Al-zn-si-mg hot-dipped steel sheet, surface-treated steel sheet, and coated steel sheet

Abstract

The purpose of the present invention is to provide an Al-Zn-Si-Mg hot-dipped steel sheet in which there are no other defects such as dross defects, in which the occurrence of wrinkle-shaped defects is suppressed, and the outer surface of which has an excellent appearance.　In order to achieve this purpose, the present invention is an Al-Zn-Si-Mg hot-dipped steel sheet comprising a plating film, said Al-Zn-Si-Mg hot-dipped steel sheet being characterized in that: the plating film has a composition comprising 45-65 mass% Al, 1.0-3.0 mass% Si, 1.0-10.0 mass% Mg, and 0.01-0.5 mass% Mn, with the remainder being Zn and unavoidable impurities; and in a range excluding 50 mm from both ends of the steel sheet, the height difference of the surface of the steel sheet per 1 mm of length is not more than 10 μm.

Description

Hot-dip Al-Zn-Si-Mg plated steel sheets, surface-treated steel sheets, and painted steel sheets

[0001] The present invention relates to a hot-dip Al-Zn-Si-Mg-plated steel sheet, a surface-treated steel sheet, and a coated steel sheet having an excellent surface appearance.

[0002] Hot-dip Al-Zn-plated steel sheets, typified by 55% Al-Zn-based alloys, are known to exhibit high corrosion resistance among various plated steel sheets because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al, as disclosed in, for example, Patent Document 1. Therefore, due to their excellent corrosion resistance, hot-dip Al-Zn-plated steel sheets are primarily used in the fields of building materials, such as roofs and walls, which are exposed outdoors for long periods of time, and in the fields of civil engineering and construction, such as guardrails, wiring and piping, and soundproof walls. In particular, the demand for materials with excellent corrosion resistance under more severe operating environments, such as acid rain caused by air pollution, the use of deicing agents to prevent road ice in snowy regions, and coastal area development, as well as maintenance-free materials, has been increasing in recent years.

[0003] The coating of hot-dip Al-Zn coated steel sheets is composed of areas where Al containing supersaturated Zn has solidified in the form of dendrites (α-Al phase) and a Zn-Al eutectic structure that exists in the spaces between the dendrites (interdendrites), and is characterized by a structure in which multiple α-Al phases are layered in the thickness direction of the coating. This unique coating structure creates a complex corrosion progression path from the surface, making it difficult for corrosion to progress easily, and it is known that hot-dip Al-Zn coated steel sheets can achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same coating thickness.

[0004] Attempts have been made to further extend the service life of hot-dip Al-Zn-coated steel sheets, and hot-dip Al-Zn-Si-Mg-coated steel sheets containing magnesium have been known. Patent Document 1, for example, discloses a hot-dip Al-Zn-Si-Mg-coated steel sheet having a coating containing an Al-Zn-Si alloy containing magnesium, the Al-Zn-Si alloy containing 45 to 60 wt. % aluminum, 37 to 46 wt. % zinc, and 1.2 to 2.3 wt. % silicon, with the magnesium concentration being 1 to 5 wt. Patent Document 2 also discloses a hot-dip Al-Zn-Si-Mg-coated steel sheet containing 2 to 10 wt. % magnesium and / or 0.01 to 10 wt. % calcium in the coating, thereby improving corrosion resistance and enhancing protection after the steel substrate is exposed. Furthermore, Patent Document 3 discloses a hot-dip Al-Zn-Si-Mg-plated steel sheet which forms a coating layer containing, by mass%, 1 to 15% Mg, 2 to 15% Si, 11 to 25% Zn, and the remainder being Al and unavoidable impurities, and which aims to improve the corrosion resistance of the flat sheet and end faces by controlling the size of intermetallic compounds such as Mg2Si phase and MgZn2 phase present in the plating film to 10 μm or less.

[0005] However, hot-dip Al-Zn-Si-Mg-plated steel sheets have a problem in that the surface appearance is significantly impaired by the occurrence of wrinkle-like irregularity defects, which will be described later.When Mg is added to a molten Al-Zn-Si bath and a steel sheet is plated, Mg is an element that oxidizes more easily than the other plating constituent elements.As a result, during the cooling and solidification process of the plated film, Mg reacts with oxygen in the outermost layer that comes into contact with the air, producing Mg-based oxides. As a result, the Mg concentration near the outermost layer decreases, and to compensate for this, Mg gradually diffuses from the interior of the coating to the outermost layer.As a result, a thick layer of Mg-based oxides forms in the outermost layer before the coating completely solidifies to its interior.This creates a difference in fluidity between the interior of the coating, which is prone to flow before solidification, and the Mg-based oxide layer, and the Mg-based oxide layer cannot keep up with the flow inside the coating, resulting in the formation of wrinkle-like irregularity defects (wrinkle defects) on the coating surface of the hot-dip Al-Zn-Si-Mg-coated steel sheet that is produced.Therefore, there has been a need for the development of a technology to improve wrinkle-like defects in hot-dip Al-Zn-Si-Mg-coated steel sheets.

[0006] Patent Documents 4 and 5 disclose techniques for suppressing the occurrence of wrinkle defects by incorporating Sr into a plating film to suppress the oxidation of Mg in the plating surface layer. These techniques utilize the phenomenon in which Sr is oxidized preferentially over Mg in the surface layer of an unsolidified plating layer released from a plating bath.

[0007] Furthermore, the above-mentioned hot-dip Al-Zn-Si-Mg-plated steel sheets have also had a problem of white rust formation due to corrosion of the plating film when used in a severely corrosive environment. Because this white rust deteriorates the appearance of the steel sheet, plated steel sheets with improved white rust resistance have been developed. Patent Documents 6 and 7 disclose surface-treated steel sheets that have improved white rust resistance by forming a chemical conversion coating containing a urethane resin on the plating film of a hot-dip Al-Zn-Si-Mg-plated steel sheet. However, even in the case of surface-treated steel sheets such as those disclosed in Patent Documents 6 and 7, the deterioration of surface appearance due to the occurrence of the above-mentioned wrinkle defects is still affected by the underlying hot-dip Al-Zn-Si-Mg-plated steel sheet, and therefore, there is still a need to improve the problem of the deterioration of surface appearance due to the occurrence of the above-mentioned wrinkle defects.

[0008] Furthermore, the above-mentioned hot-dip Al-Zn-Si-Mg-plated steel sheets are sometimes used as painted steel sheets, in which a chemical conversion coating or a primer coating is formed on the surface, and various coatings are then formed thereon. Such painted steel sheets are subjected to various processes, including 90-degree or 180-degree bending by press forming, roll forming, or embossing, and are required to have long-term coating durability after use. To meet these requirements, painted zinc-plated steel sheets have typically been applied by subjecting hot-dip Al-Zn-plated steel sheets to a chemical conversion treatment containing chromate, incorporating a chromate-based rust-preventive pigment in the primer coating, and then forming a thermosetting polyester-based resin coating or, for higher weather resistance, a fluororesin coating as a topcoat thereon. However, the use of chromate, an environmentally hazardous substance, in such painted steel sheets has recently been viewed as problematic, and chromate-free painted steel sheets have been strongly desired. For example, chromate-free painted steel sheets have been proposed, such as those disclosed in Patent Document 8.

[0009] Japanese Patent No. 5020228 Japanese Patent No. 5000039 JP 2002-12959 A JP 2000-328214 A International Publication No. 2020 / 179147 JP 2019-155872 A JP 2021-181214 A JP 2005-169765 A

[0010] However, with the techniques disclosed in both documents, it was difficult to suppress the occurrence of the wrinkle defects described above without deteriorating other physical properties.The techniques disclosed in Patent Documents 4 and 5 had a problem in that, during the production of a hot-dip Al-Zn-Si-Mg-plated steel sheet in which Sr is added to the plating film, the surface of the plating bath to which Sr is added is easily oxidized, and the resulting oxide-based dross adheres to the plating film, causing new dross defects and deteriorating the surface appearance.

[0011] Furthermore, although the techniques of Patent Documents 6 and 7 can improve white rust resistance, even in the case of surface-treated steel sheets, they are affected by the underlying hot-dip Al-Zn-Si-Mg-plated steel sheet, and there is still a need to improve the deterioration of surface appearance due to the occurrence of wrinkle-like defects. Furthermore, with the technique of Patent Document 8, it goes without saying that the surface appearance of coated steel sheets, particularly image clarity, is affected by the surface shape of the underlying plated steel sheet. The height difference of the unevenness caused by the occurrence of the above-mentioned wrinkle-like defects and dross defects can reach several tens of μm. Therefore, even if the surface is smoothed by a coating film, the unevenness cannot be completely eliminated, and improvement of the surface appearance of coated steel sheets remains a challenge. In addition, the coating film formed on the convex portions of the plating film has a thin film thickness, which raises concerns about localized deterioration in corrosion resistance. Therefore, there is a need to improve the surface appearance, particularly the surface shape, of the underlying plated steel sheet.

[0012] In view of the above circumstances, an object of the present invention is to provide a hot-dip Al-Zn-Si-Mg-plated steel sheet that is free from other defects such as dross defects, suppresses the occurrence of wrinkle defects, and has an excellent surface appearance. Another object of the present invention is to provide a surface-treated steel sheet that has an excellent surface appearance and white rust resistance, and a coated steel sheet that has an excellent surface appearance and excellent corrosion resistance.

[0013] As a result of investigations aimed at solving the above-mentioned problems, the present inventors have noticed that adding Mn to the coating film of a hot-dip Al-Zn-Si-Mg-plated steel sheet can suppress wrinkle defects without causing dross defects and the like, and have found that the occurrence of wrinkle defects can be suppressed by adjusting the amount of Mn added and minimizing the height difference on the steel sheet surface (coating film surface).

[0014] The present invention was made based on the above findings, and its gist is as follows: 1. A hot-dip Al-Zn-Si-Mg-plated steel sheet having a plating film, wherein the plating film has a composition containing 45 to 65 mass% Al, 1.0 to 3.0 mass% Si, 1.0 to 10.0 mass% Mg, and 0.01 to 0.5 mass% Mn, with the balance being Zn and unavoidable impurities, and wherein the difference in height of the steel sheet surface per mm of length within an area excluding 50 mm from both ends of the steel sheet is 10 μm or less. 2. The hot-dip Al-Zn-Si-Mg-plated steel sheet according to item 1 above, wherein the Mn content in the plating film is 0.1 to 0.3 mass%. 3. 3. The hot-dip Al-Zn-Si-Mg plated steel sheet according to 1 or 2 above, characterized in that an alloy layer containing Mn is provided at the interface between the plated film and the base steel sheet. 4. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any of 1 to 3 above, characterized in that the plated film further contains 0.01 to 3.0 mass% in total of one or more elements selected from B, Ca, Ti, V, Cr, Sr, Mo, In, Sn, Sb, Ce, and Bi. 5. A surface-treated steel sheet comprising the plating film according to any one of 1 to 4 above and a chemical conversion film formed on the plating film, wherein the chemical conversion film contains at least one resin selected from epoxy resins, urethane resins, acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins, and at least one metal compound selected from P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds.6. A coated steel sheet having a coating film formed directly or via a chemical conversion coating on the plating film described in any one of 1 to 4 above, wherein the chemical conversion coating contains a resin component containing 30 to 50 mass% in total of (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton, with the content ratio of (a):(b) being in the range of 3:97 to 60:40 by mass, and an inorganic compound containing 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound, and the coating film has at least a primer coating film containing a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide.

[0015] According to the present invention, it is possible to provide a hot-dip Al-Zn-Si-Mg-plated steel sheet that is free from other defects such as dross defects, suppresses the occurrence of wrinkle defects, and has excellent surface appearance. Furthermore, according to the present invention, it is possible to provide a surface-treated steel sheet that has excellent surface appearance and white rust resistance, and a coated steel sheet that has excellent surface appearance and excellent corrosion resistance.

[0016] <Hot-dip Al-Zn-Si-Mg-plated steel sheet> The hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention has a plating film on the surface of the steel sheet. The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 3.0 mass% Si, 1.0 to 10.0 mass% Mg, and 0.01 to 0.5 mass% Mn, with the balance being Zn and unavoidable impurities.

[0017] The Al content in the plating film is 45 to 65 mass%, preferably 50 to 60 mass%, to balance corrosion resistance and operational efficiency. This is because an Al content of at least 45 mass% in the plating film causes dendritic solidification of Al, resulting in a plating film structure primarily composed of an α-Al phase dendritic solidification structure. The dendritic solidification structure is layered in the thickness direction of the plating film, which complicates the corrosion progression path and improves the corrosion resistance of the plating film itself. Furthermore, the more α-Al phase dendritic portions are layered, the more complex the corrosion progression path becomes, making it more difficult for corrosion to reach the substrate steel sheet, thereby improving corrosion resistance. Therefore, it is preferable to set the Al content to 50 mass% or more. On the other hand, if the Al content in the plating film exceeds 65 mass%, most of the Zn will change to a structure in solid solution in the α-Al, which will make it impossible to suppress the dissolution reaction of the α-Al phase, potentially deteriorating the corrosion resistance of the hot-dip Al-Zn-Si-Mg-plated steel sheet. For this reason, the Al content in the plating film must be 65 mass % or less, and preferably 60 mass % or less.

[0018] The Si content in the plating film is primarily intended to suppress the growth of an Fe-Al and / or Fe-Al-Si interfacial alloy layer at the interface with the substrate steel sheet, thereby preventing deterioration of the adhesion between the plating film and the steel sheet. When a steel sheet is immersed in an Al-Zn plating bath containing Si, an alloying reaction occurs between the Fe on the steel sheet surface and the Al and Si in the bath, resulting in the formation of an Fe-Al and / or Fe-Al-Si intermetallic compound layer at the interface between the substrate steel sheet and the plating film. However, since the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, a higher proportion of the Fe-Al-Si alloy suppresses the growth of the entire interfacial alloy layer. Therefore, the Si content in the plating film must be 1.0 mass% or more. On the other hand, if the Si content in the plating film exceeds 4.0 mass%, not only will the effect of inhibiting the growth of the interfacial alloy layer described above reach saturation, but the presence of excess Si phase in the plating film will also promote corrosion, so the Si content in the plating film is set to 4.0 mass% or less. In addition, from the viewpoint of inhibiting the presence of excess Si phase, the Si content in the plating film is preferably set to 3.0 mass% or less.

[0019] The plating film contains 1.0 to 10.0 mass% Mg. The inclusion of Mg in the plating film allows the aforementioned Si to exist in the form of an intermetallic compound, the Mg2Si phase, thereby suppressing the acceleration of corrosion. Furthermore, the inclusion of Mg in the plating film also forms an intermetallic compound, the MgZn2 phase, in the plating film, further improving corrosion resistance. If the Mg content in the plating film is less than 1.0 mass%, sufficient corrosion resistance cannot be ensured because Mg is used to dissolve in the α-Al phase, the primary phase, rather than to form the intermetallic compounds (Mg2Si, MgZn2). On the other hand, if the Mg content in the plating film is too high, the corrosion resistance improvement effect saturates and the α-Al phase becomes brittle, resulting in reduced workability. Therefore, the content is set to 10.0 mass% or less. Furthermore, the Mg content in the plating film is preferably 5.0 mass% or less from the viewpoints of suppressing dross generation during plating formation, facilitating plating bath management, and further improving surface appearance.

[0020] Furthermore, in the hot-dip Al-Zn-Si-Mg steel sheet of the present invention, the coating contains 0.01 to 0.5 mass% Mn. When the coating contains 0.01 mass% or more Mn, a needle-shaped and / or chunky Mn-Fe-based interfacial alloy layer is formed between the coating and the steel substrate. This alloy layer has an anchoring effect that inhibits the movement of the coating solution film during complete solidification of the coating, thereby suppressing the occurrence of wrinkle-like defects due to Mg-based oxides formed on the surface of the coating. On the other hand, if the Mn content in the coating exceeds 0.5 mass%, the melting point of the coating bath increases, forcing the coating process to be performed at a high bath temperature. This causes the interfacial alloy layer to grow abnormally between the coating and the steel substrate, degrading workability and increasing the amount of dross produced, deteriorating the coating appearance, particularly the surface shape. Furthermore, if the Mn content in the plating film is 0.5% by mass or less, the amount of dross generated in the plating bath is the same as when no Mn is added, and the occurrence of dross defects can be suppressed, resulting in a stable and excellent surface appearance. Therefore, the Mn content in the plating film is set to 0.01 to 0.5% by mass. From the same perspective, the Mn content in the plating film is preferably 0.1 to 0.3% by mass.

[0021] From the viewpoint of a balance between the effects of improving corrosion resistance, suppressing wrinkle defects, and suppressing dross defects, the content of Mg and Mn in the plating film is preferably such that the ratio of the Mn content to the Mg content (Mn / Mg) is, in mass ratio, 0.02 or more (Mn / Mg≧0.02). From the same viewpoint, the ratio of the Mn content to the Mg content (Mn / Mg) is more preferably 0.03 or more (Mn / Mg≧0.03).

[0022] In addition to the aforementioned Al, Si, Mg, and Mn, the plating film also contains Zn and unavoidable impurities. Among these, the unavoidable impurities include Fe. This Fe is inevitably contained in the plating film due to the dissolution of the steel sheet or bath-immersed equipment into the plating bath, and is also supplied by diffusion from the base steel sheet during the formation of the interfacial alloy layer. The Fe content in the plating film is typically approximately 0.3 to 2.0 mass%. Other unavoidable impurities include Ni and Cu. The total content of the unavoidable impurities is not particularly limited, but because excessive content may affect various properties of the plated steel sheet, it is preferable that the total content be 5.0 mass% or less.

[0023] Furthermore, the plating film preferably further contains, as needed, one or more elements selected from B, Ca, Ti, V, Cr, Sr, Mo, In, Sn, Sb, Ce, and Bi in a total amount of 0.1 to 3 mass %. These elements have the effect of improving the stability of corrosion products when the plating film corrodes, thereby delaying the progression of corrosion, and the effect of stabilizing the spangle size on the plating surface, thereby improving the surface appearance.

[0024] The 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 subjecting the resulting solution to ICP emission spectroscopy or atomic absorption spectroscopy. This method is merely an example, and any method that can accurately quantify the composition of the plating film may be used, and is not particularly limited. Furthermore, the plating film of the hot-dip Al-Zn-Si-Mg-plated steel sheet obtained according to the present invention has a composition that is approximately equivalent to that of the plating bath as a whole. Therefore, the composition of the plating film can be controlled with high precision by controlling the composition of the plating bath.

[0025] The hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention is characterized in that the difference in elevation of the coating surface per 1 mm in length is 10 μm or less in a range excluding 50 mm from both ends of the steel sheet surface. When the difference in elevation of the steel sheet surface, i.e., the coating surface, is 10 μm or less, wrinkle-like defects are absent and an excellent surface appearance can be obtained. As described above, wrinkle-like defects are defects in which the surface of the coating film has a wrinkled, uneven shape due to Mg-based oxides, and appear as white streaks on the surface of the coating film. Therefore, suppressing wrinkle-like defects means that the streaks are not visible. Therefore, by keeping the difference in elevation within a 1 mm length of the steel sheet surface to 10 μm or less, streaks are not visible and an excellent surface appearance can be obtained. From the same perspective, it is preferable that the difference in elevation within a 1 mm length of the steel sheet surface is 5 μm or less. The steel sheet surface refers to the outermost surface of the steel sheet, and in the case of a hot-dip Al-Zn-Si-Mg plated steel sheet, refers to the surface of the plated film.

[0026] Here, the height difference on the surface of the steel sheet refers to the difference in height between the highest point and the lowest point on the steel sheet (on the plating film), where the height is defined as the direction perpendicular to the surface of the base steel sheet (in the direction of the plating film thickness). The height difference within a 1 mm length of the steel sheet surface can be obtained, for example, by using a laser microscope to measure the height differences within a 1 mm length at 100 arbitrary points on a hot-dip Al-Zn-Si-Mg-plated steel sheet on which a plating film has been formed, and then calculating the average of the measured values.

[0027] The reason for controlling the height difference on the surface of the steel sheet within a range excluding 50 mm from both ends of the surface of the steel sheet is that, in general, it is difficult to control the coating weight at both ends of a hot-dip galvanized steel sheet compared to the center of the steel sheet, and the cooling rate is also different, so wrinkle defects are likely to occur. In addition, such both ends of the steel sheet are often trimmed before use, so the surface appearance does not pose any problem.

[0028] The method for controlling the height difference of the steel sheet surface per 1 mm of the plating film length to 10 μm or less is not particularly limited, other than the Mn content (0.01 to 0.5 mass%) in the plating film described above. For example, the height difference of the film surface can be reduced by adjusting the content ratio of Mg and Mn in the plating film or by performing other surface treatments to reduce the height difference.

[0029] In addition, in the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, a Mn-containing alloy layer is preferably formed at the interface between the plating film and the steel substrate. The alloy layer is formed by an alloying reaction between Mn and Al, Fe, Si, etc. in the bath, and is primarily an Fe-Al-Mn or Fe-Al-Si-Mn intermetallic compound. The layer has an irregular shape in the direction of the plating surface, which can improve adhesion between the plating film and the steel substrate by providing an anchoring effect. Furthermore, the anchoring effect of the Mn-containing alloy layer is realized when the plating film solidifies, and the Mn-based interfacial alloy layer acts to minimize the movement of the plating solution film, thereby suppressing the occurrence of wrinkle-like defects.

[0030] Furthermore, in order to impart excellent corrosion resistance to the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, it is preferable that the diffraction intensities of Si and Mg2Si in the plating film, as measured by X-ray diffraction, satisfy the following relationship (1): Si (111) / Mg2Si (111)≦0.8 (1) Si (111): Diffraction intensity of the Si (111) plane (planar spacing d = 0.3135 nm), Mg2Si (111): Diffraction intensity of the Mg2Si (111) plane (planar spacing d = 0.3668 nm). As described above, in the present invention, it is preferable to control the abundance ratios of the Mg2Si and Si phases formed in the plating film due to the inclusion of Mg and Si to specific ratios. While ongoing investigations are currently underway into the effects of these phases on corrosion resistance, many aspects remain unclear, the following mechanism is presumed. When a hot-dip Al-Zn-Si-Mg-based plating film is exposed to a corrosive environment, the intermetallic compounds dissolve preferentially over the α-Al phase, resulting in a Mg-rich environment near the corrosion products that form. It is believed that in such a Mg-rich environment, the corrosion products that form are less likely to decompose, thereby enhancing the protective effect of the plating film. Furthermore, this enhanced protective effect of the plating film is more reliably achieved when the Si in the plating film exists as the Mg2Si phase rather than the Si phase. Therefore, it is believed that reducing the ratio of the Si phase to the Mg2Si phase is effective.

[0031] The abundance ratio of Mg2Si to Si in the plating film preferably satisfies the relationship (1): Si(111) / Mg2Si(111)≦0.8, as determined using diffraction peak intensities obtained by X-ray diffraction. However, if the abundance ratio of Mg2Si to Si in the plating film does not satisfy the relationship (1), i.e., Si(111) / Mg2Si(111)>0.8, the Si phase present in the plating film increases, making it difficult to obtain the aforementioned Mg-rich environment near the corrosion products, and thus making it difficult to achieve the improved protective effect of the plating film. From the same perspective, the abundance ratio of Si to Mg2Si (Si(111) / Mg2Si(111)) is more preferably 0.5 or less, even more preferably 0.3 or less, and particularly preferably 0.2 or less. Here, in the above relationship (1), Si (111) is the diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm), and Mg2Si (111) is the diffraction intensity of the Mg2Si (111) plane (plane spacing d = 0.3668 nm).

[0032] The Si(111) and MgSi(111) compositions can be measured by X-ray diffraction (powder X-ray diffraction measurement) by mechanically scraping a portion of the plating film and powdering it. The Si(111) / MgSi(111) ratio can be calculated by measuring the Si diffraction peak intensity corresponding to the interplanar spacing d = 0.3135 nm and the MgSi diffraction peak intensity corresponding to the interplanar spacing d = 0.3668 nm. The amount of plating film required for powder X-ray diffraction measurement (the amount of plating film scraped off) is 0.1 g or more, preferably 0.3 g or more, from the viewpoint of accurately measuring Si(111) and MgSi(111). Furthermore, when the plating film is scraped off, steel sheet components other than the plating film may be contained in the powder. However, these intermetallic compound phases are contained only in the plating film and do not affect the peak intensities described above. Furthermore, the reason why the plating film is powdered and subjected to X-ray diffraction is that if X-ray diffraction is performed on a plating film formed on a plated steel sheet, it will be affected by the plane orientation of the plating film solidification structure, making it difficult to accurately calculate the phase ratio.

[0033] Furthermore, in the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, in order to more stably improve corrosion resistance, it is preferable that the diffraction intensity of Si in the plating film by X-ray diffraction satisfy the following relationship (2): Si(111)=0 (2) Si(111): Diffraction intensity of the Si(111) plane (planar spacing d=0.3135 nm). It is generally known that the presence of Si phase as a cathode site promotes the dissolution of the surrounding α-Al phase during the dissolution reaction of an Al alloy in an aqueous solution. Therefore, reducing the Si phase is also effective in suppressing the dissolution of the α-Al phase. Among these, achieving a film free of Si phase, as shown in relationship (2) (i.e., zero diffraction peak intensity of the Si(111) plane), is the most effective for stabilizing corrosion resistance. The method for measuring the diffraction peak intensity of the Si(111) plane by X-ray diffraction is as described above.

[0034] Here, the method for satisfying the above-described relationship (1) or (2) is not particularly limited. For example, to satisfy relationship (1) or (2), the abundance ratio of MgSi and Si (diffraction intensity of MgSi(111) and Si(111)) can be controlled by adjusting the balance of the Si content, Mg content, and Al content in the plating film. Note that the balance of the Si content, Mg content, and Al content in the plating film does not necessarily satisfy relationship (1) or (2). For example, it is necessary to change the Mg and Al content ratio depending on the Si content (mass %). In addition to adjusting the balance of the Si content, Mg content, and Al content in the plating film, the diffraction intensity of MgSi(111) and Si(111) can be controlled so as to satisfy relationship (1) or (2) by adjusting the conditions during plating film formation (e.g., cooling conditions after plating).

[0035] The coating weight of the plating film is set to 45 to 120 g / m per side in order to satisfy various characteristics. 2 It is preferable that the coating weight of the plating film is 45 g / m 2 In the above cases, sufficient corrosion resistance can be obtained for applications requiring long-term corrosion resistance, such as building materials, and the coating weight of the plating film is 120 g / m 2 This is because, in the following cases, it is possible to achieve excellent corrosion resistance while suppressing the occurrence of plating cracks during processing. From the same viewpoint, the coating weight of the plating film is 45 to 100 g / m 2 It is more preferable that the coating weight of the plating film is determined by, for example, a method as specified in JIS H 0401:2013, in which a specific area of ​​the plating film is dissolved and stripped using a mixed solution of hydrochloric acid and hexamethylenetetramine, and the coating weight is calculated from the difference in weight of the steel sheet before and after stripping. To determine the coating weight per side using this method, the non-target side can be sealed with tape so that the plated surface is not exposed, and then the dissolution described above is carried out.

[0036] Furthermore, the base steel sheet constituting the hot-dip Al-Zn-Si-Mg-plated 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. The method for obtaining the base steel sheet is also not particularly limited. For example, in the case of the hot-rolled steel sheet, a steel sheet that has been subjected to a hot-rolling process and a pickling process can be used, and in the case of the cold-rolled steel sheet, a steel sheet can be produced by further adding a cold-rolling process. Furthermore, in order to obtain the desired steel sheet properties, a recrystallization annealing process or the like can also be performed before the hot-dip plating process.

[0037] The method for producing the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention is not particularly limited. For example, the steel sheet can be produced by cleaning, heating, and immersing the base steel sheet in a coating bath using continuous hot-dip galvanizing equipment. In the steel sheet heating process, recrystallization annealing or the like is performed to control the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective in preventing oxidation of the steel sheet and reducing the small amount of oxide film present on the surface.

[0038] Furthermore, as for the coating bath used in producing the hot-dip Al-Zn-Si-Mg coated steel sheet of the present invention, as described above, the coating film will have a composition that is substantially the same as the coating bath as a whole. Therefore, the coating bath may have a composition that contains 45 to 65 mass% Al, 1.0 to 3.0 mass% Si, 1.0 to 10.0 mass% Mg, and 0.1 to 0.5 mass% Mn, with the balance being Zn, Fe, and unavoidable impurities.

[0039] Furthermore, the bath temperature of the coating bath is not particularly limited, but is preferably in the range of (melting point + 20°C) to 650°C. The reason why the lower limit of the bath temperature is set to melting point + 20°C is that the bath temperature needs to be at or above the solidification point in order to perform hot-dip coating, and setting the bath temperature to melting point + 20°C prevents solidification due to a local drop in the bath temperature of the coating bath. On the other hand, the reason why the upper limit of the bath temperature is set to 650°C is that if the bath temperature exceeds 650°C, it becomes difficult to rapidly cool the coating film, and there is a risk that the interfacial alloy layer formed between the coating film and the steel sheet will become thick.

[0040] Furthermore, the temperature of the substrate steel sheet immersed in the coating bath (immersion sheet temperature) is not particularly limited, but is preferably controlled to within ±20°C of the coating bath temperature from the viewpoint of ensuring coating characteristics in continuous hot-dip coating operations and preventing changes in bath temperature. Furthermore, the immersion time of the substrate steel sheet in the coating bath is 0.5 seconds or more. This is because if it is less than 0.5 seconds, it may be difficult to form a sufficient coating film on the surface of the substrate steel sheet. There is no particular upper limit to the immersion time, but since a longer immersion time may result in a thicker interfacial alloy layer being formed between the coating film and the steel sheet, it is preferably 8 seconds or less.

[0041] Depending on the required performance, a coating film can be formed on the hot-dip Al-Zn-Si-Mg-plated steel sheet directly or via an intermediate layer. The method for forming the coating film is not particularly limited and can be appropriately selected depending on the required performance. Examples include roll coater coating, curtain flow coating, and spray coating. After applying a coating material containing an organic resin, the coating film can be formed by heating and drying the coating using means such as hot air drying, infrared heating, and induction heating. The intermediate layer is also not particularly limited, as long as it is a layer formed between the coating film of the hot-dip plated steel sheet and the coating film.

[0042] <Surface-treated steel sheet> The surface-treated steel sheet of the present invention comprises a plating film on the surface of the steel sheet and a chemical conversion film formed on the plating film. The composition of the plating film is the same as that of the plating film of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention described above. In addition, other configurations of the plating film are also the same as those of the plating film of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention described above.

[0043] The surface-treated steel sheet of the present invention has a chemical conversion coating formed on the coating. The chemical conversion coating may be formed on at least one surface of the surface-treated steel sheet, and may also be formed on both surfaces of the surface-treated steel sheet depending on the application and required performance.

[0044] The surface-treated steel sheet of the present invention is characterized in that the chemical conversion coating contains at least one resin selected from the group consisting of epoxy resins, urethane resins, acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins, and at least one metal compound selected from the group consisting of P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds. Forming the above-described chemical conversion coating on a plated coating improves affinity with the plated coating, enabling the chemical conversion coating to be formed uniformly on the plated coating, and also enhancing the rust prevention and barrier effects of the chemical conversion coating. As a result, the surface-treated steel sheet of the present invention can achieve stable corrosion resistance and white rust resistance.

[0045] Here, the resin constituting the chemical conversion coating is at least one selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin from the viewpoint of improving corrosion resistance. From the same viewpoint, the resin preferably contains at least one of urethane resin and acrylic resin. The resin constituting the chemical conversion coating also includes addition polymers of the above-mentioned resins.

[0046] Examples of the epoxy resin that can be used include glycidyl etherified epoxy resins such as bisphenol A type, bisphenol F type, and novolac type epoxy resins; glycidyl etherified bisphenol A type epoxy resins to which propylene oxide, ethylene oxide, or polyalkylene glycol has been added; aliphatic epoxy resins; alicyclic epoxy resins; and polyether-based epoxy resins.

[0047] As the urethane resin, for example, an oil-modified polyurethane resin, an alkyd-based polyurethane resin, a polyester-based polyurethane resin, a polyether-based polyurethane resin, a polycarbonate-based polyurethane resin, or the like can be used.

[0048] Examples of the acrylic resin include polyacrylic acid and copolymers thereof, polyacrylic acid esters and copolymers thereof, polymethacrylic acid and copolymers thereof, polymethacrylic acid esters and copolymers thereof, urethane-acrylic acid copolymers (or urethane-modified acrylic resins), and styrene-acrylic acid copolymers. Furthermore, these resins may be modified with other alkyd resins, epoxy resins, phenolic resins, or the like.

[0049] The acrylic silicone resin may be, for example, a resin containing a hydrolyzable alkoxysilyl group at the side chain or end of an acrylic copolymer as a main component, to which a curing agent is added. In addition, when an acrylic silicone resin is used, excellent weather resistance can be expected in addition to corrosion resistance.

[0050] Examples of the alkyd resin include oil-modified alkyd resins, rosin-modified alkyd resins, phenol-modified alkyd resins, styrenated alkyd resins, silicon-modified alkyd resins, acrylic-modified alkyd resins, oil-free alkyd resins, and high-molecular-weight oil-free alkyd resins.

[0051] The polyester resin is a polycondensate synthesized by dehydration condensation of a polycarboxylic acid and a polyalcohol to form an ester bond. Examples of the polycarboxylic acid include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of the polyalcohol include ethylene glycol, 1,3-propanediol, 1,4-butanediol, and 1,4-cyclohexanedimethanol. Specific examples of the polyester include polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, and polybutylene naphthalate. Acrylic-modified versions of these polyester resins can also be used.

[0052] Examples of the polyalkylene resin include ethylene-based copolymers such as ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, and carboxyl-modified polyolefin resins, ethylene-unsaturated carboxylic acid copolymers, and ethylene-based ionomers. Furthermore, these resins may be modified with other alkyd resins, epoxy resins, phenolic resins, or the like.

[0053] The amino resin is a thermosetting resin produced by the reaction of an amine or amide compound with an aldehyde, and examples thereof include melamine resin, guanamine resin, and thiourea resin. Melamine resin is preferred from the viewpoints of corrosion resistance, weather resistance, adhesion, etc. The melamine resin is not particularly limited, but examples thereof include butylated melamine resin, methylated melamine resin, and aqueous melamine resin.

[0054] Examples of the fluororesin include fluoroolefin polymers and copolymers of fluoroolefins with alkyl vinyl ethers, cycloalkyl vinyl ethers, carboxylic acid-modified vinyl esters, hydroxyalkyl allyl ethers, tetrafluoropropyl vinyl ethers, etc. When these fluororesins are used, not only corrosion resistance but also excellent weather resistance and excellent hydrophobicity can be expected.

[0055] Furthermore, in order to improve corrosion resistance and processability, it is particularly preferable to use a curing agent for the resin constituting the chemical conversion coating. Examples of curing agents that can be used include urea resins (butylated urea resins, etc.), melamine resins (butylated melamine resins, butyl etherified melamine resins, etc.), butylated urea-melamine resins, amino resins such as benzoguanamine resins, blocked isocyanates, oxazoline compounds, and phenolic resins.

[0056] The metal compound constituting the chemical conversion coating is at least one selected from the group consisting of P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds. From the same viewpoint, it is preferable that the metal compound contains at least one of P compounds, Si compounds, and V compounds.

[0057] The P compound contained in the chemical conversion coating can improve corrosion resistance and sweat resistance. The P compound is a compound containing P, and may contain, for example, one or more selected from inorganic phosphoric acid, organic phosphoric acid, and salts thereof.

[0058] The inorganic phosphoric acid, organic phosphoric acid, and salts thereof are not particularly limited and any compound can be used. For example, the inorganic phosphoric acid is preferably one or more selected from phosphoric acid, monophosphate, diphosphate, triphosphate, pyrophosphoric acid, pyrophosphate, tripolyphosphoric acid, tripolyphosphate, phosphorous acid, phosphite, hypophosphorous acid, and hypophosphite. The organic phosphoric acid is preferably phosphonic acid (phosphonic acid compound). The phosphonic acid is preferably one or more selected from nitrilotrismethylenephosphonic acid, phosphonobutanetricarboxylic acid, methyldiphosphonic acid, methylenephosphonic acid, and ethylidenediphosphonic acid. When the P compound is a salt, the salt is preferably a salt of an element from Groups 1 to 13 of the periodic table, more preferably a metal salt, and preferably one or more selected from alkali metal salts and alkaline earth metal salts.

[0059] When a chemical conversion treatment solution containing the above-mentioned P compound is applied to a plated steel sheet, the P compound acts to etch the surface of the plated film, forming a concentrated layer containing Al, Zn, Si, and Mg, the constituent elements of the plated film, on the side of the plated film. The formation of this concentrated layer strengthens the bond between the chemical conversion film and the plated film surface, improving the adhesion of the chemical conversion film. The concentration of the P compound in the chemical conversion treatment solution is not particularly limited, but can be 0.25% to 5% by mass. A concentration of the P compound less than 0.25% by mass results in an insufficient etching effect, reducing adhesion to the plated film interface and reducing corrosion resistance of flat surfaces. This may also reduce the corrosion resistance and sweat resistance of defects, cut edges, and damaged areas of the plated film or film caused by processing. From the same perspective, the concentration of the P compound is preferably 0.35% by mass or more, more preferably 0.50% by mass or more. On the other hand, if the concentration of the P compound exceeds 5 mass %, not only will the life of the chemical conversion treatment solution be shortened, but the appearance of the formed film will likely be uneven, and the amount of P eluted from the chemical conversion film will increase, which may reduce resistance to blackening. From the same perspective, the concentration of the P compound is preferably 3.5 mass % or less, and more preferably 2.5 mass % or less. Regarding the content of the P compound in the chemical conversion film, for example, by applying and drying a chemical conversion treatment solution with a P compound concentration of 0.25 mass % to 5 mass %, the amount of P deposited in the dried chemical conversion film will be 5 to 100 mg / m 2 It can be said that:

[0060] The Si compound is a component that forms the framework of the chemical conversion coating together with the resin, and enhances the affinity with the plating coating, enabling the formation of a uniform chemical conversion coating. The Si compound is a compound containing Si, and preferably contains one or more selected from, for example, silica, trialkoxysilane, tetraalkoxysilane, and a silane coupling agent.

[0061] The silica is not particularly limited and any silica can be used. For example, at least one of wet silica and dry silica can be used. As colloidal silica, which is a type of wet silica, for example, Snowtex O, C, N, S, 20, OS, OXS, NS, etc. manufactured by Nissan Chemical Industries, Ltd. can be suitably used. Furthermore, as the dry silica, for example, AEROSIL 50, 130, 200, 300, 380, etc. manufactured by Nippon Aerosil Co., Ltd. can be suitably used.

[0062] The trialkoxysilane is not particularly limited and any trialkoxysilane can be used. For example, it is preferable to use a trialkoxysilane represented by the general formula: R1Si(OR2)3 (wherein R1 is hydrogen or an alkyl group having 1 to 5 carbon atoms, and R2 is the same or different alkyl group having 1 to 5 carbon atoms). Examples of such trialkoxysilanes include trimethoxysilane, triethoxysilane, and methyltriethoxysilane.

[0063] The tetraalkoxysilane is not particularly limited and any tetraalkoxysilane can be used. For example, it is preferable to use a tetraalkoxysilane represented by the general formula: Si(OR)4 (wherein R is the same or different alkyl group having 1 to 5 carbon atoms). Examples of such tetraalkoxysilane include tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane.

[0064] The silane coupling agent is not particularly limited and any one can be used, for example, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropylmethyldiethoxysilane, γ-aminopropyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ-mercaptopropyltrimethoxysilane, vinyltriethoxysilane, γ-isocyanatepropyltriethoxysilane, etc.

[0065] Incorporating the Si compound into a chemical conversion coating causes the Si compound to undergo dehydration condensation, forming an amorphous chemical conversion coating with siloxane bonds that provides a high barrier effect for blocking corrosion factors. Furthermore, bonding with the resin described above results in the formation of a chemical conversion coating with even higher barrier properties. Furthermore, in a corrosive environment, dense and stable corrosion products are formed in defects or damaged areas of the plating or coating caused by processing, and the combined effect of the Si compound and the plating coating also inhibits corrosion of the substrate steel sheet. From the viewpoint of a high effect of forming stable corrosion products, it is preferable to use at least one of colloidal silica and fumed silica as the Si compound.

[0066] The concentration of the Si compound in the chemical conversion treatment solution for forming the chemical conversion coating is 0.2% by mass to 9.5% by mass. If the concentration of the Si compound in the chemical conversion treatment solution is 0.2% by mass or more, a barrier effect due to siloxane bonds can be obtained, resulting in improved corrosion resistance in flat surfaces, as well as improved corrosion resistance in defective areas, cut areas, and areas damaged by processing, etc., and sweat resistance. Furthermore, if the concentration of the Si compound is 9.5% by mass or less, the life of the chemical conversion treatment solution can be extended. By applying and drying a chemical conversion treatment solution with a Si compound concentration of 0.2% by mass to 9.5% by mass, the Si deposition amount in the chemical conversion coating after drying can be 2 to 95 mg / m 2 It can be said that:

[0067] The inclusion of the Co compound and the Ni compound in the chemical conversion coating can improve blackening resistance. This is thought to be because Co and Ni have the effect of delaying the elution of water-soluble components from the coating in a corrosive environment. Furthermore, Co and Ni are elements that are less likely to oxidize than Al, Zn, Si, Mg, etc. Therefore, by concentrating at least one of the Co compound and the Ni compound at the interface between the chemical conversion coating and the plating coating (forming a concentrated layer), the concentrated layer acts as a barrier against corrosion, thereby improving blackening resistance.

[0068] By using a chemical conversion treatment solution containing the Co compound, Co can be contained in the chemical conversion coating and incorporated into the concentrated layer. A cobalt salt is preferably used as the Co compound. It is more preferable to use one or more cobalt salts selected from the group consisting of cobalt sulfate, cobalt carbonate, and cobalt chloride. Furthermore, by using a chemical conversion treatment solution containing the Ni compound, Ni can be contained in the chemical conversion coating and incorporated into the concentrated layer. A nickel salt is preferably used as the Ni compound. It is more preferable to use one or more nickel salts selected from the group consisting of nickel sulfate, nickel carbonate, and nickel chloride.

[0069] The concentration of the Co compound and / or Ni compound in the chemical conversion treatment solution is not particularly limited, but can be 0.25% by mass to 5% by mass in total. If the concentration of the Co compound and / or Ni compound is less than 0.25% by mass, the interfacial enrichment layer will be non-uniform, reducing the corrosion resistance of flat surfaces and potentially reducing the corrosion resistance of defective areas, cut edge surfaces, and areas damaged by plating or coating due to processing. From the same perspective, the concentration is preferably 0.5% by mass or more, more preferably 0.75% by mass or more. On the other hand, if the concentration of the Co compound and / or Ni compound exceeds 5% by mass, the appearance of the formed coating is likely to be non-uniform, potentially reducing corrosion resistance. From the same perspective, the concentration is preferably 4.0% by mass or less, more preferably 3.0% by mass or less. By applying and drying a chemical conversion treatment solution having a total concentration of the Co compound and / or Ni compound of 0.25% by mass to 5% by mass, the total deposition amount of Co and Ni in the dried chemical conversion coating can be 5 to 100 mg / m 2 It can be said that:

[0070] By adding the Al compound, the Zn compound, and the Mg compound to the chemical conversion treatment solution, a concentrated layer containing at least one of Al, Zn, and Mg can be formed on the plating film side of the chemical conversion coating. The formed concentrated layer can improve corrosion resistance. The Al compound, the Zn compound, and the Mg compound are not particularly limited as long as they are compounds containing Al, Zn, and Mg, respectively, but are preferably inorganic compounds, and are preferably salts, chlorides, oxides, or hydroxides.

[0071] Examples of the Al compound include one or more selected from aluminum sulfate, aluminum carbonate, aluminum chloride, aluminum oxide, and aluminum hydroxide. Examples of the Zn compound include one or more selected from zinc sulfate, zinc carbonate, zinc chloride, zinc oxide, and zinc hydroxide. Examples of the Mg compound include one or more selected from magnesium sulfate, magnesium carbonate, magnesium chloride, magnesium oxide, and magnesium hydroxide.

[0072] The total concentration of the Al compound, Zn compound, and / or Mg compound in the chemical conversion treatment solution for forming the chemical conversion coating is preferably 0.25% by mass to 5% by mass. If the total concentration is 0.25% by mass or more, the concentrated layer can be formed more effectively, resulting in further improved corrosion resistance. On the other hand, if the total concentration is 5% by mass or less, the appearance of the chemical conversion coating becomes more uniform, and the corrosion resistance of flat areas, defective areas, and areas damaged by processing or other factors in the plating or coating is further improved.

[0073] The V compound contained in the chemical conversion coating allows the V to dissolve appropriately in a corrosive environment and bond with zinc ions and other plating components that also dissolve in a corrosive environment to form a dense protective coating. The formed protective coating can further improve corrosion resistance not only on the flat surfaces of the steel sheet, but also against defects, damaged areas in the plating coating caused by processing, and corrosion that progresses from cut edges to the flat surfaces.

[0074] The V compound is a compound containing V, and examples thereof include one or more selected from sodium metavanadate, vanadyl sulfate, and vanadium acetylacetonate.

[0075] The V compound in the chemical conversion treatment solution for forming the chemical conversion coating is preferably 0.05% by mass to 4% by mass. If the V compound concentration is 0.05% by mass or more, it will be more likely to dissolve in a corrosive environment and form a protective coating, improving the corrosion resistance of defects, cut edge portions, and damaged portions of the plating coating caused by processing. On the other hand, if the V compound concentration exceeds 4% by mass, the appearance of the formed chemical conversion coating will likely be uneven and blackening resistance will be reduced.

[0076] The Mo compound, when contained in the chemical conversion coating, can enhance the blackening resistance of the surface-treated steel sheet. The Mo compound is a compound containing Mo and can be obtained by adding one or both of molybdic acid and a molybdate to the chemical conversion treatment solution. Examples of the molybdate include one or more selected from sodium molybdate, potassium molybdate, magnesium molybdate, and zinc molybdate.

[0077] The concentration of the Mo compound in the chemical conversion treatment solution for forming the chemical conversion coating is preferably 0.01% by mass to 3% by mass. If the concentration of the Mo compound is 0.01% by mass or more, the generation of oxygen-deficient zinc oxide is further suppressed, and blackening resistance can be further improved. On the other hand, if the concentration of the Mo compound is 3% by mass or less, the life of the chemical conversion treatment solution is further extended and corrosion resistance can be further improved.

[0078] The inclusion of the Zr compound and the Ti compound in the chemical conversion coating prevents the chemical conversion coating from becoming porous and densifies the coating, making it more difficult for corrosion factors to permeate the chemical conversion coating and improving corrosion resistance.

[0079] The Zr compound is a compound containing Zr, and can be, for example, one or more selected from zirconyl acetate, zirconyl sulfate, potassium zirconyl carbonate, sodium zirconyl carbonate, and ammonium zirconyl carbonate. Among these, organic titanium chelate compounds are preferred because they densify the coating when the chemical conversion treatment liquid is dried to form the coating, thereby providing better corrosion resistance.

[0080] The Ti compound is a compound containing Ti, and for example, one or more selected from titanium sulfate, titanium chloride, titanium hydroxide, titanium acetylacetonate, titanium octylene glycolate, and titanium ethylacetoacetate can be used.

[0081] The total concentration of Zr compounds and / or Ti compounds in the chemical conversion treatment solution for forming the chemical conversion coating is preferably 0.2% by mass to 20% by mass. If the total concentration of Zr compounds and / or Ti compounds is 0.2% by mass or more, the effect of inhibiting the permeation of corrosion factors is enhanced, and the corrosion resistance of not only flat surface areas but also defective areas, cut edge surfaces, and areas of plating film damage caused by processing can be further improved. On the other hand, if the total concentration of Zr compounds and / or Ti compounds is 20% by mass or less, the life of the chemical conversion treatment solution can be further extended.

[0082] The Ca compound contained in the chemical conversion coating can exert the effect of reducing the corrosion rate.

[0083] The Ca compound is a compound containing Ca, and examples thereof include Ca oxide, Ca nitrate, Ca sulfate, and Ca-containing intermetallic compounds. More specifically, examples of the Ca compound include CaO, CaCO3, Ca(OH)2, Ca(NO3)2.4H2O, and CaSO4.2H2O. The content of the Ca compound in the chemical conversion coating is not particularly limited.

[0084] The chemical conversion coating may contain various known components commonly used in the coatings field, as needed, such as various surface conditioners such as leveling agents and antifoaming agents, various additives such as dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, silane coupling agents and titanate coupling agents, various pigments such as color pigments, extender pigments and luster materials, curing catalysts, organic solvents and lubricants.

[0085] In the surface-treated steel sheet of the present invention, the chemical conversion coating preferably does not contain harmful components such as hexavalent chromium, trivalent chromium, fluorine, etc. This is because the chemical conversion treatment solution for forming the chemical conversion coating does not contain these harmful components, resulting in high safety and a low environmental impact.

[0086] The coating weight of the chemical conversion coating is not particularly limited. For example, from the viewpoint of more reliably ensuring corrosion resistance while preventing peeling of the chemical conversion coating, the coating weight of the chemical conversion coating is set to 0.1 to 3.0 g / m 2 It is preferable to set the density to 0.5 to 2.5 g / m 2 It is more preferable that the coating weight of the chemical conversion coating is 0.1 g / m. 2 By setting the coating weight at 3.0 g / m or more, corrosion resistance can be more reliably ensured, and the coating weight of the chemical conversion coating can be set at 3.0 g / m or more. 2 The chemical conversion coating weight can be determined by an appropriate method selected from existing techniques, such as a method of measuring the amount of elements present in the coating whose contents are known in advance by performing fluorescent X-ray analysis of the coating.

[0087] The method for forming the chemical conversion coating is not particularly limited and can be selected appropriately depending on the required performance, manufacturing equipment, etc. For example, the chemical conversion coating can be formed by continuously applying a chemical conversion treatment solution to the plating film using a roll coater or the like, followed by drying at a peak metal temperature (PMT) of approximately 60 to 200°C using hot air or induction heating. In addition to roll coaters, known methods such as airless spraying, electrostatic spraying, and curtain flow coaters can also be used to apply the chemical conversion treatment solution. Furthermore, the chemical conversion coating is not particularly limited and may be either a single-layer film or a multi-layer film, as long as it contains the resin and the metal compound.

[0088] The surface-treated steel sheet of the present invention, like the surface of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, has a height difference of 10 μm or less per 1 mm of the steel sheet surface (chemical conversion coating surface) within a range excluding 50 mm from both ends. When the height difference of the chemical conversion coating surface is 10 μm or less, wrinkle-like defects are absent and an excellent surface appearance can be obtained. As described above, wrinkle-like defects are defects in the chemical conversion coating surface that are caused by Mg-based oxides, and appear as white streaks on the chemical conversion coating surface. Therefore, suppressing wrinkle-like defects means that these streaks are not visible. Therefore, by keeping the height difference of the steel sheet surface, i.e., the chemical conversion coating surface, within a 1 mm length, to 10 μm or less, streaks are not visible and an excellent surface appearance can be obtained. From the same perspective, it is preferable that the height difference of the steel sheet surface within a 1 mm length is 5 μm or less. The steel sheet surface refers to the outermost surface of the steel sheet, and in the case of a surface-treated steel sheet, refers to the surface of the chemical conversion coating.

[0089] Here, the height difference on the steel sheet surface refers to the difference between the height of the highest point on the steel sheet (on the chemical conversion coating), where the height is defined as the direction perpendicular to the surface of the base steel sheet (in the direction of the plating film thickness). The height difference within a 1 mm length of the chemical conversion coating surface can be obtained by measuring the height differences within a 1 mm length at 100 random locations on the surface-treated steel sheet using a laser microscope, as in the case of the plating film surface described above, and calculating the average of the measured values.

[0090] The reason for controlling the height difference on the surface of the steel sheet within a range excluding 50 mm from both ends of the steel sheet is that, in general, it is difficult to control the coating weight at both ends of a hot-dip galvanized steel sheet compared to the center of the steel sheet, and the cooling rate is also different, so wrinkle-like defects are likely to occur. In addition, such both ends of the steel sheet are often trimmed before use, so the surface appearance does not pose any significant problem.

[0091] Regarding the method for controlling the height difference of the coating surface per 1 mm of length on the steel sheet surface to 10 μm or less, it is important to suppress the height difference of the plating coating surface as described above. The height difference of the coating surface can be suppressed by adjusting the content ratio of Mg and Mn in the plating coating or by performing other surface treatments to suppress the height difference.

[0092] Furthermore, in the surface-treated steel sheet of the present invention, a paint film can be formed on the chemical conversion coating, if necessary.

[0093] The present invention provides a method for producing a surface-treated steel sheet comprising a plating film and a chemical conversion film formed on the plating film, wherein the chemical conversion film contains at least one resin selected from the group consisting of epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin, and at least one metal compound selected from the group consisting of P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds, and the plating film is formed by a hot-dip galvanizing process in which a substrate steel sheet is immersed in a plating bath containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the balance being Zn and unavoidable impurities.

[0094] The conditions for the hot dip coating process are the same as those explained in the description of the hot dip Al-Zn-Si-Mg coated steel sheet of the present invention.

[0095] <Coated Steel Sheet> The coated steel sheet of the present invention is a coated steel sheet having a coating film formed on a plating film, either directly or via a chemical conversion coating. The composition of the plating film is the same as that of the plating film of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention described above. Other configurations of the plating film are also the same as those of the plating film of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention described above.

[0096] The coated steel sheet of the present invention can have a chemical conversion coating formed on the plating film. The chemical conversion coating may be formed on at least one side of the coated steel sheet, or may be formed on both sides of the coated steel sheet depending on the application and required performance.

[0097] Chemical conversion coating: The coated steel sheet of the present invention is characterized in that the chemical conversion coating contains a resin component containing a total of 30 to 50 mass% of (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton, with the content ratio of (a):(b) being in the range of 3:97 to 60:40 by mass, and an inorganic compound containing 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound. Forming the above-mentioned chemical conversion coating on a plating coating can increase the strength and adhesion of the chemical conversion coating while also improving corrosion resistance.

[0098] The resin components constituting the chemical conversion coating include (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton.

[0099] The anionic polyurethane resin (a) having an ester bond may be a resin obtained by copolymerizing a reaction product of a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups with a dimethylol alkyl acid. A chemical conversion treatment solution can be obtained by dispersing the resin in a liquid such as water by a known method.

[0100] Examples of the polyester polyol include polyesters obtained by a dehydration condensation reaction between 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 a cyclic ester compound such as ε-caprolactone, and copolymer polyesters thereof. Examples of the polyisocyanate include aromatic polyisocyanates, aliphatic polyisocyanates, and alicyclic polyisocyanates. Examples of the aromatic polyisocyanate 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.).

[0101] When synthesizing a urethane by reacting the polyester polyol with the diisocyanate or polyisocyanate, the anionic polyurethane resin having an ester bond (a) can be obtained by copolymerizing a dimethylol alkyl acid, self-emulsifying it, and dissolving it in water (dispersing it in water). In this case, examples of the dimethylol alkyl acid include dimethylol alkyl acids having 2 to 6 carbon atoms, and more specifically, dimethylol ethanoic acid, dimethylol propanoic acid, dimethylol butanoic acid, dimethylol heptanoic acid, and dimethylol hexanoic acid.

[0102] Furthermore, known epoxy resins can be used as the (b) epoxy resin having a bisphenol skeleton. Examples include bisphenol A-type epoxy resins, bisphenol F-type epoxy resins, bisphenol AD-type epoxy resins, and bisphenol S-type epoxy resins. These epoxy resins can be obtained by reacting a bisphenol compound such as bisphenol A, bisphenol F, bisphenol AD, or bisphenol S with epichlorohydrin in the presence of an alkali catalyst. Among these, component [A] preferably contains a bisphenol A-type epoxy resin or a bisphenol F-type epoxy resin, and more preferably contains a bisphenol A-type epoxy resin. A chemical conversion treatment solution can be obtained by dispersing the (b) epoxy resin having a bisphenol skeleton in a liquid such as water using a known method.

[0103] The resin component acts as a binder for the chemical conversion coating. The (a) anionic polyurethane resin with ester bonds, which constitutes the binder, is flexible and therefore less likely to break (peel) during processing. The (b) epoxy resin with a bisphenol skeleton improves adhesion to the underlying zinc-plated steel sheet and the overlying primer coating. The resin component is contained in the chemical conversion coating 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 coating is reduced. If the resin component content exceeds 50% by mass, the functions of the inorganic components described below, such as their inhibitory action, are reduced. From the same perspective, the resin component content in the chemical conversion coating is preferably 35 to 45% by mass.

[0104] Furthermore, the resin component must have a mass ratio ((a):(b)) of the (a) anionic polyurethane resin having an ester bond to the (b) epoxy resin having a bisphenol skeleton of 3:97 to 60:40. If the (a):(b) ratio is outside this range, the chemical conversion coating will have reduced flexibility and adhesion, resulting in insufficient corrosion resistance. From the same perspective, the (a):(b) ratio is preferably 10:90 to 55:45.

[0105] The resin component may contain other resins (other resin components) besides the above-described (a) anionic polyurethane resin having an ester bond and (b) epoxy resin having a bisphenol skeleton, depending on the required performance. The other resin components are not particularly limited, and may be, for example, at least one or a combination of two or more selected from acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins. When the resin component contains other resins, the total content of the (a) anionic polyurethane resin having an ester bond and the (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 more reliably achieve reduced flexibility and adhesion as a chemical treatment coating.

[0106] The chemical conversion coating also contains, as inorganic compounds, 2 to 10 mass % of a vanadium compound, 40 to 60 mass % of a zirconium compound, and 0.5 to 5 mass % of a fluorine compound, which can enhance the corrosion resistance of the chemical conversion coating.

[0107] The vanadium compound is added to a chemical conversion treatment solution and acts as a rust inhibitor. The inclusion of the vanadium compound in the chemical conversion coating allows the vanadium compound to dissolve appropriately in a corrosive environment and bond with zinc ions and other plating components that also dissolve in a corrosive environment, forming a dense protective coating. The formed protective coating can further enhance corrosion resistance not only to the flat surface of the steel sheet, but also to defects, damaged areas in the plating coating caused by processing, and corrosion progressing from the cut edge to the flat surface. Examples of the vanadium compound include vanadium pentoxide, metavanadate, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. Among these, it is particularly desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

[0108] The content of the vanadium compound in the chemical conversion coating is 2 to 10 mass %. If the content of the vanadium compound in the chemical conversion coating is less than 2 mass %, the inhibitor effect is insufficient, resulting in a decrease in corrosion resistance, while if the content of the vanadium compound exceeds 10 mass %, the moisture resistance of the chemical conversion coating decreases.

[0109] The zirconium compound contained in the chemical conversion coating is expected to improve the strength and corrosion resistance of the chemical conversion coating by reacting with the plating metal and coexisting with the resin component, and furthermore, the zirconium compound itself contributes to the formation of a dense chemical conversion coating and is expected to provide a barrier effect due to its excellent coating properties. Examples of the zirconium compound include neutral salts of zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, and zirconium chloride.

[0110] The content of the zirconium compound in the chemical conversion coating is 40 to 60 mass %. If the content of the zirconium compound in the chemical conversion coating is less than 40 mass %, the strength and corrosion resistance of the chemical conversion coating will decrease, and if the content of the zirconium compound exceeds 60 mass %, the chemical conversion coating will become embrittled, causing the chemical conversion coating to break or peel when subjected to severe processing.

[0111] The fluorine compound is contained in the chemical conversion coating and acts as an adhesive agent for the plating coating. As a result, the corrosion resistance of the chemical conversion coating can be improved. Examples of the fluorine compound that can be used include fluoride salts such as ammonium salts, sodium salts, and potassium salts, and fluorine compounds such as ferrous fluoride and ferric fluoride. Among these, it is preferable to use fluoride salts such as ammonium fluoride, sodium fluoride, and potassium fluoride.

[0112] The content of the fluorine compound in the chemical conversion coating is 0.5 to 5% by mass, because if the content of the fluorine compound in the chemical conversion coating is less than 0.5% by mass, sufficient adhesion cannot be obtained at the processed portion, and if the content of the fluorine compound exceeds 5% by mass, the moisture resistance of the chemical conversion coating decreases.

[0113] The coating weight of the chemical conversion coating is not particularly limited. For example, from the viewpoint of improving the adhesion of the chemical conversion coating while more reliably ensuring corrosion resistance, the coating weight of the chemical conversion coating is set to 0.025 to 0.5 g / m. 2 It is preferable that the coating weight of the chemical conversion coating is 0.025 g / m 2 By setting the coating weight at 0.5 g / m or more, corrosion resistance can be more reliably ensured, and the coating weight of the chemical conversion coating can be set at 0.5 g / m or more. 2 The chemical conversion coating weight can be determined by an appropriate method selected from existing techniques, such as a method of measuring the amount of elements present in the coating whose contents are known in advance by fluorescent X-ray analysis of the coating.

[0114] The method for forming the chemical conversion coating is not particularly limited and can be selected appropriately depending on the required performance, manufacturing equipment, etc. For example, the chemical conversion coating can be formed by continuously applying a chemical conversion treatment solution to the plating film using a roll coater or the like, followed by drying at a peak metal temperature (PMT) of approximately 60 to 200°C using hot air or induction heating. In addition to roll coaters, known methods such as airless spraying, electrostatic spraying, and curtain flow coaters can also be used to apply the chemical conversion treatment solution. Furthermore, the chemical conversion coating is not particularly limited and may be either a single-layer film or a multi-layer film, as long as it contains the resin and the metal compound.

[0115] Coating Film As described above, the coated steel sheet of the present invention has a coating film formed on the plating film directly or via a chemical conversion film, and the coating film has at least a primer coating film.

[0116] In the present invention, the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide. By containing the polyester resin having a urethane bond and the inorganic compound, the primer coating film can improve adhesion of the coating film and corrosion resistance.

[0117] The primer coating contains a polyester resin having a urethane bond as a main component. The polyester resin having a urethane bond is flexible and strong, which provides the effect of preventing cracks from occurring in the primer coating when processed, and has a high affinity with chemical conversion coatings containing a urethane resin, which contributes to improving the corrosion resistance of processed parts in particular. Note that the "main component" here means the component that is contained in the greatest amount among the components in the primer coating.

[0118] The polyester resin having a urethane bond may be a known resin such as a resin obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups. Alternatively, a resin obtained by reacting the polyester polyol with the diisocyanate or polyisocyanate in an excess hydroxyl group state (urethane-modified polyester resin) with a blocked polyisocyanate may be cured.

[0119] The polyester polyol can be obtained by a known method utilizing a dehydration condensation reaction between a polyhydric alcohol component and a polybasic acid component. Examples of the polyhydric alcohol include glycols and trihydric or higher polyhydric alcohols. Examples of the glycols include ethylene glycol, propylene glycol, diethylene glycol, 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, and 3,3-diethyl-1,5-pentanediol. Examples of the trihydric or higher polyhydric alcohols include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, and dipentaerythritol. These polyhydric alcohols can be used alone or in combination. As the polybasic acid, a polycarboxylic acid is usually used, but a monobasic fatty acid or the like can be used in combination if necessary. Examples of the polycarboxylic acid 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 the like, as well as acid anhydrides thereof, 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.

[0120] Examples of the polyisocyanate 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); alicyclic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI; and adducts, biurets, and isocyanurates thereof. These polyisocyanates can be used alone or in combination of two or more.

[0121] The hydroxyl value of the polyester resin having a urethane bond is not particularly limited, but is preferably 5 to 120 mgKOH / g, more preferably 7 to 100 mgKOH / g, and even more preferably 10 to 80 mgKOH / g, from the viewpoints of solvent resistance, processability, etc. Furthermore, the number average molecular weight of the polyester resin having a urethane bond is preferably 500 to 15,000, more preferably 700 to 12,000, and even more preferably 800 to 10,000, from the viewpoints of solvent resistance, processability, etc.

[0122] The content of the polyester resin having a urethane bond in the primer coating is preferably 40 to 88% by mass. If the content of the polyester resin having a urethane bond is less than 40% by mass, the binder function of the primer coating may be reduced, whereas if the content of the polyester resin having a urethane bond exceeds 88% by mass, the functions of the inorganic substance described below, such as the inhibitor action, may be reduced.

[0123] Vanadium compounds, which are one of the inorganic compounds, act as inhibitors. Examples of vanadium compounds include vanadium pentoxide, metavanadate, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. Among these, it is particularly desirable to use tetravalent vanadium compounds or tetravalent vanadium compounds obtained by reduction or oxidation. The vanadium compound added to the primer coating film may be the same or different from the vanadium compound added to the chemical conversion coating film. It is believed that vanadate ions, which gradually dissolve in moisture entering from the outside, react with ions on the surface of the zinc-based plated steel sheet to form a highly adhesive passive film, protecting exposed metal areas and providing rust prevention.

[0124] The content of the vanadium compound in the primer coating film is not particularly limited, but is preferably 4 to 20 mass % from the viewpoint of achieving both corrosion resistance and moisture resistance. If the content of the vanadium compound is less than 4 mass %, the inhibitor effect may be reduced, which may lead to a decrease in corrosion resistance, while if the content of the vanadium compound is more than 20 mass %, the moisture resistance of the primer coating film may be reduced.

[0125] Phosphate compounds, which are one of the inorganic compounds, also act as inhibitors. Examples of the phosphate compounds that can be used include phosphoric acid, ammonium salts of phosphoric acid, alkali metal salts of phosphoric acid, and alkaline earth metal salts of phosphoric acid. In particular, alkali metal salts of phosphoric acid, such as calcium phosphate, are preferably used.

[0126] The content of the phosphate compound in the primer coating film 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 content of the phosphate compound is less than 4% by mass, the inhibitor effect may be reduced, resulting in a decrease in corrosion resistance, while if the content of the phosphate compound is more than 20% by mass, the moisture resistance of the primer coating film may be reduced.

[0127] Magnesium oxide, one of the inorganic compounds, produces a product containing Mg during initial corrosion, and as a sparingly soluble magnesium salt, it has the effect of stabilizing the material and improving corrosion resistance.

[0128] The content of the magnesium oxide 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 corrosion resistance of the processed portion. If the content of the magnesium oxide is less than 4% by mass, the above effect may be reduced, resulting in a decrease in corrosion resistance, while if the content of the magnesium oxide exceeds 20% by mass, the flexibility of the primer coating may be reduced, resulting in a decrease in corrosion resistance of the processed portion.

[0129] The primer coating film may also contain components other than the polyester resin having a urethane bond and the inorganic compound described above. Examples include crosslinking agents used in forming the primer coating film. The crosslinking agent reacts with the polyester resin having a urethane bond to form a crosslinked coating film. Examples include oxazoline compounds, epoxy compounds, melamine compounds, isocyanate compounds, carbodiimide compounds, and silane coupling compounds. Two or more crosslinking agents can also be used in combination. Among these, blocked polyisocyanate compounds are preferred from the viewpoint of corrosion resistance of the processed portion of the resulting coated steel sheet. Examples of blocked polyisocyanates include those in which the isocyanate group of a polyisocyanate compound is blocked with, for example, alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactams, diketones such as acetoacetic acid diesters, imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

[0130] Furthermore, the primer coating film may contain various known components commonly used in the coatings field, as needed, such as various surface conditioners such as leveling agents and antifoaming 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, luster materials, curing catalysts, organic solvents, etc.

[0131] The thickness of the primer coating film is preferably 1.5 μm or more, because by making the thickness of the primer coating film 1.5 μm or more, it is possible to more reliably obtain the effect of improving corrosion resistance and the effect of improving adhesion with the chemical conversion coating film and the top coating film formed on the primer coating film.

[0132] The method for forming the primer coating film is not particularly limited. The coating composition constituting the primer coating film can be preferably applied by a method such as roll coater coating or curtain flow coating. After applying the coating composition, the primer coating film can be obtained by baking the coating composition using a heating method such as hot air heating, infrared heating, or induction heating. The baking treatment is typically carried out at a maximum plate temperature of about 180 to 270°C for about 30 seconds to 3 minutes.

[0133] Furthermore, it is preferable that the coating film constituting the coated steel sheet of the present invention further comprises a topcoat coating film formed on the primer coating film, which not only imparts aesthetic appearance such as color, gloss, and surface condition to the coated steel sheet, but also improves various performance properties such as processability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance.

[0134] The composition of the topcoat film is not particularly limited, and the material, thickness, etc. can be appropriately selected depending on the required performance. For example, the topcoat film can be formed using a polyester resin paint, a silicone polyester resin paint, a polyurethane resin paint, an acrylic resin paint, a fluororesin paint, etc. Furthermore, the topcoat film can contain appropriate amounts of titanium oxide, red iron oxide, mica, carbon black, or other coloring pigments; metallic pigments such as aluminum powder and mica; extender pigments such as 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.

[0135] From the viewpoint of achieving both good appearance and good processability, the thickness of the top coat film is preferably 5 to 30 μm. When the thickness of the top coat film is 5 μm or more, it is possible to more reliably stabilize the color appearance, and when the thickness of the top coat film is 30 μm or less, it is possible to more reliably prevent a decrease in processability (the occurrence of cracks in the top coat film).

[0136] The method for applying the coating composition to form the topcoat film is not particularly limited. For example, the coating composition can be applied by roll coater coating, curtain flow coating, or other methods. After applying the coating composition, the topcoat film can be formed by baking using a heating method such as hot air heating, infrared heating, or induction heating. The baking treatment is typically carried out at a maximum plate temperature of about 180 to 270°C for about 30 seconds to 3 minutes.

[0137] The coated steel sheet of the present invention is characterized in that, like the surface of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention described above, the difference in elevation of the steel sheet surface (coating surface) per 1 mm of length is 10 μm or less within an area excluding 50 mm from both ends of the steel sheet. When the difference in elevation of the coating surface is 10 μm or less, wrinkle-like defects are absent and an excellent surface appearance can be obtained. As described above, wrinkle-like defects are defects in which the surface of the coating film has a wrinkled, uneven shape due to Mg-based oxides, and appear as white streaks on the coating film surface. Therefore, suppressing wrinkle-like defects means that these streaks are not visible. Therefore, by keeping the difference in elevation of the steel sheet surface, i.e., the coating surface, within a 1 mm length, to 10 μm or less, streaks are not visible and an excellent surface appearance can be obtained. From the same perspective, it is preferable that the difference in elevation of the steel sheet surface within a 1 mm length is 5 μm or less. The steel sheet surface means the outermost surface of the steel sheet, and in the case of a coated steel sheet, means the surface of the coating film.

[0138] Here, the height difference on the steel sheet surface refers to the difference in height between the highest point and the lowest point on the steel sheet (on the coating film), where the height is defined as the direction perpendicular to the surface of the base steel sheet (the direction of the coating film thickness). The height difference within a 1 mm length of the coating film surface can be determined by measuring the height differences within a 1 mm length of 100 arbitrary points on the coated steel sheet using a laser microscope, as in the case of the coating film surface described above, and calculating the average of the measured values.

[0139] The reason for controlling the height difference on the surface of the steel sheet within a range excluding 50 mm from both ends of the steel sheet is that, in general, it is difficult to control the coating weight at both ends of a hot-dip galvanized steel sheet compared to the center of the steel sheet, and the cooling rate is also different, so wrinkle-like defects are likely to occur. In addition, such both ends of the steel sheet are often trimmed before use, so the surface appearance does not pose any significant problem.

[0140] Regarding the method for controlling the height difference of the coating surface per mm of the steel sheet surface to 10 μm or less, it is important to suppress the height difference of the plating film surface as described above, and the height difference of the film surface can be suppressed by adjusting the content ratio of Mg and Mn in the plating film or by performing other surface treatments to suppress the height difference.

[0141] The method for producing a coated steel sheet of the present invention is a method for producing a coated steel sheet in which a coating film is formed on a plating film directly or via a chemical conversion coating film, and the chemical conversion coating film contains a resin component containing 30 to 50 mass% in total of (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton, the content ratio of (a):(b) being in the range of 3:97 to 60:40 by mass, and an inorganic compound containing 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound, and the coating film has at least a primer coating film containing a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide, The formation of the plating film includes a hot-dip galvanizing treatment step of immersing a substrate steel sheet in a plating bath having 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 balance being Zn and unavoidable impurities.

[0142] The conditions for the hot dip coating process are the same as those explained in the description of the hot dip Al-Zn-Si-Mg coated steel sheet of the present invention.

[0143] [Example 1: Samples 1 to 28] A cold-rolled steel sheet having a thickness of 0.8 mm, produced by a conventional method, was used as a base steel sheet, and was subjected to degreasing treatment, annealing treatment and plating treatment in a continuous hot-dip galvanizing facility, thereby producing Samples 1 to 28 of hot-dip Al-Zn-Si-Mg-plated steel sheets under the conditions shown in Table 1. The composition of the coating bath used to produce the hot-dip Al-Zn-Si-Mg coated steel sheets was varied over the following ranges: Al: 45 to 65 mass%, Si: 1.5 to 2.5 mass%, Mg: 1.0 to 4.5 mass%, Mn: 0.00 to 1.0 mass%, Sr: 0.00 to 1.0 mass%, B: 0.00 to 0.05 mass%, Ca: 0.00 to 1.0 mass%, Cr: 0.00 to 0.2 mass%, Ti: 0.00 to 0.2 mass%, and V: 0.00 to 0.2 mass%, so as to obtain the coating composition of each sample shown in Table 1. The temperature of the coating bath was 590°C for 45 to 55 mass% Al and 630°C for 65 mass% Al, and was controlled so that the temperature of the substrate steel sheet entering the coating was the same as the temperature of the coating bath. Furthermore, the plating process was carried out under the condition that the sheet temperature was cooled to a temperature range of 520 to 500°C in 3 seconds. The coating weight of the plating film was 85±5 g / m per side for samples 1 to 23 and samples 27 to 28. 2 , Samples 24 to 26: 50 to 125 g / m per side 2 It was controlled so that

[0144] <Evaluation> The following evaluations were performed on each sample of hot-dip Al-Zn-Si-Mg-plated steel sheet obtained as described above. The evaluation results are shown in Table 1. (1) Structure of the Plating Film (Deposition Weight, Composition, and X-ray Diffraction Intensity) A 100 mm diameter punch was punched out from each plated sample, and the non-measurement surface was sealed with tape. The plating was then dissolved and stripped using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The deposition weight of the plating film was calculated from the mass difference between the sample before and after stripping. The resulting deposition weights of the plating film are shown in Table 1. The stripping solution was then filtered, and the filtrate and solid content were analyzed. Specifically, the filtrate was subjected to ICP atomic emission spectroscopy to quantify components other than insoluble Si. The solid content was dried and ashed in a heating furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate. The melt was then dissolved in hydrochloric acid, and the solution was subjected to ICP atomic emission spectroscopy to quantify insoluble Si. The silicon concentration in the plating film was calculated by adding the soluble silicon concentration obtained by filtrate analysis to the insoluble silicon concentration obtained by solids analysis. The composition of the resulting plating film is shown in Table 1. Each sample was then sheared to a 100 mm x 100 mm size, and the plating film on the target surface was mechanically scraped off until the steel substrate was exposed. The resulting powder was thoroughly mixed and 0.3 g of the powder was extracted. Qualitative analysis of the powder was performed using an X-ray diffraction analyzer (Rigaku Corporation, "SmartLab") under the following conditions: Cu-Kα X-ray (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°, and a high-speed one-dimensional detector (D / teX Ultra). The intensity obtained by subtracting the base intensity from each peak intensity was used as the diffraction intensity (cps). The diffraction intensity of the Mg2Si (111) plane (planar spacing d = 0.3668 nm) and the diffraction intensity of the Si (111) plane (planar spacing d = 0.3135 nm) were measured.

[0145] (2) Surface Appearance (2-1) Wrinkle Defects The surface appearance of each sample of the obtained hot-dip Al-Zn-Si-Mg-plated steel sheet was visually inspected to determine whether wrinkle defects had occurred. Furthermore, for each sample, 100 locations were randomly selected from the area excluding 50 mm from both ends of the strip, and the height difference of the plating film surface within a 1 mm length range was measured using a laser microscope (Keyence Corporation, "VK-X3000"), and the average value was quantified as the surface shape. The occurrence of wrinkle defects and the surface shape were evaluated according to the following criteria: ◎: No wrinkle defects were observed (height difference of 5 μm or less); ○: No wrinkle defects were observed (height difference of 10 μm or less); ×: Wrinkle defects were observed (height difference of more than 10 μm).

[0146] (2-2) Dross Defects The surface appearance of each sample of the obtained hot-dip Al-Zn-Si-Mg coated steel sheet was visually inspected, and the presence or absence of dross defects was evaluated according to the following criteria: ○: No adhesion of granular dross was observed ×: Adhesion of granular dross was observed

[0147] (3) Corrosion Resistance Evaluation Each sample of the obtained hot-dip Al-Zn-Si-Mg-plated steel sheet was sheared to a size of 120 mm × 120 mm, and a 10 mm area from each edge of the surface to be evaluated, as well as the end faces and the non-evaluation surface, were sealed with tape to expose a 100 mm × 100 mm area of ​​the surface to be evaluated. Three identical evaluation samples were prepared. Each of the three evaluation samples prepared as described above was subjected to the Japan Automotive Standards Combined Cyclic Test (JASO-CCT). The accelerated corrosion test was conducted from wet to 300 cycles, and then the corrosion weight loss of each sample was measured using the methods specified in JIS Z 2383 and ISO 8407. Evaluation was based on the following criteria: ⊚: The corrosion weight loss of all three samples was 45 g / m 2 ○: The corrosion weight loss of all three samples was 90 g / m 2 Less than ×: Corrosion loss of one or more samples is 90 g / m 2 Overcoming

[0148]

[0149] The results in Table 1 show that the samples of the invention have superior surface appearance compared to the samples of the comparative examples.

[0150] [Example 2: Samples 1 to 34] (1) A cold-rolled steel sheet having a thickness of 0.8 mm, produced by a conventional method, was used as a base steel sheet, and was subjected to degreasing treatment, annealing treatment and plating treatment in a continuous hot-dip galvanizing facility, thereby producing Samples 1 to 34 of hot-dip Al-Zn-Si-Mg-plated steel sheets under the conditions shown in Table 3. The composition of the coating bath used to produce the hot-dip Al-Zn-Si-Mg coated steel sheets was varied over the following ranges: Al: 45 to 65 mass%, Si: 1.5 to 2.5 mass%, Mg: 1.0 to 4.5 mass%, Mn: 0.00 to 1.0 mass%, Sr: 0.00 to 0.00 to 1.0 mass%, B: 0.00 to 0.05 mass%, Ca: 0.00 to 1.0 mass%, Cr: 0.00 to 0.2 mass%, Ti: 0.00 to 0.2 mass%, and V: 0.00 to 0.2 mass%, so as to obtain the coating composition of each sample shown in Table 3. The bath temperature of the coating bath was 590°C for 45 to 55 mass% Al and 630°C for 65 mass% Al, and was controlled so that the temperature of the substrate steel sheet entering the coating was the same as the coating bath temperature. Furthermore, the plating process was carried out under the condition that the sheet temperature was cooled to a temperature range of 520 to 500°C in 3 seconds. The coating weight of the plating film was 85±5 g / m per side for samples 1 to 29 and samples 33 to 34. 2 , Samples 30 to 32, 50 to 125 g / m per side 2 It was controlled so that

[0151] (2) Then, a chemical conversion treatment solution was applied to the plating film of each sample of hot-dip Al-Zn-Si-Mg-plated steel sheet using a bar coater, and the resulting steel sheet was dried in a hot air oven (heating rate: 60°C / s, PMT: 120°C) to form a chemical conversion coating, producing the surface-treated steel sheet samples shown in Table 2. Surface treatment solutions A to F were prepared by dissolving each component in water as a solvent. The types of components (resin, metal compounds) contained in the surface treatment solutions are as follows: (Resin) Urethane resin: Superflex 130, Superflex 126 (Dai-ichi Kogyo Seiyaku Co., Ltd.) Acrylic resin: Boncoat EC-740EF (DIC Corporation) (Metal compounds) P compound: aluminum dihydrogen tripolyphosphate Si compound: silica V compound: sodium metavanadate Mo compound: molybdic acid Zr compound: potassium zirconyl carbonate The compositions of the prepared chemical conversion treatment solutions A to F and the coating weights of the formed chemical conversion coatings are shown in Table 1. The concentration of each component in Table 1 of this specification is the concentration (mass %) of the solid content.

[0152]

[0153] <Evaluation> The following evaluations were performed on each sample of the surface-treated steel sheet obtained as described above. The evaluation results are shown in Table 3. (1) Structure of the Plating Film (Adhesion Weight, Composition, and X-ray Diffraction Intensity) A 100 mm diameter punch was made from each plated sample, and the non-measurement surface was sealed with tape. The plating was then dissolved and stripped using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The deposition weight of the plating film was calculated from the mass difference between the sample before and after stripping. The resulting deposition weights of the plating film are shown in Table 3. The stripping solution was then filtered, and the filtrate and solid content were analyzed. Specifically, the filtrate was subjected to ICP atomic emission spectroscopy to quantify components other than insoluble Si. The solid content was dried and incinerated in a heating furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate. The melt was then dissolved in hydrochloric acid, and the solution was subjected to ICP atomic emission spectroscopy to quantify insoluble Si. The silicon concentration in the plating film was calculated by adding the soluble silicon concentration obtained by filtrate analysis to the insoluble silicon concentration obtained by solids analysis. The composition of the resulting plating film is shown in Table 3. Each sample was then sheared to a 100 mm x 100 mm size, and the plating film on the target surface was mechanically scraped off until the steel substrate was exposed. The resulting powder was thoroughly mixed and 0.3 g of the powder was extracted. Qualitative analysis of the powder was performed using an X-ray diffraction analyzer (Rigaku Corporation, "SmartLab") under the following conditions: Cu-Kα X-ray (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°, and a high-speed one-dimensional detector (D / teX Ultra). The intensity obtained by subtracting the base intensity from each peak intensity was taken as the diffraction intensity (cps), and the diffraction intensity of the Mg2Si (111) plane (plane spacing d = 0.3668 nm) and the diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm) were measured. The measurement results are shown in Table 3.

[0154] (2) Surface Appearance (2-1) Wrinkle Defects The surface appearance of each sample of the obtained hot-dip Al-Zn-Si-Mg-plated steel sheet and surface-treated steel sheet was visually inspected to determine the presence or absence of wrinkle defects. Furthermore, for each sample, 100 locations were randomly selected from the area excluding 50 mm from both ends of the strip, and the height difference of the plating film surface within a 1 mm length was measured using a laser microscope (Keyence Corporation, "VK-X3000"), and the average value was quantified as the surface shape. The occurrence of wrinkle defects and the surface shape were evaluated according to the following criteria: ◎: No wrinkle defects were observed (height difference of 5 μm or less); ○: No wrinkle defects were observed (height difference of 10 μm or less); ×: Wrinkle defects were observed (height difference of more than 10 μm).

[0155] (2-2) Dross Defects The surface appearance of each sample of the obtained hot-dip Al-Zn-Si-Mg coated steel sheet was visually inspected, and the presence or absence of dross defects was evaluated according to the following criteria: ○: No adhesion of granular dross was observed ×: Adhesion of granular dross was observed

[0156] (3) White Rust Resistance Each sample of hot-dip Al-Zn-Si-Mg plated steel sheet and surface-treated steel sheet was sheared to a size of 120 mm x 120 mm, and then a 10 mm area from each edge of the surface to be evaluated, as well as the sample edge and the surface not to be evaluated, were sealed with tape, leaving a 100 mm x 100 mm area of ​​the surface to be evaluated, which was used as the evaluation sample. Using the above evaluation samples, a salt spray test as specified in JIS Z 2371 was conducted for 90 hours and evaluated according to the following criteria: ◎: No white rust on the flat plate ○: White rust occurred on less than 10% of the flat plate area ×: White rust occurred on 10% or more of the flat plate area

[0157] (4) Corrosion Resistance Evaluation Each sample of the obtained hot-dip Al-Zn-Si-Mg-plated steel sheet and surface-treated steel sheet was sheared to a size of 120 mm × 120 mm. A 10 mm area from each edge of the evaluation surface, as well as the sample edge and the non-evaluation surface, were sealed with tape to expose a 100 mm × 100 mm area of ​​the evaluation surface, which was used as the evaluation sample. Three identical evaluation samples were prepared. Each of the three evaluation samples prepared as described above was subjected to the Japan Automotive Standards Combined Cyclic Test (JASO-CCT). After 300 cycles of accelerated corrosion testing, starting with wet corrosion, the corrosion weight loss of each sample was measured using the methods specified in JIS Z 2383 and ISO 8407, and the results were evaluated according to the following criteria: ⊚: The corrosion weight loss of all three samples was 30 g / m 2 ○: The corrosion weight loss of all three samples was 70 g / m 2 Less than ×: Corrosion loss of one or more samples is 70 g / m 2 Overcoming

[0158]

[0159] The results in Table 3 show that the samples of the invention have superior surface appearance compared to the samples of the comparative examples.

[0160] [Example 3: Samples 1 to 37] (1) A cold-rolled steel sheet having a thickness of 0.8 mm, produced by a conventional method, was used as a base steel sheet, and was subjected to degreasing treatment, annealing treatment and plating treatment in a continuous hot-dip galvanizing facility, thereby producing Samples 1 to 37 of hot-dip Al-Zn-Si-Mg-plated steel sheets under the conditions shown in Table 5. The composition of the coating bath used to produce the hot-dip Al-Zn-Si-Mg coated steel sheets was varied over the following ranges: Al: 45 to 65 mass%, Si: 1.5 to 2.5 mass%, Mg: 1.0 to 4.5 mass%, Mn: 0.00 to 1.0 mass%, Sr: 0.00 to 1.0 mass%, B: 0.00 to 0.05 mass%, Ca: 0.00 to 1.0 mass%, Cr: 0.00 to 0.2 mass%, Ti: 0.00 to 0.2 mass%, and V: 0.00 to 0.2 mass%, so as to obtain the coating composition of each sample shown in Table 5. The bath temperature of the coating bath was 590°C for 45 to 55 mass% Al and 630°C for 65 mass% Al, and was controlled so that the temperature of the substrate steel sheet entering the coating was the same as the coating bath temperature. Furthermore, the plating process was carried out under the condition that the sheet temperature was cooled to a temperature range of 520 to 500°C in 3 seconds. The coating weight of the plating film was 85±5 g / m per side for samples 1 to 32 and samples 36 to 37. 2 , Samples 33 to 35: 50 to 125 g / m per side 2 It was controlled so that

[0161] (2) Then, the chemical conversion treatment solution shown in Table 4 was applied to the plating film of each sample of the prepared hot-dip galvanized steel sheet using a bar coater, and the coating was dried in a hot-air drying oven (achieved sheet temperature: 90°C) until the coating weight reached 0.1 g / m. 2The chemical conversion coating film was formed as follows. The chemical conversion treatment solution used was one with a pH of 8 to 10, prepared by dissolving each component in water as a solvent. The types of components (resin components, inorganic compounds) contained in the chemical conversion treatment solution are as follows. (Resin Components) Resin A: (a) an anionic polyurethane resin having an ester bond (Superflex 210, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) and (b) an epoxy resin having a bisphenol skeleton (Yukarejin RE-1050, manufactured by Yoshimura Oil Chemical Co., Ltd.) mixed at a mass ratio of (a):(b) = 50:50. Resin B: acrylic resin (Boncoat EC-740EF, manufactured by DIC Corporation). (Inorganic Compounds) Vanadium compound: organic vanadium compound chelated with acetylacetone. Zirconium compound: ammonium zirconium carbonate. Fluorine compound: ammonium fluoride.

[0162] (3) A primer coating was then applied to the chemical conversion coating formed as described above using a bar coater and baked at a steel plate temperature of 230°C for a baking time of 35 seconds to form a primer coating film having the component composition shown in Table 4. A topcoat coating composition was then applied to the primer coating formed as described above using a bar coater and baked at a steel plate temperature of 230°C to 260°C for a baking time of 40 seconds to form a topcoat coating film having the resin composition and film thickness shown in Table 4, thereby producing each sample coated steel plate. The primer coating was obtained by mixing the components and then stirring in a ball mill for approximately 1 hour. The resin components and inorganic compounds constituting the primer coating were as follows: (Resin components) Resin α: Urethane-modified polyester resin (obtained by reacting 455 parts by mass of polyester resin and 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) cured with blocked isocyanate. The polyester resin to be urethane-modified was prepared under the following conditions: A flask equipped with a stirrer, a rectification column, a water separator, a condenser, and a thermometer was charged with 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 cyclohexanedimethanol. The mixture was heated and stirred, and the resulting condensation water was distilled out of the system. The temperature was raised from 160°C to 230°C at a constant rate over 4 hours. After the temperature reached 230°C, 20 parts by mass of xylene was gradually added, and the condensation reaction was continued while maintaining the temperature at 230°C. The reaction was terminated when the acid value reached 5 or less. The mixture was then cooled to 100°C, and 120 parts by mass of Solvesso 100 (a high-boiling aromatic hydrocarbon solvent, manufactured by ExxonMobil Corporation) and 100 parts by mass of butyl cellosolve were added to obtain a polyester resin solution. Resin β: urethane-cured polyester resin (Evaclad 4900 manufactured by Kansai Paint Co., Ltd.) (inorganic compounds) Vanadium compound: magnesium vanadate Phosphate compound: calcium phosphate Magnesium oxide compound: magnesium oxide Furthermore, the following paints were used for the resins used in the topcoat coatings shown in Table 4.Resin I: Melamine-cured polyester paint ("Precolor HD0030HR" manufactured by BASF Japan Ltd.) Resin II: Organosol-based bake-type fluororesin paint ("Precolor No. 8800HR" manufactured by BASF Japan Ltd.) containing polyvinylidene fluoride and acrylic resin in a mass ratio of 80:20.

[0163]

[0164] <Evaluation> The following evaluations were performed on each sample of coated steel sheet obtained as described above. The evaluation results are shown in Table 5. (1) Plating Film Structure (Deposition Weight, Composition, X-ray Diffraction Intensity) A 100 mm diameter punch was made from each plated sample, and the non-measurement surface was sealed with tape. The plating was then dissolved and stripped using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The deposition weight of the plating film was calculated from the mass difference between the sample before and after stripping. The resulting deposition weights of the plating film are shown in Table 5. The stripping solution was then filtered, and the filtrate and solid content were analyzed. Specifically, the filtrate was subjected to ICP atomic emission spectroscopy to quantify components other than insoluble Si. The solid content was dried and ashed in a heating furnace at 650 °C, and then melted by adding sodium carbonate and sodium tetraborate. The melt was then dissolved in hydrochloric acid, and the solution was subjected to ICP atomic emission spectroscopy to quantify insoluble Si. The silicon concentration in the plating film was calculated by adding the soluble silicon concentration obtained by filtrate analysis to the insoluble silicon concentration obtained by solids analysis. The composition of the resulting plating film is shown in Table 5. Each sample was then sheared to a 100 mm x 100 mm size, and the plating film on the evaluation surface was mechanically scraped off until the steel substrate was exposed. The resulting powder was thoroughly mixed and 0.3 g of the powder was extracted. Qualitative analysis of the powder was performed using an X-ray diffraction analyzer (Rigaku Corporation, "SmartLab") under the following conditions: Cu-Kα X-ray (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°, and a high-speed one-dimensional detector (D / teX Ultra). The intensity obtained by subtracting the base intensity from each peak intensity was used as the diffraction intensity (cps). The diffraction intensity of the Mg2Si (111) plane (planar spacing d = 0.3668 nm) and the diffraction intensity of the Si (111) plane (planar spacing d = 0.3135 nm) were measured.

[0165] (2) Surface Appearance (2-1) Wrinkle Defects The surface appearance of each sample of the obtained coated steel sheet was visually inspected to determine whether wrinkle defects had occurred. Furthermore, for each sample, 100 locations were randomly selected from the area excluding 50 mm from both ends of the strip, and the height difference of the plating film surface within a 1 mm length range was measured using a laser microscope (Keyence Corporation, "VK-X3000"), and the average value was quantified as the surface shape. Based on the presence or absence of wrinkle defects and the surface shape, the state of wrinkle defects was evaluated according to the following criteria: ◎: No wrinkle defects were observed (height difference of 5 μm or less) ○: No wrinkle defects were observed (height difference of 10 μm or less) ×: Wrinkle defects were observed (height difference of more than 10 μm)

[0166] (2-2) Dross Defects The surface appearance of each sample of the obtained coated steel sheet was visually inspected, and the presence or absence of dross defects was evaluated according to the following criteria: ○: No adhesion of granular dross was observed ×: Adhesion of granular dross was observed

[0167] (3) Corrosion Resistance Evaluation: Each coated steel sheet sample was sheared to a size of 120 mm x 120 mm. Three of the four edges, including the edge and the non-edge, were sealed with tape within 10 mm of the edge. One edge was left unsealed, leaving the sheared edge exposed. This was used as the evaluation sample. The shearing was performed so that the burr on the sheared edge faced the evaluation surface. The above evaluation samples were subjected to the Japan Automotive Standards Combined Cyclic Test (JASO-CCT). The accelerated corrosion test began with wetting, and every 20 cycles, the samples were removed, washed with water, dried, and visually inspected. The number of cycles at which red rust was observed on the sheared edge of the unsealed edge was evaluated according to the following criteria: ◎: Number of cycles at which red rust appeared on three samples ≥ 600 cycles ○: Number of cycles at which red rust appeared on three samples ≥ 400 cycles ×: Number of cycles at which red rust appeared on one or more samples < 400 cycles

[0168]

[0169] The results in Table 5 show that the samples of the invention are superior in both surface appearance and corrosion resistance to the samples of the comparative examples.

[0170] According to the present invention, it is possible to provide a hot-dip Al-Zn-Si-Mg-plated steel sheet that is free from other defects such as dross defects, suppresses the occurrence of wrinkle defects, and has excellent surface appearance. Furthermore, according to the present invention, it is possible to provide a surface-treated steel sheet that has excellent surface appearance and white rust resistance, and a coated steel sheet that has excellent surface appearance and excellent corrosion resistance.

Claims

1. A hot-dip Al-Zn-Si-Mg-plated steel sheet having a plating film, the plating film having a composition containing 45-65 mass% Al, 1.0-3.0 mass% Si, 1.0-10.0 mass% Mg, 0.01-0.5 mass% Mn, with the balance being Zn and unavoidable impurities, and characterized in that within an area excluding 50 mm from both ends of the steel sheet, the difference in height of the steel sheet surface per mm in length is 10 μm or less.

2. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1, characterized in that the Mn content in the plated film is 0.1 to 0.3 mass %.

3. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1 or 2, characterized in that an alloy layer containing Mn is provided at the interface between the plated film and the base steel sheet.

4. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 3, characterized in that the plating film further contains 0.01 to 3.0 mass% in total of one or more elements selected from B, Ca, Ti, V, Cr, Sr, Mo, In, Sn, Sb, Ce, and Bi.

5. A surface-treated steel sheet comprising the plating film according to any one of claims 1 to 4 and a chemical conversion film formed on the plating film, characterized in that the chemical conversion film contains at least one resin selected from the group consisting of epoxy resins, urethane resins, acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins and fluororesins, and at least one metal compound selected from the group consisting of P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds and Ca compounds.

6. A coated steel sheet having a coating film formed directly or via a chemical conversion coating film on the plating film according to any one of claims 1 to 4, wherein the chemical conversion coating film contains a resin component containing 30 to 50 mass% in total of (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton, the content ratio of (a):(b) being within the range of 3:97 to 60:40 by mass, and an inorganic compound containing 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound, and the coating film has at least a primer coating film containing a polyester resin having a urethane bond, and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide.

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