Plated Steel

Abstract

This plated steel material comprises a steel material and a plating layer disposed on the surface of the steel material, wherein: the average chemical composition of the plating layer is a prescribed composition; and a cross-section of the plating layer perpendicular to the surface of the steel material has a structure, as observed by a scanning electron microscope, in which, in terms of area proportions, an MgZn2 phase is 10% to 50%, the total of an Al phase, an Al-Zn phase, and a Zn-Al phase is 15% to 75%, and an Al / MgZn2 / Zn ternary eutectic structure is 0% to 65%, with the remainder being 0% to 5.0%, and the total area ratio of the Al phase and the Zn-Al phase with respect to the Al-Zn phase ([Al]+[Zn-Al]) / [Al-Zn]) is 0.8 or more.

Description

[Technical field]

[0001] The present invention relates to a plated steel material. This application claims priority based on Japanese Patent Application No. 2022-136497, filed on August 30, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] It is widely known that the corrosion resistance of steel materials can be improved by applying Zn plating to the surface of the steel material, and Zn-Al-Mg-plated steel sheets, which have improved corrosion resistance by adding Al and Mg to Zn, are widely used in the fields of automobiles, home appliances, building materials, and civil engineering. Among the environments in which these applications are used, there are wet environments where rainwater etc. constantly accumulates, and running water environments where water flows, and in these environments, there is a problem that even Zn-Al-Mg-plated steel sheets do not have sufficient corrosion resistance.

[0003] For example, Patent Document 1 discloses a steel sheet having an alloy layer containing Fe and Si, and a plating layer, the plating layer and the alloy layer having average compositions, in mass%, of Al: 45.0 to 65.0%, Si: 0.50 to 5.00%, Mg: 1.00 to 10.00%, with the balance being Zn, Fe and impurities, the plating layer contains an Mg-Si phase with a volume fraction of 0.1 to 20.0%, and when a range of 1 μm from the surface of the plating layer in the thickness direction of the plating layer is defined as a surface layer portion of the plating layer, the plating layer in the surface layer portion is averaged. The document describes a Zn-Al-Mg-based plated steel sheet in which the average equivalent circle diameter of the Mg-Si phase in a planar direction is 0.1 to 15.0 μm, and when the Si content is measured throughout the entire thickness of the plating layer, the integrated value of the Si content from the surface of the plating layer to the center of the plating layer thickness is 0.55 times or more the integrated value of the Si content from the surface of the plating layer to the interface between the plating layer and the alloy layer, where the center of the plating layer thickness is defined as a position that is half the thickness of the plating layer from the surface of the plating layer toward the interface between the plating layer and the alloy layer.

[0004] Patent Document 2 describes a hot-dip Al-Zn-Mg-Si plated steel sheet having a plating film on the steel sheet surface, the plating film consisting of an interfacial alloy layer present at the interface with the base steel sheet and a main layer present on the alloy layer, the plating film containing 25 to 80 mass% Al, more than 0.6 to 15 mass% Si, and more than 0.1 to 25 mass% Mg, and the area ratio of Mg2Si on the surface of the main layer is 10% or more.

[0005] US Patent No. 5,399,633 describes a method of forming a metal-coated steel strip, comprising the steps of: (a) passing the strip through a hot dip coating bath containing Al, Zn, Si and Mg, and optionally other elements, to form a molten Al-Zn-Si-Mg alloy coating on the strip; (b) cooling the coated strip to solidify the molten Al-Zn-Si-Mg alloy on the strip to form a solidified coating having a microstructure having alpha-Al phase dendrites, a Zn-rich phase in the interdendritic regions and Mg2Si phase particles in the interdendritic regions; (c) heat treating the coated strip at a temperature and for a time to form an Al-Zn phase solid solution from an as-cast microstructure of alpha-Al phase dendrites and Zn-rich interdendritic phase to promote spheroidization of the Mg2Si phase particles dispersed in the coating; and (d) cooling the heat treated strip.

[0006] Patent Document 4 describes a highly heat-resistant hot-dip Zn-Al alloy-plated steel sheet in which an alloy plating layer consisting of 3.0 to 7.0 mass % Al, 0.05 to 0.5 mass % Si, 0.01 to 0.5 mass % Mg, and the balance Zn is formed on the surface of the steel sheet, and an Fe-Al-Si alloy layer in which Si is concentrated is formed between the alloy plating layer and the steel sheet base.

[0007] Patent Document 5 discloses a coating layer formed on the surface of a steel material, the coating layer containing, in average composition, 1-10 mass% Mg, 4-22 mass% Al, with the remainder being Zn and impurities, the coating layer containing an [Al·Zn mixed structure] with an area ratio of 10-70% in a cross section of the coating layer in a matrix of an [Al / Zn / MgZn2 ternary eutectic structure], the [Al·Zn mixed structure] including a first region having a Zn concentration in the range of 75 mass% or more and less than 85 mass%, and a second region inside the first region having a Zn concentration in the range of 67 mass% or more and less than 75 mass%, the second region containing a mixture of Zn phase and Al phase, and the area of ​​the second region in the cross section of the coating layer being 1 μm 2 This document describes a hot-dip Zn-Al-Mg plated steel material in which the interface length between the Zn phase and the Al phase per unit area is 20 μm or less.

[0008] However, Patent Documents 1 to 5 do not consider corrosion resistance in a water-wet environment or in a flowing water environment (hereinafter referred to as water-wet corrosion resistance or flowing water corrosion resistance). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2019 / 049307 [Patent Document 2] JP 2016-166414 A [Patent Document 3] Special Publication No. 2012-528244 [Patent Document 4] JP 2000-265255 A [Patent Document 5] Patent Publication No. 2021-195564 Summary of the Invention [Problem to be solved by the invention]

[0010] 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 which has excellent water-wet corrosion resistance and running water corrosion resistance, and also has excellent sacrificial corrosion resistance. [Means for solving the problem]

[0011] In order to solve the above problems, the following configuration is adopted. [1] A plated steel product having a steel material and a plating layer disposed on a surface of the steel material, The average chemical composition of the plating layer is, in mass%, Al: 10.0%~40.0%, Mg: more than 4.0%~8.5%, Si: 0% to 5.0%, Ca: 0%~3.00%, Sn: 0%~3.00%, Bi: 0% to 1.00%, In:0%~1.00%, Y: 0%~0.50%, La: 0% to 0.50%, Ce: 0%~0.50%, Sr: 0%~0.50%, B: 0%~1.00%, P: 0% to 0.50%, Cr: 0%~0.25%, Ti: 0% to 0.25%, V: 0%~0.25%, Zr: 0% to 0.25%, Ni: 0%~1.00%, Co: 0% to 0.25%, Nb: 0% to 0.25%, Cu: 0%~1.00%, Mn: 0% to 0.25%, Mo: 0% to 0.25%, W: 0% to 0.25%, Ag: 0%~1.00%, Li: 0%~0.50%, Na: 0% to 0.05%, K: 0% to 0.05%, Fe: 0%~5.00%, Sb: 0% to 0.50%, Pb: 0%~0.50%, Ba: 0%~0.25%, The balance is Zn and impurities. In a cross section of the plating layer perpendicular to the surface of the steel material, the structure observed with a scanning electron microscope has an area fraction of: MgZn2 phase: 10% or more and 50% or less, The sum of Al phase, Al-Zn phase and Zn-Al phase: 15% or more and 75% or less, [Al / MgZn2 / Zn ternary eutectic structure]: 0% to 65% Remainder: 0% or more and 5.0% or less; A plated steel material, in which the area ratio (([Al]+[Zn-Al]) / [Al-Zn]) of the total of the Al phase and the Zn-Al phase to the Al-Zn phase is 0.8 or more. [2] In the average chemical composition of the plating layer, Si is 0.05% to 5.0%; The plated steel material according to [1], wherein an integrated intensity value of each element in a surface layer from the surface of the plated layer to a depth of 0.5 μm, when a depth analysis is performed using a GDS method, satisfies the following formula (1): IΣ(Si) / (IΣ(Zn)+IΣ(Al))≧0.005 …(1) where IΣ(Si), IΣ(Zn), and IΣ(Al) in equation (1) are the integrated intensity values ​​of Si, Zn, and Al detected by the GDS method, respectively. [3] In the average chemical composition of the plating layer, Ca is 0.01% to 3%; In the plating layer, Ca3Al2Si2 phase, CaAl2Si2 phase, CaAl2Si2 phase, CaAl 1-x S 1+x phase (where x = 0 to 0.2), The plated steel material according to [1] or [2], wherein an integrated intensity value of each element in a surface layer from the surface of the plated layer to a depth of 0.5 μm, when a depth analysis is performed using a GDS method, satisfies the following formula (2): IΣ(Ca) / IΣ(Si)≧2.0 … (2) Here, IΣ(Ca) and IΣ(Si) in equation (2) are the integrated intensities of Ca and Si detected by the GDS method, respectively. Effect of the Invention

[0012] According to the present invention, it is possible to provide a plated steel material which has excellent water-wet corrosion resistance and running water corrosion resistance, as well as excellent sacrificial corrosion resistance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Conventionally, pure aluminum plating materials called Type 2 for corrugated pipes are known as plating materials for wet and running water environments. However, aluminum has poor sacrificial corrosion protection, and there is a problem with appearance due to red rust generation from the end face of the steel material. In addition, as the thickness of the steel material increases, the progress of corrosion from the end face becomes more noticeable, which shortens the product life.

[0014] On the other hand, Zn-Al-Mg-based plated materials have sufficient sacrificial corrosion protection performance and superior corrosion resistance compared to Zn plating in normal environments. However, there is an issue that in wet or running water environments, corrosion products are easily washed away, making it difficult to demonstrate high corrosion resistance compared to normal environments.

[0015] The Zn-Al-Mg based coating layer is formed from a number of constituent phases. The details will be described later, but they are Al phase, Al-Zn phase, Zn-Al phase, Mg2Zn phase, Mg2Zn 11 Phases and structures such as the Al phase, Zn phase, and the ternary eutectic structure of Al / MgZn2 / Zn account for the majority of the phase composition.

[0016] The present inventors have focused on the relationship between the wet corrosion resistance and running water corrosion resistance of Zn-Al-Mg-based plating in a water-wet environment and the constituent phases and constituent structures of the plating layer, and as a result of extensive investigations, have found that the wet corrosion resistance and running water corrosion resistance can be improved by optimizing the ratio of the constituent phases and constituent structures.

[0017] It is known that Al, a plating component, precipitates as multiple phases during solidification, namely, Al phase, Al-Zn phase, and Zn-Al phase. Of these, the Al phase has the best water resistance and running water resistance, but has poor sacrificial corrosion protection. The Al-Zn phase has the best sacrificial corrosion protection among the three phases, but is poor in wet corrosion resistance and running water corrosion resistance. Furthermore, the Zn-Al phase has intermediate properties compared to the previous two phases in terms of wet corrosion resistance, running water corrosion resistance, and sacrificial corrosion protection.

[0018] Therefore, the inventors have considered that a plated steel material with excellent corrosion resistance can be obtained by appropriately adjusting the Al phase ratio in a Zn-Al-Mg-based plating layer, and as a result of extensive research, have found a plated steel material with excellent wet corrosion resistance, running water corrosion resistance, and sacrificial corrosion protection by controlling the phase ratio in the plating layer. A steel structure manufactured from the plated steel material according to the present invention has excellent wet corrosion resistance, running water corrosion resistance, and sacrificial corrosion protection. Hereinafter, a plated steel material according to an embodiment of the present invention will be described.

[0019] The plated steel material of the present embodiment is a plated steel material having a steel material and a plating layer disposed on the surface of the steel material, and the average chemical composition of the plating layer is, in mass%, Al: 10.0% to 40.0%, Mg: more than 4.0% to 8.5%, Si: 0% to 5.0%, Ca: 0% to 3.00%, Sn: 0% to 3.00%, Bi: 0% to 1.00%, In: 0% to 1.00%, Y: 0% to 0.50%, La: 0%. ~0.50%, Ce:0%~0.50%, Sr:0%~0.50%, B:0%~1.00%, P:0%~0.50%, Cr:0%~0.25%, Ti:0%~0.25%, V:0%~0. 25%, Zr:0%~0.25%, Ni:0%~1.00%, Co:0%~0.25%, Nb:0%~0.25%, Cu:0%~1.00%, Mn:0%~0.25%, Mo:0%~0.2 5%, W: 0%-0.25%, Ag: 0%-1.00%, Li: 0%-0.50%, Na: 0%-0.05%, K: 0%-0.05%, Fe: 0%-5.00%, Sb: 0%-0.50%, Pb: 0%-0.50%, Ba: 0%-0.25%, balance: Zn and impurities, and the structure observed with a scanning electron microscope in a cross section of the plating layer perpendicular to the surface of the steel material is In terms of area fraction, the MgZn2 phase: 10% or more and 50% or less, the total of the Al phase, Al-Zn phase and Zn-Al phase: 15% or more and 75% or less, [Al / MgZn2 / Zn ternary eutectic structure]: 0% or more and 65% or less, and the balance: 0% or more and 5.0% or less. The area ratio of the total of the Al phase and Zn-Al phase to the Al-Zn phase (([Al] + [Zn-Al]) / [Al-Zn]) is 0.8 or more. In addition, in the plated steel material of this embodiment, the average chemical composition of the plating layer contains 0.05% to 5.0% Si, and in the surface layer from the surface of the plating layer to a depth of 0.5 μm, it is preferable that the integrated intensity value of each element when a depth analysis is performed by the GDS method satisfies the following formula (1). IΣ(Si) / (IΣ(Zn)+IΣ(Al))≧0.005 …(1) where IΣ(Si), IΣ(Zn), and IΣ(Al) in equation (1) are the integrated intensity values ​​of Si, Zn, and Al detected by the GDS method, respectively. Furthermore, in the plated steel material of this embodiment, the average chemical composition of the plated layer contains 0.01% to 3% Ca, and the plated layer contains Ca3Al2Si2 phase, CaAl2Si2 phase 、C aAl 1-x S 1+x It is preferable that the plating layer contains any one of the phases (where x = 0 to 0.2), and that in a surface layer from the surface of the plating layer to a depth of 0.5 μm, the integrated intensity value of each element when a depth analysis is performed by the GDS method satisfies the following formula (2). IΣ(Ca) / IΣ(Si)≧2.0 … (2) Here, IΣ(Ca) and IΣ(Si) in equation (2) are the integrated intensities of Ca and Si detected by the GDS method, respectively.

[0020] In the following description, the "%" for the content of each element in the chemical composition means "mass %." Furthermore, a numerical range expressed using "~" means a range that includes the numerical values ​​before and after "~" as the lower and upper limits. When the numerical values ​​before and after "~" are followed by "more than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit.

[0021] "Corrosion resistance" refers to the property of the plating layer itself to be resistant to corrosion. Zn-based plating layers have a sacrificial corrosion protection effect on steel materials, so the plating layer corrodes and turns to white rust before the steel material corrodes, and after the white rusted plating layer disappears, the steel material corrodes and turns to red rust. This is the corrosion process of plated steel sheets. In addition, "sacrificial corrosion protection" refers to the property of suppressing corrosion of steel at exposed parts of the steel (for example, cut end surfaces of plated steel, or parts where the steel is exposed due to cracking of the hot-dip plating layer during processing). Furthermore, "water-wet corrosion resistance, running water corrosion resistance" refers to the property of the plating layer itself being resistant to corrosion when water is dripped onto the surface of the plating layer and water is allowed to flow over the plating layer.

[0022] There is no particular restriction on the quality of the steel material. Various types of steel material can be used, such as general steel, Ni pre-plated steel, Al killed steel, extra low carbon steel, high carbon steel, various high tensile steels, and some high alloy steels (steels containing strengthening elements such as Ni and Cr). There are also no particular restrictions on the conditions of the steel material manufacturing method and the steel sheet manufacturing method (hot rolling method, pickling method, cold rolling method, etc.). Furthermore, the steel material can be made of Zn, Ni, Sn, or alloys thereof with a concentration of 3 g / m 2 Steel materials on which the following metal or alloy films are formed may be used.

[0023] Next, the plating layer will be described. The plating layer according to this embodiment includes a Zn-Al-Mg alloy layer. The plating layer may also include an Al-Fe alloy layer. The Zn-Al-Mg alloy layer, in which alloy elements such as Al and Mg are added to Zn, has improved corrosion resistance compared to a normal Zn plating layer. For example, even if the Zn-Al-Mg alloy layer is about half the thickness of a normal Zn plating layer, it has the same corrosion resistance as the Zn plating layer. Therefore, the plating layer according to this embodiment also has corrosion resistance equal to or higher than that of the Zn plating layer.

[0024] The Zn-Al-Mg alloy layer is made of a Zn-Al-Mg alloy, which means a ternary alloy containing Zn, Al, and Mg.

[0025] The Al-Fe alloy layer is an interface alloy layer between the steel material and the Zn-Al-Mg alloy layer.

[0026] That is, the plating layer according to the present embodiment may be a single-layer structure of a Zn-Al-Mg alloy layer, or may be a laminated structure including a Zn-Al-Mg alloy layer and an Al-Fe alloy layer. In the case of a laminated structure, the Zn-Al-Mg alloy layer is preferably a layer that constitutes the surface of the plating layer. However, although an oxide film of the plating layer constituent elements is formed on the outermost surface of the plating layer with a thickness of less than 1 μm, it is thin compared to the thickness of the entire plating layer and is therefore often ignored by the main body of the plating layer.

[0027] The total thickness of the plating layer is preferably 5 to 70 μm. The total thickness of the plating layer is not limited to the range of 5 to 70 μm because it depends on the plating conditions. In a normal hot-dip plating method, the total thickness of the plating layer is affected by the viscosity and specific gravity of the plating bath. The total thickness of the plating layer is adjusted by the drawing speed of the steel material (original plate) and the strength of wiping.

[0028] The Al-Fe alloy layer is formed on the surface of the steel material (specifically, between the steel material and the Zn-Al-Mg alloy layer), and the Al5Fe2 phase is the main phase in the structure. The Al-Fe alloy layer is formed by mutual atomic diffusion between the base steel (steel material) and the plating bath. When the hot-dip plating method is used as the manufacturing method, the Al-Fe alloy layer is likely to be formed in the plating layer containing the Al element. Since the plating bath contains more than a certain concentration of Al, the Al5Fe2 phase is formed most frequently. However, atomic diffusion takes time, and there are also parts where the Fe concentration is high near the base steel. Therefore, the Al-Fe alloy layer may partially contain small amounts of the AlFe phase, Al3Fe phase, Al2Fe phase, etc., since the plating bath also contains a certain concentration of Zn.

[0029] When the plating layer contains Si, Si is particularly likely to be incorporated into the Al-Fe alloy layer, and may become an Al-Fe-Si intermetallic compound phase. The intermetallic compound phase identified is the AlFeSi phase, and isomers include α, β, q1, q2-AlFeSi phases. Therefore, these AlFeSi phases may be detected in the Al-Fe alloy layer. An Al-Fe alloy layer containing these AlFeSi phases is also called an Al-Fe-Si alloy layer.

[0030] Next, the average chemical composition of the plating layer will be described. When the plating layer has a single-layer structure of Zn-Al-Mg alloy layer, the average chemical composition of the entire plating layer is the average chemical composition of the Zn-Al-Mg alloy layer. When the plating layer has a laminated structure of Al-Fe alloy layer and Zn-Al-Mg alloy layer, the average chemical composition of the entire plating layer is the average chemical composition of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer combined.

[0031] Usually, in the hot-dip plating method, the chemical composition of the Zn-Al-Mg alloy layer is almost the same as that of the plating bath, since the formation reaction of the plating layer is almost completed in the plating bath. In the hot-dip plating method, the Al-Fe alloy layer is formed and grows instantly immediately after immersion in the plating bath. The formation reaction of the Al-Fe alloy layer is completed in the plating bath, and the thickness of the Al-Fe alloy layer is often sufficiently small compared to that of the Zn-Al-Mg alloy layer. Therefore, unless special heat treatment such as a heat alloying treatment is performed after plating, the average chemical composition of the entire plating layer is substantially the same as that of the Zn-Al-Mg alloy layer, and the components of the Al-Fe alloy layer and the like can be ignored.

[0032] Al: 10.0% or more, 40.0% or less Like Zn, Al is an element that constitutes the main part of the plating layer. Although Al has a small sacrificial corrosion protection effect, the inclusion of Al in the plating layer improves the wet corrosion resistance, running water corrosion resistance, and flat surface corrosion resistance. Furthermore, without Al, Mg cannot be stably maintained in the plating bath, so Al is included in the plating bath as an element essential for manufacturing. The Al content of 10.0% or more is the content required to contain a large amount of Mg, which will be described later, or the content required to ensure a certain degree of corrosion resistance. If the content is less than this amount, it is difficult to prepare the plating bath, and it is further difficult to ensure corrosion resistance. Furthermore, the Al content is set to 40.0% or less because Al has a weak sacrificial corrosion protection effect on the steel sheet, and if the content is higher than this amount, sufficient sacrificial corrosion protection cannot be obtained, so the upper limit is set to 40.0% or less.

[0033] Mg: more than 4.0%, less than 8.5% Mg has a sacrificial anticorrosive effect and is an element that enhances the corrosion resistance of the plating layer. When Mg is contained at a certain level or more, the MgZn2 phase is formed in the plating layer. The higher the Mg content in the plating layer, the more the MgZn2 phase is formed. The MgZn2 phase is known to have a structure called the Laves phase, and is known to have high hardness. The Mg content of more than 4.0% is the concentration required to exhibit corrosion resistance, and if it is 4.0% or less, sufficient corrosion resistance cannot be obtained. In addition, the MgZn2 phase is not sufficiently formed in the plating layer, and the corrosion resistance of the plating layer itself is also low. If the Mg content is excessive, it becomes difficult to manufacture the plating layer and the workability of the plating layer decreases, so the upper limit is 8.5% or less. A more preferable Mg content is 5.0% or more and 7.0% or less.

[0034] Si: 0% to 5.0% When Si is contained in the plating bath, Si single phase or Mg2Si precipitates in the plating layer, and when Ca is further contained together with Si, Al-Ca-Si compounds precipitate. Since Si has excellent wet corrosion resistance and running water corrosion resistance, the wet corrosion resistance and running water corrosion resistance can be further improved by precipitating Si or Si-based compounds on the surface of the plating layer. Since Si is an optional additive element, 0% is acceptable, but when 0.05% or more is contained, the wet corrosion resistance and running water corrosion resistance are further improved. However, when Si exceeds 5.0%, a large amount of dross is generated and non-plating occurs frequently. Therefore, the Si concentration is 0 to 5.0%, may be 0.05% to 5.0%, may be 0.05% to less than 1.0%, or may be 0.10% to 0.50%.

[0035] Ca: 0% to 3.00% Ca is easily oxidized in the atmosphere, and when present in the plating bath, it forms a dense oxide film on the bath surface, which has the effect of preventing oxidation of Mg. The above effect stabilizes the Mg concentration, facilitating the production of plated steel sheets with a target composition. Since Ca is an optional additive element, it may be 0%, but in order to favorably exert the above-mentioned effects, the Ca content is made to be more than 0%, more preferably 0.01% or more. In addition, when Ca is contained at 0.01% or more, Al-Ca-Si compounds or Ca-Al-Zn compounds are easily formed. Of these, Al-Ca-Si compounds have excellent water wet corrosion resistance and running water corrosion resistance, so that these compounds are concentrated on the plating surface layer, thereby improving water wet corrosion resistance and running water corrosion resistance. The upper limit of Ca is 3.00% or less.

[0036] Element group A Sn: 0% to 3.00% Bi: 0% to 1.00% In:0%~1.00% The elements in element group A have the function of improving sacrificial corrosion resistance. However, Mg tends to bond stronger than Zn, and the effect of the contained Mg is small, so there is an upper limit to the content of these elements. If the upper limit is exceeded, adhesion of dross etc. increases, and water wet corrosion resistance, running water corrosion resistance, workability, and weldability all tend to deteriorate. Therefore, Sn is 0 to 3.00%, more preferably more than 0 and less than 3.00%. Bi is 0% to 1.00%, more preferably more than 0 and less than 1.00%. In is 0% to 1.00%, more preferably more than 0 and less than 1.00%.

[0037] Element group B Y: 0%~0.50% La: 0% to 0.50% Ce: 0% to 0.50% Sr: 0%~0.50% The elements of element group B, Y, La, Ce, and Sr, are easily oxidized in the atmosphere, and when present in a plating bath, they form a dense oxide film on the bath surface, which has the effect of preventing oxidation of Mg. The above effect stabilizes the Mg concentration, facilitating the production of a plated steel sheet of a target composition. In order to favorably exert such an effect, the content of these elements is set to more than 0%, more preferably 0.01% or more. However, the content of each element has an upper limit, and if the upper limit of the content is exceeded, the preparation of the plating bath tends to become difficult. In addition, adhesion of dross and the like increases, and the water-wet corrosion resistance, running water corrosion resistance, workability, and weldability also tend to deteriorate. Therefore, the content of Y, La, Ce, and Sr is set to 0% to 0.50%, preferably more than 0 and less than 0.50%, and more preferably 0.01% or more and less than 0.50%.

[0038] Element group C B: 0%~1.00% P: 0%~0.50% B and P, which are in element group C, are elements belonging to metalloids. Generally, these elements do not affect wet corrosion resistance or running water corrosion resistance, but there is an upper limit to the content of each element, and if the upper limit is exceeded, adhesion of dross etc. increases and corrosion resistance tends to deteriorate. Therefore, B and P are set to 0% to 1.0% and 0% to 0.50%, respectively.

[0039] Element group D Cr: 0%~0.25% Ti: 0% to 0.25% V: 0%~0.25% Zr: 0% to 0.25% Ni: 0% to 1.00% Cobalt: 0% to 0.25% Nb: 0% to 0.25% Cu: 0% to 1.00% Mn: 0% to 0.25% Mo: 0% to 0.25% W: 0%~0.25% Ag: 0%~1.00% Li: 0% to 0.50% Sodium: 0% to 0.05% K: 0% to 0.05% Fe: 0% to 5.00% The elements of element group D, Cr, Ti, V, Zr, Ni, Co, Nb, Cu, Mn, Mo, W, Ag, Li, Na, K and Fe, are metal elements, and when these elements are incorporated into the plating layer, they form substitution solid solutions or new high-melting intermetallic compounds. This improves water-wet corrosion resistance and running water corrosion resistance. There is an upper limit to the content of each element, and if the upper limit of the content is exceeded, adhesion of dross and the like tends to increase. Therefore, Na and K are each 0% to 0.05%, preferably more than 0% and less than 0.05%. Cr, Ti, V, Zr, Co, Nb, Mn, Mo and W are each 0% to 0.25%, preferably more than 0% and less than 0.25%. Li is 0% to 0.50% or less, preferably more than 0% and less than 0.50%. Ni, Cu and Ag are each 0% to 1.00% or less, preferably more than 0% and less than 1.00%. Furthermore, Fe may be inevitably contained in the coating layer because it may diffuse from the base steel into the coating layer during coating production. Therefore, the Fe content is 0% to 5.00% or less, and may be more than 0% and less than 5.00%.

[0040] Element group E Sb: 0% to 0.50% Pb: 0%~0.50% Ba: 0% to 0.25% The elements in element group E, Sb, Pb, and Ba, are similar in properties to Zn. Therefore, the inclusion of these elements hardly produces any special effects, but they do have the effect of making it easier to form a spangle pattern on the appearance of the plating. However, excessive inclusion of these elements may reduce corrosion resistance. Therefore, the content of Sb and Pb is set to 0% to 0.50%, and preferably to more than 0% and less than 0.50%. The content of Ba is set to 0% to 0.25%, and preferably to more than 0% and less than 0.25%.

[0041] Remainder: Zn and impurities Zn is a low melting point metal and exists as the main phase of the plating layer on the steel material. Zn is an element necessary for ensuring corrosion resistance and obtaining sacrificial corrosion protection for the steel material. Zn is the balance, but it is preferable that Zn is contained at 50.00% or more. If Zn is less than 50.00%, the main part of the metal structure of the Zn-Al-Mg alloy layer is the Al phase, and the Zn phase that exhibits sacrificial corrosion protection may be insufficient. More preferably, it is 65.00% or more, or 70.00% or more. The upper limit of the Zn content is the amount that is the balance other than elements other than Zn and impurities.

[0042] Furthermore, impurities in the plating layer refer to components contained in the raw materials or components mixed in during the manufacturing process, but not intentionally added. 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.

[0043] To identify the average chemical composition of the plating layer, the plating layer is stripped and dissolved using an acid containing an inhibitor that suppresses corrosion of the base steel (steel material) to obtain an acid solution. The resulting acid solution is then measured using ICP atomic emission spectroscopy or ICP-MS to obtain the chemical composition. There are no particular restrictions on the type of acid, so long as it is an acid that can dissolve the plating layer. If the area and weight are measured before and after stripping, the plating adhesion amount (g / m 2 ) can also be obtained at the same time.

[0044] Next, the structure of the plating layer will be described. The proportion of phases and structures contained in the plating layer greatly affects the wet corrosion resistance and running water corrosion resistance of plated steel. Even if the plating layer has the same component composition, the phases or structures contained in the metal structure change depending on the manufacturing method, resulting in different properties. The metal structure of the plating layer can be easily confirmed by a scanning electron microscope with an energy dispersive X-ray analyzer (SEM-EDS). By obtaining, for example, a reflected electron image in any vertical cross section of the mirror-finished plating layer, the approximate state of the metal structure of the plating layer can be confirmed. The vertical cross section of the plating layer is a cross section in the thickness direction of the plating layer perpendicular to the surface of the steel material. Since the thickness of the plating layer in this embodiment is 3 to 80 μm, preferably 5 to 70 μm, it is preferable to confirm the metal structure in a field of view of 500 to 5000 times with SEM. The reason why the lower limit of the thickness of the plating layer is set to 3 μm or more is that it may be difficult to ensure corrosion resistance at a thickness less than this. The reason why the upper limit of the plating layer thickness is set at 80 μm is that if the thickness exceeds this limit, there is a concern that the plating may peel off when the plated steel material is processed, and it may not be practical. For example, when a plating layer with a thickness of 25 μm is observed at a magnification of 2000 times, the field of view is 25 μm (plating thickness) × 40 μm (SEM field of view width) = 1000 μm 2 In the present embodiment, since the SEM field of view for the plating layer may be a local field of view, 25 fields of view may be selected from any cross section to obtain average information on the plating layer. 2 The metal structure in the above field of view can be observed to determine the area ratio of the phase or structure that constitutes the metal structure of the plating layer.

[0045] Backscattered electron images taken by SEM are preferable because they allow easy identification of the phases or structures contained in the plating layer. Elements with small atomic numbers, such as Al, appear black, while elements with large atomic numbers, such as Zn, appear white, making it easy to read the proportions of these structures.

[0046] To confirm each phase, the composition of the phase can be pinpointed in EDS analysis, and then the phase can be identified by reading phases with roughly equivalent components using elemental mapping, etc. For items for which EDS analysis can be used, elemental mapping can be used to distinguish phases with roughly the same composition. If a phase with roughly the same composition can be identified, it is possible to know the area of ​​that crystal phase in the observation field. Once the area is known, the average crystal grain size can be calculated by determining the equivalent circular diameter.

[0047] In addition, the area ratio of each phase in the observed field of view can be determined. The area ratio of a specific phase to the plating layer corresponds to the volume ratio of that phase in the plating layer.

[0048] Al phase The Al phase in this embodiment is a region in the plating layer where the Al content is more than 35 mass%. This Al phase may contain Zn, but the Zn content is less than 65%. The Al phase can be clearly distinguished from other phases and structures in a SEM backscattered electron image. That is, the Al phase is often shown as the darkest in a SEM backscattered electron image. In this embodiment, the Al phase takes various forms in any cross section, such as a block, or a dendritic cross section such as a circular or flat shape. In this embodiment, the Al contained in the [Al / MgZn2 / Zn ternary eutectic structure] is not included in the Al phase. The Al phase is excellent in water-wet corrosion resistance and running water corrosion resistance. The Al in the bath precipitates as the Al phase, the Al-Zn phase, and the Zn-Al phase described below, but it is preferable to precipitate it as the Al phase in order to ensure water-wet corrosion resistance and running water corrosion resistance.

[0049] Al-Zn phase The Al-Zn phase in this embodiment is a phase containing 65 to 75 mass% Zn and Al. The Al-Zn phase is an aggregate of a fine Zn phase with a grain size of about 1 μm (hereinafter referred to as a fine Zn phase) and a fine Al phase with a grain size of less than 1 μm (hereinafter referred to as a fine Al phase). In the coating layer in a molten state, Al has a structure different from the crystal structure at room temperature, and is capable of dissolving a large amount of Zn phase, and exists as a high-temperature stable phase containing the Zn phase. On the other hand, at room temperature, the content of the Zn phase in this high-temperature stable phase is extremely reduced, and Al and Zn are equilibrium separated and exist as an Al-Zn phase containing a fine Al phase and a fine Zn phase. That is, the Al-Zn phase is a phase containing a fine Zn phase at a ratio of 65 to 75 mass%. This Al-Zn phase is a phase with a composition equivalent to the β phase on the Al-Zn phase diagram, and has different properties from the Al phase and Zn-Al phase contained in the coating layer, so it can be distinguished on a backscattered electron SEM image or wide-angle X-ray diffraction. Therefore, in this embodiment, a phase in which the Al component is 25 to 35 mass % and the Zn component is 65 to 75 mass % is defined as an Al-Zn phase.

[0050] In addition, since the Al-Zn phase is inferior to the Al phase and the Zn-Al phase in terms of wet corrosion resistance and running water corrosion resistance, it is desirable to prevent the crystallization of the Al-Zn phase as much as possible in order to ensure the running water resistance of the coating. Therefore, the relationship between the area ratios of the Al-Zn phase, the Al phase, and the Zn-Al phase in the structure must satisfy the following formula (A).

[0051] ([Al]+[Zn-Al]) / [Al-Zn] is ≧0.8…(A)

[0052] In the formula (A), [Al] is the area fraction (%) of the Al phase, [Zn-Al] is the area fraction (%) of the Zn-Al phase, and [Al-Zn] is the area fraction (%) of the Al-Zn phase.

[0053] Zn-Al phase The Zn-Al phase in this embodiment is a phase containing 75 to 85 mass % of Zn and Al. The Zn-Al phase is an aggregate of a fine Zn phase with a grain size of about 1 μm (hereinafter referred to as a fine Zn phase) and a fine Al phase with a grain size of less than 1 μm. The β phase, which is a high-temperature stable phase, separates into Zn and Al phases at 250° C. or less, and at that time, it takes in the surrounding Zn to form the Zn-Al phase. The Zn-Al phase is inferior to the Al phase, but is superior to the Al-Zn phase in terms of water-wet corrosion resistance and running water corrosion resistance. In addition, if the Al concentration in the plating bath is 15% or less, the Al phase is difficult to precipitate, so by controlling the precipitation of the Zn-Al phase, the water-wet corrosion resistance and running water corrosion resistance can be improved.

[0054] In order to ensure sacrificial corrosion protection while improving wet corrosion resistance and running water corrosion resistance, the total of the Al phase, Al-Zn phase, and Zn-Al phase must be 15% or more and 75% or less in area fraction. Note that it is not necessarily required that all of the Al phase, Al-Zn phase, and Zn-Al phase are crystallized, and any of the phases may be 0%.

[0055] MgZn2 phase The MgZn2 phase according to this embodiment is a region in the coating layer where Mg is 11 to 21 mass%, preferably 16 mass%, and Zn is 81 to 89 mass%, preferably 84 mass%. The MgZn2 phase is often photographed in a backscattered electron image of an SEM as a gray intermediate color between Al and Zn. In the backscattered electron image of an SEM, the MgZn2 phase can be clearly distinguished from the Al-Zn phase, the Al phase, the [ternary eutectic structure of Al / MgZn2 / Zn], etc.

[0056] The MgZn2 phase has higher sacrificial corrosion protection than the Al phase, but is inferior in wet corrosion resistance and running water corrosion resistance. Therefore, from the viewpoint of sacrificial corrosion protection, the more the MgZn2 phase, the better. However, as the MgZn2 phase increases, the wet corrosion resistance and running water corrosion resistance decrease, so there is an upper limit to the area ratio of the MgZn2 phase. The proportion of the MgZn2 phase in the coating layer needs to balance both properties, so the area ratio of the MgZn2 phase needs to be 10% or more and 50% or less.

[0057] [Al / MgZn2 / Zn ternary eutectic structure] The [Al / MgZn2 / Zn ternary eutectic structure] is a eutectic structure consisting of an Al phase, an MgZn2 phase, and a Zn phase, and is clearly distinguished from the MgZn2 phase contained as the main phase of the coating layer and the above-mentioned Al phase in a backscattered electron image of an SEM.

[0058] By allowing a certain amount of [Al / MgZn2 / Zn ternary eutectic structure] including Zn phase to exist, sacrificial corrosion protection can be ensured and end surface corrosion resistance can be improved. On the other hand, water wet corrosion resistance and running water corrosion resistance are reduced, so considering water wet corrosion resistance and running water corrosion resistance, the upper limit of the area ratio of [Al / MgZn2 / Zn ternary eutectic structure] is set to 65 area % or less, preferably 40 area % or less. There is no particular limit on the lower limit of the area ratio of [Al / MgZn2 / Zn ternary eutectic structure], and it may be 0%.

[0059] Mg2Si phase When the plating layer contains Si, the Si may precipitate as the Mg2Si phase. The Mg2Si phase has excellent wet corrosion resistance and running water corrosion resistance, so if the Mg2Si phase is present in the surface layer, it acts as a barrier layer against water and has the effect of suppressing wear of the plating layer. For this reason, it is preferable to precipitate Mg2Si in the surface layer.

[0060] The presence of Mg2Si can be confirmed by X-ray diffraction measurement. In the X-ray diffraction measurement, Cu-Kα radiation is used, and the X-ray output is 50 kV and 300 mA. In the X-ray diffraction pattern of the plating layer surface, the presence of Mg2Si phase can be confirmed by checking whether a diffraction peak of Mg2Si phase appears. There is no particular limitation on the X-ray diffraction device, but for example, a horizontal sample type high-power X-ray diffraction device RINT-TTR III manufactured by Rigaku Corporation can be used.

[0061] The concentration of Si in the surface layer of the plating layer can be confirmed by GDS (glow discharge optical emission spectrometry). To improve wet corrosion resistance and running water corrosion resistance, it is preferable that there is a Si compound in the surface layer of the plating layer, specifically, Si is present in the range from the surface of the plating layer to a depth of 0.5 μm. To convert the depth direction in GDS, it may be derived from the relationship between the sputtering seconds of Zn and the depth. The amount of Si required to improve wet corrosion resistance and running water corrosion resistance can be grasped by comparing the intensity ratio with the main components Al and Zn. Specifically, when the integrated intensity value of element X when GDS measurement is performed from the surface of the plating layer to a depth of 0.5 μm is taken as IΣ(X), it is preferable to satisfy the following formula (1). This further improves wet corrosion resistance and running water corrosion resistance. The strength integrated value of element X in the present invention refers to the strength of the plating layer, and does not include the strength integrated value of the paint coating or chemical conversion coating. To confirm the GDS strength integrated value of the plating layer of a plated steel material that has been painted or chemically treated, the paint coating or the chemical conversion coating, or both, may be removed by chemical treatment, grinding, or the like, and then measured using GDS. If it is difficult to remove the coating, the surface of the plating layer is defined based on the GDS measurement results. Specifically, measurements are performed using GDS from the surface of the plated steel material to the steel material, and the first position where the intensities of all the elements Zn, Al, and Mg, which are the main components of the plating layer of the present invention, satisfy 1 / 3 or more of their respective peak intensity values, is considered to be the plating surface layer.

[0062] In the following formula (1), IΣ(Si), IΣ(Zn), and IΣ(Al) are the integrated intensity values ​​of Si, Zn, and Al detected within a range of 0.5 μm deep from the surface of the plating layer by the GDS method, respectively.

[0063] IΣ(Si) / (IΣ(Zn)+IΣ(Al))≧0.005 …(1)

[0064] The Si intensity appears as a peak in the range from the surface of the plating layer to a depth of 0.5 μm, and at the interface between the plating layer and the steel material. The Si peak intensity on the surface side of the plating layer is more than five times stronger than the Si intensity at a position half the thickness of the plating layer.

[0065] Al-Ca-Si phase When Ca and Si are contained in the plating bath, Al-Ca-Si phases are precipitated as Si compounds in the plating layer. 1-x S 1+x phase (where x=0 to 0.2). The Al-Ca-Si phase is more excellent in wet corrosion resistance and running water corrosion resistance than the above-mentioned Mg2Si phase. Therefore, when the Al-Ca-Si phase exists within a range from the surface of the plating layer to a depth of 0.5 μm, it acts as a barrier layer against water and has the effect of suppressing consumption of the plating layer. More specifically, it is preferable to satisfy the following formula (2).

[0066] In the following formula (2), IΣ(Ca) and IΣ(Si) are the integrated intensities of Ca and Si detected by the GDS method within a range of 0.5 μm deep from the surface of the plating layer.

[0067] IΣ(Ca) / IΣ(Si)≧2.0 … (2)

[0068] The above phases and structures constitute the main phase of the plating layer, and account for 90% or more of the area fraction of the plating layer. On the other hand, when elements other than Zn, Mg, and Al are contained in the plating layer, other metal phases are formed. Although some of these are effective in improving water wetting resistance, running water resistance, and corrosion resistance, their effects are not significant. Because of the composition of the plating layer, it is difficult for the balance of these to exceed 5.0% by area in total, so it is 5.0% by area or less. The balance may be 0%.

[0069] The method for measuring the area ratio of phases and structures in the coating layer is, as already mentioned above, to expose a cross section of the coating layer in the thickness direction perpendicular to the surface of the steel material, and confirm the metal structure in a field of view of 500 to 5000 times. 25 points of view are selected from any cross section, and the average information is obtained. That is, 2 The metal structure in this field is observed to determine the area ratio of the phases or structures that make up the metal structure of the plating layer. To confirm each phase, the composition of the phase is pinpointed using EDS analysis, and phases with roughly equivalent components are identified using element mapping or other methods. When EDS analysis can be used, element mapping makes it possible to distinguish phases with roughly the same composition. If phases with roughly the same composition can be identified, it is possible to know the area of ​​that crystalline phase in the observation field. Once the area is known, the equivalent circular diameter can be calculated to calculate the average crystal grain size, and the area ratio of each phase in the observation field can be determined.

[0070] When performing a depth-wise component analysis of the inside of the plating layer by GDS analysis, a glow discharge optical emission spectrometer (GDS) is used. For example, a LECO Japan 850A may be used as the glow discharge optical emission spectrometer, but the measuring device is not limited to this. When performing a depth-wise analysis by argon sputtering, the analysis conditions are argon pressure: 0.27 MPa, output power: 30 W, output voltage: 1000 V, and discharge area: within a circular area with a diameter of 4 mm. Measurements are performed by GDS analysis, which analyzes while etching from the surface of the plating layer to a depth of 0.5 μm, and obtains an integrated value of the emission intensity of each element. Based on this integrated value, it is determined whether the above conditions (1) and (2) are met.

[0071] In addition, Al-Ca-Si phase (Ca3Al2Si2 phase, CaAl2Si2 phase, CaAl2Si2 phase, CaAl 1-x S 1+xThe presence of the Al-Ca-Si phase can be confirmed by X-ray diffraction measurement in addition to SEM and ESD measurement. In the X-ray diffraction measurement, Cu-Kα radiation is used, and the presence can be confirmed by whether or not a diffraction peak of the Al-Ca-Si phase appears in the X-ray diffraction pattern of the plating layer surface measured under conditions of an X-ray output of 50 kV and 300 mA. There is no particular limitation on the X-ray diffraction device, and for example, a horizontal sample type high-power X-ray diffraction device RINT-TTR III manufactured by Rigaku Corporation can be used, but is not limited thereto.

[0072] The presence of Mg2Si can be confirmed by X-ray diffraction measurement in addition to SEM and ESD measurement. In the X-ray diffraction measurement, Cu-Kα radiation is used and the X-ray output is 50 kV and 300 mA. The presence can be confirmed by whether or not a diffraction peak of the Mg2Si phase appears in the X-ray diffraction pattern of the plating layer surface. As an X-ray diffraction device, for example, a horizontal sample type high-power X-ray diffraction device RINT-TTR III manufactured by Rigaku Corporation can be used, but is not limited to this.

[0073] 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 can be produced by either an immersion type hot-dip plating method (batch type hot-dip plating method) or a continuous type hot-dip plating method.

[0074] There are no particular restrictions on the size, shape, surface form, etc., of the steel material to be plated. Any steel material, including ordinary steel, high-tensile steel, stainless steel, etc., can be used. More specifically, various steel materials, such as general steel, Ni pre-plated steel, Al killed steel, extra-low carbon steel, high carbon steel, various high-tensile steels, and some high alloy steels (steels containing strengthening elements such as Ni and Cr, etc.), can be used. Steel strips of general structural steel are most preferable.

[0075] The steel material may be surface-finished in advance using shot blasting, abrasive brushes, etc., and the surface may be plated with Ni, Zn, Sn, etc. at a concentration of 3 g / m. 2There is no problem if the following metal or alloy film is applied before plating. As a pretreatment of the steel material, it is preferable to thoroughly clean the steel material by degreasing and pickling.

[0076] After the steel surface is sufficiently heated and reduced with a reducing gas such as H2, the steel is immersed in a plating bath prepared with a prescribed composition. For high-tensile steels, the atmosphere during annealing is generally humidified and internal oxidation methods are used to ensure plating adhesion on high-Si, Mn steels, etc., and such treatment makes it possible to plate plated steel with few unplated areas and few appearance defects in the same way as ordinary steel. In such steels, a steel surface with fine crystal grains and an internal oxide coating layer are observed on the base steel side, but this does not affect the performance of the present invention.

[0077] In the case of a hot-dip plating method, the components of the plating layer can be controlled by the components of the plating bath to be prepared. The plating bath is prepared by mixing predetermined amounts of pure metals, for example, by a melting method in an inert atmosphere to prepare an alloy of the plating bath components.

[0078] By immersing the steel material with a reduced surface in a plating bath maintained at a specified concentration, a plating layer with almost the same composition as the plating bath is formed. If the immersion time is extended or if it takes a long time to complete solidification, the formation of the interface alloy layer becomes active, and the Fe concentration may become high, but at temperatures below 500°C, the reaction with the plating layer slows down rapidly, so the Fe concentration in the plating layer is usually less than 5.00%.

[0079] To form the hot-dip plating layer, the plating bath is preferably kept at a temperature of 450°C to 550°C. The reduced steel material is then preferably immersed in the bath for several seconds. On the surface of the reduced steel material, Fe diffuses into the plating bath and reacts with the plating bath, and an interfacial alloy layer (mainly an Al-Fe alloy layer) may form at the interface between the plating layer and the steel material. When an interfacial alloy layer is formed, the steel material below the interfacial alloy layer and the plating layer above are bonded metal-chemically more firmly.

[0080] After immersing the steel material in the plating bath for a predetermined time, the steel material is pulled out of the plating bath, and while the metal attached to the surface is in a molten state, N2 wiping is performed to adjust the plating layer to a predetermined thickness. The thickness of the plating layer is preferably adjusted to 3 to 80 μm. This is converted into the amount of adhesion of the plating layer, and is 10 to 500 g / m 2 The thickness of the plating layer may be adjusted to 5 to 70 μm. This corresponds to a coating weight of about 20 to 400 g / m 2 (One-sided).

[0081] After adjusting the coating weight of the plating layer, the deposited molten metal is solidified. The cooling means for solidifying the molten metal may be spraying nitrogen, air, or a mixed gas of hydrogen and helium, mist cooling, or submersion in water. Mist cooling is preferred, and mist cooling in which water is contained in nitrogen is preferred. The cooling rate may be adjusted by the water content in the mist.

[0082] Under normal plating solidification conditions, for example, cooling from the plating bath temperature to 150°C at an average cooling rate of 5 to 20°C / sec, it may be difficult to control the structure, making it difficult to achieve the desired performance. Therefore, the cooling conditions that enable the plating layer of this embodiment to be obtained are described below. As described below, it is preferable to adjust the average cooling rate for each temperature range under these cooling conditions.

[0083] Average cooling rate between bath temperature and 380℃: 10℃ / sec to 20℃ / sec Between the bath temperature and 380°C, Si compounds are precipitated when Si is contained. If the cooling rate is fast, the solidification of the plating proceeds without sufficient Si being precipitated, and Si tends to be uniformly dispersed in the plating layer. On the other hand, if the cooling rate is slow, the Si compounds are coarsened, Si is concentrated at the interface between the plating layer and the steel material, and the amount of Si compounds precipitated on the surface layer of the plating layer tends to be reduced. Therefore, in order to precipitate Si compounds on the surface layer, it is necessary to set the average cooling rate between the bath temperature and 380°C to 10°C / s to 20°C / s. Even if the plating layer does not contain Si, it is preferable to set the average cooling rate between the bath temperature and 380°C to 10°C / s to 20°C / s. The preferred lower limit of the average cooling rate is 15°C / s, and the preferred upper limit is 20°C / s. By setting the average cooling rate to the preferred lower limit or more, the Si compounds tend to be concentrated on the surface, and the MgZn2 structure is refined, resulting in an effect of improving workability. By setting the average cooling rate to the preferred upper limit or less, the Si compound tends to concentrate on the surface, and the Mg2Zn that precipitates when the degree of supercooling is large is reduced. 11 Thus, the effect of suppressing the precipitation of Cr and improving the workability is obtained.

[0084] Average cooling rate between 380℃ and 250℃: 20℃ / sec or more Between 380°C and 250°C, the Al phase precipitates from the liquid phase of the molten metal attached to the steel material, and a ternary eutectic reaction of the Zn-Al-MgZn2 phase occurs, and finally the liquid phase disappears and the coating layer completely solidifies. In addition, in the temperature range between 400°C and 300°C, the Al phase changes to the Al-Zn phase, which is a high-temperature stable phase, that is, the precipitated Al phase absorbs the Zn phase formed as the [Al / MgZn2 / Zn ternary eutectic structure], and the [Al / MgZn2 / Zn ternary eutectic structure] decreases. In order to improve the water-wet corrosion resistance and the running water corrosion resistance, the average cooling rate between 380°C and 250°C is at least 20°C / s or more. More preferably, the average cooling rate is 30°C / s or more. By setting the average cooling rate to 30°C / s or more, the area ratio of the Al phase tends to increase. In other words, it is preferable to set the average cooling rate between 380° C. and 250° C. faster than the average cooling rate between the bath temperature and 380° C. There is no particular upper limit to the average cooling rate, but it may be, for example, 50 / sec.

[0085] Average cooling rate between 250℃ and 150℃: 20℃ / sec or less The Al-Zn phase is unstable in the temperature range of 250°C to 150°C, and separates into the Zn phase and the Al phase. That is, the Al-Zn phase transforms into the Al phase or the Zn-Al phase. Therefore, in order to improve the water-wet corrosion resistance and the running water corrosion resistance, it is preferable to cool slowly in this temperature range and promote the precipitation of the Al phase. Therefore, the cooling rate between 250°C to 150°C is set to 20°C / sec or less. As a result, the Al-Zn phase separates into the Al phase and the Zn phase, and transforms into the Al phase or the Zn-Al phase. This tendency is particularly strong when the Al concentration is high. More preferably, it is set to 10°C / s or less, and more preferably, it is set to less than 5°C / sec. By setting the average cooling rate to 10°C / sec or less, the area ratio of the Al-Zn phase is reduced, and stable and good water-wet corrosion resistance and running water corrosion resistance are obtained. The lower limit of the average cooling rate is not particularly limited, but it may be 2.5°C / sec. If the average cooling rate is less than 2.5°C, the MgZn2 phase in the plating structure tends to coarsen, resulting in poor workability.

[0086] Average cooling rate in the temperature range below 150℃ The average cooling rate in the temperature range below 150°C during the solidification process does not affect the constituent phases in the plating layer, so there is no need to particularly limit the rate, and natural cooling may be used.

[0087] By undergoing the above cooling conditions, the plating layer of this embodiment is formed.

[0088] After cooling the plating layer, various chemical conversion treatments and painting treatments may be performed. In order to further improve corrosion resistance, touch-up paint application for repair or thermal spraying treatment may be performed on welded parts, processed parts, etc.

[0089] 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 the type of coating directly on the plating layer include a chromate coating, a phosphate coating, and a chromate-free coating. The chromate treatment, phosphate treatment, and chromate-free treatment for forming these coatings can be performed by known methods. However, since many chromate treatments may deteriorate the weldability on the plating layer surface, it is preferable to keep the thickness of the chromate treatment less than 1 μm.

[0090] Chromate treatment includes electrolytic chromate treatment, which forms a chromate film by electrolysis, reactive chromate treatment, which forms a film by utilizing a reaction with the material and then washes away excess treatment liquid, and coating chromate treatment, which applies a treatment liquid to the substrate and then dries it without rinsing with water to form a film. Any of these treatments may be used.

[0091] 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 latexes, diisopropanolamine-modified epoxy resins, etc.), and hard silica.

[0092] Examples of the phosphate treatment include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

[0093] 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 coating-type 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.

[0094] Furthermore, one or more organic resin films may be provided on the film directly on the plating layer. The organic resin is not limited to a specific type, and examples thereof include polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, and modified products of these resins. The modified product here refers to a resin obtained by reacting a reactive functional group contained in the structure of these resins with another compound (monomer, crosslinking agent, etc.) containing a functional group capable of reacting with the functional group in the structure.

[0095] As such an organic resin, one or more organic resins (unmodified) may be mixed and used, or one or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin may be mixed and used. In addition, any coloring pigment or rust-preventive pigment may be contained in the organic resin film. Aqueous resins that have been dissolved or dispersed in water may also be used. EXAMPLES

[0096] The plated steel original sheets were cut out to a size of 180 mm x 100 mm from cold-rolled steel sheets with a thickness (t) of 3.2 mm and a thickness (t) of 0.8 mm. Both sheets were SS400 (general steel). Using a batch-type hot-dip galvanizing simulator (manufactured by Rhesca Co., Ltd.), a K thermocouple was attached to a part of the steel sheet, and the steel sheet surface was sufficiently reduced by annealing at 800°C in a reducing atmosphere of N2 containing 5% H2, and then immersed in a plating bath of various compositions for 3 seconds, and then pulled out and adjusted to a plating thickness of 10 to 30 μm by N2 gas wiping. However, in Examples 55 to 58, the plating thickness was 3 μm, 5 μm, 70 μm, and 80 μm. The plating thickness on the front and back of the original sheet was the same. After pulling out from the plating bath, plated steel sheets were produced under various cooling conditions A to F below.

[0097] The average chemical composition of the plating layer was measured as follows. The plating layer was peeled off and dissolved with an acid containing an inhibitor that suppresses corrosion of the base steel (steel material) to obtain an acid solution. The obtained acid solution was then measured using ICP atomic emission spectrometry or ICP-MS to obtain the average chemical composition of the plating layer.

[0098] Condition A: After the steel material was pulled out of the plating bath, the average cooling rate between the bath temperature and 150° C. was set to any value less than 5° C. / sec (comparative condition).

[0099] Condition B: After the steel material was pulled out of the plating bath, the average cooling rate between the bath temperature and 150° C. was set to any value between 5° C. / sec and 20° C. / sec (comparative conditions).

[0100] Condition C: After the steel material was pulled out of the plating bath, the average cooling rate between the bath temperature and 150° C. was set to any value of 20° C. / sec or more (comparative conditions).

[0101] Condition D: After the steel material was pulled out of the plating bath, the average cooling rate between the bath temperature and 380°C (range T1) was set to any value between 10°C / sec and 20°C / sec, the average cooling rate between 380°C and 250°C (range T2) was set to any value between 20°C / sec and 50°C / sec, and the average cooling rate between 250°C and 150°C (range T3) was set to any value less than 5°C / sec (preferred conditions). Therefore, in condition D, the average cooling rate was adjusted for each temperature range. Condition D-2: After the steel material was pulled out of the plating bath, the average cooling rate between the bath temperature and 380°C (range T1) was set to any value between 10°C / sec and 20°C / sec, the average cooling rate between 380°C and 250°C (range T2) was set to any value between 20°C / sec and 50°C / sec, and the average cooling rate between 250°C and 150°C (range T3) was set to any value less than 10°C / sec. Therefore, in condition D-2, the average cooling rate was adjusted for each temperature range.

[0102] Condition E: After the steel material was pulled out of the plating bath, the average cooling rate between the bath temperature and 380°C (range T1) was set to any value less than 10°C / sec, the average cooling rate between 380°C and 250°C (range T2) was set to any value between 20°C / sec and 50°C / sec, and the average cooling rate between 250°C and 150°C (range T3) was set to any value less than 5°C / sec (comparative conditions). Therefore, in condition E, the average cooling rate was adjusted for each temperature range.

[0103] Condition F: After the steel material was pulled out of the plating bath, the average cooling rate between the bath temperature and 380°C (range T1) was set to any value of 20°C / sec or more, the average cooling rate between 380°C and 250°C (range T2) was set to any value less than 20°C / sec, and the average cooling rate between 250°C and 150°C (range T3) was set to any value of 5°C / sec or more (comparative conditions). Therefore, in condition F, the average cooling rate was adjusted for each temperature range.

[0104] The method for measuring the area ratio of the phases and structures (MgZn2 phase, Al-containing phase, [Al / MgZn2 / Zn ternary eutectic structure], Mg2Si phase, Al-Ca-Si phase, and the remainder) in the coating layer was as described above, in which a cross section of the coating layer in the thickness direction perpendicular to the surface of the steel material was exposed, and the metal structure was confirmed in a field of view of 500 to 5000 times.2 The metal structure was observed in the field of view to determine the area ratio of the phases or structures that make up the metal structure of the plating layer. To confirm each phase, the composition of the phase was pinpointed using EDS analysis, and phases with roughly equivalent components were identified by reading them using element mapping, etc. Element mapping made it possible to distinguish phases with roughly the same composition.

[0105] The presence of Mg2Si was also confirmed by X-ray diffraction measurement. In the X-ray diffraction measurement, Cu-Kα radiation was used, and the X-ray output was set at 50 kV and 300 mA. The presence of Mg2Si was determined by whether or not the diffraction peak of the Mg2Si phase appeared in the X-ray diffraction pattern of the plating layer surface. The X-ray diffraction device used was a horizontal sample type high-power X-ray diffractometer RINT-TTR III manufactured by Rigaku Corporation.

[0106] IΣ(Si) / (IΣ(Zn)+IΣ(Al)) and IΣ(Ca) / IΣ(Si) were measured by performing GDS analysis in the depth direction of the plating layer. Specifically, the analysis conditions for depth direction analysis using argon sputtering were argon pressure: 0.27MPa, output power: 30W, output voltage: 1000V, and discharge area: within a circular area with a diameter of 4mm. The plating layer was etched to a depth of 0.5μm from the surface while GDS measurement was performed, and the integrated intensity values ​​of Si, Zn, and Al were obtained to calculate IΣ(Si) / (IΣ(Zn)+IΣ(Al)) and IΣ(Ca) / IΣ(Si).

[0107] Al-Ca-Si phase (Ca3Al2Si2 phase, CaAl2Si2 phase, CaAl2Si2 phase, CaAl 1-x S 1+x The presence of the Al-Ca-Si phase was confirmed by X-ray diffraction measurement. The X-ray diffraction measurement was performed using Cu-Kα radiation at an X-ray output of 50 kV and 300 mA. The X-ray diffraction pattern of the plating layer surface was measured, and the Al-Ca-Si phase was identified based on whether a diffraction peak of the Al-Ca-Si phase appeared. The X-ray diffraction device used was a horizontal sample type high-power X-ray diffractometer RINT-TTR III manufactured by Rigaku Corporation.

[0108] (Sacrificial corrosion protection evaluation) A test piece with a plate thickness (t) of 3.2 mm was cut to 120 mm x 50 mm, and one end surface that contacted the stand was painted, while the other cut end surface was left uncured. The JASO M609 test was then carried out to evaluate the sacrificial corrosion protection. The occurrence of red rust on the flat surface was confirmed in a combined cycle test. Red rust was considered to have occurred when the red rust area ratio was 5% or more. The sacrificial corrosion protection was evaluated as follows: "B" was a failure, and "A" to "S" were passes.

[0109] Red rust was observed in less than 200 cycles: "B" Red rust occurred after 200 cycles: "A" Red rust occurred within 200 to 350 cycles: "AA" Red rust occurs within 350 to 500 cycles: "AAA" No red rust occurred after 500 cycles: "S"

[0110] (Evaluation of wet corrosion resistance and running water corrosion resistance) A test piece with a thickness (t) of 0.8 mm was cut to 120 mm x 60 mm, and a vinyl plate was attached to create a flow path with a width of 20 mm and a length of 100 mm. The sample was set at an inclination of 15° from the horizontal, and artificial rainwater (Cl) was intermittently injected at 6 ml / min from the upstream of the flow path through a droplet nozzle. - : 10 ppm, SO4 2- : 40 ppm, NO3 - (Adjusted to a concentration of 20 ppm. Further, an aqueous NaOH solution was added to adjust the pH to 5). The amount of droplets was adjusted to 100 to 200 μl per drop. When red rust was observed on 5% or more of the flow path area, it was considered that red rust had occurred, and the time it took for red rust to occur was evaluated as follows. The pass / fail result of water corrosion was determined as follows: "B" was a failure, and "A" to "S" were passes.

[0111] Red rust occurred in less than 8 weeks: "B" Red rust occurred within 8 to 12 weeks: "A" Red rust occurred within 12 to 14 weeks: "AA" Red rust occurred within 14 to 16 weeks: "AAA" No red rust has occurred for 16 weeks or more: "S"

[0112] (Processability evaluation) A test piece with a plate thickness (t) of 0.8 mm was cut to 30 mm x 100 mm, and subjected to 2t180° bending (a test in which a spacer twice the plate thickness is sandwiched and bending is performed at 180°), then bent back to make it flat, and a Cellophane Tape (registered trademark) peeling test was performed on the inner bent part. After the peeling test, the peeled area of ​​the plating on the inner bent surface was measured. The inner bent part was defined as an area within 3 mm from the apex of the bent part on the inner bent surface, i.e., an area of ​​30 mm x 6 mm, and the workability was evaluated based on the area ratio of the peeled plating area to the inner bent part. In the workability evaluation, "B" below was deemed to be a failure, and "A" to "S" were deemed to be a pass.

[0113] Peeling area is over 10%: "B" Peeling area is between 5% and 10%: "A" Peeling area 5% or less: "AA" No peeling: "S"

[0114] As shown in Tables 1A to 5B, Nos. 4 to 25, 27, 28, 30, 32, 33, 35 to 37, 39, 40, 43 to 45, 47, 48, and 53 to 58 (all examples) were manufactured under the preferred cooling conditions of the present invention, and the average chemical composition of the plating layer was within the appropriate range, and the area ratio of the phase and structure of the plating layer was within the appropriate range. As a result, the sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability were excellent. The Zn content in the plating layer of the examples was 50% or more.

[0115] Nos. 1 to 3 (comparative examples) had low Al and Mg contents, and the area ratios of the MgZn2 phase and the [Al / MgZn2 / Zn ternary eutectic structure] were outside the appropriate range. In addition, the manufacturing conditions were outside the preferred range. As a result, the sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability were inferior.

[0116] No. 26 (comparative example) had a low Mg content and the area ratio of the MgZn2 phase was outside the appropriate range, which resulted in inferior sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability.

[0117] No. 29 (comparative example) had an excessive amount of Sn, which resulted in inferior sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability.

[0118] No. 31 (comparative example) had an excessive amount of Bi, which resulted in inferior sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability.

[0119] No. 34 (comparative example) had an excessive amount of In. In addition, the manufacturing conditions were outside the preferred range. Furthermore, ([Al] + [Zn-Al]) / [Al-Zn] was less than 0.8. As a result, the sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability were inferior.

[0120] No. 38 (comparative example) had excessive La and Ce contents. In addition, the manufacturing conditions were outside the preferred range. Furthermore, ([Al] + [Zn-Al]) / [Al-Zn) was less than 0.8. As a result, the sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability were inferior.

[0121] No. 41 (comparative example) had an excessive amount of Sr. In addition, the manufacturing conditions were outside the preferred ranges. As a result, the sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability were inferior.

[0122] No. 42 (comparative example) had an excessive amount of Mg. Furthermore, the area ratio of the MgZn2 phase was outside the appropriate range. This resulted in inferior sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability.

[0123] No. 46 (comparative example) had an excessive amount of P. This resulted in inferior sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability.

[0124] No. 49 (comparative example) had excessive amounts of Al and Ca. In addition, the manufacturing conditions were outside the preferred range. Furthermore, ([Al] + [Zn-Al]) / [Al-Zn] was less than 0.8. As a result, the sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability were inferior.

[0125] No. 50 (Comparative Example) had excessive amounts of Al and Mg, and the manufacturing conditions were outside the preferred ranges, which resulted in inferior sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability.

[0126] No. 51 (comparative example) had an excessive amount of Al, which resulted in inferior sacrificial corrosion resistance, wet corrosion resistance, running water corrosion resistance, and workability.

[0127] No. 52 (comparative example) had an excessive amount of Al. In addition, the manufacturing conditions were outside the preferred range. Furthermore, the total of the Al phase, the Al-Zn phase, and the Zn-Al phase exceeded 75%. As a result, the sacrificial corrosion resistance, the wet corrosion resistance, the running water corrosion resistance, and the workability were inferior.

[0128] [Table 1A]

[0129] [Table 1B]

[0130] [Table 1C]

[0131] [Table 2A]

[0132] [Table 2B]

[0133]

Table 2C

[0134]

Table 3A

[0135]

Table 3B

[0136]

Table 3C

[0137]

Table 4A

[0138]

Table 4B

[0139]

Table 5A

[0140]

Table 5B

Claims

1. A plated steel product having a steel material and a plating layer disposed on a surface of the steel material, The average chemical composition of the plating layer is, in mass%, Al: 10.0% to 40.0%, Mg: more than 4.0% to 8.5%, Si: 0% to 5.0%, Ca: 0% to 3.00%, Sn: 0% to 3.00%, Bi: 0% to 1.00%, In: 0% to 1.00%, Y: 0% to 0.50%, La: 0% to 0.50%, Ce: 0% to 0.50%, Sr: 0% to 0.50%, B: 0% to 1.00%, P: 0% to 0.50%, Cr: 0% to 0.25%, Ti: 0% to 0.25%, V: 0% to 0.25%, Zr: 0% to 0.25%, Ni: 0% to 1.00%, Co: 0% to 0.25%, Nb: 0% to 0.25%, Cu: 0% to 1.00%, Mn: 0% to 0.25%, Mo: 0% to 0.25%, W: 0% to 0.25%, Ag: 0% to 1.00%, Li: 0% to 0.50%, Na: 0% to 0.05%, K: 0% to 0.05%, Fe: 0% to 5.00%, Sb: 0% to 0.50%, Pb: 0% to 0.50%, Ba: 0% to 0.25%, The balance is Zn and impurities. In a cross section of the plating layer perpendicular to the surface of the steel material, the structure observed with a scanning electron microscope has an area fraction of: MgZn ２ Phase: more than 10% and less than 50%, Sum of Al phase, Al-Zn phase and Zn-Al phase: 15% or more and 75% or less, [Al / MgZn ２ / Zn ternary eutectic structure]: 0% or more and 65% or less, Remainder: 0% or more and 5.0% or less; A plated steel material in which an area ratio (([Al]+[Zn-Al]) / [Al-Zn]) of a total of the Al phase and the Zn-Al phase to the Al-Zn phase is 0.8 or more.

2. In the average chemical composition of the plating layer, Si is 0.05% to 5.0%; 2. The plated steel material according to claim 1, wherein an integrated intensity value of each element in a surface layer from the surface of the plating layer to a depth of 0.5 μm, when a depth analysis is performed by a GDS method, satisfies the following formula (1): IΣ(Si) / (IΣ(Zn)+IΣ(Al))≧0.005…(1) Here, IΣ(Si), IΣ(Zn) and IΣ(Al) in formula (1) are the integrated intensity values ​​of Si, Zn and Al detected by the GDS method, respectively.

3. In the average chemical composition of the plating layer, Ca is 0.01% to 3%; In the plating layer, Ca ３ A ２ S ２ Phase, CaAl ２ S ２ Phase, CaAl １－ｘ S １＋ｘ phase (where x = 0 to 0.2), 3. The plated steel material according to claim 1, wherein an integrated intensity value of each element in a surface layer from the surface of the plating layer to a depth of 0.5 μm, when a depth analysis is performed by a GDS method, satisfies the following formula (2): IΣ(Ca) / IΣ(Si)≧2.0…(2) Here, IΣ(Ca) and IΣ(Si) in formula (2) are the integrated intensity values ​​of Ca and Si detected by the GDS method, respectively.

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