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)≤0.3. Relation (2): (b)>(c)=0 and (b) / (a)≤0.5.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] A molten Al-Zn-based plated steel sheet containing 20 to 95% by mass of Al in the plating layer, typified by an Al-Zn-based plated steel sheet, has excellent corrosion resistance, and thus, in recent years, the demand has been increasing mainly in the building materials field. In addition, it is common to form a coating film on a molten Al-Zn-based plated 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-based plated 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 molten plating facility. The structure of the plating layer of an Al-Zn-based plated steel sheet manufactured by such a continuous molten plating facility mainly consists of a portion where Al containing Zn in supersaturation dendritically solidifies (dendrite phase) and a portion of the remaining interdendritic gap (interdendritic phase), and the dendrite phase is laminated in the film thickness direction of the plating layer. Due to such a characteristic film structure, the Al-Zn-based plated steel sheet can achieve good corrosion resistance.

[0004] However, while a molten Al-Zn-based plated steel sheet such as an Al-Zn-based plated 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 plating layer of the processed portion, which may cause deterioration of corrosion resistance due to the cracked portion and may pose a problem in use.

[0005] Regarding such a problem, for example, Patent Document 1 discloses a technique for improving the ductility of an Al-Zn-based plated 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 or wall members, and there has still been a desire to develop 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, although it is possible to cover the cracks generated in the plating layer with the coating film and thus expect an improvement in the appearance after processing, 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 becomes yellow 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 based on a polyester resin and containing at least a pyrazole-based blocked isocyanate curing agent as a curing agent, thereby improving processability and suppressing yellowing during coating. 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 generation 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 processability 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, 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 temperature of -20 to 60°C, a curing agent, and organic spherical aggregates, wherein 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 in the paint satisfy the following relationship (1) or (2), (b) > (c) > 0 and (b) / (a) ≤ 0.3 ··· (1) (b) > (c) = 0 and (b) / (a) ≤ 0.5 ··· (2) The colored coating film has an L value in the Hunter Lab color space on the surface of 60 or more, and a color tone variation Δb of 0.25 or less (Δb≦0.25) when baking paint is applied with a continuous color line represented by the following (3). Δb = b(T)― b(E) ···(3) b(T): The b value in the Hunter Lab color space measured by a spectrocolorimeter on the surface of the colored coating film at the start of continuous baking paint application b(E): The b value in the Hunter Lab color space measured by a spectrocolorimeter on the surface of the colored coating film during continuous baking paint application 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 the formula (4), and the maximum elongation rate El (limiting elongation rate) at which no crack generation is observed in the test piece is 10% or more (El≧10). El(%)=t / (2r - t)×100(%) ···(4) t: The thickness of the steel plate (mm), r: The outer R of the test piece subjected to bending processing 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 is characterized by this.

[0013] 4. 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 optional additive components, and the balance is composed of Zn and inevitable impurities. The painted steel sheet according to any one of 1 to 3 above is characterized by this.

[0014] 5. 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 The painted steel sheet according to any one of 1 to 4 above is characterized by this.

[0015] 6. The coated steel sheet according to 4 above, wherein the plating layer further contains 0.01 to 5% by mass of Mg 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 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 the curing agent, and the content of (c) the melamine-based curing agent satisfy the following relationship (1) when the in-furnace time t (min) in the temperature range exceeding the dissociation temperature T (°C) of the blocked isocyanate in the paint during baking is 0.2 or more and less than 0.3, and satisfy the following relationship (2) when the in-furnace time t is 0.30 or more (b) > (c) > 0 and (b) / (a) ≤ 0.3 ··· (1) (b) > (c) = 0 and (b) / (a) ≤ 0.5 ··· (2) This is a method for manufacturing a coated steel sheet, characterized by the above.

Advantages 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

Embodiments for Carrying Out the Invention

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

[0020] (Hot-dip Al-Zn alloy coated steel sheet) The hot-dip Al-Zn alloy coated steel sheet is a steel sheet on which an Al-Zn alloy coating layer is formed. The Al-Zn alloy coating 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 20 to 95% by mass of Al, 1 to 3% by mass of Si, and 5% by mass or less of optional additive components, with the balance being Zn and unavoidable impurities. By having the coating layer of the hot-dip Al-Zn alloy coated steel sheet have the above-described composition, a dendrite phase and an interdendrite phase surrounding the dendrite phase in a network shape can be formed in the coating layer, and the corrosion resistance can be improved.

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

[0022] Here, the Al content in the coating layer is 20 to 95% by mass, preferably 50 to 60% by mass, in view of the balance between corrosion resistance and operation. If the Al content in the coating layer is at least 20% by mass, sufficient dendritic solidification of Al occurs. As a result, the coating layer mainly contains Zn in a supersaturated state, and is composed of a portion where Al dendritically solidifies (α-Al dendrite phase) and the remaining interdendritic gap portion (interdendrite phase), and a structure excellent in corrosion resistance in which the dendrite phase is laminated in the film thickness direction of the coating layer can be realized. In addition, the more the dendrite portions of the α-Al phase are stacked, the more complex 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 effect on Fe decreases, and the corrosion resistance deteriorates. Therefore, the Al content in the plating layer is set to 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 the molten Al-Zn-based plated steel sheet used in the coated steel sheet of the present invention, when Si is contained in the plating bath and the molten 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. And 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 deteriorated and the Si phase serving as a cathode site is likely to precipitate. Therefore, the Si content in the plating layer is set to 3% by mass or less.

[0024] The plating layer contains Zn as the main component of the plating layer. 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, it is preferable in that the content of Al can be ensured and the corrosion resistance by the above-described dendrite phase and interdendrite phase can be realized.

[0025] Furthermore, in addition to the above-described Al, Si, and Zn, the plating layer can contain an optional additive component of 5% by mass or less. 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, and B can be mentioned. Although these optional additive components can achieve effects such as further improving corrosion resistance, the workability of the plating layer may decrease, and there is a risk of deteriorating the elongation limit of the painted steel sheet. 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, improving the stability of the corrosion product and delaying the progress of corrosion. As a result, the effect of improving corrosion resistance can be 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 carried out. Also, the plating layer preferably contains at least Mg. This is because when the plating layer contains Mg, Mg2Si can be generated together with Si as 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 Ca and Mg as the optional additive components, since they can improve the stability of the corrosion product and delay the progress of corrosion, the plating layer can also contain, as the optional additive component, one or more selected from Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B, with a total content of 5% by mass or less, preferably 0.01 to 5% by mass.

[0028] Incidentally, the plating layer contains components of the base steel plate incorporated during plating due to the reaction between the plating bath and the base steel plate during the plating process, as well as inevitable impurities in the plating bath. As the components of the base steel plate incorporated during 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 present 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 total amount of inevitable impurities excluding Fe is preferably 1% by mass or less.

[0029] Incidentally, the interfacial alloy layer is a layer that exists 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 that is inevitably formed by the alloying reaction of Fe on the steel plate surface with Al or Si in the bath. Since this interfacial alloy layer is hard and brittle, if it grows thick, it will become a starting point for crack generation during processing. Therefore, it is preferably made as thin as possible. The thickness of the interfacial alloy layer is preferably 2 μm or less, more preferably 1 μm or less.

[0030] Incidentally, the means for forming the plating layer on the base steel plate is not particularly limited, and ordinary continuous melting plating equipment can be used. For example, the base steel plate is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere. After removing rolling oil, etc. adhering to the steel plate surface and reducing and removing the oxide film simultaneously with annealing, it is immersed in a molten zinc plating bath containing Al and Zn at a predetermined concentration through a snout whose lower end is immersed in the plating bath. Then, the steel plate 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 plate from a gas wiping nozzle arranged above the plating bath. Subsequently, the plating layer is formed by cooling with a cooling device.

[0031] Also, 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 alloy 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 to 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 affecting.

[0032] Furthermore, in the structure of the plating layer, there are a dendrite phase and an interdendrite phase, and the Vickers hardness of the dendrite phase is preferably 10 to 110 Hv. 0.01 By reducing the Vickers hardness of the dendrite phase to 10 to 110 Hv, the workability of the painted steel sheet can be enhanced, and the corrosion resistance after processing can be further improved. If the Vickers hardness of the dendrite phase exceeds 110 Hv, there is a risk that sufficient workability cannot be obtained. On the other hand, if the Vickers hardness of the dendrite phase is less than 10 Hv, there is a risk of degrading the scratch resistance of the plating layer surface. From the same perspective, the Vickers hardness of the dendrite phase is preferably 20 to 100 Hv, and more preferably 30 to 90 Hv. 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 formation of the colored coating film can be increased to approximately 25% or more. 0.01 0.01 0.01 0.01 0.01 Note that the Vickers hardness test is carried out with a test force of 10 g (Hv 0.01 ).

[0033] ​​​​​(Intermediate layer) The painted steel sheet of the present invention can form a chemical conversion coating as an intermediate layer formed between the plating layer of the hot-dip plated 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 painted 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 washing with water. In addition, when considering the labor 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] Also, the painted steel sheet of the present invention can also form a primer layer (undercoat coating film) as an intermediate layer formed between the plating layer of the hot-dip 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 coating film) described later to the hot-dip plated steel sheet can be further enhanced, and the corrosion resistance and rust prevention properties can also be further improved. In the painted steel sheet, when the chemical conversion coating is formed on the hot-dip Al-Zn based plated steel sheet, the primer layer is formed on the chemical conversion coating.

[0036] There are no particular limitations 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 can be obtained, while when it is 15 μm or less, sufficient workability can be ensured.

[0037] Here, from the viewpoint of improving the rust prevention property of the coated steel sheet, the primer layer can contain a rust preventive agent. Regarding the rust preventive agent, either a chromate type containing chromate or a chromate-free type not using chromate can be used. However, when considering the labor working environment, etc., the chromate-free type (that is, not containing chromium in the primer layer) is preferable. The following will explain 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] Also, regarding the polyester resin, it is preferable to mainly contain a polyester resin having a urethane bond. Here, as the polyester resin having a urethane bond, known resins such as those obtained by the reaction of a polyester polyol and a diisocyanate or polyisocyanate having two or more isocyanate groups can be used. Also, a resin obtained by reacting a polyester polyol and a diisocyanate or polyisocyanate having two or more isocyanate groups in a hydroxyl group-excess state (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. The polyhydric alcohol includes glycol and polyhydric alcohols having three or more hydroxyl groups. 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 hydroxyl groups include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol, and the like. These polyhydric alcohols can be used alone or in combination of two or more thereof. The polybasic acid is usually a polyvalent carboxylic acid, but a monovalent fatty acid or the like 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, and acid anhydrides thereof, 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, and the like. These polybasic acids can be used alone or in combination of two or more thereof.

[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), and 4,4-diphenylmethane diisocyanate (MDI); 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 kinds.

[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 containing a urethane resin, and particularly contributes to the improvement of the corrosion resistance of the processed part. Here, the hydroxyl value of the polyester resin having a urethane bond is 30 to 150 mgKOH / g, preferably 30 to 120 mgKOH / g, and more preferably 40 to 100 mgKOH / g from the viewpoints of solvent resistance, processability, etc.

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

[0044] In addition, the polyester resin is preferably contained in the primer layer at 40 to 88% by mass. If it is less than 40% by mass, the binder function as the primer layer decreases, and if it exceeds 88% by mass, the function by 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 different types from the vanadium compound added to the chemical conversion coating film. It is considered that the vanadate compound reacts with the vanadate ions gradually eluted from the moisture invading from the outside and the 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, and the rust prevention action appears.

[0046] Also, 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 and the corrosion resistance may decrease, and if it exceeds 20% by mass, the moisture resistance of the primer layer may decrease.

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

[0048] Also, the content of the phosphoric acid 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 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 product generated by initial corrosion as an insoluble magnesium salt. 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 reduction in corrosion resistance. If it exceeds 20% by mass, the flexibility of the primer layer may decrease, particularly reducing the corrosion resistance of the processed part.

[0050] In addition, 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 group of the polyisocyanate compound is 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 for the present invention along with the above-described plating layer. By optimizing the coating film composition, good workability, corrosion resistance, scratch resistance, etc. can be achieved.

[0052] In the painted steel sheet of the present invention, the colored coating film is formed using a paint containing 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 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) ≤ 0.3 ··· (1) (b) > (c) = 0 and (b) / (a) ≤ 0.5 ··· (2) 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 ≤ 0.25) when baking paint is applied by the continuous color line represented by the following (3) is 0.25 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 the start of continuous baking paint. 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 paint.

[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 workability and suppress yellowing during painting.

[0054] Here, the polyester resin is a resin serving as a base material and can contribute to improving the workability of the painted steel sheet. Examples of the polyester resin include those containing a polyester resin, a silicon-modified polyester resin, and an acrylic-modified polyester resin, and it can be used together with a curing agent described later.

[0055] The number average molecular weight of the polyester resin is 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, the viscosity becomes high, an excessive diluting solvent is required, the ratio of the resin in the paint decreases, and thus an appropriate paint film cannot be obtained, and the compatibility with other compounding components may also decrease. In addition, the glass transition point of the polyester resin is -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 paint film can be increased, and by setting it to 60 °C or lower, the flexibility of the colored paint film can be increased and the paint film cracks 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. 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. Examples of the polybasic acid component include phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, maleic acid, adipic acid, fumaric acid, etc. Examples of the polyhydric alcohol include 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.

[0057] Examples of commercially available polyester resins include, for example, Armatex (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.), and the like.

[0058] The polyester resin, which is 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 of the polyester resin as a curing agent can be exerted, 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 (conventional yellowing) in which the coating film yellowes over time in response to moisture, heat, or ultraviolet rays after the coating film is formed, but rather refers to the phenomenon in which the colored coating film yellowes due to the blocking agent during baking painting of the paint, leading to poor color tone stability.

[0060] Examples of the pyrazole-based blocked isocyanate curing agent include, for example, 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, and the like. 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. 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 contents of (a) polyester resin, (b) pyrazole-based blocked isocyanate curing agent, and (c) melamine-based curing agent in the paint satisfy the following relationship (1) or (2). (b) > (c) > 0 and (b) / (a) ≤ 0.3 ···(1) (b) > (c) = 0 and (b) / (a) ≤ 0.5 ···(2) When using a pyrazole-based blocked isocyanate curing agent and a melamine-based curing agent as the curing agent, by satisfying the relationship of (1), and when using only the pyrazole-based blocked isocyanate curing agent as the curing agent, by satisfying the relationship of (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, 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 0.3, or when (b) / (a) in the above (2) exceeds 0.5, 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, there is a possibility that the strength and processability of the coating film may decrease. Therefore, (b) / (a) in the above (2) is preferably 0.2 or more. From the same perspective, (b) / (a) in the above (1) is preferably 0.1 ≤ (b) / (a) ≤ 0.3, and (b) / (a) in the above (2) is preferably 0.2 ≤ (b) / (a) ≤ 0.5.

[0063] Regarding the organic spherical aggregates contained in the paint, the relaxation effect and reinforcement effect of the concentrated stress of the coating film can be enhanced, and by being contained in the coating film, improvement in bending processability, improvement in scratch resistance, and improvement in indentation resistance can be achieved. 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 aggregate include acrylic resin, nylon resin, urethane resin, and the like.

[0064] And, 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 0.25 or less (Δb ≦ 0.25) when baking painting is performed with a continuous color line represented 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 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

[0065] Since 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. Note that the L value on the surface of the colored coating film is not particularly limited as long as it is 60 or more, and 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 baking painting is performed 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 painting and the b value (b(E)) measured by a spectrophotometer on the surface of the colored coating film during continuous baking painting as small as 0.25 or less, yellowing during painting 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 0.2 or less, more preferably 0.15 or less, and particularly preferably 0.13 or less. Incidentally, 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. Incidentally, in a general continuous color line, galvanized steel sheets of 1 ton to several hundred tons per color are continuously processed in a single painting.

[0067] Incidentally, the method for suppressing the color tone variation Δb to 0.25 or less when baking paint is applied in the continuous color line of the paint film is not particularly limited. For example, in addition to using a pyrazole-based blocked isocyanate curing agent as a curing agent in the paint, as described in the manufacturing method of the painted 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 painting conditions such as the line speed of the color line, the size of the steel sheet, and the exhaust gas volume of the painting apparatus, whereby yellowing during painting can be suppressed. In addition, as long as the color tone variation Δb can be suppressed low by changing the conditions of the plating film, the intermediate layer, etc. in addition to the paint and painting conditions, that method can also be used.

[0068] Incidentally, the colored paint film can also be appropriately blended with titanium oxide, lead white, 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, various fine particles such as glass beads, curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate, waxes, and other additives according to the purpose and use.

[0069] Also, the method of applying 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 coating and curtain flow coating. After applying the paint composition, baking can be performed by heating means such as hot air heating, infrared heating, and induction heating to form a topcoat film. The temperature of the baking process is usually about 180 to 270 °C for the maximum reach plate temperature, and this temperature range is maintained 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 better 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, better 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, in order to improve 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] In the painted steel sheet of the present invention, 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 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 the 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 due to 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 the 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 is applied in the direction parallel to the interface between the steel sheet and the plating layer, stress (dashed arrow) is generated in the coating film formed on the plating layer, which serves as 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, enabling improvement in workability and, consequently, corrosion resistance after processing. p 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, enabling improvement in workability and, consequently, corrosion resistance after processing.

[0074] And in the painted steel sheet of the present invention, when a bending test is conducted in accordance with the plating adhesion test described in JIS G 3321 (2019), 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] Regarding the elongation rate El of the painted steel sheet, the obtained elongation rate is used as a result of conducting a bending test in accordance with the plating adhesion test described in JIS G 3321 (2019) (the "plating adhesion test" in 13.3.3 of 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. Instead, plates of any thickness can be used. In the present invention, by adopting the elongation rate shown in the following formula (4), the limiting elongation rate El (%) of the molten Al-Zn alloy coated 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 sheet of the present invention may be calculated from the above formula (4). As the inner interval during 180° bending, tests (2T bending, 3T bending) with two or three steel plates sandwiched can be used, 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, so plates of any thickness can 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 that can surely confirm cracks. For example, from the point that cracks in the test piece can surely be confirmed, a magnifying glass can be used to observe the bent portion at a magnification of 10 times. Here, the crack in the test piece is a crack that can be confirmed from the surface layer of the coated steel sheet, 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 the heat treatment conditions, etc. in a complex manner, the desired limiting elongation rate El can be obtained.

[0079] <Manufacturing method of coated steel sheet> 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-based 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. Since the paint constituting the colored coating film contains 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 improve workability and suppress yellowing during painting.

[0080] And, in the manufacturing method of the painted 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 (1) relationship when the in-furnace time t (min) in the temperature range exceeding the dissociation temperature T (°C) of the blocked isocyanate in the paint during baking is 0.2 or more and less than 0.3, and satisfy the following (2) relationship when the in-furnace time t is 0.30 or more. (b) > (c) > 0 and (b) / (a) ≤ 0.3 ···(1) (b) > (c) = 0 and (b) / (a) ≤ 0.5 ···(2)

[0081] By 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 satisfying formula (1) or (2), the content of the blocked isocyanate in the curing agent can be kept within a range where yellowing is unlikely to occur. Therefore, even when baking is performed at a certain speed, the workability of the colored coating film can be maintained well while suppressing yellowing of the colored coating film. When (b) / (a) in the above (1) exceeds 0.3, or when (b) / (a) in the above (2) exceeds 0.5, the content ratio of blocked isocyanate in the curing agent increases, and it may not be possible to suppress the yellowing during the painting of the colored coating film. Further, 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, it is preferable that (b) / (a) in the above (1) is 0.1 ≦ (b) / (a) ≦ 0.3, and it is preferable that (b) / (a) in the above (2) is 0.2 ≦ (b) / (a) ≦ 0.5.

[0082] In addition, regarding "the in-furnace time t (min) in the temperature range exceeding the dissociation temperature T (°C) of the blocked isocyanate in the paint during baking", which is a condition for selecting the relationship in the above (1) or (2), it is related to the volatilization rate of the blocking agent in the furnace for baking, and it is considered that the shorter the in-furnace time from when the dissociation temperature of the blocking agent is reached until the end of baking of the coating film, the higher the volatilization rate of the blocking agent and the higher the concentration of the blocking agent in the furnace. That is, "the in-furnace time t (min) in the temperature range exceeding the dissociation temperature T (°C) of the blocked isocyanate in the paint during baking" is an index for grasping how much of the blocking agent volatilized from the paint remains in the coating apparatus (furnace) during baking. The reason for dividing the in-furnace time t (min) into cases where it is 0.30 or more, or 0.2 or more and less than 0.3 is that when the in-furnace time t (min) is 0.30 or more, the blocking agent volatilized from the paint during baking is relatively less, so the (b) pyrazole-based blocked isocyanate curing agent in the paint can be contained in a larger amount. On the other hand, when the coating condition X is 0.2 or more and less than 0.3, it is considered that it remains to a certain extent, so the (b) pyrazole-based blocked isocyanate curing agent is suppressed to more surely suppress the yellowing of the coating film. In addition, the reason for setting the in-furnace time t (min) to 0.2 or more and less than 0.3 and setting the lower limit (0.2 min) is that when the in-furnace time t is less than 0.2 min, a large amount of the blocking agent volatilized from the paint during baking exists in the furnace, so the generation of yellowing cannot be sufficiently suppressed.

[0083] Regarding the dissociation temperature T of the blocked isocyanate, it is generally 120°C or higher, and it varies depending on the type of blocking agent, the type of isocyanate, the type of reaction partner, and the type and amount of catalyst. The pyrazole-based blocking agent has a lower dissociation temperature than other blocking agents, which is advantageous for yellowing from the perspective of the above-mentioned in-furnace time t. The dissociation temperature T can be measured by DSC (differential scanning calorimetry).

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

[0085] Also, regarding the conditions for forming the intermediate layer on the molten Al-Zn-based plated steel sheet, there are no particular limitations, and known chemical conversion film and / or primer coating and curing conditions can be appropriately used.

Examples

[0086] <Samples 1 to 29 of Coated Steel Sheets> According to the conditions of (1) molten Al-Zn-based plated steel sheet, (2) chemical conversion treatment film, (3) undercoat film, and (4) colored coating film shown below, each sample of the coated steel sheet was manufactured.

[0087] (1) Molten Al-Zn-based plated steel sheet The following molten Al-Zn-based plated steel sheets were used. The plating types used for each sample are shown in Table 1. The plate widths of plating types 1 to 3 are all 1166 mm. Plating type 1: Plate thickness 0.35 mm, plating adhesion amount 80 g / m per side 2 , Molten Al-Zn-based plated steel sheet having a plating layer with 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 , Molten Al-Zn-based plated steel sheet having a plating layer with a composition of Zn-55%Al-1.6%Si, and heat-treated at 200°C in an atmosphere for 4 hours after the formation of the plating layer Plating type 3: Plate thickness 0.35 mm, plating adhesion amount 80 g / m per side 2 Hot-dip Al-Zn plated steel sheet having a plating layer with 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. After mixing these raw materials, they were 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 For Samples 1 to 22, 25, and 30 to 37, the paint constituting the colored coating film contains, as the main component, a polyester resin (number average molecular weight 3000, glass transition temperature 25°C), a blocked isocyanate containing a pyrazole-based blocking agent (b) as the curing agent (dissociation temperature 120°C), and (c) methylated melamine, and is 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 coating film surface would be 5 to 10%. For Samples 23 and 24, paints were prepared under the same conditions as Samples 1 to 22, except that a blocked isocyanate containing an oxime-based blocking agent (dissociation temperature 150°C) was used instead of the blocked isocyanate containing a pyrazole-based blocking agent. For Sample 26, paints were prepared under the same conditions as Samples 1 to 22, 25, and 30 to 37, except that a polyester resin with a number average molecular weight of 3000 and a glass transition temperature of 70°C was used. For Sample 28, paints were prepared under the same conditions as Samples 1 to 22, 25, and 30 to 37, except that a polyester resin with a number average molecular weight of 3000 and a glass transition temperature of 40°C was used. For Sample 29, paints were prepared under the same conditions as Samples 1 to 22, 25, and 30 to 37, except that no organic spherical resin was contained. (4-2) Baking of the Coating Film Thereafter, the prepared paint was applied using a continuous color line in the 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, and the in-furnace time t (min) in the temperature region exceeding the dissociation temperature of pyrazole in the paint of 120 (°C) or that of oxime of 150 (°C) during baking was measured. The in-furnace time t (min) is shown in Table 1.

[0091] <Evaluation> For each sample of the painted steel sheet obtained as described above, the following evaluations were carried out.

[0092] (1) L value For the 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 the painted steel sheet of each sample, the b value in the Hunter Lab color space was measured using a spectrophotometer (Hunter Lab: Ultrascan VIS). Regarding the measurement of the b value, the b value at the position where 30 m was 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 where 500 m was continuously painted 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 0.25 or less, very good (◎) if it was 0.2 or less, and defective (×) if it exceeded 0.25.

[0094] (3) Pencil hardness evaluation Regarding the surface of the painted steel sheet of each sample, the pencil hardness was measured in accordance with the provisions specified in JIS G3322. After the measurement, if the pencil hardness was H or higher, it was considered qualified (〇), and if it was F or lower, it was considered unqualified (×), and the evaluation was carried out. The evaluation results are shown in Table 2.

[0095] (4) Elongation at break before and after film formation (El1, El0) Regarding the measurement of the elongation at break El1 after film formation, the painted steel sheet of each sample was bent 180 degrees with a width of 50 mm in accordance with the bending test based on the adhesion test of plating described in JIS G 3321 (2019). The surface of the bending part within a width of 30 mm excluding 10 mm at both ends was observed at a magnification of 10 times using a magnifying glass, and the maximum elongation rate (El value) at which no crack was observed was measured. The obtained values are shown in Table 1. Regarding the maximum elongation rate (El value), the elongation test was carried out and measured under the conditions of 20 °C and 40 °C.

[0096] (5) Sliding test For each painted steel plate of the samples, a sliding test was carried out using a surface property measuring machine 14FW manufactured by Shin-Toyo Kagaku Co., Ltd. An Erichsen was used to produce a 2 mm extruded slider on the front surface of the painted steel plate, and the presence or absence of coating defects on the slider side was confirmed for every 100 reciprocations under the conditions of a load of 200 gf, a sliding speed of 5000 mm / min, and a stroke of 3 mm between the flat front surface of the painted steel plate and the slider, and the number of reciprocations until the plating was exposed was measured. When the number of reciprocations until the plating surface was exposed was 3000 or more, it was rated excellent (◎), when it was 1000 or more, it was rated good (〇), and when it was less than 1000, it was rated poor (×).

[0097] (6) Weather resistance For the surface of each painted steel plate of the samples, a continuous irradiation test was carried out using a Metalining Weather Meter M6T manufactured by Suga Test Instruments Co., Ltd. to evaluate the weather resistance. The continuous irradiation test was carried out for 240 h at an irradiation illuminance of 1000 mW / cm 2 . Samples with a gloss retention rate of 30% or more after the test were considered qualified (〇), and samples with a gloss retention rate of less than 30% were considered unqualified (×).

[0098]

Table 1

[0099] From the results in Table 1, it can be seen that each sample of the present invention example is superior in yellowing property compared to Samples 23, 24, 27, 28, 30 to 37 of the comparative example. Also, it can be seen that each sample of the present invention example is superior in workability compared to Samples 25 and 26 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 25 of the comparative example and superior in pencil hardness compared to Sample 29 of the comparative example.

Industrial applicability

[0100] According to the present invention, it is possible to provide a painted steel plate with suppressed yellowing during painting and excellent workability, and a method for manufacturing the painted steel plate.

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 as the 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) ≤ 0.3... (1) (b) > (c) = 0 and (b) / (a) ≤ 0.5... (2) 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 ≤ 0.25) when baked and painted with a continuous color line represented by the following (3) is 0.25 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 painted in the longitudinal direction from the start of painting 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 painted in the longitudinal direction from the start of painting When a bending test is conducted 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 obtained from the formula (4) is such that 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 characterized by a coated steel sheet.

2. The coated steel sheet according to claim 1, wherein the organic spherical aggregates are acrylic resins having an average particle diameter 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 coated steel sheet according to claim 1 or 2 is characterized in that.

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, with the balance being 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 series 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, during baking, the in-furnace time t (min) in a temperature range exceeding the dissociation temperature T (°C) of blocked isocyanate in the paint is 0.2 or more, in the paint, the content of (a) polyester resin, the content of (b) pyrazole-based blocked isocyanate curing agent as a curing agent, and the content of (c) melamine-based curing agent satisfy the following relationship (1) when the in-furnace time t is 0.2 or more and less than 0.3, and satisfy the following relationship (2) when the in-furnace time t is 0.30 or more (b) > (c) > 0 and (b) / (a) ≤ 0.3... (1) (b) > (c) = 0 and (b) / (a) ≤ 0.5... (2) A method for manufacturing a coated steel sheet, characterized by the above.

Images