Painted steel plate

Abstract

The present invention provides a coated steel sheet that exhibits excellent corrosion resistance, coating adhesion, and corrosion resistance at cut edges, even when it does not contain chromate compounds. [Solution] To achieve the above objective, the present invention provides a coated steel sheet comprising a zinc-plated steel sheet, a chemical conversion coating, and a coating film, wherein the coating film contains a thermosetting resin obtained by heat-curing a polyamide resin (A) having a dimer structure derived from at least one of dimer acid and dimer amine, and an epoxy resin (B), wherein the weight-average molecular weight of the polyamide resin (A) is 3000 to 20000, and the glass transition temperature (Tg The epoxy resin (A) has an acid value of 5 to 120°C, an acid value of 5 to 15 (mgKOH / g), an amine value of 0.2 to 0.7, an epoxy equivalent of 130 to 450, a content ratio of 70:30 to 95:5 between the polyamide resin (A) and the epoxy resin (B), a glass transition temperature (Tg) of 25 to 90°C, and a dynamic storage modulus of 1 × 10⁻¹⁰°C at the glass transition temperature + 100°C (Tg + 100°C). 7 Pa super, 2×10 8 It is less than Pa. It is characterized by the following:

Description

[Technical Field]

[0001] The present invention relates to a painted steel sheet that exhibits excellent corrosion resistance even when it does not contain chromate compounds, as well as excellent coating adhesion and corrosion resistance at cut edges. [Background technology]

[0002] Painted steel sheets, which are steel sheets plated with zinc or a zinc-containing alloy (hereinafter referred to as "zinc-plated steel sheets") and have a chemical conversion coating and a primer coating formed on their surface, and then various coatings formed on top of that, are widely used as exterior materials for building roofs and walls, interior materials for partitions, and components for electrical equipment products, etc., due to their many advantages such as stable quality and rationalization by eliminating the painting process at the customer's end. Given that such a wide range of applications are anticipated, and the operating environments will vary, high corrosion resistance is required, particularly from the perspective of extending maintenance intervals.

[0003] To meet these requirements, a known technique involves applying a chemical conversion treatment containing chromate to zinc-plated steel sheets, forming a primer coating containing chromate-based rust-preventive pigments, and then applying a topcoat of a thermosetting polyester resin coating, or, for those requiring higher weather resistance, a fluororesin coating, to the coated steel sheets. However, in recent years, the use of chromate, an environmentally harmful substance, has become a concern, and there is a strong demand for chromate-free painted steel sheets. As a result, many chromate-free painted steel sheets have been developed.

[0004] For example, Patent Document 1 discloses a painted galvanized steel sheet obtained by applying a paint composition containing a non-chromate rust-preventive pigment such as a molybdenum compound to a galvanized steel sheet via a chemical conversion treatment film. However, the applicable galvanized steel sheets were limited to those containing 94% or more zinc, and since chromate was applied as the chemical conversion coating, they could not be considered chromate-free painted steel sheets.

[0005] Furthermore, Patent Document 2 discloses a surface treatment agent (chemical conversion coating) consisting of a resin and a chromate-free rust-preventive pigment. Furthermore, Patent Document 3 discloses a painted zinc-plated steel sheet using a surface treatment agent (chemical conversion coating) consisting of a resin and a chromate-free rust-preventive pigment. Furthermore, Patent Document 4 discloses a technology aimed at improving the overall rust prevention performance of a painted zinc-plated steel sheet that has been treated with a chemical conversion coating and a primer coating, both containing specific non-chromate rust-preventive pigments. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2008-291162 [Patent Document 2] Japanese Patent Publication No. 2009-127057 [Patent Document 3] Japanese Patent Publication No. 2014-214315 [Patent Document 4] Japanese Patent Publication No. 2005-169765 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, all of the technologies described in Patent Documents 2 to 4 had the problem of not being able to obtain sufficient corrosion resistance, particularly corrosion resistance at the cut ends. Furthermore, with conventional painted steel sheets, there is a risk of peeling of the paint film formed on the chemical conversion coating or primer coating, and further improvement in paint film adhesion was desired.

[0008] In view of these circumstances, the present invention aims to provide a painted steel sheet that exhibits excellent corrosion resistance, coating adhesion, and corrosion resistance at cut edges, even when it does not contain chromate compounds. [Means for solving the problem]

[0009] The present inventors have investigated a painted steel sheet comprising a zinc-plated steel sheet, a chemical conversion coating that does not contain chromate compounds formed on at least one side of the zinc-plated steel sheet, and a top-surface coating that does not contain chromate compounds, formed directly or via an intermediate layer on the chemical conversion coating. This investigation aims to solve the above-mentioned problems. As a result, we found that by incorporating a thermosetting resin into the coating film, which is obtained by mixing a polyamide resin (A) having a dimer structure derived from at least one of dimer acid and dimer amine with an epoxy resin (B) in a specific range of ratios and then heat-curing it, and by specifying the glass transition temperature (Tg) and dynamic storage modulus in the high-temperature region of the formed coating film within a specific range, it is possible to maintain good corrosion resistance even without containing chromate compounds, and to improve the balance between the strength and flexibility of the coating film, thereby improving coating adhesion and corrosion resistance at cut edges.

[0010] This invention is based on the above findings, and its gist is as follows. 1. Zinc-plated steel sheet and A chemical conversion coating that does not contain a chromate compound is formed on at least one side of the zinc-plated steel sheet, A coating film, which is the outermost layer and does not contain a chromate compound, is formed directly or via an intermediate layer on the aforementioned chemical conversion treatment film. A painted steel sheet having the following features: The aforementioned chemical conversion coating contains C and O, and one or more elements selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. The coating film contains a thermosetting resin obtained by thermally curing a polyamide resin (A) having a dimer structure derived from at least one of dimer acid and dimer diamine and an epoxy resin (B). The polyamide resin (A) has a weight average molecular weight of 3,000 to 20,000, a glass transition temperature (Tg) of 5 to 120°C, an acid value of 5 to 15 (mgKOH / g), and an amine value (mgKOH / g) of 0.2 to 0.7. The epoxy resin (B) has an epoxy equivalent of 130 to 450. The content ratio of the polyamide resin (A) and the epoxy resin (B) is 70:30 to 95:5. The coating film has a glass transition temperature (Tg) of 25 to 90°C and a dynamic storage modulus at glass transition temperature + 100°C (Tg + 100°C) of more than 1×10 ,

[0014] , , Pa and less than 2×10 8 Pa. The coated steel sheet is characterized by the above.

[0011] 2. The coated steel sheet according to 1 above, further comprising a primer coating film as the intermediate layer between the chemical conversion coating film and the coating film. The primer coating film contains C and O and one or more selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F.

Effects of the Invention

[0012] According to the present invention, even when not containing a chromate-based compound, a coated steel sheet excellent in corrosion resistance, coating film adhesion, and corrosion resistance at the cut end can be provided.

Brief Description of the Drawings

[0013] [Figure 1] It is a diagram schematically showing an aspect of a sliding jig using SKD11 used for evaluating the post-sliding corrosion resistance of an example.

Modes for Carrying Out the Invention

[0014] The coated steel sheet of the present invention is a zinc-based plated steel sheet and A chemical conversion coating that does not contain a chromate compound is formed on at least one side of the zinc-plated steel sheet, A coating film, which is the outermost layer and does not contain a chromate compound, is formed directly or via an intermediate layer on the aforementioned chemical conversion treatment film. It is a painted steel sheet equipped with [a specific feature / feature].

[0015] (Zinc-plated steel sheet) The zinc-plated steel sheet mentioned above is not particularly limited as long as it contains zinc in the plating layer, but it is possible to use zinc-plated steel sheets such as hot-dip galvanized steel sheet (GI) or alloyed hot-dip galvanized steel sheet (GA) or electro-galvanized steel sheet (EG), Zn-Ni plated steel sheet, Zn-Al plated steel sheet (for example, Zn-5 mass%Al alloy plated steel sheet, Zn-5 mass%Al alloy plated steel sheet), Zn-Al-Mg plated steel sheet, etc. Among these, from the viewpoint of corrosion resistance, it is preferable to use Zn-Al-Mg plated steel sheets. It is more preferable that the composition contains 50-60% by mass of Al, 1-3% by mass of Si, and 0-6% by mass of optional additives, with the remainder being Zn and unavoidable impurities. By having the above-described composition in the plating layer of the molten Al-Zn plated steel sheet, a dendrite phase and an interdendrite phase surrounding the dendrite phase in a network-like structure can be formed in the plating layer, thereby improving corrosion resistance.

[0016] Here, the Al content in the plating layer of the Zn-Al-Mg plated steel sheet is 50-60% by mass, considering the balance between corrosion resistance and operational surface. If the Al content of the plating layer is at least 50% by mass, sufficient dendrite solidification of Al occurs. As a result, the plating layer mainly contains supersaturated Zn and consists of a portion where Al has dendrite-solidified (α-Al dendrite phase) and the remaining portion of the dendrite gaps (interdendrite phase), and the dendrite phase is stacked in the direction of the thickness of the plating layer, thereby realizing a structure with excellent corrosion resistance. Furthermore, the stable presence of an Al surface oxide film on the surface of the plating layer can enhance corrosion resistance. On the other hand, if the Al content in the plating layer exceeds 60% by mass, the amount of Zn, which has a sacrificial corrosion protection effect against Fe, decreases, and the corrosion resistance deteriorates. For this reason, the Al content in the plating layer should be 60% by mass or less. Also, from the above viewpoint, it is preferable that the Al content in the plating layer be about 55% by mass.

[0017] Furthermore, the Si in the plating layer of the Zn-Al-Mg plated steel sheet is added to the plating bath with the aim of improving workability and corrosion resistance by suppressing the growth of the interfacial alloy layer that forms at the interface with the substrate steel sheet. Since the interfacial alloy layer is hard and brittle, if it grows thick, it can become the starting point for crack formation during processing, so it is preferable to make it as thin as possible. In the case of a molten Al-Zn plated steel sheet, when molten plating is performed with Si contained in the plating bath, the substrate steel sheet is immersed in the plating bath, and at the same time, the Fe on the surface of the steel sheet and the Al and Si in the bath undergo an alloying reaction, producing an alloy consisting of Fe-Al and / or Fe-Al-Si compounds. The formation of this Fe-Al-Si interfacial alloy layer can suppress the growth of the interfacial alloy layer. When the Si content in the plating layer is 1% by mass or more, the growth of the interfacial alloy layer can be sufficiently suppressed. On the other hand, when the Si content in the plating layer exceeds 3% by mass, the workability of the plating layer decreases and the Si phase that becomes the cathode site is more likely to precipitate. The precipitation of this Si phase can be suppressed by increasing the Mg content, but this leads to increased manufacturing costs, reduced workability due to the increased amount of Mg2Si, and makes it more difficult to control the composition of the plating bath. For this reason, the Si content in the plating layer should be 3% by mass or less. From a similar viewpoint, the Si content in the plating layer is preferably 1 to 2% by mass.

[0018] Furthermore, in addition to the Al, Si, and Zn mentioned above, the plating layer may contain 0 to 6% by mass of any optional additive components. Here, the optional additives can be appropriately selected according to the performance required for the plating layer. Examples include alkaline earth metals such as Ca and Mg, and additives such as Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, W, Sn, and B. While these optional additives can improve the appearance of the plating and enhance corrosion resistance, they may also reduce the workability of the plating layer. Therefore, the content of these optional additives must be 6% by mass or less, and preferably 5% by mass or less.

[0019] Furthermore, in order to further enhance the corrosion resistance of the cut end in particular, it is preferable that the plating layer contains 1 to 5% by mass of Mg among the optional additive components. When the plating layer corrodes, Mg will be included in the corrosion products, improving the stability of the corrosion products deposited on the exposed steel surface of the cut end, delaying the progression of corrosion, and resulting in improved corrosion resistance of the cut surface. In this invention, since Mg is also included in the primer coating, this effect is even more pronounced. This is because the continuous supply of Mg from the primer improves the stability of the corrosion products and suppresses corrosion of the plating. By setting the Mg content to 1% by mass or more, a sufficient corrosion delay effect can be obtained, while by setting the Mg content to 5% by mass or less, the workability does not decrease significantly, and the corrosion resistance of the processed part can also be maintained at a high level. From a similar viewpoint, it is more preferable that the Mg content in the plating layer be 2 to 5% by mass, and even more preferable that be 3 to 5% by mass.

[0020] Furthermore, the aforementioned plating layer contains components of the underlying steel sheet that are incorporated into the plating layer through the reaction between the plating bath and the underlying steel sheet during the plating process, as well as unavoidable impurities in the plating bath. The underlying steel sheet components incorporated into the plating layer may include up to approximately 2% by mass of Fe. Examples of unavoidable impurities in the plating bath include Fe, Cu, and the like. It is not possible to distinguish between the Fe incorporated from the underlying steel sheet and the Fe present in the plating bath and quantify it in the aforementioned plating layer. While there are no particular limitations on the total content of unavoidable impurities, from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, it is preferable that the total amount of unavoidable impurities excluding Fe be 1% by mass or less.

[0021] The means for forming the zinc-based plating layer on the base steel sheet are not particularly limited, and a conventional continuous hot-dip galvanizing system can be used. For example, the base steel sheet is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere, and while annealing is performed, rolling oil and other substances adhering to the surface of the steel sheet are removed, and the oxide film is reduced and removed. Then, the steel sheet is immersed in a hot-dip galvanizing bath containing a predetermined concentration of optional additives such as Al, Zn, Si, and Mg, passing through a snout whose lower end is immersed in the plating bath. After that, the steel sheet immersed in the plating bath is pulled up to the top of the plating bath via a sink roll, and the amount of plating deposited is adjusted by spraying pressurized gas onto the surface of the steel sheet from a gas wiping nozzle placed on the plating bath. Subsequently, the plating layer is formed by cooling with a cooling device.

[0022] (Chemical conversion film) The painted steel sheet of the present invention has a chemical conversion coating that does not contain a chromate compound formed on at least one side of the zinc-plated steel sheet. By forming the chemical conversion coating on the zinc-plated steel sheet, the adhesion to the primer coating or the coating can be improved, and the corrosion resistance of the painted steel sheet can be further enhanced.

[0023] Furthermore, the chemical conversion coating contains C and O, and one or more elements selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. The presence of these elements in the aforementioned chemical conversion coating makes it possible to suppress the occurrence of white rust and blistering of the coating, particularly in areas where zinc is exposed, such as cut edges and bent parts, thus improving corrosion resistance.

[0024] Furthermore, the chemical conversion coating is preferably a composite of a resin component and an inorganic component. Examples of the resin component include anionic urethane resin, epoxy resin having a bisphenol skeleton, acrylic resin, urethane resin, polyester resin, phenolic resin, etc. The anionic urethane resin having an ester bond has the effect of making the chemical conversion coating less likely to break (peel off) when processed due to its flexibility, and the epoxy resin has the effect of improving adhesion to the underlying zinc-plated steel sheet and the upper primer coating. These resins may be water-soluble resins, or they may be resins that are originally water-insoluble but can be finely dispersed in water like an emulsion or suspension (water-dispersible resins). Furthermore, it is preferable that the resin component be present in the chemical conversion coating in a total of 30 to 50% by mass. If the amount is less than 30% by mass, the binder effect of the chemical conversion coating will decrease, and if it exceeds 50% by mass, the function of the inorganic component described later, such as the inhibitory effect, may decrease.

[0025] As the anionic urethane resin having ester bonds, a resin obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups, and copolymerizing it with dimethylol alkyl acid, can be used. The chemical treatment solution can be obtained by dispersing it in a liquid such as water by a known method.

[0026] Examples of the polyester polyol include polyesters obtained by a dehydration condensation reaction from 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, and alicyclic polyisocyanates. Examples of the aromatic polyisocyanate include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, m-xylene diisocyanate, diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate, 2,2-diphenylmethane diisocyanate, triphenylmethane triisocyanate, polymethylene polyphenyl polyisocyanate, naphthalene diisocyanate, and derivatives thereof (e.g., prepolymers obtained by reaction with polyols, modified polyisocyanates such as carbodiimide compounds of diphenylmethane diisocyanate, etc.). Specifically, examples include mixtures in which a portion of diphenylmethane diisocyanate is converted to carbodiimide. When synthesizing urethane by reacting these polyester polyols with diisocyanate or polyisocyanate, copolymerizing with dimethylol alkyl acid and self-emulsifying to make it water-soluble (water-dispersible) can be used to obtain an anionic urethane resin having ester bonds for use in the present invention.

[0027] The epoxy resin having the bisphenol skeleton can be a known resin, and a chemical treatment solution can be obtained by dispersing it in a known liquid such as water.

[0028] Furthermore, the blending ratio of the anionic urethane resin having ester bonds and the epoxy resin having a bisphenol skeleton is preferably within the range of 3:97 to 60:40 by mass, when the total resin components are considered as 100. Outside this range, there is a risk of reduced adhesion due to a decrease in the flexibility of the chemical conversion treated film.

[0029] The inorganic compound in the chemical conversion coating contains one or more selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. For example, the inorganic compound may include vanadium oxide, zirconium oxide, fluorine compounds, phosphoric acid compounds, etc. By including these inorganic compounds, the corrosion resistance, strength, processability, etc., of the chemical conversion coating can be improved.

[0030] The vanadium oxide acts as a rust inhibitor in the chemical conversion treatment film. Examples of vanadium compounds added to the chemical conversion treatment solution to generate vanadium oxide in the chemical conversion treatment film include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. Particularly desirable are pentavalent vanadium compounds, or trivalent or tetravalent vanadium compounds obtained by reducing pentavalent vanadium compounds.

[0031] The vanadium oxide content in the chemical conversion coating is not particularly limited, but is preferably 2 to 10% by mass. If the vanadium oxide content is less than 2% by mass, the inhibitor effect will decrease, leading to a decrease in corrosion resistance, and if the vanadium oxide content exceeds 10% by mass, it may lead to a decrease in the moisture resistance of the chemical conversion coating. From a similar viewpoint, the vanadium oxide content in the chemical conversion coating is more preferably 4 to 10% by mass, and even more preferably 6 to 10% by mass.

[0032] The zirconium oxide forms a dense film, thereby increasing the strength and corrosion resistance of the chemical conversion treatment film, as well as improving adhesion to the plating layer, and contributing to improved coverage and barrier effect. Examples of zirconium compounds added to the chemical conversion treatment solution to generate zirconium oxide in the chemical conversion treatment film include neutralized salts of zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, zirconium chloride, etc.

[0033] The zirconium oxide content in the chemical conversion coating is not particularly limited, but is preferably 40 to 60% by mass. If the zirconium oxide content is less than 40% by mass, it will lead to a decrease in the strength and corrosion resistance of the chemical conversion coating, and if the zirconium oxide content exceeds 60% by mass, the chemical conversion coating will become brittle, and there is a risk of the chemical conversion coating breaking or peeling when subjected to severe processing. From a similar viewpoint, the zirconium oxide content in the chemical conversion coating is more preferably 38 to 54% by mass, and more preferably 38 to 50% by mass.

[0034] The fluorine compound is added to the chemical conversion treatment solution and acts as an adhesion agent to the molten Al-Zn plated steel sheet. Examples of the fluorine compound include fluoride salts such as ammonium salts, sodium salts, and potassium salts, or fluorine compounds such as ferrous fluoride and ferric fluoride. In particular, the use of ammonium fluoride, sodium fluoride, and potassium fluoride is preferred. Note that the fluorine compound added to the chemical conversion treatment solution may decompose or react with other compounds due to drying or over time, and may exist in the chemical conversion treatment film as fluorine ions or as fluorine compounds different from those added to the chemical conversion treatment solution. The content of the fluorine compound in the chemical conversion coating is not particularly limited, but it is preferably 0.5 to 5% by mass as fluorine atoms. If the content is less than 0.5% by mass, sufficient adhesion to the processed area cannot be obtained, and if the content of the fluorine compound exceeds 5% by mass, the moisture resistance of the chemical conversion coating may decrease. From a similar viewpoint, it is more preferable that the content of the fluorine compound in the chemical conversion coating is 0.7 to 3% by mass.

[0035] Furthermore, the preferred amount of the chemical conversion coating is 0.025 to 0.5 g / m². 2 The amount of the chemical conversion treatment film attached is 0.025 g / m². 2If the amount is less than 0.5 g / m², there is a risk of reduced adhesion between the underlying hot-dip Al-Zn plated steel sheet and the upper primer coating, as well as a decrease in corrosion resistance. 2 If this amount is exceeded, the chemical conversion coating may be more prone to damage (peeling) when subjected to severe bending, potentially reducing the corrosion resistance of the processed area. From a similar viewpoint, the amount of the chemical conversion coating should be 0.1 to 0.3 g / m². 2 It is preferable that it be so.

[0036] The chemical conversion coating is obtained by continuously applying a chemical conversion solution to the zinc-plated steel sheet using a roll coater or the like, and then drying it at a peak metal temperature (PMT) of approximately 60 to 200°C using hot air or induction heating. These chemical conversion coatings may be single-layer or multi-layer; in the case of multi-layer coatings, multiple chemical conversion treatments can be performed sequentially.

[0037] (Primer coating) The painted steel sheet of the present invention preferably further comprises a primer coating film that does not contain a chromate compound, formed on the chemical conversion treatment film. By forming a primer coating on the aforementioned chemical conversion coating, the adhesion between the chemical conversion coating and the topcoat coating described later can be further enhanced, thereby improving corrosion resistance and rust prevention.

[0038] Furthermore, the primer coating contains C and O, and one or more elements selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. The inclusion of these elements in the primer coating provides corrosion resistance, particularly in areas where zinc is exposed, such as cut edges and bent sections, suppressing the occurrence of white rust and blistering of the coating.

[0039] Furthermore, the primer coating preferably contains a resin component. Examples of the resin component include polyester resin, acrylic resin, epoxy resin, urethane resin, and fluororesin. These resins can also be crosslinked with butylated melamine resin, methylated melamine resin, butylmethyl mixed melamine resin, urea resin, isocyanate resin, or a crosslinking agent component of a mixture thereof. In addition, various electron beam curable resins and ultraviolet curable resins can also be used as the resin. This is because they can achieve excellent corrosion resistance and edge corrosion resistance. These resins may be used individually or in mixtures of two or more types. Among the resins mentioned above, the primer coating preferably contains at least one of polyester resin, epoxy resin, acrylic resin, and urethane resin.

[0040] The inorganic compound in the primer coating film includes one or more selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. For example, the inorganic compound can include vanadium oxide, phosphate compounds, and magnesium oxide. By including these inorganic compounds, the corrosion resistance, strength, and processability of the primer coating film can be improved.

[0041] The vanadium compound contained as the inorganic compound acts as an inhibitor. Examples of the vanadium compound include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. Among these, it is particularly preferable to contain a pentavalent vanadium compound, or a trivalent or tetravalent vanadium compound obtained by reducing a pentavalent vanadium compound.

[0042] The vanadium compound contained in the primer coating may be the same as or different from the vanadium compound contained in the chemical conversion coating. The vanadate compound is thought to react with vanadate ions that gradually dissolve in response to moisture entering from the outside, and with ions on the surface of the aluminum-zinc alloy plated steel sheet, forming a highly adhesive passive film that protects the exposed metal and exhibits a rust-preventive effect. This effect is particularly pronounced as the aluminum content in the plating layer increases, so using an Al-Zn plated steel sheet with a high aluminum content as a base coat can produce a very excellent effect.

[0043] The vanadium compound content in the primer coating is 4 to 20% by mass. If the vanadium compound content in the primer coating is less than 4% by mass, the inhibitory effect decreases, leading to a decrease in corrosion resistance, and if it exceeds 20% by mass, it leads to a decrease in the moisture resistance of the primer coating. From a similar viewpoint, the vanadium compound content in the primer coating is preferably 6 to 16% by mass.

[0044] The phosphate compounds contained as the inorganic compounds also act as inhibitors. Examples of suitable phosphate compounds include ammonium phosphate, alkali metal phosphate, and alkaline earth metal phosphate. Among these, alkali metal phosphates such as calcium phosphate are particularly preferred. It is believed that these phosphate compounds form stable corrosion products with zinc and aluminum ions leached from the aluminum-zinc alloy plated steel sheet by corrosion, thereby protecting the exposed metal and suppressing corrosion.

[0045] The content of the phosphate compound in the primer coating is 4 to 20% by mass. If the content of the phosphate compound in the primer coating is less than 4% by mass, the inhibitor effect decreases, leading to a decrease in corrosion resistance, and if it exceeds 20% by mass, it leads to a decrease in the moisture resistance of the primer coating. From a similar viewpoint, it is preferable that the content of the phosphate compound in the primer coating is 6 to 16% by mass.

[0046] The magnesium oxide contained as the inorganic compound has the effect of stabilizing corrosion products generated by initial corrosion as sparingly soluble magnesium salts. Furthermore, the amount of magnesium oxide added to the primer coating is 4 to 20% by mass. If the amount of magnesium oxide added to the primer coating is less than 4% by mass, the above effect will decrease, leading to a decrease in corrosion resistance, and if it exceeds 20% by mass, the flexibility of the primer coating will decrease, which will particularly reduce the corrosion resistance of the processed area. From a similar viewpoint, it is preferable that the magnesium oxide content in the primer coating is 4 to 10% by mass.

[0047] Furthermore, the preferred thickness of the primer coating is 1.5 μm or more. If it is thinner than this, it will lead to a decrease in corrosion resistance and a decrease in adhesion with the chemical conversion coating and the topcoat coating. From the same viewpoint, it is more preferable that the thickness of the primer coating is 3 μm or more. However, if the thickness of the primer coating is too thick, it may lead to a decrease in manufacturing cost and processability, so it is preferable to keep it at 12 μm or less.

[0048] The resin composition for the primer coating film may, if necessary, contain various known components commonly used in the paint industry. Examples of known components include various surface modifiers such as leveling agents and defoaming agents, various additives such as dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as coloring pigments, extender pigments, and heat-shielding pigments, glossing agents, curing catalysts, and organic solvents.

[0049] There are no particular restrictions on the method of applying the paint composition for forming the primer coating, but it is preferable to apply the paint composition by methods such as roll coater coating or curtain flow coating. After applying the paint composition, the primer coating is obtained by baking it using heating means such as hot air heating, infrared heating, or induction heating. The baking process is usually carried out for about 30 seconds to 3 minutes at a maximum plate temperature of about 180 to 270°C.

[0050] (coating film) The painted steel sheet of the present invention further comprises a top-surface coating film that does not contain chromate compounds, formed directly or via an intermediate layer (such as a primer coating) on ​​the above-mentioned chemical conversion treatment film. By forming the aforementioned coating film, an aesthetic appearance can be enhanced, and various properties such as processability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance can be improved.

[0051] The coating film comprises a thermosetting resin obtained by heat-curing a polyamide resin (A) having a dimer structure derived from at least one of a dimer acid and a dimer amine, and an epoxy resin (B). The polyamide resin (A) has a weight-average molecular weight of 3000 to 20000, a glass transition temperature (Tg) of 5 to 120°C, an acid value of 5 to 15 (mgKOH / g), and an amine value (mgKOH / g) of 0.2 to 0.7. The epoxy resin (B) has an epoxy equivalent of 130 to 450, and the content ratio of the polyamide resin (A) to the epoxy resin (B) is 70:30 to 95:5.

[0052] • Polyamide resin (A) The polyamide resin (A) is a polymer containing an amide group, and is a resin having a dimer structure derived from at least one of a dimer acid and a dimer amine. The polyamide resin (A) is a polymer of a polybasic acid compound, a polyamine compound, and optionally other monomers. The introduction of a dimer structure into the polyamide resin (A) can be achieved by using a monomer having a dimer structure, and dimer acid, which is a polybasic acid compound, and / or dimer amine, which is a polyamine compound, are preferably used as such monomers.

[0053] The total amount of dimer acid and dimer amine in 100% by mass of the total monomers used in the polymerization of the polyamide resin (A) can be 50 to 100% by mass. Since the amount of monomers used in the polymerization of the polyamide resin (A) substantially matches the proportion of constituent components derived from the monomers of the polyamide resin (A), the effect of the dimer structure can be fully exhibited by setting the amount of monomers to 50 to 100% by mass. The total content of dimer acid and dimer amine is preferably 60 to 95% by mass, and more preferably 70 to 90% by mass. The content of the dimer structure can be determined in the case of a polymer from the content (mass%) of raw material monomers having a dimer structure out of the total 100% by mass of all raw material monomers used when synthesizing the polyamide resin (A). In the case of a modified product, it can be determined by considering a hypothetical monomer having the modified structure, and using the raw material monomers for the unmodified monomer, from the content (mass%) of raw material monomers having a dimer structure out of the total 100% by mass of monomers. The content of the hypothetical monomer having the modified structure can be determined by considering the reaction rate with respect to the polymer. For example, when modifying the side groups derived from monomer a after obtaining a polymer, the amount of monomer (mass) X, determined by "amount of monomer a charged (moles) × modification rate of the side groups / 100 × molecular weight of a hypothetical monomer having the structure after the side groups have been modified", and the amount of monomer (mass) Y, determined by "amount of monomer a charged (moles) × (1 - modification rate of the side groups / 100) × molecular weight of monomer a", can be used to determine the content of the dimer structure for other monomers in the same way as the polymer method described above.

[0054] Here, the dimer structure is a structure having hydrocarbon chains or ring structures, and is less polar compared to the epoxy resins that are blended. By melting the resin to a solid state, the polyamide resin (A) and epoxy resin (B) readily undergo phase separation at the micron level, forming a microphase separation structure. The dimer structure of the polyamide resin (A) acts as a flexible component, while the hydrogen bonds derived from the amide bonds of the polyamide resin (A) and the parts that readily mix with the epoxy resin (B) act as restrictive components. It is believed that a microphase separation structure is formed by combining such a polyamide resin (A) with the rigid epoxy resin (B) and going through a thermal melting process. By using a resin component that readily forms a microphase separation structure through melt molding as the resin component of a curable composition, it is believed that the thermal cycle resistance and flexural strength of the cured product of this curable composition can be improved. Furthermore, the cured product has strong hydrogen bonds derived from the amide bonds of the polyamide resin (A). As a result, flexural strength can be increased, and the decrease in adhesive strength and moist heat resistance after repeated high-temperature and low-temperature cycles can be effectively suppressed. As a result, the adhesion of the coating film can be improved, and the corrosion resistance of the cut edges of the painted steel sheet can be enhanced.

[0055] Furthermore, it is preferable that the polyamide resin (A) has functional groups that can be crosslinked with the epoxy groups of the epoxy resin (B) by heat. Examples of such functional groups include carboxyl groups, amino groups, and hydroxyl groups. These may be functional groups derived from the monomer of the polyamide resin (A), or they may be introduced as a modified product after the polymer has been obtained. Functional groups can be located at the ends of the polymer, as well as on side groups and / or side chains. As a preferred example, a polymer may have functional groups such as carboxyl groups or amino groups at its ends. Alternatively, a polymer may have at least one of functional groups such as carboxyl groups, amino groups, or hydroxyl groups on its side groups or side chains. Furthermore, if the functional group has a photopolymerizable group, the curable composition containing the polyamide resin may become over-crosslinked, resulting in reduced adhesion, or a thermal radical reaction may occur when the curable composition is heated and melted for molding, resulting in reduced moldability. Therefore, it is preferable that the functional group does not have a photopolymerizable group.

[0056] Furthermore, if the polyamide resin (A) has hydroxyl groups, phenolic hydroxyl groups are preferred. By having phenolic hydroxyl groups, a crosslinked structure with the epoxy resin (B) can be formed, and a cured product with excellent durability can be obtained. Phenolic hydroxyl groups can be easily introduced by using a polybasic acid compound having phenolic hydroxyl groups and / or a polyamine compound having phenolic hydroxyl groups. It is preferable that the aromatic ring of this phenolic hydroxyl group is included in the main chain skeleton of the polyamide resin (A). Also, from the viewpoint of durability, it is preferable to use a polybasic acid compound having phenolic hydroxyl groups as the monomer of the polyamide resin (A).

[0057] The polyamide resin (A) may be a polyamide-imide having some imide groups, or a polyamide-ester having some ester groups, without departing from the spirit of the present invention.

[0058] The polybasic acid compound is a carboxylic acid with two or more basic acids. The polybasic acid compound may also be partially an acid anhydride. Examples of polybasic acid compounds include dimer acids and other polybasic acid compounds other than dimer acids. Polycarboxylic acids are typically used, but monovalent fatty acids can be used in combination as needed. Examples of polycarboxylic acids include phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, 4-methylhexahydrophthalic acid, bicyclo[2,2,1]heptane-2,3-dicarboxylic acid, trimellitic acid, adipic acid, sebacic acid, succinic acid, azelaic acid, fumaric acid, maleic acid, itaconic acid, pyromellitic acid, dimer acid, and their acid anhydrides, as well as 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, and hexahydroterephthalic acid. These polybasic acids can be used alone or in combination of two or more.

[0059] The aforementioned dimer acid is a polybasic acid compound having a dimer structure, and is a fatty acid dimer (hereinafter referred to as a fatty acid dimer). The fatty acid dimer is preferably a compound having 20 to 60 carbon atoms, more preferably a compound having 24 to 56 carbon atoms, even more preferably a compound having 28 to 48 carbon atoms, and particularly preferably a compound having 36 to 44 carbon atoms. The fatty acid dimer is preferably a dicarboxylic acid compound having a branched structure obtained by a Diels-Alder reaction of a fatty acid. The branched structure is preferably one that includes a fatty chain or a fatty chain and a ring structure, and more preferably one that includes a fatty chain and a ring structure. The ring structure is preferably one or more aromatic rings or alicyclic structures, and more preferably an alicyclic structure. The alicyclic structure may have one double bond in the ring, or it may not have a double bond, etc.

[0060] Examples of polybasic acid compounds having the dimer structure include the structures shown in the following chemical formulas (1) to (4). However, polybasic acid compounds having a dimer structure are not limited to the structures shown below.

[0061] [ka] [ka] [ka] [ka]

[0062] The fatty acid is preferably an unsaturated fatty acid having 10 to 30 carbon atoms, and more preferably an unsaturated fatty acid having 10 to 24 carbon atoms. The unsaturated fatty acid has one or more carbon double bonds or carbon triple bonds. Examples of the fatty acid include natural fatty acids such as soybean oil fatty acid, tall oil fatty acid, and rapeseed oil fatty acid, as well as oleic acid, linoleic acid, linolenic acid, and erucic acid obtained by refining these. When synthesizing the aforementioned fatty acid dimers, in addition to fatty acid dimers, fatty acid trimers and, in some cases, tetramers are also produced. Therefore, polybasic acid compounds containing a dimer skeleton are mixtures that include not only the main component fatty acid dimer, but also fatty acid trimers and, in some cases, the raw fatty acids.

[0063] Because the aforementioned dimer acid uses unsaturated fatty acids as raw materials, unsaturated bonds may remain. In such cases, hydrogenation (also called a hydrogenation reaction) can be performed to suppress the number of unsaturated bonds. This improves the reaction stability when synthesizing polyamide resin (A), and further improves the high-temperature resistance of the cured product of the curable composition containing polyamide resin (A). Dimer acids can be used alone or in combination of two or more types.

[0064] Examples of commercially available dimer acids include "Prepol 1004," "Prepol 1006," "Prepol 1009," "Prepol 1013," "Prepol 1015," "Prepol 1017," "Prepol 1022," "Prepol 1025," and "Prepol 1040" from Croda Japan; and "Empole 1008," "Empole 1012," "Empole 1016," "Empole 1026," "Empole 1028," "Empole 1043," "Empole 1061," and "Empole 1062" from Henkel Japan. Among these, "Prepol 1009," which has 36 carbon atoms, is preferred.

[0065] Other polybasic acid compounds include polybasic acid compounds other than dimer acids that are bifunctional or multifunctional compounds. Polybasic acid compounds can be used alone or in combination of two or more types.

[0066] Examples of dibasic acid compounds include aromatic dibasic acids such as phthalic acid, isophthalic acid, terephthalic acid, 1,4-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, benzophenone-4,4'-dicarboxylic acid, and 4,4'-biphenyldicarboxylic acid; aliphatic dibasic acids such as oxalic acid, malonic acid, methylmalonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, malic acid, tartaric acid, thiomalic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanediic acid, hexadecanedionic acid, and diglycolic acid; and alicyclic dibasic acids such as 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and 1,3-cyclopentanedicarboxylic acid. Among these, isophthalic acid and 1,4-cyclohexanedicarboxylic acid are preferred as dibasic acid compounds.

[0067] Examples of polybasic acid compounds with three or more functions include trimellitic acid, hydrogenated trimellitic acid, pyromellitic acid, hydrogenated pyromellitic acid, trimesic acid, and 1,4,5,8-naphthalenetetracarboxylic acid. By using polybasic acid compounds with three or more functions, a branched structure can be introduced into the polyamide resin (A), thereby improving the cohesive strength of the cured product and enhancing its thermal cycling properties and dimensional stability.

[0068] Other suitable examples of polybasic acid compounds include polybasic acid compounds having a phenolic hydroxyl group. Polybasic acid compounds having a phenolic hydroxyl group are compounds that, like phenol, have a hydroxyl group (also called a phenolic hydroxyl group) directly bonded to an aromatic ring, and have two or more acidic functional groups. Examples of acidic functional groups include carboxyl groups.

[0069] The polyamine compound is a compound having two or more amino groups. Preferred examples of polyamine compounds include dimer amines and other polyamine compounds.

[0070] The aforementioned dimer amine is a compound having two amino groups with a dimer structure, and a compound obtained by converting the carboxyl group of the aforementioned dimer acid to an amino group can be used. Examples of conversion methods include amidation of the carboxylic acid, amination by Hoffmann rearrangement, and further distillation and purification. The dimer amine is preferably a compound having 20 to 60 carbon atoms, and more preferably a compound having 24 to 56 carbon atoms.

[0071] Examples of commercially available dimer amines include "Priamine 1071," "Priamine 1073," "Priamine 1074," and "Priamine 1075" manufactured by Croda Japan Co., Ltd. Dimer amines can be used alone or in combination of two or more types.

[0072] Other polyamine compounds include polyamine compounds other than dimer amines, such as diamine compounds and polyamine compounds with three or more functionalities.

[0073] The diamine compounds include, for example, 1,4-diaminobenzene, 1,3-diaminobenzene, 1,2-diaminobenzene, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 2,3-diaminonaphthalene, 2,6-diaminotoluene, 2,4-diaminotoluene, 3,4-diaminotoluene, 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 4,4'-diamino-1,2-diphenylethane, 3,3'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 4,4'-diaminobenzophenone, and 4,4'-diamino Examples include aromatic diamines such as diphenylsulfone, 3,3'-diaminobenzophenone, and 3,3'-diaminodiphenylsulfone; aliphatic diamines such as ethylenediamine, 1,3-propanediamine, 1,4-butanediamine, 1,6-hexanediamine, 1,7-heptanediamine, 1,9-nonanediamine, 1,12-dodecamethylenediamine, and metaxylenediamine; and alicyclic diamines such as isophoronediamine, norbornanediamine, 1,2-cyclohexanediamine, 1,3-cyclohexanediamine, 1,4-cyclohexanediamine, 4,4'-diaminodicyclohexylmethane, and piperazine.

[0074] The polyamide resin (A) can be synthesized by, for example, melt polymerization, interfacial polymerization, solution polymerization, bulk polymerization, and solid-phase polymerization, as well as combinations thereof. Among these, solution polymerization is preferred. The polymer of the polyamide resin (A) can be produced using the polybasic acid compound, polyamine compound, and other monomers as needed, with or without a catalyst. For example, a predetermined amount of dimer acid, other acid monomers, dimer amine, other amine monomers, and deionized water is charged into a nitrogen-filled flask and heated and stirred at 20 to 100°C to uniformly dissolve or disperse the mixture. Then, the temperature is gradually raised to 230°C while removing the deionized water and water produced by the reaction. Once 230°C is reached, the pressure is reduced to about 15 mmHg, and this state is maintained for about 1 hour to obtain the polyamide resin (A). The heating temperature can be, for example, 150 to 300°C, and the heating time can be about 1 to 24 hours. To accelerate the synthesis reaction, it is preferable to perform a dehydration or dealcoholization reaction, and to avoid discoloration and decomposition reactions at high temperatures, it is preferable to carry out the reaction under reduced pressure at 180 to 270°C.

[0075] In addition to the polyamines mentioned above, monoamines may also be used as other monomers. Monoamines act as reaction stoppers, making it easier to adjust the molecular weight of the polyamide resin (A). Furthermore, since some of the main chain ends of the polyamide resin (A) are not reactive functional groups, the stability over time is improved. Examples of monoamines include aniline, benzylamine, 4-aminophenol, and 2-ethylhexylamine.

[0076] Furthermore, the glass transition temperature (Tg) of the polyamide resin (A) is preferably 5 to 120°C. If the glass transition temperature (Tg) is less than 5°C, the coating may soften and become easily scratched, and if it exceeds 120°C, the coating may harden, causing cracks to occur in the coating during bending, which may result in a decrease in corrosion resistance. Here, the Tg of the polyamide resin (A) is the temperature at which the value obtained by dividing the viscosity term by the elastic term (tanδ), measured by a dynamic viscoelasticity measuring device, shows a maximum.

[0077] Furthermore, the weight-average molecular weight of the polyamide resin (A) is preferably 3,000 to 20,000. This is because a molecular weight of 3,000 or higher has a greater effect in suppressing blistering of the coating film. However, if the weight-average molecular weight exceeds 20,000, the adhesion of the coating film may decrease.

[0078] Furthermore, the acid value of the polyamide resin (A) is 5 to 15 mgKOH / g. This is because when the polyamide resin (A) is cured (formed as a coating), an appropriate crosslinking density is obtained, improving adhesion to the plated surface and corrosion resistance. If the acid value is less than 5 mgKOH / g, the adhesion of the coating may decrease, and if it exceeds 15 mgKOH / g, corrosion resistance may decrease due to excessive hydrophilization. From a similar viewpoint, the acid value of the polyamide resin (A) is preferably 7 to 11 mgKOH / g.

[0079] Furthermore, the amine value of the polyamide resin (A) is preferably 0.2 to 0.7 mgKOH / g. This is from the viewpoint of production efficiency and resin stability. If the amine value is less than 0.2 mgKOH / g, the reactivity with the epoxy resin will decrease, the curing time will be prolonged, and production efficiency may decrease. If it is greater than 0.7 mgKOH / g, the stability of the amide resin will decrease, and the resin may aggregate. The amine value of the polyamide resin (A) is preferably 0.4 to 0.6 mgKOH / g.

[0080] • Epoxy resin (B) The epoxy resin (B) is a thermosetting resin that can be heat-cured by having epoxy groups. If the epoxy resin (B) itself has reactive functional groups such as hydroxyl groups, the epoxy resin (B) alone can form a crosslinked structure. In addition to or instead of single crosslinking, it is also preferable to thermally crosslink the polyamide resin (A) and the epoxy resin (B). The three-dimensional crosslinked structure formed by thermal crosslinking of the polyamide resin (A) and the epoxy resin (B) provides excellent adhesion and even better corrosion resistance.

[0081] Furthermore, in the present invention, the content ratio of the polyamide resin (A) to the epoxy resin (B) is (A):(B) = 70:30 to 95:5 in the paint composition that forms the coating film. By including the polyamide resin (A) and the epoxy resin (B) within this range, excellent erosion resistance under repeated sliding is achieved. From a similar viewpoint, the content ratio of the polyamide resin (A) to the epoxy resin (B) is preferably 40:60 to 60:40.

[0082] Furthermore, the epoxy equivalent (grams of resin containing epoxy groups per gram equivalent) of the epoxy resin (B) is preferably 130 to 450 g / eq. This is because excellent corrosion resistance can be obtained. If the epoxy equivalent of the epoxy resin (B) exceeds 450 g / eq., the resin becomes hard, and cracks are more likely to occur in the coating film. If it is less than 130 g / eq., the adhesion to the plating is poor, and blistering is more likely to occur in the coating film.

[0083] Examples of epoxy resins (B) include bisphenol-type epoxy resins such as bisphenol A-type epoxy resin, bisphenol F-type epoxy resin, and bisphenol S-type epoxy resin; novolac-type epoxy resins such as o-cresol novolac-type epoxy resin; biphenyl-type epoxy resin, naphthalene-type epoxy resin, naphthalene-containing novolac-type epoxy resin, dicyclopentadiene-type epoxy resin, phenol aralkyl-type epoxy resin, trisphenolmethane-type epoxy resin, and phenol-modified xylene resin-type epoxy resin. Note that epoxy resin (B) may be used alone or in combination of two or more types.

[0084] The thermosetting temperature between the polyamide resin (A) and the epoxy resin (B) is preferably 200 to 300°C.

[0085] Furthermore, the coating film may contain, depending on the purpose and application, various coloring pigments such as titanium dioxide, composite oxides, red iron oxide, carbon black, and other pigments; metallic pigments such as aluminum powder and coated mica; heat-shielding pigments; extender pigments such as mica, carbonates, and sulfates; various fine particles such as silica fine particles, nylon resin beads, and acrylic resin beads; curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate; waxes; leveling agents; defoaming agents; dispersants; anti-settling agents; ultraviolet absorbers; light stabilizers; anti-staining agents; and other additives in appropriate amounts.

[0086] The aforementioned coating may contain a rust inhibitor, and examples of such rust inhibitors include vanadates, tungstates, silicates, and phosphates. Examples of vanadates include calcium vanadate, magnesium vanadate, ammonium metavanadate, potassium vanadate, sodium vanadate, ammonium vanadate, phosphorus vanadate, and vanadium oxide. Examples of the tungstate salts include sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, and magnesium tungstate. Examples of the aforementioned silicates include sodium silicate, potassium silicate, lithium silicate, and calcium ion-exchange silica. Examples of the aforementioned phosphates include sodium dihydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, aluminum tripolyphosphate, magnesium tripolyphosphate, sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate, and calcium hypophosphite. The content of the rust inhibitor in the aforementioned paint composition is not particularly limited, but it is preferably 10% by mass or more and less than 25% by mass.

[0087] Furthermore, the thickness of the coating film is not particularly limited and can be adjusted as appropriate according to the required performance. For example, from the perspective of obtaining better corrosion resistance at the edges without deteriorating productivity, the film thickness of the coating film is preferably 2 to 25 μm, and more preferably 3 to 18 μm. When the film thickness of the coating film is 2 μm or more, better corrosion resistance at the edges can be achieved. On the other hand, when it is 25 μm or less, it does not cause manufacturing complexity or an increase in manufacturing cost.

[0088] The coating method of the coating composition for forming the coating film is not particularly limited. For example, the coating composition that becomes the material of the coating film can be applied by methods such as roll coater coating and curtain flow coating. After applying the coating composition, baking can be performed by heating means such as hot air heating, infrared heating, and induction heating to form a coating film. The temperature of the baking treatment is usually set such that the maximum reachable plate temperature is about 180 to 270 °C, and this temperature range is maintained for about 30 seconds to 3 minutes.

[0089] In the present invention, the glass transition temperature (Tg) of the coating film is 25 to 90 °C, and the dynamic storage modulus at the glass transition temperature + 100 °C (Tg + 100 °C) is 1×10 7 Pa or more and less than 2×10 8 Pa. Since the balance between the strength and flexibility of the coating film can be enhanced, the coating film adhesion and the corrosion resistance of the cut edges can be improved.

[0090] The glass transition temperature (Tg) of the coating film is set to 25 to 90 °C because when it is less than 25 °C, the permeability of the corrosion factor increases, resulting in a decrease in corrosion resistance and an increased tendency for cracks to occur during processing. When it exceeds 90 °C, the coating film is hard and cracks are likely to occur during processing.

[0091] The dynamic storage modulus at the glass transition temperature + 100 °C (Tg + 100 °C) of the coating film is an index of the crosslink density of the cured coating film. When the value increases, it indicates that the molecular weight between crosslinks is small, that is, the crosslink density is high. The dynamic storage modulus at this Tg + 100 °C is 1×10 7 Pa or more and less than 2×10 8The reason for setting the value below Pa is that this range provides the best coating adhesion and corrosion resistance at the cut edges, and the dynamic storage modulus at Tg + 100°C is 2 × 10⁻⁶. 8 Above Pa, internal stress increases, making paint film peeling more likely, resulting in reduced corrosion resistance, and also 1 × 10 7 Below Pa, the coating's shielding properties decrease, making it more prone to blistering and other problems.

[0092] Furthermore, there are no particular limitations on the method for achieving the above-mentioned dynamic storage modulus at the glass transition temperature + 100°C (Tg + 100°C) of the coating film. For example, the dynamic storage modulus can be achieved within the above range by appropriately adjusting the materials constituting the coating film (ratio of polyamide resin to epoxy resin, added pigments, etc.) and the conditions during coating film formation (drying temperature, drying time, etc.). [Examples]

[0093] <Examples 1-10, Comparative Examples 1-8> Samples of painted steel sheets were manufactured according to the following conditions: (1) hot-dip galvanized steel sheet, (2) chemical conversion coating, (3) primer coating, and (4) coating.

[0094] (1) Zinc-plated steel sheet The following zinc-plated steel sheets were used. The type of plated steel sheet used for each sample is shown in Table 1. • Plated steel sheet 1: Thickness 0.35 mm, plating adhesion amount 80 g / m² per side 2 Hot-dip Al-Zn plated steel sheet (JIS G3321, AZ150) having a plating layer with a Zn-55%Al composition. • Plated steel sheet 2: Thickness 0.35 mm, plating adhesion amount 130 g / m² per side 2 Hot-dip Zn-Al plated steel sheet (JIS G3317, Y25) having a plating layer with a Zn-5%Al composition. • Plated steel sheet 3: Thickness 0.35 mm, plating adhesion amount 130 g / m per side 2 Hot-dip zinc-plated steel sheet (JIS G3312, Z25)

[0095] (2) Chemical conversion coating A chemical conversion treatment solution is applied to the surface of a zinc-plated steel sheet using a roll coater, and then dried in a hot air drying oven at a final plate temperature of 90°C for 60 seconds, resulting in various chemical conversion coatings shown in Table 1 at a density of 150 mg / m². 2 It was formed to achieve the desired amount of adhesion.

[0096] Furthermore, the resin components (resin components containing C and O) used in the chemical conversion coating were "Superflex 210" manufactured by Daiichi Kogyo Seiyaku Co., Ltd., an anionic urethane resin, and "Yukaresin RE-1050" manufactured by Yoshimura Yukagaku Co., Ltd., an epoxy resin with a bisphenol skeleton. Other resin components used were acrylic resin and polyester resin. The mixing ratio (mass ratio) of these resins was 50:50. Furthermore, the inorganic compounds contained in the chemical conversion coating are shown in Table 1. Zirconium ammonium carbonate was used as the zirconium compound, and aluminum phosphate was used as the phosphate compound. These raw materials were mixed to obtain a chemical conversion treatment solution. The pH of the chemical conversion treatment solution was set to 8-10. The content of each component in the chemical conversion treatment film is shown in Table 1.

[0097] (3) Primer coating As the resin component of the primer coating, a urethane-cured polyester resin containing 10% by mass of a vanadium compound, or a urethane-cured epoxy resin containing 10% by mass of a phosphoric acid compound was used. Furthermore, magnesium vanadate was used as the vanadium compound for the rust-preventive pigment, and calcium phosphate was used as the phosphate compound. Furthermore, isophorone diisocyanate was used as a curing agent for the urethane-cured polyester resin. After adding a solvent and a rust-preventive pigment to the above components, 0.3 parts of dibutyltin dilaurine (DBTDL) was added as a reaction catalyst and uniformly mixed to prepare a chromium-free primer coating. The types of resin components in each sample are shown in Table 1. The resulting primer coating was applied to the chemical conversion treated film using a roll coater, baked at a steel plate temperature of 220°C for 35 seconds, and weighed 7g / m². 2 A coating film was formed to achieve the desired adhesion amount.

[0098] (4) Coating The coating used was a paint containing the following components. (4-1) Polyamide resin The polyamides used were those described later (PA-1 to PA-8). The types of polyamides used are shown in Table 1. (4-2) Epoxy resin The following epoxy resins (EP-1 to EP-4) were used. The types of epoxy resins used are shown in Table 1. EP-1: DIC Corporation's "EPICLON HP4032D", epoxy equivalent 134 EP-2: DIC Corporation's "EPICLON 860", epoxy equivalent 212 EP-3: DIC Corporation's "EPICLON 153", epoxy equivalent 389 EP-4: DIC Corporation's "EPICLON 1055", epoxy equivalent 472 (4-3) Anti-corrosion pigments The following rust-preventive pigments were used. The types and contents of the rust-preventive pigments used are shown in Table 1. Zirconium compounds: Zirconium carbonate ammonium Examples of fluorine compounds include ammonium fluoride. Phosphate compounds: Aluminum phosphate, zinc phosphate, magnesium phosphate Silica: Manufactured by Nissan Chemical Corporation, "Snowtex® O-40" Ion exchange silica: NoelsonChem, "Noelson (registered trademark) NSC-400" (4-4) Coloring Pigments Carbon black and titanium dioxide were used as coloring pigments. The total content of the coloring pigments was adjusted to 10% by mass in the coating film. The coating paint was applied to the undercoat using a roll coater, baked at a steel plate temperature of 240°C for 45 seconds, and formed into a coating film with the thickness shown in Table 1 after baking.

[0099] (Polyamide PA-1 to PA-8) (1) Synthesis of polyamide PA-1 In a flask equipped with a stirrer, a reflux condenser with a Dean-Stark apparatus, a nitrogen inlet tube, and a thermometer, 90 g of "Oleochemical Dimer Acid" as a dibasic dimer acid of a C36 dicarboxylic acid, 600.0 g of "Priamine 1075" (Croda) as a C36 dimeramine, 230 g of 3,4'-diaminodiphenyl ether as another diamine, 30 g of adipic acid and 50 g of sebacic acid as polybasic acids were charged and stirred. Once the heat generation subsided, the reaction was gradually increased. While removing the water generated as the reaction progressed, the internal temperature was raised to 230°C, and the reaction was maintained at that temperature for 4 hours. Next, the reaction was completed by reducing the pressure to approximately 2 kPa and holding the mixture at the same temperature for 2 hours to obtain a polyamide resin (PA-1) with a weight-average molecular weight of 5800, an acid value of 9.0 mg KOH / g, an amine value of 0.6 mg KOH / g, and a Tg of 70°C. Note that the amounts of each component shown in Table 2 represent the amounts of solids.

[0100] (2) Synthesis of polyamides PA-2 to PA-8 Polyamide resins (PA-2 to PA-8) were obtained under the same conditions as PA-1, except for the blending of various components, as shown in Table 2.

[0101] (3) Measurement of acid value The acid value of the obtained polyamide resin was measured using the following procedure. The measurement results are shown in Table 2. Precisely weigh 10 g of the oil sample into a stoppered flask and dissolve it in 100 mL of ethanol-ether mixture. Add a few drops of phenolphthalein reagent as an indicator and titrate with 0.1 mol / L potassium hydroxide solution in ethanol until a pale pink color persists for 30 seconds. The acid value was calculated from the volume of 0.1 mol / L ethanol potassium hydroxide solution required for titration using the following formula. Acid value = a × F × 5.611 / Amount of oil / fat sample (g) a: Volume (mL) of 0.1 mol / L ethanol potassium hydroxide solution F: Potency of 0.1 mol / L ethanol potassium hydroxide solution

[0102] (4) Measurement of amine value The amine value of the obtained polyamide resin was measured using the following procedure. The measurement results are shown in Table 2. Precisely weigh approximately 1 g of the sample into a stoppered Erlenmeyer flask and dissolve it in 100 mL of cyclohexanone solvent. Add 2-3 drops of an indicator prepared by mixing 0.20 g of methyl orange dissolved in 50 mL of distilled water and 0.28 g of xylene cyanol FF dissolved in 50 mL of methanol, and hold for 30 seconds. Then, titrate with 0.1 N alcoholic hydrochloric acid solution until the solution turns bluish-gray. The amine value was determined by the following formula (unit: mgKOH / g). Amine value (mgKOH / g) = (5.611 × a × F) / S however, S: Sample volume (g) a: Amount of 0.1N alcoholic hydrochloric acid solution consumed (mL) F: Potency of 0.1N alcoholic hydrochloric acid solution

[0103] (5) Measurement of weight-average molecular weight (Mw) The weight-average molecular weight (Mw) of the obtained polyamide resin was measured using the following procedure. The measurement results are shown in Table 2. A GPC (gel permeation chromatography) system "GPC-101" manufactured by Showa Denko Corporation was used. GPC is a liquid chromatography system that separates and quantifies substances dissolved in a solvent (THF; tetrahydrofuran) based on differences in their molecular size. In this invention, measurements were performed using two "KF-805L" columns (Showa Denko Corporation: GPC column: 8mm ID × 300mm size) connected in series, under the conditions of a sample concentration of 1% by mass, a flow rate of 1.0 mL / min, a pressure of 3.8 MPa, and a column temperature of 40°C. The weight-average molecular weight (Mw) was determined in terms of polystyrene equivalent. For data analysis, calibration curves, molecular weight, and peak area were calculated using the manufacturer's built-in software. The weight-average molecular weight was determined for the analysis target range of retention times from 17.9 to 30.0 minutes.

[0104] (6) Measurement of glass transition temperature The glass transition temperature of the obtained polyamide resin was measured using the following procedure. The measurement results are shown in Table 2. A polyamide resin varnish was prepared by dissolving polyamide resin in cyclohexanone to achieve a non-volatile content of 35%. This varnish was applied to a heat-resistant release film using a 10 mil doctor blade and dried at 130°C for 10 minutes to obtain a 25 μm thick polyamide resin film, which was used as a sample for glass transition temperature measurement. The glass transition temperature was determined by measuring tanδ in the temperature range of -50 to 200°C using a dynamic viscoelasticity analyzer. Dynamic viscoelasticity measuring device: DVA-200 (manufactured by IT Measurement & Control Co., Ltd.) Heating rate: 10℃ / min Measurement frequency: 10Hz Grip length: 15mm Width: 5mm

[0105] <Rating> The following measurements and evaluations were performed on each sample of painted steel sheet obtained as described above. The results are shown in Table 1.

[0106] (1) Glass transition temperature of the coating film, and dynamic storage modulus at glass transition temperature + 100°C. The coating paint used in the production of each sample was applied to a release film using a bar coater, dried at a temperature of 180°C, and coated to a thickness of 20 μm to prepare a measurement film. For each measurement film, it was placed in a dynamic viscoelasticity measuring device (DMA device) and the loss tangent (tanδ) and dynamic storage modulus E' were measured for each temperature. Then, the peak of tanδ was defined as the glass transition temperature, and the value of the storage modulus at a temperature 100°C higher than the Tg point was read. The specifications for the DMA device are as follows: DMA device: DVA-200 (manufactured by IT Measurement & Control Co., Ltd.) Temperature range: -50 to 200°C Heating rate: 10℃ / min Measurement frequency: 10Hz Grip length: 15mm Width: 5mm

[0107] (2) Blistering of the paint film at the cut end Test specimens measuring 70 cm x 150 cm were prepared from each sample. Polyester tape was then attached to the back of each specimen, and a salt spray test in accordance with JIS Z2371 was conducted for 500 hours. Cutting was performed in the downward direction, and the five specimens with the largest coating blister widths at each cut were selected, and the average value was calculated. The evaluation was conducted according to the following criteria. Results from ◎ to ○ are considered passing. ◎: Blistering width of the coating is less than 1 mm. ○: Blistering width of the coating is 1 mm or more and less than 2 mm. △: Maximum width of occurrence is 2 mm or more, but less than 3 mm. ×: Maximum width of occurrence is 3 mm or more

[0108] (3) Adhesion of the coating film Test specimens measuring 70 cm × 150 cm were prepared from each sample. Subsequently, a grid test was performed according to the following procedures a) to d) in accordance with JIS G 3325. a) Cut a grid pattern into the coating of the test specimen using a cutter knife or similar tool, reaching the plated surface. b) The grid squares are spaced 1 mm apart, and 11 lines are drawn vertically and horizontally, intersecting at right angles. c) Immerse in boiling water for 2 hours, remove, air dry, attach cellophane tape to the painted surface, and then immediately peel it off. d) The presence or absence of peeling in the grid-like squares (1 mm x 1 mm) was visually confirmed, the number of squares was counted, and then the following criteria were used for evaluation. ○: No peeling in any of the squares. △: The number of detached squares is 10% or less. ×: The number of detached squares exceeds 10%.

[0109] [Table 1]

[0110] [Table 2]

[0111] The results in Table 1 show that a thermosetting coating film with an optimal dimer acid and epoxy resin that possesses a predetermined dynamic storage modulus exhibits excellent sliding resistance and corrosion resistance after sliding. [Industrial applicability]

[0112] According to the present invention, even when chromate compounds are not included, it is possible to provide a coated steel sheet that exhibits excellent corrosion resistance, coating adhesion, and corrosion resistance at the cut edges.

Claims

1. Zinc-plated steel sheet and A chemical conversion coating that does not contain a chromate compound is formed on at least one side of the zinc-plated steel sheet, A coating film, which is the outermost layer and does not contain a chromate compound, is formed directly or via an intermediate layer on the aforementioned chemical conversion treatment film. A painted steel sheet having the following features: The aforementioned chemical conversion coating contains C and O, and one or more elements selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. The coating film comprises a thermosetting resin obtained by thermosetting a polyamide resin (A) having a dimer structure derived from at least one of a dimer acid and a dimer amine, and an epoxy resin (B), wherein the polyamide resin (A) has a weight-average molecular weight of 3000 to 20000, a glass transition temperature (Tg) of 5 to 120°C, an acid value of 5 to 15 (mgKOH / g), and an amine value (mgKOH / g) of 0.2 to 0.7, the epoxy equivalent of the epoxy resin (B) is 130 to 450, and the content ratio of the polyamide resin (A) to the epoxy resin (B) is 70:30 to 95:

5. The aforementioned coating film has a glass transition temperature (Tg) of 25 to 90°C, and a dynamic storage modulus of 1 × 10⁻¹⁰°C at the glass transition temperature + 100°C (Tg + 100°C). 7 Pa super, 2×10 8 It is less than Pa. A painted steel sheet characterized by the following features.

2. The primer coating film, which serves as an intermediate layer, is further provided between the chemical conversion treatment film and the coating film. The painted steel sheet according to claim 1, characterized in that the primer coating contains C and O, and one or more selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F.

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