Painted steel sheet

Abstract

Provided is a coated steel sheet that is excellent in corrosion resistance even when not containing a chromate compound and is excellent in sliding resistance of the coating film and corrosion resistance after sliding. 【Solution means】To achieve the above object, the present invention is a coated steel sheet including a zinc-based plated steel sheet, a chemical conversion coating film, and a coating film, wherein the coating film is obtained by thermosetting 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 weight average molecular weight of the polyamide resin (A) is 3,000 to 20,000, the glass transition temperature (Tg) is 5 to 120 ° C, the acid value is 5 to 15 (mgKOH / g), and the amine value (mgKOH / g) is 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) and the epoxy resin (B) is 20:80 to 70:30. The dynamic storage modulus of the coating film at 0 to 50 ° C is more than 2×10 9 Pa and less than 8×10 9 Pa, which is characterized by this.

Description

Technical Field

[0001] The present invention relates to a coated steel sheet that is excellent in corrosion resistance even when not containing a chromate compound, and is excellent in sliding resistance of the coating film and corrosion resistance after sliding.

Background Art

[0002] A coated steel sheet in which a chemical conversion film and a primer coating film are formed on the surface of a steel sheet plated with zinc or an alloy containing zinc (hereinafter referred to as "zinc-based plated steel sheet"), and various coating films are formed thereon, has many advantages such as quality stability and rationalization by omitting the coating process for customers. Therefore, it is widely used as exterior materials such as roofs and walls of buildings, interior materials such as partitions, and various members of electrical equipment products. When assuming such multi-purpose use, since the use environment is also under various conditions, high corrosion resistance is required from the viewpoint of extending the maintenance period.

[0003] To meet these requirements, a technique is known in which a zinc-based plated steel sheet is subjected to a chemical conversion treatment containing chromate, a primer coating film containing a chromate-based rust preventive pigment is formed, and a thermosetting polyester-based resin coating film or a fluorine-based resin coating film is formed as a topcoat coating film for higher weather resistance requirements. However, recently, the use of chromate, which is an environmental load substance, has been regarded as a problem, and since a coated steel sheet not containing chromate is strongly desired, many chromate-free coated steel sheets have been developed.

[0004] For example, Patent Document 1 discloses a coated galvanized steel sheet in which a paint composition containing a non-chromate-based rust preventive pigment such as a molybdenum compound is applied to a galvanized steel sheet via a chemical conversion film.

[0005] However, the applicable galvanized steel sheet is limited to those containing 94% or more of zinc, and since chromate is applied as the chemical conversion film, it cannot be said to be a chromate-free coated steel sheet. In addition, Patent Document 2 discloses a substrate treatment agent (chemical conversion coating film) composed of a resin and a chromium-free rust preventive pigment. Furthermore, Patent Document 3 discloses a painted zinc-plated steel sheet using a substrate treatment agent (chemical conversion coating film) composed of a resin and a chromium-free rust preventive pigment. Moreover, Patent Document 4 discloses a painted zinc-plated steel sheet in which a chemical conversion coating film and a primer coating film, both containing non-chromate rust preventive pigments, are formed, and a technique aiming at improving the overall rust prevention performance is disclosed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, for any of the technologies in Patent Documents 2 to 4, there is a problem that sufficient corrosion resistance of the processed part cannot be obtained. In addition, in conventional painted steel sheets, when they are subjected to repeated sliding, there is a problem that the mechanical strength of the coating film is weak and it is easily worn out. Especially in members such as shutter slats that are repeatedly opened and closed, the coating film wears out early, the steel sheet base surface is exposed, and practical durability cannot be achieved. Therefore, further improvement in this regard has been desired.

[0008] In view of such circumstances, an object of the present invention is to provide a coated steel sheet that is excellent in corrosion resistance even when not containing a chromate compound, and is excellent in sliding resistance of the coating film and corrosion resistance after sliding.

Means for Solving the Problems

[0009] The present inventors have studied a coated steel sheet including a zinc-based plated steel sheet, a chemical conversion coating film formed on at least one surface of the zinc-based plated steel sheet and not containing a chromate compound, and a coating film as the outermost layer formed directly or via an intermediate layer on the chemical conversion coating film and not containing a chromate compound, in order to solve the above problems. As a result, with respect to the coating film, by containing a thermosetting resin obtained by mixing a polyamide resin (A) having a dimer structure derived from at least one of dimer acid and dimer diamine and an epoxy resin (B) in a specific ratio range and thermally curing them, and by regulating the dynamic storage elastic modulus of the formed coating film within a specific range, it is possible to maintain good corrosion resistance even when not containing a chromate compound, and to enhance the balance between the mechanical strength and flexibility of the coating film. Therefore, it has been found that the sliding resistance of the coating film and the corrosion resistance after sliding can be improved.

[0010] The present invention has been made based on the above findings, and the gist thereof is as follows. 1. A zinc-based plated steel sheet, a chemical conversion coating film formed on at least one surface of the zinc-based plated steel sheet and not containing a chromate compound, a coating film as the outermost layer formed directly or via an intermediate layer on the chemical conversion coating film and not containing a chromate compound, and comprising a coated steel sheet, wherein the chemical conversion coating film contains C and O and at least one 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 thermosetting 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 weight average molecular weight of the polyamide resin (A) is 3,000 to 20,000, the glass transition temperature (Tg) is 5 to 120 °C, the acid value is 5 to 15 (mgKOH / g), and the amine value (mgKOH / g) is 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 20:80 to 70:30. The dynamic storage modulus of the coating film at 0 to 50 °C is greater than 2×10 9 Pa and less than 8×10 9 Pa. The coated steel sheet is characterized by this.

[0011] 2. A primer coating film as an intermediate layer is further provided between the chemical conversion coating film and the coating film. The primer coating film contains C and O and at least one selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. The coated steel sheet according to 1 above is characterized by this.

Effect 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, sliding resistance of the coating film, and corrosion resistance after sliding can be provided.

Brief Description of the Drawings

[0013]

Figure 1

Mode 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 film formed on at least one side of the zinc-based plated steel sheet and not containing a chromate-based compound. A coating film, which is the outermost layer and does not contain a chromate-based compound, formed directly or via an intermediate layer on the formation treatment film. It is a coated steel sheet provided with .

[0015] (Zinc-based plated steel sheet) The zinc-based plated steel sheet only needs to contain zinc in the plating layer and is not particularly limited. However, zinc-plated steel sheets such as hot-dip galvanized steel sheet (GI) or alloyed hot-dip galvanized steel sheet (GA) obtained by alloying it, electro-galvanized steel sheet (EG), Zn-Ni-based plated steel sheet, Zn-Al-based plated steel sheet (for example, Zn-5 mass% Al alloy plated steel sheet, Zn-55 mass% Al alloy plated steel sheet), Zn-Al-Mg-based plated steel sheet, etc. can be used. Among these, from the viewpoint of corrosion resistance, it is preferable to use a Zn-Al-Mg-based plated steel sheet. It more preferably has a composition containing 50 to 60 mass% of Al, 1 to 3 mass% of Si, and 0 to 6 mass% of optional additive components, with the balance being Zn and unavoidable impurities. By the plating layer of the hot-dip Al-Zn-based plated steel sheet having the above-described composition, a dendrite phase and an interdendrite phase surrounding the dendrite phase in a network shape can be formed in the plating layer, and the corrosion resistance can be improved.

[0016] Here, the Al content in the plating layer of the Zn-Al-Mg plated steel sheet is 50 to 60% by mass from the balance between corrosion resistance and operation. If the Al content in the plating layer is at least 50% by mass, dendrite solidification of Al occurs sufficiently. As a result, the plating layer mainly contains Zn in supersaturation, and is composed of a portion where Al dendrite solidifies (α-Al dendrite phase) and a portion of the remaining interdendrite gap (interdendrite phase), and a structure excellent in corrosion resistance in which the dendrite phase is laminated in the film thickness direction of the plating layer can be realized. In addition, the corrosion resistance can be enhanced by the stable presence of the surface oxide film of Al on the surface of the plating layer. On the other hand, when the Al content in the plating layer exceeds 60% by mass, the content of Zn having a sacrificial corrosion prevention effect on Fe decreases, and the corrosion resistance deteriorates. Therefore, the Al content in the plating layer is set to 60% by mass or less. Further, from the above-described viewpoints, the Al content in the plating layer is preferably about 55% by mass.

[0017] In addition, Si in the plating layer of the Zn-Al-Mg plated steel sheet is added to the plating bath for the purpose of improving workability and corrosion resistance by suppressing the growth of the interfacial alloy layer formed at the interface with the base steel sheet. The interfacial alloy layer is hard and brittle, and when it grows thick, it becomes a starting point for crack generation during processing. Therefore, it is preferably made as thin as possible. In the case of a molten Al-Zn plated steel sheet, when Si is contained in the plating bath and a hot-dip plating treatment is performed, when the base steel sheet is immersed in the plating bath, Fe on the steel sheet surface and Al and Si in the bath alloy-react to form an alloy composed of Fe-Al-based and / or Fe-Al-Si-based compounds. By the formation of this Fe-Al-Si-based interfacial alloy layer, the growth of the interfacial alloy layer can be suppressed. 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 of the plating layer exceeds 3% by mass, in the plating layer, the workability is lowered, and an Si phase serving as a cathode site is likely to precipitate. The precipitation of this Si phase can be suppressed by increasing the Mg content, but in that case, the manufacturing cost increases and Mg 2An increase in the amount of Si causes a decrease in workability and makes the composition control of the plating bath more difficult. Therefore, the Si content in the plating layer should be 3% by mass or less. From the same perspective, the Si content in the plating layer is preferably 1 to 2% by mass.

[0018] Furthermore, in addition to the above-mentioned Al, Si, and Zn, the plating layer can contain 0 to 6% by mass of optional additive components. Here, the optional additive components can be appropriately selected according to the performance required for the plating layer. For example, alkaline earth metals such as Ca and Mg, and additive components such as Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, W, Sn, and B can be mentioned. Regarding these optional additive components, although effects such as improving the plating appearance and further improving the corrosion resistance can be obtained, there is also a possibility that the workability of the plating layer will decrease. Therefore, the content of the optional additive should be 6% by mass or less, and preferably 5% by mass or less.

[0019] Also, in order to particularly enhance the corrosion resistance of the cut end, in the plating layer, among the optional additive components, it is preferable to contain 1 to 5% by mass of Mg. When the plating layer corrodes, Mg will be contained in the corrosion product, improving the stability of the corrosion product deposited on the exposed surface of the steel at the cut end face, delaying the progress of corrosion, and as a result, the corrosion resistance of the cut surface is further improved. In the present invention, since Mg is also contained in the primer coating film, such an effect becomes more prominent. This is because Mg is continuously supplied from the primer, improving the stability of the corrosion product and suppressing the corrosion of the plating. By setting the content of Mg to 1% by mass or more, a sufficient corrosion delay effect can be obtained. On the other hand, by setting the content of Mg to 5% by mass or less, the workability will not decrease significantly, so the corrosion resistance of the processed part can also be maintained at a high level. From the same perspective, the Mg content in the plating layer is more preferably 2 to 5% by mass, and even more preferably 3 to 5% by mass.

[0020] In addition, the plating layer contains components of the base steel sheet incorporated into the plating layer due to the reaction between the plating bath and the base steel sheet during the plating process, as well as inevitable impurities in the plating bath. As the components of the base steel sheet incorporated into the plating layer, Fe may be contained up to about 2% by mass at most. Examples of the types of inevitable impurities in the plating bath include Fe, Cu, and the like. Regarding Fe in the plating layer, it is impossible to distinguish and quantify the Fe taken in from the base steel sheet and the Fe in the plating bath. Although the total content of inevitable impurities is not particularly limited, from the viewpoints of maintaining the corrosion resistance and uniform solubility of the plating, the amount of inevitable impurities excluding Fe is preferably 1% by mass or less in total.

[0021] In addition, the means for forming the zinc-based plating layer on the base steel sheet is not particularly limited, and ordinary continuous melting plating equipment 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 after removing rolling oil and the like adhering to the steel sheet surface and reductively removing the oxide film simultaneously with annealing, it passes through a snout with the lower end immersed in the plating bath and is immersed in a molten zinc plating bath containing optional additive components such as Al, Zn, Si, and Mg at a predetermined concentration. Thereafter, the steel sheet immersed in the plating bath is pulled up above the plating bath via a sink roll, and then the plating adhesion amount is adjusted by injecting a pressurized gas toward the surface of the steel sheet from a gas wiping nozzle disposed above the plating bath, and then the plating layer is formed by cooling with a cooling device.

[0022] (Conversion treatment film) In the coated steel sheet of the present invention, a conversion treatment film containing no chromate-based compound is formed on at least one surface of the zinc-based plated steel sheet. By forming a conversion treatment film on the zinc-based plated steel sheet, the adhesion to the primer coating film or the coating film can be enhanced, and the corrosion resistance of the coated steel sheet can be further improved.

[0023] And the chemical conversion 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. When the chemical conversion coating contains these elements, it is possible to suppress the occurrence of white rust and coating film swelling especially when zinc is exposed at the cut end or the bent portion, etc.

[0024] Also, the chemical conversion coating is preferably a composite of a resin component and an inorganic component. As the resin component, for example, an anionic urethane resin, an epoxy resin having a bisphenol skeleton, an acrylic resin, a urethane resin, a polyester resin, a phenol resin, etc. can be used. The anionic urethane resin having an ester bond has flexibility, so it has the effect of making the chemical conversion coating difficult to break (peel) when processed. The epoxy resin has the effect of improving the adhesion with the underlying zinc-based electroplated steel sheet and the upper primer coating film. These resins may be water-soluble resins, or may be resins that are originally water-insoluble but can be finely dispersed in water in the form of an emulsion or a suspension (water-dispersible resins). Also, the resin component is preferably contained in the chemical conversion coating in a total amount of 30 to 50% by mass. If it is less than 30% by mass, the binder effect of the chemical conversion coating will decrease, and if it exceeds 50% by mass, the functions of the inorganic components described later, such as the inhibitor action, may decrease.

[0025] As the anionic urethane resin having an ester bond, a resin obtained by reacting a polyester polyol with a diisocyanate or a polyisocyanate having two or more isocyanate groups and copolymerizing dimethylolalkanoic acid can be applied, and a chemical conversion treatment liquid 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 copolyester polyols 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, a mixture obtained by carbodiimidizing a part of diphenylmethane diisocyanate can be mentioned. When synthesizing urethane by reacting these polyester polyols with diisocyanate or polyisocyanate, a dimethylol alkanoic acid is copolymerized and self-emulsified to be solubilized (water-dispersed) in water, thereby obtaining an anionic urethane resin having an ester bond used in the present invention.

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

[0028] In addition, when the total resin components are set to 100, the blending ratio of the anionic urethane resin having an ester bond and the epoxy resin having a bisphenol skeleton is preferably in the range of 3:97 to 60:40 by mass%. This is because outside this range, the adhesion may decrease due to a decrease in flexibility as a chemical conversion treatment film.

[0029] Regarding the inorganic compound in the chemical conversion coating, it contains one or more selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. For example, as the inorganic compound, vanadium oxide, zirconium oxide, fluorine compound, phosphate compound, etc. can be contained. By containing these inorganic compounds, the corrosion resistance, strength, workability, etc. of the chemical conversion coating can be enhanced.

[0030] The vanadium oxide acts as a rust inhibitor (inhibitor) in the chemical conversion coating. As the vanadium compound added to the chemical conversion treatment solution to generate vanadium oxide in the chemical conversion coating, vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, vanadium acetylacetonate, etc. can be mentioned. In particular, it is preferably a pentavalent vanadium compound or a trivalent or tetravalent vanadium compound obtained by reducing a pentavalent vanadium compound.

[0031] The content of vanadium oxide in the chemical conversion coating is not particularly limited, but it is preferably 2 to 10% by mass. If the content of vanadium oxide is less than 2% by mass, the inhibitor effect decreases, leading to a decrease in corrosion resistance. If the content of vanadium oxide exceeds 10% by mass, there is a possibility of reducing the moisture resistance of the chemical conversion coating. From the same perspective, the content of vanadium oxide in the chemical conversion coating is more preferably 4 to 10% by mass, and even more preferably 6 to 10% by mass.

[0032] Since the zirconium oxide forms a dense film, it enhances the strength and corrosion resistance of the chemical conversion coating, improves the adhesion with the plating layer, and contributes to the improvement of the coating property and barrier effect. As the zirconium compound added to the chemical conversion treatment solution to generate zirconium oxide in the chemical conversion coating, neutral salts such as zirconium sulfate, zirconium carbonate, zirconium nitrate, zirconium lactate, zirconium acetate, zirconium chloride, etc. can be mentioned.

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

[0034] The fluorine compound is added to the chemical conversion treatment solution and acts as an adhesion promoter to the molten Al-Zn alloy plated steel sheet. As the fluorine compound, for example, fluoride salts such as ammonium salts, sodium salts, potassium salts, or fluorine compounds such as ferrous fluoride and ferric fluoride can be used. In particular, it is preferable to use fluoride salts such as ammonium fluoride, sodium fluoride, and potassium fluoride. It should be noted that the fluorine compound added to the chemical conversion treatment solution may be decomposed or react with other compounds by drying or over time, and may exist as a fluorine compound different from the fluorine ions or the fluorine compound added to the chemical conversion treatment solution in the chemical conversion coating. 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 it is less than 0.5% by mass, sufficient adhesion in the processed part cannot be obtained. If the content of the fluorine compound exceeds 5% by mass, the moisture resistance of the chemical conversion coating may decrease. From the same perspective, the content of the fluorine compound in the chemical conversion coating is more preferably 0.7 to 3% by mass.

[0035] Also, the preferable coating amount of the chemical conversion coating is 0.025 to 0.5 g / m 2 is. If the coating amount of the chemical conversion coating is 0.025 g / m 2If it is less than this value, there is a risk of a decrease in the adhesion between the underlying molten Al-Zn plated steel sheet and the upper primer coating film, as well as a decrease in corrosion resistance. Also, when the adhesion amount of the chemical conversion coating film exceeds 0.5 g / m 2 when it exceeds 2 , the chemical conversion coating film is likely to break (peel off) when subjected to severe bending, and the corrosion resistance of the processed part may decrease. From the same perspective, the adhesion amount of the chemical conversion coating film is more preferably 0.1 to 0.3 g / m 2 .

[0036] Incidentally, the chemical conversion coating film is obtained by continuously coating the zinc-based plated steel sheet with a chemical conversion treatment liquid using a roll coater or the like, and then drying it at a peak metal temperature (PMT) of about 60 to 200 °C using hot air, induction heating, or the like. These chemical conversion coating films may be single-layer or multi-layer, and in the case of multi-layer, a plurality of chemical conversion treatments may be performed sequentially.

[0037] (Primer coating film) The coated steel sheet of the present invention preferably further includes a primer coating film containing no chromate-based compound, formed on the chemical conversion coating film. By forming a primer coating film on the chemical conversion coating film, the adhesion between the chemical conversion coating film and the topcoat coating film described later can be further enhanced, and the corrosion resistance and rust prevention performance can be further improved.

[0038] Also, 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. When the primer coating film contains these elements, the occurrence of white rust and coating film swelling can be suppressed particularly in the case where zinc is exposed, such as at the cut end or the bent part.

[0039] Also, the primer coating film preferably contains a resin component. Examples of the resin component include polyester resin, acrylic resin, epoxy resin, urethane resin, fluororesin, etc. These resins can also be those crosslinked with a crosslinking agent component such as butylated melamine resin, methylated melamine resin, butylmethyl mixed melamine resin, urea resin, isocyanate resin, or a mixed system thereof. Furthermore, as the resin, various electron beam curable resins, ultraviolet curable resins, etc. can also be used. This is because excellent corrosion resistance and edge corrosion resistance can be achieved. Note that these resins may be used alone or in combination of two or more. Among the above-mentioned resins, the primer coating film preferably contains at least one of polyester resin, epoxy resin, acrylic resin, and urethane resin.

[0040] Regarding the inorganic compound in the primer coating film, it contains one or more selected from the group consisting of P, Mg, Ca, V, Zr, Zn, Al, Si, and F. For example, as the inorganic compound, vanadium oxide, phosphate compound, and magnesium oxide can be contained. By containing these inorganic compounds, the corrosion resistance, strength, processability, etc. of the primer coating film can be enhanced.

[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, vanadium acetylacetonate, etc. 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 film may be of the same type or different types from the vanadium compound contained in the chemical conversion coating film. It is considered that the vanadic acid compound reacts with vanadate ions that gradually elute into moisture invading from the outside and ions on the surface of the aluminum-zinc alloy plated steel sheet to form a passive film with good adhesion, protecting the metal exposed part and exhibiting a rust prevention effect. In particular, this effect is more remarkable as the aluminum content in the plating layer is higher. Therefore, by using an Al-Zn based plated steel sheet with a high aluminum content as a base, a very excellent effect can be exerted.

[0043] In addition, the content of the vanadium compound in the primer coating film is 4 to 20% by mass. If the content of the vanadium compound in the primer coating film is less than 4% by mass, the inhibitor effect decreases, leading to a decrease in corrosion resistance. If it exceeds 20% by mass, the moisture resistance of the primer coating film decreases. From the same perspective, the content of the vanadium compound in the primer coating film is preferably 6 to 16% by mass.

[0044] The phosphate compound contained as the inorganic compound also acts as an inhibitor. As the phosphate compound, ammonium salts of phosphoric acid, alkali metal salts of phosphoric acid, alkaline earth metal salts of phosphoric acid, etc. can be used. Among these, in particular, alkali metal salts of phosphoric acid such as calcium phosphate are preferably used. It is considered that the phosphate compound forms stable corrosion products with zinc ions and aluminum ions eluted from the aluminum-zinc alloy plated steel sheet due to corrosion, protecting the metal exposed part and suppressing corrosion.

[0045] In addition, the content of the phosphate compound in the primer coating film is 4 to 20% by mass. If the content of the phosphate compound in the primer coating film is less than 4% by mass, the inhibitor effect decreases, leading to a decrease in corrosion resistance. If it exceeds 20% by mass, the moisture resistance of the primer coating film decreases. From the same perspective, the content of the phosphate compound in the primer coating film is preferably 6 to 16% by mass.

[0046] Magnesium oxide contained as the inorganic compound has the effect of stabilizing corrosion products generated by initial corrosion as hardly soluble magnesium salts. In addition, the addition amount of magnesium oxide in the primer coating film is 4 to 20% by mass. If the addition amount of magnesium oxide in the primer coating film is less than 4% by mass, the above effect is reduced, leading to a decrease in corrosion resistance. If it exceeds 20% by mass, the flexibility of the primer coating film decreases, particularly reducing the corrosion resistance of the processed part. From the same perspective, the content of magnesium oxide in the primer coating film is preferably 4 to 10% by mass.

[0047] In addition, the preferred thickness of the primer coating film 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 the adhesion to the chemical conversion coating film and the topcoat film. From the same perspective, the thickness of the primer coating film is more preferably 3 μm or more. Note that if the thickness of the primer coating film is too thick, it may lead to a decrease in terms of manufacturing cost and processability, so it is preferably 12 μm or less.

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

[0049] Note that there are no particular restrictions on the coating method of the paint composition for forming the primer coating film, but preferably, the paint composition is applied by methods such as roll coater coating and curtain flow coating. After the paint composition is applied, it is baked by heating means such as hot air heating, infrared heating, and induction heating to obtain a primer coating film. The baking treatment usually sets the maximum reaching plate temperature to about 180 to 270 °C and is performed in this temperature range for about 30 seconds to 3 minutes.

[0050] (Coating film) The coated steel sheet of the present invention further includes a coating film, which is the outermost layer and does not contain a chromate-based compound, formed directly or via an intermediate layer (such as a primer coating film) on the above-mentioned chemical conversion coating film. By forming the coating film, aesthetics can be imparted, and various properties such as processability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance can be enhanced.

[0051] And the coating film contains a thermosetting resin obtained by thermosetting 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 weight average molecular weight of the polyamide resin (A) is 3000 to 20000, the glass transition temperature (Tg) is 5 to 120 °C, the acid value is 5 to 15 (mgKOH / g), the amine value (mgKOH / g) is 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 20:80 to 70:30.

[0052] · Polyamide resin (A) The polyamide resin (A) is a polymer containing an amide group, and among them, it is a resin having a dimer structure derived from at least one of dimer acid and dimer diamine. The polyamide resin (A) is a polymer of a polybasic acid compound, a polyamine compound, and, if necessary, other monomers. To introduce the dimer structure into the polyamide resin (A), a monomer having a dimer structure may be used, and as the monomer, dimer acid, which is a polybasic acid compound, and / or dimer diamine, which is a polyamine compound, are preferably used.

[0053] The total charging ratio of dimer acid and dimer diamine in 100% by mass of all monomers used for the polymerization of the polyamide resin (A) can be 50 to 100% by mass. Since the charging ratio of the monomers used for the polymerization of the polyamide resin (A) substantially coincides with the ratio of the constituent components derived from the monomers of the polyamide resin (A), by setting the charging ratio to 50 to 100% by mass, the effect of the dimer structure can be fully exerted. The total content of dimer acid and dimer diamine is preferably 60 to 95% by mass, more preferably 70 to 90% by mass. In addition, the content of the dimer structure can be determined from the content ratio (mass%) of the raw material monomer having a dimer structure in 100% by mass of the total raw material monomers used when synthesizing the polyamide resin (A) in the case of a polymer. Further, in the case of a modified product, a virtual monomer having the structure after modification is used, and for the monomer that is not modified, it can be determined from the content ratio (mass%) of the raw material monomer having a dimer structure in 100% by mass of all monomers using the raw material monomer. The content of the virtual monomer having the structure after modification can be determined in consideration of the reaction rate with respect to the polymer. For example, when modifying the side group derived from monomer a after obtaining the polymer, the monomer amount (mass) X obtained by "the charged amount (mol) of monomer a × the modification rate of the side group / 100 × the molecular weight of the virtual monomer having the structure after the side group is modified" and the monomer amount (mass) Y obtained by "the charged amount (mol) of monomer a × (1 - the modification rate of the side group / 100) × the molecular weight of monomer a" are used, and for the other monomers, the content ratio of the dimer structure can be determined in the same manner as the method for determining the polymer described above.

[0054] Here, the dimer structure is a structure having a hydrocarbon chain or a ring structure, and is of low polarity compared to the epoxy resins to be blended. By becoming solid through the melting process, the polyamide resin (A) and the epoxy resin (B) are likely to phase-separate at the micron level to form a microphase separation structure. The dimer structure part of the polyamide resin (A) becomes a flexible component, and the hydrogen bond derived from the amide bond of the polyamide resin (A) and the site that is easily compatible with the epoxy resin (B) become constraint components. It is considered that by combining such a polyamide resin (A) and the hard epoxy resin (B) and undergoing a hot melting process, a microphase separation structure is formed. By using a resin component that easily forms a microphase separation structure by melt molding as the resin component of the curable composition, it is considered that the thermal cycle and bending strength of the cured product of this curable composition can be increased. Furthermore, strong hydrogen bonds derived from the amide bonds of the polyamide resin (A) exist in the cured product. Due to these, the bending strength can be increased, and the decrease in adhesive strength and hygrothermal properties after repeating high-temperature and low-temperature cycles can be effectively suppressed. As a result, the adhesion of the coating film can be enhanced, and the corrosion resistance of the cut end of the coated steel sheet can be improved.

[0055] Moreover, the polyamide resin (A) preferably has a functional group that can crosslink with the epoxy group of the epoxy resin (B) by heat. Examples of the functional group include a carboxy group, an amino group, and a hydroxy group. These may be functional groups derived from the monomers of the polyamide resin (A), or functional groups may be introduced as modified products after obtaining the polymer. The functional group has a form not only at the end of the polymer but also as a side group and / or a side chain. As a preferred example, a form having a functional group such as a carboxy group or an amino group at the end of the polymer can be exemplified. Also, a form having at least one of functional groups such as a carboxy group, an amino group, and a hydroxy group in the side group or side chain can be exemplified. Note that if it has a photopolymerizable group as the functional group, the curable composition containing the polyamide resin may undergo over-crosslinking and the adhesiveness may decrease, or when the curable composition is melt-melted and molded, a thermal radical reaction may occur and the moldability may decrease. Therefore, it is preferably free of a photopolymerizable group.

[0056] Furthermore, when the polyamide resin (A) has a hydroxyl group, a phenolic hydroxyl group is preferred. By having a phenolic hydroxyl group, a crosslinked structure with the epoxy resin (B) can be constructed, and a cured product with excellent durability can be obtained. The phenolic hydroxyl group can be easily introduced by using a polybasic acid compound having a phenolic hydroxyl group and / or a polyamine compound having a phenolic hydroxyl group. The aromatic ring of this phenolic hydroxyl group is preferably 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 a phenolic hydroxyl group as a monomer of the polyamide resin (A).

[0057] The polyamide resin (A) may be a polyamideimide having an imide group in part or a polyamide ester having an ester group in part within the scope not departing from the gist of the present invention.

[0058] The polybasic acid compound is a carboxylic acid having two or more carboxyl groups. A part of the polybasic acid compound may be an acid anhydride. Examples of the polybasic acid compound include dimer acid and other polybasic acid compounds other than dimer acid. Usually, polyvalent carboxylic acids are used, but monovalent fatty acids etc. can be used in combination as 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, adipic acid, sebacic acid, succinic acid, azelaic acid, fumaric acid, maleic acid, itaconic acid, pyromellitic acid, dimer acid, etc., and acid anhydrides thereof, and 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, etc. These polybasic acids can be used alone or in combination of two or more.

[0059] The dimer acid is a polybasic acid compound having a dimer structure and is a dimer of a fatty acid (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, still 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 subjecting a fatty acid to a Diels-Alder reaction. The branched structure preferably includes an aliphatic chain or an aliphatic chain and a ring structure, and more preferably includes an aliphatic chain and a ring structure. The ring structure preferably includes 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 may not have a double bond.

[0060] Examples of the polybasic acid compound having the dimer structure include structures represented by the following chemical formulas (1) to (4). Note that the polybasic acid compound having the dimer structure is not limited to the following structures.

[0061]

Chemical formula

Chemical formula

Chemical formula

Chemical formula

[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, and oleic acid, linoleic acid, linolenic acid, and erucic acid obtained by purifying these. When synthesizing the fatty acid dimer, in addition to the fatty acid dimer, a trimer or, in some cases, a tetramer of the fatty acid is generated. Therefore, the polybasic acid compound containing the dimer skeleton is a mixture containing not only the main component fatty acid dimer but also a trimer of the fatty acid and, in some cases, the raw material fatty acid.

[0063] Since unsaturated fatty acids are used as raw materials for the dimer acid, unsaturated bonds may remain. In such a case, hydrogenation (also referred to as a hydrogenation reaction) can be carried out to suppress the number of unsaturated bonds. Thereby, the reaction stability during the synthesis of the polyamide resin (A) is improved, and furthermore, the resistance at high temperatures of the cured product of the curable composition containing the polyamide resin (A) is improved. The dimer acid can be used alone or in combination of two or more kinds.

[0064] Examples of commercially available products of the dimer acid include "Pripol 1004", "Pripol 1006", "Pripol 1009", "Pripol 1013", "Pripol 1015", "Pripol 1017", "Pripol 1022", "Pripol 1025", "Pripol 1040" manufactured by Croda Japan; "Empol 1008", "Empol 1012", "Empol 1016", "Empol 1026", "Empol 1028", "Empol 1043", "Empol 1061", "Empol 1062" manufactured by Henkel Japan. Among these, it is preferable to use "Pripol 1009" having 36 carbon atoms.

[0065] Other polybasic acid compounds are polybasic acid compounds other than dimer acid and are compounds having two or more functional groups. The polybasic acid compounds can be used alone or in combination of two or more kinds.

[0066] Examples of the dibasic acid compound 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, 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, dodecanedioic acid, hexadecanedioic acid, diglycolic acid; and alicyclic dibasic acids such as 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid. Among these, isophthalic acid and 1,4-cyclohexanedicarboxylic acid are preferred as the dibasic acid compound.

[0067] Examples of the polybasic acid compound having three or more functional groups include trimellitic acid, hydrogenated trimellitic acid, pyromellitic acid, hydrogenated pyromellitic acid, trimesic acid, 1,4,5,8-naphthalenetetracarboxylic acid. When a polybasic acid compound having three or more functional groups is used, a branched structure can be introduced into the polyamide resin (A), so that the cohesive force of the cured product is improved, and the thermal cycle resistance and dimensional stability can be improved.

[0068] Preferable examples of the other polybasic acid compound include a polybasic acid compound having a phenolic hydroxyl group. The polybasic acid compound having a phenolic hydroxyl group has a hydroxyl group directly bonded to an aromatic ring (also referred to as a phenolic hydroxyl group) similar to phenol, and is a compound having two or more acidic functional groups. Examples of the acidic functional group include a carboxy group.

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

[0070] The dimer diamine is a compound having two amino groups with a dimer structure, and a compound obtained by converting the carboxy group of the aforementioned dimer acid into an amino group can be used. Examples of the conversion method include amidating a carboxylic acid, aminating it by Hofmann rearrangement, and further performing distillation and purification. The dimer diamine is preferably a compound having 20 to 60 carbon atoms, and more preferably a compound having 24 to 56 carbon atoms.

[0071] Commercially available products of the dimer diamine include, for example, "Priamine 1071", "Priamine 1073", "Priamine 1074", "Priamine 1075", etc. manufactured by Croda Japan Co., Ltd. The dimer diamine can be used alone or in combination of two or more.

[0072] Other polyamine compounds are polyamine compounds other than the dimer diamine, and examples thereof include diamine compounds and polyamine compounds having three or more functional groups.

[0073] The diamine compound is, for example, an aromatic diamine such as 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, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminobenzophenone, 3,3'-diaminodiphenyl sulfone; an aliphatic diamine such as ethylenediamine, 1,3-propanediamine, 1,4-butanediamine, 1,6-hexanediamine, 1,7-heptanediamine, 1,9-nonanediamine, 1,12-dodecamethylenediamine, metaxylenediamine; an alicyclic diamine such as isophoronediamine, norbornanediamine, 1,2-cyclohexanediamine, 1,3-cyclohexanediamine, 1,4-cyclohexanediamine, 4,4'-diaminodicyclohexylmethane, piperazine, etc.

[0074] As a method for producing the polyamide resin (A), for example, melt polymerization, interfacial polymerization, solution polymerization, bulk polymerization, and solid-phase polymerization, and combinations thereof can be used for synthesis. Among these, solution polymerization is preferred. The polymer of the polyamide resin (A) can be produced by using the above-described polybasic acid compound, polyamine compound, and, if necessary, other monomers, in the presence or absence of a catalyst. For example, a nitrogen-filled flask is charged with a predetermined amount of dimer acid, other acid monomers, dimer diamine, other amine monomers, and ion-exchanged water, and heated and stirred at 20 to 100 °C to achieve uniform dissolution or dispersion. Thereafter, while removing the ion-exchanged water and the water generated by the reaction, the temperature is gradually raised to 230 °C, and as soon as 230 °C is reached, the pressure is reduced to about 15 mmHg, and the polyamide resin (A) can be obtained by maintaining this state for about 1 hour. The heating temperature is, for example, 150 to 300 °C, and the heating time can be about 1 to 24 hours. To promote the synthesis reaction, it is preferable to perform a dehydration or dealcoholization reaction, and to avoid coloring and decomposition reactions due to high temperature, it is preferable to perform the reaction at 180 to 270 °C under reduced pressure.

[0075] As other monomers, a monoamine may be used in combination with the polyamine. Since the monoamine acts as a reaction terminator, it is easy to adjust the molecular weight of the polyamide resin (A). In addition, since a part of the main chain end of the polyamide resin (A) is not a reactive functional group, the stability over time is improved. Examples of the monoamine include aniline, benzylamine, 4-aminophenol, 2-ethylhexylamine, and the like.

[0076] The glass transition temperature (Tg) of the polyamide resin (A) is preferably 5 to 120 °C. When the glass transition temperature (Tg) is less than 5 °C, the coating film may be softened and prone to scratches. When it exceeds 120 °C, the coating film becomes hard, and cracks may occur in the coating film during bending, resulting in a possible decrease in corrosion resistance. Here, the Tg of the polyamide resin (A) is the temperature at which the value (tanδ) obtained by dividing the viscous term measured by a dynamic viscoelasticity measuring device by the elastic term shows a maximum.

[0077] Moreover, the weight average molecular weight of the polyamide resin (A) is preferably 3000 to 20000. This is because when it is 3000 or more, the effect of suppressing coating film swelling is significant. However, if the weight average molecular weight exceeds 20000, the adhesion of the coating film may decrease.

[0078] In addition, the acid value of the polyamide resin (A) is 5 to 15 mgKOH / g. This is because when the polyamide resin (A) is made into a cured product (when made into a coating film), an appropriate crosslinking density can be obtained, improving the adhesion to the plating surface and corrosion resistance. However, if the acid value is less than 5 mgKOH / g, the adhesion of the paint may decrease, and if it exceeds 15 mgKOH / g, the corrosion resistance may decrease due to excessive hydrophilicity. From the same perspective, the acid value of the polyamide resin (A) is preferably 7 to 11 mgKOH / g.

[0079] Also, the amine value of the polyamide resin (A) is preferably 0.2 to 0.7 mgKOH / g, from the viewpoints of production efficiency and resin stability. However, if the amine value is less than 0.2 mgKOH / g, the reactivity with the epoxy resin may decrease, and the curing time may become longer, potentially reducing production efficiency. If it exceeds 0.7 mgKOH / g, the stability of the amide resin may be low, 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 thermally cured by having an epoxy group. If the epoxy resin (B) itself has reactive functional groups such as hydroxyl groups, the epoxy resin (B) can form a crosslinked structure alone. In addition to or instead of self-crosslinking, a mode of thermally crosslinking the polyamide resin (A) and the epoxy resin (B) is also suitable. Due to the three-dimensional crosslinked structure formed by the thermal crosslinking of the polyamide resin (A) and the epoxy resin (B), excellent adhesion and even better corrosion resistance can be obtained.

[0081] In the present invention, the content ratio of the polyamide resin (A) to the epoxy resin (B) is (A):(B) = 20:80 to 70:30 in terms of mass ratio in the coating composition for forming the coating film. By containing the polyamide resin (A) and the epoxy resin (B) within these ranges, excellent erosion resistance against repeated sliding can be achieved. From the same perspective, the content ratio of the polyamide resin (A) to the epoxy resin (B) is preferably 40:60 to 60:40.

[0082] Further, the epoxy equivalent (the number of grams of resin containing 1 gram equivalent of epoxy groups) 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 likely to occur in the coating film. If it is less than 130 g / eq., the adhesion to plating is low and swelling is likely to occur in the coating film.

[0083] Examples of the epoxy resin (B) include bisphenol type epoxy resins such as bisphenol A type epoxy resin, bisphenol F type epoxy resin, and bisphenol S type epoxy resin; novolak type epoxy resins such as o-cresol novolak type epoxy resin; biphenyl type epoxy resin, naphthalene type epoxy resin, naphthalene-containing novolak type epoxy resin, dicyclopentadiene type epoxy resin, phenol aralkyl type epoxy resin, trisphenol methane type epoxy resin, phenol-modified xylene resin type epoxy resin, etc. The epoxy resin (B) can be used alone or in combination of two or more.

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

[0085] The coating film can contain a rust inhibitor, and examples of the rust inhibitor include vanadate, tungstate, silicate, phosphate, etc. Here, examples of the vanadate include calcium vanadate, magnesium vanadate, ammonium metavanadate, potassium vanadate, sodium vanadate, ammonium vanadate, phosphorus vanadate, vanadium oxide, and the like. Examples of the tungstate include sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, magnesium tungstate, and the like. Examples of the silicate include sodium silicate, potassium silicate, lithium silicate, calcium ion-exchanged silica, and the like. Examples of the phosphate include sodium dihydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, aluminum tripolyphosphate, magnesium tripolyphosphate, sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate, calcium hypophosphite, and the like. The content of the rust inhibitor in the paint composition is not particularly limited, but is preferably 10% by mass or more and less than 25% by mass.

[0086] Note that the film thickness of the coating film is not particularly limited and can be appropriately adjusted according to the required performance. For example, from the viewpoint 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 realized. On the other hand, when it is 25 μm or less, it does not cause complexity in manufacturing or an increase in manufacturing cost.

[0087] The coating method of the paint composition for forming the coating film is not particularly limited. For example, the paint composition serving as the material of the coating film can be applied by methods such as roll coater coating and curtain flow coating. After the paint composition is applied, it can be baked 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 such that the maximum reach plate temperature is about 180 to 270 °C, and this temperature range is maintained for about 30 seconds to 3 minutes.

[0088] And in the present invention, the dynamic storage modulus of the coating film at 0 to 50 °C is more than 2×10 9 Pa and less than 8×10 9 Pa. Since the balance between the strength and flexibility of the coating film can be enhanced, the sliding resistance and the corrosion resistance after sliding can be improved without reducing the coating film adhesion and the like. When the dynamic storage modulus is 8×10 9 Pa or more, the internal stress increases and coating film peeling is likely to occur, resulting in a decrease in corrosion resistance. Also, when it is 2×10 9 Pa or less, the sliding resistance decreases due to a decrease in the strength of the coating film. From the same viewpoint, the dynamic storage modulus of the coating film at 0 to 50 °C is preferably 4×10 9 to 7×10 9 Pa. Note that "the dynamic storage modulus at 0 to 50 °C is more than 2×10 9 Pa and less than 8×10 9 Pa" means that "all the dynamic storage moduli from 0 °C to 50 °C fall within the range of more than 2×10 9 Pa and less than 8×10 9 Pa".

[0089] Note that the method for making the dynamic storage modulus of the coating film within the above range is not particularly limited. For example, by appropriately adjusting the materials constituting the coating film (the ratio of polyamide resin and epoxy resin, added pigments, etc.) and the conditions during coating film formation (drying temperature, drying time, etc.), the dynamic storage modulus within the above range can be obtained.

Examples

[0090] <Examples 1 to 10, Comparative Examples 1 to 7> According to the conditions of (1) hot dip galvanized steel sheet, (2) chemical conversion coating film, (3) primer coating film, and (4) coating film shown below, each sample of the coated steel sheet was manufactured.

[0091] (1) Zinc-based galvanized steel sheet The following zinc-based plated steel sheets were used. Table 1 shows the types of plated steel sheets used for each sample. · Plated steel sheet 1: thickness 0.35 mm, plating adhesion amount 80 g / m per side 2 , hot-dip Al-Zn plated steel sheet with a plating layer having a composition of Zn-55% Al (JIS G3321, AZ150) · Plated steel sheet 2: thickness 0.35 mm, plating adhesion amount 130 g / m per side 2 , hot-dip Zn-Al plated steel sheet with a plating layer having a composition of Zn-5% Al (JIS G3317, Y25) · Plated steel sheet 3: thickness 0.35 mm, plating adhesion amount 130 g / m per side 2 Hot-dip Zn plated steel sheet (JIS G3312, Z25)

[0092] (2) Conversion coating On the surface of the zinc-based plated steel sheet, a conversion treatment liquid was applied by a roll coater and dried at a plate temperature of 90 °C for 60 seconds using a hot air drying furnace, thereby forming various conversion coatings shown in Table 1 to have an adhesion amount of 150 mg / m. 2

[0093] In addition, as the resin components (resin components containing C and O) in the conversion coating, "Superflex 210" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., which is an anionic urethane resin, and "Yukarezine RE-1050" manufactured by Yoshimura Oil Chemical Co., Ltd., which is an epoxy resin having a bisphenol skeleton, were used. As other resin components, acrylic resins and polyester resins were used. The mixing ratio (mass ratio) of these resins is 50:50. In addition, the inorganic compounds contained in the conversion coating are shown in Table 1. As the zirconium compound, ammonium zirconium carbonate was used, and as the phosphate compound, aluminum phosphate was used. These raw materials were mixed to obtain a conversion treatment liquid. The pH of the conversion treatment liquid was set to 8 - 10. The content of each component in the conversion coating was as shown in Table 1.

[0094] (3) Primer coating As the resin component of the primer coating film, 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. Also, magnesium vanadate was used as the vanadium compound of the rust preventive pigment, and calcium phosphate was used as the phosphoric acid compound. Furthermore, isophorone diisocyanate was used as the curing agent for the urethane-cured polyester resin. After adding a solvent and a rust preventive pigment to the above components, 0.3 part of dibutyltin dilaurate (DBTDL) was added as a reaction catalyst and mixed uniformly to prepare a chromium-free primer coating film paint. The types of resin components of each sample are shown in Table 1. The obtained primer coating film paint was applied onto the chemical conversion coating film with a roll coater, baked at a steel plate reaching temperature of 220 °C for a baking time of 35 seconds, and a coating film was formed to have an adhesion amount of 7 g / m 2

[0095] (4) Coating film As the coating film, a coating film paint containing the following components was used. (4-1) Polyamide resin The polyamides (PA-1 to PA-8) described later were used. 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: "EPICLON HP4032D" manufactured by DIC Corporation, epoxy equivalent 134 EP-2: "EPICLON 860" manufactured by DIC Corporation, epoxy equivalent 212 EP-3: "EPICLON 153" manufactured by DIC Corporation, epoxy equivalent 389 EP-4: "EPICLON 1055" manufactured by DIC Corporation, epoxy equivalent 472 (4-3) Rust preventive pigment The following were used as the rust preventive pigment. The types and contents of the rust preventive pigments used are shown in Table 1. ​Vanadium compound: An organic vanadium compound chelated with acetylacetone Zirconium compound: Ammonium zirconium carbonate As the fluorine compound, ammonium fluoride Phosphate compound: Aluminum phosphate, zinc phosphate, magnesium phosphate Silica: Manufactured by Nissan Chemical Industries, Ltd., "Snowtex (registered trademark) O-40" Ion-exchanged silica: Manufactured by NoelsonChem, "Noelson (registered trademark) NSC-400" (4-4) Coloring pigment As the coloring pigment, carbon black and titanium oxide were used. The total content of the coloring pigment was adjusted to 10% by mass in the coating film. In addition, the paint for the coating film was applied on the undercoat film with a roll coater, baked at a steel plate reaching temperature of 240 °C for 45 seconds, and a coating film was formed so that the film thickness after baking became the film thickness shown in Table 1.

[0096] (Polyamide PA-1 to PA-8) (1) Synthesis of polyamide PA-1 As shown in Table 2, 90 g of "oleochemical dimer acid" as a dibasic acid type dimer acid of dicarboxylic acid having 36 carbon atoms, 600.0 g of "Priamine 1075" (manufactured by Croda) as a dimer diamine having 36 carbon atoms, 230 g of 3,4'-diaminodiphenyl ether as other diamine, 30 g of adipic acid as polybasic acid, and 50 g of sebacic acid were charged into a flask equipped with a stirrer, a reflux condenser equipped with a Dean-Stark apparatus, a nitrogen inlet tube, and a thermometer, and stirred. When the heat generation subsided, heating was gradually carried out to proceed the reaction. While removing the water generated with the progress of the reaction out of the system, the internal temperature was raised to 230 °C and the temperature was maintained as it was, and the reaction was continued for 4 hours. Next, the pressure was reduced to about 2 kPa and maintained at the same temperature for 2 hours to complete the reaction, and a polyamide resin (PA-1) having a weight average molecular weight of 5800, an acid value of 9.0 mgKOH / g, an amine value of 0.6 mgKOH / g, and a Tg of 70 °C was obtained. The amount of each component shown in Table 2 is the amount of solid content.

[0097] (2) Synthesis of polyamides PA-2 to PA-8 Polyamide resins (PA-2 to PA-8) were obtained under the same conditions as for PA-1, except that the various components were blended under the conditions shown in Table 2.

[0098] (3) Acid value measurement The acid value of the resulting polyamide resin was measured by the following procedure. The measurement results are shown in Table 2. Accurately weigh 10 g of fat or oil sample into a stoppered flask, add 100 mL of ethanol-ether mixture to dissolve, add a few drops of phenolphthalein TS as an indicator, and titrate with 0.1 mol / L ethanolic potassium hydroxide solution until a pale pink color appears that lasts for 30 seconds. The acid value was calculated from the amount of 0.1 mol / L ethanolic potassium hydroxide solution required for titration using the following formula. Acid value = a x F x 5.611 / amount of fat or oil sample (g) a: Amount of 0.1 mol / L ethanolic potassium hydroxide solution (mL) F: Potency of 0.1 mol / L ethanolic potassium hydroxide solution

[0099] (4) Amine value measurement The amine value of the resulting polyamide resin was measured by the following procedure. The measurement results are shown in Table 2. Approximately 1 g of sample is precisely weighed and placed in a stoppered Erlenmeyer flask, and 100 mL of cyclohexanone solvent is added to dissolve it. A few drops of indicator, which is 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, are added to the sample and held for 30 seconds. The solution is then titrated with 0.1 N alcoholic hydrochloric acid until it turns blue-gray. The amine value is calculated using the following formula (unit: mgKOH / g). Amine number (mgKOH / g) = (5.611 x a x F) / S however, S: Sampling amount of sample (g) a: Consumption of 0.1N alcoholic hydrochloric acid solution (mL) F: Titer of 0.1N alcoholic hydrochloric acid solution

[0100] (5) Measurement of weight-average molecular weight (Mw) The weight-average molecular weight (Mw) of the obtained polyamide resin was measured according to the following procedure. The measurement results are shown in Table 2. GPC (gel permeation chromatography) "GPC-101" manufactured by Showa Denko KK was used. GPC is a liquid chromatography that separates and quantifies substances dissolved in a solvent (THF; tetrahydrofuran) based on the difference in their molecular sizes. In the measurement of the present invention, two columns of "KF-805L" (manufactured by Showa Denko KK: GPC column: 8 mm ID × 300 mm size) were connected in series and used. The measurement was carried out 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 determination of the weight-average molecular weight (Mw) was carried out in terms of polystyrene conversion. Data analysis was performed using the built-in software of the manufacturer to calculate the calibration curve, molecular weight, and peak area, and the weight-average molecular weight was determined by analyzing the range of retention time from 17.9 to 30.0 minutes.

[0101] (6) Measurement of glass transition temperature The glass transition temperature of the obtained polyamide resin was measured according to the following procedure. The measurement results are shown in Table 2. The polyamide resin was dissolved in cyclohexanone to a non-volatile content of 35% to prepare a polyamide resin varnish. This varnish was coated on a heat-resistant release film with a doctor blade of 10 mil and dried at 130 °C for 10 min to obtain a polyamide resin film with a thickness of 25 μm, which was used as a sample for measuring the glass transition temperature. The measurement of tanδ was carried out in the temperature range of -50 to 200 °C using a dynamic viscoelasticity measuring device to obtain the glass transition temperature. Dynamic viscoelasticity measuring device: DVA-200 (manufactured by IT Measurement & Control Co., Ltd.) Heating rate: 10 °C / min Measurement frequency: 10 Hz Grip length: 15 mm Width: 5 mm

[0102] <Evaluation> For each sample of the painted steel sheet obtained as described above, the following measurements and evaluations were carried out. The results are shown in Table 1.

[0103] (1) Glass transition temperature and dynamic storage modulus of the coating film The coating paint for the coating film used in the production of each sample was applied on a release film with a bar coater, dried at a temperature of 180 °C, and coated to a film thickness of 20 μm to prepare a measurement film. For each measurement film, it was set in a dynamic viscoelasticity measuring device (DMA device), and the loss tangent (tanδ) and dynamic storage modulus E' for each temperature were measured. Then, the peak of tanδ was taken as the glass transition temperature, and the numerical value of the dynamic storage modulus was read. Note that the conditions of the DMA device are as follows. DMA device: DVA-200 (manufactured by IT Measurement and Control Co., Ltd.) Temperature range: -50 to 200 °C Heating rate: 10 °C / min Measurement frequency: 10 Hz Grip length: 15 mm Width: 5 mm

[0104] (2) Corrosion resistance after sliding On the surface of each sample, a jig using SKD11 shown in Fig. 1 was applied with a pressure of 10 Kg / cm 2 , and the same surface was slid 10 times under the conditions of a speed of 50 mm / min. Then, SST500h (salt spray test for 500 hours) was carried out, and the rusting state of the sliding part was confirmed. Regarding the confirmed rusting state, it was evaluated according to the following criteria. 〇: Generation area of white rust is 0% △: Generation area of white rust is more than 0% and less than 20% ×: Generation area of white rust is 20% or more

[0105] (3) Adhesion of the coating film Test pieces measuring 70 cm × 150 cm were prepared from each sample. Subsequently, a checkerboard test was conducted according to the following procedures a) to d) in accordance with JIS G 3325. a) Cut checkerboard-shaped lines that reach the plating surface on the coating film of the test piece using a cutter knife or the like. b) Set the interval between the checkerboard lines to 1 mm and make 11 lines intersect perpendicularly both vertically and horizontally. c) Immerse in boiling water for 2 hours, take out, air-dry, attach cellophane tape to the coating film surface, and then immediately peel it off. d) Visually check the presence or absence of peeling in the checkerboard squares (1 mm × 1 mm), count the number of squares, and then evaluate according to the following criteria. ○: No peeling in all squares △: The number of peeled squares is 10% or less ×: The number of peeled squares exceeds 10%

[0106]

Table 1

[0107]

Table 2

[0108] From the results in Table 1, it can be seen that a coating film having a predetermined dynamic storage modulus in the optimal thermosetting coating film of dimer acid and epoxy resin has excellent sliding resistance and corrosion resistance after sliding. Furthermore, by optimizing the dynamic storage modulus at 0 to 50°C, the sliding resistance and corrosion resistance after sliding can be improved.

Industrial Applicability

[0109] According to the present invention, even when not containing a chromate-based compound, a coated steel sheet excellent in corrosion resistance, sliding resistance of the coating film, and corrosion resistance after sliding can be provided.

Claims

1. A zinc-based plated steel sheet; a chemical conversion coating film that does not contain a chromate-based compound and is formed on at least one surface of the zinc-based plated steel sheet; a coating film which is an outermost layer and does not contain a chromate-based compound, formed directly or via an intermediate layer on the chemical conversion coating; A coated steel sheet comprising: The chemical conversion coating contains C and O, and at least one element 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 thermally curing a polyamide resin (A) having a dimer structure derived from at least one of a dimer acid and a 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, and a content ratio of the polyamide resin (A) to the epoxy resin (B) is 20:80 to 70:30, The coating has a dynamic storage modulus of 2×10 at 0 to 50°C. 9 Pa super, 8×10 9 The coated steel sheet has a compressive strength of less than 100 Pa.

2. A primer coating film is further provided as an intermediate layer between the chemical conversion coating film and the coating film, 2. The coated steel sheet according to claim 1, wherein the primer coating film 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.

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