alloy layer will now be described. Whether the metal structure of the intermediate alloy layer is a mixed phase structure or a homogeneous structure can be determined from the components by area analysis. That is, in the case of a mixed phase structure, the Fe concentration is less than 20%, which is lower than that of a homogeneous structure, so the determination can be made based on the Fe concentration.

[0119] Next, it is desirable that the metal coating layer of the plated steel material of this embodiment contains as few halides as possible. Specifically, it is desirable that the fluorescent X-ray intensity of halogen elements in the metal coating layer measured by fluorescent X-ray analysis be in the range of 0 to 0.6 kcps.

[0120] As described below, the plated steel material of this embodiment is subjected to a flux treatment on the surface of the steel material, and then the steel material is immersed in a plating bath to form a metal coating layer. However, if the plating bath contains 5% or more of Al and Mg in total, significant flux residue adhesion may occur. If excessive flux residue remains on the metal coating layer, it may cause bare spots and reduce the corrosion resistance of the metal coating layer. Possible flux residues include magnesium chloride and aluminum chloride.

[0121] Therefore, in this embodiment, as a measure to reduce the flux residue that causes non-plating, NH ４ A flux that does not contain Cl is selected. This reduces the likelihood of flux residue remaining on the metal coating layer, suppressing the occurrence of bare spots and preventing a decrease in the corrosion resistance of the metal coating layer. The flux residue contained in the metal coating layer can be evaluated by the fluorescent X-ray intensity of the halogen elements in the metal coating layer. If the fluorescent X-ray intensity of the halogen elements in the metal coating layer measured by fluorescent X-ray analysis is in the range of 0 to 0.6 kcps, the amount of flux residue will be extremely small, reducing the possibility of adversely affecting corrosion resistance.

[0122] The fluorescent X-ray intensity of halogen elements in the metal coating layer is measured as follows. Fluorescent X-ray analysis is performed on the surface of the plated steel material (surface of the metal coating layer) using a RIGAKU ZSX Prism III+ analyzer. The analysis range is within a circle with a diameter of 30 mm. Measurements are performed with top irradiation using an EZ-scan program (target: Rh, 30 kV-80 mA, analyzing crystal: Ge, detector: proportional counter, measurement peak: 92.8°, detector speed: 10 deg / min).

[0123] Next, a method for producing a plated steel material according to an embodiment of the present invention will be described. The plated steel material according to this embodiment is produced by cleaning the surface of the steel material by degreasing and pickling, and then by flux treatment, followed by an immersion-type hot-dip galvanizing method (batch-type hot-dip galvanizing method).

[0124] There are no particular restrictions on the size, shape, surface configuration, etc. of the steel material to be plated. Regular steel, high-tensile steel, stainless steel, and other steel materials are all applicable. More specifically, various steel materials are applicable, such as general steel, Ni-preplated steel, Al-killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing strengthening elements such as Ni and Cr). Steel strips of general structural steel are most preferred.

[0125] The steel material may be subjected to a surface finishing process in advance using shot blasting, abrasive brushes, etc., and the surface may be plated with Ni plating, Zn plating, Sn plating, etc. at a concentration of 3 g / m ２ There is no problem if plating is performed after the following metal or alloy film is attached.

[0126] As a pretreatment of the steel material, the steel material is thoroughly cleaned by degreasing and pickling. There are no particular restrictions on the conditions for degreasing and pickling.

[0127] Next, the surface of the cleaned steel material is subjected to a flux treatment. For example, the steel material is immersed in an aqueous flux solution at about 60°C for 20 seconds or more, then pulled out and dried. The flux used is ZnCl ２ Based on NaCl, NaF, KCl, SnCl ２ , SnCl ４、 BiCl ３ A solution containing various salts such as ammonium hydroxide, surfactants, etc., and acidified with hydrochloric acid as necessary is used. By applying the flux to the steel material before plating, a metal coating layer in which Ni is segregated in the intermediate alloy layer is obtained.

[0128] The flux is NH ４ It is preferable to use a flux that does not contain Cl, since it does not generate flux residue. For example, the flux may contain 5 to 14 mass % of NaCl and 5 to 14 mass % of BiCl. ３ 1.2 to 5 mass %, KCl 7 to 18 mass %, and the balance ZnCl ２ Another example is a flux in which the flux components having the following composition are dissolved in water at a concentration of 150 to 300 g / L and the pH is adjusted to 1.5 or less. ２ 65 to 85%, (b) NaF, KF, MgF ２ , Na２ SiF ６ (c) one or more chlorides of alkali metal elements or alkaline earth metal elements in a total amount of 5 to 25%; and (d) one or more chlorides of Sn, In, Tl, Sb, and Bi in a total amount of more than 5% but not more than 20%.

[0129] The above-mentioned flux reduces the risk of flux residue remaining on the steel material or plating surface when plated steel is produced by a one-stage plating method, thereby enabling plated steel to have sufficient corrosion resistance.

[0130] Next, the steel material after flux treatment is immersed in a plating bath prepared with a predetermined composition. The composition of the plating bath may be approximately the same as the target composition of the upper plating layer. However, since Al, Ni, and other constituent elements of the plating bath are concentrated in the lower alloy layer and the intermediate alloy layer, the composition of the plating bath may be based on the target composition of the upper plating layer, with the amounts of Al and Ni increased from the target composition of the upper plating layer. For example, the Al concentration of the plating bath should be increased by approximately 1.5 to 5 mass% relative to the Al concentration (mass%) of the upper plating layer, and the Ni concentration of the plating bath should be 5 to 20 times the Ni concentration (mass%) of the upper plating layer.

[0131] Specifically, to obtain the alloy layer of the present invention, the Al concentration of the plating bath components must be in the range of 2.0 to 84.98 mass%, the Zn concentration must be in the range of 15.0% to 97.98 mass%, and the Ni concentration must be in the range of 0.02 to less than 3.0 mass%. Furthermore, to form an upper plating layer in the preferred ranges, the plating bath components preferably have an Al concentration in the range of 7.5 to 30.0 mass%, a Zn concentration in the range of more than 50% to less than 89.48 mass%, a Ni concentration in the range of 0.02 to less than 3.0 mass%, and a Mg concentration in the range of more than 3.0% to 12.5 mass%.

[0132] The temperature of the plating bath is set to 440°C to 600°C, preferably 460°C to 520°C. A high bath temperature results in faster growth of the lower alloy layer, resulting in superior productivity, but there is a risk of the Al-Fe reaction becoming uncontrollable. A low bath temperature is more likely to result in plating defects such as bare spots, particularly when the Mg concentration is high. Furthermore, the Al-Fe reaction is suppressed, preventing the formation of an alloy layer of appropriate thickness. Therefore, the bath temperature of the plating bath is set to 440°C to 600°C. Note that a higher bath temperature may increase the Fe concentration in the upper plating layer, but at 600°C or below, the Fe concentration in the upper plating layer is usually kept below 5.00%.

[0133] The immersion time of the steel material in the plating bath is 50 to 1,000 seconds, preferably 100 to 600 seconds. The immersion time is directly related to productivity, but it goes without saying that, due to the nature of flux-type immersion plating, it cannot be strictly determined. The bath temperature and immersion time are selected taking into account the heat capacity of the object to be plated and balancing productivity and quality, and cannot be determined uniquely.

[0134] When plating steel materials of a specified shape, such as bolts and screws, the material is removed from the plating bath and centrifuged to adjust the amount of molten metal attached and adjust the metal coating layer to a specified thickness.

[0135] After adjusting the thickness of the metal coating layer, the molten metal is immediately cooled. The cooling method for solidifying the molten metal is natural cooling or air cooling. The average cooling rate from the start to the end of cooling is adjusted as follows depending on the temperature of the plating bath.

[0136] When the temperature of the plating bath exceeds 500°C, the average cooling rate from the bath temperature to 500°C is set to 4 to 6°C / s, and the average cooling rate from 500 to 300°C is set to 2 to 5°C / s. When the temperature of the plating bath is 500°C or less, the average cooling rate from the bath temperature to 300°C is set to 2 to 5°C / s.

[0137] When the temperature of the plating bath exceeds 500°C, if the average cooling rate from the bath temperature to 500°C exceeds 6°C / s, the area ratio of the quasicrystalline phase may increase, and an alloy layer having the desired component concentrations may not be obtained. Also, if the average cooling rate from the bath temperature to 500°C is less than 4°C / s, the Al-Fe reaction may not be suppressed, and an alloy layer having the desired component concentrations may not be obtained.

[0138] Furthermore, if the average cooling rate between 500 and 300°C exceeds 5°C / s, an alloy layer having the desired component concentrations may not be obtained. If the average cooling rate between 500 and 300°C is less than 2°C / s, the Al-Fe reaction is not suppressed, and an alloy layer having the desired component concentrations may not be obtained.

[0139] When the temperature of the plating bath is 500°C or less, if the average cooling rate from the bath temperature to 300°C exceeds 5°C / s, an alloy layer having the desired component concentrations may not be obtained. Also, if the average cooling rate from the bath temperature to 300°C is less than 2°C / s, the Al-Fe reaction is not suppressed, and an alloy layer having the desired component concentrations may not be obtained.

[0140] The plated steel material may be cooled naturally or by air to a temperature of 300°C or less, and then water-cooled. When water-cooling is performed, it is desirable to submerge the upper plated layer after it has been solidified, from the viewpoint of preventing surface irregularities of the metal-coated multilayer.

[0141] In this manner, the plated steel material of this embodiment is manufactured.

[0142] After cooling the plating layer, various chemical conversion treatments and painting treatments may be performed. In addition, to further improve corrosion resistance, welded parts, processed parts, etc. may be subjected to touch-up paint application or thermal spraying treatment.

[0143] In the plated steel material of this embodiment, a coating may be formed on the plating layer. The coating may be one layer or two or more layers. Examples of types of coatings that may be formed directly on the plating layer include chromate coatings, phosphate coatings, and chromate-free coatings. The chromate treatment, phosphate treatment, and chromate-free treatment for forming these coatings can be performed by known methods. However, since many chromate treatments can deteriorate the weldability of the plating layer surface, it is preferable that the thickness be less than 1 μm in order to fully utilize the weldability improvement effect in the plating layer.

[0144] Chromate treatments include electrolytic chromate treatments that form a chromate film by electrolysis, reactive chromate treatments that form a film by utilizing a reaction with the material and then wash away excess treatment solution, and paint-on chromate treatments that apply a treatment solution to the substrate and dry it without rinsing with water to form a film. Any of these treatments may be used.

[0145] Examples of electrolytic chromate treatments include electrolytic chromate treatments using chromic acid, silica sol, resins (phosphoric acid, acrylic resins, vinyl ester resins, vinyl acetate acrylic emulsions, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resins, etc.), and hard silica.

[0146] Examples of the phosphate treatment include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

[0147] Chromate-free treatments are particularly suitable because they do not place a burden on the environment. Chromate-free treatments include electrolytic chromate-free treatments that form a chromate-free film by electrolysis, reactive chromate-free treatments that form a film by utilizing a reaction with the material and then wash away excess treatment liquid, and paint-on chromate-free treatments that apply a treatment liquid to the substrate and dry it without rinsing with water to form a film. Any of these treatments may be used.

[0148] Furthermore, one or more organic resin coatings may be provided on the coating directly on the plating layer. The organic resin is not limited to a specific type, and examples thereof include polyester resins, polyurethane resins, epoxy resins, acrylic resins, polyolefin resins, and modified versions of these resins. Here, the term "modified version" refers to a resin obtained by reacting a reactive functional group contained in the structure of these resins with another compound (such as a monomer or a crosslinking agent) containing a functional group capable of reacting with the functional group.

[0149] Such organic resins may be a mixture of one or more unmodified organic resins, or a mixture of one or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin. The organic resin film may also contain any coloring pigment or anti-rust pigment. Water-based organic resins obtained by dissolving or dispersing them in water may also be used.

[0150] Examples of the present invention will be described below. The original plate of the plated steel material was cut into a size of 200 mm x 100 mm from a hot-rolled steel sheet with a thickness of 1.6 mm. All of the plates were SS400 (general steel with black scale). After cleaning the surface with a commercially available alkaline degreasing agent, the steel sheet was pickled with 10% hydrochloric acid to remove surface scale. After pickling, the steel sheet was washed with hot water at 60°C and then washed with flux (ZnCl ２ / NaCl / KCl / SnCl ２ The steel sheets were immersed in a galvanizing bath (mixture of 200 (g / L) / 20 (g / L) / 40 (g / L) / 6 (g / L), pH = 1.0) for approximately 1 minute, and then heated and dried in an air atmosphere at 200°C for 5 minutes in a heating furnace. The steel sheets were immersed in the galvanizing bath for 60 to 900 seconds to be coated, then pulled out and allowed to cool naturally. After the coating had completely solidified, they were water-cooled. When the bath temperature was above 500°C, the average cooling rate between the bath temperature and 500°C was 4 to 8°C / s, and between 500 and 300°C was 2 to 5°C / s. When the bath temperature was 500°C or less, the average cooling rate between the bath temperature and 300°C was 2 to 5°C. In this manner, plated steel sheets Nos. 1 to 31 and 37 were produced by the single-stage coating method.

[0151] For plated steel materials Nos. 32 to 36, the base steel sheets were subjected to surface cleaning and pickling in the same manner as described above, followed by flux treatment in the same manner as described above, and then immersed in a zinc plating bath to form a 50 μm-thick zinc plating layer on the surface of the steel material. The steel sheets with the formed zinc plating layer were then immersed in the plating bath for 60 to 120 seconds to be plated in the same manner as described above, then pulled out, allowed to cool naturally, and water-cooled after the plating had completely solidified. In this way, various plated steel materials were produced by the two-stage plating method. The conditions for natural cooling were as shown in the table.

[0152] The plating bath compositions were almost the same as those of the upper plating layer shown in Tables 1A and 1B, but the Al concentration of the plating bath was increased by approximately 0 to 5 mass% relative to the Al concentration (mass%) of the plating layer, and the Ni concentration of the plating bath was 5 to 20 times the Ni concentration (mass%) of the upper plating layer. However, plating baths Nos. 25 to 28, 31, and 37 were excluded from the preferred concentrations.

[0153] (Measurement of Chemical Composition and Thickness of Upper Plating Layer) The composition of the upper plating layer was measured by ICP atomic emission spectroscopy. Specifically, the test material was immersed in a 5% NaOH aqueous solution containing an inhibitor to dissolve the upper plating layer, obtaining a solution in which the upper plating layer was peeled and dissolved. The end point of the alkaline dissolution was determined as the point at which foaming from the surface of the immersed test material stopped and 90% or more of the surface turned black due to the exposure of the alloy layer. The obtained solution was acid-treated by adding HCl, and then its composition was analyzed by atomic emission spectroscopy.

[0154] The thickness of the upper plating layer was calculated by subtracting the thickness of the alloy layer (described later) from the thickness of the metal coating layer. The thickness of the metal coating layer was calculated by observing the cross-sectional structure. Specifically, in an SEM backscattered electron image of the cross section of the metal coating layer, the distance between the outermost surface of the metal coating layer and the steel material was measured at 10 points, and the average value was calculated as the thickness of the metal coating layer. The distance between the measurement points was approximately 100 μm. The thickness of the upper plating layer was then calculated by subtracting the thickness of the alloy layer (described later) from the thickness of the metal coating layer.

[0155] (Measurement of chemical composition and thickness of alloy layer) The thickness of the alloy layer (total thickness of the lower alloy layer and the middle alloy layer) was measured as described above. However, since No. 26 had a region that did not contain Al, the region containing 5% or more of Fe was measured as the alloy layer.

[0156] The thickness of the intermediate alloy layer was measured as described above. However, in No. 26, since the alloy layer did not contain Al and was formed of an Fe-Zn alloy, the ζ phase was measured as the intermediate alloy layer and the γ phase as the lower alloy layer. The composition of the intermediate alloy layer was measured as described above.

[0157] The thickness of the lower alloy layer was calculated by dividing the thickness of the intermediate alloy layer by the thickness of the alloy layer, and the composition of the lower alloy layer was measured as described above.

[0158] The method for confirming the morphology of the intermediate alloy layer will now be described. Whether the metal structure of the intermediate alloy layer is a mixed-phase structure or a homogeneous structure was determined based on the components by elemental mapping analysis using EPMA. That is, in the case of a mixed-phase structure, the Fe concentration is less than 20%, which is lower than that of a homogeneous structure, so the determination was based on the Fe concentration.

[0159] The area fraction of the quasicrystalline phase in the upper plating layer and the chlorine (Cl) content in the metal coating layer were measured as described above.

[0160] (Corrosion Resistance Evaluation) The test material was cut into a size of 120 x 50 mm, and a test was conducted in accordance with JASO M609 to check the occurrence of red rust. Red rust was considered to have occurred when the area ratio of red rust was 5% or more of the test area. The corrosion resistance was evaluated as follows: "B" was considered a failure, and "A" to "S" were considered passing.

[0161] Red rust was observed in less than 450 cycles: "B" Red rust was observed in 450 to 600 cycles: "A" Red rust was observed in 600 to 750 cycles: "AA" Red rust was observed in 750 to 900 cycles: "AAA" No red rust was observed in 900 cycles: "S"

[0162] As shown in Tables 1A to 4B, Nos. 1 to 24, 29, and 30 (all examples) had chemical compositions of the intermediate alloy layer and lower alloy layer and thicknesses of each layer within the ranges of the present invention, and exhibited good corrosion resistance. The cross-sectional SEM photographs of the metal coating layer of No. 12 and the results of area analysis of each element are shown in Figures 2 to 7. In particular, as shown in Figure 4, it was confirmed that Ni was concentrated in the region where the intermediate alloy layer was present.

[0163] In Nos. 25 to 28 and 31, the plating bath composition was inappropriate, so the chemical compositions of the intermediate alloy layer and lower alloy layer or the thickness of each layer were outside the range of the present invention, resulting in inferior corrosion resistance.

[0164] That is, in No. 25, the Si composition of the plating bath was set to 2.5%, but because this was a relatively high composition, the growth of the middle alloy layer and the lower alloy layer was suppressed, resulting in a decrease in the thickness of these alloy layers and a decrease in corrosion resistance.

[0165] In No. 26, the Al composition of the plating bath was set to 0.1%, but because the composition was relatively low, the Fe-Al alloy was not sufficiently formed, and the Al content in the intermediate alloy layer and the lower alloy layer was insufficient, resulting in a decrease in corrosion resistance.

[0166] In No. 27, the Ni content of the plating bath was set to 3.0%, which resulted in an excessive Ni concentration in the plating bath, which in turn resulted in an excessive Ni content in the intermediate alloy layer, resulting in a decrease in corrosion resistance.

[0167] In No. 28, the Ni content of the plating bath was set to 0%, so no intermediate alloy layer was formed, resulting in a decrease in corrosion resistance.

[0168] In No. 31, the Zn content in the plating bath was insufficient, resulting in an insufficient amount of Zn in the intermediate alloy layer and the lower alloy layer, resulting in a decrease in corrosion resistance.

[0169] In No. 37, the manufacturing conditions were inappropriate, and a quasicrystalline phase was precipitated in the upper plating layer at a volume ratio of 10%, the chemical composition range of the alloy layer was outside the scope of the present invention, and the corrosion resistance was reduced.

[0170] In each of Nos. 32 and 34 to 36, plated steel materials were produced by a two-stage plating method, but the compositions of the intermediate alloy layer and the lower alloy layer did not satisfy the ranges of the present invention, resulting in reduced corrosion resistance. Furthermore, in Nos. 32 and 34 to 36, the metal structure of the intermediate alloy layer was a mesh-like mixed phase.

[0171] In No. 33, the plated steel material was produced by the two-stage plating method, but the composition of the plating bath was inappropriate, so an intermediate alloy layer was not formed.

[0172]

[0173]

[0174]

[0175]

[0176]

[0177]

[0178]

[0179]

[0180] The present invention can provide a Zn-Al-Mg plated steel material with excellent corrosion resistance, which has industrial applicability in that it can realize inexpensive steel structures that can exhibit stable corrosion resistance even in harsh corrosion environments.

[0181] 1...steel material, 2...metal coating layer, 2a...surface of plated steel material (surface of metal coating layer), 2b...upper plating layer, 2c...alloy layer

Claims

1. A steel material and a metal coating layer disposed on the surface of the steel material, wherein the metal coating layer is composed of, from the steel material side, a lower alloy layer containing 22.5% or more Fe, an intermediate alloy layer containing 20.0% or more Fe and 0.5 to 10.0% Ni, and an upper plating layer made of a Zn-based alloy containing 0 to 5.0% Fe, arranged in this order; the thickness of the lower alloy layer is 5 μm or more, the thickness of the intermediate alloy layer is 1.0 μm or more, and the total thickness of the lower alloy layer and the intermediate alloy layer is 6 μm or more; and the chemical composition of the lower alloy layer is, in mass %, Zn: 5.0 to 30.0%, Al: 30.0 to 55.0%, Fe: 22.5 to 50.0%, Si: 0 to 10.0%, Ni: 0 to less than 0.5%, Mg: 0 to 1.0%, the balance being impurities, and the chemical composition of the intermediate alloy layer is, in mass %, Zn: 5.0 to 35.0%, Al: 25.0 to 60.0%, Fe: 20.0 to 45.0%, Si: 0 to 10.0%, Ni: 0.5 to 10.0%, Mg: 0 to 1.0%, a total of one or more elements selected from the group consisting of Ca, Sn, Bi, In, Y, La, Ce, Sr, B, P, Cr, Ti, Co, V, Nb, Cu, Mn, Mo, W, Zr, Ag, Li, Na, K, and Ba: 0 to 2.0%, and the balance being impurities.

2. The chemical composition of the upper plating layer is, in mass%, Al: 6.0% to 25.0%, Mg: over 3.0% to 12.5%, Ni: 0.001% to less than 0.5%, Si: 0% to 1.5%, Ca: 0% to 3.0%, Sn: 0% to 3.0%, Bi: 0% to 1.0%, In: 0% to 1.0%, Y: 0% to 0.5%, La: 0% to 0.5%, Ce: 0% to 0.5%, Sr: 0% to 0.5%, B: 0% to 1.0%, P: 0% to 0.5%, Cr: 0% to 0.25%, Ti: 0% to 0.25%, Co: 0% to 0.25%, V: 0% to 0.25%, 2. The plated steel material according to claim 1, comprising: Nb: 0% to 0.25%, Cu: 0% to 1.0%, Mn: 0% to 0.25%, Mo: 0% to 0.25%, W: 0% to 0.25%, Zr: 0% to 0.25%, Ag: 0% to 1.0%, Li: 0% to 0.5%, Na: 0% to 0.05%, K: 0% to 0.05%, Ba: 0% to 0.25%, Fe: 0% to 5.0%, and the balance: more than 50% Zn and impurities.

3. The plated steel material according to claim 2, wherein the upper plating layer satisfies the following formulas (1) and (2): Mg / Al≦0.5 ... (1) 2×Ca+Sr+Y+La+Ce≧0.05 ... (2) where Mg, Al, Ca, Sr, Y, La, and Ce in formulas (1) and (2) are the contents (mass%) of the upper plating layer, respectively, and 0 is substituted if the element is not contained.

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