JP7583934B2 - Hot-dip Al-Zn plated steel sheet, its manufacturing method, surface-treated steel sheet and coated steel sheet - Google Patents

Description

The present invention relates to hot-dip Al-Zn plated steel sheets, their manufacturing method, surface-treated steel sheets and painted steel sheets.

Hot-dip Al-Zn-coated steel sheets, such as 55% Al-Zn, are known to have high corrosion resistance among hot-dip galvanized steel sheets, because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al. For this reason, hot-dip Al-Zn-coated steel sheets are used mainly in the field of building materials such as roofs and walls that are exposed to the outdoors for long periods of time, and in the field of civil engineering and construction such as guardrails, wiring and piping, and soundproof walls, due to their excellent corrosion resistance. In particular, the demand for materials with excellent corrosion resistance and maintenance-free materials in more severe usage environments such as acid rain caused by air pollution, the spraying of deicing agents to prevent roads from freezing in snowy areas, and coastal area development has been increasing in recent years, and the demand for hot-dip Al-Zn-coated steel sheets has been increasing in recent years.

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

In general, hot-dip Al-Zn plated steel sheet is produced by using a thin steel sheet obtained by hot-rolling or cold-rolling a slab as a base steel sheet, and subjecting the base steel sheet to recrystallization annealing and hot-dip plating treatment in an annealing furnace of a continuous hot-dip plating facility.
Here, in the coating bath used for manufacturing hot-dip Al-Zn-coated steel sheets, in addition to the predetermined concentrations of Al and Zn, Si is usually added to suppress the excessive growth of the interfacial alloy layer formed at the interface between the base steel and the coating. This action of Si makes it possible to control the thickness of the interfacial alloy layer of the hot-dip Al-Zn-coated steel sheet to about 1 to 5 μm. Furthermore, if the coating thickness is the same, the thinner the interfacial alloy layer, the thicker the upper layer that exhibits high corrosion resistance will be, so it is believed that suppressing the growth of the interfacial alloy layer leads to improved corrosion resistance.

It is also known that impurities inevitably get mixed into the plating bath used in the production of hot-dip galvanized steel sheets due to impurities contained in the plating raw materials or leaching from the base steel sheet or bath-immersed equipment. The inevitable mixing of impurities into the bath is no exception for plating baths used in the production of hot-dip Al-Zn-plated steel sheets, and components such as Fe, Cr, Ni, Cu, Co, and W are inevitably included in the plating film.

Impurities contained in such a plating bath can cause deterioration of the properties of hot-dip plated steel sheets, such as their appearance, corrosion resistance, and workability, and the presence or absence of the effects of impurities depends on the composition of the plating film and the impurity concentration. In other words, even impurities of the same composition may or may not be harmful to the properties of the plated steel sheet. For this reason, the effects of impurities on the properties of various hot-dip plated steel sheets have been investigated, and technologies for controlling the impurity concentration have been developed to stably obtain the required properties.
For example, Patent Document 2 discloses a hot-dip Al-Zn plated steel sheet having a plating film containing, in mass%, 0.10 to 0.6% Al, 0.03 to 0.3% Bi, and the balance being Zn and unavoidable impurities, with each content of Pb, Sn, and Cd as the unavoidable impurities controlled to 0.002%.
Furthermore, Patent Document 3 discloses a hot-dip Zn-Al-Mg plated steel sheet having a plating film containing, in mass%, 4.4 to 5.6% Al, 0.3 to 0.56% Mg, and the balance being Zn and unavoidable impurities, in which the unavoidable impurities are controlled so that Ni is not contained therein.

On the other hand, the above-mentioned hot-dip Al-Zn-plated steel sheets have a problem that, when used in a severe corrosive environment, the plating film corrodes and white rust occurs. Since this white rust deteriorates the appearance of the steel sheet, various technologies have been developed to improve the white rust resistance.
For example, Patent Document 4 discloses a surface-treated steel sheet having improved white rust resistance by providing a chemical conversion coating on a hot-dip Al-Zn-plated steel sheet, the chemical conversion coating containing a water-soluble urethane resin having a carboxyl group and an acid amide bond, N-methylpyrrolidone, a zirconium metal compound, and a silane coupling agent.
Furthermore, for example, Patent Document 5 discloses a surface-treated steel sheet having improved white rust resistance by providing, on a hot-dip Al-Zn-plated steel sheet, a chemical conversion coating containing a resin having a glass transition temperature of −10° C. or higher and selected from an acrylic resin, a polyester resin, and a urethane resin, and a compound selected from a Si compound, a Zr compound, and a triazole or tetrazole.

Furthermore, for the above-mentioned hot-dip Al-Zn plated steel sheets, coated steel sheets on whose surfaces chemical conversion coatings, primer coatings, top coatings, etc. have been formed are subjected to various processes such as 90-degree or 180-degree bending by press forming, roll forming, or embossing, and long-term coating durability performance is also required. To meet these requirements, hot-dip Al-Zn plated steel sheets are known to have chemical conversion coatings containing chromate, primer coatings containing chromate-based rust-preventive pigments, and top coatings with excellent weather resistance such as thermosetting polyester resin coatings or fluororesin coatings formed thereon.

However, in recent years, the use of chromate, an environmentally hazardous substance, has come to be seen as problematic, and there is a strong demand for the development of chromate-free painted steel sheets that can improve corrosion resistance.
As a technique for responding to these demands, for example, Patent Document 6 discloses a coated steel sheet having improved corrosion resistance at cut end faces, in which an undercoat coating film containing a rust-preventive pigment containing Mg is formed on the surface of a hot-dip Al-Zn-plated steel sheet, and a topcoat coating film having a contact angle with water of 80 degrees or more and less than 13 degrees is further formed on the undercoat coating film.
Patent Document 7 discloses a coated steel sheet having improved corrosion resistance at processed portions and cut end faces, in which a chromate-free chemical conversion coating containing a urethane resin, an epoxy resin, a vanadium compound, a zirconium compound, and a fluorine compound is formed on the surface of a hot-dip Al-Zn-plated steel sheet, a primer coating containing an ester resin as a main component and a vanadium compound, a phosphate compound, and magnesium oxide is formed thereon, and a melamine-cured polyester topcoat coating is further formed thereon.

Special Publication No. 46-7161 JP 2004-285387 A Special table 2016-540885 publication JP 2003-201578 A JP 2013-237874 A JP 2009-172553 A JP 2016-176118 A

However, in the case of hot-dip Zn-Al-plated steel sheets as disclosed in Patent Documents 2 and 3, the effect of unavoidable impurities on the corrosion resistance has not been fully elucidated, and there has been a demand for the development of a technology that can more reliably achieve excellent corrosion resistance.
In addition, control of impurities in the production of hot-dip galvanized steel sheets, not limited to hot-dip Al-Zn-coated steel sheets, is often limited to concentration control, and no technology has been established to control the morphology, such as size or distribution, and there has been a demand for the development of technology that can achieve more stable and excellent corrosion resistance.

With regard to technology for improving corrosion resistance while taking into account the effects of the above-mentioned unavoidable impurities, improvements are desired not only for plated steel sheets, but also for surface-treated steel sheets which further comprise a chemical conversion coating formed on the plated film, and for painted steel sheets which further comprise a coating formed directly on the plated film or via a chemical conversion coating.

In addition, although a certain level of corrosion resistance and white rust resistance can be obtained by using the surface-treated steel sheets described in Patent Documents 4 and 5, the effect may not be sufficient in some cases, and therefore a higher level of corrosion resistance and white rust resistance has been required.
Furthermore, although the use of the coated steel sheets of Patent Documents 6 and 7 provides a certain level of corrosion resistance in processed areas and cut end faces, the effect is sometimes insufficient, and a higher level of corrosion resistance and corrosion resistance in processed areas has been required.

In view of the above circumstances, an object of the present invention is to provide a hot-dip Al-Zn coated steel sheet having excellent corrosion resistance reliably and stably, and a method for producing the same.
Another object of the present invention is to provide a surface-treated steel sheet having reliably and stably excellent corrosion resistance and white rust resistance, and a coated steel sheet having reliably and stably excellent corrosion resistance and corrosion resistance in processed areas.

As a result of investigations conducted by the inventors to solve the above problems, they noticed that it is important not only to control the concentrations of Al, Zn and Si in the composition of the plating film of hot-dip Al-Zn plated steel sheet, but also to control the concentrations of elements contained as impurities, and discovered that deterioration of corrosion resistance can be effectively suppressed by appropriately controlling the Ni content, and further that deterioration of corrosion resistance can be more stably suppressed by appropriately controlling the size and distribution state of Ni-based compounds present as impurities in the plating film.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A hot-dip Al-Zn-plated steel sheet having a plating film,
The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and the remainder being Zn and unavoidable impurities,
A hot-dip Al-Zn plated steel sheet, characterized in that the Ni content in the unavoidable impurities is 0.010 mass% or less with respect to a total mass of the plating film.

2. The hot-dip Al-Zn plated steel sheet according to 1 above, characterized in that the plated film contains Ni-based compounds, and the Ni-based compounds have a major axis of 4.0 μm or less.
3. The hot-dip Al-Zn plated steel sheet according to 1 or 2 above, characterized in that the plating film contains Ni-based compounds, and the number of the Ni-based compounds present in a direction parallel to the surface of the base steel sheet is 5/mm or less.
4. The hot-dip Al-Zn plated steel sheet according to 1 above, characterized in that the plated coating does not contain any Ni-based compound.

5. The hot-dip Al-Zn plated steel sheet described in any one of 1 to 4, characterized in that the Al content in the plating film is 50 to 60 mass %.

6. The hot-dip Al-Zn plated steel sheet according to any one of 1 to 5, characterized in that the Co content in the unavoidable impurities contained in the plating film is 0.080 mass% or less relative to the total mass of the plating film.

7. A method for producing a hot-dip Al-Zn-plated steel sheet having a plating film, comprising the steps of:
The formation of the plating film includes a hot-dip plating process in which a base steel sheet is immersed in a plating bath having a composition containing 45 to 65 mass% Al and 1.0 to 4.0 mass% Si, with the balance being Zn and unavoidable impurities;
A method for producing a hot-dip Al-Zn plated steel sheet, characterized in that a Ni content in unavoidable impurities in the plating bath is controlled to 0.010 mass% or less with respect to a total mass of the plating bath.

8. A surface-treated steel sheet comprising the plating film according to any one of claims 1 to 6 and a chemical conversion film formed on the plating film,
The chemical conversion coating film is a surface-treated steel sheet characterized in that it contains at least one resin selected from epoxy resins, urethane resins, acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins, and at least one metal compound selected from P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds.

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

According to the present invention, it is possible to provide a hot-dip Al-Zn plated steel sheet having excellent corrosion resistance reliably and stably.
Furthermore, according to the present invention, it is possible to provide a surface-treated steel sheet having reliably and stably excellent corrosion resistance and white rust resistance, and a coated steel sheet having reliably and stably excellent corrosion resistance and corrosion resistance in processed areas.

This is a diagram to explain the flow of the Japanese Automotive Standard Combined Cycle Test (JASO-CCT).

(Hot-dip Al-Zn plated steel sheet)
The hot-dip Al-Zn plated steel sheet of the present invention has a plating film on the surface of the steel sheet.
The plating film has a composition containing 45 to 65 mass % Al, 1.0 to 4.0 mass % Si, and the remainder being Zn and unavoidable impurities.

The Al content in the plating film is 45 to 65 mass%, preferably 50 to 60 mass%, in consideration of the balance between corrosion resistance and operational aspects. This is because if the Al content in the plating film is at least 45 mass%, dendritic solidification of Al occurs, and a plating film structure mainly composed of α-Al phase dendritic solidification structure can be obtained. The dendritic solidification structure is layered in the thickness direction of the plating film, which complicates the corrosion progression path and improves the corrosion resistance of the plating film itself. In addition, the more the α-Al phase dendrite parts are layered, the more complex the corrosion progression path becomes, making it difficult for corrosion to reach the base steel sheet, and therefore the corrosion resistance improves, so it is preferable to set the Al content to 50 mass% or more. On the other hand, if the Al content in the plating film exceeds 65 mass%, most of the Zn changes to a structure in which it is solid-solved in α-Al, and the dissolution reaction of the α-Al phase cannot be suppressed, and the corrosion resistance of the Al-Zn-Si-Mg plating deteriorates. For this reason, the Al content in the plating film must be 65 mass % or less, and is preferably 60 mass % or less.

The Si in the plating film is added mainly to suppress the growth of the Fe-Al and/or Fe-Al-Si interfacial alloy layer formed at the interface with the base steel sheet, and to prevent the deterioration of the adhesion between the plating film and the steel sheet. In fact, when a steel sheet is immersed in an Al-Zn plating bath containing Si, the Fe on the steel sheet surface reacts with the Al and Si in the bath to form an Fe-Al and/or Fe-Al-Si intermetallic compound layer at the interface between the base steel sheet and the plating film. At this time, the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, so the higher the ratio of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film must be 1.0 mass% or more. On the other hand, if the Si content in the plating film exceeds 4.0 mass%, not only will the effect of suppressing the growth of the interfacial alloy layer described above saturate, but the presence of an excess Si phase in the plating film will promote corrosion, so the Si content is set to 4.0 mass% or less. Furthermore, the Si content in the plating film is preferably 3.0 mass % or less from the viewpoint of suppressing the presence of an excessive Si phase.

The plating film also contains Zn and unavoidable impurities. Among these, the unavoidable impurities include Fe. This Fe is inevitably contained in the plating film as a result of dissolution of the steel sheet or bath-immersed equipment into the plating bath, and as a result of being supplied by diffusion from the base steel sheet during the formation of the interface alloy layer. The Fe content in the plating film is usually about 0.3 to 2.0 mass%.
Other unavoidable impurities include Cr, Ni, Cu, Co, W, etc. These elements are inevitably contained in the plating film due to the fact that they are dissolved in the plating bath from the WC-based or Co-Cr-W-based thermal spray coating applied to the base steel sheet or the bath-immersing equipment made of stainless steel, that they are contained as impurities in the metal ingots that are the raw materials for the plating bath, and that they are produced using pots or bath-immersing equipment that were used in the production of plated steel sheets to which these elements were intentionally added.

The hot-dip Al-Zn-plated steel sheet of the present invention is characterized in that the Ni content in the unavoidable impurities is 0.010% by mass or less with respect to the total mass of the plating film. Since Ni contained in the plating film may deteriorate the corrosion resistance of the hot-dip Al-Zn-plated steel sheet, the deterioration of corrosion resistance can be suppressed by appropriately controlling the contents of Al, Zn, and Si in the plating film described above and further suppressing the Ni content as an unavoidable impurity. From the same viewpoint, the Ni content in the unavoidable impurities is preferably 0.005% by mass or less with respect to the total mass of the plating film.

In addition, when the inevitable impurities contain Ni, Ni-based compounds may be contained as impurities in the coating of the hot-dip Al-Zn-plated steel sheet. Here, the Ni-based compounds mainly refer to Ni-based compounds such as binary intermetallic compounds such as Ni-Al compounds and ternary intermetallic compounds such as Ni-Al-Fe compounds. An example of the Ni-Al compound is an intermetallic compound such as NiAl ₃ , and an example of the Ni-Al-Fe compound is an intermetallic compound such as (Ni,Fe)Al ₃ in which part of the Ni in NiAl ₃ is replaced with Fe, but the compounds are not limited to these compounds.

Here, the presence of Ni-based compounds in the plating film can be confirmed, for example, by using a scanning electron microscope to observe the plating film from the surface or cross section using secondary electron images or backscattered electron images, and analyzing it using energy dispersive X-ray spectroscopy (EDS). For example, 5 to 10 locations on a 100 μm plating cross section can be arbitrarily selected, and observation and element mapping analysis can be performed at an accelerating voltage of 5 kV or less, and further point analysis can be performed on the parts where Ni is detected to confirm the composition of Ni-based inclusions. This method is merely one example, and any method that can confirm the presence of Ni-based compounds can be used, and is not particularly limited.

Furthermore, when the plating film contains a Ni-based compound, the Ni-based compound preferably has a major axis of 4.0 μm or less.
The Ni-based compounds present in the plating film function as a cathode in a corrosive environment and form a local battery with the surrounding solidified structure, which may cause deterioration of corrosion resistance. In particular, when coarse Ni-based compounds are present in the plating film, the corrosion resistance of the hot-dip Al-Zn-plated steel sheet may be significantly reduced. Therefore, in order to obtain a hot-dip Al-Zn-plated steel sheet having better corrosion resistance, it is effective to control the size of the Ni-based compounds contained as impurities in the plating film to be small. Specifically, the major axis of the Ni-based compounds is preferably 4.0 μm or less, more preferably 3.0 μm or less, and even more preferably 2.0 μm or less.
The major axis of the Ni-based compound can be measured, for example, by observing a cross-section of the plating film using a scanning electron microscope, observing a backscattered electron image, confirming that the Ni-based compound is a Ni-based compound using EDS, and then observing a backscattered electron image with an enlarged observation field including the Ni-based compound. The major axis of the Ni-based compound is the maximum major axis of the Ni-based compound observed in the observation field of the plating film.

Furthermore, when the plating film contains Ni-based compounds, it is also effective to reduce the amount of the Ni-based compounds that serve as the starting points of corrosion in order to obtain high corrosion resistance more stably. Specifically, the number of particles of Ni-based compounds in the plating film is preferably 5 particles/mm or less in the direction parallel to the surface of the base steel sheet, more preferably 2 particles/mm or less, and most preferably 0 particles/mm (non-existent).
Therefore, by reducing the amount of Ni-containing compounds present in the plating film, deterioration of the corrosion resistance of the hot-dip 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 Ni-based compounds), it is important to reduce the Ni content in the unavoidable impurities, specifically, to make the Ni content 0.005 mass% or less with respect to the total mass of the plating film.
The number of particles of the Ni-based compounds can be calculated, for example, by using a scanning electron microscope to continuously observe a cross section of the plating film parallel to the surface of the base steel sheet over a length of 1 mm or more using backscattered electron images, and dividing the number of Ni-based compounds confirmed by EDS by the measured length (mm) to calculate the number of Ni-based compounds present within a length range of 1 mm.

In addition, in the hot-dip Al-Zn plated steel sheet of the present invention, the Co content in the unavoidable impurities is preferably 0.080 mass% or less with respect to the total mass of the plated film. Since Co contained in the plated film may deteriorate the corrosion resistance of the hot-dip Al-Zn plated steel sheet like Ni described above, it is effective in suppressing deterioration of corrosion resistance to appropriately control the Ni contents in the Al, Zn, Si, and unavoidable impurities in the plated film and further suppress the Co content as an unavoidable impurity.
From the same viewpoint, the Co content in the unavoidable impurities is preferably 0.020 mass % or less, and more preferably 0.010 mass % or less, with respect to the total mass of the plating film.

Furthermore, the total content of unavoidable impurities in the plating film is not particularly limited, but if contained in excess, it may affect various characteristics of the plated steel sheet, so it is preferable that the total content be 5.0 mass% or less.

The plating film preferably 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 the corrosion product when the plating film corrodes, and have the effect of delaying the progress of corrosion.
The reason why the total content of the above-mentioned components is set to 0.01 to 10 mass % is that a sufficient corrosion retardation effect can be obtained and the effect does not become saturated.

In addition, in the hot-dip Al-Zn plated steel sheet of the present invention, since the concentrations of Al, Zn, Si, and inevitable impurities such as Ni and Co can be controlled and the corrosion resistance can be improved more stably, it is preferable that the diffraction intensity of Si in the plated film, as determined by an X-ray diffraction method, satisfies the following relationship (1):
Si (111)=0...(1)
Si (111): Diffraction intensity of the Si (111) plane (planar spacing d = 0.3135 nm) It is generally known that in the dissolution reaction of an Al alloy in an aqueous solution, the presence of a Si phase as a cathode site promotes the dissolution of the surrounding α-Al phase. Therefore, reducing the amount of Si phase is also effective in terms of suppressing the dissolution of the α-Al phase. Among these, a coating without a Si phase, as shown in relationship (1) (making the diffraction peak intensity of the Si (111) zero) is the most excellent in terms of stabilizing corrosion resistance.
The method for measuring Si (111) by X-ray diffraction involves mechanically scraping off a portion of the plating film, pulverizing it into powder, and subjecting it to X-ray diffraction (powder X-ray diffraction measurement method). The diffraction intensity can be calculated by measuring the Si diffraction peak intensity corresponding to the interplanar spacing d = 0.3135 nm.
The amount of the plating film required for carrying out the powder X-ray diffraction measurement (the amount of the plating film to be scraped off) is 0.1 g or more, preferably 0.3 g or more, from the viewpoint of measuring Si (111) with high accuracy. When the plating film is scraped off, steel sheet components other than the plating film may be contained in the powder. However, these intermetallic compound phases are contained only in the plating film, and do not affect the above-mentioned peak intensity. Furthermore, the plating film is powdered and X-ray diffraction is carried out because, when X-ray diffraction is carried out on the plating film formed on the plated steel sheet, it is difficult to calculate the correct phase ratio due to the influence of the plane orientation of the plating film solidification structure.

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

From the viewpoint of satisfying various characteristics, the coating weight of the plating film is preferably 45 to 120 g/ ^m2 per side. When the coating weight of the plating film is 45 g/ ^m2 or more, sufficient corrosion resistance is obtained for applications requiring long-term corrosion resistance, such as building materials, while when the coating weight of the plating film is 120 g/m2 or ^less , excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. From the same viewpoint, the coating weight of the plating film is more preferably 45 to 100 g/ ^m2 .

The coating weight of the plating film can be calculated, for example, by dissolving and peeling off a specific area of the plating film in a mixture of hydrochloric acid and hexamethylenetetramine, as specified in JIS H 0401:2013, and calculating the difference in the weight of the steel sheet before and after peeling. To determine the coating weight per side using this method, the plating surface of the non-target side is sealed with tape so as not to be exposed, and then the dissolution described above is carried out.

The composition of the plating film can be confirmed by immersing the plating film in hydrochloric acid or the like to dissolve it, and then subjecting the solution to ICP emission spectrometry, atomic absorption spectrometry, etc., just as with the Ni content described above. This method is merely one example, and any method that can accurately quantify the composition of the plating film may be used, and is not particularly limited.

The plating film of the hot-dip Al-Zn-plated steel sheet obtained by the present invention has a composition that is almost equivalent to that of the plating bath as a whole. Therefore, the composition of the plating film can be controlled with high precision by controlling the plating bath composition.

Furthermore, the base steel sheet constituting the hot-dip Al-Zn-plated steel sheet of the present invention is not particularly limited, and cold-rolled steel sheet, hot-rolled steel sheet, etc. can be used as appropriate depending on the required performance and standards.
Furthermore, the method for obtaining the base steel sheet is not particularly limited. For example, in the case of the hot-rolled steel sheet, one that has been subjected to a hot rolling process and a pickling process can be used, and in the case of the cold-rolled steel sheet, it can be manufactured by further adding a cold rolling process. Furthermore, in order to obtain the properties of the steel sheet, it is also possible to pass through a recrystallization annealing process or the like before the hot-dip plating process.

(Manufacturing method of hot-dip Al-Zn coated steel sheet)
The method for producing a hot-dip Al-Zn-plated steel sheet of the present invention is a method for producing a hot-dip Al-Zn-plated steel sheet provided with a plating film, and the formation of the plating film includes a hot-dip galvanizing treatment step of immersing a base steel sheet in a plating bath having a composition containing 45 to 65 mass % Al and 1.0 to 4.0 mass % Si, with the balance being Zn and unavoidable impurities.
The hot-dip galvanizing process is not particularly limited except for the conditions of the galvanizing bath described below. For example, the steel sheet can be produced by cleaning, heating, and immersing the base steel sheet in a galvanizing bath in a continuous hot-dip galvanizing facility. In the steel sheet heating process, recrystallization annealing or the like is performed to control the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective in preventing oxidation of the steel sheet and reducing a small amount of oxide film present on the surface.

As for the plating bath used in the hot-dip plating process, as described above, the composition of the plating film as a whole will be approximately the same as the composition of the plating bath, and therefore a plating bath containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, with the remainder being Zn and unavoidable impurities can be used.

The method for producing a hot-dip Al-Zn-plated steel sheet according to the present invention is characterized in that the Ni content in the inevitable impurities of the coating bath is controlled to 0.010 mass% or less with respect to the total mass of the coating bath. Since Ni contained in the coating film may deteriorate the corrosion resistance of the hot-dip Al-Zn-plated steel sheet as described above, the Al, Zn, and Si contents in the coating bath are appropriately controlled, and further the Ni content as an inevitable impurity is reduced, thereby preventing deterioration of the corrosion resistance.
The content of Ni as an unavoidable impurity in the coating bath needs to be controlled to 0.010 mass% or less, and is preferably 0.005 mass% or less, based on the total mass of the coating bath. If the Ni content in the coating bath exceeds 0.005 mass%, the corrosion resistance of the produced hot-dip Al-Zn-coated steel sheet may deteriorate, and if it exceeds 0.010%, the corrosion resistance may deteriorate significantly. There is no lower limit for the Ni content, which has an adverse effect on corrosion resistance.

Here, the means for reducing the Ni content in the plating bath is not particularly limited.
For example, since it is effective to suppress the dissolution of stainless steel bath equipment into the plating bath, it is preferable to treat the surface of the bath equipment with a thermal spray coating or the like. The formation of the thermal spray coating or the like can impart corrosion resistance to the bath equipment against the plating bath, and it is possible to suppress the dissolution of the bath equipment into the plating bath. The type of the thermal spray coating is not particularly limited, but a coating having heat resistance and corrosion resistance such as a WC-based or MoB-based coating can be selected. It is also more effective to use bath equipment made of a heat-resistant material that does not contain Ni. In this case, even if the bath equipment dissolves, the increase in the Ni content can be prevented.

As another means for reducing the Ni content in the plating bath, it is preferable to use a metal block having a low Ni content among impurities as a raw material for the plating bath.
Furthermore, it is also effective not to use pots or bath equipment used in the production of plated steel sheets to which Ni is intentionally added in the production of hot-dip Al-Zn plated steel sheets, because this can prevent metal lumps containing Ni adhering to the pots or bath equipment from dissolving and being mixed into the plating bath.

In addition, the content of Co as an unavoidable impurity in the coating bath is preferably controlled to 0.080 mass% or less, more preferably 0.020 mass% or less, and even more preferably 0.001 mass% or less, based on the total mass of the coating bath. If the Co content in the coating bath is 0.080 mass% or less, the produced hot-dip Al-Zn-plated steel sheet can have sufficiently excellent corrosion resistance, if it is 0.020 mass% or less, it can have even better corrosion resistance, and if it is 0.010 mass% or less, it can have particularly excellent corrosion resistance. Thus, the lower the Co content in the coating bath, the better the corrosion resistance of the hot-dip Al-Zn-plated steel sheet, so there is no particular lower limit for the Co content.

Here, the means for reducing the Co content in the plating bath is not particularly limited.
For example, since it is effective to prevent the elution of a Co-Cr-W based thermal spray coating applied to a bath-immersed device into the plating bath, it is preferable to use a thermal spray coating that does not contain Co.

As another means for reducing the Co content in the plating bath, it is preferable to use a metal block having a low Co content among impurities as a raw material for the plating bath.
Furthermore, it is also effective not to use pots or bath-immersed equipment used in the production of plated steel sheets to which Co is intentionally added in the production of hot-dip Al-Zn-plated steel sheets, because this can prevent metal lumps containing Co adhering to the pots or bath-immersed equipment from dissolving and being mixed into the plating bath.

The temperature of the plating bath is not particularly limited, but is preferably in the range of (melting point + 20°C) to 650°C.
The reason why the lower limit of the bath temperature is set to the melting point + 20° C. is that the bath temperature needs to be equal to or higher than the solidification point in order to perform hot-dip plating, and by setting the temperature to the melting point + 20° C., solidification due to a local drop in the bath temperature of the plating bath is prevented. On the other hand, the reason why the upper limit of the bath temperature is set to 650° C. is that if the bath temperature exceeds 650° C., 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 becomes thick.

Furthermore, the temperature of the base steel sheet immersed in the coating bath (immersion sheet temperature) is not particularly limited, but from the standpoint of ensuring the coating characteristics in continuous hot-dip coating operations and preventing changes in the bath temperature, it is preferable to control it to within ±20°C of the coating bath temperature.

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

Depending on the required performance, hot-dip Al-Zn plated steel sheets can also have a coating formed directly or via an intermediate layer on top of the plating film.

The method for forming the coating film is not particularly limited and can be appropriately selected depending on the required performance. Examples include roll coater coating, curtain flow coating, spray coating, etc. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying using means such as hot air drying, infrared heating, and induction heating.

Furthermore, there are no particular limitations on the intermediate layer, so long as it is a layer formed between the plating film of the hot-dip galvanized steel sheet and the coating film.

(Surface-treated steel sheet)
The surface-treated steel sheet of the present invention comprises a plating film on the surface of the steel sheet, and a chemical conversion film formed on the plating film.
The structure of the plating film is the same as that of the plating film of the hot-dip Al-Zn plated steel sheet of the present invention described above.

The surface-treated steel sheet of the present invention has a chemical conversion coating formed on the above-mentioned plating coating.
The chemical conversion coating may be formed on at least one side of the surface-treated steel sheet, but may also be formed on both sides of the surface-treated steel sheet depending on the application and required performance.

In the surface-treated steel sheet of the present invention, the chemical conversion coating is characterized by containing at least one resin selected from epoxy resins, urethane resins, acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins, and at least one metal compound selected from P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds.
By forming the above-mentioned chemical conversion film on the plating film, it is possible to increase the affinity with the plating film, and to form the chemical conversion film uniformly on the plating film, and it is also possible to increase the rust prevention effect and barrier effect of the chemical conversion film, thereby making it possible to realize stable corrosion resistance and white rust resistance of the surface-treated steel sheet of the present invention.

Here, from the viewpoint of improving corrosion resistance, at least one resin selected from epoxy resin, urethane resin, acrylic resin, acrylic silicone resin, alkyd resin, polyester resin, polyalkylene resin, amino resin, and fluororesin is used as the resin constituting the conversion coating. From the same viewpoint, it is preferable that the resin contains at least one of urethane resin and acrylic resin. Note that the resin constituting the conversion coating also includes addition polymers of the above-mentioned resins.

As for the epoxy resin, for example, glycidyl etherified epoxy resins such as bisphenol A type, bisphenol F type, and novolac type, glycidyl etherified bisphenol A type epoxy resins with propylene oxide, ethylene oxide, or polyalkylene glycol added thereto, aliphatic epoxy resins, alicyclic epoxy resins, polyether-based epoxy resins, etc. can be used.

As the 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. can be used.

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

Examples of the acrylic silicone resin include a resin having hydrolyzable alkoxysilyl groups on the side chains or ends of an acrylic copolymer as the base resin to which a curing agent has been added. In addition, when an acrylic silicone resin is used, excellent weather resistance can be expected in addition to corrosion resistance.

Examples of the alkyd resin include oil-modified alkyd resin, rosin-modified alkyd resin, phenol-modified alkyd resin, styrenic alkyd resin, silicon-modified alkyd resin, acrylic-modified alkyd resin, oil-free alkyd resin, and high molecular weight oil-free alkyd resin.

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

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

The amino resin is a thermosetting resin produced by the reaction of an amine or amide compound with an aldehyde, and examples of such resins include melamine resin, guanamine resin, and thiourea resin. From the viewpoints of corrosion resistance, weather resistance, and adhesion, it is preferable to use melamine resin. There are no particular limitations on the melamine resin, but examples of such resins include butylated melamine resin, methylated melamine resin, and aqueous melamine resin.

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

Furthermore, it is particularly preferable to use a curing agent in order to improve corrosion resistance and workability. As the curing agent, urea resin (butylated urea resin, etc.), melamine resin (butylated melamine resin, butyl etherified melamine resin, etc.), butylated urea-melamine resin, amino resin such as benzoguanamine resin, blocked isocyanate, oxazoline compound, phenolic resin, etc. can be appropriately used.

The metal compound constituting the chemical conversion coating is at least one selected from P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds. From a similar perspective, it is preferable that the metal compound contains at least one of P compounds, Si compounds, and V compounds.

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

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

When the chemical conversion solution containing the above-mentioned P compound is applied to a hot-dip Al-Zn-plated steel sheet, the surface of the plating film is etched by the action of the P compound, and a concentrated layer containing Al, Zn, Si, and Mg, which are the constituent elements of the plating film, is formed on the plating film side of the chemical conversion film. The formation of the concentrated layer strengthens the bond between the chemical conversion film and the plating film surface, improving the adhesion of the chemical conversion film.
The concentration of the P compound in the chemical conversion treatment solution is not particularly limited, but can be 0.25 to 5 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 is reduced, and not only the corrosion resistance of the flat surface is reduced, but also the corrosion resistance and sweat resistance of defective parts, cut end faces, and damaged parts of the plating and film caused by processing may be reduced. From the same viewpoint, the concentration of the P compound is preferably 0.35 mass% or more, more preferably 0.50 mass% or more. On the other hand, if the concentration of the P compound exceeds 5 mass%, not only the life of the chemical conversion treatment solution is shortened, but also the appearance of the film when formed is likely to be non-uniform, and the amount of P eluted from the chemical conversion film may increase, which may reduce the resistance to blackening. From the same viewpoint, the concentration of the P compound is preferably 3.5 mass% or less, more preferably 2.5 mass% or less. Regarding the content of the P compound in the chemical conversion coating, for example, by applying a chemical conversion treatment solution with a P compound concentration of 0.25 to 5 mass % and drying it, the amount of P attached in the chemical conversion coating after drying can be set to 5 to 100 mg/ ^m2 .

The Si compound is a component that becomes the framework for forming the chemical conversion coating together with the resin, and can increase the affinity with the plating coating and form a uniform chemical conversion coating. The Si compound is a compound that contains Si, and preferably contains one or more selected from, for example, silica, trialkoxysilane, tetraalkoxysilane, and a silane coupling agent.

The silica is not particularly limited and any type can be used. For example, at least one of wet silica and dry silica can be used. As 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. can be suitably used. As the dry silica, for example, AEROSIL 50, 130, 200, 300, 380, etc. manufactured by Nippon Aerosil Co., Ltd. can be suitably used.

The trialkoxysilane is not particularly limited and any trialkoxysilane can be used. For example, it is preferable to use a trialkoxysilane represented by the general formula: R ₁ Si(OR ₂ ) ₃ (wherein R ₁ is hydrogen or an alkyl group having 1 to 5 carbon atoms, and R ₂ is the same or different alkyl group having 1 to 5 carbon atoms). Examples of such trialkoxysilane include trimethoxysilane, triethoxysilane, and methyltriethoxysilane.

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

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

In addition, by incorporating the Si compound into the chemical conversion film, the Si compound undergoes dehydration condensation to form an amorphous chemical conversion film having siloxane bonds that have a high barrier effect of blocking corrosion factors. In addition, by bonding with the above-mentioned resin, a chemical conversion film with higher barrier properties is formed. Furthermore, in a corrosive environment, dense and stable corrosion products are formed in defective parts and damaged parts of the plating film caused by processing, etc., and the combined effect with the plating film also has the effect of suppressing corrosion of the base steel sheet. From the viewpoint of a high effect of forming stable corrosion products, it is preferable to use at least one of colloidal silica and dry silica as the Si compound.

The concentration of the Si compound in the chemical conversion treatment liquid for forming the chemical conversion film is 0.2 to 9.5 mass%. If the concentration of the Si compound in the chemical conversion treatment liquid is 0.2 mass% or more, a barrier effect due to siloxane bonds can be obtained, and as a result, in addition to the corrosion resistance of the flat surface, the corrosion resistance of defective parts, cut parts, and damaged parts caused by processing, etc., and sweat resistance are improved. Furthermore, if the concentration of the Si compound is 9.5 mass% or less, the life of the chemical conversion treatment liquid can be extended. By applying and drying a chemical conversion treatment liquid with a concentration of Si compound of 0.2 to 9.5 mass%, the Si adhesion amount in the chemical conversion film after drying can be set to 2 to 95 mg/ ^m2 .

The Co compound and the Ni compound can improve the blackening resistance by being contained in the chemical conversion film. This is believed to be because Co and Ni have the effect of delaying the elution of water-soluble components from the film in a corrosive environment. In addition, the Co and Ni are elements that are less likely to be oxidized than Al, Zn, Si, Mg, etc. Therefore, by concentrating at least one of the Co compound and the Ni compound (forming a concentrated layer) at the interface between the chemical conversion film and the plating film, the concentrated layer becomes a barrier against corrosion, and as a result, the blackening resistance can be improved.

By using a chemical conversion treatment solution containing the Co compound, Co can be contained in the chemical conversion coating and incorporated into the concentrated layer. As the Co compound, a cobalt salt is preferably used. As the cobalt salt, one or more selected from cobalt sulfate, cobalt carbonate, and cobalt chloride are more preferably used.
In addition, by using a chemical conversion treatment solution containing the Ni compound, Ni can be contained in the chemical conversion coating and incorporated into the concentrated layer. As the Ni compound, a nickel salt is preferably used. As the nickel salt, it is more preferable to use one or more selected from nickel sulfate, nickel carbonate, and nickel chloride.

The concentration of the Co compound and/or Ni compound in the chemical conversion treatment liquid is not particularly limited, but can be 0.25 to 5 mass% in total. If the concentration of the Co compound and/or Ni compound is less than 0.25 mass%, the interface thickening layer becomes nonuniform, and not only the corrosion resistance of the flat portion decreases, but also the corrosion resistance of the defective portion, the cut end surface portion, the plating film damaged portion due to processing, etc. may decrease. From the same viewpoint, it is preferably 0.5 mass% or more, more preferably 0.75 mass% or more. On the other hand, if the concentration of the Co compound and/or Ni compound exceeds 5 mass%, the appearance of the formed film is likely to become nonuniform, and the corrosion resistance may decrease. From the same viewpoint, it is preferably 4.0 mass% or less, more preferably 3.0 mass% or less. By applying and drying the chemical conversion treatment liquid having a total concentration of the Co compound and/or Ni compound of 0.25 to 5 mass%, the total adhesion amount of Co and Ni in the chemical conversion film after drying can be 5 to 100 mg/m ² .

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

The Al compound may be, for example, one or more selected from aluminum sulfate, aluminum carbonate, aluminum chloride, aluminum oxide, and aluminum hydroxide.
The Zn compound may be, for example, one or more selected from zinc sulfate, zinc carbonate, zinc chloride, zinc oxide, and zinc hydroxide.
The Mg compound may be, for example, one or more selected from magnesium sulfate, magnesium carbonate, magnesium chloride, magnesium oxide, and magnesium hydroxide.

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

The V compound contained in the chemical conversion film allows the V to dissolve appropriately in a corrosive environment and bind with zinc ions and other plating components that also dissolve in a corrosive environment to form a dense protective film. The protective film formed can further increase corrosion resistance not only to the flat surface of the steel sheet, but also to defects, damaged areas of the plating film caused by processing, and corrosion that progresses from the cut end surface to the flat surface.

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

The V compound in the chemical conversion treatment solution for forming the chemical conversion film is preferably 0.05 to 4 mass%. If the concentration of the V compound is 0.05 mass% or more, it will be easier to dissolve in a corrosive environment to form a protective film, and the corrosion resistance of defects, cut end surfaces, and damaged areas of the plating film caused by processing will be improved. On the other hand, if the concentration of the V compound exceeds 4 mass%, the appearance of the formed chemical conversion film will be easily non-uniform, and resistance to blackening will also decrease.

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

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

The Zr compound and the Ti compound contained in the chemical conversion coating prevent the chemical conversion coating from becoming porous and densify the coating. As a result, corrosion factors are less likely to permeate the chemical conversion coating, and corrosion resistance can be improved.

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

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

The concentration of Zr compounds and/or Ti compounds in the chemical conversion treatment solution for forming the chemical conversion 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 more, the effect of inhibiting the permeation of corrosion factors is enhanced, and the corrosion resistance of not only flat surface parts but also defective parts, cut end faces, and parts of the plating film damaged 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 less, the life of the chemical conversion treatment solution can be further extended.

The Ca compound, when contained in the chemical conversion coating, can exert the effect of reducing the corrosion rate.

The Ca compound is a compound containing Ca, and examples of the Ca compound include Ca oxide, Ca nitrate, Ca sulfate, and intermetallic compounds containing Ca. More specifically, examples of the Ca compound include CaO, _CaCO3 , Ca( _OH ) ₂ , Ca _{(NO3)2.4H2O , and CaSO4.2H2O} . The content of the Ca compound in the chemical conversion coating is not particularly limited.

The conversion coating may contain various known components commonly used in the coating field, as necessary. Examples include various surface conditioners such as leveling agents and defoamers, various additives such as dispersants, anti-settling agents, UV absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as color pigments, extender pigments, and lubricants, curing catalysts, organic solvents, and lubricants.

In addition, in the surface-treated steel sheet of the present invention, it is preferable that the chemical conversion coating does not contain harmful components such as hexavalent chromium, trivalent chromium, fluorine, etc. This is because the chemical conversion treatment liquid used to form the chemical conversion coating does not contain these harmful components, resulting in high safety and low environmental impact.

The coating weight of the chemical conversion coating is not particularly limited. For example, from the viewpoint of preventing peeling of the chemical conversion coating while more reliably ensuring corrosion resistance, the coating weight of the chemical conversion coating is preferably 0.1 to 3.0 g/ ^m2 , and more preferably 0.5 to 2.5 g/ ^m2 . By setting the coating weight of the chemical conversion coating to 0.1 g/m2 or ^more , corrosion resistance can be more reliably ensured, and by setting the coating weight of the chemical conversion coating to 3.0 g/m2 or ^less , cracking and peeling of the chemical conversion coating can be prevented.
The coating weight of the chemical conversion coating may be determined by a method appropriately selected from existing techniques, such as a method of subjecting the coating to fluorescent X-ray analysis to measure the amount of an element present in the coating whose content is known in advance.

The method for forming the chemical conversion coating is not particularly limited, and can be appropriately selected according to the required performance, manufacturing equipment, etc. For example, the chemical conversion coating can be formed by continuously applying a chemical conversion treatment liquid onto the plating film using a roll coater or the like, and then drying 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 can be appropriately used to apply the chemical conversion treatment liquid. Furthermore, the chemical conversion coating may be either a single-layer film or a multi-layer film as long as it contains the resin and the metal compound, and is not particularly limited.

In addition, if necessary, the surface-treated steel sheet of the present invention can have a coating film formed on the chemical conversion coating.

(painted steel plate)
The coated steel sheet of the present invention is a coated steel sheet in which a coating film is formed on a plating film directly or via a chemical conversion film.
The structure of the plating film is the same as that of the plating film of the hot-dip Al-Zn plated steel sheet of the present invention described above.

In the coated steel sheet of the present invention, a chemical conversion film can be formed on the plating film.
The chemical conversion 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 and required performance.

The coated steel sheet of the present invention is characterized in that the chemical conversion coating contains a resin component that contains 30 to 50 mass% in total of (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton, the content ratio of (a) to (b) ((a):(b)) being in the range of 3:97 to 60:40 in terms of mass ratio, and also contains inorganic compounds that include 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.
By forming the above-mentioned chemical conversion film on the plating film, it is possible to increase the strength and adhesion of the chemical conversion film while improving the corrosion resistance.

Here, the resin components constituting the chemical conversion coating contain (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton.

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

Examples of the polyester polyol include polyesters obtained by a dehydration condensation reaction between a glycol component and an acid component such as an ester-forming derivative of a hydroxyl carboxylic acid, polyesters obtained by a ring-opening polymerization reaction of a cyclic ester compound such as ε-caprolactone, and copolymer polyesters thereof.
Examples of the polyisocyanate include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, etc. Examples of the aromatic polyisocyanate include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, m-xylylene diisocyanate, diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate, 2,2-diphenylmethane diisocyanate, triphenylmethane triisocyanate, polymethylene polyphenyl polyisocyanate, naphthalene diisocyanate, and derivatives thereof (for example, prepolymers obtained by reaction with polyols, modified polyisocyanates such as carbodiimide compounds of diphenylmethane diisocyanate, etc.).

When the polyester polyol is reacted with the diisocyanate or polyisocyanate to synthesize urethane, the anionic polyurethane resin having the ester bond (a) can be obtained by, for example, copolymerizing dimethylol alkyl acid and self-emulsifying it to be water-soluble (water-dispersed). In this case, examples of the dimethylol alkyl acid include dimethylol alkyl acids having 2 to 6 carbon atoms, more specifically, dimethylol ethanoic acid, dimethylol propanoic acid, dimethylol butanoic acid, dimethylol heptanoic acid, and dimethylol hexanoic acid.

For the epoxy resin having a bisphenol skeleton (b), a known epoxy resin can be used. Examples include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxy resin, and bisphenol S type epoxy resin. These epoxy resins can be obtained by reacting a bisphenol compound such as bisphenol A, bisphenol F, bisphenol AD, or bisphenol S with epichlorohydrin in the presence of an alkaline catalyst. Among them, component [A] preferably contains a bisphenol A type epoxy resin or a bisphenol F type epoxy resin, and more preferably contains a bisphenol A type epoxy resin. The epoxy resin having a bisphenol skeleton (b) can be dispersed in a liquid such as water by a known method to obtain a chemical conversion treatment liquid.

The resin components act as binders for the chemical conversion coating, and the (a) anionic polyurethane resin having ester bonds that constitutes the binder is flexible and therefore has the effect of making the chemical conversion coating less likely to be destroyed (peeled off) when processed, and the (b) epoxy resin having a bisphenol skeleton has the effect of improving adhesion to the underlying Al-Zn-plated steel sheet and the overlying primer coating.
The resin components are contained in the chemical conversion coating in a total amount of 30 to 50% by mass. If the content of the resin components is less than 30% by mass, the binder effect of the chemical conversion coating is reduced, and if it exceeds 50% by mass, the functions of the inorganic components described below, such as the inhibitor action, are reduced. From the same viewpoint, the content of the resin components in the chemical conversion coating is preferably 35 to 45% by mass.

Furthermore, the resin component requires that the content ratio ((a):(b)) of the (a) anionic polyurethane resin having an ester bond and the (b) epoxy resin having a bisphenol skeleton is in the range of 3:97 to 60:40 by mass. If the (a):(b) ratio is outside the above range, the flexibility and adhesion of the chemical conversion coating will decrease, and sufficient corrosion resistance will not be obtained. From the same perspective, it is preferable that the (a):(b) ratio is 10:90 to 55:45.

The resin component may contain resins (other resin components) other than the above-mentioned (a) anionic polyurethane resin having an ester bond and (b) epoxy resin having a bisphenol skeleton, depending on the required performance. The other resin components are not particularly limited, and may be, for example, at least one selected from acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins, or a combination of two or more selected therefrom.
When the resin component contains other resins, the total content of the (a) anionic polyurethane resin having an ester bond and the (b) epoxy resin having a bisphenol skeleton is preferably 50% by mass or more, and more preferably 75% by mass or more, in order to more reliably prevent a decrease in flexibility and obtain adhesion as a conversion coating.

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

The vanadium compound is added to the chemical conversion solution and acts as a rust inhibitor. The vanadium compound is contained in the chemical conversion coating, so that the vanadium compound is appropriately dissolved in a corrosive environment and combines with zinc ions of the plating components that are also dissolved in a corrosive environment to form a dense protective coating. The formed protective coating can further increase the corrosion resistance not only of the flat surface of the steel sheet, but also of defects, damaged parts of the plating film caused by processing, corrosion that progresses from the cut end surface to the flat surface, and the like.
Examples of the vanadium compound include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, vanadium acetylacetonate, etc. Among these, it is particularly desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.

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

The zirconium compound is contained in the chemical conversion coating, and is expected to improve the strength and corrosion resistance of the chemical conversion coating through its reaction with the plating metal and its coexistence with a resin component. Furthermore, the zirconium compound itself contributes to the formation of a dense chemical conversion coating, and because of its excellent covering properties, a barrier effect can be expected.
Examples of the zirconium compound include neutral salts of zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, zirconium chloride, and the like.

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

The fluorine compound is contained in the chemical conversion coating and acts as an adhesive agent for the plating coating, thereby making it possible to improve the corrosion resistance of the chemical conversion coating.
As the fluorine compound, for example, fluoride salts such as ammonium salts, sodium salts, potassium salts, or fluorine compounds such as ferrous fluoride, ferric fluoride, etc. Among these, it is preferable to use fluoride salts such as ammonium fluoride, sodium fluoride, and potassium fluoride.

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

The coating weight of the chemical conversion coating is not particularly limited. For example, from the viewpoint of improving the adhesion of the chemical conversion coating while more reliably ensuring corrosion resistance, it is preferable to set the coating weight of the chemical conversion coating to 0.025 to 0.5 g/m2. By setting the coating weight of the chemical conversion coating to 0.025 g/ ^m2 or more, corrosion resistance can be more reliably ensured, and by setting the coating weight of the chemical conversion coating to 0.5 g/ ^{m2 or less} , peeling of the chemical conversion coating can be suppressed.
The coating weight of the chemical conversion coating may be determined by a method appropriately selected from existing techniques, such as a method of subjecting the coating to fluorescent X-ray analysis to measure the amount of an element present in the coating whose content is known in advance.

The method for forming the chemical conversion coating is not particularly limited, and can be appropriately selected according to the required performance, manufacturing equipment, etc. For example, the chemical conversion coating can be formed by continuously applying a chemical conversion treatment liquid onto the plating film using a roll coater or the like, and then drying 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 can be appropriately used to apply the chemical conversion treatment liquid. Furthermore, the chemical conversion coating may be either a single-layer film or a multi-layer film as long as it contains the resin and the metal compound, and is not particularly limited.

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

In the present invention, the primer coating film contains a polyester resin having a urethane bond, and an inorganic compound including a vanadium compound, a phosphate compound, and magnesium oxide.
The primer coating film contains the polyester resin having a urethane bond and the inorganic compound, and therefore the adhesion of the coating film can be increased and the corrosion resistance can be improved.

The primer coating contains a polyester resin having a urethane bond as a main component. The polyester resin having a urethane bond is flexible and strong, so that the primer coating is less likely to crack when processed, and has a high affinity with a chemical conversion coating containing a urethane resin, so that the primer coating can particularly contribute to improving the corrosion resistance of processed parts.
The term "main component" used herein means the component that is contained in the greatest amount among all the components in the primer coating film.

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

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

Examples of the polyisocyanate include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate; aromatic diisocyanates such as xylylene diisocyanate (XDI), metaxylylene diisocyanate, tolylene diisocyanate (TDI), and 4,4-diphenylmethane diisocyanate (MDI); and cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI, as well as adducts, biurets, and isocyanurates thereof. These polyisocyanates can be used alone or in combination of two or more.

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

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

Vanadium compounds, which are one of the inorganic compounds, act as inhibitors. Examples of the vanadium compounds include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, vanadium acetylacetonate, etc. In particular, among these, it is desirable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation.
The vanadium compound added to the primer coating may be the same or different from the vanadium compound added to the chemical conversion coating. It is believed that vanadate ions, which are gradually dissolved by moisture entering from the outside, react with ions on the surface of the zinc-based plated steel sheet to form a passive film with good adhesion, protecting the exposed metal parts and exerting a rust-preventing effect.

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

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

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

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

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

The primer coating film may also contain components other than the above-mentioned polyester resin having a urethane bond and inorganic compound.
For example, a crosslinking agent used in forming a primer coating film can be mentioned. The crosslinking agent reacts with the polyester resin having the urethane bond to form a crosslinked coating film, and examples thereof 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 them, from the viewpoint of corrosion resistance of the processed part of the obtained coated steel plate, blocked polyisocyanate compounds and the like can be preferably used. Examples of the blocked polyisocyanate include those in which the isocyanate group of a polyisocyanate compound is blocked with, for example, alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactams, diketones such as acetoacetic acid diester, imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

Furthermore, the primer coating may contain various known components commonly used in the paint industry, as necessary. Specific examples include various surface conditioners such as leveling agents and defoamers, various additives such as dispersants, anti-settling agents, UV absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as color pigments and extender pigments, luster materials, curing catalysts, and organic solvents.

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

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

In addition, with regard to the coating film constituting the coated steel plate of the present invention, it is preferable that a top coating film is further formed on the primer coating film.
The topcoat coating film can impart beauty to the coated steel sheet, such as color, gloss, and surface condition, and can also improve various performance properties, such as processability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance.

The configuration of the topcoat coating is not particularly limited, and the material, thickness, etc. can be appropriately selected depending on the required performance.
For example, the topcoat film can be formed using a polyester resin paint, a silicon polyester resin paint, a polyurethane resin paint, an acrylic resin paint, a fluororesin paint, or the like.
Furthermore, the topcoat coating film may contain appropriate amounts of titanium oxide, red iron oxide, mica, carbon black or other various coloring pigments; metallic pigments such as aluminum powder and mica; extender pigments consisting of carbonates, sulfates, etc.; various fine particles such as silica fine particles, nylon resin beads, acrylic resin beads, etc.; curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate; wax; and other additives.

From the viewpoint of achieving both good appearance and workability, the thickness of the topcoat film is preferably 5 to 30 μm. If the thickness of the topcoat film is 5 μm or more, it is possible to more reliably stabilize the color appearance, and if the thickness of the topcoat film is 30 μm or less, it is possible to more reliably suppress a decrease in workability (the occurrence of cracks in the topcoat film).

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

<Example 1: Samples 1 to 47>
A cold-rolled steel sheet having a thickness of 0.8 mm produced by a conventional method was used as a base steel sheet, and annealing and plating were performed using a hot-dip plating simulator manufactured by Rhesca Corporation to produce hot-dip plated steel sheet samples 1 to 47 under the conditions shown in Table 1.
The composition of the plating bath used in producing the hot-dip plated steel sheets was changed in various ranges within the following ranges so as to obtain the composition of the plating film of each sample shown in Table 1: Al: 5 to 70 mass%, Si: 0.0 to 4.4 mass%, Ni: 0.000 to 0.025 mass%, Co: 0.001 to 0.092 mass%, V: 0.0 to 0.1 mass%, Cr: 0.0 to 0.2 mass%, Mn: 0.0 to 0.1 mass%, Mg: 0.0 to 4.0 mass%, Ca: 0.0 to 1.0 mass%, and Sr: 0.0 to 1.0 mass%. The bath temperature of the coating bath used was 450°C for 5% Al, 480°C for 18% Al, 600°C for 36-55% Al, and 660°C for over 60% Al, and was controlled so that the temperature of the base steel sheet entering the coating was the same as the coating bath temperature. Furthermore, when the Al content was 30-60% by mass, the coating process was carried out under the condition that the sheet temperature was cooled to a temperature range of 520-500°C in 3 seconds.
In addition, the coating weight of the plating film 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.

(evaluation)
The following evaluations were carried out on each sample of the hot-dip galvanized steel sheet obtained as described above. The evaluation results are shown in Table 1.

(1) Plating film (composition, coating amount, Ni-based compounds)
After plating, a 100 mm diameter cut was made from each sample, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013. The adhesion weight of the plating film was calculated from the difference in mass of the sample before and after removal. The calculated adhesion weights of the plating film obtained are shown in Table 1.
The stripper solution was then filtered, and the filtrate and solid content were analyzed. Specifically, the filtrate was analyzed by ICP atomic emission spectroscopy to quantify the components other than insoluble Si.
The solids were dried and incinerated in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was dissolved in hydrochloric acid, and the solution was analyzed by ICP emission spectroscopy to quantify the insoluble silicon. The silicon concentration in the plating film was calculated by adding the soluble silicon concentration obtained by filtrate analysis to the insoluble silicon concentration obtained by solid content analysis. The composition of the plating film obtained as a result of the calculation is shown in Table 1.
Furthermore, each sample was sheared to a size of 15 mm x 15 mm, embedded in a conductive resin so that the cross section of the steel sheet could be observed, and mechanically polished. Then, using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss), backscattered electron images were continuously taken at a width of 100 μm for 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 base steel sheet at an acceleration voltage of 3 kV. Furthermore, in the same device, elemental mapping analysis (Al, Zn, Si, Fe, and Ni) of each cross section was performed at 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 in this analysis, point analysis was performed using the same spectrometer at an acceleration voltage of 3 kV, and the substance was identified from the semi-quantitative value of the obtained component. The major axis of all Ni-based compounds confirmed in the observation field was measured to determine the maximum major axis. In addition, the number of Ni-based compound particles per mm in the direction parallel to the surface of the substrate steel was calculated by counting the number of Ni-based compound particles present in the continuous cross-sections observed and dividing the number by the observed cross-section length (mm). For the areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer at an accelerating voltage of 3 kV, and the substances were identified from the semi-quantitative values of the components obtained. The analysis results are shown in Table 1.

(2) Evaluation of corrosion resistance Each sample of the obtained hot-dip galvanized steel sheet was sheared to a size of 120 mm × 120 mm, and then a range of 10 mm from each edge of the surface to be evaluated, as well as the end faces and the surface not to be evaluated, were sealed with tape to expose the surface to be evaluated in a size of 100 mm × 100 mm, which was used as a sample for evaluation. Three identical samples for evaluation were prepared.
Accelerated corrosion tests were carried out on the three evaluation samples prepared as described above, with the cycle shown in Figure 1. The accelerated corrosion tests were carried out starting from wetting and continuing for 150 cycles, after which the corrosion weight loss of each sample was measured by 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.
◎: The corrosion loss of all three samples is 60g/m2 or less ^. ○: The corrosion loss of all three samples is 75g/m2 ^or less. ×: The corrosion loss of one or more samples exceeds 75g/ ^m2.

(3) Surface appearance Each sample of the obtained hot-dip galvanized steel sheet was sheared to a size of 70 mm x 150 mm, and then eight sheets of the same thickness were sandwiched inside and bent 180° (8T bend). Cellophane tape (registered trademark) was firmly attached to the outer surface of the bent part after bending, and then peeled off. The surface condition of the plating film on the outer surface of the bent part and the presence or absence of adhesion (peeling) of the plating film on the surface of the tape used were visually observed, and the workability was evaluated according to the following criteria. The evaluation results are shown in Table 1.
◯: Neither cracks nor peeling are observed in the plating film. △: There are cracks in the plating film, but no peeling is observed. ×: Both cracks and peeling are observed in the plating film.

From the results in Table 1, it can be seen that each sample of the present invention has a well-balanced and excellent corrosion resistance and processability compared to each sample of the comparative example.

Example 2: Samples 1 to 118
(1) A cold-rolled steel sheet having a thickness of 0.8 mm produced by a conventional method was used as a base steel sheet, and annealing and plating were performed using a hot-dip plating simulator manufactured by Rhesca Corporation to prepare samples of hot-dip plated steel sheets having the plating film conditions shown in Tables 3 to 5.
The composition of the coating bath used in the production of hot-dip galvanized steel sheets was varied in the ranges 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 as to obtain the composition of the coating film of each sample shown in Table 1. The bath temperature of the coating bath was 450°C for Al: 5 mass%, 480°C for Al: 18 mass%, 600°C for Al: 36-55 mass%, and 660°C for Al: over 60 mass%, and was controlled so that the temperature of the substrate steel sheet entering the coating was the same as the coating bath temperature. Furthermore, when the Al content was 30 to 60 mass %, the plating process was carried out under conditions in which the sheet temperature was cooled to a temperature range of 520 to 500°C in 3 seconds.
In addition, the coating weight of the plating film 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.

(2) Thereafter, a chemical conversion coating was formed on the plating film of each sample of the prepared hot-dip plated steel sheet by using a bar coater, and the plated film was dried in a hot air furnace (heating rate: 60°C/s, PMT: 120°C), thereby preparing each sample of the surface-treated steel sheet shown in Tables 3 to 5.
The chemical conversion treatment solutions were prepared by dissolving each component in water as a solvent to prepare surface treatment solutions A to F. The types of each component (resin, metal compound) contained in the surface treatment solutions are as follows:
(resin)
Urethane resin: Superflex 130, Superflex 126 (Dai-ichi Kogyo Seiyaku Co., Ltd.)
Acrylic resin: Boncoat EC-740EF (DIC Corporation)
(Metal Compounds)
P compound: Aluminum dihydrogen tripolyphosphate
Si compound: Silica
V compound: Sodium metavanadate
Mo compound: Molybdic acid
Zr compound: potassium zirconyl carbonate The compositions of the prepared chemical conversion treatment solutions A to F and the adhesion weights of the formed chemical conversion coatings are shown in Table 2. The concentration of each component in Table 2 of this specification is the concentration (mass%) of the solid content.

(evaluation)
The following evaluations were carried out on each sample of the hot-dip plated steel sheet and the surface-treated steel sheet obtained as described above. The evaluation results are shown in Tables 3 to 5.
(1) Plating film (composition, coating amount, Ni-based compounds)
For each sample of hot-dip galvanized steel sheet, a 100 mm diameter was punched out, the non-measurement surface was sealed with tape, and the plating was then dissolved and stripped off with a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013, and the adhesion weight of the plating film was calculated from the difference in mass of the sample before and after stripping. The calculated adhesion weights of the plating film obtained are shown in Tables 3 to 5.
The stripper solution was then filtered, and the filtrate and solid content were analyzed. Specifically, the filtrate was analyzed by ICP atomic emission spectrometry to quantify the components other than insoluble Si.
The solids were dried and incinerated in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the solution was analyzed by ICP emission spectroscopy to quantify the insoluble silicon. The silicon concentration in the plating film was calculated by adding the soluble silicon concentration obtained by filtrate analysis to the insoluble silicon concentration obtained by solid content analysis. The compositions of the plating films obtained as a result of calculation are shown in Tables 3 to 5.
Furthermore, each sample was sheared to a size of 15 mm x 15 mm, embedded in a conductive resin so that the cross section of the steel sheet could be observed, and mechanically polished. Then, using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss), backscattered electron images were continuously taken at a width of 100 μm for 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 base steel sheet at an acceleration voltage of 3 kV. Furthermore, in the same device, elemental mapping analysis (Al, Zn, Si, Fe, and Ni) of each cross section was performed at 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 in this analysis, point analysis was performed using the same spectrometer at an acceleration voltage of 3 kV, and the substance was identified from the semi-quantitative value of the obtained component. The major axis of all Ni-based compounds confirmed in the observation field was measured to determine the maximum major axis. In addition, the number of Ni-based compound particles per mm in the direction parallel to the surface of the substrate steel was calculated by counting the number of Ni-based compound particles present in the continuous cross-sections observed and dividing the number by the observed cross-section length (mm). For the areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer at an accelerating voltage of 3 kV, and the substances were identified from the obtained semi-quantitative values of the components. The analysis results are shown in Tables 3 to 5.

(2) Evaluation of corrosion resistance 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 evaluation target surface and the end face and the non-evaluation target surface of the sample were sealed with tape to expose the evaluation target surface in a size of 100 mm × 100 mm, which was used as the evaluation sample. Three identical evaluation samples were prepared.
Accelerated corrosion tests were carried out on the three evaluation samples prepared as described above, with the cycle shown in Figure 1. The accelerated corrosion tests were started from wetting and continued for 150 cycles, after which the corrosion weight 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.
◎: The corrosion loss of all three samples is 40g/m2 or less ^. ○: The corrosion loss of all three samples is 60g/m2 ^or less. ×: The corrosion loss of one or more samples is more than 60g/ ^m2.

(3) White rust resistance 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, as well as the end faces and non-evaluation surfaces of the sample were sealed with tape to expose the surface to be evaluated in a size of 100 mm × 100 mm, and these were used as evaluation samples.
The above evaluation samples were subjected to a salt spray test according to JIS Z 2371 for 90 hours and evaluated according to the following criteria. The evaluation results are shown in Tables 3 to 5.
◎: No white rust on the flat plate ○: White rust occurs on less than 10% of the flat plate ×: White rust occurs on 10% or more of the flat plate

(4) Workability Each hot-dip galvanized steel sheet sample was sheared to a size of 70 mm x 150 mm, and then eight sheets of the same thickness were sandwiched inside and bent 180° (8T bend). Cellophane tape (registered trademark) was firmly attached to the outer surface of the bent part after bending, and then peeled off. The surface condition of the plating film on the outer surface of the bent part and the presence or absence of adhesion (peeling) of the plating film on the surface of the tape used were visually observed, and the workability was evaluated according to the following criteria. The evaluation results are shown in Tables 3 to 5.
◯: Neither cracks nor peeling are observed in the plating film. △: There are cracks in the plating film, but no peeling is observed. ×: Both cracks and peeling are observed in the plating film.

From the results in Tables 3 to 5, it can be seen that the samples of the invention are excellent in terms of corrosion resistance, white rust resistance, and workability in a well-balanced manner compared to the samples of the comparative examples.
Furthermore, it can be seen from the results in Table 5 that the samples subjected to the chemical conversion treatments A to D exhibited particularly excellent white rust resistance.

<Example 3: Samples 1 to 47>
(1) A cold-rolled steel sheet having a thickness of 0.8 mm produced by a conventional method was used as a base steel sheet, and annealing and plating were performed using a hot-dip galvanizing simulator manufactured by Rhesca Corporation to produce hot-dip galvanized steel sheet samples 1 to 47 having the conditions shown in Table 7.
The composition of the coating 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%. The temperature of the coating bath was 450°C for Al: 5 mass%, 480°C for Al: 18 mass%, 600°C for Al: 36-55 mass%, and 660°C for Al: over 60 mass%, and the temperature of the substrate steel sheet entering the coating was controlled to be the same as the coating bath temperature. Furthermore, when the Al content was 30 to 60 mass %, the plating process was carried out under conditions in which the sheet temperature was cooled to a temperature range of 520 to 500°C in 3 seconds.
In addition, the coating weight of the plating film 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.

(2) Thereafter, the chemical conversion coating film of each sample of the prepared hot-dip galvanized steel sheet was coated with the chemical conversion coating solution shown in Table 6 using a bar coater, and then dried in a hot air drying furnace (ultimate sheet temperature: 90°C) to form a chemical conversion coating film with a coating weight of 0.1 g/ ^m2 .
The chemical conversion treatment solution used was prepared by dissolving each component in water as a solvent, and had a pH of 8 to 10. The types of each component (resin component, inorganic compound) contained in the chemical conversion treatment solution are as follows:
(Resin Component)
Resin A: (a) an anionic polyurethane resin having an ester bond ("Superflex 210" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and (b) an epoxy resin having a bisphenol skeleton ("Yukarejin RE-1050" manufactured by Yoshimura Oil Chemical Co., Ltd.) mixed in a mass ratio of (a):(b) = 50:50. Resin B: an acrylic resin ("Boncoat EC-740EF" manufactured by DIC Corporation).
(Inorganic Compounds)
Vanadium compounds: organic vanadium compounds chelated with acetylacetone Zirconium compounds: ammonium zirconium carbonate Fluorine compounds: ammonium fluoride

(3) A primer paint was applied onto the chemical conversion coating film formed as described above using a bar coater, and baked under conditions of a steel plate temperature of 230°C and a baking time of 35 seconds, thereby forming a primer coating film having the component composition shown in Table 6. Thereafter, a topcoat paint composition was applied onto the primer coating film formed as described above using a bar coater, and baked under conditions of a steel plate temperature of 230°C to 260°C and a baking time of 40 seconds, thereby forming a topcoat coating film having the resin conditions and film thickness shown in Table 6, and each sample coated steel plate was produced.
The primer coating was obtained by mixing the components and then stirring for about 1 hour in a ball mill. The following resin components and inorganic compounds were used to form the primer coating film.
(Resin Component)
Resin α: A urethane-modified polyester resin (obtained by reacting 455 parts by mass of polyester resin and 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) cured with a blocked isocyanate was used.
The polyester resin to be urethane-modified was prepared 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 cyclohexanedimethanenol were charged into a flask equipped with a stirrer, a distillation column, a water separator, a cooling tube, and a thermometer, and the mixture was heated and stirred, and the temperature was raised from 160°C to 230°C at a constant rate over 4 hours while distilling off the generated condensed water outside the system. After the temperature reached 230°C, 20 parts by mass of xylene was gradually added, and the condensation reaction was continued while maintaining the temperature at 230°C. The reaction was terminated when the acid value became 5 or less, and the mixture was cooled to 100°C, and then 120 parts by mass of Solvesso 100 (trade name, high-boiling aromatic hydrocarbon solvent, manufactured by Exxon Mobil Corp.) and 100 parts by mass of butyl cellosolve were added to obtain a polyester resin solution.
Resin β: urethane-cured polyester resin ("Evaclad 4900" manufactured by Kansai Paint Co., Ltd.)
(Inorganic Compounds)
Vanadium compound: magnesium vanadate Phosphate compound: calcium phosphate Magnesium oxide compound: magnesium oxide Furthermore, for the resins used in the top coat films shown in Table 6, the following paints were used.
Resin I: Melamine-cured polyester paint (BASF Japan Ltd. "Precolor HD0030HR")
Resin II: An organosol-based baking-type fluororesin-based paint having a mass ratio of polyvinylidene fluoride and acrylic resin of 80:20 (BASF Japan Ltd. "Precolor No. 8800HR")

(evaluation)
The following evaluations were carried out on each of the coated steel sheet samples obtained as described above. The evaluation results are shown in Table 7.

(1) Plating film (composition, coating amount, Ni-based compounds)
For each sample of hot-dip galvanized steel sheet, a 100 mm diameter was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and peeled off with a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013, and the adhesion weight of the plating film was calculated from the difference in mass of the sample before and after peeling. The calculated adhesion weights of the plating film obtained are shown in Table 7.
The stripper solution was then filtered, and the filtrate and solid content were analyzed. Specifically, the filtrate was analyzed by ICP atomic emission spectroscopy to quantify the components other than insoluble Si.
The solids were dried and incinerated in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was dissolved in hydrochloric acid, and the solution was analyzed by ICP emission spectroscopy to quantify the insoluble silicon. The silicon concentration in the plating film was calculated by adding the soluble silicon concentration obtained by filtrate analysis to the insoluble silicon concentration obtained by solid content analysis. The composition of the plating film obtained as a result of the calculation is shown in Table 7.
Furthermore, each sample was sheared to a size of 15 mm x 15 mm, embedded in a conductive resin so that the cross section of the steel sheet could be observed, and mechanically polished. Then, using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss), backscattered electron images were continuously taken at a width of 100 μm for 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 base steel sheet at an acceleration voltage of 3 kV. Furthermore, in the same device, elemental mapping analysis (Al, Zn, Si, Mg, Fe, Sr, and Ni) was performed on each cross section at 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 in this analysis, point analysis was performed using the same spectrometer at an acceleration voltage of 3 kV, and the substance was identified from the semi-quantitative value of the obtained component. The major axis of all Ni-based compounds confirmed in the observation field was measured to determine the maximum major axis. In addition, the number of particles of Ni-based compounds present in the observed continuous cross section was counted and divided by the observed cross section length (mm) to calculate the number of particles of Ni-based compounds per mm in the direction parallel to the surface of the base steel sheet (particles/mm). For the parts where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer at an accelerating voltage of 3 kV, and the substances were identified from the obtained semi-quantitative values of the components. The analysis results are shown in Table 7.

(2) Evaluation of corrosion resistance Each coated steel sheet sample was sheared to a size of 120 mm x 120 mm, and the area of 10 mm from each edge of the surface to be evaluated, as well as the end faces and non-evaluation surfaces of the sample, were sealed with tape to expose the surface to be evaluated in a size of 100 mm x 100 mm, which was used as the sample for evaluation. Three identical samples for evaluation were prepared.
An accelerated corrosion test was carried out on all three evaluation samples prepared as described above, following the cycle shown in Figure 1. The accelerated corrosion test started with wetting, and the sample was taken out every 20 cycles, washed with water, dried, and then visually inspected to see if red rust had formed on the sheared edge of one side that was not sealed with tape.
The number of cycles at which red rust was observed was evaluated according to the following criteria. The evaluation results are shown in Table 7.
◎: Number of cycles in which red rust appeared on three samples ≥ 300 cycles ○: Number of cycles in which red rust appeared on three samples > 200 cycles ×: Number of cycles in which red rust appeared on at least one sample < 200 cycles

(3) Workability after painting Each coated steel plate sample was sheared to a size of 70 mm x 150 mm, and then eight plates of the same thickness were sandwiched inside and bent 180° (8T bending). Cellophane tape (registered trademark) was firmly attached to the outer surface of the bent part after bending, and then peeled off. The surface condition of the coating film on the outer surface of the bent part and the presence or absence of adhesion (peeling) of the coating film on the surface of the tape used were visually observed, and the workability was evaluated according to the following criteria. The evaluation results are shown in Table 7.
◯: Neither cracks nor peeling are observed in the coating film. △: There are cracks in the coating film, but no peeling is observed. ×: Both cracks and peeling are observed in the coating film.

From the results in Table 7, it can be seen that each sample of the present invention has a well-balanced and excellent corrosion resistance and workability after painting compared to each sample of the comparative example.

According to the present invention, it is possible to provide a hot-dip Al-Zn plated steel sheet having excellent corrosion resistance reliably and stably.
Furthermore, according to the present invention, it is possible to provide a surface-treated steel sheet having reliably and stably excellent corrosion resistance and white rust resistance, and a coated steel sheet having reliably and stably excellent corrosion resistance and corrosion resistance in processed areas.

Claims (6)

A hot-dip Al-Zn-plated steel sheet having a plating film,
The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and the remainder being Zn and unavoidable impurities,
The Ni content in the unavoidable impurities is 0.010% by mass or less with respect to the total mass of the plating film,
the plating film contains Ni-based compounds resulting from Ni in the unavoidable impurities, the Ni-based compounds have a major axis of 4.0 μm or less, and the number of the Ni-based compounds present in a direction parallel to a surface of the base steel sheet is 5 pieces/mm or less . 2. The hot-dip Al-Zn plated steel sheet according to claim 1 , wherein the plating film has an Al content of 50 to 60 mass %. The hot-dip Al-Zn-plated steel sheet according to claim 1 or 2, characterized in that the Co content in the unavoidable impurities contained in the plating film is 0.080 mass% or less with respect to the total mass of the plating film. A method for producing a hot-dip Al-Zn-plated steel sheet having a plating film, comprising:
The formation of the plating film includes a hot-dip plating process in which a base steel sheet is immersed in a plating bath having a composition containing 45 to 65 mass% Al and 1.0 to 4.0 mass% Si, with the balance being Zn and unavoidable impurities;
The Ni content in unavoidable impurities in the plating bath is controlled to 0.010 mass% or less based on the total mass of the plating bath ;
the plating film contains Ni-based compounds resulting from Ni in the unavoidable impurities, the Ni-based compounds have a major axis of 4.0 μm or less, and the number of the Ni-based compounds present in a direction parallel to a surface of the base steel sheet is 5/mm or less . A surface-treated steel sheet comprising the plating film according to claim 1 or 2 and a chemical conversion film formed on the plating film,
The chemical conversion coating film is a surface-treated steel sheet characterized in that it contains at least one resin selected from epoxy resins, urethane resins, acrylic resins, acrylic silicone resins, alkyd resins, polyester resins, polyalkylene resins, amino resins, and fluororesins, and at least one metal compound selected from P compounds, Si compounds, Co compounds, Ni compounds, Zn compounds, Al compounds, Mg compounds, V compounds, Mo compounds, Zr compounds, Ti compounds, and Ca compounds. A coated steel sheet having a coating film formed directly or via a chemical conversion coating on the plating film according to claim 1 or 2,
The chemical conversion coating contains a resin component that contains (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton in a total amount of 30 to 50 mass % relative to 100 mass % of the chemical conversion coating , with the content ratio of (a) and (b) ((a):(b)) being in the range of 3:97 to 60:40 in terms of mass ratio, and also contains inorganic compounds that include 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;
The coating film has at least a primer coating film, and the primer coating film contains a polyester resin having a urethane bond, and an inorganic compound including a vanadium compound, a phosphate compound, and magnesium oxide.

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