Coated plated steel sheet

Abstract

This coated plated steel sheet is provided with a steel sheet, a plating layer, and a protective layer. The plating layer contains aluminum, zinc, and magnesium. The average value of the ratio of the atomic concentration of aluminum to the total of the atomic concentration of magnesium and the atomic concentration of zinc in the outermost layer at the depth of 5 nm in the plating layer is 0.40 to 1.50 inclusive. The ratio of the mass of a resin (A1) in the protective layer to the total mass of the protective layer is 0.6 or more. The ratio of the mass of silicon atoms in terms of SiO2 in the protective layer to the total mass of the protective layer is 0.07 to 0.19 inclusive. The ratio of the mass of phosphorus atoms in the protective layer to the total mass of the protective layer is 0.011 to 0.075 inclusive. The ratio of the mass of cobalt atoms in the protective layer to the total mass of the protective layer is 0.0015 to 0.02 inclusive.

Description

Coated and plated steel sheets

[0001] The present disclosure relates to a coated plated steel sheet in which a protective layer is formed on a plated steel sheet having a plating layer containing aluminum, zinc, and magnesium.

[0002] Patent Document 1 describes a chromium-free surface-treated plated steel sheet in which a protective film is formed on the surface of a hot-dip Zn—Al alloy-plated steel sheet having a hot-dip Zn—Al alloy plating layer containing 1.0 to 10% Al, 0.2 to 1.0% Mg, and 0.005 to 0.1% Ni, the protective film being made of a treatment composition containing a specific titanium-containing aqueous solution, a nickel compound and / or a cobalt compound, a fluorine-containing compound, and an aqueous organic resin in predetermined proportions. Patent Document 1 also describes that this chromium-free surface-treated plated steel sheet is excellent in corrosion resistance, blackening resistance, paint adhesion, and plating appearance.

[0003] Japanese Patent Application Laid-Open No. 2009-132952

[0004] An object of the present disclosure is to ensure resistance to blackening of a coated-plated steel sheet having a plating layer containing aluminum, zinc, and magnesium and a protective layer.

[0005] A coated-plated steel sheet according to one embodiment of the present disclosure comprises a steel sheet, a plating layer, and a protective layer laminated in this order. The plating layer contains aluminum, zinc, and magnesium. In the outermost layer of the plating layer having a depth of 5 nm, the average ratio of the atomic concentration of the aluminum to the sum of the atomic concentrations of the magnesium and the zinc is 0.40 or more and 1.50 or less. The protective layer contains a resin (A1), a silicon compound (B1), a phosphorus compound (C1), and a cobalt compound (D1). The ratio of the mass of the resin (A1) in the protective layer to the total mass of the protective layer is 0.60 or more. The ratio of the SiO of silicon atoms in the protective layer to the total mass of the protective layer is 0.50 or more. ２ The ratio of the mass of phosphorus atoms in the protective layer to the total mass of the protective layer is 0.07 or more and 0.19 or less. The ratio of the mass of cobalt atoms in the protective layer to the total mass of the protective layer is 0.0015 or more and 0.0200 or less.

[0006] According to one aspect of the present disclosure, it is possible to provide a coated-plated steel sheet that includes a plating layer containing aluminum, zinc, and magnesium and a protective layer, and that can ensure resistance to blackening.

[0007] FIG. 1 is a schematic diagram illustrating an example of a plating processing apparatus according to an embodiment of the present disclosure.

[0008] Conventionally, plated steel sheets have been produced by plating steel sheets for the purpose of improving the long-term corrosion resistance of the steel sheets, etc. Typical compositions of the plating layer in plated steel sheets include, for example, alloys containing 1 to 75 mass % aluminum, the majority of the remainder being zinc, and further containing trace amounts of third elements such as Si, Mg, and Ce—La.

[0009] It has been known that when such plated steel sheets are used as building components, the surface of the plated layer tends to become discolored black (blackened) over time due to the effects of acid rain in recent years.

[0010] Therefore, various surface treatment techniques have been proposed to prevent blackening of plated steel sheets. In particular, surface treatment techniques that do not use hexavalent chromium (chromium-free techniques) have been developed.

[0011] For example, Patent Document 1 (JP 2009-132952 A) discloses a chromium-free surface-treated plated steel sheet in which a protective film is formed on the surface of a hot-dip Zn—Al alloy-plated steel sheet having a hot-dip Zn—Al alloy plating layer containing 1.0 to 10% Al, 0.2 to 1.0% Mg, and 0.005 to 0.1% Ni, using a treatment composition containing a specific titanium-containing aqueous solution, a nickel compound and / or a cobalt compound, a fluorine-containing compound, and an aqueous organic resin in predetermined proportions. The document describes the resulting steel sheet as having excellent corrosion resistance, blackening resistance, paint adhesion, and plating appearance.

[0012] However, according to the inventor's research, such coated-plated steel sheets are required to be resistant to blackening under a wide range of environments. In particular, when the plating layer contains aluminum, zinc, and magnesium, not only is blackening resistance evaluated by an accelerated test required, but also blackening resistance under, for example, mild conditions (normal environments) under which the coated-plated steel sheets are actually stored.

[0013] Therefore, the present disclosure provides a coated-plated steel sheet that includes a steel sheet, a plating layer containing aluminum, zinc, and magnesium, and a protective layer, and that can ensure resistance to blackening under a wide range of environments.

[0014] Hereinafter, embodiments of the present disclosure will be described. The following embodiments are merely a portion of various embodiments of the present disclosure. Furthermore, the following embodiments can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved.

[0015] The coated-plated steel sheet according to the embodiment includes a steel sheet, a plating layer, and a protective layer laminated in this order. The average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of the plating layer having a depth of 5 nm is 0.40 to 1.50. Here, the outermost layer of the plating layer having a depth of 5 nm refers to a region of the plating layer extending from the interface with the protective layer to a depth of 5 nm.

[0016] The protective layer contains a resin (A1), a silicon compound (B1), a phosphorus compound (C1), and a cobalt compound (D1). The ratio of the mass of the resin (A1) in the protective layer to the total mass of the protective layer is 0.60 or more. The ratio of the SiO of silicon atoms in the protective layer to the total mass of the protective layer is 0.60 or more. ２ The converted mass ratio is 0.07 or more and 0.19 or less. The mass ratio of phosphorus atoms in the protective layer to the total mass of the protective layer is 0.011 or more and 0.075 or less. The mass ratio of cobalt atoms in the protective layer to the total mass of the protective layer is 0.0015 or more and 0.020 or less.

[0017] Therefore, in the embodiment, a coated plated steel sheet can be provided which includes a plating layer containing aluminum, zinc, and magnesium and a protective layer, and which can ensure resistance to blackening in a wide range of environments.

[0018] Here, "a wide range of environments" refers to environments ranging from normal environments in which the coated-plated steel sheet is actually stored outdoors to high-temperature and high-humidity environments, and the coated-plated steel sheet according to the embodiment can ensure resistance to blackening under such a wide range of environments.

[0019] The protective layer may be directly superimposed on the plating layer, or a functional layer may be interposed between the protective layer and the plating layer. The functional layer is a layer for surface modification, such as a chemical conversion treatment layer. The functional layer may or may not be superimposed on the protective layer. However, in this embodiment, the effects of this embodiment are achieved even when a functional layer is not interposed between the protective layer and the plating layer, or when a functional layer is not superimposed on the protective layer.

[0020] The coated plated steel sheet may also have a coating layer on the protective layer.

[0021] The coated plated steel sheet will now be described in more detail.

[0022] [Steel Plate] The steel plate may be a thin steel plate or a thick steel plate, etc. The steel plate is formed from steel such as carbon steel, alloy steel, stainless steel, nickel-chromium steel, nickel-chromium-molybdenum steel, chromium steel, chromium-molybdenum steel, or manganese steel.

[0023] [Plating Layer] The plating layer will be described.

[0024] As described above, the plating layer contains aluminum, zinc, and magnesium. In other words, the plating layer may contain an alloy of aluminum, zinc, and magnesium. In particular, when the plating layer is produced by hot-dip galvanizing, it may contain a molten Zn-Al-Mg alloy. In other words, an alloy layer formed by alloying the steel sheet and the components of the plating bath during the plating process may be present between the steel sheet and the plating layer. In this embodiment, the alloy layer and the plating layer are separate layers.

[0025] The thickness of the plating layer may be, for example, 5 μm or more and 40 μm or less.

[0026] In an embodiment, the metal atomic concentration composition in the outermost layer of the plating layer at a specific depth is adjusted to be within a specific range, thereby ensuring the adhesion and corrosion resistance of the plating layer and enhancing the blackening resistance of the plating layer under normal conditions. Specifically, the average ratio of the aluminum atomic concentration to the sum of the magnesium atomic concentration and the zinc atomic concentration in the outermost layer of the plating layer at a depth of 5 nm is 0.40 or more and 1.50 or less, thereby ensuring the adhesion and corrosion resistance of the plating layer and enhancing the blackening resistance of the plating layer under normal conditions.

[0027] To explain this in more detail, in an embodiment, the average ratio of the aluminum atomic concentration to the sum of the magnesium atomic concentration and the zinc atomic concentration in the 5-nm-deep outermost layer of the plating layer is adjusted to 0.40 or more. In this case, the magnesium atomic concentration and the zinc atomic concentration in the 5-nm-deep outermost layer of the plating layer are not excessively high, thereby improving the blackening resistance of the plating layer under normal conditions. Furthermore, the average ratio of the aluminum atomic concentration to the sum of the magnesium atomic concentration and the zinc atomic concentration in the 5-nm-deep outermost layer of the plating layer is adjusted to 1.50 or less. In this case, the aluminum atomic concentration is not excessively high, thereby improving the reactivity of the plating layer with the phosphorus compound (C2) contained in the aqueous surface treatment agent described below. This strengthens the bond between the plating layer and the phosphorus compound (C1) contained in the protective layer. This ensures the corrosion resistance and film adhesion of the plating layer. The average ratio is preferably 0.45 or more, more preferably 0.5 or more, and even more preferably 0.6 or more. The average value of this ratio is preferably 1.40 or less, more preferably 1.20 or less, and even more preferably 1.00 or less.

[0028] Furthermore, research and development has revealed that the average ratio of the atomic concentrations of metals in the outermost layer of the plating layer, which is 5 nm deep, can be adjusted by making appropriate changes to the plating layer. In other words, the inventors have focused on the atomic concentrations of each metal in the outermost layer of the plating layer, the composition of the metals in the plating layer, and the like, and have found that by appropriately adjusting the numerical ranges of these, the average ratio can be adjusted to a specific numerical range of 0.4 to 1.5.

[0029] Specifically, it was found that the average value of the ratio can be adjusted to a specific range of 0.40 or more and 1.50 or less by adjusting the average atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of a 5-nm depth of the plating layer to a specific range. The effect on the average value of the ratio when the average atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of a 5-nm depth of the plating layer are adjusted to a specific range will be described in detail in "Atomic concentration of each metal in the outermost layer of the plating layer."

[0030] In addition, it has been found that the average value of the above ratio can be adjusted to a specific range of 0.40 or more and 1.50 or less by adjusting the proportions of each of the metals, such as aluminum, zinc, and magnesium, in the plating layer to specific ranges. The proportion of aluminum to the total of zinc and magnesium in the plating layer, and the effect on the average value of the above ratio when the proportions of aluminum, zinc, and magnesium are adjusted to specific ranges will be described in detail in the section "Composition of the entire plating layer."

[0031] Furthermore, the inventors have found that the average value of the ratio tends to be adjusted to a specific numerical range by appropriately changing the method and conditions for forming a plating layer by plating a steel sheet. Specifically, with regard to the method for forming a plating layer, it has been found that when a method for forming a plating layer by hot dip plating is adopted, the average value of the ratio tends to be adjusted to a specific numerical range of 0.40 to 1.50.

[0032] In addition, the inventors have found that when a method of producing a coating layer by hot dip coating is employed, the average value of the ratio can be specifically adjusted to a specific numerical range of 0.40 or more and 1.50 or less by satisfying specific conditions for the temperature of the coating bath, the temperature of the steel sheet when immersed in the coating bath, the cooling conditions for the coating metal adhered to the steel sheet, etc. Note that the effect on the average value of the ratio when the temperature of the coating bath, the temperature of the steel sheet when immersed in the coating bath, the cooling conditions for the coating metal adhered to the steel sheet, etc., satisfy specific conditions will be described in detail in the section "Conditions for carrying out coating treatment" in the section "Method of producing a coating layer."

[0033] In the present disclosure, the average atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of the plating layer having a depth of 5 nm are values ​​obtained by taking into account the ratio of each element to the total of aluminum, zinc, and magnesium, which are the main elements contained in the plating layer, as the whole. Based on the average values ​​of aluminum, zinc, and magnesium thus obtained, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of the plating layer having a depth of 5 nm is calculated.

[0034] <Atomic concentration of each metal in the outermost layer of the plating layer> As described above, it is also preferable that the average atomic concentration of each of aluminum, zinc, and magnesium in the outermost layer of the plating layer having a depth of 5 nm is adjusted to a specific numerical range.

[0035] The average atomic concentration of aluminum in the outermost layer of the plating layer having a depth of 5 nm is preferably 20 atomic % or more. When this average value is 20 atomic % or more, the atomic concentration of aluminum in the outermost layer does not become excessively low, and the average value of the ratio is appropriately increased. Therefore, the corrosion resistance of the plating layer can be maintained while further improving the blackening resistance of the plating layer under normal conditions. This average value is more preferably 27 atomic % or more, even more preferably 30 atomic % or more, and particularly preferably 35 atomic % or more.

[0036] The average atomic concentration of aluminum in the outermost layer of the plating layer, which is 5 nm deep, is preferably 60 atomic % or less. When this average value is 60 atomic % or less, the atomic concentration of aluminum in the outermost layer does not become excessively high, and the average value of the ratio is appropriately reduced. As a result, the decrease in the blackening resistance of the plating layer is suppressed, and the film adhesion and corrosion resistance can be further ensured. This average value is more preferably 55 atomic % or less, and even more preferably 50 atomic % or less.

[0037] The average atomic concentration of zinc in the outermost layer of the plating layer, which is 5 nm deep, is preferably 1 atomic % or more. When this average value is 1 atomic % or more, the atomic concentration of zinc in the outermost layer does not become excessively low, and the average value of the ratio is appropriately reduced. This can further improve the corrosion resistance and adhesion of the plating layer. This average value is more preferably 2 atomic % or more, even more preferably 2.5 atomic % or more, and particularly preferably 5 atomic % or more.

[0038] The average atomic concentration of zinc in the outermost layer of the plating layer having a depth of 5 nm is preferably 40 atomic % or less. When this average value is 40 atomic % or less, the atomic concentration of zinc in the outermost layer does not become excessively high, and the average value of the ratio is appropriately increased. This can further improve the blackening resistance of the plating layer. This average value is preferably 10 atomic % or less, more preferably 8 atomic % or less, and particularly preferably 6 atomic % or less.

[0039] The average atomic concentration of magnesium in the outermost layer of the plating layer, which is 5 nm deep, is preferably 10 atomic % or more. When this average value is 10 atomic % or more, the atomic concentration of magnesium in the outermost layer does not become excessively low, and the average value of the ratio is appropriately reduced. As a result, the blackening resistance of the plating layer is ensured, and the corrosion resistance and adhesion of the plating layer can be further improved. This average value is more preferably 30 atomic % or more, even more preferably 40 atomic % or more, and particularly preferably 45 atomic % or more.

[0040] The average atomic concentration of magnesium in the outermost layer of the plating layer having a depth of 5 nm is preferably 75 atomic % or less. When this average value is 75 atomic % or less, the atomic concentration of magnesium in the outermost layer does not become excessively high, and the average value of the ratio is appropriately increased. Therefore, the decrease in the blackening resistance of the plating layer under normal conditions is suppressed, and the bond between the plating layer and the phosphorus compound (C1) contained in the protective layer is strengthened, thereby ensuring the film adhesion of the plating layer. This average value is more preferably 70 atomic % or less, even more preferably 60 atomic % or less, and particularly preferably 65 atomic % or less.

[0041] As described above, it is preferable that the average atomic concentration of aluminum in the outermost layer having a depth of 5 nm in the plating layer is 20 atomic % or more and 60 atomic % or less, the average atomic concentration of magnesium in the outermost layer having a depth of 5 nm in the plating layer is 10 atomic % or more and 75 atomic % or less, and the average atomic concentration of zinc in the outermost layer having a depth of 5 nm in the plating layer is 1 atomic % or more and 40 atomic % or less.

[0042] In another embodiment, the average atomic concentration of aluminum in the outermost layer of a 5 nm depth of the plating layer is preferably 20 atomic % or more and 60 atomic % or less, the average atomic concentration of magnesium is 30 atomic % or more and 75 atomic % or less, and the average atomic concentration of zinc is 1 atomic % or more and 10 atomic % or less. When the atomic concentrations of each metal in the outermost layer of the plating layer are within these ranges, there is an advantage that a relatively large amount of aluminum is present in the outermost layer, making it easier to obtain good blackening resistance. In the atomic concentrations in the outermost layer of this plating layer of a 5 nm depth, the particularly preferred lower limit of the average atomic concentration of aluminum is 25 atomic %, 30 atomic %, or 35 atomic %. The particularly preferred upper limit of the average atomic concentration of aluminum is 55 atomic % or 50 atomic %. The lower limit of the average atomic concentration of magnesium is 30 atomic %, 35 atomic %, or 40 atomic %. The upper limit of the average atomic concentration of magnesium is 65 atomic %, 60 atomic %, 55 atomic %, or 50 atomic %. The lower limit of the average value of the zinc atomic concentration is 1 atomic %, 1.5 atomic %, or 2 atomic %, and the upper limit of the average value of the zinc atomic concentration is 10 atomic %, 8 atomic %, or 5 atomic %.

[0043] In another embodiment, it is also preferable that the average atomic concentration of aluminum in the outermost layer of a 5 nm depth of the plating layer is 25 atomic % or more and 60 atomic % or less, the average atomic concentration of magnesium is 10 atomic % or more and 60 atomic % or less, and the average atomic concentration of zinc is 5 atomic % or more and 40 atomic % or less. When the atomic concentrations of each metal in the outermost layer of the plating layer are within these ranges, there is an advantage that a relatively large amount of zinc is present in the outermost layer, which improves adhesion with the protective layer and makes it easier to obtain good corrosion resistance. In the atomic concentrations in the outermost layer of this plating layer of a 5 nm depth, the particularly preferred lower limit of the average atomic concentration of aluminum is 30 mass % or 35 mass %. The particularly preferred upper limit of the average atomic concentration of aluminum is 55 mass % or 50 mass %. The particularly preferred lower limit of the average atomic concentration of magnesium is 12 mass % or 17 mass %. The particularly preferred upper limit of the average atomic concentration of magnesium is 55 mass % or 50 mass %. A particularly preferred lower limit of the average zinc atomic concentration is 7% by mass or 10% by mass, and a particularly preferred upper limit of the average zinc atomic concentration is 38% by mass or 35% by mass.

[0044] The average atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of the plating layer, which is 5 nm deep, can be measured by X-ray photoelectron spectroscopy (XPS). Specifically, the atomic concentrations of aluminum, zinc, and magnesium in the plating layer can be measured by quantifying the peak intensities of Al2p, Zn2p3, and Mg1s obtained by measurement. A more specific measurement method will be described in the Examples below.

[0045] <<Composition of the Entire Plating Layer>> As described above, in the embodiment, it is preferable that the composition of the metal contained in the plating layer is appropriately adjusted.

[0046] The aluminum content in the plating layer is preferably 5.0% by mass or more. In this case, the aluminum content in the plating layer is appropriately increased, so that the average atomic concentration of aluminum in the outermost layer of the plating layer does not become excessively low. Therefore, the average value of the ratio is appropriately increased. This allows the corrosion resistance of the plating layer to be maintained while further improving the blackening resistance of the plating layer under normal conditions. This content is more preferably 45.0% by mass or more, and more preferably 50.0% by mass or more.

[0047] The proportion of aluminum in the plating layer is preferably 65.0% by mass or less. In this case, since the proportion of aluminum in the plating layer is appropriately reduced, the average atomic concentration of aluminum in the outermost layer of the plating layer does not become excessively high. Therefore, the average value of the ratio is appropriately reduced. This can suppress hardening of the plating layer while further ensuring the adhesion and corrosion resistance of the plating layer. This proportion is more preferably 60.0% by mass or less, and even more preferably 25.0% by mass or less.

[0048] The magnesium content in the plating layer is preferably 0.5% by mass or more. In this case, the magnesium content in the plating layer is appropriately increased, so that the average atomic concentration of magnesium in the outermost layer of the plating layer does not become excessively low. Therefore, the average value of the ratio is appropriately reduced. This ensures the blackening resistance of the plating layer while further improving the corrosion resistance and adhesion of the plating layer. This content is more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more.

[0049] The magnesium content in the plating layer is preferably 10.0% by mass or less. In this case, the magnesium content in the plating layer is appropriately reduced, so that the average atomic concentration of magnesium in the outermost layer of the plating layer does not become excessively high. Therefore, the average value of the ratio is appropriately increased. This improves the appearance of the plating layer while further enhancing the blackening resistance of the plating layer under normal conditions. This content is preferably 5.0% by mass, and more preferably 3.0% by mass or less.

[0050] The coating layer may also contain silicon. The proportion of silicon in the coating layer is preferably 1.0 mass% or more. In this case, alloying between aluminum in the coating bath and components of the steel sheet is particularly suppressed when the coating layer is produced by hot-dip coating. The proportion of silicon in the coating layer is preferably 3.0 mass% or less. In this case, the generation of dross due to silicon can be particularly suppressed when the coating layer is produced by hot-dip coating.

[0051] When the plating layer does not contain silicon, the total proportion of aluminum, magnesium, and zinc in the plating layer is preferably 95.0 mass% or more relative to the plating layer. When the plating layer contains silicon, the total proportion of aluminum, magnesium, silicon, and zinc in the plating layer is preferably 95.0 mass% or more relative to the plating layer.

[0052] The plating layer may further contain one or more elements selected from Ni, Ce, Cr, Fe, Ca, Sr, rare earth elements, etc. In particular, it is preferable that the plating layer contains one or more of Ni and Cr. When the plating layer contains Ni, the proportion of Ni in the plating layer is preferably in the range of more than 0 mass% to 1 mass% or less. It is more preferable that this proportion is 0.01 mass% or more to 0.5 mass% or less. When the plating layer contains Cr, it is preferably in the range of more than 0 mass% to 1 mass% or less. It is more preferable that this proportion is 0.01 mass% or more to 0.5 mass% or less. In these cases, the coated-plated steel sheet has excellent corrosion resistance.

[0053] It is also preferable that the plating layer contains one or more of Ca, Sr, Y, La, and Ce. When the plating layer contains Ca, the proportion of Ca in the plating layer is preferably in the range of more than 0 mass% to 0.5 mass% or less. This proportion is more preferably 0.001 mass% to 0.1 mass% or less. When the plating layer contains Sr, the proportion of Sr in the plating layer is preferably in the range of more than 0 mass% to 0.5 mass% or less. This proportion is more preferably 0.001 mass% to 0.1 mass% or less. When the plating layer contains Y, the proportion of Y in the plating layer is preferably in the range of more than 0 mass% to 0.5 mass% or less. This proportion is more preferably 0.001 mass% to 0.1 mass% or less. When the plating layer contains La, the proportion of La in the plating layer is preferably in the range of more than 0 mass% to 0.5 mass% or less. This proportion is more preferably 0.001 mass% to 0.1 mass% or less. When the plating layer contains Ce, the proportion of Ce in the plating layer is preferably in the range of more than 0 mass % to 0.5 mass % or less, and more preferably 0.001 mass % to 0.1 mass % or less.

[0054] Furthermore, the proportion of Ca in the plating layer is particularly preferably 50 ppm by mass or more and 400 ppm by mass or less. By adjusting the proportion of Ca contained in the plating layer to a specific numerical range, the concentration of magnesium in the outermost layer of the plating layer can be suppressed. As a result, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of the plating layer at a depth of 5 nm can be particularly adjusted to a specific numerical range of 0.40 to 1.50. The proportion of Ca in the plating layer is preferably 70 ppm by mass or more, more preferably 90 ppm by mass or more. Furthermore, the proportion of Ca in the plating layer is preferably 300 ppm by mass or less, more preferably 250 ppm by mass or less.

[0055] Of course, the plating layer may contain elements other than those mentioned above that are inevitably mixed in.

[0056] As described above, the aluminum content in the plating layer is preferably 5.0 mass % or more and 65.0 mass % or less, and the magnesium content in the plating layer is preferably 0.5 mass % or more and 10.0 mass % or less.

[0057] In particular, it is preferable that the aluminum content in the plating layer be 45.0% by mass or more and 65.0% by mass or less, the magnesium content in the plating layer be 0.5% by mass or more and 5.0% by mass or less, and the Ca content in the plating layer be 50 ppm by mass or more and 400 ppm by mass or less. Having the composition of the entire plating layer within these ranges is advantageous in that a relatively large amount of aluminum is present in the outermost layer, making it easier to obtain good blackening resistance. In this composition of the entire plating layer, particularly preferred lower limits for the aluminum content in the plating layer are 46.0% by mass, 48.0% by mass, or 50.0% by mass. Particularly preferred upper limits for the aluminum content in the plating layer are 63.0% by mass, 60.0% by mass, or 58% by mass. Particularly preferred lower limits for the magnesium content in the plating layer are 0.8% by mass, 1.0% by mass, or 1.5% by mass. Particularly preferred upper limits for the magnesium content in the plating layer are 4.0% by mass, 3.5% by mass, or 3.0% by mass.

[0058] In particular, it is preferable that the aluminum content in the plating layer be 5.0% by mass or more and 25.0% by mass or less, and that the magnesium content in the plating layer be 1.5% by mass or more and 10.0% by mass or less. When the composition of the entire plating layer is within these ranges, the magnesium and zinc content in the outermost layer is relatively high, which has the advantage of improving adhesion to the protective layer and making it easier to obtain good corrosion resistance. In this composition of the entire plating layer, the lower limit of the aluminum content in the plating layer is particularly preferably 6.0% by mass, 9.0% by mass, or 10.0% by mass. The upper limit of the aluminum content in the plating layer is particularly preferably 24.0% by mass, 22.0% by mass, or 20.0% by mass. The lower limit of the magnesium content in the plating layer is particularly preferably 1.8% by mass, 2.0% by mass, or 2.2% by mass. The upper limit of the magnesium content in the plating layer is particularly preferably 9.0% by mass, 8.0% by mass, or 7.0% by mass.

[0059] As with the average ratio of atomic concentrations of metals in the outermost layer of the plating layer at a depth of 5 nm, the composition of the metals contained in the plating layer can be adjusted to a desired value by devising the plating process method, i.e., by adopting the "method for producing the plating layer" described below.

[0060] The proportions of metals contained in the plating layer, i.e., the proportions of each atom in the plating layer, can be measured by X-ray photoelectron spectroscopy (XPS). Specifically, the proportions of aluminum, zinc, and magnesium in the plating layer can be measured by quantifying the peak intensities of Al2p, Zn2p3, and Mg1s obtained by measurement. More specific measurement methods will be described in the examples below.

[0061] <Chemical State of Cobalt Present in the Plating Layer> In the outermost layer of the plating layer having a depth of 5 nm, when the average atomic concentration of aluminum is 25 atomic % or more and 60 atomic % or less, the average atomic concentration of magnesium is 10 atomic % or more and 60 atomic % or less, and the average atomic concentration of zinc is 5 atomic % or more and 40 atomic % or less, or when the proportion of aluminum in the plating layer is 5.0 mass % or more and the proportion of magnesium in the plating layer is 1.5 mass % or more and 10.0 mass % or less, it is preferable that 5 mol % or more of the chemical state of cobalt present at the interface between the plating layer and the protective layer is metallic cobalt. The proportion of metallic cobalt present at the interface between the plating layer and the protective layer is more preferably 7 mol % or more, 10 mol % or more, 15 mol % or more, or 20 mol % or more. Here, the amount of cobalt present at the interface between the plating layer and the protective layer is 0.1 to 20.0 mg / m ２ is preferred.

[0062] A plating layer with the above composition has a low aluminum content, which tends to result in a relatively high zinc atomic concentration in the outermost layer of the plating layer, which can lead to variations in the degree of blackening. Therefore, if a trace amount of cobalt is present in a metallic state at the interface between the plating layer and the protective layer, the blackening resistance-enhancing effect of the metallic cobalt and the synergistic effect of the protective layer can provide stable blackening resistance. Therefore, it is recommended to subject the plating layer to an immersion treatment in an aqueous solution of cobalt sulfate (e.g., an aqueous solution prepared by dissolving cobalt sulfate heptahydrate in water and adjusting the pH to approximately 2.0 to 3.0 with sulfuric acid). This treatment results in the deposition of metallic cobalt on the plating layer surface. Although metallic cobalt becomes an ionic compound or oxide when the protective layer is applied, some of the metallic cobalt remains, improving blackening resistance. Therefore, when the plating layer has the above composition, the proportion of metallic cobalt present at the interface between the plating layer and the protective layer should be 5 mol% or more (preferably 7 mol% or more, 10 mol% or more, 15 mol% or more, or 20 mol% or more). However, if the proportion of metallic cobalt is high, the corrosion resistance tends to deteriorate due to the potential difference at the plating layer surface. Therefore, the proportion is preferably 65 mol% or less, and particularly preferably 45 mol% or less.

[0063] The method for measuring the proportion (mol %) of metallic cobalt present at the interface between the plating layer and the protective layer will be described in the Examples below. Similarly, the method for measuring the amount of cobalt present will be described in the Examples below.

[0064] <Method for Producing Plated Layer> A method for producing a plated layer will be described.

[0065] Any suitable method can be used to prepare the plating layer, but as described above, a hot-dip galvanizing method is preferred. In the hot-dip galvanizing method, the plating layer is prepared, for example, by immersing the steel sheet in a plating bath prepared by melting a metal. The plating layer prepared in this manner tends to have a metal atomic concentration composition adjusted to a specific numerical range at a specific depth in the plating layer, specifically, within the outermost layer at a depth of 5 nm. The plating layer has high resistance to blackening under normal conditions, and therefore, the coated-plated steel sheet can maintain its resistance to blackening under a wide range of environments.

[0066] <<Plating Bath Composition>> The plating bath used contains aluminum, zinc, and magnesium as constituent elements, and is prepared so that the atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of the plating layer having a depth of 5 nm can be adjusted. That is, the plating bath contains aluminum, zinc, and magnesium as constituent elements, and by adjusting the composition of the constituent elements in the plating bath, a plating layer having the above composition can be produced. This provides the plating layer with high resistance to blackening under normal conditions.

[0067] The plating bath used contains aluminum, zinc, and magnesium as constituent elements, and is prepared such that the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of a 5-nm depth of the plating layer can be adjusted to a specific numerical range. That is, the plating bath contains aluminum, zinc, and magnesium as constituent elements, and by adjusting the composition of the constituent elements in the plating bath, a plating layer can be formed in which the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of a 5-nm depth of the plating layer falls within a specific numerical range. This provides the plating layer with high resistance to blackening under normal conditions.

[0068] The plating bath may also contain silicon. The plating bath may further contain one or more elements selected from Ni, Ce, Cr, Fe, Ca, Sr, rare earth elements, etc. In particular, the plating bath preferably contains one or more of Ni and Cr. It is also preferable that the plating bath contains one or more of Ca, Sr, Y, La, and Ce.

[0069] In particular, the atomic concentration of metal in the outermost layer having a depth of 5 nm can be adjusted by adjusting the proportion of Ca present in the plating bath within a specific range. Specifically, the proportion of Ca in the plating bath is preferably 50 ppm by mass or more and 400 ppm by mass or less relative to the plating bath. By adjusting the proportion of Ca contained in the plating bath within the above specific numerical range, the concentration of magnesium in the outermost layer of the plating layer can be suppressed. As a result, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer having a depth of 5 nm in the plating layer can be adjusted within a specific numerical range of 0.40 to 1.50. The proportion of Ca in the plating bath is preferably 70 ppm by mass or more, more preferably 90 ppm by mass or more, relative to the plating bath. Furthermore, the proportion of Ca in the plating bath is preferably 300 ppm by mass or less, more preferably 250 ppm by mass or less, relative to the plating bath.

[0070] <<Plating Conditions>> The atomic concentration of the metal at a specific depth in the plating layer, specifically in the outermost layer at a depth of 5 nm, can also be achieved by appropriately adjusting the plating conditions. Specifically, by performing plating under appropriately adjusted plating conditions using a plating apparatus such as that shown in FIG. 1, a plating layer can be produced in which the atomic concentration of the metal in the outermost layer is adjusted to a desired numerical range.

[0071] The plating processing apparatus shown in Fig. 1 includes a transport device that continuously transports a steel sheet 1. This transport device is composed of a payout machine 3, a winder 12, and a plurality of transport rolls 15. In this transport device, the payout machine 3 holds a coil 13 (first coil 13) of the long steel sheet 1. This first coil 13 is unwound by the payout machine 3, and the steel sheet 1 is transported to the winder 12 while being supported by the transport rolls 15. Furthermore, the winder 12 winds the steel sheet 1, and this winder 12 holds a coil 14 (second coil 14) of the steel sheet 1.

[0072] In this coating processing apparatus, a heating furnace 4, an annealing / cooling section 5, a snout 6, a pot 7, a spray nozzle 9, a cooling device 10, and a temper rolling / shape correction device 11 are provided in this order from the upstream side of the transport path of the steel sheet 1 by the transport device. The heating furnace 4 heats the steel sheet 1. This heating furnace 4 is composed of a non-oxidizing furnace or the like. The annealing / cooling section 5 heats and anneals the steel sheet 1, and then cools it. This annealing / cooling section 5 is connected to the heating furnace 4, and an annealing furnace is provided upstream and a cooling zone (cooler) is provided downstream. A reducing atmosphere is maintained within this annealing / cooling section 5. The snout 6 is a cylindrical member through which the steel sheet 1 is transported. One end of the snout 6 is connected to the annealing / cooling section 5, and the other end is located in the coating bath 2 in the pot 7. The snout 6 is maintained in a reducing atmosphere, similar to the annealing / cooling section 5. The pot 7 is a container for storing the coating bath 2, and a sink roll 8 is disposed inside the pot 7. The injection nozzle 9 injects gas toward the steel sheet 1. The injection nozzle 9 is disposed above the pot 7. The injection nozzle 9 is disposed in a position where it can inject gas toward both sides of the steel sheet 1 pulled up from the pot 7. The cooling device 10 cools the coating metal adhering to the steel sheet 1. As the cooling device 10, an air cooler, a mist cooler, or the like is provided, and the steel sheet 1 is cooled by the cooling device 10. The temper rolling / shape straightening device 11 cools the steel sheet 1 on which the coating layer has been formed. The temper rolling / shape straightening device 11 is equipped with a skin pass mill or the like for performing temper rolling on the steel sheet 1, and a tension leveler or the like for performing shape straightening on the steel sheet 1 after temper rolling.

[0073] In a plating process using a plating processing apparatus such as that shown in Figure 1, a steel sheet 1 is first unwound and continuously fed from a feeder 3. After being heated in a heating furnace 4, the steel sheet 1 is transported to an annealing and cooling section 5 having a reducing atmosphere, and while being annealed in the annealing furnace, the steel sheet 1 passes through a snout 6 and further enters a pot 7 where it is immersed in a plating bath 2 contained in the pot 7.

[0074] In this case, when the aluminum concentration of the plating bath 2 exceeds 50% by mass, the temperature of the plating bath 2 is preferably in the range of 550° C. to 700° C. When the aluminum concentration of the plating bath 2 is 50% by mass or less, the temperature is preferably in the range of 420° C. to 600° C. In these cases, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of the plating layer having a depth of 5 nm tends to be adjusted to a specific numerical range of 0.40 to 1.50.

[0075] When the aluminum concentration of the coating bath 2 is greater than 50% by mass and the temperature of the coating bath 2 is 670°C or lower, the proportion of Al contained in the alloy layer formed by alloying the steel sheet 1 with the components of the coating bath is appropriately reduced, and therefore the proportion of Al present in the outermost layer of the coating layer tends to be appropriately increased. This appropriately increases the average value of the ratio, and as a result, the ratio tends to be more easily adjusted to a specific numerical range. When the aluminum concentration of the coating bath 2 is greater than 50% by mass and the temperature of the coating bath 2 is 550°C or higher, the proportion of Al contained in the alloy layer at the interface formed by alloying the steel sheet 1 with the components of the coating bath is appropriately increased, and therefore the proportion of Al present in the outermost layer of the coating layer tends to be appropriately reduced. This appropriately decreases the average value of the ratio, and as a result, the ratio tends to be more easily adjusted to a specific numerical range. The temperature of the coating bath 2 is more preferably 570°C or higher, and even more preferably 585°C or higher. The temperature of the plating bath 2 is more preferably 650°C or less, and even more preferably 630°C or less.

[0076] When the aluminum concentration of the coating bath 2 is 50% by mass or less and the temperature of the coating bath 2 is 600°C or less, the proportion of Al contained in the alloy layer formed by alloying the steel sheet 1 with the components of the coating bath is appropriately reduced, and therefore the proportion of Al present in the outermost layer of the coating layer tends to be appropriately increased. This appropriately increases the average value of the ratio, and as a result, the ratio tends to be more easily adjusted to a specific numerical range. When the aluminum concentration of the coating bath 2 is 50% by mass or less and the temperature of the coating bath 2 is 420°C or higher, the proportion of Al contained in the alloy layer at the interface formed by alloying the steel sheet 1 with the components of the coating bath is appropriately increased, and therefore the proportion of Al present in the outermost layer of the coating layer tends to be appropriately reduced. This appropriately decreases the average value of the ratio, and as a result, the ratio tends to be more easily adjusted to a specific numerical range. The temperature of the coating bath 2 is more preferably 450°C or higher, and even more preferably 470°C or higher. The temperature of the plating bath 2 is more preferably 550°C or less, and even more preferably 530°C or less.

[0077] The steel sheet 1 is then supported by sink rolls 8 in the pot 7, and its conveying direction is changed upward, and the steel sheet 1 is drawn out of the plating bath 2. As a result, the plating metal is deposited on the steel sheet 1.

[0078] Next, gas is sprayed onto both sides of the steel sheet 1 from the spray nozzles 9 to adjust the amount of plating metal adhered to the steel sheet 1. This method of adjusting the amount of adhesion by spraying gas is called a gas wiping method. The amount of plating metal adhered to both sides of the steel sheet 1 combined is 40 g / m ２ More than 200g / m ２ It is preferable to adjust the gas content to the following range: In the gas wiping method, the type of gas (wiping gas) sprayed onto the steel sheet 1 may be, for example, air, nitrogen, argon, helium, or water vapor. These wiping gases may be preheated before being sprayed onto the steel sheet 1.

[0079] The method for adjusting the coating weight is not limited to the gas wiping method described above, and various other coating weight control methods can be applied. Examples of coating weight control methods other than gas wiping include a roll squeezing method in which the steel sheet 1 is passed between a pair of rolls arranged directly above the surface of the coating bath 2, a method in which a shielding plate is arranged close to the steel sheet 1 drawn out of the coating bath 2 and the coating metal is wiped off with this shielding plate, an electromagnetic force wiping method in which an electromagnetic force is used to apply a downward moving force to the coating metal adhering to the steel sheet 1, and a method in which the coating weight is adjusted by utilizing natural gravity without applying an external force. Furthermore, two or more of these adjustment methods may be combined to adjust the coating weight of the steel sheet 1.

[0080] Next, the steel sheet 1 is transported further above the position where the injection nozzle 9 is disposed, and then is transported so as to turn back downward while being supported by two transport rolls 15. That is, the steel sheet 1 is transported along an inverted U-shaped path. Along this inverted U-shaped path, the steel sheet 1 is cooled by air cooling, mist cooling, or the like in a cooling device 10. As a result, the plating metal attached to the surface of the steel sheet 1 solidifies, and a plating layer is formed.

[0081] In addition, by adjusting the cooling conditions when solidifying the plating metal to form the plating layer, the atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of the plating layer having a depth of 5 nm can be specifically adjusted to fall within a specific numerical range.

[0082] In order to completely complete the solidification of the plating metal, the surface of the steel sheet 1 is cooled by a cooling device 10. At this time, it is preferable that the surface temperature of the plating metal (or plating layer) is cooled to 300°C or less. The surface temperature of the plating metal is measured, for example, by a radiation thermometer or the like.

[0083] The cooling device 10 preferably has a temperature control function for adjusting the temperature of the steel sheet 1 in the conveying direction and the sheet width direction in order to control the cooling rate of the steel sheet 1. The cooling device 10 may be divided into multiple units along the conveying direction of the steel sheet 1. That is, a primary cooling device 101 that cools the steel sheet 1 in a conveying path further above the position of the injection nozzles 9, and a secondary cooling device 102 that cools the steel sheet 1 downstream of the primary cooling device 101 may be provided. When the primary cooling device 101 and the secondary cooling device 102 are provided in this manner, it is preferable that the primary cooling device 101 cools the steel sheet 1 until the surface of the plated metal reaches a temperature of 300°C or less. Furthermore, it is preferable that the secondary cooling device 102 cools the steel sheet 1 so that the temperature thereof when introduced into the temper rolling / shape correction device 11 is 100°C or less.

[0084] Furthermore, during the cooling process of the steel sheet 1, the cooling rate of the surface of the plated metal during the time when the surface temperature of the plated metal attached to the steel sheet 1 decreases from 500°C to 400°C is preferably 10°C / sec or more and 70°C / sec or less. In this case, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of the plated layer having a depth of 5 nm tends to be adjusted within a specific numerical range of 0.40 to 1.50. To explain this in more detail, the solidification range of zinc contained in the plated metal is in the range from 400°C to 500°C. Therefore, by setting the cooling rate to 10°C / sec or more, the concentration of zinc on the surface of the plated metal can be suppressed. This appropriately increases the average value of the ratio, making it easier to adjust the average value of the ratio to a specific numerical range. The cooling rate is more preferably 15°C / sec or more, and even more preferably 20°C / sec or more. The cooling rate is preferably 70°C / sec or less, more preferably 60°C / sec or less, and even more preferably 50°C / sec or less, in which case the beautiful appearance of the coated / plated steel sheet can be ensured.

[0085] When the steel sheet 1 is withdrawn from the coating bath 2, it may be withdrawn into a non-oxidizing atmosphere or a low-oxidizing atmosphere, and further, the coating weight of the plating metal on the steel sheet 1 may be adjusted by a gas wiping method in this non-oxidizing atmosphere or low-oxidizing atmosphere.

[0086] The cooled steel sheet 1 is subjected to temper rolling in a temper rolling / shape straightening device 11, and then subjected to shape straightening. The reduction rate in temper rolling is preferably 0.3% or more and 3.0% or less. The elongation rate of the steel sheet 1 due to shape straightening is preferably 3.0% or less.

[0087] The steel sheet 1 is then wound up by a winder 12, which holds a coil 14 of the steel sheet 1.

[0088] In order to solve the problems of the present disclosure, these devices may be appropriately designed by adding, removing, or replacing various functions as necessary. In other words, the configurations of the plating processing device, heating device, etc. can be variously designed depending on the shape of the steel sheet 1, etc.

[0089] <Protective Layer> The protective layer will be described.

[0090] As described above, the protective layer contains the resin (A1), the silicon compound (B1), the phosphorus compound (C1), and the cobalt compound (D1).

[0091] The protective layer can be prepared from an aqueous surface treatment agent containing, for example, a resin component (A2), a silane coupling agent (B2), a phosphorus compound (C2), and a cobalt compound (D2). In this case, the resin component (A2) is, for example, the resin (A1) in the protective layer itself, or a component that becomes the resin (A1) upon curing. The protective layer can also contain a silicon compound (B1), a phosphorus compound (C1), and a cobalt compound (D1) derived from the silane coupling agent (B2), the phosphorus compound (C2), and the cobalt compound (D2), respectively, in the aqueous surface treatment agent. The phosphorus compound (C2) in the aqueous surface treatment agent and the phosphorus compound (C1) in the protective layer may be compounds of the same structure or different structures. The same applies to the relationship between the cobalt compound (D2) and the cobalt compound (D1).

[0092] The protective layer may contain at least one of a titanium compound (E1) and a vanadium compound (F1) in addition to the resin (A1), the silicon compound (B1), the phosphorus compound (C1), and the cobalt compound (D1). In this case, the protective layer may be prepared from an aqueous surface treatment agent containing, for example, a resin component (A2), a silane coupling agent (B2), a phosphorus compound (C2), and a cobalt compound (D2), as well as a titanium compound (E2) and a vanadium compound (F2). The titanium compound (E2) in the aqueous surface treatment agent and the titanium compound (E1) in the protective layer may be compounds of the same structure or different structures. The same applies to the relationship between the vanadium compound (F2) and the vanadium compound (F1).

[0093] The resin component (A2) may contain any of a polymer, an oligomer, and a monomer.

[0094] The resin component (A2) is preferably aqueous, i.e., it is preferably water-dispersible, an emulsion, or water-soluble. The resin component (A2) is particularly preferably water-dispersible. In this case, the protective layer is less susceptible to whitening due to condensation and can have water resistance. When the resin component (A2) is water-dispersible, it is preferable that the resin component (A2) and the resin (A1) each contain an amine-modified cationic resin or a carboxy-modified anionic resin on the surface. In this case, the resin component (A2) and the silane coupling agent (B2) can coexist stably in the aqueous surface treatment agent.

[0095] The particle size of the resin component (A2) particles is, for example, 9 nm to 200 nm. The larger the particle size, the more likely it is that the adverse effects of hydrophilization due to amine modification or carboxy modification can be reduced. However, if the particle size is too large, film formation tends to be hindered during the process of drying the aqueous surface treatment agent to form a protective layer.

[0096] The resin component (A2) contains, for example, at least one selected from the group consisting of polyurethane resins, phenolic resins, and (meth)acrylic resins. In this case, the resin (A1) in the protective layer contains at least one selected from the group consisting of polyurethane resins, phenolic resins, and (meth)acrylic resins. These resins are preferred because they have adequate adhesion to the plating layer and therefore facilitate improved corrosion resistance.

[0097] Here, the resin component (A2) in the aqueous surface treatment agent and the resin (A1) in the protective layer may each be a cationic resin or an anionic resin, but from the viewpoint of the stability of the treatment liquid when it coexists with other treatment liquid components, a cationic resin is preferred.

[0098] Preferred cationic resins will be described in detail below.

[0099] When the resin component (A2) in the aqueous surface treatment agent and the resin (A1) in the protective layer each contain a cationic polyurethane resin, the processability and adhesion of the protective layer become good.

[0100] The cationic polyurethane resin contains, for example, a polycarbonate-based water-dispersible cationic polyurethane resin containing a structural unit represented by the following general formula (1).

[0101]

[0102] In formula (1), R is an aliphatic alkylene group having 4 to 9 carbon atoms, and n is an integer indicating the number of structural units in the molecule. Preferably, n is an integer corresponding to a number average molecular weight of 500 to 5,000 of the carbonate-based polyol, which is a raw material for the polycarbonate-based water-dispersible cationic polyurethane resin.

[0103] When the resin component (A2) in the aqueous surface treatment agent and the resin (A1) in the protective layer contain a cationic phenol resin, the stability of the aqueous surface treatment agent is improved and the water-resistant secondary adhesion of the protective layer to the topcoat is also improved.

[0104] The cationic phenol resin has, for example, a repeating unit represented by formula (2). In this case, the average degree of polymerization of the repeating unit in the cationic phenol resin is preferably 2 to 50. When the average degree of polymerization is within this range, the water resistance of the protective layer can be improved. The average degree of polymerization of the repeating unit of formula (2) is １ It can be determined from the integral ratio by H-NMR.

[0105]

[0106] In formula (2), Y １ , and Y ２ are each independently hydrogen or a substituent. The substituent is represented by formula (3) or formula (4). The number of substituents per benzene ring in the repeating unit of formula (2) is an average of 0.2 to 1.0. The number of substituents is １ It can be determined from the integral ratio by H-NMR.

[0107]

[0108] In each of formulas (3) and (4), R1, R2, R3, R4, and R5 are each a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, or a hydroxyalkyl group having 1 to 10 carbon atoms. － is a hydroxide ion or an ion of an oxoacid (for example, nitrate, sulfate, phosphate, carbonate, or carboxylate).

[0109] The cationic (meth)acrylic resin contains, for example, a polymer containing, as a constituent unit, a monomer having a cationic functional group. In other words, the cationic (meth)acrylic resin contains, as a constituent unit, at least one selected from the group consisting of a cationic polymerizable monomer (hereinafter referred to as polymerizable monomer (M1)) and another polymerizable monomer (hereinafter also referred to as polymerizable monomer (M2)).

[0110] The polymerizable monomer (M1) has at least one of an amino group, an alkylamino group, and a quaternary ammonium group.

[0111] The polymerizable monomer (M1) contains at least one selected from the group consisting of, for example, N,N-dialkylaminoalkyl (meth)acrylates such as N,N-dimethylaminoethyl (meth)acrylate and N,N-dimethylaminopropyl (meth)acrylate; N-alkylaminoalkyl (meth)acrylates such as N-methylaminoethyl (meth)acrylate and N-methylaminopropyl (meth)acrylate; and cationic polymerizable monomers having a quaternary ammonium group such as methacryloyloxyethyltrimethylammonium chloride.

[0112] As described above, the cationic polymerizable monomer having an amino group has an alkylamino group. For example, when the alkylamino group is a primary alkylamino group, the number of carbon atoms in the alkyl group is preferably 1 to 4, particularly preferably 1 to 2. When an alkyl group is further bonded to the alkylamino group, the number of carbon atoms in the further bonded alkyl group is preferably 1 to 6, particularly preferably 1 to 4.

[0113] The polymerizable monomer (M2) contains at least one selected from the group consisting of (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate; and aromatic vinyl monomers such as styrene, α-methylstyrene, and vinyltoluene. When the polymerizable monomer (M2) contains an alkyl (meth)acrylate, the alkyl group in the alkyl (meth)acrylate preferably has 1 or more and 18 or less carbon atoms.

[0114] In addition, when obtaining a polymer, the above-mentioned monomers may be used alone or two or more kinds may be used and polymerized.

[0115] Preferred anionic resins will be described in detail below.

[0116] Examples of anionic polyurethane resins include water-soluble or water-dispersible urethane resins obtained by reacting an organic polyisocyanate compound with a polyol compound. Among these, self-emulsifying urethane resins are preferred. Examples of organic polyisocyanate compounds include aliphatic diisocyanates such as phenyl diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, and naphthalene diisocyanate; and alicyclic diisocyanates such as cyclohexane diisocyanate, isophorone diisocyanate, norbornane diisocyanate, xylylene diisocyanate, and tetramethyl xylylene diisocyanate. Examples of polyol compounds include polyester polyols, polyether polyols, polycarbonate polyols, polyacetal polyols, polyacrylate polyols, polyesteramide polyols, and polyolefin polyols such as polybutadiene-based polyols. Self-emulsifying urethane resins are prepared by incorporating a hydrophilic component, such as a carboxylic acid-containing compound, into the molecule. Anionic polyurethane resins can be obtained by introducing hydroxyl groups into the polymer backbone.

[0117] When the resin component (A2) in the aqueous surface treatment agent and the resin (A1) in the protective layer contain an anionic phenol resin, the stability of the aqueous surface treatment agent is improved and the water-resistant secondary adhesion of the protective layer to the topcoat is also improved.

[0118] Anionic phenolic resins include novolac-type phenolic resins obtained by reacting phenols with aldehydes in the presence of an acidic catalyst, and resol-type phenolic resins obtained by reacting phenols with aldehydes in the presence of a basic catalyst. These may be used alone or in combination. Anionic phenolic resins can be obtained by adding a water-soluble polymer having an anionic group as an additional polymer to the phenolic resin.

[0119] Anionic (meth)acrylic resins include, for example, polymers containing, as a structural unit, a monomer having an anionic functional group. In other words, the anionic (meth)acrylic resin contains a polymer containing, as a structural unit, at least one selected from the group consisting of an anionic polymerizable monomer (hereinafter referred to as polymerizable monomer (M3)) and other polymerizable monomers (hereinafter also referred to as polymerizable monomer (M4)). The polymerizable monomer (M3) has at least one of a carboxyl group and a sulfone group. Examples of carboxyl group-containing monomers include unsaturated carboxylic acids such as the unsaturated carboxylic acid shown in the following formula, esters of the unsaturated carboxylic acid, and acid anhydrides of the unsaturated carboxylic acid:

[0120]

[0121] In the formula, R １ , R ２ and R ３ Each of the groups is a hydrogen atom, an alkyl group, a carboxyl group, or an ester group, and n is an integer ranging from 0 to 20. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, vinylacetic acid, crotonic acid, maleic acid, maleic acid monoester, 5-hexenoic acid, 5-heptenoic acid, and oleic acid.

[0122] Examples of sulfonic acid group-containing monomers include vinyl sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, styrene sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-methacryloyloxybutanesulfonic acid, allyloxybenzenesulfonic acid, methallyloxybenzenesulfonic acid, etc. In obtaining a polymer, the above-mentioned monomers may be used alone or in combination of two or more.

[0123] The resin component (A2) in the aqueous surface treatment agent and the resin (A1) in the protective layer may each be used in combination with a cationic resin or an anionic resin and a nonionic resin. The use of a nonionic resin improves the stability of the treatment solution when it coexists with other treatment solution components. However, the content of the nonionic resin is preferably 50% by mass or less relative to the resin component (A2) in the aqueous surface treatment agent or the resin (A1) in the protective layer. Examples of nonionic resins include nonionic urethane resins, nonionic phenol resins, nonionic acrylic resins, and nonionic epoxy resins.

[0124] When the resin component (A2) contains a cationic resin, the aqueous surface treatment agent preferably contains a counter anion of the cationic resin component. The counter anion preferably volatilizes during the process of preparing the protective layer by drying the aqueous surface treatment agent. In this case, the amine in the amine-modified cationic resin contained in the resin component (A2) loses its charge during the process of preparing the protective layer, making the particles of the resin component (A2) hydrophobic and prone to aggregation.

[0125] Specifically, the counter anion preferably contains at least one of a formate ion and an acetate ion. In this case, when the counter anion volatilizes during the process of drying the aqueous surface treatment agent to prepare a protective layer, the pH of the aqueous surface treatment agent increases, accelerating silanol formation due to hydrolysis of the silane coupling agent (B2), and promoting gelation and condensation of the silane coupling agent (B2). Furthermore, the reaction between the side chain of the cationic resin component and the silane coupling agent (B2) and the reaction between silanol and the plating layer surface are promoted, and a network of cationic resin component-siloxane-metalloxane bonds can be formed. As a result, the adhesion, corrosion resistance, and solvent resistance of the protective layer can be improved.

[0126] When the resin component (A2) is an anionic resin component, the aqueous surface treatment agent preferably contains a counter cation of the anionic resin component. The counter cation preferably volatilizes during the process of preparing the protective layer by drying the aqueous surface treatment agent. In this case, the carboxyl groups in the carboxy-modified anionic resin contained in the anionic resin component lose their charge during the process of preparing the protective layer, making the particles of the anionic resin component hydrophobic and prone to aggregation.

[0127] Specifically, the counter cation preferably contains at least one of ammonium ions and carbonate ions. In this case, when the counter cation volatilizes during the process of drying the aqueous surface treatment agent to form a protective layer, the pH of the aqueous surface treatment agent decreases, promoting silanol formation due to hydrolysis of the silane coupling agent, and promoting gelation and condensation of the silane coupling agent. Furthermore, the reaction between the side chain of the anionic resin component and the silane coupling agent, and the reaction between silanols and the plating layer surface, can be promoted, forming a network of anionic resin component, siloxane, and metasiloxane bonds. As a result, the adhesion, corrosion resistance, and solvent resistance of the protective layer can be improved.

[0128] During the process of drying the aqueous surface treatment agent to prepare a protective layer, the silane coupling agent (B2) undergoes hydrolysis and condensation to form a siloxane compound having three-dimensional crosslinking by siloxane bonds. As a result, the protective layer can contain a siloxane compound, which is a silicon compound (B1) derived from the silane coupling agent (B2). The siloxane compound can improve the corrosion resistance, adhesion, and solvent resistance of the protective layer.

[0129] The silane coupling agent (B2) contains at least one of an alkoxysilane having, for example, two or more, preferably three or more alkoxy groups, and a partial hydrolyzate thereof.

[0130] The alkoxy group of the silane coupling agent (B2) can be hydrolyzed to form silanol (—Si—OH) in the aqueous surface treatment agent. Dispersion stability of the silanol is achieved at a pH of 6.5 or less. If the pH of the treatment agent exceeds 6.5, gelation may occur, reducing the storage stability of the aqueous surface treatment agent.

[0131] The silane coupling agent (B2) contains at least one selected from the group consisting of, for example, N-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, vinyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, and 3-methacryloxypropyltrimethoxysilane, etc. The silane coupling agent (B2) may contain a commercially available product.

[0132] The silane coupling agent (B2) preferably has a functional group reactive with the resin component (A2). In this case, the type of reaction between the silane coupling agent (B2) and the resin component (A2) may be a polymerization reaction, a condensation reaction, an addition reaction, or the like, and is not particularly limited. The silane coupling agent (B2) preferably contains at least one selected from the group consisting of 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, and the like, and particularly preferably contains at least one of 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane.

[0133] The phosphorus compound (C1) in the protective layer reacts with the metal components contained in the plating layer, forming a layer consisting of a reaction product of the phosphorus compound (C1) and the metal components contained in the plating layer at the interface between the plating layer and the protective layer, which can improve the corrosion resistance and film adhesion of the coated-plated steel sheet.

[0134] The phosphorus compound (C1) in the protective layer and the phosphorus compound (C2) in the aqueous surface treatment agent each contain, for example, a phosphoric acid component. The phosphoric acid component contains at least one selected from the group consisting of inorganic phosphoric acids such as phosphoric acid, and inorganic phosphoric acid compounds such as ammonium phosphate, potassium phosphate, sodium phosphate, and monosodium dihydrogen phosphate. The phosphoric acid component preferably contains at least one of an inorganic phosphoric acid compound and inorganic phosphoric acid, and more preferably contains phosphoric acid.

[0135] The cobalt compound (D1) in the protective layer can effectively suppress discoloration, particularly blackening, of the coated-plated steel sheet, and in particular can enhance the corrosion resistance of the coated-plated steel sheet under high-temperature and high-humidity conditions.

[0136] The cobalt compound (D2) in the aqueous surface treatment agent and the cobalt compound (D1) in the protective layer each contain at least one compound selected from the group consisting of, for example, cobalt sulfate, cobalt hydrochloride, cobalt carbonate, cobalt phosphate, cobalt acetate, and cobalt nitrate. More specifically, the cobalt compound (D2) and the cobalt compound (D1) each contain at least one compound selected from the group consisting of, for example, cobalt(II) nitrate, cobalt(II) sulfate, cobalt(II) chloride, cobalt(II) carbonate, cobalt(II) phosphate, cobalt acetylacetonate, cobalt ethylenediaminetetraacetate, cobalt(II) acetate, cobalt(II) oxalate, cobalt(III) oxalate, cobalt(III) oxide, and cobalt(IV) oxide.

[0137] Each of the cobalt compound (D2) and the cobalt compound (D1) preferably contains at least one selected from the group consisting of cobalt sulfate, cobalt hydrochloride, and cobalt nitrate, and more preferably contains cobalt(II) nitrate.

[0138] The titanium compound (E1) in the protective layer can enhance the corrosion resistance of the protective layer. The aqueous surface treatment agent contains, as the titanium compound (E2), at least one selected from the group consisting of, for example, a titanium chelate compound, titanium hydrofluoric acid, and ammonium titanium fluoride.

[0139] The titanium compound (E2) preferably contains a titanium chelate compound, which can suppress whitening of the protective layer due to condensation or the like caused by fluorine liberated from the titanium compound (E2).

[0140] The vanadium compound (F1) in the protective layer can enhance the corrosion resistance of the protective layer. The vanadium compound (F1) is, for example, an oxide or an acetylacetone complex of vanadium.

[0141] The aqueous surface treatment agent contains, as the vanadium compound (F2), at least one selected from the group consisting of, for example, strong electrolyte-free vanadium pentoxide, metavanadic acid and its salts (e.g., ammonium metavanadate), vanadium trioxide, vanadium dioxide, vanadium oxyacetylacetonate, vanadium acetylacetonate, etc., and vanadium acetate, which is a salt with a volatile acid, etc. In particular, when the vanadium compound (F2) contains an acetylacetone complex of vanadium such as vanadium acetylacetonate or vanadium oxyacetylacetonate, the corrosion resistance of the protective layer can be further improved.

[0142] The aqueous surface treatment agent may further contain an acetic acid component (G). The acetic acid component (G) is a component consisting of at least one of acetic acid and an acetate salt. The acetic acid component (G) can enhance the storage stability of the aqueous surface treatment agent. This is presumably because the pH buffering action of the acetic acid component (G) stabilizes the pH of the aqueous surface treatment agent at approximately 3.5 to 4.0, thereby inhibiting the progress of the condensation reaction of the silane coupling agent (B2) in the aqueous surface treatment agent. According to studies by the inventors, the condensation reaction of the silane coupling agent (B2) progresses most slowly when the pH of the aqueous surface treatment agent is around 3.5 to 4.0.

[0143] The acetic acid component (G) contains, for example, at least one selected from the group consisting of acetic acid, ammonium acetate, potassium acetate, sodium acetate, etc. The acetic acid component (G) preferably contains acetic acid. In this case, the boiling point of acetic acid is 118°C, which is low for an organic acid having a buffering effect, and therefore acetic acid is unlikely to remain when the aqueous surface treatment agent is dried to form a protective layer.

[0144] The aqueous surface treatment agent may contain an olefin wax (H). In this case, deformation due to loosening of the coil when the coated-plated steel sheet is wound is unlikely to occur, and the coated-plated steel sheets are unlikely to collapse when stacked.

[0145] The olefin wax (H) is preferably surface-modified with a silane coupling agent (I). In this case, the olefin wax (H) can be well dispersed in the protective layer. The olefin wax (H) contains, for example, at least one selected from the group consisting of polyethylene wax, oxidized polyethylene wax, and oxidized polypropylene wax. Examples of the olefin wax (H) surface-modified with a silane coupling agent (I) include a surface-modified olefin wax obtained by directly mixing a silane coupling agent containing an epoxy group (e.g., glycidylpropyltrimethoxysilane) with a polyethylene wax emulsion having a carboxyl group that is stable in an acidic treatment solution. The mass ratio of the silane coupling agent (I) to the mass of the olefin wax (H) is preferably 0.025 to 0.035. Furthermore, the amount of the silane coupling agent (I) added is preferably at least equimolar to the acid value of the wax dispersion.

[0146] The average particle size of the olefin wax (H) is preferably 0.05 μm or more and 0.15 μm or less. If the wax particle size is 0.15 μm or less, the volume of the wax present on the coating surface is reduced, improving handleability, and if it is 0.05 μm or more, processability is improved and the adverse effect on the stability of the treatment agent due to wax aggregation is minimized. Note that the average particle size of the olefin wax (H) is a value measured by a laser diffraction / scattering method.

[0147] The aqueous surface treatment agent may contain an aqueous solvent. The aqueous solvent is water or a mixed solvent consisting of water and a water-soluble organic solvent. The water-soluble organic solvent is, for example, an alcohol. The proportion of the water-soluble organic solvent relative to the total amount of the aqueous solvent is preferably 30 mass% or less.

[0148] The aqueous surface treatment agent may contain appropriate additives such as a leveling agent and an antifoaming agent.

[0149] The aqueous surface treatment agent preferably does not contain colloidal silica, in which case the stability of the aqueous surface treatment agent can be prevented from decreasing due to a reaction between the silane coupling agent (B2) and colloidal silica.

[0150] The aqueous surface treatment agent preferably does not contain a basic alkali silicate, in which case the aqueous surface treatment agent can maintain good stability.

[0151] The aqueous surface treatment agent and the protective layer may contain at least one of chromium and a chromium compound, but preferably contain neither chromium nor a chromium compound. When the aqueous surface treatment agent contains at least one of chromium and a chromium compound, it is preferably contained only as an unavoidable impurity.

[0152] The blending ratios of the components in the protective layer and the aqueous surface treatment agent are described below. Note that the solid content of the aqueous surface treatment agent below refers to the total of the components in the aqueous surface treatment agent excluding components that volatilize during the process of forming the protective layer, specifically, solvents, volatile acids, etc.

[0153] The mass ratio of resin (A1) to the protective layer and the mass ratio of resin component (A2) to the solids content of the aqueous surface treatment agent are each 0.60 or more. In this case, the protective layer provides the effects of improving corrosion resistance and improving blackening resistance under normal conditions. This mass ratio is preferably 0.65 or more, and more preferably 0.70 or more. The mass ratio of resin (A1) to the protective layer and the mass ratio of resin component (A2) to the solids content of the aqueous surface treatment agent are each, for example, 0.85 or less. In this case, the effects of improving corrosion resistance and blackening resistance provided by the protective layer may not be saturated.

[0154] The ratio of silicon atoms (Si) in the protective layer to the total mass of the protective layer ２ The mass ratio of the silicon compound (B1) to the protective layer is 0.07 or more and 0.19 or less. ２ The mass ratio of the silane coupling agent (B2) to the solid content of the aqueous surface treatment agent is calculated by the conversion of SiO ２ The mass ratios calculated as SiO of silicon atoms in the protective layer relative to the total mass of the protective layer are preferably 0.07 or more and 0.19 or less. In this case, the protective layer can improve corrosion resistance and blackening resistance. ２The converted mass ratio is preferably 0.1 or more, more preferably 0.12 or more, and is preferably 0.18 or less, more preferably 0.16 or less.

[0155] The mass ratio of phosphorus atoms (P) in the protective layer to the total mass of the protective layer is 0.011 or more and 0.075 or less. That is, the mass ratio of the phosphorus compound (C1) to the protective layer in terms of P, and the mass ratio of the phosphorus compound (C2) to the solid content of the aqueous surface treatment agent in terms of P, are each preferably 0.011 or more and 0.075 or less. When this mass ratio is 0.011 or more, the aqueous surface treatment agent can have higher stability. When this mass ratio is 0.075 or less, a decrease in water resistance due to excessive addition can be suppressed. This mass ratio is preferably 0.015 or more, and more preferably 0.025 or more. This mass ratio is more preferably 0.070 or less.

[0156] The ratio of the mass of cobalt atoms (Co) in the protective layer to the total mass of the protective layer is 0.0015 or more and 0.0200 or less. That is, the mass ratio of the cobalt compound (D1) to the protective layer in terms of Co, and the mass ratio of the cobalt compound (D2) to the solid content of the aqueous surface treatment agent in terms of Co, are each preferably 0.0015 or more and 0.0200 or less. If the mass ratio of cobalt atoms (Co) in the protective layer to the total mass of the protective layer is 0.0015 or more, the blackening resistance under high-temperature and high-humidity conditions can be further improved, and if it is 0.02 or less, the adhesion of the protective layer can be further improved. This mass ratio is preferably 0.0030 or more, and more preferably 0.0050 or more. This mass ratio is preferably 0.0150 or less, and more preferably 0.0100 or less.

[0157] The mass ratio of titanium atoms (Ti) in the protective layer to the total mass of the protective layer is, for example, 0.005 or more and 0.0240 or less. That is, the mass ratio of the titanium compound (E1) to the protective layer in terms of Ti, and the mass ratio of the titanium compound (E2) to the solid content of the aqueous surface treatment agent in terms of Ti, are each, for example, 0.0050 or more and 0.0240 or less. If this mass ratio is 0.0050 or more, the corrosion resistance can be further improved, and if it is 0.240 or less, the adhesion of the protective layer can be further improved. This mass ratio may be 0.0070 or more. This mass ratio may be 0.0230 or less.

[0158] The mass ratio of vanadium atoms (V) in the protective layer to the total mass of the protective layer is, for example, 0.0003 or more and 0.0090 or less. That is, the mass ratio of the vanadium compound (F1) to the protective layer in terms of V, and the mass ratio of the vanadium compound (F2) to the solid content of the aqueous surface treatment agent in terms of V, are each, for example, 0.0030 or more and 0.0090 or less. If this mass ratio is 0.0030 or more, the corrosion resistance can be further improved, and if it is 0.0090 or less, the adhesion of the protective layer can be further improved. The mass ratio may be 0.004 or more.

[0159] Here, the atomic concentration ratio of cobalt to silicon in the protective layer will be described. The atomic concentration ratio of cobalt to silicon in the protective layer (atomic concentration of cobalt / atomic concentration of silicon) is preferably 0.01 or more and 0.38 or less. Here, the "atomic concentration ratio of cobalt to silicon" is also referred to as the "Co / Si ratio." A Co / Si ratio of 0.01 or more in the protective layer is preferable because blackening can be suppressed by the effect of Co. The Co / Si ratio in the protective layer is more preferably 0.10 or more. A Co / Si ratio of 0.38 or less in the protective layer is preferable because sufficient corrosion resistance can be ensured. The Co / Si ratio in the protective layer is more preferably 0.20 or less.

[0160] The Co / Si ratio in the protective layer can be adjusted by the ratio of the silicon compound (B1) to the cobalt compound (D1) in the aqueous surface treatment solution for forming the protective layer. The method for measuring the Co / Si ratio in the protective layer will be described in the Examples below.

[0161] MgZn contained in the plating layer ２ In the region of the protective layer in contact with the MgZn phase, the atomic concentration ratio of cobalt to silicon (atomic concentration of cobalt / atomic concentration of silicon) at a depth of 10 nm from the interface between the plating layer and the protective layer is preferably 1.15 to 2.50 times the atomic concentration ratio of cobalt to silicon (atomic concentration of cobalt / atomic concentration of silicon) at a depth of 60 nm from the interface between the plating layer and the protective layer. ２ The ratio of the atomic concentration ratio of cobalt to silicon at a position 10 nm deep from the interface between the plating layer and the protective layer to the atomic concentration ratio of cobalt to silicon at a position 60 nm deep from the interface between the plating layer and the protective layer in the region of the protective layer in contact with the MgZn phase is defined as "MgZn ２ It is also called "Co enrichment ratio of the protective layer on the plating layer." If the protective layer does not contain cobalt, there is no enrichment ratio. ２ "In the region of the protective layer in contact with the MgZn phase" means that the protective layer is ２ In other words, the region in question refers to the part of the protective layer that is parallel to the thickness direction of the protective layer from the part that is in contact with the MgZn phase. ２ The term "protective layer" refers to the area of ​​the protective layer that overlaps the protective layer.

[0162] Cobalt is effective in improving the blackening resistance of a plating layer containing aluminum, zinc, and magnesium. However, excessive cobalt in the protective layer deteriorates the corrosion resistance. The mechanism of blackening of a plating layer containing aluminum, zinc, and magnesium is oxygen deficiency ZnO (ZnO １－δ： The inventors have investigated the mechanism of blackening of the plating layer containing aluminum, zinc, and magnesium in detail and have found the following: The plating layer containing aluminum, zinc, and magnesium is formed by the formation of MgZn ２ The plating layer contains MgZn phase, Zn phase, Al phase, etc. ２ If it contains a phase, MgZn ２At least a part of the MgZn phase is exposed on the surface of the plating layer and is in contact with the protective layer. ２ It was found that the MgZn phase is the most susceptible to blackening. ２ By concentrating cobalt in the area of ​​the protective layer that contacts the MgZn phase, the corrosion resistance is not deteriorated. ２ The blackening of the phase is suppressed, and the blackening resistance can be effectively improved.

[0163] Therefore, MgZn ２ It is preferable that the Co enrichment ratio of the protective layer on the MgZn phase is 1.15 times or more and 2.50 times or less. ２ The Co enrichment ratio of the protective layer on the phase is more preferably 1.20 times or more and 2.00 times or less, and further preferably 1.20 times or more and 1.50 times or less. ２ The lower limit of the Co enrichment ratio of the protective layer on the MgZn phase is more preferably 1.20 times. ２ The upper limit of the Co enrichment ratio of the protective layer on the phase is more preferably 2.00 times, and even more preferably 1.50 times.

[0164] MgZn ２ In order to set the Co enrichment ratio of the protective layer on the phase within the above range, when forming the protective layer, it is advisable to ensure that the time (treatment solution retention time) from when the aqueous surface treatment agent is applied to the plating layer until the applied film is baked and dried is at least a certain time. This lengthens the time that the aqueous surface treatment agent is in contact with the plating layer in a liquid state. Since cobalt is an electrically noble metal, it is difficult to obtain a Co enrichment ratio of the protective layer on the MgZn phase, which is an electrically noble phase in the plating layer. ２ Cobalt ions can be concentrated on the MgZn phase. ２ In general, the coating film is baked and dried immediately after the application of the aqueous surface treatment agent, so that the cobalt in the protective layer is not concentrated in the MgZn ２ It cannot be thickened onto the phase.

[0165] MgZn ２ The method for measuring the Co enrichment ratio of the protective layer on the phase will be explained in the examples below.

[0166] The components of the aqueous surface treatment agent can be mixed to prepare the aqueous surface treatment agent.

[0167] A coated plated steel sheet can be produced by forming a protective layer from a water-based surface treatment agent on the plating layer of a plated steel sheet comprising a steel sheet and a plating layer.

[0168] Before forming the protective layer, it is preferable to subject the surface of the plating layer of the plated steel sheet to a treatment of applying compressive pressure, such as by using a skin pass mill or a tension leveler.

[0169] Before forming a protective layer on the plating layer, the surface of the plating layer may be subjected to a surface preparation process, such as washing with pure water or various organic solvents, or washing with various organic solvents or aqueous solutions optionally containing acids and various etching agents, etc. When the surface of the plating layer is washed in this manner, even if a small amount of Mg-based oxide film is present on the surface of the plating layer or inorganic and organic contaminants are attached to the surface of the plating layer, these Mg-based oxide film and contaminants are removed from the plating layer, thereby improving the adhesion between the plating layer and the protective layer.

[0170] A protective layer can be formed by applying an aqueous surface treatment agent to the surface of the plating layer of a plated steel sheet and then drying it, thereby producing a coated plated steel sheet.

[0171] The method for applying the aqueous surface treatment agent includes, for example, a dipping method, a spraying method, a roll coating method, etc., but is not limited to these.

[0172] The heating temperature for drying the aqueous surface treatment agent is selected so that the volatile components in the aqueous surface treatment agent (e.g., solvent, volatile acid, etc.) volatilize. It is preferable to heat and dry the aqueous surface treatment agent so that the maximum temperature reached (PMT) of the plated steel sheet is within the range of 60 to 150°C. The aqueous surface treatment agent can be dried by hot air drying or oven drying.

[0173] The amount of the protective layer attached is, for example, 100 mg / m ２ When no coating is applied on the protective layer, the coating amount of the protective layer is 300 mg / m ２The amount of the protective layer is preferably 3000 mg / m or more. ２ When spot welding is performed on coated plated steel sheets, the coating weight of the protective layer is 1500 mg / m or less. ２ The thickness of the protective layer is preferably 0.1 μm or more and 5.0 μm or less, for example.

[0174] [Coating Layer] The coated-plated steel sheet according to the embodiment may include a coating layer. That is, the coated-plated steel sheet may further include a coating layer covering the protective layer. This coating layer can be formed using a paint, a water-based surface treatment agent, a film, or the like. However, it may be formed using only a paint or only a film, or may be formed by laminating a layer formed from a paint and a layer formed from a film. That is, the coated-plated steel sheet may have at least one coating layer. The thickness of such a coating layer is preferably 0.1 μm or more. In this case, the corrosion resistance of the coated-plated steel sheet is improved. Furthermore, from the viewpoint of economy, the thickness of the coating layer is preferably 2.0 mm or less.

[0175] Furthermore, the coating layer formed from paint, film, etc. can be formed by any suitable method.

[0176] When the coating layer is formed from a paint, examples of the paint that can be used include polyester resin paint, epoxy resin paint, acrylic resin paint, fluororesin paint, silicone resin paint, amino resin paint, urethane resin paint, vinyl chloride resin paint, and composite paints made by combining these paints.

[0177] As the coating method for the paint, an appropriate method such as roll coating, curtain coating, spraying, dipping, electrolytic treatment, or air knife method can be used. After the paint is applied, a coating film layer is formed by drying at room temperature or with a heating device, or by baking. Furthermore, when an energy ray-curable paint is used, the applied paint may be cured by irradiating it with energy rays such as infrared rays, ultraviolet rays, or electron beams to form a coating film layer. The temperature and drying time for drying the paint are determined appropriately depending on the type of paint used, etc. The coating film layer is a continuous or discontinuous coating film.

[0178] When a coating film layer is formed using a paint, an undercoat coating film layer and a topcoat coating film layer may be formed on the protective layer. When forming the undercoat coating film layer, for example, an epoxy resin-based paint is used. When forming the topcoat coating film layer, for example, a polyester-based paint is used. A coated-plated steel sheet having such a topcoat coating film layer and an undercoat coating film layer can be suitably used as an outdoor building material. In this case, the thickness of the undercoat protective layer is preferably 1.0 μm or more and 10.0 μm or less, and the thickness of the topcoat protective layer is preferably 5.0 μm or more and 50.0 μm or less.

[0179] When the coating layer is formed from a film, examples of the film include vinyl chloride films, polyester resin films, acrylic resin films, fluororesin films, composite films made of these resins, and laminate films made of these films stacked together. Such films are bonded to the protective layer or the like by, for example, heat fusion or adhesive, to form the coating layer.

[0180] Furthermore, when the coating layer is formed from a film, its thickness is preferably 1.0 μm or more and 2.0 mm or less. A coated-plated steel sheet having such a coating layer can be suitably used for outdoor building materials.

[0181] The coating layer may be made of a clear paint. In this case, the plating layer can be seen from the outside through the coating layer, and the plating layer can have improved resistance to blackening, thereby suppressing deterioration in the appearance of the coated-plated steel sheet.

[0182] The coated-plated steel sheet according to the embodiment may not have a coating layer, and the outermost layer of the coated-plated steel sheet may be a protective layer. In this case, the blackening resistance of the plating layer can be improved, and deterioration of the appearance of the coated-plated steel sheet can be suppressed.

[0183] Specific examples of this embodiment will be presented below, but this embodiment is not limited to the following examples.

[0184] 1. Preparation of Plated Steel Sheet A long steel sheet 1 (low-carbon aluminum-killed steel) having a thickness of 0.8 mm and a width of 1000 mm was used. This steel sheet 1 was subjected to a plating treatment using the plating treatment device shown in FIG.

[0185] The bath temperatures during plating are shown in Tables 1 to 4.

[0186] The temperature of the steel sheet 1 when it entered the coating bath 2 was 530° C. The values ​​are shown in Tables 1 to 4.

[0187] The steel sheet 1 was pulled out into an air atmosphere when it was pulled out from the coating bath 2. Furthermore, the steel sheet 1 pulled out into the air atmosphere was subjected to gas wiping in the air atmosphere.

[0188] In the cooling device 10, the steel sheet 1 was cooled until the surface temperature of the plated metal reached 300° C. The cooling rates of the plated metal from 500° C. to 400° C. are shown in Tables 1 to 4.

[0189] 2-1. Metal atomic concentration in the outermost layer to a depth of 5 nm in the plating layer For the plated steel sheets listed in Tables 1 to 4, the average atomic concentrations of aluminum, zinc, and magnesium in the plating layer up to a depth of 5 nm were measured by X-ray photoelectron spectroscopy. A VersaProbe III model manufactured by ULVAC-PHI was used for the measurement. The measurement was performed under the following conditions:

[0190] <Measurement conditions> X-ray source: mono-Al Kα (hν: 1486.6 eV) X-ray diameter: approximately 200 μmφ, acceptance angle: 45° Sputtering conditions: Ar+, acceleration voltage: 1 kV, sputtering rate: 2 nm / min

[0191] The average atomic concentrations of aluminum, zinc, and magnesium in the plating layer up to a depth of 5 nm were calculated as follows. First, a sample was obtained from the plated steel sheet as follows. The target plated steel sheet was cut into 20 mm × 20 mm pieces to prepare an analysis sample. When the measurement target was a coated plated steel sheet, the protective layer was removed from the coated plated steel sheet in the depth direction by sputtering, and the outermost layer of the plating layer was analyzed. The atomic concentrations of each metal in the region from the surface of the plating layer to a depth of 5 nm of the obtained sample were measured while sputtering. Then, the atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of the plating layer to a depth of 5 nm were measured. This operation was performed on three samples randomly collected from the plated steel sheet. The total atomic concentrations of aluminum, zinc, and magnesium in the outermost layer of the obtained plating layer to a depth of 5 nm were arithmetically averaged. The arithmetic mean values ​​were shown in Tables 1 to 4 as the average atomic concentrations of each metal in the outermost layer of the plating layer to a depth of 5 nm. The units are atomic percentages (atomic %).

[0192] The average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc was calculated from the average atomic concentrations of each metal obtained above. The obtained values ​​are shown in Tables 1 to 4.

[0193] In the above measurement, the atomic concentrations of aluminum, zinc, and magnesium were obtained by checking the peak intensities of Al2p, Zn2p3, and Mg1s, respectively.

[0194] 2-2. Metal Proportions in the Plating Layer For the plated steel sheets listed in Tables 1 to 4, the proportions of aluminum, zinc, and magnesium in the plating layer were measured using inductively coupled plasma atomic emission spectrometry (ICP). Specifically, samples measuring 50 mm x 50 mm were cut from the plated steel sheets, and the edges and back surfaces were sealed with repair paint to prepare samples for plating component measurement. The plating layer on the surface of the sample was dissolved in an acidic solution to remove the protective coating. The removed solution was then adjusted to a volume of 100 mL, and the absolute amounts of aluminum, magnesium, and zinc contained in the solution were measured using ICP mass spectrometry (ICP analyzer used: Agilent Technologies, Model 8900). The proportions of each metal were calculated from the absolute amounts of each metal.

[0195]

[0196]

[0197]

[0198]

[0199] 3. Water-based surface treatment agents Water-based surface treatment agents were prepared to have the compositions shown in Tables 5 to 8. The raw materials used in the preparation of the water-based surface treatment agents are listed below. The composition of the protective layer formed using the water-based surface treatment agent was consistent with the composition of the solid content of the water-based surface treatment agent. - Cationic urethane resin: Polycarbonate-based cationic polyurethane resin. Manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. Product name: Superflex 650. - Cationic phenol resin: Having repeating units of formula (2), the average degree of polymerization of the repeating units is 5, and Y in formula (2) １ Ga-CH ２ N (CH ３ ) ２ , Y ２Cationic phenolic resin in which ≡H is H and the number of substituents per benzene ring is 0.5. - Cationic acrylic resin: Murayama Chemical Laboratory Co., Ltd., product name ACE-6. - Anionic urethane resin: ADEKA HUX-380. - Nonionic urethane resin: ADEKA HUX830. - Nonionic epoxy resin: ADEKA EM-0465. - Nonionic phenolic resin: DIC TD-4305, solids content 40% by mass. - Anionic acrylic resin: Murayama Chemical Laboratory Co., Ltd., Sunacrylic No. 60, solids content 32% by mass. - Silane coupling agent #1: 3-aminopropyltriethoxysilane. - Silane coupling agent #2: 3-glycidoxypropyltrimethoxysilane. - Phosphorus compound: phosphoric acid. - Cobalt compound: cobalt(II) nitrate hexahydrate. - Titanium compound: titanium diisopropoxybisacetylacetonate - Vanadium compound: vanadium acetylacetonate

[0200]

[0201]

[0202]

[0203]

[0204] 4. Production of Coated Plated Steel Sheet The plating layer of the plated steel sheet was immersed for 60 seconds in an alkaline degreasing solution (FC-4336L (manufactured by Nippon Parkerizing Co., Ltd.)) with a bath temperature of 55°C and a pH of 12, and then rinsed with pure water and dried by applying warm air.

[0205] An aqueous surface treatment agent was applied onto the plating layer of this plated steel sheet using a bar coater, and then heated in a hot air drying furnace under the heating conditions shown in Tables 9 to 21, thereby producing a protective layer with the adhesion amount shown in Tables 9 to 21.

[0206] However, in the examples in Table 21 where the column for "Co treatment" is marked "Yes," the plating layer of the plated steel sheet was subjected to the following immersion treatment before the protective layer was formed. - Immersion treatment - An aqueous solution of cobalt sulfate was prepared by dissolving cobalt sulfate heptahydrate at a concentration of 2.5 g / L and adjusting the pH to 2.4 with sulfuric acid. Next, the plated steel sheet was immersed in the aqueous cobalt sulfate solution for 4 seconds, followed by rinsing with water and drying with warm air.

[0207] 6. Various Measurements of Coated-Plated Steel Sheets 6-1. Mass Ratio of Each Atom in the Protective Layer to the Total Mass of the Protective Layer The mass ratio of each atom in the protective layer to the total mass of the protective layer corresponds to the mass ratio of each atom to the solid content of the treatment agent. However, the method for measuring the mass ratio of each atom in the protective layer from a coated-plated steel sheet is as follows. The coated-plated steel sheet is processed using a cryo-FIB (Focused Ion Beam)-μ sampling method using a focused ion beam device to obtain a sample with a cross section along the thickness direction of the protective layer as the measurement surface. The focused ion beam device used is the "NB5000" manufactured by Hitachi High-Technologies. A scanning transmission electron microscope (STEM) is used to set an observation range of 15 μm width along the surface of the coated-plated steel sheet, and the measurement surface of the sample is observed. Within the observation area, elemental analysis was performed using an energy dispersive X-ray spectrometer (EDS) on a rectangular area with a short side of 100 nm and a long side of 500 nm, centered at 2 / 1 of the thickness of the protective layer and with the short side aligned in the thickness direction of the coated-plated steel sheet, to calculate the concentration of each atom. This operation was performed at five arbitrary locations spaced at least 300 nm apart in the direction along the surface of the coated-plated steel sheet, and the arithmetic average of each atomic concentration (atomic %) was calculated. The analyzed elements were C, Si, P, and Co, excluding oxygen, and elements present in an amount of 0.1 atomic % or more as determined by elemental analysis using EDS. The arithmetic average of the C concentrations (atomic %) was then converted into mass, and the mass ratio of C to the analyzed elements was taken as the ratio of the mass of the resin (A1) in the protective layer to the total mass of the protective layer. From the arithmetic average of the Si concentrations (atomic %), SiO ２ The mass ratio of C to the analyzed elements was calculated as the SiO2 ratio of silicon atoms in the protective layer relative to the total mass of the protective layer. ２The arithmetic mean value of the P concentration (atomic %) or the Co concentration (atomic %) is converted into a mass ratio, and the mass ratio of P or Co to the analyzed element is taken as the ratio of the mass of phosphorus atoms or cobalt atoms in the protective layer to the total mass of the protective layer, respectively.

[0208] The observation conditions using a scanning transmission electron microscope (STEM) and the elemental analysis conditions using an energy dispersive X-ray spectrometer (EDS) are as follows.

[0209] - Observation conditions using a scanning transmission electron microscope (STEM) - Apparatus: "Spherical aberration corrected transmission electron microscope (Cs-TEM): NEOARM" manufactured by JEOL Ltd. Magnification: 100,000 times Acceleration voltage: 200 keV Other conditions: A cryoholder was used for STEM observation, and observation was performed at -160°C.

[0210] -Conditions for elemental analysis using energy dispersive X-ray spectroscopy (EDS)- Apparatus: "JED-2300T" manufactured by JEOL Ltd. Acceleration voltage: 200 keV Other conditions: A cryoholder was used during analysis, and observation was carried out at -160°C.

[0211] 6-2. Co / Si Ratio in the Protective Layer The Co / Si ratio in the protective layer (the atomic concentration ratio of cobalt to silicon) corresponds to the ratio of the mass ratio of cobalt atoms and the mass ratio of silicon atoms relative to the solid content in the treatment agent. However, the method for measuring the mass ratio of each atom in the protective layer from a coated / plated steel sheet is as described above. The mass ratio of cobalt atoms and the mass ratio of silicon atoms are determined in the same manner as for the mass ratio of each atom in the protective layer relative to the total mass of the protective layer, and the Co / Si ratio in the protective layer is calculated.

[0212] 6-3. MgZn ２ Co enrichment ratio of the protective layer on the MgZn phase ２ Co enrichment ratio of the protective layer on the layer MgZn contained in the plating layer ２In the region of the protective layer in contact with the plating layer, the atomic concentration ratio of cobalt to silicon (atomic concentration of cobalt / atomic concentration of silicon) at a position 10 nm deep from the interface between the plating layer and the protective layer, and the atomic concentration ratio of cobalt to silicon (atomic concentration of cobalt / atomic concentration of silicon) at a position 60 nm deep from the interface between the plating layer and the protective layer were measured as follows.

[0213] The coated plated steel sheet was processed by a cryo-FIB (Focused Ion Beam)-μ sampling method using a focused ion beam device to obtain a sample with a cross section along the thickness direction of the protective layer as the measurement surface. The focused ion beam device used was the "NB5000" manufactured by Hitachi High-Technologies. A scanning transmission electron microscope (STEM) was used to set an observation range of 15 μm along the surface of the coated plated steel sheet, and the measurement surface of the sample was observed. Within the observation range, elemental analysis and electron diffraction analysis were performed using an energy dispersive X-ray spectrometer (EDS) to identify the phase in the plating layer in contact with the protective layer, and MgZn ２ The long side is the protective layer and the MgZn ２ Elemental analysis was performed on a rectangular area of ​​10 nm on the short side and 50 nm on the long side adjacent to the interface with the phase using an energy dispersive X-ray spectrometer (EDS), and the atomic concentrations A of cobalt atoms and A of Si atoms were calculated. Then, the atomic concentration ratio A of the atomic concentrations A of cobalt atoms and A of Si atoms was calculated from the atomic concentrations A of cobalt atoms and A of Si atoms. ２Elemental analysis was performed using an energy dispersive X-ray spectrometer (EDS) on a rectangular area having a short side of 10 nm and a long side of 50 nm, centered at a position 60 nm away from the interface between the phase and the protective layer, and having a shape with its short side aligned in the thickness direction of the coated-plated steel sheet, to calculate the atomic concentration B of cobalt atoms and the atomic concentration B of Si atoms. Then, the atomic concentration ratio B of the atomic concentration B of cobalt atoms and the atomic concentration B of Si atoms was calculated from the atomic concentrations B of cobalt atoms and B of Si atoms. This operation was performed on a total of 10 locations, two locations on each of five arbitrary samples separated by 300 μm or more in the direction along the surface of the coated-plated steel sheet, to determine the arithmetic mean values ​​of the atomic concentration ratio A and the atomic concentration ratio B. From the obtained arithmetic mean values, the atomic concentration ratios A and B of MgZn and MgZn were calculated. ２ The Co concentration ratio of the protective layer on the phase was determined.

[0214] The observation conditions using a scanning transmission electron microscope (STEM) and the elemental analysis conditions using an energy dispersive X-ray spectrometer (EDS) were the same as those used to measure the mass ratio of each atom in the protective layer.

[0215] 6-4. Proportion (mol %) of metallic cobalt present at the interface between the plating layer and the protective layer The proportion of metallic cobalt present at the interface between the plating layer and the protective layer was determined by obtaining an XAFS spectrum by X-ray absorption fine structure (XAFS) analysis (hereinafter referred to as XAFS analysis) and performing a fitting process on the XAFS spectrum. Specifically, the procedure is as follows. A sample measuring 15 mm x 15 mm and including the plating layer was cut out from the coated / plated steel sheet. XAFS analysis was performed on the plating layer of the sample, and an XAFS spectrum was obtained. The measurement conditions for the XAFS analysis were as follows: Measurement method: Fluorescence method Measurement absorption edge: Co K-absorption edge Measurement atmosphere: Air Measurement temperature: Room temperature Detector: I: 7-element SDD (transmission method: ionization chamber), I ０ : Ionization chamber Energy range: 7700 eV to 7780 eV (step: 0.2 eV)

[0216] On the other hand, XAFS analysis was performed on the following pellet-shaped standard Co samples to obtain XAFS spectra. However, the measurement method was changed from the fluorescence method to the transmission method among the measurement conditions of the XAFS analysis. - Standard Co samples - Co: manufactured by Kojundo Chemical Co., Ltd. CoO: manufactured by Kojundo Chemical Co., Ltd. Co ３ O ４ : Kojundo Kagaku ・CoO(OH): Kojundo Kagaku ・Co(OH): Kojundo Kagaku ・CoCO ３ : High-purity chemicals

[0217] The obtained XAFS spectrum was subjected to linear combination fitting analysis (Linear Combination Fitting: LCF analysis) using Athena (analysis software) with the spectra of each Co standard sample to identify the compound. The XAFS spectrum obtained from the sample was fitted with the linear combination of the spectra of each Co standard sample to quantify the chemical state of cobalt and determine the proportion (mol%) of metallic cobalt. This operation was performed on five samples, and the proportion (mol%) of metallic cobalt was arithmetically averaged.

[0218] 6-5. Amount of cobalt present at the interface between the plating layer and the protective layer (mg / m ２ ) The amount of cobalt present at the interface between the plating layer and the protective layer was measured as follows. The amount of cobalt present at the interface between the plating layer and the protective layer was measured using inductively coupled plasma mass spectrometry (ICP-MS). Specifically, a sample measuring 50 mm x 50 mm was cut from a plated steel sheet coated with a protective layer, and the edges and back surface were sealed with repair paint to prepare a sample for cobalt measurement. The plating layer on the surface of the sample was dissolved in an acidic solution and peeled off together with the protective coating. The peeled solution was then adjusted to a volume of 100 mL, and the absolute amount of cobalt contained in the solution was measured using ICP mass spectrometry (ICP analyzer used: Agilent Technologies, Model 8900).

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[0220]

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[0232] 6. Evaluation Tests (1) Blackening Resistance (40°C, 95% RH) 0.1 cc (100 μl) of pure water was dropped onto the surface of the protective layer of a coated, plated steel sheet (hereinafter also referred to as the "treated sheet"), and the protective layer of another treated sheet was placed on top of this protective layer. A 6 kg weight was placed on the stacked treated sheets so that the load was evenly distributed. With the weight still attached, the stacked treated sheets were left to stand in a constant temperature and humidity chamber at 40°C and 95% relative humidity for 120 hours. The discoloration of the area where the pure water was dropped was then visually observed and evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 9 to 21. A rating of 4 or higher was considered good, and a rating of 3 or lower was deemed unsuitable for practical use. 5: No discoloration; 4: Very slight discoloration; 3: Slight discoloration; 2: Clear discoloration; 1: Clear blackening or discoloration was observed, and unevenness was present.

[0233] (2) Blackening Resistance (20°C, 60% RH) The same test as in "(1) Blackening Resistance (40°C)" above was carried out, except that the temperature of the thermo-humidistat chamber was changed to 20°C and the relative humidity to 60%. The discoloration of the area where pure water was dropped was visually observed and evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 9 to 21. A score of 4 or higher was considered good, and a score of 3 or lower was considered impractical. 5: No discoloration 4: Very slight discoloration was observed 3: Slight discoloration was observed 2: Clear discoloration was observed 1: Clear blackening or discoloration was observed, and unevenness occurred.

[0234] (3) Weather Resistance A 24-week outdoor exposure test was conducted on the treated panels from autumn (around September) to spring (around March), and the color difference of the protective layer surface on the treated panels before and after the test was measured using a color difference meter (ColorMeterZE2000, manufactured by Nippon Denshoku Industries Co., Ltd.). From the measurement results, the difference in L value and the difference in b value before and after the test were determined and evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 9 to 21. Note that for the evaluation results in Tables 6 to 11, the lower of the difference in L value and the difference in b value is given priority. Furthermore, evaluation results rated 3 or higher were considered to be at a practical level.

[0235] Difference in L value: 4: 0 or more and less than 2. 3: 2 or more and less than 4. 2: 4 or more and less than 7. 1: 7 or more.

[0236] Difference in b value: 4: 0 or more and less than 1. 3: 1 or more and less than 4. 2: 4 or more and less than 7. 1: 7 or more.

[0237] (4) Corrosion Resistance (SST) After subjecting the protective layer of the treated plate to neutral salt spray for 240 hours in accordance with the salt spray test method JIS-Z-2371, the percentage of the area on the treated plate where white rust occurred was determined visually and evaluated according to the following evaluation criteria. The evaluation results are shown in Tables 9 to 21. Note that an evaluation result of 3 or higher is at a practical level. 5: White rust occurred on less than 1% of the area. 4: White rust occurred on 1% or more but less than 3%. 3: White rust occurred on 3% or more but less than 10%. 2: White rust occurred on 10% or more but less than 30%. 1: White rust occurred on 30% or more of the area.

[0238] (5) Adhesion The test plate was immersed in boiling water for 2 hours, left for 24 hours, and then 100 1 mm square grids were cut with a cutter. A peeling test was then conducted using cellophane tape, and the results were evaluated according to the following criteria. The evaluation results are shown in Tables 9 to 21. An evaluation result of 3 or higher is considered to be at a practical level: 5: 0 peeled particles; 4: 1 to less than 10 peeled particles; 3: 10 to less than 30% peeled particles; 2: 30 to less than 70 peeled particles; 1: 70 or more peeled particles.

[0239] 7. Aspect A As is clear from the above-described embodiments, the present disclosure includes the following Aspect A. A coated-plated steel sheet in first Aspect A of the present disclosure comprises a steel sheet, a plating layer, and a protective layer laminated in this order, the plating layer containing aluminum, zinc, and magnesium, the average ratio of the atomic concentration of the aluminum to the sum of the atomic concentrations of the magnesium and the zinc in an outermost layer having a depth of 5 nm in the plating layer is 0.40 or more and 1.50 or less, the protective layer contains a resin (A1), a silicon compound (B1), a phosphorus compound (C1), and a cobalt compound (D1), the ratio of the mass of the resin (A1) in the protective layer to the total mass of the protective layer is 0.60 or more, and the ratio of the SiO of silicon atoms in the protective layer to the total mass of the protective layer is 0.50 or more. ２ the ratio of the mass of phosphorus atoms in the protective layer to the total mass of the protective layer is 0.011 or more and 0.075 or less; and the ratio of the mass of cobalt atoms in the protective layer to the total mass of the protective layer is 0.0015 or more and 0.0200 or less.

[0240] According to a first aspect A of the present disclosure, it is possible to provide a coated-plated steel sheet that includes a plating layer containing aluminum, zinc, and magnesium and a protective layer, and that can ensure resistance to blackening.

[0241] In a second aspect A of the present disclosure, the coated-plated steel sheet is the same as in the first aspect A, in which the resin (A1) is a cationic resin.

[0242] According to the second aspect A of the present disclosure, better resistance to blackening can be ensured.

[0243] In a third aspect A of the present disclosure, the coated-plated steel sheet is the first or second aspect A, wherein the resin (A1) contains at least one resin selected from the group consisting of a urethane resin, a phenolic resin, and a (meth)acrylic resin.

[0244] According to the third aspect A of the present disclosure, cracking of the protective layer during processing can be suppressed.

[0245] In a fourth aspect A of the present disclosure, the coated-plated steel sheet is any one of the first to third aspects A, wherein the atomic concentration ratio of the atomic concentration of cobalt to the atomic concentration of silicon (atomic concentration of cobalt / atomic concentration of silicon) in the protective layer is 0.01 or more and 0.38 or less.

[0246] According to the fourth aspect A of the present disclosure, better resistance to blackening can be ensured.

[0247] In a fifth aspect A of the present disclosure, the coated plated steel sheet is any one of the first to fourth aspects A, wherein the plating layer is MgZn ２ phase, ２ At least a portion of the MgZn phase is in contact with the protective layer, ２ In a region of the protective layer in contact with the plating layer, the atomic concentration ratio of the cobalt atomic concentration to the silicon atomic concentration (atomic concentration of cobalt / atomic concentration of silicon) at a position 10 nm deep from the interface between the plating layer and the protective layer is 1.15 to 2.50 times the atomic concentration ratio of the cobalt atomic concentration to the silicon atomic concentration (atomic concentration of cobalt / atomic concentration of silicon) at a position 60 nm deep from the interface between the plating layer and the protective layer.

[0248] According to the fifth aspect A of the present disclosure, better resistance to blackening can be ensured.

[0249] A sixth aspect A of the present disclosure is a coated-plated steel sheet according to any one of the first to fifth aspects A, wherein the average atomic concentration of the aluminum in the outermost layer of the 5 nm deep plating layer is 20 atomic % or more and 60 atomic % or less, the average atomic concentration of the magnesium in the outermost layer of the 5 nm deep plating layer is 10 atomic % or more and 75 atomic % or less, and the average atomic concentration of the zinc in the outermost layer of the 5 nm deep plating layer is 1 atomic % or more and 40 atomic % or less. A seventh aspect A of the present disclosure is a coated-plated steel sheet according to any one of the first to fifth aspects A, wherein the average atomic concentration of the aluminum in the outermost layer of the 5 nm deep plating layer is 20 atomic % or more and 60 atomic % or less, the average atomic concentration of the magnesium in the outermost layer of the 5 nm deep plating layer is 30 atomic % or more and 75 atomic % or less, and the average atomic concentration of the zinc in the outermost layer of the 5 nm deep plating layer is 1 atomic % or more and 10 atomic % or less. In an eighth aspect A of the present disclosure, the coated-plated steel sheet is any one of the first to fifth aspects A, wherein the average atomic concentration of the aluminum in the outermost layer of the plating layer having a depth of 5 nm is 25 atomic % or more and 60 atomic % or less, the average atomic concentration of the magnesium in the outermost layer of the plating layer having a depth of 5 nm is 10 atomic % or more and 60 atomic % or less, and the average atomic concentration of the zinc in the outermost layer of the plating layer having a depth of 5 nm is 5 atomic % or more and 40 atomic % or less.

[0250] According to the sixth to eighth aspects A of the present disclosure, the aluminum atomic concentration can maintain the corrosion resistance of the plating layer and further enhance the blackening resistance of the plating layer under normal conditions. Furthermore, the decrease in blackening resistance of the plating layer can be suppressed, and film adhesion and corrosion resistance can be further ensured. According to the sixth to eighth aspects A of the present disclosure, the magnesium atomic concentration can ensure the blackening resistance of the plating layer and further enhance the corrosion resistance and adhesion of the plating layer. Furthermore, the decrease in blackening resistance of the plating layer under normal conditions can be suppressed, and the bond between the plating layer and the phosphorus compound (C1) contained in the protective layer can be strengthened, thereby ensuring the corrosion resistance and film adhesion of the plating layer. According to the sixth to eighth aspects A of the present disclosure, the average value of the zinc atomic concentration can further enhance the corrosion resistance and adhesion of the plating layer. Furthermore, the decrease in blackening resistance of the plating layer can be suppressed, and film adhesion and corrosion resistance can be further ensured. Furthermore, the blackening resistance of the plating layer can be further enhanced.

[0251] A ninth aspect A of the present disclosure is a coated-plated steel sheet according to any one of the first to eighth aspects A, wherein the aluminum content in the plating layer is 5.0 mass% or more and 65.0 mass% or less, and the magnesium content in the plating layer is 0.5 mass% or more and 10.0 mass% or less. A tenth aspect A of the present disclosure is a coated-plated steel sheet according to any one of the first to eighth aspects A, wherein the aluminum content in the plating layer is 45.0 mass% or more and 65.0 mass% or less, the magnesium content in the plating layer is 0.5 mass% or more and 5.0 mass% or less, and the Ca content in the plating layer is 50 mass ppm or more and 400 mass ppm or less. In an eleventh aspect A of the present disclosure, the coated-plated steel sheet is any one of the first to eighth aspects A, wherein the proportion of the aluminum in the plating layer is 5.0 mass% or more and 25.0 mass% or less, and the proportion of the magnesium in the plating layer is 1.5 mass% or more and 10.0 mass% or less.

[0252] According to the ninth to eleventh aspects A of the present disclosure, the proportion of aluminum can maintain the corrosion resistance of the plating layer and further enhance its resistance to blackening under normal conditions. Furthermore, the adhesion and corrosion resistance of the plating layer can be further ensured while suppressing hardening of the plating layer. According to the ninth to eleventh aspects A of the present disclosure, the proportion of magnesium can ensure the blackening resistance of the plating layer and further enhance its corrosion resistance and adhesion. Furthermore, the appearance of the plating layer can be improved while further enhancing its resistance to blackening under normal conditions. According to the tenth aspect A of the present disclosure, by adjusting the proportion of Ca in the plating layer to a specific numerical range, Mg concentration in the outermost layer of the plating layer can be suppressed. As a result, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of the plating layer to a depth of 5 nm can be particularly adjusted to a specific numerical range of 0.40 or more and 1.50 or less.

[0253] In a twelfth aspect A of the present disclosure, the coated-plated steel sheet of the eighth or eleventh aspect A is such that 5 mol % or more of the cobalt present in the plating layer is in a chemical state of metallic cobalt.

[0254] According to the twelfth aspect A of the present disclosure, better resistance to blackening can be ensured.

[0255] 8. Aspect B As is apparent from the above-described embodiment, the present disclosure further includes the following aspect B.

[0256] In a first aspect B of the present disclosure, the coated-plated steel sheet comprises a steel sheet, a plating layer, and a protective layer laminated in this order. The plating layer contains aluminum, zinc, and magnesium. In the outermost layer of the plating layer having a depth of 5 nm, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc is 0.4 or more and 1.5 or less. The protective layer contains a cationic resin (A1), a silicon compound (B1), a phosphorus compound (C1), and a cobalt compound (D1). The ratio of the mass of the cationic resin (A1) in the protective layer to the total mass of the protective layer is 0.6 or more. The ratio of the SiO of silicon atoms in the protective layer to the total mass of the protective layer is 0.5 or more. ２The ratio of the mass of phosphorus atoms in the protective layer to the total mass of the protective layer is 0.015 to 0.075, and the ratio of the mass of cobalt atoms in the protective layer to the total mass of the protective layer is 0.0015 to 0.020.

[0257] According to the first aspect B, it is possible to provide a coated-plated steel sheet that has a plating layer containing aluminum, zinc, and magnesium and a protective layer, and that can ensure resistance to blackening.

[0258] In a second aspect B of the present disclosure, the coated-plated steel sheet is the first aspect B, wherein the average atomic concentration of aluminum in the outermost layer of the plating layer having a depth of 5 nm is 20 atomic % or more and 60 atomic % or less.

[0259] According to the second aspect B, the corrosion resistance of the plating layer can be maintained and the blackening resistance of the plating layer under normal conditions can be further improved. In addition, the deterioration of the blackening resistance of the plating layer can be suppressed and the film adhesion and corrosion resistance can be further ensured.

[0260] In a third aspect B of the present disclosure, the coated-plated steel sheet is the first or second aspect B, wherein the average atomic concentration of zinc in the outermost layer of the plating layer having a depth of 5 nm is 1 atomic % or more and 10 atomic % or less.

[0261] According to the third aspect B, the corrosion resistance and adhesion of the plating layer can be further improved. In addition, the deterioration of the blackening resistance of the plating layer can be suppressed, and the film adhesion and corrosion resistance can be further ensured. In addition, the blackening resistance of the plating layer can be further improved.

[0262] In a fourth aspect B of the present disclosure, the coated-plated steel sheet is any one of the first to third aspects B, wherein the average atomic concentration of magnesium in the outermost layer of the plating layer having a depth of 5 nm is 30 atomic % or more and 75 atomic % or less.

[0263] According to the fourth aspect B, the corrosion resistance and adhesion of the plating layer can be further improved while ensuring the blackening resistance of the plating layer. Furthermore, the decrease in the blackening resistance of the plating layer under normal conditions is suppressed, and the bond between the plating layer and the phosphorus compound (C1) contained in the protective layer is strengthened, so that the corrosion resistance and film adhesion of the plating layer can be ensured.

[0264] In a fifth aspect B of the present disclosure, the coated-plated steel sheet is any one of the first to fourth aspects B, wherein the proportion of aluminum in the plating layer is 45.0 mass % or more and 65.0 mass % or less.

[0265] According to the fifth aspect B, the corrosion resistance of the plating layer can be maintained and the blackening resistance of the plating layer under normal conditions can be further improved. Furthermore, the hardening of the plating layer can be suppressed and the adhesion and corrosion resistance of the plating layer can be further ensured.

[0266] In a sixth aspect B of the present disclosure, the coated-plated steel sheet is any one of the first to fifth aspects B, wherein the proportion of magnesium in the plating layer is 0.5 mass % or more and 5.0 mass % or less.

[0267] According to the sixth aspect B, the blackening resistance of the plating layer can be ensured and the corrosion resistance and adhesion of the plating layer can be further improved. In addition, the appearance of the plating layer can be improved and the blackening resistance of the plating layer under normal conditions can be further improved.

[0268] In a seventh aspect B of the present disclosure, the coated-plated steel sheet is any one of the first to sixth aspects B, wherein the proportion of Ca in the plating layer is 50 ppm by mass or more and 400 ppm by mass or less.

[0269] According to the seventh aspect B, the concentration of Mg in the outermost layer of the plating layer can be suppressed by adjusting the proportion of Ca contained in the plating layer to a specific range, and as a result, the average ratio of the atomic concentration of aluminum to the sum of the atomic concentrations of magnesium and zinc in the outermost layer of the plating layer to a depth of 5 nm can be adjusted to a specific range of 0.4 to 1.5.

[0270] The disclosure of Japanese Patent Application No. 2023-102000 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A steel sheet, a plating layer, and a protective layer are laminated in this order, the plating layer contains aluminum, zinc, and magnesium, the average value of the ratio of the atomic concentration of the aluminum to the sum of the atomic concentrations of the magnesium and the zinc in an outermost layer having a depth of 5 nm in the plating layer is 0.40 to 1.50, the protective layer contains a resin (A1), a silicon compound (B1), a phosphorus compound (C1), and a cobalt compound (D1), the ratio of the mass of the resin (A1) in the protective layer to the total mass of the protective layer is 0.60 or more, and the SiO of silicon atoms in the protective layer to the total mass of the protective layer is 0.50 or more. ２ a mass ratio calculated from the converted mass of phosphorus atoms in the protective layer to a total mass of the protective layer is 0.07 or more and 0.19 or less, a mass ratio of phosphorus atoms in the protective layer to a total mass of the protective layer is 0.011 or more and 0.075 or less, and a mass ratio of cobalt atoms in the protective layer to a total mass of the protective layer is 0.0015 or more and 0.0200 or less.

2. The coated plated steel sheet according to claim 1, wherein the resin (A1) is a cationic resin.

3. The coated plated steel sheet according to claim 1, wherein the resin (A1) contains at least one resin selected from the group consisting of a urethane resin, a phenolic resin, and a (meth)acrylic resin.

4. The coated plated steel sheet according to claim 1, wherein the atomic concentration ratio of cobalt to silicon in the protective layer (atomic concentration of cobalt / atomic concentration of silicon) is 0.01 or more and 0.38 or less.

5. The plating layer is MgZn ２ phase, ２ At least a portion of the MgZn phase is in contact with the protective layer, ２ 2. The coated-plated steel sheet according to claim 1, wherein in a region of the protective layer in contact with a Cr phase, an atomic concentration ratio of cobalt to silicon at a position 10 nm deep from the interface between the plating layer and the protective layer (atomic concentration of cobalt / atomic concentration of silicon) is 1.15 to 2.50 times an atomic concentration ratio of cobalt to silicon at a position 60 nm deep from the interface between the plating layer and the protective layer (atomic concentration of cobalt / atomic concentration of silicon).

6. The coated-plated steel sheet according to claim 1, wherein the average atomic concentration of the aluminum in the outermost layer having a depth of 5 nm in the plating layer is 20 atomic % or more and 60 atomic % or less, the average atomic concentration of the magnesium in the outermost layer having a depth of 5 nm in the plating layer is 10 atomic % or more and 75 atomic % or less, and the average atomic concentration of the zinc in the outermost layer having a depth of 5 nm in the plating layer is 1 atomic % or more and 40 atomic % or less.

7. The coated plated steel sheet according to claim 1, wherein the average atomic concentration of the aluminum in the outermost layer having a depth of 5 nm in the plating layer is 20 atomic % or more and 60 atomic % or less, the average atomic concentration of the magnesium in the outermost layer having a depth of 5 nm in the plating layer is 30 atomic % or more and 75 atomic % or less, and the average atomic concentration of the zinc in the outermost layer having a depth of 5 nm in the plating layer is 1 atomic % or more and 10 atomic % or less.

8. The coated plated steel sheet according to claim 1, wherein the average atomic concentration of the aluminum in the outermost layer 5 nm deep in the plating layer is 25 atomic % or more and 60 atomic % or less, the average atomic concentration of the magnesium in the outermost layer 5 nm deep in the plating layer is 10 atomic % or more and 60 atomic % or less, and the average atomic concentration of the zinc in the outermost layer 5 nm deep in the plating layer is 5 atomic % or more and 40 atomic % or less.

9. The coated plated steel sheet according to claim 1, wherein the proportion of said aluminum in said plating layer is 5.0 mass% or more and 65.0 mass% or less, and the proportion of said magnesium in said plating layer is 0.5 mass% or more and 10.0 mass% or less.

10. The coated-plated steel sheet according to claim 1, wherein the proportion of said aluminum in said plating layer is 45.0 mass% or more and 65.0 mass% or less, the proportion of said magnesium in said plating layer is 0.5 mass% or more and 5.0 mass% or less, and the proportion of Ca in said plating layer is 50 mass ppm or more and 400 mass ppm or less.

11. The coated plated steel sheet according to claim 1, wherein the proportion of said aluminum in said plating layer is 5.0 mass% or more and 25.0 mass% or less, and the proportion of said magnesium in said plating layer is 1.5 mass% or more and 10.0 mass% or less.

12. The coated plated steel sheet according to claim 8 or 11, wherein 5 mol % or more of the cobalt present at the interface between the plating layer and the protective layer is in the chemical state of metallic cobalt.

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