Painted steel sheet and its manufacturing method

Abstract

Provided is a coated steel sheet having stable and excellent corrosion resistance and corrosion resistance in processed areas. [Solution] In order to achieve the above object, the present invention provides a coated steel sheet in which a coating film is formed on a plating film directly or via a chemical conversion coating, The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the remainder consisting of Zn and unavoidable impurities, the Ni content of the unavoidable impurities being 0.010 mass% or less relative to the total mass of the plating film, the chemical conversion film containing a specific component, and the coating film containing a specific component.

Description

[Technical Field]

[0001] This invention relates to a painted steel sheet having consistently excellent corrosion resistance and corrosion resistance of processed parts, and a method for manufacturing the same. [Background technology]

[0002] Among the plated steel sheets used as a base for painted steel sheets, hot-dip Al-Zn plated steel sheets, typified by the 55% Al-Zn system, are a prime example. These hot-dip Al-Zn plated steel sheets are known to exhibit high corrosion resistance among hot-dip galvanized steel sheets because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al. For this reason, due to their excellent corrosion resistance, hot-dip Al-Zn plated steel sheets are mainly used in building materials such as roofs and walls that are exposed to the outdoors for long periods, as well as in civil engineering and construction fields such as guardrails, wiring and piping, and sound barriers. In particular, there is a growing demand for materials with superior corrosion resistance and maintenance-free materials in harsher operating environments, such as those affected by acid rain due to air pollution, the application of de-icing agents to prevent road freezing in snowy areas, and coastal development. As a result, the demand for hot-dip Al-Zn plated steel sheets has been increasing in recent years.

[0003] The plating film of hot-dip Al-Zn plated steel sheets is characterized by a structure in which multiple α-Al phases are stacked in the direction of the plating film thickness, consisting of a portion where Al containing supersaturated Zn has solidified in a dendrite-like manner (α-Al phase) and a Zn-Al eutectic structure present in the interdendrite gaps. Due to this characteristic film structure, the corrosion progression path from the surface becomes complex, making it difficult for corrosion to progress easily. As a result, hot-dip Al-Zn plated steel sheets are known to achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same plating film thickness.

[0004] Attempts have been made to further extend the lifespan of such molten Al-Zn plated steel sheets, and molten Al-Zn-Si-Mg plated steel sheets with added Mg have been put into practical use. As an example of such a hot-dip Al-Zn-Si-Mg plated steel sheet, Patent Document 1 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which the plating film contains an Al-Zn-Si alloy containing Mg, the Al-Zn-Si alloy is an alloy containing 45-60% by weight of elemental aluminum, 37-46% by weight of elemental zinc, and 1.2-2.3% by weight of Si, and the concentration of Mg is 1-5% by weight. Furthermore, Patent Document 2 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet that aims to improve corrosion resistance and enhance the protective effect after the underlying steel sheet is exposed by containing 2-10% Mg and 0.01-10% Ca in the plating film. Furthermore, Patent Document 3 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet that improves the corrosion resistance of flat plates and end faces by forming a coating layer containing Mg: 1-15%, Si: 2-15%, and Zn: 11-25% by mass, with the remainder being Al and unavoidable impurities, and by making the size of intermetallic compounds such as Mg2Si phase and MgZn2 phase present in the plating film 10 μm or less.

[0005] As mentioned above, hot-dip molten Al-Zn plated steel sheets have a beautiful appearance with a white metallic spangle pattern, and are often used without painting. Therefore, there is a strong demand for a high-quality appearance. Consequently, technologies to improve the appearance of hot-dip molten Al-Zn plated steel sheets have been developed. For example, Patent Document 4 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which wrinkle-like unevenness defects are suppressed by containing 0.01 to 10% Sr in the plating film. Furthermore, Patent Document 5 also discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which mottled defects are suppressed by containing 500 to 3000 ppm of Sr in the plating film.

[0006] Furthermore, painted steel sheets, which have a chemical conversion coating, primer coating, topcoat coating, etc. formed on the surface of a hot-dip Al-Zn plated steel sheet, can be bent at 90 degrees by press forming, roll forming, or embossing. Various processing such as 180-degree bending is required, and long-term coating durability is also demanded. To meet these demands, hot-dip Al-Zn plated steel sheets are known to have a chromate-containing chemical conversion coating, and the primer coating also contains a chromate-based rust-preventive pigment. On top of this, a weather-resistant topcoat such as a thermosetting polyester resin coating or a fluororesin coating is formed. However, in recent years, the use of chromate, an environmentally harmful substance, in such painted steel sheets has become a concern, and there is a strong demand for the development of painted steel sheets that can improve corrosion resistance and surface appearance even without chromate. As a technology to address these requirements, for example, Patent Document 6 discloses a surface-treated hot-dip galvanized steel material in which an aluminum-zinc alloy plating layer (α) containing Al, Zn, Si, and Mg, with the content of these elements adjusted, is plated onto the surface of the steel material, and a film (β) is formed on top of it, with at least one compound (A) selected from titanium compounds and zirconium compounds as the film-forming component, and the mass ratio of the Si-Mg phase in the aluminum-zinc alloy plating layer (α) to the total amount of Mg in the plating layer is adjusted to 3% or more. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 5020228 [Patent Document 2] Patent No. 5000039 [Patent Document 3] Japanese Patent Publication No. 2002-12959 [Patent Document 4] Patent No. 3983932 [Patent Document 5] Special Publication No. 2011-514934 [Patent Document 6] Japanese Patent Publication No. 2005-169765 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the technique of incorporating Mg into the plating film, as disclosed in Patent Documents 1 to 3, does not necessarily guarantee an improvement in corrosion resistance. In the molten Al-Zn-Si-Mg plated steel sheets disclosed in Patent Documents 1 to 3, corrosion resistance is improved solely by including Mg in the plating components. However, the influence of components other than the four elements mentioned above (Al, Zn, Si, Mg) and the characteristics of the metallic phase and intermetallic compound phase constituting the plating film are not considered, making it impossible to uniformly discuss the superiority or inferiority of corrosion resistance. Therefore, even when molten Al-Zn-Si-Mg plated steel sheets are manufactured using a plating bath composition with equivalent content of the four elements mentioned above, corrosion acceleration tests show variability in corrosion resistance, and there is a problem in that they are not necessarily superior to Al-Zn plated steel sheets without added Mg. Similarly, in improving the appearance of the plating, simply adding Sr to the plating film does not necessarily eliminate wrinkle-like uneven defects, and the molten Al-Zn-Si-Mg plated steel sheets disclosed in Patent Documents 4 and 5 sometimes failed to achieve both corrosion resistance and appearance. In addition, because Mg is an element that oxidizes easily, the Mg contained in the plating bath can generate oxides (top dross) near the bath surface, and in the case of molten plating, over time, Fe-Al compounds containing iron (bottom dross) may be generated, which are unevenly distributed in the bath or at the bottom of the plating bath. These drosses can adhere to the surface of the plating film, causing convex defects and potentially impairing the appearance of the plating film surface. Furthermore, it is known that when steel plates are plated using a molten Al-Zn-Si bath to which Mg has been added, in addition to the α-Al phase, Mg2Si phase, MgZn2 phase, and Si phase precipitate in the plating film. However, the influence of the amount and relative abundance of each phase on corrosion resistance had not been clarified.

[0009] Furthermore, as mentioned above, painted steel sheets can be press-formed, roll-formed, and embossed. Depending on the shape and other factors, various processing such as 90-degree bends and 180-degree bends are required, and long-term coating durability is necessary. However, the technology described in Patent Document 6 did not necessarily guarantee stable corrosion resistance or surface appearance after processing. Needless to say, the corrosion resistance of painted steel sheets is influenced by the corrosion resistance of the underlying plated steel sheet. Furthermore, regarding the surface appearance, the height difference of wrinkle-like defects can reach tens of micrometers, so even if the surface is smoothed by the coating, the unevenness cannot be completely eliminated, and it is considered that improvement in the appearance of painted steel sheets cannot be expected. In addition, there is a concern that the corrosion resistance will be locally reduced in the raised areas where the coating is thinner. For this reason, in order to obtain painted steel sheets with excellent corrosion resistance and surface appearance, it is important to improve the corrosion resistance and surface appearance of the underlying plated steel sheet.

[0010] In view of these circumstances, the present invention aims to provide a painted steel sheet and a method for manufacturing the same that have consistently excellent corrosion resistance and corrosion resistance of processed parts. [Means for solving the problem]

[0011] As a result of their investigations to solve the above problems, the inventors focused on the fact that, in order to control the composition of the plating film of a molten Al-Zn-Si-Mg plated steel sheet, it is important not only to control the concentrations of Al, Zn, Si, and Mg, but also to control the concentration of elements included as impurities. They found that by appropriately controlling the Ni content, the deterioration of corrosion resistance can be effectively suppressed, and furthermore, by appropriately controlling the size and distribution of Ni-based compounds present as impurities in the plating film, the deterioration of corrosion resistance can be suppressed even more effectively. Furthermore, regarding the Mg2Si, MgZn2, and Si phases formed in the plating film of molten Al-Zn-Si-Mg plated steel sheets, the amount of deposition increases or decreases depending on the balance of each component in the plating film and the conditions for the formation of the plating film, changing their relative abundance. Depending on the balance of the composition, one of the phases may not precipitate at all. It was investigated that the corrosion resistance of molten Al-Zn-Si-Mg plated steel sheets changes depending on the relative abundance of these phases, and that corrosion resistance stably improves, especially when the MgZn2 phase is more abundant than the Mg2Si and Si phases. However, it is known that it is extremely difficult to distinguish between these Mg2Si, MgZn2, and Si phases using general methods, such as observing the plating film from the surface or cross-section using a scanning electron microscope to obtain secondary electron images or backscattered electron images. While it is possible to obtain microscopic information by observing with a transmission electron microscope, it was not possible to grasp the relative abundance of the Mg2Si, MgZn2, and Si phases, which affect macroscopic information such as corrosion resistance and appearance. Therefore, the inventors continued their diligent research and, focusing on X-ray diffraction, discovered that the phase ratios can be quantitatively defined by utilizing the intensity ratios of specific diffraction peaks of the Mg2Si phase, MgZn2 phase, and Si phase. Furthermore, they found that when the Mg2Si phase and MgZn2 phase satisfy specific ratios in the plating film, stable and excellent corrosion resistance can be achieved, and dross generation can be suppressed, ensuring a good surface appearance. In addition, the inventors have found that by controlling the Ni content and film structure in the plating film as described above, and by controlling the Sr concentration in the plating bath, it is possible to reliably suppress the occurrence of wrinkle-like unevenness defects and obtain a plated steel sheet with excellent surface appearance.

[0012] Furthermore, the inventors also investigated the chemical conversion coating and primer coating formed on the plated film and found that by composing the chemical conversion coating from a specific resin and a specific inorganic compound, and the primer coating from a specific polyester resin and an inorganic compound, the barrier properties and adhesion of the coating can be improved, and excellent post-processing corrosion resistance can be achieved even without chromate.

[0013] The present invention has been made based on the above findings, and the gist thereof is as follows. 1. A coated steel sheet having a coating film formed directly or via a chemical conversion film on a plating film, wherein the plating film has a composition containing Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass, and Mg: 1.0 to 10.0% by mass, and the balance consisting of Zn and unavoidable impurities, the Ni content in the unavoidable impurities is 0.010% by mass or less based on the total mass of the plating film, the chemical conversion film contains a total of 30 to 50% by mass of (a) an anionic polyurethane resin having an ester bond and (b) an epoxy resin having a bisphenol skeleton, and the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass ratio, a resin component, 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and an inorganic compound containing 0.5 to 5% by mass of a fluorine compound, the coating film has at least a primer coating film, and the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound, and magnesium oxide, and is characterized by the above, a coated steel sheet.

[0014] <00001 of the above. 2. The coated steel sheet according to 1, wherein the plating film contains a Ni-based compound, and the major axis of the Ni-based compound is 4.0 μm or less. 3. The coated steel sheet according to 1 or 2, wherein the plating film contains a Ni-based compound, and the number of the Ni-based compounds present in a direction parallel to the surface of the base steel sheet is 5 or less per mm. 4. The coated steel sheet according to 1, wherein the plating film does not contain a Ni-based compound. 5. The coated steel sheet according to any one of 1 to 4, wherein the diffraction intensities of Mg2Si and MgZn2 in the plating film by X-ray diffraction satisfy the following relationship (1). Mg2Si (111) / MgZn2(100)≦2.0 (1) Mg2Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Mg2Si. MgZn2(100): Diffraction intensity of the (100) plane (interplanar spacing d=0.4510nm) of MgZn2 6. The painted steel sheet according to any one of items 1 to 5, characterized in that the diffraction intensity of Si in the plating film by X-ray diffraction satisfies the following relationship (2). Si (111) = 0 ···(2) Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3135 nm) of Si

[0015] 7. The painted steel sheet according to any one of claims 1 to 6, characterized in that the plating film further contains Sr: 0.01 to 1.0% by mass.

[0016] 8. The painted steel sheet according to any one of 1 to 7, characterized in that the Al content in the plating film is 50 to 60% by mass.

[0017] 9. The painted steel sheet according to any one of 1 to 8, characterized in that the Si content in the plating film is 1.0 to 3.0% by mass.

[0018] 10. A painted steel sheet according to any one of 1 to 9, characterized in that the Mg content in the plating film is 1.0 to 5.0% by mass.

[0019] 11. A method for manufacturing a painted steel sheet in which a coating film is formed directly on a plating film or via a chemical conversion film, The aforementioned chemical conversion film contains a resin component comprising (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton in total at a concentration of 30 to 50% by mass, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. The aforementioned coating film comprises at least a primer coating film, the primer coating film containing a polyester resin having urethane bonds, and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide. A method for manufacturing a coated steel sheet, comprising a hot-dip plating step of immersing a base steel sheet in a plating bath having a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities, characterized in that the Ni content in the unavoidable impurities of the plating bath is controlled to 0.010% by mass or less relative to the total mass of the plating bath. 12. The method for manufacturing a painted steel sheet according to 11, characterized in that the plating bath further contains Sr: 0.01 to 1.0% by mass. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a painted steel sheet and a method for manufacturing the same that have consistently excellent corrosion resistance and corrosion resistance of processed parts. [Brief explanation of the drawing]

[0021] [Figure 1] This diagram illustrates the flow of the combined cycle test (JASO-CCT) according to Japanese automotive standards. [Modes for carrying out the invention]

[0022] <Painted steel sheet> The painted steel sheet of the present invention is a painted steel sheet in which a coating film is formed directly on a plating film or via a chemical conversion film.

[0023] (Plating film) In the coated steel sheet of the present invention, the plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities.

[0024] The Al content in the plating film is 45 to 65% by mass, preferably 50 to 60% by mass, considering the balance between corrosion resistance and operational aspects. This is because if the Al content in the plating film is at least 45% by mass, Al dendrite solidification occurs, and a plating film structure mainly consisting of a dendrite solidified α-Al phase can be obtained. When this dendrite solidified structure is stacked in the direction of the thickness of the plating film, the corrosion progression path becomes complex, and the corrosion resistance of the plating film itself is improved. Furthermore, the more of these α-Al phase dendrites that are stacked, the more complex the corrosion progression path becomes, making it more difficult for corrosion to easily reach the underlying steel plate, thus improving corrosion resistance. For this reason, it is preferable to have an Al content of 50% by mass or more. On the other hand, if the Al content in the plating film exceeds 65% by mass, the structure changes to one in which most of the Zn is solid-dissolved in α-Al, the dissolution reaction of the α-Al phase cannot be suppressed, and the corrosion resistance of the Al-Zn-Si-Mg plating deteriorates. Therefore, the Al content in the plating film must be 65% by mass or less, preferably 60% by mass or less.

[0025] The Si in the aforementioned plating film is added primarily to suppress the growth of Fe-Al and / or Fe-Al-Si interfacial alloy layers that form at the interface with the underlying steel sheet, thereby preventing deterioration of the adhesion between the plating film and the steel sheet. In fact, when a steel sheet is immersed in an Al-Zn plating bath containing Si, Fe on the surface of the steel sheet reacts with Al and Si in the bath to form an alloy, and Fe-Al and / or Fe-Al-Si intermetallic compound layers are formed at the underlying steel sheet / plating film interface. However, since the Fe-Al-Si alloy grows more slowly than the Fe-Al alloy, the higher the proportion of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. For this reason, the Si content in the plating film must be 1.0% by mass or more. On the other hand, if the Si content in the plating film exceeds 4.0% by mass, not only does the aforementioned effect of suppressing the growth of the interfacial alloy layer saturate, but the presence of an excess Si phase in the plating film promotes corrosion, so the Si content should be 4.0% or less. Furthermore, the Si content in the plating film is preferably 3.0% or less, from the viewpoint of suppressing the presence of an excess Si phase. In addition, from the viewpoint of easily satisfying the relationship (1) described below in relation to the Mg content, it is preferable that the Si content be 1.0 to 3.0% by mass.

[0026] The aforementioned plating film contains 1.0 to 10.0% Mg. By including Mg in the plating film, the aforementioned Si can be present in the form of an intermetallic compound of the Mg2Si phase, thereby suppressing the acceleration of corrosion. Furthermore, when Mg is included in the plating film, an intermetallic compound, the MgZn2 phase, is also formed in the plating film, resulting in an improved corrosion resistance. If the Mg content in the plating film is less than 1.0% by mass, sufficient corrosion resistance cannot be ensured because Mg is used for solid solution into the α-Al phase, which is the main phase, rather than for the formation of the intermetallic compounds (Mg2Si, MgZn2). On the other hand, if the Mg content in the plating film is too high, the effect of improving corrosion resistance saturates, and the processability decreases due to the weakening of the α-Al phase; therefore, the content should be 10.0% or less. Moreover, from the viewpoint of suppressing dross generation during plating formation and facilitating plating bath management, the Mg content in the plating film is preferably 5.0% by mass or less. Furthermore, in relation to the Si content, from the viewpoint of easily satisfying the relationship (1) described below, it is preferable to set the Mg content to 3.0 mass%, and from the viewpoint of compatibility with dross suppression, it is more preferable to set the Mg content to 3.0 to 5.0 mass%.

[0027] Furthermore, the plating film contains Zn and unavoidable impurities. Of these, the unavoidable impurities include Fe. This Fe is inevitably included in the plating film as a result of the elution of steel plates and equipment in the plating bath, and as a result of diffusion from the underlying steel plate during the formation of the interfacial alloy layer. The Fe content in the plating film is usually about 0.3 to 2.0 mass%. Other unavoidable impurities include Cr, Ni, and Cu. These components inevitably become present in the plating film when the underlying steel plate or stainless steel bath equipment dissolves into the plating bath, when they are present as impurities in the metal ingots that serve as the raw materials for the plating bath, and when the plated steel sheets are manufactured using pots and bath equipment that were used in the production of plated steel sheets that had these components intentionally added.

[0028] Furthermore, the coated steel sheet of the present invention is characterized in that the Ni content in the unavoidable impurities is 0.010% by mass or less relative to the total mass of the plating film. Since Ni contained in the plating film may degrade the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet, the degradation of corrosion resistance can be suppressed by appropriately controlling the content of Al, Zn, Si, and Mg in the plating film as described above, and further suppressing the Ni content as an unavoidable impurity. From a similar viewpoint, it is preferable that the Ni content in the unavoidable impurities be 0.005% by mass or less relative to the total mass of the plating film.

[0029] Furthermore, if Ni is included in the aforementioned unavoidable impurities, Ni-based compounds may be present as impurities in the plating film of the molten Al-Zn-Si-Mg plated steel sheet. Here, the Ni-based compounds mainly refer to binary intermetallic compounds such as Ni-Al compounds, and ternary intermetallic compounds such as Ni-Al-Fe compounds. Examples of Ni-Al compounds include intermetallic compounds such as NiAl3, and examples of Ni-Al-Fe compounds include intermetallic compounds such as (Ni,Fe)Al3 in which some of the Ni in NiAl3 is replaced with Fe, but the compound is not limited to these. The presence of Ni-based compounds in the plating film can be confirmed, for example, by using a scanning electron microscope to observe the plating film from the surface or cross-section using secondary electron images or backscattered electron images, and then analyzing it by energy-dispersive X-ray spectroscopy (EDS). For example, by selecting 5 to 10 locations on a 100 μm cross-section of the plating, observing and performing elemental mapping analysis on each location at an accelerating voltage of 5 kV or less, and then performing point analysis on the areas where Ni is detected, the composition of the Ni-based content can be confirmed. This method is merely one example, and any method that can confirm the presence of Ni-based compounds is acceptable and is not particularly limited.

[0030] Furthermore, if the plating film contains a Ni-based compound, the major axis of the Ni-based compound is 4.0 μm or less. It is preferable to have one. Ni-based compounds present in the aforementioned plating film can function as cathodes in corrosive environments, forming local galvanic cells with the surrounding solidified tissue, which can lead to a deterioration in corrosion resistance. In particular, the presence of coarse Ni-based compounds in the plating film can significantly reduce the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet. Therefore, in order to obtain a molten Al-Zn-Si-Mg plated steel sheet with superior corrosion resistance, it is effective to control the size of the Ni-based compounds contained as impurities in the plating film. Specifically, it is preferable to have a major axis of Ni-based compounds of 4.0 μm or less, more preferably 3.0 μm or less, and even more preferably 2.0 μm or less. The major axis of the Ni-based compound can be measured, for example, by using a scanning electron microscope to observe the plating film in cross-section using a backscattered electron image, confirming that it is a Ni-based compound using EDS, and then observing a magnified backscattered electron image of the observation field containing the Ni-based compound. The major axis of the Ni-based compound is defined as the maximum major axis of the Ni-based compound confirmed within the observation field of the plating film.

[0031] Furthermore, when the plating film contains a Ni-based compound, it is also effective to reduce the amount of the Ni-based compound present, which acts as the starting point for corrosion, from the viewpoint of obtaining more stable and high corrosion resistance. Specifically, it is preferable to have 5 or fewer Ni-based compound particles in the plating film in the direction parallel to the surface of the underlying steel sheet, more preferably 2 or fewer, and most preferably 0 or none (absent). Therefore, by suppressing the amount of Ni-containing compounds present in the plating film, the deterioration of the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet can be more reliably suppressed. In order to obtain such a film structure (a film structure that does not contain Ni-based compounds), it is important to reduce the Ni content in the unavoidable impurities, specifically, to set the Ni content to 0.005% by mass or less relative to the total mass of the plating film. Furthermore, the number of Ni-based compound particles can be determined, for example, by using a scanning electron microscope to continuously observe a cross-section parallel to the surface of the underlying steel sheet with a plating film using backscattered electron images for a length of 1 mm or more, and then dividing the number of Ni-based compounds confirmed by EDS by the measured length (mm) to calculate the number of Ni-based compounds present within a 1 mm length range.

[0032] Furthermore, while there are no particular limitations on the total content of unavoidable impurities in the plating film, it is preferable to keep the total content at 5.0% by mass or less, as excessive impurities may affect various properties of the plated steel sheet.

[0033] Furthermore, in the coated steel sheet of the present invention, from the viewpoint of being able to more stably improve corrosion resistance while controlling the concentrations of Al, Zn, Si, Mg, and Ni as an unavoidable impurity as described above, it is preferable that the diffraction intensity of Mg2Si and MgZn2 in the plating film, measured by X-ray diffraction, satisfies the following relationship (1). Mg2Si (111) / MgZn2(100)≦2.0 (1) Mg2Si (111): Diffraction intensity of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si, MgZn2(100): Diffraction intensity of the (100) plane (interplanar spacing d=0.4510 nm) of MgZn2

[0034] As described above, in this invention, it is important to control the proportion of intermetallic compounds such as Mg2Si and MgZn2 that are formed in the plating film due to the inclusion of Mg to a specific ratio. The effects of these on corrosion resistance are currently under investigation and many aspects remain unclear, but the following mechanism is hypothesized.

[0035] When painted steel sheets are exposed to a corrosive environment, the intermetallic compounds mentioned above dissolve preferentially over the α-Al phase, resulting in a magnesium-rich environment near the formed corrosion products. In such a magnesium-rich environment, the formed corrosion products are less likely to decompose, and as a result, the plating It is estimated that the protective effect of the coating will be enhanced. Furthermore, since the protective effect of the plating film is greater with MgZn2 than with Mg2Si, it is considered effective to increase the proportion of MgZn2 in the intermetallic compounds present in the plating film.

[0036] Furthermore, it is preferable that the ratio of Mg2Si to MgZn2 in the plating film satisfies the relationship (1): Mg2Si (111) / MgZn2(100) ≤ 2.0, using the diffraction peak intensity obtained by X-ray diffraction. If the ratio of Mg2Si and MgZn2 in the plating film does not satisfy relationship (1), that is, if Mg2Si (111) / MgZn2(100) > 2.0, then there is a large amount of Mg2Si in the intermetallic compounds present in the plating film, making it difficult to obtain the aforementioned Mg-rich environment near the corrosion products, and thus the protective effect of the plating film becomes less effective.

[0037] Here, in relation (1) above, Mg2Si (111) is the diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Mg2Si, and MgZn2 (100) is the diffraction intensity of the (100) plane (interplanar spacing d = 0.4510 nm) of MgZn2. As a method for measuring Mg2Si (111) and MgZn2 (100) by the aforementioned X-ray diffraction, a portion of the plating film is mechanically scraped off, and X-ray diffraction is performed on the powdered material (powder X-ray diffraction measurement method). For the measurement of diffraction intensity, the diffraction peak intensity of Mg2Si corresponding to the interplanar spacing d = 0.3668 nm and the diffraction peak intensity of MgZn2 corresponding to the interplanar spacing d = 0.4510 nm are measured, and the ratio of these can be calculated to obtain Mg2Si (111) / MgZn2 (100). Furthermore, the amount of plating film required for powder X-ray diffraction measurement (the amount of plating film to be removed) is 0.1g or more, and preferably 0.3g or more, from the viewpoint of accurately measuring Mg2Si (111) and MgZn2 (100). In addition, when removing the plating film, steel sheet components other than the plating film may be included in the powder, but these intermetallic compound phases are contained only in the plating film and do not affect the peak intensity mentioned above. Moreover, the reason for performing X-ray diffraction on the plating film in powder form is that if X-ray diffraction is performed on the plating film formed on the plated steel sheet, it is difficult to calculate the correct phase ratio due to the influence of the surface orientation of the solidification structure of the plating film.

[0038] Furthermore, in the coated steel sheet of the present invention, it is preferable that the diffraction intensity of Si in the plating film, measured by X-ray diffraction, satisfies the following relationship (2), since the corrosion resistance can be improved more stably by controlling the concentrations of Al, Zn, Si, Mg, and Ni as an unavoidable impurity as described above. Si (111) = 0 ···(2) Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3135 nm) of Si Generally, in the dissolution reaction of Al alloys in aqueous solutions, the presence of the Si phase as a cathode site is known to promote the dissolution of the surrounding α-Al phase. Therefore, reducing the amount of Si phase is effective in suppressing the dissolution of the α-Al phase, and among these methods, creating a film without any Si phase (making the diffraction peak intensity of Si(111) zero), as shown in relation (2), is the best for stabilizing corrosion resistance. Furthermore, the method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction can be the same as the method for measuring Mg2Si (111) and MgZn2 (100) described above.

[0039] Here, the method for satisfying the above-mentioned relationships (1) and (2) is not particularly limited. For example, in order to satisfy relationships (1) and (2), the relative abundance of Mg2Si, MgZn2, and Si (diffraction intensities of Mg2Si (111), MgZn2 (100), and Si (111)) can be controlled by adjusting the balance of the Si content, Mg content, and Al content in the plating film. The balance of the Si content, Mg content, and Al content in the plating film does not necessarily satisfy relationships (1) and (2) by setting the content ratio to a constant ratio; for example, it may be necessary to change the content ratio of Mg and Al depending on the Si content (mass%). Furthermore, the balance of Si content, Mg content, and Al content in the plating film is adjusted. In addition, by adjusting the conditions during plating film formation (for example, cooling conditions after plating), the diffraction intensities of Mg2Si(111), MgZn2(100), and Si(111) can be controlled to satisfy relationships (1) and (2).

[0040] Furthermore, in the painted steel sheet of the present invention, it is preferable that the plating film contains 0.01 to 1.0% by mass of Sr. By including Sr in the plating film, the occurrence of surface defects such as wrinkle-like unevenness defects can be more reliably suppressed, and a good surface appearance can be achieved. The aforementioned wrinkle-like defects are defects that appear as wrinkle-like irregularities on the surface of the plating film, and are observed as whitish streaks on the surface of the plating film. Such wrinkle-like defects are more likely to occur when a large amount of Mg is added to the plating film. Therefore, in the hot-dip galvanized steel sheet, by including Sr in the plating film, Sr is preferentially oxidized over Mg on the surface layer of the plating film, thereby suppressing the oxidation reaction of Mg and making it possible to suppress the occurrence of the aforementioned wrinkle-like defects.

[0041] Furthermore, in the coated steel sheet of the present invention, it is preferable that the ratio of Mg2Si and MgZn2 in the plating film satisfies relation (1) and that the plating film contains 0.01 to 1.0 mass% of Sr. This is because the effect of improving the surface appearance due to Sr described above can be enjoyed to a greater extent. Although the reason for this is not clear, it is presumed that if the amount of Mg2Si in the plating film is high, oxidation of the plating surface layer is not easily suppressed in the first place, which affects the effect of improving the appearance when Sr is added. If the Sr content in the plating film is less than 0.01 mass%, it is difficult to obtain the effect of suppressing the occurrence of the wrinkle-like defects described above, and if the Sr content in the plating film exceeds 1.0 mass%, Sr may be excessively incorporated into the interface alloy layer, which may affect the plating adhesion and other aspects more than the effect of improving the appearance. Therefore, it is preferable that the Sr content in the plating film be 0.01 to 1.0 mass%.

[0042] Furthermore, the plating film, like the Mg mentioned above, can improve the stability of corrosion products and slow down the progression of corrosion. Therefore, it is preferable that it further contains one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Co, Sb, and B in a total amount of 0.01 to 10% by mass. The reason for setting the total content of the above-mentioned components to 0.01 to 10% by mass is that a sufficient corrosion delaying effect can be obtained without the effect becoming saturated.

[0043] Furthermore, the amount of the plating film applied is 45 to 120 g / m² per side, from the viewpoint of satisfying various characteristics. 2 Preferably, the amount of the plating film is 45 g / m². 2 In the above case, sufficient corrosion resistance can be obtained even for applications requiring long-term corrosion resistance, such as building materials, and the amount of the plating film attached is 120 g / m². 2 In the following cases, excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. From a similar viewpoint, the amount of plating film attached is 45-100 g / m². 2 It is preferable that it be so.

[0044] The amount of plating film attached can be determined, for example, by dissolving and peeling the plating film from a specific area using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and calculating the amount from the difference in steel plate weight before and after peeling. To determine the amount of plating attached to one side using this method, the plated surface of the non-target side can be sealed with tape to prevent exposure, and then the aforementioned dissolution process can be carried out.

[0045] Furthermore, the component composition of the plating film can be determined, for example, by immersing the plating film in hydrochloric acid or the like to dissolve it, and then confirming the solution using ICP emission spectrometry or atomic absorption spectrometry. This method is merely one example, and any method that can accurately quantify the component composition of the plating film is acceptable and is not particularly limited.

[0046] Furthermore, the plating film on the painted steel sheet obtained by the present invention is, overall, a product of the plating bath composition and They become almost equivalent. Therefore, the composition of the plating film can be controlled with high precision by controlling the composition of the plating bath.

[0047] Furthermore, the base steel sheet constituting the painted steel sheet of the present invention is not particularly limited, and cold-rolled steel sheets, hot-rolled steel sheets, etc., can be used as appropriate depending on the required performance and specifications.

[0048] Furthermore, there are no particular limitations on the method for obtaining the base steel sheet. For example, in the case of hot-rolled steel sheets, those that have undergone a hot-rolling process and a pickling process can be used, and in the case of cold-rolled steel sheets, a cold-rolling process can be added to the manufacturing process. In addition, it is possible to go through a recrystallization annealing process or the like before the hot-dip galvanizing process in order to obtain the properties of the steel sheet.

[0049] Furthermore, the conditions for forming the plating film when manufacturing the painted steel sheet of the present invention are not particularly limited to performing a hot-dip galvanizing process in which the base steel sheet is immersed in a plating bath. The hot-dip galvanizing process can be carried out, for example, by washing, heating, and immersing the base steel sheet in a plating bath using a continuous hot-dip galvanizing facility. In the heating process of the steel sheet, it is effective to perform recrystallization annealing or the like to control the structure of the base steel sheet itself, and to heat in a reducing atmosphere such as a nitrogen-hydrogen atmosphere in order to prevent oxidation of the steel sheet and reduce the trace amount of oxide film present on the surface.

[0050] Furthermore, as mentioned above, since the composition of the plated film is approximately the same as that of the plating bath, a plating bath containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities, can be used for the plating bath used in the molten plating process.

[0051] Furthermore, in the hot-dip plating process, the Ni content in the unavoidable impurities of the plating bath must be controlled to 0.010% by mass or less relative to the total mass of the plating bath. As mentioned above, Ni contained in the plating film may degrade the corrosion resistance of the hot-dip Al-Zn-Si-Mg plated steel sheet. Therefore, by appropriately controlling the content of Al, Zn, Si, and Mg in the plating bath, and further suppressing the Ni content as an unavoidable impurity, the degradation of corrosion resistance can be suppressed. Furthermore, the Ni content in the plating bath as an unavoidable impurity must be controlled to 0.010% by mass or less relative to the total mass of the plating bath, and preferably to 0.005% by mass or less. If the Ni content in the plating bath exceeds 0.005% by mass, the corrosion resistance of the manufactured molten Al-Zn-Si-Mg plated steel sheet may deteriorate, and if it exceeds 0.010%, a significant deterioration in corrosion resistance may occur. There is no lower limit for the Ni content that adversely affects corrosion resistance.

[0052] The means for reducing the Ni content in the plating bath are not particularly limited. For example, since it is effective to suppress the elution of stainless steel bath equipment into the plating bath, it is preferable to treat the surface of the bath equipment with a thermal spray coating or the like. This is because the formation of the thermal spray coating or the like can impart corrosion resistance to the plating bath to the bath equipment, thereby suppressing the elution of the bath equipment into the plating bath. The type of thermal spray coating is not particularly limited, but a coating with heat resistance and corrosion resistance such as WC-based or MoB-based coatings can be selected. Furthermore, it is more effective to use bath equipment made of heat-resistant material that does not contain Ni. In this case, even if the bath equipment elutes, an increase in Ni content can be prevented.

[0053] Furthermore, as another means of reducing the Ni content in the plating bath, it is preferable to use a metal ingot with a low Ni content among impurities as the raw material for the plating bath. Furthermore, it is also effective not to use pots and bath equipment used in the production of plated steel sheets with intentionally added Ni for the production of molten Al-Zn-Si-Mg plated steel sheets. This is because it prevents metal lumps containing Ni that are attached to the surface from dissolving and mixing into the plating bath.

[0054] Furthermore, while the temperature of the plating bath is not particularly limited, it is preferable to set it in the temperature range of (melting point + 20°C) to 650°C. The reason the lower limit of the bath temperature is set to the melting point + 20°C is that in order to perform the molten plating process, it is necessary to raise the bath temperature above the solidification point, and setting it to the melting point + 20°C prevents solidification due to a localized drop in the bath temperature of the plating bath. On the other hand, the reason the upper limit of the bath temperature is set to 650°C is that if it exceeds 650°C, rapid cooling of the plating film becomes difficult, and there is a risk that the interfacial alloy layer formed between the plating film and the steel sheet will become thicker.

[0055] Furthermore, while there are no particular limitations on the temperature of the base steel plate that enters the plating bath (entry plate temperature), it is preferable to control it to within ±20°C of the temperature of the plating bath from the viewpoint of ensuring plating characteristics and preventing changes in the bath temperature during continuous hot-dip galvanizing operations.

[0056] Furthermore, the immersion time of the steel plate in the plating bath is 0.5 seconds or more. This is because if it is less than 0.5 seconds, there is a risk that a sufficient plating film may not be formed on the surface of the base steel plate. There is no particular upper limit to the immersion time, but since a longer immersion time may result in a thicker interfacial alloy layer formed between the plating film and the steel plate, it is preferable to keep it within 8 seconds.

[0057] (Chemical conversion coating) The painted steel sheet of the present invention can have a chemical conversion film formed on the above-described plating film. Furthermore, the chemical conversion coating only needs to be formed on at least one side of the painted steel sheet, but it can also be formed on both sides of the painted steel sheet depending on the application and required performance.

[0058] Furthermore, the painted steel sheet of the present invention is characterized in that the chemical conversion film contains a resin component comprising (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton in total at a concentration of 30 to 50% by mass, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass ratio, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. By forming the aforementioned chemical conversion coating on the plated film, the strength and adhesion of the chemical conversion coating can be increased while also improving corrosion resistance.

[0059] Here, the resin components constituting the chemical conversion film include (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton.

[0060] (a) The anionic polyurethane resin having an ester bond mentioned above includes resins obtained by copolymerizing a dimethylol alkyl acid with a reaction product of a polyester polyol and a diisocyanate or polyisocyanate having two or more isocyanate groups. Furthermore, a chemical treatment solution can be obtained by dispersing it in a liquid such as water by a known method.

[0061] Examples of the aforementioned polyester polyols include polyesters obtained by a dehydration condensation reaction from a glycol component and an acid component such as an ester-forming derivative of a hydroxylcarboxylic acid, polyesters obtained by a ring-opening polymerization reaction of cyclic ester compounds such as ε-caprolactone, and copolymer polyesters thereof. The aforementioned polyisocyanates include aromatic polyisocyanates and aliphatic polyisocyanates. Examples of aromatic polyisocyanates include alicyclic polyisocyanates. Examples of the aromatic polyisocyanates include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, m-xylene diisocyanate, diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate, 2,2-diphenylmethane diisocyanate, triphenylmethane triisocyanate, polymethylene polyphenyl polyisocyanate, naphthalene diisocyanate, and derivatives thereof (e.g., prepolymers obtained by reaction with polyols, modified polyisocyanates such as carbodiimide compounds of diphenylmethane diisocyanate, etc.).

[0062] Furthermore, when synthesizing urethane by reacting the polyester polyol with the diisocyanate or polyisocyanate, for example, by copolymerizing a dimethylol alkyl acid and self-emulsifying it to make it water-soluble (water-dispersible), the anionic polyurethane resin having an ester bond (a) can be obtained. In this case, examples of dimethylol alkyl acids include dimethylol alkyl acids having 2 to 6 carbon atoms, and more specifically, dimethylolethane acid, dimethylolpropanoic acid, dimethylolbutanoic acid, dimethylolheptanoic acid, and dimethylolhexanoic acid.

[0063] Furthermore, known epoxy resins can be used for the epoxy resin having the bisphenol skeleton (b) described above. Examples include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD ​​type epoxy resin, and bisphenol S type epoxy resin. These epoxy resins can be obtained by reacting bisphenol compounds such as bisphenol A, bisphenol F, bisphenol AD, and bisphenol S with epichlorohydrin in the presence of an alkaline catalyst. In particular, component [A] preferably contains bisphenol A type epoxy resin or bisphenol F type epoxy resin, and more preferably contains bisphenol A type epoxy resin. The epoxy resin having the bisphenol skeleton (b) can be dispersed in a liquid such as water by known methods to obtain a chemical conversion solution.

[0064] The resin component acts as a binder for the chemical conversion coating. The (a) anionic polyurethane resin having ester bonds that constitutes the binder is flexible, which helps to prevent the chemical conversion coating from being damaged (peeled) when processed. The (b) epoxy resin having a bisphenol skeleton improves adhesion to the underlying zinc-plated steel sheet and the upper primer coating. The resin component is present in the chemical conversion film in a total amount of 30 to 50% by mass. If the resin component content is less than 30% by mass, the binder effect of the chemical conversion film decreases, and if it exceeds 50% by mass, the function of the inorganic component shown below, such as the inhibitory effect, decreases. From a similar viewpoint, the resin component content in the chemical conversion film is preferably 35 to 45% by mass.

[0065] Furthermore, the resin component must have a mass ratio ((a):(b)) of (a) anionic polyurethane resin having an ester bond and (b) epoxy resin having a bisphenol skeleton in the range of 3:97 to 60:40. If (a):(b) is outside this range, sufficient corrosion resistance cannot be obtained due to a decrease in flexibility and adhesion as a chemical conversion treatment film. From a similar viewpoint, it is preferable that (a):(b) be 10:90 to 55:45.

[0066] Furthermore, the resin component may include resins other than (a) anionic polyurethane resin having ester bonds and (b) epoxy resin having a bisphenol skeleton, depending on the required performance (other resin components). The other resin component is not particularly limited and may be selected from, for example, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin. At least one or more of these can be used in combination. If the resin component includes other resins, the total content of (a) anionic polyurethane resin having ester bonds and (b) epoxy resin having a bisphenol skeleton is preferably 50% by mass or more, and more preferably 75% by mass or more. This is to ensure a reduction in flexibility and adhesion as a treated film.

[0067] Furthermore, the chemical conversion coating contains, as inorganic compounds, 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. By including these compounds, the corrosion resistance of the chemical conversion coating can be improved.

[0068] The vanadium compound is added to the chemical conversion treatment solution and acts as a rust inhibitor. The presence of the vanadium compound in the chemical conversion coating allows it to dissolve appropriately in a corrosive environment, combining with zinc ions and other plating components that also dissolve in the corrosive environment to form a dense protective film. This formed protective film further enhances corrosion resistance not only to the flat surfaces of the steel plate, but also to defects, damaged areas of the plating film caused by processing, and corrosion progressing from the cut edges to the flat surfaces. Examples of the vanadium compounds include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. In particular, it is desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

[0069] Furthermore, the vanadium compound content in the chemical conversion treatment film is 2 to 10% by mass. If the vanadium compound content in the chemical conversion treatment film is less than 2% by mass, the inhibitory effect is insufficient, leading to a decrease in corrosion resistance. On the other hand, if the vanadium compound content exceeds 10% by mass, it leads to a decrease in the moisture resistance of the chemical conversion treatment film.

[0070] The zirconium compound is contained in the aforementioned chemical conversion coating, and through its reaction with the plating metal and coexistence with the resin component, it is expected to improve the strength and corrosion resistance of the chemical conversion coating. Furthermore, the zirconium compound itself contributes to the formation of a dense chemical conversion coating, and its excellent covering properties allow for a barrier effect. Examples of the aforementioned zirconium compounds include neutralized salts of zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, and zirconium chloride.

[0071] Furthermore, the zirconium compound content in the chemical conversion coating is 40 to 60% by mass. If the zirconium compound content in the chemical conversion coating is less than 40% by mass, it will lead to a decrease in the strength and corrosion resistance of the chemical conversion coating, and if the zirconium compound content exceeds 60% by mass, the chemical conversion coating will become brittle, and when subjected to severe processing, the chemical conversion coating may break or peel off.

[0072] The fluorine compound is contained in the chemical conversion coating and acts as an adhesion enhancer to the plating film. As a result, the corrosion resistance of the chemical conversion coating can be improved. As the fluorine compound, for example, fluoride salts such as ammonium salts, sodium salts, and potassium salts, or fluorine compounds such as ferrous fluoride and ferric fluoride can be used. Among these, it is preferable to use ammonium fluoride or fluoride salts such as sodium fluoride and potassium fluoride.

[0073] Furthermore, the fluorine compound content in the chemical conversion coating is 0.5 to 5% by mass. If the fluorine compound content in the chemical conversion coating is less than 0.5% by mass, sufficient adhesion cannot be obtained in the processed area. Furthermore, if the content of the fluorine compound exceeds 5% by mass, the moisture resistance of the chemical conversion coating decreases.

[0074] Furthermore, the amount of the chemical conversion coating applied is not particularly limited. For example, from the viewpoint of ensuring corrosion resistance more reliably while improving the adhesion of the chemical conversion coating, the amount of the chemical conversion coating applied may be 0.025 to 0.5 g / m². 2 It is preferable to have the amount of the chemical conversion coating attached to be 0.025 g / m². 2 By doing so, corrosion resistance can be more reliably ensured, and the amount of the chemical conversion coating to adhere is 0.5 g / m². 2 The peeling of the chemical conversion coating can be suppressed by doing the following. The amount of the chemical conversion coating can be determined by a method appropriately selected from existing techniques, such as measuring the amount of elements whose content in the coating is known in advance by analyzing the coating with X-ray fluorescence.

[0075] The method for forming the chemical conversion coating is not particularly limited and can be appropriately selected according to the required performance and manufacturing equipment. For example, the coating can be formed by continuously applying a chemical conversion solution to the plating film using a roll coater or the like, and then drying it at a peak metal temperature (PMT) of about 60 to 200°C using hot air or induction heating. In addition to a roll coater, known methods such as airless spraying, electrostatic spraying, and curtain flow coating can be appropriately used for applying the chemical conversion solution. Furthermore, the chemical conversion coating may be a single layer or a multi-layer film, as long as it contains the resin and the metal compound, and is not particularly limited.

[0076] (coating film) As described above, the painted steel sheet of the present invention has a coating film formed on the plating film either directly or via a chemical conversion film, and the coating film has at least a primer coating film.

[0077] Furthermore, the present invention provides that the primer coating film contains a polyester resin having urethane bonds and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide. By including the urethane bonded polyester resin and the inorganic compound in the primer coating, the adhesion of the coating film can be improved while simultaneously enhancing corrosion resistance.

[0078] The aforementioned primer coating contains a polyester resin having urethane bonds as its main component. Because the polyester resin having urethane bonds possesses both flexibility and strength, it is less likely to crack in the primer coating when processed, and its high affinity with chemical conversion coatings containing urethane resin contributes to improving the corrosion resistance of the processed area in particular. In this context, "main component" refers to the component that is present in the highest amount among all the components in the primer coating.

[0079] As the polyester resin having the urethane bond, known resins can be used, such as resins obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups. In addition, a resin obtained by reacting the polyester polyol with the diisocyanate or polyisocyanate in an excess of hydroxyl groups (urethane-modified polyester resin) and curing it with a blocked polyisocyanate can also be used.

[0080] Furthermore, the polyester polyol can be obtained by a known method that utilizes a dehydration condensation reaction between a polyhydric alcohol component and a polybasic acid component. Examples of the polyhydric alcohols include glycols and polyhydric alcohols of three or more valents. Examples of the glycols include ethylene glycol, propylene glycol, and diethylene Examples include glycols, triethylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, neopentyl glycol, hexylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-butyl-2-ethyl-1,3-propanediol, methylpropanediol, cyclohexanedimethanol, 3,3-diethyl-1,5-pentanediol, and the like. Examples of polyhydric alcohols with a valency of 3 or higher include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, and dipentaerythritol. These polyhydric alcohols can be used individually or in combination of two or more. The aforementioned polybasic acid is usually a polycarboxylic acid, but monovalent fatty acids may be used in combination as needed. Examples of the aforementioned polycarboxylic acids include phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, 4-methylhexahydrophthalic acid, bicyclo[2,2,1]heptane-2,3-dicarboxylic acid, trimellitic acid, adipic acid, sebacic acid, succinic acid, azelaic acid, fumaric acid, maleic acid, itaconic acid, pyromellitic acid, dimer acid, and their acid anhydrides, as well as 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, and the like. These polybasic acids can be used alone or in combination of two or more.

[0081] Examples of the polyisocyanates include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate; aromatic diisocyanates such as xylylene diisocyanate (XDI), metaxylylene diisocyanate, tolylene diisocyanate (TDI), and 4,4-diphenylmethane diisocyanate (MDI); cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI; and their adducts, biuretes, and isocyanurates. These polyisocyanates can be used individually or in combination of two or more types.

[0082] Furthermore, the hydroxyl value of the polyester resin having the urethane bond is not particularly limited, but from the viewpoint of solvent resistance, processability, etc., it is preferably 5 to 120 mgKOH / g, more preferably 7 to 100 mgKOH / g, and even more preferably 10 to 80 mgKOH / g. Furthermore, the number-average molecular weight of the polyester resin having the urethane bond is preferably 500 to 15,000, more preferably 700 to 12,000, and even more preferably 800 to 10,000, from the viewpoint of solvent resistance and processability.

[0083] The content of the polyester resin having urethane bonds in the primer coating is preferably 40 to 88% by mass. If the content of the polyester resin having urethane bonds is less than 40% by mass, the binder function of the primer coating may be reduced, while if the content of the polyester resin having urethane bonds exceeds 88% by mass, the function of the inorganic substances shown below, such as inhibitory activity, may be reduced.

[0084] One of the aforementioned inorganic compounds, a vanadium compound, acts as an inhibitor. Examples of the vanadium compound include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. In particular, it is desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation. The vanadium compound added to the primer coating may be the same as or different from the vanadium compound added to the chemical conversion treatment coating. The vanadium compound is absorbed from the outside. It is believed that vanadate ions, which gradually dissolve in the incoming moisture, react with ions on the surface of the zinc-plated steel sheet, forming a highly adhesive passive film that protects the exposed metal and provides rust prevention.

[0085] The vanadium compound content in the primer coating is not particularly limited, but is preferably 4 to 20% by mass from the viewpoint of achieving both corrosion resistance and moisture resistance. If the vanadium compound content is less than 4%, the inhibitor effect may decrease, leading to a decrease in corrosion resistance, and if the vanadium compound content exceeds 20% by mass, it may lead to a decrease in the moisture resistance of the primer coating.

[0086] One of the aforementioned inorganic compounds, a phosphate compound, also acts as an inhibitor. Examples of the phosphate compound include phosphoric acid, ammonium phosphate, alkali metal phosphate, and alkaline earth metal phosphate. In particular, alkali metal phosphates such as calcium phosphate can be preferably used.

[0087] The content of the phosphate compound in the primer coating is not particularly limited, but from the viewpoint of achieving both corrosion resistance and moisture resistance, it is preferably 4 to 20% by mass. If the content of the phosphate compound is less than 4% by mass, the inhibitor effect may decrease, leading to a decrease in corrosion resistance, and if the content of the phosphate compound exceeds 20% by mass, it may lead to a decrease in the moisture resistance of the primer coating.

[0088] Magnesium oxide, one of the aforementioned inorganic compounds, produces a product containing Mg through initial corrosion. This product, as a sparingly soluble magnesium salt, helps to stabilize the compound and improve its corrosion resistance.

[0089] The magnesium oxide content in the primer coating is not particularly limited, but from the viewpoint of achieving both corrosion resistance and corrosion resistance of the processed part, it is preferably 4 to 20% by mass. If the magnesium oxide content is less than 4% by mass, the above effect may decrease, leading to a decrease in corrosion resistance, and if the magnesium oxide content exceeds 20% by mass, the flexibility of the primer coating may decrease, which may reduce the corrosion resistance of the processed part.

[0090] Furthermore, the primer coating may also contain components other than the polyester resin and inorganic compound having the urethane bond described above. For example, a crosslinking agent used when forming a primer coating film can be mentioned. The crosslinking agent reacts with the polyester resin having the urethane bond to form a crosslinked coating film, and examples include oxazoline compounds, epoxy compounds, melamine compounds, isocyanate compounds, carbodiimide compounds, silane coupling compounds, etc., and it is also possible to use two or more types of crosslinking agents in combination. In particular, from the viewpoint of the corrosion resistance of the processed part of the resulting coated steel sheet, blocked polyisocyanate compounds can be used. Examples of blocked polyisocyanates include those obtained by blocking the isocyanate group of a polyisocyanate compound with, for example, alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactam, diketones such as acetoacetate diester, imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

[0091] Furthermore, the primer coating may, if necessary, contain various known components commonly used in the paint industry. Specifically, examples include various surface modifiers such as leveling agents and defoaming agents, various additives such as dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as coloring pigments and extender pigments, glossing agents, curing catalysts, and organic solvents.

[0092] The thickness of the primer coating is preferably 1.5 μm or more. This is because a thickness of 1.5 μm or more of the primer coating makes it possible to more reliably obtain the effect of improving corrosion resistance and the effect of improving adhesion with the topcoat coating formed on the chemical conversion coating or primer coating.

[0093] The method for forming the primer coating is not particularly limited. Furthermore, the coating method for the paint composition constituting the primer coating is preferably one in which the paint composition is applied by methods such as roll coater coating or curtain flow coating. After coating the paint composition, the primer coating can be obtained by baking it using heating means such as hot air heating, infrared heating, or induction heating. The baking process is usually carried out for about 30 seconds to 3 minutes, with a maximum plate temperature of approximately 180 to 270°C.

[0094] Furthermore, it is preferable that the coating film constituting the painted steel sheet of the present invention has a topcoat film formed on top of the primer coating film. In addition to imparting aesthetic qualities such as color, gloss, and surface condition to the painted steel sheet, the aforementioned topcoat film can also enhance various properties such as processability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance.

[0095] The composition of the topcoat film is not particularly limited, and materials, thickness, etc., can be appropriately selected according to the required performance. For example, the topcoat film can be formed using polyester resin-based paints, silicone polyester resin-based paints, polyurethane resin-based paints, acrylic resin-based paints, fluororesin-based paints, and the like. Furthermore, the topcoat film may contain appropriate amounts of titanium dioxide, red iron oxide, mica, carbon black or other coloring pigments; metallic pigments such as aluminum powder and mica; extender pigments consisting of carbonates and sulfates; various fine particles such as silica fine particles, nylon resin beads, and acrylic resin beads; curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate; wax; and other additives.

[0096] Furthermore, the thickness of the topcoat film is preferably 5 to 30 μm from the viewpoint of achieving both good appearance and workability. When the thickness of the topcoat film is 5 μm or more, it becomes possible to more reliably stabilize the color tone and appearance, and when the thickness of the topcoat film is 30 μm or less, it becomes possible to more reliably suppress the decrease in workability (crack formation in the topcoat film).

[0097] The method for applying the paint composition to form the topcoat film is not particularly limited. For example, the paint composition can be applied by methods such as roll coater coating or curtain flow coating. After applying the paint composition, the topcoat film can be formed by baking using heating means such as hot air heating, infrared heating, or induction heating. The baking process can usually be performed for about 30 seconds to 3 minutes at a maximum plate temperature of about 180 to 270°C.

[0098] <Manufacturing method for painted steel sheets> The present invention provides a method for manufacturing a painted steel sheet in which a coating film is formed directly on a plating film or via a chemical conversion film. Furthermore, in the manufacturing method of the present invention, the chemical conversion film contains a resin component comprising (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton in total at a concentration of 30 to 50% by mass, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. The aforementioned coating film has at least a primer coating film, and the primer coating film provides urethane bonding It contains a polyester resin and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide. The formation of the aforementioned plating film comprises a hot-dip plating process in which a base steel plate is immersed in a plating bath having a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities, and the Ni content in the unavoidable impurities of the plating bath is controlled to 0.010% by mass or less.

[0099] As described above, Ni contained in the plating film may deteriorate the corrosion resistance of the molten Al-Zn-Si-Mg alloy plated steel sheet. Therefore, after appropriately controlling the contents of Al, Zn, Si, and Mg in the plating bath, the content of Ni as an inevitable impurity is further suppressed to suppress the deterioration of corrosion resistance.

[0100] The conditions of the molten plating process are the same as those described for the plating film of the coated steel sheet of the present invention. Also, the compositions of the chemical conversion film and the coating film are the same as those described for the chemical conversion film of the coated steel sheet of the present invention.

Examples

[0101] (Samples 1 to 62) Cold-rolled steel sheets with a thickness of 0.8 mm manufactured by a conventional method were used as base steel sheets, and samples 1 to 62 of molten-plated steel sheets with the conditions shown in Table 1 were produced by performing annealing treatment and plating treatment using a molten plating simulator manufactured by Resca Co., Ltd. Regarding the composition of the plating bath used for manufacturing the molten-plated steel sheet, the composition of the plating bath was variously changed in the range of Al: 5 to 75% by mass, Si: 0.0 to 4.5% by mass, Mg: 0 to 10% by mass, and Ni: 0.000 to 0.025% by mass so as to obtain the composition of the plating film of each sample shown in Tables 2 and 3. The bath temperature of the plating bath was 450 °C when Al was 5% by mass, 480 °C when Al was 15% by mass, 590 °C when Al was 30 to 60% by mass, and 630 °C when Al exceeded 60% by mass, and the plating immersion plate temperature of the base steel sheet was controlled to be the same as the bath temperature. Further, when Al was 30 to 60% by mass, the plating treatment was carried out under the condition of cooling in a temperature range of 520 to 500 °C for 3 seconds. Also, the coating amount of the plating film was 85 ± 5 g / m per side for Samples 1 to 59 2 , 50 ± 5 g / m per side for Sample 60 2 , 100 ± 5 g / m per side for Sample 61 2 , 125 ± 5 g / m per side for Sample 62 2 and was controlled to be such.

[0102] (2) Subsequently, the chemical conversion treatment solution shown in Table 1 was applied to the plating film of each sample of the prepared hot-dip galvanized steel sheet using a bar coater, and dried in a hot air drying oven (final plate temperature: 90°C) to achieve an adhesion amount of 0.1 g / m². 2 A chemical conversion coating was formed. The chemical treatment solution used was prepared by dissolving each component in water as a solvent, resulting in a pH of 8-10. The types of components (resin components, inorganic compounds) contained in the chemical treatment solution are as follows. (Resin components) Resin A: A mixture of (a) anionic polyurethane resin having ester bonds ("Superflex 210" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and (b) epoxy resin having a bisphenol skeleton ("Yukarezin RE-1050" manufactured by Yoshimura Yukagaku Co., Ltd.), with a mass ratio of (a):(b) = 50:50. Resin B: Acrylic resin (DIC Corporation's "Boncoat EC-740EF") (Inorganic compound) Vanadium compounds: Organovanadium compounds chelated with acetylacetone Zirconium compounds: Zirconium carbonate ammonium Fluorine compounds: Ammonium fluoride

[0103] (3) Then, a primer coating was applied to the chemical conversion film formed as described above using a bar coater, and the film was baked at a steel plate temperature of 230°C and a baking time of 35 seconds to form a primer coating film having the component composition shown in Table 1. Subsequently, a topcoat coating composition was applied to the primer coating film formed as described above using a bar coater, and the film was baked at a steel plate temperature of 230°C to 260°C and a baking time of 40 seconds to form a topcoat coating film having the resin conditions and film thickness shown in Table 1, thereby producing painted steel plates for each sample. The primer coating was obtained by mixing the components and then stirring them in a ball mill for approximately one hour. The resin components and inorganic compounds that make up the primer coating film are as follows. (Resin components) Resin α: A urethane-modified polyester resin (obtained by reacting 455 parts by mass of polyester resin with 45 parts by mass of isophorone diisocyanate, with a resin acid value of 3, a number average molecular weight of 5,600, and a hydroxyl value of 36) was cured with a blocked isocyanate. The polyester resin to be modified for urethane was prepared under the following conditions: In a flask equipped with a stirrer, rectification column, water separator, condenser, and thermometer, 320 parts by mass of isophthalic acid, 200 parts by mass of adipic acid, 60 parts by mass of trimethylolpropane, and 420 parts by mass of cyclohexanedimethane were charged. The mixture was heated and stirred, and while distilling off the condensation water produced, the temperature was raised from 160°C to 230°C at a constant rate over 4 hours. After reaching 230°C, 20 parts by mass of xylene were gradually added, and the condensation reaction was continued while maintaining the temperature at 230°C. The reaction was terminated when the acid value became 5 or less, and after cooling to 100°C, 120 parts by mass of Solvesso 100 (manufactured by ExxonMobil, trade name, high-boiling-point aromatic hydrocarbon solvent) and 100 parts by mass of butyl cellosolve were added to obtain the polyester resin solution. Resin β: Urethane-cured polyester resin ("Evaclad 4900" manufactured by Kansai Paint Co., Ltd.) (inorganic compound) Vanadium compounds: Magnesium vanadate Phosphate compounds: Calcium phosphate Magnesium oxide compounds: Magnesium oxide Furthermore, the following paints were used for the topcoat coating shown in Table 1. Resin I: Melamine-cured polyester paint (BASF Japan Ltd. "Precolor HD0030HR") Resin II: An organosol-based bake-on fluoropolymer coating (BASF Japan Ltd.'s "Precolor No. 8800HR") in which polyvinylidene fluoride and acrylic resin are in a mass ratio of 80:20.

[0104] [Table 1]

[0105] (evaluation) Each sample of painted steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 2.

[0106] (1) Plating film (composition, amount of coating, Ni-based compound, X-ray diffraction intensity) For each sample of hot-dip galvanized steel sheet, a 100 mm diameter hole was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The amount of plating film attached was calculated from the difference in mass of the sample before and after removal. The calculated amount of plating film attached is shown in Table 2. Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, the filtrate was analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. Furthermore, the solid components were dried and ashed in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the insoluble Si was quantified by ICP emission spectroscopy analysis of the solution. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from solid content analysis to the soluble Si concentration obtained from filtrate analysis. The resulting composition of the plating film is shown in Table 2. Furthermore, after shearing each sample to a size of 15 mm × 15 mm and embedding it in conductive resin so that the cross-section of the steel plate could be observed, mechanical polishing was performed. Then, using a scanning electron microscope (Carl Zeiss ULTRA55), continuous backscattered electron images were taken at a width of 100 μm under an acceleration voltage of 3 kV for a continuous cross-section of an arbitrarily selected plating film with a length of 2 mm or more in the direction parallel to the surface of the underlying steel plate. Furthermore, elemental mapping analysis (Al, Zn, Si, Mg, Fe, Sr, and Ni) of each cross-section was performed using an energy-dispersive X-ray spectrometer (Oxford Instruments Ultim Extreme) under an acceleration voltage of 3 kV within the same apparatus. For areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer under an acceleration voltage of 3 kV, and the substance was identified from the semi-quantitative values ​​of the obtained components. The major axis was measured for all Ni-based compounds confirmed in the observation field, and the largest major axis was determined. Furthermore, the number of Ni-based compound particles present in the observed continuous cross-section was counted and divided by the observed cross-sectional length (mm) to calculate the number of Ni-based compound particles per mm parallel to the surface of the underlying steel plate (particles / mm). For areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer under an acceleration voltage of 3kV, and the substance was identified from the semi-quantitative values ​​of the obtained components. The analysis results are shown in Table 2. In addition, for each sample, after shearing it to a size of 100 mm x 100 mm, the plating film on the evaluation symmetric surface was mechanically scraped off until the underlying steel plate was exposed. After thoroughly mixing the resulting powder, 0.3 g was taken out and qualitative analysis of the powder was performed using an X-ray diffractometer (Rigaku Corporation's "SmartLab") under the following conditions: X-ray used: Cu-Kα (wavelength = 1.54178 Å), Kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4° / min, sampling interval: 0.020°, divergence slit: 2 / 3°, solar slit: 5°, detector: high-speed one-dimensional detector (D / teX Ultra). The diffraction intensity (cps) was calculated by subtracting the base intensity from each peak intensity. The diffraction intensities of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si, the (100) plane (interplanar spacing d=0.4510 nm) of MgZn2, and the (111) plane (interplanar spacing d=0.3135 nm) of Si were measured. The measurement results are shown in Table 2.

[0107] (2) Corrosion resistance evaluation For each sample of painted steel sheet, after shearing to a size of 120 mm x 120 mm, the area within 10 mm of each edge of the surface to be evaluated, as well as the end face of the sample and the surface not to be evaluated, were sealed with tape, leaving the surface to be evaluated exposed at a size of 100 mm x 100 mm. Three identical evaluation samples were prepared. The three evaluation samples prepared as described above were all subjected to accelerated corrosion tests using the cycle shown in Figure 1. The accelerated corrosion test started in a wet state, and every 20 cycles, the sample was removed, washed with water, dried, and then visually inspected to check for the occurrence of red rust on the shear end face of one side that was not tape-sealed. The number of cycles at which red rust was detected was then evaluated according to the following criteria. The evaluation results are shown in Table 2. ◎: Number of cycles for red rust formation in 3 samples ≥ 600 cycles ○: 600 cycles > Number of cycles for rust formation in 3 samples ≥ 380 cycles ×: Number of rust-forming cycles in at least one sample < 380 cycles

[0108] (3) Appearance after painting Each sample of painted steel sheet was observed visually. The observation results were then evaluated according to the following criteria. The evaluation results are shown in Table 2. ◎: No wrinkle-like defects were observed at all. ○: Wrinkle-like defects were observed only within a 50mm radius from the edge. ×: Wrinkle-like defects were observed outside the 50mm range from the edge.

[0109] (5) Workability after painting Each sample of painted steel sheet was sheared to a size of 70 mm x 150 mm, then bent 180° (8T bend) with eight sheets of the same thickness sandwiched inside. After bending, cellophane tape (registered trademark) was firmly applied to the outer surface of the bent section and then peeled off. The surface condition of the coating on the outer surface of the bent section and the presence or absence of coating adhesion (peeling) on ​​the surface of the tape used were visually observed, and the processability was evaluated according to the following criteria. The evaluation results are shown in Table 2. ○: No cracks or peeling are observed in the plating film. △: Cracks are present in the plating film, but no peeling is observed. ×: Both cracks and peeling are observed in the plating film.

[0110] (5) Bath stability During the hot-dip plating process, the condition of the plating bath surface was visually inspected and compared with the plating bath surface used when manufacturing hot-dip Al-Zn plated steel sheets (a bath surface free of Mg-containing oxides). The evaluation was performed according to the following criteria, and the evaluation results are shown in Table 2. ○: Similar to a molten Al-Zn plating bath (55% by mass Al - remainder Zn - 1.6% by mass bath) △: Contains more white oxides compared to molten Al-Zn plating baths (55% by mass Al - remainder Zn - 1.6% by mass bath). ×: Formation of black oxide is observed during plating.

[0111] [Table 2]

[0112] From the results in Table 2, each sample of the present invention compared to each sample of the comparative example showed superior corrosion resistance and coating properties. It is evident that the coating offers a well-balanced and excellent appearance after application, processability after painting, and bath stability. [Industrial applicability]

[0113] According to the present invention, it is possible to provide a painted steel sheet and a method for manufacturing the same that have consistently excellent corrosion resistance and corrosion resistance of processed parts.

Claims

1. A painted steel sheet in which a coating film is formed directly or via a chemical conversion film on a plated film, The aforementioned plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities. The Ni content in the aforementioned unavoidable impurities is 0.010% by mass or less relative to the total mass of the plating film. The plating film contains a Ni-based compound, and the number of Ni-based compounds present in a direction parallel to the surface of the underlying steel sheet is 5 or less per mm. Mg in the aforementioned plating film 2 Si and MgZn 2 The diffraction intensity obtained by X-ray diffraction satisfies the following relationship (1): Mg 2 Si(111) / MgZn 2 (100)≦2.0 ・・・(1) Mg 2 Si (111): Mg 2 Diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Si, MgZn 2 (100): MgZn 2 Diffraction intensity of the (100) plane (interplanar spacing d = 0.4510 nm) The aforementioned chemical conversion film contains a resin component comprising (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton in total at a concentration of 30 to 50% by mass, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. The aforementioned coating film comprises at least a primer coating film, the primer coating film containing a polyester resin having urethane bonds and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide, and is characterized by this, in the painted steel sheet.

2. The painted steel sheet according to claim 1, characterized in that the major axis of the Ni-based compound is 4.0 μm or less.

3. The painted steel sheet according to claim 1 or 2, characterized in that the diffraction intensity of Si in the plating film, measured by X-ray diffraction, satisfies the following relationship (2). Si (111)=0...(2) Si (111): Diffraction intensity of the (111) plane of Si (interplanar spacing d = 0.3135 nm)

4. The painted steel sheet according to any one of claims 1 to 3, characterized in that the plating film further contains Sr: 0.01 to 1.0% by mass.

5. The painted steel sheet according to any one of claims 1 to 4, characterized in that the Al content in the plating film is 50 to 60% by mass.

6. The painted steel sheet according to any one of claims 1 to 5, characterized in that the Si content in the plating film is 1.0 to 3.0% by mass.

7. The painted steel sheet according to any one of claims 1 to 6, characterized in that the Mg content in the plating film is 1.0 to 5.0% by mass.

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