HOT DIP Al-Zn COATED STEEL SHEET, METHOD OF PRODUCING SAME, SURFACE-TREATED STEEL SHEET, AND PREPAINTED STEEL SHEET

Abstract

The present invention provides a molten Al-Zn plated steel sheet having excellent corrosion resistance in a reliable and stable manner. To achieve the above objective, the present invention provides a molten Al-Zn plated steel sheet having a plating film, wherein the plating film has a composition comprising Al: 45 to 65 mass% and Si: 1.0 to 4.0 mass%, with the remainder being Zn and unavoidable impurities, and the Ni content among the unavoidable impurities is 0.010 mass% or less relative to the total mass of the plating film.

Description

Technology Field

[0001] The present invention relates to a molten Al-Zn plated steel sheet, a method for manufacturing the same, a surface-treated steel sheet, and a coated steel sheet. Background Technology

[0002] Hot-dip Al-Zn galvanized steel sheets, represented by the 55% Al-Zn type, are known to exhibit high corrosion resistance among hot-dip galvanized steel sheets because they combine the sacrificial protection property of Zn with the high corrosion resistance of Al. For this reason, due to their excellent corrosion resistance, hot-dip Al-Zn galvanized steel sheets are mainly used in construction materials such as roofs and walls exposed outdoors for long periods, as well as in civil engineering and construction fields such as guardrails, wiring piping, and sound barriers. In particular, the demand for materials with excellent corrosion resistance and maintenance-free properties has recently increased, as there is a growing demand for materials with excellent corrosion resistance and maintenance-free properties under stricter usage 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.

[0003] The plating film of a hot-dip Al-Zn plated steel sheet is characterized by being composed of a portion (α-Al phase) in which Al containing supersaturated Zn is solidified in a dendrite shape and a Zn-Al eutectic structure existing in the interdendritic spaces, and having a structure in which the α-Al phase is stacked multiple times in the thickness direction of the plating film. It is also known that due to this characteristic film structure, the corrosion progression path from the surface becomes complex, making it difficult for corrosion to proceed easily, and thus the hot-dip Al-Zn plated steel sheet can achieve superior corrosion resistance compared to a hot-dip galvanized steel sheet with the same plating film thickness.

[0004] Generally, molten Al-Zn galvanized steel sheets are manufactured by using a thin steel sheet obtained by hot-rolling or cold-rolling a slab as a base steel sheet, and performing recrystallization annealing and molten plating treatment on the base steel sheet in an annealing furnace of a continuous molten plating facility.

[0005] Here, in the plating bath used to manufacture molten Al-Zn-plated steel sheets, in addition to Al and Zn at predetermined concentrations, it is common practice to add Si to suppress the excessive growth of the interfacial alloy layer formed at the interface between the steel substrate and the plating. Due to the function of this Si, the thickness of the interfacial alloy layer of the molten Al-Zn-plated steel sheet can be controlled to approximately 1 to 5 μm. Furthermore, it is believed that suppressing the growth of the interfacial alloy layer leads to improved corrosion resistance, as the upper layer exhibiting high corrosion resistance becomes thicker as the interfacial alloy layer becomes thinner when the plating film thickness is the same.

[0006] In addition, regarding the plating bath used for manufacturing hot-dip galvanized steel sheets, it is known that impurities are inevitably incorporated into the bath due to impurities contained in the plating raw materials, leaching from the underlayer steel sheet or bath equipment. Furthermore, the inevitability of impurities incorporating into the bath is no exception for the plating bath used for manufacturing hot-dip Al-Zn galvanized steel sheets, and components such as Fe, Cr, Ni, Cu, Co, and W are inevitably incorporated into the plating film.

[0007] Impurities contained in such plating baths can cause deterioration in properties such as the appearance, corrosion resistance, and workability of hot-dip galvanized steel sheets, and the presence or absence of such influence is determined by the composition of the plating film and the impurity concentration. In other words, even impurities of the same composition may be harmful to the properties of the galvanized steel sheet or harmless. Therefore, the influence of impurities on the properties of various hot-dip galvanized steel sheets is being investigated, and technologies to control impurity concentration are being developed to reliably obtain the required properties.

[0008] For example, Patent Document 2 discloses a molten Al-Zn plated steel sheet having a plating film composed of, in mass%, Al: 0.10 to 0.6%, Bi: 0.03 to 0.3%, and the remainder being Zn and unavoidable impurities, wherein the content of each of Pb, Sn, and Cd as unavoidable impurities is controlled to 0.002%.

[0009] In addition, Patent Document 3 discloses a molten Zn-Al-Mg plated steel sheet having a plating film that is controlled so as not to include Ni among the unavoidable impurities, wherein, in mass%, Al: 4.4 to 5.6%, Mg: 0.3 to 0.56%, and the remainder is Zn and unavoidable impurities.

[0010] Meanwhile, regarding hot-dip Al-Zn plated steel sheets such as those mentioned above, there was a problem that white rust occurred due to the corrosion of the plating film when used in harsh corrosive environments. Since this white rust causes a deterioration in the appearance of the steel sheet, various technologies have been developed to improve white rust resistance.

[0011] For example, Patent Document 4 discloses a surface-treated steel sheet that improves white rust resistance by forming a chemical conversion layer containing a water-soluble urethane resin having carboxyl groups and acid amide bonds, N-methylpyrrolidone, a zirconium metal compound, and a silane coupling agent on a molten Al-Zn plated steel sheet.

[0012] In addition, for example, Patent Document 5 discloses a surface-treated steel sheet that improves white rust resistance by forming a chemical coating containing a resin selected from acrylic resin, polyester resin, and urethane resin having a glass transition temperature of -10°C or higher, and a compound selected from Si compounds, Zr compounds, triazoles, and tetrazols on a molten Al-Zn plated steel sheet.

[0013] In addition, regarding hot-dip Al-Zn plated steel sheets such as those described above, the coated steel sheets, which have a chemical conversion film, a primer film, and a top coating layer formed on their surface, are subjected to various processing such as 90-degree bending or 180-degree bending by press forming, roll forming, or embossing, and thus long-term coating durability performance is also required. To meet these requirements, a coated steel sheet is known in which the hot-dip Al-Zn plated steel sheet forms a chemical conversion film containing chromate, contains a chromate-based anti-corrosion pigment in the primer film, and forms a top coating film with excellent weather resistance, such as a thermosetting polyester resin film or a fluoropolymer resin film, on top thereof.

[0014] However, recently, the use of chromate, an environmentally harmful substance, has become a concern, leading to a strong demand for the development of coated steel sheets that can improve corrosion resistance even when chromate-free.

[0015] As a technology responding to these requirements, for example, Patent Document 6 discloses a coated steel sheet that improves corrosion resistance in the cut section by forming an undercoat layer having an anti-corrosion pigment containing Mg on the surface of a molten Al-Zn plated steel sheet, and additionally forming an overcoat layer having a contact angle with water of 80 degrees or more and less than 130 degrees thereon.

[0016] In addition, Patent Document 7 discloses a coated steel sheet that improves corrosion resistance in processed parts or cut sections by forming a chromate-free conversion film containing a urethane resin, an epoxy resin, a vanadium compound, a zirconium compound, and a fluorine compound on the surface of a molten Al-Zn plated steel sheet, forming a primer film containing a vanadium compound, a phosphate compound, and magnesium oxide with an ester resin as the main component thereon, and additionally forming a melamine-cured polyester-based overcoat film thereon. Prior art literature

[0017] Japanese Patent Publication No. Sho 46-7161, Japanese Published Patent Publication No. 2004-285387, Japanese Published Patent Publication No. 2016-540885, Japanese Published Patent Publication No. 2003-201578, Japanese Published Patent Publication No. 2013-237874, Japanese Published Patent Publication No. 2009-172553, Japanese Published Patent Publication No. 2016-176118 The problem to be solved

[0018] However, in hot-dip Zn-Al plated steel sheets as disclosed in Patent Documents 2 and 3, the influence of inevitable impurities on corrosion resistance has not been sufficiently elucidated, so there was a desire to develop a technology that could more certainly achieve superior corrosion resistance.

[0019] Furthermore, not limited to molten Al-Zn-based plated steel sheets, the control of impurities in the manufacturing of molten-dip plated steel sheets has mostly been limited to concentration control, and technology for controlling their shape, such as size or distribution, has not been established; therefore, there was a demand for the development of technology capable of realizing superior corrosion resistance more stably.

[0020] Furthermore, regarding the technology for improving corrosion resistance after considering the influence of the aforementioned unavoidable impurities, improvements are similarly desired not only for plated steel sheets but also for surface-treated steel sheets having a chemical conversion film additionally formed on the plating film, and for coated steel sheets having a coating film additionally formed directly on the plating film or through a chemical conversion film.

[0021] In addition, although certain corrosion resistance and white rust resistance are obtained by using the surface-treated steel plates of Patent Documents 4 and 5, there are cases where the effect is not sufficient, so a higher level of corrosion resistance and white rust resistance is required.

[0022] In addition, although corrosion resistance in certain processed parts or cut sections is obtained by using the coated steel sheets of Patent Documents 6 and 7, there are cases where the effect is not sufficient, so there was a need for corrosion resistance and corrosion resistance in processed parts at a higher level.

[0023] The present invention aims to provide a molten Al-Zn plated steel sheet having excellent corrosion resistance that is reliable and stable, and a method for manufacturing the same, in consideration of these circumstances.

[0024] In addition, the present invention aims to provide a surface-treated steel sheet having excellent corrosion resistance and white rust resistance that is reliable and stable, and a painted steel sheet having excellent corrosion resistance and corrosion resistance of the processed part that is reliable and stable. means of solving the problem

[0025] As a result of conducting a review to solve the above problem, the inventors noted that regarding the composition of the plating film of a molten Al-Zn plated steel sheet, it is important not only to control the concentrations of Al, Zn, and Si, but also to control the concentration of elements included as impurities. They discovered that the deterioration of corrosion resistance can be effectively suppressed by appropriately controlling the content of Ni, and furthermore, the deterioration of corrosion resistance can be suppressed more stably by appropriately controlling the size and distribution state of Ni-based compounds present as impurities in the plating film.

[0026] The present invention is based on the above understanding, and its gist is as follows.

[0027] 1. A molten Al-Zn plated steel sheet having a plating film,

[0028] The above plating film has a composition comprising Al: 45–65 mass% and Si: 1.0–4.0 mass%, with the remainder consisting of Zn and unavoidable impurities, and

[0029] A molten Al-Zn plated steel sheet characterized in that the Ni content among the above-mentioned unavoidable impurities is 0.010 mass% or less with respect to the total mass of the plating film.

[0030] 2. A molten Al-Zn plated steel sheet described in 1, characterized in that the plating film comprises a Ni-based compound, and the major axis length of the Ni-based compound is 4.0 μm or less.

[0031] 3. A molten Al-Zn plated steel sheet as described in 1 or 2, characterized in that the plating film contains a Ni-based compound, and the number of said Ni-based compounds existing in a direction parallel to the surface of the underlying steel sheet is 5 or less.

[0032] 4. A molten Al-Zn plated steel sheet described in 1, characterized in that the plating film does not contain a Ni-based compound.

[0033] 5. A molten Al-Zn plated steel sheet as described in any one of 1 to 4, characterized in that the Al content in the plating film is 50 to 60 mass%.

[0034] 6. A molten Al-Zn plated steel sheet as described in any one of 1 to 5, characterized in that the Co content among the unavoidable impurities included in the plating film is 0.080 mass% or less with respect to the total mass of the plating film.

[0035] 7. A method for manufacturing a molten Al-Zn plated steel sheet having a plating film,

[0036] The formation of the above plating film comprises a molten plating treatment process in which an underlayer steel plate is immersed in a plating bath having a composition containing Al: 45 to 65 mass% and Si: 1.0 to 4.0 mass%, with the remainder consisting of Zn and unavoidable impurities.

[0037] A method for manufacturing a molten Al-Zn plated steel sheet, characterized by controlling the Ni content among the inevitable impurities of the plating bath to 0.010 mass% or less relative to the total mass of the plating bath.

[0038] 8. A surface-treated steel sheet having a plating film described in any one of claims 1 to 6 above and a chemical conversion film formed on said plating film, wherein

[0039] The surface-treated steel sheet is characterized in that the above-mentioned chemical 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 fluoropolymer resin, 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.

[0040] 9. A coated steel sheet having a coating formed on a plating film described in any one of claims 1 to 6 above, either directly or through a chemical conversion film,

[0041] The above-mentioned chemical conversion film comprises a resin component containing (a): an anionic polyurethane resin having ester bonds and (b): an epoxy resin having a bisphenol backbone in a total amount of 30 to 50 mass%, wherein the content ratio ((a):(b)) of said (a) and said (b) is in the range of 3:97 to 60:40 in mass ratio, and an inorganic compound comprising 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound.

[0042] A coated steel sheet characterized in that the above coating film has at least a primer coating film, and said primer coating film contains a polyester resin having urethane bonds and an inorganic compound including a vanadium compound, a phosphate compound, and magnesium oxide. Effects of the invention

[0043] According to the present invention, it is possible to provide a molten Al-Zn plated steel sheet having excellent corrosion resistance that is reliable and stable.

[0044] In addition, according to the present invention, it is possible to provide a surface-treated steel sheet having excellent corrosion resistance and white rust resistance in a reliable and stable manner, and a painted steel sheet having excellent corrosion resistance and corrosion resistance of the processed part in a reliable and stable manner. Brief explanation of the drawing

[0045] Figure 1 is a diagram illustrating the flow of the Japanese Automobile Standard Combined Cycle Test (JASO-CCT). Specific details for implementing the invention

[0046] (Form for carrying out the invention)

[0047] (Hot-dip Al-Zn plated steel sheet)

[0048] The molten Al-Zn-based plated steel sheet of the present invention has a plating film on the surface of the steel sheet.

[0049] In addition, the plating film has a composition comprising Al: 45 to 65 mass% and Si: 1.0 to 4.0 mass%, with the remainder consisting of Zn and unavoidable impurities.

[0050] The Al content in the plating film is 45 to 65 mass%, preferably 50 to 60 mass%, based on the balance between corrosion resistance and operational performance. This is because if the Al content in the plating film is at least 45 mass%, Al dendrite solidification occurs, and a plating film structure consisting mainly of α-Al phase dendrite solidification structures can be obtained. By adopting a structure in which the said dendrite solidification structures are stacked in the film thickness direction of the plating film, the corrosion progression path becomes complex, thereby improving the corrosion resistance of the plating film itself. Furthermore, as the α-Al phase dendrite portions are stacked more extensively, the corrosion progression path becomes complex, making it difficult for corrosion to easily reach the steel plate, thus improving corrosion resistance; therefore, it is desirable to have an Al content of 50 mass% or more. Meanwhile, if the Al content in the plating film exceeds 65 mass%, the structure changes to one in which most of the Zn is dissolved in α-Al, and the dissolution reaction of the α-Al phase cannot be suppressed, causing the corrosion resistance of the Al-Zn-Si-Mg-based plating to deteriorate. For this reason, the Al content in the plating film needs to be 65 mass% or less, and preferably 60 mass% or less.

[0051] The Si in the plating film is added primarily to suppress the growth of the Fe-Al and / or Fe-Al-Si intermetallic alloy layer formed at the interface with the underlying steel plate, thereby preventing deterioration of the adhesion between the plating film and the steel plate. In fact, when a steel plate is immersed in an Al-Zn plating bath containing Si, the Fe on the surface of the steel plate reacts with the Al or Si in the bath to form an alloying reaction, and an Fe-Al and / or Fe-Al-Si intermetallic compound layer is formed at the interface between the underlying steel plate and the plating film. However, since the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, the growth of the entire intermetallic alloy layer is suppressed as the proportion of the Fe-Al-Si alloy increases. Therefore, it is necessary to have a Si content of 1.0 mass% or more in the plating film. Meanwhile, if the Si content in the plating film exceeds 4.0 mass%, the growth inhibition effect of the aforementioned interfacial alloy layer is saturated, and corrosion is promoted due to the presence of an excess Si phase in the plating film; therefore, the Si content is set to 4.0 mass% or less. In addition, the Si content in the plating film is preferably set to 3.0 mass% or less from the perspective of suppressing the presence of an excess Si phase.

[0052] In addition, the plating film contains Zn and unavoidable impurities. Among these, the unavoidable impurities contain Fe. This Fe is inevitably included in the plating film as a result of being supplied by diffusion from the underlying steel plate during the formation of the interfacial alloy layer, as well as by the steel plate or equipment in the bath dissolving into the plating bath. The Fe content in the plating film is typically about 0.3 to 2.0 mass%.

[0053] Other unavoidable impurities include Cr, Ni, Cu, Co, W, etc. These components are inevitably included in the plating film by dissolving WC-based or Co-Cr-W-based thermal spray coatings applied to underlay steel sheets or stainless steel bath equipment, by being contained as impurities in the metal ingots that serve as raw materials for the plating bath, or by using pots or bath equipment used in the manufacture of plated steel sheets in which these components are intentionally added.

[0054] Furthermore, the molten Al-Zn plated steel sheet of the present invention is characterized in that the Ni content among the unavoidable impurities is 0.010 mass% or less with respect to the total mass of the plating film. Since Ni contained in the plating film may degrade the corrosion resistance of the molten Al-Zn plated steel sheet, the deterioration of corrosion resistance can be suppressed by appropriately controlling the content of Al, Zn, and Si in the plating film and then further suppressing the Ni content as an unavoidable impurity. In the same regard, it is preferable that the Ni content among the unavoidable impurities be 0.005 mass% or less with respect to the total mass of the plating film.

[0055] In addition, when Ni is included among the above-mentioned unavoidable impurities, Ni-based compounds may be included as impurities in the plating film of the molten Al-Zn plated steel sheet. Here, the above-mentioned Ni-based compounds are mainly Ni-based compounds such as binary intermetallic compounds like Ni-Al compounds or ternary intermetallic compounds such as Ni-Al-Fe compounds. Examples of Ni-Al compounds include intermetallic compounds such as NiAl3, and examples of Ni-Al-Fe compounds include intermetallic compounds such as (Ni,Fe)Al3 in which some of the Ni in NiAl3 is substituted with Fe, but are not limited to these compounds.

[0056] Here, the presence of Ni-based compounds in the plating film can be confirmed, for example, by utilizing a scanning electron microscope to observe the plating film from the surface or cross-section using a secondary electron image or a reflection electron image, and by analyzing it using energy-dispersive X-ray spectroscopy (EDS). For example, the composition of Ni-based inclusions can be confirmed by arbitrarily selecting about 5 to 10 plating cross-sections of 100 μm, performing observation and elemental mapping analysis at each with an acceleration voltage of 5 kV or less, and further performing point analysis on the parts where Ni was detected. This method is merely an example, and any method capable of confirming the presence of Ni-based compounds is acceptable and is not particularly limited.

[0057] In addition, when the plating film contains a Ni-based compound, it is preferable that the major axis of the Ni-based compound be 4.0 μm or less.

[0058] Ni-based compounds present in the plating film may cause a deterioration in corrosion resistance because they function as a cathode under corrosive environments and form local cells with the surrounding solidification structure. In particular, if coarse Ni-based compounds are present in the plating film, there is a risk that the corrosion resistance of the molten Al-Zn plated steel sheet will be significantly reduced. Therefore, in order to obtain a molten Al-Zn plated steel sheet with superior corrosion resistance, it is effective to control the size of the Ni-based compounds included as impurities in the plating film to be small. Specifically, it is preferable to make the major axis of the Ni-based compounds 4.0 μm or less, more preferable to make it 3.0 μm or less, and even more preferable to make it 2.0 μm or less.

[0059] In addition, the major axis of the above-mentioned Ni-based compound can be measured, for example, by utilizing a scanning electron microscope to observe the plating film from a cross-section using a reflected electron image, confirming that it is a Ni-based compound using EDS, and then observing the reflected electron image with an enlarged field of view containing the Ni-based compound. The major axis of the above-mentioned Ni-based compound is defined as the maximum major axis of the Ni-based compound identified within the field of view of the plating film.

[0060] In addition, when the plating film contains a Ni-based compound, it is effective to reduce the amount of the Ni-based compound present, which serves as a starting point for corrosion, from the perspective of obtaining high corrosion resistance more stably. Specifically, it is preferable to reduce the number of Ni-based compound particles in the plating film to 5 particles / mm or less in a direction parallel to the surface of the underlying steel plate, more preferable to reduce it to 2 particles / mm or less, and most preferable to reduce it to 0 particles / mm (not present).

[0061] Therefore, by suppressing the amount of a compound containing Ni in the plating film, the deterioration of the corrosion resistance of the molten Al-Zn plated steel sheet can be more reliably suppressed. In order to obtain such a film structure (a film structure that does not contain a Ni-based compound), it is important to reduce the Ni content among the inevitable impurities, specifically, to make the Ni content 0.005 mass% or less relative to the total mass of the plating film.

[0062] In addition, regarding the number of particles of the above-mentioned Ni-based compounds, for example, by utilizing a scanning electron microscope to continuously observe a cross-section parallel to the surface of the underlayer steel plate of the plating film with a reflected electron image for a length of 1 mm or more, and by dividing the number of Ni-based compounds confirmed by EDS by the measured length (mm), the number of Ni-based compounds present within a length range of 1 mm can be calculated.

[0063] In addition, the molten Al-Zn plated steel sheet of the present invention preferably has a Co content among the unavoidable impurities of 0.080 mass% or less relative to the total mass of the plating film. Since Co contained in the plating film may degrade the corrosion resistance of the molten Al-Zn plated steel sheet, just like Ni mentioned above, it is effective to suppress the Co content as an unavoidable impurity after appropriately controlling the content of Al, Zn, Si, and Ni among the unavoidable impurities in the plating film mentioned above, in order to suppress the deterioration of corrosion resistance.

[0064] In the same regard, it is preferable that the Co content among the above-mentioned unavoidable impurities be 0.020 mass% or less with respect to the total mass of the plating film, and more preferable that it be 0.010 mass% or less.

[0065] In addition, regarding the total content of unavoidable impurities in the plating film, there is no particular limitation, but since an excessive amount may affect various properties of the plated steel sheet, it is desirable to keep the total amount at 5.0 mass% or less.

[0066] In addition, it is preferable that the plating film further contains 0.01 to 10 mass% in total of one or more elements selected from V, Cr, Mn, Mg, Ca, and Sr. These elements can improve the stability of corrosion products when the plating film corrodes, thereby having the effect of delaying the progression of corrosion.

[0067] The total content of the aforementioned components is set to 0.01 to 10 mass% because it allows for a sufficient corrosion retardation effect to be obtained without the effect becoming saturated.

[0068] In addition, in the molten Al-Zn plated steel sheet of the present invention, it is preferable that the diffraction intensity of Si in the plating film by X-ray diffraction satisfies the following relationship (1) in order to more stably improve corrosion resistance after controlling the concentrations of Al, Zn, Si and Ni or Co as unavoidable impurities as described above.

[0069] Si(111)=0 ···(1)

[0070] Si(111): Diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135 nm).

[0071] Generally, regarding the dissolution reaction of an Al alloy into an aqueous solution, it is known that the presence of a Si phase as a cathode site promotes the dissolution of the surrounding α-Al phase. Therefore, reducing the Si phase is effective from the perspective of suppressing the dissolution of the α-Al phase, and among them, making a film in which no Si phase exists as in relation (1) (making the diffraction peak intensity of the Si (111) zero) is the best for stabilizing corrosion resistance.

[0072] As a method for measuring Si (111) by the above X-ray diffraction, it can be calculated by mechanically cutting off a part of the plating film and performing X-ray diffraction in a powdered state (powder X-ray diffraction measurement method). For measuring the diffraction intensity, it is preferable to measure the diffraction peak intensity of Si corresponding to the plane spacing d = 0.3135 nm.

[0073] In addition, when performing powder X-ray diffraction measurements, the amount of plating film required (the amount of plating film removed) is preferably 0.1g or more and preferably 0.3g or more from the perspective of measuring Si (111) with high precision. Also, when removing the plating film, steel plate components other than the plating film may be included in the powder, but these intermetallic compound phases are included only in the plating film and do not affect the peak intensity mentioned above. In addition, the reason for performing X-ray diffraction with the plating film as a powder is that if X-ray diffraction is performed on the plating film formed on the plated steel plate, it is difficult to calculate the correct phase ratio due to the influence of the plane orientation of the plating film solidification structure.

[0074] Here, the method for satisfying the aforementioned relationship (1) is not specifically limited. For example, the ratio of Si (diffraction intensity of Si (111)) can be controlled by adjusting the balance of Si content and Al content in the plating film. The balance of Si content, Mg content, Ca content, Sr content and Al content in the plating film does not necessarily satisfy relationship (1) by setting it to a constant ratio; for example, it is necessary to change the ratio of Al content according to the Si content (mass%). In addition, in addition to adjusting the balance of Si content and Al content in the plating film, the diffraction intensity of Si (111) can be controlled to satisfy relationship (1) by adjusting the conditions during plating film formation (for example, cooling conditions after plating).

[0075] In addition, from the perspective of satisfying various characteristics, it is preferable that the amount of the plating film applied be 45 to 120 g / m² per side. This is because if the amount of the plating film applied is 45 g / m² or more, sufficient corrosion resistance is obtained even for applications requiring long-term corrosion resistance, such as building materials, and if the amount of the plating film applied is 120 g / m² or less, excellent corrosion resistance can be realized while suppressing the occurrence of plating cracks during processing. From the same perspective, it is more preferable that the amount of the plating film applied be 45 to 100 g / m².

[0076] The amount of the plating film attached above can be determined by a method, for example, as shown in JIS H 0401: 2013, by dissolving and peeling off a plating film of a specific area with a mixture of hydrochloric acid and hexamethylenetetramine, and calculating the amount from the difference in weight of the steel sheet before and after peeling. To obtain the amount of plating attached per side using this method, the plating surface of the non-target side can be sealed with tape so that it is not exposed, and then the dissolution described above can be performed.

[0077] In addition, the component composition of the plating film can be determined by dissolving the plating film in hydrochloric acid, etc., with the same content as the aforementioned Ni, and analyzing the solution using ICP emission spectroscopic analysis or atomic absorption analysis. This method is merely an example, and any method capable of accurately quantifying the component composition of the plating film is acceptable and is not specifically limited.

[0078] In addition, the plating film of the molten Al-Zn-based plated steel sheet obtained by the present invention is, overall, nearly equivalent to the composition of the plating bath. Therefore, the control of the composition of the plating film can be performed with high precision by controlling the composition of the plating bath.

[0079] In addition, regarding the underlayer steel sheet constituting the molten Al-Zn-plated steel sheet of the present invention, there are no particular limitations, and cold-rolled steel sheets or hot-rolled steel sheets can be appropriately used depending on the required performance or specifications.

[0080] Furthermore, the method for obtaining the above-mentioned lower steel sheet is not particularly limited. For example, in the case of the above-mentioned hot-rolled steel sheet, one that has undergone a hot rolling process and an acid cleaning process may be used, and in the case of the above-mentioned cold-rolled steel sheet, it may be manufactured by additionally adding a cold rolling process. In addition, to obtain the characteristics of the steel sheet, it is also possible to undergo a recrystallization annealing process, etc., before the hot-dip galvanizing process.

[0081] (Method for manufacturing molten Al-Zn plated steel sheets)

[0082] The method for manufacturing a molten Al-Zn plated steel sheet according to the present invention is a method for manufacturing a molten Al-Zn plated steel sheet having a plating film, wherein the formation of the plating film comprises a molten plating treatment process in which a steel sheet is immersed in a plating bath having a composition comprising Al: 45 to 65 mass% and Si: 1.0 to 4.0 mass%, with the remainder being Zn and unavoidable impurities.

[0083] Furthermore, the above-mentioned molten plating process is not particularly limited, except for the conditions of the plating bath described below. For example, the above-mentioned underlayer steel plate can be manufactured by cleaning, heating, and immersing it in a plating bath using a continuous molten plating facility. In the heating process of the steel plate, in addition to performing recrystallization annealing to control the microstructure of the underlayer steel plate itself, heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective to prevent oxidation of the steel plate and to reduce trace oxide films present on the surface.

[0084] In addition, regarding the plating bath used in the above-mentioned molten plating process, as described above, a composition containing Al: 45 to 65 mass% and Si: 1.0 to 4.0 mass%, with the remainder consisting of Zn and unavoidable impurities, can be used in such a way that the composition of the plating film is almost equivalent to the composition of the plating bath overall.

[0085] In addition, the method for manufacturing a molten Al-Zn-plated steel sheet according to the present invention is characterized by controlling the Ni content among the inevitable impurities of the plating bath to 0.010 mass% or less relative to the total mass of the plating bath. Since Ni contained in the plating film may degrade the corrosion resistance of the molten Al-Zn-plated steel sheet as described above, the deterioration of corrosion resistance can be suppressed by appropriately controlling the content of Al, Zn, and Si in the plating bath and then further suppressing the Ni content as an inevitable impurity.

[0086] In addition, the content of Ni as an unavoidable impurity in the plating bath needs to be controlled to 0.010 mass% or less relative to the total mass of the plating bath, and it is preferable to keep it 0.005 mass% or less. This is because if the Ni content in the plating bath exceeds 0.005 mass%, there is a risk that the corrosion resistance of the manufactured molten Al-Zn plated steel sheet will deteriorate, and if it exceeds 0.010%, there is a possibility that significant deterioration in corrosion resistance will occur. Furthermore, there is no lower limit for the Ni content that adversely affects corrosion resistance.

[0087] Here, the means for reducing the Ni content in the plating bath is not particularly limited.

[0088] For example, it is desirable to treat the surface of the equipment in the bath with a thermal spray film or the like, as this is effective in suppressing the leaching of the stainless steel equipment into the plating bath. This is because the formation of the thermal spray film or the like can impart corrosion resistance to the plating bath to the equipment in the bath, thereby enabling the suppression of the equipment's leaching into the plating bath. The type of thermal spray film is not particularly limited, but a film having heat resistance and corrosion resistance, such as a WC-based or MoB-based film, can be selected. In addition, it is more effective to use equipment in the bath made of a heat-resistant material that does not contain Ni. In this case, even if the equipment in the bath leaches out, an increase in the Ni content can be prevented.

[0089] In addition, as another means of reducing the Ni content in the plating bath, it is preferable to use a metal ingot with a low Ni content among impurities as a raw material for the plating bath.

[0090] In addition, it is also effective not to use the pot or bath equipment used in the manufacture of plated steel sheets with intentionally added Ni in the manufacture of molten Al-Zn plated steel sheets. This is because it is possible to prevent the Ni-containing metal ingot attached to the pot or bath equipment from melting and being mixed into the plating bath.

[0091] In addition, the content of Co as an inevitable impurity in the plating bath is preferably controlled to 0.080 mass% or less relative to the total mass of the plating bath, more preferably to 0.020 mass% or less, and even more preferably to 0.010 mass% or less. If the Co content in the plating bath is 0.080 mass% or less, the manufactured molten Al-Zn plated steel sheet can have sufficiently excellent corrosion resistance; if it is 0.020 mass% or less, it can have better corrosion resistance; and if it is 0.010 mass% or less, it can have particularly excellent corrosion resistance. As such, since the corrosion resistance of the molten Al-Zn plated steel sheet is superior as the Co content in the plating bath decreases, there is no specific lower limit for the Co content.

[0092] Here, the means for reducing the content of Co in the plating bath is not particularly limited.

[0093] For example, it is desirable to apply a thermal spray film that does not contain Co, as it is effective to suppress the leaching of a Co-Cr-W-based thermal spray film applied to a device in the bath into the plating bath.

[0094] In addition, as another means of reducing the Co content in the plating bath, it is preferable to use a metal ingot with a low Co content among impurities as a raw material for the plating bath.

[0095] In addition, it is also effective not to use the pot or bath equipment used in the manufacture of plated steel sheets in which Co is intentionally added for the manufacture of molten Al-Zn plated steel sheets. This is because it is possible to prevent the Co-containing metal ingot attached to the pot or bath equipment from melting and being mixed into the plating bath.

[0096] In addition, the temperature of the plating bath is not particularly limited, but it is preferable to have a temperature range of (melting point + 20°C) to 650°C.

[0097] The reason the lower limit of the above bath temperature is set to melting point + 20°C is that in order to perform molten plating treatment, it is necessary to raise the bath temperature above the solidification point, and by setting it to melting point + 20°C, solidification caused by a local drop in the bath temperature of the plating bath is prevented. On the other hand, the reason the upper limit of the above 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 thick.

[0098] In addition, regarding the temperature of the underlay steel sheet entering the plating bath (entering sheet temperature), although not specifically limited, it is desirable to control it within ±20℃ relative to the temperature of the plating bath in order to secure plating characteristics in the continuous molten plating operation and to prevent changes in the bath temperature.

[0099] Additionally, regarding the immersion time of the lower steel plate in the plating bath, it is preferable that it be 0.5 seconds or longer. This is because if it is less than 0.5 seconds, there is a concern that a sufficient plating film cannot be formed on the surface of the lower steel plate. Although there is no specific upper limit for the immersion time, it is more preferable to keep it within 8 seconds, as there is a concern that the interfacial alloy layer formed between the plating film and the steel plate may become thicker if the immersion time is prolonged.

[0100] In addition, depending on the required performance, the molten Al-Zn plated steel sheet may form a coating film directly on the plating film or with an intermediate layer interposed therein.

[0101] The method for forming the above coating film is not particularly limited and can be appropriately selected according to the required performance. Examples of forming methods include roll coater coating, curtain flow coating, and spray coating. It is possible to form a coating film by applying a paint containing an organic resin and then heating and drying it using means such as hot air drying, infrared heating, or induction heating.

[0102] In addition, regarding the above intermediate layer, it is not particularly limited to a layer formed between the plating film of the hot-dip galvanized steel sheet and the coating film.

[0103] (Surface-treated steel plate)

[0104] 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 said plating film.

[0105] Among these, the composition of the above-mentioned plating film is the same as the plating film of the molten Al-Zn-based plated steel sheet of the present invention described above.

[0106] The surface-treated steel sheet of the present invention has a chemical conversion film formed on the aforementioned plating film.

[0107] In addition, the above-mentioned chemical coating may 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 or required performance.

[0108] In addition, in the surface-treated steel sheet of the present invention, the chemical conversion film 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 fluoropolymer resin, 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.

[0109] By forming the aforementioned chemical conversion film on the plating film, it becomes possible to increase the affinity with the plating film and to uniformly form the chemical conversion film on the plating film, in addition to increasing the anti-corrosion effect or barrier effect of the chemical conversion film. As a result, stable corrosion resistance and white rust resistance of the surface-treated steel sheet of the present invention can be realized.

[0110] Here, regarding the resin constituting the chemical conversion film, at least one selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluoropolymer is used for the purpose of improving corrosion resistance. For the same purpose, it is preferable that the resin contains at least one of urethane resin and acrylic resin. In addition, regarding the resin constituting the chemical conversion film, an addition polymer of the aforementioned resin is also included.

[0111] For the above epoxy resin, for example, a glycidyl etherified epoxy resin of the bisphenol A type, bisphenol F type, novolak type, etc., a glycidyl etherified epoxy resin of the bisphenol A type by adding propylene oxide, ethylene oxide, or polyalkylene glycol to the epoxy resin of the bisphenol A type, an aliphatic epoxy resin, a dicycloic epoxy resin, a polyether-based epoxy resin, etc. may be used.

[0112] For the above urethane resin, for example, oil-modified polyurethane resin, alkyd-based polyurethane resin, polyester-based polyurethane resin, polyether-based polyurethane resin, polycarbonate-based polyurethane resin, etc. may be used.

[0113] Examples of the above acrylic resins include polyacrylic acid and its copolymer, polyacrylic acid ester and its copolymer, polymethacrylic acid and its copolymer, polymethacrylic acid ester and its copolymer, urethane-acrylic acid copolymer (or urethane-modified acrylic resin), styrene-acrylic acid copolymer, etc. Additionally, these resins may be modified by other alkyd resins, epoxy resins, phenolic resins, etc.

[0114] Examples of the above acrylic silicone resins include a resin having hydrolyzable alkoxysilyl groups on the side chains or terminals of an acrylic copolymer used as a main component, to which a curing agent has been added. Furthermore, when an acrylic silicone resin is used, excellent weather resistance can be expected in addition to corrosion resistance.

[0115] Examples of the above alkyd resins include rheological alkyd resin, rosin-modified alkyd resin, phenol-modified alkyd resin, styrene-modified alkyd resin, silicone-modified alkyd resin, acrylic-modified alkyd resin, oil-free alkyd resin, high molecular weight oil-free alkyd resin, etc.

[0116] The above 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 used include terephthalic acid, 2,6-naphthalenedicarboxylic acid, etc. Examples of polyalcohols used include ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,4-cyclohexanedimethanol, etc. Specifically, the above polyester may include polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, etc. Additionally, an acrylic modified version of these polyester resins may be used.

[0117] Regarding the above polyalkylene resin, examples include ethylene-based copolymers such as ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, and carboxyl-modified polyolefin resin, ethylene-unsaturated carboxylic acid copolymer, and ethylene-based ionomer. Additionally, resins modified by other alkyd resins, epoxy resins, phenolic resins, etc., may be used.

[0118] Regarding the above amino resin, it 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, but it is preferable to use melamine resin from the perspective of corrosion resistance, weather resistance, and adhesion. Melamine resin is not particularly limited, but examples include butylated melamine resin, methylated melamine resin, and aqueous melamine resin.

[0119] Examples of the above fluoropolymers include fluoroolefin-based polymers or copolymers of fluoroolefins with alkyl vinyl ethers, cycloalkyl vinyl ethers, carboxylic acid-modified vinyl esters, hydroxyalkylallyl ethers, tetrafluoropropyl vinyl ethers, etc. When these fluoropolymers are used, excellent weather resistance and excellent hydrophobicity can be expected in addition to corrosion resistance.

[0120] In addition, it is particularly desirable to use a curing agent for the purpose of improving corrosion resistance or processability. As a curing agent, urea resin (butylated urea resin, etc.), melamine resin (butylated melamine resin, butyl etherified melamine resin, etc.), amino resins such as butylated urea-melamine resin and benzoguanamine resin, block isocyanates, oxazoline compounds, phenol resin, etc. can be appropriately used.

[0121] In addition, regarding the metal compound constituting the above-mentioned chemical film, at least one 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 is used. In the same regard, it is preferable that the metal compound contains at least one of P compound, Si compound, and V compound.

[0122] Here, the above P compound can improve corrosion resistance or perspiration resistance by being included in the above chemical coating. The above P compound is a compound containing P, and may contain, for example, one or more selected from inorganic phosphoric acid, organic phosphoric acid and salts thereof.

[0123] As for the above inorganic phosphoric acid, organic phosphoric acid, and salts thereof, any compound may be used without being particularly limited. For example, as the above inorganic phosphoric acid, it is preferable to use one or more selected from phosphoric acid, primary phosphate, secondary phosphate, tertiary phosphate, pyrophosphate, pyrophosphate, tripolyphosphoric acid, tripolyphosphoric acid, phosphoric acid, phosphoric acid, hypophosphoric acid, and hypophosphoric acid. In addition, as the above organic phosphoric acid, it is preferable to use phosphonic acid (phosphonic acid compound). In addition, as the above phosphonic acid, it is preferable to use one or more selected from nitrilotrismethylenephosphonic acid, phosphonobutanetricarboxylic acid, methyldiphosphonic acid, methylenephosphonic acid, and ethylidenediphosphonic acid.

[0124] In addition, when the above P compound is a salt, the salt is preferably a salt of an element of Group 1 to Group 13 in the periodic table, more preferably a metal salt, and preferably one or more selected from alkali metal salts and alkaline earth metal salts.

[0125] When a chemical treatment solution containing the above P compound is applied to a molten Al-Zn plated steel sheet, the surface of the plating film is etched by the action of the said P compound, and an enriched layer in which Al, Zn, Si, and Mg, which are constituent elements of the plating film, are blown in is formed on the plating film side of the chemical film. As the enriched layer is formed, the bond between the chemical film and the surface of the plating film is strengthened, and the adhesion of the chemical film is improved.

[0126] The concentration of the P compound in the above chemical treatment solution is not particularly limited, but can be 0.25 to 5 mass%. If the concentration of the P compound is less than 0.25 mass%, the etching effect is insufficient, the adhesion to the plating interface decreases, and not only is the corrosion resistance of the planar part reduced, but there is also a risk that the corrosion resistance and cold resistance of the plating or film damaged parts caused by defects, cut sections, processing, etc. In the same regard, the concentration of the P compound is preferably 0.35 mass% or more, and more preferably 0.50 mass% or more. On the other hand, if the concentration of the P compound exceeds 5 mass%, not only is the lifespan of the chemical treatment solution shortened, but the appearance when the film is formed is prone to becoming uneven, and there is also a risk that the blackening resistance will decrease due to an increased amount of P leaching from the chemical film. In the same regard, the concentration of the P compound is preferably 3.5 mass% or less, and more preferably 2.5 mass% or less. Regarding the content of the P compound in the above chemical film, for example, by applying and drying a chemical treatment solution having a concentration of P compound of 0.25 to 5 mass%, the amount of P attached to the chemical film after drying can be 5 to 100 mg / m².

[0127] The above Si compound is a component that forms a framework for forming a chemical conversion film together with the resin, and can uniformly form a chemical conversion film by increasing affinity with the plating film. The above Si compound is a compound containing Si, and it is preferable to contain one or more selected from, for example, silica, trialkoxysilane, tetraalkoxysilane, and silane coupling agents.

[0128] As the above silica, any type may be used without being particularly limited. As the above silica, for example, at least one of wet silica and dry silica may be used. As a colloidal silica, which is a type of wet silica, for example, Snowtex O, C, N, S, 20, OS, OXS, NS, etc. manufactured by Nissan Chemical Co., Ltd. may be suitably used. In addition, as the above dry silica, for example, AEROSIL 50, 130, 200, 300, 380, etc. manufactured by Japan Aerosil Co., Ltd. may be suitably used.

[0129] As for the above trialkoxysilane, any one may be used without being particularly limited. 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, methyltriethoxysilane, etc.

[0130] As for the above tetraalkoxysilane, any one may be used without being particularly limited. 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, tetrapropoxysilane, etc.

[0131] As the above silane coupling agent, any one can be used without being particularly limited. 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-isocyanatepropyltriethoxysilane, etc.

[0132] In addition, by incorporating the above Si compound into the chemical conversion film, the Si compound undergoes dehydration condensation to form an amorphous chemical conversion film having siloxane bonds with a high barrier effect that shields corrosion agents. Furthermore, by combining with the aforementioned resin, a chemical conversion film with even higher barrier properties is formed. Moreover, under corrosive conditions, dense and stable corrosion products are formed in the defective parts or damaged parts of the plating film caused by processing, and through a composite effect with the plating film, there is also an effect of suppressing corrosion of the underlying steel plate. From the perspective of having a high effect of forming stable corrosion products, it is preferable to use at least one of colloidal silica and dry silica as the above Si compound.

[0133] The concentration of the Si compound in the chemical treatment solution for forming the above chemical film is set to 0.2 to 9.5 mass%. If the concentration of the Si compound in the above chemical treatment solution is 0.2 mass% or higher, a barrier effect due to siloxane bonding can be obtained, and as a result, in addition to corrosion resistance in planar parts, corrosion resistance in defective parts, cut parts, and damaged parts caused by processing, and cold resistance are improved. Furthermore, if the concentration of the Si compound is 9.5 mass% or lower, the lifespan of the chemical treatment solution can be extended. By applying and drying the chemical treatment solution having a Si compound concentration of 0.2 to 9.5 mass%, the amount of Si attached to the chemical film after drying can be set to 2 to 95 mg / m².

[0134] The above Co compound and the above Ni compound can improve resistance to blackening by being included in the chemical conversion film. This is believed to be because Co or Ni has the effect of slowing down the leaching of water-soluble components from the film under corrosive environments. Furthermore, the above Co and the above Ni are elements that are difficult to oxidize compared to Al, Zn, Si, Mg, etc. Therefore, by concentrating at least one of the above Co compound and the above Ni compound at the interface between the chemical conversion film and the above plating film (forming a concentration layer), the concentration layer acts as a barrier against corrosion, thereby improving resistance to blackening.

[0135] By using a chemical treatment solution containing the above Co compound, Co can be incorporated into the chemical film and blown into the enrichment layer. It is preferable to use a cobalt salt as the above Co compound. It is more preferable to use one or more selected from cobalt sulfate, cobalt carbonate, and cobalt chloride as the above cobalt salt.

[0136] In addition, by using a chemical treatment solution containing the above Ni compound, Ni can be incorporated into the chemical film and blown into the enrichment layer. It is preferable to use a nickel salt as the above Ni compound. It is more preferable to use one or more selected from nickel sulfate, nickel carbonate, and nickel chloride as the above nickel salt.

[0137] The concentration of Co compounds and / or Ni compounds in the above chemical treatment solution is not particularly limited, but can be 0.25 to 5 mass% in total. If the concentration of the above Co compounds and / or Ni compounds is less than 0.25 mass%, the interfacial thickening layer becomes non-uniform, and there is a risk that not only will the corrosion resistance of the planar portion decrease, but the corrosion resistance of the portions damaged in the plating film due to defects, cut sections, processing, etc., will also decrease. In this regard, it is preferably 0.5 mass% or more, and more preferably 0.75 mass% or more. On the other hand, if the concentration of the above Co compounds and / or Ni compounds exceeds 5 mass%, the appearance when the film is formed is prone to becoming non-uniform, and there is a risk that the corrosion resistance will decrease. In this regard, it is preferably 4.0 mass% or less, and more preferably 3.0 mass% or less. By applying and drying a chemical treatment solution in which the total concentration of the above Co compound and / or Ni compound is 0.25 to 5 mass%, the total amount of Co and Ni attached to the chemical film after drying can be 5 to 100 mg / m².

[0138] With respect to the above Al compound, the above Zn compound, and the above Mg compound, by incorporating them into the chemical treatment solution, a richness layer comprising at least one of Al, Zn, and Mg can be formed on the plating film side of the chemical film. The formed richness layer can improve corrosion resistance.

[0139] In addition, the above Al compound, the above Zn compound, and the above Mg compound are not particularly limited as long as they are compounds containing Al, Zn, and Mg, respectively, but are preferably inorganic compounds, and preferably are salts, chlorides, oxides, or hydroxides.

[0140] As the above Al compounds, for example, one or more selected from aluminum sulfate, aluminum carbonate, aluminum chloride, aluminum oxide, and aluminum hydroxide may be cited.

[0141] Examples of the above Zn compounds include one or more selected from zinc sulfate, zinc carbonate, zinc chloride, zinc oxide, and zinc hydroxide.

[0142] Examples of the above Mg compounds include one or more selected from magnesium sulfate, magnesium carbonate, magnesium chloride, magnesium oxide, and magnesium hydroxide.

[0143] The concentration of Al compounds, Zn compounds, and / or Mg compounds in the phosphating solution for forming the above phosphating film is preferably 0.25 to 5 mass% in total. If the total concentration is 0.25 mass% or higher, the enriched 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 mass% or lower, the appearance of the phosphating film becomes more uniform, and the corrosion resistance of flat parts, defective parts, or damaged parts of the plating film caused by processing is further improved.

[0144] The above V compound is included in the above chemical conversion film, thereby allowing V to properly leach out under corrosive conditions and combine with zinc ions of plating components that leach out under the same corrosive conditions to form a dense protective film. Due to the formed protective film, corrosion resistance can be further enhanced not only against flat surfaces of the steel sheet but also against defects, damaged plating film caused by processing, and corrosion progressing from the cut surface to the flat surface.

[0145] Regarding the above V compound, one or more compounds containing V may be selected from sodium metavanadate, vanadyl sulfate, and vanadium acetylacetonate.

[0146] It is preferable that the V compound in the phosphating solution for forming the above phosphating film be 0.05 to 4 mass%. If the concentration of the V compound is 0.05 mass% or higher, it becomes easier to leach out under a corrosive environment and form a protective film, thereby improving the corrosion resistance of defects, cut sections, and damaged parts of the plating film caused by processing. On the other hand, if the concentration of the V compound exceeds 4 mass%, the appearance of the phosphating film formed is prone to becoming non-uniform, and the resistance to blackening also decreases.

[0147] The above Mo compound can increase the resistance to blackening of the surface-treated steel sheet by being included in the chemical conversion film. The above Mo compound is a compound containing Mo and can be obtained by adding one or both of molybdic acid and molybdate salt to the chemical conversion treatment solution.

[0148] In addition, as the above molybdate salt, one or more selected from sodium molybdate, potassium molybdate, magnesium molybdate, and zinc molybdate may be cited.

[0149] The concentration of the Mo compound in the phosphating solution for forming the above phosphating film is preferably 0.01 to 3 mass%. If the concentration of the Mo compound is 0.01 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 mass% or lower, the lifespan of the phosphating solution is further extended, and corrosion resistance can be further improved.

[0150] The above Zr compound and the above Ti compound are included in the chemical conversion film to prevent the film from becoming porous, thereby densifying the film. As a result, it becomes difficult for corrosion agents to penetrate the chemical conversion film, which can increase corrosion resistance.

[0151] Regarding the above Zr compound, one or more compounds containing Zr may be used, for example, selected from zirconyl acetate, zirconyl sulfate, potassium zirconyl carbonate, sodium zirconyl carbonate, and zirconyl ammonium carbonate. Among these, an organic titanium chelate compound is suitable because, when a film is formed by drying the chemical treatment solution, it densifies the film, thereby obtaining superior corrosion resistance.

[0152] As for the above Ti compound, one or more compounds containing Ti may be used, for example, selected from titanium sulfate, titanium chloride, titanium hydroxide, titanium acetylacetonate, titanium octylene glycolate, and titanium ethylacetoacetate.

[0153] The concentration of Zr compounds and / or Ti compounds in the phosphating solution for forming the above phosphating film is preferably 0.2 to 20 mass% in total. If the total concentration of the Zr compounds and / or Ti compounds is 0.2 mass% or higher, the effect of inhibiting the penetration of corrosion factors is increased, and the corrosion resistance of not only the planar part but also the defect part, the cut section part, and the plating film damaged part caused by processing can be further improved. On the other hand, if the total concentration of the Zr compounds and / or Ti compounds is 20 mass% or lower, the life of the phosphating solution can be further extended.

[0154] The above Ca compound can exhibit the effect of reducing the corrosion rate by being included in the above chemical coating.

[0155] Regarding the above Ca compound, examples of compounds containing Ca include calcium oxides, calcium nitrates, calcium sulfates, and intermetallic compounds containing Ca. More specifically, examples of the above Ca compounds include CaO, CaCO3, Ca(OH)2, Ca(NO3)2·4H2O, CaSO4·2H2O, etc. The content of the above Ca compound in the chemical conversion film is not particularly limited.

[0156] In addition, the above-mentioned chemical film may contain various known components commonly used in the paint field, as needed. Examples include various surface modifiers such as leveling agents and defoaming agents, various additives such as dispersants, anti-settling agents, UV absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as coloring pigments, extender pigments, and optical brightening agents, curing catalysts, organic solvents, and lubricants.

[0157] In addition, in the surface-treated steel sheet of the present invention, it is preferable that the chemical conversion film does not contain harmful components such as hexavalent chromium, trivalent chromium, and fluorine. This is because the chemical conversion treatment solution for forming the chemical conversion film does not contain these harmful components, thereby increasing safety and reducing adverse effects on the environment.

[0158] In addition, the amount of the chemical conversion film applied is not particularly limited. For example, from the perspective of ensuring corrosion resistance more reliably while preventing peeling of the chemical conversion film, it is preferable to set the amount of the chemical conversion film applied to 0.1 to 3.0 g / m², and more preferable to set it to 0.5 to 2.5 g / m². By making the amount of the chemical conversion film applied 0.1 g / m² or more, it can be more reliably ensured, and by making the amount of the chemical conversion film applied 3.0 g / m² or less, cracking or peeling of the chemical conversion film can be prevented.

[0159] The amount of the above-mentioned chemical film attached can be determined by a method appropriately selected from existing methods, such as a method of measuring the presence of an element whose content is known in advance by analyzing the film with fluorescent X-rays.

[0160] In addition, the method for forming the above-mentioned chemical film is not particularly limited and can be appropriately selected depending on the required performance or manufacturing equipment. For example, it can be formed by continuously applying a chemical treatment solution onto the above-mentioned plating film using a roll coater, 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 spray, electrostatic spray, and curtain flow coater may be appropriately employed for applying the above-mentioned chemical treatment solution. Furthermore, the above-mentioned chemical film may be either a single layer or a multilayer film as long as it contains the above-mentioned resin and the above-mentioned metal compound, and is not particularly limited.

[0161] In addition, the surface-treated steel plate of the present invention may form a coating film on the chemical conversion film as needed.

[0162] (Painted steel plate)

[0163] The coated steel sheet of the present invention is a coated steel sheet having a coating formed on a plating film, either directly or through a chemical conversion film.

[0164] Among these, the composition of the above-mentioned plating film is the same as the plating film of the molten Al-Zn-based plated steel sheet of the present invention described above.

[0165] The coated steel sheet of the present invention can form a chemical conversion film on the plating film.

[0166] In addition, the above-mentioned chemical coating may be formed on at least one side of the coated steel sheet, and may also be formed on both sides of the coated steel sheet depending on the application or required performance.

[0167] In addition, the coated 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 backbone, in a total amount of 30 to 50 mass%, wherein the ratio of (a):(b) to (b) is in the range of 3:97 to 60:40 in mass ratio, and an inorganic compound comprising 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound.

[0168] By forming the aforementioned chemical conversion film on the plating film, the strength and adhesion of the chemical conversion film can be increased, and corrosion resistance can also be improved.

[0169] Herein, regarding the resin component constituting the above-mentioned chemical film, it contains (a): an anionic polyurethane resin having ester bonds and (b): an epoxy resin having a bisphenol backbone.

[0170] Regarding the above (a) anionic polyurethane resin having ester bonds, examples include a resin obtained by copolymerizing a dimethylolalkyl acid with a reaction product of a polyester polyol and a diisocyanate or polyisocyanate having two or more isocyanate groups. Additionally, a chemical treatment solution can be obtained by dispersing it in a liquid such as water by a known method.

[0171] Examples of the above 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 a cyclic ester compound such as ε-caprolactone, and copolymers thereof.

[0172] Examples of the above polyisocyanates include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, etc. Examples of the above 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.).

[0173] In addition, when synthesizing a urethane by reacting the above polyester polyol with the above diisocyanate or polyisocyanate, for example, by copolymerizing a dimethylol alkyl acid and self-emulsifying to make it water-soluble (water-dispersible), an anionic polyurethane resin having the above (a) ester bond can be obtained. In this case, as the dimethylol alkyl acid, for example, a dimethylol alkyl acid having 2 to 6 carbon atoms can be cited, and more specifically, dimethylol ethanoic acid, dimethylol propanoic acid, dimethylol butanoic acid, dimethylol heptanoic acid, and dimethylol hexanoic acid can be cited.

[0174] In addition, regarding the above (b) epoxy resin having a bisphenol backbone, known epoxy resins may be used. Examples include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD ​​type epoxy resin, bisphenol S type epoxy resin, etc. These epoxy resins can be obtained by reacting a bisphenol compound, such as bisphenol A, bisphenol F, bisphenol AD, or bisphenol S, with epichlorohydrin in the presence of an alkali catalyst. Among these, it is preferable that component (b) includes a bisphenol A type epoxy resin or a bisphenol F type epoxy resin, and it is more preferable that it includes a bisphenol A type epoxy resin. A chemical treatment solution can be obtained by dispersing the said (b) epoxy resin having a bisphenol backbone in a liquid such as water by a known method.

[0175] The above resin component acts as a binder for the chemical conversion film, but the (a) anionic polyurethane resin having ester bonds constituting the binder is flexible, so it can have the effect of making it difficult for the chemical conversion film to be destroyed (peeled off) when processed, and the (b) epoxy resin having a bisphenol backbone can have the effect of improving adhesion between the Al-Zn plated steel sheet of the substrate and the primer film of the upper layer.

[0176] The above resin component is included in the above conversion film in a total of 30 to 50 mass%. If the content of the above resin component is less than 30 mass%, the binder effect of the conversion film is reduced, and if it exceeds 50 mass%, the function of the inorganic component described below, for example, inhibitor action, is reduced. In the same regard, it is preferable that the content of the above resin component in the above conversion film is 35 to 45 mass%.

[0177] In addition, the resin component requires that the content ratio ((a):(b)) of the anionic polyurethane resin having (a) ester bonds and the epoxy resin having (b) a bisphenol backbone be in the range of 3:97 to 60:40 by mass ratio. This is because if (a):(b) is outside the above range, sufficient corrosion resistance is not obtained, accompanied by a decrease in flexibility or adhesion as a chemical treatment film. In the same regard, it is preferable that (a):(b) be 10:90 to 55:45.

[0178] In addition, regarding the resin component, depending on the required performance, it may include a resin other than the aforementioned (a) anionic polyurethane resin having ester bonds and (b) epoxy resin having a bisphenol backbone (other resin component). The other resin component is not particularly limited and, for example, at least one or two or more selected from acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluoropolymer resin may be used in combination.

[0179] When the above resin component includes other resins, it is preferable that the total content of the (a) anionic polyurethane resin having ester bonds and the (b) epoxy resin having a bisphenol backbone is 50 mass% or more, and more preferable that it is 75 mass% or more. This is to ensure a reduction in flexibility or adhesion as a chemical treatment film more reliably.

[0180] In addition, the above-mentioned chemical film comprises, as inorganic compounds, 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound.

[0181] By including these compounds, the corrosion resistance of the chemical coating can be increased.

[0182] The above vanadium compound is added to the chemical treatment solution and acts as a rust inhibitor. By including the above vanadium compound in the chemical film, the vanadium compound is properly leached out under a corrosive environment and combines with zinc ions of the plating components that are leached out under the same corrosive environment to form a dense protective film. Due to the formed protective film, corrosion resistance can be further enhanced not only on the flat surface of the steel sheet but also on defects, damaged parts of the plating film caused by processing, and corrosion progressing from the cut surface to the flat surface.

[0183] Examples of the above vanadium compounds include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadylacetylacetonate, vanadium acetylacetonate, etc. In particular, among these, it is preferable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

[0184] In addition, the content of the vanadium compound in the chemical treatment film is 2 to 10 mass%. This is because if the content of the vanadium compound in the chemical treatment film is less than 2 mass%, the inhibitor effect is insufficient, leading to a decrease in corrosion resistance, while if the content of the vanadium compound exceeds 10 mass%, it leads to a decrease in the moisture resistance of the chemical treatment film.

[0185] The zirconium compound is contained in the above-mentioned chemical conversion film, and through reaction with the plating metal or coexistence with the resin component, it is expected that the strength and corrosion resistance of the chemical conversion film will be improved. Furthermore, the zirconium compound itself contributes to the formation of a dense chemical conversion film, and a barrier effect can be expected due to its excellent coverage.

[0186] Examples of the above zirconium compounds include zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, zirconium chloride, and other neutral salts.

[0187] In addition, the content of the zirconium compound in the above chemical treatment film is 40 to 60 mass%. This is because if the content of the zirconium compound in the above chemical treatment film is less than 40 mass%, it results in a decrease in strength or corrosion resistance as a chemical treatment film, and if the content of the zirconium compound exceeds 60 mass%, the chemical treatment film becomes embrittled, and if subjected to harsh processing, the chemical treatment film may break or peel off.

[0188] The above fluorine compound is contained in the chemical conversion film and acts as an adhesion agent with the plating film. As a result, it is possible to increase the corrosion resistance of the chemical conversion film.

[0189] As the above 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 may be used. Among these, it is preferable to use fluoride salts such as ammonium fluoride, sodium fluoride, and potassium fluoride.

[0190] In addition, the content of the fluorine compound in the chemical treatment film is 0.5 to 5 mass%. This is because if the content of the fluorine compound in the chemical treatment film is less than 0.5 mass%, sufficient adhesion at the processed part is not obtained, and if the content of the fluorine compound exceeds 5 mass%, the moisture resistance of the chemical treatment film decreases.

[0191] In addition, the amount of the chemical conversion film applied is not particularly limited. For example, from the perspective of improving the adhesion of the chemical conversion film while ensuring corrosion resistance more reliably, it is preferable to set the amount of the chemical conversion film applied to 0.025 to 0.5 g / m². By making the amount of the chemical conversion film applied 0.025 g / m² or more, corrosion resistance can be more reliably secured, and by making the amount of the chemical conversion film applied 0.5 g / m² or less, peeling of the chemical conversion film can be suppressed.

[0192] The amount of the above-mentioned chemical film attached can be determined by a method appropriately selected from existing methods, such as a method of measuring the presence of an element whose content is known in advance by analyzing the film with fluorescent X-rays.

[0193] In addition, the method for forming the above-mentioned chemical film is not particularly limited and can be appropriately selected depending on the required performance or manufacturing equipment. For example, it can be formed by continuously applying a chemical treatment solution onto the above-mentioned plating film using a roll coater, 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 spray, electrostatic spray, and curtain flow coater may be appropriately employed for applying the above-mentioned chemical treatment solution. Furthermore, the above-mentioned chemical film may be either a single layer or a multilayer film as long as it contains the above-mentioned resin and the above-mentioned metal compound, and is not particularly limited.

[0194] As described above, the coated steel sheet of the present invention has a coating formed on a plating film directly or via a chemical conversion film, and the coating film has at least a primer coating film.

[0195] In addition, the present invention comprises a primer film having a urethane bond, a polyester resin, and an inorganic compound including a vanadium compound, a phosphate compound, and magnesium oxide.

[0196] The above primer film can improve corrosion resistance while increasing the adhesion of the film by containing the above urethane bonded polyester resin and the above inorganic compound.

[0197] The above primer film contains a polyester resin having urethane bonds as a main component. Since the polyester resin having urethane bonds possesses both flexibility and strength, effects such as preventing cracks from forming in the primer film when subjected to processing are obtained, and in terms of high affinity with a chemical treatment film containing urethane resin, it can particularly contribute to improving the corrosion resistance of the processed part.

[0198] In addition, the term "main component" as used herein refers to the component with the highest content among the various components in the primer film.

[0199] As the polyester resin having the above urethane bond, known resins may be used, such as a resin 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 above polyester polyol with the above diisocyanate or the above polyisocyanate in a state of excess hydroxyl groups (urethane-modified polyester resin) and curing it with a blocked polyisocyanate may also be used.

[0200] In addition, the above polyester polyol can be obtained by a known method using a dehydration condensation reaction between a polyhydric alcohol component and a polybasic acid component.

[0201] Examples of the above polyhydric alcohols include glycols and polyhydric alcohols of trihydric or higher hydric acid. Examples of the above 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, etc. Additionally, examples of the above polyhydric alcohols of trihydric or higher hydric acid include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol, etc. These polyhydric alcohols may be used individually or in combination of two or more types.

[0202] The above polybasic acid is usually a polycarboxylic acid, but monovalent fatty acids, etc., may be used in combination as needed. Examples of the above 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, etc., and their acid anhydrides, as well as 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, etc. These polybasic acids may be used alone or in combination of two or more types.

[0203] Examples of the above 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), as well as cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI, and their adducts, biurets, and isocyanurates. These polyisocyanates may be used alone or in combination of two or more types.

[0204] In addition, the hydroxyl group value of the polyester resin having the above urethane bond is not particularly limited, but is preferably 5 to 120 mg KOH / g, more preferably 7 to 100 mg KOH / g, and even more preferably 10 to 80 mg KOH / g from the perspective of solvent resistance, processability, etc.

[0205] In addition, the number average molecular weight of the polyester resin having the above urethane bond is preferably 500 to 15,000 in terms of solvent resistance, processability, etc., more preferably 700 to 12,000, and even more preferably 800 to 10,000.

[0206] In the above primer film, it is preferable that the content of the polyester resin having urethane bonds be 40 to 88 mass%. If the content of the polyester resin having urethane bonds is less than 40 mass%, there is a risk that the binder function as a primer film will be reduced, and on the other hand, if the content of the polyester resin having urethane bonds exceeds 88 mass%, there is a risk that the function of the inorganic material described below, such as inhibitor action, will be reduced.

[0207] One of the above inorganic compounds, a vanadium compound, acts as an inhibitor. Examples of the above vanadium compounds include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadylacetylacetonate, vanadium acetylacetonate, etc. In particular, among these, it is preferable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

[0208] The vanadium compound added to the above primer film may be of the same type or different type from the vanadium compound added to the above chemical treatment film. It is believed that the vanadic acid compound reacts with the ions on the surface of the zinc-plated steel sheet and the vanadic acid ions, which gradually leach out due to moisture entering from the outside, to form a passivation film with good adhesion, thereby protecting the exposed metal parts and exhibiting an anti-corrosion effect.

[0209] The content of the vanadium compound in the primer film is not particularly limited, but from the perspective of achieving both corrosion resistance and moisture resistance, it is preferable to be 4 to 20 mass%. If the content of the vanadium compound is less than 4 mass%, the inhibitor effect may decrease, potentially leading to a decrease in corrosion resistance, and if the content of the vanadium compound exceeds 20 mass%, there is a risk of a decrease in the moisture resistance of the primer film.

[0210] It also acts as an inhibitor for a phosphoric acid compound, which is one of the inorganic compounds mentioned above. Examples of the phosphoric acid compounds that can be used include phosphoric acid, ammonium salts of phosphoric acid, alkali metal salts of phosphoric acid, alkaline earth metal salts of phosphoric acid, etc. In particular, alkali metal salts of phosphoric acid, such as calcium phosphate, can be suitably used.

[0211] The content of the phosphoric acid compound in the primer film is not particularly limited, but from the perspective of achieving both corrosion resistance and moisture resistance, it is preferable to be 4 to 20 mass%. If the content of the phosphoric acid compound is less than 4 mass%, the inhibitor effect may decrease, potentially leading to a decrease in corrosion resistance, and if the content of the phosphoric acid compound exceeds 20 mass%, there is a risk of a decrease in the moisture resistance of the primer film.

[0212] Magnesium oxide, one of the above inorganic compounds, produces a product containing Mg upon initial corrosion, and as a sparingly soluble magnesium salt, it promotes stabilization and has the effect of improving corrosion resistance.

[0213] The content of magnesium oxide in the primer film is not particularly limited, but from the perspective of achieving both corrosion resistance and corrosion resistance of the processed part, it is preferable to be 4 to 20 mass%. If the content of magnesium oxide is less than 4 mass%, the above effect is reduced and there is a risk of causing a decrease in corrosion resistance, and if the content of magnesium oxide exceeds 20 mass%, the flexibility of the primer film is reduced, which may result in a decrease in corrosion resistance of the processed part.

[0214] In addition, the primer film may contain components other than the polyester resin having the aforementioned urethane bond and inorganic compound.

[0215] For example, a crosslinking agent used when forming a primer film can be cited. The crosslinking agent reacts with the polyester resin having the urethane bond to form a crosslinked 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. Among these, from the perspective of corrosion resistance of the processed part of the resulting coated steel sheet, a blocked polyisocyanate compound, etc., can preferably be used. Examples of the blocked polyisocyanates include those in which the isocyanate group of a polyisocyanate compound is blocked by, for example, alcohols such as butanol, oximes such as methyl ethyl keto-oxime, lactams such as ε-caprolactams, diketones such as acetoacetic acid diesters, imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

[0216] In addition, the above primer film may, if necessary, contain various known components commonly used in the paint field. Specifically, examples include various surface modifiers such as leveling agents and defoaming agents, various additives such as dispersants, anti-settling agents, UV absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as coloring pigments and extender pigments, gloss agents, curing catalysts, and organic solvents.

[0217] It is preferable that the thickness of the primer film be 1.5 μm or more. This is because by making the thickness of the primer film 1.5 μm or more, the effect of improving corrosion resistance or the effect of improving the adhesion of the overcoat film formed on the chemical treatment film or the primer film can be obtained more reliably.

[0218] The method for forming the primer film is not particularly limited. Furthermore, regarding the coating method of the paint composition constituting the primer film, the paint composition may preferably be applied by a roll coater coating, curtain flow coating, etc. After coating the paint composition, the primer film may be obtained by baking using a heating means such as hot air heating, infrared heating, or induction heating. The baking treatment can typically be performed for about 30 seconds to 3 minutes at a maximum plate temperature of about 180 to 270°C.

[0219] In addition, regarding the coating film constituting the coated steel plate of the present invention, it is preferable that an additional overcoat film is formed on the primer film.

[0220] In addition to imparting aesthetic qualities such as color, gloss, and surface condition to the coated steel plate, the above-mentioned overcoat film can improve various performances such as processability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance.

[0221] The composition of the above-mentioned overlay film is not specifically limited, and the material or thickness can be appropriately selected according to the required performance.

[0222] For example, the above-mentioned overcoat film can be formed using polyester resin-based paint, silicone polyester resin-based paint, polyurethane resin-based paint, acrylic resin-based paint, fluoropolymer-based paint, etc.

[0223] In addition, the above-mentioned overcoat film may contain a suitable amount of titanium oxide, Bengala, mica, carbon black, or various other coloring pigments; metallic pigments such as aluminum powder or mica; extender pigments composed of carbonates or sulfates; various fine particles such as silica fine particles, nylon resin beads, acrylic resin beads; curing catalysts such as p-toluenesulfonic acid, dibutyltin dilaurate; wax; and other additives.

[0224] In addition, from the perspective of achieving compatibility between appearance and processability, the thickness of the overcoat film is preferably 5 to 30 μm. When the thickness of the overcoat film is 5 μm or more, it is possible to stabilize the color tone appearance more reliably, and when the thickness of the overcoat film is 30 μm or less, the deterioration of processability (occurrence of cracks in the overcoat film) can be suppressed more reliably.

[0225] The method of applying the paint composition to form the overcoat film is not particularly limited. For example, the paint composition may be applied by a method such as a roll coater or a curtain flow coating. After applying the paint composition, the overcoat film may be formed by baking using a heating means such as hot air heating, infrared heating, or induction heating. The baking treatment can typically be performed with a maximum plate temperature of about 180 to 270°C and within this temperature range for about 30 seconds to 3 minutes.

[0226] Examples

[0227] <Example 1: Samples 1–47>

[0228] Samples 1 to 47 of hot-dip galvanized steel sheets under the conditions shown in Table 1 were produced by using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by the normal method as the base steel sheet, and performing annealing and plating treatments using a hot-dip galvanizing simulator manufactured by Reska Co., Ltd.

[0229] In addition, regarding the composition of the plating bath used to manufacture the hot-dip galvanized steel sheet, the composition of the plating bath was varied in the range of Al: 5–70 mass%, Si: 0.0–4.4 mass%, Ni: 0.000–0.025 mass%, Co: 0.001–0.092 mass%, V: 0.0–0.1 mass%, Cr: 0.0–0.2 mass%, Mn: 0.0–0.1 mass%, Mg: 0.0–4.0 mass%, Ca: 0.0–1.0 mass%, and Sr: 0.0–1.0 mass% so that the composition of the plating film of each sample shown in Table 1 would be obtained. In addition, the bath temperature of the plating bath used was set to 450°C for Al: 5 mass%, 480°C for Al: 18 mass%, 600°C for Al: 36 to 55 mass%, and 660°C for Al: over 60 mass%, and was controlled so that the plating penetration temperature of the underlying steel sheet was the same as the plating bath temperature. In addition, for Al: 30 to 60 mass%, the plating treatment was carried out under conditions of cooling for 3 seconds in a temperature range where the plate temperature was 520 to 500°C.

[0230] In addition, the amount of plating film applied was controlled to be 85±5 g / m² per side for samples 1 to 44, 50±5 g / m² per side for sample 45, 100±5 g / m² per side for sample 46, and 125±5 g / m² per side for sample 47.

[0231] (evaluation)

[0232] For each sample of the hot-dip galvanized steel sheet obtained as described above, the following evaluation was performed. The evaluation results are shown in Table 1.

[0233] (1) Plating film (composition, amount of coating, Ni-based compound)

[0234] For each sample after plating, a 100 mmφ was punched, and the non-measuring surface was sealed with tape. Then, the plating was dissolved and peeled off using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013, and the amount of the plating film attached was calculated from the difference in mass between the samples before and after peeling. The amount of the plating film attached obtained as a result of the calculation is shown in Table 1.

[0235] Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, components other than insoluble Si were quantified by analyzing the filtrate using ICP emission spectroscopy.

[0236] In addition, the solids were dried and incinerated in a furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate. Furthermore, the molten material was dissolved in hydrochloric acid, and the insoluble Si was quantified by analyzing the solution using ICP emission spectroscopic analysis. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from the solids analysis to the soluble Si concentration obtained from the filtrate analysis. The composition of the obtained plating film is shown in Table 1.

[0237] In addition, for each sample, after shearing it to a size of 15 mm × 15 mm, the steel plate was embedded in a conductive resin to observe the cross-section, and after mechanical polishing, a continuous cross-section of an arbitrarily selected plating film having a length of 2 mm or more in a direction parallel to the surface of the underlying steel plate was continuously captured with a width of 100 μm under an acceleration voltage of 3 kV using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss). Additionally, within the same apparatus, an elemental mapping analysis (Al, Zn, Si, Fe, and Ni) of each cross-section was performed under an acceleration voltage of 3 kV using an energy-dispersive X-ray spectrometer (Ultim Extreme manufactured by Oxford Instruments). For the parts where high Ni intensity was detected by this analysis, point analysis was performed using the spectrometer under an acceleration voltage of 3 kV, and the material was identified from the semi-quantitative values ​​of the obtained components. The major axis of all Ni-based compounds identified within the field of view was measured to determine the maximum major axis. Additionally, the number of all Ni-based compound particles present in the observed continuous cross-section was counted and divided by the length of the cross-section (mm) to calculate the number of Ni-based compound particles per 1 mm (particles / mm) oriented parallel to the surface of the underlay steel plate. For the areas where high Ni intensity was detected through this analysis, point analysis was performed using a dynamic spectrometer under conditions of an acceleration voltage of 3 kV, and the material was identified from the semi-quantitative values ​​of the obtained components. The analysis results are shown in Table 1.

[0238] (2) Corrosion resistance evaluation

[0239] For each sample of the obtained hot-dip galvanized steel sheet, after shearing it to a size of 120 mm × 120 mm, the area of ​​10 mm from each edge of the surface to be evaluated, the cross-section 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, and this was used as an evaluation sample. In addition, three identical evaluation samples were produced.

[0240] For the three evaluation samples prepared as described above, a cyclic corrosion acceleration test as shown in Fig. 1 was performed on all of them. The corrosion acceleration test was started from a wet state and continued until 150 cycles were completed. Afterward, the corrosion loss of each sample was measured using the methods described in JIS Z 2383 and ISO 8407 and evaluated according to the following criteria. The evaluation results are shown in Table 1.

[0241] ◎: Corrosion loss of all 3 samples is 60 g / ㎡ or less.

[0242] ○: Corrosion loss of all 3 samples is 75 g / ㎡ or less.

[0243] ×: Corrosion loss of one or more samples exceeds 75 g / m²

[0244] (3) Surface appearance

[0245] For each sample of the obtained hot-dip galvanized steel sheet, after shearing to a size of 70 mm × 150 mm, eight copper-thick plates were inserted inside, and a 180° bending process (8T bend) was performed. After bending, cellophane tape (registered trademark) was strongly adhered to the outer surface of the bent section and then peeled off. The surface condition of the plating film on the outer surface of the bent section and the presence or absence of adhesion (peeling) of the plating film on the surface of the tape used were visually observed, and the processability was evaluated according to the following criteria. The evaluation results are shown in Table 1.

[0246] ○: Neither cracks nor peeling are observed in the plating film.

[0247] △: There are cracks in the plating film, but no peeling is observed.

[0248] ×: Both cracks and peeling are observed in the plating film.

[0249]

[0250] From the results of Table 1, it can be seen that each sample of the present invention example has excellent corrosion resistance and processability in good balance compared to each sample of the comparative example.

[0251] <Example 2: Samples 1–118>

[0252] (1) A cold-rolled steel sheet with a thickness of 0.8 mm manufactured by the commercial method was used as the base steel sheet, and an annealing treatment and plating treatment were performed using a hot-dip plating simulator manufactured by Reska Co., Ltd., thereby producing samples of hot-dip plating steel sheets with plating film conditions as shown in Tables 3 to 5.

[0253] In addition, regarding the composition of the plating bath used to manufacture the hot-dip galvanized steel sheets, the composition of the plating bath was varied in the range of Al: 5–70 mass%, Si: 0.0–4.4 mass%, Ni: 0.000–0.025 mass%, V: 0.0–0.1 mass%, Cr: 0.0–0.2 mass%, Mn: 0.0–0.1 mass%, Mg: 0.0–4.0 mass%, Ca: 0.0–1.0 mass%, and Sr: 0.0–1.0 mass% so that the composition of the plating film of each sample shown in Tables 3 to 5 would be obtained. In addition, the bath temperature of the plating bath was set to 450°C for Al: 5 mass%, 480°C for Al: 18 mass%, 600°C for Al: 36 to 55 mass%, and 660°C for Al: over 60 mass%, and controlled so that the plating penetration temperature of the underlying steel sheet was the same as the plating bath temperature. In addition, for Al: 30 to 60 mass%, the plating treatment was carried out under conditions of cooling for 3 seconds in a temperature range where the plate temperature was 520 to 500°C.

[0254] In addition, the amount of plating film applied was controlled to be 85±5 g / m² per side for samples 1 to 88 and 95 to 118, 50±5 g / m² per side for samples 89 to 90, 100±5 g / m² per side for samples 91 to 92, and 125 g / m²±5 g / m² per side for samples 93 to 94.

[0255] (2) After that, a chemical treatment solution was applied to the plating film of each sample of the manufactured hot-dip galvanized steel sheet using a bar coater, and a chemical treatment film was formed by drying in a hot air furnace (heating rate: 60℃ / s, PMT: 120℃), and each sample of the surface-treated steel sheet shown in Tables 3 to 5 was manufactured.

[0256] In addition, surface treatment solutions A to F were prepared by dissolving each component in water as a solvent. The types of each component (resin, metal compound) contained in the surface treatment solutions are as follows.

[0257] (profit)

[0258] Urethane resin: Super Flex 130, Super Flex 126 (Daiichi Kogyo Seiyaku Co., Ltd.)

[0259] Acrylic Resin: Boncoat EC-740 EF (DIC Co., Ltd.)

[0260] (metal compounds)

[0261] P Compound: Aluminum dihydrogen tripolyphosphate

[0262] Si compounds: Silica

[0263] V Compound: Sodium metavanadate

[0264] Mo Compound: Molybdic Acid

[0265] Zr Compound: Potassium Zirconyl Carbonate

[0266] The composition of the prepared chemical treatment solutions A to F and the amount of the formed chemical film attached are shown in Table 2. In addition, the concentration of each component in Table 2 of this specification is the concentration of solids (mass%).

[0267]

[0268] (evaluation)

[0269] For each sample of the hot-dip galvanized steel sheet and surface-treated steel sheet obtained as described above, the following evaluation was performed. The evaluation results are shown in Tables 3 to 5.

[0270] (1) Plating film (composition, amount of coating, Ni-based compound)

[0271] For each sample of the hot-dip galvanized steel sheet, a 100 mmφ was punched, and the non-measuring surface was sealed with tape. Then, the plating was dissolved and peeled off using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013, and the amount of the plating film attached was calculated from the difference in mass of the sample before and after peeling. The amount of the plating film attached obtained as a result of the calculation is shown in Tables 3 to 5.

[0272] Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, components other than insoluble Si were quantified by analyzing the filtrate using ICP emission spectroscopy.

[0273] In addition, the solids were dried and incinerated in a furnace at 650°C, and then dissolved by adding sodium carbonate and sodium tetraborate. Furthermore, the molten material was dissolved with hydrochloric acid, and the insoluble Si was quantified by analyzing the solution using ICP emission spectroscopic analysis. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from the solids analysis to the soluble Si concentration obtained from the filtrate analysis. The composition of the obtained plating film is shown in Tables 3 to 5.

[0274] In addition, for each sample, after shearing to a size of 15 mm × 15 mm, the steel plate was embedded in a conductive resin to observe the cross-section, and after mechanical polishing, a continuous cross-section of an arbitrarily selected plating film having a length of 2 mm or more in a direction parallel to the surface of the underlying steel plate was continuously captured using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss) at a width of 100 μm under an acceleration voltage of 3 kV. Furthermore, within the same apparatus, an elemental mapping analysis (Al, Zn, Si, Fe, and Ni) of each cross-section was performed using an energy-dispersive X-ray spectrometer (Ultim Extreme manufactured by Oxford Instruments) under an acceleration voltage of 3 kV. For the parts where high Ni intensity was detected by this analysis, point analysis was performed using the spectrometer under an acceleration voltage of 3 kV, and the material was identified from the semi-quantitative values ​​of the obtained components. The major axis was measured for all Ni-based compounds identified within the field of view, and the maximum major axis was determined. Additionally, the number of all Ni-based compound particles present in the observed continuous cross-section was counted and divided by the observed cross-sectional length (mm) to calculate the number of Ni-based compound particles per 1 mm (particles / mm) oriented parallel to the surface of the underlay steel plate. For the areas where high Ni intensity was detected through this analysis, point analysis was performed using a dynamic spectrometer under conditions of an acceleration voltage of 3 kV, and the material was identified from the semi-quantitative values ​​of the obtained components. The analysis results are shown in Tables 3 to 5.

[0275] (2) Corrosion resistance evaluation

[0276] For each sample of the hot-dip galvanized steel sheet and surface-treated steel sheet, after shearing to a size of 120 mm × 120 mm, a 10 mm range from each edge of the surface to be evaluated, as well as the cross-section 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, and used as an evaluation sample. In addition, three identical evaluation samples were produced.

[0277] For the three evaluation samples prepared as described above, a cyclic corrosion acceleration test as shown in Fig. 1 was performed on all of them. The corrosion acceleration test was started from a wet state and continued until 150 cycles were completed. Afterward, the corrosion loss of each sample was measured using the methods described in JIS Z 2383 and ISO 8407 and evaluated according to the following criteria. The evaluation results are shown in Tables 3 to 5.

[0278] ◎: Corrosion loss of all 3 samples is 40 g / ㎡ or less.

[0279] ○: Corrosion loss of all 3 samples is 60 g / ㎡ or less.

[0280] ×: Corrosion loss of one or more samples exceeds 60 g / m²

[0281] (3) Inner white blue

[0282] For each sample of hot-dip galvanized steel sheet and surface-treated steel sheet, after shearing to a size of 120 mm × 120 mm, a range of 10 mm from each edge of the surface to be evaluated, and the cross-section 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, and used as an evaluation sample.

[0283] 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. The evaluation results are shown in Tables 3 to 5.

[0284] ◎: No white rust on the flat surface

[0285] ○: Area of ​​white rust on the flat surface less than 10%

[0286] ×: Area of ​​white rust on the flat surface exceeding 10%

[0287] (4) Processability

[0288] For each sample of hot-dip galvanized steel sheet, after shearing to a size of 70 mm × 150 mm, eight copper-thick plates were inserted inside, and a 180° bending process (8T bend) was performed. After bending, cellophane tape (registered trademark) was strongly adhered to the outer surface of the bent section and then peeled off. The surface condition of the plating film on the outer surface of the bent section and the presence or absence of adhesion (peeling) of the plating film on the surface of the tape used were visually observed, and processability was evaluated according to the following criteria. The evaluation results are shown in Tables 3 to 5.

[0289] ○: Neither cracks nor peeling are observed in the plating film.

[0290] △: There are cracks in the plating film, but no peeling is observed.

[0291] ×: Both cracks and peeling are observed in the plating film.

[0292]

[0293]

[0294]

[0295] From the results of Tables 3 to 5, it can be seen that each sample of the present invention is excellent in terms of corrosion resistance, white rust resistance, and processability in good balance compared to each sample of the comparative example.

[0296] In addition, from the results of Table 5, it can be seen that each sample subjected to chemical treatments A to D exhibits particularly excellent white rust resistance.

[0297] <Example 3: Samples 1–47>

[0298] (1) Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by the commercial method as the base steel sheet, and by performing annealing and plating treatments using a hot-dip plating simulator manufactured by Reska Co., Ltd., samples 1 to 47 of hot-dip plating steel sheets with conditions shown in Table 7 were produced.

[0299] The composition of the plating bath was varied in the range of Al: 5–70 mass%, Si: 0.0–4.4 mass%, Ni: 0.000–0.025 mass%, Co: 0.001–0.092 mass%, V: 0.0–0.1 mass%, Cr: 0.0–0.2 mass%, Mn: 0.0–0.1 mass%, Mg: 0.0–4.0 mass%, Ca: 0.0–1.0 mass%, and Sr: 0.0–1.0 mass%. In addition, the bath temperature of the plating bath was set to 450°C for Al: 5 mass%, 480°C for Al: 18 mass%, 600°C for Al: 36 to 55 mass%, and 660°C for Al: over 60 mass%, and controlled so that the plating penetration temperature of the underlying steel sheet was the same as the plating bath temperature. In addition, for Al: 30 to 60 mass%, the plating treatment was carried out under conditions of cooling for 3 seconds in a temperature range where the plate temperature was 520 to 500°C.

[0300] In addition, the amount of plating film applied was controlled to be 85±5 g / m² per side for samples 1 to 44, 50±5 g / m² per side for sample 45, 100±5 g / m² per side for sample 46, and 125±5 g / m² per side for sample 47.

[0301] (2) Afterwards, a chemical treatment solution shown in Table 6 was applied to the plating film of each sample of the manufactured hot-dip galvanized steel sheet using a bar coater, and dried in a hot-air drying oven (reached plate temperature: 90℃) to form a chemical treatment film with an adhesion amount of 0.1 g / m².

[0302] In addition, the phosphating solution used was a phosphating solution with a pH of 8 to 10 prepared by dissolving each component in water as a solvent. The types of each component (resin component, inorganic compound) contained in the phosphating solution are as follows.

[0303] (Resin component)

[0304] Resin A: (a) anionic polyurethane resin having ester bonds ("Super Flex 210" manufactured by Daiichi Kokyose Iyaku Co., Ltd.) and (b) epoxy resin having a bisphenol backbone ("Yuka Resin RE-1050" manufactured by Yoshimura Yukagaku Co., Ltd.), mixed in a mass ratio (a):(b) = 50:50.

[0305] Resin B: Acrylic resin (Manufactured by DIC Co., Ltd. "Boncoat EC-740 EF")

[0306] (Inorganic compound)

[0307] Vanadium compounds: Organic vanadium compounds chelated with acetylacetone

[0308] Zirconium Compounds: Ammonium Zirconium Carbonate

[0309] Fluorine compounds: Ammonium fluoride

[0310] (3) Then, a primer paint was applied to the phosphating film formed as described above using a bar coater, and a primer film having the composition shown in Table 6 was formed by baking under conditions of a steel plate reaching temperature of 230°C and a baking time of 35 seconds. After that, an overcoat paint composition was applied to the primer film formed as described above using a bar coater, and a overcoat film having the resin conditions and film thickness shown in Table 6 was formed by baking under conditions of a steel plate reaching temperature of 230°C to 260°C and a baking time of 40 seconds. Then, each sample of coated steel plate was produced.

[0311] In addition, the primer paint was obtained by mixing each component and stirring with a ball mill for about 1 hour. The resin component and inorganic compound constituting the primer film were as follows.

[0312] (Resin component)

[0313] 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, having a resin acid value of 3, a number average molecular weight of 5,600, and a hydroxyl value of 36) was used after curing with a blocked isocyanate.

[0314] In addition, the urethane-modified polyester resin was manufactured under the following conditions. 320 parts by mass of isophthalic acid, 200 parts by mass of adipic acid, 60 parts by mass of trimethylolpropane, and 420 parts by mass of cyclohexanedimethanol are added to a flask equipped with a stirrer, a rectification column, a water separator, a condenser, and a thermometer, and heated and stirred, while distilling off the resulting condensation water from the system, the temperature is increased from 160°C to 230°C at a constant rate over 4 hours, and after reaching 230°C, 20 parts by mass of xylene are slowly added, and the condensation reaction is continued while maintaining the temperature at 230°C, and the reaction is terminated when the acid value becomes 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 butylcellosolve are added, A polyester resin solution was obtained.

[0315] Resin β: Urethane-cured polyester resin ("Ibaeclad 4900" manufactured by Kansai Paint Co., Ltd.)

[0316] (Inorganic compound)

[0317] Vanadium Compounds: Magnesium Vanadate

[0318] Phosphate compounds: Calcium phosphate

[0319] Magnesium oxide compounds: Magnesium oxide

[0320] In addition, regarding the resin used for the overlay film shown in Table 6, the following paints were used.

[0321] Resin I: Melamine-cured polyester paint ("Free Color HD0030HR" manufactured by BASF Japan, Inc.)

[0322] Resin II: Organozol-based baking-type fluoropolymer paint in which polyvinylidene fluoride and acrylic resin are mixed in a mass ratio of 80:20 ("Free Color No. 8800HR" manufactured by BASF Japan Inc.)

[0323]

[0324] (evaluation)

[0325] For each sample of the coated steel sheet obtained as described above, the following evaluation was performed. The evaluation results are shown in Table 7.

[0326] (1) Plating film (composition, amount of coating, Ni-based compound)

[0327] For each sample of the hot-dip galvanized steel sheet, a 100 mmφ was punched, and the non-measuring surface was sealed with tape. Then, the plating was dissolved and peeled off using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013, and the amount of the plating film attached was calculated from the difference in mass of the sample before and after peeling. The amount of the plating film attached obtained as a result of the calculation is shown in Table 7.

[0328] Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, components other than insoluble Si were quantified by analyzing the filtrate using ICP emission spectroscopy.

[0329] In addition, the solids were dried and incinerated in a furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate. Furthermore, the molten material was dissolved in hydrochloric acid, and the insoluble Si was quantified by analyzing the solution using ICP emission spectroscopic analysis. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from the solids analysis to the soluble Si concentration obtained from the filtrate analysis. The composition of the obtained plating film is shown in Table 7.

[0330] In addition, for each sample, after shearing to a size of 15 mm × 15 mm, the steel plate was embedded in a conductive resin to observe the cross-section, and after mechanical polishing, a continuous cross-section of an arbitrarily selected plating film having a length of 2 mm or more in a direction parallel to the surface of the underlying steel plate was continuously captured using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss) at a width of 100 μm under an acceleration voltage of 3 kV. Furthermore, within the same apparatus, an elemental mapping analysis (Al, Zn, Si, Mg, Fe, Sr, and Ni) of each cross-section was performed using an energy-dispersive X-ray spectrometer (Ultim Extreme manufactured by Oxford Instruments) under an acceleration voltage of 3 kV. For the parts where high Ni intensity was detected by this analysis, point analysis was performed using the spectrometer under an acceleration voltage of 3 kV, and the material was identified from the semi-quantitative values ​​of the obtained components. The major axis was measured for all Ni-based compounds identified within the field of view, and the maximum major axis was determined. Additionally, the number of all Ni-based compound particles present in the observed continuous cross-section was counted and divided by the length of the observed cross-section (mm) to calculate the number of Ni-based compound particles per 1 mm (particles / mm) oriented parallel to the surface of the underlay steel plate. For the areas where high Ni intensity was detected through this analysis, point analysis was performed using a dynamic spectrometer under conditions of an acceleration voltage of 3 kV, and the material was identified from the semi-quantitative values ​​of the obtained components. The analysis results are shown in Table 7.

[0331] (2) Corrosion resistance evaluation

[0332] For each sample of the coated steel plate, after shearing it to a size of 120 mm × 120 mm, the area of ​​10 mm from each edge of the surface to be evaluated, the cross-section 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, and this was used as an evaluation sample. In addition, three identical evaluation samples were produced.

[0333] For the three evaluation samples prepared as described above, a corrosion acceleration test was performed on all of them using the cycle shown in Fig. 1. The corrosion acceleration test was started from a wet state, and samples were removed every 20 cycles, washed with water, and dried. Afterward, the samples were visually inspected to check for the occurrence of red rust on the shear cross-section of one side that was not tape-sealed.

[0334] In addition, the number of cycles when red-blue was confirmed was evaluated according to the following criteria. The evaluation results are shown in Table 7.

[0335] ◎: Number of red-blue generation cycles for 3 samples ≥ 300 cycles

[0336] ○: 300 cycles > Number of cycles for red-blue generation of 3 samples ≥ 200 cycles

[0337] ×: Number of red-blue occurrence cycles of at least one sample < 200 cycles

[0338] (3) Processability after painting

[0339] For each sample of the coated steel plate, after shearing to a size of 70 mm × 150 mm, eight copper-thick plates were inserted inside, and a 180° bending process (8T bend) was performed. After bending, cellophane tape (registered trademark) was strongly adhered to the outer surface of the bent section and then peeled off. The surface condition of the coating on the outer surface of the bent section and the presence or absence of adhesion (peeling) of the coating on the surface of the tape used were visually observed, and the processability was evaluated according to the following criteria. The evaluation results are shown in Table 7.

[0340] ○: Neither cracks nor peeling are observed in the coating.

[0341] △: There are cracks in the coating, but no peeling is observed.

[0342] ×: Both cracks and peeling are observed in the coating.

[0343]

[0344] From the results of Table 7, it can be seen that each sample of the present invention example has excellent, well-balanced corrosion resistance and processability after coating compared to each sample of the comparative example.

[0345] Industrial applicability

[0346] According to the present invention, it is possible to provide a molten Al-Zn plated steel sheet having excellent corrosion resistance that is reliable and stable.

[0347] In addition, according to the present invention, it is possible to provide a surface-treated steel sheet having excellent corrosion resistance and white rust resistance in a reliable and stable manner, and a painted steel sheet having excellent corrosion resistance and corrosion resistance of the processed part in a reliable and stable manner.

Claims

Claim 1 A molten Al-Zn plated steel sheet having a plating film, wherein the plating film has a composition comprising Al: 45 to 65 mass% and Si: 1.0 to 4.0 mass%, with the remainder being Zn and unavoidable impurities, wherein the Ni content among the unavoidable impurities is greater than 0 to 0.010 mass% with respect to the total mass of the plating film, wherein the plating film contains a Ni-based compound, wherein the major axis of the Ni-based compound is greater than 0 to 4.0 μm, and furthermore, the number of said Ni-based compounds existing in a direction parallel to the surface of the base steel sheet is greater than 0 to 5 / mm. Claim 2 A molten Al-Zn plated steel sheet according to claim 1, characterized in that the Al content in the plating film is 50 to 60 mass%. Claim 3 A molten Al-Zn plated steel sheet according to claim 1 or 2, characterized in that the Co content among the unavoidable impurities included in the plating film is 0.080 mass% or less with respect to the total mass of the plating film. Claim 4 A method for manufacturing a molten Al-Zn plated steel sheet having a plating film, wherein the formation of the plating film comprises a molten plating treatment process in which a base steel sheet is immersed in a plating bath having a composition in which Al: 45 to 65 mass% and Si: 1.0 to 4.0 mass% is contained, and the remainder is Zn and unavoidable impurities; wherein the Ni content among the unavoidable impurities of the plating bath is controlled to be greater than 0 to 0.010 mass% relative to the total mass of the plating bath; wherein the plating film contains a Ni-based compound, the major axis of said Ni-based compound is greater than 0 to 4.0 μm, and furthermore, the number of said Ni-based compounds existing in a direction parallel to the surface of the base steel sheet is greater than 0 to 5 / mm. Claim 5 A surface-treated steel sheet having a plating film described in claim 1 or 2 and a chemical conversion film formed on said plating film, wherein the chemical conversion film comprises at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluoropolymer resin, 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. Claim 6 A coated steel sheet having a coating formed on a plating film described in claim 1 or 2, either directly or through a conversion film interposed therebetween, wherein the conversion film comprises a resin component containing (a): an anionic polyurethane resin having ester bonds and (b): an epoxy resin having a bisphenol backbone in a total amount of 30 to 50 mass%, wherein the ratio of (a):(b) to (b) is in the range of 3:97 to 60:40 in mass ratio, and an inorganic compound comprising 2 to 10 mass% of a vanadium compound, 40 to 60 mass% of a zirconium compound, and 0.5 to 5 mass% of a fluorine compound, wherein the coating film comprises at least a primer coating film, wherein the primer coating film comprises a polyester resin having urethane bonds and an inorganic compound comprising a vanadium compound, a phosphate compound, and magnesium oxide. Claim 7 delete Claim 8 delete Claim 9 delete

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