Plated steel material

Abstract

A plating layer of the plated steel material contains, in mass%, 10.0%-40.0% of Al, 5.0%-12.5% of Mg, 0%-1.0% of Ti, 0%-1.0% of Zr, and 50.0% or more of Zn and impurities, the total of Ti and Zr being 0.001% or more, and the diffraction intensity obtained from the X-ray diffraction measurement result of the plating layer satisfying formula (1). I (200) Al / {(I (111) Al + I (220) Al + I (200) Al + I (311) Al} > = 0.40 (1) In formula (1), I (200) Al is the diffracted intensity of Al (200), I (111) Al is the diffracted intensity of Al (111), I (220) Al is the diffracted intensity of Al (220), and I (311) Al is the diffracted intensity of Al (311).

Description

Technical Field

[0001] This invention relates to coated steel.

[0002] This application claims priority based on Japanese Patent Application No. 2023-131415 filed on August 10, 2023, the contents of which are incorporated herein by reference. Background Technology

[0003] Coated steel is used in the building materials and civil engineering fields due to its excellent corrosion resistance. Hot-dip Zn-Al-Mg based steel, in particular, is widely used in these fields because of its superior corrosion resistance. Hot-dip Zn-Al-Mg based steel is sometimes processed into various shapes before becoming various final products.

[0004] In hot-dip Zn-Al-Mg coatings, representative phase structures include the MgZn2 phase, the [Al / Zn / MgZn2] ternary eutectic structure, and primary Al crystals. Primary Al crystals are considered to be Zn-containing Al dendrites. The MgZn2 phase and the [Al / Zn / MgZn2] ternary eutectic structure are considered to have low plastic deformation capacity. While primary Al crystals are considered to have higher plastic deformation capacity than the MgZn2 phase and the [Al / Zn / MgZn2] ternary eutectic structure, this is not considered sufficient. Therefore, hot-dip Zn-Al-Mg steel coatings become poorly machinable. Consequently, hot-dip Zn-Al-Mg steel coatings suffer from the following problems: inability to follow processing deformation, coating cracking at processed areas, and lower corrosion resistance at processed areas compared to flat surfaces.

[0005] Patent Document 1 discloses a technique that precipitates MgZn2 in Al primary crystals to inhibit the corrosion of the Al primary crystals. Patent Document 1 aims to improve the corrosion resistance of the processed part by inhibiting the corrosion rate of the Al primary crystals, but does not investigate improving the processability of the coating itself.

[0006] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2005-336546 Summary of the Invention

[0007] The problem that the invention aims to solve The present invention was made in view of the above circumstances, and the objective is to provide coated steel with excellent corrosion resistance and excellent processability of the coating.

[0008] Methods for solving problems To solve the above problems, the following configuration is adopted.

[0009] [1] A plated steel material comprising steel and a coating on said steel. The average chemical composition of the coating, expressed as a percentage by mass, comprises: Al: 10.0%~40.0% Mg: 5.0%~12.5%, Ti: 0%~1.0% Zr: 0%~1.0% Si: 0%~5.00% Ca: 0%~3.00% Y: 0%~0.50% La: 0%~0.50% Ce: 0%~0.50% Sr: 0%~0.50% Sn: 0%~3.00% Bi: 0%~1.00% In: 0%~1.00%, B: 0%~1.00% P: 0%~0.50%, Cr: 0%~0.25% V: 0%~0.25% Ni: 0%~1.0% Co: 0%~0.25%, Nb: 0%~0.25%, Cu: 0%~1.0%, Mn: 0%~0.25%, Mo: 0%~0.25%, W: 0%~0.25% Ag: 0%~1.00%, Li: 0%~0.50% Na: 0%~0.05% Ba: 0%~0.25% K: 0%~0.05% Fe: 0%~5.0%, Sb: 0%~0.50% Pb: 0%~0.50%, The remaining portion consists of over 50.0% Zn and impurities. The total content of Ti and Zr in the coating is 0.001% or more. The diffraction intensity obtained from the X-ray diffraction measurement of the coating satisfies the following formula (1).

[0010] I (200) Al / {(I(111) Al +I (220)Al +I (200) Al +I (311) Al ≥0.40 (1) In equation (1), I (200) Al Let I be the diffraction intensity of Al (200), and I be the diffraction intensity of Al (111). Al Let I be the diffraction intensity of Al (111), and let I be the diffraction intensity of Al (220). Al Let I be the diffraction intensity of Al (220), and I be the diffraction intensity of Al (311). Al Let be the diffraction intensity of Al (311).

[0011] [2] According to the plated steel described in [1], wherein the total content of Ti and Zr in the coating is 0.020% or more.

[0012] [3] According to the coated steel described in [1] or [2], in the cross section of the coating in the thickness direction, the total area of ​​the Al phase with an equivalent circle diameter of less than 20 μm is more than 50% of the total area of ​​the Al phase.

[0013] Invention Effects According to the present invention, it is possible to provide coated steel with excellent corrosion resistance and excellent processability of the coating. Attached Figure Description

[0014] Figure 1 This is a cross-sectional schematic diagram of the plated steel material according to an embodiment of the present invention. Detailed Implementation

[0015] When plating steel is bent, if cracks occur in the coating, exposing the base metal, the corrosion-resistant properties of the coating in the surrounding area come into play, causing the leaching of the coating's constituent elements and partial consumption of the coating itself. Therefore, the coating near the workpiece may have lower corrosion resistance compared to the coating outside the workpiece. To prevent this, it is necessary to prevent cracking of the coating at the workpiece. Regarding the coating, the greater its plastic deformation capacity, the less prone it is to cracking.

[0016] Zn-Al-Mg coatings have a phase structure containing multiple phases and structures, such as the MgZn2 phase, a ternary eutectic structure of Al / Zn / MgZn2, and primary Al crystals. The primary Al crystals have a crystal structure containing Zn Al dendrites. The MgZn2 phase and the ternary eutectic structure of Al / Zn / MgZn2 are considered to have low plasticity and poor workability. On the other hand, it is known that the workability of Al varies depending on the crystal orientation. Therefore, if the orientation of the Al crystals, mainly contained in the primary Al crystals, can be controlled, the overall workability of the coating can be expected to improve, and cracking at the processing area can be reduced.

[0017] The inventors of this invention conducted research with the aim of improving the processability of the coating by controlling the crystal orientation of the Al phase. They found that if the coating contains one or both of Zr or Ti, the proportion of the (100) plane in the crystal orientation plane of Al at the surface of the coating increases, thereby improving the processability of the coating.

[0018] Furthermore, Zr and Ti form compounds with Al in the coating, specifically intermetallic compounds such as Al3Zr and Al3Ti. If Si is further present, Al is formed. 2.7 Si 0.3 Zr, Al 2.5 Si 0.5 Ti. These intermetallic compounds act as solidification nuclei for primary Al crystals, resulting in the refinement of the primary Al crystals. Consequently, the processability of the coating is further improved.

[0019] Steel structures made with such coated steel exhibit excellent workability and corrosion resistance.

[0020] The coated steel material according to embodiments of the present invention will be described below.

[0021] The coated steel of this embodiment includes steel and a coating on the surface of the steel. The average chemical composition of the coating, in mass percent, includes Al: 10.0%–40.0%, Mg: 5.0%–12.5%, Ti: 0%–1.0%, Zr: 0%–1.0%, Si: 0%–5.00%, Ca: 0%–3.00%, Y: 0%–0.50%, La: 0%–0.50%, Ce: 0%–0.50%, Sr: 0%–0.50%, Sn: 0%–3.00%, Bi: 0%–1.00%, In: 0%–1.00%, B: 0%–1.00%, P: 0%–0.50%, Cr: 0%–0.25%, V: 0%–0.25%. Ni: 0%~1.0%, Co: 0%~0.25%, Nb: 0%~0.25%, Cu: 0%~1.0%, Mn: 0%~0.25%, Mo: 0%~0.25%, W: 0%~0.25%, Ag: 0%~1.00%, Li: 0%~0.50%, Na: 0%~0.05%, Ba: 0%~0.25%, K: 0%~0.05%, Fe: 0%~5.0%, Sb: 0%~0.50%, Pb: 0%~0.50%, the remainder being 50.0% or more of Zn and impurities, and the total Ti and Zr in the coating being 0.001% or more. The diffraction intensity obtained from the X-ray diffraction measurement of the coating satisfies the following formula (1). Here, the surface of the steel refers to the interface between the coating and the steel. The coating on the surface of the steel refers to the coating on the steel.

[0022] I (200) Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al ≥0.40 (1) In equation (1), I (200) Al Let I be the diffraction intensity of Al (200), and I be the diffraction intensity of Al (111). Al Let I be the diffraction intensity of Al (111), and let I be the diffraction intensity of Al (220). Al Let I be the diffraction intensity of Al (220), and I be the diffraction intensity of Al (311). Al Let be the diffraction intensity of Al (311).

[0023] In addition, the total content of Ti and Zr in the coating is preferably 0.020% or more.

[0024] Furthermore, in this embodiment, it is preferable that, in the cross-section along the thickness direction of the coating, the total area of ​​the Al phase with an equivalent circle diameter of 20 μm or less accounts for 50% or more of the total area of ​​the Al phase.

[0025] In the following explanation, the percentage (%) for the content of each element in the chemical composition refers to "mass %". Additionally, the numerical range indicated by "~" refers to the range including the values ​​before and after the "~" as lower and upper limits. It should be noted that when the values ​​before and after the "~" are marked "exceeding" or "below", the numerical range does not include these values ​​as lower or upper limits.

[0026] The term "corrosion resistance" refers to the property of the coating itself not being easily corroded. Because Zn-based coatings have a chemically protective effect on steel, the coating corrodes and turns white rust before the steel corrodes. After the white rust coating disappears, the steel corrodes and produces red rust. This is the corrosion process of coated steel plates.

[0027] The term "processability" refers to the property of plating that makes it less prone to cracking when the plating is bent.

[0028] like Figure 1As shown, the plated steel 1 of this embodiment has a steel material 11. There are no particular limitations on the shape of the steel material 11; one example is a steel plate. Alternatively, the steel material 11 can also be a base steel material that has undergone forming processes, such as steel pipes, civil engineering materials (fences, corrugated pipes, drainage covers, sand-proof boards, bolts, metal mesh, guardrails, waterproof walls, etc.), appliance components (outdoor unit housings of air conditioners, etc.), or automotive parts (running components, etc.). Forming processes include various plastic forming methods such as pressing, roll forming, and bending.

[0029] There are no particular restrictions on the material of steel 11. For example, steel 11 can be made of various types of steel, such as general steel, Ni-plated 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, there are no particular restrictions on the manufacturing methods of steel 11, such as hot rolling, pickling, cold rolling, etc. Moreover, steel 11 can also be made of steel with a metal or alloy film of less than 1 μm formed on it, such as Zn, Ni, Sn, or their alloys.

[0030] Next, the coating 12 will be described. The plated steel 1 of this embodiment has a coating 12 disposed on the surface of the steel 11. When the steel 11 is a steel plate, the coating 12 is disposed on at least one side of the steel plate and the other side opposite to the first side. In addition, the coating 12 is also disposed on the end face between one side and the other side of the steel plate.

[0031] The coating 12 in this embodiment, due to its chemical composition described later, is mainly composed of a Zn-Al-Mg alloy layer. The Zn-Al-Mg alloy layer is formed from a Zn-Al-Mg alloy. A Zn-Al-Mg alloy refers to a ternary alloy containing Zn, Al, and Mg. The Zn-Al-Mg alloy layer, with alloying elements such as Al and Mg added to Zn, exhibits improved corrosion resistance compared to a conventional Zn coating. For example, even at half the thickness of a conventional Zn coating, the Zn-Al-Mg alloy layer possesses corrosion resistance equivalent to that of a Zn coating. Therefore, the coating of this embodiment also possesses corrosion resistance equal to or greater than that of a Zn coating.

[0032] Alternatively, the coating 12 of the plated steel 1 in this embodiment may also include an Fe-Al interfacial alloy layer (hereinafter referred to as an Al-Fe alloy layer) between the steel 11 and the Zn-Al-Mg alloy layer. Furthermore, the Al-Fe alloy layer is an interfacial alloy layer located between the steel and the Zn-Al-Mg alloy layer.

[0033] The coating in this embodiment can be a single-layer structure of a Zn-Al-Mg alloy layer, or a stacked structure comprising a Zn-Al-Mg alloy layer and an Al-Fe alloy layer. In the case of a stacked structure, it is preferable to designate the Zn-Al-Mg alloy layer as the surface layer constituting the coating. However, an oxide film of coating constituent elements is formed on the outermost surface of the coating with a thickness of less than about 1 μm, but since it is thin relative to the overall thickness of the coating, it can be ignored from the main body of the coating.

[0034] Generally, a thinner coating improves processability, but considering coating loss due to corrosion, a thicker coating makes it easier to ensure corrosion resistance. Therefore, the overall coating thickness is preferably set to 10–70 μm. It should be noted that the overall coating thickness is influenced by the plating conditions and is not limited to the range of 10–70 μm. In typical hot-dip plating methods, the overall coating thickness is affected by the viscosity and specific gravity of the plating bath. Furthermore, the overall coating thickness is adjusted by the speed at which the steel (coating substrate) is retrieved and the intensity of wiping.

[0035] The Al-Fe alloy layer is formed on the surface of steel (specifically, between the steel and the Zn-Al-Mg alloy layer) and is predominantly composed of the Al5Fe2 phase in terms of its microstructure. The Al-Fe alloy layer is formed through atomic diffusion between the base metal (steel) and the plating bath. When using a hot-dip galvanizing method, an Al-Fe alloy layer is easily formed in plating containing Al. Because the plating bath contains a certain concentration or higher of Al, the Al5Fe2 phase is formed most frequently. However, atomic diffusion takes time, and there are regions with higher Fe concentrations near the base metal. Therefore, the Al-Fe alloy layer may also contain small amounts of AlFe, Al3Fe, Al2Fe, etc. Furthermore, because the plating bath also contains a certain concentration of Zn, the Al-Fe alloy layer also contains small amounts of Zn.

[0036] When the coating contains Si, Si is particularly readily incorporated into the Al-Fe alloy layer, potentially forming an Al-Fe-Si intermetallic compound phase. Among the identified intermetallic compound phases is the AlFeSi phase, which exists as isomers such as α, β, q1, and q2-AlFeSi phases. Therefore, these AlFeSi phases can be detected in Al-Fe alloy layers. Al-Fe alloy layers containing these AlFeSi phases are also referred to as Al-Fe-Si alloy layers.

[0037] Next, the average chemical composition of the coating will be explained. The overall average chemical composition of the coating is the average chemical composition of the Zn-Al-Mg alloy layer when the coating is a single-layer structure. Furthermore, when the coating is a laminated structure of Al-Fe alloy layer and Zn-Al-Mg alloy layer, the average chemical composition is the combined average chemical composition of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer.

[0038] Typically, in hot-dip plating, since the coating formation reaction is essentially completed within the plating bath, the chemical composition of the Zn-Al-Mg alloy layer becomes roughly the same as that of the plating bath. Furthermore, in hot-dip plating, the Al-Fe alloy layer forms and grows instantaneously immediately after immersion in the plating bath. The Al-Fe alloy layer then completes its formation reaction within the plating bath, and its thickness is generally significantly smaller than that of the Zn-Al-Mg alloy layer. Therefore, unless special heat treatments such as alloying are performed after plating, the overall average chemical composition of the coating is substantially equal to that of the Zn-Al-Mg alloy layer, and the composition of the Al-Fe alloy layer, etc., can be ignored.

[0039] The chemical composition of the coating in this embodiment includes Zn, other alloying elements, and impurities. Alternatively, the chemical composition of the coating in this embodiment may also consist of Zn, other alloying elements, and impurities. The chemical composition of the coating will be described in detail below. It should be noted that elements with a concentration lower limit of 0% are optional elements; they are not essential for solving the problems of the coated steel in this embodiment, but are permitted to be included in the coating for the purpose of improving properties, etc.

[0040] Al: 10.0%~40.0% Al, like Zn, is a major component of the coating. Although Al has a small role in corrosion resistance, its presence in the coating improves the corrosion resistance of planar surfaces. Furthermore, without Al, Mg cannot be stably maintained in the plating bath; therefore, Al is an indispensable element in the manufacturing process and is included in the plating bath.

[0041] The Al content is set at 10.0% or higher to ensure a high content of Mg (described later) and to guarantee processability. If the content is below this level, it becomes difficult to prepare the plating bath, making it difficult to ensure both the processability and corrosion resistance of the coating. Furthermore, the Al content is set at 40.0% or lower because Al has a weak corrosion-resistant effect on steel; if the content exceeds this level, sufficient corrosion resistance cannot be achieved. Therefore, the upper limit is set at 40.0% or lower.

[0042] Mg: 5.0%~12.5% Mg is an element that provides corrosion resistance and improves the corrosion resistance of coatings. By containing a certain amount of Mg, a MgZn2 phase is formed in the coating. The higher the Mg content in the coating, the more MgZn2 phase is formed. The MgZn2 phase is known to have a structure called the Laves phase, and it is known to have high hardness. Setting the Mg content to 5.0% or more is the concentration required to achieve corrosion resistance; when the Mg content is below 5.0%, sufficient corrosion resistance is not obtained. Furthermore, the MgZn2 phase will not form sufficiently in the coating, and the corrosion resistance of the coating itself will also decrease. If the Mg content becomes excessive, the coating manufacturing becomes difficult, and the processability of the coating decreases; therefore, its upper limit is 12.5% ​​or less. A more preferred Mg content is 6.0% to 8.0%.

[0043] Ti: 0%~1.0% Zr: 0%~1.0% If the plating bath contains one or both of Ti and Zr, intermetallic compounds Al3Zr and Al3Ti will be formed, respectively. Additionally, if Si is present, Al will be formed. 2.7 Si 0.3 Zr, Al 2.5 Si 0.5 Ti. These have good lattice matching with the crystal structure of the Al phase and function as solidification nuclei for primary Al crystals. When intermetallic compounds containing Ti and Zr function as solidification nuclei for primary Al crystals, the (100) planes of Al preferentially align parallel to the surface of the coating on the surface of the coated steel. If the total concentration of Ti and Zr is 0.001% or more, formation begins in such a way that the (100) planes of Al become parallel to the surface of the coating, and there is a tendency for the orientation of the (100) planes to increase with increasing concentrations of Ti and Zr. Therefore, the lower limit of the total concentration of Ti and Zr is 0.001% or more, and more preferably 0.020% or more.

[0044] Furthermore, if the combined concentration of Ti and Zr is 0.020% or higher, the primary Al crystals begin to refine, exhibiting a tendency for the primary Al crystals to refine with increasing concentration. The grain refinement effect saturates when the combined concentration of Ti and Zr is 0.5%. Refining the crystal structure improves processability; therefore, for the refinement of the primary Al crystals, a combined concentration of Ti and Zr of 0.020% or higher is preferable. On the other hand, if the concentrations of Ti and Zr become high, the plating bath construction tends to become difficult. Additionally, if the concentrations of Ti and Zr each exceed 1.0%, a large amount of slag is generated, leading to frequent uncoated areas and a deterioration in appearance and corrosion resistance. Therefore, the concentrations of Ti and Zr are each set to 1.0% or less. The combined concentration of Ti and Zr is 0.001% to 2.0%, more preferably 0.001% to 1.5% or 0.010% to 0.5%, and even more preferably 0.1% to 0.5%.

[0045] Furthermore, if the plating bath contains one or both of Ti and Zr, it will inhibit the growth of Mg2Zn. 11 The tendency for phase precipitation improves processability.

[0046] Si: 0%~5.00% Si is an optional additive element, but if Si is included in the plating bath, a single-phase Si or Mg2Si will precipitate in the coating. If Ca is further included, Al-Ca-Si compounds will precipitate. By precipitating these Si or Si-based compounds in the surface layer of the coating, the water resistance and water flow resistance are improved. In addition, Si forms an Al-Fe-Si phase by being incorporated into the Al-Fe alloy layer, thereby inhibiting the growth of the Al-Fe alloy layer, thus improving bending workability and coating adhesion. It is preferable that the Si content is 0.05% or more. However, if the Si content exceeds 5.00%, a large amount of slag will be generated, and incomplete coating will occur frequently. Therefore, the Si concentration is set to 5.00% or less. The preferred Si concentrations are 0% to 5.00%, 0.05% to 3.00%, 0.05% to 1.0%, or 0.10% to 0.50%.

[0047] Element group A Ca: 0%~3.00% Y: 0%~0.50% La: 0%~0.50% Ce: 0%~0.50% Sr: 0%~0.50% Ca, Y, La, Ce, and Sr, which are in element group A, are optional additive elements. However, these elements are easily oxidized in the atmosphere. If present in the plating bath, they can form a dense oxide film on the bath surface and prevent the oxidation of Mg. Through the above-mentioned effect, the Mg concentration is stabilized, making it easier to manufacture the coated steel sheet with the target composition. To achieve this effect appropriately, it is preferable to set the content of these elements to more than 0%, and more preferably to more than 0.01%. In addition, there is an upper limit to the content of each element. If the upper limit is exceeded, it tends to make the construction of the plating bath more difficult. Furthermore, there is a tendency for more slag and other deposits to adhere, and for the appearance and corrosion resistance to deteriorate.

[0048] Therefore, Ca is set to 0% to 3.00%, preferably to more than 0% and less than 2.00%, more preferably to more than 0.01% and less than 2.00%, and even more preferably to 0.01% to 1.50%. In addition, Ca can be less than 1.00%, less than 0.60%, or less than 0.50%.

[0049] Furthermore, Y, La, Ce, and Sr are each set to 0% to 0.50%, preferably more than 0% and less than 0.50%, and more preferably more than 0.01% and less than 0.50%. Moreover, each element can be more than 0.01%, less than 0.40%, or less than 0.30%. Element group A forms compounds with Al and Zn in the plating structure. Taking Ca as an example, when Ca is present at more than 0%, preferably more than 0.01%, an Al-Ca-Zn compound is formed; if Si is further present, an Al-Ca-Si compound is more easily formed. Furthermore, if Y, La, Ce, and Sr are present, the Ca forming the above compounds is replaced by substances of each element.

[0050] Element group B Sn: 0%~3.00% Bi: 0%~1.00% In: 0%~1.00% The elements in element group B are optional additions, but these elements have the effect of improving corrosion resistance. However, these elements tend to bind more strongly to Mg than to Zn, resulting in a smaller effect from the Mg content. Therefore, there is an upper limit to the content of these elements. If the upper limit is exceeded, there is a tendency for more slag and other deposits to adhere and for corrosion resistance to deteriorate. Therefore, Sn is set to 0 to 3.00%, more preferably to more than 0% and less than 3.00%. Sn can also be 0.01% or more, 0.05% or more, 2.50% or less, 2.00% or less, or 1.50% or less. Bi is set to 0% to 1.00%, more preferably to more than 0% and less than 1.00%. Bi can also be 0.01% or more, 0.05% or more, 0.80% or less, 0.50% or less, or 0.40% or less. In is set to 0% to 1.00%, more preferably to more than 0% and less than 1.00%. In can also be above 0.01%, above 0.05%, below 0.80%, below 0.50%, or below 0.40%.

[0051] Element group C B: 0%~1.00% P: 0%~0.50% B and P, belonging to element group C, are semi-metallic elements. These elements are optional additives, but B acts as a solidification nucleus for Al primary crystals in the form of AlB2, thereby refining the Al primary crystals and improving their processability. However, its effect is not significant and does not reach the level of Ti and Zr. Although P does not refine the Al primary crystals, it improves corrosion resistance. There are upper limits to the content of each element. If the upper limit is exceeded, there is a tendency for more slag to adhere, and for the appearance and corrosion resistance to deteriorate. Therefore, B is set to 0% to 1.00%, preferably more than 0% and less than 0.50%, and more preferably more than 0% and less than 0.10%. B can also be more than 0.01% or more, or more than 0.05%. P is set to 0% to 0.50%, more preferably more than 0% and less than 0.50%, and more preferably more than 0% and less than 0.01%. P can be greater than 0.001% or greater than 0.005%.

[0052] Element group D Cr: 0%~0.25% V: 0%~0.25% Ni: 0%~1.0% Co: 0%~0.25% Nb: 0%~0.25% Cu: 0%~1.0% Mn: 0%~0.25% Mo: 0%~0.25% W: 0%~0.25% Ag: 0%~1.00% Li: 0%~0.50% Na: 0%~0.05% Ba: 0%~0.25% K: 0%~0.05% Fe: 0%~5.0% Element group D consists of metallic elements, which are optional additions. These elements are incorporated into the coating, thereby improving corrosion resistance. There are upper limits to the content of each element; exceeding these limits can lead to increased adhesion of slag and other contaminants. Therefore, Cr, V, Co, Nb, Mn, Mo, W, and Ba are set to 0%–0.25%, preferably more than 0% and less than 0.25%, more than 0.01% and less than 0.20%, or more than 0% and less than 0.10%, respectively. Ni, Cu, and Ag are set to 0–1.0%, preferably more than 0% and less than 1.0%, more than 0% and less than 0.5%, more than 0% and less than 0.20%, or more than 0% and less than 0.10%. Ni, Cu, and Ag can be more than 0.01% or more, or more than 0.05%. Li is set to 0%–0.50%, preferably more than 0% and less than 0.10%. Li can be 0.01% or more. Na and K are set to 0% to 0.05%, preferably more than 0% and less than 0.03%. Na and K can also be 0.01% or more. In addition, Fe may inevitably be contained in the coating. This is because it may diffuse from the base metal into the coating during the coating manufacturing process. Therefore, the Fe content is 0% to 5.0%, preferably more than 0% and less than 2.0%, more than 0% and less than 1.5%, more than 0% and less than 1.2%, or more than 0% and less than 1.0%. Fe can also be 0.1% or more, 0.3% or more, 0.5% or more, or 0.9% or less.

[0053] Element group E Sb: 0%~0.50% Pb: 0%~0.50% Sb and Pb, which are in element group E, are optional additive elements and have properties similar to Zn. Therefore, the presence of these elements facilitates the formation of zinc flower patterns and other effects on the appearance of the coating. However, excessive amounts of these elements may reduce corrosion resistance. Therefore, Sb and Pb are set to 0% to 0.50%, preferably more than 0% and less than 0.50%, more than 0% and less than 0.40%, or more than 0% and less than 0.10%. Sb and Pb can also be 0.01% or more, or 0.05% or more, respectively.

[0054] Remaining portion: Zn: 50.0% or more and impurities Zinc (Zn) is a low-melting-point metal that exists as the main phase in steel coatings. Zn is an element necessary to ensure corrosion resistance and provide corrosion protection for the steel. If the Zn content is less than 50.0%, the main metallic structure of the Zn-Al-Mg alloy layer becomes the Al phase, resulting in insufficient Zn phase to provide corrosion protection. Therefore, the Zn content is set to 50.0% or more. More preferably, it is set to 60.0% or more or 70.0% or more. It should be noted that the upper limit of the Zn content refers to the amount remaining after considering other elements and impurities. The Zn content can also be 85.0% or less.

[0055] Furthermore, impurities in the coating refer to components contained in the raw materials, components mixed in during the manufacturing process, and components other than the optional added elements mentioned above, which are included to a extent that does not affect the effects of the present invention. For example, in the coating, due to the mutual atomic diffusion between the steel (base metal) and the plating bath, it is possible for trace amounts of components other than Fe to be mixed in as impurities.

[0056] It should be noted that, in the coating of this embodiment, elements other than those listed above are not excluded, as long as they do not affect the effect of the present invention. "Does not affect the effect of the present invention" refers to obtaining an A rating or higher in the corrosion resistance and processability evaluations described later.

[0057] Furthermore, in the coating of this embodiment, the total content of Al, Mg and Zn is preferably 83.7% or more, or it may be 90.0% or more, or it may be 94.7% or more.

[0058] To determine the average chemical composition of the coating, an acid solution is obtained by peeling and dissolving the coating with an acid containing an inhibitor that suppresses corrosion of the base metal (steel). The chemical composition is then determined by ICP-MS or ICP-luminescence spectrophotometry. Any acid capable of dissolving the coating can be used; there are no particular restrictions. If the area and weight before and after peeling are measured, the coating adhesion weight (g / m²) can also be obtained simultaneously. 2 ).

[0059] The structure of the coating will be described next.

[0060] The proportion of phases contained in a coating has a significant impact on its performance. Even coatings with the same composition can exhibit different phases or structures in their metallic microstructure depending on the manufacturing method, resulting in varying properties. The metallic microstructure of a coating can be easily confirmed using a scanning electron microscope (SEM-EDS) with an energy-dispersive X-ray analyzer. In any vertical cross-section (thickness direction) of the mirror-finished coating, for example, by obtaining a reflected electron image, the approximate state of the metallic microstructure can be confirmed. In this embodiment, the coating thickness is approximately 10–70 μm; therefore, in the SEM, a field of view of 500–5000x is preferred for confirming its metallic microstructure. For example, when confirming a 25 μm thick coating at 2000x magnification, each field of view can be confirmed as 25 μm (coating thickness) × 40 μm (SEM field width) = 1000 μm. 2 The cross-section of the coating in the region. In this embodiment, since the SEM field of view for the coating may be observed in a localized area, in order to obtain average information about the coating, the field of view of 25 points selected from any cross-section can be used as the average information. That is, for a total area of ​​25000 μm... 2 The area ratio and size of the metal structure that constitutes the coating can be determined by observing the metal structure in the field of view.

[0061] The reflectance electron image obtained using SEM is preferred in that it allows for easy identification of the phases or structures contained in the coating. Since elements with small atomic numbers, such as Al, are imaged darker, while elements with large atomic numbers, such as Zn, are imaged lighter, the proportions of these structures can be easily read.

[0062] To identify individual phases, EDS analysis precisely determines the phase composition by using elemental mapping and other methods to identify phases with approximately identical compositions. EDS analysts can use elemental mapping to distinguish phases with roughly the same composition. If phases with roughly the same composition can be identified, the area of ​​their crystalline phase in the field of view can be determined. If the area is known, the equivalent circle diameter can be calculated.

[0063] Furthermore, it is possible to determine the area proportion of each phase in the field of view. The area proportion of a given phase in the coating becomes equivalent to the volume proportion of that phase in the coating.

[0064] In this embodiment, the coating comprises a MgZn2 phase, Al primary crystals comprising an Al phase, and a ternary eutectic structure of Al / Zn / MgZn2. Furthermore, the coating may also contain additional structures. The Al primary crystals are composed solely of the Al phase, or are formed with the Al phase occupying the central portion of the dendrites and the Al-Zn phase occupying the outer periphery.

[0065] The preferred proportions of phases and structures in the coating are as follows: in any vertical cross-section (thickness direction) of the coating, based on the area fraction observed in the field of view of a scanning electron microscope, the area fraction of the MgZn2 phase is 15% to 50%, the combined area fraction of the Al phase and Al-Zn phase is 15% to 70%, the area fraction of the [Al / Zn / MgZn2 ternary eutectic structure] is 0% to 60%, and the area fraction of other phases is 0% to 10%. It should be noted that as long as the coating of this embodiment satisfies the above chemical composition and the above formula (1), it has excellent corrosion resistance and processability. Therefore, the area fractions of phases and structures in the coating are not limited to the above ranges.

[0066] MgZn2 phase In this embodiment, the MgZn2 phase is the region in the coating where Mg is 16% (±5%) by mass and Zn is 84% ​​(±5%). The MgZn2 phase is mostly captured as a grayish hue, an intermediate color between Al and Zn, in SEM reflectance electron microscopy images. In SEM reflectance electron microscopy images, the MgZn2 phase can be clearly distinguished from the Al phase, Al-Zn phase, and [the ternary eutectic structure of Al / Zn / MgZn2].

[0067] A higher proportion of the MgZn2 phase in the coating improves corrosion resistance. However, the MgZn2 phase, known as the Laffers phase, may reduce the coating's workability. Therefore, from a corrosion resistance perspective, a higher proportion of the MgZn2 phase is preferable, but an upper limit can be set to ensure workability. To achieve a balance between corrosion resistance and workability, the proportion of the MgZn2 phase in the coating can be 15% to 50% in terms of area fraction. The area fraction of the MgZn2 phase can be 20% or more, 25% or more, 45% or less, or 40% or less.

[0068] Al phase In this embodiment, the Al phase constituting the primary Al crystal is located in the region of the coating where the Al content exceeds 40% by mass. Zn may also be present in the Al phase, but the Zn content is less than 60% by mass. The Al phase can be clearly distinguished from other phases and structures in the SEM reflectance electron image. That is, the Al phase is usually displayed as the darkest part in the SEM reflectance electron image. In this embodiment, the Al phase takes various forms, such as appearing as blocky, circular, or flat tree-like cross-sections in any cross-section. It should be noted that in this embodiment, the Al contained in the [Al / Zn / MgZn2 ternary eutectic structure] is not included in the Al phase.

[0069] There are no particular restrictions on the area ratio of the Al phase in the coating, but it can be 5% or more, 10% or more, 20% or more, or 25% or more. Alternatively, the Al phase can be less than 60%, 55% or less, or less than 50%.

[0070] Compared to the MgZn2 phase and the ternary eutectic structure of Al / Zn / MgZn2, the Al phase exhibits superior plasticity. It is known that the Al phase adopts a face-centered cubic crystal structure, which is orientation-dependent on processing. In bending processes, if the preferred orientation at the surface of the coating is the (100) plane, the plasticity of the coating increases, and the coating extends with the processing, thus reducing cracking in the processed portion. Furthermore, the tendency for the preferred orientation to develop varies depending on the Al content of the coating. If the Al content is low and the Zn content is high, the (110) plane becomes the preferred orientation; as the Al content increases, the preferred orientation changes to the (100) plane.

[0071] The preferred orientation of the Al phase at the surface of the coating can be derived by performing X-ray diffraction. For example, when X-ray diffraction is measured using a Cu source with an output condition of 50 kV-300 mA, the (111) plane appears near 38.47°, the (200) plane near 44.74°, the (220) plane near 65.13°, and the (311) plane near 78.23° in the 2θ peak. Since the (200) plane is parallel to the (100) plane and the (220) plane is parallel to the (110) plane, the (200) plane can be replaced by the (100) plane and the (220) plane can be replaced by the (110) plane when evaluating the preferred orientation at the surface of the coating by X-ray diffraction.

[0072] Whether the (100) plane appears as a preferred orientation on the surface of the coating can be confirmed by the ratio of the peak intensity of the (200) plane to the peak intensity of other orientation planes. It can be said that the higher the peak intensity of the (200) plane, the more likely the (100) plane will appear as a preferred orientation. In this embodiment, when the diffraction intensity obtained from the X-ray diffraction measurement of the coating satisfies the following formula (1), the (100) plane becomes a preferred orientation relative to the surface of the coating, which can improve the processability of the coating.

[0073] I (200) Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al ≥0.4 (1) In equation (1), I (200) Al Let I be the diffraction intensity of Al (200), and I be the diffraction intensity of Al (111). AlLet I be the diffraction intensity of Al (111), and let I be the diffraction intensity of Al (220). Al Let I be the diffraction intensity of Al (220), and I be the diffraction intensity of Al (311). Al The diffraction intensity of Al (311). In the coating composition of the present invention, the value of formula (1) may also be 0.7 or less.

[0074] (1) The I (200) specified in the formula Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al The determination method for} is as follows. First, the surface of the coating is mechanically ground and then chemically ground as needed to set the surface of the coating to a mirror state. Next, for example, an X-ray diffraction apparatus (Rigaku Corporation, model RINT-TTR III) is used with the following settings: X-ray output of 50kV, 300mA, copper target, goniometer TTR (horizontal goniometer), slit width of Kβ filter of 0.05mm, length limiting slit width of 2mm, light-receiving slit width of 8mm, and light-receiving slit 2 open. X-ray diffraction is performed with a scanning speed of 5deg. / min, a step size of 0.01deg, and a scanning axis of 2θ (5~90°). Then, the diffraction intensity of Al (200) is measured. The diffraction intensities of Al(111) (maximum intensity within the range of 44.74°±0.20°), Al(220) (maximum intensity within the range of 65.13°±0.20°), and Al(311) (maximum intensity within the range of 78.23°±0.20°) were determined. The diffraction intensities were set to the values ​​after removing the background intensity. I(200) was then calculated from the obtained diffraction intensities. Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al}

[0075] Furthermore, in any perpendicular cross-section (thickness direction) of the coating, the total area of ​​Al phases with an equivalent circular diameter of 20 μm or less in the Al phase contained in the coating is preferably 50% or more in proportion to the total area of ​​all Al phases. That is, in the cross-section along the thickness direction of the coating, the total area of ​​Al phases with an equivalent circular diameter of 20 μm or less is preferably 50% or more in proportion to the total area of ​​the Al phase. This results in a finer Al primary crystal structure, further improving the processability of the coating. Since finer Al primary crystals tend to improve the processability of the coating, the ratio of the total area of ​​Al phases with an equivalent circular diameter of 20 μm or less to the total area of ​​the Al phase in the thickness direction can also be 70% or more. An upper limit of 100% is also possible.

[0076] The method for determining the area ratio of Al phase with an equivalent circle diameter of less than 20 μm involves extracting the Al phase using image extraction within a specified observation field, measuring the area of ​​each Al phase, and thus deriving the equivalent circle diameter. The specific measurement method is explained below.

[0077] The observation field of view was set to a total of 25,000 μm to confirm the phases and microstructure of the aforementioned coating. 2 The observation field of view is defined. In EDS analysis of this field of view, the phase composition is precisely identified by using SEM image contrast and elemental mapping to determine the Al phase by identifying approximately equal compositional phases. Since Al is a light element, it can be represented as a black area in the SEM reflectance electron image. Alternatively, elemental mapping can also be used to identify the Al phase in EDS analysis. The method for identifying the Al phase is not limited to the above; for example, EPMA mapping can also be used. Next, image processing is used to extract the Al phases in the observation field of view, and the area of ​​each phase is measured. If the area is determined, the equivalent circle diameter of each Al phase can be calculated. Then, Al phases with an equivalent circle diameter of 20 μm or less are extracted, and their total area is calculated. Finally, the area ratio (%) of Al phases with an equivalent circle diameter of 20 μm or less relative to the total area of ​​Al phases in the observation field of view is calculated.

[0078] Al-Zn phase In this embodiment, the Al-Zn phase is a phase containing 60% by mass or more of Zn and Al. The Al-Zn phase is an aggregate of fine Zn phase with a particle size of approximately 1 μm (hereinafter referred to as fine Zn phase) and fine Al phase with a particle size of less than 1 μm (hereinafter referred to as fine Al phase). In the molten coating, Al acquires a structure different from its crystal structure at room temperature (e.g., 25°C), becoming capable of dissolving a large amount of Zn phase, and exists as a high-temperature stable phase containing approximately 60% Zn phase. On the other hand, at room temperature, the Zn phase content in this high-temperature stable phase is extremely reduced, and Al and Zn separate in equilibrium, resulting in the Al-Zn phase, which contains both fine Al and fine Zn phases. In other words, the Al-Zn phase contains 60% by mass or more of fine Zn phase. Because its properties differ from both the Al and Zn phases contained in the coating, this Al-Zn phase can be distinguished in reflected electron SEM images and wide-angle X-ray diffraction. It is believed that: in wide-angle X-ray diffraction, for example as Al 0.403 Zn 0.597 (JCODF #00-052-0856), Al 0.71 Zn 0.29 (PDF#00-019-0057) and other inherent diffraction peaks. Therefore, in this embodiment, the phase with an Al content of 15 to 40% by mass and a Zn content of 60 to 85% by mass is used as the Al-Zn phase.

[0079] As mentioned above, the Al-Zn phase, being an aggregate of fine Al and Zn phases, exhibits superior plasticity compared to the MgZn2 phase and the ternary eutectic structure of Al / Zn / MgZn2. However, it is believed to have inferior corrosion resistance compared to the MgZn2 phase, the ternary eutectic structure of Al / Zn / MgZn2, and the Al phase.

[0080] There are no particular restrictions on the area fraction of the Al-Zn phase, but it can be above 5% or 10%. Additionally, the area fraction of the Al-Zn phase can be below 20% or below 15%.

[0081] In this invention, the total area ratio of Al primary crystals, i.e., Al phase and Al-Zn phase, can also be 15% to 70%. The total area ratio of Al phase and Al-Zn phase can also be 20% or more, 25% or more, or 30% or more, or it can be less than 65%, 60%, 55% or less, or 50%.

[0082] [Ternary eutectic structure of Al / Zn / MgZn2] The ternary eutectic structure of Al / Zn / MgZn2 is a eutectic structure containing Al, MgZn2 and Zn phases. In reflected electron SEM images, it is clearly distinguished from the MgZn2 phase, which is the main phase of the coating, and the aforementioned Al phase.

[0083] By ensuring the presence of a certain degree of the [Al / Zn / MgZn2 ternary eutectic structure] containing the Zn phase, the corrosion resistance of the end face is improved by ensuring the substitution corrosion resistance. On the other hand, water resistance and water flow resistance decrease. Therefore, if water resistance and water flow resistance are taken into consideration, the [Al / Zn / MgZn2 ternary eutectic structure] can also be 60% or less in area. Furthermore, it can also be 40% or less, 35% or less, or 30% or less in area. There is no particular limitation on the lower limit of the area percentage of the [Al / Zn / MgZn2 ternary eutectic structure], which can be set to 0%, or it can be set to 5% or more, 10% or more, or 15% or more in area.

[0084] Mg2Zn 11 Mutually Mg2Zn in this embodiment 11 The phase is a region where Mg is 5% (±3%) by mass and Zn is 93% (±4%). Mg2Zn 11 In most cases, the phase is captured as a grayish color between Al and Zn in the reflectance electron images of SEM, and is brighter than the MgZn2 phase.

[0085] Mg2Zn 11 Since the Mg2Zn2 phase lacks plastic deformation capability compared to the MgZn2 phase, it is preferable that it does not precipitate in the coating structure. That is, the preferred form of Mg in the coating structure is the MgZn2 phase, [a ternary eutectic structure of Al / Zn / MgZn2], or the Mg2Si phase described later. Specifically, in X-ray diffraction intensity, Mg2Zn... 11 The diffraction intensity of the (322) plane of phase (maximum intensity within the range of 43.60° ± 0.20°. Hereinafter set as I(322)). Mg2Zn11 The diffraction intensity of the (201) plane of the MgZn2 phase is the maximum intensity within the range of 41.30° ± 0.20°. Hereinafter, it is set as I(201). MgZn2 The ratio of ) can also satisfy I(322) Mg2Zn11 / I (201) MgZn2 / ≤0.2, or more preferably 0.1 or less. The composition of the plating bath and the cooling rate after being retrieved from the plating bath affect Mg2Zn 11 The precipitation of the phase, in this embodiment, the galvanized steel sheet produces a product that satisfies I (322). Mg2Zn11 / I (201) MgZn2 / ≤0.2, the proportion of Mg2Zn in the coated structure11 The proportion of the phase is less than 1% of the area.

[0086] The phases and microstructures described above constitute the main phases of the coating, accounting for over 90% of the coating's area fraction. On the other hand, other metallic phases are formed by including elements other than Zn, Mg, and Al in the coating. For example, Si forms the Mg₂Si phase, and Ca forms the Al-Zn-Ca phase. Representative components of the remaining microstructure may include the Mg₂Si phase, AlZnCa phase, and AlCaSi phase. While these phases improve weldability and corrosion resistance, their effects are not significant. Given the coating's composition, it is difficult to set their combined area fraction to exceed 10%, therefore it is generally acceptable to set it to less than 10%.

[0087] Next, the case of manufacturing the coated steel of this embodiment by hot-dip galvanizing will be described. The coated steel of this embodiment can be manufactured by either an immersion galvanizing method (intermittent) or a continuous galvanizing method.

[0088] There are no particular restrictions on the size, shape, or surface morphology of the steel to be coated. Even ordinary steel, high-tensile steel, stainless steel, etc., can be used, as long as it is steel. Steel strips of general structural steel are preferred. Alternatively, surface finishing can be performed beforehand using shot peening, grinding brushes, etc., to apply a Ni, Fe, Zn, Sn coating of 3g / m. 2 There is no problem with plating after the following metal or alloy film. In addition, as a pretreatment of steel, it is preferable to thoroughly wash the steel by degreasing and pickling.

[0089] After the steel surface is thoroughly heated and reduced using reducing gases such as H2, the steel is immersed in a plating bath with a specified composition. For high-tensile steels, the annealing atmosphere is generally humidified, and internal oxidation methods are used to ensure coating adhesion for high-Si and Mn steels. This treatment allows unplated steels with few appearance defects to be plated in the same way as ordinary steels. Such steels exhibit a fine-grained steel surface and internal oxide film on the base metal side, but this does not affect the performance of the present invention.

[0090] In the case of hot-dip plating, the composition of the coating can be controlled by the composition of the plating bath. The plating bath is prepared by mixing pure metals in a specified amount, for example by melting in an inert atmosphere to create an alloy of plating bath components.

[0091] By immersing the reduced steel surface in a plating bath maintained at a specified concentration, a coating with approximately the same composition as the plating bath is formed. When the immersion time is long, and solidification takes an extended period, the formation of the interfacial alloy layer becomes more active, and the Fe concentration may increase. However, below 500°C, the reaction with the coating slows down rapidly, so the Fe concentration in the coating typically remains below 5.0%.

[0092] To form a hot-dip coating, it is preferable to maintain the plating bath at 450°C to 550°C. Then, it is preferable to immerse the reduced steel in the bath for several seconds. On the surface of the reduced steel, Fe may diffuse into the plating bath and react with it to form an interfacial alloy layer at the interface between the coating and the steel. This interfacial alloy layer is primarily an Al-Fe intermetallic compound layer (Al-Fe alloy layer). With the formation of this interfacial alloy layer (Al-Fe alloy layer), the steel beneath the Al-Fe alloy layer and the coating above it are more chemically bonded together.

[0093] After immersing the steel in the plating bath for a specified time, the steel is removed from the bath and wiped with N2 while the metal adhering to the surface is still in a molten state, thereby adjusting the coating to the specified thickness. The coating thickness is preferably adjusted to 10–70 μm. This translates to a coating adhesion weight of 40–450 g / m³. 2 (Single-sided).

[0094] After adjusting the coating adhesion amount, the adhered molten metal is allowed to solidify. Cooling during solidification can be achieved by blowing nitrogen, air, or a mixture of hydrogen and helium, or by mist cooling or water immersion. Preferably, mist cooling is used, and more preferably, mist cooling with water contained in nitrogen is employed. The cooling rate is adjusted by the water content ratio.

[0095] In this embodiment, if the plating solidification conditions are normal, such as cooling at an average cooling rate of 5 to 20°C / second between the plating bath temperature and 150°C, the microstructure may not be controllable, and therefore the specified performance may not be met. Therefore, the cooling process for obtaining the plating layer of this embodiment will be described below.

[0096] Average cooling rate between bath temperature and 380°C: Over 20°C / second and below 50°C / second Between bath temperatures and 380°C, the Al phase precipitates as primary crystals, followed by the precipitation of the MgZn2 phase. Even within the coating composition range of this invention, if the supercooling is high, there is a tendency for the Al phase to grow more readily on orientation planes other than the (100) plane. Therefore, in order to prioritize the growth of the Al phase on the (100) plane, the average cooling rate needs to be set to at least 50°C / second. On the other hand, if the cooling rate is 20°C / second or less in terms of average cooling rate, there is a tendency for the Al primary crystals to coarsen and for processability to decrease. Therefore, in the region between bath temperatures and 380°C, the average cooling rate needs to be set to more than 20°C / second and less than 50°C / second.

[0097] Average cooling rate between 380℃ and 300℃: 5℃ / second or more but less than 15℃ / second Between 380℃ and 300℃, Al-Zn phase precipitation from the liquid phase and a ternary eutectic reaction of Zn-Al-MgZn2 occur, causing the liquid phase to disappear and the coating to completely solidify. Within this temperature range of 380℃ to 300℃, if the supercooling is significant, Mg2Zn may form. 11 Phase. If Mg2Zn precipitates. 11 The Al phase tends to deteriorate in processability, therefore the average cooling rate between 380°C and 300°C is preferably set to less than 15°C / second. On the other hand, if the average cooling rate is slower than 5°C / second, recrystallization of the Al phase occurs, increasing the proportion of orientation planes other than the (100) plane, thus tending to deteriorate in processability. Therefore, the average cooling rate is preferably set to 5°C / second or more and less than 15°C / second, and more preferably to 5°C / second to 10°C / second.

[0098] Average cooling rate between 300℃ and 150℃: exceeding 10℃ / second but less than 20℃ / second In the temperature range of 300℃ to 150℃, the fine Zn phase incorporated into the Al-Zn phase is rapidly expelled from the Al-Zn phase. Therefore, if cooling is carried out slowly in this temperature range, the proportion of the Al phase in the primary Al crystals increases. This tendency is particularly strong if the Al concentration is high. If the cooling rate between 300℃ and 150℃ is less than 20℃ / second, the Al-Zn phase separates into Al and Zn phases. On the other hand, if the cooling rate is less than 10℃ / second, the ternary eutectic structure undergoes grain growth, forming coarse MgZn2 and Mg2Zn phases. 11 This phase tends to deteriorate in processability. Therefore, in the temperature range of 300°C to 150°C, it is preferable to cool with an average cooling rate of more than 10°C / second and less than 20°C / second.

[0099] Temperature range below 150℃ During solidification, the cooling rate in the temperature range below 150°C will not affect the constituent phases in the coating, so there is no need to limit the cooling conditions, and natural cooling is also acceptable.

[0100] After the coating cools, various chemical conversion treatments and coating processes can be performed. In addition, to further improve corrosion resistance, repair coatings and spray plating can be applied to welded areas, machining areas, etc.

[0101] For the plated steel of this embodiment, a film can also be formed on the plating layer. One or more films can be formed. Examples of films directly above the plating layer include chromate films, phosphate films, and chromate-free films. The chromate treatment, phosphate treatment, and chromate-free treatment used to form these films can be performed using known methods. However, chromate treatments often deteriorate weldability on the plating surface; therefore, to fully utilize the weldability improvement effect of the plating layer, its thickness is preferably set to less than 1 μm.

[0102] For chromate treatment, there are the following types of chromate treatment: electrolytic chromate treatment, which forms a chromate film through electrolysis; reactive chromate treatment, which forms a film by reacting with the raw material and then rinsing off excess treatment solution; and coating-type chromate treatment, which forms a film by applying the treatment solution to the substrate and drying it without washing. Any of these treatments can be used.

[0103] Examples of electrolytic chromate treatments include those using chromic acid, silica sol, resins (phosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene-butadiene latex, diisopropanolamine modified epoxy resin, etc.) and hard silica.

[0104] Examples of phosphate treatments include zinc phosphate treatment, calcium zinc phosphate treatment, and manganese phosphate treatment.

[0105] Chromate-free treatment is preferred, especially since it has no environmental impact. Chromate-free treatments include: electrolytic chromate-free treatment, which forms a chromate-free film through electrolysis; reactive chromate-free treatment, which forms a film by reacting with the raw material and then rinsing off excess treatment solution; and coating-type chromate-free treatment, which forms a film by applying the treatment solution to the substrate and drying it without washing. Any of these treatments can be used.

[0106] Furthermore, one or more layers of organic resin film may be present on the film directly above the coating. The organic resin is not limited to specific types; examples include polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, or modified forms of these resins. Here, "modified form" refers to a resin obtained by reacting reactive functional groups contained in the structure of these resins with other compounds (monomers, crosslinking agents, etc.) containing functional groups in their structure that can react with those functional groups.

[0107] As such an organic resin, one or more organic resins (unmodified resins) can be used in combination, or one or more organic resins obtained by modifying at least one other organic resin can be used in combination in the presence of at least one organic resin. Additionally, any coloring pigments or anti-rust pigments can be included in the organic resin film. Aqueous resins obtained by dissolving or dispersing in water can also be used.

[0108] <Corrosion Resistance Evaluation> The corrosion resistance of a coating can be evaluated using accelerated corrosion tests such as JASO. Specifically, the number of cycles (time) until red rust appears is compared; a longer cycle time indicates good corrosion resistance, while a shorter cycle time indicates poor corrosion resistance. Since the number of cycles until red rust appears also varies depending on the coating's adhesion, it is preferable to compare coated steels with a consistent adhesion.

[0109] <Processability Evaluation> By performing a bending process and measuring the number of cracks in the coating at the processed section, the processability of the coating can be evaluated. Generally speaking, the fewer the cracks in the coating, the better the processability. If there are no cracks in the coating, the consumption of the coating due to corrosion protection can be suppressed, thus making the corrosion resistance of the processed section equal to that of the flat section.

[0110] Example The base sheet for the coated steel was designed as a 180mm x 100mm cut from a 0.8mm thick cold-rolled steel sheet. All sheets were SS400 (general steel). Using an intermittent hot-dip galvanizing simulation apparatus (manufactured by RHESCA), a K thermocouple was mounted on a portion of the steel sheet. The steel surface was thoroughly reduced by annealing at 800°C in a reducing atmosphere containing 5% H2 and N2. The sheet was then immersed in the plating bath for 3 seconds, removed, and wiped with N2 gas to achieve a plating thickness of 20μm (±1μm). The plating thickness was the same on both the front and back sides. After removal from the plating bath, the coated steel was manufactured under various cooling conditions (A to E) described below.

[0111] Condition A: After the steel is retrieved from the plating bath, the average cooling rate is set to 30℃ / s for the bath temperature range of ~380℃, 5℃ / s for the temperature range of 380℃ to 300℃, and 15℃ / s for the temperature range of 300℃ to 150℃. Cooling below 150℃ is set to release.

[0112] Condition B (Comparison Condition): After the steel is retrieved from the plating bath, the average cooling rate between the bath temperature and 150°C is set to 20°C / s. Cooling below 150°C is set to exothermic cooling.

[0113] Condition C (Comparison Condition): After the steel is retrieved from the plating bath, the average cooling rate between the bath temperature and 150°C is set to 2°C / s. Cooling below 150°C is set to exothermic cooling.

[0114] Condition D (Comparison Condition): After the steel is retrieved from the plating bath, the average cooling rate between the bath temperature and 150°C is set to 60°C / s. Cooling below 150°C is set to exothermic cooling.

[0115] Condition E (Comparison Condition): After the steel is retrieved from the plating bath, the average cooling rate is set to 30℃ / s for the bath temperature range of ~380℃, 1℃ / s for the temperature range of 380℃ to 300℃, and 15℃ / s for the temperature range of 300℃ to 150℃. Cooling below 150℃ is set to release.

[0116] The average chemical composition of the coating was determined as follows: An acid solution was obtained by stripping and dissolving the coating with an acid containing an inhibitor that suppresses corrosion of the base metal (steel). The average chemical composition of the coating was then determined by ICP-MS or ICP-luminescence spectrophotometry. The results are shown in Tables 1A to 1F.

[0117] The method for determining the area fraction of phases and microstructures (MgZn2 phase, Al phase, Al-Zn phase, [ternary eutectic structure of Al / Zn / MgZn2], and residual microstructure) in the coating is as described above. A section perpendicular to the thickness direction of the coating is exposed, and the metallic microstructure is confirmed using a field of view of 500 to 5000 times. Specifically, for a total thickness of 25000 μm... 2 The metallic microstructure within the field of view is observed to determine the phases or area fractions of the metallic microstructure constituting the coating. To identify individual phases, EDS analysis precisely confirms their composition by using elemental mapping to identify approximately identical phases. Elemental mapping allows for the identification of phases with approximately the same composition.

[0118] Furthermore, regarding the area ratio of Al phase with an equivalent circle diameter of less than 20 μm, in relation to the aforementioned 25000 μm... 2In the EDS analysis of the observation field, the phase composition was precisely identified by using elemental mapping to read approximately equivalent compositional phases to determine the Al phase. Next, image processing was used to extract the Al phase from the observation field, the area of ​​each crystal was measured, and the equivalent circle diameter of each crystal was calculated. Then, Al phases with an equivalent circle diameter of 20 μm or less were extracted, and their total area was calculated. Finally, the area ratio (%) of Al phases with an equivalent circle diameter of 20 μm or less relative to the total area of ​​Al phases in the observation field was calculated.

[0119] Furthermore, in the X-ray diffraction pattern of the coating surface obtained by measuring with Cu-Kα rays under conditions of 50kV and 300mA X-ray output, I(200) as specified in the above formula (1) is determined. Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al The measurement method was set as follows. First, the surface of the coating was mechanically ground and then chemically ground as needed to achieve a mirror finish. Next, an X-ray diffraction apparatus (Rigaku Corporation, model RINT-TTR III) was used with the following settings: X-ray output 50kV, 300mA, copper target, TTR goniometer (horizontal goniometer), Kβ filter slit width 0.05mm, length limiting slit width 2mm, light-receiving slit width 8mm, and light-receiving slit 2 open. X-ray diffraction measurements were performed with a scanning speed of 5 deg. / min, a step size of 0.01 deg, and a scanning axis of 2θ (5–90°). Then, the diffraction intensity of Al (200) was measured. The diffraction intensities of Al(111) (maximum intensity within the range of 44.74°±0.20°), Al(220) (maximum intensity within the range of 65.13°±0.20°), and Al(311) (maximum intensity within the range of 78.23°±0.20°) were determined. The diffraction intensities were set to the values ​​after removing the background intensity. I(200) was then calculated from the obtained diffraction intensities. Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al The results are shown in Tables 2A and 2B.

[0120] (Corrosion resistance evaluation) Cut the test material into 50×100mm pieces, leaving the end faces uncoated and unsealed. Perform a composite cycle test according to JASO M609 and M610 to evaluate corrosion resistance. Specifically, conduct repeated salt spraying, drying, and wetting cycles. In the salt-dry-wet cycle test, for the test material, spraying with a 5% NaCl aqueous solution (2 hours at 35°C), drying (4 hours at 60°C with 30% relative humidity), and wetting (2 hours at 50°C with 95% relative humidity) constitutes one cycle. After each cycle, wash with water and allow to dry, observe the surface of the test material, and calculate the red rust area ratio. To calculate the red rust area ratio, photograph the surface of the test material after the salt-dry-wet cycle test. Perform image analysis to binarize the photograph, calculate the area of ​​each pixel, and count the pixels of the rusted areas. The red rust area ratio is calculated using the following formula.

[0121] Red rust area percentage (%) = area of ​​rusted part (mm²) 2 ) / Total area of ​​the observation section (mm) 2 )×100 A red rust area ratio of 5% or more is defined as the occurrence of red rust. The corrosion resistance evaluation is set as follows: "B" is set as unacceptable, and "A" to "S" are set as acceptable. The results are shown in Tables 3A and 3B.

[0122] B: Red rust was observed at fewer than 200 cycles.

[0123] A: Red rust appeared after 200 cycles.

[0124] AA: Red rust appeared when the number of cycles exceeded 200 but was less than 350.

[0125] AAA: Red rust appeared at 350 to 500 cycles.

[0126] S: No red rust was produced after 500 cycles.

[0127] (Process evaluation) The test material was cut into pieces measuring 30 mm (C direction) × 100 mm (L direction) and subjected to a 5t 180° bending process. That is, while bending the test material, five sheets of the same thickness as the test material were clamped together and bent 180°. Afterwards, the number of cracks within a 30 mm × 1.6 mm area at the top of the bent portion was measured using a stereomicroscope at 40x magnification, and the results were determined as follows. The results are shown in Table 3.

[0128] B: The number of cracks is 30 or more. A: The number of cracks is more than 20 but less than 30. AA: The number of cracks is 10 or more but less than 20 AAA: Number of cracks is 5 or more but less than 10 S: Number of cracks less than 5 As shown in Tables 1A to 3B, Examples No. 3 to 43, 53 and 54 of the present invention appropriately control the chemical composition and metallic structure of the coating, resulting in excellent corrosion resistance and processability. It does not contain Mg2Zn. 11 Mutually.

[0129] In Comparative Example No. 1, the hot-dip coating had insufficient Al and Mg content, and the content of Ti and Zr was 0%. Additionally, I (200) Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al The value drops below 0.40. Therefore, for No.1, both corrosion resistance and processability are insufficient.

[0130] Comparative Example No. 2 had insufficient Al content in its hot-dip coating, and the contents of Ti and Zr were 0%. I (200) Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al The value becomes below 0.40. Therefore, for No.2, the processability is insufficient.

[0131] The hot-dip coating of Comparative Example No. 44 has excessive Al and Mg content. Therefore, No. 44 is deficient in both corrosion resistance and processability.

[0132] The hot-dip coating of Comparative Example No. 45 has an excessive amount of Al. In the case of No. 45, the corrosion resistance is insufficient.

[0133] The Ti and Zr content of the hot-dip coating in Comparative Example No. 46 was 0%. Additionally, I (200) Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al The value becomes below 0.40. Therefore, for No. 46, the processability is insufficient.

[0134] The hot-dip coating of Comparative Example No. 47 contains excessive Ca. Furthermore, the manufacturing conditions are outside the preferred range. In the case of No. 47, both corrosion resistance and processability are insufficient.

[0135] The manufacturing conditions of comparative examples No. 48, 49, and 50 are outside the range of preferred conditions. Additionally, I (200) Al / {(I(111) Al +I (220) Al +I (200) Al +I (311) Al The value becomes below 0.40. Therefore, the processability of Nos. 48, 49, and 50 decreases.

[0136] The hot-dip coating of Comparative Example No. 51 has an excessive amount of Zr. Therefore, the corrosion resistance of No. 51 is insufficient.

[0137] The hot-dip coating of Comparative Example No. 52 has an excessive amount of Ti. Therefore, the corrosion resistance of No. 52 is insufficient.

[0138] [Table 1A] [Table 1B] [Table 1C] [Table 1D] [Table 1E] [Table 1F] [Table 2A] [Table 2B] [Table 3A] [Table 3B] Industrial availability This invention has industrial applicability in that it can provide coated steel with excellent corrosion resistance and excellent processability of the coating.

[0139] Symbol Explanation 1. Coated steel, 11. Steel, 12. Coating.

Claims

1. A plated steel material, comprising steel and a coating on said steel, The average chemical composition of the coating, expressed as a percentage by mass, comprises: Al：10.0%～40.0%、 Mg: 5.0%~12.5%, Ti: 0%~1.0% Zr：0%～1.0%、 Si: 0%~5.00% Ca: 0%~3.00% Y：0%～0.50%、 La: 0%~0.50% Ce: 0%~0.50% Sr：0%～0.50%、 Sn: 0%~3.00% Bi: 0%~1.00% In: 0%~1.00%, B：0%～1.00%、 P：0%～0.50%、 Cr：0%～0.25%、 V：0%～0.25%、 Ni: 0%~1.0% Co: 0%~0.25%, Nb: 0%~0.25%, Cu: 0%~1.0%, Mn: 0%~0.25%, Mo: 0%~0.25%, W：0%～0.25%、 Ag: 0%~1.00%, Li: 0%~0.50% Na: 0%~0.05% Ba: 0%~0.25% K：0%～0.05%、 Fe: 0%~5.0%, Sb: 0%~0.50% Pb: 0%~0.50%, The remaining portion consists of over 50.0% Zn and impurities. The total content of Ti and Zr in the coating is 0.001% or more. The diffraction intensity obtained from the X-ray diffraction measurement of the coating satisfies the following formula (1). I(200) Al / {(I(111) Al +I(220) Al +I(200) Al +I(311) Al }≥0.40 (1), in, In equation (1), I (200) Al Let I be the diffraction intensity of Al (200), and I be the diffraction intensity of Al (111). Al Let I be the diffraction intensity of Al (111), and let I be the diffraction intensity of Al (220). Al Let I be the diffraction intensity of Al (220), and I be the diffraction intensity of Al (311). Al Let be the diffraction intensity of Al (311).

2. The plated steel according to claim 1, wherein, The total content of Ti and Zr in the coating is 0.020% or more.

3. The plated steel according to claim 1 or claim 2, wherein in the cross section along the thickness direction of the coating, the total area of ​​the Al phase with an equivalent circle diameter of 20 μm or less accounts for more than 50% of the total area of ​​the Al phase.

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