Precoated steel sheet and formed product

Abstract

Provided is a Zn-Al-Mg alloy plated steel sheet as a base sheet, which does not cause cracking and peeling in a coating film even due to 0T bending processing, and which is capable of further improving corrosion resistance with respect to under-coating corrosion of a 0T bending processed portion and its surroundings. A pre-coated plated steel sheet of the present invention, which has a plated layer composed of a Zn-Al-Mg alloy plating layer on one side or both sides of a steel sheet, a chemical conversion treatment coating film layer on the plated layer, and a coating film layer on the chemical conversion treatment coating film, the coating film layer having a primer coating film layer, and an upper layer coating film layer on the primer coating film layer, satisfies at least any one of the following conditions (a-1) and (a-2), condition (b), and condition (c).

Description

TECHNICAL FIELD

[0001] The present application relates to a pre-coated plated steel sheet and a shaped product. BACKGROUND

[0002] A pre-coated plated steel sheet is used instead of a post-coating product which is coated after a shaping process for home electric appliances, building materials, automobiles, and the like. This pre-coated plated steel sheet has a colored organic coating film on a plated steel sheet subjected to a rust-proof treatment, and has the following characteristics: a beautiful appearance, and sufficient processability and good corrosion resistance.

[0003] A pre-coated plated steel sheet is mostly used for applications requiring deep drawing, bending of a drawn portion, and the like, which are extremely strict in processability. Therefore, many techniques have been studied as a method for improving the processability.

[0004] For example, Patent Literature 1 below discloses a technique in which a layer formed of an amino-based and / or mercapto-based silane coupling agent is provided on a chemical conversion treatment layer, and a primer layer is provided on the layer, the primer layer being formed of (A) a linear linear high-molecular-weight polyester resin having a number average molecular weight of 5000 or more, (B) an epoxy resin having an epoxy equivalent of 2500 or less, a hydroxyl group content of 3% or more, and a number average molecular weight of 800 or more, mixed at a weight ratio of (A) / (B) = 50 / 50 to 95 / 5, and further containing (C) one or two or more of a melamine-based resin, a urea-based resin, a blocked isocyanate resin, and a phenol-based resin at a weight ratio of (A)+(B) / (C) = 60 / 40 to 95 / 5, thereby ensuring processability and corrosion resistance of a processed portion.

[0005] In addition, Patent Literature 2 below discloses a coated metal sheet which is a coated metal sheet in which a primer coating film, an intermediate coating film, and a top coating film are sequentially provided on a surface of a metal sheet, the intermediate coating film is formed by curing a polyester-based paint containing a polyester resin having a number average molecular weight of 5000 to 15000 and a glass transition temperature of -5 to 50°C, and a fibrous filler at 5 to 50% by weight, and an elastic strain energy accumulated in the intermediate coating film is set to 20% or less with respect to a ratio of a mechanical energy applied to the intermediate coating film when a tensile stress is applied to the intermediate coating film itself until a tensile ratio is 50%.

[0006] In addition, Patent Document 3 below discloses a coated steel sheet which is a coated steel sheet having a steel sheet, a chemical conversion treatment coating film formed on the surface of the steel sheet, a primer coating film formed on the surface of the chemical conversion treatment coating film, and a top coating film formed on the surface of the primer coating film, the chemical conversion treatment coating film containing a fluoride of titanium or a fluoride of zirconium, an oxide of titanium or a hydroxide of titanium, and an oxide of zirconium or a hydroxide of zirconium, the primer coating film containing an organic resin, and a pigment A containing one or two compounds selected from the group consisting of magnesium phosphate and zirconium phosphate, and a pigment B containing barium sulfate, the content of the pigment A and the content of the pigment B in the primer coating film being 20 to 100 parts by mass, respectively, with respect to 100 parts by mass of the organic resin, and the total content of the pigment A and the pigment B in the primer coating film being 40 to 160 parts by mass with respect to 100 parts by mass of the organic resin.

[0007] In addition, Patent Document 4 below discloses a water-based primer composition and a coated article in which the above water-based primer composition is provided on a metal raw material, the water-based primer composition being a water-based primer composition containing a modified epoxy resin containing a carbonyl group (A), a crosslinking agent (B), and an antirust pigment (C), the modified epoxy resin containing a carbonyl group (A) being produced by reacting an amine compound (al) having at least one active hydrogen bonded to a nitrogen atom and having a carbonyl group in one molecule, and an epoxy resin (a2) having two or more epoxy groups in one molecule.

[0008] In addition, Patent Document 5 below discloses a pre-coated metal sheet which is a pre-coated metal sheet having a metal sheet, a base coating film layer not containing a chromate provided on the metal sheet, a primer coating film layer not containing a chromate provided on the base coating film layer, and a top coating film layer provided on the primer coating film layer, the breaking limit tensile rate of the coating film layer in which the primer coating film layer and the top coating film layer are combined together being 10% or more and 25% or less, the glass transition temperature of the primer coating film layer exceeding 50°C and being 100°C or less, the top coating film layer containing a resin component, the resin component containing an acrylic resin or a polyester-modified acrylic resin, when the pre-coated metal sheet is subjected to 2T close bending processing, a total of 5 or more and 30 or less cracks having a width of 0.1 mm or less occur in the top coating film in a range of 1 mm in length in a direction perpendicular to the bending direction with the top of the bend as the center.

[0009] In addition, Patent Document 6 below discloses a pre-coated steel sheet provided with: a steel sheet; a zinc alloy plated layer provided on the steel sheet, containing 1 to 25 mass% of Al, 0.1 to 13 mass% of Mg, and 0 to 2.0 mass% of Si, with the balance being Zn and impurities; and a colored coating film layer provided on the zinc alloy plated layer, containing at least one of a phosphoric acid compound and a vanadium compound, a coloring pigment, and a binder resin. The following gist is disclosed therein: in the case where the colored coating film layer contains the phosphoric acid compound, the concentration of the phosphoric acid compound is 0.3 to 5.0 mass% in terms of the amount of P with respect to the total solid content mass of the colored coating film layer, and when a cross section in the thickness direction of the colored coating film layer is observed by mapping observation using FE-EPMA, there are 1 to 15 first regions in which the concentration of phosphorus element is 3% or more in a range of 2 μm in the thickness direction and 200 μm in a direction parallel to the interface of the colored coating film layer and the zinc alloy plated layer. In addition, the following gist is disclosed: in the case where the colored coating film layer contains the vanadium compound, the vanadium compound is a vanadic acid compound or a vanadium oxide, the concentration of the vanadium compound is 0.5 to 8.0 mass% in terms of the amount of V with respect to the total solid content mass of the colored coating film layer, and when a cross section in the thickness direction of the colored coating film layer is observed by mapping observation using FE-EPMA, there are 1 to 10 second regions in which the concentration of vanadium element is 3% or more in a range of 1 μm in the thickness direction and 200 μm in a direction parallel to the interface from the interface toward the surface side.

[0010] Prior Art Documents

[0011] Patent Documents

[0012] Patent Document 1: Japanese Patent Application Laid-Open (JP A) No. 63-97267

[0013] Patent Document 2: Japanese Patent Application Laid-Open (JP A) No. 2008-12734

[0014] Patent Document 3: International Publication No. 2012 / 001981

[0015] Patent Document 4: International Publication No. 2012 / 039217

[0016] Patent Document 5: International Publication No. 2016 / 174746

[0017] Patent Document 6: International Publication No. 2020 / 100286 SUMMARY

[0018] PROBLEMS TO BE SOLVED BY THE INVENTION

[0019] In recent years, the use of pre-coated steel sheets for heavy salt environment applications has increased, and the corrosion resistance of the pre-coated steel sheets is required to be higher than ever. As a base sheet for the pre-coated steel sheet, a hot-dip galvanized steel sheet (GI) or an electro-galvanized steel sheet (EG) has been used as in the past, but the corrosion resistance is not sufficient. Therefore, a Zn-Al-Mg alloy plated steel sheet having excellent corrosion resistance is used as the base sheet.

[0020] Here, the Zn-Al-Mg alloy plated layer is harder than a zinc plated layer of a conventional GI or EG, and the plated layer is not uniform in composition. Thus, when the plated steel sheet is subjected to drawing processing, cracks are locally generated in the plated layer, and various problems are caused by the cracks.

[0021] For example, due to T-bending processing as an example of drawing processing, a large number of fine cracks are generated in the resin coating film on the upper layer at the same time as the plated layer, and this becomes a cause of reduction in corrosion resistance. In the case of a pre-coated steel sheet using a conventional GI, EG or the like as a base sheet, the drawing ratio of the coating film when nT-bending processing (n = 0, 1, 2,...) is performed is 1 / (1 + n). For example, in the case of 0T-bending processing, the coating film is calculated to be 1 / (1 + 0) = 1, that is, 100% drawing. At this time, if the coating film has a drawing ratio of 100% or more, no cracks are generated in the coating film. This is because the plated layer and the coating film are drawn substantially uniformly. However, in the case of a base sheet of a Zn-Al-Mg alloy plated steel sheet, cracks are locally generated in the plated layer due to T-bending processing as described above, and the coating film on the upper layer thereof is locally subjected to a large deformation. Therefore, in the case of a coating film having the same drawing ratio as in the case of a GI, EG or the like, the cracks in the coating film are significantly increased. In order to prevent cracks from being formed in the coating film due to T-bending processing, it is considered that a coating film having excellent drawing properties can be selected, but a coating film having excellent drawing properties generally has a large stress due to elasticity at the time of processing. Thus, in heating and actual use over time, the following problems are caused: peeling of the coating film from the T-bending processed portion (a phenomenon called "blooming"), or further, under-film corrosion (under-film swelling) of the coating film from the T-bending processed portion to the flat portion due to actual use in a salt environment, and the like.

[0022] Blooming is considered to be caused by the fact that the stress of the coating film is concentrated at the interface between the coating film and the base sheet in the state after bending processing, the concentrated stress is increased due to the influence of the entropic elasticity of the coating film by heating and over time, and thus the coating film is peeled. As for under-film swelling, it is considered that the stress with which the coating film attempts to recover acts as a force to peel the coating film, accelerates corrosion at the interface portion having weak adhesion, and thus under-film swelling of the coating film is caused. That is, by using a Zn-Al-Mg alloy plated steel sheet as a base sheet for a pre-coated steel sheet, the corrosion resistance is improved, but new problems are caused in the workability and corrosion resistance of the pre-coated steel sheet. These problems will be hereinafter referred to as "problems of the drawing processed portion".

[0023] The technical idea of the above-described Patent Document 1 is to improve adhesion by using a coupling agent for pretreatment and forming a specific primer film that can react with the coupling agent. In this case, the steel sheet contemplated in the above-described Patent Document 1 is a zinc-based plated steel sheet such as a cold-rolled steel sheet, a hot-dip galvanized steel sheet, an electrogalvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a zinc-nickel alloy plated steel sheet, and is different from a Zn-Al-Mg alloy. As a result of our studies, it has been found that if the method of the above-described Patent Document 1 is actually applied to a primer film layer of a Zn-Al-Mg alloy plated steel sheet, damage to the primer film layer and stress concentration at the interface of the plated layer during processing cannot be avoided. From the above-described viewpoint, there is room for improvement in the above-described Patent Document 1.

[0024] The technical idea of the above-described Patent Document 2 is to suppress "flowering" of the T-bent portion by using a coating film having a low elastic strain energy for the intermediate coating film. In this case, the metal sheet contemplated in the above-described Patent Document 2 is an aluminum-plated steel sheet, a zinc-plated steel sheet, an aluminum-zinc plated steel sheet, and a stainless steel sheet, and is different from a Zn-Al-Mg alloy. As a result of our studies, it has been found that if the method of the above-described Patent Document 2 is actually applied to a primer film layer of a Zn-Al-Mg alloy plated steel sheet, damage to the primer film layer and stress concentration at the interface of the plated layer during processing cannot be avoided. From the above-described viewpoint, there is room for improvement in the above-described Patent Document 2.

[0025] The technical idea of the above-described Patent Document 3 is to ensure corrosion resistance by a rust-preventive pigment contained in the primer film. As a result of our studies, it has been found that if the method of the above-described Patent Document 3 is actually applied to a primer film layer of a Zn-Al-Mg alloy plated steel sheet, damage to the primer film layer and stress concentration at the interface of the plated layer during processing cannot be avoided. From the above-described viewpoint, there is room for improvement in the above-described Patent Document 3.

[0026] The technical idea of the above-described Patent Document 4 is to ensure processability and corrosion resistance of the processed portion by forming a primer film from a specific water-based epoxy coating material. As a result of our studies, it has been found that if the method of the above-described Patent Document 4 is actually applied to a primer film layer of a Zn-Al-Mg alloy plated steel sheet, the performance level is insufficient because it is a water-based coating material, and it remains at the level of 4T bending processing. From the above-described viewpoint, there is room for improvement in the above-described Patent Document 4.

[0027] The technical idea of the above-described Patent Document 5 is to suppress peeling of the coating film of the processed portion by causing cracks in the hard top coating film to relax stress. As a result of our studies, it has been found that if the method of the above-described Patent Document 5 is actually applied to a Zn-Al-Mg alloy plated steel sheet, the glass transition temperature of the primer film is high, and there is room for improvement in terms of processability improvement.

[0028] The technology of the above-described Patent Document 6 intends to improve various characteristics including workability by providing a single-layer thin film containing a vanadium-based or phosphorus-based pigment (anti-rust pigment) on the surface of a Zn-Al-Mg alloy plated steel sheet, controlling the distribution of the pigment in the coating film, thereby improving the corrosion resistance. However, the results of the studies by the present inventors and others have revealed that the method of the above-described Patent Document 6 improves the corrosion resistance by the effect of the anti-rust pigment, and thus there is room for improvement in terms of the resistance to blooming.

[0029] The present application has been made in view of the above-described problems, and an object of the present application is to provide a pre-coated plated steel sheet and a formed product in which a Zn-Al-Mg alloy plated steel sheet is used as a base sheet, in which even if 0T bending processing is performed, cracking and peeling in the coating film do not occur, and the corrosion resistance with respect to undercoating corrosion of the 0T bending processed portion and the surrounding thereof can be further improved.

[0030] Solution to the problem

[0031] In order to solve the above-described problems, the present inventors and others have conducted intensive studies, and as a result, the present inventors and others have conceived conditions that the coating film should satisfy. In addition, further studies have been conducted on the constitution of the coating film that can more easily achieve the above-described conditions, and as a result, it has been found that in the case where the base sheet is a Zn-Al-Mg alloy plated steel sheet, the appropriate coating film properties are different from those in the case where a conventional zinc plated steel sheet is used as the base sheet. That is, it has been found that the method of lowering the stress of the coating film at the time of processing by setting the Tg of the resin to be high (i.e., designing the coating film to be hard, and easily plastically deformed with respect to strain), and reducing the amount of addition of the crosslinking agent to thereby suppress the crosslinking density to be low, is not effective as in the conventional idea.

[0032] That is, the present inventors and others have found that, unlike the conventional idea, by forming a coating film in which a resin having a low Tg, a high molecular weight, and a good elongation rate is sufficiently cured without reducing the amount of addition of the crosslinking agent (i.e., without reducing the crosslinking density), and the stress due to the deformation of the coating film can be dispersed, the problem of the processing portion can be more easily improved without reducing the performance of the coating film.

[0033] The gist of the present application completed based on the above-described findings is as described below.

[0034] (1) A pre-coated plated steel sheet provided with a plated layer composed of a Zn-Al-Mg alloy plating layer on one side or both sides of a steel sheet, a chemical conversion treatment coating film layer on the plated layer, and a coating film layer on the chemical conversion treatment coating film, the coating film layer having a primer coating film layer, and an upper coating film layer on the primer coating film layer, the pre-coated plated steel sheet satisfying at least any one of the following conditions among the conditions (a-1) and (a-2); condition (b); and condition (c) among the conditions shown below.

[0035] (a-1) The interface between the aforementioned coating film layer and the layer on the steel sheet side than the coating film layer is cut by the SAICAS method and the cutting strength obtained by the measurement is 1.00 kN / m or more on average, and 5% or less of the cutting area is in the interface peeling mode, and the remaining cutting area is in the cohesive failure mode within the aforementioned coating film layer.

[0036] (a-2) When the part of the pre-coated steel sheet subjected to uniaxial stretching processing, the thickness of which is reduced by 20% or more compared to the non-processed part, is cut by the SAICAS method, the cutting strength for the interface between the aforementioned coating film layer and the layer on the steel sheet side than the coating film layer is 1.00 kN / m or more on average, and 20% or less of the cutting area is in the interface peeling mode, and the remaining cutting area is in the cohesive failure mode within the aforementioned coating film layer.

[0037] (b) In the cross section obtained by cutting the aforementioned primer coating film layer in the thickness direction, the penetration load when a four-pyramid type indenter for micro hardness measurement is inserted at a speed of 2 μm per minute to a depth of 10 μm from the position 1 μm in the direction of the aforementioned primer coating film layer from the interface between the aforementioned primer coating film layer and the layer on the steel sheet side than the primer coating film layer is less than 1000 mN.

[0038] (c) In the cross section obtained by cutting the aforementioned primer coating film layer in the thickness direction, after a four-pyramid type indenter for micro hardness measurement is inserted to a depth of 10 μm at a speed of 10 μm per second from the position 1 μm in the direction of the aforementioned primer coating film layer from the interface between the aforementioned primer coating film layer and the layer on the steel sheet side than the primer coating film layer, the load is unloaded at a speed of 2 μm per minute, the load curve at that time with respect to displacement is plotted, and the area of the range enclosed by the straight line corresponding to displacement = 10 μm, the straight line corresponding to load = 0, and the aforementioned load curve at the time of unloading in the plane defined by displacement and load is less than 500 μm·mN.

[0039] (2) The pre-coated steel sheet according to (1), wherein the aforementioned plated layer contains 0.5 mass% or more and 60.0 mass% or less of aluminum, 0.5 mass% or more and 15.0 mass% or less of magnesium, and the balance being zinc and impurities, the thickness of the aforementioned primer coating film layer is 2 to 10 μm, the total thickness of the aforementioned primer coating film layer and the aforementioned upper coating film layer is 10 to 30 μm, the aforementioned primer coating film layer contains a polyester-based resin having an average molecular weight of 19000 or more and a glass transition temperature Tg of less than 40°C, and a curing agent, and the aforementioned curing agent contains a blocked isocyanate or a blocked isocyanate resin.

[0040] (3) The pre-coated steel sheet according to (2), wherein the content of the aforementioned curing agent is 5 to 15% by mass with respect to the total content of the aforementioned polyester-based resin and the aforementioned curing agent.

[0041] (4) The pre-coated steel sheet according to (2) or (3), wherein the aforementioned curing agent further contains melamine or a melamine derivative.

[0042] (5) The pre-coated steel sheet according to (4), wherein the content of the aforementioned melamine or melamine derivative is 20 to 50% by mass, relative to the total content of the aforementioned blocked isocyanate or blocked isocyanate resin and the aforementioned melamine or melamine derivative.

[0043] (6) The pre-coated steel sheet according to any one of (2) to (5), wherein the average molecular weight of the aforementioned polyester-based resin is 23,000 to 25,000.

[0044] (7) The pre-coated steel sheet according to any one of (2) to (6), wherein the glass transition temperature Tg of the aforementioned polyester-based resin is 0 to 20°C.

[0045] (8) The pre-coated steel sheet according to any one of (2) to (7), wherein when the number of substituents in the aforementioned blocked isocyanate or blocked isocyanate resin, which can react with OH groups in the aforementioned polyester-based resin, is denoted as na, and the number of OH groups in the aforementioned polyester-based resin is denoted as nb, and (na + nb) is 5.5 or more and 8.0 or less.

[0046] (9) The pre-coated steel sheet according to any one of (2) to (8), wherein the aforementioned plated layer is composed of a Zn-11%Al-3%Mg-0.2%Si alloy plated layer further containing Si instead of a part of the balance of Zn.

[0047] (10) A shaped product which is a shaped product formed from a pre-coated steel sheet having a plated layer composed of a Zn-Al-Mg-based alloy plated layer on one side or both sides of a steel sheet, a chemical conversion treatment coating layer on the aforementioned plated layer, and a coating film layer on the aforementioned chemical conversion treatment coating, the coating film layer having a primer coating film layer and an upper layer coating film layer on the aforementioned primer coating film layer, in a portion of the plated steel sheet in the aforementioned shaped product in which the thickness is reduced by 20% or more compared to a non-shaping processed portion, the cutting strength obtained by cutting and measuring the interface between the coating film layer and the layer on the steel sheet side of the coating film layer using the SAICAS method is 1.00 kN / m or more on average, and 20% or less of the cutting area is in an interface peeling form, and the remaining portion of the cutting area is in a cohesive failure form within the aforementioned coating film layer.

[0048] (11) The shaped product according to (10), wherein the plated layer in the aforementioned shaped product contains 5 mass% or more and 15 mass% or less of aluminum, and contains 2 mass% or more and 4 mass% or less of magnesium.

[0049] Effects of the Invention

[0050] As explained above, according to the present application, the Zn-Al-Mg alloy plated steel sheet is used as a base sheet, and even if 0T bending processing is performed, cracking and peeling in the coating film do not occur, and the corrosion resistance with respect to undercoating corrosion of the 0T bending processed portion and the surrounding thereof can be further improved. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1A A drawing for schematically showing the structure of a plated steel sheet of the embodiment of the present application.

[0052] Figure 1B A drawing for schematically showing the structure of a plated steel sheet of the same embodiment.

[0053] Figure 2A A drawing for schematically showing the structure of a pre-coated plated steel sheet of the same embodiment.

[0054] Figure 2B A drawing for schematically showing the structure of a pre-coated plated steel sheet of the same embodiment.

[0055] Figure 3 A drawing for explaining a measurement method using a four pyramid type indenter for micro hardness measurement.

[0056] Figure 4 A drawing for explaining a measurement method using a four pyramid type indenter for micro hardness measurement.

[0057] Figure 5 A drawing for explaining a molded product of the same embodiment. DETAILED DESCRIPTION

[0058] Hereinafter, a preferred embodiment of the present application will be explained in detail with reference to the accompanying drawings. Note that in the present specification and drawings, constituent elements having substantially the same function are attached with the same reference numerals, and repeated explanation is omitted.

[0059] (Research by the Inventor et al.)

[0060] Hereinafter, before explaining the pre-coated plated steel sheet and the molded product of the embodiment of the present application, various research contents performed with respect to the portion where the above-mentioned tensile processed portion problem occurs will be explained in detail.

[0061] The inventors conducted cross-sectional observation of the portion of the coating film in which cracking occurs in the T-bent processed portion of the pre-coated plated steel sheet, and as a result, cracking was confirmed at any one portion of the plated layer of the plated steel sheet, and at the position of the portion of the coating film in which cracking of the coating film was confirmed, cracking was also confirmed in the plated layer. It was considered that, due to the tensile processing such as T-bending, the plated layer is first broken and cracking occurs, and according to the tensile properties of the coating film, cracking occurs in a portion of several percent thereof. If the thickness of the plated steel sheet portion in the above-mentioned portion is measured, it is reduced compared to the thickness before forming. From the above-mentioned results, it was known that the problem of the tensile processed portion is a problem that occurs in the portion of the plated steel sheet that is stretched and the thickness of which is reduced.

[0062] It is generally considered that, with the deformation of the plated steel sheet due to the tensile processing, the following things occur. First, many fine cracks occur in the resin coating film on the upper layer at the same time as the plated layer, and this becomes a route for a corrosion factor to penetrate from the outside to the coating film below, and thus the corrosion resistance is reduced. Next, due to the elastic stress due to the processing of the coating film, peeling of the coating film occurs in the heated processed portion, and over time in actual use (a phenomenon called "blooming"). Even if the coating film does not crack, more "blooming" occurs when there is no crack and the stress is large. Furthermore, corrosion (expansion under the coating film) from the T-bent processed portion to the flat portion under the coating film occurs due to actual use in a salt-affected area. This is because, the stress with which the coating film attempts to recover acts as a force that peels the coating film, and corrosion selectively and rapidly occurs at the interface portion of the plated layer and the coating film, which is weak in adhesion due to thinning caused by the mold, even in the deformed portion and the non-deformed portion of the steel sheet.

[0063] As a result of the research by the inventors, as a factor that determines the occurrence or non-occurrence of the problem of the above-mentioned tensile processed portion, it was thought that the following (1) to (3) should be researched. It was considered that, if these influence factors are in a good state in a comprehensive manner, the problem of the tensile processed portion is suppressed.

[0064] (1) The compactness of the plated surface after the processing with stretching (presence or absence of unevenness, cracking)

[0065] (2) The adhesion of the plated layer and the primer coating film by the chemical conversion treatment coating film after the processing with stretching

[0066] (3) The state of the entire coating film including the top layer coating film that is deformed due to the processing with stretching (presence or absence of cracking, internal stress)

[0067] The inventors conducted further research focusing on the coating film properties that affect the above-mentioned (3), since the problem of the above-mentioned tensile processed portion easily occurs in the pre-coated plated steel sheet in which a Zn-Al-Mg alloy plated steel sheet is used as the original sheet, and the easiness of the occurrence of the problem of the tensile processed portion differs depending on the type of the coating film.

[0068] As a result, the present inventors and others have conceived that at least either of the following 2 points is required for a resin coating film (primer coating film) of a pre-coated plated steel sheet of a Zn-Al-Mg alloy plated steel sheet as a base sheet in contact with the base sheet.

[0069] The first point is that a fracture due to cohesive failure does not occur due to the above-mentioned processing. A coating film fracture due to processing means that the coating film is insufficient in stretchability. A coating film having sufficient stretchability in which a crack does not occur in the coating film even at a portion subjected to large processing accompanying deformation of the steel sheet is required.

[0070] The second point is that the stress of the coating film in a state subjected to the above-mentioned deformation due to processing is sufficiently small. If the stress of the coating film in a state subjected to deformation is large, the stress is concentrated at the interface of the coating film and the plated layer. If the concentrated stress exceeds the interfacial adhesion force of the processed portion, peeling of the coating film occurs.

[0071] The present inventors and others have conducted a study on a means capable of determining the above-mentioned first point, and as a result, have reached the following conclusion: observation of the peeling pattern and measurement of the cutting strength based on cutting of the coating film using the SAICAS method (Surface and Interfacial Cutting Analysis System) is effective. By the peeling pattern of the primer coating film not becoming a measurement object being cohesive failure of the primer coating film layer, and the cutting strength being a sufficiently high value, it is possible to determine whether or not the above-mentioned first point is satisfied.

[0072] The SAICAS method is a method of cutting from the surface of a sample to the adhesive interface of an adhesive body using a sharp blade at an ultralow speed, and measuring the cutting strength. Therefore, it is possible to observe the cutting strength and the peeling state at the interface of a specific layer of a laminated multi-layer film which is difficult to measure using a conventional method.

[0073] Specifically, both of the following 2 conditions (i), (ii) are satisfied as conditions regarding the above-mentioned first point.

[0074] (i) When a flat portion of the pre-coated plated steel sheet is cut using the SAICAS method, the cutting strength for the interface of the coating film layer and the layer on the steel sheet side than the coating film layer (more specifically, the interface of the primer coating film layer and the chemical conversion treatment coating film layer or the plated layer on the steel sheet side than the primer coating film layer) is 1.00 kN / m or more on average, and 5% or less of the cutting area is an interface peeling pattern, and the remaining portion of the cutting area is a cohesive failure pattern in the coating film layer (more specifically, in the primer coating film layer).

[0075] (ii) When cutting the portion of the pre-coated steel sheet subjected to uniaxial stretching processing, in which the thickness is reduced by 20% or more compared to the non-processed portion, using the SAICAS method, the cutting strength is 1.00 kN / m or more on average for the interface of the coating film layer and the layer on the steel sheet side of the coating film layer (more specifically, the interface of the primer coating film layer and the chemical conversion treatment coating film layer or the plated layer on the steel sheet side of the primer coating film layer), and 20% or less of the cutting area is in the interface peeling form, and the remaining portion of the cutting area is in the cohesive failure form within the coating film layer (more specifically, within the primer coating film layer).

[0076] In addition, research was conducted on a means for determining the above-mentioned point 2, and as a result, the following conclusion was reached: a method of micro hardness measurement by pressing a pyramid-shaped indenter, represented by Vickers hardness measurement, is effective.

[0077] Either (iii) drawing a displacement-load curve at a sufficiently low speed at which the indenter is pressed, measuring the value of the load at a prescribed displacement (at this time, the force with which the coating film is pushed back), or (iv) at least one of the elastic strain energy is found from the load curve at the time of unloading and the displacement of the recovery of the coating film after the indenter is pressed at a high speed to a prescribed depth, can determine whether the above-mentioned point 2 is satisfied.

[0078] Specifically, (iii) in a cross section obtained by cutting the primer coating film layer in the thickness direction, the pressing load at the time when a pyramid-shaped indenter for micro hardness measurement is inserted at a speed of 2 μm per minute to a depth of 10 μm from a position 1 μm in the direction of the primer coating film layer from the interface of the primer coating film layer and the layer on the steel sheet side of the primer coating film layer is less than 1000 mN; or (iv) in a cross section obtained by cutting the primer coating film layer in the thickness direction, after a pyramid-shaped indenter for micro hardness measurement is inserted to a depth of 10 μm at a speed of 10 μm per second from a position 1 μm in the direction of the primer coating film layer from the interface of the primer coating film layer and the layer on the steel sheet side of the primer coating film layer, the indenter is unloaded at a speed of 2 μm per minute, the load curve with respect to displacement at this time is drawn, and in the plane defined by displacement and load, at least one of the area of the range enclosed by the line corresponding to displacement = 10 μm, the line corresponding to load = 0, and the load curve at the time of unloading is less than 500 μm·mN, which becomes a condition for satisfying the above-mentioned point 2.

[0079] Here, in the above-mentioned three determination methods, the "interface with the layer on the steel sheet side" is because the thickness of the chemical conversion treatment coating film layer is usually extremely thin, around 0.1 μm, and thus the chemical conversion treatment coating film layer can be integrated with the primer coating film layer and the interface of the primer coating film layer and the chemical conversion treatment coating film layer cannot be specified.

[0080] As above, it was found that, as a primer coating film layer of a pre-coated plated steel sheet in which the original sheet is a Zn-Al-Mg alloy plated steel sheet, a coating film that at least satisfies either of the content indicated in the above 1st point or the content indicated in the 2nd point (most preferably a coating film that satisfies both the content indicated in the above 1st point and the content indicated in the 2nd point) is used, whereby it is possible to suppress the problem of the stretch-processed portion.

[0081] Further, the present inventors and others studied the cutting strength, peeling form of the interface of the chemical conversion treatment coating film layer, coating film layer (more specifically, primer coating film layer) and the plated layer for a shaped body obtained by forming processing the above-described pre-coated plated steel sheet. When using the conventional peeling test, it is possible to measure the peeling strength of the coating film of the pre-coated steel sheet, but it is not possible to correctly measure the peeling strength, peeling form of the stretch portion in the plated steel sheet constituting the shaped body. The present inventors and others used the SAICAS method as a method that can simultaneously measure these portions, and evaluated the cutting strength, peeling form.

[0082] The present inventors and others performed sample production of the stretch portion of the plated steel sheet of the shaped body, and, as a shaped portion of the stretch advantage, focused on a portion in which the thickness is reduced compared to the thickness of the plated steel sheet before shaping, and measured this portion using the SAICAS method, whereby the following findings were obtained.

[0083] That is, it was found that, for a shaped body formed from the above-described pre-coated plated steel sheet in which no cracks of the stretch portion, peeling of the coating film occur, the peeling form of the coating film is not interfacial peeling of the interface of the chemical conversion treatment coating film layer, coating film layer (more specifically, primer coating film layer) and the plated layer, cohesive failure of the coating film layer (more specifically, primer coating film layer) is formed, and a cutting strength of at least a certain level is formed.

[0084] As a result, it was found that, when the cutting strength of the stretch portion of the plated steel sheet (i.e., the portion of the plated steel sheet of the shaped product in which the thickness is reduced by 20% or more compared to the thickness before shaping (which can also be considered to be a non-shaping processed portion)) obtained using the SAICAS method is 1.00 kN / m or more on average, and 20% or less of the cutting area is an interfacial peeling form, and the cutting area of the remaining portion is a cohesive failure form within the coating film (for example, within the primer coating film in the case where the coating film includes multiple layers), it is possible to obtain a shaped product formed from a pre-coated plated steel sheet in which the crack resistance, coating film adhesion, and corrosion resistance of the coating film of the stretch-processed portion are excellent.

[0085] Here, the interfacial peeling morphology refers to any one or a combination of cohesive failure of the chemical conversion treatment coating layer, interfacial peeling of the chemical conversion treatment coating layer from the coating film layer (more specifically, the primer coating film layer), or interfacial peeling of the chemical conversion treatment coating layer from the plated layer. However, the chemical conversion treatment coating film is extremely thin as described above, and thus is integrated with the plated layer, the coating film layer (more specifically, the primer coating film layer), and the like, and the above-described peeling morphology cannot be visually distinguished in most cases.

[0086] The present inventors and others further researched the primer coating film layer under the above-described conditions based on the above-described findings, and as a result, further found the following. That is, it was conceived that the primer coating film layer of the pre-coated plated steel sheet in contact with the original sheet, which is a Zn-Al-Mg alloy plated steel sheet as the original sheet, is preferably a coating film in which a resin having a low glass transition temperature (Tg), a high molecular weight, and a good tensile rate is sufficiently cured without reducing the crosslinker addition amount (i.e., without reducing the crosslinking density), and the stress is dispersed at the time of deformation of the coating film without applying excessive stress to the adhesion interface with the original sheet. By using such a coating film, it is expected that the problem of the tensile processing portion can be improved without reducing the coating film performance such as chemical resistance.

[0087] In the case where the original sheet is a Zn-Al-Mg alloy plated steel sheet, unlike the case where a zinc plated steel sheet is the original sheet, it is considered that the design idea of the conventional coating film in which the crosslinking density is suppressed to be low by setting the Tg of the resin to be high (i.e., designing the coating film to be hard and easily plastically deformed with respect to strain) and reducing the crosslinker addition amount to reduce the coating film stress at the time of processing is not effective for two reasons.

[0088] The first reason is that the barrier effect of the coating film is reduced due to the low crosslinking density, and a corrosion factor easily penetrates the coating film and is immersed, so that the chemical resistance and corrosion resistance cannot be maintained. The Zn-Al-Mg alloy plating layer is superior in corrosion resistance to a pure zinc plating layer, but contains Mg, which is chemically unstable, and thus is easily subjected to corrosion due to chemicals. If the chemicals penetrate the coating film and reach the plated layer of the original sheet, it becomes a cause of coating film swelling due to poor chemical resistance. Even with respect to corrosion resistance, the Zn-Al-Mg alloy plating layer cannot exhibit its inherent superior corrosion resistance due to a decrease in corrosion resistance caused by the immersion of the corrosion factor.

[0089] The second reason is that suppressing the crosslinking density too low prevents the maintenance of the coating's cohesive force. In the past, when zinc-plated steel sheets were used as the base material, the coating was uniformly stretched and compressed during processing, resulting in uniform deformation of the coating, which could be followed. However, Zn-Al-Mg alloy coatings are hard and non-uniform, leading to localized and severe deformation (cracking, compressive strain) during processing. If the coating also deforms severely along with this severe deformation of the coating, the coating's cohesive force is insufficient, resulting in cohesive failure within the coating layer and ultimately coating peeling.

[0090] As mentioned above, when the original plate is a Zn-Al-Mg alloy coated steel plate, a different coating design is required compared to that of the previous zinc-coated steel plate.

[0091] The inventors have studied the design of a new primer coating layer, which will be described in detail below. As a result, it can be seen that the above-mentioned conditions can be achieved by using an appropriate resin with a Tg below 40°C, an appropriate average molecular weight of 19,000 or more, and appropriate crosslinking using end-capped isocyanate as a curing agent.

[0092] For the coatings described above, and for the steel sheets coated with them, the requirement is that when the coating deforms following the deformation of the original sheet caused by pressure processing, the stress caused by the deformation of the coating is not large, and the stress is dispersed and does not concentrate at the interface between the original sheet and the coating. Therefore, it has been determined that using a coating based on the design principles described above as a primer layer and as a lower layer under an upper coating layer is effective.

[0093] The following provides a detailed description of the pre-coated steel sheet and the molded article according to embodiments of the present invention based on the findings described above.

[0094] (For Zn-Al-Mg alloy coated steel sheets that become the original plates)

[0095] First refer to Figure 1A and Figure 1B At the same time, a detailed description is given of the Zn-Al-Mg alloy coated steel sheet of the original plate of the pre-coated steel sheet used as an embodiment of the present invention.

[0096] Figure 1A This is an explanatory diagram illustrating an example of the structure of the Zn-Al-Mg alloy-coated steel sheet of this embodiment. Figure 1B An explanatory diagram illustrating another example of the structure of the Zn-Al-Mg alloy-coated steel sheet of this embodiment.

[0097] like Figure 1AAs schematically shown, the Zn-Al-Mg alloy plated steel sheet 10 of the present embodiment has a steel sheet 101 that is a base material, and a Zn-Al-Mg alloy plated layer 103 that is an example of a plated layer and is present on one surface of the steel sheet. In addition, the Zn-Al-Mg alloy plated steel sheet 10 of the present embodiment can also have, as shown in FIG. 2, a Zn-Al-Mg alloy plated layer 103 on both surfaces of the steel sheet 101 that is a base material. Figure 1B As schematically shown, the Zn-Al-Mg alloy plated steel sheet 10 of the present embodiment has a steel sheet 101 that is a base material, and a Zn-Al-Mg alloy plated layer 103 that is an example of a plated layer and is present on one surface of the steel sheet. In addition, the Zn-Al-Mg alloy plated steel sheet 10 of the present embodiment can also have, as shown in FIG. 2, a Zn-Al-Mg alloy plated layer 103 on both surfaces of the steel sheet 101 that is a base material.

[0098] <For the steel sheet 101>

[0099] The steel sheet 101 used as a base material of the Zn-Al-Mg alloy plated steel sheet 10 of the present embodiment can use various steel sheets depending on the mechanical strength and the like required of the Zn-Al-Mg alloy plated steel sheet 10. As such steel sheets 101, various steel sheets such as an Al-killed steel, an ultra-low carbon steel containing Ti, Nb, or the like, a high-strength steel containing a strengthening element such as P, Si, Mn, or the like in addition to the ultra-low carbon steel, and the like can be cited.

[0100] In addition, the thickness (d0) of the steel sheet 101 of the present embodiment can be appropriately set depending on the mechanical strength and the like required of the Zn-Al-Mg alloy plated steel sheet 10, and can be set to, for example, about 0.2 mm to 2.0 mm. Figure 1A Figure 1B <For the Zn-Al-Mg alloy plated layer 103>

[0101] The Zn-Al-Mg alloy plated layer 103 of the present embodiment (hereinafter sometimes referred to simply as "plated layer 103"), as schematically shown, is a layer formed on at least one surface of the steel sheet 101, and is provided in order to improve the corrosion resistance of the Zn-Al-Mg alloy plated steel sheet 10. First, the chemical composition of the Zn-Al-Mg alloy plated layer 103 of the present embodiment will be described below.

[0102] The Zn-Al-Mg alloy plated layer 103 of the present embodiment, for example, is a plated layer containing, in terms of mass %, aluminum (Al): 0.5% or more and 60.0% or less, magnesium (Mg): 0.5% or more and 15.0% or less, and the balance being zinc (Zn) and impurities. Figure 1A Figure 1B [Al: 0.5 to 60.0 mass%]

[0103] [Al: 0.5 to 60.0 mass%]

[0104] [Al: 0.5 to 60.0 mass%]

[0105] ​​The Zn-Al-Mg alloy plated layer 103 of the present embodiment preferably contains 0.5 mass% or more and 60.0 mass% or less of Al. By setting the content of Al to 0.5 mass% or more and 60.0 mass% or less, the corrosion resistance of the Zn-Al-Mg alloy plated steel sheet 10 of the present embodiment is improved, and the adhesion of the Zn-Al-Mg alloy plated layer 103 (more specifically, the adhesion to the steel sheet 101) can be ensured. In a case where the content of Al is less than 0.5 mass%, there is a possibility that the Zn-Al-Mg alloy plated layer 103 becomes brittle and the adhesion of the Zn-Al-Mg alloy plated layer 103 is reduced. The content of Al is more preferably 5.0 mass% or more. On the other hand, in a case where the content of Al exceeds 60.0 mass%, there is a possibility that the effect of improving the corrosion resistance of the Zn-Al-Mg alloy plated steel sheet 10 is saturated. The content of Al is more preferably 15.0 mass% or less.

[0106] [Mg: 0.5 to 15.0 mass%]

[0107] The Zn-Al-Mg alloy plated layer 103 of the present embodiment preferably contains 0.5 mass% or more and 15.0 mass% or less of Mg. By setting the content of Mg to 0.5 mass% or more and 15.0 mass% or less, the corrosion resistance of the Zn-Al-Mg alloy plated steel sheet 10 of the present embodiment is improved, and the adhesion of the Zn-Al-Mg alloy plated layer 103 (more specifically, the adhesion to the steel sheet 101) can be ensured. In a case where the content of Mg is less than 0.5 mass%, there is a possibility that the effect of improving the corrosion resistance of the Zn-Al-Mg alloy plated steel sheet 10 is insufficient. The content of Mg is more preferably 2.0 mass% or more. On the other hand, in a case where the content of Mg exceeds 15.0 mass%, there is a possibility that the Zn-Al-Mg alloy plated layer 103 becomes brittle and the adhesion of the Zn-Al-Mg alloy plated layer 103 is reduced. The content of Mg is more preferably 4.0 mass% or less.

[0108] [Balance: Zn and impurities]

[0109] In the Zn-Al-Mg alloy plated layer 103 of the present embodiment, the balance other than the above components is Zn and impurities. In addition, in the Zn-Al-Mg alloy plated layer 103 of the present embodiment, silicon (Si) can be contained in a content of 0 mass% or more and 2.0 mass% or less in place of a part of the Zn of the balance.

[0110] [Si: 0 to 2.0 mass%]

[0111] The Zn-Al-Mg alloy coating 103 of this embodiment can contain 0% by mass or more and 2.0% by mass or less Si, replacing a portion of the remaining Zn. By setting the Si content to 0% by mass or more and 2.0% by mass or less, the adhesion of the Zn-Al-Mg alloy coating 103 can be further improved. If the Si content exceeds 2.0% by mass, the effect of improving the adhesion of the Zn-Al-Mg alloy coating 103 may become saturated. The Si content is more preferably 1.6% by mass or less.

[0112] Furthermore, in the Zn-Al-Mg alloy coating 103 of this embodiment, a portion of the remaining Zn can be replaced, and elements such as Fe, Sb, and Pb, either alone or in combination, can be present in amounts of 1% by mass or less.

[0113] As for the coated steel sheet 10 having a Zn-Al-Mg alloy coating 103 with the chemical composition described above, examples include molten zinc-aluminum-magnesium-silicon alloy coated steel sheets (such as "SuperDyma" and "ZAM" manufactured by Nippon Steel Corporation) that have a Zn-11%Al-3%Mg-0.2%Si alloy coating.

[0114] [Regarding the average film thickness of coating 103]

[0115] In the Zn-Al-Mg alloy coated steel sheet 10 of this embodiment, the average film thickness of the Zn-Al-Mg alloy coating layer 103 is ( Figure 1A and Figure 1B The average film thickness d1 of the Zn-Al-Mg alloy coating 103 is preferably 6 μm or more, more preferably 9 μm or more. This average film thickness further improves the corrosion resistance of the Zn-Al-Mg alloy coated steel sheet 10. It should be noted that when the average film thickness d1 of the Zn-Al-Mg alloy coating 103 exceeds 45 μm, the increased coating cost outweighs the cost of improving corrosion resistance. Therefore, from an economic point of view, it is preferable that the average film thickness d1 of the Zn-Al-Mg alloy coating 103 is 45 μm or less.

[0116] It should be noted that the average film thickness d1 of the Zn-Al-Mg alloy coating 103 can be calculated, for example, using a gravimetric method as follows: A Zn-Al-Mg alloy-coated steel sheet with a specified area (e.g., 50 mm × 50 mm) is dissolved in hydrochloric acid with added inhibitors, and the weight of the dissolved material is calculated from the weight difference before and after dissolution. Furthermore, the weight ratio of elements such as Al, Zn, and Fe in the solution is determined / calculated using inductively coupled plasma (ICP) emission spectroscopy, and the average specific gravity of the Zn-Al-Mg alloy coating is calculated from this ratio. The average film thickness d1 of the Zn-Al-Mg alloy coating 103 is calculated by dividing the dissolved weight by the average specific gravity, and then by the area (or area × 2 in the case of double-sided coating).

[0117] Reference above Figure 1A and Figure 1B At the same time, the Zn-Al-Mg alloy coated steel sheet 10 of this embodiment will be described in detail.

[0118] The Zn-Al-Mg alloy-coated steel sheet 10 of this embodiment, as described above, can be manufactured as follows. First, the surface of the prepared steel sheet 101 is subjected to pretreatment such as washing and degreasing as needed. Then, the steel sheet 101, which has undergone pretreatment as needed, is subjected to a conventional hot-dip galvanizing method in an oxidation-free furnace to form a coating layer.

[0119] Here, a hot-dip galvanizing bath with the desired chemical composition is prepared (i.e., a hot-dip galvanizing bath containing at least Al: 0.5–60.0% by mass, Mg: 0.5–15.0% by mass, and the balance being Zn and impurities), and the bath temperature is controlled at approximately 450°C. The resulting steel sheet 101 is then immersed in the galvanizing bath, and hot-dip galvanization is applied to the surface of the steel sheet in a manner that forms a desired average film thickness. The cooling rate after galvanizing is then controlled at 10°C / second or higher. This allows the formation of a Zn-Al-Mg alloy coating.

[0120] (For pre-coated steel sheets)

[0121] Next, refer to Figure 1A and Figure 1B At the same time, a detailed description is given of the pre-coated steel sheet 20 that uses the Zn-Al-Mg alloy coated steel sheet 10 as described above.

[0122] Figure 2A This is an explanatory diagram illustrating an example of the structure of the pre-coated steel sheet according to this embodiment. Figure 2B An explanatory diagram illustrating another example of the structure of the pre-coated steel sheet of this embodiment.

[0123] As Figure 2A As Figure 2B As

[0124] In the pre-coated plated steel sheet 20 of the present embodiment, the steel sheet 101 has the same structure as the steel sheet 101 in the Zn-Al-Mg alloy plated steel sheet 10 described above, and exerts the same effects. Therefore, detailed description will be omitted below.

[0125] In addition, with respect to the Zn-Al-Mg alloy plated layer 201 in the pre-coated plated steel sheet 20 of the present embodiment, as the chemical conversion treatment coating layer 203 described later is formed, interdiffusion of atoms and the like contained in each layer and the like can occur in the vicinity of the interface between the Zn-Al-Mg alloy plated layer 201 and the chemical conversion treatment coating layer 203. However, with respect to the average chemical composition of the Zn-Al-Mg alloy plated layer 201, the same effects as the Zn-Al-Mg alloy plated layer 103 in the Zn-Al-Mg alloy plated steel sheet 10 described above are exerted. Therefore, detailed description will be omitted below.

[0126] <Chemical Conversion Treatment Coating Layer 203>

[0127] The chemical conversion treatment coating layer 203 of the present embodiment is a coating layer on the Zn-Al-Mg alloy plated layer 201, and is a layer formed by chemical conversion treatment after impurities such as oil and surface oxides attached to the surface of the Zn-Al-Mg alloy plated steel sheet 10 are removed using a publicly known degreasing process and a washing process.

[0128] The chemical conversion treatment coating layer 203 of the present embodiment can contain, for example, any one or more selected from the group consisting of a resin, a silane coupling agent, a zirconium compound, silicon dioxide, phosphoric acid and a salt thereof, a fluoride, a vanadium compound, and tannin or tannic acid. By containing these substances, the film-forming properties after coating of the chemical conversion treatment liquid, the barrier properties (denseness) of the coating film against moisture, corrosive ions, and the like, and the adhesion of the coating film to the Zn-Al-Mg alloy plated surface are improved, and the level of corrosion resistance of the coating film is improved.

[0129] In particular, if the chemical conversion treatment coating layer 203 contains any one or more of a silane coupling agent or a zirconium compound, a crosslinked structure is formed within the coating layer 203, and the bonding to the Zn-Al-Mg alloy plated surface is strengthened, and thus the adhesion and barrier properties of the coating film can be further improved.

[0130] In addition, if the chemical conversion treatment coating layer 203 contains any one or more of silicon dioxide, phosphoric acid and a salt thereof, a fluoride, or a vanadium compound, it functions as an inhibitor, and a precipitated coating film or a passive coating film is formed on the Zn-Al-Mg alloy plated layer or the steel surface, and thus the corrosion resistance can be further improved.

[0131] The following describes each of the constituent components that the chemical conversion treatment coating layer 203 can contain, with examples.

[0132] [Resin]

[0133] There is no particular limitation on the resin, and known organic resins such as polyester resins, polyurethane resins, epoxy resins, phenol resins, acrylic resins, and polyolefin resins can be used. In order to further improve the adhesion to the plated steel sheet for a pre-coated steel sheet, at least one of a resin having a strong site and a polar functional group in the molecular chain (polyester resins, polyurethane resins, epoxy resins, acrylic resins, and the like) is preferably used. The resin can be used alone or in combination of two or more.

[0134] The content of the resin in the chemical conversion treatment coating layer 203 is, for example, preferably 0% by mass or more and 85% by mass or less with respect to the solid content of the coating film. The content of the resin is more preferably 0% by mass or more and 60% by mass or less, and further preferably 1% by mass or more and 40% by mass or less. If the content of the resin exceeds 85% by mass, the ratio of other coating film constituent components decreases, and the performance required of the coating film other than corrosion resistance decreases.

[0135] [Silane Coupling Agent]

[0136] As the silane coupling agent, for example, γ-(2-aminoethyl)aminopropyltrimethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-(2-aminoethyl)aminopropyltriethoxysilane, γ-(2-aminoethyl)aminopropylmethyldiethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, γ-methacryloyloxypropylmethyldimethoxysilane, γ-methacryloyloxypropyltriethoxysilane, γ-methacryloyloxypropylmethyldiethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropylmethyldimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltriethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-mercaptopropylmethyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, vinyltriacetoxysilane, γ-chloropropyltrimethoxysilane, γ-chloropropylmethyldimethoxysilane, γ-chloropropyltriethoxysilane, γ-chloropropylmethyldiethoxysilane, hexamethyldisilazane, γ-anilinopropyltrimethoxysilane, γ-anilinopropylmethyldimethoxysilane, γ-anilinopropyltriethoxysilane, γ-anilinopropylmethyldiethoxysilane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldimethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(triethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldiethoxysilyl)propyl]ammonium chloride, γ-chloropropylmethyldimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, and the like can be mentioned. The amount of the silane coupling agent added in the chemical conversion treatment agent for forming the chemical conversion treatment coating layer 203 can be set to 2 to 80 g / L, for example. In the case where the amount of the silane coupling agent added is less than 2 g / L, adhesion to the plated surface is insufficient, and the processing adhesion of the coating film can possibly be reduced. Further, in the case where the amount of the silane coupling agent added exceeds 80 g / L, cohesion of the chemical conversion treatment coating layer is insufficient, and the processing adhesion of the coating film layer can possibly be reduced.The silane coupling agent exemplified above can be used singly or in combination of two or more.

[0137] [Zirconium compound]

[0138] As the zirconium compound, for example, zirconium n-propoxide, zirconium n-butyrate, zirconium tetraacetylacetonate, zirconium monoacetylacetonate, zirconium bisacetylacetonate, zirconium monoethylacetoacetate, zirconium acetylacetonate bisethylacetoacetate, zirconium acetate, zirconium monostearate, zirconium carbonate, zirconium ammonium carbonate, zirconium potassium carbonate, zirconium sodium carbonate, and the like can be exemplified. The addition amount of the zirconium compound in the chemical conversion treatment agent for forming the chemical conversion treatment film layer 203 can be set to, for example, 2 to 80 g / L. In the case where the addition amount of the zirconium compound is less than 2 g / L, adhesion to the plated surface is insufficient, and the processing adhesion of the coating film can possibly be reduced. In addition, in the case where the addition amount of the zirconium compound exceeds 80 g / L, cohesion of the chemical conversion treatment film layer is insufficient, and the processing adhesion of the coating film layer can possibly be reduced. The above zirconium compound can be used singly or in combination of two or more.

[0139] [Silica]

[0140] As the silica, for example, commercially available silica gels such as "Snowtex N", "Snowtex C", "Snowtex UP", "Snowtex PS" manufactured by Nissan Chemical Industries, Ltd., "ADELITE AT-20Q" manufactured by ADEKA Co., Ltd., or powder silica such as Aerosil #300 manufactured by Nippon Aerosil Co., Ltd. can be used. The silica can be appropriately selected depending on the desired properties of the pre-coated plated steel sheet. The addition amount of the silica in the chemical conversion treatment agent for forming the chemical conversion treatment film layer 203 is preferably set to 1 to 40 g / L. In the case where the addition amount of the silica is less than 1 g / L, the processing adhesion of the coating film layer can possibly be reduced, and in the case where the addition amount of the silica exceeds 40 g / L, the effects of the processing adhesion and the corrosion resistance are likely to be saturated, and thus it is not economical.

[0141] [Phosphoric acid and salts thereof]

[0142] As the phosphoric acid and salts thereof, for example, phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, and salts thereof, ammonium salts such as ammonium phosphate, diammonium hydrogen phosphate, phosphonic acids such as amino tris(methylene phosphonic acid), 1-hydroxyethylidene-1, 1-diphosphonic acid, ethylenediamine tetra(methylene phosphonic acid), diethylenetriamine penta(methylene phosphonic acid), and salts thereof, organic phosphoric acids such as phytic acid, and salts thereof, and the like can be exemplified. Note that, as the salts of phosphoric acid other than the ammonium salts, metal salts with Na, Mg, Al, K, Ca, Mn, Ni, Zn, Fe, and the like can be exemplified. The phosphoric acid and salts thereof can be used singly or in combination of two or more.

[0143] Note that the content of phosphoric acid and its salt is preferably 0 mass% or more and 20 mass% or less with respect to the solid content of the coating film. In a case where the content of phosphoric acid and its salt exceeds 20 mass%, the coating film becomes brittle, and the process adhesion of the coating film at the time of forming processing of the pre-coated plated steel sheet can possibly be reduced. The content of phosphoric acid and its salt is more preferably 1 mass% or more and 10 mass% or less.

[0144] [Fluoride]

[0145] As the fluoride, for example, ammonium fluorozirconate, ammonium fluorosilicate, ammonium fluorotitanate, sodium fluoride, potassium fluoride, calcium fluoride, lithium fluoride, fluorotitanic acid, fluorozirconic acid, and the like can be exemplified. The above-mentioned fluorides can be used alone or in combination of two or more.

[0146] Note that the content of fluoride is preferably 0 mass% or more and 20 mass% or less with respect to the solid content of the coating film. In a case where the content of fluoride exceeds 20 mass%, the coating film becomes brittle, and the process adhesion of the coating film at the time of forming processing of the pre-coated plated steel sheet can possibly be reduced. The content of fluoride is more preferably 1 mass% or more and 10 mass% or less.

[0147] [Vanadium compound]

[0148] As the vanadium compound, for example, vanadium compounds obtained by reducing vanadium compounds of valence 5 such as vanadium pentoxide, metavanadic acid, ammonium metavanadate, sodium metavanadate, vanadium trioxynitrate, to valence 2 to 4 with a reducing agent, vanadium trioxide, vanadium dioxide, vanadyl sulfate, vanadyl oxalate, vanadyl acetylacetonate, vanadium acetylacetonate, vanadium trichloride, phosphomolybdovanadic acid, vanadic sulfate, vanadic dichloride, vanadic oxide, and the like can be exemplified. The above-mentioned vanadium compounds can be used alone or in combination of two or more.

[0149] Note that the content of vanadium compound is preferably 0 mass% or more and 20 mass% or less with respect to the solid content of the coating film. In a case where the content of vanadium compound exceeds 20 mass%, the coating film becomes brittle, and the process adhesion of the coating film at the time of forming processing of the pre-coated plated steel sheet can possibly be reduced. The content of vanadium compound is more preferably 1 mass% or more and 10 mass% or less.

[0150] [Tannin or tannic acid]

[0151] The tannin or tannic acid can use any one of tannins which can be hydrolyzed, condensed tannins. As examples of the tannin and tannic acid, gallic tannin, gallnut tannin, gallotannin, chebulic tannin, leafflower tannin, algarovilla tannin, oak tannin, catechin, etc. can be listed. The addition amount of the tannin or tannic acid in the chemical conversion treatment agent for forming the chemical conversion treatment film 203 can be set to 2 to 80 g / L. In the case where the addition amount of the tannin or tannic acid is less than 2 g / L, adhesion to the plated surface is insufficient, and the processing adhesion of the coating film can be reduced. In addition, in the case where the addition amount of the tannin or tannic acid exceeds 80 g / L, cohesion of the chemical conversion treatment film is insufficient, and the processing adhesion of the coating film can be reduced.

[0152] In addition, in the chemical conversion treatment agent for forming the chemical conversion treatment film 203, in a range not impairing the properties, in order to adjust the pH, an acid, a base, etc. can be added.

[0153] The chemical conversion treatment agent containing the various components as described above is applied to one side or both sides of the Zn-Al-Mg alloy plated steel sheet 10, and then dried to form the chemical conversion treatment film layer 203. The pre-coated steel sheet of the present embodiment is preferably formed with the chemical conversion treatment film layer 203 of 10 to 1000 mg / m 2 on each side of the Zn-Al-Mg alloy plated steel sheet. The adhesion amount of the chemical conversion treatment film layer 203 is more preferably 20 to 800 mg / m 2 , and most preferably 50 to 600 mg / m 2 . Note that the film thickness (thickness d2 in Figure 2A and Figure 2B ) of the chemical conversion treatment film layer 203 corresponding to the above adhesion amount is approximately 0.01 to 1 μm or so, although depending on the components contained in the chemical conversion treatment agent.

[0154] <Coating film layer 205>

[0155] The coating film layer 205 of the present embodiment is a layer formed on the chemical conversion treatment film layer 203 as described above. The coating film layer 205 as described above is composed of a plurality of layers such as a primer coating film layer and an upper coating film layer, as schematically shown in Figure 2A and Figure 2B

[0156] ​Here, the coating film layer 205 in contact with the chemical conversion treatment coating film layer 203 is also referred to as a primer coating film layer. This primer coating film layer ensures adhesion and corrosion resistance of the entire coating film layer 205 to the chemical conversion treatment coating film layer 203 and is positioned as the most important factor that affects the problem of the stretch-processed portion. On the other hand, an upper coating film layer of the coating film that is positioned above the primer coating film layer described above is also referred to as a top coating film, and is provided mostly to ensure designability, barrier properties, and other surface functionality achieved by coloring. This upper coating film layer is positioned as a factor related to the physical properties of the entire coating film layer 205 that affect the problem of the stretch-processed portion.

[0157] The coating film layer 205 described above contains at least a resin. In addition, the coating film layer 205 described above preferably contains a pigment. In the coating film layer 205, various additives such as a leveling agent, a defoaming agent, a colorant, a viscosity modifier, and an ultraviolet absorber can be further contained in addition to these components. Note that a coating liquid for forming the coating film layer 205 is preferably obtained by dispersing or dissolving the components described above in a solvent.

[0158] The preferred mode of the primer coating film layer and the upper coating film layer of the coating film layer 205 of the present embodiment will be described in more detail below.

[0159] [Primer Coating Film Layer]

[0160] The primer coating film layer of the present embodiment is implemented in accordance with the design policy described above. The primer coating film layer described above preferably has a polyester-based resin having an average molecular weight of 19000 or more and a glass transition temperature Tg of less than 40°C, and a curing agent.

[0161] In the case where the average molecular weight (more specifically, the number average molecular weight) of the polyester-based resin is less than 19000, the stretchability of the primer coating film layer formed is insufficient. As a result, in the case where the coating film layer 205 is deformed, the coating film layer 205 cannot follow the deformation and large stress is generated in the coating film layer 205. By setting the average molecular weight of the polyester-based resin to 19000 or more, the deformation generated in the coating film layer 205 can be followed and the stress generated in the coating film layer 205 can be suppressed to a small value. The average molecular weight of the polyester-based resin is more preferably 20000 or more and further preferably 23000 or more.

[0162] On the other hand, in the case where the average molecular weight of the polyester-based resin exceeds 25000, the crosslinking density cannot be sufficiently increased and it is likely that scratches of the coating film are easily generated. Therefore, the average molecular weight of the polyester-based resin is more preferably 25000 or less.

[0163] Note that the average molecular weight of the polyester-based resin can be measured using gel permeation chromatography (GPC).

[0164] In addition, when the glass transition temperature Tg of the polyester-based resin is 40°C or higher, the primer coating film layer formed is too hard and easily plastically deforms in response to strain. By lowering the glass transition temperature Tg of the polyester-based resin to below 40°C, a primer coating film layer having an appropriate hardness can be achieved. The glass transition temperature Tg of the polyester-based resin is more preferably 30°C or lower, and further preferably 20°C or lower. On the other hand, when the glass transition temperature Tg of the polyester-based resin is below 0°C, there is a possibility that the corrosion resistance will decrease due to insufficient cohesion of the coating film. Therefore, the glass transition temperature Tg of the polyester-based resin is more preferably 0°C or higher.

[0165] Note that the glass transition temperature Tg of the polyester-based resin can be specified by performing differential thermal analysis on the resin of interest using a differential scanning calorimeter, from the resulting DTA curve.

[0166] In addition, the primer coating film layer of the present embodiment contains a blocked isocyanate or a blocked isocyanate resin as a curing agent. By containing a blocked isocyanate or a blocked isocyanate resin as a curing agent in the primer coating film layer, the polyester-based resin can be crosslinked in an appropriate state, and a high level of coating film extensibility, and a balance of adhesion and hardness performance of the primer coating film layer can be achieved.

[0167] As the blocked isocyanate described above, for example, TDI (toluene diisocyanate), MDI (diphenylmethane diisocyanate), XDI (xylene diisocyanate), NDI (naphthalene diisocyanate), HDI (hexamethylene diisocyanate), IPDI (isophorone diisocyanate), hydrogenated MDI, and hydrogenated XDI, etc. can be exemplified. Any one of the blocked isocyanates can be used, and if at least any one of HDI, IPDI, hydrogenated MDI, or hydrogenated XDI, which are aliphatic compounds or alicyclic compounds, is used, the reaction proceeds comparatively smoothly and uniform crosslinking is achieved, and thus is particularly preferable for the above purpose. In addition, a blocked isocyanate resin formed by pre-polymerization using these blocked isocyanates as raw materials can also be used. At this time, by resin design, the number of functional groups per molecule (NCO group number) of the blocked isocyanate resin can be changed, and thus the coating film properties after curing can be more easily controlled.

[0168] In addition, as the blocking agent of the blocked isocyanate or the blocked isocyanate resin, ε-caprolactam, MEK oxime, benzotriazole, etc. can be used, but MEK oxime, which has a comparatively low dissociation temperature, is preferable for the above purpose.

[0169] In addition, in the primer coating film layer of the present embodiment, if at least any one of the following conditions (a-1) and (a-2); condition (b); and condition (c) is satisfied, as a curing agent, a curing agent other than blocked isocyanate or blocked isocyanate resin can also be used.

[0170] By the primer coating film layer of the present embodiment having the above-described configuration, the formed primer coating film layer at least satisfies any one of the first point or the second point mentioned earlier. As a result, the primer coating film layer of the present embodiment at least satisfies any one of the following conditions: conditions (a-1) and (a-2); condition (b); and condition (c).

[0171] (a-1) The interface between the coating film layer and the layer on the steel sheet side of the coating film layer is cut using the SAICAS method, and the cutting strength obtained is 1.00 kN / m or more on average, and 5% or less of the cutting area is in the interface peeling mode, and the remaining cutting area is in the cohesive failure mode within the coating film layer.

[0172] (a-2) When a portion of a pre-coated steel sheet subjected to uniaxial stretching processing, the thickness of which is reduced by 20% or more compared to the non-processed portion, is cut using the SAICAS method, for the interface between the coating film layer and the layer on the steel sheet side of the coating film layer (more specifically, the interface between the primer coating film layer and the chemical conversion treatment film layer or the plating layer on the steel sheet side of the primer coating film layer), the cutting strength is 1.0 kN / m or more on average, and 20% or less of the cutting area is in the interface peeling mode, and the remaining cutting area is in the cohesive failure mode within the coating film layer (more specifically, within the primer coating film layer).

[0173] (b) In a cross section obtained by cutting the primer coating film layer in the thickness direction, when a four-pyramid type indenter for micro hardness measurement is inserted at a speed of 2 μm per minute to a depth of 10 μm at a position 1 μm from the interface between the primer coating film layer and the layer on the steel sheet side of the primer coating film layer in the direction of the primer coating film layer, the indentation load at that time is less than 1000 mN.

[0174] (c) In a cross section obtained by cutting the primer coating film layer in the thickness direction, when a four-pyramid type indenter for micro hardness measurement is inserted at a speed of 10 μm per second to a depth of 10 μm at a position 1 μm from the interface between the primer coating film layer and the layer on the steel sheet side of the primer coating film layer in the direction of the primer coating film layer, and then the indenter is unloaded at a speed of 2 μm per minute, the area of the range enclosed by the line corresponding to displacement = 10 μm, the line corresponding to load = 0, and the load curve at the time of unloading in a plane defined by displacement and load is less than 500 μm·mN.

[0175] Note that the "1 μm position in the direction of the primer coating film layer from the interface of the primer coating film layer and the layer on the steel sheet side of the primer coating film layer" in the above-described conditions (b) and (c) is, as schematically shown, Figure 2A as schematically shown, can be regarded as the 1 μm position in the direction of the primer coating film layer from the lower end (end portion on the steel sheet side) of the primer coating film layer toward the upper layer coating film layer.

[0176] Further, the area in condition (c) is, as schematically shown, the area corresponding to the region depicted by the diagonal line in the figure. In this case, in the case of the primer coating film layer on which the present embodiment focuses, the maximum load P Figure 2B MAX is usually formed to be around 1000 to 2000 mN, and the displacement D MAX at the time of the maximum load is usually also more than 10 μm. The above-described area is formed as an index of the strain energy remaining in the resin at the time of unloading after applying a strain to the resin on which attention is focused, and it can be considered that the larger the above-described area, the larger the energy (i.e., residual stress) remaining in the resin.

[0177] In this case, the cutting strength and the peeling form of the primer coating film layer using the SAICAS method in conditions (a-1) and (a-2) can be measured at the same time using a measuring device capable of cutting using the SAICAS method (for example, DN-GS type manufactured by DAIPLA WINTES CO., LTD.).

[0178] The cutting conditions using the SAICAS method are as described below.

[0179] As the cutting blade, a diamond blade (0.3 mm width) was used, and after performing oblique cutting at a constant speed mode of a horizontal speed of 1 μm / sec. and a vertical speed of 0.1 μm / sec., the horizontal movement was switched to only the horizontal movement to cut at a length of 200 μm in the vicinity of the interface, and the average cutting strength at the time of the horizontal movement was measured. The depth position at which the horizontal movement of the cutting blade was switched was set by specifying the interface position (position at which the plating layer was not cut) using a preliminary experiment. In the case where the plating layer surface was cut due to the unevenness of the plating layer in the horizontal movement of the cutting blade, the cutting strength was instantaneously abnormally increased, and thus this could be distinguished. This case was excluded as an outlier, and the average cutting strength was calculated. Note that the average value of three values of the average cutting strength was set as the cutting strength, with the number of measurements n = 3.

[0180] The measurement method of the ratio of the interface peeling form and the cohesive failure form in the cutting portion at the time of the horizontal movement is as described below.

[0181] ​If the surface of the cut portion by the SAICAS method is observed with an optical microscope, the difference in the peeling mode of each portion can be clearly distinguished. (A) In the case where a very thin coating film remains on the cut portion, coloring due to the resin and pigment in the coating film is found, and it can be determined that the peeling mode is cohesive failure in the thin layer within the coating film. (B) In the case where the cut portion is interfacial peeling, the appearance of the surface of the plated layer of the substrate is observed. Even if light is irradiated on the above portion, no strong reflection is found, and a blackened appearance is formed. In addition, in the case of interfacial peeling, the cutting strength obtained by the SAICAS method is locally reduced, which becomes a criterion for the material. This is because the moving position of the cutting tool is within the coating film directly above the interface, but the adhesion of the interface is lower than the cohesion of the coating film, and thus the peeling position shifts to the interface, resulting in interfacial peeling. (C) In the case where the cut portion is cohesive failure of the plated layer, metallic luster is observed, and if light is irradiated on the above portion, it is strongly reflected, and thus it is easily distinguished from interfacial peeling. In addition, the cutting strength obtained by the SAICAS method is locally increased, which becomes a criterion for the material.

[0182] An optical microscope photograph of the horizontal cut range (size 300 μm x 200 μm) by the SAICAS method is taken, and the positions of the cohesive failure of the coating film, the interfacial peeling, and the cohesive failure of the plated layer within the same range are specified by the above criteria, and their areas are measured using image processing software or transparent grid paper. Next, the ratio of the interfacial peeling area to the area excluding the cohesive failure of the plated layer from the horizontal cut range by the SAICAS method is calculated.

[0183] In the primer coating film layer of the present embodiment, the content of the above curing agent is more preferably 5 to 15 mass% relative to the total content of the polyester-based resin and the curing agent. By containing the curing agent at this content, the crosslinked state of the polyester-based resin can be made more appropriate. The content of the curing agent is further preferably 8 to 12 mass% relative to the total content of the polyester-based resin and the curing agent.

[0184] In addition, when the number of substituents in the blocked isocyanate or the blocked isocyanate resin that can react with the OH group in the polyester-based resin is denoted as na, and the number of OH groups in the polyester-based resin is denoted as nb, and (na + nb) is preferably 5.5 or more and 8.0 or less. By satisfying the above condition, the crosslinked state of the polyester-based resin can be formed in a further appropriate state. (na + nb) is more preferably 6.0 or more. In addition, (na + nb) is more preferably 7.5 or less.

[0185] By setting the conditions as above, a resin having a low Tg, a high molecular weight, and a good stretchability can be sufficiently cured without reducing the amount of the crosslinking agent added (i.e., without reducing the crosslinking density), and a coating film in which stress caused by deformation of the coating film is dispersed can be obtained. As a result, the problem of the stretch-processed portion can be more easily improved without reducing the properties of the coating film.

[0186] Note that the number of the above substituents can be calculated as follows.

[0187] That is, regarding na, 2.0 in the case of diisocyanate for blocked isocyanate. In addition, in the case of blocked isocyanate resin, it can be calculated from the catalog value of the NCO content of the isocyanate used as a raw material and the molecular weight after prepolymerization (measured by gel permeation chromatography). Regarding nb, it can be calculated from the number average molecular weight and the OH value (both are catalog values) of the polyester-based resin used.

[0188] In addition, for the primer coating film layer of the present embodiment, it is more preferable that, as the curing agent, in addition to the above blocked isocyanate, melamine or a melamine derivative is contained. By containing melamine or a melamine derivative as the curing agent, the crosslinking state of the primer coating film layer can be more preferably made. As such melamine or a melamine derivative, for example, methylation melamine typified by fully alkylated melamine, methylol melamine, imino melamine, and the like, butylation melamine typified by fully alkylated melamine, imino butylated melamine, and the like, mixed etherification melamine typified by fully alkylated mixed etherification melamine, methylol mixed etherification melamine, imino mixed etherification melamine, and the like can be listed. In addition, as the melamine or the melamine derivative, in particular, imino methylated melamine is used, whereby the crosslinking state of the primer coating film layer is dense and has high uniformity, and the crosslinking state can be further improved.

[0189] Here, the content of the melamine or the melamine derivative is more preferably 20 to 60% by mass with respect to the total content of the blocked isocyanate or the blocked isocyanate resin and the melamine or the melamine derivative. By containing the melamine or the melamine derivative at such a content, the crosslinking state of the polyester-based resin can be more appropriately made. The content of the melamine or the melamine derivative is further preferably 30 to 50% by mass with respect to the total content of the blocked isocyanate or the blocked isocyanate resin and the melamine or the melamine derivative.

[0190] The base coating of the primer coating film can be appropriately selected according to the use environment and purpose of the pre-coated plated steel sheet, provided that it is within the range of the primer coating described above. The base coating contains the polyester-based resin and the curing agent described above, and any of the organic solvent-based, water-based, or powder-based systems can be used.

[0191] The base coating preferably contains an anti-rust pigment (particularly, a non-chromate anti-rust pigment). The non-chromate anti-rust pigment in the base coating can use any of calcium ion-exchanged silica (also called calcium silicate, at times), aluminum tripolyphosphate, phosphorus-vanadium pigment (PV pigment), zinc phosphate, iron phosphate, aluminum phosphate, calcium molybdate, aluminum molybdate, barium molybdate, vanadium oxide, water-dispersed silica, fumed silica, orthophosphoric acid, pyrophosphoric acid, metaphosphoric acid, hypophosphoric acid, phosphorous acid, hypophosphorous acid, and salts thereof. The content of the anti-rust pigment described above is preferably 20 to 60% by mass, for example, with respect to the solid content of the coating film. If the content of the anti-rust pigment is less than 20% by mass, the corrosion resistance effect can not be sufficiently ensured, and the rigidity and cohesion of the coating film can decrease, which can cause the coating film to peel off (coating film scratches) when the plated steel sheet is pressed against a mold. If the content of the anti-rust pigment exceeds 60% by mass, the workability can decrease. From the viewpoint of the balance between corrosion resistance, chemical resistance, and workability, the content of the anti-rust pigment is more preferably 30 to 55% by mass, and further preferably 35 to 50% by mass.

[0192] The thickness of the primer coating film layer before forming is preferably 2 to 10 μm. If the thickness of the primer coating film layer is less than 2 μm, the corrosion resistance and the like required of the pre-coated plated steel sheet can not be sufficiently exhibited. On the other hand, if the thickness of the primer coating film layer exceeds 10 μm, the workability of the coating film can decrease. The thickness of the primer coating film layer before forming is more preferably 3 to 8 μm, and further preferably 4 to 6 μm.

[0193] After the primer coating composition containing the components that constitute the primer coating film layer described above is applied, it is sintered and dried at a temperature of 150°C or higher and lower than 300°C. If the sintering temperature is lower than 150°C, the adhesion can not be sufficiently ensured, and if the sintering temperature is 300°C or higher, thermal degradation of the resin components can occur, which can decrease the workability.

[0194] Note that the application of the primer coating composition described above can be performed using a generally known application method, such as roll coating, curtain flow coating, air spray, airless spray, dipping, bar coating, brush coating, and the like. In order to solve the problem of the stretch-processed portion described above, it is advantageous to uniformly apply and dry the primer coating film layer, and in order to uniformly apply within the film thickness range described above, roll coating or curtain flow coating is preferably performed.

[0195] [upper coating film layer]

[0196] The base coating of the upper coating film layer can be appropriately selected according to the use environment and purpose of the pre-coated plated steel sheet, within the range of the upper coating film coating prescribed in the present application. As the type of the resin of the base coating, a generally known resin type can be used. As such a resin, for example, a polyacrylic resin, a polyolefin-based resin, a polyurethane-based resin, an epoxy-based resin, a polyester-based resin, a polybutyral-based resin, a melamine-based resin, a silicone resin, a fluorine resin, an acrylic resin, and the like can be listed, and these resins can be used directly or in combination. In addition, these resins can be cured with an arbitrary curing agent. As the base coating, any one of an organic solvent-based, water-based, or powder-based, and the like can be used. Note that, as the resin contained in the base coating of the upper coating film layer, the type can be the same as or different from the resin contained in the base coating film of the primer coating film layer, but, in consideration of the adhesion between the primer coating film layer and the upper coating film layer, it is preferable to use the same type of resin as each other.

[0197] In a use purpose where the formability is more severe, the base coating preferably contains a high-molecular polyester resin and a curing agent. As the high-molecular polyester resin, any one of a high-molecular polyester-based resin used as a solvent-based coating can be used, according to the purpose of the pre-coated plated steel sheet. The above high-molecular polyester-based resin is preferably a high-molecular polyester resin in which a main resin is constituted by an ester bond of two or more resin monomers.

[0198] As the curing agent for forming a thermosetting resin coating film by reacting with the above high-molecular polyester resin, for example, an amino resin such as a melamine resin, a urea resin, a benzoguanamine resin, or an isocyanate compound and a blocking body thereof can be used. The mass ratio of these curing agents and resins in the dried coating film is preferably 10 to 35 mass parts of the amount of the curing agent, with respect to 100 mass parts of the total amount of the resin and the curing agent. In the case where the amount of the curing agent is less than 10 mass parts, it can not be possible to sufficiently ensure the adhesion, corrosion resistance, solvent resistance, and the like, and in the case where the amount exceeds 35 mass parts, the processability, chemical resistance, and impact resistance can be reduced.

[0199] In addition, the upper coating film layer can further contain, as necessary, a pigment, a metal powder subjected to surface modification, a glass powder, a dispersant, a leveling agent, a wax, an aggregate, a fluorine resin bead, and the like, an additive, a dilution solvent, and the like.

[0200] In this case, in the case where the upper coating film layer further contains a pigment, the content of the above pigment is, for example, preferably 60% or less in terms of pigment weight concentration (PWC).

[0201] The total thickness of the primer coating film layer and the upper coating film layer in the coating film layer 205 of the present embodiment is preferably 10 to 30 μm. In the case where the total thickness is less than 10 μm, the corrosion resistance can be decreased in addition to the increase in the cracking due to processing, and thus is not preferred. The total thickness is more preferably 13 μm or more, and further preferably 20 μm or more. On the other hand, in the case where the total thickness of the primer coating film layer and the upper coating film layer exceeds 30 μm, the residual stress at the time of processing is increased, and various performance deteriorations of the stretched processed portion such as the blooming phenomenon can occur, and thus is not preferred. The total thickness is more preferably 28 μm or less, and further preferably 25 μm or less.

[0202] Here, the thickness of the primer coating film layer and the total thickness of the primer coating film layer and the upper coating film layer of the present embodiment can be measured by observing the cross section of the coating film layer 205 of the present embodiment using an optical microscope. More specifically, a pre-coated plated steel sheet containing the coating film layer 205 of interest is embedded in a thermosetting resin such as an epoxy resin, and the sample is cut at the portion to be observed using a cutting machine such as a precision cutter in a manner parallel to the thickness direction, and the obtained cross section is observed using an optical microscope. The shortest distance from an arbitrary position of a plurality of positions (for example, 5 positions) on the interface between the embedded resin and the coating film layer to the interface between the coating film layer and the layer on the steel sheet side of the coating film layer is measured (that is, the distance is measured in the direction perpendicular to the interface), and the obtained measured values are averaged. The average thickness of the coating film thus obtained can be set as the total thickness of the primer coating film layer and the upper coating film layer. The thickness of the primer coating film layer can also be measured in the same manner by measuring the shortest distance from the interface between the primer coating film layer and the upper coating film layer to the interface between the coating film layer and the layer on the steel sheet side of the coating film layer.

[0203] After the coating composition for the upper coating film layer is applied, sintering and drying are performed at a temperature of 150°C or higher and lower than 300°C. In the case where the sintering temperature is lower than 150°C, the adhesion of the coating film can not be sufficiently ensured, and in the case where the sintering temperature is 300°C or higher, thermal degradation of the resin components represented by the polyester resin component can occur, and the processability can be decreased.

[0204] Note that the application of the coating for forming the upper coating film layer can be performed using a generally known coating method such as roll coating, curtain flow coating, air spray, airless spray, dipping, bar coating, brush coating, and the like. In order to solve the problems of the stretched processed portion described above, it is advantageous to uniformly apply and dry the upper coating film layer as with the primer coating film layer, and in order to perform uniform application within the above film thickness range, roll coating or curtain flow coating is preferred.

[0205] The pre-coated plated steel sheet 20 of the present embodiment will be described in detail with reference to Figure 3 the above.

[0206] (for a molded product)

[0207] Next, a molded product using the pre-coated plated steel sheet 20 as explained above will be described in detail with reference to Figure 4 Figures 2A-4 is a diagram for schematically showing an example of a structure of the molded product of the present embodiment.

[0208] As schematically shown in the example shown in Figure 5 The molded product 30 of the present embodiment is formed by subjecting the pre-coated plated steel sheet 20 as explained above to 0T bending processing to form a desired shape.

[0209] Here, the average chemical composition of the plated layer possessed by the molded product 30 of the present embodiment is the same as that of the plated layer 201 possessed by the pre-coated plated steel sheet 20 that is the origin. Therefore, the average chemical composition of the plated layer possessed by the molded product 30 contains, for example, 0.5 to 60.0 mass% of aluminum and 0.5 to 15.0 mass% of magnesium. Among them, the plated layer possessed by the molded product 30 of the present embodiment preferably contains 5 mass% or more and 15 mass% or less of aluminum and 2 mass% or more and 4 mass% or less of magnesium. By containing aluminum and magnesium in the plated layer of the molded product 30 in the above-described amounts, the desired corrosion resistance can be further improved. Note that the balance of the above-described aluminum and magnesium in the plated layer of the molded product 30 is elements derived from the external environment, zinc, and impurities.

[0210] As a specific shape of the molded product 30 of the present embodiment, various shapes possessed by various components, typified by articles that are mainly used outdoors such as air conditioner outdoor units and water heaters, can be cited.

[0211] The processing method for processing the pre-coated plated steel sheet 20 of the present embodiment to form a molded product is not particularly limited, and as for the processing conditions thereof, appropriate settings can be made in accordance with the processing method used, the shape of the molded product, and the like.

[0212] When manufacturing the molded product as described above, 0T bending processing can be said to be a severe forming process for the pre-coated plated steel sheet 20. By the above-described 0T bending processing, a portion subjected to stretching processing is formed near the top of the head of the portion subjected to bending processing. In the case of using a general pre-coated plated steel sheet, peeling of the coating film frequently occurs at the above-described top of the head.

[0213] However, in the case of using the pre-coated plated steel sheet 20 of the present embodiment as a raw material, the stress of the coating film due to processing is not large due to the properties of the coating film, and in addition, the cohesion of the primer coating layer is also sufficient, so that the occurrence of peeling of the coating film in the portion subjected to stretching processing can be more reliably suppressed.

[0214] ​For the measured value obtained by the SAICAS method

[0215] For the shaped product formed from the pre-coated plated steel sheet of the present embodiment, the cutting strength obtained by measuring a specific portion using the SAICAS method, and the peeling state of the portion are specified.

[0216] Figure 5 In the shaped product of the present embodiment, which is schematically shown as an example, the portion in which the thickness of the pre-coated plated steel sheet of the shaped product is reduced by 20% or more compared to the thickness before shaping (which can also be regarded as a non-shaped processed portion) (when the thickness is set as d', the portion in which the relationship of (d-d') / d ≥ 0.20 is satisfied), is a portion in which the plated steel sheet is compressed and stretched by processing, and the stretching is greater than the compression. Hereinafter, this portion in which the thickness is reduced by 20% or more will be referred to as a "reduced portion". Such a reduced portion is a portion in which peeling of the coating film is likely to occur in the shaped product.

[0217] For the portion in which the thickness of the pre-coated plated steel sheet in the shaped product of the present embodiment is reduced by 20% or more compared to before shaping (i.e., the stretched portion), the cutting strength between the chemical conversion treatment film layer and the coating film layer (more specifically, the primer coating film layer) is 1.00 kN / m or more on average, as measured using the SAICAS method. Furthermore, when the stretched portion is cut using the SAICAS method, 20% or less of the cut area forms an interfacial peeling pattern, and the remaining portion of the cut area forms a cohesive failure pattern within the coating film layer (more specifically, within the primer coating film layer). In the case where the stretched portion does not satisfy all of the above conditions regarding the cutting strength and the peeling area, the chemical conversion treatment film is damaged due to the processing accompanied by stretching, and thus the adhesion is reduced. Furthermore, the internal stress of the coating film is concentrated at the interface between the chemical conversion treatment film layer and the coating film layer (more specifically, the primer coating film layer), or at the interface between the chemical conversion treatment film layer and the plated layer, and as a result, the adhesion of the coating film during the stretching processing of the portion in which the adhesion strength is the lowest is insufficient, and peeling of the coating film occurs. Furthermore, in the case where the cutting strength measured using the SAICAS method does not satisfy 1.00 kN / m on average, even though a sufficient area forms a cohesive failure within the coating film layer (more specifically, within the primer coating film layer), the cohesive force of the primer coating film layer is not sufficient, and as a result, peeling of the coating film due to cohesive failure of the primer coating film layer occurs.

[0218] Note that, in the case of the portion in which the thickness of the pre-coated plated steel sheet in the shaped product is reduced by 20% or more compared to before shaping, no significant difference was found in the measurement results using the SAICAS method due to the difference in %.

[0219] The cutting strength of the formed product obtained by the SAICAS method is preferably 1.10 kN / m or more, and more preferably 1.20 kN / m or more. Note that the higher the cutting strength, the better, but the upper limit is substantially 1.30 kN / m.

[0220] In addition, the ratio of the portion in which the interfacial separation state is formed is preferably 15% or less, and more preferably 10% or less, with respect to the cutting area of the stretched portion of the formed product obtained by the SAICAS method. Note that the smaller the ratio of the portion in which the interfacial separation state is formed, the better, and the lower limit is 0%.

[0221] [Method for measuring cutting strength and separation form by SAICAS method]

[0222] The cutting strength and separation form of the formed product in question obtained by the SAICAS method using the pre-coated plated steel sheet were measured as follows.

[0223] First, with respect to the formed product in question, three or more flat portions 3 of the non-formed processed portion were specified, and with respect to each flat portion, the total thickness (plated steel sheet including the substrate, coating film on the front and back surfaces) was measured three times using a micrometer, and the average value was calculated. This measurement was performed at multiple specified portions, and the average value between each portion was further calculated. The average value between the multiple portions thus obtained was set as the thickness d of the pre-coated plated steel sheet before forming in the formed product in question.

[0224] In addition, a measurement sample (approximately 20 mm x 20 mm or more in size) was cut out from a portion considered to be subjected to various forming processes such as unidirectional stretching processing, and was smoothed by a steel sheet straightener (straightening machine). With respect to the measurement sample thus obtained, the total thickness (plated steel sheet including the substrate, coating film on the front and back surfaces) was measured using a micrometer, and based on the measurement value thus obtained and the thickness of the pre-coated plated steel sheet before forming as described above, the reduction ratio was calculated. The portion in which the reduction ratio thus obtained showed a value of 20% or more was set as the stretched portion of the formed product.

[0225] With respect to the stretched portion thus specified, the cutting strength and separation form of the coating film in the interface between the coating film layer and the layer on the steel sheet side of the coating film layer were simultaneously cut and measured using a measurement device capable of the SAICAS method (for example, DN-GS manufactured by DAIPLA WINTESCO., LTD.).

[0226] In addition, even with an unprocessed pre-coated plated steel sheet, by stretch working, a portion having a thickness reduced by 20% or more compared to before forming can be obtained. A pre-coated plated steel sheet cut into a long strip shape (for example, 40 x 200 mm) is stretched in a uniaxial direction using a tensile testing machine until the maximum stretched portion is stretched by approximately 60%, and a steel sheet around the maximum stretched portion is cut out to a sufficient size (approximately 20 x 20 mm or more). A portion having a thickness reduced by 20% or more compared to before forming (the method of specifying is described later) is specified from among the steel sheet pieces, and the cutting strength and peeling form of the coating film of the portion are measured using the SAICAS method. The cutting direction is set to be perpendicular to the stretching direction of the steel sheet.

[0227] The reduction ratio in thickness of the plated steel sheet of the formed product compared to before forming is measured as follows. The total thickness (including the plated steel sheet of the base plate, the coating film on the front and back surfaces) of each position within the steel sheet piece cut from the formed product is measured using a micrometer, and the reduction ratio from the total thickness (including the plated steel sheet of the base plate, the coating film on the front and back surfaces) of the pre-coated plated steel sheet before forming is calculated. Note that the value of the ratio calculated using this method can be confirmed to show no difference compared to the value calculated from the thickness of the base plate measured in a state where the coating film is removed from the front and back surfaces of the pre-coated plated steel sheet before and after forming using a coating film peeling agent.

[0228] Note that the value of the reduction ratio calculated using the method described above can be confirmed to show no difference compared to the value calculated from the thickness of the base plate measured in a state where the coating film is removed from the front and back surfaces of the pre-coated plated steel sheet before and after forming using a coating film peeling agent.

[0229] The cutting conditions using the SAICAS method, and the measurement method of the ratio of the interfacial peeling form and the cohesive failure form of the cut portion are omitted from detailed description below as previously described.

[0230] The formed product of the present embodiment is described in detail below with reference to Figure 5 Figure 5 Figure 5 .

[0231] As described above, according to the embodiment of the present application, by using the pre-coated plated steel sheet of the embodiment of the present application, even if a Zn-Al-Mg alloy plated steel sheet is used as the original sheet, a formed product in which the coating film does not crack even if 0T bend working is performed, the coating film of the 0T bend working portion does not peel due to heating or over time, and the corrosion resistance with respect to undercoating corrosion of the 0T bend working portion and the surrounding coating film is also excellent can be obtained.

[0232] Example

[0233] The pre-coated plated steel sheet and shaped article of the present application are specifically described below while showing examples and comparative examples. Note that the examples shown below are only one example of the pre-coated plated steel sheet and shaped article of the present application, and the pre-coated plated steel sheet and shaped article of the present application are not limited to the following examples.

[0234] Test Example 1

[0235] (1. Plated steel sheet used as a base sheet)

[0236] As a base sheet of a pre-coated steel sheet, the following four kinds of zinc-based plated steel sheets were used. The plated layer of the following zinc-based plated steel sheets was subjected to 5-second spray treatment using a commercially available original sodium silicate-based alkali cleaning solution, and was subjected to water washing, drying.

[0237] G-A: Zn-11% Al-3% Mg-0.2% Si molten zinc alloy plated steel sheet (sheet thickness 0.60 mm, plated adhesion amount 40 g / m 2 )

[0238] G-B: Zn-1% Al-1% Mg molten zinc alloy plated steel sheet (sheet thickness 0.60 mm, plated adhesion amount 40 g / m 2 )

[0239] G-C: Zn-6% Al-3% Mg molten zinc alloy plated steel sheet (sheet thickness 0.6 mm, plated adhesion amount 40 g / m 2 )

[0240] G-D: Zn-55% Al-2% Mg-1.6% Si molten zinc alloy plated steel sheet (sheet thickness 0.35 mm, plated adhesion amount 75 g / m 2 )

[0241] (2. Film formation of chemical conversion treatment coating film layer)

[0242] The following were used as a coating composition for forming a chemical conversion treatment coating film layer. Note that the addition amount of each component in each coating composition was adjusted in a range within the previously described addition amount.

[0243] S1: water-based coating composition containing tannic acid, silane coupling agent, fine particles of silicon dioxide, polyester resin

[0244] S2: water-based coating composition containing silane coupling agent, phosphate, acrylic resin

[0245] S3: water-based coating composition containing silane coupling agent, fluorotitanic acid, fluorozirconic acid, polyurethane resin

[0246] The coating compositions of S1 to S3 described above were bar coated on the plated steel sheet for the pre-coated steel sheet in such a manner that the dry film thickness of the prescribed dry film thickness was formed, and then dried and air-dried at a temperature of 70°C on the metal surface using a hot air oven.

[0247] (3-1. Film formation of the primer coating film layer)

[0248] As the coating composition for forming the primer coating film, primer paints in which the kind and compounding ratio of the polyester-based resin, the curing agent, and the rust-preventive pigment shown in Table 1-1 and Table 1-2 were variously changed were prepared.

[0249] < Polyester-based resin >

[0250] Polyester resin (number average molecular weight: 6000, Tg: 50°C)

[0251] Polyester resin (number average molecular weight: 8000, Tg: 5°C)

[0252] Polyester resin (number average molecular weight: 15000, Tg: 5°C)

[0253] Polyester resin (number average molecular weight: 19000, Tg: 5°C)

[0254] Polyester resin (number average molecular weight: 20000, Tg: 15°C)

[0255] Polyester resin (number average molecular weight: 20000, Tg: 35°C)

[0256] Polyester resin (number average molecular weight: 23000, Tg: 5°C)

[0257] Polyester resin (number average molecular weight: 25000, Tg: 5°C)

[0258] Polyester resin (number average molecular weight: 30000, Tg: 5°C)

[0259] < Curing agent >

[0260] BI-A: MEK oxime-terminated HDI

[0261] BI-B: epsilon-caprolactam-terminated MDI

[0262] MF-A: methylated melamine

[0263] MF-B: butylated melamine

[0264] < Rust-preventive pigment >

[0265] Compounded product of equal mass of calcium ion-exchanged silica and aluminum tripolyphosphate

[0266] After the above-mentioned chemical conversion treatment was carried out, the primer coating having the components shown in Table 1-1, Table 1-2 below was bar-coated on the plated steel sheet in such a manner as to form a dried thickness shown in Table 1-1, Table 1-2, and dried at a temperature of 215°C on the metal surface using a hot air furnace.

[0267] (3-2. Film formation of the upper coating film layer)

[0268] As a coating composition for film formation of the upper coating film layer, the following was used.

[0269] T-A: High molecular polyester / melamine resin curing system composition (FLC7000 manufactured by Nippon Paint Co., Ltd.)

[0270] T-B: Polyester / melamine resin curing system composition (FLC7100 manufactured by Nippon Paint Co., Ltd.)

[0271] After the above-mentioned T-A or T-B was bar-coated on the upper layer on which the above-mentioned primer coating film was formed in such a manner as to form a total thickness at the time of drying shown in Table 1-1, Table 1-2, drying was carried out at a temperature of 230°C on the metal surface using a hot air furnace.

[0272] (4-1. State of the unprocessed coating film)

[0273] The state of the coating film of the pre-coated plated steel sheet produced by the above-mentioned method was confirmed by SAICAS and indenter insertion.

[0274] (1) SAICAS

[0275] The interface of the chemical conversion treatment coating film layer and the primer coating film layer, or the interface of the chemical conversion treatment coating film layer and the plated layer of the pre-coated plated steel sheet was cut using the SAICAS method, and the cutting strength and the peeling form were measured. Note that the device used in the SAICAS method was DN-GS manufactured by DAIPLA WINTES CO., LTD.

[0276] < Cutting strength >

[0277] After the oblique cutting is performed at a constant speed mode with a diamond blade (0.3 mm width) at a horizontal speed of 1 μm / sec and a vertical speed of 0.1 μm / sec, the average cutting strength during the horizontal movement alone is measured for a length of 200 μm. The depth position at which the movement is switched to the horizontal movement is set by specifying the interface position (position at which the limit of the plated layer is not cut) using a preliminary experiment. In the case where the plated layer surface is cut due to the unevenness of the plated layer during the horizontal movement of the cutting blade, the cutting strength momentarily increases abnormally, and thus can be distinguished. This case is excluded as an abnormal value, and the average cutting strength is calculated. The number of measurements is set to n = 3, and the average of the three values of the average cutting strength is set as the cutting strength.

[0278] The case where the obtained cutting strength is 1.00 kN / m or more is set as a score "A", and the case where it is less than 1.00 kN / m is set as a score "B".

[0279] <Peeling form>

[0280] An optical microscope photograph of the horizontal cutting range (300 x 200 μm) of the SAICAS is taken, and the positions of the primer cohesion failure, the interface peeling, and the plated layer cohesion failure in the same range are specified using the aforementioned judgment criteria. The areas thereof are measured using transparent grid paper. Next, the ratio of the interface peeling area to the area excluding the cohesion failure of the plated layer from the horizontal cutting range of the SAICAS is calculated.

[0281] The case where the obtained ratio is 5% or less is set as a score "A", and the case where it exceeds 5% is set as a score "B".

[0282] (2) Indenter insertion

[0283] The indenter insertion load and the value equivalent to the elastic strain energy are measured using a four-pyramid type indenter for micro hardness measurement.

[0284] <Indenter insertion load and value equivalent to elastic strain energy>

[0285] In the cross section obtained by cutting the primer coating layer in the thickness direction, the four-pyramid type indenter for micro hardness measurement is inserted at a speed of 2 μm / min from the position 1 μm in the direction of the primer coating layer from the interface of the primer coating layer and the chemical conversion treatment coating layer or the interface of the primer coating layer and the plated layer, and the load at the time of reaching a depth of 10 μm is measured. This value is set as the indenter insertion load. Then, the load is unloaded at a speed of 2 μm / min, and the load curve with respect to displacement at this time is measured. Using the obtained load curve, the area of the range enclosed by the line corresponding to displacement = 10 μm, the line corresponding to load = 0, and the load curve at the time of unloading in the plane defined by displacement and load is calculated, and this is set as the value equivalent to the elastic strain energy.

[0286] (4-2. State of the coated film after forming (compactness))

[0287] As described above, the interface between the chemical conversion treatment coating layer and the primer coating layer, or the interface between the chemical conversion treatment coating layer and the plated layer, of the portion in which 20% or more of the film thickness was reduced (including the portion produced by the uniaxial stretching process) of the pre-coated plated steel sheet subjected to the forming process was cut using the SAICAS method, and the cutting strength and the peeling pattern were measured.

[0288] < Cutting strength >

[0289] After performing the oblique cutting using a diamond blade (0.3 mm width) at a constant speed mode of a horizontal speed of 1 μm / sec and a vertical speed of 0.1 μm / sec, the horizontal movement was switched in the vicinity of the interface, and the average cutting strength during the horizontal movement was measured by cutting at a length of 200 μm. The depth position at which the horizontal movement was switched was set by specifying the interface position (the limit position at which the plated layer was not cut) using a preliminary experiment. In the case where the plated layer surface was cut due to the unevenness of the plated layer in the horizontal movement of the cutting blade, the cutting strength was instantaneously abnormally increased, and thus it could be distinguished. Such a case was excluded as an abnormal value, and the average cutting strength was calculated. The number of measurements was set to n = 3, and the average value of the three values of the average cutting strength was set as the cutting strength.

[0290] The case where the obtained cutting strength was 1.10 kN / m or more was set as the score "Al", the case where it was 1.00 kN / m or more and less than 1.10 kN / m was set as the score "A2", and the case where it was less than 1.00 kN / m was set as the score "B".

[0291] < Peeling pattern >

[0292] An optical microscope photograph of the horizontal cutting range (300 x 200 μm) of the SAICAS was taken, and the positions of the primer cohesive failure, the interface peeling, and the plated layer cohesive failure in the same range were specified using the aforementioned judgment criteria, and the areas thereof were measured using transparent grid paper. Next, the ratio of the interface peeling area to the area excluding the cohesive failure of the plated layer from the horizontal cutting range of the SAICAS was calculated.

[0293] The case where the obtained ratio was 15% or less was set as the score "Al", the case where it was more than 15% and 20% or less was set as the score "A2", and the case where it was more than 20% was set as the score "B".

[0294] (5. Performance evaluation)

[0295] The performance of the portion subjected to the stretching process of the pre-coated plated steel sheet produced using the above-described method was evaluated using the following method.

[0296] (1) 0T bending processed portion

[0297] The pre-coated plated steel sheet was subjected to 0T bending processing at 20°C, and the following evaluations were performed.

[0298] <Presence or absence of cracking>

[0299] The observation was performed using a magnifying glass with a magnification of 10 times, and the case where no cracking was found at all in the coating film was rated as "A1", the case where cracking was found but the number of cracks was 1 or less per 1 cm was rated as "A2", and the case where cracking was found beyond that was rated as "B".

[0300] <Adhesive tape peeling test>

[0301] When an adhesive tape (trade name: CELLOPHANE TAPE (registered trademark)) was attached to the 0T bending processed portion and then peeled off with force, the case where no peeling of the coating film of the pre-coated plated steel sheet was found at all was rated as "A1", the case where peeling of the coating film was found but the peeling area was 5% or less was rated as "A2", and the case where peeling of the coating film was found beyond that was rated as "B".

[0302] <Peeling (blooming) due to boiling water immersion>

[0303] After the pre-coated plated steel sheet subjected to 0T bending processing was immersed in boiling water for 1 hour and then dried at room temperature, the processed portion was observed using a magnifying glass with a magnification of 10 times.

[0304] The case where no cracking was found at all in the coating film or the condition of cracking was not changed at all compared to before the boiling water immersion was rated as "A1", the case where the progress of peeling of the coating film due to cracking or the generation of new cracks in the coating film was found due to the boiling water immersion but the number of cracks was 1 or less per 1 cm was rated as "A2", and the case where the progress of peeling of the coating film due to cracking or the coating film cracking that was not found before the boiling water immersion was found after the boiling water immersion was rated as "B".

[0305] <Salt water spray test (SST) corrosion resistance>

[0306] After the pre-coated plated steel sheet subjected to 0T bending processing was immersed in a salt water spray device for 480 hours with the 0T bending portion facing upward, the following three evaluations were performed.

[0307] ◆Crown rust

[0308] The case where no white rust or red rust was found at all at the curvature portion of the 0T bending portion (referred to as the crown) was rated as "A1", the case where white rust or red rust was found but the generation area was 5% or less was rated as "A2", and the case where white rust or red rust was found beyond that was rated as "B".

[0309] ◆Crown portion expansion

[0310] The case where no coating film expansion was found at the curvature portion of the 0T bend portion (referred to as the crown portion) was rated "Al", the case where coating film expansion was found, but the area of the peeling expansion was within 5% was rated "A2", and the case where coating film expansion was found beyond that was rated "B".

[0311] ◆Bend ~ flat portion expansion

[0312] The coating film expansion from the curvature portion of the 0T bend portion (referred to as the crown portion) toward the flat portion was observed using a magnifying glass with a magnification of 10 times. The case where the maximum expansion width of the flat portion was within 0.2 mm was rated "Al", the case where it was more than 0.2 mm and within 0.5 mm was rated "A2", and the case where it was more than 0.5 mm was rated "B".

[0313] (2) Erichsen 7 mm extrusion processed portion

[0314] The pre-coated plated steel sheet was subjected to Erichsen 7 mm extrusion processing at 20°C, and the following evaluations were performed.

[0315] <Presence or absence of cracking>

[0316] The Erichsen processed portion was observed using a magnifying glass with a magnification of 10 times, and the case where no coating film cracking was found was rated "Al", the case where coating film cracking was found but the number of cracks was within 10 was rated "A2", and the case where coating film cracking was found beyond that was rated "B".

[0317] <Stripping test>

[0318] When an adhesive tape (trade name: CELLOPHANE TAPE (registered trademark)) was attached to the Erichsen processed portion and then strongly peeled off, the case where no coating film peeling of the pre-coated plated steel sheet was found was rated "Al", the case where coating film peeling was found but the peeling area was within 5% was rated "A2", and the case where coating film peeling was found beyond that was rated "B".

[0319] <Corrosion resistance in salt spray test (SST)>

[0320] After the pre-coated plated steel sheet subjected to Erichsen processing was immersed in a salt spray device for 480 hours in a direction in which the Erichsen processed portion was oriented upward, the following two evaluations were performed.

[0321] ◆Processed portion rust

[0322] The case where no white rust or red rust was found at all at the Erichsen cupping processed portion was rated "Al", the case where white rust or red rust was found, but the area of the rust was 5% or less was rated "A2", and the case where white rust or red rust was found beyond that was rated "B".

[0323] ♦ Processed portion expansion

[0324] The case where no coating film expansion was found at all at the Erichsen cupping processed portion was rated "Al", the case where coating film expansion was found, but the area of the expansion was 5% or less was rated "A2", and the case where coating film expansion was found beyond that was rated "B".

[0325] Note that the 0T bending processing and the evaluation thereof were based on the method described in JIS K5400. In addition, the Erichsen cupping 7 mm extrusion processing and the evaluation thereof were performed using a tester described in JIS B7729, based on the method described in JIS K5400. Further, the salt spray test (SST) was performed based on the method described in JIS Z2371.

[0326] The results obtained are summarized in Table 1-1 and Table 1-2 below.

[0327] [Table 1-1]

[0328]

[0329] [Table 1-2]

[0330]

[0331] The coating film of the 0T bending portion of the example did not have problems such as cracking, peeling, coating film peeling (blooming) due to boiling water immersion, and poor corrosion resistance due to SST, and the like, because the coating film properties obtained by the method of inserting a microhardness measuring pyramid indenter, and the cutting strength and peeling form of the stretched portion of the molded article obtained by SAICAS all satisfied the criteria.

[0332] On the other hand, the comparative example had problems such as the stretched processed portion because the coating film properties obtained by the method of inserting a microhardness measuring pyramid indenter, and the cutting strength and peeling form of the compressed portion of the molded article obtained by SAICAS all did not satisfy the criteria.

[0333] 《Test Example 2》

[0334] Further verification was made focusing on the number of substituents in the blocked isocyanate resin capable of reacting with OH groups in the polyester resin, and the number of OH groups in the polyester resin.

[0335] (1. Plated steel sheet used as a base plate)

[0336] The following base plate among the base plates used in Test Example 1 was used.

[0337] G-A: Zn-11%Al-3%Mg-0.2%Si molten zinc alloy plated steel sheet (plate thickness 0.60 mm, plated adhesion amount 40 g / m 2 )

[0338] (2. Film formation of chemical conversion treatment coating layer)

[0339] The following coating composition among the coating compositions used in Test Example 1 for film formation of the chemical conversion treatment coating layer was used.

[0340] S1: water-based coating composition containing tannic acid, silane coupling agent, fine silica particles, and polyester resin

[0341] (3-1. Film formation of primer coating layer)

[0342] As the coating composition for film formation of the primer coating, a primer paint in which the kind and compounding ratio of the polyester resin, curing agent, and rust-preventive pigment shown in Table 2 were variously changed was prepared.

[0343] < Polyester resin >

[0344] Pes-A: polyester resin (number of OH groups in molecule 2.0)

[0345] Pes-B: polyester resin (number of OH groups in molecule 3.5)

[0346] Pes-C: polyester resin (number of OH groups in molecule 5.0)

[0347] Each of these resins had a number average molecular weight of 23,000 and a Tg of 5°C.

[0348] < Curing agent >

[0349] BI-C: MEK oxime-terminated hydrogenated XDI resin (number of NCO groups in molecule 2.5)

[0350] BI-D: MEK oxime-terminated hydrogenated XDI resin (number of NCO groups in molecule 3.0)

[0351] BI-E: MEK oxime-terminated hydrogenated XDI resin (number of NCO groups in molecule 3.5)

[0352] These curing agents all use hydrogenated xylene diisocyanate as a raw material, and have a number average molecular weight of 1500 to 2000.

[0353] As the melamine derivative, the following melamine derivative was used among the melamine derivatives used in Test Example 1.

[0354] MF-A: methylated melamine

[0355] <Anti-rust pigment>

[0356] A compounded product of the same mass of calcium ion-exchanged silica and aluminum tripolyphosphate as the anti-rust pigment used in Test Example 1 was used.

[0357] A primer coating material having the components shown in Table 2 below was bar-coated onto the plated steel sheet on which the above chemical conversion treatment had been performed, in such a manner as to form a dry thickness shown in Table 2, and dried at a temperature of 215°C on the metal surface using a hot air furnace.

[0358] (3-2. Film formation of the upper coating film layer)

[0359] As the coating composition for film formation of the upper coating film layer, the following coating composition was used among the coating compositions used in Test Example 1.

[0360] T-A: high-molecular polyester / melamine resin curing system composition (FLC7000 manufactured by Nippon Paint Co., Ltd.)

[0361] The above T-A was bar-coated onto the upper layer on which the above primer coating film had been formed, in such a manner as to form a total thickness shown in Table 2 at the time of drying, and dried at a temperature of 230°C on the metal surface using a hot air furnace.

[0362] (4-1. State of the coating film at the time of non-working)

[0363] The state of the coating film of the pre-coated plated steel sheet produced by the above method was confirmed by SAICAS and indenter penetration, as in Test Example 1 above. Note that the evaluation method and evaluation criteria were the same as in Test Example 1 above, and therefore detailed explanations are omitted below.

[0364] (5. Performance evaluation)

[0365] The performance of the stretch worked portion of the pre-coated plated steel sheet produced by the above method was evaluated, as in Test Example 1 above. Note that the evaluation method and evaluation criteria were the same as in Test Example 1 above, and therefore detailed explanations are omitted below.

[0366] The results obtained are summarized in Table 2 below.

[0367] [Table 2]

[0368]

[0369] Examples C to H, since the values of na+nb are in the range of 5.5 to 8.0, i.e. the state of the resin which is in a low Tg, high molecular weight and well stretched and cured with sufficient crosslinking density, and the coating film which can disperse the stress due to deformation of the coating film, the coating film properties obtained by the method of inserting a four-pyramid type indenter for micro hardness measurement, and the cutting strength and peeling form of the stretched portion of the molded article obtained by SAICAS satisfy the criteria, and no cracking, peeling of the coating film of the 0T curved portion, peeling (blooming) of the coating film due to boiling water immersion and poor corrosion resistance due to SST, cracking, peeling of the coating film of the 7 mm processed portion of the Erichsen cupping, and poor corrosion resistance due to SST, etc. problems of the stretched processed portion were found.

[0370] On the contrary, Examples A and B, since the values of na+nb are less than 5.5, the crosslinking is not so much, and thus although the cracking of the coating film due to processing is suppressed, the cohesion of the coating film is low, the cutting strength obtained by SAICAS is slightly low, and as a result, it was found that the processed portion tends to easily cause peeling and reduced corrosion resistance. In addition, Example I, since the values of na+nb exceed 8.0, the crosslinking is a little more, and thus it was found that the cracking of the coating film due to processing tends to increase. It was known that these examples as a result have a tendency that the corrosion resistance of the processed portion is also reduced. In addition, it was known that the stress due to the insertion of the indenter is slightly large, the residual stress of the coating film at the time of processing is increased, and thus there is a tendency that the problem of blooming is not easily suppressed in the stretched processed portion.

[0371] The preferred embodiments of the present application are described in detail with reference to the accompanying drawings, but the present application is not limited to the above examples. It is obvious to those skilled in the art having ordinary knowledge in the technical field to which the present application pertains that various modifications or corrections can be conceived within the scope of the technical idea recited in the claims, and for them, it is understood that they certainly belong to the technical scope of the present application.

[0372] Explanation of Reference Signs

[0373] 10 Zn-Al-Mg alloy plated steel sheet

[0374] 20 pre-coated plated steel sheet

[0375] 30 molded article

[0376] 101 steel sheet

[0377] 103, 201 Zn-Al-Mg alloy plated layer

[0378] 203 chemical conversion coating layer

[0379] 205 coating layer

Claims

1. A pre-coated steel sheet, comprising: a coating layer composed of a Zn-Al-Mg alloy coating layer on one or both sides of the steel sheet, a chemical conversion treatment coating layer on the coating layer, and a film layer on the chemical conversion treatment coating layer. The coating layer has a primer coating layer and an upper coating layer located on the primer coating layer. The coating contains 0.5% by mass and 60.0% by mass of aluminum, 0.5% by mass and 15.0% by mass of magnesium, with the balance being zinc and impurities. The thickness of the primer coating layer is 2–10 μm. The total thickness of the primer coating and the upper coating is 10–30 μm. The primer coating layer comprises: a polyester resin with an average molecular weight of 19,000–25,000 and a glass transition temperature (Tg) below 40°C; and a curing agent. The curing agent contains end-capped isocyanate or end-capped isocyanate resin. The pre-coated steel sheet shall satisfy at least one of the following conditions: condition (a-1) and condition (a-2); condition (b); condition (c). (a-1) Using the SAICAS method, the interface between the coating layer and the layer on the steel plate side of the coating layer was cut and the cutting strength was measured. The average cutting strength was above 1.00 kN / m, and less than 5% of the cutting area was of the interfacial peeling mode, while the remaining cutting area was of the cohesive failure mode within the coating layer. (a-2) When cutting a portion of a pre-coated steel sheet that has undergone uniaxial stretching with a thickness reduced by more than 20% compared to the non-formed portion using the SAICAS method, the cutting strength at the interface between the coating layer and the layer closer to the steel sheet than the coating layer is an average of 1.00 kN / m or more, and less than 20% of the cutting area is in the interface peeling mode, while the remaining cutting area is in the cohesive failure mode within the coating layer. (b) In a cross-section formed by cutting the primer coating layer along its thickness direction, the indentation load when a microhardness testing pyramidal indenter is inserted at a speed of 2 μm per minute to a depth of 10 μm from the interface between the primer coating layer and the layer closer to the steel plate than the primer coating layer along the direction of the primer coating layer is less than 1000 mN. (c) In a cross section formed by cutting the primer coating layer along the thickness direction, for a position 1 μm from the interface between the primer coating layer and the layer closer to the steel plate than the primer coating layer, a microhardness measuring pyramidal indenter is inserted to a depth of 10 μm at a speed of 10 μm per second, and then the indenter is unloaded at a speed of 2 μm per minute. The load curve relative to the displacement at this time is plotted. In a plane defined by displacement and load, the area enclosed by the straight line corresponding to displacement = 10 μm, the straight line corresponding to load = 0, and the load curve at the unloading time is less than 500 μm·mN.

2. The pre-coated steel sheet according to claim 1, wherein, The content of the curing agent, relative to the total content of the polyester resin and the curing agent, is 5% to 15% by mass.

3. The pre-coated steel sheet according to claim 1 or 2, wherein, The curing agent also contains melamine or melamine derivatives.

4. The pre-coated steel sheet according to claim 3, wherein, The content of melamine or melamine derivative is 20-50% by mass, relative to the total content of the terminated isocyanate or terminated isocyanate resin and the melamine or melamine derivative.

5. The pre-coated steel sheet according to any one of claims 1 to 4, wherein, The average molecular weight of the polyester resin is 23,000 to 25,000.

6. The pre-coated steel sheet according to any one of claims 1 to 5, wherein, The glass transition temperature (Tg) of the polyester resin is 0–20 °C.

7. The pre-coated steel sheet according to any one of claims 1 to 6, wherein, When the number of substituents in the terminated isocyanate or terminated isocyanate resin that can react with the OH group in the polyester resin is recorded as na, and the number of OH groups in the polyester resin is recorded as nb, the sum of (na+nb) is 5.5 or more and 8.0 or less.

8. The pre-coated steel sheet according to any one of claims 1 to 7, wherein, The coating is composed of a Zn-11%Al-3%Mg-0.2%Si alloy coating, which also contains Si to replace a portion of the balance Zn.

9. A molded article formed from a pre-coated steel sheet, the pre-coated steel sheet comprising: a plating layer composed of a Zn-Al-Mg alloy coating on one or both sides of the steel sheet, a chemical conversion treatment coating layer on the plating layer, and a coating layer on the chemical conversion treatment coating layer, the coating layer comprising a primer coating layer and an upper coating layer on the primer coating layer. The coating contains 0.5% by mass and 60.0% by mass of aluminum, 0.5% by mass and 15.0% by mass of magnesium, with the balance being zinc and impurities. The thickness of the primer coating layer is 2–10 μm. The total thickness of the primer coating and the upper coating is 10–30 μm. The primer coating layer comprises: a polyester resin with an average molecular weight of 19,000–25,000 and a glass transition temperature (Tg) below 40°C; and a curing agent. The curing agent contains end-capped isocyanate or end-capped isocyanate resin. In the formed article, for the portion of the coated steel sheet whose thickness is reduced by more than 20% compared to the non-formed portion, the cutting strength obtained by cutting the interface between the coating layer and the layer on the steel sheet side of the coating layer using the SAICAS method is 1.00 kN / m or more on average, and less than 20% of the cutting area is in the interface peeling mode, while the remaining cutting area is in the cohesive failure mode within the coating layer.

10. The molded article according to claim 9, wherein, The coating layer in the molded article contains 5% by mass and less than 15% by mass of aluminum, and 2% by mass and less than 4% by mass of magnesium.

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