JP7097493B2 - Painted steel sheet - Google Patents

Description

The present invention relates to a coated steel sheet having stable and excellent corrosion resistance and corrosion resistance of a processed portion.

Examples of the plated steel sheet used as the base of the coated steel sheet include a hot-dip Al-Zn-based plated steel sheet typified by 55% Al-Zn-based, and the hot-dip Al-Zn-based plated steel sheet has Zn sacrificial corrosion resistance and Al. It is known that it exhibits high corrosion resistance among hot-dip zinc-plated steel sheets because it has both high corrosion resistance. Therefore, due to its excellent corrosion resistance, hot-dip Al-Zn plated steel sheets are mainly used in the field of building materials such as roofs and walls that are exposed to the outdoors for a long period of time, and in the field of civil engineering and construction such as guard rails, wiring pipes, and soundproof walls. .. In particular, there is an increasing demand for materials with excellent corrosion resistance and maintenance-free materials under harsher usage environments such as acid rain due to air pollution, spraying of snowmelt to prevent road freezing in snowy areas, and coastal area development. In recent years, the demand for molten Al-Zn-based plated steel sheets has been increasing.

The plating film of the molten Al-Zn-based plated steel sheet is composed of the portion where Al containing Zn in hypersaturation is solidified into a dendrite (α-Al phase) and the Zn-Al co-crystal structure existing in the dendrite gap (interdendrite). It is characterized by having a structure in which a plurality of α-Al phases are laminated in the film thickness direction of the plating film. Due to such a characteristic film structure, the corrosion progress path from the surface becomes complicated, so that the corrosion does not easily proceed, and the hot-dip Al-Zn-based plated steel sheet becomes a hot-dip galvanized steel sheet having the same plating film thickness. It is also known that better corrosion resistance can be achieved.

Attempts have been made to further extend the life of such hot-dip Al-Zn-based plated steel sheets, and molten Al-Zn-Si-Mg-based plated steel sheets to which Mg has been added have been put into practical use.
As such a molten Al-Zn-Si-Mg-based plated steel sheet, for example, Patent Document 1 contains an Al-Zn-Si alloy containing Mg in a plating film, and the Al-Zn-Si alloy is 45 to An alloy containing 60% by weight elemental aluminum, 37-46% by weight elemental zinc and 1.2-2.3% by weight Si, wherein the Mg concentration is 1-5% by weight, molten Al-Zn-Si-. Mg-based plated steel sheets are disclosed.
Further, in Patent Document 2, the plating film contains at least one of 2 to 10% Mg and 0.01 to 10% Ca to improve corrosion resistance and protect the base steel sheet after it is exposed. A hot-dip Al-Zn-Si-Mg-based plated steel sheet for the purpose of enhancing the above is disclosed.
Further, in Patent Document 3, Mg: 1 to 15%, Si: 2 to 15%, Zn: 11 to 25% are contained in mass%, and the balance forms a coating layer composed of Al and unavoidable impurities. , Molten Al-Zn-Si-Mg system with improved corrosion resistance of flat plate and end face by reducing the size of intermetallic compounds such as Mg ₂ Si phase and Mg Zn ₂ phase present in the plating film to 10 μm or less. Plated steel plates are disclosed.

Since the above-mentioned molten Al-Zn-based plated steel sheet has a beautiful appearance with a spangle pattern of white metallic luster, it is often used without painting, and the actual situation is that there is a strong demand for its appearance. be. Therefore, techniques for improving the appearance of hot-dip Al-Zn-based plated steel sheets have also been developed.
For example, Patent Document 4 discloses a molten Al-Zn-Si-Mg-Sr-based plated steel sheet in which wrinkle-like unevenness defects are suppressed by containing 0.01 to 10% Sr in the plating film.
Further, Patent Document 5 also discloses a molten Al-Zn-Si-Mg-Sr-based plated steel sheet in which mottled defects are suppressed by containing 500 to 3000 ppm of Sr in the plating film.
Techniques for improving the view are disclosed.
For example, Patent Document 4 discloses a molten Al-Zn-Si-Mg-based plated steel sheet in which wrinkle-like unevenness defects are suppressed by containing 0.01 to 10% Sr in the plating film.
Further, Patent Document 5 also discloses a molten Al-Zn-Si-Mg-based plated steel sheet in which mottled defects are suppressed by containing 500 to 3000 ppm of Sr in the plating film.
Further, Patent Document 6 discloses a molten Al-Zn-Si-Mg-based plated steel sheet in which 0.001 to 1.0% of Sr is contained in the plating film to achieve both surface appearance and corrosion resistance.
Further, Patent Document 7 also describes a molten Al-Zn-Si-Mg-based plated steel sheet in which 0.001 to 1.0% of Sr is contained in the plating film to achieve both surface appearance and corrosion resistance between the flat plate portion and the processed portion. Is disclosed.
Furthermore, Patent Document 8 also discloses a molten Al-Zn-Si-Mg-based plated steel sheet that achieves both surface appearance and corrosion resistance by containing 0.01 to 0.2% Sr in the plating film. ..
Further, Patent Document 9 discloses a molten Al-Zn-Si-Mg-based plated steel sheet having improved corrosion resistance by controlling the Si and Mg concentrations in the plating film at a specific ratio.

In addition, a coated steel sheet in which a chemical conversion film, primer coating film, topcoat coating film, etc. is formed on the surface of a molten Al-Zn-based plated steel sheet can be bent 90 degrees or 180 degrees by press forming, roll forming, or embossing. Various processing is applied, and long-term coating film durability is required. In order to meet these demands, the molten Al-Zn-based plated steel sheet forms a chemical conversion film containing chromate, the primer coating film also contains a chromate-based rust preventive pigment, and a heat-curable polyester-based steel sheet is added. A coated steel sheet on which a topcoat coating film having excellent weather resistance, such as a resin coating film or a fluororesin coating film, is formed is known.
However, recently, the use of chromate, which is an environmentally hazardous substance, has been regarded as a problem for such coated steel sheets, and the development of coated steel sheets that can improve corrosion resistance and surface appearance even if they are chromate-free is strongly desired. ..
As a technique for meeting these requirements, for example, Patent Document 10 describes aluminum-zinc alloy plating containing Al, Zn, Si and Mg on the surface of a steel material and adjusting the content of these elements. The layer (α) is plated, and a film (β) containing at least one compound (A) selected from a titanium compound and a zirconium compound as a film-forming component is formed as an upper layer thereof, and an aluminum-zinc alloy plated layer (β) is formed. A surface-treated hot-dip plated steel material in which the mass ratio of the Si—Mg phase in α) to the total amount of Mg in the plating layer is adjusted to 3% or more is disclosed.

Japanese Patent No. 5020228 Japanese Patent No. 5000039 Japanese Unexamined Patent Publication No. 2002-12959 Japanese Patent No. 3983932 Special Table 2011-514934 International Publication No. 2020/179147 International Publication No. 2020/179148 Japanese Unexamined Patent Publication No. 2020-143370 International Publication No. 2016/140370 Japanese Unexamined Patent Publication No. 2005-169765

However, the technique of containing Mg in the plating film as disclosed in Patent Documents 1 to 3 does not always bring about the improvement of corrosion resistance uniquely.
In the molten Al-Zn-Si-Mg-based plated steel sheets disclosed in Patent Documents 1 to 3, the corrosion resistance is improved only by containing Mg in the plating component. The characteristics of the intermetallic phase were not considered, and it was not possible to uniformly talk about the superiority or inferiority of corrosion resistance. Therefore, even when a molten Al-Zn-Si-Mg-based plated steel sheet is manufactured using the same plating bath composition, the corrosion resistance varies when the corrosion acceleration test is carried out, and the Al-Zn-based plated steel sheet to which Mg is not added is present. There was a problem that it did not necessarily have an advantage over.
Similarly, in order to improve the appearance of the plating, it is not always possible to eliminate the wrinkle-like unevenness defect only by adding Sr to the plating film, and the molten Al-disclosed in Patent Documents 4 to 8 can be eliminated. In some cases, the Zn-Si-Mg-based plated steel sheet could not achieve both corrosion resistance and appearance. In addition, since Mg is an element that easily oxidizes, Mg contained in the plating bath generates an oxide (top dross) near the bath surface, and in the case of hot-dip plating, the bath of the plating bath over time. FeAl-based compounds (bottom dross) containing iron that are unevenly distributed in the middle or bottom may be generated, and these dross adhere to the surface of the plating film and cause convex defects, which impairs the appearance of the surface of the plating film. There was also a fear.

In addition, when the steel sheet is plated with a bath in which Mg is added to the molten Al-Zn-Si bath, Mg ₂ Si phase, Mg Zn ₂ phase, and Si phase are deposited in the plating film in addition to the α-Al phase. It is known. However, the effect of the precipitation amount and abundance ratio of each phase on the corrosion resistance has not been clarified.
In the molten Al-Zn-Si-Mg-based plated steel sheet disclosed in Patent Document 9, the concentration of Si and Mg is controlled at a specific ratio, and the corrosion resistance is improved by eliminating the precipitation of the Si phase in the plating film. However, it cannot always be said that the Si phase can be suppressed, and even if the formation of the Si phase in the plating film can be suppressed, excellent corrosion resistance may not be obtained, which is technically incomplete. Met.

Further, as described above, the coated steel sheet has long-term coating film durability performance in a state of being subjected to various processing such as 90 degree bending and 180 degree bending by press forming, roll forming, embossing and the like. Although required, it cannot be said that the technique of Patent Document 10 can always obtain stable corrosion resistance and surface appearance after processing.
It goes without saying that the corrosion resistance of the coated steel sheet is affected by the corrosion resistance of the plated steel sheet used as the base, and as for the surface appearance, the height difference of the unevenness of the wrinkle-like defect is as much as several tens of μm. Even if the surface is smoothed, the unevenness is not completely eliminated, and it is considered that the appearance of the coated steel sheet cannot be improved. Further, since the coating film becomes thin in the convex portion, there is a concern that the corrosion resistance is locally deteriorated. Therefore, in order to obtain a coated steel sheet having excellent corrosion resistance and surface appearance, it is important to improve the corrosion resistance and surface appearance of the underlying plated steel sheet.

In view of such circumstances, it is an object of the present invention to provide a coated steel sheet having stable and excellent corrosion resistance and corrosion resistance of a processed portion.

As a result of studies to solve the above problems, the present inventors have made into the Mg ₂ Si phase, Mg Zn ₂ phase and Si phase formed in the plating film of the molten Al-Zn-Si-Mg-based plated steel sheet. It was found that the amount of precipitation increases or decreases depending on the balance of each component in the plating film and the formation conditions of the plating film, the abundance ratio changes, and one of the phases may not precipitate depending on the balance of the composition. In addition, it was clarified that the corrosion resistance of the coated steel sheet changes depending on the abundance ratio of these phases, and that the corrosion resistance and the corrosion resistance of the processed part are stably improved especially when the MgZn ₂ phase is larger than that of the Mg ₂ Si phase and the Si phase. did.
However, for these Mg ₂ Si phase and Si phase, a general method such as a scanning electron microscope is used to observe the secondary electron image or backscattered electron image of the plating film from the surface or cross section. It is known that it is very difficult to distinguish the difference between the phases. As a method for more detailed analysis, it is possible to obtain microscopic information by observing with a transmission electron microscope, but Mg ₂ Si phase and Si phase that influence macroscopic information such as corrosion resistance and appearance. It was not possible to grasp the abundance ratio of.
Therefore, as a result of further diligent research, the present inventors focused on the X-ray diffraction method and quantified the phase abundance ratio by using the intensity ratio of specific diffraction peaks for the Mg ₂ Si phase and the Si phase. Furthermore, if the Mg ₂ Si phase and the Mg Zn ₂ phase satisfy a specific abundance ratio in the plating film, stable and excellent corrosion resistance and corrosion resistance of the processed part can be realized, and the occurrence of dross is suppressed. It was found that good surface appearance can be ensured.
In addition, the present inventors controlled the abundance ratio of the above-mentioned Mg ₂ Si phase, Si phase, etc., and then controlled the Sr concentration in the bath to surely suppress the occurrence of wrinkle-like uneven defects on the surface. It was also found that a plated steel sheet with excellent appearance can be obtained.

Furthermore, the present inventors have also studied the chemical conversion film and the primer coating film formed on the plating film, and while forming the chemical conversion film with a specific resin and a specific inorganic compound, the primer coating film is formed. It has also been found that, by being composed of a specific polyester resin and an inorganic compound, the barrier property and adhesion of the coating film can be enhanced, and excellent post-processing corrosion resistance can be realized even if it is chromate-free.

The present invention has been made based on the above findings, and the gist thereof is as follows.
1. 1. A coated steel sheet in which a coating film is formed directly or via a chemical conversion film on a plating film.
The plating film contains Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass, Mg: 1.0 to 10.0% by mass and Sr: 0.01 to 1.0% by mass, and the balance is composed of Zn and unavoidable impurities. Have,
The diffraction intensity of Si and Mg ₂ Si in the plating film by X-ray diffraction satisfies the following relationship (1).
Si (111) / Mg ₂ Si (111) ≤ 0.8 ・ ・ ・ (1)
Si (111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction strength of Mg ₂ Si (111) plane (plane spacing d = 0.3668 nm) The chemical conversion film is: (a): anionic polyurethane resin having an ester bond and (b): bisphenol skeleton. Contains 30 to 50% by mass of the epoxy resin having It contains 2 to 10% by mass of vanadium compound, 40 to 60% by mass of zirconium compound, and 0.5 to 5% by mass of an inorganic compound containing a fluorine compound.
The coating film has at least a primer coating film, and the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide. , Painted steel plate.

2. 2. The coated steel sheet according to 1 above, wherein the diffraction intensity of Si in the plating film by the X-ray diffraction method satisfies the following relationship (2).
Si (111) = 0 ・ ・ ・ (2)

4. The coated steel sheet according to 1 or 2, wherein the Al content in the plating film is 50 to 60% by mass.

4. The coated steel sheet according to any one of 1 to 3, wherein the content of Si in the plating film is 1.0 to 3.0% by mass.

5. The coated steel sheet according to any one of 1 to 4, wherein the content of Mg in the plating film is 1.0 to 5.0% by mass.

According to the present invention, it is possible to stably provide a coated steel sheet having excellent corrosion resistance and corrosion resistance of a processed portion.

It is a figure for demonstrating the flow of the Japanese Automotive Standards Organization compound cycle test (JASO-CCT).

The coated steel sheet of the present invention is a coated steel sheet in which a coating film is formed directly on the plating film or via a chemical conversion film.

(Plating film)
In the coated steel sheet of the present invention, the plating film contains Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass and Mg: 1.0 to 10.0% by mass, and the balance consists of Zn and unavoidable impurities. Have.

The Al content in the plating film is 45 to 65% by mass, preferably 50 to 60% by mass, in view of the balance between corrosion resistance and operational aspects. This is because if the Al content in the plating film is at least 45% by mass, dendrite solidification of Al occurs, and a plating film structure mainly composed of the dendrite solidified structure of the α-Al phase can be obtained. By adopting a structure in which the dendrite solidified structure is laminated in the film thickness direction of the plating film, the corrosion progress path becomes complicated and the corrosion resistance of the plating film itself is improved. Further, the more the dendrite portions of the α-Al phase are laminated, the more complicated the corrosion progress path becomes, and the more difficult it is for corrosion to reach the base steel sheet. Therefore, the corrosion resistance is improved, and the Al content is 50 mass. % Or more is preferable. On the other hand, when the Al content in the plating film exceeds 65% by mass, most of Zn changes to a structure that is solid-dissolved in α-Al, and the dissolution reaction of the α-Al phase cannot be suppressed, so that Al-Zn -The corrosion resistance of Si-Mg plating deteriorates. Therefore, the Al content in the plating film needs to be 65% by mass or less, preferably 60% by mass or less.

Si in the plating film mainly suppresses the growth of Fe-Al-based and / or Fe-Al-Si-based interfacial alloy layers generated at the interface with the underlying steel sheet, and does not deteriorate the adhesion between the plating film and the steel sheet. Added for the purpose. When the steel plate is actually immersed in an Al-Zn-based plating bath containing Si, Fe on the surface of the steel plate and Al or Si in the bath undergo an alloying reaction, and Fe-Al-based and / or Fe-Al-Si-based. The metal-metal compound layer is formed at the base steel plate / plating film interface. At this time, the Fe-Al-Si alloy has a slower growth rate than the Fe-Al alloy, so the ratio of the Fe-Al-Si alloy is high. The higher the value, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film needs to be 1.0% by mass or more. On the other hand, when the Si content in the plating film exceeds 4.0% by mass, not only the growth suppressing effect of the above-mentioned interfacial alloy layer is saturated, but also the presence of an excess Si phase in the plating film promotes corrosion. Therefore, the Si content should be 4.0% or less. Further, the content of Si in the plating film is preferably 3.0% or less from the viewpoint of suppressing the presence of an excessive Si phase. The Si content is preferably 1.0 to 3.0% by mass from the viewpoint of easily satisfying the relational expression (1) described later in relation to the Mg content described later.

The plating film contains 1.0 to 10.0% of Mg. By containing Mg in the plating film, the above-mentioned Si can be present in the form of an intermetallic compound of Mg ₂ Si phase, and the promotion of corrosion can be suppressed.
Further, when Mg is contained in the plating film, MgZn ₂ phase, which is an intermetallic compound, is also formed in the plating film, and the effect of further improving the corrosion resistance can be obtained. When the Mg content in the plating film is less than 1.0% by mass, Mg is used for solid solution to the α-Al phase, which is the main phase, rather than the formation of the intermetallic compound (Mg ₂ Si, MgZn ₂ ). Therefore, sufficient corrosion resistance cannot be ensured. On the other hand, when the Mg content in the plating film is large, the effect of improving the corrosion resistance is saturated and the workability is lowered due to the weakening of the α-Al phase, so the content is set to 10.0% or less. Further, the Mg content in the plating film is preferably 5.0% by mass or less from the viewpoint of suppressing the generation of dross during plating formation and facilitating the management of the plating bath. From the viewpoint of easily satisfying the relational expression (1) described later in relation to the Si content, the Mg content is preferably 3.0% by mass, and compatibility with dross suppression is taken into consideration. Then, it is more preferable to set the Mg content to 3.0 to 5.0% by mass.

In the surface-treated steel sheet of the present invention, the diffraction intensities of Si and Mg ₂ Si in the plating film by the X-ray diffraction method need to satisfy the following relationship (1).
Si (111) / Mg ₂ Si (111) ≤ 0.8 ・ ・ ・ (1)
Si (111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135 nm), Mg ₂ Si (111): Diffraction strength of Mg ₂ Si (111) plane (plane spacing d = 0.3668 nm)

As described above, in the present invention, it is important to control the abundance ratio of the Mg ₂ Si phase and the Si phase generated in the plating film due to the inclusion of Mg and Si to a specific ratio. The effects of these on corrosion resistance are currently being investigated and there are many unclear points, but the following mechanisms are presumed.

When the coated steel sheet is exposed to a corrosive environment, the above-mentioned intermetallic compound dissolves preferentially over the α-Al phase, resulting in an environment rich in Mg in the vicinity of the corrosive product formed. It is estimated that in such an Mg-rich environment, the corrosion products formed are less likely to be decomposed, and as a result, the protective effect of the plating film is enhanced. In addition, since MgZn ₂ has a greater protective effect than Mg ₂ Si in this plating film, it is considered effective to increase the abundance ratio of MgZn ₂ in the intermetallic compound present in the plating film. Be done.

The abundance ratio of Mg ₂ Si and Si in the plating film has a relationship (1): Si (111) / Mg ₂ Si (111) ≤ 0.8 using the diffraction peak intensity obtained by the X-ray diffractometry. It is necessary to satisfy, but when the abundance ratio of Mg ₂ Si and Si in the plating film does not satisfy the relationship (1), that is, when Si (111) / Mg ₂ Si (111)> 0.8, the plating is performed. Since the amount of Si phase present in the film increases, the above-mentioned environment rich in Mg cannot be obtained in the vicinity of the corrosion product, and it becomes difficult to obtain the effect of improving the protective effect of the plating film. From the same viewpoint, the abundance ratio of Si to Mg ₂ Si (Si (111) / Mg ₂ Si (111)) is preferably 0.5 or less, more preferably 0.3 or less, and preferably 0.2 or less. Especially preferable.
Regarding the abundance ratio of Mg ₂ Si and Si in the plating film, the composition of the plating film tentatively satisfies the range of the present invention (Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass, Mg: Even if it contains 1.0 to 10.0% by mass and Sr: 0.01 to 1.0% by mass, and the balance consists of Zn and unavoidable impurities), the abundance ratio of Mg ₂ Si and Si does not satisfy the relationship (1). However, the effect of improving the protective effect of the plating film according to the present invention cannot be sufficiently obtained.

Here, in the above relationship (1), Si (111) is the diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135 nm), and Mg ₂ Si (111) is (111) of Mg ₂ Si. ) Diffraction intensity of the plane (plane spacing d = 0.3668 nm).
As a method of measuring Si (111) and Mg ₂ Si (111) by the X-ray diffraction, a part of the plating film is mechanically scraped off and X-ray diffraction is performed in a powdered state (powder X). It can be calculated by the line diffraction measurement method). For the measurement of the diffraction intensity, the diffraction peak intensity of Si corresponding to the surface spacing d = 0.3135 nm and the diffraction peak intensity of Mg ₂ Si corresponding to the surface spacing d = 0.3668 nm are measured, and these ratios are calculated. Si (111) / Mg ₂ Si (111) can be obtained.
The amount of plating film (amount to scrape off the plating film) required for powder X-ray diffraction measurement is 0.1 g or more from the viewpoint of accurately measuring Si (111) and Mg ₂ Si (111). It may be sufficient, preferably 0.3 g or more. Further, when the plating film is scraped off, steel plate components other than the plating film may be contained in the powder, but these intermetallic compound phases are contained only in the plating film, and the above-mentioned peak strength can be obtained. It has no effect. Further, the reason why the plating film is powdered and X-ray diffraction is performed is that when X-ray diffraction is performed on the plating film formed on the plated steel sheet, it is affected by the plane orientation of the solidification structure of the plating film and has the correct phase ratio. This is because it is difficult to perform the calculation.

Further, in the coated steel sheet of the present invention, it is preferable that the diffraction intensity of Si in the plating film by the X-ray diffraction method satisfies the following relationship (2) from the viewpoint that the corrosion resistance can be improved more stably. ..
Si (111) = 0 ・ ・ ・ (2)
Generally, in the dissolution reaction of an Al alloy in an aqueous solution, it is known that the presence of the Si phase as a cathode site promotes the dissolution of the surrounding α-Al phase, so that the Si phase is reduced. This is also effective from the viewpoint of suppressing the dissolution of the α-Al phase, and among them, a film having no Si phase as in the relationship (2) is used (the diffraction peak intensity of the Si (111) is set to zero. That) is the best for stabilizing corrosion resistance.
The method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction is as described above.

Here, the method for satisfying the above-mentioned relationship (1) and relationship (2) is not particularly limited. For example, in order to satisfy the relationship (1) and the relationship (2), the balance between the Si content, the Mg content and the Al content in the plating film is adjusted to obtain Mg ₂ Si and Si. The abundance ratio (diffraction intensity of Mg ₂ Si (111) and Si (111)) can be controlled. It should be noted that the balance between the Si content, the Mg content and the Al content in the plating film does not necessarily satisfy the relationship (1) and the relationship (2) if the content ratio is set to a certain level. For example, it is necessary to change the content ratio of Mg and Al depending on the content of Si (% by mass).
In addition to adjusting the balance between the Si content, Mg content, and Al content in the plating film, the conditions for forming the plating film (for example, cooling conditions after plating) can be adjusted. , The diffraction intensity of Mg ₂ Si (111) and Si (111) can be controlled so as to satisfy the relationship (1) and the relationship (2).

Further, in the coated steel sheet of the present invention, the plating film contains 0.01 to 1.0% by mass of Sr. When the plating film contains Sr, it is possible to more reliably suppress the occurrence of surface defects such as wrinkle-like uneven defects, and it is possible to realize good surface appearance.
It can be reliably suppressed and good surface appearance can be realized.
The wrinkle-like defect is a wrinkle-like uneven defect formed on the surface of the plating film, and is observed as a whitish streak on the surface of the plating film. Such wrinkle-like defects are likely to occur when a large amount of Mg is added to the plating film. Therefore, in the hot-dip plated steel sheet, by containing Sr in the plating film, Sr is preferentially oxidized over Mg in the surface layer of the plating film, and the oxidation reaction of Mg is suppressed, thereby causing the wrinkle-like defect. It is possible to suppress the occurrence of.

The plating film contains 0.01 to 1.0% by mass of Sr, but when the Sr content in the plating film is less than 0.01% by mass, the effect of improving the appearance cannot be obtained, and the Sr in the plating film is not obtained. If the content exceeds 1.0% by mass, Sr is excessively incorporated into the interfacial alloy layer, which adversely affects the plating adhesion and the like. From the same viewpoint, the Sr content in the plating film is preferably 0.05 to 0.5% by mass, more preferably 0.1 to 0.4% by mass.
In the coated steel sheet of the present invention, the abundance ratios of Si and Mg ₂ Si in the above-mentioned plating film satisfy the relationship (1), and the plating film contains 0.01 to 1.0% by mass of Sr. Thereby, the effect of improving the surface appearance by the above-mentioned Sr can be further enjoyed. Although the cause of this is not clear, it is presumed that when the amount of Si in the plating film increases, the oxidation of the plating surface layer is difficult to be suppressed in the first place, which affects the effect of improving the appearance when Sr is added. ..

In the coated steel sheet of the present invention, the plating film contains Zn and unavoidable impurities.
Of these, the unavoidable impurity contains Fe. This Fe is inevitably contained in the plating bath due to elution of the steel sheet and the equipment in the bath, and is supplied by diffusion from the base steel sheet at the time of forming the interfacial alloy layer, and as a result, it is inevitably contained in the plating film. Will be included. The Fe content in the plating film is usually about 0.3 to 2.0% by mass. Other unavoidable impurities include Cr, Ni, Cu and the like. The total content of the unavoidable impurities is not particularly limited, but if it is excessively contained, it may affect various characteristics of the plated steel sheet, so that the total content is preferably 5.0% by mass or less.

Further, since the plating film can improve the stability of the corrosion product and delay the progress of corrosion in the same manner as the above-mentioned Mg, Cr and Mn in total of 0.01 to 10% by mass. , V, Mo, Ti, Ca, Ni, Co, Sb and B are preferably selected from one or more. The total content of the above-mentioned components is set to 0.01 to 10% by mass because a sufficient corrosion delay effect can be obtained and the effect is not saturated.

The amount of the plating film adhered is preferably 45 to 120 g / m ² per side from the viewpoint of satisfying various characteristics. When the adhesion amount of the plating film is 45 g / m ² or more, sufficient corrosion resistance can be obtained even for applications that require long-term corrosion resistance such as building materials, and the adhesion amount of the plating film is 120 g / m / m. This is because when m ² or less, excellent corrosion resistance can be realized while suppressing the occurrence of plating cracks during processing. From the same viewpoint, the amount of the plating film adhered is more preferably 45 to 100 g / m ² .

The amount of the plating film adhered is calculated from, for example, the difference in the weight of the steel sheet before and after peeling by dissolving and peeling the plating film of a specific area with a mixed solution of hydrochloric acid and hexamethylenetetramine shown in JIS H 0401: 2013. Can be derived. In order to determine the amount of plating adhesion per one surface by this method, it can be determined by sealing with tape so that the plating surface of the non-target surface is not exposed, and then performing the above-mentioned dissolution.

Further, the component composition of the plating film can be confirmed, for example, by immersing the plating film in hydrochloric acid or the like to dissolve the plating film, and confirming the solution by ICP emission spectroscopic analysis, atomic absorption spectroscopy or the like. This method is merely an example, and any method can be used as long as the component composition of the plating film can be accurately quantified, and the method is not particularly limited.

The plating film of the coated steel sheet obtained by the present invention has almost the same composition as the plating bath as a whole. Therefore, the composition of the plating film can be controlled accurately by controlling the composition of the plating bath.

Further, the base steel sheet constituting the coated steel sheet of the present invention is not particularly limited, and a cold-rolled steel sheet, a hot-rolled steel sheet, or the like can be appropriately used according to the required performance and specifications.

Further, the method for obtaining the base steel plate is not particularly limited. For example, in the case of the hot-rolled steel sheet, a steel sheet that has undergone a hot-rolling step and a pickling step can be used, and in the case of the cold-rolled steel sheet, it can be manufactured by further adding a cold-rolling step. Further, in order to obtain the characteristics of the steel sheet, it is possible to go through a recrystallization annealing step or the like before the hot-dip plating step.

The conditions for forming the plating film when manufacturing the coated steel sheet of the present invention are not particularly limited. For example, it can be manufactured by washing, heating, and immersing the base steel sheet in a plating bath in a continuous hot-dip plating facility. In the heating step of the steel sheet, recrystallization baking is performed to control the structure of the base steel sheet itself, and in order to prevent oxidation of the steel sheet and reduce a trace amount of oxide film existing on the surface, a nitrogen-hydrogen atmosphere, etc. It is effective to heat in the reducing atmosphere of.

As for the plating bath used for forming the plating film, as described above, the composition of the plating film as a whole is almost the same as the composition of the plating bath. Therefore, Al: 45 to 65% by mass. Those containing Si: 1.0 to 4.0% by mass, Mg: 1.0 to 10.0% by mass and Sr: 0.01 to 1.0% by mass, and the balance having a composition of Zn, Fe and unavoidable impurities can be used.

Further, the bath temperature of the plating bath is not particularly limited, but is preferably in the temperature range of (melting point + 20 ° C.) to 650 ° C.
The lower limit of the bath temperature is set to a melting point of + 20 ° C., because the bath temperature must be equal to or higher than the freezing point in order to perform the hot-dip plating treatment. This is to prevent coagulation due to a local decrease in bath temperature. On the other hand, the upper limit of the bath temperature is set to 650 ° C. If the temperature exceeds 650 ° C, rapid cooling of the plating film becomes difficult, and the interfacial alloy layer formed between the plating film and the steel sheet may become thick. Because.

The temperature of the base steel plate that penetrates into the plating bath (penetration plate temperature) is also not particularly limited, but from the viewpoint of ensuring the plating characteristics in the continuous hot-dip plating operation and preventing changes in the bath temperature, the plating bath is used. It is preferable to control the temperature within ± 20 ° C.

Further, the immersion time of the steel sheet in the plating bath is 0.5 seconds or more. This is because if it takes less than 0.5 seconds, a sufficient plating film may not be formed on the surface of the base steel sheet. The upper limit of the dipping time is not particularly limited, but it is preferably within 8 seconds because the interfacial alloy layer formed between the plating film and the steel sheet may become thicker if the dipping time is lengthened.

Furthermore, in the cooling process after plating, it is preferable to cool the plate over 3 seconds or more from 520 ° C to 500 ° C. The precipitation start temperature of the simple substance Si phase and Mg2Si in the plating film is 500 to 490 ° C for the simple substance Si phase and 520 to 500 ° C for Mg2Si. Therefore, by increasing the residence time in the temperature range of 520 to 500 ° C where only Mg2Si precipitates, the precipitation of Mg2Si is promoted and the precipitation of the simple substance Si phase is suppressed. This is because it is easier to fill.

(Chemical film)
The coated steel sheet of the present invention can form a chemical conversion film on the above-mentioned plating film.
The chemical conversion film may be formed on at least one side of the coated steel sheet, and may be formed on both sides of the coated steel sheet depending on the application and required performance.

In the coated steel plate of the present invention, the chemical conversion film contains (a): an anionic polyurethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton in a total of 30 to 50% by mass. A resin component in which the content ratio ((a): (b)) of a) and the (b) is in the range of 3:97 to 60:40 by mass ratio, and 2 to 10% by mass of vanadium compound, 40 to It is characterized by containing an inorganic compound containing 60% by mass of a zirconium compound and 0.5 to 5% by mass of a fluorine compound.
By forming the above-mentioned chemical conversion film on the plating film, it is possible to improve the strength and adhesion of the chemical conversion film and also improve the corrosion resistance.

Here, the resin component constituting the chemical conversion film contains (a): an anionic urethane resin having an ester bond and (b): an epoxy resin having a bisphenol skeleton.

As for the anionic polyurethane resin having the (a) ester bond, a resin obtained by copolymerizing a polyester polyol with a diisocyanate or a polyisocyanate having two or more isocyanate groups and a dimethylolalkyl acid is obtained. Can be mentioned. Further, a chemical conversion treatment liquid can be obtained by dispersing it in a liquid such as water by a known method.

The polyester polyol includes a polyester obtained by a dehydration condensation reaction from a glycol component and an acid component such as an ester-forming derivative of hydroxylcarboxylic acid, a polyester obtained by a ring-opening polymerization reaction of a cyclic ester compound such as ε-caprolactone, and these. Examples thereof include the copolymerized polyester of.
Examples of the polyisocyanate include aromatic polyisocyanates, aliphatic polyisocyanates, and alicyclic polyisocyanates. Examples of the aromatic polyisocyanate include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, m-xylene diisocyanate, diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate, 2,2-diphenylmethane diisocyanate, and triphenyl. Examples thereof include methanetriisocyanate, polymethylene polyphenyl polyisocyanate, naphthalenediocyanate, and derivatives thereof (for example, prepolymers obtained by reaction with polyols, modified polyisocyanates such as carbodiimide compounds of diphenylmethane diisocyanate, etc.). ..

When synthesizing urethane by reacting the polyester polyol with the diisocyanate or polyisocyanate, for example, by copolymerizing dimethylolalkyl acid, self-emulsifying and water-solubilizing (water-dispersing) the above (water dispersion). a) An anionic polyurethane resin having an ester bond can be obtained. In this case, examples of the dimethylolalkyl acid include dimethylolalkyl acids having 2 to 6 carbon atoms, and more specifically, dimethylol ethanoic acid, dimethylol propanoic acid, dimethylolbutanoic acid, and dimethylol heptane. Acids and dimethylolhexanoic acid and the like can be mentioned.

Further, as the epoxy resin having the (b) bisphenol skeleton, a known epoxy resin can be used. For example, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxy resin, bisphenol S type epoxy resin and the like can be mentioned. These epoxy resins can be obtained by reacting bisphenol compounds such as bisphenol A, bisphenol F, bisphenol AD, and bisphenol S with epichlorohydrin in the presence of an alkaline catalyst. Among them, the 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 the (b) bisphenol skeleton can be dispersed in a liquid such as water by a known method to obtain a chemical conversion treatment liquid.

The resin component acts as a binder for the chemical conversion film, but the anionic polyurethane resin having the (a) ester bond constituting the binder is flexible, so that the chemical conversion film is destroyed when processed (). The effect of making it difficult to peel off) can be achieved, and the epoxy resin having the (b) bisphenol skeleton can have the effect of improving the adhesion to the underlying zinc-based plated steel plate and the upper primer coating film.
The resin component is contained in the chemical conversion film in a total amount of 30 to 50% by mass. If the content of the resin component is less than 30% by mass, the binder effect of the chemical conversion film is lowered, and if it exceeds 50% by mass, the function of the inorganic component shown below, for example, the inhibitory action is lowered. From the same viewpoint, the content of the resin component in the chemical conversion film is preferably 35 to 45% by mass.

Further, in the resin component, the content ratio ((a): (b)) of the anionic polyurethane resin having the (a) ester bond and the epoxy resin having the (b) bisphenol skeleton is 3:97 by mass ratio. It must be in the range of ~ 60: 40. This is because when the above (a): (b) is out of the above range, sufficient corrosion resistance cannot be obtained due to a decrease in flexibility and adhesion as a chemical conversion coating film. From the same viewpoint, the above (a): (b) is preferably 10:90 to 55:45.

The resin component includes a resin (other resin component) other than the above-mentioned (a) anionic polyurethane resin having an ester bond and (b) an epoxy resin having a bisphenol skeleton, depending on the required performance. be able to. The other resin components are not particularly limited, and are, for example, at least one or two or more selected from acrylic resin, acrylic silicon resin, alkyd resin, polyester resin, polyalkylene resin, amino resin and fluororesin. Can be used in combination.
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. It is more preferably 75% by mass or more. This is to ensure that the flexibility and adhesion of the chemical conversion coating film are reduced.

Further, the chemical conversion film contains 2 to 10% by mass of vanadium compound, 40 to 60% by mass of zirconium compound and 0.5 to 5% by mass of fluorine compound as inorganic compounds.
By including these compounds, the corrosion resistance of the chemical conversion film can be enhanced.

The vanadium compound is added to the chemical conversion treatment liquid and acts as a rust preventive (inhibitor). When the vanadium compound is contained in the chemical conversion film, the vanadium compound is appropriately eluted in a corrosive environment and combined with zinc ions or the like of a plating component that also elutes in a corrosive environment to form a dense protective film. .. The formed protective film can further enhance the corrosion resistance not only to the flat surface portion of the steel sheet but also to the defective portion, the damaged portion of the plating film caused by processing, the corrosion progressing from the cut end face to the flat surface portion, 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, and vanadium acetylacetonate. In particular, among these, it is 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 treatment film 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 inhibitory effect is not sufficient, resulting in a decrease in corrosion resistance. On the other hand, if the content of the vanadium compound exceeds 10% by mass, the moisture resistance of the chemical conversion coating is reduced. This is because it causes a decrease in.

The zirconium compound is contained in the chemical conversion coating, and can be expected to improve the strength and corrosion resistance of the chemical conversion coating by reacting with the plating metal and coexisting with the resin component. Furthermore, the zirconium compound itself is a dense chemical conversion coating. It contributes to the formation of zirconium and is rich in coating properties, so a barrier effect can be expected.
Examples of the zirconium compound include neutralizing salts such as zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, and zirconium chloride.

The content of the zirconium compound in the chemical conversion treatment film 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 are deteriorated, and if the content of the zirconium compound exceeds 60% by mass, the chemical conversion coating is brittle. This is because the chemical conversion treatment film is destroyed or peeled off when it is subjected to severe processing.

The fluorine compound is contained in the chemical conversion film and acts as an adhesive-imparting agent with the plating film. As a result, it is possible to improve the corrosion resistance of the chemical conversion film.
As the fluorine compound, for example, a fluoride salt such as an ammonium salt, a sodium salt or a potassium salt, or a fluorine compound such as ferrous fluoride or ferrous fluoride can be used. Among these, it is preferable to use ammonium fluoride or fluoride salts such as sodium fluoride and potassium fluoride.

The content of the fluorine compound in the chemical conversion treatment film is 0.5 to 5% by mass. If the content of the fluorine compound in the chemical conversion treatment film is less than 0.5% by mass, sufficient adhesion at the processed portion cannot be obtained, and if the content of the fluorine compound exceeds 5% by mass, the moisture resistance of the chemical conversion treatment film becomes high. This is because it decreases.

Further, the amount of the chemical conversion film adhered is not particularly limited. For example, from the viewpoint of improving the adhesion of the chemical conversion film while ensuring the corrosion resistance more reliably, it is preferable to set the adhesion amount of the chemical conversion film to 0.025 to 0.5 g / m ² . By setting the adhesion amount of the chemical conversion film to 0.025 g / m ² or more, corrosion resistance can be ensured more reliably, and by setting the adhesion amount of the chemical conversion film to 0.5 g / m ² or less, peeling of the chemical conversion film is suppressed. be able to.
The amount of the chemical conversion film adhered may be obtained by a method appropriately selected from existing methods such as a method of measuring the abundance of an element whose content in the film is known in advance by fluorescent X-ray analysis of the film. ..

The method for forming the chemical conversion film is not particularly limited, and can be appropriately selected depending on the required performance, manufacturing equipment, and the like. For example, a chemical conversion treatment liquid is continuously applied onto the plating film with a roll coater or the like, and then, using hot air or induction heating, the plate temperature reaches about 60 to 200 ° C. (Peak Metal Temperature: PMT). It can be formed by drying. For the application of the chemical conversion treatment liquid, a known method such as an airless spray, an electrostatic spray, a curtain flow coater or the like can be appropriately adopted in addition to the roll coater. Further, the chemical conversion film may be either a single-layer film or a multi-layer film as long as it contains the resin and the metal compound, and is not particularly limited.

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

In the present invention, the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide.
By containing the polyester resin having a urethane bond and the inorganic compound in the primer coating film, it is possible to improve the adhesion of the coating film and the corrosion resistance.

The primer coating film contains a polyester resin having a urethane bond as a main component. Since the polyester resin having a urethane bond has both flexibility and strength, it is possible to obtain an effect that cracks are less likely to occur in the primer coating film when it is processed, and a chemical conversion treatment film containing a urethane resin is obtained. Since it has a high affinity with, it can contribute to the improvement of corrosion resistance of the processed portion.
The term "main component" as used herein means that the component has the highest content among 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 a polyisocyanate having two or more isocyanate groups can be used. Further, a resin obtained by reacting the polyester polyol with the diisocyanate or the polyisocyanate in a state of excess hydroxylate (urethane-modified polyester resin) and cured with a blocked polyisocyanate can also be used.

The polyester polyol can be obtained by a known method using 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. The glycols include, for example, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, neopentyl glycol, hexylene glycol, 1,3-butanediol, 1,4. -Butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-butyl-2-ethyl-1,3-propanediol, methylpropanediol, cyclohexanedimethanol, 3,3-diethyl-1,5 -Centane diol and the like can be mentioned. Examples of the trihydric or higher polyhydric alcohol 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 if necessary. Examples of the polyvalent carboxylic acid include phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, 4-methylhexahydrophthalic acid, bicyclo [2,2,1] heptane-2,3-dicarboxylic acid, trimellitic acid, and the like. Adipic acid, sebacic acid, succinic acid, azelaic acid, fumaric acid, maleic acid, itaconic acid, pyromellitic acid, dimer acid, etc., and their acid anhydrides, as well as 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid. Acids, hexahydroisophthalic acid, hexahydroterephthalic acid and the like can be mentioned. These polybasic acids can be used alone or in combination of two or more.

Regarding the polyisocyanate, for example, aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimerate diisocyanate, and xylylene diisocyanate (XDI), metaxylylene diisocyanate, tolylene diisocyanate (TDI), 4, 4 -Aromatic diisocyanates such as diphenylmethane diisocyanate (MDI), cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydride XDI, hydride TDI, hydride MDI, and adducts, biurets, isocyanurates and the like thereof. Will be. These polyisocyanates can be used alone or in combination of two or more.

The hydroxyl value of the polyester resin having a urethane bond is not particularly limited, but is preferably 5 to 120 mgKOH / g, more preferably 7 to 100 mgKOH / g from the viewpoint of solvent resistance, processability and the like. It is g, more preferably 10 to 80 mgKOH / g.
Further, 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, still more preferably 800 to 10,000 from the viewpoint of solvent resistance, processability and the like. Is.

The content of the polyester resin having a urethane bond in the primer coating film 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 as a primer coating film may be deteriorated, while if the content of the polyester resin having a urethane bond exceeds 88% by mass, the binder function may be deteriorated. The functions of the following inorganic substances, such as inhibitory action, may be reduced.

The vanadium compound, which is one of the inorganic compounds, acts as an inhibitor. Examples of the vanadium compound include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. In particular, 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 film may be the same as or different from the vanadium compound added to the chemical conversion treatment film. In the vanazic acid compound, the vanazic acid ion that gradually elutes with the moisture that invades from the outside reacts with the ion on the surface of the zinc-based plated steel sheet to form a passivation film with good adhesion, and protects and prevents exposed metal parts. It is believed that rusting will occur.

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

A phosphoric acid compound, which is one of the inorganic compounds, also acts as an inhibitor. As the phosphoric acid compound, for example, phosphoric acid, an ammonium salt of phosphoric acid, an alkali metal salt of phosphoric acid, an alkaline earth metal salt of phosphoric acid and the like can be used. In particular, an alkali metal salt of phosphoric acid such as calcium phosphate can be preferably used.

The content of the phosphoric acid compound in the primer coating film is not particularly limited, but is preferably 4 to 20% by mass from the viewpoint of achieving both corrosion resistance and moisture resistance. If the content of the phosphoric acid compound is less than 4% by mass, the inhibitory effect may be lowered and the corrosion resistance may be lowered, and if the content of the phosphoric acid compound is more than 20% by mass, the moisture resistance of the primer coating film may be lowered. May be invited.

Magnesium oxide, which is one of the inorganic compounds, produces a product containing Mg by initial corrosion, and has the effect of stabilizing as a sparingly soluble magnesium salt and improving corrosion resistance.

The content of the magnesium oxide in the primer coating film is not particularly limited, but is preferably 4 to 20% by mass from the viewpoint of achieving both corrosion resistance and corrosion resistance of the processed portion. If the magnesium oxide content is less than 4% by mass, the effect may be lowered and the corrosion resistance may be lowered, and if the magnesium oxide content is more than 20% by mass, the flexibility of the primer coating film may be deteriorated. The corrosion resistance of the processed portion may decrease due to the decrease in the amount of

Further, the primer coating film may contain components other than the polyester resin having a urethane bond and the inorganic compound described above.
For example, a cross-linking agent used when forming a primer coating film can be mentioned. The cross-linking agent reacts with the polyester resin having a urethane bond to form a cross-linked coating film, and for example, an oxazoline compound, an epoxy compound, a melamine compound, an isocyanate-based compound, a carbodiimide-based compound, a silane coupling compound and the like. It is also possible to use two or more kinds of cross-linking agents in combination. Among them, a blocked polyisocyanate compound or the like can be preferably used from the viewpoint of corrosion resistance of the processed portion of the obtained coated steel sheet. Examples of the blocked polyisocyanate include alcohols such as butanol, oximes such as methylethylketooxime, lactams such as ε-caprolactam, and diketones such as acetoacetic acid diester, using the isocyanate group of the polyisocyanate compound. Examples thereof include those blocked by imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

Further, the primer coating film may contain various known components usually used in the coating field, if necessary. Specifically, for example, various surface modifiers such as leveling agents and defoamers, dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, silane coupling agents, various additives such as titanate coupling agents, etc. Examples thereof include various pigments such as coloring pigments and extender pigments, bright materials, curing catalysts, and organic solvents.

The thickness of the primer coating film is preferably 1.5 μm or more. By setting the thickness of the primer coating film to 1.5 μm or more, it is possible to more reliably obtain the effect of improving corrosion resistance and the effect of improving the adhesion with the chemical conversion treatment film or the top coat film formed on the primer coating film. Because it can be done.

The method for forming the primer coating film is not particularly limited. Further, as for the coating method of the coating composition constituting the primer coating film, the coating composition can be preferably applied by a method such as roll coater coating or curtain flow coating. After the coating composition is coated, it can be baked by heating means such as hot air heating, infrared heating, and induction heating to obtain a primer coating film. The baking treatment is usually carried out at a maximum plate temperature of about 180 to 270 ° C., and can be performed in this temperature range for about 30 seconds to 3 minutes.

Further, with respect to the coating film constituting the coated steel sheet of the present invention, it is preferable that a topcoat coating film is further formed on the primer coating film.
The topcoat coating film can impart aesthetics such as color, luster, and surface condition to the coated steel sheet, and also has various performances such as workability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance. Can be enhanced.

The composition of the topcoat coating film is not particularly limited, and the material, thickness, and the like can be appropriately selected according to the required performance.
For example, the topcoat coating film can be formed by using a polyester resin-based paint, a silicon polyester resin-based paint, a polyurethane resin-based paint, an acrylic resin-based paint, a fluororesin-based paint, or the like.
Further, the top coat is a titanium oxide, a valve handle, mica, carbon black or various other coloring pigments; metallic pigments such as aluminum powder and mica; extender pigments composed of carbonates and sulfates; silica fine particles and nylon resins. Various fine particles such as beads and acrylic resin beads; a curing catalyst such as p-toluenesulfonic acid and dibutyltin dilaurate; wax; and other additives can be contained in an appropriate amount.

Further, the thickness of the topcoat coating film is preferably 5 to 30 μm from the viewpoint of achieving both appearance and processability. When the thickness of the topcoat coating film is 5 μm or more, the color tone appearance can be more reliably stabilized, and when the thickness of the topcoat coating film is 30 μm or less, the workability is deteriorated (topcoat coating). The generation of cracks in the film) can be suppressed more reliably.

The coating method of the coating composition for forming the topcoat coating 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 coated, it can be baked by heating means such as hot air heating, infrared heating, and induction heating to form a topcoat coating film. The baking treatment is usually carried out at a maximum plate temperature of about 180 to 270 ° C., and can be performed in this temperature range for about 30 seconds to 3 minutes.

(Samples 1-44)
(1) The plating film conditions shown in Table 2 are obtained by using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as a base steel sheet and performing an annealing treatment and a plating treatment with a hot-dip plating simulator manufactured by Resuka Co., Ltd. A sample of the hot-dip plated steel sheet was prepared.
Regarding the composition of the plating bath used for manufacturing the hot-dip plated steel sheet, the composition of the plating bath was Al: 30 to 75% by mass and Si: 0.5 to 4.5 so as to be the composition of the plating film of each sample shown in Table 1. It was varied in the range of% by mass, Mg: 0 to 10% by mass, and Sr: 0.00 to 0.15% by mass. The bath temperature of the plating bath is 590 ° C when Al: 30 to 60% by mass and 630 ° C when Al: more than 60% by mass, and the plating penetration plate temperature of the base steel sheet is the same as the plating bath temperature. It was controlled to be. Furthermore, the plating treatment was carried out under the condition that the plate temperature was cooled to a temperature range of 520 to 500 ° C. in 3 seconds.
The amount of the plating film adhered was controlled to be 85 ± 5 g / m ² per side for the samples 1 to 41 and 52 to 128 g / m ² per side for the samples 42 to 44.

(2) After that, the chemical conversion treatment liquid shown in Table 1 was applied on the plating film of each sample of the prepared hot-dip galvanized steel sheet with a bar coater, and dried in a hot air drying furnace (reached plate temperature: 90 ° C.). A chemical conversion-treated film having an adhesion of 0.1 g / m ² was formed.
As the chemical conversion treatment liquid used, a chemical conversion treatment liquid having a pH of 8 to 10 prepared by dissolving each component in water as a solvent was used. The types of each component (resin component, inorganic compound) contained in the chemical conversion treatment liquid are as follows.
(Resin component)
Resin A: (a) Urethane-modified polyester resin having an ester bond ("Superflex 210" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and (b) Epoxy resin having a bisphenol skeleton ("Yukaresin RE" manufactured by Yoshimura Oil Chemical Co., Ltd. -1050 ") mixed at a content mass ratio (a): (b) = 50: 50 Resin B: Acrylic resin ("Boncoat EC-740EF "manufactured by DIC Co., Ltd.)
(Inorganic compound)
Vanadium compound: Organic vanadium compound chelated with acetylacetone Zirconium compound: Zirconium ammonium carbonate Fluorine compound: Ammonium fluoride

(3) Then, a primer paint is applied on the chemical conversion film formed as described above with a bar coater, and the steel sheet is baked under the conditions of an ultimate temperature of 230 ° C. and a baking time of 35 seconds. A primer coating film having the above was formed. Then, the topcoat coating composition is applied on the primer coating film formed as described above with a bar coater, and the steel sheet is baked under the conditions of reaching temperature of 230 ° C to 260 ° C and baking time of 40 seconds, as shown in Table 1. A topcoat coating film having the resin conditions and film thickness shown was formed, and a coated steel sheet of each sample was produced.
The primer paint was obtained by mixing each component and then stirring with a ball mill for about 1 hour. The following resin components and inorganic compounds were used to form the primer coating film.
(Resin component)
Resin α: Urethane-modified polyester resin (a product 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). , The one cured with blocked isocyanate was used.
The urethane resin to be urethane-modified was produced under the following conditions. A flask equipped with a stirrer, a rectification tower, a water separator, a cooling tube and a thermometer was charged with 320 parts by mass of isophthalic acid, 200 parts by mass of adipic acid, 60 parts by mass of trimethylolpropane, and 420 parts by mass of cyclohexanedimethanenol. Heat and stir, and while distilling the generated condensed water out of the system, raise the temperature from 160 ° C to 230 ° C over 4 hours at a constant rate, and after reaching the temperature of 230 ° C, gradually add 20 parts by mass of xylene. After adding, the condensation reaction is continued with the temperature maintained at 230 ° C, the reaction is terminated when the acid value becomes 5 or less, and after cooling to 100 ° C, Solbesso 100 (manufactured by Exxon Mobile Co., Ltd., trade name, high) A polyester resin solution was obtained by adding 120 parts by mass of a boiling aromatic hydrocarbon solvent and 100 parts by mass of butyl cellosolve.
Resin β: Urethane-cured polyester resin ("Evaclad 4900" manufactured by Kansai Paint Co., Ltd.)
(Inorganic compound)
Vanadium compound: Magnesium vanadate Phosphoric acid compound: Calcium calcium phosphate Magnesium oxide compound: Magnesium oxide The following paints were used as the resin used for the top coat.
Resin I: Melamine-cured polyester paint ("Precolor HD0030HR" manufactured by BASF Japan Ltd.)
Resin II: Organosol-based baking-type fluororesin-based paint with a mass ratio of polyvinylidene fluoride and acrylic resin of 80:20 ("Precolor No. 8800HR" manufactured by BASF Japan Ltd.)

(evaluation)
The following evaluations were performed on each sample of the coated steel sheet obtained as described above. The evaluation results are shown in Table 2.

(1) Composition of plating film (adhesion amount, composition, X-ray diffraction intensity)
For each sample of hot-dip plated steel sheet, 100 mmφ is punched out, the non-measurement surface is sealed with tape, and then the plating is melted and peeled off with a mixed solution of hydrochloric acid and hexamethylenetetramine shown in JIS H 0401: 2013. The amount of adhesion of the plating film was calculated from the mass difference. Table 2 shows the amount of adhesion of the plating film obtained as a result of the calculation.
Then, the stripping solution was filtered, and the filtrate and solid content were analyzed respectively. Specifically, components other than insoluble Si were quantified by ICP emission spectroscopic analysis of the filtrate.
The solid content was dried and incinerated in a heating furnace at 650 ° C., and then melted by adding sodium carbonate and sodium tetraborate. Furthermore, insoluble Si was quantified by dissolving the melt with hydrochloric acid and ICP emission spectroscopic analysis of the solution. The Si concentration in the plating film is the sum of the soluble Si concentration obtained by the filtrate analysis and the insoluble Si concentration obtained by the solid content analysis. Table 2 shows the composition of the plating film obtained as a result of the calculation.
Furthermore, for each sample, after shearing to a size of 100 mm × 100 mm, the plating film on the evaluation symmetric plane is mechanically scraped until the underlying steel plate appears, the obtained powder is mixed well, and then 0.3 g is taken out and X is taken out. Using a line diffraction line device (“SmartLab” manufactured by Rigaku Co., Ltd.), X-ray used: Cu-Kα (wavelength = 1.54178Å), Kβ ray removal: Ni filter, tube voltage: 40kV, tube current: 30mA, scanning・ Speed: 4 ° / min, sampling interval: 0.020 °, divergence slit: 2/3 °, solar slit: 5 °, detector: high-speed one-dimensional detector (D / teX Ultra), under the conditions of the above powder A qualitative analysis was performed. The intensity obtained by subtracting the base intensity from each peak intensity is defined as each diffraction intensity (cps), and the diffraction intensity of the (111) plane (plane spacing d = 0.3668 nm) of Mg ₂ Si and the (111) plane (plane spacing) of Si. The diffraction intensity of d = 0.3135 nm) was measured. The measurement results are shown in Table 2.

(2) Corrosion resistance evaluation After shearing each sample of coated steel sheet to a size of 120 mm × 120 mm, it is evaluated as a range of 10 mm from the edges of three arbitrarily selected sides of the evaluation target surface and the end faces of the same three sides of the sample. The target surface was sealed with tape and the evaluation target surface was exposed in a size of 100 mm × 100 mm, which was used as an evaluation sample. Three identical evaluation samples were prepared.
Corrosion acceleration tests were carried out in the cycle shown in FIG. 1 for all three evaluation samples prepared as described above. The corrosion acceleration test was started from wetting, samples were taken out every 20 cycles, washed with water and dried, and then visually observed to confirm the occurrence of red rust on the sheared end face on one side that was not tape-sealed.
Then, the number of cycles when red rust was confirmed was evaluated according to the following criteria. The evaluation results are shown in Table 2.
⊚: Number of red rust generation cycles of 3 samples ≧ 600 cycles ○: 600 cycles ＞ Number of red rust generation cycles of 3 samples ≧ 500 cycles ×: Number of red rust generation cycles of at least 1 sample <500 cycles

(3) Appearance after painting The surface of each sample of painted steel sheet was visually observed.
Then, the observation results were evaluated according to the following criteria. The evaluation results are shown in Table 2.
⊚: No wrinkle-like defect was observed ○: Wrinkle-like defect was observed only in the range of 50 mm from the edge ×: Wrinkle-like defect was observed in the range other than 50 mm from the edge.

(5) Workability after painting Each sample of painted steel plate was sheared to a size of 70 mm × 150 mm, and then eight plates of the same thickness were sandwiched inside and bent by 180 ° (8T bending). After the cellophane tape was strongly attached to the outer surface of the bent portion after bending, it was peeled off. The surface condition of the coating film on the outer surface of the bent portion and the presence or absence of adhesion (peeling) of the coating film on the surface of the used tape were visually observed, and the processability was evaluated according to the following criteria. The evaluation results are shown in Table 2.
〇: Both cracks and peeling are not observed in the plating film △: There are cracks in the plating film, but peeling is not observed ×: Both cracks and peeling are observed in the plating film

(5) Bath stability During hot-dip plating, the condition of the hot-dip bath surface is visually confirmed, and the hot-dip Al-Zn-based plated steel sheet is used to manufacture the hot-dip Al-Zn-based plated steel sheet. ). The evaluation is performed according to the following criteria, and the evaluation results are shown in Table 2.
〇: Similar to the molten Al-Zn-based plating bath (55% by mass Al-remaining Zn-1.6% by mass bath) △: For the molten Al-Zn-based plating bath (55% by mass Al-remaining Zn-1.6% by mass) Compared to more white oxide ×: Formation of black oxide is observed in the plating bath

From the results in Table 2, each sample of the example of the present invention is superior to each sample of the comparative example in terms of corrosion resistance, appearance after painting, processability after painting, and bath stability in a well-balanced manner. I understand.

According to the present invention, it is possible to stably provide a coated steel sheet having excellent corrosion resistance and corrosion resistance of a processed portion.

Claims (5)

A coated steel sheet in which a coating film is formed directly or via a chemical conversion film on a plating film.
The plating film contains Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass, Mg: 1.0 to 10.0% by mass and Sr: 0.01 to 1.0% by mass, and the balance is composed of Zn and unavoidable impurities. Have,
The diffraction intensities of Si and Mg ₂ Si in the plating film, which were measured in a state where a part of the plating film was mechanically cut into powder until the underlying steel plate appeared, had the following relationship. Satisfy (1)
Si (111) / Mg ₂ Si (111) ≤ 0.8 ・ ・ ・ (1)
Si (111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction strength of Mg ₂ Si (111) plane (plane spacing d = 0.3668 nm) The chemical conversion film is: (a): anionic polyurethane resin having an ester bond and (b): bisphenol skeleton. Contains 30 to 50% by mass of the epoxy resin having It contains 2 to 10% by mass of vanadium compound, 40 to 60% by mass of zirconium compound, and 0.5 to 5% by mass of an inorganic compound containing a fluorine compound.
The coating film has at least a primer coating film, and the primer coating film contains a polyester resin having a urethane bond and an inorganic compound containing a vanadium compound, a phosphoric acid compound and magnesium oxide. , Painted steel plate. The coated steel sheet according to claim 1, wherein the diffraction intensity of the Si in the plating film due to X-ray diffraction satisfies the following relationship (2).
Si (111) = 0 ・ ・ ・ (2) The coated steel sheet according to claim 1 or 2, wherein the Al content in the plating film is 50 to 60% by mass. The coated steel sheet according to any one of claims 1 to 3, wherein the content of Si in the plating film is 1.0 to 3.0% by mass. The coated steel sheet according to any one of claims 1 to 4, wherein the content of Mg in the plating film is 1.0 to 5.0% by mass.

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