Coated Steel Sheet and Method for Manufacturing the Same

Abstract

To provide: a coated steel plate which can suppress yellow discoloration in coating and is excellent in workability; and a method for manufacturing a coated steel plate.SOLUTION: There is provided a coated steel plate in which a coloring coated film is formed on a molten Al-Zn-based plated steel plate directly or via an inter-layer, wherein the coloring coated film is formed using a coating material containing at least a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a curing agent and an organic spherical aggregate, (a) a content of a polyester resin in the coating material, and (b) a content of a pyrazole-based block isocyanate curing agent and (c) a content of a melamine-based curing agent as curing agents satisfy the following relation (1) or (2). Relation (1): (b)>(c)>0 and (b) / (a)≤0.4. Relation (2): (b)>(c)=0 and 0.2≤(b) / (a)≤0.8.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a coated steel sheet capable of suppressing yellowing during coating and having excellent workability, and a method for manufacturing the coated steel sheet.

Background Art

[0002] The hot-dip Al-Zn alloy coated steel sheet represented by the Al-Zn alloy coated steel sheet, which contains 20 to 95% by mass of Al in the coating layer, has excellent corrosion resistance, and thus the demand has been increasing in recent years, mainly in the building materials field. In addition, it is common to form a coating film on the hot-dip Al-Zn alloy coated steel sheet and use it as a coated steel sheet for roofs and walls of factories, commercial facilities, houses, etc.

[0003] Here, for the above-mentioned Al-Zn alloy coated steel sheet, a pickled and descaled hot-rolled steel sheet or a cold-rolled steel sheet obtained by further cold-rolling this can be used as a base steel sheet, and it can be manufactured by a continuous hot-dip coating facility. The structure of the coating layer of the Al-Zn alloy coated steel sheet manufactured by such a continuous hot-dip coating facility mainly consists of a part where Al containing Zn in supersaturation dendritically solidifies (dendrite phase) and the remaining part of the dendrite gaps (interdendrite phase), and the dendrite phase is laminated in the film thickness direction of the coating layer. Due to such a characteristic film structure, the Al-Zn alloy coated steel sheet can achieve good corrosion resistance.

[0004] However, although the hot-dip Al-Zn alloy coated steel sheet such as the Al-Zn alloy coated steel sheet has good corrosion resistance, it tends to be inferior in mechanical properties, particularly elongation properties. Therefore, when performing processing such as bending, depending on the degree of processing, cracks may occur in the coating layer of the processed part, which may cause deterioration of corrosion resistance due to the cracked part, and may cause problems in use.

[0005] For such problems, for example, Patent Document 1 discloses a technique for improving the ductility of an Al-Zn alloy coated steel sheet by subjecting it to a predetermined heat treatment. However, although the technique of Patent Document 1 can obtain a certain ductility improvement effect, it is difficult to sufficiently suppress cracks in the plating layer for complex processing such as processing when formed into roof and wall members, and there has still been a desire for the development of a technique capable of improving workability.

[0006] In addition, Patent Document 2 discloses a technique for improving workability by forming a specific coating film on a molten Al-Zn-based plated steel sheet. However, regarding the technique of providing a coating film on a plating layer as in Patent Document 2, when the coating film has flexibility, the cracks generated in the plating layer can be covered by the coating film, so although an improvement in the appearance after processing can be expected, there are cracks in the plating layer under the coating film, and when used in a severe corrosion environment, there is a problem that sufficient corrosion resistance after processing cannot be obtained, and when the coating film formed on the plating layer is made flexible, there is a problem that the surface hardness decreases and the scratch resistance deteriorates. Furthermore, when a urethane-based curing agent is used as a curing agent for the resin constituting the coating film formed on the plated steel sheet, there is also a problem that the coating film formed when the paint is baked is yellowed due to the influence of the blocking agent contained in the curing agent.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] In view of such circumstances, an object of the present invention is to provide a coated steel sheet capable of suppressing yellowing during coating and having excellent workability, and a method for manufacturing the coated steel sheet.

Means for Solving the Problems

[0009] In order to solve the above problems, the inventors of the present invention studied a coated steel sheet in which a colored coating film was formed directly or via an intermediate layer on a molten Al-Zn-based plated steel sheet. As a result, it was found that the colored coating film uses a paint containing at least a pyrazole-based blocked isocyanate curing agent as a curing agent with a polyester resin as a base material, thereby improving workability and suppressing yellowing during coating. Then, by optimizing the content of (b) the pyrazole-based blocked isocyanate curing agent with respect to the content of (a) the polyester resin and the content of (c) the melamine-based curing agent in the paint, it was found that yellowing during coating can be more reliably suppressed. In addition, the inventors of the present invention focused on the fact that the occurrence of cracks in the plating layer is closely related to the elongation and deformation of the plating layer during processing, and that the forming of the coated steel sheet is mainly by bending. By quantitatively evaluating the stress relaxation effect of the coating film in terms of the elongation rate (deformation rate) of the plating layer based on bending and the relationship with the cracks, it was also found that excellent workability suitable for the actual use environment can be realized.

[0010] The present invention has been made based on the above findings, and the gist thereof is as follows. 1. A coated steel sheet in which a colored coating film is formed directly or via an intermediate layer on a molten Al-Zn-based plated steel sheet, The colored coating film is formed using a paint containing at least a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a curing agent, and organic spherical aggregates, In the paint, the content of (a) the polyester resin, the content of (b) the pyrazole-based blocked isocyanate curing agent as a curing agent, and the content of (c) the melamine-based curing agent satisfy the following relationship (1) or (2), (b) > (c) > 0 and (b) / (a) ≤ 0.4 ··· (1) (b) > (c) = 0 and 0.2 ≤ (b) / (a) ≤ 0.8 ··· (2) The colored coating film has an L value of 60 or more in the Hunter Lab color space on the surface, and when baking painting is performed with a continuous color line represented by the following (3), the color tone variation Δb is 0.3 or less (Δb≦0.3). Δb = b(T)― b(E) ···(3) b(T): The b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film at the start of continuous baking painting b(E): The b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film during continuous baking painting When a bending test is performed in accordance with the adhesion test of plating described in JIS G 3321 (2019), the elongation rate El by the bending test is obtained from the formula (4), and the maximum elongation rate El (limiting elongation rate) at which no cracks are observed in the test piece is 10% or more (El≧10). El(%)=t / (2r-t)×100(%) ···(4) t: The thickness of the steel sheet (mm), r: The outer R of the bent test piece A painted steel sheet, characterized by the above.

[0011] 2. The painted steel sheet according to 1 above, wherein the organic spherical aggregate is an acrylic resin having an average particle size of 8 to 40 μm.

[0012] 3. The El is the limiting elongation rate El at a plate temperature of 40°C 40 is 14% or more (El 40 ≧14), and the painted steel sheet according to 1 or 2 above, characterized by the above.

[0013] 4. The painted steel sheet according to any one of 1 to 3 above, wherein the plating layer of the hot-dip Al-Zn alloy plated steel sheet contains 50 to 60% by mass of Al, 1 to 3% by mass of Si, and 5% or less of an optional additive component, and the balance is composed of Zn and inevitable impurities.

[0014] 5. The painted steel sheet according to any one of 1 to 4 above, wherein the Vickers hardness of the dendrite phase in the plating layer of the hot-dip Al-Zn alloy plated steel sheet is 10 to 110 HV 0.01 and the painted steel sheet is characterized by the above.

[0015] 6. The coated steel sheet according to 4 above, wherein the plating layer further contains Mg: 0.01 to 5% by mass as the optional additive component.

[0016] 7. A method for manufacturing a coated steel sheet in which a colored coating film is formed directly or via an intermediate layer on a molten Al-Zn-based plated steel sheet, wherein the colored coating film is formed using a paint containing at least a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a curing agent, and an organic spherical aggregate, in the paint, the content of (a) the polyester resin, the content of (b) the pyrazole-based blocked isocyanate curing agent, and the content of (c) the melamine-based curing agent satisfy the following relationship (1) when the coating condition X shown in the following (5) is 9 to 18 (X = 9 to 18 g / Nm 3 ), and satisfy the following relationship (2) when the coating condition X shown in the following (5) is less than 9 (X < 9 g / Nm 3 ): X (g / Nm 3 ) = line speed (m / min) × plate width (m) × coating amount (g / m 2 ) / exhaust gas amount of the coating apparatus (Nm 3 / min) ··· (5) (b) > (c) > 0 and (b) / (a) ≤ 0.4 ··· (1) (b) > (c) = 0 and 0.2 ≤ (b) / (a) ≤ 0.8 ··· (2) The method for manufacturing a coated steel sheet is characterized by the above.

Effect of the Invention

[0017] According to the present invention, it is possible to provide a coated steel sheet that can suppress yellowing during coating and has excellent workability, and a method for manufacturing the coated steel sheet.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0019] The painted steel sheet of the present invention is a painted steel sheet in which a colored coating film is formed directly or via an intermediate layer on a molten Al-Zn-based plated steel sheet. Hereinafter, each member constituting the painted steel sheet of the present invention will be described.

[0020] (Molten Al-Zn-based plated steel sheet) The molten Al-Zn-based plated steel sheet is a steel sheet on which an Al-Zn-based plating layer is formed. The Al-Zn-based plating layer is not particularly limited as long as it contains Al and Zn as main components. For example, from the viewpoint of corrosion resistance, it can have a composition containing Al: 20 to 95% by mass, Si: 1 to 3% by mass, and optional additive components: 5% by mass or less, with the balance being Zn and inevitable impurities. By having the above-described composition for the plating layer of the molten Al-Zn-based plated steel sheet, 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.

[0021] Also, from the same viewpoint, the plating layer preferably has a composition defined in "Plating Bath Components" in JIS G 3321 (2019) 5.1, specifically, containing Al: 50 to 60% by mass, Si: 1 to 3% by mass, and optional additive components: 5% by mass or less, with the balance being Zn and inevitable impurities.

[0022] Here, the Al content in the plating layer is 20 to 95% by mass, preferably 50 to 60% by mass, in view of the balance between corrosion resistance and operability. If the Al content in the plating layer is at least 20% 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 interdendritic gaps (interdendritic 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 more the dendrite portions of the α-Al phase are laminated, the more complicated the corrosion progress path becomes, and it becomes difficult for corrosion to easily reach the base steel plate, so the corrosion resistance is improved. On the other hand, when the Al content in the plating layer exceeds 95% by mass, the content of Zn having a sacrificial anticorrosion action with respect to Fe decreases, and the corrosion resistance deteriorates. Therefore, the Al content in the plating layer is 95% by mass or less.

[0023] Also, Si in the plating layer is added to the plating bath for the purpose of suppressing the growth of the interfacial alloy layer formed at the interface with the base steel plate and for the purpose of improving corrosion resistance and workability, and is inevitably contained in the plating layer. In the case of a hot-dip Al-Zn-based plated steel sheet used for the coated steel sheet of the present invention, when Si is contained in the plating bath and hot-dip plating treatment is performed, the base steel plate is immersed in the plating bath, and at the same time, Fe on the steel plate 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, the workability is lowered in the plating layer, and an Si phase serving as a cathode site is likely to precipitate. Therefore, the Si content in the plating layer is 3% by mass or less.

[0024] The plating layer contains Zn as a main component thereof. By containing Zn in the plating layer, a sacrificial anticorrosion effect can be obtained, and it becomes possible to improve the corrosion resistance. On the other hand, when the content of Zn is 80% by mass or less, the content of Al can be ensured, and it is preferable in that the corrosion resistance due to the dendrite phase and interdendrite phase described above can be realized.

[0025] Furthermore, in addition to Al, Si, and Zn described above, the plating layer can contain an optional additive component in an amount of 5% by mass or less. Here, the optional additive component 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, and B can be mentioned. Regarding these optional additive components, although effects such as further improving the corrosion resistance can be obtained, there is a possibility that the workability of the plating layer deteriorates and the elongation limit of the coated steel sheet deteriorates. Therefore, the content of the optional additive is preferably 5% by mass or less.

[0026] The plating layer can contain Mg and / or Ca as the optional additive component. When the plating layer corrodes, Mg and / or Ca will be contained in the corrosion product, the stability of the corrosion product is improved, and as a result, the progress of corrosion is delayed, and the effect of improving the corrosion resistance is obtained. The total content of Ca and / or Mg is not particularly limited as long as it is 5% by mass or less, but it is preferably 0.01 to 5% by mass. By setting the content to 0.01% by mass or more, a sufficient corrosion delay effect can be obtained. On the other hand, by setting the content to 5% by mass or less, the effect does not saturate, the increase in manufacturing cost can be suppressed, and the composition management of the plating bath can be easily performed. Moreover, the plating layer preferably contains at least Mg. This is because when the plating layer contains Mg, Mg2Si can be generated together with Si described above, and a corrosion delay effect can be obtained. Here, the content of Mg in the plating layer is preferably 0.01 to 5% by mass, and more preferably 2 to 4.9% by mass.

[0027] Furthermore, similar to the alkaline earth metals such as Ca and Mg as the optional additive components, since it has the effect of improving the stability of the corrosion product and delaying the progress of corrosion, the plating layer may further contain, as the optional additive component, one or more selected from Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B, in a total amount of 5% by mass or less, preferably 0.01 to 5% by mass.

[0028] The plating layer contains components of the base steel plate taken into the plating during the plating process due to the reaction between the plating bath and the base steel plate, as well as inevitable impurities in the plating bath. As the components of the base steel plate taken into the plating, Fe may be contained up to about 2% at most. Examples of the types of inevitable impurities in the plating bath include Fe, Cu, Zr, etc. Regarding Fe in the plating layer, it is impossible to distinguish and quantify the Fe taken in from the base steel plate and the Fe in the plating bath. Although the total content of inevitable impurities is not particularly limited, from the viewpoint 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.

[0029] The interface alloy layer is a layer existing at the interface with the base steel plate in the plating layer. As described above, it is a Fe-Al-based and / or Fe-Al-Si-based compound inevitably formed by the alloying reaction of Fe on the steel plate surface with Al and Si in the bath. Since this interface alloy layer is hard and brittle, if it grows thick, it will become the starting point for crack generation during processing. Therefore, it is preferably made as thin as possible. Specifically, the thickness of the interface alloy layer is preferably 2 μm or less, more preferably 1 μm or less.

[0030] In addition, as a means for forming the plating layer on the base steel sheet, there is no particular limitation, and a normal continuous melting plating facility can be used. For example, the base steel sheet is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere. After removal of rolling oil and the like adhering to the steel sheet surface and reduction removal of the oxide film are carried out simultaneously with annealing, it passes through a snout with its lower end immersed in a plating bath and is immersed in a molten zinc plating bath containing Al and Zn 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 arranged above the plating bath, and then the plating layer is formed by cooling with a cooling device.

[0031] Further, when the plating layer does not contain any optional additive components, for example, by performing heat treatment at about 200°C for 24 hours, the elongation at break of the molten Al-Zn-based plated steel sheet before film formation can be improved to about 20% or more. This is presumably because Zn supersaturated and solid-dissolved in the Al-rich dendrite phase is discharged by the above heat treatment, resulting in softening of the plating layer. On the other hand, in the case of a plating layer having a composition containing optional additive components, the effect of improving workability by such heat treatment is small. For example, when 2 - 4.9% of Mg is added as the optional additive component, the elongation at break after heat treatment remains less than 5%. The reason for this is not yet clear, but it is presumed that factors such as remaining as a solid solution element in the Al-rich dendrite phase even after heat treatment due to its high solubility in Al are influential.

[0032] Furthermore, the structure of the plating layer has a dendrite phase and an interdendrite phase, and it is preferable that the Vickers hardness of the dendrite phase is 10 - 110 Hv. 0.01 By making the Vickers hardness of the dendrite phase as small as 10 - 110 Hv, the workability of the painted steel sheet can be enhanced, and the corrosion resistance after processing can be further enhanced. When the Vickers hardness of the dendrite phase is 110 Hv 0.01 0.01 ​If it exceeds this value, there is a risk that sufficient workability cannot be obtained. On the other hand, when the Vickers hardness of the dendrite phase is less than 10 Hv 0.01 this is because there is a risk of degrading the scratch resistance of the surface of the plating layer. From the same perspective, the Vickers hardness of the dendrite phase is preferably 20 to 100 Hv 0.01 more preferably 30 to 90 Hv 0.01 In such a case, the elongation at break during processing of the plating layer alone can be increased to approximately 20% or more, and the elongation at break after forming the colored coating film can be increased to approximately 25% or more. Regarding the Vickers hardness, the test is carried out with a test force of 10 g (Hv 0.01 ).

[0033] (Intermediate layer) In the coated steel sheet of the present invention, a chemical conversion coating can be formed as an intermediate layer formed between the plating layer of the hot-dip galvanized steel sheet and the colored coating film. By forming a chemical conversion film between the plating layer and the colored coating film, the corrosion resistance of the coated steel sheet can be further enhanced.

[0034] There are no particular limitations on the type and formation conditions of the chemical conversion coating, and it can be appropriately selected according to the required performance. For example, it can be formed by chromate treatment or chromium-free chemical conversion treatment in which a chromate treatment solution or a chromium-free chemical conversion treatment solution is applied and dried at a steel sheet temperature of 80 to 300 °C without water washing. When considering the working environment, etc., it is preferable to form the chemical conversion coating by chromium-free treatment (that is, the chemical conversion coating does not contain chromium).

[0035] In addition, for the painted steel sheet of the present invention, a primer layer (undercoat film) can be formed as an intermediate layer formed between the plating layer of the molten Al-Zn-based plated steel sheet and the colored coating film. By forming a primer layer between the plating layer and the colored coating film, the adhesion of the colored coating film (topcoat film) described later to the molten plated steel sheet can be further enhanced, and the corrosion resistance and rust prevention property can also be further improved. In the case where the chemical conversion film is formed on the molten Al-Zn-based plated steel sheet in the painted steel sheet, the primer layer is formed on the chemical conversion film.

[0036] There is no particular limitation on the film thickness of the primer layer, and it can be appropriately adjusted according to the required performance. For example, from the viewpoint of achieving both rust prevention and workability, the film thickness of the primer layer can be set to 2 to 15 μm. When the film thickness is 2 μm or more, sufficient rust prevention property can be obtained, while when it is 15 μm or less, sufficient workability can be ensured.

[0037] Here, the primer layer can contain a rust preventive agent from the viewpoint of improving the rust prevention property of the painted steel sheet. As the rust preventive agent, either a chromate type containing chromate or a chromate-free type not using chromate can be used. However, considering the working environment, etc., the chromate-free type (that is, not containing chromium in the primer layer) is preferable, and the following will describe the chromate-free type primer layer.

[0038] The resin constituting the matrix component of the primer layer is not particularly limited, but from the viewpoints of workability, corrosion resistance, rust prevention property, etc., it is preferable to use a polyester resin and / or an epoxy resin.

[0039] In addition, for the polyester resin, it is preferable to mainly contain a polyester resin having a urethane bond. Here, as the polyester resin having the urethane bond, known resins such as those obtained by the reaction of a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups can be used. Also, a resin obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups in a state of excess hydroxyl group (urethane-modified polyester resin) and cured with a blocked polyisocyanate can also be used.

[0040] The polyester polyol can be obtained by a known method using a dehydration condensation reaction between a polyhydric alcohol component and a polybasic acid component. Examples of the polyhydric alcohol include glycols and polyhydric alcohols having three or more valences. Examples of the glycol include ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, neopentyl glycol, hexylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-butyl-2-ethyl-1,3-propanediol, methylpropanediol, cyclohexanedimethanol, 3,3-diethyl-1,5-pentanediol, and the like. Examples of the polyhydric alcohol having three or more valences include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol, and the like. These polyhydric alcohols can be used alone or in combination of two or more. The polybasic acid is usually a polyvalent carboxylic acid, but a monovalent fatty acid or the like can be used in combination as needed. 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.

[0041] Examples of the polyisocyanate compound include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate, aromatic diisocyanates such as xylylene diisocyanate (XDI), metaxylylene diisocyanate, tolylene diisocyanate (TDI), 4,4-diphenylmethane diisocyanate (MDI), and cycloaliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI, and adducts, biuret compounds, isocyanurate compounds, etc. of these. These polyisocyanate compounds can be used alone or in combination of two or more.

[0042] The polyester resin having a urethane bond has both flexibility and strength, and when processed, effects such as suppressing the generation of cracks in the primer layer can be obtained. In addition, it has a high affinity with the chemical conversion coating film containing a urethane resin, and particularly contributes to the improvement of the corrosion resistance of the processed part. Here, from the viewpoints of solvent resistance, processability, etc., the hydroxyl value of the polyester resin having a urethane bond is preferably 5 to 120 mgKOH / g, more preferably 7 to 100 mgKOH / g, and still more preferably 10 to 80 mgKOH / g.

[0043] In addition, from the viewpoints of solvent resistance, processability, etc., the number average molecular weight of the polyester resin having the urethane bond is preferably 500 to 15,000, more preferably 700 to 12,000, and even more preferably 800 to 10,000.

[0044] In addition, the polyester resin is preferably contained in the primer layer in an amount of 40 to 88% by mass. If it is less than 40% by mass, the binder function as the primer layer will decrease. If it exceeds 88% by mass, the function of the inorganic substances shown below, for example, the inhibitor action may decrease. The inorganic substances contained in the primer layer may include vanadium compounds, phosphate compounds, magnesium oxides, etc. that function as inhibitors.

[0045] Examples of the types of vanadium compounds that act as the inhibitor include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, vanadium acetylacetonate, etc. Among these, as the vanadium compound, it is preferable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation. The vanadium compound added to the primer layer may be of the same type or a different type from the vanadium compound added to the chemical conversion coating film. It is considered that the vanadate compound reacts with vanadate ions that gradually elute from moisture invading from the outside and ions on the surface of the zinc-based electroplated steel sheet to form a passive film with good adhesion and protect the metal exposed part, thereby exhibiting a rust prevention effect.

[0046] In addition, the content of the vanadium compound in the primer layer is preferably 4 to 20% by mass. If it is less than 4% by mass, the inhibitor effect may decrease, leading to a possible decrease in corrosion resistance. If it exceeds 20% by mass, the moisture resistance of the primer layer may decrease.

[0047] Regarding the types of phosphate compounds that act as the inhibitor, for example, phosphoric acid, ammonium salts of phosphoric acid, alkali metal salts of phosphoric acid, alkaline earth metal salts of phosphoric acid, etc. can be used. Among these, it is preferable to use alkali metal salts of phosphoric acid such as calcium phosphate.

[0048] Also, the content of the phosphate compound in the primer layer is preferably 4 to 20% by mass. If it is less than 4% by mass, the inhibitor effect may decrease, leading to a possible reduction in corrosion resistance. If it exceeds 20% by mass, the moisture resistance of the primer layer may decrease.

[0049] Magnesium oxide that acts as the inhibitor has the effect of stabilizing the products generated by initial corrosion as poorly soluble magnesium salts. The addition amount of magnesium oxide in the primer layer is preferably 4 to 20% by mass. If it is less than 4% by mass, the above effect may decrease, leading to a possible reduction in corrosion resistance. If it exceeds 20% by mass, the flexibility of the primer layer may decrease, particularly leading to a possible reduction in the corrosion resistance of the processed part.

[0050] The crosslinking agent used when forming the primer layer reacts with the polyester resin having the urethane bond to form a crosslinked coating film, and is preferably a blocked polyisocyanate compound. Examples of the blocked polyisocyanate include those in which the isocyanate groups of the polyisocyanate compound are blocked by alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactam, diketones such as diethyl acetoacetate, imidazoles such as imidazole and 2-ethylimidazole, or phenols such as m-cresol.

[0051] (Colored coating film) The coated steel sheet of the present invention further includes a colored coating film in addition to the above-described hot-dip Al-Zn alloy coated steel sheet and the intermediate layer. The colored coating film is one of the components that play an important role in the present invention along with the above-described plating layer. By optimizing the coating film composition, good processability, corrosion resistance, scratch resistance, etc. can be achieved.

[0052] In the coated steel sheet of the present invention, the colored coating film is formed using a paint containing at least a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a curing agent, and organic spherical aggregates. In the paint, the contents of (a) the polyester resin, (b) the pyrazole-based blocked isocyanate curing agent, and (c) the melamine-based curing agent satisfy the following relationship (1) or (2): (b) > (c) > 0 and (b) / (a) ≤ 0.4 ···(1) (b) > (c) = 0 and 0.2 ≤ (b) / (a) ≤ 0.8 ···(2) The colored coating film has an L value of 60 or more in the Hunter Lab color space on the surface, and when baking coating is performed with a continuous color line represented by the following (3), the color tone variation Δb is 0.3 or less (Δb ≤ 0.3), Δb = b(T) ― b(E) ···(3) b(T): The b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film at the start of continuous baking coating b(E): The b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film during continuous baking coating.

[0053] By including a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a curing agent containing at least a pyrazole-based blocked isocyanate curing agent, and organic spherical aggregates in the paint constituting the colored coating film, it is possible to improve processability and suppress yellowing during coating.

[0054] Here, the polyester resin is a resin serving as a base material and can contribute to the improvement of the processability of the coated steel sheet. As the polyester resin, for example, those containing a polyester resin, a silicon-modified polyester resin, and an acrylic-modified polyester resin can be used, and they can be used together with a curing agent described later.

[0055] The polyester resin has a number average molecular weight of 1,000 to 30,000, preferably 1,000 to 20,000. If the number average molecular weight of the polyester resin is less than 1,000, the processability may decrease. On the other hand, if the number average molecular weight exceeds 30,000, the weather resistance decreases, and a high viscosity results in the need for an excessive diluting solvent, and the ratio of the resin in the paint decreases, so that an appropriate coating film cannot be obtained, and the compatibility with other compounding components may also decrease. Also, the polyester resin has a glass transition point of -20 to 60°C. The reason for setting the glass transition point of the polyester resin to -20 to 60°C is that by setting the glass transition point to -20°C or higher, the hardness of the colored coating film can be increased, and by setting it to 60°C or lower, the flexibility of the colored coating film can be increased, and cracking during processing can be suppressed. Furthermore, the hydroxyl groups in the molecule of the polyester resin may be located at either the terminal or the side chain in the molecule. Here, the hydroxyl value of the polyester resin is 10 to 150 mgKOH / g, preferably 20 to 140 mgKOH / g, and more preferably 30 to 130 mgKOH / g. Note that the number average molecular weight of the polyester resin is the polystyrene-equivalent molecular weight measured by GPC.

[0056] The polyester resin is a copolymer obtained by heating and reacting a polybasic acid and a polyhydric alcohol by a conventional method. As the polybasic acid component, for example, phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, maleic acid, adipic acid, fumaric acid, etc. can be used. As the polyhydric alcohol, for example, ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, triethylene glycol, glycerin, pentaerythritol, trimethylolpropane, trimethylolethane, etc. can be used.

[0057] Examples of commercially available polyester resins include, for example, Almatex (trade name, manufactured by Mitsui Chemicals, Inc.), Desmophen (trade name, manufactured by Sumika Covestro Urethane Co., Ltd.), Baylon (trade name, manufactured by Toyobo Co., Ltd.), etc.

[0058] Further, the polyester resin as the main resin is used in combination with a curing agent. Here, as the curing agent, a pyrazole-based blocked isocyanate curing agent is used, and as an optional component, a melamine-based curing agent can also be used in combination. By using a pyrazole-based blocked isocyanate curing agent as the curing agent, the function as a curing agent of the polyester resin can be exhibited, and compared with the case of using other curing agents, yellowing of the colored coating film during painting due to the blocking agent can be suppressed.

[0059] Note that the "yellowing of the coating film during painting" described above does not refer to the phenomenon that the coating film yellows over time (conventional yellowing) by reacting with moisture, heat, or ultraviolet rays after the coating film is formed, but refers to the phenomenon that the colored coating film yellows due to the blocking agent during baking painting of the paint, resulting in poor color tone stability.

[0060] Here, examples of the pyrazole-based blocked isocyanate curing agent include pyrazole, 3,5-dimethylpyrazole, 3-methylpyrazole, 4-benzyl-3,5-dimethylpyrazole, 4-nitro-3,5-dimethylpyrazole, 4-bromo-3,5-dimethylpyrazole, 3-methyl-5-phenylpyrazole, etc. Incidentally, the pyrazole-based blocked isocyanate curing agent can be used alone or in combination of two or more kinds.

[0061] Examples of the melamine-based curing agent include methylated melamine resin, n-butylated melamine resin, iso-butylated melamine resin, and the like. Incidentally, the melamine-based curing agent can be used alone or in combination of two or more kinds.

[0062] In the coated steel sheet of the present invention, the content of (a) polyester resin, the content of (b) pyrazole-based blocked isocyanate curing agent, and the content of (c) melamine-based curing agent in the paint satisfy the following relationship (1) or (2). (b) > (c) > 0 and (b) / (a) ≤ 0.4 ··· (1) (b) > (c) = 0 and 0.2 ≤ (b) / (a) ≤ 0.8 ··· (2) When a pyrazole-based blocked isocyanate curing agent and a melamine-based curing agent are used as the curing agent, by satisfying the relationship (1), and when only a pyrazole-based blocked isocyanate curing agent is used as the curing agent, by satisfying the relationship (2), the content of blocked isocyanate in the curing agent can be kept within a range where yellowing is less likely to occur. Therefore, even when baking painting is performed at a normal speed, the processability of the colored coating film can be maintained well while suppressing the yellowing of the colored coating film. When (b) / (a) in the above (1) exceeds 0.4, or when (b) / (a) in the above (2) exceeds 0.8, the content ratio of blocked isocyanate in the curing agent increases, and thus the yellowing during coating of the colored coating film may not be suppressed. Also, when (b) / (a) in the above (2) is less than 0.2, the strength and processability of the coating film may decrease. From the same viewpoint, (b) / (a) in the above (1) is preferably 0.2 ≤ (b) / (a) ≤ 0.4, and (b) / (a) in the above (2) is preferably 0.3 ≤ (b) / (a) ≤ 0.6.

[0063] Regarding the organic spherical aggregates contained in the paint, by enhancing the stress relaxation effect and reinforcement effect of the coating film and being contained in the coating film, it is possible to improve the bending processability, scratch resistance, and indentation resistance. The organic spherical aggregates are not particularly limited, but preferably have an average particle size of 8 to 40 μm, and more preferably 10 to 35 μm. Here, examples of the organic spherical aggregates include acrylic resin, nylon resin, and urethane resin.

[0064] The formed colored coating film has an L value (hereinafter sometimes simply referred to as "L value") in the Hunter Lab color space on the surface of 60 or more, and a color tone variation Δb of 0.3 or less (Δb ≦ 0.3) when baked and painted with a continuous color line as shown by the following (3). Δb = b(T)― b(E) ···(3) b(T): The b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film at the start of continuous baking and painting b(E): The b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film during continuous baking and painting

[0065] When the L value on the surface of the colored coating film is 60 or more, the lightness is high, and the yellowing suppression effect according to the present invention can be more significantly confirmed. When the L value on the surface of the colored coating film is less than 60, it is difficult to determine the presence or absence of yellowing, and it becomes difficult to grasp the effect according to the present invention. Regarding the L value on the surface of the colored coating film, there is no particular limitation as long as it is 60 or more, and it can be appropriately changed according to the performance required for the colored coating film. The L value is the L value in the Hunter Lab color space, and it can be grasped by using a commercially available spectrophotometer corresponding to the measurement of the Hunter Lab color space.

[0066] The above (3) is a relational expression for showing the color tone variation Δb when baked and painted with a continuous color line. By making the b value (b(T)) measured by a spectrophotometer on the surface of the colored coating film at the start of continuous baking coating and the b value (b(E)) measured by a spectrophotometer on the surface of the colored coating film during continuous baking coating as small as 0.3 or less, yellowing during coating can be suppressed, and an excellent surface appearance of the colored coating film can be obtained. From the same viewpoint, the color tone variation Δb is preferably 0.25 or less, more preferably 0.2 or less, and particularly preferably 0.15 or less. The b value is the b value in the Hunter Lab color space, and it can be grasped by using a commercially available spectrophotometer corresponding to the measurement in the Hunter Lab color space. In a general continuous color line, 1 to several hundred tons of hot-dip galvanized steel sheets per color are continuously processed in a single coating.

[0067] There is no particular limitation on the method for suppressing the color tone variation Δb to 0.3 or less when baking coating is applied in the continuous color line of the coating film. For example, in addition to using a pyrazole-based blocked isocyanate curing agent as the curing agent in the paint, as described in the manufacturing method of the coated steel sheet described later, in the paint, (a) the content of the polyester resin, (b) the content of the pyrazole-based blocked isocyanate curing agent, and (c) the content of the melamine-based curing agent are adjusted according to the coating conditions such as the line speed of the color line, the size of the steel sheet, and the exhaust gas volume of the coating device, so that yellowing during coating can be suppressed. In addition to the conditions of the paint and coating, if the color tone variation Δb can be suppressed low by changing the conditions of the plating film, intermediate layer, etc., that method can also be used.

[0068] In the colored coating film, appropriate amounts of titanium oxide, mica, mica, carbon black, other various coloring pigments, metallic pigments such as aluminum powder and mica, pigments such as carbonates and sulfates, silica fine particles, nylon resin beads, acrylic resin beads, glass fibers, glass beads and other various fine particles, curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate, waxes, and other additives can also be blended according to the purpose and use.

[0069] In addition, the painting method of the paint is not particularly limited. For example, the paint that becomes the material of the colored coating film can be applied by methods such as roll coater painting and curtain flow painting. 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 topcoat film. The temperature of the baking treatment is usually about 180 to 270°C for the maximum reach plate temperature, and this temperature range is carried out for about 30 seconds to 3 minutes.

[0070] Note that the film thickness of the colored coating film is not particularly limited and can be appropriately adjusted according to the required performance. For example, from the viewpoint of obtaining more excellent scratch resistance and corrosion resistance after processing without deteriorating productivity, the film thickness of the colored coating film is preferably 5 to 30 μm, more preferably 10 to 25 μm, and even more preferably 12 to 22 μm. When the film thickness of the colored coating film is 5 μm or more, more excellent scratch resistance and corrosion resistance after processing can be realized. On the other hand, when it is 30 μm or less, it does not cause manufacturing complexity or an increase in manufacturing cost.

[0071] (Workability of the painted steel sheet) For the painted steel sheet of the present invention, for the purpose of improving workability and corrosion resistance after processing, it is very effective to set the elongation at break, which can also be said to be the effect index of the coating film related to the workability of the plating layer, to an appropriate value.

[0072] And in the painted steel sheet of the present invention, when a bending test is conducted in accordance with the adhesion test of plating described in JIS G 3321 (2019), the elongation rate El by the bending test is obtained from Formula (4), and the maximum elongation rate El (elongation at break) at which no cracks are observed in the test piece is 10% or more (El ≧ 10). El (%) = t / (2r - t) × 100 (%) ···(4) t: Thickness of the steel sheet (mm), r: Outer R of the bent test piece

[0073] Here, FIG. 1 is a photograph obtained by observing a cross-section of the plating layer of the processed part when the painted steel sheet of the present invention is bent, at a magnification of 300 times by SEM. It can be seen that the generation of cracks in the interdendritic phase is suppressed by the stress relaxation effect of the coating film formed on the plating layer. Also, regarding the coating film formed on the plating layer, it can be seen that local elongation occurs due to the stress generated during bending, and the film thickness becomes smaller. On the other hand, FIG. 2 schematically shows a cross-section of the coating film and the plating layer of the processed part when the painted steel sheet of the present invention is bent. When tensile stress σ p is applied in a direction parallel to the interface between the steel sheet and the plating layer where the plating layer is bent, stress (dashed arrow) is generated in the coating film formed on the plating layer, and this becomes the stress relaxation effect of the plating layer. As a result, compared with the conventional molten Al-Zn based plated steel sheet, the stress (solid arrow) generated in the plating layer becomes smaller, and it becomes possible to improve workability and, consequently, the corrosion resistance after processing.

[0074] And in the painted steel sheet of the present invention, when a bending test compliant with the plating adhesion test described in JIS G 3321 (2019) is performed, the maximum elongation rate El (limit elongation rate) at which no cracks are observed in the test piece is 10% or more (El≧10). By having this limit elongation rate, the stress concentrated in the interdendrites in the plating layer can be relaxed, and excellent corrosion resistance after processing can be realized. From the same perspective, it is preferable that the maximum elongation rate El is 12% or more (El≧12), more preferably 14% or more (El≧14), and even more preferably 20% or more (El≧20).

[0075] Note that for the elongation rate El of the painted steel sheet, the obtained elongation rate is used as a result of performing a bending test compliant with the plating adhesion test (the "plating adhesion test" in JIS G 3321 (2019) 13.3.3) described in JIS G 3321 (2019). In the above JIS G 3321 (2019), the steel plates sandwiched inside are specified by the number of plates with the indicated thickness. In this case, only a fixed elongation rate can be obtained. However, instead of this, plates with arbitrary thicknesses can also be used. In the present invention, by adopting the elongation rate represented by the following formula (4), the limiting elongation rate El (%) of the molten Al-Zn alloy plated steel sheet before film formation can be measured. El (%) = t / (2r - t) × 100 (%) ··· (4) (t: thickness of the steel plate (mm), r: outer R of the bent test piece) Note that the method for measuring the outer R of the test piece is not particularly limited. For example, measurement by an R gauge, measurement by a laser microscope, measurement by a 3D microscope, etc. can be mentioned.

[0076] Note that the measurement conditions of the limiting elongation rate El in the coated steel plate of the present invention may be calculated from the above formula (4). As the inner spacing during 180° bending, tests (2T bending, 3T bending) with two or three steel plates sandwiched can also be performed, or 180° bending can be performed without sandwiching anything. Also, although the steel plates sandwiched inside are specified by the number of plates with the indicated thickness, only a fixed elongation rate can be obtained in that case, so plates with arbitrary thicknesses can also be used.

[0077] Also, regarding the means for confirming whether cracks have occurred in the test piece when the bending test is performed, it is not particularly limited as long as it is a method capable of reliably confirming cracks. For example, from the viewpoint of being able to reliably confirm the cracks in the test piece, the bent part can be observed at a magnification of 10 times using a loupe. Here, the crack in the test piece is a crack that can be confirmed from the surface layer of the coated steel plate, and is a state where the plating layer or the steel plate substrate is exposed and the metal surface can be seen.

[0078] Note that the method for adjusting the limiting elongation rate El is not particularly limited. By combining the composition of the plating layer, the composition of the colored coating film, the adjustment of heat treatment conditions, etc. in a complex manner, a desired limiting elongation rate El can be obtained.

[0079] <Manufacturing method of coated steel plate> The manufacturing method of the painted steel sheet of the present invention is a method for manufacturing a painted steel sheet in which a colored coating film is formed directly or via an intermediate layer on a molten Al-Zn alloy plated steel sheet. Here, the colored coating film is formed using a paint containing at least a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a pyrazole-based blocked isocyanate curing agent, and an organic spherical aggregate. By including, in the paint constituting the colored coating film, a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a pyrazole-based blocked isocyanate curing agent, a melamine-based curing agent, and an organic spherical aggregate, it is possible to suppress yellowing during painting while improving workability.

[0080] And, in the manufacturing method of the painted steel sheet of the present invention, the content of (a) the polyester resin, the content of (b) the pyrazole-based blocked isocyanate curing agent, and the content of (c) the melamine-based curing agent in the paint satisfy the following relationship when the painting condition X shown in the following (5) is 9 to 18 (X = 9 to 18 g / Nm 3 ), and satisfy the following relationship (2) when the painting condition X shown in the following (5) is less than 9 (X < 9 g / Nm 3 ). X (g / Nm 3 ) = line speed (m / min) × plate width (m) × coating amount (g / m 2 ) / exhaust gas amount of the coating apparatus (Nm 3 / min) ···(5) (b) > (c) > 0 and (b) / (a) ≤ 0.4 ···(1) (b) > (c) = 0 and 0.2 ≤ (b) / (a) ≤ 0.8 ···(2)

[0081] In the paint, by satisfying the content of (a) polyester resin, the content of (b) pyrazole-based blocked isocyanate curing agent, and the content of (c) melamine-based curing agent in Formula (1) or (2), the content of blocked isocyanate in the curing agent can be kept within a range where yellowing is less likely to occur. Therefore, even when baking painting is performed at a certain speed, while maintaining good processability of the colored coating film, yellowing of the colored coating film can be suppressed. When (b) / (a) in the above (1) exceeds 0.4, or when (b) / (a) in the above (2) exceeds 0.8, the content ratio of blocked isocyanate in the curing agent increases, so it may not be possible to suppress yellowing during painting of the colored coating film. Also, when (b) / (a) in the above (2) is less than 0.2, the strength and processability of the coating film may decrease. From the same perspective, (b) / (a) in the above (1) is preferably 0.2 ≦ (b) / (a) ≦ 0.4, and (b) / (a) in the above (2) is preferably 0.3 ≦ (b) / (a) ≦ 0.6.

[0082] The painting condition X is a condition indicating the coating level of the paint with respect to the exhaust gas amount of the painting apparatus when baking and curing the paint, and is an index for grasping how much of the blocking agent volatilized from the paint remains in the painting apparatus (in the furnace) during baking painting. The painting condition X is 9 g / Nm 3 less than or 9 - 18 g / Nm 3 The reason for dividing by cases is that when the painting condition X is less than 9 g / Nm 3 , since the amount of the blocking agent volatilized from the paint during baking painting is relatively small, more of the (b) pyrazole-based blocked isocyanate curing agent can be contained in the paint. On the other hand, when the painting condition X is 9 - 18 g / Nm 3 , since it is considered that it remains to some extent, the (b) pyrazole-based blocked isocyanate curing agent is suppressed to more surely suppress yellowing of the coating film. Note that the painting condition X is set to 9 - 18 g / Nm 3 and the upper limit (18 g / Nm 3The reason for defining 3 is that when the painting condition X exceeds 18 g / Nm, a large amount of the blocking agent volatilized from the paint exists in the furnace during baking painting, so the occurrence of yellowing cannot be sufficiently suppressed.

[0083] The line speed refers to the passing speed of the continuous color line (painting apparatus). Also, the plate width refers to the plate width of the steel plate to which the paint is applied. Furthermore, the coating amount refers to the coating amount of the coating film formed by curing the paint. Furthermore, the exhaust gas amount of the painting apparatus refers to the amount of gas discharged (gas discharge performance) from the furnace in which baking painting is performed in the painting apparatus within a unit time (1 minute).

[0084] In addition, regarding the configurations of the hot-dip Al-Zn alloy coated steel sheet, the intermediate layer, the colored coating film, and the paint other than the above-described conditions in the method for manufacturing a painted steel sheet of the present invention are the same as those described in the painted steel sheet of the present invention described above.

[0085] Also, the conditions for forming the intermediate layer on the hot-dip Al-Zn alloy coated steel sheet are not particularly limited, and known chemical conversion film and / or primer application and curing conditions can be appropriately used.

Examples

[0086] <Samples 1 to 29 of painted steel sheets> Samples of painted steel sheets were manufactured according to the conditions of (1) hot-dip Al-Zn alloy coated steel sheet, (2) chemical conversion treatment film, (3) undercoat film, and (4) colored coating film shown below.

[0087] (1) Hot-dip Al-Zn alloy coated steel sheet The following hot-dip Al-Zn alloy coated steel sheets were used. The plating types used for each sample are shown in Table 1. The plate width of plating types 1 to 3 is 1166 mm in all cases. Plating type 1: Plate thickness 0.35 mm, plating adhesion amount 80 g / m per side 2、A hot-dip Al-Zn plated steel sheet with a plating layer having a composition of Zn-55%Al-1.6%Si Plating type 2: Plate thickness 0.35 mm, plating adhesion amount 80 g / m per side 2 、A hot-dip Al-Zn plated steel sheet with a plating layer having a composition of Zn-55%Al-1.6%Si, and heat-treated at 200°C in an atmosphere for 4 hours after the plating layer was formed Plating type 3: Plate thickness 0.35 mm, plating adhesion amount 80 g / m per side 2 、A hot-dip Al-Zn plated steel sheet with a plating layer having a composition of Zn-55%Al-4%Mg-2%Si

[0088] (2) Conversion coating As the resin component of the conversion coating, "Superflex 210" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., an anionic urethane resin having an ester bond, and "Yukarezine RE-1050" manufactured by Yoshimura Oil Chemical Co., Ltd., an epoxy resin having a bisphenol skeleton, were mixed. As the rust preventive components contained in the conversion coating, an organic vanadium compound chelated with acetylacetone, ammonium zirconium carbonate, and ammonium fluoride were used. These raw materials were mixed to obtain a conversion treatment solution. The pH of the conversion treatment was set to 8 - 10. The obtained conversion treatment solution was applied onto the hot-dip Al-Zn plated steel sheet in a continuous color line, dried at a steel sheet reaching temperature of 90°C and a baking time of 10 seconds, and a conversion coating was formed so that the adhesion amount became 0.2 g / m 2 .

[0089] (3) Primer coating film As the main component of the primer coating film, a urethane-modified polyester resin cured with blocked isocyanate, which is a polyester resin having a urethane bond, was used. As the rust preventive components, magnesium vanadate and calcium phosphate were used. These raw materials were mixed and then stirred in a ball mill for about 1 hour to obtain a paint for the primer coating film. The obtained paint for the primer coating film was applied onto the conversion coating in a continuous color line, baked at a steel sheet reaching temperature of 230°C and a baking time of 35 seconds, and a coating film was formed so that the film thickness after baking became 4 μm.

[0090] (4) Colored coating film (4-1) Preparation of paint In Samples 1 to 26, the paint constituting the colored coating film contains, as main components, a polyester resin (number average molecular weight 3000, glass transition temperature 25°C), a blocked isocyanate (b) containing a pyrazole-based blocking agent as a curing agent, and (c) methylated melamine, which are formulated as shown in Table 1. Thereafter, acrylic resins with different average particle diameters (see Table 1) as organic spherical resins were used in the paint. In addition, in the paint, titanium oxide pigment (white) and iron chromium composite oxide pigment (black) were contained in the range of 15 to 40% by mass as coloring pigments to adjust the L value. Furthermore, 2 to 10% by mass of a matting agent (silica) was contained in the paint so that the gloss value at 60° of the mirror surface of the coating film would be 5 to 10%. In Sample 27, a paint was prepared under the same conditions as Samples 1 to 26, except that a polyester resin with a number average molecular weight of 3000 and a glass transition temperature of 70°C was used. In Sample 28, a paint was prepared under the same conditions as Samples 1 to 26, except that a polyester resin with a number average molecular weight of 3000 and a glass transition temperature of 40°C was used. In Sample 29, a paint was prepared under the same conditions as Samples 1 to 26, except that no organic spherical resin was contained. (4-2) Baking of coating film Thereafter, the prepared paint was applied using a continuous color line in a roll coater method to form a coating film. It was applied on the primer coating film with a roll coater and baked at a steel plate reaching temperature of 240°C. The line speed (m / min) of the continuous color line, the exhaust gas volume (Nm 3 / min) of the coating device, the adhesion amount (g / m 2 ) of the formed colored coating film, and the value of the coating condition X (g / Nm 3 ) obtained from formula (5) are shown in Table 1.

[0091] <Evaluation> For each sample of the painted steel plate obtained as described above, the following evaluation was carried out.

[0092] (1) L value For each painted steel sheet of each sample, the L value in the Hunter Lab color space was measured using a spectrophotometer (Hunter Lab: Ultrascan VIS).

[0093] (2) Yellowing For each painted steel sheet of each sample, the b value in the Hunter Lab color space was measured using a spectrophotometer (Hunter Lab: Ultrascan VIS). For the measurement of the b value, the b value at the position 30 m painted in the longitudinal direction from the start of painting of the continuous color line coating was defined as b(T), and the b value at the position continuously painted 500 m in the longitudinal direction from b(T) was defined as b(E), and Δb was calculated according to the following formula. Δb = b(T) - b(E) Δb was considered good (〇) if it was 0.3 or less, very excellent (◎) if it was 0.2 or less, and defective (×) if it exceeded 0.3.

[0094]

Table 1

[0095] From the results in Table 1, it can be seen that each sample of the present invention example is superior in yellowing compared to Samples 22, 24 to 26 of the comparative example. Also, it can be seen that each sample of the present invention example is superior in workability compared to Samples 23, 27 of the comparative example. Furthermore, it can be seen that each sample of the present invention example is superior in weather resistance compared to Sample 28 of the comparative example and superior in pencil hardness compared to Sample 29 of the comparative example.

Industrial Applicability

[0096] According to the present invention, it is possible to provide a painted steel sheet that can suppress yellowing during painting and is excellent in workability, and a method for manufacturing the painted steel sheet.

Claims

1. A coated steel sheet having a colored coating film formed directly or via an intermediate layer on a molten Al-Zn alloy plated steel sheet, wherein the colored coating film is formed using a paint containing at least a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a curing agent, and organic spherical aggregates, wherein the contents of (a) the polyester resin, (b) the pyrazole-based blocked isocyanate curing agent, and (c) the melamine-based curing agent in the paint satisfy the following relationship (1) or (2): (b) > (c) > 0 and (b) / (a) ≤ 0.4... (1) (b) > (c) = 0 and 0.2 ≤ (b) / (a) ≤ 0.8... (2) wherein the colored coating film has an L value of 60 or more in the Hunter Lab color space on the surface, and the color tone variation Δb (Δb ≤ 0.3) when baked with a continuous color line represented by the following (3) is 0.3 or less, Δb = b(T) - b(E)... (3) b(T): the b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film at a position 30 m longitudinally coated from the start of coating b(E): the b value in the Hunter Lab color space measured by a spectrophotometer on the surface of the colored coating film at a position 500 m longitudinally coated from the start of coating When a bending test is performed in accordance with the adhesion test of plating described in JIS G 3321 (2019), the elongation rate El at a plate temperature of 20°C in the bending test is obtained from the formula (4), and the maximum elongation rate El (limiting elongation rate) at which no cracks are observed in the test piece is 10% or more (El ≥ 10) El (%) = t / (2r - t) × 100 (%)... (4) t: the thickness of the steel sheet (mm), r: the outer R of the bent test piece A coated steel sheet characterized by the above.

2. The coated steel sheet according to claim 1, wherein the organic spherical aggregates are acrylic resins having an average particle size of 8 to 40 μm.

3. El, which is the elongation at break at a plate temperature of 40°C ４０ is 14% or more (El ４０ ≧ 14), and the painted steel sheet according to claim 1 or 2 is characterized thereby.

4. The coated steel sheet according to claim 1 or 2, wherein the plating layer of the molten Al-Zn alloy plated steel sheet has a composition containing 50 to 60% by mass of Al, 1 to 3% by mass of Si, and 5% or less of optional additive components, and the balance consists of Zn and inevitable impurities.

5. The Vickers hardness of the dendrite phase in the plating layer of the molten Al-Zn-based plated steel sheet is 10 to 110 HV 0.01 The coated steel sheet according to claim 1 or 2, characterized in that it is as described above.

6. The coated steel sheet according to claim 4, wherein the plating layer further contains 0.01 to 5% by mass of Mg as the optional additive component.

7. A method for manufacturing a coated steel sheet in which a colored coating film is formed directly or via an intermediate layer on a molten Al-Zn alloy coated steel sheet, wherein the colored coating film is formed using a paint containing at least a polyester resin having an average molecular weight of 1,000 to 30,000 and a glass transition point of -20 to 60°C, a curing agent, and organic spherical aggregates, The painting condition X shown in the following (5) is 18 or less (X ≦ 18 g / Nm 3 ) and In the said paint, the content of (a) the polyester resin, the content of (b) the pyrazole-based blocked isocyanate curing agent as a curing agent, and the content of (c) the melamine-based curing agent satisfy the following coating condition X shown in (5) when X = 9 to 18 (X = 9 to 18 g / Nm 3 ) and satisfy the following relationship (1). When the coating condition X shown in (5) is less than 9 (X < 9 g / Nm 3 ), the following relationship (2) is satisfied X (g / Nm 3 ) = Line speed (m / min) × Sheet width (m) × Adhesion amount (g / m 2 ) / Exhaust gas volume of coating equipment (Nm 3 / min) ··· (5) (b) > (c) > 0 and (b) / (a) ≤ 0.4... (1) (b) > (c) = 0 and 0.2 ≤ (b) / (a) ≤ 0.8... (2) characterized by the above. A method for manufacturing a coated steel sheet.

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