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

Abstract

To provide a hot-dip Al-Zn-Si-Mg-plated steel sheet, which is free of dross and other defects, suppresses the occurrence of wrinkle-shaped defects, and exhibits a superior external appearance.SOLUTION: The present invention provides a hot-dip Al-Zn-Si-Mg-plated steel sheet with a plated coating. The plated coating has a composition including 45-65 mass% Al, 1.0-3.0 mass% Si, 1.0-10.0 mass% Mg, and 0.01-0.5 mass% Mn, the remainder consisting of Zn and unavoidable impurities. In a range excluding 50 mm from both ends of the steel sheet, the height difference of the steel sheet surface per 1 mm of length is 10 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to hot-dip Al-Zn-Si-Mg plated steel sheets, surface-treated steel sheets, and painted steel sheets having excellent surface appearance. [Background technology]

[0002] Hot-dip Al-Zn plated steel sheets, typified by 55% Al-Zn, 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 shown in Patent Document 1, for example. Therefore, due to their excellent corrosion resistance, hot-dip Al-Zn plated steel sheets are mainly used in building materials such as roofs and walls that are exposed to the outdoors for long periods, as well as in civil engineering and construction fields such as guardrails, wiring and piping, and soundproof walls. In particular, the demand for materials with superior corrosion resistance in harsher operating environments, such as acid rain caused by air pollution, the application of de-icing agents to prevent road freezing in snowy areas, and coastal development, as well as maintenance-free materials, has been increasing in recent years. As a result, the demand for hot-dip Al-Zn plated steel sheets has been growing.

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

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

[0005] However, molten Al-Zn-Si-Mg plated steel sheets had a problem in that their surface appearance was significantly degraded due to the occurrence of wrinkle-like irregularities, as described later. When Mg is added to a molten Al-Zn-Si bath and plated onto a steel sheet, Mg is an element that oxidizes more easily than other plating components. Therefore, during the cooling and solidification process of the plating film, Mg reacts with oxygen in the outermost layer that is in contact with the atmosphere to produce Mg-based oxides. Consequently, the Mg concentration near the outermost layer decreases, and to compensate for this, Mg diffuses sequentially from the inside of the plating film to the outermost layer. As a result, before the plating film completely solidifies to its interior, a thick layer of Mg-based oxide forms on the outermost layer. This creates a difference in fluidity between the fluid interior of the plating film before solidification and the Mg-based oxide layer. The Mg-based oxide layer cannot keep up with the flow inside the plating film, resulting in wrinkle-like irregularities (wrinkle-like defects) on the plated surface of the manufactured molten Al-Zn-Si-Mg plated steel sheet. Therefore, there was a need for the development of technologies to improve the wrinkle-like defects in molten Al-Zn-Si-Mg plated steel sheets.

[0006] Furthermore, Patent Documents 4 and 5 disclose a technique for suppressing the oxidation of Mg on the plating surface and preventing the occurrence of wrinkle-like defects by incorporating Sr into the plating film. These techniques utilize the phenomenon in which Sr is preferentially oxidized over Mg on the surface of the unsolidified plating layer after it has been removed from the plating bath.

[0007] Furthermore, the aforementioned hot-dip Al-Zn-Si-Mg plated steel sheets had a problem: when used in harsh corrosive environments, white rust would occur due to corrosion of the plating film. Since this white rust degrades the appearance of the steel sheet, development is underway to improve the resistance to white rust in plated steel sheets. Therefore, Patent Documents 6 and 7 disclose surface-treated steel sheets in which the resistance to white rust is improved by forming a chemical conversion film containing urethane resin on the plating film of a molten Al-Zn-Si-Mg plated steel sheet. However, even in the case of surface-treated steel sheets such as those described in Patent Documents 6 and 7, the surface appearance is still affected by the underlying molten Al-Zn-Si-Mg plated steel sheet, and therefore, the deterioration of the surface appearance caused by the occurrence of the aforementioned wrinkle-like defects still needed to be improved.

[0008] Furthermore, the aforementioned hot-dip Al-Zn-Si-Mg plated steel sheets are sometimes used as painted steel sheets by forming a chemical conversion coating or a primer coating on the surface, and then forming various coatings on top of that. Such painted steel sheets are subjected to various processes including 90-degree bending or 180-degree bending by press forming, roll forming, or embossing, and long-term coating durability performance after use is required. To meet these requirements, a chemical conversion treatment containing chromate is applied to a molten Al-Zn-based plated steel sheet, a chromate-based rust-preventive pigment is contained in the primer coating film, and a thermosetting polyester-based resin coating film or a fluorine-based resin coating film for higher weather resistance requirements is formed as the topcoat coating film on top of it. Generally, painted zinc-based plated steel sheets are applied. However, recently, the use of chromate, which is an environmental load substance, has been regarded as a problem for such painted steel sheets, and chromate-free painted steel sheets that do not contain chromate are strongly desired. For example, chromate-free painted steel sheets as disclosed in Patent Document 8 have been proposed.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, for any of the technologies disclosed in the literature, it has been difficult to suppress the occurrence of the above-mentioned wrinkle-like defects without degrading other physical properties. In the technologies disclosed in Patent Document 4 and Patent Document 5, during the production of a hot-dip Al-Zn-Si-Mg-based plated steel sheet with Sr added to the plating film, the surface of the plating bath with Sr added is liable to oxidize, and there has been a problem that dross defects caused by the oxide-based dross adhering to the plating film newly occur, leading to deterioration of the surface appearance.

[0011] Also, although the technologies of Patent Document 6 and Patent Document 7 can improve the whitening resistance, even in the case of a surface-treated steel sheet, it is still desired to improve the deterioration of the surface appearance due to the occurrence of wrinkle-like defects, which is affected by the underlying hot-dip Al-Zn-Si-Mg-based plated steel sheet. Furthermore, regarding the technology of Patent Document 8, it is needless to say that the surface appearance of the coated steel sheet, particularly the sharpness of imaging, is affected by the surface shape of the underlying plated steel sheet. Since the height difference of the unevenness associated with the occurrence of the above-mentioned wrinkle-like defects and dross defects reaches dozens of μm, even if the surface is smoothed by the coating film, the unevenness cannot be completely eliminated, and the improvement of the surface appearance as a coated steel sheet has still been an issue. In addition, since the coating film formed on the convex portion of the plating film has a reduced film thickness, there is also a concern that the corrosion resistance locally decreases. Therefore, it has been desired to improve the surface appearance of the underlying plated steel sheet, particularly the surface shape.

[0012] In view of such circumstances, an object of the present invention is to provide a hot-dip Al-Zn-Si-Mg-based plated steel sheet having excellent surface appearance with no other defects such as dross defects and with the occurrence of wrinkle-like defects suppressed. Another object of the present invention is to provide a surface-treated steel sheet having excellent surface appearance and whitening resistance, and a coated steel sheet having excellent surface appearance and corrosion resistance.

Means for Solving the Problems

[0013] As a result of their investigation to solve the above problems, the inventors focused on the fact that by adding Mn to the plating film of a molten Al-Zn-Si-Mg plated steel sheet, wrinkle-like defects can be suppressed without causing dross defects, etc. They found that by adjusting the amount of Mn added and keeping the height difference on the steel sheet surface (plating film surface) small, the occurrence of wrinkle-like defects can be suppressed.

[0014] This invention is 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, The aforementioned plating film has a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, Mg: 1.0-10.0% by mass, and Mn: 0.01-0.5% by mass, with the remainder being Zn and unavoidable impurities. A hot-dip Al-Zn-Si-Mg plated steel sheet, characterized in that the difference in height of the steel sheet surface per 1 mm in length is 10 μm or less in the area excluding 50 mm from both ends of the steel sheet. 2. The hot-dip Al-Zn-Si-Mg plated steel sheet according to item 1, characterized in that the Mn content in the plating film is 0.1 to 0.3% by mass. 3. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1 or 2 above, characterized in that it comprises an alloy layer containing Mn at the interface between the plating film and the underlying steel sheet. 4. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of 1 to 3 above, characterized in that the plating film further contains a total of 0.01 to 3.0% by mass 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 a plating film as described in any of items 1 to 4 above, and a chemical conversion film formed on the plating film, The surface-treated steel sheet is characterized in that the chemical conversion coating contains at least one resin selected from 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 P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound. 6. A painted steel sheet having a coating film formed directly or via a chemical conversion coating on a plating film as described in any of items 1 to 4 above, The aforementioned chemical conversion film contains a resin component comprising (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton in total at a concentration of 30 to 50% by mass, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. The aforementioned coating film comprises at least a primer coating film, the primer coating film containing a polyester resin having urethane bonds and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide, and is characterized by this, in the painted steel sheet. [Effects of the Invention]

[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-like defects, and has an excellent surface appearance. Furthermore, according to the present invention, it is possible to provide a surface-treated steel sheet having excellent surface appearance and resistance to white rust, as well as a painted steel sheet with excellent surface appearance and corrosion resistance. [Modes for carrying out the invention]

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

[0017] The Al content in the aforementioned plating film is 45 to 65% by mass, preferably 50 to 60% by mass, taking into account the balance between corrosion resistance and operational surface. This is because if the Al content in the plating film is at least 45% by mass, Al dendrite solidification occurs, and a plating film structure mainly consisting of a dendrite solidified α-Al phase can be obtained. When this dendrite solidified structure is stacked in the direction of the thickness of the plating film, the corrosion progression path becomes more complex, and the corrosion resistance of the plating film itself is improved. Furthermore, the more of these α-Al phase dendrites that are stacked, the more complex the corrosion progression path becomes, making it more difficult for corrosion to easily reach the underlying steel sheet, thus improving corrosion resistance. For this reason, it is preferable to have an Al content of 50% by mass or more. On the other hand, if the Al content in the plating film exceeds 65% by mass, the structure changes to one in which most of the Zn is solid-dissolved in α-Al, and the dissolution reaction of the α-Al phase cannot be suppressed, which may lead to a deterioration in the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet. For this reason, the Al content in the plating film must be 65% by mass or less, and preferably 60% by mass or less.

[0018] The Si in the aforementioned plating film is added primarily to suppress the growth of Fe-Al and / or Fe-Al-Si interfacial alloy layers that form at the interface with the underlying steel sheet, thereby preventing deterioration of the adhesion between the plating film and the steel sheet. In practice, when a steel sheet is immersed in an Al-Zn plating bath containing Si, the Fe on the surface of the steel sheet reacts with the Al and Si in the bath to form an alloy, and Fe-Al and / or Fe-Al-Si intermetallic compound layers are formed at the underlying steel sheet / plating film interface. However, since the Fe-Al-Si alloy grows more slowly than the Fe-Al alloy, the higher the proportion of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film must be 1.0% by mass or more. On the other hand, if the Si content in the plating film exceeds 4.0% by mass, not only will the growth-inhibiting effect of the interfacial alloy layer described above become saturated, but corrosion will be promoted due to the presence of an excess Si phase in the plating film. Therefore, the Si content in the plating film should be 4.0% by mass or less. In addition, from the viewpoint of suppressing the presence of an excess Si phase, the Si content in the plating film should preferably be 3.0% by mass or less.

[0019] The aforementioned plating film contains 1.0 to 10.0% by mass of Mg. By including Mg in the plating film, the above-mentioned Si can be present in the form of an intermetallic compound of the Mg2Si phase, thereby suppressing the acceleration of corrosion. Furthermore, when Mg is included in the plating film, an intermetallic compound, the MgZn2 phase, is also formed in the plating film, resulting in an improved corrosion resistance. If the Mg content in the plating film is less than 1.0% by mass, sufficient corrosion resistance cannot be ensured because Mg is used for solid solution into the α-Al phase, which is the main phase, rather than for the formation of the intermetallic compounds (Mg2Si, MgZn2). On the other hand, if the Mg content in the plating film is too high, the effect of improving corrosion resistance saturates, and the processability decreases due to the weakening of the α-Al phase; therefore, the content should be 10.0% by mass or less. Furthermore, the Mg content in the plating film is preferably 5.0% by mass or less, from the viewpoint of suppressing dross generation during plating formation, facilitating plating bath management, and further improving the surface appearance.

[0020] Furthermore, in the molten Al-Zn-Si-Mg steel sheet of the present invention, the plating film contains 0.01 to 0.5 mass% of Mn. By containing 0.01 mass% or more of Mn in the plating film, a needle-shaped and / or lumpy Mn-Fe interfacial alloy layer is formed between the plating film and the base metal. This alloy layer acts as an anchor, suppressing the movement of the plating solution film while the plating is completely solidified, thereby suppressing the occurrence of wrinkle-like defects caused by Mg oxides formed on the surface of the plating film. On the other hand, if the Mn content in the plating film exceeds 0.5% by mass, the melting point of the plating bath rises, forcing plating at a high bath temperature. This causes abnormal growth of the interfacial alloy layer formed between the plating film and the base metal, degrading workability and increasing the amount of dross generated, thus degrading the appearance of the plating, especially the surface shape. However, 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 equivalent to that when no Mn is added, thus suppressing the occurrence of dross defects and allowing for 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 a similar viewpoint, it is preferable that the Mn content in the plating film is 0.1 to 0.3% by mass.

[0021] Here, from the viewpoint of balancing the effects of improving corrosion resistance, suppressing wrinkle-like defects, and suppressing dross defects, the ratio of Mn content to Mg content (Mn / Mg) is preferably 0.02 or more (Mn / Mg ≥ 0.02) by mass ratio. From the same viewpoint, it is more preferable that the ratio of Mn content to Mg content (Mn / Mg) is 0.03 or more (Mn / Mg ≥ 0.03).

[0022] Furthermore, in addition to the Al, Si, Mg, and Mn mentioned above, the plating film also contains Zn and unavoidable impurities. Of these, the aforementioned unavoidable impurities include Fe. This Fe is inevitably included in the plating film as a result of the dissolution of steel plates and equipment in the plating bath, and as a result of diffusion from the underlying steel plate during the formation of the interfacial alloy layer. The Fe content in the plating film is usually about 0.3 to 2.0 mass%. Other unavoidable impurities include Ni and Cu. The total content of the aforementioned unavoidable impurities is not particularly limited, but it is preferable that the total content be 5.0% by mass or less, as excessive impurities may affect various properties of the plated steel sheet.

[0023] Furthermore, the plating film preferably contains, if necessary, 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% by 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 also have the effect of stabilizing the spangle size on the plating surface, thereby improving the surface appearance.

[0024] The component composition of the plating film can be determined, for example, by immersing the plating film in hydrochloric acid or the like to dissolve it, and then examining the solution using ICP emission spectrometry or atomic absorption spectrometry. This method is merely one example, and any method that can accurately quantify the component composition of the plating film is acceptable and is not particularly limited. Furthermore, the plating film of the molten Al-Zn-Si-Mg plated steel sheet obtained by the present invention has an overall composition that is almost equivalent to that of the plating bath. Therefore, the composition of the plating film can be controlled with high precision by controlling the composition of the plating bath.

[0025] Furthermore, the molten Al-Zn-Si-Mg plated steel sheet of the present invention is characterized in that, in the area excluding 50 mm from both ends of the steel sheet surface, the height difference of the film surface per 1 mm length is 10 μm or less. When the height difference of the steel sheet surface, i.e., the plated film surface, is 10 μm or less, there are no wrinkle-like defects, and an excellent surface appearance can be obtained. As described above, the wrinkle-like defects are defects in which the surface of the plating film has a wrinkled, uneven shape due to Mg-based oxides, and appear as white streaks on the surface of the plating film. Therefore, suppressing wrinkle-like defects means that the streaks are not visible. For this reason, by keeping the height difference within a 1 mm length range on the surface of the steel sheet to within 10 μm, the streaks are not visible and an excellent surface appearance can be obtained. From a similar viewpoint, it is preferable that the height difference within a 1 mm length range on the surface of the steel sheet be within 5 μm. 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, it refers to the surface of the plating film. Furthermore, the method for obtaining the height difference within a 1 mm length range on the steel sheet surface can be obtained, for example, by using a laser microscope to measure the height difference within a 1 mm range at any 100 locations on a hot-dip Al-Zn-Si-Mg plated steel sheet with a plating film formed on it, and then calculating the average of the measured values.

[0026] Furthermore, the reason for controlling the height difference of the steel plate surface in the area excluding 50 mm from both ends of the steel plate surface is that, generally, it is difficult to control the amount of coating at both ends of a hot-dip galvanized steel plate compared to the center of the plate, and the cooling rate is also different, making it prone to wrinkle-like defects. In addition, these ends of the steel plate are often trimmed before use, so the surface appearance is hardly a problem.

[0027] The method for controlling the height difference on the steel plate surface per 1 mm of length of the plating film to 10 μm or less is not particularly limited to the Mn content in the plating film (0.01 to 0.5 mass%) mentioned above. For example, the height difference on the film surface can be suppressed by adjusting the ratio of Mg and Mn content in the plating film or by performing other surface treatments to suppress the height difference.

[0028] Furthermore, in the molten Al-Zn-Si-Mg plated steel sheet of the present invention, it is preferable that an alloy layer containing Mn is formed at the interface between the plating film and the base metal. The alloy layer is formed by an alloying reaction between Mn and Al, Fe, Si, etc. in the bath, and is mainly an Fe-Al-Mn or Fe-Al-Si-Mn intermetallic compound. Because it has an uneven shape with respect to the plating surface direction, the adhesion between the plating film and the base steel sheet can be improved by an anchoring effect. Furthermore, the anchoring effect of the Mn-containing alloy layer manifests itself when the plating film solidifies, and the Mn-based interfacial alloy layer minimizes the movement of the plating solution film, thus also having the effect of suppressing the occurrence of wrinkle-like defects.

[0029] Furthermore, in the molten 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, measured by X-ray diffraction, satisfy the following relationship (1) in order to provide excellent corrosion resistance. Si (111) / Mg2Si (111)≦0.8 ···(1) Si (111): Diffraction intensity of the (111) plane (interplanar spacing d=0.3135 nm) of Si, Mg2Si (111): Diffraction intensity of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si As described above, in the present invention, it is preferable to control the ratio of Mg2Si phase and Si phase formed in the plating film by the inclusion of Mg and Si to a specific ratio. The effects of these on corrosion resistance are currently under investigation and many aspects remain unclear, but the following mechanism is hypothesized. When a molten Al-Zn-Si-Mg 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 formed corrosion products. In such a Mg-rich environment, the formed corrosion products are less likely to decompose, and as a result, the protective effect of the plating film is presumed to be enhanced. Furthermore, this enhanced protective effect of the plating film is more reliably exhibited when Si in the plating film exists as an Mg2Si phase rather than a Si phase; therefore, it is considered effective to reduce the ratio of the Si phase to the Mg2Si phase.

[0030] The ratio of Mg2Si to Si in the plating film is preferably such that relation (1): Si (111) / Mg2Si (111) ≤ 0.8, using the diffraction peak intensity obtained by X-ray diffraction. However, if the ratio of Mg2Si and Si in the plating film does not satisfy relation (1), that is, if Si (111) / Mg2Si (111) > 0.8, then there is a large amount of Si phase in the plating film, making it difficult to obtain the aforementioned Mg-rich environment near the corrosion products, and thus the protective effect of the plating film is less likely to be obtained. From a similar viewpoint, the 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 relation (1) above, Si (111) is the diffraction intensity of the (111) plane of Si (interplanar spacing d = 0.3135 nm), and Mg2Si (111) is the diffraction intensity of the (111) plane of Mg2Si (interplanar spacing d = 0.3668 nm).

[0031] Furthermore, as a method for measuring Si(111) and Mg2Si(111) by X-ray diffraction, a portion of the plating film is mechanically scraped off, and X-ray diffraction is performed on the powdered material (powder X-ray diffraction measurement method). For the measurement of diffraction intensity, the diffraction peak intensity of Si corresponding to the interplanar spacing d=0.3135 nm and the diffraction peak intensity of Mg2Si corresponding to the interplanar spacing d=0.3668 nm are measured, and the Si(111) / Mg2Si(111) can be obtained by calculating the ratio of these two values. When performing powder X-ray diffraction measurements, the amount of plating film required (the amount of plating film to be removed) should be 0.1 g or more, preferably 0.3 g or more, from the viewpoint of accurately measuring Si (111) and Mg2Si (111). Furthermore, when removing the plating film, steel sheet components other than the plating film may be included in the powder, but these intermetallic compound phases are contained only in the plating film and do not affect the peak intensity mentioned above. Moreover, the reason for performing X-ray diffraction on powdered plating film is that if X-ray diffraction is performed on a plating film formed on a plated steel sheet, it is difficult to calculate the correct phase ratio due to the influence of the surface orientation of the solidification structure of the plating film.

[0032] Furthermore, in the molten Al-Zn-Si-Mg plated steel sheet of the present invention, it is preferable that the diffraction intensity of Si in the plating film, measured by X-ray diffraction, satisfies the following relationship (2), since corrosion resistance can be improved more stably. Si (111) = 0 ···(2) Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3135 nm) of Si Generally, in the dissolution reaction of Al alloys in aqueous solutions, the presence of the Si phase as a cathode site is known to promote the dissolution of the surrounding α-Al phase. Therefore, reducing the amount of Si phase is effective in suppressing the dissolution of the α-Al phase, and among these methods, creating a film without any Si phase (making the diffraction peak intensity of Si(111) zero), as shown in relation (2), is the best for stabilizing corrosion resistance. The method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction is as described above.

[0033] Here, there are no particular limitations on the methods for satisfying the above-mentioned relationships (1) and (2). For example, in order to satisfy relationships (1) and (2), the relative abundance of Mg2Si and Si (diffraction intensity of Mg2Si (111) and Si (111)) can be controlled by adjusting the balance of the Si content, Mg content, and Al content in the plating film. However, the balance of the Si content, Mg content, and Al content in the plating film does not necessarily mean that relationships (1) and (2) can be satisfied by setting the content ratio to a constant ratio; for example, it may be necessary to change the content ratio of Mg and Al depending on the Si content (mass%). In addition to adjusting the balance of Si, Mg, and Al content in the plating film, the diffraction intensity of Mg2Si(111) and Si(111) can be controlled to satisfy relationships (1) and (2) by adjusting the conditions during plating film formation (for example, cooling conditions after plating).

[0034] The amount of the aforementioned plating film applied is 45 to 120 g / m² per side, from the viewpoint of satisfying various characteristics. 2 Preferably, the amount of the plating film is 45 g / m². 2 In the above case, sufficient corrosion resistance can be obtained even for applications requiring long-term corrosion resistance, such as building materials, and the amount of the plating film attached is 120 g / m². 2 In the following cases, excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. From a similar viewpoint, the amount of plating film attached is 45-100 g / m². 2 It is preferable that it be so. The amount of plating film attached can be determined, for example, by dissolving and peeling the plating film from a specific area using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and calculating the amount from the difference in steel sheet weight before and after peeling. To determine the amount of plating attached to one side using this method, the plated surface of the non-target side is sealed with tape to prevent exposure, and then the aforementioned dissolution is performed.

[0035] Furthermore, the base steel sheet constituting the molten 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 not particularly limited. For example, in the case of hot-rolled steel sheets, those that have undergone a hot-rolling process and a pickling process can be used, and in the case of cold-rolled steel sheets, a cold-rolling process can be added to the manufacturing process. Furthermore, in order to obtain the properties of the steel sheet, it is also possible to go through a recrystallization annealing process or the like before the hot-dip galvanizing process.

[0036] The method for producing the molten Al-Zn-Si-Mg plated steel sheet of the present invention is not particularly limited. For example, it can be produced by washing, heating, and immersing the base steel sheet in a plating bath using a continuous molten plating facility. In the heating process of the steel sheet, 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 trace oxide film present on the surface.

[0037] Furthermore, as mentioned above, regarding the plating bath used when manufacturing the molten Al-Zn-Si-Mg plated steel sheet of the present invention, since the overall composition of the plating film is approximately the same as the composition of the plating bath, a plating bath containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, Mg: 1.0-10.0% by mass, and Mn: 0.1-0.5% by mass, with the remainder consisting of Zn, Fe, and unavoidable impurities, can be used.

[0038] Furthermore, while the temperature of the plating bath is not particularly limited, it is preferable to set it in the temperature range of (melting point + 20°C) to 650°C. The reason the lower limit of the bath temperature is set to the melting point + 20°C is that in order to perform the molten plating process, it is necessary to raise the bath temperature above the solidification point, and setting it to the melting point + 20°C prevents solidification due to a localized drop in the bath temperature of the plating bath. On the other hand, the reason the upper limit of the bath temperature is set to 650°C is that if it exceeds 650°C, rapid cooling of the plating film becomes difficult, and there is a risk that the interfacial alloy layer formed between the plating film and the steel sheet will become thicker.

[0039] Furthermore, while there are no particular limitations on the temperature of the base steel plate that enters the plating bath (entry plate temperature), it is preferable to control it to within ±20°C of the temperature of the plating bath from the viewpoint of ensuring plating characteristics and preventing changes in the bath temperature during continuous hot-dip galvanizing operations. Furthermore, the immersion time of the base steel plate in the plating bath is 0.5 seconds or more. This is because if it is less than 0.5 seconds, there is a risk that a sufficient plating film may not be formed on the surface of the base steel plate. There is no particular upper limit to the immersion time, but since a longer immersion time may result in a thicker interfacial alloy layer formed between the plating film and the steel plate, it is preferable to keep it within 8 seconds.

[0040] Furthermore, depending on the required performance, a coating can be formed on the hot-dip Al-Zn-Si-Mg plated steel sheet either directly or via an intermediate layer. The method for forming the coating film is not particularly limited and can be appropriately selected according to the required performance. Examples of coating methods include roll coater coating, curtain flow coating, and spray coating. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying it using means such as hot air drying, infrared heating, or induction heating. Furthermore, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the hot-dip galvanized steel sheet and the coating film.

[0041] <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. Of these, the composition of the plating film is the same as that of the plating film of the molten Al-Zn-Si-Mg plated steel sheet of the present invention described above. Furthermore, the other components of the plating film are the same as those of the plating film of the molten Al-Zn-Si-Mg plated steel sheet of the present invention described above.

[0042] The surface-treated steel sheet of the present invention has a chemical conversion coating formed on the aforementioned coating. Furthermore, the chemical conversion coating only needs to be formed on at least one side of the surface-treated steel sheet, and may also be formed on both sides of the surface-treated steel sheet depending on the application and required performance.

[0043] Furthermore, in the surface-treated steel sheet of the present invention, the chemical conversion coating is characterized by containing at least one resin selected from 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 P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound. By forming the aforementioned chemical conversion film on the plated film, the affinity with the plated film is increased, enabling the uniform formation of the chemical conversion film on the plated film. In addition, the rust-preventive and barrier effects of the chemical conversion film can be enhanced. As a result, stable corrosion resistance and white rust resistance of the surface-treated steel sheet of the present invention can be achieved.

[0044] Here, the resin constituting the chemical conversion coating is 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 a similar viewpoint, it is preferable that the resin 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.

[0045] For example, the epoxy resin can be a glycidyl ether of bisphenol A type, bisphenol F type, novolac type epoxy resin, a glycidyl ether of bisphenol A type epoxy resin with propylene oxide, ethylene oxide, or polyalkylene glycol added, aliphatic epoxy resin, alicyclic epoxy resin, polyether epoxy resin, etc.

[0046] For example, the urethane resin can be an oil-modified polyurethane resin, an alkyd polyurethane resin, a polyester polyurethane resin, a polyether polyurethane resin, a polycarbonate polyurethane resin, or the like.

[0047] Examples of the acrylic resin include polyacrylic acid and its copolymers, polyacrylic acid esters and their copolymers, polymethacrylic acid and its copolymers, polymethacrylic acid esters and their copolymers, urethane-acrylic acid copolymers (or urethane-modified acrylic resins), styrene-acrylic acid copolymers, and moreover, resins modified with other alkyd resins, epoxy resins, phenolic resins, etc. can be used.

[0048] Examples of the aforementioned acrylic silicone resin include a resin having hydrolyzable alkoxysilyl groups in the side chains or terminals of an acrylic copolymer as the main component, to which a curing agent is added. Furthermore, when an acrylic silicone resin is used, excellent weather resistance can be expected in addition to corrosion resistance.

[0049] Examples of the alkyd resins mentioned above include oil-modified alkyd resins, rosin-modified alkyd resins, phenol-modified alkyd resins, styrene-modified alkyd resins, silicon-modified alkyd resins, acrylic-modified alkyd resins, oil-free alkyd resins, and high molecular weight oil-free alkyd resins.

[0050] The polyester resin is a polycondensate synthesized by dehydrating and condensing a polycarboxylic acid and a polyalcohol to form an ester bond. Examples of polycarboxylic acids include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of polyalcohols include ethylene glycol, 1,3-propanediol, 1,4-butanediol, and 1,4-cyclohexanedimethanol. Specifically, examples of polyesters include polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, and polybutylene naphthalate. Acrylic-modified versions of these polyester resins can also be used.

[0051] Examples of the polyalkylene resins include ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, carboxyl-modified polyolefin resins, ethylene-unsaturated carboxylic acid copolymers, ethylene-ionomers, and the like. Furthermore, these resins can be modified with other alkyd resins, epoxy resins, phenolic resins, and the like.

[0052] The amino resin mentioned above is a thermosetting resin produced by the reaction of an amine or amide compound with an aldehyde, and examples include melamine resin, guanamine resin, and thiourea resin. However, from the viewpoint of corrosion resistance, weather resistance, and adhesion, it is preferable to use melamine resin. There are no particular limitations on the melamine resin, but examples include butylated melamine resin, methylated melamine resin, and aqueous melamine resin.

[0053] Examples of the aforementioned fluororesins 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.

[0054] Furthermore, the resin constituting the chemical conversion film preferably uses a curing agent in particular, with the aim of improving corrosion resistance and processability. As curing agents, urea resins (such as butylated urea resin), melamine resins (such as butylated melamine resin, butylated ether melamine resin, etc.), butylated urea / melamine resin, amino resins such as benzoguanamine resin, blocked isocyanates, oxazoline compounds, phenolic resins, etc. can be used as appropriate.

[0055] Furthermore, the metal compound constituting the chemical conversion film is at least one 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. From a similar viewpoint, it is preferable that the metal compound contains at least one of P compounds, Si compounds, and V compounds.

[0056] Here, the P compound, when included in the chemical conversion coating, can improve corrosion resistance and sweat resistance. The P compound is a compound containing P, and may contain one or more selected from, for example, inorganic phosphoric acid, organic phosphoric acid, and salts thereof.

[0057] The inorganic phosphoric acid, organic phosphoric acid, and salts thereof can be any compound without particular limitations. For example, it is preferable to use one or more selected from phosphoric acid, monophosphate, dicaphosphate, tertiary phosphate, pyrophosphate, pyrophosphate, tripolyphosphate, tripolyphosphate, phosphorous acid, phosphate, hypophosphorous acid, and hypophosphorous acid as the inorganic phosphoric acid. It is also preferable to use phosphonic acid (phosphonic acid compound) as the organic phosphoric acid. Furthermore, it is preferable to use one or more selected from nitrilotrismethylenephosphonic acid, phosphobutanetricarboxylic acid, methyldiphosphonic acid, methylenephosphonic acid, and ethylidenediphosphonic acid as the phosphonic acid. Furthermore, if the P compound is a salt, it is preferable that the salt is a salt of an element from Group 1 to Group 13 of the periodic table, more preferably a metal salt, and more preferably one or more selected from alkali metal salts and alkaline earth metal salts.

[0058] When the chemical conversion treatment solution containing the above-mentioned P compound is applied to a plated steel sheet, the surface of the plated film is etched by the action of the P compound, and a concentrated layer is formed on the plated film side of the chemical conversion film, incorporating the constituent elements of the plated film: Al, Zn, Si, and Mg. 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 between 0.25% by mass and 5% by mass. If the concentration of the P compound is less than 0.25% by mass, the etching effect will be insufficient, reducing adhesion to the plating interface and potentially decreasing corrosion resistance of flat surfaces. Furthermore, the corrosion resistance and sweat resistance of damaged areas of the plating and film caused by defects, cut edges, and processing may also decrease. From a similar viewpoint, 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% by mass, not only will the lifespan of the chemical conversion treatment solution be shortened, but the appearance of the formed film may become uneven, and the amount of P leached from the chemical conversion film may increase, potentially reducing resistance to blackening. From a similar viewpoint, the concentration of the P compound is preferably 3.5% by mass or less, more preferably 2.5% by mass or less. Regarding the content of the P compound in the aforementioned chemical conversion coating, for example, by applying and drying a chemical conversion treatment solution with a P compound concentration of 0.25% to 5% by mass, the amount of P adhering to the chemical conversion coating after drying is 5 to 100 mg / m². 2 It can be done this way.

[0059] The Si compound is a component that forms the backbone of the chemical conversion film together with the resin, and enhances its affinity with the plating film, enabling the uniform formation of the chemical conversion film. The Si compound is a compound containing Si, and preferably contains one or more selected from, for example, silica, trialkoxysilane, tetraalkoxysilane, and silane coupling agents.

[0060] The silica is not particularly limited and any type can be used. For example, at least one of wet silica and dry silica can be used. As a type of wet silica, colloidal silica, for example, Snowtex O, C, N, S, 20, OS, OXS, NS, etc. manufactured by Nissan Chemical Corporation can be suitably used. As a type of dry silica, for example, AEROSIL 50, 130, 200, 300, 380, etc. manufactured by Nippon Aerosil Co., Ltd. can be suitably used.

[0061] The trialkoxysilane mentioned above 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.

[0062] The tetraalkoxysilane mentioned above is not particularly limited and any type 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 tetraalkoxysilanes include tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane.

[0063] The silane coupling agent can be any agent without particular limitations. Examples include γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropylmethyldiethoxysilane, γ-aminopropyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-mercaptopropylmethyldimethoxysilane, and γ-mercaptopropyltrimethoxysilane, vinyltriethoxysilane, γ-isocyanatetopropyltriethoxysilane, and the like.

[0064] Furthermore, by incorporating the Si compound into the chemical conversion coating, the Si compound undergoes dehydration condensation to form an amorphous chemical conversion coating having siloxane bonds that provide a high barrier effect against corrosion factors. In addition, by bonding with the aforementioned resin, a chemical conversion coating with even higher barrier properties is formed. Moreover, in corrosive environments, dense and stable corrosion products are formed in defective areas and damaged areas of the plating or coating caused by processing, and the combined effect with the plating film also has the effect of suppressing corrosion of the underlying steel plate. From the viewpoint of having a high effect in forming stable corrosion products, it is preferable to use at least one of colloidal silica and dry silica as the Si compound.

[0065] The concentration of the Si compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is set to 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 higher, a barrier effect due to siloxane bonding can be obtained, resulting in improved corrosion resistance not only on flat surfaces but also on defective areas, cut areas, and damaged areas caused by processing, as well as improved sweat resistance. Furthermore, if the concentration of the Si compound is 9.5% by mass or lower, the lifespan 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 amount of Si adhering to the chemical conversion film after drying is 2 to 95 mg / m². 2 It can be done this way.

[0066] The Co compound and the Ni compound, when included in the chemical conversion coating, can improve resistance to blackening. 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 susceptible to oxidation than Al, Zn, Si, and Mg. 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 film (forming a concentrated layer), the concentrated layer acts as a barrier against corrosion, thereby improving resistance to blackening.

[0067] By using a chemical conversion treatment solution containing the aforementioned Co compound, Co can be incorporated into the chemical conversion coating and incorporated into the concentrated layer. It is preferable to use a cobalt salt as the Co compound. More preferably, one or more cobalt salts selected from cobalt sulfate, cobalt carbonate, and cobalt chloride are used. Furthermore, by using a chemical conversion treatment solution containing the Ni compound, Ni can be incorporated into the chemical conversion coating and into the concentrated layer. It is preferable to use a nickel salt as the Ni compound. It is even more preferable to use one or more nickel salts selected from nickel sulfate, nickel carbonate, and nickel chloride.

[0068] 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% 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 uneven, which may reduce the corrosion resistance of flat surfaces as well as the corrosion resistance of defective areas, cut edges, and areas with plating or film damage caused by processing. From a similar viewpoint, it is preferably 0.5% by mass or more, and 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 film when it is formed tends to be uneven, which may reduce corrosion resistance. From a similar viewpoint, it is preferably 4.0% by mass or less, and more preferably 3.0% by mass or less. By applying and drying a chemical conversion treatment solution in which the total concentration of the Co compound and / or Ni compound is 0.25% to 5% by mass, the total amount of Co and Ni deposited on the chemical conversion film after drying can be 5 to 100 mg / m². 2 It can be done this way.

[0069] By including the Al compound, Zn compound, and Mg compound in the chemical conversion treatment solution, a concentrated layer containing at least one of Al, Zn, and Mg can be formed on the plating 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 they are preferably inorganic compounds, and preferably salts, chlorides, oxides, or hydroxides.

[0070] 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.

[0071] The concentrations of the Al compound, Zn compound, and / or Mg compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film are preferably 0.25% by mass to 5% by mass in total. If the total concentration is 0.25% by mass or more, the concentrated layer can be formed more effectively, and as a result, corrosion resistance can be further improved. On the other hand, if the total concentration is 5% by mass or less, the appearance of the chemical conversion film becomes more uniform, and the corrosion resistance of flat surfaces, defective areas, and damaged areas of the plating or film caused by processing is further improved.

[0072] The V compound, when included in the chemical conversion coating, dissolves appropriately in a corrosive environment and combines with zinc ions and other plating components that also dissolve in a corrosive environment, forming a dense protective film. The formed protective film further enhances corrosion resistance not only to the flat surfaces of the steel plate, but also to defects, damaged areas of the plating film caused by processing, and corrosion progressing from the cut edges to the flat surfaces.

[0073] The V compound is a compound containing V, and examples include one or more selected from sodium metavanadate, vanadyl sulfate, and vanadium acetylacetonate.

[0074] The concentration of the V compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is preferably 0.05% to 4% by mass. If the concentration of the V compound is 0.05% by mass or higher, it readily dissolves in a corrosive environment to form a protective film, improving the corrosion resistance of defective areas, cut edges, and damaged areas of the plating film caused by processing. On the other hand, if the concentration of the V compound exceeds 4% by mass, the appearance of the chemical conversion film tends to become uneven, and the resistance to blackening also decreases.

[0075] The Mo compound, when included in the chemical conversion coating, can enhance the resistance of the surface-treated steel sheet to blackening. The Mo compound is a compound containing Mo and can be obtained by adding one or both of molybdic acid and / or molybdate to the chemical conversion treatment solution. Examples of the molybdate salt include one or more selected from sodium molybdate, potassium molybdate, magnesium molybdate, and zinc molybdate.

[0076] The concentration of the Mo compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is preferably 0.01% to 3% by mass. If the concentration of the Mo compound is 0.01% by mass or higher, the formation of oxygen-deficient zinc oxide is further suppressed, and resistance to blackening can be further improved. On the other hand, if the concentration of the Mo compound is 3% by mass or lower, the lifespan of the chemical conversion treatment solution is further extended, and corrosion resistance can be further improved.

[0077] The inclusion of the Zr compound and the Ti compound in the chemical conversion coating prevents the coating from becoming porous and densifies the coating. As a result, corrosion factors become less likely to penetrate the chemical conversion coating, thereby improving corrosion resistance.

[0078] The Zr compound is a Zr-containing compound, and for example, one or more selected from zirconyl acetate, zirconyl sulfate, potassium zirconyl carbonate, sodium zirconyl carbonate, and ammonium zirconyl carbonate can be used. Among these, organotitanium chelate compounds are preferred because they densify the film when the chemical treatment solution is dried to form a film, resulting in superior corrosion resistance.

[0079] The Ti compound is a Ti-containing compound, 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.

[0080] The concentration of the Zr compound and / or Ti compound in the chemical conversion treatment solution for forming the aforementioned chemical conversion film is preferably 0.2% by mass to 20% by mass in total. If the total concentration of the Zr compound and / or Ti compound is 0.2% by mass or more, the effect of suppressing the penetration of corrosive factors is enhanced, and the corrosion resistance of not only flat surfaces but also defective areas, cut edges, and areas of plating film damage caused by processing can be further improved. On the other hand, if the total concentration of the Zr compound and / or Ti compound is 20% by mass or less, the lifespan of the chemical conversion treatment solution can be further extended.

[0081] The aforementioned Ca compound, when included in the chemical conversion coating, can exert the effect of reducing the corrosion rate.

[0082] The Ca compound is a compound containing Ca, such as Ca oxides, Ca nitrates, Ca sulfates, and Ca-containing intermetallic compounds. More specifically, the Ca compound can be CaO, CaCO3, Ca(OH)2, Ca(NO3)2·4H2O, CaSO4·2H2O, etc. The content of the Ca compound in the chemical conversion film is not particularly limited.

[0083] Furthermore, the aforementioned chemical conversion film may contain various known components commonly used in the paint industry, as needed. Examples include various surface modifiers such as leveling agents and defoaming agents, various additives such as dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as coloring pigments, extender pigments, and luminescence agents, curing catalysts, organic solvents, and lubricants.

[0084] Furthermore, in the surface-treated steel sheet of the present invention, it is preferable that the chemical conversion coating does not contain harmful components such as hexavalent chromium, trivalent chromium, and fluorine. This is because the chemical conversion treatment solution used to form the chemical conversion coating does not contain these harmful components, resulting in higher safety and a smaller environmental impact.

[0085] Also, the deposition amount of the chemical conversion film is not particularly limited. For example, from the viewpoint of more reliably ensuring corrosion resistance and preventing peeling of the chemical conversion film, the deposition amount of the chemical conversion film is preferably 0.1 to 3.0 g / m 2 and more preferably 0.5 to 2.5 g / m 2 By setting the deposition amount of the chemical conversion film to 0.1 g / m 2 or more, corrosion resistance can be more reliably ensured. By setting the deposition amount of the chemical conversion film to 3.0 g / m 2 or less, cracking and peeling of the chemical conversion film can be prevented. The deposition amount of the chemical conversion film may be determined by an appropriately selected method from existing methods such as a method of measuring the abundance of elements whose content in the film is known in advance by fluorescent X-ray analysis of the film.

[0086] In addition, the method for forming the chemical conversion film is not particularly limited and can be appropriately selected according to the required performance, manufacturing equipment, etc. For example, a chemical conversion treatment solution is continuously applied onto the plating film using a roll coater or the like, and then dried at a peak metal temperature (PMT) of about 60 to 200°C using hot air, induction heating, or the like. For the application of the chemical conversion treatment solution, in addition to a roll coater, known methods such as an airless spray, an electrostatic spray, and a curtain flow coater can be appropriately employed. Furthermore, the chemical conversion film may be either a single-layer film or a multi-layer film as long as it contains the resin and the metal compound, and is not particularly limited.

[0087] And, similar to the surface of the molten Al-Zn-Si-Mg-based plated steel sheet of the present invention described above, in the range excluding 50 mm from both ends of the steel sheet surface (chemical conversion film surface), the height difference of the steel sheet surface per 1 mm is 10 μm or less for the surface-treated steel sheet of the present invention. When the height difference of the chemical conversion film surface is 10 μm or less, there are no wrinkle-like defects, and an excellent surface appearance can be obtained. As described above, the wrinkle-like defects are defects in which the surface of the chemical conversion coating has a wrinkled, uneven shape due to Mg-based oxides, and appear as white streaks on the surface of the chemical conversion coating. Therefore, suppressing wrinkle-like defects means that the streaks are not visible. For this reason, by keeping the height difference within a 1 mm range on the steel plate surface, i.e., the surface of the chemical conversion coating, to within 10 μm, the streaks are not visible, and an excellent surface appearance can be obtained. From a similar viewpoint, it is preferable that the height difference within a 1 mm range on the steel plate surface be within 5 μm. The steel plate surface refers to the outermost surface of the steel plate, and in the case of surface-treated steel plates, it refers to the chemical conversion coating surface. Furthermore, the method for obtaining the height difference within a 1 mm range on the chemical conversion coating surface can be obtained by using a laser microscope to measure the height difference within a 1 mm range at any 100 locations on the surface-treated steel plate, similar to the case of the plated coating surface described above, and calculating the average of the measured values.

[0088] Furthermore, the reason for controlling the height difference on the steel plate surface in the area excluding 50 mm from both ends of the steel plate is that, generally, it is difficult to control the amount of coating at both ends of a hot-dip galvanized steel plate compared to the center of the plate, and the cooling rate is also different, making it prone to wrinkle-like defects. In addition, these ends of the steel plate are often trimmed before use, so the surface appearance is hardly a problem.

[0089] Regarding the method for controlling the height difference of the film surface per 1 mm length on the steel plate surface to 10 μm or less, it is important to suppress the height difference of the plating film surface as described above. This can be achieved by adjusting the content ratio of Mg and Mn in the plating film or by performing other surface treatments to suppress the height difference.

[0090] Furthermore, the surface-treated steel sheet of the present invention may also have a coating film formed on the chemical conversion film as needed.

[0091] The present invention relates to a method for manufacturing a surface-treated steel sheet, comprising a plating film and a chemical conversion film formed on the plating film. Furthermore, in the manufacturing method of the present invention, the chemical conversion film contains at least one resin selected from 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 P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound. The formation of the aforementioned plating film comprises a hot-dip plating process in which a base steel plate is immersed in a plating bath having a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities.

[0092] The conditions for the hot-dip plating process are the same as those described in the section on the hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention.

[0093] <Painted steel sheet> The painted steel sheet of the present invention is a painted steel sheet in which a coating film is formed directly on a plating film or via a chemical conversion film. Of these, the composition of the plating film is the same as that of the plating film of the molten Al-Zn-Si-Mg plated steel sheet of the present invention described above. Furthermore, the other components of the plating film are the same as those of the plating film of the molten Al-Zn-Si-Mg plated steel sheet of the present invention described above.

[0094] The painted steel sheet of the present invention can have a chemical conversion film formed on the plated film. Furthermore, the chemical conversion coating only needs to be formed on at least one side of the painted steel sheet, but it can also be formed on both sides of the painted steel sheet depending on the application and required performance.

[0095] • Chemical conversion coating Furthermore, the painted steel sheet of the present invention is characterized in that the chemical conversion film contains a resin component comprising (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton in total at a concentration of 30 to 50% by mass, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass ratio, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. By forming the aforementioned chemical conversion coating on the plated film, the strength and adhesion of the chemical conversion coating can be increased while also improving corrosion resistance.

[0096] Here, the resin components constituting the chemical conversion film include (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton.

[0097] (a) The anionic polyurethane resin having an ester bond mentioned above includes resins obtained by copolymerizing a dimethylol alkyl acid with a reaction product of a polyester polyol and a diisocyanate or polyisocyanate having two or more isocyanate groups. Furthermore, a chemical treatment solution can be obtained by dispersing it in a liquid such as water by a known method.

[0098] Examples of the aforementioned polyester polyols include polyesters obtained by a dehydration condensation reaction from a glycol component and an acid component such as an ester-forming derivative of a hydroxylcarboxylic acid, polyesters obtained by a ring-opening polymerization reaction of cyclic ester compounds such as ε-caprolactone, and copolymer polyesters thereof. Examples of the aforementioned polyisocyanates include aromatic polyisocyanates, aliphatic polyisocyanates, and alicyclic polyisocyanates. Examples of the aforementioned aromatic polyisocyanates include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, m-xylene diisocyanate, diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate, 2,2-diphenylmethane diisocyanate, triphenylmethane triisocyanate, polymethylene polyphenyl polyisocyanate, naphthalene diisocyanate, and derivatives thereof (e.g., prepolymers obtained by reaction with polyols, modified polyisocyanates such as carbodiimide compounds of diphenylmethane diisocyanate, etc.).

[0099] Furthermore, when synthesizing urethane by reacting the polyester polyol with the diisocyanate or polyisocyanate, for example, by copolymerizing a dimethylol alkyl acid and self-emulsifying it to make it water-soluble (water-dispersible), the anionic polyurethane resin having an ester bond (a) can be obtained. In this case, examples of dimethylol alkyl acids include dimethylol alkyl acids having 2 to 6 carbon atoms, and more specifically, dimethylolethane acid, dimethylolpropanoic acid, dimethylolbutanoic acid, dimethylolheptanoic acid, and dimethylolhexanoic acid.

[0100] Furthermore, known epoxy resins can be used for the epoxy resin having the bisphenol skeleton (b) described above. Examples include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD ​​type epoxy resin, and bisphenol S type epoxy resin. These epoxy resins can be obtained by reacting bisphenol compounds such as bisphenol A, bisphenol F, bisphenol AD, and bisphenol S with epichlorohydrin in the presence of an alkaline catalyst. In particular, component [A] preferably contains bisphenol A type epoxy resin or bisphenol F type epoxy resin, and more preferably contains bisphenol A type epoxy resin. The epoxy resin having the bisphenol skeleton (b) can be dispersed in a liquid such as water by known methods to obtain a chemical conversion solution.

[0101] The resin component acts as a binder for the chemical conversion coating. The (a) anionic polyurethane resin having ester bonds that constitutes the binder is flexible, which helps to prevent the chemical conversion coating from being damaged (peeled) when processed. The (b) epoxy resin having a bisphenol skeleton improves adhesion to the underlying zinc-plated steel sheet and the upper primer coating. The resin component is present in the chemical conversion film in a total amount of 30 to 50% by mass. If the resin component content is less than 30% by mass, the binder effect of the chemical conversion film decreases, and if it exceeds 50% by mass, the function of the inorganic component shown below, such as the inhibitory effect, decreases. From a similar viewpoint, the resin component content in the chemical conversion film is preferably 35 to 45% by mass.

[0102] Furthermore, the resin component must have a mass ratio ((a):(b)) of (a) anionic polyurethane resin having an ester bond and (b) epoxy resin having a bisphenol skeleton in the range of 3:97 to 60:40. If (a):(b) is outside this range, sufficient corrosion resistance cannot be obtained due to a decrease in flexibility and adhesion as a chemical conversion treatment film. From a similar viewpoint, it is preferable that (a):(b) be 10:90 to 55:45.

[0103] Furthermore, depending on the required performance, the resin component may include resins other than (a) anionic polyurethane resin having ester bonds and (b) epoxy resin having a bisphenol skeleton (other resin components). The other resin components are not particularly limited, and for example, at least one or more selected from acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins can be used in combination. If the resin component includes other resins, the total content of (a) anionic polyurethane resin having ester bonds and (b) epoxy resin having a bisphenol skeleton is preferably 50% by mass or more, and more preferably 75% by mass or more. This is to ensure a reduction in flexibility and adhesion as a treated film.

[0104] Furthermore, the chemical conversion coating contains, as inorganic compounds, 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. By including these compounds, the corrosion resistance of the chemical conversion coating can be improved.

[0105] The vanadium compound is added to the chemical conversion treatment solution and acts as a rust inhibitor. The presence of the vanadium compound in the chemical conversion coating allows it to dissolve appropriately in a corrosive environment, combining with zinc ions and other plating components that also dissolve in the corrosive environment to form a dense protective film. This formed protective film further enhances corrosion resistance not only to the flat surfaces of the steel plate, but also to defects, damaged areas of the plating film caused by processing, and corrosion progressing from the cut edges to the flat surfaces. Examples of the vanadium compounds include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. In particular, it is desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

[0106] Furthermore, the vanadium compound content in the chemical conversion treatment film is 2 to 10% by mass. If the vanadium compound content in the chemical conversion treatment film is less than 2% by mass, the inhibitory effect is insufficient, leading to a decrease in corrosion resistance. On the other hand, if the vanadium compound content exceeds 10% by mass, it leads to a decrease in the moisture resistance of the chemical conversion treatment film.

[0107] The zirconium compound is contained in the aforementioned chemical conversion coating, and through its reaction with the plating metal and coexistence with the resin component, it is expected to improve the strength and corrosion resistance of the chemical conversion coating. Furthermore, the zirconium compound itself contributes to the formation of a dense chemical conversion coating, and its excellent covering properties allow for a barrier effect. Examples of the aforementioned zirconium compounds include neutralized salts of zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, and zirconium chloride.

[0108] Furthermore, the zirconium compound content in the chemical conversion coating is 40 to 60% by mass. If the zirconium compound content in the chemical conversion coating is less than 40% by mass, it will lead to a decrease in the strength and corrosion resistance of the chemical conversion coating, and if the zirconium compound content exceeds 60% by mass, the chemical conversion coating will become brittle, and when subjected to severe processing, the chemical conversion coating may break or peel off.

[0109] The fluorine compound is contained in the chemical conversion coating and acts as an adhesion enhancer to the plating film. As a result, the corrosion resistance of the chemical conversion coating can be improved. As the fluorine compound, for example, fluoride salts such as ammonium salts, sodium salts, and potassium salts, or fluorine compounds such as ferrous fluoride and ferric fluoride can be used. Among these, it is preferable to use ammonium fluoride or fluoride salts such as sodium fluoride and potassium fluoride.

[0110] Furthermore, the fluorine compound content in the chemical conversion coating is 0.5 to 5% by mass. If the fluorine compound content in the chemical conversion coating is less than 0.5% by mass, sufficient adhesion to the processed area cannot be obtained, and if the fluorine compound content exceeds 5% by mass, the moisture resistance of the chemical conversion coating decreases.

[0111] Furthermore, the amount of the chemical conversion coating applied is not particularly limited. For example, from the viewpoint of ensuring corrosion resistance more reliably while improving the adhesion of the chemical conversion coating, the amount of the chemical conversion coating applied may be 0.025 to 0.5 g / m². 2 It is preferable to have the amount of the chemical conversion coating attached to be 0.025 g / m². 2 By doing so, corrosion resistance can be more reliably ensured, and the amount of the chemical conversion coating to adhere is 0.5 g / m². 2 The peeling of the chemical conversion coating can be suppressed by doing the following. The amount of the chemical conversion coating can be determined by a method appropriately selected from existing techniques, such as measuring the amount of elements whose content in the coating is known in advance by analyzing the coating with X-ray fluorescence.

[0112] The method for forming the chemical conversion coating is not particularly limited and can be appropriately selected according to the required performance and manufacturing equipment. For example, the coating can be formed by continuously applying a chemical conversion solution to the plating film using a roll coater or the like, and then drying it at a peak metal temperature (PMT) of about 60 to 200°C using hot air or induction heating. In addition to a roll coater, known methods such as airless spraying, electrostatic spraying, and curtain flow coating can be appropriately used for applying the chemical conversion solution. Furthermore, the chemical conversion coating may be a single layer or a multi-layer film, as long as it contains the resin and the metal compound, and is not particularly limited.

[0113] · Nurihime As described above, the painted steel sheet of the present invention has a coating film formed on the plating film either directly or via a chemical conversion film, and the coating film has at least a primer coating film.

[0114] Furthermore, the present invention provides that the primer coating film contains a polyester resin having urethane bonds and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide. By including the urethane bonded polyester resin and the inorganic compound in the primer coating, the adhesion of the coating film can be improved while simultaneously enhancing corrosion resistance.

[0115] The primer coating contains a polyester resin having urethane bonds as its main component. Because the polyester resin having urethane bonds possesses both flexibility and strength, it has the effect of making the primer coating less prone to cracking when processed, and because it has high affinity with chemical conversion coatings containing urethane resin, it can contribute to improving the corrosion resistance of the processed area in particular. In this context, "main component" refers to the component that is present in the highest amount among all the components in the primer coating.

[0116] As the polyester resin having the urethane bond, known resins can be used, such as resins obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups. In addition, a resin obtained by reacting the polyester polyol with the diisocyanate or polyisocyanate in an excess of hydroxyl groups (urethane-modified polyester resin) and curing it with a blocked polyisocyanate can also be used.

[0117] Furthermore, the polyester polyol can be obtained by a known method that utilizes a dehydration condensation reaction between a polyhydric alcohol component and a polybasic acid component. Examples of polyhydric alcohols include glycols and polyhydric alcohols with a valency of three or higher. Examples of 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, 3,3-diethyl-1,5-pentanediol, and the like. Examples of polyhydric alcohols with a valency of three or higher include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, and dipentaerythritol. These polyhydric alcohols can be used individually or in combination of two or more types. The aforementioned polybasic acid is usually a polycarboxylic acid, but monovalent fatty acids may be used in combination as needed. Examples of the aforementioned polycarboxylic acids include phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, 4-methylhexahydrophthalic acid, bicyclo[2,2,1]heptane-2,3-dicarboxylic acid, trimellitic acid, adipic acid, sebacic acid, succinic acid, azelaic acid, fumaric acid, maleic acid, itaconic acid, pyromellitic acid, dimer acid, and their acid anhydrides, as well as 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, and the like. These polybasic acids can be used alone or in combination of two or more.

[0118] Examples of the polyisocyanates include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate; aromatic diisocyanates such as xylylene diisocyanate (XDI), metaxylylene diisocyanate, tolylene diisocyanate (TDI), and 4,4-diphenylmethane diisocyanate (MDI); cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI; and their adducts, biuretes, and isocyanurates. These polyisocyanates can be used individually or in combination of two or more types.

[0119] Furthermore, the hydroxyl value of the polyester resin having the urethane bond is not particularly limited, but from the viewpoint of solvent resistance, processability, etc., it is preferably 5 to 120 mgKOH / g, more preferably 7 to 100 mgKOH / g, and even more preferably 10 to 80 mgKOH / g. Furthermore, the number-average molecular weight of the polyester resin having the urethane bond is preferably 500 to 15,000, more preferably 700 to 12,000, and even more preferably 800 to 10,000, from the viewpoint of solvent resistance and processability.

[0120] The content of the polyester resin having urethane bonds in the primer coating is preferably 40 to 88% by mass. If the content of the polyester resin having urethane bonds is less than 40% by mass, the binder function of the primer coating may be reduced, while if the content of the polyester resin having urethane bonds exceeds 88% by mass, the function of the inorganic substances shown below, such as inhibitory activity, may be reduced.

[0121] One of the aforementioned inorganic compounds, a vanadium compound, acts as an inhibitor. Examples of the vanadium compound include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. In particular, it is desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation. The vanadium compound added to the primer coating may be the same as or different from the vanadium compound added to the chemical conversion coating. It is believed that the vanadate compound reacts with ions on the surface of the zinc-plated steel sheet as vanadate ions gradually dissolve in response to moisture entering from the outside, forming a highly adhesive passive film that protects the exposed metal and exhibits rust prevention properties.

[0122] The vanadium compound content in the primer coating is not particularly limited, but is preferably 4 to 20% by mass from the viewpoint of achieving both corrosion resistance and moisture resistance. If the vanadium compound content is less than 4%, the inhibitor effect may decrease, leading to a decrease in corrosion resistance, and if the vanadium compound content exceeds 20% by mass, it may lead to a decrease in the moisture resistance of the primer coating.

[0123] One of the aforementioned inorganic compounds, a phosphate compound, also acts as an inhibitor. Examples of the phosphate compound include phosphoric acid, ammonium phosphate, alkali metal phosphate, and alkaline earth metal phosphate. In particular, alkali metal phosphates such as calcium phosphate can be preferably used.

[0124] The content of the phosphate compound in the primer coating is not particularly limited, but from the viewpoint of achieving both corrosion resistance and moisture resistance, it is preferably 4 to 20% by mass. If the content of the phosphate compound is less than 4% by mass, the inhibitor effect may decrease, leading to a decrease in corrosion resistance, and if the content of the phosphate compound exceeds 20% by mass, it may lead to a decrease in the moisture resistance of the primer coating.

[0125] Magnesium oxide, one of the aforementioned inorganic compounds, produces a product containing Mg through initial corrosion. This product, as a sparingly soluble magnesium salt, helps to stabilize the compound and improve its corrosion resistance.

[0126] The magnesium oxide content in the primer coating is not particularly limited, but from the viewpoint of achieving both corrosion resistance and corrosion resistance of the processed part, it is preferably 4 to 20% by mass. If the magnesium oxide content is less than 4% by mass, the above effect may decrease, leading to a decrease in corrosion resistance, and if the magnesium oxide content exceeds 20% by mass, the flexibility of the primer coating may decrease, which may reduce the corrosion resistance of the processed part.

[0127] Furthermore, the primer coating may also contain components other than the polyester resin and inorganic compound having the urethane bond described above. For example, a crosslinking agent used when forming a primer coating film can be mentioned. The crosslinking agent reacts with the polyester resin having the urethane bond to form a crosslinked coating film, and examples include oxazoline compounds, epoxy compounds, melamine compounds, isocyanate compounds, carbodiimide compounds, silane coupling compounds, etc., and it is also possible to use two or more types of crosslinking agents in combination. In particular, from the viewpoint of the corrosion resistance of the processed part of the resulting coated steel sheet, blocked polyisocyanate compounds can be used. Examples of blocked polyisocyanates include those obtained by blocking the isocyanate group of a polyisocyanate compound with, for example, alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactam, diketones such as acetoacetate diester, imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

[0128] Furthermore, the primer coating may, if necessary, contain various known components commonly used in the paint industry. Specifically, examples include various surface modifiers such as leveling agents and defoaming agents, various additives such as dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as coloring pigments and extender pigments, glossing agents, curing catalysts, and organic solvents.

[0129] The thickness of the primer coating is preferably 1.5 μm or more. This is because a thickness of 1.5 μm or more of the primer coating makes it possible to more reliably obtain the effect of improving corrosion resistance and the effect of improving adhesion with the topcoat coating formed on the chemical conversion coating or primer coating.

[0130] The method for forming the primer coating is not particularly limited. Furthermore, the coating method for the paint composition constituting the primer coating is preferably one in which the paint composition is applied by methods such as roll coater coating or curtain flow coating. After coating the paint composition, the primer coating can be obtained by baking it using heating means such as hot air heating, infrared heating, or induction heating. The baking process is usually carried out for about 30 seconds to 3 minutes, with a maximum plate temperature of approximately 180 to 270°C.

[0131] Furthermore, it is preferable that the coating film constituting the painted steel sheet of the present invention has a topcoat film formed on top of the primer coating film. In addition to imparting aesthetic qualities such as color, gloss, and surface condition to the painted steel sheet, the aforementioned topcoat film can also enhance various properties such as processability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance.

[0132] The composition of the topcoat film is not particularly limited, and materials, thickness, etc., can be appropriately selected according to the required performance. For example, the topcoat film can be formed using polyester resin-based paints, silicone polyester resin-based paints, polyurethane resin-based paints, acrylic resin-based paints, fluororesin-based paints, and the like. Furthermore, the topcoat film may contain appropriate amounts of titanium dioxide, red iron oxide, mica, carbon black or other coloring pigments; metallic pigments such as aluminum powder and mica; extender pigments consisting of carbonates and sulfates; various fine particles such as silica fine particles, nylon resin beads, and acrylic resin beads; curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate; wax; and other additives.

[0133] Furthermore, the thickness of the topcoat film is preferably 5 to 30 μm from the viewpoint of achieving both good appearance and workability. When the thickness of the topcoat film is 5 μm or more, it becomes possible to more reliably stabilize the color tone and appearance, and when the thickness of the topcoat film is 30 μm or less, it becomes possible to more reliably suppress the decrease in workability (crack formation in the topcoat film).

[0134] The method for applying the paint composition to form the topcoat film is not particularly limited. For example, the paint composition can be applied by methods such as roll coater coating or curtain flow coating. After applying the paint composition, the topcoat film can be formed by baking using heating means such as hot air heating, infrared heating, or induction heating. The baking process can usually be performed for about 30 seconds to 3 minutes at a maximum plate temperature of about 180 to 270°C.

[0135] Furthermore, the painted steel sheet of the present invention is characterized in that, similar to the hot-dip Al-Zn-Si-Mg plated steel sheet surface of the present invention described above, the height difference of the steel sheet surface (coating surface) per 1 mm length is 10 μm or less in the area excluding 50 mm from both ends of the steel sheet. When the height difference of the coating film surface is 10 μm or less, wrinkle-like defects are absent, and an excellent surface appearance can be obtained. As described above, the 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 the streaks are not visible. For this reason, by keeping the height difference within a 1 mm length range of the steel plate surface, i.e., the coating film surface, to within 10 μm, the streaks are not visible, and an excellent surface appearance can be obtained. From a similar viewpoint, it is preferable that the height difference within a 1 mm length range of the steel plate surface be within 5 μm. The steel plate surface refers to the outermost surface of the steel plate, and in the case of painted steel plates, it refers to the paint film surface. Furthermore, the method for obtaining the height difference within a 1 mm range on the paint film surface can be obtained by using a laser microscope to measure the height difference within a 1 mm range at any 100 locations on the painted steel plate, similar to the case of the plated film surface described above, and calculating the average of the measured values.

[0136] Furthermore, the reason for controlling the height difference on the steel plate surface in the area excluding 50 mm from both ends of the steel plate is that, generally, it is difficult to control the amount of coating at both ends of a hot-dip galvanized steel plate compared to the center of the plate, and the cooling rate is also different, making it prone to wrinkle-like defects. In addition, these ends of the steel plate are often trimmed before use, so the surface appearance is hardly a problem.

[0137] Regarding the method for controlling the height difference of the coating surface per 1 mm length on the steel plate surface to 10 μm or less, it is important to suppress the height difference of the plating film surface as described above. This can be achieved by adjusting the content ratio of Mg and Mn in the plating film or by performing other surface treatments to suppress the height difference.

[0138] The present invention provides a method for manufacturing a painted steel sheet in which a coating film is formed directly on a plating film or via a chemical conversion film. Furthermore, in the manufacturing method of the present invention, the chemical conversion film contains a resin component comprising a total of 30 to 50% by mass of (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. The aforementioned coating film comprises at least a primer coating film, the primer coating film containing a polyester resin having urethane bonds, and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide. The formation of the aforementioned plating film comprises a hot-dip plating process in which a base steel plate is immersed in a plating bath having a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities.

[0139] The conditions for the hot-dip plating process are the same as those described in the section on the hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention. [Examples]

[0140] [Example 1: Samples 1-28] Samples 1 to 28 of hot-dip Al-Zn-Si-Mg plated steel sheets were prepared under the conditions shown in Table 1 by using 0.8 mm thick cold-rolled steel sheets manufactured by conventional methods as the base steel sheet, and performing degreasing, annealing, and plating treatments in a continuous hot-dip galvanizing facility. The composition of the plating bath used in the production of the molten Al-Zn-Si-Mg plated steel sheets was varied within the range of Al: 45-65 mass%, Si: 1.5-2.5 mass%, Mg: 1.0-4.5 mass%, Mn: 0.00-1.0 mass%, Sr: 0.00-1.0 mass%, B: 0.00-0.05 mass%, Ca: 0.00-1.0 mass%, Cr: 0.00-0.2 mass%, Ti: 0.00-0.2 mass%, and V: 0.00-0.2 mass% to match the composition of the plating film for each sample shown in Table 1. The bath temperature of the plating bath was set to 590°C for Al: 45-55 mass%, and to 630°C for Al: 65 mass%, and was controlled so that the plate temperature of the base steel sheet upon plating penetration was the same as the plating bath temperature. Furthermore, the plating process was carried out under conditions that the plate temperature was cooled to a temperature range of 520-500°C in 3 seconds. Furthermore, the amount of plating film adhering to samples 1-23 and 27-28 was 85±5 g / m² per side. 2 In samples 24-26, the g / m² per side was 50-125 g / m². 2 It was controlled to achieve this.

[0141] <Rating> Each sample of the molten Al-Zn-Si-Mg plated steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 1. (1) Composition of the plated film (amount of coating, composition, X-ray diffraction intensity) For each plated sample, a 100 mm diameter hole was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The amount of plating film attached was calculated from the difference in mass of the sample before and after removal. The calculated amount of plating film attached is shown in Table 1. Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, the filtrate was analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. Furthermore, the solid components were dried and ashed in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the insoluble Si was quantified by ICP emission spectroscopy analysis of the solution. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from solid content analysis to the soluble Si concentration obtained from filtrate analysis. The resulting composition of the plating film is shown in Table 1. Furthermore, for each sample, after shearing it to a size of 100 mm x 100 mm, the plating film on the surface to be evaluated was mechanically scraped off until the underlying steel plate was exposed. After thoroughly mixing the resulting powder, 0.3 g was taken out and qualitative analysis of the powder was performed using an X-ray diffractometer (Rigaku Corporation's "SmartLab") under the following conditions: X-ray used: Cu-Kα (wavelength = 1.54178 Å), Kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4° / min, sampling interval: 0.020°, divergence slit: 2 / 3°, solar slit: 5°, detector: high-speed one-dimensional detector (D / teX Ultra). The diffraction intensity (cps) was calculated by subtracting the base intensity from each peak intensity. The diffraction intensities of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si and the (111) plane (interplanar spacing d=0.3135 nm) of Si were measured.

[0142] (2) Surface appearance (2-1) Wrinkle-like defects For each sample of the obtained molten Al-Zn-Si-Mg plated steel sheet, the surface appearance was visually inspected to check for the presence or absence of wrinkle-like defects. Furthermore, for each sample, 100 locations were arbitrarily selected from the portion excluding 50 mm from both ends of the strip, and the height difference of the plated film surface over a length of 1 mm 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-like defects and the surface shape, the state of wrinkle-like defect occurrence was evaluated according to the following criteria. ◎: No wrinkle-like defects are observed (height difference of 5 μm or less) ○: No wrinkle-like defects are observed (height difference of 10 μm or less) ×: Wrinkle-like defects are observed (height difference exceeds 10 μm)

[0143] (2-2) Dross defect For each sample of the obtained molten Al-Zn-Si-Mg plated steel sheet, the surface appearance was visually inspected, and the presence or absence of dross defects was evaluated according to the following criteria. ○: No granular dross is observed to be present. ×: Granular dross is observed to be present.

[0144] (3) Corrosion resistance evaluation Each sample of the obtained molten Al-Zn-Si-Mg plated steel sheet was sheared to a size of 120 mm × 120 mm. Then, a 10 mm area from each edge of the surface to be evaluated, as well as the end face of the sample and the non-evaluation surface, were sealed with tape, leaving a 100 mm × 100 mm area of ​​the surface to be evaluated exposed. This exposed area was used as the evaluation sample. Three identical evaluation samples were prepared. The three evaluation samples prepared as described above were all subjected to the Japanese Automotive Standards Combined Cycle Test (JASO-CCT). The accelerated corrosion test was started in a wet state and continued until 300 cycles had passed. After that, the corrosion loss of each sample was measured according to the methods described in JIS Z 2383 and ISO 8407, and evaluated according to the following criteria. ◎: The corrosion loss for all three samples was 45g / m².2 below ○: The corrosion loss for all three samples was 90g / m². 2 below ×: Corrosion loss of 90g / m² or more than one sample. 2 cross

[0145] [Table 1]

[0146] The results in Table 1 show that each sample of the present invention has superior surface appearance compared to each sample of the comparative example.

[0147] [Example 2: Samples 1-34] (1) Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as the base steel sheet, samples 1 to 34 of hot-dip Al-Zn-Si-Mg plated steel sheets were prepared under the conditions shown in Table 3 by degreasing, annealing, and plating in a continuous hot-dip galvanizing facility. The composition of the plating bath used in the production of the molten Al-Zn-Si-Mg plated steel sheets was varied within the range of Al: 45-65 mass%, Si: 1.5-2.5 mass%, Mg: 1.0-4.5 mass%, Mn: 0.00-1.0 mass%, Sr: 0.00-1.0 mass%, B: 0.00-0.05 mass%, Ca: 0.00-1.0 mass%, Cr: 0.00-0.2 mass%, Ti: 0.00-0.2 mass%, and V: 0.00-0.2 mass% to achieve the plating film composition of each sample shown in Table 3. The bath temperature was set to 590°C for Al: 45-55 mass%, and 630°C for Al: 65 mass%, and controlled so that the plate temperature of the substrate steel sheet upon plating penetration was the same as the plating bath temperature. Furthermore, the plating process was carried out under conditions that the plate temperature was cooled to a temperature range of 520-500°C in 3 seconds. Furthermore, the amount of plating film adhering to samples 1-29 and 33-34 was 85±5 g / m² per side. 2 In samples 30-32, the g / m² was 50-125 g / m² per side. 2 It was controlled to achieve this.

[0148] (2) Subsequently, a chemical conversion solution was applied to the plating film of each sample of the prepared molten Al-Zn-Si-Mg plated steel sheet using a bar coater, and the sheet was dried in a hot air oven (heating rate: 60°C / s, PMT: 120°C) to form a chemical conversion film, thereby producing the surface-treated steel sheet samples shown in Table 2. The chemical treatment solutions were prepared by dissolving each component in water as a solvent, creating surface treatment solutions A to F. The types of components (resins, metal compounds) contained in the surface treatment solutions are as follows. (resin) Urethane resin: Superflex 130, Superflex 126 (Daiichi Kogyo Seiyaku Co., Ltd.) Acrylic resin: Boncoat EC-740EF (DIC Corporation) (metal compound) P compound: Aluminum dihydrogen tripolyphosphate Si compounds: Silica Compound V: Sodium metavanadate Mo compound: Molybdic acid Zr compound: Potassium zirconyl carbonate Table 1 shows the composition of the prepared chemical conversion treatment solutions A to F and the amount of the resulting chemical conversion film attached. Note that the concentrations of each component in Table 1 of this specification represent the solid content concentration (mass%).

[0149] [Table 2]

[0150] <Rating> Each sample of surface-treated steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 3. (1) Composition of the plated film (amount of coating, composition, X-ray diffraction intensity) For each plated sample, a 100 mm diameter hole was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The amount of plating film attached was calculated from the difference in mass of the sample before and after removal. The calculated amount of plating film attached is shown in Table 3. Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, the filtrate was analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. Furthermore, the solid components were dried and ashed in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the insoluble Si was quantified by ICP emission spectroscopy analysis of the solution. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from solid content analysis to the soluble Si concentration obtained from filtrate analysis. The resulting composition of the plating film is shown in Table 3. Furthermore, for each sample, after shearing it to a size of 100 mm x 100 mm, the plating film on the surface to be evaluated was mechanically scraped off until the underlying steel plate was exposed. After thoroughly mixing the resulting powder, 0.3 g was taken out and qualitative analysis of the powder was performed using an X-ray diffractometer (Rigaku Corporation's "SmartLab") under the following conditions: X-ray used: Cu-Kα (wavelength = 1.54178 Å), Kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4° / min, sampling interval: 0.020°, divergence slit: 2 / 3°, solar slit: 5°, detector: high-speed one-dimensional detector (D / teX Ultra). The diffraction intensity (cps) was calculated by subtracting the base intensity from each peak intensity. The diffraction intensities of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si and the (111) plane (interplanar spacing d=0.3135 nm) of Si were measured. The measurement results are shown in Table 3.

[0151] (2) Surface appearance (2-1) Wrinkle-like defects For each sample of the obtained molten Al-Zn-Si-Mg plated steel sheet and surface-treated steel sheet, the surface appearance was visually inspected to check for the presence or absence of wrinkle-like defects. Furthermore, for each sample, 100 locations were arbitrarily selected from the portion excluding 50 mm from both ends of the strip, and the height difference of the plated film surface over a length of 1 mm 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-like defects and the surface shape, the state of wrinkle-like defect occurrence was evaluated according to the following criteria. ◎: No wrinkle-like defects are observed (height difference of 5 μm or less) ○: No wrinkle-like defects are observed (height difference of 10 μm or less) ×: Wrinkle-like defects are observed (height difference exceeds 10 μm)

[0152] (2-2) Dross defect For each sample of the obtained molten Al-Zn-Si-Mg plated steel sheet, the surface appearance was visually inspected, and the presence or absence of dross defects was evaluated according to the following criteria. ○: No granular dross is observed to be present. ×: Granular dross is observed to be present.

[0153] (3) White rust resistance For each sample of hot-dip Al-Zn-Si-Mg plated steel sheet and surface-treated steel sheet, after shearing to a size of 120 mm × 120 mm, the area within 10 mm from each edge of the surface to be evaluated, and the end face of the sample and the surface not to be evaluated were sealed with tape, and the surface to be evaluated was exposed to a size of 100 mm × 100 mm, which was used as the evaluation sample. Using the above evaluation samples, a salt spray test as described in JIS Z 2371 was conducted for 90 hours and evaluated according to the following criteria. ◎: No white rust on the flat surface. ○: White rust area on flat surface is less than 10% ×: White rust area on flat surface exceeds 10%

[0154] (4) Corrosion resistance evaluation Each sample of the obtained molten Al-Zn-Si-Mg plated steel sheet and surface-treated steel sheet was sheared to a size of 120 mm × 120 mm. Then, a 10 mm area from each edge of the surface to be evaluated, as well as the end face of the sample and the surface not to be evaluated, were sealed with tape, and the surface to be evaluated was exposed to a size of 100 mm × 100 mm. This exposed surface was used as the evaluation sample. Three identical evaluation samples were prepared. The three evaluation samples prepared as described above were all subjected to the Japanese Automotive Standards Combined Cycle Test (JASO-CCT). The accelerated corrosion test was started in a wet state and continued until 300 cycles had passed. After that, the corrosion loss of each sample was measured according to the methods described in JIS Z 2383 and ISO 8407, and evaluated according to the following criteria. ◎: The corrosion loss for all three samples was 30g / m². 2 below ○: The corrosion loss for all three samples was 70g / m². 2 below ×: Corrosion loss of 70g / m² or more than one sample. 2 cross

[0155] [Table 3]

[0156] The results in Table 3 show that each sample of the present invention has superior surface appearance compared to each sample of the comparative example.

[0157] [Example 3: Samples 1-37] (1) Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as the base steel sheet, samples 1 to 37 of hot-dip Al-Zn-Si-Mg plated steel sheets were prepared under the conditions shown in Table 5 by degreasing, annealing, and plating in a continuous hot-dip galvanizing facility. Regarding the composition of the plating bath used in the production of molten Al-Zn-Si-Mg plated steel sheets, the composition of the plating bath was varied within the range of Al: 45-65 mass%, Si: 1.5-2.5 mass%, Mg: 1.0-4.5 mass%, Mn: 0.00-1.0 mass%, Sr: 0.00-1.0 mass%, B: 0.00-0.05 mass%, Ca: 0.00-1.0 mass%, Cr: 0.00-0.2 mass%, Ti: 0.00-0.2 mass%, and V: 0.00-0.2 mass%, so that the plating bath temperature was 590°C for Al: 45-55 mass%, and 630°C for Al: 65 mass%, so that the plate temperature of the base steel sheet upon plating penetration was the same as the plating bath temperature. Furthermore, the plating process was carried out under conditions that the plate temperature was cooled to a temperature range of 520-500°C in 3 seconds. Furthermore, the amount of plating film adhering to samples 1-32 and 36-37 was 85±5 g / m² per side. 2 In samples 33-35, the g / m² per side was 50-125 g / m². 2 It was controlled to achieve this.

[0158] (2) Subsequently, 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 dried in a hot air drying oven (final plate temperature: 90°C) to achieve an adhesion amount of 0.1 g / m². 2 A chemical conversion coating was formed. The chemical treatment solution used was prepared by dissolving each component in water as a solvent, resulting in a pH of 8-10. The types of components (resin components, inorganic compounds) contained in the chemical treatment solution are as follows. (Resin components) Resin A: A mixture of (a) anionic polyurethane resin having ester bonds ("Superflex 210" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and (b) epoxy resin having a bisphenol skeleton ("Yukarezin RE-1050" manufactured by Yoshimura Yukagaku Co., Ltd.), with a mass ratio of (a):(b) = 50:50. Resin B: Acrylic resin (DIC Corporation's "Boncoat EC-740EF") (Inorganic compound) Vanadium compounds: Organovanadium compounds chelated with acetylacetone Zirconium compounds: Zirconium carbonate ammonium Fluorine compounds: Ammonium fluoride

[0159] (3) Then, a primer coating was applied to the chemical conversion film formed as described above using a bar coater, and the film was baked at a steel plate temperature of 230°C and a baking time of 35 seconds to form a primer coating film having the component composition shown in Table 4. Subsequently, a topcoat coating composition was applied to the primer coating film formed as described above using a bar coater, and the film was baked at a steel plate temperature of 230°C to 260°C and a baking time of 40 seconds to form a topcoat coating film having the resin conditions and film thickness shown in Table 4, thereby producing painted steel plates for each sample. The primer coating was obtained by mixing the components and then stirring them in a ball mill for approximately one hour. The resin components and inorganic compounds that make up the primer coating film are as follows. (Resin components) Resin α: A urethane-modified polyester resin (obtained by reacting 455 parts by mass of polyester resin with 45 parts by mass of isophorone diisocyanate, with a resin acid value of 3, a number average molecular weight of 5,600, and a hydroxyl value of 36) was cured with a blocked isocyanate. The polyester resin to be modified for urethane was prepared under the following conditions: In a flask equipped with a stirrer, rectification column, water separator, condenser, and thermometer, 320 parts by mass of isophthalic acid, 200 parts by mass of adipic acid, 60 parts by mass of trimethylolpropane, and 420 parts by mass of cyclohexanedimethanen were charged, heated, and stirred, and while distilling off the condensation water produced, the temperature was raised from 160°C to 230°C at a constant rate over 4 hours. After reaching 230°C, 20 parts by mass of xylene were gradually added, and the condensation reaction was continued while maintaining the temperature at 230°C. The reaction was terminated when the acid value became 5 or less, and after cooling to 100°C, 120 parts by mass of Solvesso 100 (manufactured by ExxonMobil, trade name, high-boiling-point aromatic hydrocarbon solvent) and 100 parts by mass of butyl cellosolve were added to obtain a polyester resin solution. Resin β: Urethane-cured polyester resin ("Evaclad 4900" manufactured by Kansai Paint Co., Ltd.) (Inorganic compound) Vanadium compounds: Magnesium vanadate Phosphate compounds: Calcium phosphate Magnesium oxide compounds: Magnesium oxide Furthermore, the following paints were used for the topcoat coating shown in Table 4. Resin I: Melamine-cured polyester paint (BASF Japan Ltd. "Precolor HD0030HR") Resin II: An organosol-based bake-on fluoropolymer coating (BASF Japan Ltd.'s "Precolor No. 8800HR") in which polyvinylidene fluoride and acrylic resin are in a mass ratio of 80:20.

[0160] [Table 4]

[0161] <Rating> Each sample of painted steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 5. (1) Composition of the plated film (amount of coating, composition, X-ray diffraction intensity) For each plated sample, a 100 mm diameter hole was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The amount of plating film attached was calculated from the difference in mass of the sample before and after removal. The calculated amount of plating film attached is shown in Table 5. Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, the filtrate was analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. Furthermore, the solid components were dried and ashed in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the insoluble Si was quantified by ICP emission spectroscopy analysis of the solution. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from solid content analysis to the soluble Si concentration obtained from filtrate analysis. The resulting composition of the plating film is shown in Table 5. Furthermore, for each sample, after shearing it to a size of 100 mm x 100 mm, the plating film on the surface to be evaluated was mechanically scraped off until the underlying steel plate was exposed. After thoroughly mixing the resulting powder, 0.3 g was taken out and qualitative analysis of the powder was performed using an X-ray diffractometer (Rigaku Corporation's "SmartLab") under the following conditions: X-ray used: Cu-Kα (wavelength = 1.54178 Å), Kβ ray removal: Ni filter, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4° / min, sampling interval: 0.020°, divergence slit: 2 / 3°, solar slit: 5°, detector: high-speed one-dimensional detector (D / teX Ultra). The diffraction intensity (cps) was calculated by subtracting the base intensity from each peak intensity. The diffraction intensities of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si and the (111) plane (interplanar spacing d=0.3135 nm) of Si were measured.

[0162] (2) Surface appearance (2-1) Wrinkle-like defects For each sample of the obtained coated steel sheet, the surface appearance was visually inspected to check for the presence or absence of wrinkle-like defects. Furthermore, for each sample, 100 locations were arbitrarily selected from the portion 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. Based on the presence or absence of wrinkle-like defects and the surface shape, the state of wrinkle-like defect occurrence was evaluated according to the following criteria. ◎: No wrinkle-like defects are observed (height difference of 5 μm or less) ○: No wrinkle-like defects are observed (height difference of 10 μm or less) ×: Wrinkle-like defects are observed (height difference exceeds 10 μm)

[0163] (2-2) Dross defect Each sample of painted steel sheet obtained was visually inspected for surface appearance, and the presence or absence of dross defects was evaluated according to the following criteria. ○: No granular dross is observed to be present. ×: Granular dross is observed to be present.

[0164] (3) Corrosion resistance evaluation Each of the obtained painted steel sheet samples was sheared to a size of 120 mm x 120 mm. Three of the four edges of the surface to be evaluated were sealed with tape, extending 10 mm from the edges, and the end face and non-evaluation face were sealed with tape. Only one edge was left unsealed, exposing the sheared end face, and this was used as the evaluation sample. The shearing was done so that the burrs on the sheared end face were facing the surface to be evaluated. The above evaluation samples were used to conduct the Japanese Automotive Standards Combined Cycle Test (JASO-CCT). The corrosion acceleration test started in a wet state, and samples were taken out every 20 cycles. After washing and drying, they were visually inspected, and the number of cycles at which red rust was observed on the shear end face of one side that was not tape-sealed was evaluated according to the following criteria. ◎: Number of cycles for red rust formation in 3 samples ≥ 600 cycles ○: Number of cycles for red rust formation in 3 samples ≥ 400 cycles ×: Number of cycles for red rust occurrence in one or more samples < 400 cycles

[0165] [Table 5]

[0166] The results in Table 5 show that each sample of the present invention exhibits superior surface appearance and corrosion resistance compared to each sample of the comparative examples. [Industrial applicability]

[0167] 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-like defects, and has an excellent surface appearance. Furthermore, according to the present invention, it is possible to provide a surface-treated steel sheet having excellent surface appearance and resistance to white rust, as well as a painted steel sheet with excellent surface appearance and corrosion resistance.

Claims

1. A hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film, The aforementioned plating film has a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, Mg: 1.0-10.0% by mass, and Mn: 0.01-0.5% by mass, with the remainder being Zn and unavoidable impurities, and the ratio of the Mn content to the Mg content (Mn / Mg) is 0.02 or more by mass (Mn / Mg ≥ 0.02). A hot-dip Al-Zn-Si-Mg plated steel sheet, characterized in that the difference in height of the steel sheet surface per 1 mm in length is 10 μm or less in the area excluding 50 mm from both ends of the steel sheet.

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

3. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1, characterized in that it comprises a Mn-containing alloy layer at the interface between the plating film and the underlying steel sheet.

4. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 2, characterized in that it comprises a Mn-containing alloy layer at the interface between the plating film and the underlying steel sheet.

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

6. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 2, characterized in that the plating film further contains 0.01 to 3.0% by mass of one or more elements selected from B, Ca, Ti, V, Cr, Sr, Mo, In, Sn, Sb, Ce, and Bi.

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

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

9. A surface-treated steel sheet having a chemical conversion coating formed on a molten Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 8, The surface-treated steel sheet is characterized in that the chemical conversion coating contains at least one resin selected from 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 P compound, Si compound, Co compound, Ni compound, Zn compound, Al compound, Mg compound, V compound, Mo compound, Zr compound, Ti compound, and Ca compound.

10. A painted steel sheet having a coating film formed directly or via a chemical conversion film on a molten Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 8, The aforementioned chemical conversion film contains a resin component comprising (a) an anionic polyurethane resin having ester bonds and (b) an epoxy resin having a bisphenol skeleton in total at a concentration of 30 to 50% by mass, wherein the content ratio of (a) to (b) ((a):(b)) is in the range of 3:97 to 60:40 by mass ratio, and an inorganic compound comprising 2 to 10% by mass of a vanadium compound, 40 to 60% by mass of a zirconium compound, and 0.5 to 5% by mass of a fluorine compound. The aforementioned coating film comprises at least a primer coating film, the primer coating film containing a polyester resin having urethane bonds and an inorganic compound containing a vanadium compound, a phosphate compound, and magnesium oxide, and is characterized by this, in the painted steel sheet.

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