Molten-plated steel material and method for producing molten-plated steel material

Abstract

A molten plated steel material having a steel material and a plating layer disposed on the surface of the steel material, the plating layer having a prescribed chemical composition and a thickness of 5.0 [mu] m or more, the surface of the plating layer being provided with a region A and a region B, the region B having a thickness of 0.02 [mu] m or more formed with a black oxide layer containing a Zn oxide, and the region B having a thickness of 0.02 [mu] m or more. When PA is the mass concentration ratio (O / Zn) of O and Zn in the region (A) of the surface of the plating layer and PB is the mass concentration ratio (O / Zn) of O and Zn in the region (B) of the surface of the plating layer, the following formulae (1)-(3) are satisfied. 0 lt; palt; 0.80... (1) 0.80 < = PB... (2) 0.40 < = PB-PA... (3)

Description

Technical Field

[0001] This disclosure relates to molten coated steel and a method for manufacturing molten coated steel.

[0002] This application claims priority based on Japanese Patent Application No. 2023-191957, filed in Japan on November 10, 2023, the contents of which are incorporated herein by reference. Background Technology

[0003] In the building materials, civil engineering, and automotive industries, fused-coated steel is widely used to manufacture steel structures through various processing methods. For example, in streets, there are many opportunities to directly see the metallic fused-coated steel used in guardrails, windbreaks, electrical panels, cable trays, and other similar applications. Unlike ordinary coated steel, steel structures that form part of roads, railways, and streetscapes often require aesthetic considerations. For example, anti-glare measures for safety in road and railway infrastructure, black-based color schemes in recent urban spaces, and logos representing advertising and brand names all increase the demand for color and appearance design in fused-coated steel. While painting is the most common method, the need for regular repairs due to deterioration and the cost proportional to the construction area present significant bottlenecks. From a durability perspective, stainless steel and aluminum are sometimes used, but their use is often difficult due to material costs and strength considerations.

[0004] As a means of solving these problems, there are techniques for marking the plated metal itself without coating when the molten-coated steel leaves the factory. For example, Patent Documents 1-2 provide examples of molten-coated steel sheets that impart an appearance design by controlling the internal constituent phases of the coating as a metal film and combining it with a resin-based coating. In these existing examples, the appearance design is formed by utilizing the differences in the constituent phases of the metal in the coating. However, in these examples, there is room for improvement in achieving clarity similar to that of handwriting and durability against corrosion accompanying the coating.

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 7107474 Patent Document 2: Japanese Patent Application Publication No. 2021-85084 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] This disclosure was made in view of the above circumstances, and its object is to provide a melt-coated steel with an appearance design that imparts clarity and durability to the coating, and a method for manufacturing the same.

[0008] means for solving problems

[0009] The inventors have solved the above-mentioned problems and discovered that when developing coated steel with a clear and durable appearance design, a specific highly corrosion-resistant coating is formed on the surface of the steel, and an oxide layer with a specific elemental composition is formed on the surface of the coating. By partially irradiating the oxide layer with a laser, a pattern of a desired shape can be formed on the surface of the coating.

[0010] To address the aforementioned issues, this disclosure adopts the following structure.

[0011] [1] One aspect of the present disclosure is a molten-coated steel having a steel and a coating disposed on the surface of the steel, wherein the average chemical composition of the coating, by mass%, comprises Al: more than 10.0% and less than 40.0%, Mg: more than 4.0% and less than 15.0%, Si: more than 0% and less than 1.00%, Sn: more than 0% and less than 0.7%, Bi: more than 0% and less than 0.3%, In: more than 0% and less than 0.3%, the total amount of Sn, Bi and In ΣX: more than 0% and less than 0.7%, Ca: more than 0% and less than 0.60%, Y: more than 0% And below 0.30%, La: above 0% and below 0.30%, Ce: above 0% and below 0.30%, Sr: above 0% and below 0.30%, Li: above 0% and below 0.30%, the combined amount of Ca, Y, La, Ce, Sr and Li ΣYa: above 0% and below 0.60%, Cr: above 0% and below 1.00%, Ni: above 0% and below 1.00%, Mo: above 0% and below 0.25%, Cu: above 0% and below 1.00%, Ag: above 0% and below 0.25%, Sb: above 0% and below 0.25%. The following are the concentrations of Pb: 0% or more and 0.25% or less; the combined concentrations of Cr, Ni, Mo, Cu, Ag, Sb, and Pb: ΣYb: 0% or more and 1.00% or less; B: 0% or more and 0.50% or less; P: 0% or more and 0.50% or less; the combined concentrations of B and P: ΣYc: 0% or more and 0.50% or less; Ti: 0% or more and 0.25% or less; Co: 0% or more and 0.25% or less; V: 0% or more and 0.25% or less; Nb: 0% or more and 0.25% or less; Mn: 0% or more and 0.25% or less; Zr: 0% or more and 0.25% or less. The coating contains 25% or less of the following components: W: 0% or more and 0.25% or less; Ti, Co, V, Nb, Mn, Zr, and W; Z: 0% or more and 0.25% or less; Fe: 0% or more and 5.0% or less; and Zn: 40.0% or more and 86.0% or less. The coating has a thickness of 5.0 μm or more. Region A and Region B are formed on the surface of the coating. A black oxide layer containing Zn oxide is formed in Region B with a thickness of 0.02 μm or more. The mass concentration ratio of O to Zn (O / Zn) in Region A on the surface of the coating is set as P. A Let the mass concentration ratio (O / Zn) of O in region B on the surface of the coating be P. B Under the condition that the following equations (1) to (3) are satisfied. 0 <P A <0.80…(1) 0.80≤P B …(2) 0.40≤P B -PA …(3)

[0012] [2] The molten coated steel described in [1] can also be, satisfying the following formula (4). 0.50≤P B -P A …(4)

[0013] [3] According to the melt-coated steel described in [1] or [2], the Mg mass concentration in region B of the surface of the coating may be 3.2% by mass or more.

[0014] [4] The molten coated steel according to any one of [1] to [3] may also be such that the difference (ZB-ZA) between the height ZA of the surface of region A and the height ZB of the surface of region B is 0.1 μm or more and less than 5.0 μm.

[0015] [5] The molten steel according to any one of [1] to [4] may also include a region C adjacent to the region A in the coating, wherein the mass concentration ratio (O / Zn) of O in the region C is set to P. C Under the condition that the following equations (5) and (6) are satisfied. 0 <P A <P C <0.80…(5) P C <0.40+P A …(6)

[0016] [6] According to the molten coated steel described in [5], the arithmetic mean roughness Ra of the surface of the region C in units of μm may also satisfy the following formula (7). Ra<2.0…(7)

[0017] [7] In any of the following methods, the molten coated steel may be provided with a pattern portion including the region A on the surface of the coating, and the pattern portion may be provided with an intentional shape.

[0018] [8] In any of the following methods, the molten coated steel may be provided with a pattern portion including the region A on the surface of the coating. The pattern portion may be any one of straight lines, curves, dots, graphics, numbers, symbols, patterns or text, or a combination of two or more of them.

[0019] [9] The molten coated steel according to any one of [1] to [6] may also have a pattern portion including the region A provided on the surface of the coating, wherein the pattern portion is an intentional shape composed of any one or more of the following: a straight line portion, a curved portion, a dot portion, a graphic, a number, a mark, a pattern or a character.

[0020]

[10] In the molten coated steel according to [7], the patterned portion may include the region A, the region B and the region C that exists between the region A and the region B.

[0021]

[11] In the molten steel according to [8], the patterned portion may include the region A, the region B and the region C that exists between the region A and the region B.

[0022]

[12] In the molten steel according to [9], the patterned portion may include the region A, the region B and the region C that exists between the region A and the region B.

[0023]

[13] The molten coated steel according to any one of [1] to

[12] may also have a non-patterned portion formed by the region B provided on the surface of the coating.

[0024]

[14] Another aspect of the present disclosure describes a method for manufacturing molten coated steel, which is described in [1] and includes the following steps: a molten coating step, wherein the average chemical composition, by mass %, comprises Al: more than 10.0% and less than 40.0%, Mg: more than 4.0% and less than 15.0%, Si: more than 0% and less than 1.00%, Sn: more than 0% and less than 0.7%, Bi: more than 0% and less than 0.3%, In: more than 0% and less than 0.3%, the total amount of Sn, Bi and In ΣX: more than 0% and less than 0.7%, and Ca: more than 0% and less than 0.60%. The following are the concentrations of Ca, Y, La, Ce, Sr, and Li: ΣYa: 0% to 0.30%, 0% to 0.30%, 0% to 0.30%, 0% to 0.30%, 0% to 0.30%, 0% to 0.30%, and the combined concentrations of Ca, Y, La, Ce, Sr, and Li: ΣYa: 0% to 0.60%, 0% to 1.00%, 0% to 1.00%, 0% to 1.00%, 0% to 1.00%, 0% to 0.25%, 0% to 1.00%, 0% to 0.25%, 0% to 0.25%, 0% to 0.25%, and 0% to 0.25%, respectively. The following are the concentrations of the following elements: 0% and below, Cr, Ni, Mo, Cu, Ag, Sb, and Pb; ΣYb: 0% and below, B: 0% and below, P: 0% and below, ΣYc: 0% and below, Ti: 0% and below, Co: 0% and below, V: 0% and below, Nb: 0% and below, Mn: 0% and below, Zr: 0% and below, W: 0% and below, ΣYc ... The coating comprises the following components: a total amount of Ti, Co, V, Nb, Mn, Zr, and W ΣZ: 0% or more and 0.25% or less, Fe: 0% or more and 5.0% or less, and Zn: 40.0% or more and 86.0% or less; a heating process in which the coated steel obtained by the melt coating process is heated under conditions of relative humidity of 90% or more and temperature of 100°C or more to form a black oxide layer on the surface of the coating; and a laser treatment process in which a portion of the surface of the coating after the heating process is irradiated with a laser so that the mass concentration ratio of O to Zn (O / Zn) at the irradiated area is less than 0.80.

[0025]

[15] The manufacturing method of molten coated steel described in

[14] may also be that the irradiated part is set as any one of the following or a combination of two or more of the following: a straight part, a curved part, a dot part, a graphic, a number, a mark, a pattern or a character.

[0026] Invention Effects

[0027] According to the above-described method of this disclosure, it is possible to provide molten-coated steel with a clear, durable, and aesthetically pleasing finish, as well as a method for manufacturing the same. This disclosure provides an inexpensive and aesthetically pleasing material, thus contributing to industrial development. Attached Figure Description

[0028] Figure 1 This is a top view schematic diagram showing an example of a pattern formed on the surface of a coating on molten steel.

[0029] Figure 2 yes Figure 1 A magnified top view of region M in the diagram.

[0030] Figure 3 Is with Figure 2 A schematic diagram of the cross-section corresponding to the NN line in the diagram.

[0031] Figure 4 This is a magnified top-view diagram showing regions A and B. Detailed Implementation

[0032] The inventors have conducted in-depth research on methods for displaying text, designs, etc., in the coating of molten steel and improving the clarity and durability of the text, designs, etc.

[0033] In order to display the design in specific areas of a metal surface, it is considered to make the areas representing text, design, etc. different from the areas that become the background in terms of hue, gloss, reflectivity, etc.

[0034] Among these factors, color tone is perhaps the most crucial for determining the overall appearance. While the surface of metallic materials generally exhibits a uniform color tone, except for variations caused by the oxide coating of titanium or stainless steel, or anodized aluminum, alloys can alter the surface color by changing the composition of their chemical components. However, in molten-coated steel with a Zn-Al-Mg system coating where the chemical composition is consistently determined, it is difficult to partially change the chemical composition of the coating during manufacturing. Therefore, it is typically difficult to intentionally alter the color tone in specific areas of the coating.

[0035] Regarding gloss, there is a concern about the durability of the appearance design during long-term use of metallic materials. That is, as corrosion progresses over time, the gloss difference between the area displaying the design and the background area decreases, potentially making the design less noticeable. Furthermore, gloss is also related to properties such as anti-glare, thus requiring careful control. Moreover, gloss, like hue, is affected by the composition ratio of the alloy's chemical composition; however, for the same reasons mentioned above, it is difficult to intentionally alter the gloss in specific areas of a Zn-Al-Mg based coating.

[0036] Regarding reflectivity, it can be varied by controlling the surface roughness of the metallic material. Therefore, it is believed that even Zn-Al-Mg coatings, where some chemical composition is difficult to control, can have their appearance displayed by partially changing their reflectivity. In this case, to ensure the clarity of the appearance, it is necessary to increase the difference in surface roughness. As a means of partially controlling the surface roughness of the coating, machining is generally considered. However, machining the coating may cause cracking, raising concerns about reduced corrosion resistance.

[0037] The inventors have studied the difference in hue, that is, how to display an appearance design through color difference. On the surface of a plating layer, in order to display an appearance design by forming a pattern, it is believed that by setting the pattern portion that will become the appearance design to a bright hue (e.g., white) and the background portion to a dark hue (e.g., black), the appearance design can be displayed on the surface of the plating layer. However, since the surface of the plating layer has a metallic luster, even if a partially bright pattern portion is provided, it is not easy to increase the color difference with the background portion, resulting in insufficient clarity of the appearance design. To solve this problem, it is believed that it is necessary to change the hue of the plating layer surface.

[0038] The inventors attempted to form an oxygen-deficient oxide layer on the surface of a Zn-Al-Mg based coating, making the entire coating a dark color (black), and then partially irradiating the oxide layer with a laser to create areas with a bright color (e.g., white).

[0039] Typically, on the surface of Zn-Al-Mg based coatings, oxides such as ZnO, Al2O3, and MgO are formed as oxides, but their hue is usually white or colorless and transparent. However, since the oxide layer produced by heat treatment in a low-oxygen atmosphere contains oxygen-deficient oxides, it sometimes appears black. When such an oxygen-deficient oxide layer is uniformly formed on the surface of the coating with a thickness of 0.02 μm or more, the surface of the coating becomes uniformly black. By irradiating this oxide layer with a laser, the oxide layer is removed, and the original metallic luster of the coating appears. Thus, the inventors discovered that a large color difference is obtained between the laser-irradiated area and other areas, enabling the display of a design with excellent clarity.

[0040] The appearance design achieved by shaping the laser irradiation area to a desired form is revealed through the color difference between the hue of the oxide layer on the surface of the coating and the hue of the coating itself. However, in general, in Zn-plated steel (steel with a Zn-based coating), corrosion occurs within the coating over time. As the coating corrodes, white rust forms. White rust not only forms in the coating but also in the black oxide layer, turning gray, thus diminishing the clarity of the appearance design. To improve the durability of the appearance design, it is necessary to reduce the amount of white rust forming in the coating.

[0041] Alternatively, mechanical methods such as grinding and polishing can be used to remove the oxide layer from the plated surface. However, when removing the black oxide layer from the plated surface mechanically, the removal area becomes relatively wide, making it difficult to obtain fine patterns. Furthermore, when removing the black oxide layer mechanically, the base plate is cut away, resulting in a thinner plate. Therefore, corrosion easily occurs in the areas where the black oxide layer has been removed. Thus, laser irradiation is the preferred method for removing the black oxide layer.

[0042] Furthermore, the chemical composition of the coating after removing the oxide layer is preferably one that minimizes the formation of white rust by forming a strong natural oxide film after the oxide layer is removed. Therefore, it is determined that a Zn-Al-Mg system coating containing a high concentration of both Al and Mg is preferred.

[0043] The following describes the melt-coated steel according to the embodiments of this disclosure.

[0044] The melt-coated steel of this disclosure comprises steel and a coating disposed on the surface of the steel. The average chemical composition of the coating, by mass%, includes Al: more than 10.0% and less than 40.0%, Mg: more than 4.0% and less than 15.0%, Si: more than 0% and less than 1.00%, Sn: more than 0% and less than 0.7%, Bi: more than 0% and less than 0.3%, In: more than 0% and less than 0.3%, the total amount of Sn, Bi and In ΣX: more than 0% and less than 0.7%, Ca: more than 0% and less than 0.60%, Y: more than 0% and less than 0%. The following are the percentages of different compounds: La: 0% and below; Ce: 0% and below; Sr: 0% and below; Li: 0% and below; Combined amount of Ca, Y, La, Ce, Sr, and Li (ΣYa): 0% and below; Cr: 0% and below; Ni: 0% and below; Mo: 0% and below; Cu: 0% and below; Ag: 0% and below; Sb: 0% and below. Below 5%, Pb: above 0% and below 0.25%, total amount of Cr, Ni, Mo, Cu, Ag, Sb and Pb ΣYb: above 0% and below 1.00%, B: above 0% and below 0.50%, P: above 0% and below 0.50%, total amount of B and P ΣYc: above 0% and below 0.50%, Ti: above 0% and below 0.25%, Co: above 0% and below 0.25%, V: above 0% and below 0.25%, Nb: above 0% and below 0.25%, Mn: above 0% and below 0.25%, Zr: 0% and below 0%. The coating has a content of 5.0 μm or more, including: % or more and 0.25% of Ti, Co, V, Nb, Mn, Zr, and W; ΣZ: 0% or more and 0.25% or less; Fe: 0% or more and 5.0% or less; and Zn: 40.0% or more and 86.0% or less. The coating surface has regions A and B. Region B has a black oxide layer containing Zn oxide with a thickness of 0.02 μm or more. The mass concentration ratio of O to Zn (O / Zn) in region A of the coating surface is set as P. A Let the mass concentration ratio (O / Zn) of O in region B of the coating surface be P. B Under the condition that the following equations (1) to (3) are satisfied. 0 <P A <0.80…(1) 0.80≤P B …(2) 0.40≤P B -P A …(3)

[0045] In addition, the molten steel in this embodiment preferably satisfies the following formula (4). 0.50≤P B -P A …(4)

[0046] In addition, in this embodiment, the Mg mass concentration in region B of the surface of the molten steel coating is preferably 3.2% by mass or more.

[0047] In addition, in this embodiment, the difference (ZB-ZA) between the surface height ZA of region A and the surface height ZB of region B in the molten steel is preferably 0.1 μm or more and less than 5.0 μm.

[0048] Furthermore, in this embodiment, the molten steel preferably includes a region C adjacent to region A in the coating, and the mass concentration ratio of O to Zn (O / Zn) in region C is set to P. C Under the condition that the following equations (5) and (6) are satisfied. 0 <P A <P C <0.80…(5) P C <0.40+P A …(6)

[0049] In addition, the flatness index of the preferred region C of the molten steel in this embodiment, namely the arithmetic mean roughness Ra, satisfies the following formula (7). Ra<2.0…(7)

[0050] In addition, the molten steel in this embodiment preferably has a patterned portion including region A on the surface of the coating, and the patterned portion is set in an intentional shape.

[0051] In addition, the molten steel in this embodiment preferably has a pattern portion including region A on the surface of the coating. The pattern portion is any one of straight lines, curves, dots, graphics, numbers, symbols, patterns or text, or a combination of two or more of them.

[0052] In addition, the molten steel in this embodiment preferably has a pattern portion including region A on the surface of the coating. The pattern portion is any one of straight lines, curves, dots, graphics, numbers, symbols, patterns or text, or a combination of two or more of them, and is an intentional shape.

[0053] Here, the intentional shape is an area with a size of 1.0 mm × 1.0 mm or more when viewed from a direction perpendicular to the coating surface. As described later, the patterned area is formed by laser irradiation, and in this case, the size of the patterned area is at least 1.0 mm × 1.0 mm or more.

[0054] Furthermore, the patterned portion of the molten steel in this embodiment preferably includes region A, region B, and region C existing between region A and region B.

[0055] In addition, it is preferable that the surface of the coating of the molten steel in this embodiment is provided with a non-patterned portion formed by region B.

[0056] In the following explanation, the percentage (%) of the content (concentration) 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 "~" as lower and upper limits. When the values ​​before and after "~" are marked "more than" or "less than", the range does not include these values ​​as lower or upper limits.

[0057] The steel to be plated is described.

[0058] Steel materials, primarily steel plates, steel wires, or steel rods, are not particularly limited in size. For example, steel plates are acceptable as long as they can be used in a typical molten zinc plating process. Specifically, steel plates used in processes such as continuous molten zinc plating lines (CGLs), where the metal is immersed in molten metal and then solidifies, are classified here. Regarding the size of the steel plates, for example, plates with a thickness of 10 mm or less and a width of 2000 mm or less can be used, but the size of the steel plates is not limited to these dimensions. Regarding the shape of the steel plates, this also includes steel plates with pre-defined embossed or recessed surfaces, such as striped steel plates.

[0059] For steel wire or steel wire, it is acceptable as long as it can be used in the usual molten metal coating process.

[0060] In addition, components, angle steel, L-angle steel and other shaped steel manufactured by processing steel plates are also included in steel products.

[0061] There are no particular restrictions on the type of steel used. For example, ordinary steel, pre-plated steel with a thin coating of various metals, Al-killed steel, very low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing corrosion-resistant strengthening elements such as Ni and Cr, etc.) can be used.

[0062] In addition, as a steel manufacturing process, common processes such as ironmaking and steelmaking in blast furnaces or electric furnaces, hot rolling, pickling, cold rolling, and heat treatment can be cited. However, the steel in this embodiment can undergo any process, and the processing conditions for each process are not limited.

[0063] Next, the coating will be described. The coating of this embodiment includes a Zn-Al-Mg alloy layer and a black oxide layer formed on a portion of the surface of the Zn-Al-Mg alloy layer (the location of region B, etc., described later). The reason for including the Zn-Al-Mg alloy layer in the coating other than the oxide layer is that Zn-Al-Mg alloys have high corrosion resistance, which is excellent from the viewpoint of durable appearance design.

[0064] The coating may also include an Al-Fe interfacial alloy layer.

[0065] In this embodiment, the thickness of the coating is the sum of the thicknesses of the Zn-Al-Mg alloy layer without an oxide layer and the Al-Fe layer. This is because, in order to form a black oxide layer on the surface of the coating and then create the desired appearance using laser irradiation, a specific thickness is required for the coating formed through the plating process. Furthermore, a certain level of corrosion resistance is ensured even in areas where no oxide layer exists.

[0066] Considering the above aspects, the coating thickness needs to be 5.0 μm or more. On the other hand, for steel plates, steel wires, or steel wires, the thickness of the coating formed by the conventional melt-coating method is affected by the drawing speed of the steel from the plating bath and the wiping conditions, and the maximum thickness is mostly 100.0 μm or less. Therefore, the coating thickness of the melt-coated steel in this embodiment can, for example, be 100.0 μm or less.

[0067] The Zn-Al-Mg alloy layer is composed of Zn-Al-Mg alloys. Zn-Al-Mg alloys refer to ternary alloys whose main components are Zn, Al, and Mg, and which may contain any other elements. The total amount of elements other than Fe constituting the coating is over 95%.

[0068] The Al-Fe interfacial alloy layer is an interfacial alloy layer located between the steel and the Zn-Al-Mg alloy layer, and it is in contact with the surface of the steel.

[0069] The thickness of the Al-Fe interfacial alloy layer is preferably less than 2.0 μm, but can be less than 1.0 μm, less than 0.7 μm, more preferably less than 0.5 μm, or less than 0.3 μm. Typically, the Al-Fe interfacial alloy layer is thinner than the Zn-Al-Mg alloy layer, accounting for less than 10% of the total coating thickness.

[0070] The coating can be formed from a black oxide layer and a Zn-Al-Mg alloy layer, or it can be a stacked structure comprising a black oxide layer, a Zn-Al-Mg alloy layer, and an Al-Fe interfacial alloy layer. In the case of a stacked structure, the Zn-Al-Mg alloy layer exists on the surface side of the coating relative to the Al-Fe interfacial alloy layer, and in areas where no oxide layer is present, it can be designated as a layer constituting the coating surface. In areas where an oxide layer is present, the oxide layer can be designated as a layer constituting the coating surface.

[0071] The coating thickness is calculated based on the coating adhesion amount. The weight change is measured when the coating is dissolved by acid. There are no particular restrictions on the type of acid, as long as it can dissolve the coating. It is preferable to use an acid containing an inhibitor that suppresses corrosion of the iron-based (steel) material. By measuring the area and weight before and after dissolution, the coating adhesion amount (g / m²) can be obtained. 2 Coating adhesion amount (g / m) 2 The calculation of the coating thickness can be performed not only on steel plates but also on steel wires or bars. The coating thickness can be calculated based on the surface area (diameter × π × length) of the steel wire or bar, according to the weight of the dissolved coating. This is achieved by dividing the coating thickness by the coating specific gravity (g / m³). 3 The thickness of the coating is then determined. The specific gravity of the coating can be calculated by determining the chemical composition of the coating and then calculating the specific gravity based on the chemical composition determination results. The determination of the chemical composition of the coating, as described later, involves using an acid-dissolved solution to determine the content of each element by ICP-MS or ICP-luminescence spectrophotometry.

[0072] Next, the average chemical composition of the coating will be explained. When the coating is formed of an oxide layer and a Zn-Al-Mg alloy layer, the overall average chemical composition of the coating is the average chemical composition of the oxide layer and the Zn-Al-Mg alloy layer. Furthermore, when the coating is a stacked structure of an Al-Fe interfacial alloy layer, a Zn-Al-Mg alloy layer, and an oxide layer, the average chemical composition is the average chemical composition of the oxide layer, the Al-Fe interfacial alloy layer, and the Zn-Al-Mg alloy layer. However, in the coating defined in this disclosure, it is preferable that the thickness of the Al-Fe interfacial alloy layer is less than 10% of the overall thickness of the coating, and therefore the Fe content of the coating is mostly less than 5%. Furthermore, since the oxide layer is relatively thin compared to the coating, its influence on the average chemical composition is almost negligible. Therefore, it is acceptable to consider the average chemical composition of the coating as approximately the composition of the Zn-Al-Mg alloy layer. Furthermore, traces of the original plating material are unlikely to be considered as residual chemical components of the coating. Therefore, the average chemical composition of the coating can be considered to be approximately the same as the composition of the plating bath used in the manufacturing process.

[0073] The average chemical composition of the coating in this embodiment needs to be within a range that can improve corrosion resistance and form a black oxide layer on the surface of the coating. That is, it needs to contain Al: more than 10.0% and less than 40.0%, Mg: more than 4.0% and less than 15.0%, Si: more than 0% and less than 1.00%, the total amount of Sn, Bi and In ΣX: more than 0% and less than 0.7%, the total amount of Ca, Y, La, Ce, Sr and Li ΣYa: more than 0% and less than 0.60%, the total amount of Cr, Ni, Mo, Cu, Ag, Sb and Pb ΣYb: more than 0% and less than 1.00%, the total amount of B and P ΣYc: more than 0% and less than 0.50%, the total amount of Ti, Co, V, Nb, Mn, Zr and W ΣZ: more than 0% and less than 0.25%, Fe: more than 0% and less than 5.0%, and Zn: more than 40.0% and less than 86.0%.

[0074] As mentioned above, Al and Mg are components related to the durability of the appearance design. In addition, Al and Mg are essential elements for the formation of oxygen-deficient oxides. When these elements are in low amounts, the black color becomes lighter, and white oxides called ZnO (Zincite) are easily formed.

[0075] Other elements are arbitrary elements that form new intermetallic compounds, substitutions for major elements, etc., within the compositional range disclosed in this disclosure. If these elements are outside the aforementioned compositional range, it is difficult to form a black oxide in the coating, resulting in poor clarity or significantly reduced durability. Within the compositional range of this disclosure, the formation state of the black oxide layer has been confirmed to be normal.

[0076] The following is a detailed explanation of the content (concentration) of each element.

[0077] A1: Above 10.0% and below 40.0% Al, along with Zn, is the main element constituting the coating. In Zn-Al-Mg coatings, the Al phase mainly forms in the coating. When the Al content is below 10.0%, the corrosion resistance of the coating is low, and the hardness of the coating deviates from an appropriate range, so it is not preferred. When the Al content exceeds 40.0%, the Zn content is relatively low, resulting in poor corrosion resistance, so it is also not preferred. The Al content is preferably 15.0% or more or 19.0% or more. Furthermore, the Al content is preferably 35.0% or less or 30.0% or less.

[0078] Mg: ≥4.0% and ≤15.0% Mg, along with Al and Zn, is a major element constituting the coating. Insufficient Mg results in poor sacrificial corrosion resistance of the coating; therefore, the Mg content is set to exceed 4.0%. When the Mg content exceeds 15.0%, the corrosion resistance of the coating deteriorates. Therefore, the Mg content is set to 15.0% or less. The Mg content is preferably 5.0% or more, or 6.0% or more. Furthermore, the Mg content is preferably 8.0% or less, or 7.0% or less.

[0079] The elements described below are all elements that can be added arbitrarily.

[0080] Si: 0% or more and less than 1.00% Si may or may not be present in the coating, but when Si is present, intermetallic compounds form within the coating. The coating composition in this embodiment has a high melting point, thus the operating temperature during melt coating is around 500°C. At this operating temperature, when steel is immersed in the plating bath, Al, Zn, and Fe undergo active interdiffusion to form Fe-based intermetallic compounds, but Si inhibits this excessive reaction. Therefore, when Si is present, if the content is 0.01% or more, the diffusion reaction of Fe is significantly suppressed, making it easy to control the formation of Fe-based intermetallic compounds in the coating. On the other hand, when the Si content is excessive, the effect becomes saturated, therefore the Si content is kept below 1.00%. The Si content is preferably 0.05% or more or 0.25% or more. Furthermore, the Si content is preferably 0.75% or less.

[0081] Element group X Sn: 0% or more and 0.7% or less Bi: 0% or more and 0.3% or less In: 0% or more and 0.3% or less The total amount of Sn, Bi, and In, ΣX: above 0% and below 0.7%. Since the elements in element group X (Sn, Bi, In) can be included arbitrarily, their respective contents are set to 0% or higher. Including these elements sacrifices the improvement in corrosion resistance. Each element has an upper limit on its content; even with high concentrations, the effect saturates. Therefore, the upper limit for these elements is set below 0.7% in Sn, below 0.3% in Bi and In, and consequently, their total content ΣX is also limited to below 0.7%.

[0082] Element group Ya Ca: 0% or more and 0.60% or less Y: Above 0% and below 0.30% La: 0% or more and 0.30% or less Ce: 0% or more and 0.30% or less Sr: 0% or more and 0.30% or less Li: 0% or more and 0.30% or less The combined amount of Ca, Y, La, Ce, Sr, and Li, ΣYa: ≥0% and ≤0.60% These elements, along with Si, control the reaction rate of the coating and also control the diffusion of Fe in the plating bath. Furthermore, the formation reaction of intermetallic compounds containing these elements between the iron-based (steel) and the interfacial alloy layer ensures the adhesion between the iron-based and Al-Fe alloy layers. To achieve this effect, Ca can be present at 0.03% or more, preferably 0.10% or more. However, when Ca is excessive, various scum forms in the plating bath, increasing plating defects, making the plating bath extremely viscous, reducing the amount of molten metal adhering to the steel when it is lifted from the bath, resulting in an extremely thin coating and poor corrosion resistance. Additionally, the hardness of the coating becomes excessively high. Therefore, the upper limit for Ca is set to 0.60% or less. The elements in element group Ya, excluding Ca, can achieve approximately the same effect as Ca, and therefore can be included as substitutes for Ca. However, they cannot be included in the same large quantities as Ca. That is, the content of each element in element group Ya, excluding Ca, is set to 0~0.30%, preferably 0.01~0.30%. In addition, the total amount ΣYa of element group Ya containing Ca is set to 0~0.60%.

[0083] element group Yb Cr: 0% or more and less than 1.00% Ni: 0% or more and less than 1.00% Mo: 0% or more and 0.25% or less Cu: 0% or more and less than 1.00% Ag: 0% or more and 0.25% or less Sb: 0% or more and 0.25% or less Pb: above 0% and below 0.25% The total amount of element group Yb of Cr, Ni, Mo, Cu, Ag, Sb and Pb, ΣYb: ≥0% and ≤1.00%. Since the elements in element group Yb can be included arbitrarily, their respective contents are set to 0% or more. The elements in element group Yb have properties similar to Zn, allowing for relatively large amounts to be included. When these elements are included in the above-mentioned concentration range, they have an effect on improving corrosion resistance. This effect is observed when they are included at approximately 0.10%. However, the effect saturates when the total content of these elements is excessive. Therefore, the content of Cr is set to 0~1.00%, preferably 0.01~1.00%; the content of Ni and Cu is set to 0~1.00%; and the content of Mo, Ag, Sb, and Pb is set to 0~0.25%. The total ΣYb content is set to 0% or more and 1.00% or less.

[0084] Element group Yc B: Above 0% and below 0.50% P: Above 0% and below 0.50% The combined amount of B and P, ΣYc: ≥0% and ≤0.50% Since the elements in element group Yc can be included arbitrarily, their respective contents are set to 0% or higher. When these elements are present in the above-mentioned concentration range, they have an effect on improving corrosion resistance. The effect is observed when their combined content is around 0.05%. On the other hand, even with large amounts of these elements, the effect saturates. Therefore, the content of each element in element group Yc is set to 0~0.50%. The total content ΣYc is set to 0% or higher and 0.50% or lower.

[0085] Element group Z Ti: 0% or more and 0.25% or less Co: 0% or more and less than 0.25% V: Above 0% and below 0.25% Nb: 0% or more and 0.25% or less Mn: 0% or more and 0.25% or less Zr: 0% or more and 0.25% or less W: Above 0% and below 0.25% The summation ΣZ of element group Z of Ti, Co, V, Nb, Mn, Zr and W: above 0% and below 0.25% Since the elements in element group Z can be included arbitrarily, their individual contents are set to 0% or higher. The presence of elements from element group Z in the coating improves corrosion resistance. This effect is observed when the total concentration is around 0.10%. However, even with high concentrations of these elements, the effect becomes saturated. Therefore, the content of each element in element group Z is set to 0-0.25%. The total concentration ΣZ is set to 0% or higher and 0.25% or lower.

[0086] Fe: 0% or more and 5.0% or less The molten steel in this embodiment is manufactured by a molten coating method, therefore, during manufacturing, Fe sometimes diffuses from the coating material into the coating layer. As described above, in this embodiment, the Al concentration of the coating is high, sometimes forming an Al-Fe interfacial alloy layer, but its thickness is thin. As a result, the coating sometimes contains a maximum of 5.0% Fe, but if the Fe concentration is limited to below 5.0%, it has no effect on the frequency of crack formation in the coating. Therefore, the Fe content is set to 0~5.0%. The Fe content can exceed 0%.

[0087] Zn: 40.0% or higher and 86.0% or lower The molten steel used in this embodiment is a highly versatile Zn-based coated steel, therefore, Zn is the main phase element constituting the coating. If the Zn content is less than 40.0%, the corrosion resistance is insufficient; if it exceeds 86.0%, the improved corrosion resistance effect brought by other elements such as Al and Mg cannot be obtained. Therefore, the Zn content is set to be 40.0% or more and 86.0% or less.

[0088] The balance other than those mentioned above can also be impurities. That is, it can also contain the elements mentioned above, and the balance includes impurities. Impurities refer to components contained in the raw materials or components mixed in during the manufacturing process, and are not intentionally present. For example, in the coating, due to the mutual atomic diffusion between the steel (iron-based) and the plating bath, components other than Fe are sometimes mixed in in trace amounts as impurities.

[0089] The average chemical composition of the coating can be determined using an acid solution prepared when measuring the thickness of the coating. Specifically, an acid solution is obtained by peeling and dissolving the coating with an acid containing an inhibitor that suppresses corrosion of the iron-based (steel) material. The chemical composition can then be determined by measuring the obtained acid solution using ICP-MS or ICP-luminescence spectrophotometry.

[0090] Next, regions A, B, and C on the surface of the coating will be described. At least regions A and B are provided on the surface of the coating in this embodiment. Region A is the region where the black oxide layer on the coating surface is removed, and region B is the region where the black oxide layer remains. Region A is formed by partially removing the black oxide layer by laser irradiation after it has been formed on the surface of the coating. Region C is the region where the black oxide layer is not completely removed due to the heat effect of laser irradiation. Region C sometimes forms around region A. Region C is defined as a region where the O / Zn mass concentration ratio (O / Zn) is set to P. C The region that satisfies equation (5) as described later.

[0091] Regions A and B can be easily distinguished by microscopic observation. For example, an optical microscope can be used to observe the coating surface at 200x magnification. Region C can also be visually identified by microscopic observation, but the precise boundaries (the boundaries between regions A and C, and between regions C and B) can be defined by measuring the mass concentration ratio of the coating surface, as described later.

[0092] Furthermore, in this embodiment, the surface of the plating layer has patterned areas and non-patterned areas. The patterned areas are arranged on the surface of the plating layer in a predetermined shape. More specifically, the patterned areas are any one or a combination of two or more of the following: straight lines, curves, dots, graphics, numbers, symbols, patterns, or text. The non-patterned areas are the areas other than the patterned areas. The shape of the patterned areas is permissible even if it is partially missing, such as a missing dot, as long as it can be identified as a whole. In addition, the non-patterned areas may also be shapes that border the boundaries of the patterned areas.

[0093] The patterned area includes at least region A. Additionally, the patterned area may include either region B or region C. Conversely, the non-patterned area includes region B but not region A. Thus, when observing the patterned area containing region A and the non-patterned area containing region B with the naked eye, the difference in light and shadow between them can be identified. Therefore, the boundary between the patterned and non-patterned areas can be determined with the naked eye. Alternatively, the boundary between the patterned and non-patterned areas can be determined based on magnified images obtained using optical microscopes, magnifying glasses, etc.

[0094] The patterned portion, including region A, can be formed to a size and degree that allows the presence of the patterned portion to be discerned with the naked eye, under a magnifying glass, or under a microscope. Furthermore, the non-patterned portion constituting region B occupies most of the surface of the coating (the molten coating).

[0095] Sometimes, patterned portions are arranged within non-patterned areas. In this case, the patterned portion encompassing region A is arranged in a predetermined shape within the non-patterned portion constituting region B. Specifically, a patterned portion is a shape formed within a non-patterned area by any one or a combination of two or more of the following: straight lines, curves, dots, graphics, numbers, symbols, patterns, or text. By adjusting the shape of the patterned portion, a shape formed by any one or a combination of two or more of the following—straight lines, curves, dots, graphics, numbers, symbols, patterns, or text—is displayed on the surface of the plating layer. For example, strings of text, strings of numbers, symbols, marks, line drawings, designs, or combinations thereof formed by patterned portions are displayed on the surface of the plating layer. This shape is intentionally or artificially formed by the manufacturing method described later, and is not naturally formed.

[0096] Region A is the exposed surface area of ​​the Zn-Al-Mg alloy layer constituting the coating. Therefore, Region A has a pear-skin-like appearance similar to the coating itself. This pear-skin-like appearance is achieved by the mixture of tiny white areas and tiny areas exhibiting a metallic luster. Region A is formed by partially removing the black oxide layer after it has been formed on the surface of the coating using laser irradiation, thus exposing the coating (Zn-Al-Mg alloy layer).

[0097] On the other hand, in region B, a black oxide layer with a thickness of 0.02 μm or more is formed on the surface of the Zn-Al-Mg alloy layer that mainly constitutes the coating. The black oxide layer contains at least Zn oxide. Some or all of the Zn oxide is oxygen-deficient Zn oxide. Therefore, the oxide layer is black. In addition to Zn oxide, this black oxide layer may also contain oxides of one or both of Al and Mg. Furthermore, the black oxide layer may also contain hydroxides of any one or more of Zn, Al, and Mg. As described later, this oxide layer is formed by heating to 100°C or higher in an atmosphere with a relative humidity of 90% or higher. Thus, it contains oxygen-deficient Zn oxide.

[0098] The thickness of the black oxide layer is 0.02 μm or more. When the thickness of the oxide layer is less than 0.02 μm, the hue of region B becomes gray, or it exhibits the pear-skin appearance of the original coating, resulting in an appearance similar to the patterned area including region A. Therefore, when the thickness of the oxide layer is less than 0.02 μm, it is difficult to distinguish between the patterned and non-patterned areas, and the visual recognizability of the patterned area is low, which is not preferable. There is no upper limit, but if it is too thick, it is difficult to remove and difficult to form a pattern. Therefore, the thickness of the black oxide layer is preferably 7.00 μm or less.

[0099] Region A is the area where the surface of the coating is exposed. Therefore, oxygen (O) is relatively scarce on the surface of region A. Thus, the mass concentration ratio of O to Zn (O / Zn) in region A of the coating surface is set as P. A In the case shown in equation (1) below, P A The range is greater than 0 and less than 0.80. P A When the value is above 0.80, the hue of region A is close to that of region B, resulting in low visual recognizability of the pattern area, and therefore it is not preferred. 0 <P A <0.80…(1)

[0100] In region A, the appearance is pear-skin shaped as described above, but to improve the clarity of the design, a near-white metallic color is preferred. When the metallic color is close to white, the clarity of the design is significantly improved. The metallic color of the coating depends on the content of the metal elements contained in the coating. In particular, a high Al concentration tends to result in a stronger white color, leading to Al combining with Zn to form an Al-Zn phase, thereby orienting the residual Zn. When this state is achieved, P... A Displays small values, and the clarity of the design is significantly improved. P A More preferably, it is 0.50% or less, and even more preferably 0.30% or less. In this case, as the average chemical composition of the coating, the Al content is preferably set to 15.0 to 35.0% by mass, and more preferably limited to 19 to 30% by mass.

[0101] On the other hand, region B contains a large amount of oxygen (O) on its surface due to the presence of a black oxide layer. Let P be the mass concentration ratio of O to Zn (O / Zn) in region B on the surface of the coating. B In the case shown in equation (2) below, P B It should be 0.80 or higher. Preferably, it should be 0.90 or higher. P B When the value is less than 0.80, the hue of region B is close to that of region A, resulting in poor visual recognizability of the pattern area. Additionally, P... B When the value exceeds 1.40, the oxygen deficiency is eliminated, resulting in situations where the oxide layer does not appear black, the hue of region B is close to that of region A, and the visual recognizability of the patterned area is low. Therefore, P B Preferably, it should be below 1.40. 0.80≤P B …(2)

[0102] In order to improve P B The coating may contain a certain amount of Mg. Preferably, the Mg content in the coating is set to 6.0~8.0% by mass.

[0103] To further improve the durability of the coating, the Mg mass concentration in region B is preferably 3.2% or higher. A concentration of 4.6% or higher further improves durability and is therefore more preferable. Therefore, setting the Mg content to 6.0-7.0% by mass is sufficient as the average chemical composition of the coating. In this case, while the MgZn2 phase exists in a certain amount as a component of the coating beneath the black oxide layer, it is believed that this MgZn2 phase improves the corrosion resistance of the coating surface after the black oxide layer is removed, thereby suppressing the formation of white rust and improving the durability of the coating. The Mg mass concentration in region B is determined by performing elemental analysis on the surface of region B using an energy-dispersive elemental analyzer (EDS).

[0104] Furthermore, as shown in equation (3) below, P in region B B P in region A A The difference (P) B -P A The value needs to be above 0.40. (P) B -P A The concentration of P in region A is above 0.40, indicating a significant reduction in the amount of oxides present in region A compared to region B. This results in a greater difference in brightness between regions A and B, making them visually distinguishable. B -P A When the P value is less than 0.40, regions A and B cannot be identified by the naked eye. More preferably, as shown in equation (4), (P... B -P A The content of Al and Mg in the coating is 0.50 or higher, and more preferably 0.60 or higher. By controlling the Al and Mg content in the coating and controlling the amount of Zn oxide formed, it is removed when forming region A, thereby obtaining indicators that meet these clarity requirements. 0.40≤P B -P A …(3) 0.50≤P B -P A …(4)

[0105] When a coating with a certain chemical composition and an oxide layer of a certain thickness are formed, the blackness of the oxide layer depends on the chemical composition of the underlying coating. Furthermore, the hue of the coating also depends on its chemical composition. To form a patterned area (region A) by laser irradiation, and to impart an appearance design formed by the patterned area to the surface of the coating, it is necessary to appropriately consider the factors determining the blackness of the oxide layer in region B, namely the Al and Mg content of the coating, and the range of other additive elements. Additionally, after laser irradiation, the surface layer of the coating slightly evaporates, resulting in a new surface that reacts with the atmosphere (a new oxide film surface that reacts with oxygen in the atmosphere). Therefore, P... A P B It becomes an inherent value of the metal's composition.

[0106] Next, region C will be explained. Region C sometimes forms around region A. Region C is formed when the mass concentration ratio of O to Zn (O / Zn) is set to P. CThe region that satisfies the following equation (5) is as follows. As described above, region A is formed by partially removing the black oxide layer after forming a black oxide layer on the surface of the coating using laser irradiation. The region where the black oxide layer is removed is called region A, and the region where the black oxide layer remains is called region B. At this time, region C is sometimes formed between region A and region B. Region C is the region where the black oxide layer is not completely removed. It is assumed that region C is the region affected by heat during laser irradiation.

[0107] That is, as shown in equation (5), the P of region C C Although the oxygen content is higher than that of region A (P) A More, but becoming more than P as region B B The lower limit value is 0.80.

[0108] Furthermore, in this embodiment, region C preferably satisfies equation (6). If the difference between the oxygen content of region A and region C is less than 0.40, the difference in oxygen content between region C and the adjacent region A is suppressed, thus preventing the difference in corrosion resistance between region C and region A from widening.

[0109] By having a region C that satisfies equations (5) and (6), the height difference between region A and region B becomes smaller, which can suppress the low corrosion resistance in region A. 0 <P A <P C <0.80…(5) P C <0.40+P A …(6)

[0110] Furthermore, in this embodiment, the flatness index of the surface of region C, i.e., the arithmetic mean roughness Ra in units of μm, preferably satisfies Ra < 2.0. In this embodiment, the patterned portion is formed by laser irradiation rather than mechanical grinding, thereby achieving Ra < 2.0 on the surface of region C. By ensuring that Ra on the surface of region C is less than 2.0, localized corrosion of the surface of region C can be prevented, and the appearance design of the patterned portion can be maintained for a long time. On the other hand, when region A is formed by mechanically removing the black oxide layer, the Ra of region C typically exceeds 2.0.

[0111] Ra can be determined using a non-contact white light interferometer. Specifically, for example, using a white light interferometer CONTOUR GT-I (manufactured by Bruker), in vertical scanning low coherence interferometry (CSI), the Ra of region C in an arbitrary 150×150 mm field of view containing region C is measured at three locations, and the Ra of region C in each field of view is calculated. The average value of these values ​​is taken as the Ra of region C in this embodiment.

[0112] The difference (ZB-ZA) between the surface height ZA of region A and the surface height ZB of region B is preferably 0.1 μm or more and less than 5.0 μm. In this embodiment, after the black oxide layer is formed, it is removed by laser irradiation, so the surface height ZA of region A is lower than the surface height ZB of region B. If this difference (ZB-ZA) is too large, the thickness of the coating in region A becomes thinner, and the corrosion resistance may be low. Therefore, (ZB-ZA) is preferably less than 5.0 μm. (ZB-ZA) is more preferably less than 3.0 μm. On the other hand, when the surface height ZA of region A is close to the surface height ZB of region B, a large amount of oxide layer may remain in region A, and the clarity of the appearance design may be low. Therefore, (ZB-ZA) is preferably 0.1 μm or more.

[0113] The determination method for (ZB-ZA) is as follows.

[0114] In the molten coated steel of interest, the region with the patterned area was cut along the thickness direction of the molten coated steel (the direction perpendicular to the surface of the molten coated steel). This yielded cross-sectional samples of the Zn-Al-Mg coating and oxide layer.

[0115] Next, the obtained sample is embedded in room-temperature drying epoxy resin in a cross-section with visible thickness, and the cross-section is ground. Then, a field of view is selected within this cross-section sample, showing both region A and region B, and observed using a scanning electron microscope (SEM, such as the JEOL JSM-7000F). The observation conditions are set, for example, accelerating voltage: 15kV, irradiation current: 1.0nA, number of scans: 10, and magnification: 5000x.

[0116] Then, images are taken of the observed area, and ZB-ZA within the field of view is measured.

[0117] More specifically, in the (ZB-ZA) measurement method, firstly, regions A and B are selected that can be observed within the same field of view, and this region is observed at 1000x magnification using a scanning electron microscope. Next, the height difference between the highest point on the surface of region B within the field of view and the lowest point on the surface of region A within the field of view is calculated. Here, the "highest point" refers to the part of region B within the field of view that is furthest from the steel plate surface towards the plated surface in the direction of the coating thickness, i.e., perpendicular to the coating surface. Similarly, the "lowest point" refers to the part of region A within the field of view that is closest to the steel plate surface in the direction of the coating thickness. The same measurement is performed in 10 fields of view, and the average value of the height differences measured in each field of view is calculated. This average value is set as the difference between the height ZA of the surface of region A and the height ZB of the surface of region B in this embodiment (ZB-ZA).

[0118] The patterned area includes region A, and the non-patterned area includes region B. The patterned area can be visually identified by the difference in brightness between region A and region B.

[0119] Furthermore, the patterned area can include area C together with area A, or it can further include area B. When the patterned area includes both area A and area B, the difference in brightness between the patterned area and the non-patterned area is relatively small, but by including area A in the patterned area, the patterned area can be visually identified with the naked eye.

[0120] When the pattern section includes regions A and B, the pattern section can be exemplified by having multiple linear regions A arranged approximately parallel to each other within the pattern section, with regions B between the linear regions A. Furthermore, when the pattern section includes region C, region C is positioned at the boundary between regions A and B. The width of each region A can be, for example, in the range of 10 to 250 μm. Additionally, the spacing between regions A, which is the center of the width direction of region A, can be in the range of 50 to 800 μm. By giving regions A and B the above-described configuration, the pattern section can be visually identifiable to the naked eye.

[0121] exist Figures 1-4 An example of a patterned section is shown. Figure 1 The middle section indicates that the Japanese hiragana character "め" is represented as a pattern on the surface of the coating. This hiragana character "め" consists of area A, area B, and area C (without the illustration).

[0122] Figure 2 Indicates by Figure 1 An enlarged top-view diagram of the area M enclosed by a single-dotted line. Additionally, Figure 3 express Figure 2 A schematic diagram of the cross-section of the arrow NN line. (See diagram below.) Figure 2As shown, the pattern section is composed of multiple regions A arranged approximately parallel to each other at predetermined intervals. Figure 2 For convenience, region C is omitted. Therefore, in Figure 2 In this context, the area outside of region A is region B.

[0123] In addition, such as Figure 3 As shown, the molten steel 1 of this embodiment has a steel material 2 and a coating 3 formed on the surface of the steel material. Region A and region B are formed in the coating 3. In region A, the surface of the Zn-Al-Mg alloy layer 3A constituting the coating 3 is exposed. Alternatively, in region A, the outermost layer of the Zn-Al-Mg alloy layer 3A may be removed by laser irradiation. On the other hand, in region B, a black oxide layer 4 is present on the Zn-Al-Mg alloy layer 3A in the coating 3.

[0124] in addition, Figure 4 This is an enlarged top view of region A. Region C is located around region A. Region C is situated at the boundary between region A and region B.

[0125] Figures 2-4 The pattern shown includes regions A, B, and C; however, the pattern in this embodiment is not limited to this configuration and may also include regions A and B. Alternatively, the entire pattern area may be formed from region A, or the entire pattern area may be formed from regions A and C.

[0126] Next, for P in regions A, B, and C... A and P B and P C The determination method is described below. Elemental analysis is preferably performed using an energy-dispersive elemental analyzer (EDS) mounted on a scanning electron microscope. The measurement area for region A and region B is 100 μm in each region. 2 The measurement area of ​​region C is located 3 μm away from the edge of region A identified by SEM image. Elemental analysis of a 10 μm × 10 μm region was performed by EDS, and compared with P... A P B Similarly, this is calculated. In this embodiment, region A is the area where the Zn-Al-Mg alloy layer 3A is exposed. Therefore, the boundary between region C, which is the residue of oxide layer 4, and region A can be clearly identified by SEM images.

[0127] The above measurement area consists of three arbitrarily isolated locations. The average value of the measurements at these three locations is calculated. The mass concentration is calculated using ZAF correction. The conditions at this point are as follows. Accelerating voltage: 15kV • Beam diameter: 2nm • Measurement interval: 0.1 nm • Determine the starting point: the location of the edge of region A in the observed image. • Measurement endpoint: 100 nm from the edge of region A in the observed image, in a direction parallel to the coating surface.

[0128] By performing EDS analysis on the surface of the coating under the above conditions, the mass concentrations of O and Zn were determined in regions A, B, and C, respectively. Based on the results, P, representing the mass concentration ratio, was calculated. A =O / Zn、P B =O / Zn and P C =O / Zn.

[0129] Next, the method for measuring the thickness of the oxide layer will be explained. Regarding the thickness of the oxide layer, the molten coated steel is cut to expose the cross-section of the coating, and the cross-section is mechanically ground to a mirror finish. The sample prepared in this way is observed using a scanning electron microscope with reflected electron imaging, and the thickness of the layer with black contrast existing on the outermost surface of the coating is measured.

[0130] Specifically, in the molten coated steel of interest, the region where region B is formed is cut along the thickness direction of the molten coated steel (the direction perpendicular to the surface of the molten coated steel) to obtain a cross-sectional sample of the oxide layer. Then, the obtained sample is embedded in room-temperature drying epoxy resin with a cross-section visible in the thickness direction, and the cross-section is ground.

[0131] Next, any part of the cross-sectional sample is observed using a scanning electron microscope (SEM, such as the JEOL JSM-7000F). The observation conditions can be set as follows: accelerating voltage: 15 kV, irradiation current: 1.0 nA, number of scans: 10, and magnification: 5000x. Then, images are taken of the observed area, and the thickness of the oxide layer within the field of view is measured.

[0132] Similarly, 10 randomly selected fields of view within the sample are observed, and the average thickness of the oxide layer measured in each field of view is taken as the average thickness of the oxide layer in this embodiment.

[0133] (Evaluation of clarity)

[0134] Next, the method for confirming the sharpness of the pattern area will be explained. In confirming the sharpness, a computer-based character recognition function, which utilizes machine learning for character recognition and similar techniques, will be used. For example, the case of a laser-irradiated area forming a predetermined shape by laser irradiating the surface of a coating with a black oxide layer will be explained.

[0135] For the laser irradiation area, a photograph is taken from the vertical direction under the same conditions such as specified distance, illumination distance, and angle. The pixels constituting the area including the laser irradiation area are mosaicked and binarized by image analysis.

[0136] In addition, paper was used as the printing medium to print a black-and-white image of a pattern area depicting the same pattern area formed on the plating layer by laser irradiation. The image was photographed under the same conditions as in the case of the plated steel sheet, and the pixels constituting the area including the aforementioned pattern area were pixelated and binarized using image analysis.

[0137] When comparing the two binarization judgment results, the black-and-white consistency rate of the binarization judgment of the laser-irradiated part of the plated steel plate on the pattern part on the paper is measured and evaluated according to the following judgment criteria. S, AA, A, and B are set as qualified. S: Black and white consistency rate is above 98% and below 100%. AA: Black and white consistency rate is above 96% and less than 98%. A: The black-and-white consistency rate is above 93% and less than 96%. B: The black-and-white consistency rate is above 90% and less than 93%. C: Black and white consistency rate is less than 90%.

[0138] (Persistence evaluation)

[0139] The durability of the appearance design is evaluated based on the decrease in clarity of the appearance design after corrosion. White rust often forms due to corrosion of the coating; however, when the amount of white rust is large, forming raindrop-like patterns or accumulating on the coated surface, it becomes difficult to identify the appearance design. Cyclic corrosion testing (CCT) is used to obtain a good correlation with the domestic exposure environment. Unlike salt spray testing (SST), which is a corrosion-promoting test, CCT involves repeated salt spraying, drying, and wetting processes, thus closely resembling the corrosion conditions in the atmospheric environment and confirming a certain degree of correlation. Here, the JASO cycle (M609-91) is used as the CCT. These 30 cycles are equivalent to 10 years in a typical domestic corrosion environment. The clarity of the appearance design before and after CCT is evaluated according to the aforementioned clarity evaluation method. Furthermore, durability is determined based on the change in clarity evaluation before and after CCT.

[0140] The plated steel sheet with the appearance design is cut into 100×50mm size, the cut end face is coated with epoxy resin coating, and the evaluation surface of 70×40mm is set in the center of the steel sheet to make a test piece.

[0141] A laser irradiation section of arbitrary size, which applies laser processing, is positioned at the center of the evaluation surface. The outer side of the laser irradiation section is the non-application area for laser processing.

[0142] Prepare 5 test pieces and conduct a corrosion test of 30 cycles of CCT (JASO cycle) on each test piece.

[0143] For both the pre- and post-CCT test specimens, sharpness was evaluated using a computer's character recognition function (which incorporates machine learning for character recognition), similar to the sharpness evaluation described above, and the black-and-white consistency rate was calculated. Then, the difference between the black-and-white consistency rate of the pre- and post-CCT test specimens was calculated. Finally, an evaluation was conducted based on the following criteria. S, A, and B were set as acceptable. S: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is less than 5 points. A: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is worse than 5 points but less than 10 points. B: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is lower than 10 points but lower than 15 points. C: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is lower than 15 points.

[0144] Next, the manufacturing method of the melt-coated steel according to this embodiment will be described.

[0145] The molten-coated steel of this embodiment is obtained by annealing the steel used as the coating material, performing molten coating, cooling, reheating to a specified humidity and temperature atmosphere, and then performing laser treatment. These will be explained separately below.

[0146] (Melt-coating process)

[0147] For steel materials to be used as the base material for plating, it is preferable to anneal in a reducing atmosphere before immersion in the molten plating bath. The reducing atmosphere and annealing conditions are not particularly limited, but it is preferable to remove oxides present on the surface of the steel as much as possible through this annealing.

[0148] Next, the freshly annealed steel is immersed in a molten plating bath. The chemical composition of the plating bath can be adjusted appropriately to obtain the chemical composition of the coating described above. Furthermore, the temperature of the plating bath is not particularly limited; a suitable temperature for molten plating can be selected. For example, the plating bath temperature can be set to a value approximately 20°C higher than the melting point of the plating bath.

[0149] Next, the steel is lifted from the plating bath. The amount of plating can be controlled by adjusting the lifting speed of the steel sheet. Alternatively, the amount of plating can be controlled by wiping the coated steel sheet as needed. There are no particular limitations on the amount of plating; for example, it can be set within the range described above.

[0150] Next, the coating is cooled. There are no particular restrictions on the cooling conditions; it can be done by blowing cooling gas or mist, or by natural cooling.

[0151] (Heating process)

[0152] In the heating process, the steel to be plated is heated (and held) for at least 10 minutes under conditions of relative humidity of 90%RH or higher and ambient temperature of 100°C or higher, thereby forming a black oxide layer on the surface of the coating with a thickness of 0.02 μm or higher, preferably 0.10 μm or higher. Because it is formed with a thickness of 0.02 μm or higher, the coating has a black appearance. By heating in an atmosphere with a relative humidity of 90%RH or higher, an oxygen-deficient oxide layer with less oxygen is formed compared to heating in the atmosphere. This oxygen-deficient oxide layer is black.

[0153] The higher the ambient temperature, the faster the oxide layer forms. However, when the oxide layer reaches a certain thickness (e.g., 5 μm or more), the oxygen diffusion rate on the surface of the coating is limited, thus halting the growth of the blackness and thickness. Therefore, when the ambient temperature of the heating process is excessively raised to a high temperature, the white oxide and coating begin to melt. Therefore, it is preferable to set the temperature to below 200°C.

[0154] The processing time is preferably set within the range of 10 minutes to 80 hours. When the processing time is less than 10 minutes, the black oxide layer cannot be sufficiently formed. In addition, when the processing time exceeds 80 hours (4800 minutes), the black oxide layer is formed too thickly, making it impossible to form the pattern in the subsequent laser irradiation process.

[0155] (Laser processing process)

[0156] In the laser processing step, after forming a black oxide layer, a portion of the coating surface is irradiated with a laser to reduce the O / Zn mass concentration ratio (O / Zn) at the irradiated area to less than 0.80. While the aforementioned oxygen-deficient black oxide layer can be removed by mechanical means such as grinding and polishing, or by evaporation and decomposition, in this embodiment, the oxide layer on the coating surface is partially removed by laser irradiation. By focusing the laser light on a specific area of ​​the oxide layer on the coating surface, a locally high-temperature region can be generated at the laser focal point, thereby facilitating the removal of the oxide layer. The irradiated area can be controlled to obtain the desired pattern.

[0157] The type of laser can be a gas laser such as a CO2 laser or a solid-state laser such as a YAG laser; there are no particular restrictions. The laser output needs to be set according to the thickness of the oxide layer. Laser irradiation conditions also depend on the type of laser. The output power should be set to 1.5~20.0W, the laser spot diameter to 0.01~0.20mm, and the laser scanning speed to 200~1000mm / second. There is no particular limit to the number of irradiations, for example, 1~30 times. The atmosphere during laser irradiation is not particularly limited and can be atmospheric. If the laser output is too high or the laser scanning speed is too slow, the coating may melt; therefore, it is preferable to irradiate the coating under conditions that do not cause melting. Furthermore, if the oxide layer cannot be removed in a single scan, multiple scans can be performed.

[0158] In this embodiment, the molten steel can form a film on the coating after laser irradiation. One or more films can be formed. Examples of films directly above the coating include chromate films and chromate-free films. The chromate treatment and chromate-free treatment for forming these films can be performed using known methods.

[0159] Chromate treatments include: 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 applies the treatment solution to the substrate and dries it without washing to form a film. Any treatment can be used. For aesthetic purposes, near-colorless and transparent materials are preferred.

[0160] 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.

[0161] Chromate-free treatment is preferred as it does not impose a significant environmental burden. Types of 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 materials and then rinsing off excess treatment solution; and coating-type chromate-free treatment, which involves applying the treatment solution to the substrate and drying it without washing to form a film. Any treatment method can be used.

[0162] 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 a specific type; examples include polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, or modified forms of these resins. Here, a 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 capable of reacting with those functional groups.

[0163] As such an organic resin, one or more organic resins (unmodified organic resins) can be used in combination, or at least one organic resin modified from another organic resin can be used in combination in the presence of at least one organic resin. Furthermore, the organic resin film can contain any coloring pigments or anti-rust pigments. Aqueous substances that are dissolved or dispersed in water can also be used.

[0164] Example

[0165] For use in laser irradiation applications, the following coated steel sheets were prepared. The coated steel sheets were coated using a Rhesca melt coating simulator, where an alloy containing specified amounts of pure metals was prepared, melted, and then melt-coated.

[0166] The base plate used for plating is a 200×100×1.2mm cold-rolled steel sheet (equivalent to SPCC JIS G3141:2021). Some base plates (No.3, 6, 14, 15, 27) use 200×100×2.0mm hot-rolled steel sheets (equivalent to SPHC JIS G3193:2019).

[0167] Before plating, the steel sheet surface is fully reduced by holding it at 800°C for 1 minute in a 5% N2-H2 atmosphere (dew point -40°C). Then, it is immersed in a plating bath (melting point +30°C) for 3 seconds and lifted off, and wiped with N2 gas to adjust the plating thickness to 17~40μm. Immediately after wiping, it is cooled to room temperature at an average cooling rate of 10°C / second. The manufactured plating steel sheet is then subjected to surface finishing rolling with a reduction rate of less than 1% of the steel sheet thickness, resulting in a surface roughness Ra in the range of 0.8~1.5μm.

[0168] The coated steel sheets are placed at a relative humidity of 53~100%RH, an ambient temperature of 90~180℃, and atmospheric pressure for 5~6000 minutes (100 hours) to form a black oxide layer on the coated surface. Except for No.1~7, 9, 35, 36, and 42~45, a black oxide layer of 0.02μm or larger is formed.

[0169] Laser processing involves irradiating with a CO2 laser (carbon dioxide laser). As shown in Table 2, the irradiation input heat is set to 5.0W, the spot size to 0.05mm, the scanning speed to 600.0mm / second, and the number of scans to 2-30. The black oxide layer is removed by laser irradiation. The patterned area is composed of multiple linear regions A spaced apart. Furthermore, as... Figure 1 As shown, the overall shape of the pattern section is set to the shape of the character "め" representing the hiragana. The size of the character is 10mm in length and 10mm in width. Thus, the plated steel of the embodiment and the comparative example were manufactured. The area where the black oxide layer was removed by laser irradiation is designated as region A, and the area not irradiated by laser is designated as region B. Furthermore, a region C is formed between region A and region B where part of the black oxide layer was removed due to the influence of laser irradiation.

[0170] The average chemical composition of the coating was determined as follows. The coating was dissolved using an acid containing an inhibitor that suppresses corrosion of the iron-based (steel) material. The chemical composition of the coating was determined by analyzing the resulting acid solution using ICP-MS or ICP-luminescence spectrophotometry. The results are shown in Tables 1A to 1C. Σ in the tables represents the sum of the element groups.

[0171] Regarding the coating thickness, firstly, to determine the average chemical composition of the coating, the weight change was measured when the coating was dissolved in acid. The area of ​​the coating before dissolution and the weight change before and after dissolution were measured to obtain the coating adhesion amount (g / m²). 2 Then, divide the coating adhesion amount by the coating specific gravity (g / m³). 3 The thickness of the coating is obtained from this. The coating specific gravity is calculated by determining the chemical composition of the coating and then calculating the coating specific gravity based on the chemical composition determination results.

[0172] In addition, P was measured in regions A, B, and C. A and P B and P C The determination was performed using an energy-dispersive elemental analyzer (EDS) mounted on a scanning electron microscope. The measurement area for regions A and B was 100 μm in each region. 2 The measurement area of ​​region C is located 20 μm away from the edge of region A identified by SEM image. Elemental analysis of a 10 μm × 10 μm region was performed by EDS, and compared with P... A P BSimilarly, the values ​​were calculated. The measurement areas were arbitrarily isolated at three locations, and the average of the three measurements was calculated. ZAF correction was used to calculate the mass concentration. EDS analysis was performed on the surface of the coating under an accelerating voltage of 15 kV, and the mass concentrations of O and Zn were determined in regions A, B, and C, respectively. Based on the results, P was calculated as the mass concentration ratio. A =O / Zn、P B =O / Zn and P C =O / Zn.

[0173] In addition, the arithmetic mean roughness Ra of the surface in region C, expressed in μm, is calculated. The details of the calculation method for the arithmetic mean roughness Ra of the surface in region C are the same as those described above, and therefore are omitted here.

[0174] The Mg mass concentration in region B was determined by performing elemental analysis on the surface of region B using an energy-dispersive elemental analyzer (EDS).

[0175] The difference between the surface height ZA of region A and the surface height ZB of region B (ZB-ZA) is calculated by observing the cross-sections of both regions simultaneously using a scanning electron microscope at 1000x magnification. The difference is calculated based on the average of the differences between the highest point of region B and the lowest point of region A. The detailed calculation method for ZB-ZA is the same as described above, and therefore will not be explained further.

[0176] The clarity was confirmed using a computer-generated character recognition function, which incorporates machine learning for character recognition and other techniques. For the pattern formed on the galvanized steel sheet, photographs were taken vertically under identical conditions (distance, lighting distance, angle, etc.). Image analysis was used to perform mosaic processing on the pixels of the 10mm x 10mm area constituting the character "め" and then binarized. Additionally, the same pattern was printed in black and white on paper, under the same conditions as the photograph taken on the galvanized steel sheet. The same mosaic processing was applied to this print, and then binarized.

[0177] When comparing the two binarization judgment results, the black-and-white consistency rate of the binarization judgment of the pattern area on the paper and the plated steel plate is measured, and evaluated according to the following judgment criteria. S, AA, A, and B are set as qualified. S: Black and white consistency rate is above 98% and below 100%. AA: Black and white consistency rate is above 96% and less than 98%. A: The black-and-white consistency rate is above 93% and less than 96%. B: The black-and-white consistency rate is above 90% and less than 93%. C: Black and white consistency rate is less than 90%.

[0178] The durability of the appearance design is evaluated based on the decrease in clarity of the appearance design after corrosion. White rust often forms due to corrosion of the coating; however, when the amount of white rust is large, forming raindrops or accumulating on the coated surface, it becomes difficult to identify the appearance design. Cyclic corrosion testing (CCT) is used to obtain a good correlation with the domestic exposure environment. Unlike salt spray testing (SST), which is a corrosion-promoting test, CCT involves repeated salt spraying, drying, and wetting processes, thus closely resembling the corrosion conditions in the atmospheric environment and confirming a certain degree of correlation. Here, the JASO cycle (M609-91) is used as the CCT. These 30 cycles are equivalent to 10 years in a typical domestic corrosion environment. The clarity of the appearance design before and after CCT is evaluated according to the aforementioned clarity evaluation method. Furthermore, durability is determined based on the decrease in clarity of the appearance design before and after CCT.

[0179] The plated steel sheet with the appearance design is cut into 100×50mm size, the cut end face is coated with epoxy resin coating, and the evaluation surface of 70×40mm is set in the center of the steel sheet to make a test piece.

[0180] In the center of the evaluation surface, a 10mm x 10mm hiragana character "め" (me) has been laser-processed. The area outside the 10mm square is the non-application section for laser processing.

[0181] Prepare 5 test pieces and perform a 30-cycle CCT (JASO cycle) corrosion test on each test piece. Then, prepare 5 test pieces before the CCT test.

[0182] Before and after the test, for sharpness, the computer's character recognition function, which has undergone machine learning for character recognition, was used to evaluate sharpness in the same way as the sharpness evaluation mentioned above. The black-and-white consistency rate was calculated for the test images before and after the CCT test. The black-and-white consistency rate was the average of the five test images. Then, the difference between the black-and-white consistency rate of the test images before and after the CCT test was calculated. Finally, the evaluation was carried out according to the following evaluation criteria. S, A, and B were set as acceptable. S: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is less than 5 points. A: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is worse than 5 points but less than 10 points. B: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is lower than 10 points but lower than 15 points. C: The clarity (black-and-white consistency) of the evaluation surface before and after corrosion is lower than 15 points.

[0183] As shown in Tables 1A to 3, Examples 8, 10-33 and 38-41, 47, and 48 formed black oxide layers containing Zn oxide with a thickness of 0.02 μm or more. Furthermore, as shown in Tables 1A to 3, the chemical composition of the coating, the thickness of the coating, and the relationship between regions A and B are all within the scope of this disclosure, resulting in good clarity and durability. However, in Example 48, instead of laser treatment, the oxide layer was removed with sandpaper, resulting in an arithmetic mean roughness Ra of region C exceeding 2.0, and both clarity and durability were rated as B.

[0184] On the other hand, as shown in Tables 1A to 3, in Comparative Examples 1 to 7, 9, 34 to 37, 42 to 46, any one or more of the chemical composition of the coating, the thickness of the coating, the thickness of the black oxide on the surface of the coating, and the relationship with regions A and B are outside the scope of this disclosure, resulting in a disadvantage in clarity or durability.

[0185] In Comparative Examples 1-7, 9, and 34-37, the Al, Mg, or Si content of the coating deviates from the scope of this disclosure. As a result, except for Comparative Example 34, a black oxide layer is not sufficiently formed, resulting in poor clarity or durability.

[0186] In Comparative Examples 42-46, since the conditions of the heating process deviated from the preferred range, P in region A... A or P in region B B Beyond the scope of this disclosure, clarity and durability are reduced.

[0187]

[0188]

[0189]

[0190]

[0191]

[0192] Industrial availability According to this disclosure, a melt-coated steel with excellent clarity and durability of the coating and a method for manufacturing the same are provided. This disclosure also provides an inexpensive and aesthetically pleasing material, thus contributing to industrial development.

[0193] Explanation of reference numerals in the attached figures 1…Melt-coated steel 2…steel 3… Coating 3A…Zn-Al-Mg alloy layer 4…Oxide layer

Claims

1. A molten-coated steel material, comprising a steel material and a coating disposed on the surface of said steel material, The average chemical composition of the coating, expressed as a percentage by mass, includes A1: Above 10.0% and below 40.0% Mg: ≥4.0% but ≤15.0% Si: 0% or more and less than 1.00% Sn: 0% or more and 0.7% or less Bi: 0% or more and less than 0.3% In: 0% or more and less than 0.3% The total amount of Sn, Bi, and In, ΣX: above 0% and below 0.7%, Ca: 0% or more and 0.60% or less Y: Above 0% and below 0.30% La: 0% or more and 0.30% or less Ce: 0% or more and 0.30% or less Sr: 0% or more and 0.30% or less Li: 0% or more and less than 0.30% The total amount of Ca, Y, La, Ce, Sr and Li, ΣYa: ≥0% and ≤0.60%, Cr: 0% or more and less than 1.00% Ni: 0% or more and less than 1.00% Mo: 0% or more and 0.25% or less Cu: 0% or more and less than 1.00% Ag: 0% or more and 0.25% or less Sb: 0% or more and 0.25% or less Pb: above 0% and below 0.25% The total amount of Cr, Ni, Mo, Cu, Ag, Sb and Pb, ΣYb: ≥0% and ≤1.00%. B: Above 0% and below 0.50% P: Above 0% and below 0.50% The combined amount of B and P, ΣYc: ≥0% and ≤0.50%, Ti: 0% or more and 0.25% or less Co: 0% or more and less than 0.25% V: Above 0% and below 0.25% Nb: 0% or more and 0.25% or less Mn: 0% or more and 0.25% or less Zr: 0% or more and 0.25% or less W: Above 0% and below 0.25% The total amount of Ti, Co, V, Nb, Mn, Zr, and W, ΣZ: above 0% and below 0.25%. Fe: 0% or more and 5.0% or less Zn: 40.0% or higher and 86.0% or lower The thickness of the coating is 5.0 μm or more. Region A and region B are provided on the surface of the coating. A black oxide layer containing Zn oxide is formed in region B with a thickness of 0.02 μm or more. Let P be the mass concentration ratio (O / Zn) of O to Zn in region A on the surface of the coating. A Let the mass concentration ratio (O / Zn) of O in region B on the surface of the coating be P. B Under the condition that the following equations (1) to (3) are satisfied: 0<P A <0.80…（1） 0.80≤P B …（2） 0.40≤P B -P A …(3)。 2. The molten steel according to claim 1, which satisfies the following formula (4): 0.50≤P B -P A …(4)。 3. The molten steel according to claim 1, wherein, The Mg mass concentration in region B on the surface of the coating is 3.2% by mass or more.

4. The molten steel according to claim 1, wherein, The difference (ZB-ZA) between the surface height ZA of region A and the surface height ZB of region B is greater than 0.1 μm and less than 5.0 μm.

5. The molten steel according to claim 1, wherein, The coating includes a region C adjacent to region A, where the mass concentration ratio of O to Zn (O / Zn) in region C is set to P. C Under the condition that the following equations (5) and (6) are satisfied: 0<P A <P C <0.80…(5) P C <0.40+P A …(6)。 6. The molten steel according to claim 5, wherein, The arithmetic mean roughness Ra of the surface of region C, measured in μm, satisfies the following equation (7): Ra<2.0…(7) 7. The molten steel coated according to any one of claims 1 to 6, wherein, A pattern portion including the region A is provided on the surface of the coating, and the pattern portion is set in an intentional shape.

8. The molten steel coated according to any one of claims 1 to 6, wherein, A pattern portion including region A is provided on the surface of the coating. The pattern part is set as any one of the following, or a combination of two or more of the following: straight line part, curved line part, dot part, graphic, number, symbol, pattern or text.

9. The molten steel coated according to any one of claims 1 to 6, wherein, A pattern portion including region A is provided on the surface of the coating. The pattern portion is set as any one of the following, or a combination of two or more of them: a straight line portion, a curved line portion, a dot portion, a graphic, a number, a symbol, a pattern, or a word.

10. The molten steel according to claim 7, wherein, The pattern portion includes region A, region B, and region C located between region A and region B.

11. The molten steel according to claim 8, wherein, The pattern portion includes region A, region B, and region C located between region A and region B.

12. The molten steel according to claim 9, wherein, The pattern portion includes region A, region B, and region C located between region A and region B.

13. The molten steel coated according to any one of claims 1 to 6, wherein, A non-patterned portion formed by region B is provided on the surface of the coating.

14. A method for manufacturing molten-coated steel, which is the method for manufacturing molten-coated steel as described in claim 1, comprising the following steps: The melt plating process forms an average chemical composition, by mass % of [missing information]. A1: Above 10.0% and below 40.0% Mg: ≥4.0% but ≤15.0% Si: 0% or more and less than 1.00% Sn: 0% or more and 0.7% or less Bi: 0% or more and less than 0.3% In: 0% or more and less than 0.3% The total amount of Sn, Bi, and In, ΣX: above 0% and below 0.7%, Ca: 0% or more and 0.60% or less Y: Above 0% and below 0.30% La: 0% or more and 0.30% or less Ce: 0% or more and 0.30% or less Sr: 0% or more and 0.30% or less Li: 0% or more and less than 0.30% The total amount of Ca, Y, La, Ce, Sr and Li, ΣYa: ≥0% and ≤0.60%, Cr: 0% or more and less than 1.00% Ni: 0% or more and less than 1.00% Mo: 0% or more and 0.25% or less Cu: 0% or more and less than 1.00% Ag: 0% or more and 0.25% or less Sb: 0% or more and 0.25% or less Pb: above 0% and below 0.25% The total amount of Cr, Ni, Mo, Cu, Ag, Sb and Pb, ΣYb: ≥0% and ≤1.00%. B: Above 0% and below 0.50% P: Above 0% and below 0.50% The combined amount of B and P, ΣYc: ≥0% and ≤0.50%, Ti: 0% or more and 0.25% or less Co: 0% or more and less than 0.25% V: Above 0% and below 0.25% Nb: 0% or more and 0.25% or less Mn: 0% or more and 0.25% or less Zr: 0% or more and 0.25% or less W: Above 0% and below 0.25% The total amount of Ti, Co, V, Nb, Mn, Zr, and W, ΣZ: above 0% and below 0.25%. Fe: 0% or more and 5.0% or less Zn: A coating of 40.0% or more but less than 86.0%; The heating process involves heating the plated steel obtained through the melt plating process at a relative humidity of 90% or higher and a temperature of 100°C or higher, thereby forming a black oxide layer on the surface of the plating layer; and In the laser processing step, a portion of the surface of the coating after the heating step is irradiated with a laser so that the mass concentration ratio of O to Zn (O / Zn) at the irradiated area is less than 0.

80.

15. The method for manufacturing melt-coated steel according to claim 14, wherein, The irradiated area is set as any one of the following, or a combination of two or more of them: a straight line, a curved line, a dot, a graphic, a number, a mark, a pattern, or a character.

Images