Plated steel sheet and method for manufacturing same

Abstract

Provided is a plated steel sheet which has low lightness from immediately after being produced and in which detachment of a plating layer is suppressed. A plated steel sheet according to the present invention has a plating layer comprising 1.0-22.0 mass% of Al and 1.5-10.0 mass% of Mg, the remaining portion being Zn and an optionally added trace additive. The plated steel sheet has a surface lightness (L*) of 50 or less, and has an X-ray diffraction spectrum, obtained through X-ray diffraction, satisfying all of (1)-(3).　(1): A / B is 0.001-0.015. (2): A / C is 1.2 or more. (3): The half width d of a peak indicated by A is 0.30-1.00 deg. A represents the peak intensity of a peak indicating the 100 plane of ZnO at which 2θ appears in 31.27-32.27 deg, B represents the peak intensity of a peak indicating the 101 plane of Zn at which 2θ appears in 42.73-43.73 deg, and C represents the intensity when 2θ is 30.00 deg.

Description

Coated steel sheet and its manufacturing method

[0001] The present invention relates to a plated steel sheet and a method for producing the same.

[0002] There is a demand for steel sheets with reduced external brightness (e.g., blackened) to enhance designability for use in roofing materials and exterior materials for buildings, home appliances, automobiles, etc. For example, Patent Documents 1 and 2 describe black-plated steel sheets in which a hot-dip Al, Mg-containing Zn plating layer formed on the surface of the steel sheet is subjected to a water vapor treatment to form a black oxide, thereby giving the steel sheet a black appearance.

[0003] It should be noted that Patent Document 3 describes that hot-dip galvanized steel sheets that have been subjected to a skin pass after plating suffer from blackening during storage or transportation.

[0004] JP 2013-241665 A International Publication No. 2016 / 157665 JP 11-335866 A

[0005] Plated steel sheets with low surface brightness as described in Patent Document 1 and Patent Document 2 are being considered for application in various applications due to their high designability. Patent Document 3 describes that hot-dip galvanized steel sheets that have been skin-passed turn black over time, but this document considers blackening to be undesirable and has investigated ways to suppress blackening. Furthermore, for use in applications requiring designability, it is preferable that the brightness of the plated steel sheet is low immediately after production.

[0006] Incidentally, the plated steel sheets described in Patent Documents 1 and 2 are thought to have reduced brightness due to the formation of oxides such as zinc oxide (ZnO) by steam treatment (see Patent Document 2). Furthermore, the plated steel sheet described in Patent Document 1 is said to be able to maintain a black appearance even in processed areas by forming black oxides inside the plating. However, in recent years, plated steel sheets are sometimes required to be processed under more severe conditions, and it is therefore required that the oxides formed on the surface do not easily peel off even when processed under such severe conditions.

[0007] The present invention has been made in consideration of the above points, and has an object to provide a plated steel sheet that has low brightness immediately after production and in which peeling of the plating layer is suppressed more than conventionally, and a method for producing the plated steel sheet.

[0008] One aspect of the present invention for solving the above problems relates to the following plated steel sheets [1] to [4]: ​​[1] A plated steel sheet comprising a steel sheet and a plating layer having an Al content of 1.0 mass% or more and 22.0 mass% or less, an Mg content of 1.5 mass% or more and 10.0 mass% or less, and the balance consisting of Zn and optionally added trace additives, and having a surface brightness (L ＊ ) is 50 or less, and an X-ray diffraction spectrum obtained by subjecting the plating layer to X-ray diffraction using a Cu tube at a voltage of 50 kV and a current of 300 mA satisfies all of the following (1) to (3): (1) A / B is 0.001 or more and 0.015 or less. (2) A / C is 1.2 or more. (3) The half-width d of the peak representing A is 0.30 deg or more and 1.00 deg or less. Here, A is the peak intensity of a peak representing a 100 plane of ZnO appearing between 2θ of 31.27 deg or more and 32.27 deg or more. B is the peak intensity of a peak representing a 101 plane of Zn appearing between 2θ of 42.73 deg or more and 43.73 deg or more. C is the intensity at 2θ of 30.00 deg. [2] The plated steel sheet according to [1], wherein the plating layer includes a phase including an amorphous phase containing Mg, Zn, and oxygen, and a ZnO phase. [3] The plated steel sheet according to [1] or [2], wherein, when a cross section of the plating layer having a length of 15 μm is observed at any 10 points in a direction parallel to the surface of the plated steel sheet, the plating layer contains a metallic Zn phase or a metallic Al phase within a depth range of 1 μm from the surface on the side opposite to the steel sheet. [4] The plated steel sheet according to any one of [1] to [3], wherein the plating layer has a chemical conversion coating on the side opposite to the steel sheet.

[0009] One aspect of the present invention for solving the above-mentioned problems relates to a method for producing a plated steel sheet as described in [5] below. [5] A method for producing a plated steel sheet, comprising the steps of: preparing a material-plated steel sheet having a steel sheet and a plating layer containing 1.0 to 22.0 mass% Al, 1.5 to 10.0 mass% Mg, and the balance consisting of Zn and optional trace additives; applying a skin-pass to the plated steel sheet using a skin-pass roll having an arithmetic mean roughness Ra of 2.1 μm to 2.8 μm under conditions such that the elongation of the material-plated steel sheet is 0.8 to 1.3% and the rolling load (tons) is 4.0 to 7.0 times the product of the sheet thickness (mm) and the sheet width (m), and steam treating the material-plated steel sheet having the strained plating layer.

[0010] According to the present invention, there are provided a plated steel sheet that has low brightness immediately after production and in which peeling of the plating layer is suppressed, and a method for producing the plated steel sheet.

[0011] FIG. 1A is a cross-sectional scanning transmission electron microscope (STEM) bright-field image of a cross section of a plated steel sheet according to an embodiment of the present invention, captured with an STEM, and FIG. 1B is a schematic diagram showing the metallic Zn phase and metallic Al phase in FIG. 1A .

[0012] 1. Plated Steel Sheet One embodiment of the present invention relates to a plated steel sheet having a steel sheet and a plating layer.

[0013] 1-1. Steel Plate The type of steel plate is not particularly limited, and any steel plate can be used, such as hot-rolled mild steel plate and steel strip described in JIS G 3131 (2018), hot-rolled steel plate and steel strip for automobiles described in JIS G 3113 (2018), cold-rolled steel plate and steel strip described in JIS G 3141 (2017), and various stainless steel plates (including austenitic, martensitic, ferritic, and ferrite-martensite dual-phase).

[0014] 1-2. Plating layer The plating layer is a so-called Al-, Mg-containing zinc-based plating layer, which has an aluminum (Al) content of 1.0 mass % or more and 22.0 mass % or less, a magnesium (Mg) content of 1.5 mass % or more and 10.0 mass % or less, and the balance consisting of Zn and optionally added trace additives. Note that these contents are the ratios of the contents of these elements to the total mass of the plating layer.

[0015] The plating layer may contain optional trace additives and unavoidable impurities. Trace additives are, for example, elements added to enhance the corrosion resistance of the plating layer. Trace additives may be elements such as Sn, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, and P. The content of these trace additives may be from the detection limit to 1% by mass.

[0016] Al in the coating layer passivates the plated steel sheet, improving its corrosion resistance and suppressing the generation of dross during production. Furthermore, the presence of a moderate amount of Al in the coating layer makes it easier for the brightness of the coating layer to be reduced by steam treatment. When the Al content is 1.0% by mass or more, the corrosion resistance of the plated steel sheet can be sufficiently improved, the generation of dross can be sufficiently suppressed, and the coating layer can be easily reduced in brightness. When the Al content is 22.0% by mass or less, the adhesion of the coating layer can be sufficiently improved, the sacrificial corrosion protection effect of Zn can be fully exerted, and the coating layer can be easily reduced in brightness.

[0017] Mg in the coating layer uniformly forms dense corrosion products on the surface of the coating layer in a corrosive environment, preventing erosion by corrosion factors and improving the corrosion resistance of the plated steel sheet. Furthermore, the presence of an appropriate amount of Mg in the coating layer makes it easier for the brightness of the coating layer to decrease upon steam treatment. A Mg content of 1.5 wt% or more sufficiently generates the dense and uniform corrosion products, thereby sufficiently improving the corrosion resistance of the plated steel sheet and further facilitating the reduction of the brightness of the coating layer. A Mg content of 10.0 wt% or less fully exhibits the sacrificial corrosion protection effect of Zn and also makes it easier to suppress the generation of dross.

[0018] By subjecting a plated steel sheet having the above-described composition to steam treatment, oxides such as zinc oxide (ZnO) are formed on the surface and inside of the plating layer, thereby reducing the surface brightness of the plated steel sheet. However, if excessive ZnO is formed, peeling of the plating layer due to peeling of the ZnO may occur during processing under more severe conditions (e.g., 2T bending or processing under more severe conditions). Therefore, based on the following findings, the present inventors have developed a plated steel sheet that is less likely to peel off the plating layer during processing under more severe conditions, while also achieving a sufficient reduction in brightness.

[0019] ZnO is typically a semiconductor, and its bandgap energy is greater than that of visible light, allowing it to transmit visible light. Therefore, it is transparent as a substance, while in powder form, light is diffusely reflected and appears white. In other words, simply forming ZnO does not sufficiently reduce the lightness of a plated steel sheet. The present inventors investigated this point and found that the degree of blackening of ZnO in plated steel sheets whose lightness has been reduced by steam treatment can be evaluated by the half-width of the peak representing ZnO (near 2θ = 31.77 deg) in the X-ray diffraction spectrum. A large half-width in the X-ray diffraction spectrum generally indicates a large number of lattice defects in the crystals or a small crystal size. This indicates that plated steel sheets whose lightness has been reduced by steam treatment contain ZnO with a large number of lattice defects, a small crystal size, or both. These lattice defects or the crystal size effect create an energy level within the bandgap of ZnO, which absorbs visible light, resulting in the reduced lightness of the plated steel sheets treated with steam.

[0020] Based on this finding, the present inventors considered that if ZnO having more lattice defects or a smaller crystal size is formed in a larger amount than conventionally, the lightness of the plated steel sheet would be sufficiently reduced even if the amount of ZnO is reduced to make the plating layer less likely to peel off during processing, etc.

[0021] Specifically, the plated steel sheet in this embodiment has a surface brightness (L ＊ ) is 50 or less. By subjecting a plated steel sheet having the above composition to a steam treatment, oxides such as zinc oxide (ZnO) are formed inside the plating layer, and the lightness of the surface of the plated steel sheet can be reduced. ＊ ) is preferably 45 or less, more preferably 40 or less.

[0022] The surface brightness of the plating layer (L ＊ The color difference value is a value measured using a color difference meter (CR-400 manufactured by Konica Minolta, Inc.) under a diffuse illumination and perpendicular light receiving method including specular reflection light in accordance with JIS Z8722: 2009. The illumination diameter during measurement is 11 mmΦ, and the measurement diameter is 8 mmΦ.

[0023] Furthermore, in the plated steel sheet of this embodiment, the X-ray diffraction spectrum of the plating layer obtained by X-ray diffraction using a Cu tube at a voltage of 50 kV and a current of 300 mA satisfies all of the following (1) to (3): (1) A / B is 0.001 or more and 0.015 or less, (2) A / C is 1.2 or more, and (3) the half-width d of the peak representing A is 0.30 deg or more and 1.00 deg or less, where A is the peak intensity of the peak representing the 100 plane of ZnO that appears between 2θ of 31.27 deg and 32.27 deg, B is the peak intensity of the peak representing the 101 plane of Zn that appears between 2θ of 42.73 deg and 43.73 deg, and C is the intensity at 2θ of 30.00 deg.

[0024] A is the intensity (unit: cps) of the peak at 2θ of approximately 31.77 degrees, which indicates the 100 plane of ZnO, and B is the intensity (unit: cps) of the peak at 2θ of approximately 43.23 degrees, which indicates the 101 plane of zinc (Zn).

[0025] Regarding condition (1), when A / B is 0.001 or more, it can be said that Zn is oxidized to form a sufficient amount of ZnO. Also, when A / B is 0.015 or less, ZnO is not formed in excess, so peeling of the plating layer during processing due to peeling of ZnO is unlikely to occur. From the viewpoint of balancing these, A / B is preferably 0.001 or more and 0.012 or less, more preferably 0.001 or more and 0.011 or less, and even more preferably 0.001 or more and 0.008 or less.

[0026] Regarding condition (2), when A / C is 1.2 or more, it can be said that a sufficient amount of ZnO is formed. The upper limit of A / C is preferably 3.5 or less from the viewpoint of preventing peeling of the plating layer during processing due to peeling of ZnO. From the viewpoint of balancing these factors, A / C is preferably 1.5 or more and 3.0 or less, and more preferably 1.5 or more and 2.7 or less.

[0027] Regarding condition (3), if the half-width d of the peak representing A (100 plane of ZnO) is 0.30 deg or more, it can be said that a large number of lattice defects are generated or that fine ZnO is formed. If the half-width d is 1.00 deg or less, it can be said that the peak is not too broad and a sufficient amount of ZnO is formed. From the viewpoint of balancing these, the half-width d is preferably 0.40 to 0.90, and more preferably 0.50 to 0.80.

[0028] In this way, in this embodiment, an appropriate amount of ZnO is formed, but the lightness of the plating layer is sufficiently reduced even with a smaller amount of ZnO by increasing the lattice defects of the ZnO or by reducing the crystal size of the ZnO (i.e., increasing the half-value width d).This keeps the amount of ZnO at an appropriate level, and prevents peeling of the plating layer during processing due to peeling of the ZnO.

[0029] The plating layer preferably has an amorphous phase containing Mg, Zn, and oxygen, and a phase containing ZnO. The amorphous phase is the MgZn phase that was present in the plating layer before the steam treatment. ２ This phase is formed by transformation due to steam treatment. ２ The phase oxidizes (blackens) relatively quickly. ２ By steam treating a plated steel sheet containing the MgZn phase, a plated steel sheet with low brightness can be produced in a short time. ２ The phase is distributed relatively uniformly throughout the plating layer. ２ When a plated steel sheet containing the phase is subjected to steam treatment, a plated steel sheet with little unevenness in brightness can be obtained.

[0030] FIG. 1A is a cross-sectional STEM image of the cross section of the plated steel sheet of this embodiment taken with a scanning transmission electron microscope (STEM), and FIG. 1B is a schematic diagram showing the metallic Zn phase and metallic Al phase in FIG. 1A (note that since elements with large atomic weights appear dark in a cross-sectional STEM bright-field image, the actual color tone of the cross section is not reflected in FIG. 1A ). The plated steel sheet of this embodiment can sufficiently reduce the brightness of the plating layer even if the time of steam treatment is shortened. Therefore, as shown in FIGS. 1A and 1B , the plating layer may contain a metallic Zn phase or metallic Al phase, which is an unoxidized phase. In this embodiment, the efficiency of reducing brightness is increased by increasing the number of lattice defects in ZnO or reducing the crystal size of ZnO. Therefore, even if an unoxidized metallic Zn phase or metallic Al phase is present near the plating surface, the brightness (L ＊ Furthermore, if the plating layer contains a predetermined amount of metallic Zn phase or metallic Al phase, these phases are softer than ZnO, and therefore the plating layer can be made less likely to peel off during processing, etc.

[0031] In this case, from the viewpoint of ensuring peeling resistance of the plating in the processed portion, it is preferable that the metallic Zn phase and the metallic Al phase are uniformly distributed on the surface of the plating layer. Specifically, when a cross section of a 15 μm long range in a direction parallel to the surface of the plated steel sheet is observed at any 10 positions, it is preferable that the plating layer contains a metallic Zn phase or a metallic Al phase within a depth range of 1 μm from the surface on the side opposite to the steel sheet in each cross section.

[0032] The metallic Zn phase or metallic Al phase contained in the plating layer is measured on a cross section obtained by processing an average location on the surface of the plated steel sheet using a focused ion beam device with a cryo-FIB-μ sampling method. Within the obtained cross section, a 15 μm-wide observation range is set along the surface of the plated steel sheet, and this observation range is observed with a transmission electron microscope (TEM). The phase of the plating surface layer is identified through elemental analysis by energy dispersive X-ray spectroscopy (EDS) and electron diffraction analysis. A determination range of 1 μm in depth is then set along the surface of the plating layer, and the presence or absence of a metallic Zn phase or metallic Al phase is determined within this determination range. The surface of the plating layer is typically not linear but has an irregular, uneven shape. Therefore, a determination range of 1 μm in depth is set along this uneven shape. Ten average locations on the surface of the plated steel sheet are arbitrarily set, and the cross section is observed to determine the presence or absence of a metallic Zn phase or metallic Al phase.

[0033] The average thickness of the plating layer is preferably 3 μm or more and 100 μm or less. The larger the average thickness of the plating layer, the less likely scratches occurring during handling will reach the steel sheet, improving appearance retention and corrosion resistance. The smaller the average thickness of the plating layer, the more effectively it can suppress peeling of the plating layer during processing due to differences in ductility between the steel sheet and the plating layer.

[0034] 1-3. Chemical Conversion Coating The plated steel sheet may have a chemical conversion coating on the outer side (the side opposite the steel sheet). The chemical conversion coating improves the adhesion between the zinc-based plating layer and the primer coating and the corrosion resistance of the steel sheet. The type of chemical conversion coating is not particularly limited, and can be a layer formed by a known chemical conversion treatment such as a chromate treatment, a chromate-free treatment, or a phosphate treatment. Of these, a chromate-free chemical conversion coating formed by a chromate-free treatment is preferred.

[0035] The chromate-free chemical conversion coating can be a layer containing, for example, a binder resin, a silane coupling agent, a zirconium compound, silica, phosphoric acid or its salt, a fluoride, a valve metal compound, an organic acid, etc. Chemical conversion coatings containing these materials tend to improve adhesion to the zinc-based plating layer and also tend to improve the corrosion resistance of the plated steel sheet because they have high barrier properties (density) against corrosion factors such as moisture and corrosive ions.

[0036] The type of binder resin is not particularly limited, and known organic resins such as polyester resin, polyurethane resin, epoxy resin, phenolic resin, acrylic resin, and polyolefin resin can be used. From the viewpoint of improving adhesion to the zinc-based plating layer, it is preferable to use polyester resin, urethane resin, epoxy resin, and acrylic resin, which have polar moieties or polar functional groups in the molecular chain. These resins may be used alone or in combination of two or more.

[0037] The content of the binder resin is preferably 1% by mass or more and 85% by mass or less, more preferably 1% by mass or more and 60% by mass or less, and even more preferably 1% by mass or more and 40% by mass or less, relative to the total mass of the chemical conversion treatment film.

[0038] The valve metal compound is a compound containing one or more elements selected from the group consisting of V, Nb, Ta, and W. Of these, V and Nb are preferred. The valve metal compound contributes to improving the corrosion resistance of the chemically treated steel sheet or suppressing excessive gloss in the chemically treated steel sheet. Examples of the valve metal compound include oxides, hydroxides, and fluorides of the above valve metals.

[0039] The content of the valve metal compound in the chemical conversion coating is preferably 0.005 to 2.0 mass % in terms of valve metal atoms, from the viewpoint of improving corrosion resistance.

[0040] Examples of phosphoric acid or salts thereof include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, and tetraphosphoric acid, or salts thereof (e.g., ammonium salts such as triammonium phosphate and diammonium hydrogen phosphate), phosphonic acids such as aminotri(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra(methylenephosphonic acid), and diethylenetriaminepenta(methylenephosphonic acid), or salts thereof, and organic phosphoric acids such as phytic acid or salts thereof. In addition to ammonium salts, metal salts with Na, Mg, Al, K, Ca, Mn, Ni, Zn, Fe, and the like may also be used. These phosphoric acids or salts thereof may be used alone or in combination of two or more.

[0041] The content of phosphoric acid or its salt is preferably 1 mass % or more and 20 mass % or less, and more preferably 1 mass % or more and 10 mass % or less, relative to the total mass of the chemical conversion coating. By setting the content within this range, it is possible to further improve the adhesion between the chemical conversion coating and the zinc-based plating layer, and further improve the corrosion resistance of the plated steel sheet.

[0042] Examples of organic acids include tartaric acid, tannic acid, citric acid, oxalic acid, malonic acid, lactic acid, acetic acid, and ascorbic acid.

[0043] The amount of organic acid added in the chemical conversion treatment solution for forming the chemical conversion treatment film can be from 2 g / L to 80 g / L, inclusive. By setting the amount in this range, it is possible to further improve the adhesion between the chemical conversion treatment film and the zinc-based plating layer, and further improve the corrosion resistance of the plated steel sheet.

[0044] The silane coupling agent and zirconium compound form a crosslinked structure in the chemical conversion coating, thereby improving the adhesion of the chemical conversion coating to the zinc-based plating layer and improving the barrier properties, thereby further enhancing the corrosion resistance of the plated steel sheet. In addition, silica, phosphoric acid or its salt, fluoride, and vanadium compound function as inhibitors to form precipitated films and passive films, thereby further enhancing the corrosion resistance of the plated steel sheet.

[0045] Examples of the silane coupling agent include γ-(2-aminoethyl)aminopropyltrimethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-(2-aminoethyl)aminopropyltriethoxysilane, γ-(2-aminoethyl)aminopropylmethyldiethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-methacryloxypropyl Pyrmethyldiethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropylmethyldimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltriethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropyltriethoxysilane silane, γ-glycidoxypropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-mercaptopropylmethyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, vinyltriacetoxysilane, γ-chloropropyltrimethoxysilane, γ-chloropropylmethyldimethoxysilane, γ-chloropropyltriethoxysilane, γ-chloropropyl methyldiethoxysilane, hexamethyldisilazane, γ-anilinopropyltrimethoxysilane, γ-anilinopropylmethyldimethoxysilane, γ-anilinopropyltriethoxysilane, γ-anilinopropylmethyldiethoxysilane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldimethoxysilyl)propyl]ammonium chloride,Examples of suitable silane coupling agents include octadecyldimethyl[3-(triethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldiethoxysilyl)propyl]ammonium chloride, γ-chloropropylmethyldimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, methyltrichlorosilane, dimethyldichlorosilane, and trimethylchlorosilane. These silane coupling agents may be used alone or in combination of two or more.

[0046] The amount of the silane coupling agent added to the chemical conversion treatment solution for forming the chemical conversion treatment film can be from 2 g / L to 80 g / L, inclusive. By setting the amount within this range, the adhesion between the chemical conversion treatment film and the zinc-based plating layer can be further improved, and the corrosion resistance of the plated steel sheet can be further improved.

[0047] Examples of the zirconium compound include zirconium normal propylate, zirconium normal butylate, zirconium tetraacetylacetonate, zirconium monoacetylacetonate, zirconium bisacetylacetonate, zirconium monoethylacetoacetate, zirconium acetylacetonate bisethylacetoacetate, zirconium acetate, zirconium monostearate, zirconium carbonate, zirconium ammonium carbonate, zirconium potassium carbonate, and zirconium sodium carbonate, etc. These zirconium compounds may be used alone or in combination of two or more.

[0048] The amount of zirconium compound added in the chemical conversion treatment solution for forming the chemical conversion treatment film can be from 2 g / L to 80 g / L, inclusive. By setting the amount within this range, the adhesion between the chemical conversion treatment film and the zinc-based plating layer can be further improved, and the corrosion resistance of the plated steel sheet can be further improved.

[0049] As the silica, commercially available silica gel, powdered silica, and the like can be widely used.

[0050] The amount of silica added in the chemical conversion treatment solution for forming the chemical conversion coating can be 1 g / L or more and 40 g / L or less. By setting the amount in this range, the corrosion resistance of the plated steel sheet can be further improved.

[0051] Examples of fluorides include ammonium zirconium fluoride, ammonium silicofluoride, ammonium titanium fluoride, sodium fluoride, potassium fluoride, calcium fluoride, lithium fluoride, titanium hydrofluoric acid, and zirconium hydrofluoric acid, etc. These fluorides may be used alone or in combination of two or more.

[0052] The fluoride content is preferably 1 mass % or more and 20 mass % or less, and more preferably 1 mass % or more and 10 mass % or less, relative to the total mass of the chemical conversion coating. By setting the fluoride content within this range, it is possible to further improve the adhesion between the chemical conversion coating and the zinc-based plating layer, and further improve the corrosion resistance of the plated steel sheet.

[0053] Examples of vanadium compounds include vanadium compounds obtained by reducing pentavalent vanadium compounds such as vanadium pentoxide, metavanadate, ammonium metavanadate, sodium metavanadate, and vanadium oxytrichloride with a reducing agent to divalent to tetravalent vanadium compounds, as well as divalent to tetravalent vanadium compounds such as vanadium trioxide, vanadium dioxide, vanadium oxysulfate, vanadium oxyoxalate, vanadium oxyacetylacetonate, vanadium acetylacetonate, vanadium trichloride, vanadium phosphomolybdic acid, vanadium sulfate, vanadium dichloride, and vanadium oxide. These vanadium compounds may be used alone or in combination of two or more.

[0054] The content of the vanadium compound is preferably 1 mass % or more and 20 mass % or less, and more preferably 1 mass % or more and 10 mass % or less, relative to the total mass of the chemical conversion coating. By setting the content within this range, the adhesion between the chemical conversion coating and the zinc-based plating layer can be further improved, and the corrosion resistance of the plated steel sheet can be further improved.

[0055] The amount of chemical conversion coating applied was 10 mg / m ２ More than 1000mg / m２ Preferably, it is 20 mg / m or less. ２ 800mg / m or more ２ More preferably, it is 50 mg / m or less. ２ 600mg / m or more ２ It is more preferable that the coating weight of the inorganic chemical conversion coating film that does not contain a binder resin is 20 mg / m or less in terms of the coating weight of the valve metal. ２ 100mg / m or more ２ Preferably, it is 40 mg / m or less. ２ 80mg / m or more ２ The coating weight of the organic chemical conversion coating containing a binder resin is preferably from 0.3 μm to 2.5 μm, more preferably from 0.8 μm to 1.5 μm.

[0056] The chemical conversion coating can be formed by applying a chemical conversion treatment solution containing the above-mentioned components to the surface of the zinc-based plating layer and drying it. The chemical conversion treatment solution may also contain an acid or alkali for adjusting the pH.

[0057] 2. Manufacturing Method of Plated Steel Sheet One example of a method for manufacturing the above-mentioned plated steel sheet includes preparing a material-plated steel sheet having a steel sheet and a plating layer having an Al content of 1.0 mass % or more and 22.0 mass % or less, an Mg content of 1.5 mass % or more and 10.0 mass % or less, with the balance being Zn and optional trace additives (hereinafter, the prepared plated steel sheet before steam treatment will also be simply referred to as the "material-plated steel sheet"), imparting strain to the plating layer of the material-plated steel sheet, and subjecting the material-plated steel sheet having the strained plating layer to steam treatment.

[0058] 2-1. Preparation of a material-plated steel sheet First, a material-plated steel sheet is prepared. The material-plated steel sheet may be prepared by forming a plating layer having the above-described composition on a steel sheet, or may be prepared by purchasing a material-plated steel sheet having the above-described plating layer on the surface of the steel sheet.

[0059] The method for forming the coating layer is not limited, and the coating layer can be formed, for example, by a hot-dip coating method in which a steel sheet is immersed in a coating bath containing 1.0 to 22.0 mass% Al, 1.5 to 10.0 mass% Mg, and the balance being Zn and optionally added trace additives, and then pulled out and cooled. In this case, the average film thickness of the coating layer can be adjusted by changing the immersion time and the volume and pressure of the gas blown during cooling.

[0060] Before being immersed in the plating bath, the steel sheet may be subjected to pretreatment such as washing and degreasing, as required.

[0061] 2-2. Applying strain Strain can be applied to the plated steel sheet by rolling or other methods.

[0062] The rolling method is not particularly limited, and may be, for example, a method in which the material-plated steel sheet is passed between skin-pass rolls rotating in opposite directions to apply pressure to the material-plated steel sheet.

[0063] In this embodiment, the roughness of the surface of the skin-pass roll is preferably set to an arithmetic mean roughness Ra of more than 2.1 μm and not more than 2.8 μm, and more preferably to be more than 2.3 μm and not more than 2.6 μm. By setting the arithmetic mean roughness Ra of the surface of the skin-pass roll to 2.1 μm or more, sufficient strain can be formed, allowing for the production of more ZnO with more lattice defects or smaller crystal size. By setting the arithmetic mean roughness Ra of the surface of the skin-pass roll to 2.8 μm or less, it is possible to suppress a deterioration in the design of the plated steel sheet due to excessive unevenness of the plated surface.

[0064] Furthermore, the elongation of the material-plated steel sheet obtained by skin-pass rolling is preferably 0.8% or more and 1.3% or less, and more preferably 0.8% or more and 1.1% or less. By setting the elongation of the material-plated steel sheet obtained by skin-pass rolling to 0.8% or more, sufficient strain can be formed, allowing for the production of more ZnO with more lattice defects or smaller crystal size. By setting the elongation of the material-plated steel sheet obtained by skin-pass rolling to 1.3% or less, it is possible to suppress a decrease in the adhesion of the plating layer due to the excessive production of ZnO.

[0065] Furthermore, the rolling load (tons) during the skin pass is preferably set to 4 to 7 times the product of the sheet thickness (mm) and the sheet width (m). The load during the skin pass increases as the sheet thickness to be rolled increases. By setting the pressure applied by the skin pass to 4 times or more the product of the sheet thickness (mm) and the sheet width (m), appropriate strain can be imparted to the plated steel sheet, resulting in the production of more ZnO with more lattice defects or smaller crystal size. By setting the rolling load (tons) during the skin pass to 7 times or less the product of the sheet thickness (mm) and the sheet width (m), cracks in the coating layer can be made less likely to occur. Furthermore, by suppressing the release of strain due to cracks, it is possible to easily achieve both low brightness and suppression of peeling of the coating layer during processing.

[0066] 2-3. Steam Treatment Steam treatment can be carried out by placing the plated steel sheet in a high-temperature, high-humidity atmosphere and bringing the plating layer into contact with steam. Steam treatment is preferably carried out in a sealed container.

[0067] The steam treatment can oxidize phases containing any of Zn, Al, and Mg in the coating layer and cause the volume of these phases to expand, thereby forming a plurality of fine protrusions on the surface of the coating layer, reducing the gloss of the plated steel sheet, and reducing the lightness of the plated steel sheet due to the oxidized black phase.

[0068] The temperature of the steam treatment is preferably 105°C or higher and 210°C or lower, and more preferably 105°C or higher and 150°C or lower. The higher the temperature, the faster the oxidation of the plating layer progresses and the faster ZnO is formed, so the lightness of the plated steel sheet can be reduced in a shorter time. Furthermore, the lower the temperature, the more effectively the occurrence of cracks due to rapid oxidation can be suppressed.

[0069] The relative humidity of the atmosphere during the steam treatment is preferably 60% or more and 100% or less, and more preferably 85% or more and less than 100%. The higher the relative humidity, the shorter the time it takes to reduce the gloss and brightness of the plated steel sheet. On the other hand, if the relative humidity is less than 100%, adhesion of condensed water to the surface of the plated steel sheet is suppressed, and the gloss and brightness of the plated steel sheet can be reduced more uniformly.

[0070] The oxygen concentration in the atmosphere during the water vapor treatment is preferably 13% by volume or less. If the oxygen concentration is high, white ZnO is likely to be formed, making it difficult to obtain a black appearance. The oxygen concentration can be reduced by increasing the concentration (relative humidity) of water vapor, introducing an inert gas into the atmosphere (replacing the air in the sealed container with an inert gas), or removing the air in the sealed container using a vacuum pump or the like. Examples of the inert gas include Ar, N ２ and He, etc. can be used.

[0071] The treatment time (soaking time) for the steam treatment is preferably 1 hour or more and 35 hours or less, and more preferably 5 hours or more and 25 hours or less. The longer the treatment time, the more likely it is that the gloss and brightness of the plated steel sheet will be reduced. The shorter the treatment time, the more likely it is that cracks will occur in the plating layer due to volume expansion during the steam treatment, and that this will cause peeling of the plating layer during processing.

[0072] At this time, by adjusting the conditions of the steam treatment, such as the temperature and treatment time, depending on the composition (proportion of each phase) of the material-plated steel sheet, the gloss and brightness of the plated steel sheet to be produced can be adjusted to the desired level.

[0073] A chemical conversion coating or the like may be formed on the plated steel sheet produced in this manner by a known method.

[0074] The plated steel sheets described above have low surface brightness, excellent design properties, and are resistant to peeling of the plating layer during processing. Therefore, they are useful as plated steel sheets for use in roofing and exterior materials for buildings, home appliances, automotive components, and the like.

[0075] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0076] [Test 1] 1. Preparation of Plated Steel Sheets Using cold-rolled steel sheets (JIS G3141:2021) with thicknesses of 0.6 to 1.2 mm and widths of 1.05 m as the substrate, multiple hot-dip Al- and Mg-containing Zn-plated steel sheets (material-plated steel sheets) with different coating layer compositions and thicknesses were prepared. Furthermore, skin-pass treatment was performed using a skin-pass rolling mill located downstream of the plating device (downstream in the sheet passing direction). During this process, strain was imparted to the material-plated steel sheets under different conditions by varying the surface roughness of the skin-pass rolls of the skin-pass rolling mill, the elongation of the material-plated steel sheets, and the rolling load.

[0077] The plated steel sheet was then placed in an ultra-accelerated life testing apparatus (PC-304R8, manufactured by Hirayama Manufacturing Co., Ltd.) and water vapor was brought into contact with the plating layer to obtain a plated steel sheet with a reduced brightness of the plating layer. At this time, the temperature, relative humidity, and soaking time of the water vapor treatment were changed, and the plated steel sheet was subjected to the water vapor treatment under different conditions.

[0078] X-ray diffraction measurements were carried out on the prepared plated steel sheets. An X-ray diffraction analyzer RINT-TTR III (manufactured by Rigaku Corporation) was used, and the obtained X-ray diffraction patterns were analyzed using analysis software PDXL2 (manufactured by Rigaku Corporation). ２ The presence or absence of diffraction peaks due to the MgZn phase was confirmed. ２ When the phase is confirmed, the plated steel sheet contains MgZn. ２ I decided there was a phase.

[0079] The plating compositions, plating layer thicknesses, skin pass conditions, and steam treatment conditions of the plated steel sheets 1 to 26 thus obtained are shown in Tables 1 and 2.

[0080]

[0081]

[0082] 2. Measurement and Evaluation 2-1. X-ray Diffraction (XRD) Measurement For the plating layers of Coated Steel Sheets 1 to 26, an X-ray diffraction analyzer, RINT-TTR III (manufactured by Rigaku Corporation), was used, and the obtained X-ray diffraction patterns were analyzed using analysis software PDXL2 (manufactured by Rigaku Corporation). Measurement conditions included a Cu tube, a voltage of 50 kV, a current of 300 mA, a divergence slit of 1 / 2°, a divergence vertical limiting slit of 5 mm, open scattering slit and receiving slit, a monochrome receiving slit, and a horizontal goniometer with a goniometer radius of 280 mm. A D / teX Ultra Full detector was used, and X-ray diffraction measurements were performed at a scan speed of 2 deg / min, a step width of 0.01 deg, a scan axis of 2θ / θ, and a scan range of 5 to 100 deg to obtain X-ray diffraction patterns. From the obtained X-ray diffraction pattern, the peak intensity (A) of the peak representing the 100 plane of ZnO appearing between 2θ of 31.27 deg and 32.27 deg, the peak intensity (B) of the peak representing the 101 plane of Zn appearing between 2θ of 42.73 deg and 43.73 deg, and the intensity (C) at 2θ of 30.00 deg were determined. Then, the values ​​of A / B and A / C were determined. In addition, the obtained X-ray diffraction pattern was subjected to background processing using a fitting method, smoothed using a B-spline, and then Kα2 removal was performed using the Rachinger method (intensity ratio of Kα1 to Kα2: 0.4970). After this processing, the peak shape was fitted with a split pseudo-Voigt function, and the half-width d of the peak representing A was determined on the analysis software. For ZnO, data from PDF card number 00-036-1451 was used, and for Zn, data from PDF card number 00-004-0831 was used.

[0083] 2-2. STEM Measurements Using a focused ion beam (FIB) system, STEM measurements were performed on cross sections obtained by processing an average location on the surface of the plated steel sheet using the cryo-FIB-μ sampling method. The sample was prepared using a Hitachi High-Technologies NB5000 FIB system. Specifically, a 15 μm-wide observation range was set along the surface of the plated steel sheet within the obtained cross section. This observation range was then observed using a transmission electron microscope (TEM). The phases of the plating surface were identified through elemental analysis using energy dispersive X-ray spectroscopy (EDS) and electron diffraction analysis. The electron microscope used was a JEOL 200 kV-field emission transmission electron microscope (JEM-2100F), and the EDS analyzer used was a JEOL JED-2300T. A 1 μm-deep determination range was set along the surface of the plating layer, and the presence or absence of metallic Zn or Al phases within this determination range was determined. The 1 μm-deep determination range was measured 1 μm from each surface, following the uneven surface shape of the plating layer. Ten average locations on the surface of the plated steel sheet were arbitrarily selected and observed and identified, and the presence of either a metallic Zn phase or a metallic Al phase was confirmed in all cross sections. If either a metallic Zn phase or a metallic Al phase was present in one or more of the 10 selected cross sections, it was evaluated as "present," and if it was present in less than one location, it was evaluated as "absent."

[0084] 2-3. Evaluation of lightness Using a color difference meter (Konica Minolta, Inc., CR-400), the lightness (L ＊ The illumination diameter during measurement was 11 mmΦ, and the measurement diameter was 8 mmΦ. The obtained lightness (L ＊ Based on the measured value, peeling of the plating layer was evaluated according to the following criteria: ＊ The value is 40 or less. ＊ The value is greater than 40 and less than 50. ×: L ＊ The value is greater than 50

[0085] 2-4. Evaluation of peeling of plating layer Test pieces 0.8 mm thick and 50 mm x 50 mm obtained from each plated steel sheet were subjected to 2T bending. Then, adhesive tape (cellophane tape, manufactured by Nichiban Co., Ltd.) was applied to the apex of the bent part and peeled off, and the peeled adhesive tape was observed with a magnifying glass. Peeling of the plating layer was evaluated based on the maximum size of the peeled plating pieces adhering to the adhesive tape (maximum distance between the apexes of the peeled pieces) using the following criteria: ◎: The maximum size of the peeled plating pieces was less than 0.2 mm ○: The maximum size of the peeled plating pieces was 0.2 mm or more and less than 0.5 mm △: The maximum size of the peeled plating pieces was 0.5 mm or more and less than 1 mm ×: The maximum size of the peeled plating pieces was 1 mm or more

[0086] The results of the measurements and evaluations are shown in Table 3.

[0087]

[0088] As shown in Tables 1 to 3, by subjecting a material-plated steel sheet having a plating layer containing an Al content of 1.0 mass % or more and 22.0 mass % or less, an Mg content of 1.5 mass % or more and 10.0 mass % or less, and the balance being Zn and optionally added trace additives, to steam treatment, plated steel sheets with low lightness could be obtained (Plated Steel Sheet 1 to Plated Steel Sheet 18).

[0089] In this case, by applying an appropriate amount of strain to the material plated steel sheets before the steam treatment, it was possible to obtain plated steel sheets (Plated Steel Sheets 1 to 18) in which the lightness of the plating layer was sufficiently reduced and the plating layer was less likely to peel even in a 2T bending test, which is more severe than conventional tests.

[0090] On the other hand, the plating layers of plated steel sheet 19, which had a low amount of Al in the plating layer, plated steel sheet 20, which had a low amount of MgZn in the plating layer, and plated steel sheet 22, which had an excessive amount of Al, did not blacken sufficiently even after steam treatment.

[0091] Furthermore, plated steel sheet 21, which had an excessive elongation rate during skin-pass treatment, produced a large amount of ZnO during steam treatment, resulting in significant peeling of the coating layer during the 2T bending test. Plated steel sheets 23 and 24, which had small roll roughness, a small elongation rate, and a small load during the skin-pass test, resulting in a small amount of applied strain, did not produce sufficient ZnO, which had more lattice defects or a smaller crystal size. Therefore, a large amount of ZnO must be produced to achieve sufficient blackening, resulting in significant peeling of the coating layer during the 2T bending test. Similarly, plated steel sheet 25, which had a small elongation rate and a small load during the skin-pass test, resulting in a small amount of applied strain, did not produce sufficient blackening of the coating layer when steam treatment was performed to a degree that did not significantly peel off the coating layer during the 2T bending test. Furthermore, in the plated steel sheet 26 in which an excessive load was applied during the skin pass test, cracks occurred and strain was released, resulting in more lattice defects or in insufficient production of ZnO with a smaller crystal size. Therefore, when the plated steel sheet 26 was subjected to steam treatment to the extent that the plated layer did not peel off significantly in the 2T bending test, the plated layer did not blacken sufficiently.

[0092] Furthermore, for each of the plated steel sheets 1 to 18, a focused ion beam device was used to process an average portion of the surface using a cryo-FIB-μ sampling method, and STEM measurements were performed on the cross sections obtained. This observation area was observed with a transmission electron microscope (TEM), and the phases of the plated surface layer were identified by elemental analysis using energy dispersive X-ray spectroscopy (EDS) and electron diffraction analysis. As a result, for all of the plated steel sheets, MgZn ２ It was confirmed that the phase had changed to a phase containing an amorphous phase containing Mg, Zn and O, and a ZnO phase.

[0093] Furthermore, in the case of plated steel sheet 26, in which the rolling load during skin pass was increased, cracks were found in the plated layer when the plated surface was observed with a scanning electron microscope. The cracks probably released strain, and therefore the plated layer could not be sufficiently blackened.

[0094] [Experiment 2] 1. Preparation of plated steel sheets Plated steel sheets A and B were obtained by forming a plating layer, performing a skin-pass treatment, and performing a steam treatment in the same manner as in Experiment 1. The plating compositions, plating layer thicknesses, skin-pass treatment conditions, and steam treatment conditions of plated steel sheets A and B are shown in Table 4.

[0095]

[0096] The materials shown in Table 5 were added to water in the amounts shown in the table and mixed to prepare treatment solutions 1 to 18 for chemical conversion treatment.

[0097]

[0098] Each treatment solution was applied to the surface of plated steel sheet A or plated steel sheet B, and the resulting sheet was heated and dried at a temperature of 120°C to form a chemical conversion coating. The amount of treatment solution applied was adjusted so that the amount of valve metal in the chemical conversion coating would be the amount shown in Table 5 for inorganic treatment solutions 1 to 9, and so that the thickness of the chemical conversion coating would be the amount shown in Table 6 for organic treatment solutions 10 to 18.

[0099] 2. Measurement and Evaluation 2-1. X-ray Diffraction (XRD) Measurement X-ray diffraction measurement was performed in the same manner as in Experiment 1 to obtain an X-ray diffraction pattern. From the obtained X-ray diffraction pattern, the values ​​of A / B and A / C were determined. In addition, the half-value width d of the peak indicating A was determined.

[0100] STEM measurement was performed in the same manner as in Experiment 1. Then, it was determined whether or not a metallic Zn phase or a metallic Al phase was present within a determination range of 1 μm in depth along the surface of the plating layer.

[0101] 2-3. Evaluation of brightness As in Experiment 1, the brightness (L ＊ value) was measured.

[0102] 2-4. Evaluation of peeling of plating layer Peeling of the plating layer was evaluated in the same manner as in Experiment 1.

[0103] 2-5. Evaluation of Corrosion Resistance After sealing the end faces of 0.8 mm thick, 50 mm x 100 mm test pieces obtained from each plated steel sheet, a salt spray test was conducted in accordance with JIS Z2371:2015, in which a 35°C NaCl aqueous solution was sprayed for 12 hours. After 12 hours of spraying, the area rate of white rust that had developed on the test pieces was measured, and the ratio of the area where white rust had developed to the total area of ​​the test piece (area where white rust had developed / total area of ​​test piece: white rust development rate) was determined, and corrosion resistance was evaluated according to the following criteria: ◎: White rust development rate was 0% ○: White rust development rate was more than 0% but less than 5% △: White rust development rate was 5% or more

[0104] The results of the measurements and evaluations are shown in Tables 6 and 7.

[0105]

[0106]

[0107] As shown in Tables 3 to 7, even in the case of plated steel sheets that had been subjected to a chemical conversion treatment, it was possible to obtain plated steel sheets in which the lightness of the plating layer was sufficiently reduced and the plating layer was made less likely to peel even in a 2T bending test that was more severe than conventional tests.

[0108] This application claims priority from Japanese Patent Application No. 2023-211053, filed December 14, 2023. The entire disclosures of the specification, claims, and drawings of that application as originally filed are incorporated herein by reference.

[0109] The plated steel sheet of the present invention has low surface brightness, excellent designability, and is resistant to peeling of the plating layer during processing, making it suitable for use in a variety of fields where designability is required.

Claims

1. A steel sheet having a plating layer with an Al content of 1.0 mass% or more and 22.0 mass% or less, an Mg content of 1.5 mass% or more and 10.0 mass% or less, and the balance being Zn and optionally added trace additives, and having a surface brightness (L ＊ A plated steel sheet, wherein the X-ray diffraction spectrum obtained by subjecting the plated layer to X-ray diffraction at a voltage of 50 kV and a current of 300 mA using a Cu tube satisfies all of the following (1) to (3): (1) A / B is 0.001 or more and 0.015 or less, (2) A / C is 1.2 or more, and (3) the half-width d of the peak showing A is 0.30 deg or more and 1.00 deg or less, wherein A is the peak intensity of a peak showing the 100 plane of ZnO appearing between 2θ of 31.27 deg or more and 32.27 deg or more, B is the peak intensity of a peak showing the 101 plane of Zn appearing between 2θ of 42.73 deg or more and 43.73 deg or more, and C is the intensity at 2θ of 30.00 deg.

2. The plated steel sheet according to claim 1, wherein the plating layer has an amorphous phase containing Mg, Zn, and oxygen, and a phase containing ZnO.

3. The plated steel sheet according to claim 1 or 2, wherein, when cross sections of the plating layer are observed at any ten points within a range of 15 μm in length in a direction parallel to the surface of the plated steel sheet, each cross section contains a metallic Zn phase or metallic Al phase within a range of 1 μm in depth from the surface on the side opposite the steel sheet.

4. The plated steel sheet according to any one of claims 1 to 3, further comprising a chemical conversion coating on the opposite side of the plated layer to the steel sheet.

5. A method for manufacturing a plated steel sheet, comprising the steps of: preparing a material-plated steel sheet having a steel sheet and a plating layer having an Al content of 1.0% by mass or more and 22.0% by mass or less, an Mg content of 1.5% by mass or more and 10.0% by mass or less, with the remainder being Zn and optionally added trace additives; performing a skin pass using a skin pass roll having an arithmetic mean roughness Ra of 2.1 μm or more and 2.8 μm or less under conditions such that the elongation of the material-plated steel sheet is 0.8% or more and 1.3% or less and the rolling load (tons) is 4.0 times or more and 7.0 times or less the product of the sheet thickness (mm) and the sheet width (m), thereby imparting strain to the plating layer; and steam treating the material-plated steel sheet having the plating layer to which the strain has been imparted.

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